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Fall 2014

Output limitations to single stage and cascaded 2-2.5μm light Output limitations to single stage and cascaded 2-2.5 m light

emitting diodes emitting diodes

Andrew Ian Hudson University of Iowa

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Part of the Physics Commons

Copyright 2014 Andrew Hudson

This thesis is available at Iowa Research Online: https://ir.uiowa.edu/etd/1468

Recommended Citation Recommended Citation Hudson, Andrew Ian. "Output limitations to single stage and cascaded 2-2.5μm light emitting diodes." MS (Master of Science) thesis, University of Iowa, 2014. https://doi.org/10.17077/etd.y6510eyy

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OUTPUT LIMITATIONS TO SINGLE STAGE AND CASCADED 2-2.5 m LIGHT EMITTING DIODES

by

Andrew Ian Hudson

A thesis submitted in partial fulfillment of the requirements for the Master of

Science degree in Physics in the Graduate College of

The University of Iowa

December 2014

Thesis Supervisor: Professor John Prineas

Copyright by

ANDREW IAN HUDSON

2014

All Rights Reserved

Graduate College The University of Iowa

Iowa City, Iowa

CERTIFICATE OF APPROVAL

_______________________

MASTER’S THESIS

_______________

This is to certify that the Master’s thesis of

Andrew Ian Hudson

has been approved by the Examining Committee for the thesis requirement for the Master of Science degree in Physics at the December 2014 graduation.

Thesis Committee: ________________________________ John P. Prineas, Thesis Supervisor

______________________________ Thomas F. Boggess Jr

________________________________ Mark A. Arnold

To Julie: thank you for your support and encouragement in all things

ii

ACKNOWLEDGEMENTS

I would like to acknowledge the patient guidance of Professor John Prineas. His concern for

the education of his research assistants is always appreciated, as are his standards for student

research. The time I have spent as an investigator for his group has been a fascinating and

educational experience which has prepared me for future employment.

I would like to thank Professor Thomas Boggess for the privilege of working in his

laboratory. The characterization facility administered by his research group has yielded data

crucial to this thesis. I would also like to thank both Professor Boggess and Professor Mark

Arnold for generously agreeing to serve on my thesis committee.

I would also like to express my gratitude to my fellow student researchers. Asli Yilderim

generously provided assistance concerning the use of the carrier lifetime experiment and the

processing of lifetime data. Russell Ricker patiently offered guidance concerning the use of the

Boggess Lab characterization facility, in addition to valuable training in processing methods

such as flip chipping. He also provided other resources such as the device traveler sheets found

in Appendix F. Sydney Provence supplied tutorial guidance concerning the use of the MBE

facility, as well as information concerning the growth sample preparation steps. Kailing Zhang

also generously offered instruction concerning the MBE facility.

I would finally like to thank Professor Mary Hall-Reno, Christine Stevens, Heather Mineart

and Jeanne Mullen, who have given so much useful guidance during my career at the University

of Iowa. I am grateful for everything they do.

iii

ABSTRACT

Since the advent of precise semiconductor engineering techniques in the 1960s,

considerable effort has been devoted both in academia and private industry to the fabrication

and testing of complex structures. In addition to other techniques, molecular beam epitaxy

(MBE) has made it possible to create devices with single mono-layer accuracy. This facilitates

the design of precise band structures and the selection of specific spectroscopic properties for

light source materials.

The applications of such engineered structures have made solid state devices common

commercial quantities. These applications include solid state lasers, light emitting diodes and

light sensors. Band gap engineering has been used to design emitters for many wavelength

bands, including the short wavelength (SWIR) infrared region which ranges from 1.5 to 2.5m

[1]. Practical devices include sensors operating in the 2-2.5m range. When designing such a

device, necessary concerns include the required bias voltage, operating current, input

impedance and especially for emitters, the wall-plug efficiency. Three types of engineered

structures are considered in this thesis. These include GaInAsSb quaternary alloy bulk active

regions, GaInAsSb multiple quantum well devices (MQW) and GaInAsSb cascaded light emitting

diodes.

The three structures are evaluated according to specific standards applied to emitters of

infrared light. The spectral profiles are obtained with photo or electro-luminescence, for the

purpose of locating the peak emission wavelength. The peak wavelength for these specimens is

in the 2.2-2.5m window. The emission efficiency is determined by employing three empirical

techniques: current/voltage (IV), radiance/current (LI), and carrier lifetime measurements. The

first verifies that the structure has the correct electrical properties, by

measuring among other parameters the activation voltage. The second is used to determine the

energy efficiency of the device, including the wall-plug and quantum efficiencies. The last

provides estimates of the relative magnitude of the Shockley Read Hall, radiative and Auger

iv

coefficients. These constants illustrate the overall radiative efficiency of the material, by noting

comparisons between radiative and non-radiative recombination rates.

v

TABLE OF CONTENTS

LIST OF TABLES .................................................................................................................... ix LIST OF FIGURES ................................................................................................................... x CHAPTER

1. SEMICONDUCTOR STRUCTURES ............................................................................ 1 1.1 Band Structure ................................................................................................ 1

1.1.1 Kronig-Penney Model ............................................................................ 1 1.1.2 k

.p Method ............................................................................................. 3

1.2 Carrier Dynamics ............................................................................................ 5 1.3 Conduction and Valence Bands ...................................................................... 6 1.4 pn Junctions .................................................................................................... 7 1.5 Schottky Contacts and Barriers .................................................................... 12 1.6 Electroluminescent Diodes ........................................................................... 16 1.7 Molecular Beam Epitaxy ............................................................................... 17 1.8 Engineered Semiconductor Structures......................................................... 19

2. CARRIER LIFETIME THEORY .................................................................................. 23

2.1 Recombination Mechanisms ....................................................................... 23 2.2 Recombination Rate .................................................................................... 25 2.3 Empirical Determination of Recombination Rate ........................................ 27 2.4 Background Carrier Density and Recombination Coefficients ..................... 31

3. PREPARATION OF RESEARCH SAMPLES ............................................................... 33

3.1 LED Structures .............................................................................................. 33 3.2 Sample Pre-Processing ................................................................................. 33 3.3 Sample Growth, IAG 300 Series .................................................................... 34 3.4 Device Processing, IAG 300 Series ................................................................ 36 3.5 Device Preparation, IAG 300 Series .............................................................. 36

4. IV AND LI TEST SYSTEMS ...................................................................................... 38

4.1 MQW Test Lab .............................................................................................. 38 4.2 Single Stage and Cascaded LED Test Lab ...................................................... 41 4.2.1 IV Test .................................................................................................. 41 4.2.2 EL Test ................................................................................................. 42 4.2.3 LI Test ................................................................................................... 44

vi

5. CARRIER LIFETIME MEASUREMENT SYSTEM ....................................................... 46

5.1 Photoluminescence ...................................................................................... 46 5.2 Time Resolved Photoluminescence Measurement System ......................... 47 5.3 Peak Carrier Density Calculations ................................................................. 53

6. EXPERIMENTAL RESULTS ..................................................................................... 55

6.1 Test Devices and Mesa Variability ................................................................ 55 6.2 IV Tests ......................................................................................................... 57

6.2.1 Activation Voltage ................................................................................ 57 6.2.2 Dependence of Current Density on Mesa Size .................................... 57 6.3 LI Tests .......................................................................................................... 59 6.3.1 Radiance Profile Characteristics........................................................... 59 6.3.2 Wall-plug Efficiency .............................................................................. 63 6.3.3 Quantum Efficiency .............................................................................. 68 6.3.4 Device Heating ..................................................................................... 69 6.4 Recombination Coefficient Results .............................................................. 71 6.4.1 e-2 Spot Size Estimates .......................................................................... 71 6.4.2 System Impulse Response and Carrier Lifetimes ................................. 72 6.4.3 Recombination Rate Coefficients for IAG 337 ..................................... 75 6.4.4 Recombination Rate Error Analysis ..................................................... 77 6.4.5 Optimal Carrier Density for Radiative Output ..................................... 77

7. CONCLUSIONS ...................................................................................................... 82

7.1 Principal Findings .......................................................................................... 82

7.2 Avenues for Future Research ........................................................................ 83 7.3 Final Thoughts ............................................................................................... 84 APPENDIX A. ADDITIONAL SINGLE STAGE AND CASCADED LED PERFORMANCE ANALYSIS ..... 85

A.1 Temperature Dependence of the IAG 300 Series Performance ................... 85 A.1.1 Activation Voltage ................................................................................ 85 A.1.2 Radiant Output ..................................................................................... 87 A.2 Leakage Currents ........................................................................................... 89 B. IA2300 DEVICE SERIES, SINGLE STAGE AND MQW PERFORMANCES .................................................................................................. 92 B.1 Spectral Output of IA2300 Series Bulk and MQW Devices .......................... 92 B.2 Bulk and MQW Comparisons for the IA2300 Series .................................... 94 C. SYSTEM NOISE REDUCTION .................................................................................. 98

vii

C.1 General Noise Reduction ............................................................................... 98 C.2 Operation of the MQW Test Lab Amplifier .................................................... 99 C.3 Dark Currents ............................................................................................... 101 C.4 Data Averaging for the Carrier Lifetime Measurement System .................. 102 C.5 Vibrations and Optical Systems ................................................................... 103 C.6 Data Smoothing ............................................................................................ 105

D. FOURIER ANALYSIS AND DATA FILTERING ......................................................... 106 D.1 Continuous Time Fourier Series .................................................................. 106 D.2 Continuous Time Fourier Transform ........................................................... 107 D.3 Discrete Time Fourier Series ....................................................................... 108 D.4 Fast Fourier Transform (FFT) ....................................................................... 109 D.5 Data Filtering ............................................................................................... 109 E. CONVOLUTION ................................................................................................... 113 E.1 Continuous Time Convolution ..................................................................... 113 E.2 Continuous Time Impulse Response ........................................................... 113 F. IAG 300 SERIES DEVICE PROCESSING TRAVELER DOCUMENTS ......................... 115

REFERENCES .................................................................................................................... 119

viii

LIST OF TABLES

Table

6.1 Current Densities for 2.5m Single Stage Device 100 m by 100 m

Mesas .................................................................................................................. 80

6.2 Optimal Carrier Density Predictions .................................................................... 80

C.1 Current Amplifier Pulse Transients .................................................................... 100

ix

LIST OF FIGURES

Figure

1.1 One Dimensional Periodic Array of Atoms ............................................................ 1

1.2 Periodic Potential Approximation ......................................................................... 2

1.3 Extended Zone Band Structure ............................................................................. 3

1.4 Reduced Zone Band Structure ............................................................................... 4

1.5 Photon Induced Direct Transition ......................................................................... 7

1.6 Fermi Level ............................................................................................................ 8

1.7 Effect of Doping on Fermi Level ............................................................................ 9

1.8 pn Junction Band Structure ................................................................................. 10

1.9 Forward Biased pn Junction ................................................................................ 11

1.10 Ideal pn Junction IV Profile .................................................................................. 11

1.11 Metallic Energy Band Diagram ............................................................................ 12

1.12 n-Type Semiconductor Energy Band Diagram..................................................... 13

1.13 MS Diode with Schottky Barrier .......................................................................... 13

1.14 Forward Biased MS Diode ................................................................................... 14

1.15 Forward Biased MS Diode Interface .................................................................... 15

1.16 Reverse Biased MS Diode Interface .................................................................... 16

1.17 MBE Growth System ............................................................................................ 18

1.18 RHEED System ..................................................................................................... 18

1.19 Type I and Type II MQW Band Structures ........................................................... 20

1.20 Superlattice with Minibands .............................................................................. 21

1.21 Potential Diagram of a Cascaded LED Tunnel Junction ....................................... 22

2.1 Recombination Mechanisms ............................................................................... 24

2.2 Temporally Resolved Photoluminescence Profile ............................................... 25

x

2.3 Theoretical Recombination Rate Plot for Quaternary GaInAsSb Alloy Material 27

2.4 Peak Pulse Carrier Density/Peak PL Plot with Linear Fit ..................................... 28

2.5 Determination of Peak Carrier Lifetime .............................................................. 29

2.6 Determination of Temporally Resolved Peak PL Signal ........................................ 30

3.1 Stack Diagrams for Bulk and Cascaded LED Devices ............................................ 35

3.2 LED Flip Chipping to SVSM Header ....................................................................... 37

4.1 IV and LI Experiment Schematic ........................................................................... 39

4.2 IV and LI Test Station ............................................................................................ 40

4.3 Radiance Plot for a Quantum Well Device Mesa .................................................. 41

4.4 IV Experiment Schematic ..................................................................................... .42

4.5 EL Experiment Schematic ...................................................................................... 43

4.6 Cryostat, Nicolet and MCT-10 Detector Configuration for EL Test ...................... 43

4.7 LI Experiment Schematic....................................................................................... 45

4.8 Cryostat and MCT-10 Detector Configuration for LI Test ..................................... 45

5.1 Generation of PL Photons ..................................................................................... 47

5.2 Tsunami Model 3960C Femtosecond Configuration ............................................ 48

5.3 Wavelength Spectrum of a Tsunami Ti: Sapphire Pulse ....................................... 49

5.4 PL Signal and Chopped Pulse Noise ...................................................................... 51

5.5 Lifetime Measurement System ............................................................................ 52

5.6 Lifetime Measurement System Schematic ........................................................... 52

5.7 Gaussian Fit for Beam Profile Convolution ........................................................... 54

6.1 IV Profiles for Device Mesas ................................................................................. 58

6.2 LI Plots for Multiple Mesa Sizes ............................................................................ 61

6.3 Carrier Recombination Mechanisms .................................................................... 62

6.4 Radiance and Input Power.................................................................................... 63

6.5 WE for 2.5 m Single Stage Device ...................................................................... 64

xi

6.6 WE for Cascaded LED ............................................................................................ 65

6.7 WE for 100 m by 100 m Mesas ........................................................................ 65

6.8 WE as a Function of Current Density .................................................................... 66

6.9 WE Efficiency for the 500 nm Single Stage Device ............................................... 67

6.10 QE as a Function of Input Power .......................................................................... 69

6.11 Dependence of Radiant Output on Current Duty Cycle at 85 K ........................... 70

6.12 Dependence of Radiant Output on Pulse Width for a Cascaded LED

400 m by 400 m Mesa at 85 K ......................................................................... 71

6.13 Beam Profile Convolutions and Gaussian Data Fits ............................................. 73

6.14 Lifetime System Impulse Response with Gaussian Fit ......................................... 74

6.15 Model of Original and Convolved Peak Carrier Density Recombination Rates ... 75

6.16 Recombination Rate/Carrier Density Quadratic Fit ............................................. 76

6.17 QE for IAG 337 Using Fit Coefficients of Fig. 6.16 ................................................ 78

A.1 IV Temperature Dependence for the 2.5m Single Stage Emitter ...................... 85

A.2 Temperature Dependence of Current Activation for the 2.5 m Single Stage

Emitter .................................................................................................................. 86

A.3 Temperature Dependence of EL Spectra for a 400 m by 400 m Mesa ........... 87

A.4 Radiant Output Temperature Dependence for a 2.5 m Single Stage Device .... 88

A.5 Quantum Efficiency Temperature Dependence for a 200 m by 200 m Mesa,

2.5 m Single Stage Device ...................................................................................... 88

A.6 Temperature Dependent Radiant Output of Cascaded LED ................................. 89

A.7 IV Profiles for 500 nm Single Stage Device, 200 s Current Pulse, 1% Duty Cycle 90

B.1 PL Spectra of Specimens IA 2344 and IA 2363 at 293 K ........................................ 92

B.2 Stack Diagram for Single Stage and MQW IA 2300 Series LEDs ............................ 93

B.3 Radiant Output of IA 2300 Series Single Stage Devices at 293 K .......................... 96

B.4 Radiant Output of IA 2300 Series QW Devices at 293 K ....................................... 97

C.1 Square Pulse Profile of Amplifier Output, 2 % Duty Cycle .................................. 100

xii

C.2 Photograph of Square Pulse Profile .................................................................... 100

C.3 FGA20 Dark Current Plot ..................................................................................... 101

C.4 Polynomial Fit for Dark Current Compensation in Data ...................................... 102

C.5 Signal to Noise for Signals Collected by a Tektronix TDS5032B Oscilloscope ..... 104

C.6 Vibration Test Carrier Recombination Data ........................................................ 104

C.7 Effect of Smoothing on Random Noise ............................................................... 105

D.1 Temporally Resolved PL Profiles with Two FFT Selection Windows ................... 110

D.2 FFT Spectrum of Data in Fig. D.1 ......................................................................... 110

D.3 80 MHz Noise in Fig. D.2 FFT Spectrum ............................................................... 111

D.4 Modified FFT Spectrum for Data in Fig. D.3 ........................................................ 111

D.5 Filtered PL Signal ................................................................................................. 112

xiii

1

CHAPTER 1

SEMICONDUCTOR STRUCTURES

The majority of solid state devices in use today as emitters and sensors of

electromagnetic radiation are fabricated from a category of materials called

semiconductors. The constituent atoms are bonded into a repeating structure called a

crystal. The periodic arrangement of the atoms in this crystal determines the material

band structure. This band structure is the collection of accessible energy states for

carriers of electric charge within the material.

1.1 Band Structure

To understand the physical origin of a crystalline semiconductor band structure it is

preferable to see how it is derived from first principles. Two common approaches are

the Kronig-Penney model and the k.p method. The latter is more general, and more

easily applicable to multidimensional materials.

1.1.1Kronig-Penney Model

The simplest example is a one dimensional arrangement of identical atoms. The

derivation that follows is the Kronig-Penney model. Assume that the atoms are spaced

uniformly, as in Fig. 1.1. One can approximate the atomic array as a series of potential

wells as in Fig. 1.2.

Figure 1.1: One Dimensional Periodic Array of Atoms

2

Figure 1.2: Periodic Potential Approximation

Here “a” and “b” represent the widths of the barrier and the well, and V is the potential

depth. If one assumes a static system, then it can be represented by Eq. 1.1.1, the time

independent Schrodinger equation. The solution to this is a modulated free particle

wavefunction with wavenumber k and energy eigenvalue

(1.1.1)

(1.1.2)

Eq. 1.1.2 is called the Bloch function. The crystal is assumed in this simplified case to be

infinite, so the wavenumber k must be a real number to keep well behaved. Both

the atomic potential V(x) and the function are assumed to be periodic over

integer n atomic separations, or

(1.1.3)

(1.1.4)

and its first derivative are continuous and periodic. These boundary conditions

when applied to the wavefunction yield four equations, which can be combined to yield

the transcendental equation,

(1.1.5)

3

for which – and

(1.1.6)

and and are the potential barrier and well widths. The solutions Eq. 1.1.5 for specific

wavenumber and energy eigenvalues constitute the “band structure”, the collection of

permissible particle states. The values that do not satisfy Eq. 1.1.5 are the inaccessible

states, or the “band gaps.” The band structure appears in Fig. 1.3.

Figure 1.3: Extended Zone Band Structure

This model can be extended to two and three dimensions, to give the band structures

of multidimensional pure crystals. The spacial periodicity extends into the frequency

domain above as well. All of the bands may be folded back into a zone centered on the

origin and of width 2/(a+b), referred to as the first Brillouin zone. This is called the

“reduced zone” representation, as illustrated in Fig. 1.4.

1.1.2 k.p Method

A second and more generally applicable approach for deriving band structure is

accomplished through what is called the k.p method. This assumes the presence of a

4

periodic potential and the applicability of the Bloch function. When Eq. 1.1.2 is inserted

into the time independent Schrodinger Equation, an equation for the periodic function

can be derived,

(1.1.7)

Here, is the particle momentum and is the mass.

Figure 1.4: Reduced Zone Band Structure

The solutions to Eq. 1.1.7 form a complete, orthonormal set of basis functions. When

the solutions and are known at the Brillouin zone center, one can treat ħk2/2m

as a perturbation for small k values, using non-degenerate or degenerate perturbation

theory. This approach can be used to derive first and second order corrections to the

zone center band gap energy . This technique is referred to as the k.p method. It can

be used to derive the band structure near zone center, but if a number of band energies

5

are spectroscopically measured for a semiconductor specimen, then this approach can

be expanded to derive extended band structure portions.

1.2 Carrier Dynamics

An electron in any kind of material is a quantum mechanical object which is

described in terms of probabilities. Consider a wavefunction (x) for a generic particle

moving in one dimension. | (x)|2 represents a probability per unit length. Here

| (x)|2 = (x)* (x), where (x)* is the complex conjugate of the original wavefunction.

Integrating this product between two locations gives the probability P of finding the

particle in that interval,

(1.2.1)

Just as a beat frequency represents a superposition of distinct frequencies in classical

physics, in quantum mechanics a particle moving in a semiconductor lattice can be

represented as an integral sum of distinct wavefunctions. This sum may be written as

the Fourier integral, or transform, of frequency domain Bloch states.

(1.2.2)

As for any wave phenomenon, this superposition of Bloch states has a group velocity,

(1.2.3)

where represents the angular frequency of one Bloch component of f(x). Applying

Planck’s formula for the energy quantum,

E = ħ (1.2.4)

one can write the group velocity of a particle as

6

ħ

(1.2.5)

Assuming the application of Newton’s second law,

(1.2.6)

and the group velocity relationship,

(1.2.7)

one can derive a formula for the mass of the wave packet,

(1.2.8)

Equation 1.2.8 illustrates that the particle behaves in the lattice as if it has a mass

determined by the band structure. This is called the “effective mass.” For many band

structures, the effective mass is a constant quantity for states near the band edges and

the center of the first Brillouin zone. These band regions are referred to as “parabolic

regions”, because there the band structure can be fit with a second order polynomial,

which has a constant second derivative. This derivation for Eq. 1.2.8 is that presented by

Pierret [2].

1.3 Conduction and Valence Bands

A semiconductor such as that described in Fig. 1.5 is referred to as a “direct”

semiconductor. In the reduced zone scheme, electrons can be promoted from the peak

of the top occupied band to the trough of the first vacant band by absorbing a photon.

Photons carry little momentum, have very small k values, and essentially can’t change

the momentum of a carrier. In other words, for photon induced transitions, the k value

of the carrier must remain constant. From Eq. 1.2.5, it can be seen that if one adds the

7

group velocities of the carriers in a filled band, the sum is zero. Carriers with equal and

oppositely signed k values will have velocity contributions which cancel. The nearly full

band in Fig.1.5 behaves physically like a single positively charged carrier with a group

velocity equal in magnitude but opposite in direction to that of the electron which had

occupied that band location. This is referred to as a “hole”. The single electron in the

nearly empty band also has a group velocity. One can describe the behavior of a

semiconductor with excited electrons as a host material with of two kinds of carriers,

electrons and holes. The band which accepts the excited electrons is called the

“conduction” band, and the level from which the electrons are removed is the “valence”

band.

Figure 1.5: Photon Induced Direct Transition

1.4 pn Junctions

At equilibrium, a material can be characterized by a Fermi level, and at absolute

zero charge carriers will occupy energies below or at the Fermi level. The Fermi level can

be defined as an energy state which has a 50% occupation probability at all

8

temperatures. However, in semiconductors, the Fermi level will often lie in the material

band gap, the energy divide between the valence and the conduction bands. The

carriers are forbidden to occupy energies within the band gap, so here the Fermi level

does not represent an actual carrier energy state, but a quantity which can be used to

characterize the distribution of the carrier energy states.

Figure 1.6: Fermi Level

Raising the temperature above absolute zero thermally excites electrons above the

Fermi level. Atomic vibrations in semiconductors are also quantized, and are referred to

as “phonons”. Unlike photons, phonons have non-zero momentum, so the electron

transitions above the Fermi level may be either direct or indirect, depending on the

circumstances. Raising the temperature also shifts the Fermi level [3],

(1.4.1)

9

Here, Ef is the Fermi level, Ec is the conduction band minimum, Ev is the valence band

maximum, is the effective mass of the valence band hole, and

is the effective

mass of the conduction band electron. Note that the semiconductor energy states are

assumed to be in a state of equilibrium. The semiconductor is also assumed to be un-

doped, to have equal concentrations of electrons and holes. Such a semiconductor is

also called intrinsic.

In the case of intrinsic semiconductors, the Fermi level will lie near the band gap

center. Adding dopants to a semiconductor will change the Fermi level dramatically.

Consider a crystal of pure silicon. If a phosphorus atom is added to it, the phosphorus

contains one electron which will not be covalently bonded to the nearby atoms. It will

contribute to the electron current. Such a semiconductor is referred to as “n-type”. If an

atom of gallium is added to silicon, it contains one less electron than the other atoms,

and behaves as if it has an unbound hole. This kind of semiconductor is referred to as a

“p-type”. Doping semiconductors shifts the position of the Fermi level from the near

center intrinsic value, lowering it for p-type and raising it for n-type.

, n-type (1.4.2)

p-type (1.4.3)

Here, n refers to the concentration of electron donating dopants, p to the concentration

of hole donating dopants, and ni is the concentration of the intrinsic material charges.

Figure 1.7: Effect of Doping on Fermi Level

10

When adjacent layers of oppositely doped material are grown in contact with one

another a pn junction is formed. Electrons migrate into the p-type material, and holes

into the n-type. The result is the formation of a so called “depletion zone”, or a zone of

reduced carrier concentration, due to the recombination of the holes and electrons. At

equilibrium the Fermi level will have a constant value across the spacial expanse of the

device. This is illustrated in Fig. 1.8. The gradient in the band structure generates an

electric field in the depletion region, and hence an internal bias voltage. Forward biasing

the device in Fig. 1.9 reduces the strength of this electric field, which opposes the

motion of both electrons in the conduction band and holes in the valence band. The

electrons will flow from right to left in Fig. 1.9 and must overcome the conduction band

potential barrier to reach the anode, while holes must overcome the valence band

barrier. Note that the junction is not in equilibrium once an external bias is applied. It is

now operating under steady state conditions, and the n-type and the p-type layers have

distinct Fermi levels.

Figure 1.8: pn Junction Band Structure

11

Figure 1.9: Forward Biased pn Junction

When an applied forward bias is equal in magnitude to the built in potential, the carriers

flow freely through the device. The current voltage characteristic (IV) of such an ideal pn

junction is exponential in character, and is described by Eq. 1.4.2.

(1.4.2)

This is Schottky’s ideal diode equation. Io is the reverse saturation current, caused by the

drift of minority carriers from the neutral to the depletion region. The typical profile

appears in Fig. 1.10.

0 2 4 6 8 10 12 140

1x105

2x105

3x105

4x105

Cu

rre

nt

(mA

)

Applied Bias (V)

Figure 1.10: Ideal pn Junction IV Profile

12

1.5 Schottky Contacts and Barriers

A metal-semiconductor (MS) contact is incorporated into a diode device at the

location where a semiconductor material meets a deposited metallic region. The

discussion that follows is presented by Pierret [3]. This metallic deposit usually performs

the function of an electrical anode or cathode contact. The ideal MS contact has three

properties. The metal and semiconductor are in intimate contact on the atomic level,

with no intervening layers between them. This assumes the complete absence of oxide

layers. Also, there is no diffusion of the metallic atoms into the semiconductor. Finally,

there are no absorbed impurities or surface charges at the MS interface.

To illustrate the formation of a MS contact, first consider a metallic interface in the

absence of a semiconductor. Here, EFM is the Fermi energy of the metal and Eo is the

vacuum energy, the minimal energy state a carrier achieves once ionized from the

metal. WM is the work function for the metal, the energy input required to ionize the

carrier. The work function is unique to each metal.

Figure 1.11: Metallic Energy Band Diagram

The generic semiconductor contains Ev and Ec, the valence and conduction bands as

discussed earlier. Ei is the Fermi level for the intrinsic semiconductor, and EFS is the

semiconductor Fermi level for a doped specimen. Figure 1.12 assumes an n-type

specimen, and if the sample were p-type, then EFS would lie below Ei. Ws is the

13

semiconductor work function and is the electron affinity of the semiconductor, the

ionization energy for conduction band carriers. When the metallic and semiconductor

interfaces are brought into contact, electrons will transfer to the metal, creating a

surface depletion region. This will distort the semiconductor valence and conduction

bands, and once the process has reached equilibrium, both materials will share a

common Fermi level. The potential barrier present at the surface depletion region is

called a Schottky barrier, designated below by Ws.

Figure 1.12: n-Type Semiconductor Energy Band Diagram

Figure 1.13: MS Diode with Schottky Barrier

14

Figure 1.13 assumes that the work function of the metal is greater than that of the

semiconductor. Vb is the magnitude of the built in voltage across the MS diode interface

under equilibrium conditions. It represents a potential to be exceeded to drive electrons

past the junction, and can be ideally calculated from Eq. 1.5.1.

Vb =

[Ws - (Ec – EF) ] (1.5.1)

When the device containing this interface is forward or reverse biased, the effect of

the barrier on carrier migration through the interface will either be reduced or

enhanced. The potential and current polarities for forward biasing the MS diode are

illustrated in Fig. 1.14.

Figure 1.14: Forward Biased MS Diode

Here I is the conventional current, with negatively charged electrons actually migrating

primarily in the opposite direction. The band structure will flatten as the device is

forward biased, but the magnitude of the Schottky barrier step will increase due to

enhanced electron migration into the metallic contact. Compared to the unbiased case,

a greater number of electrons will be able to climb the reduced potential barrier Vb.

Dark currents will be reduced due to the presence of the enlarged Schottky barrier. The

result of both these effects is an increased device current. Note that under steady state

forward biasing conditions, the Fermi levels of the metal and the semiconductor once

again become distinct. When the device is reverse biased, the band bending near the

15

interface becomes more extreme, resulting in a reduced device current. Dark currents

persist, but will be negligible compared to the currents operating in the forward biased

device.

The presence of the built in voltage discussed earlier affects device performance

under forward biasing conditions. As will be seen later, the devices considered in this

thesis have n–type metallic interfaces at the cathode and p–type metallic interfaces at

the anode. Each represents a MS diode with an associated built in voltage, and these

will unavoidably reduce current throughput.

Figure 1.15: Forward Biased MS Diode Interface

16

Figure 1.16: Reverse Biased MS Diode Interface

1.6 Electroluminescent Diodes

The samples examined in this thesis have pin junctions. This is simply a layered

structure consisting of a positively doped, intrinsic and negatively doped material.

Doping serves the purpose of creating device layers which contribute to the electrical

contacts. The forward biased potential which needs to be applied to the device injects

electrons into the conduction band at the n-side contact, and holes into the valence

band at the p-side contact. When the electron/hole pairs recombine they radiate light

proportional to the direct band gap energy. This is the basic functionality of a light

emitting diode (LED). The approximate relationship between the bias voltage at which

the current rises above negligible levels and the band gap energy is

V ~ Eg/q (1.6.1)

Here q is the electric charge quantum and Eg is the band gap energy. This bias flattens

the pin band structure so that the carriers move freely, encountering little resistance by

17

the material. This ignores the other factors which serve to increase the bias needed to

activate an LED, such as the presence of Schottky barriers at the metallic contacts.

1.7 Molecular Beam Epitaxy

The growth of semiconductor devices operates at such a level of control that

mono-layers of atoms can be deposited with high levels of precision. Various methods

of such growth, called “epitaxy”, exist. The one utilized for the growth of the devices

evaluated for this thesis is called solid source molecular beam epitaxy. High purity

samples of growth elements are placed in Knudson source cells in both the Gen-20 and

the EPI 930 MBE systems in use at the University of Iowa. A semiconductor substrate is

placed on a target which is radiatively heated. The temperature of the cells and the cell

shutters control the III and V fluxes. Also, the V fluxes have an additional valve which

provides an extra means of control. The heating of the samples in the Knudson cells

releases clusters of atoms. The III flux species then enter the growth chamber, but

before exiting the cell the flux of atomic V complexes (e.g. tetramers) must be “cracked”

into dimers or monomers by a second heat source located near the cell aperture. For V

flux cells, the first heat source is called the base, and the second the cracker. The entire

growth system is maintained under conditions of extreme vacuum (~10-10 Torr) to

ensure high growth purity and to create mean free paths for the molecular fluxes that

exceed the dimensions of the growth chamber. The base temperature controls the rate

of atomic release, and hence the layer growth rate. The substrate temperature

determines the exact parameters of mono-layer formation. The growth rates for specific

III cell temperatures and V cell shutter settings is measured by a reflection high energy

electron diffraction module, or a RHEED system. This consists of an electron source, a

photo-luminescent detection screen and a camera. The electron source directs electrons

towards the semiconductor growth surface, and these strike it at a large angle of

18

Figure 1.17: MBE Growth System

Figure 1.18: RHEED System

19

incidence relative to the surface normal vector. Some of the electrons scatter from the

surface atoms and impact the screen, generating a diffraction pattern which is recorded

by the system camera. A computer program performs a fast Fourier transform of the

time changing pattern, and the dominant spectral peak provides the growth rate

estimate. A second application for the RHEED system is to monitor the desorption

process during which service oxide layers are removed from the growth substrate by

heating.

1.8 Engineered Semiconductor Structures

The growth control provided by techniques such as MBE has enabled physicists and

engineers to prepare extremely precise structures. Three examples are the multiple

quantum well (MQW), the superlattice and the cascaded LED structure. The MQW

structure is created by combining epitaxial layers with different band gaps and band

offsets. The simplest is simply labeled the Type I structure. For this one, the potential

barriers (or wells) in the valence band and the conduction band exist around a common

layer, with the result that both carrier types will be confined to that layer. This is

presented in Fig. 1.19a. If the potential barriers (or wells) for the valence and the

conduction bands are located in different layers, than holes will be localized in one kind

of epilayer, and electrons in the other. This is a Type II structure, and appears in Fig.

1.19b.

If the well widths are comparable to the DeBroglie wavelength of the carriers, then

quantum confinement occurs, a division of the well into energy levels which are

comparable to the classic example of the particle in the infinite potential box. For MQW,

the width of the barrier layers is often designed to minimize the overlap of electron

wavefunctions in adjacent wells.

20

a) Type I

b) Type II

Figure 1.19: Type I and Type II MQW Band Structures

The key distinguishing feature of the superlattice (SL) is that the potential barriers

are reduced in thickness to enable carrier wavefunctions from adjacent wells to spacially

overlap. These interfering wavefunctions ultimately generate what are termed “mini-

bands”, or discrete bands of energy within the quantum well which carriers can occupy.

The SL may be categorized into two types which have characteristics similar to these of

the MQW. In the Type I (T1SL), both carrier species are predominantly confined to the

same epilayer, while in the Type II (T2SL) they are predominantly confined to different

21

ones [4]. The bandgap (and hence the spectral output) can be tuned by adjusting the

layer thicknesses.

Figure 1.20: Superlattice with Minibands

A cascaded LED consists of active regions which are separated by reverse bias pn

junctions. The adjacent active regions are designed to form a downward energy

staircase from the cathode to the anode, as illustrated in Fig. 1.21. To minimize the

spectral width, the active regions are designed to have the same band structure and

bandgap. The tunnel junctions prevent the carriers (both electrons and holes) from

travelling from one emission region to the next until they first undergo radiative

recombination. They then tunnel to the adjacent emission region and emit a second

photon. For N stages, a carrier is recycled N times and emits N photons. This ideally

reduces the necessary driving current compared to a single quantum well device, by 1/N

[5]. It also requires N times the driving voltage of a single stage device.

Several variables can be manipulated to achieve both electron and hole

confinement for the purpose of optimal radiative recombination and the minimization

of junction resistance [6]. One is the use of high doping in the tunnel junctions. Such

high doping will generate the potential barriers required to isolate carriers in

neighboring active regions. The p-side confines the conduction band electrons while the

22

Figure 1.21: Potential Diagram of a Cascaded LED Tunnel Junction[1]

n-side confines the valence band holes. The p and n doped epilayers might not be made

of the same material. To create a junction with minimal tunneling resistance, it has been

demonstrated that the n-side must be chosen with care [6]. Also, a broken gap band

offset at the junction where the n-side conduction band sits below the p-side valence

band ensures a spacially thin tunnel region.

0.5 1.0

-0.5

0.0

0.5

1.0

Ba

nd

Ed

ge

Po

ten

tia

l (V

)

Position (m)

23

CHAPTER 2

CARRIER LIFETIME THEORY

2.1 Recombination Mechanisms

The performance of LED devices described in Chapter 1 is dependent upon the

carrier transitions inside the active regions. When applied to optical emitters and

sensors, the radiative recombination of electrons and holes should be optimized. Carrier

lifetime measurements can be used to evaluate the relative magnitude of the radiative

and parasitic modes for a given active region material, and to determine which of the

two dominates at specific carrier densities.

There are several basic types of carrier recombination processes [2]. Only three will

be considered here.

a) Band to band recombination. An electron in the conduction band directly

combines with a hole in the valence, emitting a photon. This is also called radiative

recombination. The excitation of additional carriers induces faster radiative decay. The

radiative recombination rate is linearly dependent on carrier concentration.

b) Shockley-Reed-Hall (SRH) recombination. Impurity or defect sites permit

recombination to occur non-radiatively. The electron and the hole are attracted to the

impurity/defect site, where they recombine and release energy in the form of lattice

vibrations, or phonons. The impurity/defect accomplishes this by providing a mid-band

site for recombination. This process is dependent on the presence of the

impurities/defects, and is independent of carrier concentration.

c) Auger recombination. Two electrons and one hole (or two holes and one

electron) collide, with the result that one is excited by energy released from the other

two. The excited carrier then thermalizes, losing energy in the form of phonons to the

crystal lattice until it decays back to the conduction band minimum. The Auger

recombination rate depends quadratically on the carrier concentration.

24

Figure 2.1: Recombination Mechanisms

25

0.0 1.0x10-7

2.0x10-7

3.0x10-7

1E-3

0.01

0.1

PL

Sig

na

l (V

)

Time (s)

IAG337 805nm Pump 0.8MHz Rep Rate 77K

Figure 2.2: Temporally Resolved Photoluminescence Profile

2.2 Recombination Rate

The total recombination rate R can be written

(2.1.1)

where , and are the SRH, radiative and Auger recombination rates,

respectively. For a well engineered sensor or emitter, R is approximately equal to Rrad

and the non-radiative processes do little to influence the carrier lifetime. In this

discussion it is assumed that the majority of the sample carriers will be excited by an

incident beam of pulsed laser light, generated by a system similar to that described in

Chapter 5. Assuming that the carriers from dopants and thermal excitation are

negligible in comparison to the optically generated carriers for an optically pulsed

sample,

(2.1.2)

26

where is the dopant or thermally generated carrier density, and is the

optically generated one, then Eq. 2.1.1 may be written as a polynomial,

(2.1.3)

Here is the SRH recombination rate coefficients, which has units of inverse

seconds. is the zero-carrier density intercept, which corresponds to the limit

where the ratio of the excited carrier density to the background density is much less

than unity [7]. In this limit, the radiative and non-radiative mechanisms function

independent of the excited carrier density and make a constant contribution to the

excited carrier recombination rate [8]. This recombination effect can be expanded to

include background carrier density effects, or the non-radiative transitions induced by

unintended dopants, when the condition of Eq. 2.1.2 is not met.

In Eq. 2.1.3, represents the radiative recombination rate which depends

linearly on the carrier density is the radiative recombination coefficient

which has units of 1/(carrier density*second). is the non-radiative

Auger recombination rate and is the Auger recombination coefficient which has

units of 1/([carrier density]2*second).

In III-V compound materials, the magnitude of the bandgap energy is comparable to

the spin orbit coupling energy [9], [10]. Split off valence band holes are generated via

Auger processes with a small momentum transfer and low activation energy [11]. This

leads to an enhanced Auger recombination rate. The minimization of Auger

recombination processes is crucial for the development of efficient emitters. The

incorporation of lattice mismatch and strain between specimen epilayers can be used to

reduce the magnitude of Auger processes [7]. Also, the emission region thickness can be

manipulated to improve performance. Increasing the thickness is a method for reducing

the free carrier density, and hence the magnitude of the quadratic term in Eq. 2.1.3.

Measuring the recombination rate for various carrier concentrations permits

27

the estimation of , and by plotting the recombination rate/carrier

density data and determining the best quadratic fit for it. Figure 2.3 contains a plot of a

theoretical recombination rate. The purpose of using this model was to estimate the

necessary carrier densities (and laser pulse powers) to activate radiative and Auger

recombination effects. The model assumes that the coefficient values are, respectively,

7.7x106 s-1, 5x10-11 cm3/s and 1x10-28 cm6/s [12].

1016

1017

1018

107

108

109

Re

co

mb

ina

tio

n R

ate

(s-1

)

Carrier Density (cm-3)

Radiative: 1015 to 1017cm-3

Auger: above 1017 cm-3

SRH, possibly Auger: 1015 cm-3 and below

Figure 2.3: Theoretical Recombination Rate Plot for a

Quaternary GaInAsSb Alloy Material

The carriers considered in this thesis will be excited primarily by a pulsed Ti:Saph

oscillator configured to generate femtosecond laser pulses. The majority of these pulses

will be removed as the beam passes through an electro-optic modulator. This device is

intended to remove a selected number of pulses from the beam path, and to extend the

time between consecutive pulses incident on the sample. Removing for example 99 out

of every 100 pulses temporally separates the pulses by two orders of magnitude,

ensuring that the excited carriers decay to the valence band before experiencing

28

subsequent excitation. The instrumentation for this part of the experiment is discussed

in Chapter 5.

2.3 Empirical Determination of Recombination Rate

Considerable analysis is required to generate a recombination rate versus carrier

density data plot. The procedure used for this thesis consisted of several steps. In this

analysis it is assumed that pulsed laser light is employed to excite the carriers, and that

photoluminescent output distinct from the incident light pulses is monitored.

1) Calculate the carrier density excited by a laser pulse.

= 4ET/hcd( 1/e2)2 (2.1.4)

Here, is the peak optically excited carrier density, E is the pulse energy, is the

pump line center wavelength, T is the product of the Fresnel transmission coefficient for

the window of the cryostat which contains the specimen and the reflection coefficients

for the collecting and focusing parabolic mirrors (see Chapter 5). x1/e2 is the 1/e2 beam

spot size diameter, h is Planck’s constant, c is the speed of light in vacuum, and d is the

thickness of the specimen active region. The spot size is measured by convolving the

signal with a pinhole at the position to be occupied by the sample. This and other details

about the determination of the pulse power and spot size are discussed in Chapter 5.

The maximum PL is measured at zero time delay for multiple initial carrier densities

and a plot of the peak carrier density versus the peak PL signal is generated. Note that

only the carrier density resulting from a single exciting laser pulse is used. The imperfect

performance of the electro-optic modulator used for this thesis permits low amplitude

fragments from the chopped pulses to excite carriers (see Chapter 5). The repetition

rate of the chopped pulses far exceeds that of the unchopped ones, creating a carrier

background offset. Figure 2.4 is plotted only for the calculated carrier densities

associated with the un-chopped excitation pulses.

29

0.000 0.005 0.010 0.015 0.020 0.0251x10

15

2x1015

3x1015

4x1015

5x1015

6x1015

7x1015

8x1015

9x1015

Pe

ak P

uls

ed

Ca

rrie

r D

en

sity (

cm

-3)

Peak PL (V)

Figure 2.4: Peak Pulse Carrier Density/Peak PL Plot with Linear Fit

Note that ideally the fit should intersect the origin, corresponding to an absence of any

PL signal when there are no optically excited carriers. In reality, it will be non-zero,

corresponding to data scattering resulting from limitations concerning the calculation of

the optically excited carrier density and in the measurement of the PL signal.

2) Determine the carrier recombination rate for a given carrier density. Equation

2.1.5 describes the rate of recombination of the excited carriers.

(2.1.5)

Here is the signal strength in volts. d( )/d(PL) is evaluated from the derivative to

the numerical fit to a plot as in Fig. 2.4, at the peak optically excited carrier density.

is the total carrier density, which might contain contributions due to doping or other

sources. d( )/dt is the rate of change in the PL measurement as in Fig. 2.5. Measuring

the time for the peak PL signal to decay by 1/e yields an estimate for the peak carrier

lifetime.

(2.1.6)

30

1.0x10-7

2.0x10-7

3.0x10-7

1E-3

0.01

0.1

PL

Sig

na

l (V

)

Time (s)

PL

t

Peak Carrier Lifetime ~ t

PLo

PLo/2.72

Figure 2.5: Determination of Peak Carrier Lifetime

d( )/dt, the initial signal decay rate, as illustrated in Fig. 2.6, may be approximated as

the peak PL value divided by the peak carrier lifetime, if the signal obeys an exponential

decay rate.

(2.1.7)

(2.1.8)

This might not be the case for carrier densities in the extreme Auger regime. However, G

is a term resulting from the presence of a carrier density not associated with the optical

pulse. For this research, one such source was the residual pulses mentioned before,

which provide a near constant background carrier density. This will be discussed further

in Chapter 5. Let the offset carrier density associated with this effect be designated by

the symbol ncw. G may be estimated by the ratio of the unchopped pulse carrier

contribution to the peak carrier lifetime,

G = ncw/ (2.1.9)

31

0.0 5.0x10-8

1.0x10-7

1.5x10-7

2.0x10-7

0.00

0.01

0.02

0.03

0.04

Sig

na

l (V

)

Time (s)

IAG 337 840nm Pump 160KHz Rep Rate 77K

PLo

Figure 2.6: Determination of Temporally Resolved Peak PL Signal

3) Once the data is collected for the carrier densities of interest, a recombination rate

versus carrier density plot is generated. A quadratic fit in the form of Eq. 2.1.3

provides constants from which the SRH, radiative and Auger coefficients can be

extracted. This is explained in the next section.

2.4 Background Carrier Density and Recombination Coefficients

Unintended doping during the growth of a semiconductor can add a background

carrier density to an LED active region. This adds a term to the carrier density which

disrupts the simple estimation of the coefficients from a recombination rate versus

carrier density plot.

Rate = ASRH + Brad( nopt + nback) + CAuger( nopt + nback)2 (2.1.10)

Here, nback is the background carrier density. From Eq. 2.1.10 it is evident that the

zeroth order constant of a recombination rate versus carrier density plot will actually be

a combination of all three recombination coefficients. It can also be seen that the first

32

order fit constant will be a combination of the radiative and the Auger coefficients. Let

Rate = A + B nopt + C( nopt)2 (2.1.11)

represent the quadratic fit to an experimentally determined recombination rate versus

optically generated carrier density plot. A, B and C are the data fit coefficients.

Comparing this to Eq. 2.1.10 provides three relationships between the data fit constants

and the actual recombination rate coefficients.

ASRH = A – Brad nback – CAuger( nback)2

(2.1.11)

Brad = B – 2CAuger nback (2.1.12)

CAuger = C (2.1.13)

Only the second order coefficient of the polynomial fit can be directly linked to one

physical process, Auger recombination. The background carrier density must be known

to extract and isolate all three coefficients separately.

33

CHAPTER 3

PREPARATION OF IAG 300 SERIES RESEARCH SAMPLES

3.1 LED Structures

Samples from the IA 2300 and the IAG 300 device series were examined for this

thesis. The IA 2300 series specimens have GaInAsSb active regions which contain double

or triple quantum wells. The devices are grown strained by MBE on an n-doped GaSb

substrate. The samples from the IAG 300 series are quaternary GaInAsSb alloy single

stage and cascaded LED structures. These were grown lattice matched on lightly n-

doped GaSb (100) substrates. The carriers do not penetrate to the substrate, but the

advantage of using n-doped GaSb is that it has a larger transmission coefficient than un-

doped GaSb at the emission wavelength. The IAG 300 series is the primary subject of

this research, but the IA 2300 single stage and quantum well devices are discussed in

Appendix B.

3.2 Sample Pre-Processing

Prior to growth, the samples were cleaned and etched to remove trace chemicals

as well as to thin the oxide layer. The cleaning step consists of five minutes of an

acetone dip followed by five minutes in iso-propyl alcohol and then drying with nitrogen

gas. The etch step actually thins the oxide layer, and includes a four minute sample dip

in hydrochloric acid followed by a iso-propyl alcohol rinse and a nitrogen gas dry. The

oxide layer is thinned because the oxide attacks and roughens the growth surface when

it is thermally desorbed. The remaining oxide layer is later removed in the MBE chamber

prior to growth by thermal desorption.

34

3.3 Sample Growth

The IAG 300 series studied for this thesis was grown in the Veeco GEN20 MBE

system at the Iowa Advanced Technology Laboratories. The GEN20 is equipped with V

cell valved crackers, and the p and n dopants are provided by beryllium and tellurium

cells, respectively. The substrate is continuously rotated during the growth process to

minimize thickness gradients. An initial RHEED check was performed during the

desorption process. All were held about 20 oC above the desorption temperature for

13.5 minutes to remove the majority of the oxide layer. A second RHEED check was

performed prior to the growth of the doped cathode contact level. This was done to fine

tune the III cell temperatures for the necessary In and Ga growth rates. The substrate of

each sample was maintained at 455 oC and a constant total growth rate of 0.6ml/s was

used during the device growth.

The device structures appear in Fig. 3.1. Figure 3.1a and 3.1b represent the stacks

for the single stage devices, and 3.1c is that for the cascaded LED. Heavy (1018 cm-3) p

and n-doping at the anode and cathode GaSb contact layers acts to give the layer good

conductivity and to reduce contact resistance. The reverse biased tunnel junctions are

found only in the cascaded LED specimen. As described in Chapter 1, these junctions are

intended to block electron and hole leakage out of the active regions, and to optimize

the probability that they will experience radiative recombinations instead.

The composition of the tunnel junctions is selected to maximize carrier trapping

within an active region prior to radiative recombination. Studies have concluded that

the containment of holes in SL cascaded devices can be problematic [6]. Even though

these devices are not SL structures, hole containment is a possible issue. GaSb was used

for the p-side of the junctions because of its wide band gap. This effectively contains

conduction band electrons. Research conducted at the University of Iowa [13] suggests

that the use of n-GaInAsSb effectively contains valence band holes. It also thins the

35

a) IAG 338 b) IAG 339

b) IAG 343

Figure 3.1: Stack Diagrams for Bulk and Cascaded LED Devices

tunnel junction when combined with p-GaSb. The conduction and valence bands of

GaInAsSb lie below those of GaSb. The use of p-doping for the GaSb and graded

n-doping for GaInAsSb cause extreme junction band bending and thin the junction

region even before the application of an external bias. The doping utilized for both

regions of each junction were 5x1018 cm-3, which exceeds that used for the previous

University of Iowa study by over 300% [13]. This thinning also optimizes the probability

that carriers will tunnel through the junctions, decreases the junction resistance and

36

reduces Joule losses.

Included in Fig. 3.1 are the etched mesas and the metalized anode and cathode

contacts. Note that all of the devices are grown on n-GaSb substrates. The metallic

anode contact which covers the majority of a mesa acts to reflect light through

substrate which acts as the emitting surface for each device. The use of n-GaSb

minimizes the free carrier absorption of the light [5]. One additional incentive for using

the GaInAsSb quaternary material is that III-V compound semiconductors have lower

Auger scattering rates [13].

3.4 Device Processing

Fabrication of the samples into mesa LED devices was accomplished through the

use of standard photolithography and wet chemical etching. This process is complex and

mostly beyond the scope of this thesis. For the wet chemical etching a citric and

phosphoric acid mix is used to etch the epilayer and form the mesas. The metal contacts

are deposited by the electron beam evaporation process. These contacts consist of

layers of titanium, platinum and gold (TiPtAu), evaporated onto the doped clad layers in

that order. Gold is a good electrical conductor, but diffuses into the matrix of

semiconductors. The platinum prevents gold diffusion, and titanium promotes good

adherence of the contact to the semiconductor surface. Together these constitute a

good ohmic contact when deposited on the p-GaSb anode [15]. A Schottky barrier forms

at the metal/n-GaSb interface which adds an internal series voltage to the device [5].

This could contribute to the Joule heating of the device during operation. The

enlargement of the n-contact area is an attempt to reduce the contact resistance which

this barrier can cause.

37

3.5 Device Preparation

After processing, a thermal evaporator is used to deposit a layer of chromium and

then a layer of indium on the device titanium/platinum/gold (TiPtAu) contacts. This is

done to optimize the electrical contact between the header indium traces and the

device contacts. The LED devices of the IAG 300 series were then flip chipped onto the

header with the required mesa fan out contact pattern on it. The LED is then pressed

into place for up to 24 hours, with about 5 lb of force. This force must be carefully

selected, for if it is too low the majority of the mesa sites might not make sufficient

contact with the header and form open circuits when inserted into the cryostat. If too

much force is applied, then the anode and cathode contacts might be pressed together

forming a short circuit. During the flip chipping process an adhesive is applied to the

chip and the header to cement them together. After it is removed from the flip chipper,

the header is placed onto an LCC socket which has an indium layer underneath the

header to function as a heat sink. The header is affixed to the socket with an adhesive,

and after this dries they are wire bonded together. Devices processed in this manner

were tested as described in Chapter 4.

Figure 3.2: LED Flip Chipping to Header

38

CHAPTER 4

IV AND LI TEST SYSTEMS

Current versus voltage (IV) test data are taken to verify the basic electrical

functionality of a device. A radiance versus current (LI) test is conducted to evaluate

radiant output as a function of applied current. As discussed in Chapter 1, the bias

voltage which raises the current above negligible levels is proportional to the direct

band gap, so IV tests can also provide an approximate band gap value.

4.1 MQW Test Lab

The IA 2300 quantum well device series tested at a temperature of 20oC were mesa

etched surface emitters, with the etching completed by Dr. Jon Olesberg of the

University of Iowa Department of Chemistry. The mesa sizes included 20 m by 20 m,

50 m by 50 m, 100 m by 100 m, 200 m by 200 m and 400 m by 400 m. A

current pulser was controlled by the computer LabVIEW program via a National

Instruments DAQ board. The pulser was built by the University of Iowa Department of

Physics and Astronomy departmental electrical engineer Michael Miller. Current pulses

with the specified amplitude, duration and duty cycle are delivered to the test device

placed on a thermoelectrically cooled pad.

Micro-positioners, guided with the assistance of a video camera and monitor, are

placed on one of the device cathodes and on the anode of the mesa to be tested. The

current pulse is sent into the circuit, which appears in Figure 4.1. The potential drop is

monitored across the device, indicated by V1, and the current is directed through a

1.46 resistor. The resistor potential drop V2 is recorded by the DAQ board, and

collected by the LabVIEW program. The current passes back to the pulser and the circuit

is grounded through it. Both potentiometers are grounded through the DAQ board

which conveys the data to the computer for processing and plotting.

39

Figure 4.1: IV and LI Experiment Schematic

The device pad has an aperture made of transparent sapphire. A thermoelectric

cooler keeps the device at a specified temperature. All of the devices in this thesis were

tested at 20 oC. Light generated by the active region carriers passes down through it to a

photo-diode mounted in a grounded aluminum casing. The sensor is circular in shape

with a diameter of 1 mm. The distance from the photo-diode to the device is measured,

and this is used to calculate the solid angle occupied by the detector with respect to the

test device center. The photo-diode was typically placed about 11 cm from the

underside of the test device. For such a small solid angle, an approximation is used,

A/r2 (4.1.1)

Here A is the detector area, r is the test sample/detector separation and is the solid

angle. The photo-diode detector generates a current in response to the accepted

radiant energy, which was passed through a current amplifier. The operating setting of

the gain box was adjusted to minimize data noise, but still preserve the shape of the

square data pulse which the detector should pick up.

40

Figure 4.2: IV and LI Test Station

The voltage drop across the resister in the circuit of Fig 4.1 needs to be converted

into current density, to make the mesa performance evaluation size independent. First,

assuming the relationship is Ohmic, the current through the mesa is calculated by

dividing the resister voltage drop by the 1.46 resister value. Next, the current density

is determined by dividing the current by the mesa size, which assumes that the current

is evenly distributed over the mesa. This might not actually be true, but to attempt to

correct for this would require the use of a numerical simulation in a program such as

COMSOL Multiphysics. This was not done.

The operation of the detector at room temperature required a correction for dark

currents in the data. The procedure for this is described in Appendix C. To calculate the

radiance for a given measurement, the current induced in the detector was divided by

41

the responsivity of the detector at the peak output wavelength, and by the mesa area

and the detector solid angle with respect to the device,

L = I/A (4.1.2)

Here L is the device radiance at the detector, I is the detector current, is the detector

responsivity (in Amps/Watt), A is the mesa area and is the detector solid angle. A

sample plot of a collection of such data appears in Fig. 4.3.

0 200 4000.000

0.001

0.002

0.003

Ra

dia

nce

(W

/sr*

mm

2)

Current Density (A/cm2)

IA 2341 400mx400m Mesa

50s Pulse 1%Duty

Figure 4.3: Radiance Plot for a Quantum Well Device Mesa

4.2 Single Stage and Cascaded LED Test Lab

4.2.1 IV Test

IV tests were performed on the IAG 300 series of devices in a second lab facility,

capable of conducting performance data for low specimen temperatures. They were

placed in a Henriksen liquid nitrogen cooled cryostat equipped with a cold-finger and

electrical contacts to the fan out header. This permits the individualized testing of

the 81 mesas created by the wet etching of the device. The temperature is monitored by

a Lakeshore 331 Temperature controller. Temperature monitoring diodes are placed on

the cold-finger which is also equipped with a heater element. The operating

42

temperature is entered into the Lakeshore unit, which activates the heater in response

to linear, differential and integral feedback systems to maintain the smallest possible

difference between the set point and the measured temperature. A switcher box

controlled by the lab computer selects a mesa for testing according to a batch file

referenced by the MATLAB program written at the University of Iowa. The command to

test the device is delivered to a Keithley 2612 System Sourcemeter via a USB

connection. The Keithley voltage source then applies a bias to the sample through the

switcher. A two point contact method is used to determine the current resulting from

the known bias. The Keithley then raises the bias and repeats the test, and continues to

do this until the bias testing interval as entered into the program is achieved. The data

are then stored as a “.dat” file for later access.

Figure 4.4: IV Experiment Schematic

4.2.2 EL Test

An electroluminescent (EL) test consists of applying electric current to a

semiconductor device and monitoring the radiated spectral output. The mesas are

activated via the switcher as for the IV tests, but the radiant output is collected by a gilt

parabolic mirror situated at the cryostat ZnSe window. The device is located at the

mirror focal length, so that a collimated beam of light is delivered to a Nicolet Magna-IR

560 Spectrometer, and ultimately to a second parabolic mirror which focuses the

43

emission onto a MCT-10 cryogenic detector module. The system is configured into a

double modulated detection scheme [14], operating at a 20kHz frequency provided

Figure 4.5: EL Experiment Schematic

Figure 4.6: Cryostat, Nicolet and MCT-10 Detector Configuration for EL Test

44

by an Agilent 33220A Waveform generator. This generator delivers the modulation

frequency to the SRS Model SR830 DSP Lock-In Amplifier, which de-modulates the

radiated signal to eliminate the obscuring effects of background infrared emissions.

4.2.3 LI Test

For the collection LI data the cryostat and switcher are configured as described for

the IV and EL tests. A current pulser constructed by Michael Miller delivers pulsed

current with a period and duty cycle specified by a MATLAB program. For the majority of

the tests, quasi-DC biasing conditions were used, with a 500 s pulse period and a 50%

duty cycle. These are the operating conditions under which the devices might eventually

be operated, to achieve a near constant output. This delivers the current to the

switcher, and hence to the selected mesa. The MCT-10 is situated directly in front of the

cryostat ZnSe window, at a known distance from the LED. This and the dimensions of

the active MCT-10 detector element permit the calculation of the solid angle occupied

by the detector with respect to the LED. No focusing optics are used, and the LED total

upper hemisphere output is assumed to occupy a Lambertian distribution. This permits

the approximation of this output by multiplying the measured axial power by and

dividing this product by the detector solid angle [5].

45

Figure 4.7: LI Experiment Schematic

Figure 4.8: Cryostat and MCT-10 Detector Configuration for LI Test

46

CHAPTER 5

CARRIER LIFETIME MEASUREMENT SYSTEM

One frequently used technique for the determination of the electron/hole

recombination rate for a semiconductor is the use of a pulsed laser system. The pulse

width should be much shorter than the expected lifetime value, so that a near

instantaneous excitation is followed by a gradual recombination unaffected by the laser

itself. Also, the period between temporally adjacent pulses should far exceed the

expected lifetime, so that the electron/hole recombination is complete before the next

pulse arrives. Several methods may be used to achieve this. One is the pump/probe

method [4]. Another is the use of an ultra-fast laser in combination with a fast detector.

The fast detector has a sufficiently brief response time to resolve photoluminescent

signals emitted by the semiconductor. This is commonly referred to as time-resolved

photoluminescence (TRPL), and is frequently applied to III-V semiconductors. Before

proceeding, a brief discussion of the phenomenon of photoluminescence in

semiconductors follows.

5.1 Photoluminescence

Figure 5.1.1 illustrates the phenomenon of photoluminescence (PL), which refers to

the generation of electro-magnetic radiation by excited conduction band electrons

recombining radiatively with valence band holes. Electrons experiencing photo-

excitation by an ultra-fast system pulse will transition to the conduction band after

absorbing a photon (step 1). Photon absorption conserves carrier momentum, so the

photon energy must exceed the band gap to excite an electron.

After the excitation of the electron/hole pairs, the carriers will thermalize and lose

energy in the form of phonons, until the carriers decay to the conduction and valence

band minima. Then the electron/hole pairs will recombine and emit a photon equal in

47

energy to the minimal band gap (step 2). The energy of the radiated photon (Photon 2)

is less than that of the initial exciting one (Photon 1). Ideally, the energy difference

between them would equal the collective energy of the radiated phonons.

The radiative recombination of the electron/hole pairs is a spontaneous process,

resulting from the so called vacuum fluctuations. However, as discussed in Chapter 2,

other decay mechanisms also affect the recombination rate. Monitoring the fluoresced

light provides the 1/e carrier lifetime as discussed in Chapter 2.

K

E

2

Photon 1Absorption

1

2

1

2

Photon 2Emission

Figure 5.1: Generation of PL Photons

5.2 Time Resolved PL Measurement System

The ultra-fast system used in this thesis is for the collection of the recombination

rate PL data is based upon a mode locked ultra-fast Tsunami Ti: Sapphire (Ti:Saph)

oscillator, model 3960C arranged in the femtosecond configuration. This is driven by a

Millennia Xs diode pumped CW laser. Both these devices are manufactured by Spectra

Physics. The Millennia generates NIR light with two fiber coupled diode laser bars

(Fcbars) in the T80 Power Supply. This is sent to the laser head through a fiber optic

48

umbilical and on to a neodymium yttrium vanadate (Nd:YVO4) crystal in a laser cavity.

The crystal converts the Fcbar output into 1064 nm light. This in turn is fed into a lithium

triborate (LBO) frequency doubling crystal which converts 1064 nm into 532 nm light. A

coupler transparent to 532 nm and opaque to 1064 nm transmits only the 532 nm

output. It is this light which provides the driving energy for the Tsunami. The Tsunami

was aligned and optimized by a field technician during the summer of 2013, prior to the

collection of the data for this thesis.

Mode locking is established by the vibrations of the acousto-optic modulator near

the M10 output coupler. This operates at a frequency of 80 MHz and would be a source

of RF noise in the laboratory if it were not turned off after mode locking is achieved.

Figure 5.2: Tsunami Model 3960C Femtosecond Configuration

(Spectra Physics: all rights reserved)

The pulse bandwidth is determined by the position of prisms Pr2 and Pr3 in the beam

path, which also counteract group velocity dispersion. The line center wavelength is set

by the tuning slit between the prisms. The operation mode of the Tsunami generates

pulses at the 80 MHz repetition rate. The spectrum is measured during device

49

optimization by a Spectral Products Inc. SM-240 spectrometer. A Gaussian fit to a

sample spectrum is illustrated in Fig. 5.3.

Figure 5.3: Wavelength Spectrum of a Tsunami Ti: Sapphire Pulse

The line center of the spectrum in Fig. 5.3 is 804.7 nm and the FWHM is 13.5 nm. The fit

is a good one, with an R2 value of 0.9988. Pulses with line center wavelengths ranging

from 828 to 842 nm were used to generate the PL signals examined for this thesis. The

pulse duration is not precisely known, since there is no autocorrelation device in the lab.

However, if one treats the pulse shape as Gaussian, the time-bandwidth product may be

used to estimate the pulse FWHM.

t ~ 0.44 (5.2.1)

One estimate based on a signal with an 800 nm line center wavelength with a FWHM of

20 nm is about 7.5 fs, a reasonable value for this Ti: Saph system configuration.

Pulses separated by 12.5 ns (corresponding to an 80 MHz repetition rate) are not

sufficiently spaced to allow for the emptying of the conduction band in the excited

specimen. To obtain a complete decay profile for the determination of the rate

constants discussed in Chapter 2, the interval between the pulses must be increased.

780 785 790 795 800 805 810 815 820 825

0

1

2

3

4

5

6

7

8

9

10

11

Inte

nsity(a

u)

Wavelength (nm)

Gaussian Fit for Tsumani Spectral Output

December 2, 2013

50

This is accomplished with the use of an electro-optic modulator (EOM). The output of

the Tsunami is almost completely vertically polarized. There is a slight horizontal

component, but this is reduced by the introduction of a high power glan-air polarizing

beam splitter. This is placed in the beam path prior to the Conoptic Inc. Electro-optic

Modulator Model 350 (the EOM), which has a potassium dideuterium phosphate (KDP)

core. Applying an electric field to the EOM induces birefringence in the KDP crystal.

When activated it produces a phase shift between two circularly polarized components

of the linearly polarized electric field vector. The effect is to rotate the electric field

vector of the incident laser light by 90o when activated. The beam impinges on a

polarizing beam splitter inside the modulator chamber. The rotated horizontally

polarized pulses are removed (for the most part) from the beam path, and the vertically

polarized ones are allowed to pass. A Conoptic model 25D push/pull amplifier supplies

the necessary voltage to drive the EOM. This is in turn controlled by a Model 305

synchronous divider, which sets the chop rate. Carrier decay is an exponential process,

so at least four or five time constants should elapse to ensure that a majority of the

electron/hole pairs have recombined.

One issue which arose was the incomplete extinction of the chopped signals. It was

noticed that a small percentage of each chopped pulse passes through the EOM,

exciting carriers in the semiconductor sample. The optimized signal to noise ratio,

accomplished by adjusting the drive voltage and bias control on the model 25D voltage

driver, was initially limited to about 80 to 1. The ideal factory extinction ratio for an

optimally aligned device was quoted at 100 to 1. Adding the high power beam splitter

prior to the EOM removed most of the horizontally polarized Ti: Saph output, and

increased the extinction ratio above the manufacturer quote. The chopped pulses do

refresh the sample conduction band with excited carriers every 12.5 ns, creating a

background carrier density. This makes it difficult to achieve the low carrier densities

needed to measure the SRH recombination coefficient.

51

Figure 5.4: PL Signal and Chopped Pulse Noise

After passing through the EOM, the pulse beam is focused by a chromatic doublet

lens. The beam is directed through a hole drilled in a focusing parabolic mirror, and onto

the sample mounted in a cryostat. The cryostat is a Cryovac brand unit, which has a

removable face plate. The samples adhere to the specimen face with Apiezon grease.

Capillaries in contact with the specimen face circulate liquid nitrogen drawn by a

roughing pump, and the chamber itself is reduced to micro-Torr vacuum pressures by a

turbo pump. The PL light exits the chamber, strikes the first parabolic mirror where it is

collimated, is directed to a second parabolic, which focuses it onto an extended wave,

fast InGaAs photodetector (UPD-5N-IR2-P series ALPHALAS, 0.8-2.6m spectral

resolution). A silicon filter placed in front of the detector screens out the residual pulse

light, so that only the PL signal is monitored. The detector in the Fig. 5.5 is not the one

used for the lifetime measurements, but the ALPHALAS photodetector occupied the

same position in the experiment.

0 50 100 150 200

0.00

0.05

0.10

0.15

0.20

PL

Sig

na

l (v

)

Time (ns)

52

Figure 5.5: Lifetime Measurement System

Figure 5.6: Lifetime Measurement System Schematic

53

To estimate the Auger coefficient for the samples considered in this thesis, carrier

densities in excess of 1.0x1017 cm-3 needed to be achieved. At peak density, these could

have decay rates in the single nanoseconds range. The photodetector and the

oscilloscope need to resolve these rapid transients. The ALPHALAS photodetector has a

rise time resolution of less than 300 ps and a fall time of less than 900 ps. The spectral

resolution ranges from 0.8 to 2.6 m, with an optimal response between 2.3 and 2.5

m. The 77 K line center wavelength for the PL sample considered in this thesis, IAG

337, was adjacent to this optimal window, with a cryogenic line center wavelength of

about 2.18 m. The bandwidth of the MDO3000 series oscilloscope used for data

collection is 1 GHz, and the data were collected at full bandwidth mode.

5.3 Peak Carrier Density Calculations

Several quantities are needed in order to calculate the peak carrier density for an

incident pulse according to Eq. 2.1.3. The spot size of the laser at the sample position

needs to be determined. This is done by convolving a 100 m pinhole mounted over a

hole in the cryostat sample face, and a power meter placed in the beam path behind it.

Equation 5.3.1 is used for the Gaussian data fit.

–

(5.3.1)

Here is the peak center and is the standard deviation. The 1/e2 profile full width is

determined from Eq. 5.3.1, obtained by setting the exponent equal to two.

– = (5.3.2)

The beam width using a 100 m pinhole is obtained (in millimeters) by subtracting the

square of the pinhole diameter from = 2,

( 1/e2)2= (2)2 – 0.01 mm2 (5.3.3)

54

Treating the beam profile as Gaussian, the 1/e2 spot size diameter would contain

approximately 95 % of the beam power, and is sufficient for a reasonable estimate of

the peak carrier density. Once the pulse power has been estimated, the pulse energy

needs to be determined as well. The pulse energy can be approximated by taking the

product of the time averaged power and the period between pulses.

E = Pavg* t (5.3.4)

Here Pavg is the time averaged power, and t is the pulse repetition period. These

quantities are used to calculate the peak carrier density initially excited by the pulse, as

discussed in Chapter 2.

= 4ET/hcd( 1/e2)2 (2.1.4)

Figure 5.7: Gaussian Fit for Beam Profile Convolution

0 50 100

0.0

0.5

1.0

Norm

aliz

ed

In

tensity (

AU

)

Position (mm/100)

55

CHAPTER 6

EXPERIMENTAL RESULTS

The preceding chapters of this thesis discussed basic semiconductor theory, the

growth and processing of the IAG 300 series of devices and the experiments used for

collecting the data needed to determine their performance. Because these devices are

of interest for the development of LEDs, the analysis will focus primarily on two

subjects. The first to be considered is the radiant output of the specimens, examined

through the collection of LI test data. The processing of this information includes the

determination of accepted figures of merit such as wall-plug and quantum efficiency.

The second topic is the estimation of the recombination rate coefficients for the active

region material. These constants can be used to predict the carrier density which

optimizes the radiative output of the active region. They can also be used to estimate

the carrier densities for which Auger processes are dominant, an important topic for the

understanding of device heating.

6.1 Test Devices and Mesa Variability

The LED devices tested from the IAG 300 series were produced from single MBE

growths for the cascaded device and for the bulk emitters with 500 nm and 2.5 m

active region thicknesses. Two processing runs of wet chemical etching produced two

sets of devices. These were labeled the “A” and the “B” device sets. The time constraints

of this thesis did not permit the rigorous testing of both these sets. The 2.5 m active

region specimen IAG 338 – B, and the cascaded device sample IAG 343 - B were

characterized with the IV, EL and LI tests. Devices from the same processing run are

compared because they share similar chemical etching parameters.

After rigorous analysis of IAG 338 - B and IAG 343 - B it was determined that the

specimen for the corresponding 500 nm single stage active region device, IAG 339 - B,

56

displayed properties which imply that it is flawed. The data suggest that the specimen

has unexpectedly low electrical conductance, as evidenced by the low radiant output.

The performance of the 500 nm bulk active region device, examined in the context of

the quantum and wall-plug efficiencies, ranged from one half to one order of magnitude

lower than the 2.5 m active region specimen . The device traveler document, which

describes the various processing steps and parameters, suggests that the etching

penetrated the n - doped clad, and imply that the deposited cathode would make

proper contact with it. The source of this poor device performance must result from

some other factor. IAG 339 - A, the 500 nm bulk active region device from the first

processing run, was examined once the issue concerning the sample from the second

series was recognized. The device traveler reveals that the wet etching had penetrated

the n - doped clad, and examination of the device properties suggests that the device is

performing as expected. For these reasons this specimen from the first processing run

and the other two from the second are considered in this thesis. The traveler

documents for the three samples appear in Appendix F.

Even if a sample is processed properly, many of the mesas might not be functional

LED devices. Anode and cathode contacts can be pressed during the flip chipping

procedure until a short is created. Working devices can also be destroyed during

successive testing. Some of the metallic contacts might not sufficiently bond with the

clad layer upon which they are deposited, creating an open circuit. As a result, data is

often collected for a minority of the devices etched into each growth specimen. For the

three specimens considered here, about one quarter to one third of the mesas provided

acceptable radiance data.

Another factor to consider is the natural performance variability between working

devices. A performance norm can be established once several mesas are examined for a

growth specimen. The data for pairs of mesas of a given size are presented. More mesas

would be preferable, but legibility constraints often prevent this, especially when mesas

57

of different sizes are compared on the same plot, as is done here. The mesas selected

for this thesis typify the results for each growth specimen.

6.2 IV Tests

6.2.1 Activation Voltage

One useful piece of information provided by the current-voltage (IV)

characterization of a device is the determination of the activation voltage. The

activation voltage for a diode may be defined as the bias at which the conducted current

starts to exceed values negligible compared to those of normal operation, and will be

proportional to the band gap energy, as in Eq. 1.6.1. If a diode does not have the

expected activation voltage it will be apparent upon inspection of the IV plot.

Figure 6.1 contains the IV profiles of the devices to be considered. The activation

voltage of the cascaded LED should be about five times larger than that of both single

stage devices due to the presence of tunnel junctions. Each single stage specimen has an

activation voltage of 0.5 V at 85 K, while that of the cascaded LED at this temperature is

about 2.5 V. This suggests that the cascaded LED tunnel junctions are working properly,

at least for low input powers.

6.2.2 Dependence of Current Density on Mesa Size

The current conducted by a diode device as a function of applied bias will be

dependent on mesa size. For a pn diode, represented by Eq. 1.4.2, Io is the reverse

saturation current, caused by the drift of minority carriers from the neutral to the

depletion region.

Jo =Io/A = q

(6.2.1)

58

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60

50

100

150

200

250

100mx 100m

100mx 100m

200mx 200m

200mx 200m

400mx 400m

Cu

rre

nt

De

nsity (

A/c

m2)

Bias (V)

85K

a) 500 nm Single Stage Emitter

0.0 0.5 1.0 1.5 2.0 2.50

100

200

300

400

500

600

700 100mx 100m

100mx 100m

200mx 200m

200mx 200m

400mx 400m

Cu

rre

nt

De

nsity (

A/c

m2)

Bias (V)

85K

b) 2.5 m Single Stage Emitter

0 1 2 3 4 50

100

200

300

400

500

600 100mx 100m

100mx 100m

200mx 200m

200mx 200m

400mx 400m

Cu

rre

nt

De

nsity (

A/c

m2)

Bias (V)

85K

c) Cascaded LED

Figure 6.1: IV Profiles for Device Mesas

59

Here, q is the charge quantum, A is the cross sectional area of the current element,

are the hole and electron diffusion coefficients, are the hole and electron

carrier lifetimes, are the donor and acceptor concentrations at the n and p sides,

is the intrinsic carrier concentration in the material, and Jo is the reverse saturation

current per unit area, or current density. Equation 6.1.1 reveals that the current density

ideally is independent of mesa area. In actuality, as the IV plots in Fig. 6.1 illustrate, the

smallest mesas conduct the greatest current density, and the largest ones the least. The

fact that this pattern is present for all three devices suggests a mesa size dependent

mechanism which reduces device performance in larger mesas. One possible cause is

device heating. Larger mesas require a much greater input power for a given voltage

and current density (P = IV = AJV) and hence dissipate much more thermal energy,

causing the whole chip to heat more. This effect will be considered in the context of the

LI test data as well.

6.3 LI Tests

The radiance verses current density profile, or LI plot, provides a basic

characterization of the LED performance. The radiance represents the power emitted

per unit mesa area per unit solid angle at the photodetector surface. This quantity

facilitates performance comparisons between mesas of different sizes. The LI data in

this thesis were collected under quasi-DC conditions, with a current pulse width of

500 s at 50 % duty cycle. LI plots for the device series appear in Fig. 6.2.

6.3.1 Radiance Profile Characteristics

The devices will be compared at similar active region carrier densities, at which they

should ideally have similar radiative efficiencies. It is assumed that for the single stage

devices, each carrier experiences one radiative recombination event within the active

60

region, as indicated in Fig. 6.3. It is also assumed that each cascaded LED carrier

experiences five radiative recombinations, corresponding to perfect carrier recycling.

In accordance with these assumptions, the 500 nm thick and the 2.5 m thick single

stage devices will be compared when the current of the latter is five times greater than

that of the former, to ensure comparable carrier densities. When this is done, the

2.5 m single stage device prior to rollover displays about seven to eight times more

radiance than the 500 nm one. Note that this approximately corresponds to the

difference in active region thickness, implying that increasing the active region thickness

(and emission region volume) by a factor of five at a fixed carrier density generates a

proportionately greater amount of electromagnetic radiation.

The performance of the 2.5 m single stage device and the cascaded LED

will be considered at similar input powers to achieve similar carrier densities. Under this

condition, the 2.5 m single stage device operates at five times the current density of

the cascaded LED but has a five times thicker emission region. Also, the cascaded LED

operates at five times the single stage device voltage,

Pss =VssIss = (Vcas/5)*(5Icas) = Pcas (6.3.1)

Here, P is the input power, I is the input current, V is the device voltage and the subscripts

“ss” and “cas” refer to the 2.5m single stage and cascaded LED devices. The

proportionality between carrier density and device current is illustrated by the relation

n ~ I/Ad (6.3.2)

Here, A is the mesa area and d is the active region thickness. Comparisons made at

similar input powers are also made at similar carrier densities,

nss ~ Iss/Adss = 5Icas/A(5dcas) ~ ncas (6.3.3)

61

0 500 1000 1500 2000 2500 3000 35000.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

100mx100m

100mx100m

100mx100m

200mx200m

200mx200m

200mx200m

Ra

dia

nce

(W

/sr*

mm

2)

Current Density (A/cm2)

500s Pulse 50%Duty 85K

0 200 400 600 800 10000.00

0.01

0.02

0.03

0.04

0.05

0.06

500s Pulse 50%Duty 85K

200mx200m

200mx200m

200mx200m

400mx400mRa

dia

nce

(W

/sr*

mm

2)

Current Density (A/cm2)

a) 500 nm Single Stage Device

0 1000 2000 3000 4000 50000.00

0.05

0.10

0.15

0.20

0.25

0.30

100mx100m

100mx100m

200mx200m

200mx200m

Ra

dia

nce

(W

/sr*

mm

2)

Current Density (A/cm2)

500s Pulse 50%Duty 85K

0 500 1000 15000.00

0.02

0.04

0.06

0.08

200mx200m

200mx200m

400mx400mRa

dia

nce

(W

/sr*

mm

2)

Current Density (A/cm2)

500s Pulse 50%Duty 85K

b) 2.5 m Single Stage Device

0 500 1000 1500 2000 25000.0

0.2

0.4

0.6

0.8

100mx100m

100mx100m

200mx200m

200mx200mRa

dia

nce

(W

/sr*

mm

2)

Current Density (A/cm2)

500s Pulse 50%Duty 85K

0 200 400 600 800 10000.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

200mx200m

200mx200m

400mx400mRa

dia

nce

(W

/sr*

mm

2)

Current Density (A/cm2)

500s Pulse 50%Duty 85K

c) Cascaded LED

Figure 6.2: LI Plots for Multiple Mesa Sizes

62

a) 500 nm Single Stage Device b) 2.5 m Single Stage Device

b) Cascaded LED

Figure 6.3: Carrier Recombination Mechanisms

An additional advantage of considering radiance as a function of input power is that

device heating is the same for all mesa sizes. A disadvantage is that the carrier density

will be unique for each mesa size.

The device outputs presented in Fig. 6.4 should ideally be identical for similar

input powers (and carrier densities). But the cascaded LED generates about three to five

times the radiance of the 2.5 m single stage depending on the mesa size being

considered. The superior performance of the cascaded LED will be considered more

thoroughly in the context of the wall-plug efficiency.

63

0 1 2 3 4 50.0

0.2

0.4

0.6

0.8

100mx100m

100mx100m

200mx200m

200mx200m

Ra

dia

nce

(W

/sr*

mm

2)

Input Power (W)

500s Pulse 50% Duty 85K

a) 2.5 m Single Stage Device b) Cascaded LED

Figure 6.4: Radiance and Input Power

6.3.2 Wall-plug Efficiency

The wall-plug efficiency (WE) is defined as the ratio of the total output power (here

in the form of electromagnetic radiation) to total input power (in the form of electrical

energy),

WE = Ltot/IV (6.3.4)

Here Ltot is the total radiant energy output. The WE provides a percentage estimate of

the energy conversion capacity of the device. It is assumed here that the radiant energy

of each device occupies a Lambertian distribution. This permits the approximation of

the total emitted light in a half sphere of 2 sr as

(6.3.5)

Here is the energy radiated approximately normal to the semiconductor surface and

is the solid angle occupied by the detector with respect to the emitter. The WE will be

calculated for the data as a percentage.

0 1 2 3 4 50.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

100mx100m

100mx100m

200mx200m

200mx200m

Ra

dia

nce

(W

/sr*

mm

2)

Input Power (W)

500s Pulse 50% Duty 85K

64

The WE for the 2.5m single stage device in Fig. 6.5 is dependent upon several

factors. For high input powers, the various mesas display a similar performance. The WE

decreases more rapidly for the smallest mesas due to the fact that the device is

operating at a higher current density and carrier density for a given input power, and

might be experiencing Auger recombination effects. The divergence in performance for

low input powers could possibly be generated by the presence of Schottky barriers at

the device contacts, or potential spikes located at the interface of the active region and

the p(or n)-doped injector layers. These would increase the device resistance, and

generate Ohmic (Joule) heating. The fact that the overall performance increases with

decreasing mesa size is a possible result of mitigating edge effects which are more

prominent for small mesas with a large perimeter to area ratio.

Figure 6.5: WE for 2.5 m Single Stage Device

The various mesas of the cascaded LED displays similar WE values when considered

as a function of input power. The WE decreases more rapidly for the smallest mesa

because they are operating at higher current densities and carrier densities for a given

input power in comparison to those of the larger mesas. The tunnel junctions might

reduce device per-stage resistance, leading to high performance at low input powers.

0 1 2 3 4 5 6 70.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

100mx100m

100mx100m

200mx200m

200mx200m

400mx400m

Wa

ll-p

lug

Effic

ien

cy (

%)

Input Power (W)

500s Pulse 50% Duty 85K

65

Figure 6.6: WE for Cascaded LED

Ideally, for the reasons outlined in the last section, the 2.5 m single stage device

and the cascaded LED should exhibit similar wall-plug efficiencies for similar input

powers. In fact, as illustrated in Fig. 6.7, the cascaded LED displays a superior WE to the

2.5 m single stage device, especially at the low input powers.

Figure 6.7: WE for 100 m by 100 m Mesas

0 2 4 6 8 10 120

1

2

3

4

5

6

7500s Pulse 50% Duty 85K

100mx100m

100mx100m

200mx200m

200mx200m

400mx400m

Wa

ll-p

lug

Effic

ien

cy (

%)

Input Power (W)

0.0 0.5 1.0 1.5 2.0 2.5 3.00

1

2

3

4

5

6

2.5um Single Stage

Cascaded LED

Wa

ll-p

lug

Effic

ien

cy (

%)

Input Power (W)

500s Pulse 50% Duty 85K

66

This performance differentiation might result in part from the fact that greater electric

fields are being applied to the cascaded LED at a given input power than to the single

stage device,

Ecas ~ Vcas/dcas = 5Vss/dss ~ 5Ess (6.3.6)

This could potentially reduce the magnitude of any Schottky barriers or interfacial

potential spikes for the cascaded LED in comparison to 2.5m single stage device. At a

high input power and bias for which the Schottky barriers are diminished, the

performances become comparable.

Comparing the wall-plug efficiencies plotted as a function of current density reveals

other important details. For a given current density (and carrier density, according to Eq.

6.2.3), larger amounts of power will be put into the larger mesas, with the result that

larger mesas will generate more chip heating. This heating results in a reduction of the

device efficiency and hence radiant output. The result should be that at a fixed current

density, as the mesas grow in size they will exhibit a reduction in wall-plug efficiency.

a) 2.5 m Single Stage Device b) Cascaded LED

Figure 6.8: WE as a Function of Current Density

Figure 6.8 exhibits this pattern. Also, for a given current density, the cascaded LED will

be operating at a higher carrier density than the 2.5 m single stage device. This is due

0 1000 2000 3000 4000 50000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4 100mx100m

100mx100m

200mx200m

200mx200m

400mx400m

Wa

ll-p

lug

Effic

ien

cy (

%)

Current Density (A/cm2)

500s Pulse 50% Duty 85K

0 500 1000 1500 2000 25000

1

2

3

4

5

6

7

100mx100m

100mx100m

200mx200m

200mx200m

400mx400m

Wa

ll-p

lug

Effic

ien

cy (

%)

Current Density (A/cm2)

500s Pulse 50% Duty 85K

67

to the fact that each of the cascaded active region stages is only 500 nm thick. As a

result, the WE decreases more rapidly per unit carrier density for the cascaded LED,

possibly due to increased Auger recombination effects. The difference in performance

according to mesa size is not as extreme for the cascaded LED as for the 2.5 m single

stage device. Perhaps the tunnel junctions make the device less resistive, and reduce

the relative magnitude of any Joule heating processes.

The 500nm thick active region device represents a test for these ideas concerning

the differences in the cascaded LED and the 2.5 m single stage device performances.

Data for several mesas are presented in Fig. 6.9. The active region is the same thickness

as one of the cascaded LED stages, and yet it is a single stage device. For a given bias,

E500nm ~ V/500nm = 5V/2.5m ~ 5E2.5m (6.3.7)

The presence of electric fields in the 500 nm device which exceed those in the

2.5 m one could reduce the magnitude of any potential barriers, and the effects of

device heating. This implies that the performance of the 500nm device might be more

Figure 6.9: WE Efficiency for the 500 nm Single Stage Device

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60.0

0.2

0.4

0.6

0.8

1.0 100mx100m

100mx100m

200mx200m

200mx200m

400mx400m

Wa

ll-p

lug

Eff

icie

ncy (

%)

Input Power (W)

500s Pulse 50% Duty 85K

68

consistent across the various mesa sizes as a function of applied input power than for

the 2.5 m device. Fig. 6.9 does indicate some diverging mesa performance for small

input powers, but these are not as severe as for the 2.5 m device. The 400 m by

400 m mesa examined in Fig. 6.9 was the only one of the four for this specimen which

did not fail, so it could possibly represent an outlier. The other three specimens of this

mesa size displayed the behavior of shorted devices.

6.3.3 Quantum Efficiency

The quantum efficiency (QE) is defined as the number of photons emitted per

injected carrier. Here it is computed with the relation

(6.3.8)

is the total radiated power, calculated as before using the Lambertian assumption.

is the energy of the spectrum line center photon, used as an average for the entire

spectral bandwidth, while I is the input current and q is the charge quantum. Multiplying

by 100 converts the quantity into a percentage. This relation predicts that the QE for the

cascaded LED should be five times greater than that of the 500 nm single stage device at

a given current density (and carrier density) due to carrier recycling. The QE of the

cascaded LED will be compared to that for the 2.5 m single stage device at similar input

powers. When this is done, the radiant outputs should ideally be the same, and the

current of the single stage device will be five times greater than that of the cascaded

LED,

QEcas ~ Ltot/Icas = Ltot/(1/5)I2.5m ~ 5QE2.5m (6.3.9)

69

Hence the QE of the cascaded LED should ideally be five times greater than that of the

2.5 m single stage device. Figure 6.10 compares the QE of the two devices. Note that

for the data the QE is calculated as a percentage.

In actuality, the peak output QE of the cascaded LED exceeds that of the 2.5 m

single stage device by approximately a factor of seven. The single stage device operates

at higher currents than the cascaded LED, and could be experiencing greater energy loss

in the form of heat dissipation from Joule heating. As discussed before, it might have a

greater resistance than the cascaded device. It is also apparent that the cascaded LED

has a greater rate of decrease in performance per unit input power than the single stage

device. This could result from increased hole leakage through the tunnel junctions at

high applied biases. For both devices, the QE decreases faster for smaller mesas due to

the presence of higher carrier densities and possibly higher Auger scattering rates.

a) 2.5 m Single Stage Device b) Cascaded LED

Figure 6.10: QE as a Function of Input Power

6.3.4 Device Heating

It has already been noted that the MS contacts/injection region and the injection

region/active region interfaces could possibly generate potential barriers which add

internal resistance to the diode devices, and act as centers for device heating. The

0 1 2 3 4 5 6 70

1

2

3

4

5 100mx100m

100mx100m

200mx200m

200mx200m

400mx400m

Qu

an

tum

Effic

ien

cy (

%)

Input Power (W)

500s Pulse 50% Duty 85K

0 2 4 6 8 100

5

10

15

20

25

30

35

40

100mx100m

100mx100m

200mx200m

200mx200m

400mx400m)Qu

an

tum

Eff

icie

ncy (

%)

Input Power (W)

500s Pulse 50% Duty 85K

70

smaller current densities required by the cascaded LED in comparison to the 2.5 m

single stage device for comparable radiant output generate less thermal waste and

better device performance. Evidence of this was already seen in the magnitude of the

WE and QE for each device.

The tests conducted up to this point have been under quasi-dc biasing conditions,

with pulse durations of 500 s and duty cycles of 50 %. Changing this condition has the

potential to increase radiant output due to the reduction of heating either due to Joule

heating, Auger recombination, or both. In Fig. 6.11, the pulse width is fixed at 200 s,

but the duty cycle is varied, resulting in longer or shorter periods. Increasing the duty

cycle from 10 % to 80 % reduces the current density associated with LI plot rollover by

increase in device heating associated with an increase in duty cycle. This suggests that

over 50 % for both the 2.5 m single stage device and the cascaded LED. This steady

reduction in operating carrier density and radiant output appears to be linked to an

increased output could also be achieved by using reduced current pulse widths. The

DAQ board in use cannot measure current pulses with a width less than 100 s. But Fig.

6.12 illustrates the increase in output achievable by reducing the pulse from the quasi-

DC value of 500 s and decreasing the duty cycle.

0 100 200 300 400 5000.00

0.02

0.04

10%

20%

30%

40%

50%

60%

70%

80%Ra

dia

nce

(W

/sr*

mm

2)

Current Density (A/cm2)

400mx400m Mesa 200s Pulse

0 500 1000 15000.0

0.1

0.2

0.3

0.4

0.5

1%

10%

20%

30%

40%

50%

60%

70%

80%

Ra

dia

nce

(W

/sr*

mm

2)

Current Density (A/cm2)

200mx200m Mesa 200s Pulse

a) 2.5 m Single Stage Emitter b) Cascaded LED

Figure 6.11: Dependence of Radiant Output on Current Duty Cycle at 85 K

71

a) 200 s Pulse, 1% Duty Cycle b) 500 s Pulse, 50% Duty Cycle

Figure 6.12: Dependence of Radiant Output on Pulse Width for a Cascaded LED

400 m by 400 m Mesa at 85K

6.4 Recombination Coefficient Results

The recombination coefficients will be estimated for the GaInAsSb quaternary

active region employed for these devices. With knowledge of the recombination

coefficients, the recombination rates of radiative and non-radiative processes can be

calculated as a function of carrier density. The carrier density at which non-radiative

Auger recombination becomes important can be calculated, and its contribution to chip

heating estimated and weighed in comparison to Joule heating. Additionally, knowledge

of the radiative coefficients allows calculation of the ideal quantum efficiency, and

comparison to the measured quantum efficiency as a function of carrier density.

6.4.1 e-2 Spot Size Estimates

The beam spot size is an important parameter for determining carrier density

created by an optical pump pulse. The beam spot size at the semiconductor was varied

by changing the position of the focusing lens incorporated into the lifetime

measurement system beam path. This, coupled with the use of optical dispersion filters,

varied the density of the excited carriers in the semiconductor specimen. A 100 m

0 50 100 150 200 250 300 350 4000.00

0.02

0.04

0.06

0.08

Ra

dia

nce

(W

/sr*

mm

2)

Current Density (A/cm2)

0 100 200 300 4000.00

0.02

0.04

0.06

0.08

0.10

Ra

dia

nce

(W

/sr*

mm

2)

Current Density (A/cm2)

72

pinhole was convolved to measure the e-2 beam spot sizes as described in Chapter 5.

Normalized convolutions of the beam profiles appear in Fig. 6.13. The beam displays

some filamentation. This is not inherent to the system. Previous spot size

measurements conducted after the EOM without the focusing lens displayed a Gaussian

profile. One possibility is that the effect was created while adjusting the system to

achieve temporal stability in the extinction ratio. The lens does correct for this

filamentation as Fig. 6.13a suggests. Here the sample was placed at the theoretical value

of the lens focal length, and a satisfactory Gaussian profile is generated. The Gaussian

fits for Fig. 6.13a-c have adjusted R2 values of 0.99, 0.89 and 0.83, respectively. The spot

size estimates for the focusing lens positioned at 30 cm and at 10 cm are 0.45 mm and

0.65 mm, respectively. The unfocused beam had a spot size of 0.93 mm. The carrier

densities range from approximately 2x1015 cm-3 to 1.5x1017cm-3. If these estimates are

correct, then the carriers could possibly experience SRH, radiative and weak Auger

recombination mechanisms.

6.4.2 System Impulse Response and Carrier Lifetimes

The system response time sets the time resolution of the experimental setup, and

so determines the shortest carrier lifetimes that can be measured. The detector system

consists of an ALPHALAS UPD-5N-IR2-P photodetector in series with a pre-amplifier and

a secondary amplifier, both constructed by departmental electrical engineer Michael

Miller. The rise time of the detector is less than 300 ps, while that of the pre and

secondary amplifiers are about 6 ns and 2 ns. The oscilloscope has a bandwidth of 1

GHz, with an accompanying rise time of 333 ps. The uncertainty of the system rise time

and its possible effect on temporal resolution necessitated the acquisition of the system

impulse response. The shape and half-width of this response increases the observed PL

peak carrier lifetimes beyond those of the actual values. To estimate the response, the

Ti: Saph output was reflected off a piece of roughened copper in the cryostat.

73

a) Focusing Lens 30 cm from Sample b) Focusing Lens 10 cm from Sample

c) No Focusing Lens in Beam Path

Figure 6.13: Beam Profile Convolutions and Gaussian Data Fits

The roughened surface simulated the scattering of light by excited carriers. This light

was then focused onto the detector system by the two parabolic mirrors. The fact that

the Ti: Saph is arranged in the femtosecond configuration ensures that the pulse output

has a Gaussian profile which is about 100 fs in width or less. This represents an

adequate simulation of a delta function for a photodetector with a 300 ps rise time. The

data and Gaussian fit appear in Fig. 6.14.

0 10 20 30 40 50 60 70 80 90 1001101200.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1N

orm

aliz

ed

In

ten

sity (

AU

)

Position (mm/100)

0 10 20 30 40 50 60 70 80 90 1000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

No

rma

lize

d In

ten

sity (

AU

)

Position (mm/100)

0 50 100 150 200 250 3000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

No

rma

lize

d I

nte

nsity (

AU

)

Position (mm/100)

74

Figure 6.14: Lifetime System Impulse Response with Gaussian Fit

The 10.5 ns half width of this response is sufficient to broaden the initial recombination

lifetime measurements for every carrier density sampled. The greatest convolved

lifetime determined was about 70 ns for carrier densities on the order of 1015 cm-3, and

even this would be affected by such a system response. The true signal lifetime needs to

be de-convolved from the measured signals. If a Gaussian impulse response with signal

(6.4.1)

half width acts upon a decaying exponential signal with time constant ,

(2.1.5)

the system output will be of the form [16],

(6.4.2)

will maintain an exponential character for some ratios of / , but after a point the

profile of the original signal will be absent from the output. Origin 8 code was prepared

-5.00E-008 0.00E+000 5.00E-008-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

No

rma

lize

d A

mp

litu

de

(A

U)

Time(s)

Half width: 10.5ns

Adj. R2: 0.987

75

to model this output based on the input Gaussian half width and the exponential time

constant. Output time constants were determined for a range of input values, and a plot

was constructed. A second order polynomial fit to this provided a function which was

used to estimate the de-convolved peak carrier lifetimes. The data and the fit appear in

Fig. 6.15.

Figure 6.15: Model of Original and Convolved Peak Carrier Density Recombination Rates

6.4.3 Recombination Rate Coefficients for IAG 337

PL data was collected for sample IAG 337, the un-doped quaternary PL sample for

the IAG series LEDs. The resulting recombination rate data were fit to a quadratic

function. The quadratic fit has the form of Eq. 2.1.11,

R = A + B nopt + C( nopt )

2 (2.1.11)

in which nopt is the optically generated carrier density and A, B and C are the data fit

coefficients. It must be remembered that the SRH, radiative and Auger coefficients can

be extracted from the fit constants only when the background carrier density is known,

according to Eq.2.1.11-2.1.13.

1E-8 1E-7

1E-9

1E-8

1E-7O

rig

ina

l S

ign

al L

ife

tim

e (

s)

Convolved Output Lifetime (s)

y = a +bt +ct2

a: -1.15x10-8s +/- 0.07x10-8s

b: 1.14 +/- 0.03

c: -6.92x105s-1 +/- 2.47x105s-1

Adj. R2: 0.998

76

The fit constants are:

A: 5.62x106s-1 +/- 1.53x106s-1

B: 2.69x10-11cm3/s +/- 9.00x10-11 cm3/s

C: 6.27x10-27cm6/s +/- 0.98x10-27 cm6/s

Figure 6.16: Recombination Rate/Carrier Density Quadratic Fit

These values and their uncertainties are generated by the Origin 8 quadratic fit to the

data in Fig. 6.16. It is immediately apparent that there are problems with the first order

coefficient. The uncertainty exceeds the mean, and obviously the possibility of a

negative radiative coefficient is not applicable to a real recombination process.

Scattering in the data contributes to these uncertainties, and if the experiment were to

be repeated, error corrections would be necessary to reduce the uncertainty in the

coefficients.

1015

1016

1017

0

1x107

2x107

3x107

4x107

5x107

6x107

7x107

8x107

9x107

Re

co

mb

ina

tio

n R

ate

(s

-1)

Carrier Density (cm-3)

IAG 337 77K

Adj. R2 = 0.758

77

6.4.4 Recombination Rate Error Analysis

Scattering in the data might result from small instabilities in the pulse picker

extinction ratio during data collection, as well as observed asymmetries in the

transmission of the optical dispersion filters used to attenuate the beams. It was

observed that the filters do not transmit light equally when flipped in orientation.

Fresnel’s coefficients imply that no such difference should exist. This asymmetric

behavior could result from at least two sources:

a) Fabry-Perot cavity effects. If a pair of filters are mounted together in parallel to

achieve a greater level of attenuation, then they could constitute a Fabry-Perot cavity in

which multiple reflections exist. Such a cavity would have a transmission which would

not equal the product of the two face transmissions. Such parallel arrangements were

used during data collection.

b) Dust or oils might be present on the surfaces. Irregular distribution would ensure

that one location would have a unique transmission value.

Whatever the cause, once this asymmetry was noted, the transmission of every

orientation and every pair of filters was measured. These measurements were taken

with the filters situated in the locations used during lifetime beam attenuation. This

reduced the scattering in the data, but did not eliminate it. Accidental rotation of the

filter holders and other unknown effects might also have occurred.

6.4.5 Optimal Carrier Density for Radiative Output

Comparison of the measured QE of a device to that predicted by the recombination

coefficients is a way to estimate how close or far from ideal a device is working. QE may

be defined as the ratio of the radiative recombination rate to the total,

(6.4.3)

78

This may also be written in terms of the total carrier densitiy n and the SRH, Radiative

and Auger recombination coefficients,

(6.4.4)

The recombination coefficients obtained from Fig. 6.16 were used to generate a QE plot,

which appears in Fig. 6.17. Note that the QE here is calculated as a percentage.

Figure 6.17: QE for IAG 337 Using the Fit Coefficients of Fig. 6.16

Note the division of the carrier densities into approximately two parts. The first

represents the increasing radiative output as the radiative recombination processes

increase in magnitude relative to SRH ones. The second is the regime where Auger

processes start to become important, with the result that the QE decreases.

The optimal carrier density from Fig. 6.17 is compared to a value obtained from the

QE data for the devices. The QE plot for a single stage emitter is examined, and the

current density is determined which corresponds to a maximum QE value. The

associated carrier density is estimated from the relations

0.0 2.0x1017

4.0x1017

0

1

2

3

4

5

6

7

Qu

an

tum

Effic

ien

cy (

%)

Carrier Density (cm-3)

~ 2.99x1016cm-3

0.0 2.0x1016

4.0x1016

6.0x1016

6.0

6.5

Qu

an

tum

Eff

icie

ncy (

%)

Carrier Density (cm-3)

n ~ 2.93x1016cm-3 to 3.05x1016cm-3

79

J = qd / (6.4.5)

J = (6.4.6)

Here q is the charge quantum, d is the active region thickness, is the carrier lifetime, J

is the current density and is the carrier density. Dimensional analysis reveals that the

units Eq. 6.3.5 are correct, and instead of bearing the interpretation of unit charge

traversing a distance d in time , it may be understood to be the lifetime during which

the electron/hole pair remain uncombined in the active region of thickness d.

Earlier in the thesis QE was defined as the photons out per injected electron

(6.3.8)

Here we show that this definition is the same as that in Eq. 6.4.4. Eq. 6.3.8 may be

replaced by

(6.4.7)

where h is Planck’s constant, and υ is the spectrum linecenter photon frequency. We

may substitute for

(6.4.8)

where N is the number of generated photons. Similarly, we may substitute Eq. 6.4.6 for

the current,

(6.4.9)

where A is the area of the current element. The quantity Ad may be interpreted as the

volume element occupied by N carriers, so

N = nV = nAd (6.4.10)

80

and we can rewrite Eq. 6.4.9 as

(6.4.11)

This illustrates that the definitions of QE in Eq. 6.3.8 and 6.4.4 are the same.

To evaluate the optimal carrier density derived from the experimental QE data, four

100 m by 100 m mesa LED devices from the 2.5 m single stage sample were

examined. This mesa size was selected in an attempt to minimize the effects of Joule

heating. The optimal current density was determined for each from the quantum

efficiency plot, and these were then averaged. Eq. 6.4.6 was used to convert this into an

optimal carrier density, which was then compared to the result from Fig. 6.17.

Mesa Number Peak Quantum Efficiency (%) Current Density (A/cm2)

16 4.86 2666

35 3.27 2564

45 4.63 2415

71 5.41 2415

Average Optimal Current Density: 2515 A/cm2

Table 6.1: Current Densities for 2.5m Single Stage Device 100 m by 100 m Mesas

Table 6.2 contains the carrier density predictions determined by these two methods.

From Table 6.1 From Fig. 6.17 (from the A, B and C Coefficients)

2.13x1017cm-3 2.99x1016 cm-3

Table 6.2: Optimal Carrier Density Predictions

81

The optimal QE occurs at a carrier density nearly one order of magnitude higher

than that predicted by consideration of the A, B, and C coefficients. This is a little

surprising, because one would expect the non-ideal device performance from heating to

push the current density for optimal QE to lower carrier densities, not higher. The

process was repeated for two 100 m by 100 m mesas for the 500 nm single stage

device, with similar results. Tentatively, the Schottky barrier for the single stage devices

might be inhibiting the device performance at low carrier density, and pushing the peak

QE out to higher carrier densities.

However, these estimated optimal carrier densities must be further refined before

meaningful comparisons can be made. Because the background carrier density is still an

unknown quantity, the constants from the Fig. 6.16 data fit were used instead of the

actual SRH, radiative and Auger coefficients, as suggested by Eq. 6.4.4 and Eq. 6.4.6, and

these may differ. Additionally, the large uncertainty in the B coefficient needs to be

accounted for, and possibly reduced through the collection of additional data.

The QE referred to in Eq. 6.4.11 is the internal quantum efficiency of the active

region material, whereas the QE determined from the LI data is the external one. Due to

imperfect extraction of light, one would expect the internal QE to be much higher than

the external one. Yet comparison of Figs. 6.17 and 6.10 show the peak QE to be similar.

This further underlines the need to do additional refinement in the estimation of the

recombination coefficients, as discussed above.

Once more confidence is gained in the carrier density for peak QE as determined by

the recombination coefficients, the current density at which heating from Auger

recombination occurs can be estimated. This in turn will help us to understand why the

LED radiative output rolls over, and which mechanisms predominantly cause the roll

over. A calculation of the QE from reliable recombination coefficients will also show how

close or far the devices are working from the ideal, and how much power could be

potentially collected from the LED.

82

CHAPTER 7

CONLCUSIONS

7.1 Principal Findings

The activation voltage of the cascaded LED was expected to exceed that of the two

single stage devices by a factor proportional to the number of cascaded active regions.

This proved to be the case, and indicated that the cascaded LED tunnel junctions are

functioning properly at low input powers.

The IV profiles for the various mesa sizes plotted as a function of current density

deviated from ideal behavior. Theory suggests that the current density should only

depend upon the applied bias and the quantities presented in Eq. 6.2.1. Ideally, the IV

profiles for mesas of different sizes should be identical. Instead, for both the cascaded

LED and the single stage devices, the current density for a given applied bias increased

as a function of decreasing mesa size.

Radiative performance comparisons between the various devices were made at

similar carrier densities. When this was done, the radiative output of the 2.5 m single

stage device was observed to exceed that of the 500nm one by a factor comparable to

the ratio of active region thicknesses. The cascaded LED and the 2.5 m single stage

device should demonstrate similar radiant outputs at similar input powers. In fact the

cascaded device generated three to five times the maximum radiance of the single stage

device, depending on mesa size.

The WE was one figure of merit used for the purpose of comparing the

performance of different device types. Ideally, the 2.5 m single stage device and the

cascaded LED should demonstrate comparable WE values for similar input powers. In

actuality, the WE of the cascaded device consistently exceeded that of the single stage

device, especially at low input powers. The WE of the 2.5 m single stage device for a

given input power increased as a function of decreasing mesa size, while the differences

83

between those for the various cascaded LED mesa sizes were less pronounced. The

single stage device with the 500 nm active region thickness also exhibited this

characteristic of the cascaded LED, with the exception of the 400 m by 400 m mesa.

For all three devices, the WE decreased more rapidly as a function of increasing input

power as the mesa size was reduced.

Considering the WE for a single device type as a function of current density also

revealed important information. The WE for the 2.5 m single stage device increased as

a function of decreasing mesa size for all recorded current densities, while the WE for

the different cascaded LED mesas diverged as the current density was increased, with

the smaller mesas again demonstrating the larger values.

The QE was the other figure of merit to be considered. When examined as a

function of input current, the cascaded LED was expected to demonstrate a QE which

exceeds that of the 2.5 m single stage device by a factor of five. This prediction is

based on the cascaded LED architecture and the phenomenon of carrier recycling. In

fact the cascaded device QE exceeded that of the single stage device by at minimum a

factor of seven.

As will be discussed in Appendix A, several temperature dependent behaviors are

also observed for the devices. The activation voltages for the 2.5 m single stage and

the cascaded LED are approximately doubled as the temperature is decreased from 293

K to 85 K. The radiant output of the 2.5 m single stage device decreases by about one

order of magnitude as the temperature is increased from 85 K to 293 K, and the

radiance of the cascaded LED is reduced by nearly two orders of magnitude over the

same temperature range.

7.2 Avenues for Future Research

In any research topic there exists areas for continuing investigation. The first is to

determine the background carrier density for the specimen active regions for the reason

84

described above. Modeling based on the recombination coefficients could be used to

determine the current densities at which Auger non-radiative recombination becomes

important, and to compare the relative importance of this process to Joule heating. A

final avenue of investigation is the evaluation of the recombination coefficients at

multiple temperatures. This could be used to explain the LED radiant output

temperature dependency which will be discussed in Appendix A.

7.3 Final Thoughts

The single stage and cascaded LED devices considered in this thesis were a

fascinating topic of study. They illustrate why, in addition to being a subject of immense

practical importance, solid state physics represents a frontier for current scientific

research. Hybridized within the topic of this thesis were diverse theoretical and

experimental considerations. It was a challenging and rewarding research project that

the author enjoyed and through which learned a great deal.

85

APPENDIX A

ADDITIONAL SINGLE STAGE AND CASCADED LED PERFORMANCE ANALYSIS

Many interesting device properties were studied during the course of this

research. These reveal important information and avenues for continuing investigation

concerning both the single stage and cascaded device types. Included is an analysis of

specific temperature dependent device properties, as well as a consideration of the

topic of leakage currents.

A.1 Temperature Dependence of the IAG 300 Series Performance

A.1.1 Activation Voltage

Temperature affects the performance of semiconductor devices. As the

temperature of a sample is increased, the crystal lattice expands resulting in a

weakening of inter-atomic bonds and an associated decrease in the band gap [3]. This

will also cause a reduction in the activation voltage, according to Eq. 1.6.1. This bias shift

is illustrated in Fig. A.1. for the 2.5 m single stage emitter.

0.0 0.5 1.0 1.50

100

200

300

400

500

600

700

85K

293K

Cu

rrre

nt

De

nsity (

A/c

m2)

Bias (V)

0 1 2 30

100

200

300

400

500

85K

293K

Cu

rre

nt D

en

sity (

A/c

m2)

Bias (V)

a) 100 m by 100 m Mesa b) 200 m by 200 m Mesa

Figure A.1: IV Temperature Dependence for the 2.5 m Single Stage Emitter

86

There is a softer current turn on at the activation voltage for room in comparison to

cryogenic temperature, as illustrated in Fig. A.2. The range of thermally accessible band

energy states increases as a function of temperature. At higher temperatures more

carriers will occupy states energetic enough for them to transition over the active region

potential barriers at a given applied bias. This effect generates the softer diode turn on

profile.

0.2 0.4 0.6

0

50

100

85K

293K

Cu

rrre

nt

De

nsity (

A/c

m2)

Bias (V)

0.2 0.4 0.6

0

20

40

60

85K

293K

Cu

rre

nt D

en

sity (

A/c

m2)

Bias (V)

a) 100 m by 100 m Mesa b) 200 m by 200 m Mesa

Figure A.2: Temperature Dependence of Current Activation

for the 2.5 m Single Stage Emitter

The spectral FWHM increases as a function of temperature, as illustrated in Fig. A.3.

This is a consequence of alteration of the carrier distribution in the energy bands,

. (A.2.1)

Here is the intensity of the radiated light, is the Fermi-Dirac distribution,

is the joint density of states and is the energy of the excited carrier which

releases a photon with frequency υ upon decay. At higher temperatures, carriers occupy

87

a wider range of valence and conduction band energy states, and can emit a greater

range of frequencies as they radiatively transition to the valence band.

0.4 0.5 0.6 0.70.0

0.5

1.0

No

rma

lize

d In

ten

sity (

AU

)

Energy (eV)

85K

100K

150K

200K

250K

293K

0.45 0.50 0.55 0.60 0.65 0.70 0.750.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d I

nte

nsity (

AU

)

Energy (eV)

85K

100K

150K

200K

250K

293K

a) 2.5 m Single Stage Emitter b) Cascaded LED

Figure A.3: Temperature Dependence of EL Spectra for a 400 m by 400 m Mesa

A.1.2 Radiant Output

A strong dependence of radiant output on temperature exists for all the devices in

this study. Figure A.4 contains data for the 2.5m single stage device. The LED output

decreases by about an order of magnitude as the temperature is raised from 85 K to

293 K. This reduction in performance is suggested even more directly by Fig. A.5. Here

there is a temperature dependent shift in device current density associated with the

optimal QE. This reduction of the optimal current density (and hence of the optimal

carrier density according to Eq. 6.2.2) as a function of increasing temperature could

suggest that the recombination coefficients are temperature dependent. Recent

research indicates that the III-V T2SL Auger and radiative recombination coefficients are

temperature dependent [7]. Perhaps this is also the case for the GaInAsSb alloy.

88

0 1000 2000 3000 4000 5000

1E-3

0.01

0.1

85K

100K

150K

200K

250K

293KRa

dia

nce

(W

/sr*

mm

^2

)

Current Density (A/cm2)

500s Pulse 50%Duty

0 250 500 750 1000 1250 1500 1750

1E-4

1E-3

0.01

85K

100K

150K

200K

250K

293KRa

dia

nce

(W

/sr*

mm

^2

)

Current Density (A/cm2)

500s Pulse 50%Duty

a) 100 m by 100 m Mesa b) 200 m by 200 m Mesa

Figure A.4: Radiant Output Temperature Dependence for a 2.5 m Single Stage Device

0 250 500 750 1000 1250 1500 17500.0

0.5

1.0

1.5

2.0

2.5

3.0

85K

100K

150K

200K

250K

293KQu

an

tum

Eff

icie

ncy (

%)

Current Density (A/cm2)

500s Pulse 50% Duty

Figure A.5: Quantum Efficiency Temperature Dependence for a

200 m by 200 m Mesa, 2.5 m Single Stage Device

The reduction in radiant output associated with increasing temperature is even more

dramatic for the cascaded LED in Fig. A.6 than for the single stage device in Fig. A.4. The

100 m by 100 m mesa examined for the 2.5 m single stage device had a reduction in

peak output by about a factor of 25, but the same mesa size for the cascaded LED has

nearly a hundredfold reduction. Joule losses should not be as important a limiting factor

89

as for the single stage device because the cascaded LED operates at a lower carrier

density for a given current density due to the thicker per stage active region. This also

should reduce the importance of Auger effects. Another possible source of loss already

discussed in Chapter 6 is that of hole leakage past the tunnel junctions. The holes can

achieve higher energies at room as opposed to cryogenic temperatures. The thermal

excitation of the holes might combine with the thinning of the tunnel junctions at high

bias to increase the rate of non-radiative hole transport past the junctions at higher

temperatures.

0 500 1000 1500 2000 25001E-3

0.01

0.1

1

85K

100k

150k

200K

250K

293KRa

dia

nce

(W

/sr*

mm

2)

Current Density (A/cm2)

500s Pulse 50% Duty

0 100 200 300 400 500 600 700 800

1E-3

0.01

0.1

85K

100K

150k

200k

250k

293kRa

dia

nce

(W

/sr*

mm

2)

Current Density (A/cm2)

500s Pulse 50% Duty

a) 100 m by 100 m Mesa b) 200 m by 200 m Mesa

Figure A.6: Temperature Dependent Radiant Output of Cascaded LED

A.2 Leakage Currents

Guo et al [15] researched the impact of leakage currents on radiant output.

Leakage currents occur when carriers pass from one contact to the other along the

mesa wall without experiencing recombination in the active region. An applicable figure

of merit is the mesa perimeter to area ratio. As the mesa grows, this ratio decreases,

resulting in essentially more channels for carrier transport through the mesa than

around it. Just as this ratio decreases with increasing mesa size, so could the draining

90

effect of leakage currents. IV plots for mesas of various sizes may be used to evaluate

the possible presence leakage currents. Figure A.7 presents the IV data collected when

200 s current pulses with a 1 % duty cycle were applied to the 500 nm single stage

device. This represents a considerable reduction in heating compared to data collected

before with 500 s pulses and a 50 % duty cycle. The decrease in current density at

constant voltage from the smallest to the largest mesas suggests the possible presence

of leakage currents. The IV curves diverge for low input currents indicating that if this

physical effect is present, it occurs even at low input powers when device heating is

minimal. This is observed at both room and cryogenic temperatures.

0 1 2 3 4 50

200

400

600

800

1000

1200

100mx100m

100mx100m

100mx100m

200mx200m

200mx200m

200mx200m

400mx400m

Cu

rre

nt

De

nsity (

A/c

m^2

)

Bias (V)

0 1 2 3 4 5 6 7 8 9 100

200

400

600

800

1000

1200

1400

1600

100mx100m

100mx100m

100mx100m

200mx200m

200mx200m

200mx200m

400mx400m

Cu

rre

nt D

en

sity (

A/c

m^2

)

Bias (V)

a) 85 K b) 293 K

Figure A.7: IV Profiles for 500 nm Single Stage Device,

200 s Current Pulse, 1 % Duty Cycle

An examination of the wall-plug efficiency plots of Fig. 6.5 resolves this question.

For all of the devices, there is a considerable decrease in wall-plug efficiency as a

function of input power from the smaller to the larger mesas. This suggests that instead

of leakage currents causing the smaller mesas to short current between the contacts,

the larger mesas are experiencing power loss, possibly due to the presence of Schottky

91

barriers. The challenge then is not to insulate the small mesa sidewalls, but to reduce

power loss in the large mesas.

92

APPENDIX B

IA 2300 DEVICE SERIES, SINGLE STAGE AND

MQW PERFORMANCES

B.1 Spectral Output of IA2300 Series Bulk and MQW Devices

Chapter 1 describes how the band gap of a direct semiconductor determines the

line center of the emission spectrum. Not all transitions occur at the band gap, and if

phonon interaction is included some transitions might even be indirect, resulting in a

change of carrier momentum. Also, the lines may be broadened at room temperature as

a result of the thermal distribution of carriers. The result is an emission spectrum

expanded about the band gap transition frequency. Such spectra are illustrated in Fig.

B.1.

1.5 2.0 2.5 3.0 3.50.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d In

ten

sity (

AU

)

Wavelength (m)

1.5 2.0 2.5 3.0 3.50.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d I

nte

nsity (

AU

)

Wavelength (m)

a) IA 2344 b) IA 2363

Figure B.1: PL Spectra of Specimens IA 2344 and IA 2363 at 293 K

These spectral measurements were taken at room temperature by Dr. Jonathan

Olesberg shortly after the growth of each sample. They are for two of the devices from

the IA 2300 series of MQW and bulk specimens which were examined for this thesis.

Comparing the spectra of the IA 2300 series bulk samples with those of the MQW

devices revealed two interesting facts. The samples exhibit energy band gaps

93

a) IA 2341 b) IA 2344

c) IA 2354 d) IA 2363

e) IA 2372 f) IA 2375

Figure B.2: Stack Diagram for Single Stage and MQW IA 2300 Series LEDs

94

corresponding to a 2.4-2.5 m photon, the intended peak wavelength for the device

series. The spectra of the MQW specimens have a reduced full width half max in

comparison to the bulk specimens. The MQW sample IA 2344 has a FWHM of about 0.2

m, in comparison to about 0.3 m for IA 2363. The density of states for a QW is a step

function at the band edge, in comparison to a function which increases as the square

root of the energy for a bulk material device. The different density of states changes the

distribution of emitted light as described by Eq. A.2.1.

B.2 Bulk and MQW Comparisons for the IA 2300 Series

As illustrated in the stack diagrams of Fig. B.2, the MQW specimens contain three

QW, with a total active region thickness of 30 and 42 nm. The bulk specimens have

active regions with a thickness of 500nm. Radiance tests were used to gauge the

ultimate performance of each specimen. Because the bulk and MQW have different

active region thicknesses, comparisons at similar carrier densities are difficult to

achieve. In an attempt to address this issue, radiances will be compared as a function of

input power. The data in Fig. B.3 and B.4 were collected for 400 m by 400 m mesa

sizes, to which 50 s current pulses were applied to minimize the effect of contact

device heating. The bulk devices demonstrate superior performance to the QW

specimens at comparable input powers. The bulk had about 2.4 times the radiant

output of the QW devices. All of the device surfaces were observed to be relatively

defect free, and while substrate doping levels varied, they did not appear to significantly

impact the IR transmission. For example, IA 2363 has a lightly doped substrate, while IA

2375 has a heavily doped one, but both had comparable radiance values. Also, the

heavily doped quantum well device IA 2344 has a heavily doped substrate, but it still has

only about 75 % of the radiant output of IA 2363. With the exception of IA 2341, they

were Te doped at the cathode contact to a concentration of 5x1018 cm-3. This suggests

that most should have comparable Schottky potential barriers at the contacts.

95

The factor which might differentiate the performance of the bulk and MQW devices

is that of carrier density. Because the MQW devices have thinner active regions, they

will operate at a higher carrier density than the bulk specimens at similar input powers.

Non-radiative processes such as Auger recombination might be reducing the MQW

radiative outputs. QW devices can be engineered to suppress Auger recombination,

even at higher carrier densities. To more thoroughly understand the performance of

these devices, the recombination coefficients should be determined via TRPL at room

temperature. These constants could then be used to estimate the carrier densities

associated with the optimal QE for each device. These QE could in turn be used to

examine the relative importance of chip heating and Auger thermal effects for the

operation of the LEDs. Such an investigation would lead to a greater understanding of

the carrier dynamic processes and their relation to radiative output.

96

0.0 0.5 1.0 1.5 2.00.000

0.007

0.014

Ra

dia

nce

(W

/sr*

mm

2)

Input Power (W)

50s Pulse 1%Duty

0.0 0.2 0.4 0.60.000

0.007

0.014

Ra

dia

nce

(W

/sr*

mm

2)

Input Power (W)

50s Pulse 1%Duty

a) IA 2363 b) IA 2372

0.0 0.2 0.4 0.60.000

0.004

0.008

0.012

Ra

dia

nce

(W

/sr*

mm

2)

Input Power (W)

50s Pulse 1%Duty

c) IA 2375

Figure B.3: Radiant Output of IA 2300 Series Single Stage devices at 293 K

97

0.00 0.25 0.50 0.75 1.00 1.250.000

0.001

0.002

0.003

Rad

ian

ce (

W/s

r*m

m2

)

Input Power (W)

IA 2341 400mx400m Mesa 50s Pulse 1%Duty

0.0 0.2 0.4 0.60.000

0.003

0.006

0.009

Rad

ian

ce (

W/s

r*m

m2

)

Input Power (W)

IA 2344 400mx400m Mesa 50s Pulse 1%Duty

0.00 0.25 0.50 0.75 1.00 1.250.000

0.002

0.004

0.006

Rad

ian

ce (

W/s

r*m

m2

)

Input Power (W)

IA 2354 400mx400m Mesa 50us Pulse 1%Duty

Figure B.4: Radiant Output of IA 2300 Series QW Devices at 293 K

98

APPENDIX C SYSTEM NOISE REDUCTION

C.1 General Noise Reduction

Electromagnetic noise is a limitation for any laboratory with sensitive monitoring

devices. Noise can result from many sources, and its minimization is crucial for the

collection of high quality data. For both the LI and the carrier lifetime measurement

methods used for this thesis, noise was present in the data which was reduced through

the use of a variety of methods. Two fundamental noise source types exist.

a) Power source noise. Any piece of experimental equipment which requires large

amounts of power can potentially function as a noise source. These devices can radiate

electromagnetic waves and inject noise directly into an electrical network. Sensitive

detectors should be connected to a designated part of the lab grid, as far from that used

by high power devices as possible.

b) Electromagnetic (EM) noise. Distancing the outlet used by a detector cannot

isolate noise in the form of propagating electromagnetic waves. Once generated, these

can spread and reflect randomly throughout the lab environment. Minimizing cable

length is a basic precaution. The shorter a cable is, the less able it is to act as an

antenna, receiving and transmitting EM disturbances. Shielding can possibly correct this

problem. Coaxial cables should have proper shielding connected to the instrument

ground. Also, any exposed components, such as a breadboard with electronic parts,

should be covered with copper shielding grounded to the lab table. If instrument

grounding to a lab table is employed, this should in turn be grounded through a 180 M

resister to the electrical outlet used by the table instrumentation. The resister serves to

dissipate static electric charge.

Filtering can remove noise from data after collection if the noise is periodic in

nature. One can apply the Fourier techniques which will be described later in Appendix

D to isolate and remove noise, especially if the frequencies are known in advance. If

99

they are not known, then those same techniques will have to reveal the presence of

suspicious frequency peaks, and the filtering will have to occur at the scientist’s

discretion.

C.2 Operation of the MQW Test Lab Amplifier

One noise minimization parameter unique to this thesis is the proper use of the

trans-impedance amplifier in the IV experimental apparatus. The device was originally

constructed by Michael Miller. It accepts current from the photodetector and converts it

to a voltage signal. The gain ranges from 1000 to 1011 ohms. The RC time constant of the

amplifier must be selected to minimize the data noise, but preserve the shape of the

square pulse input as much as possible. As will be illustrated, pulse shape distortion can

reduce correlation in the LI plot, and make device performance appear less reliable. A

test was conducted to determine the optimal gain. Device IA2344 with a 400 m by 400

m mesa was chosen. Data was frequently collected from the 100 m by 100 m, 400

m by 400 m and 800 m by 800 m mesas, and the mesa selected for the test was

considered to be an intermediate value, representative of the group as a whole. The

current pulse duty cycles used for the IV and LI tests included 1 %, 2 %, and 5 % values.

An intermediate value of 2 % was selected for the test. The output of the amplifier was

passed to both the DAQ board, and hence the computer for processing, and an

oscilloscope for signal monitoring. The quality of the signal was evaluated according to

two transients. In Figure C.1, the lead transient demonstrates ringing, which is transient

T1 in Data Table C.1, and the lag is T2. The distortion is evident for gains above 100000.

This was the gain selected for data collection, providing a compromise between pulse

distortion and signal noise. This suggests that a minimal pulse width which should be

used for system pumping. At a gain of 100000, the pulse width should at least exceed 14

s, or the signal would distort and never reach a settled

100

Figure C.1: Square Pulse Profile of Amplifier Output, 2 % Duty Cycle

Figure C.2: Photograph of Square Pulse Profile

Pulse Width(s) Amplifier Gain T1 (s) T2 (s) 25 10000 1 13

100000 2 12

50 10000 1 8.5

100000 2 12.8

1000000 8 15

100 10000 1.6 11

100000 2.5 12.8

1000000 9.2 33

Table C.1: Current Amplifier Pulse Transients

101

value to preserve the square pulse. This effect reduces correlation in the data, possibly

due to reduced carrier excitation.

As was evident from the data in Table C.1, there was little change in the pulse

transients from 25 - 100 s, so device heating is not a problem given a sufficiently small

duty cycle. For the reasons stated, the IV and LI tests were conducted using a current

gain of 100000, a duty cycle of 2 % and a pulse width of 25 s.

C.3 Dark Currents

Dark currents are defined as the random generation of electron/hole pairs in a pn

junction depletion region by thermal energy sources. In the LIV characterization system

discussed in this thesis, dark currents are generated in the MQW test lab FGA20

photodetector, by its operation at room temperature. Figure C.4 is a plot of the detector

dark currents from the manufacturer data sheet. Even at very low reverse bias values,

micro-amps of current may be generated. Figure C.4 illustrates one method for

compensating for dark current effects in data. It contains a fifth degree polynomial fit of

Figure C.3: FGA20 Dark Current Plot

102

the photodetector current versus device current density for specimen IA2344. The

polynomial fit has a constant y intercept offset value, which is treated as the small

reverse bias dark current. This coefficient is subtracted from the detector current value,

resulting in an approximately zero offset in the data. The effect is small, but this

approach removes to a large extent the effects of dark currents in the LI plot. The y

intercept is -0.0016 mA. This is of the same order of magnitude as the dark current

presented in Fig. C.3. If this does represent the presence of a dark current in the initial

operation of the device, then it would be generated by an initial reverse bias of less than

0.02 V.

0.0 0.3 0.6-0.006

-0.004

-0.002

De

tecto

r C

urr

en

t (m

A)

Applied Current (A)

IA2344 Polynomial Fit to

Photo-Detector Current Data

Figure C.4: Polynomial Fit for Dark Current Compensation in Data

C.4 Data Averaging for the Carrier Lifetime Measurement System

The signal to noise ratio may be defined as the ratio of the data signal power to that

of the total noise. The noise may be random or periodic or a combination of both types.

If the noise is entirely random in nature, then data integration will reduce its magnitude.

Data integration refers to the averaging of multiple data sets collected in time. If the

103

noise is random, then signals of opposite sign will partially cancel. If the noise is truly

random, then theoretically any amount of noise reductions should be possible, as long

as enough data sets are integrated. This integration technique was utilized in the carrier

lifetime system. Three stages of averaging were employed:

a) The oscilloscope first collected and averaged 512 data waveforms. The average

was saved as a .csv file on a flash drive.

b) Depending on the quality of the signal, multiple files were saved and manually

averaged at a computer.

Note that this approach cannot eliminate periodic noise sources. The attempts to filter

these via Fourier analysis are described in Appendix D.

Theoretically, if the signal to noise is exclusively random, the signal to noise ratio

improves as the square root of the number of averaged data sets. Fig. C.5 plots the

signal to noise ratio for PL data signals collected by the lifetime measurement system. It

represents a signal to noise performance that is inferior to that predicted by theory.

Instead of improving by a factor or 100 from one data set to 10000, it improves by a

factor of 34. Also, the curve appears to be gradually trending towards a plateau. The

TDS5032B oscilloscope employed for this exercise can only average 10000 at a time, but

if more were possible, the signal to noise could possibly reach a constant value. For the

MDO3000 series oscilloscope used for lifetime data collection, the signal to noise will be

lower, due to the fact that it can average only 512 waveforms at a time.

C.5 Vibrations and Optical Systems

One preventable source of potentially random (or periodic) noise is the presence of

vibrations in an optical system. During the collection of data for this thesis, the IATL

laboratory was experiencing the effects of heavy construction associated with the

University of Iowa flood mitigation plan. A company specializing in the practice of

vibration testing was hired to evaluate the possible effect of construction vibrations on

104

0 2000 4000 6000 8000 10000

0

20

40

Number of Averaged Waveforms

Me

asu

red

Sig

na

l to

No

ise

Signal to Noise: 804nm Pump, 0.4MHz Rep Rate 77K

0

20

40

60

80

100 Th

eo

retic

al S

ign

al to

No

ise

Figure C.5: Signal to Noise for Signals Collected by a Tektronix TDS5032B Oscilloscope

the facility labs. The PL lab was included in this study. The optics tables in the lab are not

floated on air, so the outcome of the test was particularly important. The engineers

bounced medicine balls at a frequency of approximately 1 Hz and monitored the effect

on accelerometers placed on the optics table surfaces. In addition, the lifetime system

used for this thesis also collected PL carrier recombination data. The blue plot in Fig. C.6

is a scatter plot of data collected during the absence of vibrations, and the black was

0.0 1.0x10-7

2.0x10-7

3.0x10-7

4.0x10-7

-0.0005

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

With Vibrations

Without Vibrations

Time (s)

Sig

na

l (v

)

PL Lab Vibration Test: PL Experiment

0.000

0.001

0.002

0.003

Sig

na

l (v)

Figure C.6: Vibration Test Carrier Recombination Data

105

collected when the test was conducted. As the figure illustrates, the effect on the

system was minimal. It is unlikely that construction vibrations had any impact on the

lifetime data.

C.6 Data Smoothing

A common data processing technique can reduce the effect of random noise. This is

the process of data smoothing. Data from adjacent collection intervals (usually in time)

are averaged. The assumption is that if the collection rate is great enough compared to

the system transients, significant variation in the data is more likely to result from

random systemic fluctuations than from the phenomenon being examined. This may be

performed on the data as it is collected, by a gated integrator or an oscilloscope, or as a

post collection processing step.

-50 0 50 100 150 200 250 300 350

0.0

0.1

0.2

0.3

0.4

PL

Sig

na

l(v)

Time(ns)

IAG 337 77K No Data Smoothing

-50 0 50 100 150 200 250 300 350

0.0

0.1

0.2

0.3

0.4

PL

Sig

na

l(v)

Time(ns)

IAG 337 77K 1000 Point Data Smoothing

a) PL plot without data smoothing b) PL plot with 2000 point data smoothing

Figure C.7: Effect of Smoothing on Random Noise

106

APPENDIX D FOURIER ANALYSIS AND DATA FILTERING

Fourier analysis is concerned with the representation of a phenomenon in terms of

a series of periodic functions. The phenomenon may be periodic or a-periodic. The

former may be represented as a Fourier series, and the latter by a Fourier transform.

The motive for applying Fourier analysis to a signal will be to locate noise frequencies

within the spectrum and to filter them out, generating a signal with minimal systematic

noise.

D.1 Continuous Time Fourier Series

Assume that a system is linear and obeys the Principle of Superposition, so that a

disturbance in the system may be represented as a simple sum of individual

perturbations. For simplicity, also assume that it is time invariant. If the disturbance is

periodic, then there exists a fundamental time difference T such that for signal X(t),

X(t+T) = X(t) (D.1.1)

Assuming that no value in time less than T exists which preserves this quality, then T is

the fundamental period of the system, and

= 2/T (D.1.2)

is the fundamental angular frequency. A signal with period T may be represented as a

Fourier series based upon this fundamental frequency.

(D.1.3)

107

The series is a sum of periodic signals, with harmonically related frequencies, all of

which are integer multiples of the fundamental. The weighting coefficients are the

Fourier coefficents and are given by

(D.1.4)

This is said to be continuous in time because the temporal variable t is assumed to be

continuous in Eq. D.1.3.

D.2 Continuous Time Fourier Transform

For the representation of a-periodic signals, a different approach is required. The

Fourier transform does not assume a fundamental period. Another way of saying this is

that the period may be extended to infinity. A discrete sum cannot represent such a

signal, but a continuous one can. The system is still assumed to be linear and time

invariant and the generic, a-periodic disturbance may be approximated as an integral

sum.

(D.2.1)

Here, X(t) is the so called “time domain” representation of the phenomenon, and X(j )

is the “frequency domain representation”, with being the unspecified angular

frequency, which is the Fourier transform of the time domain variable

= 2/t (B.2.2)

Periodic signals may also be written in terms of a Fourier transform.

108

D.3 Discrete Time Fourier Series

To understand the Fast Fourier Transform, which is a crucial tool for modern signal

analysis, one must first encounter the discrete time Fourier series. For this, a

fundamental period is still assumed which fulfills the property

X[n+N] = X[n] (D.3.1)

but here n is assumed to be a discrete variable, with integer values. N is the

fundamental period of the system, the minimal time value which fulfills this

relationship. The fundamental angular frequency is

o = 2/N (D.3.2)

Due to its periodicity, x[n] may be represented as a complex exponential,

(D.3.3)

Since N maintains its function as the fundamental period,

= = = = ~ X[n]

This adds periodicity to the representation of a generic periodic signal as well. Instead of

an infinite series, the sum is evaluated only over 0 ≤ n ≤ N-1.

(D.3.4)

k = 0, 1, 2, 3……N-1 (D.3.5)

Again, the frequencies are integer multiples of the fundamental, but N distinct ones

exist, not an infinite. One application of the discrete Fourier series is to periodic sampled

data.

109

D.4 Fast Fourier Transform (FFT)

The Fast Fourier Transform, or FFT, is a method for rapidly computing a Fourier

transform from sampled continuous time data. It is based to a large degree on the

discrete time Fourier series and several assumptions illustrate this relationship. The FFT

method assumes

a) X(t) is periodic, and due to the sampling, this may be represented as X[n].

b) The period is equal to the data sample length. This might or might not be true

depending on how the data was collected. The qualities of the data determine how

applicable an FFT is to it.

D.5 Data Filtering

The purpose of applying an FFT to a data set is to locate any prominent noise

features in the spectrum, and to filter them out. However, it might or might not be

appropriate to apply an FFT to a given set. This may be illustrated with a PL data set

collected for an un-doped, bulk sample collected in August of 2013. This data was

collected at a rate of one point every 4ns. The fact that the sample is continuously

pumped by a mode locked Ti-Sapphire laser renders the data periodic. This fulfills the

first condition above. The second condition is dependent on the portion of the data

selected for FFT analysis. The data bordered by the red lines constitutes one complete

period, fulfilling the second condition. The blue region focuses more on the actual

transient, the portion useful for determining radiative and non-radiative coefficients,

but does not include one period. This violates the second condition. If this occurs,

“leakage”, or the distortion of the weighting coefficients assigned to each frequency by

the FFT might occur. However, a-periodic data selection is not guaranteed to generate

appreciable leakage. The user should perform the FFT on a complete period, and then

on the desired data, and a comparison of the two frequency coefficients should reveal

the severity of the effect. The FFT encoded in the Origin 8 data analysis program was

110

used to monitor for the presence of systematic noise in the carrier recombination

measurements.

0.0 1.0x10-6

2.0x10-6

0.000

0.003

0.006

0.009

Sig

na

l (V

)

Time (s)

IA2338 77K

Figure D.1: Temporally Resolved PL Profiles with Two FFT Selection Windows

The signal of particular concern was that associated with the mode locked laser

itself. The Ti: Saph laser has a repetition rate of 12.5 ns. An acousto-optic modulator is

used to initially generate mode locking, which will vibrate at the rep rate frequency of

80M Hz. It was always turned off during data collection, but it was possible that some RF

noise associated with the pulses could still be present in the system. Figure D.2 contains

a FFT of the data in Fig D.1.

Figure D.2: FFT Spectrum of Data in Fig. D.1

0.00E+000 5.00E+007 1.00E+008 1.50E+008

0.000

0.001

0.002

0.003

Frequency

Am

plit

ude

-3000

-2000

-10000.00E+000 5.00E+007 1.00E+008 1.50E+008

Frequency

Phase

111

Here, an 80 MHz peak with finite bandwidth was present. Fig. D.3 illustrates the noise

bandwidth for the data in Fig B.2.

6x107

7x107

8x107

9x107

1x108

-0.08

-0.06

-0.04

-0.02

0.00

0.02

0.04

Frequency (Hz)

Re

al (A

U)

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010Im

ag

ina

ry (A

U)

IA2338 77K

Real and Imaginary FFT Components

Figure D.3: 80 MHz Noise in Fig. D.2 FFT Spectrum

Filtering these frequencies would remove any contribution that they might actually

make to the carrier recombination lifetime profile. One alternative is to eliminating the

noise bandwidth coefficients and then interpolate over the gap.

6x107

7x107

8x107

9x107

1x108

-0.015

-0.010

-0.005

0.000

0.005

0.010

Frequency (Hz)

Re

al (A

U)

-0.008

-0.006

-0.004

-0.002

0.000

0.002

0.004

0.006

0.008

0.010

Ima

gin

ary

(AU

)

IA2338 808nm Pump 77K

Interpolated Real and Imaginary FFT Components

Figure D.4: Modified FFT Spectrum for Data in Fig. D.3

112

The noise bandwidth was taken to be about 0.8 MHz in width, spread around the 80

MHz signal. These coefficients were removed and a polynomial interpolation was used:

a linear one would not match the character of the surrounding FFT data. The inverse FFT

of the filtered signal appears in Figure D.5.

0.000000 0.000001 0.000002

-0.003

0.000

0.003

0.006

PL

Sig

na

l (V

)

Time(s)

IA2338 808nm Pump 77K

Figure D.5: Filtered PL Signal

The 80 MHz wobble in the data has been removed, with only a strange exaggeration of

it near the 2 s conclusion of the data. If the signal were to be used to estimate the peak

carrier density recombination rate, the first peak would be selected for this calculation.

113

APPENDIX E

CONVOLUTION

E.1 Continuous Time Convolution

Convolution is a mathematical operation on two functions, which may be labeled f

and g, which produces a third one which reflects the area of overlap between them as a

function of translation. This translation variable may represent translation either in

space or in time. Convolution is represented by an asterix appearing between the

convolved functions.

(E.1.1)

The limits of integration depend upon the nature of the functions. For those supported

on all values of , the convolution would be evaluated from - to . Eq. E.1.1 may be

interpreted as the sum of infinitesimally spaced weighted averages. Here, g(t- ) is the

weighting function for f( ). g(- ) shifts past f( ) by t. As the weighting function g(- )

shifts past the input f( ), it emphasizes different parts of it. These parts a summed up in

the integral to obtain the total overlap.

E.2 Continuous Time Impulse Response

The weighting function g(t- ) in Eq. E.1.1 has a specific interpretation in the field of

linear systems. The field of linear systems models physical systems as those which obey

the principles of superposition and scaling. That is, if H is an operator representing the

processes of the system, x1(t) and x2(t) are system inputs in a generic independent

variable t, y1(t) and y2(t) are system output, and and are scaling constants, then

y1(t) + y2(t) = H{ x1(t)} + H{ x2(t)} = H{ x1(t) + x2(t)} (E.2.1)

114

If an impulse is sent into a system to test the system behavior, then the output is said to

be the impulse response. For a linear system, this response will be the sum of the

convolved system reactions to the impulse. The impulse response may be represented

as a convolution between the system represented by operator g(t- ) and a Dirac delta

function ( ).

(E.2.2)

Thus, the system function g is the impulse response. This is a quantity of interest when

researchers want to better understand the behavior of a system under perturbations.

115

APPENDIX F

IAG 300 SERIES DEVICE PROCESSING TRAVELER DOCUMENTS

(Courtesy of Russell Ricker)

F.1 IAG 338 B

12/13/13 Etch:

AMI bath/rinse, N2 dry: 60°C, 3 minutes

AZ400K 1:4 DI Developer dip: 45 s

AMI bath/rinse, N2 dry: 60°C, 3 minutes

AZ1518: o Ramp Phase: RPM:2000 Ramp Time:1s Dwell:1s o Coat Phase: RPM:4000 Ramp Time:1s Dwell:30s o Bake:5 min at 100°C

Expose: Mask: 54,4-1 Duration: 8s Power: 450W Current:6.5A

Developer bath 1: 30s , Bath 2: 15s , DI, N2 dry o Looked under mic: Looks good o Bake: 5 min at 100°C

RIE descum: Time: 15s T: 20°C Pr: 100mT O2: 75sccm Power: 150W

Etch: T: 40°C Stir Bar: 4 Final Time: 9:35 Checked Times: 2:45, 4:00, 6:30, 7:15, 8:30 o Recipe: 15mL Chow, 3x860μL H2O2 o Notes: White matter in Chow solution o DI bath/rinse, N2 dry, Acetone bath/rinse, N2 dry, Veeco: 3.14μm

Notes: 1/8/14 Metallization:

AMI bath/rinse, N2 dry: 60°C, 3 mins.

LOR10B: o Ramp Phase: RPM:1000 Ramp Time:1s Dwell:1s o Coat Phase: RPM:2000 Ramp Time:1s Dwell:30s o Bake: 45s at 100°C, 4:30 at 150°C, 1 min. at 100°C

AZ1518: o Ramp Phase: RPM:2000 Ramp Time:1s Dwell:1s o Coat Phase: RPM:4000 Ramp Time:1s Dwell:30s o Bake: 5min at 100°C

Expose: Mask: 53,3-1 Duration: 8s Power: 450W Current: 6.5A

Developer bath 1: 40s , Bath 2: 20s , DI, N2 dry o Looked under mic: Put additional 10s in bath 2

116

E-Beam: 200Å Ti/300Å Pt/2000 Å Au at 3/3/5 Ås-1 o P0: 1.92e-6 T pd: Ti: 3.46e-7 T /Pt: 1.22e-6 T /Au: 1.29e-6 T o Rates @ Powers: Ti: 3Å/s @ 13.9% /Pt: 0.6 Å/s @ 40% /Au: 5Å/s @ 9.6% o Notes: Rotated Ti crucible, huge affect on rate; Pt still low rate, will return to graphite crucible. 1/18/14

Edwards: 400Å Cr/5000Å In at 3/10 Å-s-1 o P0: 2e-7mbar pd: Cr: 1.5e-5mbar /In: 6e-6mBar tfinal: Cr: 411Å In: 4945 Å o Rates @ Powers: Cr: 1.5Å/s at 98% /In: 2.0Å/s at 100% Xtal: 7% o Notes:

Liftoff: Remover PG: T:84°C Stir bar: 8.5 Final Time: 37 min. o Notes:

Isopropanol bath/rinse, N2 dry

Notes:

F.2 IAG 339 A

12/5/13

Etch:

AMI bath/rinse, N2 dry: 60°C, 3 minutes

AZ400K 1:4 DI Developer dip: 45 s

AMI bath/rinse, N2 dry: 60°C, 3 minutes

AZ1518:

o Ramp Phase: RPM:2000 Ramp Time:1s Dwell:1s

o Coat Phase: RPM:4000 Ramp Time:1s Dwell:30s

o Bake:5 min at 100°C

Expose: Mask: 54,4-1 Duration: 8s Power: 450W

Current:6.5A

Developer bath 1: 30s , Bath 2: 15s , DI, N2 dry

o Looked under mic: Looks okay, maybe a little overdeveloped

o Bake: 5 min at 100°C

12/6/2013

Etch: T: 40°C Stir Bar: 4 Final Time: 2:50 Checked Times: 1:00

o Recipe: 15mL Chow, 3x860μL H2O2

o Notes: rotated at 1:15

o DI bath/rinse, N2 dry, Acetone bath/rinse, N2 dry, Veeco: 0.849μm

Notes: RIE skipped

Metallization:

AMI bath/rinse, N2 dry: 60°C, 3 mins.

LOR10B:

o Ramp Phase: RPM:1000 Ramp Time:1s Dwell:1s

o Coat Phase: RPM:2000 Ramp Time:1s Dwell:40s

o Bake: 45s at 100°C, 4:30 at 150°C, 1 min. at 100°C

AZ1518:

117

o Ramp Phase: RPM:2000 Ramp Time:1s Dwell:1s

o Coat Phase: RPM:4000 Ramp Time:1s Dwell:30s

o Bake: 5min at 100°C

Expose: Mask: 53,3-1 Duration: 8s Power: 450 Current: 6.5A

Developer bath 1: 40s , Bath 2: 20s , DI, N2 dry

o Looked under mic: Put additional 10s in bath 2

HCl oxide removal: Time: 30s T: 20°C

E-Beam: 200Å Ti/300Å Pt/2000 Å Au at 3/3/5 Ås-1

o P0: 4.7e-7 T pd: Ti: /Pt: 2.77e-6 T /Au: 1.62e-6 T

o Rates @ Powers: Ti: 3Å/s @ 29.1% /Pt: 1.6 Å/s @ 35% /Au: 5Å/s @ 7.4%

o Notes: had to change position of beam for Pt a lot, still low

Edwards: 400Å Cr/5000Å In at 3/10 Å-s-1

o P0: 3e-7mbar pd: Cr: 1e-5mbar /In: 6e-6mBar tfinal: Cr: 402 Å In: 5008 Å

o Rates @ Powers: Cr: 2.0Å/s at 98% /In: 9.1->21.1Å/s at 96% Xtal: 39%

o Notes: Rough Pump leaking oil

Liftoff: Remover PG: T:84°C Stir bar: 8.5 Final Time: 37:57 min.

o Notes:

Isopropanol bath/rinse, N2 dry

Notes:

F.2 IAG 343 B

12/13/13 Etch:

AMI bath/rinse, N2 dry: 60°C, 3 minutes

AZ400K 1:4 DI Developer dip: 45 s

AMI bath/rinse, N2 dry: 60°C, 3 minutes

AZ1518: o Ramp Phase: RPM:2000 Ramp Time:1s Dwell:1s

o Coat Phase: RPM:4000 Ramp Time:1s Dwell:30s o Bake:5 min at 100°C

Expose: Mask: 54,4-1 Duration: 8s Power: 450W Current:6.5A

Developer bath 1: 30s , Bath 2: 15s , DI, N2 dry o Looked under mic: Looks good o Bake: 5 min at 100°C

RIE descum: Time: 15s T: 20°C Pr: 500mT O2: 75sccm Power: 150W

Etch: T: 40°C Stir Bar: 4 Final Time:11:00 Checked Times: 1:36,2:36,4:36,8:06 o Recipe: 15mL of 55 Citric acid:3 H3PO4,a.k.a. Chow, 3x860μL H2O2

118

o Notes: White matter in Chow solution o DI bath/rinse, N2 dry, Acetone bath/rinse, N2 dry, Veeco: 3.47μm

Notes: 1/8/14 Metallization:

AMI bath/rinse, N2 dry: 60°C, 3 mins.

LOR10B: o Ramp Phase: RPM:1000 Ramp Time:1s Dwell:1s o Coat Phase: RPM:2000 Ramp Time:1s Dwell:30s o Bake: 45s at 100°C, 4:30 at 150°C, 1 min. at 100°C

AZ1518: o Ramp Phase: RPM:2000 Ramp Time:1s Dwell:1s o Coat Phase: RPM:4000 Ramp Time:1s Dwell:30s o Bake: 5min at 100°C

Expose: Mask: 53,3-1 Duration: 8s Power: 450W Current: 6.5A

Developer bath 1: 45s , Bath 2: 25s , DI, N2 dry o Looked under mic: Put additional 10s in bath 1

E-Beam: 200Å Ti/300Å Pt/2000 Å Au at 3/3/5 Ås-1 o P0: 1.92e-6 T pd: Ti: 3.46e-7 T /Pt: 1.22e-6 T /Au: 1.29e-6 T o Rates @ Powers: Ti: 3Å/s @ 13.9% /Pt: 0.6 Å/s @ 40% /Au: 5Å/s @ 9.6% o Notes: Rotated Ti crucible, huge affect on rate; Pt still low rate, will return to graphite crucible. 1/18/14

Edwards: 400Å Cr/5000Å In at 3/10 Å-s-1 o P0: 2e-7mbar pd: Cr: 1.5e-5mbar /In: 6e-6mBar tfinal: Cr: 411Å In: 4945 Å o Rates @ Powers: Cr: 1.5Å/s at 98% /In: 2.0Å/s at 100% Xtal: 7% o Notes:

Liftoff: Remover PG: T:84°C Stir bar: 8.5 Final Time: 37 min. o Notes:

Isopropanol bath/rinse, N2

119

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[2] R.F. Pierret. Advanced Semiconductor Fundamentals. Volume VI, Second Edition, Prentice Hall (2003). [3] R.F. Pierret. Semiconductor device Fundamentals. Addison Wesley Longman (1996) [4] B.V. Olson. Time-Resolved Measurements of Charge Carrier Dynamics and Optical Non-Linearities in Narrow Bandgap Semiconductors. PhD Thesis, University of Iowa (2013) [5] E.J. Koerperick, J.T.Olesberg, J.T. Hicks, J.P. Prineas, T.F. Boggess. Active region cascading for improved performance in InAs-GaSb superlattice LEDs. IEEE Jour. of Quant. Elec. (44)12, (2008)

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[12] J.T. Olesberg, M.E. Flatte’. Theory of Mid-Wavelength Infrared Laser Active Regions: Intrinsic Properties and Design Strategies. Mid-Infrared Semconductor Optoelectronics, A.Krier, Ed. (Springer-Verlag, London, 2006)

[13] E.J. Koerperick, J.T Olesberg, J.L Hicks, J.P. Prineas, T.F. Boggess. High-Power MWIR Cascaded InAs-GaSb Superlattice LEDs. IEEE Jour. of Quant. Elec. 45(7) (2009) [14] D. Norton. Double Modulation Infrared Spectroscopy of InAs/GaSb Superlattice Materials and Devices. Critical Essay, Master of Science degree in Physics. University of Iowa (2011) [15] X.Guo, Y.L. Li, E.F. Schubert. Efficiency of GaN/InGaN light emitting diodes with interdigitated mesa geometry. Applied Physics Letters 79(13) (2001)

[16] Mircea Gheorghiu, Andrei Tokmakoff. 5.33 Advanced Chemical Experimentation and Instrumentation, Fall 2007. (MIT OpenCourse Ware: Massachusetts Institute of Technology), http://ocw.mit.edu/courses/chemistry/5-33-advanced-chemical-experimentation-and-instrumentation-fall-2007