Optical transitions and photoluminescence of fluorine-doped zinc tin oxide thin films prepared by ultrasonic spray pyrolysis

Thin films of fluorine-doped zinc tin oxide (FZTO) were deposited on borosilicate glass by ultrasonic spray pyrolysis. Structural and elemental investigations revealed that all films have a single-phase polycrystalline tetragonal rutile structure and proved the successful incorporation of zinc ions into tin sites and the fluorine ions into the sites of oxygen vacancies. A comprehensive analysis of the optical transitions in FZTO films was presented. All films show high optical transparency (>= 85%) in the visible range. Four direct optical transitions were observed in the range 3.8 - 5.1 eV by spectrophotometric measurements, in addition to three transitions identified by photoluminescence. From these transitions observed for the first time in FZTO, three states of point defects were determined in the bandgap and attributed to the neutral, singly- and doubly-ionized oxygen vacancies. The ascription of these states to oxygen vacancies was supported by the x-ray photoelectron spectroscopy measurements. The band diagram of FZTO was sketched, demonstrating the observed transitions. The obtained results are expected to have their imprint on understanding the optical processes in FZTO and evaluating the future utility of it in various optoelectronic applications.