Comparison of digital versus continuous growth techniques for MgCaO dielectric on GaN

The utility of crystalline ternary oxides based on magnesium oxide, MgO, as gate dielectric materials for GaN has been investigated. Deposition of these materials by gas-source molecular beam epitaxy from elemental metals and an oxygen plasma was found to produce epitaxial interfacial layers with excellent surface morphologies as evidenced by SEM and AFM. Two different growth sequences were employed, a continuous method where the shutters were open for the entire growth run and a digital alloy method where the Mg and Ca shutters were cycled. The digital approach was found to produce significant improvement in the compositional uniformity as determined by Auger electron spectroscopy (AES) and surface morphology as shown by atomic force microscopy (AFM). Neither growth method showed evidence of phase separation in the resultant films. X-ray diffraction shows a change in lattice parameter with the addition of calcium, which reduces the lattice mismatch to GaN. Structural characterization of the oxide/GaN interface as a function of oxide composition is also presented.