  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.
