Structural analysis was performed on a purple laser diode composed of In0.20Ga0.80N (3 nm)/ In0.05Ga0.95N (6 nm) multiple quantum wells, by employing transmission electron microscopy and energy-dispersive x-ray microanalysis, both of which are assessed from the cross-sectional direction. It was found that the contrast of light and shade in the well layers corresponds to the difference in In composition. The main radiative recombination was attributed to excitons localized at deep traps which probably originate from the In-rich region in the wells acting as quantum dots. Photopumped lasing was observed at the high energy side of the main spontaneous emission bands.

We demonstrate the fabrication of blue, green, and amber InGaN/GaN light-emitting diodes (LEDs) on semipolar {11-22} bulk GaN substrates. The {11-22}GaN substrates used in this study are produced by cutting out from a c-oriented GaN bulk crystal grown by hydride vapor epitaxy. The LEDs have a dimension of 320 ×320 µm2 and are packed in an epoxide resin. The output power and external quantum efficiency (EQE) at a driving current of 20 mA are 1.76 mW and 3.0%, respectively, for the blue LED, 1.91 mW and 4.1% for the green LED, and 0.54 mW and 1.3% for the amber LED. The maximum output powers obtained with a maximum current of 200 mA are 19.0 mW (blue), 13.4 mW (green), and 1.9 mW (amber), while the maximum EQEs are 4.0% at 140 mA (blue), 4.9% at 0.2 mA (green), and 1.6% at 1 mA (amber). It is confirmed that the emission light is polarized along the [1-100] direction, reflecting the low crystal symmetry of the {11-22} plane.

Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm—due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments.

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We investigated correlations between nanoscopic optical and structural properties in violet-emitting, blueemitting, and green-emitting In x Ga 1−x N / GaN quantum wells ͑QWs͒ by means of scanning near-field optical microscopy ͑SNOM͒ and atomic force microscopy. Only in the blue-emitting QW, threading dislocations were not major nonradiative recombination centers ͑NRCs͒. SNOM data indicated that NRCs in the blue-emitting QW are surrounded by energy levels higher than those for radiative recombination. Such potential distributions realize "antilocalization" of carriers to NRCs, which is the cause of high emission quantum efficiencies in blue emitters.

In x Ga 1 − x N multiple quantum wells (QWs) with [0001], ⟨112¯2⟩, and ⟨112¯0⟩ orientations have been fabricated by means of the regrowth technique on patterned GaN template with striped geometry, normal planes of which are (0001) and {112¯0}, on sapphire substrates. It was found that photoluminescence intensity of the {112¯2} QW is the strongest among the three QWs, and the internal quantum efficiency of the {112¯2} QW was estimated to be as large as about 40% at room temperature. The radiative recombination lifetime of the {112¯2} QW was about 0.38ns at low temperature, which was 3.8 times shorter than that of conventional [0001]-oriented InxGa1−xN QWs emitting at a similar wavelength of about 400nm. These findings strongly suggest the achievement of stronger oscillator strength owing to the suppression of piezoelectric fields.

We report the observation of optical polarization switching in In x Ga 1−x N / GaN quantum well active layers, using semipolar ͕1122͖ planes. When the In composition is less than ϳ30%, the emissions related to the top and second valence bands are polarized along the ͓1100͔ and perpendicular ͓1123͔ directions, respectively, similar to earlier studies. On the contrary, as the In composition increases above 30%, the polarizations switch, indicating a crossover between the two valence bands. Because the polarization degree is less sensitive to the well width, the observed polarization switch is ascribed to the InN deformation potentials.

AlN layers were grown directly on sapphire ͑0001͒ substrates using three different growth sequences based on metal-organic vapor phase epitaxy with an emphasis on initial nucleation processes. These three methods were simultaneous, alternating supply of aluminum and nitrogen sources, and a combination of the two. In all the methods, nucleation was initiated by three-dimensional ͑3D͒ islands with a typical diameter of ϳ20 nm. Enhanced migration by the alternating source supply caused highly 3D AlN ridge structures at the sapphire molecular steps. These ridge structures prevented a flattened AlN surface and, in addition, moderated lattice relaxation, suggesting the importance of controlling the initial nucleation in determining the film's properties. In fact, the hybridized method, derived from the simultaneous and alternating supply methods, was able to control the initial nucleation, and provided the best film quality; the 600-nm-thick AlN grown by this method had an atomically flat surface free of pits and particles, and the x-ray diffraction line widths were ϳ45 and ϳ250 arcsec for the ͑0002͒ and ͑1012͒ planes, respectively.