Local instability of p-type bonding makes amorphous GeTe a lone-pair semiconductor

Bonding in the crystalline phase of GeTe-based phase-change alloys is believed to be purely p type. In this work we demonstrate that in the absence of long-range order, despite the preserved bonding angles of ∼90 ^â̂̃, sp3 hybridization is more favorable with a pair of nonbonding electrons localized on a Ge sp3 orbital, i.e., in contrast to the crystalline phase, amorphous GeTe is a lone-pair (LP) semiconductor. Upon disordering, tetrahedral Ge sites are formed due to unpairing of LP electrons and subsequent formation of additional Ge-Ge bonds, a process generating additional free electrons. Recombination of these electrons during the structure relaxation may be the underlying reason for the conductivity drift in the amorphous phase making the latter process analogous to persistent photoconductivity. Implications for the stability of the amorphous phase are also discussed.