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300-nm-thick AlN layers without a nucleation layer were grown on 6H-SiC (0001) vicinal substrates with 3-bilayer-height steps by rf-plasma-assisted molecular-beam epitaxy. A Ga beam was supplied for 0 to 30 seconds just before growth of AlN. The Ga pre-irradiation for 7 seconds (∼1.6 ML) was found to be effective for realization of the AlN layer-by-layer growth mode at an earlier stage of the growth and reduction of threading dislocation densities (TDD) in the AlN layers. The screw-type and edge-type TDDs were ∼106 cm-2 and 6×108 cm-2, respectively. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

250 nm thick AlN layers without a nucleation layer were grown directly on 6H-SiC(0001) with 3-bilayer-height steps by rf-plasma-assisted molecular-beam epitaxy. The structure and morphology of the AlN layers have been studied using atomic force microscopy, X-ray diffraction and transmission electron microscopy. Two different types of unique defect structures were observed. Rows of pure-edge-type threading dislocations were observed along the pre-existing step-edges of the SiC substrate for AlN grown on as-gas-etched SiC sub- strates, while the planar defects threading through the AlN layer were observed at the step-edges of the substrate for AlN on SiC with sacrificial oxidation. We concluded that these planar defects were the stacking mismatch boundaries due to the difference in stacking sequence of AlN layers on different SiC terraces.

Impact of step height of silicon carbide (SiC) substrates on heteroepitaxial growth of aluminum nitride (AlN) was investigated. Step-and-terrace structures with various step heights, 6 monolayer (ML), 3ML and 1ML, were formed on 6H–SiC (0 0 0 1) vicinal substrates by high-temperature gas etching. 2H–AlN layers were grown on the substrate by plasma-assisted molecular-beam epitaxy (MBE) and then these layers were characterized by atomic-force microscopy (AFM) and X-ray diffraction (XRD). High-quality AlN can be grown on SiC substrates with 6ML- and 3ML-height step, while AlN grown on SiC substrates with 1ML-height step exhibited inferior crystalline quality. A model for high-quality AlN growth on SiC substrates with 3ML-height step is proposed.

We grew heavily Mg-doped GaN using ammonia molecular-beam epitaxy. The use of low growth temperature (740 °C) allows decreasing the incorporation of donor-like defects (

β-Ga2O3(010) homo-epitaxial growth was performed by plasma-assisted molecular beam epitaxy. Under Ga-rich conditions and for growth temperatures above 650 °C, the growth rate was independent of the Ga/O ratio (>1). A high growth rate of 2.2 nm/min for β-Ga2O3(010) was achieved by optimizing the O flux between 650 and 750 °C. Under Ga-rich conditions between growth temperatures of 500–900 °C, smooth surfaces with rms roughness below 1 nm were realized. We found that the slightly Ga-rich conditions between 650 and 750 °C were optimal for β-Ga2O3(010) growth with a smooth surface and a high growth rate.

N-face GaN was grown on free-standing GaN (0001¯) substrates at a growth rate of 1.5 μm/h using plasma-assisted molecular beam epitaxy. Difference in growth rate between (0001¯) and (0001) oriented GaN depends on nitrogen plasma power, and the (0001¯) oriented GaN had only 70% of the growth rate of the (0001) oriented GaN at 300 W. Unintentional impurity concentrations of silicon, carbon, and oxygen were 2 × 1015, 2 × 1016, and 7 × 1016 cm−3, respectively. A growth diagram was constructed that shows the dependence of the growth modes on the difference in the Ga and active nitrogen flux, ΦGa − ΦN*, and the growth temperature. At high ΦGa − ΦN* (ΦGa ≫ ΦN*), two-dimensional (step-flow and layer-by-layer) growth modes were realized. High growth temperature (780 °C) expanded the growth window of the two-dimensional growth modes, achieving a surface with rms roughness of 0.48 nm without Ga droplets.

To obtain a high-crystalline-quality AlN/GaN short-period superlattice with higher average GaN mole fraction, the effects of the thicknesses of AlN barrier and GaN well layers on the superlattice growth were investigated. Coherent growth with an average GaN mole fraction of 20% was realized by reducing the AlN barrier layer thickness to 8 bilayers (BL) while keeping the GaN well layer thickness at 2 BL. Further reduction in the AlN barrier layer thickness resulted in lattice relaxation and degradation of the crystalline quality. Grown layers with various well and barrier thicknesses were investigated by transmission electron microscopy, X-ray diffraction, and atomic force microscopy.

We report on the critical thickness of AlN on SiC(0001). AlN was directly grown on 6H-SiC(0001) at 650 °C by rf-plasma-assisted molecular beam epitaxy. The growth layer had a relatively low threading dislocation density of 4×108–4×109 cm-2. Although the critical thickness of AlN on SiC(0001) is estimated to be 3.5 nm using a Matthews–Blakeslee model, the critical thickness in our experiment was over 700 nm. Low dislocation density, a layer-by-layer growth mode, and low growth temperature may contribute to such a large critical thickness. Sharp and intense free exciton emission was observed in low-temperature PL measurements of the AlN layer.

We demonstrate the coherent growth of AlN/GaN short-period superlattice (SPSL) on 6H-SiC(0001) substrates by molecular beam epitaxy. A high-quality 5-nm-thick AlN layer was grown on SiC as a template layer, followed by the growth of AlN (12 BL)/GaN (2 BL) SPSL, which consists of 40 periods (total thickness: 140 nm). The SPSL was coherently grown on SiC, and its threading dislocation density (TDD) was as low as 8×108 cm-2. The SPSL, which had 3-BL-thick GaN layers, was relaxed, and the TDD increased to 8×1010 cm-2.