The γ-irradiation Effect on the Optical Properties of CdTe Thin Films Deposited by Thermal Evaporation Technique

Abstract

CdTe thin films of 50 nm thickness have been deposited on glass substrates by thermal vacuum evaporation technique and the γ-irradiation effect on their optical properties has been investigated. Transmittance and reflectance have been measured at normal incidence, in the visible region from 400 to 700 nm, and were used to evaluate the optical parameters and the band gap energy for CdTe thin films before and after γ-irradiation. Structure and surface morphology before and after γirradiation were investigated using X-ray diffraction (XRD) analysis and scanning electron microscope (SEM) respectively. XRD pattern of the as-deposited CdTe film shows an amorphous nature. Crystalline structure was observed to improve when samples were exposed to γ-irradiation at doses from 40-120 kGy in the (111) plane of the cubic zincblende structure. SEM shows a change in the surface morphology of CdTe films as a consequence of γ-irradiation compared to as-deposited sample. The optical properties such as refractive index, extinction coefficient, dielectric constant were found to decrease with the increase of the dose of γ-irradiation. Energy band gap was found to decrease from 2.47 eV for the as-deposited down to 2.20 eV for 120 kGy (blue shift compared to bulk CdTe), that was attributed to the small thin film thickness.

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