Stoichiometry control in Cu₂ZnSnS₄ thin films grown by pulsed laser deposition

We synthesized Cu₂ZnSnS₄ (CZTS) thin films on soda lime glass substrates using pulsed laser deposition. Deposition parameters and sputtering target compositions were varied in order to optimize the compositions and morphologies of the CZTS films. At room temperature, films deposited using a sputtering target composition of Cu_1.83Zn_1.16Sn_0.95S_4.05, consisting of the nominally desired metal ratios and excess sulfur, were found to be heavily Sn-rich and S-deficient. By reducing the Sn concentrations in the sputtering targets, film compositions could be adjusted to the desired stoichiometries. Laser fluence was found to strongly influence the Cu:Zn ratios in the films, with a clear laser ablation threshold visible at ∼2.5 J/cm², above which the film stoichiometry did not significantly change. Increasing the pulse repetition rate slightly increased the relative Zn and S concentrations in the films but also introduced micron-sized cracks. Substrate temperature was shown to have a significant effect on the composition and quality of the deposited films. While smoother films with larger-width grains were achieved at temperatures of 400 and 500 °C, significant losses in Zn and Sn were observed. Recommendations on the optimized deposition parameters and sputtering target compositions are presented.