Phase transformation kinetics and optical properties of Ga–Se–Sb phase-change thin films

Abstract

Phase transformation kinetics in Ga25Se75−xSbx glasses have been determined by non-isothermal differential scanning calorimetric measurements at heating rates of 5, 10, 15, 20 and 25K/min. The values of glass transition (Tg) and crystallization temperature (Tc) are found to be composition and heating rate dependent. The activation energy of crystallization and glass transition have been determined from the dependence of Tc and Tg on the heating rate. Thin films of Ga25Se75−xSbx glasses have been prepared by vacuum evaporation technique with thickness 400nm. These thin films were crystallized by thermal annealing and laser-irradiation. The phase change phenomena have been studied by measuring optical absorption of as-prepared and crystallized thin films in the wave length region 400–900nm. The optical absorption data indicate that the absorption mechanism is non-direct transition. Optical band gap values decrease with increase in Sb contents in Ga–Se as well as with increase in annealing temperature and laser-irradiation time. The optical band gap is shifted due to crystallization by annealing/laser-irradiation. As the phase of the films changes from amorphous to crystalline, a non sharp change of the optical band gap is observed. This gradual decrease in optical band gap was explained to be a result of an amorphous–crystalline phase transformation.