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 Al_Ti and Ti_Al 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.