Mg-doped p-type a-plane GaN films were grown on high-quality unintentionally doped GaN on þ0:5 -off r-plane sapphire substrates by metalorganic vapor phase epitaxy (MOVPE). A maximum hole concentration of 2:0 Â 10 18 cm À3 was reproducibly achieved at room temperature, which was higher than the maximum hole concentration of p-type c-plane GaN. The activation energy of Mg acceptors in p-type a-plane GaN with a hole concentration of 2:0 Â 10 18 cm À3 was found to be 118 meV by temperature-dependent Hall-effect measurement.

We demonstrated a UV-laser diode grown on low-dislocation-density AlGaN. The combination of a low-temperature-deposited AlN interlayer technology and heteroepitaxial lateral overgrowth yielded crack-free and partially low-dislocation-density AlGaN on a grooved GaN substrate. A ridge waveguide was fabricated in the low-dislocation-density region. The lasing wavelength under pulsed current injection at room temperature was 350.9 nm, which is the shortest wavelength ever reported.

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In this study, the anisotropically biaxial strain in a-plane AlGaN on GaN is investigated by X-ray diffraction (XRD) analysis using an AlGaN/GaN heterostructure grown on r-plane sapphire. In accordance with XRD reciprocal lattice space mapping, when the AlN molar fraction x in the AlGaN layer is 0.18, the AlGaN layer is fully strained under tensile stress and grows coherently on the underlying GaN layer. However, when x is as large as 0.31, partial relaxation is observed only in the c-axis direction. The tensile stress in the AlGaN layer is calculated taking the actual in-plane lattice constants of the underlying GaN layer into account, and it was found that the stress in the a-plane AlGaN layer in the c-axis direction is approximately 1.7 times larger than that in the m-axis direction.

Nitride-based blue light-emitting diodes (LEDs) with a moth-eye structure on the back of a 6H–SiC substrate have been developed. The moth-eye LED has a roughness less than the optical wavelength at the back surface of the SiC substrate fabricated by reactive ion etching (RIE) with CF4 gas. The light extraction efficiency and corresponding output power have been increased to 3.8 times those of a LED with a conventional structure. The experimental findings agree with the results of a theoretical analysis of the effect of the moth-eye structure.