Doping effect on the structural and optical properties of some chalcogenide semiconductors

Abstract

In the present work, we discuss the experimental results of structural, compositional and optical properties of CdS thin films prepared by spray pyrolysis technique at different substrate temperatures and different flow rates and Ag doped CdS thin films prepared by closed space sublimation (CSS) technique at different immersion time. The compositional structural and optical properties of the deposited films and Ag doped CdS films were studied using Energy Dispersive X-ray (EDX) analysis, x ray diffraction (XRD) and UV-VIS- NIR double beam spectrophotometer.

EDX analysis shows that Cd, S and Ag are present in the investigated films. Also it is seen that Cd and S for the as deposited films are very close to the ideal stoichiometry with slight sulfur deficiency, whereas, the Ag at.% for CdS films immersed in AgNO3 solutions increases as the periods of the immersion time increase.

XRD patterns indicated that the prepared films are nanocrystalline hexagonal structure with preferetional orientation along (002) plane. The deposits have a close stoichiometric composition irrespective of substrate temperature and flow rates. It was also observed that the X- ray diffraction patterns of Ag doped CdS thin films for different immersion time following by annealing at 400oC for 1 h have the same feature.

The SEM of the as deposited CdS films consist of small particles distributed uniformly through the investigated regions. It is seen from SEM of Ag doped CdS thin films that there are white regions observed on to the surface of Ag doped CdS thin films. Those regions correspond to agglomerate of Ag particles, which form due to immersed the CdS films in to AgNO3 solutions or because the thermal difference between the film and substrate. The area of those regions was found to be increase as the period of the immersion time increase. The selected area for pure as well as Ag doped films shows continuous ringes of different intensities confirming the polycrystalline structure.

The optical transmission T and reflection R spectra of pure CdS and Ag doped CdS films exhibit an oscillatory behavior, which the reflection and transmission method can be applied to calculate the refractive index (n) , extinction coefficient (κ) and dielectric constant (ε1 and ε2). The optical energy band gap of CdS films and Ag doped CdS films deposited at different substrate temperatures, different flow rates and different immersion time is found to be direct allowed band to band transition and increase from 2.42 to 2.53 eV by increasing substrate temperatures from 300 oC to 450 oC whereas decrease from 2.35 eVto 2.24 eV by increasing immersion time.

Urbach energy (Eu) which corresponds to the width of the band tail decreases from 0.220 eV to 0.181 eV for flow rate 2 ml/min and decrease from 0.202 eV to 0.140 eV for flow rate 3 ml/min by increasing temperature from 300 oC to 450 oC but the value of Eu increase from 0.245 eVto 0.299 eV with increasing immersion time.

Extiction coefficient (κ) and refractive index (n) of CdS films is found to decrease with increasing substrate temperatures and flow rate but increase with increasing immersion time.