Reversible conversion over multimillion times in bond types between metavalent and covalent bonds becomes one of the most promising bases for universal memory. As the conversions have been found in metastable states, an extended category of crystal structures from stable states via redistribution of vacancies, research on kinetic behavior of the vacancies is highly in demand. However, it remains lacking due to difficulties with experimental analysis. Herein, the direct observation of the evolution of chemical states of vacancies clarifies the behavior by combining analysis on charge density distribution, electrical conductivity, and crystal structures. Site-switching of vacancies of Sb2Te3 gradually occurs with diverged energy barriers owing to their own activation code: the accumulation of vacancies triggers spontaneous gliding along atomic planes to relieve electrostatic repulsion. Studies on the behavior can be further applied to multiphase superlattices composed of Sb2Te3 (2D) and GeTe (3D) sublayers, which represent superior memory performances, but their operating mechanisms were still under debate due to their complexity. The site-switching is favorable (suppressed) when Te-Te bonds are formed as physisorption (chemisorption) over the interface between Sb2Te3 (2D) and GeTe (3D) sublayers driven by configurational entropic gain (electrostatic enthalpic loss). Depending on the type of interfaces between sublayers, phases of the superlattices are classified into metastable and stable states, where the conversion could only be achieved in the metastable state. From this comprehensive understanding on the operating mechanism via kinetic behaviors of vacancies and the metastability, further studies toward vacancy engineering are expected in versatile materials.
Bibliographical noteFunding Information:
This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Government of Korea (MSIP) (No. 2021M3H4A1A03052566 and No. 2020M3F3A2A0108232413), the Ministry of Trade, Industry & Energy (MOTIE) of Korea (Project No. 10080625), and the Korea Semiconductor Research Consortium (KSRC) through a project for developing source technologies for future semiconductor devices. This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (Grant No. 2022M3I7A2079098). This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant No. 2022R1A6A3A03067579). Y.K. and E.S. acknowledge the support from NRF-2020R1A2C2007468. D.K. is very grateful for discussions with Peter Kerres and Professor Matthias Wuttig.
© 2022 American Chemical Society.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Physics and Astronomy(all)