Wide Bandgap Nanowires

Tuan Anh Pham is a research assistant as well as a process engineer at the Queensland Micro- and Nanotechnology Centre, Griffith University, Australia. Dr. Pham’s longstanding research interests have been focused on various fields of condensed matter physics and materials science, including 1D/2D materials, topological insulators, wide bandgap semiconductors, surface science, and wearable technology. Toan Dinh is a Senior Lecturer in the School of Engineering, University of Southern Queensland, Australia. Dr. Dinh’s research interests include micro/nano-electromechanical systems (MEMS/NEMS), physics of semiconductors, sensors for harsh environments, soft robotics, and advanced materials for healthcare and wearable applications. Nam-Trung Nguyen is a Professor and the Director of Queensland Micro- and Nanotechnology Centre at Griffith University in Brisbane, Australia, and leads a research group of over 20 postgraduate and postdoctoral researchers. Research themes of the group include micro- and nanofluidics, micro- and nanofabrication; as well as micro- electromechanical systems (MEMS). Hoang-Phuong Phan is currently an ARC DECRA Fellow at Queensland Micro- and Nanotechnology Centre, Griffith University. He will join the University of New South Wales (UNSW), Sydney, Australia from March 2022 as a Senior Lecturer. The Phan Lab’s research interest covers a broad range of semiconductor devices and applications, including MEMS/NEMS, integrated sensors, flexible electronics, and bio-sensing applications.

Chapter 1 8

Bottom-up growth methods 8

Abstract 8

1.1. Introduction 9

1.2. Bottom-up growth mechanisms 10

1.2.1. Vapor-liquid-solid growth mechanism 10

1.2.2. Vapor-solid-solid growth mechanism 16

1.2.3. Vapor-solid growth mechanism 22

1.2.4. Solution-liquid-solid growth mechanism 26

1.3. Bottom-up growth techniques 29

1.3.1. Chemical Vapor Deposition 29

1.3.2. Metal-organic chemical vapor deposition 33

1.3.3. Plasma-enhanced chemical vapor deposition 36

1.3.4. Hydride vapor phase epitaxy 38

1.3.5. Molecular Beam Epitaxy 41

1.3.6. Laser ablation 44

1.3.7. Thermal evaporation 46

1.3.8. Carbothermal reduction 48

References 51

Chapter 2 65

Top-down fabrication processes 65

Abstract 65

2.1. Introduction 66

2.2. Top-down fabrication techniques 68

2.2.1. Focused ion beam 68

2.2.2. Electron beam lithography 69

2.2.3. Reactive ion etching 72

2.2.4. Combined lithography techniques 74

References 76

Chapter 3 81

Hybrid fabrication techniques and nanowire heterostructures 81

Abstract 81

3.1. Introduction 82

3.2. Bottom-up meets top-down approaches 84

3.3. Integration of nanowires onto unconventional substrates 86

3.3.1. Transferring nanowires onto flexible substrates 86

3.3.2. Growing nanowires on graphene and layered material substrates 92

3.4. Synthesis of nanowire heterostructures 95

3.4.1. Synthesis of one-dimensional heterostructures 95

3.4.2. Synthesis of mixed dimensional heterostructures 98

References 101

Chapter 4 108

Electrical properties of wide bandgap nanowires 108

Abstract 108

4.1. Electrical properties 109

4.2. Measurement of electrical conductivity 109

4.3. Fundamental electrical properties of nanowires 112

4.3.1 Effect of doping on electrical properties 113

4.3.2 Mobility 115

4.3.3 Activation/ionization energy 116

4.3.4 Dependence of activation/ionization energy on NW dimensions 118

4.4 Electrical properties of wide bandgap nanowire based devices 118

4.4.1 Single NW electrical sensing devices 118

4.4.2 Field-effect transistors (FETs) 120

References 129

Chapter 5 132

Mechanical properties of wide bandgap nanowires 132

Abstract 132

5.1. Characterization techniques 133

5.1.1 Bending and buckling methods 133

5.1.2 Nano indenting method 138

5.1.3 Resonance testing method 139

5.2. Impact of defects and microstructures on mechanical properties of NWs 140

5.2.1. Defects 140

5.2.2 Effect of structures, dimensions and temperatures 143

5.3. Anelasticity and plasticity properties 148

5.3.1 Anelasticity 148

5.3.2 Plasticity 148

5.3.3 Brittle to ductile transition 150

References 152

Chapter 6 155

Optical properties of wide bandgap nanowires 155

Abstract 155

6.1 Optical properties of WBG NWs 156

6.1.1 Photoluminescence characterization of NWs 156

6.1.2 Size-dependent optical properties 157

6.1.3 Shape/morphology-dependent optical properties 158

6.1.4 Effect of crystal orientation 159

6.1.5 Tuning optical properties of NWs 160

6.2 Wide bangap nanowire light-emitting diodes (LEDs) 164

6.2.1 GaN nanowire based LEDs 164

6.2.2 GaN nanowire UV LEDs 169

6.2.3 ZnO nanowire based LEDs 172

References 175

Chapter 7 180

Thermal properties of wide bandgap nanowires 180

Abstract 180

7.1. Thermal conductivity 181

7.1.1 Fundamental of thermal transport and thermal conductivity 181

7.1.2 Measurement of thermal conductivity 182

7.1.3 Effect of diameters on thermal properties 183

7.1.4 Effect of orientation on thermal properties 186

7.1.5 Tenability of thermal properties 187

7.2 Thermoelectric properties 190

7.2.1 Fundamental thermoelectric properties 190

7.2.2 Thermoelectric properties of ZnO and GaN NWs 191

7.2.3 Thermoelectric properties of SiC NWs 193

7.2.4 Optimisation of the thermoelectric properties 194

References 196

Chapter 8 200

Ultraviolet sensors 200

Abstract 200

8.1. Introduction 201

8.2. Sensing mechanism 201

8.2.1. Photoconductor architectures 202

8.2.2. Schottky diode photo sensors 204

8.2.3. Semiconductor p-n junction 206

8.2.4. Field effect transistor-based UV sensors 208

8.3. Device development technologies 210

8.3.1. The choice of wide band gap materials for UV sensing 210

8.3.2 Top down fabrication of wide band gap nanowire UV sensors 216

8.3.4. Transfer process for nanowires 219

8.4. Applications of nanowire UV sensors 222

8.4.1 Flame sensors 222

8.4.2. Environmental monitoring 224

8.4.4 Biological sensors and health care applications 225

References 227

Chapter 9 233

Mechanical Sensors 233

Abstract 233

9.1. Introduction 234

9.2. Sensing mechanisms and corresponding materials 234

9.2.1. The piezoresistive effect 234

9.2.2. Piezotronics effect in nanowires 239

9.2.3 Capacitive sensing 243

9.3. Transducer configurations and fabrication technologies 244

9.3.1. Strain sensors 244

9.3.2. Pressure sensors 248

9.3.3 Tactile sensors 253

9.3.4. Acceleration and vibration sensors 256

9.3.5. Energy harvesting devices 257

9.4. Applications of mechanical sensors using wide band gap materials 261

9.4.1. Structural heath monitoring 261

9.4.2. Advanced health care 262

9.4.3 Robotics 265

References 267

Chapter 10 273

Gas sensors 273

Abstract 273

10.1. Introduction 274

10.2. Principle of gas sensing 274

10.2.1. Transconductance sensing mechanism 274

10.2.2. Field effect transistor-based gas sensors 276

10.2.3. Metal-semiconductor Schottky contact based gas sensors 277

10.2.4. Integration of nanowires with micro heaters 278

10.3. Standard physical parameters for gas sensors 280

10.3.1. Sensitivity 280

10.3.2. Selectivity 281

10.3.3. Response time 282

10.4. Materials for different types of gases 284

10.4.1 Oxygen sensors 284

10.4.2 Carbon dioxide 285

10.4.3 Organic gases 287

10.4.4 Hydrogen gas 290

References 301

Chapter 11 308

Wide band gap nanoresonators 308

Abstract 308

11.1. Introduction 309

11.2. Principle of nanoresonators 310

11.3. Actuation and measurement techniques 316

11.3.1 Electrostatic actuation 316

11.3.2 Piezoelectric actuation 318

11.3.3 Magnetomotive actuation 320

11.3.4. Thermal actuator 323

11.4. Engineering the performance of nanoresonators using wide band gap materials 325

11.4.1. Residual stress 325

11.4.2 Mechanical clamping enhancement 329

11.4.3 Tunning resonant frequency using electrically driven forces 331

11.5. Applications of nanoresonators 334

11.5.1 Logic Circuit at high temperatures 334

11.5.2 Mass sensing applications 337

11.5.3 Biosensors 338

11.5.4 Mechanical sensing 339

11.5.5 Optical devices 341

References 343