## 3 Results and Discussion
### Electrooptical Response of PIN-Diodes Sensed by Electroluminescence (EL) Spectroscopy  
The EL emission spectra of the PIN-diodes based on the GaN reference structure and the In\({}_{0.055}\)Ga\({}_{0.945}\)N/GaN, In\({}_{0.15}\)Al\({}_{0.85}\)N/GaN, In\({}_{0.18}\)Al\({}_{0.82}\)N/GaN, and In\({}_{0.21}\)Al\({}_{0.79}\)N/GaN quantum well heterostructures are presented in Figures 10 and 11; evidently, both the excitonic- and defect-related emission depend on the applied bias-voltage (DC, AC, or a combination of both DC and AC). The excitonic emission in the UV spectral range is facilitated at higher voltages, whereas at voltages slightly above the EL emission-threshold of the PIN-diodes, only the defect-related emission due to the donor-acceptor pair recombination (DAP) in the VIS range is present, as demonstrated in Figure 10a,b.  
Figure 10: EL emission spectra of PIN-diodes based on (**a**) GaN reference structure and (**b**) In\({}_{0.055}\)Ga\({}_{0.945}\)N/GaN DHS in dependence of DC bias-voltage.  
The EL emission characteristics of PIN-diodes based on the GaN reference structure and the In\({}_{0.055}\)Ga\({}_{0.945}\)N/GaN DHS exhibit a vast change by variation of the DC voltage in the range 2-4 V under maintenance of the AC voltage at 2 Vp-p. An intensity-tuning of the ratio of the excitonic- to defect-related emission in UV-VIS is thus easily achieved in conjunction with the applied forward bias. In fact, group-III nitride based QW heterostructures exhibit much larger optical gains in case the optical matrix element is largely enhanced due to the disappearance of the internal field [111]. The exact emission energies of the EL components in UV-VIS and VIS-NIR were obtained by fitting the EL spectra with Gaussians. In case of the GaN PIN-diode operated at 2.4 V DC and 2 V\({}_{\text{P-P}}\) AC, the UV emission is composed of a high intensity component at the GaN fundamental gap energy \(E_{a}\) (=\(E_{0}\)) = 3.414 eV, with particularly narrow bandwidth (FWHM) of \(\Gamma\) = 0.054 eV, and a broader low intensity component at \(E_{DAP}\) = 3.299 eV, \(\Gamma\) = 0.283 eV. In case of the PIN-diode of the In\({}_{0.055}\)Ga\({}_{0.945}\)N/GaN DHS, an additional EL component related to the excitonic emission of the InGaN/GaN quantum well heterostructure was observed. Following the trend of the UV emission of GaN, this additional EL band was enhanced as the DC voltage progressively increased.  
The EL emission characteristics of PIN-diodes based on the In\({}_{0.15}\)Al\({}_{0.85}\)N/GaN, In\({}_{0.18}\)Al\({}_{0.82}\)N/GaN, and In\({}_{0.21}\)Al\({}_{0.79}\)N/GaN DHS were shadowed by the emission of the low energy gap GaN over-layers. The excitonic- and defect-related emission of GaN in dependence of the DC voltage, in Figure 11a,b, follows the same trend, as previously discussed. By comparison of the EL emission energy of In\({}_{\text{v}}\)Al\({}_{1\text{-v}}\)N/GaN PIN-diodes with x = 0.15, 0.18, and 0.21, at a fixed bias-voltage, a red-shift of the GaN barrier emission with the increase of indium content was evidenced, which indicates that the overall emission characteristics of the In\({}_{\text{v}}\)Al\({}_{1\text{-v}}\)N/GaN DHS appear red-shifted by an increase of indium in consistency with published literature [112].