Interfacial Cation-Defect Charge Dipoles in Stacked TiO2/Al2O3 Gate Dielectrics.

Layered atomic-layer-deposited and forming-gas-annealed TiO2/Al2O3 dielectric stacks, with the Al2O3 layer interposed between the TiO2 and a p-type germanium substrate, are found to exhibit a significant interface charge dipole that causes a ∼-0.2 V shift of the flat-band voltage and suppresses the leakage current density for gate injection of electrons. These effects can be eliminated by the formation of a trilayer dielectric stack, consistent with the cancellation of one TiO2/Al2O3 interface dipole by the addition of another dipole of opposite sign. Density functional theory calculations indicate that the observed interface-dependent properties of TiO2/Al2O3 dielectric stacks are consistent in sign and magnitude with the predicted behavior of AlTi and TiAl point-defect dipoles produced by local intermixing of the Al2O3/TiO2 layers across the interface. Evidence for such intermixing is found in both electrical and physical characterization of the gate stacks.