EFFECT OF IRRADIATION DAMAGE ON GAN-BASED METAL-OXIDE

SEMICONDUCTOR HIGH ELECTRON MOBILITY TRANSISTORS AND β-GA2O3

By

SHIHYUN AHN

A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL

OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT

OF THE REQUIREMENTS FOR THE DEGREE OF

DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA

2017

© 2017 Shihyun Ahn

To my family

4

ACKNOWLEDGMENTS

First of all, I would like to thank my advisor and supervisory committee chair, Dr. Fan

Ren. His profound knowledge in the research field and passion guided me through to the right

direction in this PhD journey. Without his dedication and commitment, I would not have made

this far.

I also would like to thank the rest of the committee members, Dr. Stephen J. Pearton, Dr.

Brent P. Gila, and Dr. Peng Jiang for their kind advises and supports, widening my research

perspectives.

I could not make this happened without the support and assistance from former and

current group members, Dr. Byung-Jae Kim, Dr. Ya-hsi Hwang, Dr. Tsung Sheng Kang, Dr. Liu

Lu, Jiangcheng Yang, Patrick Carey, Chien Hsu, Lin Yuan, Yi-Hsuan Lin, Weidi Zhu, Chen

Dong, Shun Li, Yueh-Ling Hsieh, Lei Lei, Guan-Che Ting. We all had good days and bad days

but time we spent together was very meaningful and motivating. Thanks all for your helps and I

wish you all the best lucks in the future career. The support staffs from the chemical engineering

department were exceptional; Craig Smith, Preston Town, and Jim Hinnant in machine shop,

Micah Herron and Carolyn Miller in purchasing orders.

Special thanks to our collaborators; Dr. Jihyun Kim and Gwangseok Yang at Korea

University, Ivan I. Kravchenko at Oak Ridge National Laboratory, Dr. Leonid Charnyak at

University of Central Florida, Dr. Aaron G. Lind, Dr. Kevin S. Jones, Dr. David Cheney, and Dr.

Nancy J. Ruzycki in material science department, Dr. Erin Patrick and Dr. Mark E. Law in

electrical & computer engineering department, Akito Kurama in Tamura Corporation.

Finally, I thank my family for their supports and love that made a person who I am now.

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TABLE OF CONTENTS

page

ACKNOWLEDGMENTS ...............................................................................................................4

LIST OF TABLES ...........................................................................................................................8

LIST OF FIGURES .........................................................................................................................9

ABSTRACT ...................................................................................................................................13

CHAPTER

1 INTRODUCTION ..................................................................................................................15

Background .............................................................................................................................15

Dissertation Outline ................................................................................................................19

2 METHODOLOGY .................................................................................................................21

Material Characterization .......................................................................................................21 Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy .................21 Transmission Electron Microscopy .................................................................................21

Secondary Ion Mass Spectrometry ..................................................................................22 X-ray Photoelectron Spectroscopy ..................................................................................22

Ellipsometer .....................................................................................................................22

Device Fabrication ..................................................................................................................23

Photolithography .............................................................................................................23 Isolation ...........................................................................................................................23

Reactive Ion Etching .......................................................................................................24 Electron Beam Evaporation .............................................................................................24 Lift-Off ............................................................................................................................24

Rapid Thermal Annealing ...............................................................................................24 Plasma-Enhanced Chemical Vapor Deposition ...............................................................25 Atomic Layer Deposition ................................................................................................25

Device Characterization ..........................................................................................................25

Transmission Line Measurement ....................................................................................25 DC Performance ..............................................................................................................26 Off-State Drain Breakdown Voltage ...............................................................................26

Gate Lag Measurement ....................................................................................................26

3 STUDY OF THE EFFECTS OF GAN BUFFER LAYER QUALITY ON THE DC

CHARACERISTICS OF ALGAN/GAN HIGH ELECTRON MOBILITY

TRANSISTORS .....................................................................................................................28

Introduction to GaN buffer layer quality impact on dc performance .....................................28 Experimental ...........................................................................................................................29

6

Result and Discussion .............................................................................................................29

Summary .................................................................................................................................32

4 EFFECT OF PROTON IRRADIATION ENERGY ON ALGAN/GAN METAL-

OXIDE SEMICONDUCTOR HIGH ELECTRON MOBILITY TRANSISTORS ...............37

Introduction to Effect of Proton Irradiation on AlGaN/GaN MOSHEMT.............................37 Experimental ...........................................................................................................................38 Results and Discussion ...........................................................................................................38 Summary .................................................................................................................................43

5 EFFECT OF PROTON IRRADIATION DOES ON INALN/GAN METAL-OXIDE

SEMICONDUCTOR HIGH ELECTRON MOBILITY TRANSISTORS WITH AL2O3

GATE OXIDE ........................................................................................................................52

Introduction to Effect of Proton Irradiation on InAlN/GaN MOSHEMT ..............................52

Experimental ...........................................................................................................................52 Results and Discussion ...........................................................................................................53 Summary .................................................................................................................................58

6 DEUTERIUM INCORPORATION AND DIFFUSIVITY IN PLASMA-EXPOSED

BULK GA2O3 .........................................................................................................................64

Introduction to Plasma Exposed Deuterium in bulk Ga2O3....................................................64 Experimental ...........................................................................................................................65 Result and Discussion .............................................................................................................65

Summary .................................................................................................................................67

7 THERMAL STABILITY OF IMPLANTED OR PLASMA EXPOSED DEUTERIUM

IN SINGLE CRYSTAL GA2O3 .............................................................................................71

Introduction to Thermal Stability of Deuterium in bulk Ga2O3 .............................................71

Experimental ...........................................................................................................................74 Result and Discussion .............................................................................................................75 Summary .................................................................................................................................77

8 ELEVATED TEMPERATURE PERFORMANCE OF SI-IMPLANTED SOLAR

BLIND β-GA2O3 PHOTODETECTOR .................................................................................80

Introduction to Solar Blind β-Ga2O3 Photodetector ...............................................................80 Experimental ...........................................................................................................................81 Results and Discussion ...........................................................................................................82 Summary .................................................................................................................................85

9 EFFECT OF 5 MEV PROTON IRRADIATION DAMAGE ON PERFORMANCE OF

β-GA2O3 PHOTODETECTOR ..............................................................................................89

Introduction to Irradiation Damage in β-Ga2O3 Photodetector ..............................................89

7

Experimental ...........................................................................................................................90

Result and Discussion .............................................................................................................91 Summary .................................................................................................................................94

10 TERMPERATRUE-DEPENDENT CHARACTERISTICS OF NI/AU AND PT/AU

SCHOTTKY DIODES ON β-GA2O3 ...................................................................................101

Introduction to Schottky Diodes on β-Ga2O3 .......................................................................101

Experimental .........................................................................................................................102 Results and Discussion .........................................................................................................103 Summary ...............................................................................................................................107

11 EFFECT OF FRONT AND BACK GATES ON β-GA2O3 NANO-BELT FIELD-EFFECT TRANSISTORS ....................................................................................................114

Introduction to β-Ga2O3 Nano-Belt Field-Effect Transistors ...............................................114 Experimental .........................................................................................................................116 Result and Discussion ...........................................................................................................117

Summary ...............................................................................................................................119

12 CONCLUSIONS ..................................................................................................................124

LIST OF REFERENCES .............................................................................................................127

BIOGRAPHICAL SKETCH .......................................................................................................138

8

LIST OF TABLES

Table page

1-1 Comparison between common semiconductor material properties ...................................19

4-1 Effect of proton irradiation on sheet, transfer and specific contact resistance ..................50

4-2 Effect of proton irradiation on the changes ........................................................................50

4-3 Simulated potential difference ...........................................................................................51

5-1 Effect of proton irradiation ................................................................................................63

5-2 Carrier removal rate of InAlN/GaN MOSHEMTs. ...........................................................63

9

LIST OF FIGURES

Figure page

3-1 BF-XTEM showing the AlGaN/GaN interface .................................................................33

3-2 Schematics of wafer bowing for HEMT wafer ..................................................................34

3-3 Drain I-Vs of HEMTs ........................................................................................................34

3-4 Transfer characteristics of HEMTs ....................................................................................35

3-5 Gate-lag pulsed measurements ..........................................................................................36

4-1 SRIM simulation results ....................................................................................................45

4-2 Drain I-V characteristics ....................................................................................................46

4-3 Mobility and sheet carrier concentration reduction ...........................................................47

4-4 Transfer characteristics ......................................................................................................48

4-5 Gate I-V .............................................................................................................................49

5-1 Drain I-V ............................................................................................................................59

5-2 Gate I-V of MOSHEMTs before and after ........................................................................60

5-3 Sub-threshold curve. ..........................................................................................................61

5-4 Gate lag measurement ........................................................................................................62

6-1 SIMS profiles of 2H in Ga2O3 exposed to deuterium plasma ............................................69

6-2 SIMS profiles of 2H in Ga2O3 exposed to deuterium plasma ............................................69

6-3 Fraction of 2H remaining ...................................................................................................70

7-1 SIMS profile of 2H implanted into Ga2O3 (100 keV, 1015 cm-2) before and after ............78

7-2 Percentage of retained 2H implanted into Ga2O3 (100 keV, 1015 cm-2) ............................78

7-4 SIMS profile of 2H in Ga2O3 exposed to deuterium plasma at 270°C. .............................79

8-1 Schematic ...........................................................................................................................86

8-2 Real-time current of the Si implanted β-Ga2O3 photoconductor biased at 5 V .................87

8-3 Temperature dependent net photocurrent increase ............................................................87

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8-4 Temperature dependent photo-to-dark current ratio (PDCR) ............................................88

9-1 Schematics of the Si implanted β-Ga2O3 photo-diode under UV lamp. ............................95

9-2 SRIM simulation ................................................................................................................96

9-3 Real-time current of the Si implanted β-Ga2O3 photo-diode biased at 5V ........................97

9-4 Difference in photo current and dark current. ....................................................................98

9-5 Photo to dark current ratio .................................................................................................99

9-6 Responsivity of β-Ga2O3 photo-diode under 254 nm UV light .......................................100

9-7 Quantum efficiency of β-Ga2O3 photo-diode under 254 nm UV light ............................100

10-1 Schematics .......................................................................................................................108

10-2 I-V characteristics ............................................................................................................109

10-3 Schottky barrier height and ideality factor .......................................................................110

10-4 Reverse biased I-V characteristics ...................................................................................111

10-5 Reverse biased breakdown voltage and figure of merit (FOM), VBR2/Ron ......................112

10-6 Reverse recovery time measurement ...............................................................................113

11-1 Schematic .........................................................................................................................120

11-2 TEM images .....................................................................................................................121

11-3 Drain I-V of β-Ga2O3 flake based FET with front, back, and both gates at 25°C. ..........122

11-4 Typical transfer characteristics of β-Ga2O3 flake FET with front or both gates. .............122

11-5 Gate voltage dependent drain current and gate leakage current ......................................123

11

LIST OF ABBREVIATIONS

2DEG 2 Dimensional Electron Gas

AES Auger Electron Spectroscopy

ALD Atomic Layer Deposition

AlGaN Aluminum Gallium Nitride

AlN Aluminum Nitride

BOE Buffer Oxide Etchant

BSD Back Scattering Electron Detector

C-V Capacitance-Voltage

DLTS Deep-Level Transient Spectroscopy

Ec Conduction Band Edge

EDX Energy Dispersive X-Ray Spectroscopy

EV Valence Band Edge

FET Field Effect Transistor

FLOODS FLorida Object Oriented Device Simulator

GaN Gallium Nitride

HEMT High Electron Mobility Transistor

ICP Inductively Coupled Plasma

InAlN Indium Aluminum Nitride

IR Infrared

I-V Current-Voltage

LOR Lift-Off Resist

MBE Molecular Beam Epitaxy

MISHEMT Metal Insulator Semiconductor High Electron Mobility Transistor

MOCVD Metal Organic Chemical Vapor Deposition

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MOSFET Metal Oxide Semiconductor Field Effect Transistor

MOSHEMT Metal Oxide Semiconductor High Electron Mobility Transistor

NSS Surface States Density

PECVD Plasma-Enhanced Chemical Vapor Deposition

PMGI Poly(methyl glutarimide)

PMMA Poly(methyl methacrylate)

PR Photo Resist

Rc Contact Resistance

RI Refractive Index

RIE Reactive Ion Etching

Rs Sheet Resistance

RT Transfer Resistance

RTA Rapid Thermal Annealing

SBH Schottky Barrier Height

SD Secondary Electron Detector

SEM Scanning Electron Microscopy

SiC Silicon Carbide

SRIM Stopping and Range of Ions In Matter

TEM Transmission electron microscopy

TLM Transmission line measurement

UV Ultra Violet

VTH Threshold voltage

XPS X-Ray Photoelectron Spectroscopy

13

Abstract of Dissertation Presented to the Graduate School

of the University of Florida in Partial Fulfillment of the

Requirements for the Degree of Doctor of Philosophy

EFFECT OF IRRADIATION DAMAGE ON GAN-BASED METAL-OXIDE

SEMICONDUCTOR HIGH ELECTRON MOBILITY TRANSISTORS AND β-GA2O3

By

Shihyun Ahn

May 2017

Chair: Fan Ren

Major: Chemical Engineering

The effect of buffer layer quality on dc characteristics of AlGaN/GaN high electron

mobility was studied. HEMTs were fabricated on the same aluminum concentration of

AlGaN/GaN substrate with different thickness and defect densities. Their dc performances show

similar results however, one with more defects showed 71% drain current reduction in the gate-

lag pulsed measurements.

The effects of proton irradiation on AlGaN/GaN and InAlN/GaN MOS-HEMTs’ dc

performances were also studied. At lower irradiation energy, devices showed severe current

reduction upto 95.3% as well as with the reduction of transconductance upto 88%. In addition,

devices also showed more degradation in their dc performance at higher irradiation doses. These

degradations are due to the damage done by collisions between the incident protons and the

semiconductor lattice causing the reduction of electron densities and saturated carrier velocities

in the 2DEG.

The further proton irradiation study was conducted on photodetector fabricated on Si-

implanted β-Ga2O3 bulk substrate showing higher responsivity with higher irradiation doses. The

photodetectors were also characterized at the different temperature with no significant

14

degradation in its sensitivity upto 350°C, showing its robust material properties to withstand

harsh environmental conditions such as high temperature and radiation abundant atmosphere.

The thermal stability of the differently implanted deuterium into bulk β-Ga2O3 was also

studied. Ion implanted deuterium in β-Ga2O3 showed similar thermal stability in ZnO with ~60%

remaining after 500°C annealing. For plasma treated deuterium, the estimated diffusivity was 6.4

х 10-13 cm2/V∙s.

The electrical properties of the β-Ga2O3 were evaluated through fabrications of Schottky

diodes and field-effect-transistors on the β-Ga2O3 with different metallizations. The barrier

height value of 1.07 eV was obtained for Ni/Au based Schottky diode on Si doped β-Ga2O3 and

1.04 eV was obtained for Pt/Au based Schottky diode on undoped β-Ga2O3. Also, the β-Ga2O3

nano-belt FET was fabricated using Al2O3 and SiO2 as the gate insulator for the front and back

gate. Employing both gates showed its advantages in the current modulation with no electrical

breakdown upto biases of VDS = +100V and VGS = -100V.

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CHAPTER 1

INTRODUCTION

Background

With the increase in needs for higher power and higher frequency as the current

technology advances in various fields including optoelectronics, communications, power devices,

etc., the Si-based (semiconductor) transistors have grown to meet the needs however; there are

still some difficulties that cannot be overcome due to the materials’ limit. Indeed, there have

been a lot of efforts to solve those difficulties through compound semiconductor. In such, group

III-V compound semiconductors show their superior intrinsic properties shown in table 1-1.

GaAs’ higher mobility over Si promises its usage in high speed applications. Also SiC, GaN, and

β-Ga2O3 have superior breakdown field over Si, which are crucial factors for power devices. III-

V compounds semiconductors show their abilities to sustain their material properties at harsh

environment, such as radiation rich surrounding or at elevated temperature.

Transistors are semiconductor devices that are to either amplify or to switch electronic

signals and electrical powers. They are fabricated on the semiconductor material with three

electrodes; source (S), drain (D), and gate (G). Source and drain are to form Ohmic contacts to

the semiconductor forming channels for electrons to flow, and gate is to form either Schottky

contact or metal-insulator-semiconductor contact to modulate the channel. With the gate’s

modulation, the output electrical signals or power can be higher than the input power. These days,

Si is commonly used semiconductor material for its easy accessibility and matured technologies.

However, as demands for high power and frequencies as well as for ability to operate in harsh

environments increases, Si based transistors show their limits due to their intrinsic property limit.

In the other hands, the formation of heterojunctions in III-V composite materials such as

AlGaN/GaN or InAlN/GaN material with excellent physical and chemical stabilities as well as

16

outstanding electrical properties including high breakdown electric field, electron motilities, and

radiation hardness.

The AlGaN/GaN and InAlN/GaN based transistors high electron mobility transistors

(HEMTs), which have been intensely studied for their promising features in high power, high

temperature, and high frequency applications as mentioned above. These features promise its

candidacy for next generation of power transistor technology in hybrid electric vehicles inverter,

advanced radar system, and ground or in space communication system[1-3]. For those reasons,

several studies focused on pushing the limit of the HEMTs by improving material quality,

optimizing epi-layer structures, and designing and processing device fabrication[4, 5]. These

days, the-state-of-the-art HEMTs can take up to electron mobility of over 1500 cm2/V-s and

electron saturation velocity of around 2.5 × 107 cm/s. A current-gain cutoff frequency (fT) of 225

GHz with a gate length (LG) of 55 nm, and a power-gain cutoff frequency (fmax) of 300 GHz with

a gate length of 60 nm have been demonstrated[6]. Likewise, 2 dimensional electron gas (2DEG)

forms owning to the energy band gap discontinuity. The presence of piezoelectric effect and

spontaneous polarization in III-nitride induce high carrier concentration within 2DEG, which

could reach above 1013 cm-2. This can result in high current density without conventional doping

like in silicon based semiconductor business, thus potentially reduces the cost of doping

implantation. High breakdown electric field due to large band gap of GaN (3.4eV) and Ga2O3

(~4.9eV) also enabled III-V composite materials to handle high voltage which allows their usage

in high power application.

Furthermore, the radiation hardness of III-nitride based transistors highlights their

prospective usages in space missions and military applications. In case of space application,

usually satellites are orbiting above 300 km from the earth, which are within Van Allen belts.

17

The Van Allen belts consist of high energy particles ranging from few MeVs to several hundred

MeV, where its inner altitude ranges from 100s to 6,000 km and outer altitude exits up to 60,000

km from the earth[7]. In the space, high energy particles including protons, neutrons, electrons,

and heavy ions, can change the lattice atoms of the irradiated devices; likely degrade

performance of the device[8]. In order to simulate the space application environment, proton

irradiation of HEMTs devices at different MeV energies have been experimented. According to

Lu, no degradation of the device was observed at 5 MeV proton irradiation of fluence up to 2 ×

1013 cm-2, which is equivalent to over 100 years exposure to the low earth orbit’s dose[9]. Higher

energy irradiation showed less degradation in device’s dc performance was reported due to less

damage in the 2 dimensional electron gas region (2DEG)[10, 11].

Despite of those HEMTs’ superior material qualities mentioned previously, the lack of

reliability limits its potential usage. The high gate leakage and trap-related effects during the

device operation results in reduction of drain current and compromises its radio frequency device

performance. The reliability issue of the HEMTs can be accounted for hot electron induced

degradation and current collapse[12].

During the device operation with hot electron-induced degradation[13], under higher

drain bias, the electron obtain high energy in the channel to be trapped in between device’s

surface and AlGaN, InAlN barrier layer or in GaN buffer layer. Hot-electron also can induce

permanent traps in regions between gate to drain which cause surface depletion, increase sheet

resistance, and reduce gate-drain electric field. These results in threshold voltage shift and

increase in drain resistance thus reducing saturation drain current[13]. The current collapse is

another phenomenon that reduces drain current in high voltage device operation. The current

collapse can be attributed to surface trapping on the surface, in AlGaN barrier layer, in GaN

18

buffer layer, resulting in dispersion between dc and pulsed current characteristics[13]. Also,

donor-like traps induced by strong polarization of the material capture electrons tunneling from

gate to drain forming a so-called “virtual gate”, depleting electrons in the channel thus, the

reduction of the drain current is observed[14].

In order to overcome the reliability issues of GaN based HEMTs, different methods have

been employed. Surface passivation of different dielectric on the HEMTs or surface treatments to

terminate dangling bonds on surface to reduce the current dispersion showed enhancement in

device reliability [15-19]. Although, the surface passivation and surface treatments were reported

to help to enhance the reliability problems, under dc and rf stress conditions of SiNx and SiO2[20]

passivated AlGaN/GaN HEMTs, the hot-electron induced device degradation has been observed.

Instead of the surface modification, the metal-oxide-semiconductor HEMTs (MOS-HEMTs) are

studied. Different oxides have widely been studied, using SiO2[21, 22], Si3N4[20], and other

oxides[20, 23] as gate dielectrics to solve the problems mention in above. Al2O3 deposited by

atomic layer deposition (ALD) shows its advantage in perfect conformity, low defect density,

low stress, and excellent adhesion[24, 25]. Moreover, Al2O3 has high dielectric constant (k~10),

high breakdown field (5-10 MV/cm), thermal stability, and chemical stability.

Gallium oxide (Ga2O3) is another promising candidate for next generation compounds

semiconductor material with excellence in its material properties including high electric

breakdown field, and solar-blindness. With the presence of large band gap (4.8-4.9 eV), the

expected Baliga’s figure of merits (FOM) are much larger compare to the other compound

semiconductor materials as well. A capability of mass production adds more values to the

material evaluation beyond its material superiority.

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Table 1-1 Comparison between common semiconductor material properties

Material Si GaAs 4H-SiC GaN β-Ga2O3

Band gap (eV) 1.1 1.4 3.3 3.4 4.8-4.9

Electron Mobility

(cm2/Vs) 1,400 8,000 1,000 1,200 300

Breakdown Field (MV/cm) 0.3 0.4 2.5 3.3 8

Thermal Conductivity

(W/cmK) 1.5 0.5 4.9 2.0 13.6, 22.8

Relative dielectric constant 11.8 12.9 9.7 9.0 10

Baliga’s FOM (to Si) 1 15 340 870 3,444

Dissertation Outline

This dissertation covers two main topics; the effects of proton irradiations on GaN-based

metal-oxide-semiconductor high electron mobility transistors (MOS-HEMTs) and β-Ga2O3 bulk

substrates as well as the reliability study of devices fabricated on β-Ga2O3.

Chapter 1 covers the background knowledge of the III-V compound semiconductor

transistors, especially, the properties and current status of gallium nitride (GaN) based high

electron mobility transistors as well as with their reliability issues, which leads into metal-oxide

semiconductor high electron transistors. Further characteristics of β-Ga2O3 are discussed as well.

Chapter 2 provides the general overviews of the methodologies and technics that are

utilized in this dissertation. Chapter 3 covers the effect of GaN buffer layer on HEMTs dc

performance. Chapter 4 covers the effects of proton irradiation energies on AlGaN/GaN

MOSHEMTs, and chapter 5 covers the effect of proton irradiation doses on InAlN/GaN

MOSHEMT. These two chapters are to serve the effect of proton irradiation on these

MOSHEMTs’ dc characteristics and reliabilities.

Chapter 6 and 7 covers deuterium diffusion and implantation to bulk β-Ga2O3 substrates

using various methods and activation energies of differently implanted deuterium are extracted.

20

Chapter 8 covers the effect of elevated temperature on performance of Si-implanted solar blind

β-Ga2O3 photodetector. Chapter 9 covers the effect of proton irradiation damage on the

performance solar blind solar blind β-Ga2O3 photodetector. Chapter 10 covers the effect of

temperature on dc characteristics of Ni/Au and Pt/Au Schottky diodes fabricated on β-Ga2O3 and

chapter 11 covers the effect of front and back gate on β-Ga2O3 nanobelt field effect transistor.

Chapter 12 provides a summary and conclusion of all the topics that are discussed in my

dissertation.

21

CHAPTER 2

METHODOLOGY

Material Characterization

Scanning Electron Microscopy and Energy Dispersive X-ray Spectroscopy

Scanning Electron Microscopy is a tool that produces images of the target sample by

scanning the surface with beam of electrons. The electron gun of the SEM emits electrons in the

energy range of 1-40 keV and those emitted electrons collides onto the target surface. The

excited atoms of the sample emit either secondary electrons or backscattered electrons upon the

collisions. By analyzing the intensities of emitted electrons from the excited atoms, the machine

produces an image.

Energy Dispersive X-ray Spectroscopy is a technique that is embedded in the SEM

system, where it uses the X-ray intensities and energy generated upon the electron collision to

quantify the elements present in the target. Each element has its unique atomic structure that

emits its unique electromagnetic emission spectrum. Once the high energy electrons knock

elements’ electrons from their orbits, they create vacancies. At higher energy state electron in the

orbit then falls into fill the vacancy, thus emitting excessive energy. Detector monitors the

emission of kinetic energy and number of electrons during the electron irradiation. By analyzing

this electromagnetic emission spectrum, the system gives semi-quantitative elemental analysis.

Transmission Electron Microscopy

Transmission Electron Microscopy is a technique that uses a beam of electrons to

transmit through an ultra-thin sample to produce a high magnification image. As the electrons

pass through the sample, they are scattered by the electrostatic potentials set up by the

constituent elements in the sample and by analyzing the scatters, the system can provide detailed

22

images. These images can be used to study the growth of layers, composition, and defects in the

semiconductors.

Secondary Ion Mass Spectrometry

Secondary Ion Mass Spectrometry is an analytical method to examine the composition of

the solid surface with high detection limit by sputtering off the surface under an ultra-high

vacuum environment. It bombards the surface with focused beam of high energetic ions (10 – 40

keV) such as Cs+, O2+, O-, and Ar, on the target specimen’s surface. Small fraction of the ejected

atoms from the bombardment is ionized either positively or negatively, which are secondary

electrons. Those secondary electrons are detected by a mass spectrometer to determine the

elemental, isotopic, or molecular composition of the surface profile.

X-ray Photoelectron Spectroscopy

X-ray photoelectron spectroscopy is a surface-sensitive qualitative and quantitative

spectroscopic technique that can gives elemental composition, empirical formula, chemical state

and electronic state of the elements within a material. The beam of x-rays is irradiated onto the

target material and the kinetic energy and number of electrons are measured simultaneously.

Each element has its own sets of XPS peaks at the characteristic binding energies and by

monitoring the emission of kinetic energy and number of electrons during the x-ray irradiation,

the surface chemistry of the specimen can be projected.

Ellipsometer

Ellipsometer uses an optical technique to measure the dielectric properties of a thin film.

It measures the change in polarization as light reflects or transmits from a material structure and

compares it to a model. Because the signal depends on the film’s thickness and material

properties (refractive index), sample properties can be extracted by fitting an experimental curve

into a model.

23

Device Fabrication

Photolithography

Photolithography is the process of the transferring of geometric shapes on a mask (or a

reticle) to a photoresist (PR) covered surface of a semiconductor wafer. The photolithography is

widely used technique in the semiconductor fabrication process that is very critical.

Photolithography requires photoresist, reticles, exposure system, and developer.

Photoresist (PR) commonly consists with base resin, photo active compound (PAC), and

organic solvent. Depending on the solubility of the PR after the exposure, it determines negative

or positive PR. The simple steps of photolithography consist with coating, exposure, and

developing. After coating of PR, the mask with target pattern is placed in between the exposure

light source to the PR covered wafer and the PR covered wafer is exposed then developed in the

developer solution transferring the opposite tone of patterns on the mask.

Photolithography can be used as a mask for either wet or dry etching or creating patterns

for metal deposition, which are crucial process steps in the device fabrication.

Isolation

Isolation is a process to eliminate the leakage current among the devices that are

fabricated on semiconductor materials. This process can be achieved in either way; a mesa

etching or ion implantation. Mesa etching technique is to physically remove a semiconductor

material in between each device. Both wet and dry etching can be utilized, and patterned PR is

commonly used as a mask for a desired pattern. In doing so, a possible leakage path through the

semiconductor channel is removed, providing a good isolation of each device to device. Another

technique is to intentionally damage the semiconductor surface (a possible leakage path) between

devices by implanting high energy ions such as H, He, N, O, Zn. The implantation causes the

vacancies formation within the semiconductor leading towards a good isolation.

24

Reactive Ion Etching

Reactive Ion Etching (RIE) is one of dry etching techniques that utilize the plasma to etch

the target sample’s surface. The oscillating electric field created from RIE chamber electrodes

ionizes the gas molecules, move their outer shell electrons from parent atom thus igniting plasma.

Depending on the volatility of the etched product of gas precursor and target wafer, this plasma

can both chemically and physically removes particles on the wafer surface. RIE can provide very

anisotropic etch profiles but can leave the ion bombardment damages on the specimen.

Electron Beam Evaporation

Electron beam evaporation is a common deposition method to deposit metals on the

sample. In a high vacuum chamber (

25

throughout the semiconductor. The system consists of either high intensity lamps or lasers[26]

allowing system to reach the target temperature in a short period of time. For Ohmic contact

formation, the annealing temperature, time, and the flowing gas plays important roles in

achieving low resistance metal contact on semiconductor.

Plasma-Enhanced Chemical Vapor Deposition

Plasma-Enhanced Chemical Vapor Deposition is a widely used process that is used to

deposit conformal dielectrics. These dielectrics can provide passivation as well as improve

device performance by changing the semiconductor surface chemistry. The use of plasma

provides a high deposition rate of dielectric as well as its ability to operate at high enough

pressure, lowering down the equipment cost. The dielectric film formation can be controlled by

the different precursor gases, flow rate of each gas, temperature, pressure, and rf power of

plasma generation.

Atomic Layer Deposition

Atomic Layer Deposition is another dielectric film deposition technique that is based on

the sequential gas phase chemical process on a solid surface. The dielectric film is deposited

atomically layer by layer on a surface that provides high quality of film. As a result, the

thickness and quality of the deposited film is well controlled, however, due to its layer by layer

deposition method, the deposition rate is slow.

Device Characterization

Transmission Line Measurement

Transmission Line Measurement (TLM) is a method to quantify Ohmic contacts. A

typical transmission line consists with series of rectangular Ohmic contacts with different gaps

on a single mesa. By following the Ohmic law (V = IR), each resistance can be measured on

each gap. By plotting the resistance versus the gap, it provides a linear curve. The slope is the

26

sheet resistance, Rs reported in Ω/□, of the channel, which is a normalized resistance of

semiconductor material. In the ideal case of the plot, the intercept should be 0 if there is no

additional resistance besides the material. However, the contact has resistance itself and the

intercept represents either transfer resistance (Rs) in Ω/mm or contact resistance (Rc) in Ω-cm2

depending on the orientation of the device (lateral or vertical respectively).

DC Performance

DC performance is a way to characterize the performance of field effect transistor. It

consists of drain and gate current-voltage (I-V) and transfer characteristics. The voltage is

applied to the three terminals, source, gate, and drain of the FET and current is monitored. Drain

I-V shows the current modulation of the device including its pinch off and saturation current.

Gate I-V shows the blocking voltage and Schottky characteristics to evaluate the effectiveness of

gate modulation.

Off-State Drain Breakdown Voltage

Off-State Drain Breakdown Voltage is a key feature for high power switching

applications which can be operated at lower current level to reduce the diameter of cable. To

measure the device’s off-state drain breakdown voltage, the gate voltage (VG) is held constant

voltage where the current is pinched off. Meanwhile, the current is monitored as the drain

voltage (VD) is increased. Off-state breakdown voltage is a drain voltage that the current will

abruptly increases. The typical off-state drain breakdown voltage’s current set point for lateral

HEMT device is 1 mA/mm.

Gate Lag Measurement

Gat lag measurement is a technique that uses pulsed gate voltage to estimate traps on the

HEMT surface. While the drain voltage is held at constant, the gate voltage is pulsed from

beyond pinch-off to a voltage of an on-state at a frequency range. The presence traps in the

27

material will cause the device to response differently from the gate pulse to dc measurements.

When the device is pulsed from off-state to on-state at high frequency, the trapped charges does

not have enough time to responds resulting in the reduction of current as compared to the current

in dc mode operation.

28

CHAPTER 3

STUDY OF THE EFFECTS OF GAN BUFFER LAYER QUALITY ON THE DC

CHARACERISTICS OF ALGAN/GAN HIGH ELECTRON MOBILITY TRANSISTORS

Introduction to GaN buffer layer quality impact on dc performance

Owing to the large bandgaps in the AlGaN/GaN heterostructure as well as the high

electron mobility and breakdown fields, AlGaN/GaN high electron mobility transistors (HEMTs)

are promising candidates for high power and high current applications in advanced radar

systems, inverter units in hybrid electric vehicles, space and satellite communication

networks[27-29]. AlGaN/GaN HEMTs are commonly grown on silicon carbide (SiC) or

sapphire. It is well established that epitaxial films grown on sapphire or SiC typically have

dislocation densities in the order of 108-1010 cm-2 due to their thermal coefficient and lattice

mismatches[21, 30, 31]. SiC is an excellent substrate candidate for power applications because of

its superior thermal conductivity, however, the cost of SiC substrates is high. Therefore, despite

its low thermal conductivity, sapphire has an advantage in its lower price and often superior

surface quality.

A number of studies have reported that optimizing the parameters of GaN buffer layer

growth such as process pressure, precursor flow rate, additional AlN interlayer and GaN buffer

layer thickness can reduce the defect densities which are caused by lattice mismatch[21, 30, 31].

Also, the thinner GaN buffer layers on SiC have been reported to have higher threading

dislocation density in the GaN, but lead to higher off-state breakdown voltages[32, 33].

However, thick GaN buffer layers on sapphire and their impact on dc performance has not been

widely studied.

In this work, the impact of GaN buffer layer quality on dc and gate-lag pulse performance

of AlGaN/GaN HEMTs was investigated. Transmission electron microscopy (TEM) was used to

determine the dislocation density in the GaN buffer layer of the HEMT samples. Drain I-Vs,

29

transfer characteristics and gate pulsed drain I-V characteristics of the HEMTs were measured to

establish a correlation between the AlGaN/GaN HEMTs’ dc and gate-pulsed performance and

GaN buffer layer quality.

Experimental

AlGaN/GaN HEMT structures grown by metal organic chemical vapor deposition

(MOCVD) on sapphire substrates were acquired from two different vendors. Both types of

wafers had 24% Al concentration in the AlGaN barrier layer. One type of wafer had a 5 μm GaN

buffer and the other had a 2 μm GaN buffer layer. HEMT fabrication was started with mesa

patterning by conventional optical lithography. The mesa-etching was achieved using a Unaxis

Shuttle-lock Reactive Ion Etcher with Inductively Coupled Plasma Module (ICP) for device

isolation with a Cl2/Ar plasma. Ohmic metallization of electron-beam evaporated Ti/Al/Ni/Au

(25nm/125nm/45nm/100nm) was patterned by lift-off and subsequently rapid thermally annealed

at 850ºC in a flowing N2 ambient for 45seconds. Schottky gates of e-beam evaporated Ni/Au

(20nm/80nm) with 100 µm width and 1 µm length were obtained by standard lift-off patterning.

DC current-voltage (I-V) characteristics were measured using an HP4156 parameter analyzer,

and gate-lag pulse measurements were measured using an Agilent 8114A pulse generator.

Result and Discussion

Bright-field cross-sectional TEM (BF-XTEM) was used to quantify the defect density in

the HEMT wafers. Figure 3-1 displays the cross-sectional transmission electron microscopy

(XTEM) pictures of AlGaN/GaN surface for the top layers of A) HEMT structure with 2 μm

GaN buffer layer and B) HEMT structure with 5 μm GaN buffer layer as well as the

GaN/sapphire interface for the C) HEMT structure with 2 µm GaN buffer layer and D) HEMT

structure with 5 µm GaN buffer layer. Dislocations are seen shown to originate near the

30

GaN/sapphire interface and propagate up through the AlGaN/GaN interface to the surface of

both HEMT wafers. However, it is clear that the HEMT structure with 2 μm GaN buffer layer

exhibited a much higher defect density. The defect density near the surface for the two substrates

was calculated from XTEM taken along the length of the TEM lamella based on the thickness

measured during lamella formation with a focused ion beam (FIB). In the case of the 5 μm GaN

buffer layer substrate, the measured defect density was 5×108 cm-2, while an order of magnitude

higher measured defect density of 7×109 cm-2 was observed for the 2 μm GaN buffer layer

sample.

Although the HEMT wafer with 5 µm GaN buffer had one order less defect density, this

wafer was under a high strain. As shown in Figure 3-2, the HEMT wafer with 5 μm GaN buffer

layer was significantly bowed. A height difference of 70 µm across the 2” wafer was measured

with a much smaller radius of curvature of 5 m. On the contrary, the HEMT wafer with 2 μm

GaN buffer layer was flat and the wafer height difference across 3” wafer was around 21.25 µm

with a radius of curvature of 45 m. These results indicate that the HEMT wafer with 5 μm GaN

buffer layer was highly strained by with lower defect density.

Figure 3-3 shows the drain current-voltage (I-V) characteristics of HEMTs fabricated on

the different thickness of buffer layers. The HEMTs were measured by sweeping VDS from 0V to

5V while VG started from +1V with a step of -1V till reaching pinch-off. There was little

difference in the saturation current observed at VDS = +5V. Transfer characteristics of HEMTs

fabricated on different buffer layers were measured at VDS = +5V and Vg was swept from -5V to

0V, as illustrated in Figure 3-4. The thicker buffer GaN layer HEMTs showed marginally higher

drain current at VG = 0V than the thinner GaN layer HEMTs, with current densities of 305 ± 35

mA/mm and 302 ± 36 mA/mm respectively. The peak transconductance for 5 µm GaN buffer

31

layer HEMTs was 111 ± 7 mS/mm and for the thinner GaN layer HEMTs was 107 ± 4 mS/mm,

showing only a slightly higher value for the thicker GaN layer HEMT. Using the linear region of

the drain I-V curve, the mobility and carrier concentration were calculated using a charge control

model[12, 34]. The calculated mobility and carrier concentration for thicker GaN layer HEMTs

were 989 cm2/V·S and 8.45 х1012 cm-2 and for thinner GaN layer HEMTs were 907 cm2/V·S and

8.63 х 1012 cm-2. These similarities in their dc characteristics suggest that the defect density in

GaN buffer layer has a minimal influence on HEMT dc performance over this range of buffer

thicknesses.

Furthermore, gate-lag pulse measurements were applied to evaluate the effect of the

different GaN buffer layers on material electrical quality. In this method, the drain current (IDS)

was recorded while gate voltage (VG) was pulsed. The normalized drain current-gate voltage

(VG) and pulsed measurements are shown in Figure 3-5 A and B. VG was pulsed from -5V to the

voltage indicated on the x-axis in Figure 3-5 at 100 kHz frequency with 10% duty cycle while

VDS was kept +5V. As shown in Figure 3-5 A, a dramatic reduction of drain current was

observed during the gate-pulsed measurement for the HEMT with 2 µm GaN buffer layer. When

the gate was pulsed at a fixed drain voltage, some of the surface traps become charged by

trapping hot electrons[12]. If the gate is pulsed above a certain high frequency, the traps do not

have enough time to detrap. Those charged traps acted as a virtual gate and formed an additional

depletion region between gate and drain electrodes, and reduced the drain current. However,

there was no drain current reduction during the gate pulsed measurement for the HEMT with 5

µm GaN buffer layers. On the other hand, there was 79 ± 10 % of drain current reduction in the

gate pulsed measurement for the HEMT with 2 µm GaN buffer layer at VG = 0V. This indicates

32

that the excess number of defects in the thinner GaN buffer layer influenced the generation of

surface trap and induced drain current collapse at high frequency operation.

Summary

The effects of defect density on HEMT dc and rf performance were studied. HEMTs

were fabricated on 2 and 5μm of GaN buffer layers with the same composition of active layers.

TEM showed that the thinner GaN layers had an order of magnitude higher defect densities.

HEMTs on thicker GaN buffer layers showed negligible difference in the drain I-V

characteristics and showed a 6% higher value in peak transconductance. Significant reduction in

pulse measurements were observed on thinner GaN layer HEMTs. These results suggest that the

defects created in GaN buffer layer do not largely influence device dc performance but are

closely related to high frequency performance.

33

Figure 3-1. BF-XTEM showing the AlGaN/GaN interface for the A) 2 μm GaN and B) 5 μm

GaN substrates and the GaN/sapphire interface for the C) 2 μm GaN and D) 5 μm

GaN substrates.

34

Figure 3-2. Schematics of wafer bowing for HEMT wafer with A) 5 μm and B) 2 μm GaN buffer

layer.

0 1 2 3 4 50

100

200

300

400

500

600 5m GaN

2m GaN

VG = +1V Step = -1V

I DS (

mA

/mm

)

VDS

(V)

Figure 3-3. Drain I-Vs of HEMTs fabricated on different GaN buffer layer structures.

A

B

35

-5 -4 -3 -2 -1 00

100

200

300

400

500

5m GaN

2m GaN

VDS

= +5V

I DS (

mA

/mm

)

VG (V)

0

30

60

90

120

150

GM (

mS

/mm

)

Figure 3-4. Transfer characteristics of HEMTs fabricated on different GaN buffer layer

structures.

36

-5 -4 -3 -2 -1 00

20

40

60

80

100

120 DC measurement 100 kHz Pulse

VDS

= +5V

Norm

aliz

ed I

DS (

%)

VG (V)

-5 -4 -3 -2 -1 00

20

40

60

80

100

120 DC measurement 100 kHz Pulse

VDS

= +5V

No

rma

lize

d I

DS (

%)

VG (V)

Figure 3-5. Gate-lag pulsed measurements on A) HEMTs fabricated with a 2 μm GaN buffer

layer. B) Gate-lag pulsed measurements on HEMTs fabricated with a 5 μm GaN

buffer layer.

A

B

37

CHAPTER 4

EFFECT OF PROTON IRRADIATION ENERGY ON ALGAN/GAN METAL-OXIDE

SEMICONDUCTOR HIGH ELECTRON MOBILITY TRANSISTORS

Introduction to Effect of Proton Irradiation on AlGaN/GaN MOSHEMT

Due to their wide energy bandgaps, high sheet carrier concentration and high electron

mobility, AlGaN/GaN high-electron mobility transistors (HEMTs) are promising for high power

and high frequency applications, including inverter units in hybrid electric vehicles, advanced

radar systems, satellite-based communication networks and space communication systems[4, 35,

36]. However, the high gate leakage and drain current collapse in these conventional Schottky

gate metal devices limits the stability and performance of the HEMTs[37, 38]. In order to solve

these problems, oxides have been widely employed to reduce the metal-oxide-semiconductor

(MOS) diode leakage current or passivate surface traps, with Sc2O3, MgO, SiO2 and Si3N4[20,

22, 39-41], as the most effective gate dielectrics. Al2O3 deposited by atomic layer deposition

(ALD) has also been used as the gate oxide due to its advantages of excellent conformability,

low defect density, low stress, and excellent adhesion[38, 42]. Moreover, Al2O3 has a high

dielectric constant (k~10), high breakdown field (5-10 MV/cm), excellent thermal stability, and

chemical stability against reaction with AlGaN.

In this work, the effect of proton irradiation energy on dc performance of AlGaN/GaN

MOSHEMT with ALD-deposited Al2O3 as the gate dielectric was investigated. The proton

energy was varied from 5 MeV, 10 MeV to 15 MeV at a fixed dose of 5 × 1015 cm-2. The

MOSHEMTs dc characteristics, including drain and gate IV characteristics, sheet and contact

resistance, threshold voltage were compared before and after proton irradiation. The effect of

trap density in AlGaN barrier layer and GaN channel layer on the drain saturation current was

investigated with the finite-element based Florida object oriented device and process simulator

(FLOODs) technology computer aided design (TCAD) simulator.

38

Experimental

The AlGaN/GaN heterostructure consisted of 5μm GaN buffer, 21 nm of un-doped

Al0.24Ga0.76N, and a 5nm undoped GaN cap on sapphire substrate grown by metal-organic

chemical vapor deposition (MOCVD). Device isolation was achieved with mesa etching by an

inductively coupled plasma (ICP) system using Cl2/Ar based discharges. Ti/Al/Ni/Au based

metallization was used as the Ohmic metal with the standard lift-off electron-beam deposition,

with subsequently rapid thermal annealing at 850ºC for 45 sec in a N2 ambient. A 10 nm Al2O3

layer was deposited by atomic layer deposition (ALD) at 300ºC using trimethylaluminum and

water as the precursors. Electron-beam-deposited Ti/Au (20 nm/80 nm) was employed as the

gate metal. The gate width and length were 2 × 100 µm and 1 µm, respectively. Proton

irradiations were conducted at the Korean Institute of Radiological & Medical Sciences

(KIRAMS) using a MC 50 (Scanditronix) cyclotron.

MOSHEMT dc characteristics were measured with a 4156 HP parameter analyzer.

Transmission line measurements (TLM) were used to determine sheet resistance (RS), transfer

resistance (RT), and contact resistance (RC) before and after proton irradiation. Stopping and

range of ions in matter simulations (SRIMs) were used to simulate the proton penetration depth

and proton irradiation generated vacancy concentration distribution.

Results and Discussion

Figure 4-1 shows the SRIM simulation of vacancies created as a function of proton

penetration into the Al2O3/AlGaN/GaN MOSHEMT structure grown on a sapphire substrate. The

energy loss of the proton is a maximum near the end-of-range due to the nuclear stopping in that

region. This process causes atomic displacements and is the main cause of carrier loss due to

trapping into these defects. Nearer the surface, the energy loss is dominated by electronic

stopping which leads to ionization and heating. Thus, the damage induced by proton irradiation

39

is focused near the end-of-range at the penetration depths of 120, 392 and 794 μm for 5, 10 and

15 MeV, respectively. Since the wafer thickness is around 500 µm as marked in Figure 4-1A, the

15 MeV protons penetrate through the entire wafer with the least amount of damage created in

the active regions of the MOSHEMTs. The thickness of the AlGaN/GaN MOSHEMT structure

is less than 6 µm, and the 2DEG channel is located 36 nm below the Al2O3 surface of the

MOSHEMT. Therefore, the damage created by the high energy proton in the MOSHEMT

structure is uniform but much lower than in the tail region of the damage profile; the simulated

vacancy concentrations are 6.6 × 1018, 3.2 × 1018, 1.8 × 1018 cm-3 for 5, 10 and 15 MeV

irradiation, respectively, as shown in Figure 4-1B. Energetic protons transfer a part of their

kinetic energy to the Ga and N atoms through non-ionizing energy loss (NIEL), and displaces

atoms from their lattice sites to create vacancies. The minimal energies of defect formation were

found to be 18±1 eV for Ga and 22± 1 eV for nitrogen, but the average displacement energy was

much higher, 45±1 eV (Ga) and 109±2 eV (N). Thus the density of the Ga vacancy (VGa) should

be much higher than that of the VN. The VGa behaves as a compensation acceptor-like defect, and

VN is donor-like defect. As illustrated in Table 4-1, the sheet resistance, Rs, of the 5 MeV

proton-irradiated MOSHEMT was increased more than 10 times and it increased around 2.5

times for 10 and 15 MeV proton irradiated MOSHEMTs. These experimental outcomes are

consistent with the SRIM simulation results. Besides the increases of the sheet resistance, the

MOSHEMT transfer resistance, Rt, and specific contact resistance, Rc, also increased inversely

proportional to the proton energy, as shown in Table 4-1.

Figure 4-2A shows the drain current-voltage (I-V) curves of specific MOSHEMTs before

and after proton irradiation at 10MeV fluence of 5 × 1015 cm-2. The gate voltage was varied form

1 V to -3 V with a step of -0.5 V. All the proton-irradiated Al2O3/AlGaN/GaN MOSHEMTs

40

exhibited a sub-threshold drain leakage current less than 1 × 10-4 mA/mm. The amount of the

saturation drain current reduction depended on the irradiation energy. For the 10 MeV irradiated

MOSHEMTs, the reduction of saturation drain current at VG = 1V was 68.3%. A much larger

saturation drain current reduction, 95%, was observed for the MOSHEMTs irradiated with

proton energy of 5 MeV, and there was only 59.8% drain current reduction exhibited for the

MOSHEMTs irradiated with proton energy at 15 MeV, as summarized in Figure 4-2B. The

reduction of saturation drain current could be attributed both to the decrease in electron density

in the channel and in saturated carrier velocity. Defect centers are introduced as a result of

displacement damage during collisions between the incident protons and the semiconductor

lattice. These defect centers capture free carriers, resulting in reduction of carrier density and

conductivity of irradiated MOSHEMTs[43-45]. The mobility is affected both by the AlGaN/GaN

interface roughness[46, 47] and the scattering from defect centers created through proton

irradiation in the vicinity of the MOSHEMT channel[44, 48]. The decrease of initial slope of the

drain I-V curves in the linear region is also an indication that the irradiation decreased of carrier

concentration and mobility [49]. The electron mobility and sheet carrier concentration in the

2DEG channel were extracted using the low field linear region of the drain I-V curves using the

charge control model and verified with the Hall measurements[50]. The electron mobility in the

2DEG channel before and after proton irradiation at 5, 10 and 15 MeV were 1363, 1342.5, 944.5

and 865.5 cm2/V s, respectively. The sheet carrier concentration in 2DEG channel before and

after proton irradiation at 5, 10 and 15 MeV were 9.55 × 1012, 4.75 × 1012, 4.33 × 1012 and 2.87 ×

1012 cm-2, respectively. As shown in Figure 4-3 A, the ranges of the extracted percent electron

mobility and sheet carrier concentration reductions were 63.5 to 99.0% and 30.3 to 55.6%,

respectively, depending on the proton energy. The electron mobility and sheet carrier

41

concentrations of proton irradiated MOSHEMTs are summarized in Table 4-2. Figure 4-3B

shows that the carrier removal rate was inversely proportional to the irradiated proton energy,

RNC = -16.0·E (MeV) + 362.7, where RNC is carrier removal rate and E is proton energy. The

carrier removal rates were defined as the ratio of carrier concentration decrease divided by the

fluence of irradiated protons. The carrier concentration was obtained by the Hall measurements.

The lower energy proton irradiation created more non-ionizing energy loss and hence more

damage in the 2DEG, producing a higher carrier removal rate. Note that these removal rates are

similar to metal gate HEMTs irradiation under similar conditions and show that the presence of

the dielectric is not influencing the response of the devices to proton irradiation[11].

Figure 4-4A shows transfer characteristics from the MOSHEMTs before and after

irradiation at 10 MeV. The extrinsic transconductance, gm, was reduced by 54.4 % and there was

a positive shift of 1.96 V for the threshold voltage, Vth. These degradations were mainly due to

the displacement damage induced by the ion bombardment that reduced the carrier density and

electron mobility. As shown in Figure 4-4B, more severe degradation of the gm and a larger

positive Vth shift were observed for MOSHEMTs irradiated with lower irradiation energy of 5

MeV, with the gm decreased around 88% and Vth shifted by 2.89V. The lower the irradiation

energy used, then the shallower is the ion penetration depth. Thus, more damage was created

around the AlGaN/GaN interface. The FLOODS TCAD finite-element solver was employed to

simulate the potential difference between Fermi level (EF) and the ground state of sub-bands (E0)

in 2DEG channel and sheet carrier concentration in the 2DEG channel with different

concentrations of the charged traps. The sheet carrier concentration in the 2DEG channel is

proportional to EF - E0 as follows,

)()(

)()(*

2

0 xnxm

xExE sF

(4-1)

42

where ћ is the Planck’s constant divide by 2π, x is the Aluminum fraction of barrier layer, m* is

the effective mass of carriers, and is the sheet concentration in the 2DEG channel. Figure 4-4C

shows simulated conduction band-structures of the AlGaN/GaN HEMT by varying the charged

trap concentration in the GaN layer from 1×1015 to 5×1016 cm-3 and keeping charged trap

concentration constant at 1×1014 cm-3 uniformly distributed across the AlGaN barrier layer. As

more negative charges reside in the GaN layer, the E0 moves closer to EF resulted in lower sheet

concentrations in the 2DEG channel. The effect of charged traps in the AlGaN channel layer on

the sheet carrier concentration in the 2 DEG channel was also studied and a similar trend was

observed; less sheet carrier concentration presented in the 2DEG channel for more charged traps

in the AlGaN layer to induce a positive threshold voltage shift. Table 4-3 summarizes the

simulated EF - E0 and ns as a function of trap concentration in the AlGaN and GaN channel

layers. The variations of EF - E0 and ns were more sensitive to charged trap concentration in the

channel GaN layer. A similar trend was observed for the effect of irradiation energy on extrinsic

transconductance reduction, with 88.0 % and 54.4 % reductions after 5 and 10MeV irradiation,

respectively, and 40.7 % for 15 MeV. There was a positive shift of threshold voltage of 2.89 V

after 5 MeV proton irradiation, while the MOSHEMT irradiated with a 15 MeV proton

irradiation exhibited a much smaller shift of 1.26 V.

The reverse and forward gate I-V characteristics of the MOSHEMTs before and after

proton irradiation at 10 MeV are illustrated in Figure 4-5A. There was no degradation observed

on the oxide breakdown strength; around 5 MV/cm, but unexpectedly both forward and reverse

MOS gate leakage current decreased more than three orders of magnitude. The level of MOS

gate leakage current is inversely proportional to the gate oxide thickness and oxide breakdown

strength. Since there was no degradation of the oxide breakdown strength nor changes of the

43

oxide thickness after proton irradiation, the forward and reverse MOS gate leakage current

should be very similar prior to and after proton irradiation. Thus, the dramatic MOS gate leakage

current reduction must be related to other factors. It was reported that proton irradiation with a

dose of 4 × 1014 cm-2 induced voids in the Ni layer of Ni/Au based gate electrode and resulted in

decreasing gate area and gate leakage current[51]. Ferric cyanide (FeCN) and potassium iodide

(KI) based Au etch solution was used to remove the Au layer on the top of the Ti/Au based gate

electrode and no voids on Ti layer were observed, as shown in Figure 4-5B. Thus, the possibility

of gate contact area reduction induced gate leakage decrease was ruled out. The MOSHEMT

structure used in this experiment has 5 µm un-doped GaN layer. Typical keV isolation

implantation and mesa etching will not be able to isolate the thicker GaN buffer except the use of

carbon or iron doping in the GaN buffer layer. However, the penetration depth of MeV

implantation is way over 5 µm GaN layer, the traps created by proton implantation would reduce

the leakage current in the GaN buffer layer as well as MOSHEMT isolation current. As shown in

Figure 4-5C, the MOSHEMT isolation current significantly decreased after proton irradiation.

Therefore, the unexpected reduction of MOS gate leakage current after proton irradiation was

resulted from the improvement of MOSHEMT isolation current.

Summary

Fluence of 5 × 1015cm-2 protons at irradiation energies of 5 MeV, 10 MeV, and 15 MeV

were used to study the effect of proton irradiation energy on Al2O3/AlGaN/GaN MOSHEMTs.

There were significant reduction of carrier mobility, carrier concentration, and degradation of dc

characteristics observed for the lower proton energy irradiation. The severe degradation at lower

irradiation energy is due to deep trap generation near the 2DEG channel which increased the

resistance of the channel and this defect generation also influenced the positive shift in the

threshold voltage. The reverse and forward gate I-V was decreased more than an order of

44

magnitude after proton irradiation for all energies, which was due to the improvement of

isolation in the GaN buffer layer.

45

5 10 15

2x1018

4x1018

6x1018

8x1018

Va

ca

ncie

s (

cm

-3)

Proton Energy (MeV) Figure 4-1. SRIM simulation results showing A) vacancies created as a function of target depth

in the AlGaN/GaN MOSHEMT structure and B) vacancies created at the 2DEG

region as function of irradiation energy.

0 200 400 600 8000

2x1019

4x1019

6x1019

8x1019

1x1020

Va

can

cie

s (

cm

-3)

Target Depth (m)

5 MeV

10 MeV

15 MeV

(a)

Dose

5 x 1015

cm-2

A

B

46

0 2 4 6 80

100

200

300

400 Before

After

10MeV

VG = +1V

Step = -0.5V

I DS (

mA

/mm

)

VDS

(V)

5 10 150

20

40

60

80

100

I DS r

ed

uctio

n (

%)

Proton Energy (MeV) Figure 4-2. Drain I-V characteristics of A) MOSHEMT before and after proton irradiation with

fluence of 5 × 1015 cm-2 at an energy of 10 MeV. B) Reduction of saturation current

as function of proton irradiation energies for a fixed fluence of 5 × 1015 cm-2.

A

B

47

5 10 1560

70

80

90

100 Mobility

Sheet carrier

concentration

Mo

bili

ty R

ed

uctio

n (

%)

Proton Energy (MeV)

20

40

60

80

Sh

ee

t ca

rrie

r co

nce

ntr

atio

n

Re

du

ctio

n (

%)

Figure 4-3. Mobility and sheet carrier concentration reduction of A) irradiated MOSHEMTs as a

function of proton irradiation energies at a fixed fluence 5 × 1015 cm-2. B) Carrier

removal rate in MOSHEMT as a function of proton irradiation energy (cm-1).

5 10 150

100

200

300

400

500

N

c/P

roto

n D

ose

, R

Nc (

cm

-1)

Proton Energy (MeV)

RNC

= -16.0E(MeV) + 362.7

(b)

A

B

48

-5 -4 -3 -2 -1 0 10

100

200

300

400

IDS

IDS

After

GM G

M After

VDS

= +8V

I DS (

mA

/mm

)

VG (V)

10 MeV

0

30

60

90

120

150

GM (

mS

/mm

)5 10 15

0

20

40

60

80

100

Gm reduction

Vth shift

GM r

ed

uctio

n (

%)

Proton Energy (MeV)

1

2

3

Vth s

hift

(V)

0 1 2 3 4 5 6 7 8-0.5

0.0

0.5

1.0

1.5

2.0

510

16 cm-3

AlGaN GaN

EF

11016 cm

-3

En

erg

y (

eV

)

Distance (10-2 m)

11014

cm-3

Figure 4-4. Transfer characteristics of A) MOSHEMTs before and after proton irradiation with

fluence of 5 × 1015 cm-2 at 10 MeV. B) Reduction of gm and threshold voltage shift

as function of proton irradiation energies at fixed fluence of 5 × 1015 cm-2. C) FLOOD

TCAD simulation results for conduction band-structure of the AlGaN/GaN HEMT at

different charged trap concentration in the GaN layer.

A

B

C

49

-10 -8 -6 -4 -2 0 210

-9

10-7

10-5

10-3

10-1

101

Before

After

I G (

mA

/mm

)

VG (V)

10MeV

0 10 20 30 40 500

2x10-7

4x10-7

6x10-7

8x10-7

1x10-6 Reference

5MeV

10MeV

15 MeV

Curr

ent

(A)

VDS

(V)

Figure 4-5. Gate I-V of A) MOSHEMTs pre- and post- proton irradiation with 10MeV protons at

a dose of 5 × 1015 cm-2 B) Scanning electron microscopy image of proton irradiated

gate area after the removal of Au on the gate electrode. C) Change of device isolation

of MOSHEMT pre- and post- proton irradiation as a function of different energies.

A

B

C

50

Table 4-1. Effect of proton irradiation on sheet, transfer and specific contact resistance of

Al2O3/AlGaN/GaN MOSHEMTs.

Condition Rs (Ω/) Rt (Ω-mm) Rc (Ω-cm2)

5 MeV Before 493 0.48 4.7 × 10-6

After 5713 6.85 8.2 × 10-5

10 MeV Before 489 0.51 5.3 × 10-6

After 1254 1.00 7.9 × 10-6

15 MeV Before 497 0.37 2.8 × 10-6

After 1260 0.76 4.6 × 10-6

Table 4-2. Effect of proton irradiation on the changes of saturation drain current, extrinsic

transconductance, threshold voltage, electron mobility and 2DEG sheet carrier

concentration of Al2O3/AlGaN/GaN MOSHEMTs.

Proton Energy ΔIDSS

(%)

Δgm

(%)

ΔVth

(V)

ΔMobility

(%)

ΔSheet carrier

concentration (%)

5 MeV 95.3 88.0 2.89 99.0 55.6

10 MeV 68.3 54.4 1.96 69.3 45.2

15 MeV 59.8 40.7 1.26 63.5 30.3

51

Table 4-3. Simulated potential difference between Fermi level (EF) and the ground state of sub-

bands (E0) in 2DEG channel as well as the sheet carrier (ns) concentration

degradation in the 2DEG channel as a function of ionized acceptor-type trap

concentration in the AlGaN and GaN layers with FLOODS TCAD finite-element

based simulator.

Ionized accept-type

trap concentration

in AlGaN (cm-3)

Ionized accept-type

trap concentration

in GaN (cm-3)

EF-E0 (meV) ns degradation

1.0×1013

1.0×1014 97.910 0.04%

1.0×1015 97.880 0.04%

1.0×1016 97.580 4.50%

1.0×1014

1.0×1013

97.914 0.0002%

1.0×1015 97.900 0.006%

1.0×1016 97.750 0.080%

52

CHAPTER 5

EFFECT OF PROTON IRRADIATION DOES ON INALN/GAN METAL-OXIDE

SEMICONDUCTOR HIGH ELECTRON MOBILITY TRANSISTORS WITH AL2O3 GATE

OXIDE

Introduction to Effect of Proton Irradiation on InAlN/GaN MOSHEMT

The lattice mismatch between AlGaN and GaN channel/buffer layers has led to interest in

InAlN/GaN since the InAlN layer can be grown lattice matched to the GaN channel/buffer layer

at a concentration of 17% indium. These InAlN/GaN based HEMTs have a higher polarization-

induced two-dimensional electron gas (2DEG) density and have better carrier confinement of

2DEG channel with a wider energy bandgap of InAlN as compared to AlGaN[52-57]. Al2O3 or

AlN dielectric layers have been employed to reduce gate leakage current, increase gate

breakdown voltage and improve gate modulation for GaN based HEMTs[38, 58, 59]. Many

studies have been conducted on high energy proton irradiation tolerance of both AlGaN/GaN and

InAlN/GaN HEMTs as well as AlGaN/GaN metal oxide semiconductor HEMTs (MOSHEMTs)

but there has not been a study performed for proton irradiation effects on InAlN/GaN

MOSHEMTs[59-61].

In this chapter, the effect of proton irradiation dose on the characteristics of InAlN/GaN

metal-oxide semiconductor high electron transistors (MOSHEMTs) with atomic layer deposited

(ALD) Al2O3 as the gate oxide has been studied. The proton dose was from 1013-1015 cm-2 at 5

MeV energy. The sheet and contact resistance, drain and gate current-voltage characteristics,

sub-threshold swing, and gate-pulsed drain current were investigated before and after the proton

irradiations.

Experimental

The InAlN based MOSHEMT structure consisted of a 2 nm GaN cap layer, a 5.3 nm

In0.17Al0.83N barrier layer, a 1 nm AlN spacer layer, and a 0.85 μm Al0.04Ga0.96N channel/buffer

53

layer. These were grown on a sapphire substrate by metal-organic chemical vapor deposition

(MOCVD). Device isolation was achieved with an inductively coupled plasma (ICP) system

using Cl2/Ar mixture. Ohmic metallization was accomplished by standard lift-off process of

electron-beam deposited Ti/Al/Ni/Au followed with a rapid thermal annealing at 850°C for 45

seconds in flowing nitrogen ambient. 5 nm Al2O3 was deposited on the surface using atomic

layer deposition (300°C) with tri-methyl-aluminum and water as the precursors. Afterwards, the

Al2O3 was annealed at 400°C ambient nitrogen to reduce the defect density in the oxide. Gates

with a dimension of 1 μm × 100 μm were defined by standard lift-off processing of electron-

beam deposited Ni/Au. 200 nm of plasma enhanced chemical vapor deposited (PECVD) silicon

nitride was used for device passivation and electron-beam deposited Ti/Au was employed for the

final metal.

Proton irradiations were performed at the Korean Institute of Radiological & Medical

Sciences (KIRAMS) using a MC 50 Scanditronix cyclotron. The proton energy at the exit of the

cyclotron was 30 MeV. The proton energy at the sample was 5 MeV after passing through two

aluminum degraders. The thickness of each aluminum degrader was 2.7mm. The beam currents

were measured using a Faraday-cup to calculate the flux density. Proton doses of 1 × 1013, 7 ×

1013, 3 × 1014, and 1 × 1015 cm-2 were used in this study.

Results and Discussion

The sheet resistance (Rs), contact resistance (Rc), and transfer resistance (Rt) of the

InAlN/GaN MOSHEMTs were extracted by transmission line measurements before and after the

proton irradiation and are summarized in Table 5-1. There was no change of Rs observed for the

MOSHEMTs irradiated at 1 × 1013 cm-2 of protons, and 2.8, 3.3 and 9.7% increases of Rs for

MOSHEMTs irradiated with 7 × 1013, 3 × 1014 and 1 × 1015 cm-2 protons, respectively. However,

much higher degradations in Rc and Rt were exhibited for the proton irradiated MOSHEMTs,

54

and the Rc and Rt also increased linearly proportional to the irradiation doses from 27% to 114%

and from 12% to 54%, respectively, over the dose range investigated. The differences in the

degree of degradation induced by proton irradiations between Rs and Rc/Rt were a result of the

relative changes in the Ohmic metal contacts. Since the 5MeV protons have a long projected

range, the resultant nuclear energy loss is minimal around the irradiated surface area, and the

proton energy loss in this region is dominated by the electronic-stopping mechanism and not by

nuclear-stopping. Only the latter leads to lattice damage in the semiconductor and thus, minimal

damage was introduced in the 2 dimensional-electron gas (2DEG) region located 6.3nm below

the top surface of the MOSHEMT. The result is that there is only a small change in Rs. On the

contrary, in the Ohmic contacts, there is 200 nm of metal in the contact stacks on the

MOSHEMT structure, and the nuclear-stopping mechanism for the proton energy loss is now

much more significant both in that area and in the underlying semiconductor layers. The

observed near- linear increases of Rc and Rt as a function of proton dose were a result of the

higher non-ionizing energy loss creating more damage in the metal contact regions.

Figure 5-1A shows the drain I-V characteristics of InAlN/GaN MOSHEMTs before and

after the proton irradiation dose of 1 × 1015 cm-2 at 5 MeV. The drain voltage was swept from 0 V

to 10 V, while the gate bias voltage started at 0 V with a step of -1 V. Figure 5-1 B shows the

reduction of I-V after the proton irradiation as function of doses. The averaged drain current

reductions were 96, 131, 175, and 242 mA/mm for MOSHEMT irradiated with 1 × 1013, 7 ×

1013, 3 × 1014, and 1 × 1015 cm-2, respectively. The IDS reductions were proportional to the proton

irradiation dose. The displacement damage created during the collisions between the incident

protons and the semiconductor lattice causes the reduction of electron density in the channel and

in the saturated carrier velocity[7, 59-61]. The defect centers introduced in the crystal lattice lead

55

to carrier removal through carrier trapping. Figure 5-1 B shows the drain current density

reduction as a function of irradiation dose. The drain current reduction increased due to the

increase of contact resistance between the metal to the semiconductor and the displacement

damage within 2DEG, resulting in reduction of electron density and carrier mobility. As more

charged traps reside in the InAlN and GaN, the potential difference between the Fermi level (EF)

and the ground state of sub-bands (E0) in the 2DEG channel became smaller, resulting in lower

sheet carrier concentrations[50, 62]. The low field drain I-Vs (< 0.5 V) were used to extract the

carrier concentrations with the charge control model[50]. Carrier removal rates of the proton

irradiations were calculated using the change of sheet carrier concentration before and after the

proton irradiation divided by the proton dose, as shown in Table 5-2. The carrier removal rate

was independent of proton dose. Ionized traps introduced in the InAlN and GaN layers changed

conduction band bending and lowered the carrier density in the 2-DEG[62]. The average carrier

removal rate of 1287±64 cm-1 was larger than we reported for AlGaN/GaN MOSHEMTs

irradiated under similar conditions (~300)[7, 63-65] and is consistent with the lower average

bond strength of InAlN relative to AlGaN. The same trend was reported previously for standard

metal-gate InAlN/GaN and AlGaN/GaN HEMTs[7, 63-66].

Figure 5-2A and B illustrate the reverse and forward gate I-V characteristics of the

MOSHEMTs before and after proton irradiation at 1 × 1013 cm-2 and 1 × 1015 cm-2, respectively,

at 5 MeV. There was no sign of changes in oxide breakdown strength, which was around 6

MV/cm and defined as the forward voltage at the forward gate leakage current of 1 mA/mm,

after proton irradiation. However, the forward bias gate current was slightly reduced, and there

was an almost one order magnitude decrease in the reverse bias gate leakage current for the

MOSHEMTs irradiated with 1 × 1015 cm-2 of protons. Since the gate oxide breakdown strength

56

did not change, the gate leakage current should be similar to the value prior to proton irradiation.

Koehler et al. reported that Ni voids form between semiconductor and gate interface of high dose

proton irradiated AlGaN/GaN HEMTs when using Ni/Au gate as the gate electrode, which

resulted in less gate contact area and less gate leakage current[51]. However, similar work was

performed in our study on AlGaN/GaN MOSHEMT irradiated with 1 × 1015 cm-2 proton by wet-

chemically removing the Ti/Au based gate electrode and there were no voids formed on the

Al2O3 gate oxide when using this metallization scheme. The reduction of the gate leakage current

was attributed to the improvement of device isolation by reducing the conductivity of GaN buffer