Person:

Gong, Xian

We demonstrate for the first time that a singlecrystalline epitaxial MgxCa1−xO film can be deposited on gallium nitride (GaN) by atomic layer deposition (ALD). By adjusting the ratio between the amounts of Mg and Ca in the film, a lattice matched MgxCa1−xO/GaN(0001) interface can be achieved with low interfacial defect density. High-resolution X-ray diffraction (XRD) shows that the lattice parameter of this ternary oxide nearly obeys Vegard’s law. An atomically sharp interface from cross-sectional transmission electron microscopy (TEM) confirmed the high quality of the epitaxy.High-temperature capacitance−voltage characterization showed that the film with composition Mg0.25Ca0.75O has the lowest interfacial defect density. With this optimal oxide composition, a Mg0.25Ca0.75O/AlGaN/GaN metal−oxide −semiconductor high-electron-mobility (MOS-HEMT) device was fabricated. An ultrahigh on/off ratio of 1012 and a near ideal SS of 62 mV/dec were achieved with this device.

Atomic layer deposition (ALD) processes were developed for growing MgO and CaO on GaN substrates using a home-built ALD reactor with amidinate-based precursors. An optimal deposition condition was then determined to deposit pure MgxCa1–xO by tuning deposition temperature, achieving a high-quality film without carbon and hydroxyl contamination. Band offset analysis was conducted by X-ray photoelectron spectroscopy on MgxCa1–xO/GaN samples with various compositions. Mg0.25Ca0.75O/GaN demonstrated band alignments between defect-free and fully pinned, while other ratios showed near fully pinned alignments, which agree with the best electrical properties achieved in our previous studies. This result indicates that band offset analysis provides a good aid in understanding part of the electrical and interfacial properties for oxide/semiconductor structures.