## 3 Results and Discussion  
Figure 2 shows the _I-V_ characteristics of the NUV LEDs measured by a Keithley semiconductor parameter analyzer, which was used to understand forward-bias voltage (\(V_{\mathrm{F}}\)), series resistance (\(R_{\mathrm{S}}\)) and forward leakages. As the sample's In compositions increased, both \(V_{\mathrm{F}}\) and calculated \(R_{\mathrm{S}}\)[31] decreased as shown in Table 1. \(V_{\mathrm{F}}\) and \(R_{\mathrm{S}}\) decreased due to decreased bandgap energy and increased radiative recombination rates in the QWs [32]. The inset shows the ideality-factor characteristics calculated from the _I-V_ data [33]. The values of the ideality factor, \(n\), 1, 2, and \(>\)2 represent the carrier recombination mechanisms of band-to-band radiative recombination, SRH recombination through defects, and defect-assisted tunneling or surface recombination, respectively [33; 34; 35]. Both the tunneling and the SRH recombination via defects decreased from \(\sim\)2.7 to 3.0 V for all samples except #1, which suffered from high defect-assisted tunneling under all applied biases. The minimum ideality factors (\(n_{\mathrm{min}}\)), as shown in Table 1 indicate that the number of defects (especially point defects) in the QWs decreased with the sample's In composition. The carriers captured by the nonradiative recombination centers (NRCs) [25; 34] caused by the point defects can also be reflected in the light-emission distribution shown in Figure 3, where the distributions were taken at a low current (1 \(\upmu\)A), thus defects affecting emission properties. It was observed that the intensity in the EL distribution increased with the sample's In composition. The increase in EL intensity or the radiative recombination rate with the sample number indicates that the number of point defects in the QWs decreased. Therefore, the decreased \(n_{\mathrm{min}}\), and the increased EL intensity reveal the improvement of crystal quality with the sample's In composition. The _I-I_ characteristics of these NUV LEDs are shown in Figure 4a, where optical power was measured by a Si photodetector. These experiments were done in a pulse mode with 100 \(\upmu\)m pulses and a 10% duty cycle to minimize the self-heating effect. Figure 4b shows the \(S\) characteristics of the samples, where the \(S\) values, obtained from the differential slope of log \(L\) vs. log \(I\), indicate the dominant carrier-recombination mechanism at a given applied bias. The values of \(S>\) 2, 2, 1, and \(<\) 1 represent tunneling current, SRH recombination via defects, radiative recombination, and carrier leakage through overflow, respectively [36]. At a low current (10 mA), the sample's \(S\) value decreased (see Table 1) due to the decrease in point defects, which is consistent with \(n_{\mathrm{min}}\) and Figure 3. Thus, both the carrier tunneling from the QWs to the p-GaN layer and the SRH recombination through point defects decreased with the sample's In composition. Furthermore, the overflow of electrons was suppressed at high currents due to the increase in conduction-band offset (\(\Delta E_{C}\)) with the sample's In composition. Figure 2: Current-voltage (_I-V_) characteristics of the LED samples. The inset shows the calculated ideality factor (\(n\)) characteristics. Figure 4: (**a**) Light power vs. current (\(L\)-\(I\)) characteristics and (**b**) \(S\) values calculated from the measured \(L\)-\(I\) characteristics for all samples, where #1 suffers from severe overflow of electrons owing to the low band offset. Figure 3: Light-emission patterns of NUV LEDs at 1 \(\upmu\)A (\(\sim\)2.4 V) measured by a Hamamatsu PHEMOS-1000 system. Figure 5a shows the normalized EL spectra and \(\lambda_{\mathrm{p}}\) characteristics (inset) measured by the fiber-optic spectrometer (AvaSpec-2048). The sample's \(\lambda_{\mathrm{p}}\) increased owing to the decrease in bandgap energy. The decrease in defect density improved the radiative recombination rate that resulted in the sample's EL intensity increase.