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.

For AlGaN-based multi-quantum-well light emitters grown on c-plane substrates there is a tendency for the polarization of the emitted light to switch from transverse electric (TE) polarization to transverse magnetic (TM) polarization as the wavelength decreases. This transition depends on various factors that include the strain in the quantum well. Experimental results are presented that illustrate the phenomenon for nitride light emitting diodes (LEDs) grown on sapphire and on bulk AlN. Model calculations are presented which quantify the dependence of the TE/TM switch on the quantum well strain and the Al composition in the barriers surrounding the well.

Selective-area epitaxy is used to form three-dimensional (3D) GaN structures providing semipolar crystal facets. On full 2-in. sapphire wafers we demonstrate the realization of excellent semipolar material quality by introducing inverse GaN pyramids. When depositing InGaN quantum wells on such a surface, the specific geometry influences thickness and composition of the films and can be nicely modeled by gas phase diffusion processes. Various investigation methods are used to confirm the drastically reduced piezoelectric polarization on the semipolar planes. Complete electrically driven light-emitting diode test structures emitting in the blue and blue/green spectral regions show reasonable output powers in the milliwatt regime. Finally, first results of the integration of the 3D structures into a conventional laser design are presented

No abstract