Introduction of a thin GeO₂ as the interfacial layer (IL) seems to be beneficial for formation of high-k gate stacks on germanium substrate. The thickness of GeO₂/GeO_x impacts the overall equivalent oxide thickness (EOT) because of its low dielectric constant compared to high-k layer. Current trends indicate that the formation of appropriate thickness of GeO_x or GeO₂ between high-k and Ge substrate seems to have low interface state density (D_it). Since ZrO₂ showed lower leakage current than HfO₂ with similar dielectric high-k stack, ZrO₂/Al₂O₃ bilayer as gate dielectric is becoming popular. Even though excellent gate stacks on germanium substrate has been demonstrated the reliability and the impact of interfacial layer is still unknown.

This work investigates the p-Ge/Al₂O₃/ZrO₂/TiN gate stacks with thin GeO₂ or GeO_x layer, formed when the gate stack was exposed to pre- or post-deposition slot plane antenna plasma oxidation (SPAO). The post Al₂O₃/ZrO₂ deposition SPAO forms relatively thicker GeO₂ and post Al₂O₃ deposition SPAO forms thinner GeO₂ whereas pre-deposition SPAO forms thin fragmented GeO_x layer at the interface. We have observed that the relatively thicker GeO₂ interfacial layer formed after the post Al₂O₃/ZrO₂ deposition SPAO degraded faster during time dependent dielectric breakdown (TDDB) measurements on substrate injection mode compared to the thin GeO₂ or GeO_x interfacial layers (Fig. 1). The improvement in TDDB degradation for thinner interfacial layer thickness suggests that electrons tunnel through the thinner GeO_x IL, and TDDB measured the quality of Al₂O₃ rather than IL.

The TDDB characteristics in gate injection mode suggests that the SPAO can significantly affect the TDDB characteristics GEI mode (Fig. 1). The SPAO treatment effectively removes the trap centers in ZrO₂ and Al₂O₃ layers when SPAO was performed after the gate stack deposition. By employing carrier transport mechanisms as a function of temperature in both gate and substrate injection mode we were able to identify the traps that contributes to the TDDB degradation. The trap center (f_t1 = 0.13 eV) observed in the ZrO₂ when ZrO₂ is not subjected to the SPAO, i.e. pre-deposition SPAO. This trap f_t1 (0.13 eV) was eliminated for post Al₂O₃/ZrO₂ deposition SPAO. While defects in the ZrO₂ layer determines the TDDB characteristics in gate injection mode the GeO₂ or GeO_x interfacial layer thickness determines the TDDB degradation in substrate injection mode.