Zhimei Sun

Phase Change and Memory

Beihang University, Beijing, China

Email: zmsun@buaa.edu.cn

Abstract for Presentation

Accelerating Phase-Change Materials Design by Integrating High-Throughput ab initio Calculations with Experiments

   Phase-change (PC) materials, especially chalcogenide PC materials, are the foundation for phase-change random access memory (PCRAM) technology. As one pristine PC material hardly meets all the criteria of PCRAM, doping is commonly used to optimize or enhance the performance of PC materials. Nevertheless, the identification of optimal dopants for various PC material is laborious and is usually carried out by an intuition fashion of time-consuming trial and error way. In this presentation, using Sb2Te3 as an example, we show that rapid identifying optimal-dopants can be achieved by integrating high-throughput ab initio calculations with “artificial intelligence”. On the basis of high-throughput ab initio calculations Scandium (Sc) and Yttrium (Y) are identified as the optimal dopants for Sb2Te3. Based on ab initio calculations, both Sc and Y dopants increase rather linearly the band gap of Sb2Te3 with increasing doped concentrations. The underline mechanism has been analyzed by density of states, electron localization function and charge transfers. Furthermore, by the semi-classical Boltzmann transport theory, both Sc and Y can significantly decrease the electrical conductivity and thermal conductivity of Sb2Te3, and thus reduced power consumption of Sb2Te3-based PCRAM is expected. Besides, the ab initio molecular dynamics simulations demonstrated that Sc and Y can significantly improve the thermal stability of amorphous Sc-doped Sb2Te3, and hence enhanced data retention for Sb2Te3-based PCRAM. The above theoretical conclusions are confirmed by experiments. Y dopants result in a ~5.5 times increase in electrical resistivity and a ~25 times decrease in device power consumption, while the maintained crystal structure and the grain refinement provide Y doped Sb2Te3 competitive fast crystallization speed. Moreover, the metastable cubic phase of Sb2Te3 is stabilized by Y dopants, and thus a new type of multi-level data storage system, that is utilizing the reversible multi-level phase transitions between amorphous, metastable cubic, and stable hexagonal crystalline phases, is realized. The evident reversible cubic-to-hexagonal transition is attributed to the sequential and directional migration of Sb atoms, studied by the atomic-resolution HAADF, EDX images, and ab initio simulations. The present work provides fundamentals for improving the overall performance of chalcogenide phase-change materials for PCRAM, and the present methods can be extended to other similar semiconductor materials.