Influence of EDTA concentration on the structure and properties of SnS films prepared by electro-deposition

Tin sulfide (SnS) thin films were deposited onto indium tin oxide (ITO) glass substrates by cathodic electro-deposition from aqueous solution containing ethylene diamine tetraacetate acid (EDTA). Because EDTA can slow the deposition rate of Sn through formation of Sn chelates, it is possible to obtain stoichiometric SnS films with good quality by adding EDTA to the deposition bath. The deposited films were characterized with X-ray diffraction (XRD), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy (Raman), and ultraviolet–visible–near infrared (UV–VIS–NIR) spectrophotometer. The as-deposited films are mainly polycrystalline SnS with orthorhombic crystalline structure, and they show good uniformity and surface coverage with root mean square (RMS) roughness of 45.36–62.39 nm and grain sizes of 100–300 nm. Raman microscopy shows that the films have bands at around 190 and 218 cm−1 belonging to Ag mode of SnS. The concentration ratio of EDTA and Sn2+ (EDTA/Sn2+) has some influence on the structure, phase, Raman shift and optical properties of the deposited films. When the EDTA/Sn2+ is less than 0.5, the films have a Raman shift at around 306 cm−1 due to Sn2S3. XPS analysis also shows that there exists a Sn2S3 phase in the deposited films. When the EDTA/Sn2+ equals 1/1, there is only the SnS phase in the deposited films. With an increase of the EDTA/Sn2+ from 0.1 to 1, the direct band gap of the films is decreased from 1.75 eV to 1.43 eV. Therefore EDTA/Sn2+=1/1 is good for depositing SnS films.