Chalcogenide- and Oxide-based selectors for 3D stackable memory applications

Abstract

This doctoral dissertation describes an experimental study on the electrical features and development of chalcogenide- and oxide-based selectors for 3D stackable memory applications. Initially, I discussed the electrical properties of ZnTe selectors for threshold switching and expatiated on the formation of the switching filament during the forming process. The measured off-current was explained to analytic model derived from the trap-limited conduction mechanism of OTS. ZnTe selectors exhibited a rapid response within 100ns and stable switching throughout repeated cycles, featuring a high nonlinearity of 104. 1S1R measurements using ZnTe selectors with Ta2O5 ReRAM confirmed suitability for crossbar applications. Switching characteristics remained intact even after annealing at 300℃, particularly when heated in a nitrogen atmosphere to prevent the phase separation of ZnTe. Structural and compositional analyses, including transmission electron spectroscopy, X-ray diffractometry, and X-ray photoelectron spectroscopy, were employedto support these findings. Subsequently, Ta2O5 selectors were researched with grain boundaries as line defects. The initial ReRAM behavior of Ta2O5 deposited through the sputtering method transitioned to nonlinear conduction through the grain boundary of polycrystalline Ta2O5, attributed to sub-oxidized Ta atoms at the grain boundary. Asymmetric nonlinearity was minimized by adjusting the thickness of the Ta2O5 layer. Through both DC and pulse measurements, stable endurance was confirmed with minimal current distribution. Finally, Te atoms were introduced into insulating Ta2O5 thin films as point defects to induce nonlinear conductivity. Te-Ta2O5 compound films, sandwiched between electrodes, featured high resistance and nonlinearity at the appropriate Te content. In the Ta2O5-based matrix, Te functioned as an impurity, reducing the density and bandgap of Ta2O5. All efforts were directed towards the development of selectors utilizing defects. Through the aforementioned studies, selectors with nonlinear current-voltage characteristics were fabricated based on zinc telluride and tantalum oxide. The switching characteristics induced by defects in zinc telluride provided inspiration for the development of oxide-based selective materials with high performance and stability. The grain boundaries of tantalum oxide were identified as linear defects and co-sputtered tellurium played the role of point defects, enabling the realization of a selector with high resistance and nonlinear conductivity. These studies proposed new methods for the development of selectors for crossbar array applications.