Spectroscopy of Oxide-GaN Interfaces

Abstract

GaN-based devices are of interest for applications requiring high-frequency, high-power operation at elevated temperatures. As in traditional, silicon-based devices, integration of semiconducting phases with insulators is critical. Additionally, applications involving the integration of GaN with polar oxides such as perovskite ferroelectrics have been proposed, due to the coupling that may be achieved between the respective polar vector. Devices utilizing such a coupling behavior would make possible two-dimensional electron gases of high charge densities that could be modulated by the oxide’s polarization. The current status of oxide-GaN research is far behind that of oxide-Si research, and large-scale realization of GaN devices will require detailed understanding of oxide-GaN interfaces. This thesis focuses on the characterization of several oxide-GaN interfaces using x-ray photoelectron spectroscopy (XPS), as well as the identification of issues relating to the GaN surface. The rocksalt oxides MgO and CaO have been proposed as candidates for GaN MOSFET gate oxides, passivating layers, and buffer layers in GaN-ferroelectric structures. Thus, knowledge of film growth modes and band alignments is critical. Utilizing in-vacuo molecular beam epitaxy (MBE) and XPS, the growth of MgO on GaN was found to occur by the Volmer-Weber mode, with coalescence occurring at ~12 nm. This coalescence behavior was not found to affect the band alignment. As measured by XPS, the valence band offset at the MgO-GaN interface is 1.2 ± 0.2 eV, leading to a conduction band offset of 3.5 eV. A similar study was undertaken for the CaO-GaN system, in which more rapid coalescence was observed, leading to the conclusion of a Stranski-Krastanov growth mode. The difference in coalescence behavior is attributed to the increased reactivity of the CaO surface. The band offsets at the CaO-GaN interface were found to be 1.0 ± 0.2 eV at the valence band, and 2.5 eV at the conduction band. The band structures measured in this thesis are considered to be sufficient for limiting leakage current by Schottky emission for high-temperature devices. Surface chemical stability of rocksalt oxides is a known issue with respect to hydroxylation through water adsorption. XPS characterization of water uptake was performed using the O 1s photoelectron line after several in-vacuo exposures, culminating in a one-hour exposure to a water/oxygen mixture at 1 x 10-6 Torr. Characterization of polycrystalline MgO showed a saturating coverage of –OH groups at approximately 1 monolayer, regardless of exposure. CaO films exhibited increased reactivity, with hydroxyl coverage increasing to 3 monolayers, in addition to a similar amount of physisorbed water, suggesting the possibility for further reaction. Complete recovery of both oxide surfaces is shown to be achievable using mild vacuum anneals. Finally, the surface of GaN has been characterized with respect to several issues encountered during these investigations. GaN surfaces are found to be significantly Ga-rich, with surface stoichiometries routinely in excess of Ga2N. Several wet chemistries for GaN preparation were evaluated for their ability to modify the electrical behavior of subsequently grown oxide films. XPS could not unambiguously identify any change in surface chemistry that promotes these effects. Finally, p-type GaN films were noted to consistently possess greater oxide contamination in the as-grown state. Typical n-type or undoped GaN were marked by submonolayer quantities of oxide surface coverage, while p-type GaN typically exhibited coverages in the 1-2 nm scale. This difference has been found to be due to the p-type dopant activation anneal, during which GaN oxidation cannot be suppressed