Morphology and optical analysis of defect levels in ultrasonically-sprayed zinc tin oxide thin films

Abstract

We prepared zinc-doped tin oxide (ZTO) films of various concentrations of zinc using the ultrasonic spray pyrolysis technique. The surface morphology and roughness were studied by Field-Emission Scanning Electron Microscope and atomic force microscope. Systematic x-ray diffraction (XRD) measurements revealed that all films have tetragonal rutile structure and mixed preferred orientation along the (200) and (110) planes. The proper substitution of zinc ions into tin sites and the absence of any secondary phase in the films were proved by the analysis of the x-ray photoelectron spectroscopy (XPS) measurements. The detailed analysis of the optical absorption spectra revealed the presence of four direct optical transitions within the energy range 3.8–5.1 eV. The defect levels of the neutral and double-ionized oxygen vacancy states were determined within the energy bandgap by the comprehensive analysis of the photoluminescence spectra. The existence of high concentration of oxygen vacancies is supported by the XPS results. The observed transitions were used to sketch the band diagram of ZTO films.