Band gap widening with time induced by structural relaxation in amorphous Ge2Sb2Te5 films

In a phase change memory the device resistance corresponding to the amorphous phase monotonically increases with time after the reset programming operation. This phenomenon, called drift, affects the stability of the high resistive state, namely the reset state. In this work we investigate the resistance-drift process through ellipsometric measurements as a function of time in thin film of as-deposited amorphous Ge2Sb2Te5 alloy. We show a tight correlation between the resistance increase with time and the optical band gap widening extracted by ellipsometric measurements. This characterization supports the drift origin due to a structural atomic rearrangement of the amorphous network affecting the band structure that, in particular, promotes the increase of the energy gap and the reduction of localized states within the energy gap.