Extrinsic interface formation of HfO2 and Al2O3∕GeOx gate stacks on Ge (100) substrates

The extrinsic interfaces present at the HfO2∕GeOx∕Ge and Al2O3∕GeOx∕Ge gate stacks are investigated. The effective trapped charge density, estimated from hysteresis in capacitance-voltage characteristics, is higher for HfO2 than for Al2O3, implying qualitatively different charge trapping sources in each dielectric. Spectroscopic ellipsometry and medium energy ion scattering measurements reveal that HfO2 deposition induces the formation of a thicker germanate (intermixed) layer at the HfO2∕GeOx interface, where nonstoichiometric Ge-rich GeOx having significantly low bandgap (∼1.8eV) is present. In contrast, Al2O3 deposition leads to an abrupt and thinner O-rich GeOx interfacial layer without Ge-rich GeOx phase. The proposed band alignment indicates that Ge-rich GeOx layer at HfO2∕GeOx arises a significant band potential well trapping, while O-rich GeOx layer in Al2O3∕GeOx is responsible for a relatively lower charge trapping at band potential well. The combined results strongly suggest that the control of the GeOx interface layers is crucial to reduce the high charge trapping at high-κ∕GeOx∕Ge gate stacks.