Structural and optical properties of thermally evaporated cadmium thiogallate CdGa₂S₄ nanostructure films

Ain Shams University, Faculty of Education, Physics Department, Thin Film Laboratory, Cairo, Egypt

Corresponding author: amall56@yahoo.com

Received: 16 July 2009
Revised: 7 March 2010
Accepted: 30 June 2010
Published online: 22 September 2010

Abstract

Nano structure films of cadmium thiogallate CdGa₂S₄ have been prepared by a conventional thermal evaporation technique (at substrate temperature = 303 K). These films were deposited on both glass and quartz substrates. X-ray diffraction measurements showed that CdGa₂S₄ compound in the powder form has a polycrystalline nature with a tetragonal structure. The as-deposited film was annealed at 673 K for 2 h under vacuum 10^-3 Pa and was irradiated by 10 kGy γ_co rays. This resulted in a transformation to nanostructure grains of CdGa₂S₄ thin films. Transmission electron microscopy was carried out for all of the investigated films, which also confirmed that those films could be transformed to nanostructure grains. Optical properties of the CdGa₂S₄ films under investigation were examined using spectrophotometric measurements of transmittance and reflectance at normal incidence in the wavelength range 400–2500 nm. It was found that both refractive index n and absorption index k changed with the heat and irradiation treatments. The dispersion of refractive index in CdGa₂S₄ was analyzed according to the single oscillator model. Some dispersion parameters were determined for all investigated films. The calculated values of β (which is defined as the parameter used to determine the type of crystal) indicate that CdGa₂S₄ belongs to the covalent class for all films investigated. The ratio of the free carrier concentration to the effective mass N/m* was also determined. The analysis of the absorption coefficient indicated that this ternary defect chalcopyrite compound has both direct and indirect transitions in relevance to the energy gaps E_g1^dir, E_g2^dir and E_g^ind, respectively. These energy values decreased by irradiation, while they increased by annealing.