Depth-resolved and temperature dependent analysis of phase formation processes in Cu–Zn–Sn–Se films on ZnO substrates

The constitution of secondary phases in kesterite Cu2ZnSnSe4 (CZTSe) thin films is still a limiting factor for their application in solar cells. Therefore an enhanced understanding of phase formation processes during the fabrication of CZTSe films is required. In this study we present a temperature and film-depth dependent phase analysis of Zn/Sn/Cu precursors on ZnO substrates selenized at different temperatures. A special sample preparation step using a focused ion beam was applied to prepare shallow angle cross sections for depth-resolved Raman profiling of the thin films. At low selenization temperatures multiphase structures are demonstrated and a first formation of CZTSe besides secondary phases at only 250 °C is detected. At high selenization temperatures an accumulation of ZnSe at the interface of CZTSe and ZnO substrates is observed. Furthermore indications for the formation of a thin SnO2 interface layer were found by X-ray diffraction, secondary electron microscopy and energy dispersive X-ray spectrometry.