On the photoinduced phase transition in (GeTe)n(Sb2Te3)m
  We suggest a phenomenological description of the photo-conversion in Ge-Sb-Te phase-change memory alloys from amorphous to crystalline phase which explains why both photo-excitation and high temperatures T > 160C are required for the transition from hexagonal to tetrahedral phase. The position of chemical potential at high temperatures allows light induced inverse population of the nucleons of the crystalline phase which are not stable otherwise. Then, inverse population accumulates holes on neighboring Te and Ge ions and locks the photo-conversion transition by pushing Ge ions into the interstitial position to minimize the Coulomb repulsion energy. 
S. M. Yakubenya, A. S. Mishchenko
cond-mat.mtrl-sci