Sulfurization of co-evaporated Cu2ZnGeSe4 layers: Influence of the precursor cation's ratios on the properties of Cu2ZnGe(S,Se)4 thin films

Cu2ZnGe(S,Se)4 (CZGSSe) thin films were fabricated by sulfurization of co-evaporated Cu2ZnGeSe4 (CZGSe) thin films. The goal of this work is to investigate the effect of the composition of CZGSe layers on the structural, vibrational and morphological properties of CZGSSe compounds. Different CZGSe layers with different cation's ratios (Cu-poor, Zn-rich; Cu-poor, Zn-stoichiometric and Cu-poor, Ge-rich) were investigated before and after the sulfurization process. Break-off and different cooling experiments during sulfurization of CZGSe were also carried out. Break-off experiments revealed that 440 °C is the key temperature to incorporate S into CZGSe lattice, being required 480 °C to enhance the interdiffusion of the elements. Fast cooling experiments during sulfurization appeared to be a promising strategy to avoid the formation of secondary phases. All the samples showed the CZGSSe kesterite phase, as well as higher S content and different secondary phases at the surface. These experiments demonstrate that a minimum Ge content in CZGSe is required to assist the growth of CZGSSe grains and develop a compact structure. These results indicate the importance of controlling the cation's ratio of CZGSe to develop high quality wide band gap CZGSSe compounds, which can be very attractive for different applications