Effect of Al/N flux ratio during nucleation layer growth on the microstructure of GaN films grown by molecular-beam epitaxy

The microstructure of AlN nucleation layers grown on semi-insulating 4H-SiC substrates by plasma-assisted molecular beam epitaxy has a major effect on subsequent GaN and AlGaN/GaN film quality and electronic properties. Characterization by transmission electron microscopy shows that the morphology of the GaN films is critically dependent on the Al to active nitrogen flux ratio used during nucleation layer growth. Structures grown with Al/N flux ratios slightly above approximately 0.98 have many basal-plane stacking faults in the AlN nucleation layer, the GaN/AlN interfaces are wavy and ill defined, and regions of cubic GaN exist close to the GaN/AlN interfaces. In comparison, for structures grown with lower Al/N flux ratios, the AlN nucleation layers have no stacking faults and the GaN/AlN interfaces are sharp. The impact of these changes in microstructure on the electron Hall mobility and buffer leakage are discussed.