The optical and structural properties of CuGaSe2 polycrystalline thin films

The optical properties of CuGaSe₂ polycrystalline thin films deposited by multisource evaporation as a function of composition and substrate temperature are discussed in terms of the crystal structure and film morphology. Absorption coefficients were determined from total reflection and transmission measurements between 400 and 800 nm. Absorption humps in the absorption spectra are attributed to valence subband transitions, the magnitudes of which were related to the crystal structure of the films as determined by X-ray diffraction. Band splitting by crystal field and spin-orbit effects was observed at 1.741 and 1.932 eV in stoichiometric and Cu-rich films which contained mainly tetragonal chalcopyrite material. Cu-poor material, however, contained a predominance of the cubic sphalerite phase and exhibited the spin orbit effect alone at 1.992 eV. Also observed was a change in bandgap from 1.662 eV for stoichiometric to 1.718 eV for Cu-poor films.