Characterization of polycrystalline CuInSe2 thin films deposited by sputtering and evaporation as a function of composition

In this study, a series of polycrystalline single-layer CulnSe2 thin films with Cu/In atomic ratios between 1.7 and 0.5 were grown by using a physical vapor deposition system Cu, In sputtering, and Se evaporation. The variation in the microstructure (surface morphology, grain size, and defects) of the CulnSe2 thin films on gradually changing the stoichiometry from Cu-rich to Cu-poor was investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). In general, Cu-rich thin films showed large grains with relatively low defect densities, while Cu-poor thin films exhibited highly planar defects, twins and stacking faults, with small grains. Polycrystalline bi-layer CulnSe2 thin films were also grown by depositing a Cu-poor layer on the top of a Cu-rich layer. The bi-layer Cu-poor thin films exhibited large grains (>3 μm). High-resolution TEM lattice image shows a thin coherent layer (∼20 nm) at the both sides of the grain boundary for a slightly Cu-poor film. TEM energy dispersive X-ray spectroscopy (EDX) analyses indicate that the grain boundary region is more Cu-poor than the intra-grain for the slightly Cu-poor film. The coherent thin layer and more Cu-poor at the grain boundary of the slightly Cu-poor film could be related to a type inverse. The surface morphologies and the grain boundaries of the polycrystalline CuInSe2 could provide an understanding of the growth behavior of CuInSe2 and the performance of CuInSe2 devices.