Point defects in disordered and stable GeSbTe phase-change materials

Phase-change materials (PCMs) are promising candidates for efficient storage-class memory exploiting the pronounced resistivity contrast between their amorphous and crystalline phase. GeSbTe compounds, the prototypical class of PCMs, are theoretically predicted to be small-gap semiconductors in their disordered, cubic and ordered, hexagonal phase, but in experiment they show p-type conduction. While this is attributed to self-doping, the very defect types responsible have not been entirely identified. Here, we present an ab-initio study of point defects and their formation energies in GeSb2Te4 and Ge2Sb2Te5 in their disordered and ordered crystalline phases. Our simulations indicate that GeSb, rather than VGe or SbTe, is the most important defect to explain p-doping in the hexagonal structure. In the disordered phase, on the other hand, standard gradient-corrected functionals yield low-formation energies for defects associated with n-type conduction, in apparent contradiction with experimental data. We discuss possible sources for this discrepancy and argue that it stems from the underestimation of the band gap.