Nondestructive investigation of interface states in high-k oxide films on Ge substrate using X-ray absorption spectroscopy

The unoccupied electronic structures of 5 nm thick high permittivity (k) oxides (HfO ₂, ZrO ₂, and Al ₂O ₃) and SiO ₂ films on Ge substrates were examined using O K-edge X-ray absorption spectroscopy. Comparative studies with those on Si substrates showed contrasts in the conduction bands, which should be due to the formation of interface states. In the Al ₂O ₃ and SiO ₂ films, GeO ₂ layers are formed at the interface and they suppress in part the formation of detrimental germanate phases. In contrast, in the HfO ₂ and ZrO ₂ films, no signature of the Ge-oxide phase is observed but some germanate phases are expected to prevail, suggesting a degradation of the gate oxide characteristics.