Preparation and Properties of Cadmium- Zinc Chalcogenide Thin Films for Device Applications

Abstract

The present work described in this thesis involves studies on cadmium selenide, both undoped and doped along with a study of ternary compounds Cd(1-x)Zn(x)Se and CdSe(1-x)Te(x) in thin film form obtained by depositing bulk compounds on glass substrates by thermal evaporation technique. The structural, morphological, optical and electrical properties were studied in order to understand the effect of doping and alloying with the intention of using these compounds in band gap engineering. The interest in these compounds is due to the fact that they are direct band gap materials and have their band gap energies in an attractive range of values. CdSe films were obtained in stoichiometric composition by varying deposition parameters such as substrate temperature and annealing duration. These films exhibit polycrystalline hexagonal crystal structure with a preferred growth along (002) direction. The optical band gap and electrical conductivity were found to be 1.68 eV and 10-4 /Ω-cm respectively. Further, CdSe thin films were doped with pure metals such as silver, bismuth and indium separately by varying the dopant concentration from 0-3%. The maximum electrical conductivity was obtained for 3% In doped CdSe thin films. Photocurrent measurements were carried out for undoped and doped CdSe thin films. 3% Bi doped CdSe thin films showed maximum photocurrent of 1.38 μA. The effect of annealing duration on the formation of homogenous single phase ternary alloys of Cd(1- x)Zn(x)Se and CdSe(1-x)Te(x) thin films was studied. The band gap energy value was determined in both ternary systems by varying the composition x = 0, 0.2, 0.4, 0.6, 0.8 and 1. Heterojunction of undoped CdSe/p-Si, Ag doped CdSe/p-Si, Bi doped CdSe/pSi and In doped CdSe/p-Si were fabricated and studied to consider their usefulness in optoelectronic devices. The electrical conduction in all the four diodes was found to takes place by thermionic emission at lower voltages and by space charge limited conduction mechanism at higher voltages. Among four diodes Bi doped CdSe/p-Si heterojunction has high rectification ratio of 894.1at ±4 V.