Optical properties and phase transition in photodoped amorphous Ge–Sb–Te:Ag thin films

Abstract

Photodoping of Ag into amorphous Ge2Sb2Te5 thin films has been carried out by illuminating thermally evaporated Ge–Sb–Te:Ag bilayers with a halogen lamp in the inert atmosphere at room temperature. Amorphous nature of the as-deposited and photodoped films was confirmed by an X-ray diffraction study. The composition of Ge2Sb2Te5 thin films and the amount of Ag photodiffused have been gathered from electron probe micro-analyzer having a wavelength dispersive spectrometer. Amorphous-to-crystalline transition of the films was induced by thermal annealing and the structural phases were identified by X-ray diffraction. The crystalline temperature of the films was evaluated by temperature dependent sheet resistance measurements. The effects of photodoped Ag concentration on the optical properties of the Ge2Sb2Te5 films have been examined by transmission and reflection spectrometry. It was found that the optical band gap, the sheet resistance and the phase transition temperature increase while the absorption coefficient and extinction coefficient decrease upon incorporation of Ag in Ge2Sb2Te5 system. A correlation between the optical band gap and the electronegativity of the alloys indicates that the optical band gap increases with the decrease of electronegativity.