## 3 Results and Discussion
As seen in the inset, all samples suffered from redshift due to bandgap narrowing (or shrinkage) caused by heat generation. The fact that the redshift decreased with the sample's indium composition reflected the reduced heat produced by the carriers' nonradiative recombination, namely SRH recombination [33; 34] and tunneling from QWs [34; 35] via point defects. No blueshift was observed at low currents. The blueshift was caused by the screening of \(F_{\mathrm{PZ}}\) (calculated through the ER spectroscopy seen in Table 1) due to localized carriers or band filling, both of which seem to be smaller than the redshift caused by heat generation. The decrease in redshift at the current range from 1 to 350 mA was consistent with the fact that the number of point defects in QWs decreased with the sample's In composition. Figure 5b depicts the ER spectra at different biases for sample #1 used to find the flat-band voltage for calculating \(F_{\mathrm{PZ}}\). The flat-band voltage was calculated by finding the phase inversion between the spectra as seen in Figure 5b [37]. The flat-band voltages thus obtained were 0.7, 0.5, 0.1 and \(-\)1.0 V for samples #1, #2, #3, and #4, respectively. The \(F_{\mathrm{PZ}}\)'s of these samples were calculated using the method described in Reference [38] with the device structure given above. Figure 5: (**a**) Normalized electroluminescence (EL) spectra at 350 mA and room temperature, where all the samples’ peak intensities are normalized with respect to that of #4. The inset shows the peak wavelength characteristics. (**b**) Electroreflectance (ER) spectra at different biases for sample #1. Differences in carrier recombination properties can be understood directly by analyzing IQE characteristics. The IQE is defined as the ratio of the number of photons emitted from the active region per second to the number of electrons injected into the active region per second. The IQEs of NUV LED samples have been calculated by using the fitting method of calculating IQE as described in Reference [39], which is shown in Figure 6a. The LEEs of these samples have also been calculated by using Reference [39]. Since LEE measures photon loss during propagation from the active region to free space [39], which is almost equal for all samples as seen in Figure 6b, the absorption of emitted photons by AlGaN barriers and GaN clad layers were similar for all samples [11]. For currents \(<\) 200 mA, the increase in IQE was caused by the increase in the radiative recombination rate, which is directly related to carrier localization and the effective radiative-recombination area in QWs. For the case of identical epitaxial structure, the increase in the radiative recombination rate at low currents before the IQE peak was due to the decrease in defects (especially the point defects in the active MQW region) since the defects act as NRCs for carriers, inducing SRH recombination [30] as reflected in the \(A\) coefficient. Again, the calculated \(A\) coefficient [39] decreased with the sample's In composition (see Table 1), which is consistent with the PHEMOS patterns, \(n_{\text{min}}\) and \(S\) values at low currents. Thus, the effective radiative-recombination area increased with the reduction in point defects in QWs. Consequently, the SRH recombination and the tunneling from QWs through point defects decreased. As a result, IQE increased with the sample's In composition. The increase in \(\Delta E_{C}\) reduced the carrier overflow from QWs, decreasing the sample's IQE droop at high currents. Figure 6: (**a**) The IQE characteristics on a semi-log scale from 200 uA to 250 mA. The inset shows the IQE characteristics from 200 uA to 700 mA on a linear scale. (**b**) The calculated LEE efficiencies.