Composition effects on structural and optical infrared properties of CuIn0.5Ga0.5Se2

The dependence of structural parameters and force constants for Cu-Se, Ga-S e, and In-Se bonds on compositional deviations in CuIn0.5Ga0.5Se2 have been studied. The composition gradient along the ingot was obtained by a single fusion at 1150 degrees C of the components and subsequent slow cooling in a still ampoule placed in a vertical furnace. All along the sample, a singl e chalcopyrite phase is present and its composition along its length was fo und by energy dispersive analysis of x-ray measurements on slices. Unit cel l parameters, anion displacement, and Cu occupation fraction in its sublatt ice were analyzed by x-ray powder diffraction and Rietveld refinement metho ds. The anion displacement found is a function of the Cu defect in its subl attice. The existence of associated defects, i.e., two Cu vacancies and one Ga in Cu site, [2V(Cu)+Ga-Cu], is proposed to explain the Cu defect in its sublattice and the changes in lattice parameters. This leads to the existe nce of B-III vacancies (B-III=In+Ga), and interstitial Cu up to 8 at. % tha t also cause changes in the structural parameters. Infrared reflectance mea surements led to the imaginary dielectric constant determination which, fit ted to a Lorentz function, permitted to obtain atomic vibration modes. Usin g the model of Neumann for chalcopyrites, the values of force constants for Cu-Se, Ga-Se, and In-Se bonds were computed. These appear to increase when the occupation of each sublattice increases. (C) 2000 American Institute o f Physics. [S0021-8979(00)08412-7].