Al-doped ZnS thin films: Physical and electrochemical characterizations

Abstract

This study evaluated the effect of Al incorporation on the structural, optical, electrical and electrochemical properties of ZnS thin films. An electrochemical route was used to synthesize the studied un- and Al-doped ZnS thin films on indium tin oxide (ITO) glass substrates using an aqueous solution of ZnCl2 and Na2S2O3. The Al-doped ZnS samples were prepared by adding different amounts of AlCl3 to the main electrolyte. Cyclic voltammetry determined −0.9 to −1.1 V as the appropriate potential range for deposition of the ZnS thin films. In addition, appearance of a new reduction peak in the cyclic voltammograms of the Al-doped ZnS samples approved the presence of the Al dopant in the structure of the electrodeposited films. The recorded X-ray diffraction (XRD) patterns exhibited that all deposited samples contain cubic ZnS crystals and indicated that Al-doping decreases the crystallite size of the ZnS samples. Field emission scanning electron microscopy (FESEM) demonstrated that all samples are composed of big grains with 150–600 nm dimension distributed in a background phase including 15–60 nm grains. The photoluminescence (PL) spectra showed that all samples emit five PL peaks due to their different crystal defects. UV–Vis spectroscopy clarified that Al-doping decreases the band gap energy of the ZnS samples from 3.93 to 3.50 eV. Furthermore, it showed that the Al-doped ZnS samples present a lower capacitance and a shorter response time, which are suitable properties for application of these materials in fast photodetectors. The Mott-Schottky plots demonstrated n-type conductivity for all samples and revealed that the existence of the Al dopant increases the carrier concentration of the ZnS thin films. The electrochemical impedance spectroscopy (EIS) results confirmed that Al incorporation reduces the time constant and capacitance of the ZnS samples. Consequently, all results declared that Al incorporation has significant effects on the physical, optical and electrical properties of ZnS thin films.