DFT Studies of Pristine Hexagonal Ge1Sb2Te4(0001), Ge2Sb2Te5(0001), and Ge1Sb4Te7(0001) Surfaces

Abstract

Ternary alloys of germanium, antimony, and tellurium (GexSbyTez) are not only prominent phase-change data-storage materials with intriguing properties; they also form precisely ordered crystal structures if one allows them to do so. Here, we study atomically pristine surfaces of representative GexSbyTez alloys using density functional theory (DFT) and suitable plane-wave-based slab models. In particular, we look at the hexagonal (0001) surfaces of Ge1Sb2Te4, Ge2Sb2Te5, and Ge1Sb4Te7 but with the intent to draw some general conclusions beyond those explicit compositions. The conditions for thermodynamic stability of the ternary surfaces (i.e., the space of permissible chemical potentials) are delineated, and a simple general expression to estimate the latter is given. Because of weak van der Waals-type interactions between Te atomic layers, we assess how surface energy computations are affected by the choice of exchange–correlation functional and also by the use of dispersion corrections (here, Grimme’s DFT+D2 scheme). LDA and PBE+D2 predicted surface energies are similar to a degree that is rather unexpected; generally, Te terminations are lowest in surface energy regardless of the compound and methodology. Implications for further surface studies of “less ideal’’ GexSbyTez surfaces are discussed.