Epitaxial Growth of Mg_xCa_1-xO on GaN by Atomic Layer Deposition

We demonstrate for the first time that a single-crystalline epitaxial Mg_xCa_1-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 Mg_xCa_1-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 Mg_0.25Ca_0.75O has the lowest interfacial defect density. With this optimal oxide composition, a Mg_0.25Ca_0.75O/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.