Optimized Al-doped TiO2 gate insulator for a metal-oxide-semiconductor capacitor on a Ge substrate

Abstract

Atomic layer deposited TiO₂- and Al₂O₃-based high-k gate insulators (GIs) were examined for the Ge-based metal-oxide-semiconductor field effective transistor (MOSFET) application. The single-layer TiO₂ film showed a too high leakage current to be used as a useful GI. Interposing a thin Al₂O₃ layer at the TiO₂/Ge interface was very effective in decreasing the leakage current. However, the resulting Al₂O₃/TiO₂ interface induced various adverse charge effects, including the density of the slow trapping sites, fixed dipole charge, while the Al₂O₃/Ge interface had a low density of fast carrier trapping sites, especially after the forming gas annealing (FGA) at 400 °C. Therefore, the Al₂O₃/TiO₂ stacked-layer could not be a viable GI on the Ge-substrate. In contrast, the Al-doped TiO₂ (ATO) film showed highly promising performance as the GI on the Ge-substrate. The 5.8-nm-thick ATO film resulted in an equivalent oxide thickness of 1.24 nm and leakage current density of 3.2 × 10⁻² A cm⁻² at a flat band voltage of −1 V, which was lower than that of the TiO₂ film by ∼three orders of magnitude. The slow trap density, fixed charge density, and interface trap density values of the ATO/Ge structure after the FGA were ∼1 × 10¹² cm⁻², ∼1.9 × 10¹¹ cm⁻², and ∼5 × 10¹¹ cm⁻² eV⁻¹, respectively. The k value of the ATO was ∼27.