Photo-darkening in GeSe film by CW laser irradiation

Abstract

Current interest in the chalcogenide compounds and their photo-darkening and photo-bleaching phenomena under laser irradiation approximated to their band gap energy show considerable impact on making many devices [1, 2]. Most of the available data on various chalcogenide compounds originated from optical measurements [3]. The fast annealing process induced by laser irradiation is unique in the sense that the amorphous state of chalcogenide compound may be retained in the transformation process where photo-structural changes affect the associated optical constants. Recently photo-darkening effect has been reported [4] in low temperature irradiation of GeSe film where the results are comparable to room temperature irradiation process. In the present work attempt has been made in preparing very thin films of a-GeSe of various stoichiometric proportions to find optical constants viz., refractive index (n), absorption coefficient (α), and optical gap energy (E g ) and to estimate their changes under laser irradiation (514 nm) near to the bandgap energy in order to explain the feasibility of the compound formation. The structural and morphological changes of the compound formed are observed through a simple optical transmission spectra. Pellets of pure Ge and Se (99.999%) were coevaporated via the Joule effect maintaining the near stoichiometric proportions on prepared cleaned quartz substrates under a vacuum ∼1.5× 10−3 Pa housed in a vacuum coating chamber model Polaran 150. The rate of deposition, 2–4 nm s−1, was maintained by an Edward FTM-5 quartz crystal monitoring unit. Different thin films of GeSe alloy between the thicknesses 300 and 500 nm under identical condition of deposition rate, vapor temperature were prepared. The amorphous nature of the films was verified by X-ray diffractograms (XRD) which were taken by CuKα radiation from Philips 1050 model. The structural information was recorded before and after photo-irradiation by optical polarizing microscope [5]. One representative film of thickness 335 nm was photo-irradiated by means of a CW Argon Ion Laser (Spectra Physics, Stabilite 2017) beam of diameter 1.5 mm at 514 nm wavelength of 1.5 W optical power for 20 s and scanned stepwise by a micropositioner in order to get a broader irradiated zone. The optical transmission and reflection spectra were recorded at normal incident wavelength from 400–2000 nm by a Shimadzu P101 spectrophotometer for the virgin and the photo-irradiated film of GeSe. The XRD pattern confirms the amorphous state of the GeSe film. The optical transmission spectra before and after laser irradiation of the a-GeSe film is shown in Fig. 1. The transmission plot (Fig. 1) has been grouped into transparent, weak and medium absorption zones to find the optical constants. Following Manificier et al. [6] and Swanepoel [7], let us explain the steps for evolving optical parameters. The transmission T is a complex function,