Crystal Growth of Silicon for Solar Cells (eBook)

Preface 6
Contents 8
Contributors 13
1 Feedstock 15
1.1 Introduction 15
1.1.1 Main Supply Route Today 15
1.1.2 Impurities 16
1.2 Metallurgical Si 16
1.3 The Siemens Process 18
1.4 Refining of Si for the PV Applications 19
1.4.1 Removal of Boron by Oxidation 19
1.4.1.1 Slag Refining in a Ladle 22
1.4.1.2 Slag Properties 25
1.4.2 Removal of Boron by Reaction with Water Vapor 25
1.4.3 Removal of Phosphorous by Vacuum Treatment 25
1.4.4 Refining by Solidification 27
1.4.4.1 Factors that Reduces the Refining Efficiencyand the Stability of Crystallization 30
1.4.5 Solvent Refining 31
1.4.6 Removal of Impurities by Leaching 32
1.4.7 Electrolysis/Electrochemical Purification 34
1.4.8 Removal of Inclusions by Settling 34
1.4.9 Removal of Inclusions by Filtration 35
References 36
2 Czochralski Silicon Crystal Growthfor Photovoltaic Applications 38
2.1 Introduction 38
2.2 Hot-Zone Design 40
2.2.1 Power and Growth Speed 41
2.2.2 Interface Shape and Thermal Stress 43
2.2.3 Argon Consumption and Graphite Degradation 44
2.2.4 Yield Enhancement 45
2.3 Continuous Charge 46
2.3.1 Multiple Charges 46
2.3.2 Coated Crucible 48
2.3.3 Large Size and Continuous Growth 48
2.4 Crystal Quality Improvement 49
2.5 Conclusions and Comments 50
Acknowledgments 51
References 51
3 Floating Zone Crystal Growth 53
3.1 The FZ Method: Its Strengths and Weaknesses 53
3.2 Silicon Feed Rods for the FZ Method 60
3.2.1 Siemens and Monosilane Deposition Processes 60
3.2.2 Growth of Feed Rods 60
3.2.3 Granular Feed Stock 60
3.3 Doping of FZ Silicon Crystals 61
3.4 Physical and Technical Needs and Limitations 61
3.5 Growth of Quadratic FZ Crystals (qFZ) 62
3.6 Comments on the Potential of FZ Silicon for Solar Cells 64
3.7 Summary 64
References 64
4 Crystallization of Silicon by a DirectionalSolidification Method 66
4.1 Directional Solidification Method: Strengths and Weaknesses 66
4.2 Control of Crystallization Process 67
4.3 Incorporation of Impurity in Crystals 70
4.4 Three-Dimensional Effects of Solidification 76
4.5 Summary 77
Acknowledgment 79
References 79
5 Mechanism of Dendrite Crystal Growth 81
5.1 Introduction 81
5.2 Twin-Related Dendrite Growth in Semiconductor Materials 83
5.3 Formation Mechanism of Parallel Twins DuringMelt Growth Processes 84
5.4 Growth Mechanism of Si Faceted Dendrite 87
References 91
6 Fundamental Understanding of Subgrain Boundaries 93
6.1 Introduction 93
6.2 Structural Analysis of Subgrain Boundaries 95
6.3 Electrical Properties of Subgrain Boundaries 98
6.4 Origin of Generation of Subgrain Boundaries:Model Crystal Growth 100
6.5 Summary 104
References 104
7 New Crystalline Si Ribbon Materialsfor Photovoltaics 106
7.1 Ribbon Growth 106
7.2 Description of Ribbon Growth Techniques 107
7.2.1 Type I 109
7.2.1.1 Edge-Defined Film-Fed Growth (Type I Technology) 109
7.2.1.2 String Ribbon (Type I Technology) 110
7.2.2 Type II 111
7.2.2.1 Ribbon Growth on Substrate (type II) 113
7.2.3 Comparison of Growth Techniques 113
7.3 Material Properties and Solar Cell Processing 114
7.3.1 Refractory Materials 114
7.3.2 Ribbon Material Properties 116
7.3.2.1 EFG and SR 117
7.3.2.2 RGS 118
7.3.3 Ribbon Silicon Solar Cells 118
7.3.3.1 Hydrogenation in Ribbon Silicon 120
7.3.3.2 Solar Cell Processing 121
7.4 Summary 123
References 124
8 Crystal Growth of Spherical Si 129
8.1 Historical Background 129
8.2 Crystal Growth from Undercooled Melt 131
8.3 Levitation Experiments: PolycrystallinityDue to Fragmentation of Dendrites 133
8.4 Spherical Si Crystal Fabricated by Drop Tube Method 136
8.5 Summary 141
Acknowledgements 141
References 141
9 Liquid Phase Epitaxy 143
9.1 Description 144
9.2 Kinetics of Growth 144
9.3 Choice of the Solvent 147
9.3.1 Influence of the Substrate Surface 148
9.4 Experimental Results 149
9.4.1 Growth with Sn and In Solventin the 900–1,050C Range 150
9.4.2 Doping and Electrical Propertiesof Epitaxial Layers 151
9.5 Growth on Multicrystalline Si Substrates 153
9.5.1 Photovoltaic Results Obtainedwith LPE Silicon Layers 154
9.6 Low-Temperature Silicon Liquid Phase Epitaxy 156
9.7 Liquid Phase Epitaxy on Foreign Substrates 157
9.8 Epitaxial Lateral Overgrowth 159
9.9 High-Throughput LPE 160
9.10 Conclusion 162
Acknowledgments 162
References 162
10 Vapor Phase Epitaxy 166
10.1 Introduction 166
10.2 Theoretical Aspects of VPE 168
10.2.1 Notions of Hydrodynamics 168
10.2.2 Kinetics and Growth Regimes 169
10.2.2.1 Theoretical Approach 170
10.2.2.2 Limitations Due to the Surface Kinetics or Mass Transfer 171
10.2.2.3 Model of the Boundary Layer 172
10.3 Experimental Aspects of VPE 172
10.3.1 Experimental Approach of the Kinetics and Mechanismsof Silicon Growth in a SiH2Cl2/H2 System 172
10.3.2 Doping of Epitaxial Films 175
10.3.2.1 Dopant Incorporation 175
10.3.2.2 Profile of the Concentration of Dopants 176
10.4 Epitaxial Growth Equipments 178
References 182
11 Thin-Film Poly-Si Formed by Flash Lamp Annealing 183
11.1 Introduction 183
11.2 FLA Equipment 183
11.3 Thermal Diffusion Length 184
11.4 Thermal Model of FLA 186
11.5 Control of Lamp Irradiance 187
11.6 FLA for Solar Cell Fabrication 189
11.7 Microstructure of the Poly-Si Films 190
11.8 Summary 195
Acknowledgements 195
References 195
12 Polycrystalline Silicon Thin-Films Formedby the Aluminum-Induced Layer Exchange(ALILE) Process 198
12.1 Introduction 198
12.2 General Aspects of the ALILE Process 199
12.3 Kinetics of the ALILE Process 205
12.4 Structural and Electrical Properties of the Poly-Si Films 207
12.5 Influence of the Permeable Membrane 211
12.6 Model of the ALILE Process 212
12.7 Other Aspects of the ALILE Process 217
12.8 Summary and Conclusions 219
Acknowledgements 220
References 221
13 Thermochemical and Kinetic Databasesfor the Solar Cell Silicon Materials 224
13.1 Introduction 224
13.2 The Assessed Thermochemical Database 225
13.2.1 Thermodynamic Description 226
13.2.1.1 Element and Stoichiometric Compound 226
13.2.1.2 Solutions 227
13.2.1.3 Solubility 228
13.2.1.4 Equilibrium Distribution Coefficient 228
13.2.1.5 Retrograde Solubility 229
13.2.2 Typical Examples 229
13.3 The Kinetic Database 236
13.3.1 Impurity Diffusivity 236
13.3.2 Typical Examples 237
13.4 Application of the Thermochemical and Kinetic Databases 242
13.4.1 Effect of Solubility, Distribution Coefficient, and Stable Precipitates in Solar Cell Grade Silicon 242
13.4.2 Surface/Interfacial Tensions 246
13.4.3 Grain Boundary Segregation of Impurityin Polycrystalline Silicon 248
13.4.4 Determination of the Denuded Zone Width 250
13.5 Conclusions 250
Acknowledgments 252
References 252
Index 257