Deposition of zinc sulphide films from thiourea complexes and a study of their optical properties
Abstract
This work presents the results of a study of zinc sulphide films deposited by aerosol pyrolysis from aqueous solutions of thiourea complex compounds [Zn(N2H4CS)2Cl2] and [Zn(N2H4CS)2Br2] in the temperature range of 350–500 °C.
The IR and Raman spectra of zinc complexes were studied. It was determined that in the studied complexes, the thiourea molecule was coordinated to the metal cation through the sulphur atom. In the low-frequency Raman region (n < 400 cm–1), we recorded the bands characterising the vibrations of the zinc-sulphur and zinc-chlorine (bromine) bonds of the studied complex compounds in the Raman scattering spectra. The optical properties of zinc sulphide films were studied using optical spectrophotometry. Based on the absorption spectra, the optical band gap of ZnS films was determined. It was 3.67–3.74 eV and 3.63–3.70 eV for the samples deposited from [Zn(N2H4CS)2Cl2] and [Zn(N2H4CS)2Br2] complexes, respectively.
We recorded a decrease in the band gap of the synthesised layers upon an increase in the deposition temperature. It is due to changes in their defect structure.
One of the main types of defects in the ZnS films deposited from [Zn(N2H4CS)2Cl2] and [Zn(N2H4CS)2Br2] complexes is a halogen atom in the anion sublattice of the sulphide (ClS˙, BrS˙). As the deposition temperature increases, the content of these defects in the films decreases due to the complete destruction of Zn–Cl and Zn–Br bonds and volatilisation of halogen during the thermolysis of the complexes. Oxygen (OSх) occupies the vacated places of ClS˙, BrS˙. The films contained oxygen as they were synthesised in an oxidising atmosphere and due to partial hydrolysis of the initial zinc salt. An increase of oxygen content in the samples upon an increase of the deposition temperature results in a decrease of the optical band
gap of the ZnS films.
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