Structural, Optical and Photoconductive Properties of Chemically Deposited Nanocrystalline CdS Thin Films

Abstract

Nanocrystalline cadmium sulphide (CdS) thin films were prepared using chemical bath deposition (CBD), and the structural, optical and photoconductive properties were investigated. The crystal structure of CdS thin film was studied by X-ray diffraction. The crystallite size, dislocation density and lattice constant of CBD CdS thin films were investigated. The dislocation density of CdS thin films initially decreases with increasing film thickness, and it is nearly constant over the thickness of 2,500 <TEX>${\AA}$</TEX>. The dislocation density decreases with increasing the crystallite size. The Urbach energies of CdS thin films are obtained by fitting the optical absorption coefficient. The optical band gap of CdS thin films increases and finally saturates with increasing the lattice constant. The Urbach energy and optical band gap of the 2,900 A-thick CdS thin film prepared for 60 minutes are 0.24 eV and 2.83 eV, respectively. The activation energies of the 2,900 <TEX>${\AA}$</TEX>-thick CdS thin film at low and high temperature regions were 14 meV and 31 meV, respectively. It is considered that these activation energies correspond to donor levels associated with shallow traps or surface states of CdS thin film. Also, the value of <TEX>${\gamma}$</TEX> was obtained from the light transfer characteristic of CdS thin film. The value of <TEX>${\gamma}$</TEX> for the 2,900 A-thick CdS thin film was 1 at 10 V, and it saturates with increasing the applied voltage.