Smoothing of polycrystalline Cu(In,Ga)(Se,S)2 thin films by low-energy ion-beam etching

Abstract

We present a study of the smoothing process of polycrystalline CuInS2 and CuGaSe2 thin films using ion-beam etching with low-energy nitrogen ions. The evolution of the surface roughness and morphology have been studied by atomic force microscopy with respect to the ion-beam angle of incidence and the ion dose. Raman scattering and photoluminescence spectroscopy were applied to investigate the surface damage induced by the ion-beam etching process. A drastic reduction of the surface roughness for both materials has been observed for ion-beam angles of incidence larger than 70° with increasing ion doses without significant surface damage. Using this technique the initial root-mean-square roughness of, e.g., CuGaSe2 of about 87 nm could be decreased to about 25 nm. Roughness analysis over different spatial wavelength regions by using the power spectral density reveals that smoothing occurs for all spatial wavelengths. Hence, ion-beam etching with nitrogen is a suitable method to polish polycrystalline CuInS2 and CuGaSe2 thin films for surface analysis techniques and depth profiling.