Influence of Thermal Treatments on In-depth Compositional Uniformity of CuIn(S,Se)2 Thin Films

CuIn(S,Se)₂ thin films were prepared by thermal crystallization of co-sputtered Cu-In alloy precursors in S/Se atmosphere. In-depth compositional uniformity is an important prereq-uisite for obtaining device-quality CuIn(S,Se)₂ absorber thin films. In order to figure out the influence of heat treatments on in-depth composition uniformity of CuIn(S,Se)₂ thin films, two kinds of reaction temperature profiles were investigated. One process is "one step profile", referring to formation of CuIn(S,Se)₂ thin films just at elevated temperature (e.g. 500 oC). The other is "two step profile", which allows for slow diffusion of S and Se elements into the alloy precursors at a low temperature before the formation and re-crystallization of CuIn(S,Se)₂ thin films at higher temperature (e.g. first 250 oC then 500 oC). X-ray diffrac-tion studies reveal that there is a discrepancy in the shape of (112) peak. Samples annealed with "one step profile" have splits on (112) peaks, while samples annealed with "two step profile" have relatively symmetrical (112) peaks. Grazing incident X-ray diffraction and en-ergy dispersive spectrum measurements of samples successively etched in bromine methanol show that CuIn(S,Se)₂ thin films have better in-depth composition uniformity after "twostep profile" annealing. The reaction mechanism during the two thermal processing was also investigated by X-ray diffraction and Raman spectra.