Laser induced changes on a-Ga50Se50 thin films

This paper reports on the laser induced changes on the optical properties of a-Ga50Se50 thin films prepared by the thermal evaporation technique under vacuum. Thin film samples, on a glass substrate, were exposed to laser light of wavelength lambda = 532 nm for different exposure times, t(E) (t(E) = 0, 500, 1500 and 3000 s). Optical parameters such as refractive index, absorption coefficient, optical gap, extinction coefficient, real and imaginary dielectric constants, dielectric loss, optical conductivity of as-deposited thin film and their laser induced changes were studied at three different times of exposure. The mechanism of optical absorption follows the rule of the indirect allowed transition model proposed by Tauc and the optical band gap is calculated by Tauc's extrapolation. It is seen that after laser irradiation there is a shift of the optical absorption edge to larger photon energy. The value of the absorption coefficient of the GaSe thin film decreases on exposing the film to laser irradiation. A microscopic model in which heteropolar bonds are broken by absorption of high energy photons and new homopolar bonds are formed simultaneously has been used to describe the effect.