Paul Fons1 2 Alexander Kolobov1 Yuta Saito1 Kotaro Makino1 Junji Tominaga1

1, Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaragi, Japan
2, SPring8, Japan Synchrotron Radiation Institute (JASRI), Hyogo, , Japan

One of the more active research areas in phase-change memory is the concept of van der Waals (vdW)-bonded GeTe/Sb2Te3 superlattices (SL) also known as interfacial phase-change memory (iPCM). Devices based upon GeTe/Sb2Te3 SL structures have demonstrated both lower energy consumption as well as order of magnitude faster switching speeds. Unlike conventional Ge-Sb-Te alloy-based devices in which the SET and RESET states correspond to the crystalline and amorphous phases, the SET and RESET states in SL devices are both crystalline. Transmission electron microscopy studies of GeTe/Sb2Te3 SL structures structures have revealed that GeTe and Sb2Te3 blocks intermix in as-grown structures implying that the switching mechanism may be different than originally believed. In a recent work [1], it was proposed that the switching process is associated with the reconfiguration of vdW gaps along with concomitant deviations in the local stoichiometry from the GeTe/Sb2Te3 quasibinary alloys. This model offers an explanation of why the large conductivity difference between the SET and RESET states persists in the presence of intermixing. In the model, dynamic reconfigurations of the vdW gap in response to external stimuli such as heat or electric field are shown to lead to large changes in conductivity. In this work, we use density-functional theory to investigate the effects of electric field on a variety of proposal structures in the form of van der Waals blocks including the previously proposed Ferro, Kooi, Petrov, and Inverted-Petrov structures. Both 0K and finite temperature Molecular dynamics calculations were carried out using a supercell slab geometry for each structure with a vacuum gap taking advantage of the lack of dangling bonds on the outermost Te-plane that constitute one side of a vdW gap with an externally imposed sawtooth like potential that maintained periodic boundary conditions with the potential discontinuity confined to the vacuum region. The Ferro structure was found to undergo the largest structural rearrangement and the Kooi structure the least. Changes in local geometry and effective charges for each structure will be discussed. The effect of temperature will also be discussed via molecular dynamics studies using the same geometry.
This work was supported by JST-CREST (JPMJCR14F1).

[1] A. V. Kolobov, P. Fons, Y. Saito, and J. Tominaga. Atomic reconfiguration of van der Waals gaps as the key to switching in GeTe/Sb2Te3 superlattices. ACS Omega, 2(9):6223–6232, 2017.