Optical constants Cu2ZnSnS4 thin films deposited at different substrate temperatures

In this paper, optical properties of Cu₂ZnSnS₄ (CZTS) thin films grown by thermal evaporation under vacuum method were studied. The samples were deposited at different glass substrate temperatures ranging from Room temperature to 175°C. The optical constants such as refractive index, absorption and extinction coefficients were calculated using the optical measurements in the spectral range of 300-1800 nm. The Swanepoel´s envelope method was used to determine the refractive index which can be extrapolated by Cauchy dispersion relationship. The dispersion of the refractive index is discussed in terms of the single oscillator model proposed by Wemple and DiDomenico. By using this model, i.e. by plotting (n²-1)^-1 against (hv)² and fitting a straight line, oscillator parameters, E₀ the single oscillator energy and E_d the dispersion energy, were directly determined. The calculated refractive index was found to be in the range of 2.66-3.91 and the absorption coefficient was higher than 10⁴ cm^-l measured in the VIS-NIR region. The electric free carrier susceptibility (Xe) and the carrier concentration on the effective mass ratio (N/m*) were evaluated according to Spitzer-Fan model.