Phase formation in Cu2ZnSnSe4 thin films deposited with multi-stage co-evaporation processes

•Detailed investigation of film growth and phase formation processes during co-evaporation of CZTSe thin films.•Subdivision of the process in distinct growth phases with characteristic phase formations.•Chemical reaction between CZTSe and Mo hinders CZTSe growth in the initial stage of the process.

The deposition of Cu2ZnSnSe4 (CZTSe) thin films with a two-stage co-evaporation process was investigated with in situ laser light scattering (LLS) and ex situ characterization methods. As a result of the process conditions as well as intrinsic reactions, the film growth is divided into distinct growth phases with characteristic phase formation processes in each phase. In a first phase, under supply of Cu, Zn, Sn and Se, only Cu2−xSeCu2−xSe and ZnSe are deposited on the substrate. A film with several hundred nanometer thickness is necessary to initiate the CZTSe formation. Then, in a second phase, CZTSe grows by incorporation of Cu and Sn in ZnSe while Cu2−xSeCu2−xSe remains as secondary phase. In the second process stage with terminated Cu evaporation, the remaining Cu2−xSeCu2−xSe is consumed and the film stoichiometry turns from Cu-rich to Cu-poor. A substantial influence on the growth process is found for the Mo substrate, CZTSe growth is hindered by a detrimental interaction between Mo and CZTSe. During the whole process, the Cu2−xSeCu2−xSe phase plays an important role in the grain growth process and supports the formation of large CZTSe grains.