Mapping the band structure of GeSbTe phase change alloys around the Fermi level

TL;DR

This study uses photoelectron spectroscopy to directly map the electronic band structure of GeSbTe phase change alloys, revealing the valence band topology, disorder effects, and a potential topological state near the Fermi level.

Contribution

It provides the first direct experimental mapping of the band structure of GeSbTe alloys, linking electronic properties to their phase change behavior.

Findings

Valence band forms hexagonal tubes near Fermi level

Fermi level is 100 meV above valence band maximum due to disorder

Discovery of a linearly dispersing state possibly of topological origin

Abstract

Phase change alloys are used for non-volatile random access memories exploiting the conductivity contrast between amorphous and metastable, crystalline phase. However, this contrast has never been directly related to the electronic band structure. Here, we employ photoelectron spectroscopy to map the relevant bands for metastable, epitaxial GeSbTe films. The constant energy surfaces of the valence band close to the Fermi level are hexagonal tubes with little dispersion perpendicular to the (111) surface. The electron density responsible for transport belongs to the tails of this bulk valence band, which is broadened by disorder, i.e., the Fermi level is 100 meV above the valence band maximum. This result is consistent with transport data of such films in terms of charge carrier density and scattering time. In addition, we find a state in the bulk band gap with linear dispersion, which might be of topological origin.