Effect of Active Layer Thickness on Device Performance of Tungsten-Doped InZnO Thin-Film Transistor

Tungsten (~4 at. %)-doped InZnO thin-film transistors were fabricated as a function of the active layer thickness using an RF sputtering system. To explain the degradation of the device performance in relation to the changes of the active layer thickness, the correlations between the device performance and the physical properties, including the film density, surface/interface roughness, band edge state below the conduction band, refractive index, and composition along the depth direction were investigated. Tungsten-doped indium-zinc oxide (WIZO) TFTs with active layer thickness of 10 nm exhibited the highest field effect mobility of 19.57 cm2/Vs and the lowest threshold voltage shift of 0.62 V. The enhancement of the device performance is strongly correlated with the highest film density and a flat interface roughness of SiO2-WIZO. In addition, interface layer thickness and band edge states below the conduction band were changed with increasing active layer thickness. These remarkable changes in the interface layer thickness and band edge state could be correlated to changes in the device performance.