Ab Initio Modeling of α-GeTe(111) Surfaces

Density-functional theory (DFT) computations are reported for the (111) crystal surfaces of the phase-change material germanium telluride in its stable rhombohedral modification (dubbed alpha-GeTe). Atomic structures and surface energies are evaluated using a custom-tailored slab model and periodic plane-wave basis sets in the PBE-GGA approximation. Independent of the chemical surrounding, a pristine Te-covered (111) surface is energetically favorable among competing models, whereas a purely Ge-terminated surface is about 60 meV angstrom(-2) higher in energy and predicted to undergo a structural reconstruction. Several atomic motifs for such reconstructions lie closely together energetically and are expected to coexist at finite temperatures. Formation of Ge vacancies in the subsurface layers is investigated in detail. Scanning tunneling microscopy (STM) images are simulated from the DFT wave functions for comparison with experiments to be performed in the foreseeable future.