Optical properties of the ( CuIn Se 2 ) 1 − x − ( 2 ZnSe ) x system

Optical electronic transitions were studied in the ${(\mathrm{CuIn}{\mathrm{Se}}_{2})}_{1\ensuremath{-}x}\ensuremath{-}{(2\mathrm{ZnSe})}_{x}$ system from 0.5 to 14 eV. The system crystallizes in the chalcopyrite structure for $x\ensuremath{\le}0.43$ and in the zinc-blende structure for $x\ensuremath{\ge}0.48$. The absorption edge was found to be direct for all the compositions and the energy gap varies with $x$ parabolically, without discontinuity at the structural transition. This dependence is associated with the substitutional disorder in the mixtures. Reflection measurements showed substantial differences between the compositions with the chalcopyrite structure and those with the zinc-blende structure. The additional peaks observed for the compounds with the chalcopyrite structure are ascribed to pseudodirect transitions which become allowed by zone folding. The optical constants in the visible and ultraviolet regions were determined from the reflection spectra by Kramers-Kronig analysis. The dependence of the absorption bands on composition and structure is discussed and possible assignments are suggested. The effective number of electrons per atom contributing, in the energy range studied, to optical transitions was calculated and was found to depend strongly on the amount of Cu $d$ electrons in the mixture as well as on the crystal structure.