Theory of persistent, p-type, metallic conduction in c-GeTe

It has been known for over twenty years that rhombohedral c-germanium telluride is predicted to be a narrow gap semiconductor. However, it always displays p-type metallic conduction. This behaviour is also observed in other chalcogenide materials, including Ge 2 Sb 2 Te 5 , commonly used for optically and electrically switched, non-volatile memory, and so is of great interest. We present a theoretical study of the electronic structure of the perfect crystal and of the formation energies of germanium/tellurium vacancy and antisite defects in rhombohedral germanium telluride. We find that germanium vacancies are by far the most readily formed defect, independent of Fermi level and of growth ambient. Moreover, we predict that the perfect crystal is thermodynamically unstable. Thus, the predicted large equilibrium densities of the germanium vacancy of ∼5 × 10 19 cm −3 results in a partially filled valence band and in the observed p-type conductivity.