Observations on the influence of pH control on the properties of chemically deposited CdS films in an ammonia-free system

In this work we report on the properties of chemically deposited CdS thin films in an ammonia-free cadmium-sodium citrate system. We studied the influence on the properties of the films of the pH control of the reaction solution. For this, we deposited two types of CdS films employing two different types of reaction solutions. The only difference between both reaction solutions was the addition of a pH buffer in one of them in order to control its pH throughout the deposition process. Several sets of CdS films were deposited from growth solutions with different contents of Cd to determine also the influence of this parameter on the properties of the films. The CdS films were studied by X-ray diffraction, optical transmission and reflection spectroscopy and scanning electron microscopy measurements. We found that the properties of the films depend both on the amount of Cd in the growth solutions and on their pH control. The increase in Cd in the reaction solution yields to films with shorter lattice constant and then higher energy band gap. On the other hand, the pH control of the reaction produces higher deposition rate, larger final thickness and higher energy band gaps in the CdS films.