Dr. Ming Xu [RWTH Aachen University, Germany]

Title: Phase-change memory materials under high pressure

Time: 11:00 - 12:00 PM, Tuesday, January 5, 2016

Place: Conference Room 201, HPSTAR (Shanghai)

Host: Dr. Lin Wang

Abstract:

The binary phase change memories, which take advantage of the resistive contrast between the amorphous and the crystalline phases, will eventually give way to the high-capacity multi-state memories, in which both phases can reach various resistive states. Recent studies demonstrated that these multiple resistive states can be achieved in crystalline phase change materials (PCMs), by manipulating the vacancies in Ge-Sb-Te (GST) system and controlling the disorder with different annealing temperatures. In lieu of the heat treatment, our large-scale ab initio molecular dynamics (AIMD) simulations show that pressure can also tune the disorder in crystalline PCMs. Our models contain 1008 atoms of rocksalt-like GeSb2Te4 with random distribution of vacancies, and the AIMD simulations were performed at various temperatures and pressures for about 100 ps. We observed that, by lowering the migration energy for the anti-site hopping (e.g., Te ions jump to the Ge/Sb layers and Sb ions hop to the Te layers), the high pressure increases the compositional disorder due to the accumulation of these anti-site ions, resulting in some localized electronic states near the bottom of the conduction band. On the other hand, the disorder leads to the unoriented displacement of the anti-site ions, distorting and destabilizing the lattice near them. The disorder-induced electron localization triggered by pressure will pave the way for the understanding and development of the multi-state memory devices. And the random distortion of lattices due to the compositional disorder offers a new mechanism that may contribute to the amorphization of crystalline PCMs at high pressure.

Biography of the Speaker:

Ming Xu is currently a Humboldt postdoc fellow atInstitute of Physics (IA), RWTH Aachen University, Germany. He obtained B.S.(2005) and M.S. (2008) degrees in optics at Fudan University (Shanghai, China)and received the PhD degree (2013) at the Department of Materials Science andEngineering, Johns Hopkins University (Baltimore, MD, USA) under the guidanceof Prof. Dr. Evan Ma. His research primarily focuses on the structure-propertyrelations in phase-change materials and their high-pressure behaviors. Xu haspublished over 20 high-profile peer-reviewed papers including Phys. Rev. Lett.,PNAS, Advanced Science, etc.