Optical properties of the glassy thin films from Ge-Se-Ga system

Summary form only given. A study of the optical spectra of amorphous GeSe₄Ga_x and GeSe₅Ga_x chalcogenides in the spectral range 460 nm-1400 nm is performed.. Samples used were in a bulk and thin film form. Bulk glasses are prepared by monotemperature synthesis from elements with 4N purity in evacuated quartz ampoules. Thin films were deposited by vacuum evaporation at a residual pressure of 10^-3 Torr using optical glass as the film substrate. For the measurements the samples were placed in the cryostat and kept in vacuum. A 75 W halogen lamp with monochromatic light was used to illuminate the sample. The intensity of exident and transmitted light through the sample was measured with a silicon detector at the lower and a germanium detector at the longer wavelength. From the absorption spectra the optical energy gap (E_g ⁴) was determined. The optical-absorption edge is described using the non-direct transition model proposed by Tauc and the optical band gap-E_g^T is calculated from the absorption coefficient values by Tauc´s procedure. Optical absorption was described in the medium-absorption region, according to Urbach´s rule and the slope of the Urbach tail was calculated. The compositional dependence of the band gap was discussed. A relation between the average coordination number, composition and optical characteristics was obtained. From the transmission spectra, the optical constants were calculated using an optical-characterisation method based on creating the upper and the lower envelope curves of the spectrum. The values of the refractive index and absorption coefficient are consider as a function of the average coordination number (z) of covalent bonds per atom. The observed regularities are related to some structural pecularities in the ternary systems investigated