Characterization of polycrystalline CuInSe 2 thin films deposited by sputtering and evaporation as a function of composition

Abstract

In this study, a series of polycrystalline single-layer CulnSe 2 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 CulnSe 2 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 CulnSe 2 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 CuInSe 2 could provide an understanding of the growth behavior of CuInSe 2 and the performance of CuInSe 2 devices.