Surface evolution of sputtered Cu(In,Ga)Se₂ thin films under various annealing temperatures

In this work, copper indium gallium selenide (CIGS) thin films were prepared by sputtering CIGS quaternary target and subsequent annealing under Se atmosphere in the 240–550 °C range. X-ray diffraction (XRD), Grazing incidence x-ray diffraction (GIXRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were performed to investigate the crystalline structures and chemical compositions of the CIGS thin films. According to XRD, GIXRD and Raman analyses, the CIGS phase was splitted into CuSe phase and In₄Se₃ phase at the very surface when CIGS was annealed at 240 and 270 °C. In addition, XRD patterns indicated a grain size growth with increasing temperature. XPS analyses contributed more information on the film surface chemistry. Indeed, O 1 s and Na 1 s signals were observed at the film surface above 380 °C, and were even increased from 450 to 550 °C. Simultaneously, the surface appeared to exhibit a Cu and Ga poor surface chemical composition. The strong correlation between O, Na and Cu atomic contents in the CIGS surface were discussed in the paper.