FORMATION AND CHARACTERIZATION OF GaN NANOWIRES AND STUDY OF THEIR PHOTOCONDUCTION PROPERTIES MUKESH KUMAR

Transcription

1 FORMATION AND CHARACTERIZATION OF GaN NANOWIRES AND STUDY OF THEIR PHOTOCONDUCTION PROPERTIES MUKESH KUMAR DEPARTMENT OF PHYSICS INDIAN INSTITUTE OF TECHNOLOGY DELHI OCTOBER 2017

2 Indian Institute of Technology Delhi (IITD), New Delhi, 2017

3 FORMATION AND CHARACTERIZATION OF GaN NANOWIRES AND STUDY OF THEIR PHOTOCONDUCTION PROPERTIES by MUKESH KUMAR Department of Physics Submitted in fulfillment of the requirement of the degree of Doctor of Philosophy to the INDIAN INSTITUTE OF TECHNOLOGY DELHI OCTOBER 2017

4 Certificate We are satisfied the thesis entitled Formation and Characterization of GaN Nanowires and Study of their Photoconduction Properties submitted by Mr. Mukesh Kumar is worthy of consideration for the award of the degree of Doctor of Philosophy and is a record of original and bonafide research work carried out by him under our supervision. The results contained in it have not been submitted in part or full to any other university or institute for the award of any degree/diploma. Dr. Rajendra Singh Department of Physics Indian Institute of Technology Delhi New Delhi (India) Prof. Vikram Kumar Centre for Applied Research in Electronics Indian Institute of Technology Delhi New Delhi (India)

5 Acknowledgements First, I am very thankful to my supervisors Dr. Rajendra Singh and Prof. Vikram Kumar who have nurtured the research basics in me. It is great privilege for me to have worked under their proficient guidance and excellent teaching. I am grateful to Dr. Rajendra Singh for experimental suggestions, encouragement for attempting research problems, and providing freedom to execute research idea. I am grateful to Prof. Vikram Kumar for scientific discussions to extract novel findings, systematic technical writing skills, suggestions to plan research problems following experimentation and their improvements. I am always inspired by him for the research and services he has contributed for advancement of science. I am thankful to PhD SRC committee: Prof. Santanu Ghosh, Prof. J. P. Singh and Dr. Sameer Sapra for their time to time evaluation and feedbacks during the course of this research work. I am also thankful to Dr. Samaresh Das and Prof. Neeraj Khare for the helpful research interactions. I gratefully acknowledge the Physics Department, IIT Delhi for providing research fellowship and Heads of Department for providing research oriented atmosphere. I gratefully acknowledge the Nanoscale Research Facility IIT Delhi. NRF has been developed with excellent experimental facilities in last few years with financial support from Department of Electronics and Information Technology, Government of India. I would like to thank NRF for providing experimental facilities and partial financial support. I am also thankful to the Nanomission: Department of Science and Technology (DST), India (Project no.: Sr/NM/NS-1106/2012 (G)) for partial financial support for this work. I would like to thank my colleagues Dr. Uday Dadwal, Dr. Ashish Kumar, Dr. Rajesh Bagh, Dr. Sudheer Kumar, Dr. Ashutosh Kumar, Mandeep Thakur, Chandra Shakher Pathak, Manjari Garg, Veerendra Dhyani, Chandan Sharma, Ravi Pathak, Monika, Bhera Ram Tak, Kapil Narang, Aditya Singh and project fellows (Masters and Bachelors) for their academic as well as personal help. iii

6 I would like to express my thank to Prof. Silke Christiansen from Max Planck Institute for the Science of Light, Erlangen, Germany for providing MOCVD grown GaN nanowires samples and FESEM, CL, Raman measurements. I gratefully acknowledge Prof. Sakthi Kumar and Prof. Yoshikata Nakajima from Bio-Nano Electronics Research Centre, Toyo University, Japan for their support in TEM, XPS measurements and learning nanofabrication during one month research visit. Finally, words are inadequate to express thank to my family for their blessings, love, care and support. Mukesh Kumar iv

7 Abstract Semiconductor GaN nanowires are one of the potential materials for variety of nanoscale device applications. For successful establishment of GaN nanowire based nanoscale device platform and their integration in complex nanosystems require effective solutions which are still challenging. Physical dimension with high quality of GaN nanowires not only offers the advantage of tuning fundamental electronic and optical properties but also gives prospect for fabrication of high performance nanoscale devices. For example, formation of long GaN nanowires with high quality is highly desirable for nanowire based large scale device integration, catalytic functionalities, sensors, light detection applications, unification with micro scale systems and nano-bio devices due to biological macromolecules size regime. In general, GaN nanowires grown by vapor-liquid-solid (VLS) as well as non-vls method such as catalyst free growth, or top down etched nanowires have limited length up to several tens of micrometers. On the other hand, β-ga2o3 nanowires exhibit higher growth rate than VLS grown GaN nanowires. Alternate approach as the formation of GaN nanowires from β-ga2o3 nanowires is one of the effective solutions to obtain high quality long GaN nanowires for the application of high performance nanoscale devices. In this perspective, research work has been performed as highlighted below. First, growth of β-ga2o3 nanowires and their diameter tuning using chemical vapor deposition technique has been investigated under various experimental conditions. Diameter of root grown β-ga2o3 nanowires having monoclinic crystal structure is tuned by varying separation distance between metal source and substrate. Effects of gas flow rates on the morphology and diameter of nanowires have been studied. The dependence of diameter on temperature has been investigated by growing β-ga2o3 nanowires at various temperatures. With appropriate combination of growth temperature and catalyst nanoparticles size, nanowires diameter of few tens of nanometre has been demonstrated. As a result of ammonification, wurtzite GaN nanowires were formed from β-ga2o3 nanowires having monoclinic crystal structure. For obtaining GaN nanowires through ammonification, the diameter of β-ga2o3 nanowires plays a crucial role. Ammonification v

8 temperature and ammonification time is the main parameters for β-ga2o3 to GaN phase conversion. The effect of ammonification temperature and time on the formation of GaN/β- Ga2O3 nanowires has been investigated. The coaxial β-ga2o3/gan nanowires and GaN nanowires have been formed using ammonification. The enhancement of GaN formation with increase in ammonification temperature and time has been demonstrated. As-formed GaN nanowires are of high quality as inferred from various characterizations. The growth of GaN nanowires using VLS mechanism has also been investigated. GaN nanowire length has been interrogated under various experimental conditions in CVD system such as various growth temperature, different gas flow rate, and growth time. These investigation shows that VLS grown GaN nanowires exhibit the length below tens of micrometers. On the other hand, the high quality ultralong GaN nanowires up to millimeter length scale have been demonstrated using GaN formation from β-ga2o3 nanowires. The possibility of GaN growth on surface of β-ga2o3 nanowires is observed. The GaN formation and its high quality nature have been investigated by various characterization methods. To investigate the high electrical quality and photodetector application of as-formed GaN nanowires, electrical contacts on single GaN nanowire have been fabricated using multiple step lithography procedures. The photoconduction study in as-formed GaN nanowires showed ultra high responsivity (~ A/W at V) and gains (~ at V) under UV illumination confirming their high quality nature. vi

9 स र स म क ड टर GaN न न यर न न क ल डव इस अन य ग क व वधत क लए स भ वत स म य म स एक ह GaN न न व यर आध रत न न क ल डव इस ल टफ म क सफल थ पन क लए और ज टल न न स ट स म उनक एक करण क भ व सम ध न क आव यकत ह त ह ज अभ भ च न त प ण ह GaN न न यस क उ ग णव क स थ श र रक आय म न क वल म लक इल नक और ऑ टकल ग ण क नग क ल भ द न करत ह ब क उ न प दन व ल न न क ल उपकरण क नम ण क स भ वन भ द त ह उद हरण क लए, न न व यर आध रत बड़ प म न पर डव इस एक करण, उ रक य मकत, स सर, क श क पत लग न क अन य ग, स म प म न पर ण लय क स थ एक करण और ज वक अण क आक र क नयम क क रण न न -ब य उपकरण क लए उ ग णव व ल ल ब समय तक GaN न न यस क नम ण ब हद व छन य ह आमत र पर, व प-तरल-ठ स (व एलएस) क स थ-स थ ग र-व एलएस व ध ज स उ रक म व य गढ़ ई न न व यर र GaN न न यस क कई द सय म इ म टर तक स मत म म स मत कय ज त ह द सर ओर, β-ga 2 O 3 न न व यर व एलएस क त लन म उ व दर क दश त ह, β-ga 2 O 3 न न यस स GaN न न यस क गठन क प म व क पक द क ण उ न प दन व ल न न क ल उपकरण क आव दन क लए उ ग णव व ल ल ब समय तक GaN न न यस करन क लए भ व सम ध न म स एक ह इस प र य म, अन स ध न क य क न च ह इल इट कय गय ह सबस पहल, र स य नक व प जम व तकन क क उपय ग करक β-ga 2 O 3 न न यस और उनक स नग क व व भ य ग मक थ तय क तहत ज च क गई ह म न ल नक टल स रचन व ल β-ga 2 O 3 न न यस क स क स म टल त और स स ट क ब च अलग ह न क द र क अन स र अलग- अलग ह न न यस क आक रक और स पर ग स व ह दर क भ व क अ ययन कय गय ह त पम न पर स क नभ रत व भ त पम न पर बढ़ रह β-ga 2 O 3 न न यस र ज च क गई ह वक स त पम न और उ रक न न कण क आक र क उपय स य जन क स थ, क छ न न म टर क न न वर स क दश न कय गय ह अम नयन क प रण म व प, म ट क ल नक टल स रचन व ल β-ga 2 O 3 न न यस स wurtzite GaN न न यस क गठन कय गय थ अम नयन शन क म यम स GaN न न व यर करन क लए, β-ga 2 O 3 न न यस क स एक मह वप ण भ मक नभ त ह GaN चरण प तरण क लए β-ga 2 O 3 क लए एम न फक शन त पम न और एम न करण समय म य प र म टर ह ग यन GaN / β-ga 2 O 3 न न यस क vii

10 गठन पर एम न करण त पम न और समय क भ व ज च कर लय गय ह सम म दन β-ga 2 O 3 / GaN न न यस और GaN न न यस क गठन अम नयन शन स कय गय ह अम नयन त पम न और समय म व क स थ GaN गठन क व क दश न कय गय ह क प म ग ठत GaN न न यस उ ग णव व ल ह, ज व भ ल ण स अन म नत ह व एलएस त क उपय ग क ग न न न यस क व क भ ज च ह च क ह GaN न न व र ल ब ई स व ड ण ल म व भ व क त पम न, व भ ग स व ह दर, और वक स समय ज स व भ य गक शत क तहत प छत छ क गई ह इन ज च स पत चलत ह क व एलएस क व ई GaN न न व यर दस म टर स न च क म इ -म टर क दश न करत ह द सर ओर, β-ga 2 O 3 न न यस स GaN गठन क उपय ग करत ए उ ग णव व ल अ ल न GaN न न यस क मल म टर ल ब ई क प म न पर द शत कय गय ह β- Ga 2 O 3 न न य यर क सतह पर GaN वक स क स भ वन मन ई गई ह GaN गठन और इसक उ ग णव क त व भ ल ण वण न तर क स ज च क गई ह उ व त ग णव और photodetector अन य ग क गठन क प म GaN न न यस क ज च करन क लए, सगल GaN न न व यर पर व त स पक कई कदम लथ फ य क उपय ग कर गढ़ गए ह GaN न न यस क प म त य र कए गए photodetector अ ययन म य व र शन क तहत अ उ उ रद य व (~ A/W, V) और ल भ (~ , 0.3 V) ग णव क त ज च क गई ह viii

11 Table of contents Certificate Acknowledgements Abstract Table of contents List of figures List of symbols and abbreviations Page No. i iii v ix xiii xix Chapter-1: Introduction Background Properties of GaN Overview of GaN and its nanowires Requirement of ultralong GaN nanowires Growth of GaN nanowires Growth mechanism Alternate approach to obtain GaN nanowires β-ga2o3 nanowires Strategy for nanowire based devices Photoconduction Thesis organization References 15 Chapter-2: Experimental techniques Experimental background CVD systems Characterization techniques SEM XRD Raman spectroscopy TEM 24 ix

12 2.3.5 PL UV-Vis spectrophotometery XPS Cathodoluminescence Electron beam lithography References 27 Chapter-3: Growth and diameter tuning of β-ga2o3 nanowires Introduction Experimental Results and discussion Dispersion of Au nanoparticles β-ga2o3 nanowires with source to substrate distances β-ga2o3 nanowires with different gas flow rates β-ga2o3 nanowires at different growth temperatures Structural analysis of β-ga2o3 nanowires Band-gap of β-ga2o3 nanowires Conclusions References 47 Chapter-4: Formation of GaN nanowires from β-ga2o3 nanowires via ammonification process Introduction Ammonification of β-ga2o3 nanowires Experimental Results and discussion β-ga2o3 nanowires used for ammonification Ammonified nanowires with different diameters Partial GaN/β-Ga2O3 nanowires with catalyst alloy at its tip Analyses of phase conversion from β-ga2o3 to GaN through various characterization methods Effect of ammonification temperature on the formation of GaN/β-Ga2O3 nanowires 67 x

13 4.3.1 Experimental Results and discussion β-ga2o3 nanowires used for ammonification Ammonified nanowires at different temperatures Analyses of phase conversion from β-ga2o3 to GaN through various characterization methods Effect of ammonification time on the formation of GaN/β-Ga2O3 nanowires Experimental Results and discussion Ammonified nanowires at different time Analyses of phase conversion from β-ga2o3 to GaN through various characterization methods Conclusions References 87 Chapter-5: Growth of GaN nanowires and formation of high quality ultralong GaN nanowires Introduction Growth of GaN nanowires using VLS mechanism Experimental Results and discussion Au nanoparticles as catalyst Growth of GaN nanowires at different temperatures Inspection of GaN nanowires length at different growth conditions MOCVD grown nano-micro scale needles Experimental Results and discussion Triangular nano-micro scale GaN needles XPS analysis of triangular nano-micro scale GaN needles Formation of high quality ultralong GaN nanowires Experimental Results and discussion 110 xi

14 Morphological properties Structural properties Optical properties Conclusions References 116 Chapter-6: Study of photoconduction in GaN nanowires Introduction Fabrication of electrical contacts on single nanowire Photoconduction properties of GaN nanowires Experimental Results and discussion Conclusions References 130 Chapter-7: Summary and future perspective Summary Future perspective 137 Publications in International Journals 139 International/National Conference Presentations 140 Bio-data 141 xii

15 List of figures Figure 1.1: Demonstration of nanowire based device applications: (a) Multiple FET array on a single ultralong p-si nanowire, (i) schematic, (ii) optical image of fabricated device, scale bar is 100 µm (Image: (b) Lasing from single ZnO nanowire, wire acts as a laser cavity, laser emission emanates from the two ends (Image: (c) Schematic of photodetector based on metal contacted single nanowire supported on insulating layer with substrate (Image: (d) Highly integrated NW-neuron devices, (i) optical image of aligned axon crossing an array of 50 nanowire devices with a 10-mm interdevice spacing, (ii) schematic of virus binding to a nanowire sensor (Image: 3 Figure 1.2: Schematic of GaN wurtzite crystal structure... 5 Figure 1.3: Schematic of the VLS growth mechanism... 8 Figure 2.1: (a) MTI CVD single zone furnace system, (b) schematic diagram of tubular furnace with metal source and substrate kept in a same temperature zone. (c) Metrex CVD single zone furnace system Figure 2.2: Photograph of Raith e-line plus EBL system installed at IIT Delhi, India Figure 3.1: FESEM images of dispersed Au nanoparticles (a) 20 nm and (b) 50 nm Figure 3.2: FESEM images of β-ga2o3 nanowires grown at different distances between source to substrate (a, b) 2 cm, (c, d) 4 cm and (e, f) 6 cm. Circles in insets of image (b) and (d) show the tip and root of nanowire. Histograms in insets of (b, d, f) show nanowire diameter distribution and upper inset of (f) shows the plot for nanowire diameter vs separation distance between metal source to substrate Figure 3.3: Schematic illustration of gas velocity profile v(b) and corresponding developed boundary layer ( ) in the tube of CVD system Figure 3.4: FESEM images of Ga2O3 nanowires grown with (a, b) Ar:O2 total flow rate as 160 ml/min (120:40 ml/min) and (c, d) Ar/O2 flow rate ratio as 1/3(20:60 ml/min) xiii

16 Figure 3.5: FESEM images of Ga2O3 nanowires (a, b) grown at 850 C with 50 nm Au nanoparticles, (c, d) grown at 800 C with 50 nm Au nanoparticles Figure 3.6: FESEM images of Ga2O3 nanowires (a, b) grown at 800 C with 20 nm Au nanoparticles, (c, d) grown at 850 C with 20 nm Au nanoparticles Figure 3.7: Growth schematic for diameter tuning of Ga2O3 nanowires at various growth conditions in CVD system Figure 3.8: XRD patterns of β-ga2o3 nanowires grown at different distances between source and substrate (a) 2 cm (D1), (b) 4 cm (D2), (c) 6 cm (D3); (d) Zoom XRD (002) plane peak. Plots are in linear scale obtained with bulk mode XRD Figure 3.9: Raman spectra of β-ga2o3 nanowires grown at source to substrate distances of (a) 2 cm (D1), (b) 4 cm (D2) and (c) 6 cm (D3) Figure 3.10: (a) A plot between [F(R) hν] 2 versus photon energy (hν) for β-ga2o3 nanowires with decreasing diameters from D1 to D3, (b) alteration of band-gap with diameter of β-ga2o3 nanowires Figure 4.2.1: FESEM images of β-ga2o3 nanowires (a, b) grown at 900 C using 50 nm Au catalyst nanoparticles (GO1), (c, d) grown at 800 C using 50 nm Au catalyst nanoparticles (GO3). (e, f) grown at 800 C using 20 nm Au catalyst nanoparticles (GO3).Circle in inset of (b) show the tip of β-ga2o3 nanowires Figure 4.2.2: FESEM images of β-ga2o3 nanowires (having catalyst alloy at the tip of nanowires) grown on substrates (a) sapphire, (b) GaN and (c) silicon Figure 4.2.3: FESEM images of ammonified nanowires at 1000 C (a, b) GN1 resultant from GO1, (c, d) GN2 resultant from GO2 and (e, f) GN3 resultant from GO Figure 4.2.4: FESEM images of ammonified nanowires at 900 C (a) GNO4 resultant from GO4, GNO5 resultant from GO5 and (b) GNO6 resultant from GO6; (d) faceted nanostructure of catalyst alloy. (e) CL image in panchromatic mode and (f) corresponding FESEM image of GNO6 as ammonified nanowires with insets for GO6 before ammonification Figure 4.2.5: XRD patterns of (a) β-ga2o3 nanowires (GO1), (b) ammonified nanowires (GN1) at 1000 C, (c) ammonified nanowires (GNO4) at 900 C. Symbol * represents residual Ga2O3 XRD peaks. Plots are in linear scale obtained with 1 glancing angle XRD Figure 4.2.6: Raman spectra of (i) β-ga2o3 nanowires (GO1), (ii) converted GaN nanowires (GN1), (iii) partially converted GaN/β-Ga2O3 nanowires (GNO4) xiv

17 Figure 4.2.7: TEM images (a), (d), (g); HRTEM images (b), (e), (h); SAED pattern images (c), (f), (i); Images {(a), (b), (c)} - β-ga2o3 nanowires (GO1); {(d), (e), (f)} - converted GaN nanowires (GN1); and {(g), (h), (i)} - partially converted GaN/β-Ga2O3 nanowires (GNO4). TEM images have been recorded with electron microscope operated at 200 kv Figure 4.2.8: XPS core level spectra (a) Ga 2p3/2 of β-ga2o3 nanowires, (b) Ga 2p3/2 of converted GaN nanowires. Core level spectra (c) O1s and (d) N1s with (i) Ga2O3 nanowires and (ii) converted GaN nanowires Figure 4.2.9: Schematic for GaN phase formation from β-ga2o3 nanowires Figure 4.3.1: FESEM images (a), (b) of β-ga2o3 nanowires Figure 4.3.2: FESEM images of GaN/β-Ga2O3 nanowires formed at ammonification temperatures of (a, b) 900 C, (c, d) 950 C, (e, f) 1000 C and (g, h) 1050 C Figure 4.3.3: XRD patterns of GaN/β-Ga2O3 nanowires formed at ammonification temperatures of (a) 900 C, (b) 950 C, (c) 1000 C and (d) 1050 C; Insets: zoom segments with normalized intensity; Symbol '*' represents residual β-ga2o Figure 4.3.4: Arrhenious plot of XRD intensity ratios of GaN to residual β-ga2o3 against ammonification temperature Figure 4.3.5: Raman spectra of GaN/β-Ga2O3 nanowires formed at ammonification temperatures of (a) 900 C, (b) 950 C, (c) 1000 C and (d)1050 C Figure 4.3.6: Resonant Raman spectra of GaN/β-Ga2O3 nanowires formed at 1050 C ammonification temperature Figure 4.3.7: RRS spectra of GaN films (Left panel) with (a) c-plane, (b) r-plane, (c) a-plane and corresponding cross-sectional TEM images (Right panel) Figure 4.3.8: EDX spectrum of ammonified nanowires at (a) 900 C (b) 950 C, (c) 1000 C, and (d) 1050 C Figure 4.3.9: TEM images of ammonified nanowires with SAED images at (a) 900 C and (b) 1050 C. TEM images have been recorded with electron microscope operated at 200 kv 78 Figure (a) CL image in panchromatic mode and (b) spectra of 900 C ammonified nanowires. (c) CL image and (d) spectra of 1050 C ammonified nanowires. Insets of (a, c): Respective FESEM images...78 Figure : Photoluminescence spectra of GaN/β-Ga2O3 nanowires formed at ammonification temperatures of (a) 900 C, (b) 950 C, (c) 1000 C and (d)1050 C using excitation wavelength of 325 nm Figure 4.4.1: FESEM images of ammonified nanowires at 900 C for different time of (a, b) 1 h, (c, d) 5 h and (e, f) 12 h xv

18 Figure 4.4.2: XRD patterns of ammonified nanowires at 900 C for different time of (a) 1 h, (b) 5 h and (c) 12 h. Symbol * represents residual Ga2O3 XRD peaks Figure 4.4.3: Raman spectra of ammonified nanowires at 900 C for different time of (a) 1 h, (b) 5 h and (c) 12 h Figure 4.4.4: Photoluminescence spectra of ammonified nanowires at 900 C for different time of (a) 1 h, (b) 5 h and (c) 12 h using excitation wavelength of 325 nm Figure 4.4.5: (a) TEM image with (b) high resolution image of ammonified nanowires at 900 C for 5 h Figure 5.2.1: Annealing of Au film (5 nm) at different temperature under Ar flow (50 ml/min). FESEM images of (a) Au film, (b) annealed at 200 C, (c) annealed at 400 C, (d) annealed at 600 C, (e) annealed at 800 C and (f) annealed at 1000 C Figure 5.2.2: FESEM images for GaN nanowires growth at (a) 800 C for 1 h, (b) 900 C for 1 h, (c) 950 C for 1 h and (d) 1000 C for 1 h Figure 5.2.3: (a) XRD pattern for GaN nanowires grown at 1000 C for 1 h. Plots are in linear scale obtained with bulk mode XRD Figure 5.2.4: FESEM images of GaN nanowires grown at (a) 1000 C for 1 h with NH3/N2 flow rate ratio of 2/3 (40:60 cc/min), (b) 1000 C for 1 h with NH3/N2 flow rate ratio of 1/9 (50:450 cc/min), (c) 1000 C for 3 h with NH3/N2 flow rate ratio of 2/3 (200:300 cc/min) and (d) 1050 C for 3 h with NH3/N2 flow rate ratio of 2/3 (200:300 cc/min) Figure 5.2.5: XRD patterns of GaN nanowires grown at (a) 1000 C for 1 h with NH3/N2 flow rate ratio of 2/3 (40:60 cc/min), (b) 1000 C for 1 h with NH3/N2 flow rate ratio of 1/9 (50:450 cc/min), (c) 1000 C for 3 h with NH3/N2 flow rate ratio of 2/3 (200:300 cc/min) and (d) 1050 C for 3 h with NH3/N2 flow rate ratio of 2/3 (200:300 cc/min). Plots are in linear scale obtained with bulk mode XRD Figure 5.3.1: SEM images of TGN (a, c) grown at 1000 C for 12 min. and circles show the Ni catalyst present at the top of TGN. (b, d) grown at 1050 C for 7 min. Inset of c) shows a broken TGN Figure 5.3.2: XRD patterns of highly dense ensemble of TGN covered surface (grown at 1000 C) and low dense ensemble of TGN covered surface (grown at 1050 C) Figure 5.3.3: XPS deconvoluted CLS of Ga 3d for highly dense ensemble of TGN covered surface Figure 5.3.4: XPS deconvoluted CLS (a, b) Ga 2p3/2 and (c, d) N1s. (Top (a, c)) highly dense ensemble of TGN covered surface, (Bottom (b, d)) low dense ensemble of TGN covered surface xvi

19 Figure 5.3.5: XPS deconvoluted CLS (a, b) O1s and (c, d) C1s. (Top (a, c)) highly dense ensemble of TGN covered surface, (Bottom (b, d)) low dense ensemble of TGN covered surface Figure 5.4.1: (a-c) FESEM images of ultralong GaN nanowires obtained from β-ga2o3 nanowires template. Image (c) shows the ultralong single GaN nanowire Figure 5.4.2: XRD pattern of as obtained ultralong GaN nanowires showing high crystalline quality Figure 5.4.3: Raman spectra of as obtained ultralong GaN nanowires. Strong Raman E2 (high) phonon mode reflects the high quality of wurtzite GaN nanowires Figure 5.4.4: (a) TEM image with fringe pattern and (b) selected area electron diffraction (SAED) image with spot pattern of as-formed ultralong GaN nanowires. TEM images have been recorded with electron microscope operated at 200 kv Figure 5.4.5: (a) A plot between [F(R) hν] 2 versus photon energy (hν) for ultralong GaN nanowires for estimating bandgap and (b) photoluminescence spectra as obtained ultralong GaN nanowires Figure 6.2.1: Optimization of resist (PMMA A 8%) thickness on Si/SiO2 template using spin speed of spin coating method (a) 1000 rpm, (b) 2000 rpm and (c) 4000 rpm Figure 6.2.2: (a) FESEM images of fabricated metal address marks on Si/SiO2 template using electron beam lithography system; inset showing zoom view of metal dot pattern. (b) Optical image showing metal pads formed by photolithography Figure (a) Schematic for nanowire contact formation through lift off using electron beam lithography exposure. (b) Fabricated electrical contacts (Cr/Au) on single nanowire (GaN nanowire obtained from β-ga2o3 nanowires through nitridation) Figure 6.3.1: (a) FESEM image of fabricated photoconducting device of single converted GaN nanowire using Ti/Au as ohmic electrodes. (b) Its current-voltage characteristics in dark and in presence of UV light. (c) Calculated responsivity and (d) gain showing ultra high responsivity and normalized gain in this device Figure 6.3.2: (a) FESEM image of fabricated photoconducting device of single converted GaN nanowire using Cr/Au as ohmic electrodes. (b) Its current-voltage characteristics in dark and in presence of UV light. (c) Calculated responsivity and (d) gain showing ultra high responsivity and normalized gain in this device Figure 6.3.3: (a) Current-voltage characteristics of single ultralong GaN nanowire obtained from β-ga2o3 nanowires in dark and in presence of UV light. (b) Calculated responsivity and gain for the single nanowire photoconduction. Inset of (b) shows FESEM image of xvii

20 fabricated device under test. The Cr/Au contacts at the ends of nanowire are used for photoconduction study. The other contacts (Ni/Au) on device under test were meant for Schottky diode investigations and not used in the present work xviii

21 Symbols A Interfacial surface area Al d E Eg G L Li P q R Rm S s Sd ts td V V Vn v Y ΔGv ΔGs l β γ ε η List of symbols and abbreviations Light receiving area in photoconduction Interplanar spacing in crystal structure Electric field Bandgap energy Photoconductive Gain Carrier travel length between electrodes Light intensity Poisson s ratio Electronic charge Photoconductive Responsivity Minimum radius of a catalyst droplet Sensitivity for photoconduction Degree of supersaturation of the vapor Separation distances between metal source and substrate in CVD Adatom surface saturation time Adatom catalyst body diffusion time Applied voltage Molar volume of droplet Volume of nuclei Drift velocity Young s modulus Volume free energy Strain energy Boundary layer thickness in chemical vapor deposition system Beta (crystal phase of Ga2O3 having monoclinic crystal structure) Surface energy Lattice mismatch Quantum efficiency µ Mobility xix

22 σ τ τt Liquid-vapor surface energy Carrier life time Carrier transit time between electrodes Abbreviations CAD Computer aided design CL Cathodoluminescence CLS Core level spectra in XPS CVD Chemical vapor deposition DI Deionised water EBL Electron beam lithography EDX Energy dispersive X-ray spectroscopy FET Field effect transistors FWHM Full width at half maximum HVPE Hydride vapor phase epitaxy JCPDS Joint committee on powder diffraction standards LED Light emitting diode MOCVD Metal-organic chemical vapor deposition NBE Near band edge emission PL Photoluminescence PMMA Poly (methyl methacrylate) RRS Resonant Raman scattering RT Room temperature SAED Selected area electron diffraction SEM Scanning electron microscopy TEM Transmission electron microscopy UV Ultra-violet VLS Vapor-liquid-solid VSS Vapor-solid-solid XPS X ray photoelectron spectroscopy XRD X-ray diffraction xx