Deposition and characterization of threshold-switching Ge-Sb-Bi-Te and Si-As-Te thin films for the application of next-generation non-volatile memory

다국어 초록 (Multilingual Abstract)

Threshold-switching of chalcogenide – abrupt decrease of resistance when the applied electric field exceeds a critical value – has attracted wide attention since its discovery and enabled unique application for non-volatile memory. For the threshold switching, group VI elements such as Se or Te primarily constitute the chalcogenide alloy, but Se or Te alone cannot be used for non-volatile memory because these elements easily crystallizes at room temperature. To enhance the stability of the amorphous phase, group IV and V elements are added to cross-link the atomic network within amorphous phase. In this thesis, group IV and V elements are varied for different application of chalcogenides; Ge, Sb, and Bi for phase change memory and Si and As for threshold-switch application.

Bi is added to Ge2Sb2Te5 – the most intensely studied material for phase change memory – by cosputtering Bi2Te3 and Ge2Sb2Te5 compound targets to incorporate Bi atoms in Ge/Sb sites of Ge2Sb2Te5. This study revealed that incorporated Bi increased the crystallization speed of both amorphous and liquid phase. This was attributed to the decrease of the activation energy of crystallization and reduction of the thermal conductivity by doping Bi to Ge2Sb2Te5. It is suggested that Bi-doped Ge2Sb2Te5 shows a guideline for the development of future phase change memory; thermal engineering of phase change material by substitutional doping.

Amorphous Si-As-Te thin films were sputter-deposited to make threshold switch for selector devices of crossbar array of resistive switching memory. Bi-directional flow of current is possible in threshold switch, which is required for selector device of bipolar resistive switching memory. Instead of Ge and Sb used for phase-changing chalcogenides, Si and As are used for threshold-switch application because bonding enthalpy of Si-Te and As-Te are larger than that of Ge-Te and Sb-Te (bonding enthalpy: Si-Te 38.5 kcal/mol, As-Te 32.7 kcal/mol, Ge-Te 35.5 kcal/mol, and Sb-Te 31.6 kcal/mol). Higher thermal stability of Si-As-Te enabled the construction of stable threshold switch; endurance test using 200 ns-long pulse showed little degradation of the device performance after 1,000 times of ON-OFF switching. Suppression of the leakage current by serial connection of TiO2 unipolar memory and Si-As-Te threshold-switching thin film was observed, which indicates that Si-As-Te threshold switch is a promising candidate for selector devices of crossbar array of resistive switching memory.

목차 (Table of Contents)

• Abstract i
• Table of Contents iii
• List of Tables vi
• List of Figures vii
• I. Introduction
• 1.1. Overview 1
• 1.2. Issues on PRAM and RRAM 3
• 1.3. Objective and Chapters Overview 5
• 1.4. Bibliography 6
• II. Literature Review
• 2.1. Chalcogenide: general introduction 8
• 2.2. PRAM: general introduction 10
• 2.3. Device structure of PRAM 13
• 2.4. Crystallization of amorphous GST 16
• 2.5. Threshold switching of chalcogenide 27
• 2.6. Effect of doping into GST 33
• 2.6.1. (Ge1Sb2Te4)0.9(Sn1Bi2Te4)0.1 34
• 2.6.2. Ge2Sb2Te5 + Bi 35
• 2.6.3. Sb8Te2 + Bi 37
• 2.6.4. GeTe + Bi2Te3 37
• 2.6.5. GST + nitrogen 39
• 2.6.6. GST + oxygen 42
• 2.6.7. GST + SiOx or TiOx 43
• 2.6.8. GeTe + carbon 46
• 2.7. Selector devices for cross-bar structure of resistive switching memory 47
• 2.8. Thermal conductivity measurement 50
• 2.8.1. Definition of thermal conductivity 50
• 2.8.2. Thermal conduction mechanism in solids 51
• 2.8.3. Measurement of thermal conductivity of thin films: 3-w method and transient thermoreflectance 52
• 2.8.4. Thermal conductivity of GST 54
• 2.9. Bibliography 57
• III. Experimental Procedures 67
• 3.1. Apparatus of DC/RF Magnetron Sputter system for telluride deposition 67
• 3.2. Analysis Methods 69
• 3.3. The way to determine crystallization temperature and activation energy of crystallization 70
• 3.4. Apparatus of electrical test system 77
• 3.5. 3ω method for the measurement of thin-film thermal conductivity 81
• 3.6. Laser test for the measurement of crystallization of phase change materials 88
• 3.7. Bibliography 91
• IV. Results and Discussions 92
• 4.1. Bi2Te3-doped GST for the application of phase change random access memory 92
• 4.1.1. Composition and microstructure 92
• 4.1.2. Switching by laser pulses 100
• 4.1.3. Activation energy of crystallization 108
• 4.1.4. Thermal conductivity measurement by 3ω method 120
• 4.2. Si-As-Te thin films for the application of selector devices of crossbar array of resistance switching random access memory 126
• 4.2.1. Crystallization by ex-situ heating 126
• 4.2.2. Composition control to achieve threshold switching 135
• 4.2.3. Composition dependence of switching parameters 139
• 4.2.4. Serial connection of threshold switch and resistive switching memory 147
• 4.2.5. Endurance 151
• 4.2.6. Bibliography 153
• V. Conclusions 156
• List of publications 159
• Abstract (in Korean) 163