Extrinsic interface formation of Hf O₂ and Al₂ O ₃ /Ge O_x gate stacks on Ge (100) substrates

The extrinsic interfaces present at the HfO₂ /Ge O_x /Ge and Al₂ O₃ /Ge O_x /Ge gate stacks are investigated. The effective trapped charge density, estimated from hysteresis in capacitance-voltage characteristics, is higher for Hf O₂ than for Al₂ O₃, implying qualitatively different charge trapping sources in each dielectric. Spectroscopic ellipsometry and medium energy ion scattering measurements reveal that Hf O₂ deposition induces the formation of a thicker germanate (intermixed) layer at the Hf O₂ /Ge O_x interface, where nonstoichiometric Ge-rich Ge O_x having significantly low bandgap (∼1.8 eV) is present. In contrast, Al₂ O₃ deposition leads to an abrupt and thinner O-rich Ge O_x interfacial layer without Ge-rich Ge O_x phase. The proposed band alignment indicates that Ge-rich Ge O_x layer at Hf O₂ /Ge O_x arises a significant band potential well trapping, while O-rich Ge O_x layer in Al₂ O₃ /Ge O_x is responsible for a relatively lower charge trapping at band potential well. The combined results strongly suggest that the control of the Ge O_x interface layers is crucial to reduce the high charge trapping at high- κ/Ge O_x /Ge gate stacks.