Heat treatment effect on the structural and optical properties of AgInSe 2 thin films

Abstract

AgInSe 2 thin films were deposited on glass and quartz substrates at ≃300 K by the conventional thermal evaporation technique. Nearly stoichiometric films were obtained using a single source in 8×10 −4 Pa vacuum. The chemical composition of the bulk material, as-deposited films, and annealed films were determined by energy-dispersive X-ray spectrometry, where the chemical formula could be represented by Ag 1− x In 1−3 x Se 2(1+3 x) with x⩽0.045. The as-deposited films were amorphous. After annealing at 500 K in vacuum for 1 h, the films were polycrystalline and exhibited tetragonal chalcopyrite structure (1 1 2) orientation with lattice parameters a=0.601 nm, c=1.133 nm, and ( c/ a)=1.885. The optical constants (the refractive index, n, the absorption index, k, and the absorption coefficient, α) of both amorphous and polycrystalline AgInSe 2 films were determined from spectrophotometric measurements of the transmittance and reflectance carried out at normal incidence in the wavelength range 300–2500 nm.The analysis of the data gave an indirect, allowed transition energy gap ( E g i(all)=1.08 eV) and another indirect forbidden transition energy gap ( E g i(for)=0.79 eV) for the amorphous films. A direct allowed energy gap ( E g d(all)=1.25 eV) and direct forbidden transition ( E g d(for)=1.35 eV) for crystallized films were also identified. On the other hand, the refractive index behaviour of the as-deposited and crystallized films under study is analysed within the oscillator framework proposed by Wemple and DiDomenico. Graphical representations of surface and volume energy loss functions as well as the real optical conductivity as a function of photon energy show the existence of three possible optical transitions.