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Influence of Thickness on the Optical Properties of Vacuum Evaporated CdSe Thin Films

Corresponding Author : M. Hossain (rubi_2602@yahoo.com)

Authors : J. Begum (rubi_2602@yahoo.com), T. Begum , A. Hannan

Keywords : Cadmium Selenide thin films, substrate temperature, extinction coefficient,, absorption coefficient, band gap energy

Abstract :

Cadmium Selenide (CdSe) thin films were deposited onto chemically and ultrasonically cleaned glass substrates by the thermal evaporation technique in high vacuum (∼10-5 mbar). The film thickness was varied from 50 to 250 nm keeping substrate temperature and rate of evaporation fixed at 1000C and 0.2-0.3 nm/sec, respectively. The thickness of the films was measured in situ by a frequency shift of quartz crystal (FTM5, UK). Dependence of the optical properties of the developed films on film thickness has been investigated. The optical properties of the films were ascertained by an UV-VIS-NIR spectrophotometer (Shimadzu, UV-3100, Japan) in the photon wavelength ranging between 300 and 2500 nm. The transmittance spectra show interference pattern with a sharp fall of transmittance at the band edge. It has been observed that the overall transmittance decreases with the increase in the film thickness. All the films have high absorption coefficient in the UV and visible region. In the NIR region the value of absorption coefficient decreases with the increase in the films thickness. The band gap energy of the films was calculated from the optical absorption spectra. The direct band gap energy was found to be in the range of 1.654 ~ 1.698 eV depending on the film thickness. The band gap energy was the maximum when film thickness was 50 nm. The direct band gap energy decreases with the increase of film thickness. It is observed that the refractive indices of the films decrease from 3.5 to 1.5 with increasing photon energy irrespective of film thickness. The extinction coefficient increases with increasing photon energy. The high absorption coefficient, nature and extent of band gap energy inspired us to consider the developed material to be prospective for solar photovoltaic applications.  

Published on December 30th, 2012 in Volume 20, Issue 3, Applied Sciences and Technology