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

J Kellner,C Pauly,V L Deringer,R Dronskowski,V Bragaglia,G Bihlmayer,S Cecchi,E Golias,O Rader,S Otto,M Morgenstern,R N Wang,J Sánchez-Barriga,M Liebmann,J E Boschker,P Küppers,T Fauster,P Bhaskar,R Calarco

doi:10.1038/s42005-018-0005-8

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.

Highlights

Phase change alloys are used for non-volatile random-access memories exploiting the conductivity contrast between amorphous and metastable, crystalline phase
angular resolved photoelectron spectroscopy (ARPES) experiments were performed at 29 different photon energies hν = 16–31 eV with a step size of
The valence band maximum is about 100 meV below EF, such that only the tails of the disorder broadened E(k) states contribute to the conductivity

Summary

Introduction

Phase change alloys are used for non-volatile random-access memories exploiting the conductivity contrast between amorphous and metastable, crystalline phase. This contrast has never been directly related to the electronic band structure. Phase change alloys are typically chalcogenides consisting of Ge, Sb, and Te (GST) with Ge2Sb2Te5 (GST-225) being the prototype[1,4] They exhibit three different structural phases: an amorphous, a metastable rock salt, and a stable trigonal phase. The localization is lifted by annealing due to the respective continuous ordering of the Ge, Sb, and Vcs into different layers[18,19,20] This is accompanied by a shift of the Fermi level EF toward the valence band (VB)[17,21]. It was found that the epitaxial GST films are in the technologically relevant rock salt phase, but often exhibit ordering of the vacancies in separate layers[20]

Methods

Results

Conclusion