A study on the optical and microstructural characteristics of quaternary Cu(In,Ga)Se/sub 2/ polycrystalline thin films

The optical and microstructural properties of polycrystalline CuIn/sub 1-y/Ga/sub y/Se/sub 2/ (CIGS) thin film deposited by coevaporation are reported within the boundaries of an orthogonal experimental design investigating the effects of Cu flux, Ga/(Ga+In) composition. Se rate, substrate temperature, T/sub s/ and substrate type. The optical bandgaps for near-stoichiometric CuIn/sub 1-y/Ga/sub y/Se/sub 2/ are smaller and exhibit bowing behavior which follows the relationship E/sub g/=1.011+0.664y+0.249y(y-1). In comparison, Cu-poor films exhibit a linear variation with zero bowing given by E/sub g/=1.0032+0.71369y. The increase in E/sub g /with decreasing Cu may result in part from lattice shrinkage as measured by X-ray diffraction (XRD). Optical absorption below the band edge appears to be dependent upon both Cu and Ga content. Absorption coefficients of alpha >or=10/sup 3/ cm/sup -1/ within this region are indicative of Cu-rich films. Absorption or=50% Ga. The magnitude of alpha varies from equivalent to 2*10/sup 4/ near the band edge tip to 10/sup 5/ cm/sup -1/ at 1 ev above the edge for near-stoichiometric films, with the absorption in Cu-poor films being slightly less.<>