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A study on the optimization of metal-insulator transition in NbO2 thin films for selector device

강민국 (Kang Minkook, 포항공과대학교)

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초록 moremore
Correlated oxides, which undergo insulator-to-metal (IMT) transitions under external stimuli such as temperature, strain and electric field, have attracted due to their potential application in next-generation electronic devices. Especially correlated transition metal oxides, such as VO2 and NbO2, h...
Correlated oxides, which undergo insulator-to-metal (IMT) transitions under external stimuli such as temperature, strain and electric field, have attracted due to their potential application in next-generation electronic devices. Especially correlated transition metal oxides, such as VO2 and NbO2, have been regarded as one of the promising materials for selector devices in cross-point array three dimensional (3D) resistance random access memory (ReRAM) due to their high ON/OFF ratio, fast switching speed, and high uniformity. ReRAM is the most promising current-based memory to overcome the scaling issue of present charge-based memory devices such as DRAM. ReRAM with three dimensional (3D) cross-point array structure has been proposed to achieve high-density memory device, but this architecture incurs sneak-path current that flows through unselected neighboring cells and causes the read-out errors. Therefore, selector devices with high on-off ratio (selectivity) must be connected in each cells of ReRAM to eliminate read-out errors in the memory cells by blocking sneak path current. Bulk NbO2 undergoes a transition from insulating tetragonal phase to metallic rutile phase at high temperature (~1081 K). Due to sensitive metal-insulator phase transition under various external stimuli, this substance is a promising material for selector devices due to its abrupt non-linear resistive switching, i.e., insulator-to-metal transition (IMT). Selector devices that use NbO2 thin films switch quickly (~22 ns), have a consistent threshold switching, and are thermally stable at the operating temperature of the memory devices (~70 °C). However, the multivalent Nb cation can change oxidation states, so the required growth window for NbO2 is too narrow to synthesize high-quality NbO2 films with the right stoichiometry, despite the attempts using various growth processes, e.g., oxidation of Nb metals or reduction of Nb2O5 ceramics. The first part will discuss voltage-controlled insulator-to-metal transition (IMT) characteristics of the NbO2 thin films that are deposited under forming gas in the growth chamber. It is shown that the hydrogen in the forming gas gives rise to the abrupt voltage-induced IMT characteristics in NbO2 thin films that are sandwiched between top and bottom Pt electrodes. By a catalytic reaction at the triple boundary between NbO2 and Pt, hydrogen appears to be easily incorporated into the NbO2 lattice and doping significantly lowers the IMT temperature of NbO2 thin films, along with the reduction of NbO2 films. Despite the good ION in metallic NbO2, the IOFF of NbO2-based selector devices must be reduced to improve the readout margin of a cross-point array ReRAM. The second part focus on the off-state current reduction of NbO2-based selector device through ultra-thin TiO2 tunnel barrier. Significant off-state current reduction by an order of magnitude in the NbO2-based selector devices is observed by inserting an ultrathin TiO2 (~2 nm) tunneling barrier. Moreover, the ultrathin TiO2 layer improves the reliability and uniformity of voltage-induced insulator-to-metal transition (IMT) in the NbO2 selector devices by thermodynamically suppressing the formation of a surface Nb2O5 layer. In last parts, selector properties of NbO2-based selector device are discussed to realize the 1S – 1R structured ReRAM device. NbO2-based selector devices successfully block the current flow through the memory cell at half read voltage of ReRAM devices. Furthermore, annealing effect of partially crystallized NbO2 thin films on its forming process is discussed.
목차 moremore
Table of Contents
Chapter 1. Introduction
1.1. Background
...
Table of Contents
Chapter 1. Introduction
1.1. Background
1.2. Resistive Random Access Memory
1.3. Sneak path current and Selector device
1.4. References

Chapter 2. Motivation of Research
2.1. Metal-Insulator Transition
2.2. Mechanisms of Metal-Insulator Transition
2.2.1. Mott-Hubbard Model
2.2.2. Peierls Model
2.3. MIT Oxide based selector device
2.4. NbO2 for selector device
2.5. References

Chapter 3. Metal-Insulator Transition characteristics of NbO2-based selector device
3.1. Introduction
3.2. Experimental Methods
3.3. Results
3.3.1. NbO2 thin film deposited under reduce condition
3.3.2. Nano-scale NbO2-based selector devices
3.3.3. Structural analysis of NbO2 thin film
3.4. Discussions
3.5. Conclusions
3.6. References

Chapter 4. Off-state current optimization of electrical properties of NbO2-based selector device by tunnel barrier
4.1. Introduction
4.2. Experimental Methods
4.3. Results
4.3.1. Electrical properties of NbO2-based selector device with TiO2 tunnel barrier
4.3.2. Structural analysis of NbO2 thin films with TiO2 barrier
4.4. Discussions
4.5. Conclusions
4.6. References

Chapter 5. Selector properties of NbO2-based selector device for ReRAM
5.1. Introduction and Experimental Steps
5.2. Selector properties of 1S – 1R structured ReRAM
5.3. Future study on MIT Oxide-based selector device
5.4. Conclusions
5.5. References