Optical transitions near the band edge in bulk CuInxGa1−xSe2 from ellipsometric measurements

From the analysis of the variation of optical absorption coefficient α with incident photon energy between 0.8 and 2.6 eV, obtained from ellipsometric data, the energy EG of the fundamental absorption edge and EG′ of the forbidden direct transition for CuInxGa1−xSe2 alloys are estimated. The change in EG and the spin-orbit splitting ΔSO=EG′−EG with the composition x can be represented by parabolic expression of the form EG(x)=EG(0)+ax+bx2 and ΔSO(x)=ΔSO(0)+a′x+b′x2, respectively. b and b′ are called “bowing parameters”. Theoretical fit gives a=0.875 eV, b=0.198 eV, a′=0.341 eV and b′=−0.431 eV. The positive sign of b and negative sign of b′ are in agreement with the theoretical prediction of Wei and Zunger [Phys. Rev. B 39 (1989) 6279].