Nature of defects and gap states in GeTe model phase change materials

The electrical storage mechanism in GeSbTe phase change materials is discussed in terms of their gap states using GeTe as a model system. The lowest energy defect in crystalline rhombohedral GeTe phase is the Ge vacancy, because it reconstructs along the resonant bonding directions. The lowest energy in amorphous GeTe is the divalent Te atom, which creates overlapping band-tail states that pin Fermi level E-F near midgap. In contrast, the lowest cost defect in disordered phase in GeSbTe superlattices is the Te interstitial whose negative correlation energy pins E-F near midgap.