Output limitations to single stage and cascaded 2-2.5¼m light emitting diodes
Transcript of Output limitations to single stage and cascaded 2-2.5¼m light emitting diodes
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Theses and Dissertations
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
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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.
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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.
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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
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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
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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
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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
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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
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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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[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)
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