Interfacial Cation-Defect Charge Dipoles in Stacked TiO/AlO Gate Dielectrics.

Layered atomic-layer-deposited and forming-gas-annealed TiO/AlO dielectric stacks, with the AlO layer interposed between the TiO 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 TiO/AlO interface dipole by the addition of another dipole of opposite sign. Density functional theory calculations indicate that the observed interface-dependent properties of TiO/AlO dielectric stacks are consistent in sign and magnitude with the predicted behavior of Al and Ti point-defect dipoles produced by local intermixing of the AlO/TiO layers across the interface. Evidence for such intermixing is found in both electrical and physical characterization of the gate stacks.