Defect states in amorphous Ge₂Sb₂Te₅ phase change material¹

Steady-state photoconductivity measurements in the temperature range 100-300 K on amorphous Ge₂Sb₂Te₅ thin film prepared by dc sputtering are analyzed. The dark conductivity is thermally activated with a single activation energy that allocates the position of the Fermi level approximately in the middle of the energy gap relative to the valance band edge. The temperature dependence of the photoconductivity ensures the presence of a maximum normally observed in chalcogenides with low-and high-temperature slopes, which predict the location of discrete sets of localized states (recombination levels) in the gap. The presence of these defect states close to the valence and conduction band edges leaves the quasi Fermi level shifts in a continuous distribution of gap states at high temperatures, as evidenced from the Î values of the lux-ampere characteristics.