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Power Efficient and Reliable Nanoscale Phase Change Random Access Memory

Jin, Bo

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To enhance and optimize the performance of phase change random access memory, numerous studies on nanowire (NW)-based PCRAM, including device fabrication and exploration of materials such as GeTe and GeSbTe alloys, have appeared in the last decade. Also, the NW-based PCRAM technology has attracted g...
To enhance and optimize the performance of phase change random access memory, numerous studies on nanowire (NW)-based PCRAM, including device fabrication and exploration of materials such as GeTe and GeSbTe alloys, have appeared in the last decade. Also, the NW-based PCRAM technology has attracted great attention, since the programmable active volume as well as programming power can be reduced due to the reduction in melting temperature in nanoscale materials. More recently, the use of oxide-based materials have been reported for PCRAM applications. Memory devices using Ga-doped In2O3 (Ga:In2O3) thin film have successfully shown repeatable switching behavior between crystalline and amorphous phases with a reduction in reset current by controlling the Ga concentration. Furthermore, an issue that resistance drift phenomenon has been raised through the course of PCRAM development. The drift phenomenon primarily determines the reliability and multilevel capability of PCRAM, and several theoretical models have been proposed to explain the phenomenon. In this study, In2Se3 NW and Ga:In2O3 NW were synthesized via vapor-liquid-solid method, to investigate: (1) size-dependent switching behaviors and thermal resistance vs. power consumption in In2Se3 NW PCRAM; (2) Ga concentration dependent switching behaviors, resistance drift phenomenon and thermal resistance vs. power consumption in Ga:In2O3 NW PCRAM. Moreover, we proposed a new form of mechanical stress relaxation model, to study the stress relationship between a phase change material and an encapsulating layer material (ELM), and investigate the effect of ELM on stress relaxation in order to mitigate the resistance drift phenomenon.
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Contents

ABSTRACT i
...
Contents

ABSTRACT i
Contents iii
List of Figures vi
List of Tables xi


I. Introduction 1

1.1 Motivation 1

1.2 Phase Change Random Access Memory 4
1.2.1 Mechanism 4
1.2.2 Thin Film Structure PCRAM 7
1.2.3 Nanowire Structure PCRAM 9
1.2.4 Scalability and Reliability in Nanowire Structure 11

1.3 Thesis Overview 15

References 17


II. Thermally Efficient and Highly Scalable In2Se3 Nanowire Phase Change Random Access Memory 23

2.1 Introduction 23

2.2 Experimental Work 25
2.2.1 In2Se3 NW Synthesis and Device Fabrication 25
2.2.2 In2Se3 NW Characterization and Device Measurement 27

2.3 Characteristics of In2Se3 NW PCRAM Device 28
2.3.1 Phase Transition Characteristics 28
2.3.2 Thermal Resistance and Power Consumption 33
2.3.3 Resistance Drift 39

2.4 Conclusion 41

2.5 Q&A 42

References 49


III. Ga-doped Indium Oxide Nanowire Phase Change Random Access Memory Cell 56

3.1 Introduction 56

3.2 Experimental Work 59
3.2.1 Ga:In2O3 NW Synthesis and Characterization 59
3.2.2 Device Fabrication and Measurement 60

3.3 Characteristics of Ga:In2O3 Nanowire PCRAM Device 61
3.3.1 Ga:In2O3 NW Characterization 61
3.3.2 Phase Transition Characteristics 64
3.3.3 Resistance Drift and Stability 70
3.3.4 Thermal Resistance and Power Consumption 74

3.4 Conclusion 78

3.5 Q&A 79

References 82


IV. Role of an Encapsulating Layer for Reducing Resistance Drift in Phase Change Random Access Memory 88

4.1 Introduction 88

4.2 Model and Simulation 92
4.2.1 Fully Reset Case 92
4.2.2 Partially Reset Case 101

4.3 Conclusion 105

4.4 Q&A 106

References 110


V. Conclusion 115


Acknowledgement 120