Growth of ScN(111) on Sc_2O_3(111) for GaN integration on Si(111): Experiment and ab-initio calculations

摘要

Growth mechanism of ScN on Sc_2O_3 for integration of Ga-polar GaN on Si(111) is investigated by in-situ X-ray photoemission spectroscopy, ex-situ time-of-flight secondary ion mass spectrometry, atomic force microscopy, and ab-initio density functional theory (DFT) calculations. The ScN films are grown by molecular beam epitaxy from e-beam evaporated Sc and N plasma. The films grow in a layer-by-layer (Frank-van der Merwe, FM) fashion. Diffusion of nitrogen into Sc_2O_3 and segregation of oxygen onto ScN are observed. The segregated O atoms are gradually removed from the surface by N atoms from the plasma. Experiment and theory show that nitrogen cannot be efficiently incorporated into Sc_2O_3 by exposing it to N plasma alone, and calculations indicate that anion intermixing between ScN and Sc_2O_3 should be weak. On the basis of ab-initio data, the in-diffusion of N into Sc_2O_3 is attributed mostly to the effect of interaction between ScN ad-dimers on the Sc_2O_3 surface in the initial stage of growth. The segregation of O to the ScN surface is understood as driven by the tendency to compensate build-up of the electric field in the polar ScN film. This segregation is computed to be energetically favorable (by 0.4 eV per O atom) already for a monolayer of ScN; the energy gain increases to 1.0 eV and 1.6 eV per O atom for two and three ScN layers, respectively. Finally, it is verified by DFT that the ScN deposition method in which Sc metallic film is deposited first and then nitridized would lead to strong incorporation of O into the grown film, accompanied by strong reduction of the Sc_2O_3 substrate.