Structural, Electrical and Optical Characterization of ZnO:Li Thin Films Prepared by Sol-Gel Spin Coating

Abstract

Li-doped ZnO thin films prepared by sol-gel spin coating method, have been studied in this research to increase the p-type ZnO layers conduc-tivity. For this purpose, the lithium dopant concentration was changed and structural, electrical, and optical properties of the layers were in-vestigated. Structural investigations showed that the single phase layers with wurtzite hexagonal structure and 40-50 nm average grain size were formed. For obtaining p-type layers and enhancing their electrical conductivity, there was an optimum in moderate doping concentra-tions (in 0.4 Li/Zn ratio in sol). Furthermore, regarding the optical properties, it was concluded that the band gap energy is sensitive to Li con-centration and changes from 3.27 eV in pure layer to 3.16 eV in heavy Li doped layer. Besides, the refractive index of ZnO layer decreased with Li doping. This can be due to moth eye effect. Finally, a novel mechanism was developed by DFT which is in accordance with electrical and optical observations. These calculations propose the activation of a self-compensation mechanism by Li doping which can be responsible for the decrease of the conductivity at high Li concentrations. In the proposed mechanism, Zni donor defect sites, accompanying LiZn sites, cause self-compensation.