Comparative study of aluminum and nickel contact electrodes for indium–tin–zinc oxide thin film transistors using oxygen vacancy diffusion model

We fabricate amorphous indium–tin–zinc oxide (ITZO) thin film transistors (TFTs), in which the source/drain (S/D) electrodes are composed of metallic Al and nickel Ni. We experimentally investigate the contact characteristics of Al and Ni S/D electrodes using the transmission line method. The ITZO with Al exhibits low field-effect mobility (≤4.8 cm²/Vs) and behaves like a conductor at short channel lengths. Even with a short channel length, the ITZO with Ni exhibits typical field-effect TFT behavior and high field-effect mobility (≥8.1 cm²/Vs), required for a high-resolution display. We present a mechanism to dope oxygen vacancies in the S/D regions to account for the differences in the properties of the ITZO TFTs with Al and Ni electrodes. Using a technology computer-aided design (TCAD) simulation, we show that the oxygen vacancies (V_O⁺) could diffuse from the Al and Ni S/D electrodes at a substrate temperature of 350 °C in 30 min, and find that their diffusion coefficient and activation energy are 2.8 × 10⁻⁸ cm²/s and 0.45 eV, respectively. Further, the S/D doping concentrations of V_O⁺ are 3.47 × 10¹⁹ and 1.55 × 10¹⁷/cm³ at the Al and Ni interfaces, respectively. The lateral diffusion profiles of V_O⁺ are different for the Al and Ni electrodes, which caused a difference in the drain current to gate voltage transfer characteristics of each electrode.