On the optical constants of amorphous GexSe1-x thin films of non-uniform thickness prepared by plasma-enhanced chemical vapour deposition

The preparation of layers of amorphous GexSe1-x (with Ge atomic concentrati ons x = 0, 0.17, 0.25 and 0.34) by plasma-enhanced chemical vapour depositi on (PECVD) using the hydrides, GeH4 and H2Se, as precursor gases is describ ed in detail. Information concerning the structure of the films was obtaine d from Raman spectroscopy. The optical transmission was measured over the 3 00 to 2500 nm spectral region in order to derive the refractive index and e xtinction coefficient of these PECVD films. The expressions proposed by Swa nepoel, enabling the calculation of the optical constants of a thin film wi th nonuniform thickness, have successfully been applied. The refractive-ind ex dispersion data were analysed using the Wemple-DiDomenico single-oscilla tor fit. The optical-absorption edges have been all of them described using the 'non-direct transition' model proposed by Tauc. The optical gaps were calculated using Tauc's extrapolation, resulting in values ranging from 1.9 3 eV for a-Se to 2.26 eV for a-GeSe2. (C) 1998 Elsevier Science Ltd. All ri ghts reserved.