Effect of carbon doping on threshold voltage and mobility of In-Si-O thin-film transistors

In this study, a co-sputtering method with In2O3 and SiC targets was used to fabricate carbon-doped In-Si-O (In1-xSixO1-yCy) as the channel material for oxide thin-film transistors (TFTs). Three types of In1-xSixO1-yCy channels, namely, In0.88Si0.12O0.99C0.01 (Si0.12C0.01), In0.76Si0.24O0.99C0.01 (Si0.24C0.01), and In0.60Si0.40O0.98C0.02 (Si0.40C0.02), were prepared. After annealing at 300 °C, the Si0.24C0.01 and Si0.40C0.02 films retained an amorphous structure, while the Si0.12C0.01 films exhibited a body-centered-cubic structure. However, all the In1-xSixO1-yCy films maintained a smooth surface with a root-mean-square roughness of approximately 0.28 nm, despite structural differences. Results showed that the conductivities of all the In1-xSixO1-yCy films were not sensitive to the O2 partial pressure during sputtering, indicating that In1-xSixO1-yCy films exhibit more stable electrical conductivity than other InOx-based oxides. The field-effect mobility (μFE) with respect to the Si concentration of In1-xSixO1-yCy and In1-xSixO TFTs showed very similar behavior. In contrast, the threshold voltage (Vth) behavior of the two types varied dramatically, with the In1-xSixO TFTs Vth value increasing drastically from −57.7 to 9.7 V with increasing Si concentration, and the Vth of In1-xSixO1-yCy TFTs increasing only gradually from −9.2 to 2.4 V. This indicates that incorporated carbon has a significant effect on Vth at a low Si concentration due to strong C—O bond formation. The highest bond dissociative energy occurs between O and C atoms in the In1-xSixO1-yCy channel. The amount of oxygen vacancy in Si0.12C0.01, Si0.24C0.01, and Si0.40C0.02 was 18.9%, 13.3%, and 12.9%, respectively. As a result, the Si0.12C0.01 TFT exhibited superior transistor properties of Vth = −9.2 V while maintaining a μFE of 32.4 cm2/Vs. Therefore, an In1-xSixO1-yCy film is significantly advantageous as a channel material for oxide TFTs given that it can result in mobility exceeding 30 cm2/Vs.