An insight into doping mechanism in Sn-F co-doped transparent conducting ZnO films by correlating structural, electrical and optical properties

On the face of massively growing market of transparent optoelectronics, developing ZnO-based transparent conductive thin films as a promising substitute for indium-free transparent electrode is extremely important. However, the detailed function of the dopants, especially co-dopants acting on the electrical and optical properties of ZnO-based transparent conductive thin films is not clear yet. We present a detailed comparative investigation on the structural, electrical and optical properties of pulsed laser deposited ZnO thin films co-doped with Sn and F for the first time. An unexpected expansion in the lattice structure has been observed when Zn²⁺ are replaced by Sn⁴⁺ having smaller ionic radius. Electrical measurements show that there is no anticipated change in the carrier concentration with the dopant concentration. A minimum resistivity of 2.56 × 10^-3 Ohm-cm with a carrier concentration of 4.41 × 10²⁰ cm^-3 has been obtained for 1 at.% each Sn-F co-doped film. Most interestingly, a significant improvement in the ultraviolet (UV)/visible (VIS) photoluminescence peak intensity in Sn doped and Sn-F co-doped films in correlation with the structural and electrical properties allows us to propose that Sn doping into ZnO lattice causes a screening of the native Zn vacancy defects. While the presence of F co-dopant induces Sn²⁺ to occupy the lattice sites, as evidenced from the lattice expansion, an insignificant increase in the carrier concentration as well as enhanced UV emission of the co-doped films. The results obtained in this study shed light on the development of ZnO-based transparent electrodes.