## 1 Introduction
This extraordinary range of band gaps in a single kind of alloy material suggests among others its use for solar cells [8]; alloys with varying proportions of In and Ga could bracket the entire solar spectrum from the near infrared to the deep ultraviolet [13]. GaN is rather difficult to grow and to dope in order to create p-type material. Indium does not mix evenly with gallium in the alloy. Instead, it agglomerates to tiny indium-rich clusters that emit light efficiently though the material is riddled with defects. Defects usually ruin the optical properties of a semiconductor by trapping charge carriers and dissipating their energy as heat. The fact that InGaN operates as an emitter suggests that its defect tolerance may be adopted and should be of great advantage in solar cells. To exploit the alloy correspondence to the sunlight spectrum requires a multi-junction cell with layers of different composition. Lattice matching in multi-junction cells is a rather difficult task. The optoelectronic properties of nitride materials, however, show insensitivity to defect dislocations generated by the lattice mismatch. InGaN can be grown on transparent substrates including sapphire (Al\({}_{2}\)O\({}_{3}\)) or silicon carbide (SiC), has tremendous heat capacity, and as other group-III nitrides is extremely resistant to radiation. These properties are ideal for the solar arrays that power communications satellites and other spacecraft. Assuming that the production costs are of the same order of magnitude as for light emitters, inexpensive nitride based efficient solar cells are expected to revolutionize space and simultaneously play a key-role in the use of solar power for terrestrial applications. The strongest feature of group-III nitride semiconductors compared to other wide-bandgap counterparts is the heterostructure technology that it can support: quantum wells, modulation-doped heterointerfaces, and heterojunctions can all be made, in this system, giving access to new spectral regions for optical devices and new operational regimes for electronic devices [18]. The growth of InGaN quantum dots (QDs) containing multilayers appears to be profitable for pursuing higher emission rates and broader spatial and spectral resonances. High density, self-organized InGaN/GaN quantum dots grown by plasma-assisted molecular beam epitaxy exhibit strong emission ranging from 430 to 524 nm at room temperature. Most UV-LED heterostructures are grown on (0001) oriented c-plane sapphire substrates. Owing to the large volumes of sapphire substrates that are being used for blue LED production, sapphire wafers have become very inexpensive. Most importantly, sapphire is fully transparent across the entire UVA, UVB, and UVC spectral range because of its large bandgap energy of 8.8 eV. Considering that group-III nitrides are prepared on foreign substrates, since low-cost native substrates are not yet available, precise measurements of the mechanical, thermal, electrical, and optical properties of the group members are imperative for further advances. The common epitaxial growth of III-V nitrides in the direction normal to the c-plane of the hexagonal wurtzite structure results in polarization charges present at the layer interface and thus spontaneous electric fields affecting the optical and electrical properties of nitride material based devices [3; 6; 7; 32; 33; 34]. The strength and direction of the internal electric fields depend also on piezoelectric fields induced by the mismatch strain between the deposited film and the substrate [35; 36; 37; 38]. One of the most demanding physical aspects of nitride emitters is the presence of large piezoelectric fields in these materials. Because of the hexagonal lattice symmetry without a center of inversion, the piezoelectric coefficients for wurtzite nitrides are non-zero. InGaN QWs are under biaxial compressive stress due to the larger lattice constant of InGaN compared to GaN.