Growth and morphology of thin Cu(In,Ga)S2 films during reactive magnetron co-sputtering

Abstract

The roughness and the morphology of chalcopyrite Cu(In,Ga)S2 (CIGS) films on different substrates prepared by dc reactive magnetron co-sputtering with thicknesses ranging from 10 to 1000nm were investigated by X-ray diffraction, scanning electron microscopy and atomic force microscopy (AFM). One-dimensional power spectral density (1DPSD) functions, derived from the AFM profiles, were used to compare the microstructure scaling behavior of the thin films. For CIGS films on Mo coated glass substrates, the variation of the grain size (dg) obtained from the AFM measurements, with film thickness (df) in the columnar grained films was dg(AFM)~df0.33. The growth exponent β, characterizing the roughness evolution of the films, changes with the film thickness. The root-mean-square roughness (Rrms) increases only slightly as Rrms~dfβ (β=0.13±0.09) in the initial growth stage, while Rrms increases with β=0.56±0.03, when the film thickness is larger than 50nm. 1DPSD analysis showed three spatial-frequency regimes of the roughness evolution during room temperature deposition, where the intermediate region disappears when the substrate temperature is increased. The nucleation and growth mechanisms are discussed in terms of surface diffusion, grain growth and shadowing effects. The substrate morphology does not significantly affect CIGS growth, which was proved for TiN and float glass. From the viewpoint of surface roughness and film growth, TiN is comparable to molybdenum and is suited as a potential back contact for chalcopyrite solar cell applications.