Exploring the subsurface atomic structure of the epitaxially grown phase change material Ge$_2$Sb$_2$Te$_5$

TL;DR

This study combines STM, STS, and DFT to reveal that the subsurface layer of epitaxially grown Ge2Sb2Te5 is more ordered than expected, with minimal vacancies and specific bonding characteristics, indicating improved structural quality.

Contribution

It provides detailed atomic-scale insights into the subsurface structure of epitaxial Ge2Sb2Te5, highlighting the absence of vacancies and the nature of Ge bonding, which was not previously characterized.

Findings

Subsurface layer is more ordered with nearly no vacancies.

Potential fluctuations are consistent with charged acceptors, not vacancies.

Absence of tetrahedral Ge bonds as predicted by DFT.

Abstract

Scanning tunneling microscopy (STM) and spectroscopy (STS) in combination with density functional theory (DFT) calculations are employed to study the surface and subsurface properties of the metastable phase of the phase change material Ge$_{2}$Sb$_{2}$Te$_{5}$ as grown by molecular beam epitaxy. The (111) surface is covered by an intact Te layer, which nevertheless allows to detect the more disordered subsurface layer made of Ge and Sb atoms. Centrally, we find that the subsurface layer is significantly more ordered than expected for metastable Ge$_{2}$Sb$_{2}$Te$_{5}$. Firstly, we show that vacancies are nearly absent within the subsurface layer. Secondly, the potential fluctuation, tracked by the spatial variation of the valence band onset, is significantly less than expected for a random distribution of atoms and vacancies in the subsurface layer. The strength of the fluctuation is compatible with the potential distribution of charged acceptors without being influenced by other types of defects. Thirdly, DFT calculations predict a partially tetrahedral Ge bonding within a disordered subsurface layer, exhibiting a clear fingerprint in the local density of states as a peak close to the conduction band onset. This peak is absent in the STS data implying the absence of tetrahedral Ge, which is likely due to the missing vacancies required for structural relaxation around the shorter tetrahedral Ge bonds. Finally, isolated defect configurations with a low density of $~10^{-4}$/nm$^2$ are identified by comparison of STM and DFT data, which corroborates the significantly improved order in the epitaxial films driven by the build-up of vacancy layers.