Gallium Oxide Solar-blind Photodetectorsfor Harsh Environment Applications: From

Thin Film Growth to Device Packaging

BHERA RAM TAK

INDIAN INSTITUTE OF TECHNOLOGY DELHIOctober 2020

©Indian Institute of Technology Delhi (IITD), New Delhi, 2020

Gallium Oxide Solar-blind Photodetectorsfor Harsh Environment Applications: From

Thin Film Growth to Device Packaging

by

BHERA RAM TAKDepartment of Physics

Submitted

In fulfillment of the requirements of the degree of

Doctor of Philosophyto the

INDIAN INSTITUTE OF TECHNOLOGY DELHIOctober 2020

This thesis is dedicated to my parents for their unconditional love and support

Certificate

This is to certify that the thesis entitled “Gallium Oxide Solar-blind Photode-

tectors for Harsh Environment Applications: From Thin Film Growth to

Device Packaging” being submitted by Mr. Bhera Ram Tak to Indian Institute

of Technology Delhi for the award of the degree of Doctor of Philosophy is a

record of bonafide research work carried out by him. He has worked under my guidance

and supervision and has fulfilled the requirements, which to our knowledge have reached

the requisite standard for the submission of the thesis. The results contained in this

thesis have not been submitted in part or full to any other University or Institute for

the award of any degree or diploma.

Prof. Rajendra Singh

Department of Physics

Indian Institute of Technology Delhi

New Delhi-110016, India

Date:

i

Acknowledgments

My journey of Ph.D. has been tough and challenging as well as joyful. During this

time, hard luck always motivated me to be patient and work hard. The outcome of this

thesis also involves support, criticism and encouragement of lots of people. I also wish

to thank all the institutions that supported me for the accomplishment of my thesis

work.

First of all, I would like to express my sincere gratitude to my Ph.D. supervisor

Professor Rajendra Singh. It was an honor for me to work in this guidance. I am grateful

to him for his suggestions, motivation for research and giving me the freedom to work

on research ideas. It has been a great time to learn and improve myself professionally as

well as personally. I appreciate him for his valuable time and devotion for my research

work, research fundings and collaborations which made this thesis work productive and

stimulating. Finally, a special thanks to him for pushing me hard to execute device

packaging work.

Besides my advisor, I wish to thank my Ph.D. research committee members Professor

Sujeet Chaudhary, Professor J. P. Singh and Professor Samaresh Das for their time to

time evaluations, feedback, comments and appreciation during this research work. I

would like to acknowledge the Department of Physics, Indian Institute of Technology

Delhi for giving me the opportunity to work here.

I would also like to thank the funding agencies department of science and technology

(DST), India and British Council, United Kingdom for awarding me as a Newton-

Bhabha Ph.D. fellow to execute some part of work at the University of Warwick, United

Kingdom. I would like to express my gratitude to Professor Marine Alexe for hosting

me during this program. It was a learning and productive experience to work under his

guidance. I wish to thank him for providing research facilities and training me during

this program. His passion for research and his way towards dealing research problem

always impressed me. I would like to thank Dr. Mingmin Yang, Mr. Afan, Ms. Daniela-

Emilia Dogaru, Mr. Hangbo Zhang and other group members for helping and training

me on various experimental tools.

I acknowledge the department of science and technology (DST), India for awarding

me INSPIRE fellowship and contingency grant to execute my Ph.D. degree. I am also

iii

thankful to the Indian Institute of Technology Delhi for providing funds to attend an

international conference outside India. I would like to acknowledge Nano Research

Facility (NRF), IIT Delhi for providing funding support for my research visit at the

University of Kolkata and Indian Institute of Science (IISc), Bangalore.

I acknowledge the department of Physics, NRF, central research facility (CRF) IIT

Delhi for providing various experimental tools to perform my research work. I would

also like to express my gratitude to Professor Vinay Gupta and Dr. Monika Tomar,

Department of Physics and Astrophysics, University of Delhi for providing me Pulsed

Laser Deposition (PLD) system to deposite gallium oxide thin films. I wish to thank

his group members especially Sheetal Dewan and Surbhi Gupta for helping me during

my time in his lab.

I am obliged to Prof. Xiaohang Li, Advanced Semiconductor Laboratory, King Ab-

dullah University of Science and Technology (KAUST) and Professor Ying-Hao Chu

from National Chiao Tung University, Taiwan for the cross-sectional TEM measure-

ments. Professor Chu is also acknowledged for providing muscovite substrates. I am

thankful to Dr. Samaresh Das and Veerendra Dhiyani from Centre of Applied Elec-

tronics, IIT Delhi, Professor B. R. Mehta, Professor B. D. Gupta, Mr. Mujeeb Ahmad

and Mr. Vivek Semwal from Department of physics, IIT Delhi for helping in metal

deposition.

I would like to thank following collaborators for their help during my Ph.D. work:

• Dr. Ashok Kapoor from Solid state physics laboratory (SSPL), New Delhi

• Dr. Raman Kapoor from Solid state physics laboratory (SSPL), New Delhi

• Dr. Anshu Goyal from Solid state physics laboratory (SSPL), New Delhi

• Dr. K. Asokan, Inter University Accelerator Centre, India

• Dr. S. Nagarajan from Aalto University, Finland

• Dr. Ashish Kumar, Inter University Accelerator Centre, India

Working at Advanced Semiconductor Materials and Devices Laboratory, IIT Delhi,

has been an enriching experience. I would like to thank my seniors Dr. Uday Dadwal,

Dr. Ashish Kumar, Dr. Ashutosh Kumar, Dr. Sudheer Kumar, Dr. Mukesh Kumar, Dr.

iv

Chandra Shekhar Pathak, Dr. Manjari Garg, Dr. Monika Moun for their valuable sug-

gestions and help. I also express my gratitude to my colleagues Ravi Pathak, Chandan

Sharma, Aditya Singh, Kapil Narang, Shuchi Kaushik, Sahin Sorifi, Madan Pancholi,

Hardhyan, Pallavi Agarwal, T. Arundeepth, Prithu Bhatnagar, Kalyani Thakur, Anjali

Chauhan, Aarti, Sukhdeep Gill, Mohd. Danish Ali, Swapnil, Arun, Shabbin Rahiman

K., Suresh Bhambhu and Rahul Agarwal for the scientific and personal discussions.

Once again, I thank all these members for making my stay at IIT Delhi unforgettable.

This PhD would have been more challenging without the good and bad times that

I shared with my friends at IIT Delhi. All the tea and coffee sessions and parties

with Balwant Singh, Vikas Sharma, Mujeeb Ahmad, Sonu, Minakshi and Vivek Semwal

will be cherished forever. Also excursions and dinners with Manjari Garg, Monika,

Ravi Pathak, and Aditya Singh will be treasured. All the moments spent with friends

specially Nakul Jain, Rajesh Jangir, Balwant Singh, Vikas Sharma, Minakshi, Barkha,

Manisha and Dora during trekking and camping will always be cherished. My days at

IIT Delhi will be incomplete without all my friends with whom I used to play cricket

and badminton.

Finally, words are inadequate to express my deep and heart filled gratitude towards

my family for their blessings, love, care, unconditional support, encouragement, patience

and sacrifices during all the stages of this Ph.D. and in both good times and bad times.

Thank you.

Bhera Ram Tak

v

Abstract

Gallium oxide (Ga2O3) is a most promising and emerging wide bandgap semiconductor

material for optoelectronics, high-power devices and radio-frequency power electronics

applications. Owing to its wide bandgap, β-Ga2O3 is a potential contender for solar-

blind photodetector applications. The Ga2O3 technology is at the early stage of research

which provides a great opportunity to identify different challenges. A good quality sin-

gle crystalline material is inevitably required for high performance photodetectors. The

effect of oxygen growth conditions on the quality of Ga2O3 is still not investigated

that set the first challenge to begin this work. The investigation of the photodetector

performance for harsh environmental conditions such as temperature and radiation is

necessary for defense, security, environmental and space applications. The photocur-

rent transport both at high temperature and gamma irradiation environments is also not

studied for Ga2O3 material. Flexible and self-powered deep ultraviolet (UV) photode-

tectors are pivotal for future technologies. The fabrication of epitaxial β-Ga2O3 thin

films is challenging on flexible substrates due to high-temperature growth requirements.

In order to address the aforementioned problems, this thesis work is accomplished in the

direction of rigid as well as flexible photodetectors for harsh environmental conditions.

In the beginning of this work, the growth of β-Ga2O3 thin films was optimized by

varying oxygen content. The thin films deposited at 0.5 mT oxygen growth pressure

at 800 ◦C temperature possessed minimum point defects. It was also found that the

Fermi level was pinned at the mid-gap energy in both oxygen-deficient conditions and

oxygen-rich conditions which are attributed to oxygen and gallium vacancy related de-

fects. Further, the metal-semiconductor-metal (MSM) photodetectors were fabricated

on the best quality thin film sample. The photodetector exhibited an ultra-low dark

current of 8.6 ± 3.4 fA at zero bias. An ultra-low noise current of 9.1 ×10−16 A/Hz1/2

at 1 Hz was also obtained in the self-powered condition. Such ultra-low noise current

suggests the potential of this device in detecting very weak optical signals. The linear

dynamic range (LDR) of 88.5 dB was achieved which is very useful for high-resolution

imaging. The dark current, noise floor, and LDR of the photodetector are the bench-

mark for β-Ga2O3 self-powered deep UV photodetectors. We also demonstrated a 3×4

two-dimensional photodetector array with uniform dark and photocurrent across all the

vii

pixels. The outcomes of the present work are encouraging for imaging applications of

β-Ga2O3 based energy-efficient deep UV photodetectors. In subsequent work, High-

temperature operation of MSM UV photodetectors fabricated on pulsed laser deposited

β-Ga2O3 thin films has been investigated. These photodetectors were operated up to

250 ◦C temperature under 255 nm illumination. The photo to dark current (PDCR)

ratio of about 7100 was observed at room temperature (RT) and 2.3 at high temper-

ature of 250 ◦C with 10 V applied bias. A decline in photocurrent was observed until

a temperature of 150 ◦C beyond which it increased with temperature up to 250 ◦C.

The suppression of the UV and blue band was also observed in the normalized spec-

tral response curve above 150 ◦C temperature. Temperature-dependent rise and decay

times of temporal response were analyzed to understand the associated photocurrent

mechanism at high temperatures. Electron-phonon interaction and self-trapped holes

were found to influence the photoresponse in the devices. Further, thermally stimulated

current (TSC) measurements were also performed to identify deep level traps in thin

films. The deep level trap of 1.03 eV energy was found dominant trap responsible for

persistent photocurrent.

Further, the radiation hardness of Ga2O3 MSM solar-blind photodetectors has also

been investigated under the exposure of 60Co γ-source. It was observed that the metal

contacts were not degraded and the dark current of photodetector was slightly improved

from 3.27× 10−7 A to 1.88 × 10−7 A. The photo to dark current ratio (PDCR) was

observed to increase from 5.1 to 14.1 with increasing γ-radiation exposure. The apparent

Schottky barrier height (SBH) evaluated from current-voltage characteristics were found

to increase with irradiation. The increased SBH was explained using image force induced

barrier lowering. The obtained results reveal that the Ga2O3 solar-blind photodetectors

are relatively less susceptible to the radiation environment.

For flexible photodetectors, wearable solar-blind photodetector based on amorphous

gallium oxide grown at muscovite mica is reported for room temperature as well as high

temperature operations. The ultra-high photoresponsivity of 9.7 A/W is obtained for

5 V applied bias at room temperature under 75 µW/cm2 weak illumination of 270 nm

wavelength. The detector enables very low noise equivalent power (NEP) of 9×10−13

W/Hz1/2 and ultra-high detectivity of 2×1012 jones which shows the magnificent detec-

tion sensitivity. Further, bending tests are performed for robust utilization of flexible

viii

detectors up to 500 bending cycles with each bending radius of 5 mm. After 500 bending

cycles, device shows a slight photocurrent decrease. The bending performances exhibit

excellent potential for wearable applications. Moreover, photocurrent and dark current

characteristics above room temperature demonstrate the outstanding functionalities till

523K temperature which is remarkable for flexible photodetectors. Further, to improve

the performance of flexible photodetectors, β-Ga2O3 (20 1) films are hetero-epitaxially

grown on ultra-thin and environment-friendly muscovite mica which is the first time β-

Ga2O3 epitaxy growth on any flexible substrate. The integration of Gallium oxide with

muscovite enables high-temperature processing as well as excellent flexibility compared

to polymer substrates. Additionally, the metal-semiconductor-metal (MSM) photode-

tector on β-Ga2O3 layer shows an ultra-low dark current of 800 fA at zero bias. The

photovoltaic peak responsivity of 11.6 µA/W is obtained corresponding to very weak

illumination of 75 µW/cm2 of 265 nm wavelength. Thermally stimulated current (TSC)

measurements are employed to investigate the optically active trap states. Among these

traps, trap with an activation energy of 166 meV dominates the persistence photocur-

rent in the devices. Finally, photovoltaic detectors have shown excellent photocurrent

stability under bending induced stress up to 0.32%. Hence, this novel heteroepitaxy

opens the new way for flexible deep UV photodetectors. In the last, the solar-blind

photodetectors fabricated on β-Ga2O3 /sapphire were packaged in the transistor out-

line header. The packaged devices showed the same photoresponse before and after wire

bonding.

ix

xiii

सार

गलियम ऑकसाइड (Ga2O3) ऑपटोइिकरॉनिक, उचच-शककि उपकरणो और रडडयो-आवतति पावर इिकरॉनिकस क उपयोग क लिए एक सबस आशाजिक और उभरिा हआ ततवसिि बडगप समीकडकटर ह। इसक ततवसिि बडगप क कारण, β- Ga2O3 सोिरबिाइड फोटोडडटकटर क लिए एक सभाततवि दावदार ह। Ga2O3 िकिीक अिसधाि क परारलभक चरण म ह जो ततवलभनि चिौनियो की पहचाि करि का एक शािदार अवसर परदाि करिा ह। उचच परदशशि फोटोडडटकटसश क लिए एक अचछी गणविा वािी एकि करिसटिीय मटररयि अनिवायश रप स आवशयक ह। Ga2O3 की गणविा पर ऑकसीजि क परभाव की अभी भी जाच िही की गई ह जो इस काम को शर करि क लिए पहिी चिौिी निधाशररि करिी ह। रकषा, सरकषा, पयाशवरण और अिररकष अिपरयोगो क लिए कठोर पयाशवरणीय पररकसिनियो जस िापमाि और ततवकरकरण क लिए इि फोटोडडटकटसश क परदशशिो की जाच आवशयक ह। उचच िापमाि और गामा ततवकरकरण वािावरणो म फोटो करट पररवहि को Ga2O3

मटररयि क लिए समझा िही गया ह। िचीि और सव-सचालिि गहर पराबगिी (यवी) फोटोडडटकटर भततवषय की परौदयोगगकरकयो क लिए महतवपणश ह। उचच िापमाि की आवशयकिाओ क कारण िचीि सबसरटस पर एततपटककसयि β- Ga2O3 पििी करफलमो का निमाशण चिौिीपणश ह। उपयशकि समसयाओ को सबोगधि करि क लिए, कठोर पयाशवरणीय पररकसिनियो क लिए यह िीलसस कायश, अिमय और साि ही िचीि फोटोडडटकटसश की ददशा म परा करकया जािा ह।

इस काम की शरआि म, ऑकसीजि की मातरा को बदि करक β- Ga2O3 पििी करफलमो क गरोि को अिकलिि करकया गया । 800 °C िापमाि ििा 0.5 mT ऑकसीजि गरोि परशर म बिाई पििी करफलमो म नयििम बबद दोष ि। ऑकसीजि की कमी वािी कसिनियो और ऑकसीजि अगधकिा वािी कसिनियो म फमी सिर मधय-अिराि ऊजाश पर ततपि हआ पाया गया िा, कजसक लिए ऑकसीजि और गलियम ररककि सबधी दोष कजममदार ह। इसक आग, धाि-अधशचािक-धाि (MSM) फोटोडडटकटसश को सबस अचछी गणविा वािी पििी करफलम पर गढा गया िा। फोटोडडटकटर ि शनय वोलटज पर 8.6 ± 3.4 fA क अलरा-िो डाकश करट का परदशशि करकया। 1Hz पर 9.1 ×10-16 A/Hz1/2 का अलरा-िो िोइज करट भी सव-सचालिि कसिनि म परापि करकया गया िा। इस िरह क अलरा-िो िॉइज करट बहि कमजोर ऑकपटकि लसगिि का पिा िगाि म इस डडवाइस की कषमिा बिािा ह। 88.5 dB की िीनियर डायिलमक रज (LDR) हालसि की गई जो उचच-ररजॉलयशि इमकजग क लिए बहि उपयोगी ह। फोटोडडटकटर का डाकश करट, िोइज िवि, और LDR β-Ga2O3 सव-सचालिि गहर यवी फोटोडडटकटसश क लिए बचमाकश ह। हमि सभी ततपकसिो म एक समाि डाकश और फोटो करट वािी एक 3 × 4 दततव-आयामी फोटोडडटकटर सरणी को परदलशशि करकया ह। विशमाि कायश क पररणाम β-

Ga2O3 आधाररि ऊजाश कशि गहर यवी फोटोडडटकटर क इमकजग अिपरयोगो क लिए परोतसादहि करि वाि ह। बाद क काम म, पलसड िजर स बिाई β- Ga2O3 की पििी करफलमो पर गढ गए MSM यवी फोटोडडटकटर क उचच िापमाि पर सचािि की जाच की गई ह। य फोटोडडटकटर 255 nm परकाश म 250 °C िापमाि िक

xiv

सचालिि करकए गए ि। 10 V पर फोटो स डाकश करट (PDCR) का अिपाि कमर क िापमाि (RT) पर िगभग 7100 और 250 °C क उचच िापमाि पर 2.3 दखा गया। फोटो करट म 150 °C क िापमाि िक गगरावट दखी गई िी और उसक ऊपर 250 °C िापमाि िक इसम वदगध हई । यवी और बि बड का दमि भी 150 °C िापमाि स ऊपर सामानयीकि वणशिमीय परनिकरिया वि म दखा गया िा। उचच िापमाि स सबगधि फोटो करट मकनिजम को समझि क लिए टमपोरि परनिकरिया क िापमाि-निभशर वदगध और कषय समय का ततवशिषण करकया गया िा। इिकराि-फोिोि इटरकशि और सलफ-रपड होि को उपकरणो म फोटोरसपोनस को परभाततवि करि क लिए कजममदार पाया गया। इसक अिावा, पििी करफलमो म गहर सिर क रपस की पहचाि करि क लिए िमशि रप स उिकजि करट (TSC) माप भी करकए गए ि। 1.03 eV ऊजाश क गहर सिर क रप को परलससटट फोटो करट क लिए कजममदार परमख रप पाया गया।

इसक अिावा, Ga2O3 MSM सोिरबिाइड फोटोडडटकटर की ततवकरकरण कठोरिा की जाच 60Co γ- सरोि क ससगश म भी की गई ह। यह दखा गया करक धाि कोनटकट खराब िही हई िी और फोटोडडटकटर क डाकश करट म 3.27 × 10-7 A स 1.88 × 10-7 A िक िोडा सधार हआ िा। फोटो स डाकश करट (PDCR) का अिपाि γ-

रडडएशि ससगश क साि 5.1 स बढकर 14.1 हआ पाया गया। करट-वोलटज वि मलयाकि स निकािी गई सपषट शोटकी बररयर ऊचाई (SBH) को ततवकरकरण क साि बढा हआ पाया गया। वदगध हई SBH को छततव बि स परररि अवरोध कमी स समझाया गया । परापि पररणामो स पिा चििा ह करक Ga2O3 सोिरबिाइड

फोटोडडटकटर ततवकरकरण पयाशवरण क लिए अपकषाकि कम सवदिशीि ह।

िचीि फोटोडडटकटसश क लिए, मसकोवाइट माइका पर उगाए गए अमोरफस गलियम ऑकसाइड पर आधाररि ततवयरबि सोिरबिाइड फोटोडडटकटर को कमर क िापमाि क साि-साि उचच िापमाि सचािि क लिए सपाददि करकया गया ह। 9.7 A/W की अलरा-उचच फोटोररसपोलसततवटी 5V क लिए कमर क िापमाि पर 75

W/cm2 एव 270 nm िरग दधयश की कमजोर रोशिी क िहि परापि की जािी ह। डडटकटर 9 × 10-13 W/Hz1/2 क बहि कम िोइज समककष पॉवर (NEP) और 2 × 1012 Jones की अनि-उचच डडटककटततवटी क सकषम ह जो शािदार पहचाि सवदिशीििा को दशाशिा ह। इसक अिावा, 5 mm की परतयक झकाव बतरजया क साि 500 झकाव चिो िक िचीि डडटकटरो क मजबि उपयोग क लिए झकाव परीकषण करकए जाि ह। 500 झकाव चिो क बाद, डडवाइस फोटोकरट म एक मामिी कमी ददखािा ह। झकाव परदशशि क पररणाम ततवयरबि अिपरयोगो क लिए उतकषट कषमिा ददखाि ह। इसक अिावा, कमर क िापमाि क ऊपर 523K

िापमाि िक फोटोकरट और डाकश करट उतकषट कायशकषमिा परदलशशि करिी ह जो िचीि फोटोडडटकटर क लिए उलिखिीय ह। इसक अिावा, िचीि फोटोडडटकटर क परदशशि को बहिर बिाि क लिए, β- Ga2O3 (-

2 0 1) करफलम अलरा-पििी और पयाशवरण क अिकि मसकोवाइट माइका पर दहटरो-एततपटककसयिी उगाई जािी ह, जो करकसी भी िचीि सबसरट पर पहिी बार β- Ga2O3 एततपटकसी गरोि ह। मसकोवाइट क साि गलियम ऑकसाइड का एकीकरण बहिक सबसरट की िििा म उचच िापमाि परससकरण क साि-साि

xv

उतकषट िचीिापि दिा ह। इसक अनिररकि, β- Ga2O3 परि पर MSM फोटोडडटकटर शनय वोलट पर 800 fA

की एक अलरा-िो डाकश करट ददखािा ह। 11.6 μA/W की फोटोवोकलटक लशखर रसपोनस 265 nm िरग दधयश एव 75 μW /cm2 की बहि कमजोर रोशिी क ससगश म परापि की जािी ह। परकालशक सकरिय रप सिरो की जाच क लिए TSC माप काम म लिया गया ह। इि रपो म स, 166 meV की सकरियण ऊजाश का रप उपकरणो म परलससटट फोटोकरट क लिए हावी ह। अि म, फोटोवोकलटक डडटकटरो ि 0.32% िक झकाव स परररि ििाव क िहि उतकषट फोटोकरट कसिरिा ददखाई ह। इसलिए, यह िया दहटरो-एततपटकसी िचीिी गहरी यवी फोटोडडटकटसश क लिए िया रासिा खोििा ह। आखखरी म, राकजसटर आउटिाइि हडर म β- Ga2O3/सफायर पर निलमशि सोिरबिाइड फोटोडटकटर पक करकए गए ि। पक करकए गए उपकरणो ि वायर बकनडग स पहि और बाद म एक ही फोटोरसपोनस ददखाया।

Contents

Certificate i

Acknowledgments iii

Abstract vii

List of Figures xix

List of Tables xxv

Appendix xxvii

1 Introduction 1

1.1 Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Fundamentals of photodetectors: Figures of merit . . . . . . . . . . . . . 2

1.2.1 External quantum efficiency and responsivity . . . . . . . . . . . 2

1.2.2 Photo to dark current ratio (PDCR) and UV to visible rejection

ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.2.3 Transient response or speed . . . . . . . . . . . . . . . . . . . . . 4

1.2.4 Noise equivalent power (NEP) . . . . . . . . . . . . . . . . . . . . 5

1.2.5 Specific detectivity (D∗) . . . . . . . . . . . . . . . . . . . . . . . 6

1.2.6 Linear dynamic range (LDR) . . . . . . . . . . . . . . . . . . . . 6

1.3 Technology roadmap for next-generation UV detectors . . . . . . . . . . 6

1.4 Schottky MSM architecture . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.5 Material selection for Solar-blind photodetector . . . . . . . . . . . . . . 9

1.6 Organization of Thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2 Experimental and characterization methods 17

2.1 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.1.1 Material target fabrication . . . . . . . . . . . . . . . . . . . . . 17

xv

2.1.2 Pulsed laser deposition . . . . . . . . . . . . . . . . . . . . . . . . 18

2.1.3 Device fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . 20

2.2 Material characterizations . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.2.1 Structural characterizations . . . . . . . . . . . . . . . . . . . . . 21

2.2.2 Scanning probe microscopy . . . . . . . . . . . . . . . . . . . . . 22

2.2.3 Optical characterization . . . . . . . . . . . . . . . . . . . . . . . 24

2.2.4 Elemental characterization . . . . . . . . . . . . . . . . . . . . . . 25

2.3 Gamma irradiation facility . . . . . . . . . . . . . . . . . . . . . . . . . . 25

2.4 Device characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

2.4.1 Photoelectrical measurements . . . . . . . . . . . . . . . . . . . . 26

2.4.2 Noise measurements . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.4.3 Flexibility measurements of devices . . . . . . . . . . . . . . . . . 29

2.5 Thermally stimulated current spectroscopy . . . . . . . . . . . . . . . . . 29

2.6 Device packaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

2.6.1 Dicing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

2.6.2 Wire bonding and packaging . . . . . . . . . . . . . . . . . . . . 31

3 Growth of epitaxial β-Ga2O3 thin films on sapphire 33

3.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.1.1 β-Ga2O3 target fabrication . . . . . . . . . . . . . . . . . . . . . 35

3.1.2 Thin film growth . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

3.1.3 Material Characterizations . . . . . . . . . . . . . . . . . . . . . . 36

3.1.4 Kelvin Probe Force Microscopy Measurement . . . . . . . . . . . 36

3.2 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

3.2.1 Structural and elemental characterization of Ga2O3 target . . . . 36

3.2.2 Material characterizations of Ga2O3 thin films . . . . . . . . . . 37

3.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4 Solar-blind photodetectors fabricated on β-Ga2O3/sapphire 49

4.1 Room temperature performance of photodetectors arrays . . . . . . . . . 49

4.1.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

4.1.2 Material characterizations . . . . . . . . . . . . . . . . . . . . . . 52

4.1.3 Photoresponse measurements . . . . . . . . . . . . . . . . . . . . 53

xvi

4.1.4 Noise measurements . . . . . . . . . . . . . . . . . . . . . . . . . 54

4.1.5 NEP and LDR measurements . . . . . . . . . . . . . . . . . . . . 56

4.1.6 Performance of 2D-array . . . . . . . . . . . . . . . . . . . . . . . 57

4.1.7 Photocurrent transport mechanism of MSM detector . . . . . . . 59

4.1.8 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

4.2 High-temperature performance of photodetectors . . . . . . . . . . . . . 62

4.2.1 Experimental: thin film growth and MSM device fabrication . . . 63

4.2.2 Photo to dark current ratio . . . . . . . . . . . . . . . . . . . . . 64

4.2.3 Photoresponse . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

4.2.4 Spectral response . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

4.2.5 Temporal response . . . . . . . . . . . . . . . . . . . . . . . . . . 68

4.2.6 Physical mechanisms of photocurrent transport . . . . . . . . . . 69

4.2.7 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

5 Analysis of trap states using thermally stimulated current spectroscopy 73

5.1 Experimental: TSC spectroscopy . . . . . . . . . . . . . . . . . . . . . . 75

5.2 Thermally stimulated current measurements . . . . . . . . . . . . . . . . 75

5.3 Fractional emptying of traps . . . . . . . . . . . . . . . . . . . . . . . . . 76

5.4 TSC mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

6 Gamma irradiation effect on β-Ga2O3 photodetectors 83

6.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

6.2 Material characterizations . . . . . . . . . . . . . . . . . . . . . . . . . . 85

6.3 Photoresponse of γ-irradiated device . . . . . . . . . . . . . . . . . . . . 86

6.4 Schottky barrier height calculations . . . . . . . . . . . . . . . . . . . . . 89

6.5 Photoluminescence of irradiated device . . . . . . . . . . . . . . . . . . . 91

6.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

7 Gallium oxide flexible solar-blind photodetectors on muscovite mica 93

7.1 Growth of amorphous gallium oxide and photodetector performance thereon

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

7.1.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

7.1.2 Material characterizations . . . . . . . . . . . . . . . . . . . . . . 96

xvii

7.1.3 Photoresponsivity . . . . . . . . . . . . . . . . . . . . . . . . . . 97

7.1.4 Temporal response . . . . . . . . . . . . . . . . . . . . . . . . . . 99

7.1.5 Optical power dependent photoresponse . . . . . . . . . . . . . . 100

7.1.6 Photocurrent gain, detectivity and NEP . . . . . . . . . . . . . . 101

7.1.7 Effect of bending induced strain on photoresponse . . . . . . . . . 102

7.1.8 High temperature performance of flexible photodetector . . . . . 105

7.1.9 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

7.2 Growth of epitaxial gallium oxide and photodetector performance thereon 108

7.2.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

7.2.2 Structural characterizations . . . . . . . . . . . . . . . . . . . . . 110

7.2.3 Photoelectrical measurements . . . . . . . . . . . . . . . . . . . . 112

7.2.4 Investigation of traps using TSC spectroscopy . . . . . . . . . . . 115

7.2.5 Effect of bending induced strain on photoresponse . . . . . . . . . 118

7.2.6 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

8 Wire bonding and packaging of photodetectors 121

8.1 Experimental: device fabrication, dicing and packaging . . . . . . . . . . 123

8.2 Photoelectrical measurements before and after wire bonding . . . . . . . 125

8.3 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

9 Summary and future perspective of the work 129

9.1 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

9.2 Future perspectives and challenges . . . . . . . . . . . . . . . . . . . . . 130

References 133

Publications in international journals 147

International/national conference presentations 149

Bio-data 151

xviii

List of Figures

1.1 UV exposure limit for the humans . . . . . . . . . . . . . . . . . . . . . . 2

1.2 Applications of solar-blind photodetectors . . . . . . . . . . . . . . . . . 7

1.3 Technology roadmap for next generation UV photodetectors [25] . . . . . 8

1.4 Energy band diagram of MSM structure under (a) thermal equilibrium

(b) applied bias V in dark condition and (c) applied bias V upon optical

illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

1.5 Unit cell of monoclinic β-Ga2O3 . . . . . . . . . . . . . . . . . . . . . . . 11

1.6 Band structure of monoclinic β-Ga2O3 . . . . . . . . . . . . . . . . . . . 11

2.1 Process flow for pellet fabrication . . . . . . . . . . . . . . . . . . . . . . 18

2.2 Pulsed laser deposition systems at (a) University of Warwick, United

Kingdom and (b) University of Delhi, India . . . . . . . . . . . . . . . . 19

2.3 Clean room process flow for device fabrication . . . . . . . . . . . . . . . 20

2.4 Bragg’s diffraction condition on a lattice of interplanar distance d and

(b) Schematic of Bragg-Brentano geometry of XRD system . . . . . . . . 21

2.5 Gamma chamber facility at IUAC, New Delhi . . . . . . . . . . . . . . . 26

2.6 Schematic of photodetector system . . . . . . . . . . . . . . . . . . . . . 27

2.7 Camera image of complete photodetector system in the laboratory . . . . 28

2.8 Schematic of noise measurement system . . . . . . . . . . . . . . . . . . 28

2.9 Module for flexible detector measurements . . . . . . . . . . . . . . . . . 29

2.10 Process flow involved in the TSC technique . . . . . . . . . . . . . . . . 30

2.11 Camera image of the dicing system . . . . . . . . . . . . . . . . . . . . . 31

3.1 Camera image of sintered β-Ga2O3 target . . . . . . . . . . . . . . . . . 35

3.2 (a) XRD 2θ-scan and (b) EDAX of Ga2O3 target . . . . . . . . . . . . . 37

3.3 (a) XRD 2θ scans (b) XRD Φ scan of the film deposited at 10 mT

pressure (c) Bandgap variation of Ga2O3 thin films deposited at with

various growth pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

xix

3.4 ((a) Transmittance spectra of thin films (b) bandgap of thin films cacu-

lated from tauc plot . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

3.5 XPS of C 1s spectra . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.6 XPS spectra of O1s peak of β-Ga2O3 films deposited at (a) 0.2 mTorr

(b) 0.5 mTorr (c) 10 mTorr and Ga 3d peak deposited at (d) 0.2 mTorr

(e) 0.5 mTorr (f) 10 mTorr . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.7 (a) Raman phonon modes for oxygen and gallium vacancies (b) valence

band spectra for thin films grown at different oxygen pressures . . . . . . 44

3.8 (a) Surface potential mapping (b) Gaussian distribution of VCPD (c)

calculated the work function of thin films deposited at various oxygen

pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3.9 Band diagram of samples deposited at (a) 0.2 mT (b) 0.5 mT and (c) 10

mT oxygen pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4.1 Optical microscope image of three devices among 3×4 two-dimensional

array . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

4.2 X-ray diffraction (a) 2θ scan which shows (2 0 1) plane orientation of

β-Ga2O3 thin film (b) rocking curve of (2 0 1) plane (c) phi-scan of (4 0

1) plane and (d) surface topography of the thin film . . . . . . . . . . . 52

4.3 (a) Cross-sectional TEM (CS-TEM) of Ga2O3/Al2O3 interface (b) Mag-

nified TEM image of thin film from the cross-sectional area and (c) in-

terplanar distance of (2 0 1) planes of β-Ga2O3 thin film in the HRTEM

image . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

4.4 (a) Dark current and photocurrent of device exhibiting zero bias response.

(b) spectral response of the detector at zero bias. Inset is the logarithmic

plot of responsivity versus wavelength . . . . . . . . . . . . . . . . . . . . 54

4.5 (a) Noise current as a function of frequency (b) total noise current at 1Hz

as a function of power density (c) Dynamic photocurrent response at zero

bias. Greenline represents the total noise current per Hz bandwidth at

zero bias (d) State-of-the-arts linear dynamic range of self-powered solar-

blind photodetectors as a function of dark current . . . . . . . . . . . . . 56

xx

4.6 (a) Dark and photocurrent of a 2D array, histograms of (b) dark and (c)

photocurrent of pixels and (d) current image of two pixels illuminated

with 250 nm light . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

4.7 Energy band diagram at zero bias of MSM photodetector for (a) ideal

and (b) real case . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

4.8 Current-voltage fitting of thermionic emission model for both the contacts 60

4.9 Contact potential difference distribution of Ga2O3 thin film at (a) posi-

tion 1 and (b) position 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

4.10 SEM image of fabricated photodetector at 100 µm scale . . . . . . . . . 64

4.11 Variable temperature (a) dark current-voltage and (b) photocurrent-

voltage measurement of Ga2O3 photodetector . . . . . . . . . . . . . . . 65

4.12 (a) Temperature-dependent PDCR (b) photocurrent and peak respon-

sivity of β-Ga2O3 photodetector at 10V bias and 255 nm illumination . . 67

4.13 Spectral response of fabricated photodetector with temperature variation

ranging from 23 ◦C to 250 ◦C . . . . . . . . . . . . . . . . . . . . . . . . 67

4.14 Real-time current change of fabricated photodetector at different tem-

peratures under 255 nm UV illumination . . . . . . . . . . . . . . . . . . 68

4.15 (a) Rise times (τr1 and τr2) and decay times (τd1 and τd2) at 5V bias under

255 nm illumination with detector temperature (b) Arrhenius plot of slow

components of rise times shows activation energies of 73 and 205 meV

and (c) decay times depicts activation energies of 58 and 168 meV (d)

Model for photocurrent mechanism below 150 ◦C with electron capture

process having activation energies of 58 and 73 meV and (e) Model for

photocurrent mechanism above 150 ◦C with the transition of STH to a

mobile hole having activation energies of 205 and 168 meV . . . . . . . . 70

5.1 (a) heating and cooling curve of TSC at 7 K/min heating rate (b) net

TSC spectrum of the β-Ga2O3 thin film . . . . . . . . . . . . . . . . . . 76

5.2 Process flow of fractional emptying method for TSC peak resolution . . . 77

5.3 Resolved peaks of TSC spectrum for all the traps . . . . . . . . . . . . . 77

5.4 Net TSC peaks with characteristic peak teamperatue of all the traps . . 78

5.5 Arrhenius plot of dark current . . . . . . . . . . . . . . . . . . . . . . . . 79

xxi

5.6 Band diagram of (a) trap filling via optical injection of charge carriers

(b) after complete trapping and (c) under thermal release of carriers . . . 81

6.1 Schematic of the fabricated β-Ga2O3 based Ni/Au/Ni metal-semiconductor-

metal photodetector . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

6.2 (a) X-ray diffraction pattern showing (201) orientation. Inset shows the

rocking curve of (201) plane and (b) AFM image of β-Ga2O3 thin film

grown on c-plane sapphire . . . . . . . . . . . . . . . . . . . . . . . . . . 85

6.3 Current-voltage (I-V) measurements under (a) dark conditions and (b)

illumination using 245 nm wavelength, for the MSM photodetector after

gamma exposure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

6.4 (a) Photo to dark current ratio (PDCR) and (b) peak responsivity at

10 V of the fabricated photodetector with radiation dose under 245 nm

illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87

6.5 Spectral responsivity of photodetector with γ-ray irradiation at 10 V bias 88

6.6 SEM images of MSM photodetectors (a) pristine (b) after 100 kGy irra-

diation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

6.7 Logarithmic plots of exp(eV/kT) vs V. Inset shows the magnified scale

of linear fit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

6.8 Variation in dark current and Schottky barrier height (SBH) with γ-ray

irradiation dose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

6.9 Photoluminescence spectra of Ga2O3 thin film with increasing cumulative

radiation dose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

7.1 (a) XRD 2θ scan of β-Ga2O3 thin film (b) Highly smooth surface mor-

phology of thin film . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

7.2 (a) photoluminescence spectra of Ga2O3 having various defect bands of

UV, blue, green and red (b) XPS survey scan of thin film and (c) X-ray

photoelectron spectroscopy of Ga 3d and (d) O 1s core levels . . . . . . . 97

xxii

7.3 (a) Schematic of photodetection measurements on interdigitated elec-

trode structures fabricated on Ga2O3/Mica (b) variation of dark current

and photocurrent at a peak wavelength of 270 nm with an applied bias to

the detector (c) demonstration of spectral responsivity at 5V bias rang-

ing from 240 nm to 600 nm wavelength. Inset shows the camera image

of fabricated devices on highly transparent and flexible Ga2O3/Mica and

(d) time response curve of the detector at 5V bias with biexponential

fitting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

7.4 Optical power dependent (a) photocurrent and (b) responsivity at peak

wavelength of 270 nm of a photodetector which was kept at 5V external

bias during measurements . . . . . . . . . . . . . . . . . . . . . . . . . . 100

7.5 (a) Responsivity (blue curve) and gain (red curve) of the fabricated device

with varying voltages (b) variation of detectivity (blue curve) and NEP

(red curve) with voltages exhibiting sensitivity of the devices . . . . . . . 101

7.6 CCD camera image corresponding to (a) flat condition and (b) 5 mm

bending radius (c) dark and photocurrent at the various bending radius

(d) Photocurrent pulses with bending cycles of the device having 5 mm

bending radius (e) Plot of photocurrent and dark current with bending

cycles showing a small decrease in current after 500 cycles . . . . . . . . 104

7.7 (a) Dark current (blue curve) and photocurrent (red curve) at 2V bias

with temperature up to 523K (b) PDCR of the detector at elevated

temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

7.8 Comparisons of reported maximum working temperature of solar-blind

photodetectors based on flexible substrates . . . . . . . . . . . . . . . . . 107

7.9 (a) XRD 2θ-scan of β-Ga2O3 /muscovite (b) rocking curve of (201)-plane

and (c) surface morphology of β-Ga2O3 thin film . . . . . . . . . . . . . 111

7.10 Cross-sectional TEM image of β-Ga2O3 /muscovite mica interface (b)

SAED pattern of β-Ga2O3 thin film and (c) SAED pattern of muscovite

mica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

7.11 (a) Current-voltage measurements under dark and 265 nm wavelength

illumination of device (b) peak photoresponsivity (c) time-response mea-

surement at zero bias and (d) normalized spectral responsivity . . . . . 113

xxiii

7.12 TSC curve of the device with 5 ◦C/min heating and cooling rates at zero

bias (b) net TSC plot with two broad current peaks (c) and (d) are the

Gaussian fits of both peaks . . . . . . . . . . . . . . . . . . . . . . . . . 115

7.13 (a) Measurement setup for bending tests of the device (b) Tensile strain

versus bending radius of the device (c) CCD camera images of a flexi-

ble device under flat (d) 16 mm bending radius condition (e) Dark and

photocurrent of the device at zero bias with bending radius of muscovite. 118

8.1 SEM image of (a) ball bond and (b) wedge bond [247] . . . . . . . . . . 122

8.2 Wire bonding process . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123

8.3 Optical image of (a) single diced device (b) wedge bond on the device

pad (c) camera image of a mounted device with a wire loop and ball

bond and (d) packaged photodetector on TO-header with quartz optical

window . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

8.4 (a) Dark current and (b) photocurrent as a function of voltage before

and after wire bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

8.5 (a) Responsivity as a function of voltage before and after wire bonding

and (b)time response before and after bonding . . . . . . . . . . . . . . 127

xxiv

List of Tables

1.1 Material comparisions for solar-blind photodetectors application . . . . . 10

1.2 Physical properties of β-Ga2O3 . . . . . . . . . . . . . . . . . . . . . . . 12

3.1 Lorentzian width and FWHM of thin films grown at different oxygen

pressures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.2 Ga/O ratio of samples calculated from XPS spectra . . . . . . . . . . . . 42

3.3 Raman active phonon modes of all the thin films . . . . . . . . . . . . . 43

3.4 Average values of bandgap (from Tauc plot), intrinsic Fermi-level (from

theoretical calculations), work function (from KPFM) and energy gap

between VBM and Ef (from VB spectra) . . . . . . . . . . . . . . . . . . 47

4.1 Comparisons of Ga2O3 self-powered photodetector arrays . . . . . . . . . 59

5.1 Known origins of the traps in β-Ga2O3 . . . . . . . . . . . . . . . . . . . 74

5.2 List of traps and their activation energies in β-Ga2O3 thin film using

TSC method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

5.3 List of traps and their activation energies in β-Ga2O3 thin films by Ar-

rhenius plot of dark current . . . . . . . . . . . . . . . . . . . . . . . . . 80

7.1 State-of-the-arts room temperature performance parameters of flexible

solar-blind photodetectors . . . . . . . . . . . . . . . . . . . . . . . . . . 103

7.2 Comparisons of wearable photodetectors in the perspective of flexibility 105

7.3 Comparisons of photovoltaic UV-C detectors with different device struc-

tures and channel layer growth methods (Notations: TG - growth tem-

perature, Id - dark current, Ip- photocurrent and Popt - optical power

density) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

7.4 Distribution of bandgap states calculated from thermally stimulated cur-

rent spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

xxv

7.5 Comparisons of self-powered and flexible UV-C photodetectors based on

Ga2O3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

8.1 Comparisons of different wire bonding methods . . . . . . . . . . . . . . 121

8.2 Comparisons of different wire bonding methods . . . . . . . . . . . . . . 124

8.3 List of wire bonding parameters for gold wire . . . . . . . . . . . . . . . 124

xxvi

Appendix

Symbols

η Ideality factor

Ip Photocurrent

q Charge

Popt Optical power density

h Plank constant

ν Frequency of photon

c Speed of light

λ Wavelength

Rλ Spectral response at wavelength λ

Aeff Effective device area where charge carrier generates

φ Photon flux

r Reflectance of material

α Absorption coefficient of material

ξ Factor due to electron-hole recombination in material

κ Extinction coefficient of material

Id Dark current

I Current

I0 Steady state current

τ Relaxation time constant

A1 & A2 Constants

i2n Mean square of total noise current

is Shot noise

iT Thermal noise

R resistance of material

k Boltzmann constant

T Temperature

Pmax Maximum illumination power

ϕ Schottky barrier height

xxvii

Vb Built-in potential

β Beta phase

γ gamma

Angstrom

m0 Rest mass of electron

Be Beryllium

Li Lithium

Sn Tin

Si Silicon

Ge Germanium

Zr Zirconium

Ni Nickel

Au Gold

Co Cobalt

Cu Copper

Pt Platinum

Ir Iridium

Al Aluminum

Ti titanium

Fe Iron◦C Degree Celsius

K Kelvin

J Joule

θ Incident angle of x-ray with crystal plane

Φ Rotation angle with normal to the crystal plane

VCPD Contact potential difference

ϕtip Work function of the tip

ϕsample Work function of the sample

Eg bandgap

B Proportionality constant

Efi Intrinsic Fermi level

m∗p Hole effective mass

xxviii

m∗n Electron effective mass

SI Spectral density of current fluctuations

Sv Spectral density of voltage fluctuations

A Device area

ϕap Apparent Schottky barrier height

A∗ Richardson constant

Pλ Optical power density at λ wavelength

ET Activation energy of trap

Tm Characteristic peak temperature

Γ Heating rate

vth Thermal velocity

Nc Density of states in conduction band

σn Capture cross section

Efn Quasi Fermi level of electron

Efp Quasi Fermi level of hole

ϕb Barrier height

ϕif Image force induced barrier lowering

Nd Donor concentration

G Gain

D∗ Detectivity

ε Strain

tf Thickness of film

ts Thickness of substrate

Rc radius of curvature

Yf Young’s modulus of film

Ys Young’s modulus of substrate

ζtfts

kGy Kilo Gray

xxix

Abbreviations

UV Ultraviolet

EQE External quantum efficiency

PDCR Photo to dark current ratio

NEP Noise equivalent power

LDR Linear dynamic range

FET Field effect transistor

MSM Metal-semiconductor-metal

MOS Metal oxide semiconductor

SB Solar-blind

IoT Internet of things

DUV Deep ultraviolet

Ga2O3 Gallium oxide

AlGaN Aluminum gallium nitride

SiC Silicon carbide

VBM Valence band maximum

TSC Thermally stimulated current

TO Transistor outline

IDE Interdigitated electrodes

PVA Polyvinyl alcohol

PLD Pulsed laser deposition

TEM Transmission electron microscope

XRD X-ray diffraction

SPM Scanning probe microscopy

KPFM Kelvin probe force microscopy

LH Lift height

AC Alternative current

DC Direct current

PL Photoluminescence

XPS X-ray photoelectron microscopy

DLTS Deep level transient spectroscopy

TCO Transparent conducting oxide

xxx

UID Unintentionally doped

EDAX Energy dispersive X-ray spectroscopy

FWHM Full width at half maximum

DOS Density of states

VBM Valence band maxima

HOPG highly oriented pyrolytic graphite

FLP Fermi level pinning

SNR Signal to noise ratio

MBE Molecular beam epitaxy

MOCVD Metal oxide chemical vapor deposition

PECVD Plasma enhanced chemical vapor deposition

SBH Schottky barrier height

DUT Device under test

RT Room temperature

STH Self trapped holes

DLOS Deep level optical spectroscopy

TSDC Thermally stimulated depolarization current

EFG Edge-defined film-fed grown

MOSFET Metal oxide semiconductor field effect transistor

SEM Scanning electron microscope

ICNIRPInternational Commission on Non-Ionizing Radiation

Protection

PEN Polyethylene naphthalate

BN boron nitride

ZnO Zinc oxide

PET Polyethylene terephthalate

CCD Charge coupled device

HRXRD High resolution X-ray diffraction

FFT Fast Fourier transform

xxxi