Growth of High-Resistivity Wurtzite and Zincblende Structure Single Crystal Gan by Reactive-Ion Molecular Beam Epitaxy

ScAlMgO4: an Oxide Substrate for GaN Epitaxy

We report the use of ScAlMgO4 as a substrate for the epitaxial growth of wurzitic GaN. The low misfit (+1.8%) allows coherent epitaxy of GaN, as observed by RHEED. The congruent melting of ScAlMgO4 makes Czochralski growth possible, suggesting that large, high quality substrates can be realized. Boules about 17mm in diameter are reported. We have used nitrogen-plasma molecular beam epitaxy to grow GaN epitaxial films onto ScAlMgO4 substrates. Band-gap photoluminescence has been observed from some of these films, depending primarily on the deposition conditions. A 3×3 superstructure has been observed by RHEED on the GaN surfaces. Structural analysis by x-ray diffraction indicates very good in-plane alignment of the GaN films. We also report thermal expansion measurements for ScAlMgO4.

Growth and Doping of GaN Films by ECR-Assisted MBE

We report on growth, doping, and characterization studies of GaN films produced by the Electron Cyclotron Resonance microwave plasma assisted Molecular Beam Epitaxy. The films were grown heteroepitaxially on sapphire (0001), whose surface was converted into atomically smooth AIN by plasma nitridation. The GaN films were grown in two temperature steps, a process found to promote the layer-by-layer growth mode. ECR plasma conditions to grow either n-type autodoped or semi-insulating GaN film were identified. The structure and microstructure as well as the electrical properties of these two classes of films are discussed. A systematic dependence between electron mobility and net carrier concentration was found, which predicts that the mobility of GaN with a net carrier concentration of 1014cm−3 is about 104cm2 /V.s. The insulating films were intentionally doped either p-type or n-type by incorporation of Mg or Si during film growth. Hole or electron concentrations at 300K between 1018-1019cm−3 have been obtained without requiring any post-growth treatment.

Structural Defects and Their Relationship to Nucleation of Gan Thin Films

Microstructure and extended defects in α-GaN films grown by organometallic vapor phase epitaxy on sapphire substrates using low temperature AIN (or GaN) buffer layers have been studied using transmission electron microscopy. The types and distribution of extended defects were correlated with the film growth mode and the layer nucleation mechanism which was characterized by scanning force microscopy. The nature of the extended defects was directly related to the initial three-dimensional growth. It was found that inhomogeneous nucleation leads to a grain-like structure in the buffer; the GaN films then have a columnar structure with a high density of straight edge dislocations at grain boundaries which are less likely to be suppressed by common annihilation mechanisms. Layer-by-layer growth proceeds in many individual islands which is evidenced by the observation of hexagonal growth hillocks. Each growth hillock has an open-core screw dislocation at its center which emits monolayer-height spiral steps.