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

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.