Optical Investigations of Fluorine-Doped Zinc Tin Oxide Thin Films Prepared by Ultrasonic Spray Pyrolysis Technique
Tin dioxide (TO) is currently gaining an increasing interest due to its promising incorporation into many photovoltaic applications. Despite the prolific literature on TO, whether in pure state or doped with Zn or F, the quaternary content of fluorine-doped zinc tin oxide was rarely investigated. This research cared to provide an inclusive treatment of this semiconductor oxide, starting from its novel synthesis by ultrasonic spray pyrolysis, and extending to analyzing its various structural, elemental, electric and optical properties.
The structural analysis by the x-ray diffraction (XRD) has revealed two opposing effects of doping with Zn and F, in terms of crystallite sizes and dislocation densities. These effects were reflected on the trends of free charge carrier concentration. The elemental x-ray photoelectron spectroscopy (XPS) was included to verify the existence of various types of oxygen vacancies and examine their possible role in optoelectronic behavior. Three charge states of oxygen vacancies were found, and their classification was determined by photoluminescence (PL) and optical transition analysis resulting from absorption spectra. This latter analysis revealed four direct transitions in the region 3.8-5.1 eV, which assigns the found defects to the neutral, singly- and doubly-ionized oxygen vacancies. At the end, the effects of Moss-Burstein and Urbach tails were tracked in FZTO. This study supported previous inspections that singly-ionized oxygen vacancies are attributed to dopants foreigner to the ZTO lattice. The study has also set a relation between Urbach tails and optical bandgap in ZTO, which was not known for this material before.