Bath pH Dependence on the Structural and Optical Properties of Chemical Bath Deposited CdS Thin Films

Abstract

High quality CdS thin films made with ultrasonic agitation during chemical bath deposition were investigated. The change in band gap and variation of atomic stacking during the film growth was observed as a function of bath pH. The band gap of as-deposited thin films of CdS was found to be between 2.26 eV and 2.50 eV, the highest being observed when the pH was 9.0. The transition from cubic (β-CdS) to hexagonal (a-CdS) was observed with decreasing pH. The lattice parameters of CdCl₂ treated CdS were found to be a = 0.414 nm and c = 0.672 nm. Calculations based on the Sherrer formula showed significant grain growth after annealing with CdCl₂. No apparent effect of ultrasonication on crystalline structure of CdS was seen in this method, although ultrasonication was noted to produce films with a higher quality optical surface. A maximum bath temperature was fixed at 70°C in order to suppress the rate of homogeneous reaction and minimise the evaporation of ammonia from the chemical bath.