Effect of flow dynamics on the growth kinetics of CdS thin films in chemical bath deposition

During chemical bath deposition of CdS thin films, the solution is typically stirred to prevent unnecessary concentration gradient and precipitates sticking onto the film surface. Stirring rate changes the thickness of laminar boundary layer over the substrate, affecting the diffusion length of the reactants toward the substrate. It can also change hydrodynamic condition in the bath between laminar and turbulent flow. Interestingly, effect of stirring rate on the deposition kinetics of CdS thin films reported in the literature is conflicting. In this study, we showed that the diffusion-limited deposition kinetics has created a decreasing profile of film thickness along the sample as a function of boundary layer thickness when the substrate was under laminar flow and deposition temperature was between 45 °C and 63 °C. On the other hand, the deposition became feed-limited when the substrate was under turbulent flow and the deposition temperature was between 45 °C and 63 °C. While the hydrodynamic condition changed the deposition rate significantly, all the CdS films deposited showed a cubic crystal structure and the optical bandgap was slightly lower than that of single crystalline cubic CdS. The tensile stress in film probed by Raman spectroscopy can explain the low bandgap.