Growth and optical properties of Cd₁-_xZn_xS thin films

Highly transparent cadmium zinc sulfide (Cd_1-xZn_xS) thin films were deposited by chemical bath deposition (CBD) under various ammonia and thiourea concentrations, which strongly affected the optical properties of Cd_1-xZn_xS films at different deposition stages: homogeneous reaction, atom-by-atom process and cluster-by-cluster deposition. Ammonia bound metal ions and, according to complex ion dissociation and equilibrium constants, released small concentration of ions. The hydrolysis of thiourea in the alkaline medium, provided by ammonia hydrolysis, released sulfide ion S^2- resulting in the formation of Cd_1-xZn _xS thin films. In the CBD deposition process, it was important to control the ions concentration released by these two agents. Otherwise, there would be immediate precipitation or existed a large amount colloids in the reactive bath preventing the deposition of thin films. The influences of various ammonia and thiourea concentrations on the growth mechanism and the optical properties of thin films were investigated. The transmission spectra (TS) and spectra-ellipsometry (SE) were measured and the data were modeled and fitted, giving the parameters which were further to estimate the quality of thin films. The optical properties and the parameters indicated that the ammonia concentration played an important role for obtaining high quality Cd _1-xZn_xS films. All parameters showed that it was more essential to control ammonia concentration in CBD deposition of high quality thin films, compared with thiourea.