Phase Change Materials (eBook)

Foreword 5
Preface 8
Contents 10
Abbreviations 17
1. History of Phase Change Memories 21
1.1 The Discovery of Phase Change Materials 21
1.2 Early Electronic Computers and Memory Systems 22
1.3 Pioneers in Phase Change Memory 24
1.4 Early Attempts with Phase Change Memory 29
1.5 Rebirth of Phase Change Memory 30
References 34
Part I:Material Science: Theoryand Experiment 35
2. Density Functional Theory Calculations for Phase Change Materials 36
2.1 Introduction 36
2.2 The Theorem of Hohenberg and Kohn 37
2.3 The Kohn-Sham Equation 39
2.4 The Local Density Approximation 41
2.5 Beyond Density Functional Theory 42
2.6 Application of DFT in the Field of Phase Change Materials 43
2.6.1 Structure Determination 44
2.6.2 Electronic Properties 48
References 55
3. Nature of Glasses 58
3.1 Introduction 58
3.2 Thermodynamics of the Glass Transition 60
3.3 Glass Transition from Dynamics 62
3.4 Glass Forming Tendency 63
3.4.1 Compositional Trends of the Glass TransitionTemperature 65
3.5 Calorimetric Measurement of the Glass Transition Temperature and Related Thermal Properties 67
3.6 Three Generic Classifications of Glasses and Glass Transitions 70
3.7 Elastic Phases in Ionic and Super-ionic Glasses 73
3.8 Ideal Glasses and Self-organization of Networks 73
3.9 Does the View Below the Glass Transition Temperature Correlate with the View above the Glass Transition Temperature? 75
3. 10 Glass Formation in Hydrogen Bonded Networks 76
3.11 Epilogue 78
References 78
4. Structure of Amorphous Ge-Sb-Te Solids 82
4.1 Introduction 82
4.2 Structural Order in Amorphous Materials 83
4.2.1 Short-range Order 83
4.2.2 Medium-range Order 84
4.2.3 Long-range Structure 85
4.3 Experimental Structural Probes 86
4.4 Structural Modeling 87
4.5 The Structure of Amorphous Phase-change Materials 88
4.5.1 Experimental Studies 88
4.5.2 Simulational Studies 91
4.6 Summary 97
References 98
5. Experimental Methods for Material Selection in Phase-change Recording 100
5.1 Introduction 100
5.2 Reversible Switching 101
5.3 Phase-change Materials 103
5.3.1 Crystallization by Nucleation and Growth 105
5.3.2 Crystallization Dominated by Crystal Growth 107
5.4 Archival Life Stability 108
5.5 Crystallization Rate 110
5.6 Material Optimization 112
5.7 Outlook 116
References 117
6. Scaling Properties of Phase Change Materials 118
6.1 Introduction 118
6.2 Thin Films of Phase Change Materials 119
6.2.1 Crystallization Temperature as a Function of Film Thickness 120
6.2.2 Crystallization Rate as a Function of Film Thickness 124
6.2.3 Change in Optical Constants and Electrical and Thermal Parameters as a Function of Film Thickness 127
6.2.4 Limits of Storage Density in Thin Films 128
6.3 Phase Change Nanowires 130
6.4 Phase Change Nanoparticles 133
6.5 Scaling in Time – Switching Speed of Phase Change Materials 137
References 139
7. Crystallization Kinetics 144
7.1 Theory 144
7.1.1 Homogeneous Crystal Nucleation 144
7.1.1.1 Thermodynamics of Cluster Formation (Gibbs, 1878) 145
7.1.1.2 Model Based on Equilibrium Distribution of Clusters (Volmer and Weber, 1926) 146
7.1.1.3 Steady State Model (Becker and Döring, 1935) 147
7.1.1.4 The Kinetic Pre-factor of the Nucleation Rate (Turnbull and Fisher, 1949) 148
7.1.2 Heterogeneous Crystal Nucleation 152
7.1.3 Crystal Growth 154
7.1.3.1 Interface-controlled Growth 155
7.1.3.2 Growth Controlled by Long-range Diffusion 156
7.2 Measurements 157
7.2.1 Crystallization Parameters Around the Glass Transition Temperature 157
7.2.2 Crystallization Parameters Close to the Melting Temperature 161
References 164
8. Short and Long-Range Order in Phase Change Materials 168
8.1 Historical Background 168
8.1.1 Glass Formation Process 169
8.2 Long-Range Order 170
8.2.1 GeTe 171
8.2.2 Ge-Sb-Te Alloys 173
8.2.2.1 Metastable Ge-Sb-Te Alloys 173
8.2.2.2 High-Pressure Effects on Metastable Ge-Sb-Te Alloys 175
8.2.2.3 Ge-Sb-Te Equilibrium Structures 176
8.2.2.4 Sb-Te Alloys 177
8.3 Short-Range Order 179
8.3.1 X-ray Absorption 179
8.3.1.1 Short-range Order in Crystalline GeTe 183
8.3.1.2 Short-range Order in Amorphous GeTe 184
8.3.1.3 Short-range Order in Crystalline Ge2Sb2Te5 186
8.3.2 Short Range Order in Sb-Te Alloys 189
8.3.2.1 Conclusions 190
References 190
9. Optical and Electrical Properties of Phase Change Materials 194
9.1 Introduction 194
9.2 Optical Constants and Optical Bandgap 195
9.2.1 Determination of the Optical Constants and Absorption Coefficient 195
9.2.1.1 Transmission and Reflection Measurements 196
9.2.1.2 Ellipsometry 196
9.2.1.3 Optical Contrast between Amorphous and Crystalline Phases 197
9.2.2 Optical Bandgap 198
9.2.3 Infrared Absorption: Band Tails and Free Carrier Absorption 200
9.2.3.1 Urbach Edge 200
9.2.3.2 Free Carrier Absorption 201
9.2.4 Effects of Composition and Preparation Conditions 201
9.3 Photo-induced Effects 203
9.3.1 Photo-induced Current and Optical Nonlinearity 203
9.3.2 Photo-Oxidation 204
9.4 Conductivity and Phase Transformation 205
9.4.1 Temperature-dependence of Resistivity 205
9.4.2 Intermediate States: Percolation and Multilevel Recording 206
9.4.3 Effects of Composition and Processing Conditions 207
9.5 Electronic Transport Properties and Band Structure 208
9.5.1 Characterization of Transport Properties 208
9.5.1.1 Hall Measurements 208
9.5.1.2 Thermoelectric Effect 209
9.5.2 Hexagonal Ge2Sb2Te5 210
9.5.3 Face-centered-cubic Ge2Sb2Te5 212
9.5.4 Amorphous Ge2Sb2Te5 213
9.6 Perspective for the Future 213
References 214
10. Development of Materials for Third Generation Optical Storage Media 218
10.1 Introduction 218
10.2 Requirements for a Phase-change Material 219
10.3 Why Chalcogenide Semiconductors for Optical Memory? 221
10.4 Merits and Demerits of the Te Based Eutectic Alloys 222
10.5 Merits and Demerits of the Te-based Single Phase Materials 225
10.6 From Eutectic to Single Phase Compositions 227
10.7 Discovery of the GeTe-Sb2Te3 Pseudo-binary System 228
10.8 Importance of the Cubic Structure and Vacancies 232
10.9 Secrets of the Present Phase-change Materials I 234
10.10 Materials for Blue Laser and Multi-layer Applications 238
10.11 Secrets of Present Phase-change Materials II 241
10.12 Conclusions 242
References 243
11. Novel Deposition Methods 246
11.1 Chemical Vapor Phase Deposition 246
11.2 Electrodeposition 252
11.3 Solution-phase Deposition 257
11.4 Nanomaterials 260
11.5 Conclusions 262
References 263
Part II: Applications: Optical, Solid State Memory and Reconfigurable Logic 268
12. Optical Memory: From 1st to 3rd Generation and its Future 269
12.1 Introduction 269
12.2 Three Generations of Optical Media 270
12.2.1 The First Generation: Compact Discs (CDs) 271
12.2.2 The Second Generation: Digital Versatile Disks (DVDs) 271
12.2.3 The Third Generation: Blu-ray Discs (BDs) 274
12.2.3.1 Blu-ray Discs 274
12.3 The Basic Principle of Optical Recording 275
12.4 Phase-change Optical Recording and Related Technologies 278
12.4.1 Phase-Change Optical Storage 278
12.4.1.1 Principle of Phase-Change Optical Storage 278
12.4.1.2 Phase-Change Materials 281
12.4.1.3 Development of Phase-Change Optical Storage Media 282
12.4.1.4 Disc Structure of Phase-Change Optical Disc 285
12.4.1.5 Models of Phase-Change Induced by Moving Laser Beam 287
12.4.2 Techniques for Phase-Change Optical Storage 288
12.4.2.1 Short Wavelength Laser Diodes 289
12.4.2.2 Large Numerical Aperture (NA) 289
12.4.2.3 Land/Groove Recording 289
12.4.2.4 Write Strategy 290
12.4.2.5 Cross Talk 291
12.4.2.6 Super Resolution 292
12.4.2.7 Multilevel Phase-Change Recording 293
12.4.2.8 Dual Layer Phase-change Optical Recording 293
12.4.2.9 Superlattice-like Phase-change Optical Disc 294
12.4.2.10 Initialization Free Phase-change Optical Disc 295
12.4.2.11 Near-field Phase-Change Optical Storage 297
12.5 The Future of Optical Storage 297
References 300
13. 4th Generation Optical Memories Based on Super-resolution Near-field structure (Super-RENS) and Near-field Optics 303
13.1 Introduction 303
13.2 Diffraction Limit and Near-Field Optics 304
13.3 Small Aperture and Non-propagating Photons 306
13.4 Super-resolution Near-field Structure (Super-RENS) Principle to Retrieve Non-propagating Light 308
13.5 Origin of the Strong Scattered Signals for 4th Generation Super-RENS Disks 310
13.6 Beyond Super-RENS 314
References 315
14. Phase Change Memory Device Modeling 317
14.1 Introduction 317
14.2 Device Operation 318
14.3 Modeling of Electrical Conduction in the Amorphous Phase 320
14.4 Threshold Switching in the Amorphous Chalcogenide 324
14.5 Modeling the Electrical Conduction in the Crystalline Chalcogenide 326
14.6 Electro-thermal Modeling of the Programming Characteristics 327
14.7 Modeling the Amorphous to Crystalline Phase Transformation 332
14.8 Modeling the Structural Relaxation in the Amorphous Phase 338
14.9 Summary and Outlook 343
References 345
15. Phase Change Random Access Memory Advanced Prototype Devices and Scaling 348
15.1 Introduction 348
15.2 Device Scaling by Reducing the Electrode Contact Area 349
15.2.1 The Heater Structure 350
15.2.1.1 Additional Adhesion Layer 351
15.2.1.2 Size Effect of the Phase Change Material 352
15.2.1.3 Different Phase Change Materials 353
15.2.1.4 Process Integration Issues for Scaling 353
15.2.2 The Edge Contact Structure 354
15.2.3 ?Trench Structure 355
15.2.4 The Ring Structure 355
15.3 Device Scaling by Reducing the Phase Change Material Volume 356
15.3.1 The Pillar Structure 357
15.3.2 The Line Structure 358
15.3.3 The Bridge Structure 359
15.4 Other Prototype Devices 360
15.4.1 Scaling Both the Material and the Contact 361
15.4.2 Multi-level Cell 362
15.4.3 Confined Structure 362
15.5 Advanced Device Testing 364
15.6 Summary 366
References 367
16. Phase Change Memory Cell Concepts and Designs 372
16.1 Introduction 372
16.2 Technology Overview 373
16.3 Phase Change Memory Cell Electrical Characterization 378
16.4 Phase Change Memory Cell Reliability 385
16.4.1 Data Retention Characterization 386
16.4.2 Retention Behavior with Device Scaling 393
16.4.3 Cycling Endurance 394
16. 5 Summary and Outlook 395
References 396
17. Phase Change Random Access Memory Integration 398
17.1 Introduction 398
17.2 Phase Change Random Access Memory Design Basics 399
17.3 Review of Desired Phase Change Memory CellCharacteristics 403
17.4 The Access Device 407
17.5 Device Design Considerations 410
17.5.1 The Mushroom Cell without or with Bottom RingElectrode 410
17.5.2 The Pillar Cell 414
17.5.3 The ?Trench Cell 416
17.5.4 The Pore Cell 416
17.6 Multi-Level Phase Change Random Access Memory 420
17.7 Concluding Remarks 423
References 423
18. Reconfigurable Logic 426
18.1 Introduction 426
18.2 Digital System Basics 427
18.3 Simple Configurable Digital Systems 431
18.4 Considerations in Computation Architectures 436
18. 5 Multi-valued Systems 437
18.6 Threshold Logic 439
18.7 Artificial Neural Networks 442
18.8 Other Analog-domain Programmable Systems 443
18.9 Conclusions 446
References 446
Author Bios 448
Index 454