Electric-field-induced annihilation of localized gap defect states in amorphous phase-change memory materials

Structural relaxation of amorphous phase-change-memory materials has been attributed to defect-state annihilation from the band gap, leading to a time-dependent drift in the electrical resistance, which hinders the development of multi-level memory devices with increased data-storage density. In this compu-tational study, homogeneous electric fields have been applied, by utilizing a Berry-phase approach with hybrid-density-functional-theory simulations, to ascertain their effect on the atomic and electronic structures associated with the mid-gap states in models of the prototypical glassy phase-change material, Ge2Sb2Te5. Above a threshold value, electric fields remove spatially localized defects from the band gap and transform them into delocalized conduction-band-edge electronic states. A lowering of the nearest-neighbor coordination of Ge atoms in the local environment of the defect-host motif is observed, accom-panied by a breaking of 4-fold rings. This engineered structural relaxation, through electric-field tuning of electronic and geometric properties in the amorphous phase, paves the way to the design of optimized glasses. (C) 2021 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc.