## 1 Introduction  
Engineering of efficient light emitting diodes (LEDs) and laser diodes (LDs) in the deep ultraviolet (UVC) range (100-280 nm) is a main goal of nanooptoelectronics and photonics [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18]. A vast variety of applications includes water purification and availability of safe drinking water, which is a vital natural resource especially in developing countries. A proven physical disinfection method in drinking water purification is the use of ultraviolet (UV) light with wavelengths of 200-300 nm to inactivate micro-organisms. Based on the ability of UV light to function as a broad-spectrum antimicrobial agent with short contact times and minimal disinfection by-product formation, it is a viable alternative to chemical disinfectants. The wavelength of the applied UV light depends on the implemented UV emitter. Conventionally, low- and medium-pressure mercury lamps are used; however, in water purification applications, particularly in discontinuously operated, decentralized, and mobile water systems, UV light emitting diodes have certain advantages compared to conventional UV emitters: they do not contain mercury, which may contaminate the water, and need lower voltages than conventional mercury lamps, therefore offering the option to be operated with solar cells or rechargeable batteries. Extensive research efforts are thus devoted to the quantification and possible optimization of processes in ultra-violet light emitting diodes (UV LEDs) inclusive the control of electric fields [19; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31], which ultimately affect the LED properties. Except for InGaN with very high In content, the binary members of group-III nitrides, AlN and GaN, and their alloys with InN are all wide-bandgap materials and can crystallize in both wurtzite and zinc-blende polytypes. The bandgaps of the wurtzite polytypes are direct and range from \(\approx\)0.7 eV for InN to 3.4 eV for GaN and 6.1 eV for AlN. GaN alloyed with AlN and InN may span a continuous range of direct-bandgap energies throughout much of the visible and deep into the ultraviolet spectrum. The entire visible spectrum (400-700 nm) can theoretically be covered by the InAlGaN quaternary nitrides. This makes the nitride system highly attractive for optoelectronic device applications such as LEDs, LDs, and UV-detectors and sets it on focus of intensive research and development (R&D) studies. Commercialization of bright blue and green LEDs and the possibility of yellow LEDs paved the way for the development of full-color displays with compactness, long lifetime, and relatively low power consumption. Additional possible applications include the use of LEDs in agriculture as light sources for accelerated photosynthesis and in health care for diagnosis and treatment. In this context, group-III nitrides provide an incredibly expanding material source, in particular due to their impressive optical properties being much less affected by the presence of lattice defects and dislocations than arsenides and phosphides [6; 7; 9]. Lateral mismatched substrates may still lead to substantial densities of misfit and threading dislocations in broad-area epitaxially deposited GaN on foreign substrates in the range of 10\({}^{9}\)-10\({}^{10}\) cm\({}^{-2}\); InGaN, the primary material system for efficient green and blue light emitters, however, exhibits dislocation densities typically in the order of 10\({}^{7}\)-10\({}^{9}\) cm\({}^{-2}\), which in fact accelerated its commercialization. Difficulties still present in the ternary InGaN growth include the degradation of the material quality of InGaN films with increasing indium content due to the re-evaporation of indium from the instantaneous grown surface. By precise control of the relative amounts of In and Ga in indium gallium nitride (InGaN) alloy crystals, the band gap increases smoothly and continuously as the proportions shift away from In in favor of Ga until reaching the well-established value of 3.4 eV for pure GaN.