Structural characterization of thin films based on II-VI ternary compounds deposited by evaporation

Thin films of Zn(S, Se), (Zn, In)Se and Cd(S, Te) compounds, deposited by evaporation were characterized through X-ray diffraction (XRD) measurements and analyzed with the help of a simulation program. The interest in studying these materials is due to their potential for photovoltaic applications, especially as buffer materials in Cu(In,Ga)Se-2 (CIGS) and CdTe based solar cells. The XRD measurements allowed us to determine the effect of the chemical composition on the structure and lattice parameter, which must be known to predict an optimum mechanical match between the buffer and absorber layers; a good mechanical match improves the hetero-interface of the solar cell. The studies revealed that In-rich Zn(x)ln(1-x)Se films and Te-rich CdSxTe1-x films grow with hexagonal structure; however, their structure is changed to cubic when they become Zn-rich and Te-rich, respectively. On the contrary, the ZnxIn1-xSe films grow with cubic structure, independently of its chemical composition. It was also found that the variation of the chemical composition leads to a significant variation of the optical gap Eg, which was determined by extrapolation of the curve (alpha hv)(2) vs. hv, assuming that, for this type of compounds, the relation alpha hv = A(hv - Eg)(1/2) is valid. It was observed, in the three type of compounds studied, that their Eg values increase with the decreasing of the lattice constant, which in turn varies according to Vegard's Law. Comparing the lattice parameters of the ZnSxSe1-x and ZnxIn1-xSe compounds, with those reported in the literature for Cu(In1-x,Ga-x)Se-2 thin films, helpful information was found to achieve a good lattice match between the studied II-VI compounds and the CIGS film. (c) 2005 Elsevier B.V. All rights reserved.