Temperature dependence of the structural and optical properties of the amorphous-to-crystalline transition in CdGa2Se4 thin films

Nearly stoichiometric thin films of CdGa2Se4 were deposited, by conventional thermal evaporation of presynthesized ingot material, onto quartz and Corning glass substrates. The chemical composition of the samples was determined by means of energy-dispersive X-ray spectrometry. X-ray diffraction studies revealed that the as-deposited and annealed films at Ta=473 and 573 K have an amorphous phase, while those annealed at Ta≥673 K are crystalline with a single phase of a defective chalcopyrite structure similar to that of the synthesized material. The unit cell lattice parameters of both the synthesized material and the crystalline films were determined and compared with reported data. The optical constants (n, k) and the film thickness (t) of the films were determined from optical transmittance data using the Swanepoel method. The dispersion parameters were determined from an analysis of the refractive index. An analysis of the optical absorption spectra revealed a non-direct energy gap characterizing the amorphous films, while a direct energy gap characterized the crystalline films. The annealing temperature dependence of the corresponding energy gaps was found to fit a polynomial equation. (copyright 2004 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)