Effect of co-doping on performance of solution-processed InLiAlO thin film transistors

Abstract

Indium oxide thin film transistors have a moderate bandgap, high carrier concentration, and mobility, enabling their use in high currents. But they may be hard to turn off and have high leakage current, threshold voltage, and power consumption. These can be solved by co-doping, which can control the defects and quality of the films. In this study, InLiAlO thin films and TFTs with Li:Al ratios of 1:0, 2:1, 1:1, 1:2, and 0:1 (Li + Al = 10 at%) were prepared using spin-coating. It is found that both dopants significantly reduced defects and oxygen vacancies, and increased compactness. Moreover, with the increase of Al addition, the crystallization was inhibited. Therefore, the corresponding device performance should be improved. The undoped device had moderate mobility, low I_on/I_off, and high subthreshold swing (SS). The TFT doped with 10 at% Li exhibited higher mobility and lower SS, while the one with 10 at% Al doping showed decreased mobility, and improved I_on/I_off and SS. It is inferred that lithium doping promoted the transformation of hydroxides to oxides, forming more oxides and reducing hydroxide-related defects, but may generate oxygen vacancies. The aluminum doping suppressed the formation of oxygen vacancies and inhibited crystallization. After optimization, Li:Al = 2:1 is the best doping ratio, featuring a saturation mobility of 5.30 cm² V⁻¹·s⁻¹, a current on/off ratio of 7.48 × 10⁵, and a sub-threshold swing of 0.25 V/decade. The corresponding films have high smoothness and density, indicating high quality and fewer internal voids. It is suggested that co-doping is a promising strategy for improving the performance of indium oxide-based TFTs.