Smooth, high quality N-polar GaN films were realized by metal organic chemical vapor deposition (MOCVD) through growth on misoriented (0001) sapphire substrates and the development of a high temperature nucleation process. Misorientation angles from 0.5° to 4° toward the $a$ and the $m$ plane of the sapphire substrate were investigated. Whereas GaN films grown on substrates with a misorientation angle of only 0.5° or 1° exhibited high densities of hexagonal surface features as commonly observed for N-polar GaN films grown by MOCVD, smooth GaN layers were obtained on sapphire substrates with misorientation angles of 2° or larger. In addition, the structural and optical properties of the GaN films significantly improved with increasing misorientation angle, as evaluated by high resolution x-ray diffraction, atomic force microscopy, transmission electron microscopy, and photoluminescence measurements. The properties of GaN layers grown on (0001) sapphire with a misorientation of 4° were comparable to Ga-polar GaN films grown in the same reactor.

The properties of N-polar $\mathrm{In}\mathrm{Ga}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ multiquantum wells (MQWs) grown by metal-organic chemical vapor deposition were investigated. Samples grown under optimized conditions exhibited distinct quantum well related emission, smooth surfaces, and abrupt interfaces as evaluated by room temperature photoluminescence, atomic force microscopy, and x-ray diffraction. Enhanced incorporation of indium into N-polar compared to Ga-polar MQW samples was observed for MQWs simultaneously deposited onto the (0001) and $\left(000\overline{1}\right)$ GaN-on-sapphire base layers using trimethylindium-to-trimethylgallium-flow-ratios larger than 1.2 during growth. Necessary adjustments of the growth procedure for N polar in comparison with Ga-polar MQWs are described.

Smooth N-polar $\text{GaN}/{\text{Al}}_x{\text{Ga}}_{1-x}\text{N}/\text{GaN}$ heterostructures with a different Al mole fraction were grown by metalorganic chemical vapor deposition on (0001) sapphire substrates with a misorientation angle of $4°$ toward the $a$ -sapphire plane. The sheet electron density of the two-dimensional electron gas (2DEG), which formed at the upper $\text{GaN}/{\text{Al}}_x{\text{Ga}}_{1-x}\text{N}$ interface increased with an increasing Al-mole fraction in the ${\text{Al}}_x{\text{Ga}}_{1-x}\text{N}$ layer and increasing silicon modulation doping, similar to the observations for Ga-polar heterostructures. The transport properties of the 2DEG, however, were anisotropic. The growth on vicinal substrates led to the formation of well ordered multiatomic steps during ${\text{Al}}_x{\text{Ga}}_{1-x}\text{N}$ growth and the sheet resistance of the 2DEG parallel to the steps was about $25\%$ lower than the resistance measured in the perpendicular direction. The fabricated devices exhibited a drain-source current, $I_{\text{DS}}$ , of 0.9 A/mm at a gate-source voltage $+1\text{V}$ . At a drain-source voltage of 10 V and $I_{\text{DS}}=300\text{mA}/\text{mm}$ , current-gain and maximum oscillation frequencies of 15 and 38 GHz, respectively, were measured.

GaN nanopillar and nanostripe arrays with embedded $\mathrm{In}\mathrm{Ga}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ multi-quantum wells (MQWs) were fabricated by holographic lithography and subsequent reactive ion etching. Etch related damage of the nanostructures was successfully healed through annealing in $\mathrm{N}{\mathrm{H}}_3∕{\mathrm{N}}_2$ mixtures under optimized conditions. The nanopatterned samples exhibited enhanced luminescence in comparison to the planar wafers. X-ray reciprocal space maps recorded around the asymmetric $\left(10\overline{1}5\right)$ reflection revealed that the MQWs in both nanopillars and nanostripes relaxed after nanopatterning and adopted a larger in-plane lattice constant than the underlying GaN layer. The pillar relaxation process had no measurable effect on the Stokes shift typically observed in MQWs on $c$ -plane GaN, as evaluated by excitation power dependent photoluminescence (PL) measurements. Angular-resolved PL measurements revealed the extraction of guided modes from the nanopillar arrays.