## 3 Results and Discussion
Nevertheless, we will need to further investigate whether the mechanism behind the degradation of LOPs during aging could be attributed only to the cracking phenomenon or to the decrease in transparency of the encapsulated silicone [16; 18; 20]. Moreover, as the aging time increased from 700 to 1500 h, the relative LOPs of samples S1-S4 were 50.3, 45.3, 48.6, and 56.6% of their initial values, respectively. In this case, however, the rates of degradation of samples S1-S3 (\(-18.1\), \(-16.6\), and \(-17.5\)%, respectively) were significantly faster than that of sample S4 (\(-11.6\)%). Furthermore, the rate of degradation of sample S1 was faster than those of samples S2 and S3. In contrast, the degradation of samples S2 (\(-13.9\)%) and S3 (\(-17.1\)%) was accelerated in comparison with that of samples S1 (\(-5.6\)%) and S4 (\(-3.4\)%) during operation from 1500 to 3500 h. At the end of aging (after 3500 h), the absolute LOPs of samples S1-S4 were 2.94, 2.67, 2.87, and 3.41 mW, respectively. Thus, the relative LOPs of samples S1-S4 had decreased to 47.5, 39, 40.3, and 54.7% of their initial values, respectively. Figure 3: Relative LOPs measured at 100 mA during aging tests of samples S1–S4. In addition, in the absence of attached quartz glass, we observed no significant cracks in the encapsulated silicone filling the cavity of the AlN-DPC-LF over an aging time of 2000 h. Similar results during 1000 h of operation at 40 mA have been discussed previously [27]. Thus, excluding any mechanisms responsible for degradation in the UVB-LED chips, it might be possible that the silicone present between the UVB-LED chip and the attached quartz glass had undergone serious deterioration as a result of thermal stress and concomitant decay by UVB irradiation in samples S1-S3. The induced thermal stresses in samples S2 and S3 were more obvious than that in sample S1 due to the silicone touching the attached quartz glass. Thus, the degradation of the silicone in samples S2 and S3 was faster than that in sample S1, despite a lack of cracks prior to the aging time reaching 700 h. During the aging period from 700 to 1500 h, we suspect that the rate of degradation of sample S1 was faster than that of samples S2 and S3, because the appearance of cracks in the silicone of sample S1 was obviously retarded relative to that in samples S2 and S3. During operation from 1500 to 3500 h, the rates of degradation of all four of the samples decreased simultaneously. In other words, the influence of the cracked encapsulated silicone on the rate of degradation became a minor factor. An alternative mechanism might be that the silicone was slightly destroyed by the UVB irradiation. As displayed in Figure 3, however, the LOPs decreased noticeably between the onset of operation and 200 h, but the degradation slowed thereafter during operation between 200 and 3500 h. Such time-dependence of the LOPs of AlGaN-based LEDs has been reported previously, but without considering the effect of an encapsulant [28; 29; 30]. The mechanism behind this faster degradation rate is usually associated with diffusion of the metal contact or dopant atoms in the AlGaN-based LED chip. Interestingly, while the LOPs decreased upon increasing the aging time in this study, no significant changes occurred in the peak wavelengths or FWHMs after 3500 h of aging, within an accuracy of \(\pm 1\) nm, in the EL spectra (Figure 5a-d). This behavior suggests that the properties of the QWs did not change over time. Nevertheless, the effect of aging on sample S4 in Figure 3, without any encapsulating silicone, followed neither an exponential nor a square root time-dependence. Figure 6 presents the time-dependence of the forward voltages, measured at a value of \(I_{\text{f}}\) of 100 mA, for all of the samples during the aging tests. After 48 h aging,  
Figure 4: Photographs of samples S1–S4 taken after 3500 h of aging. the forward voltages of samples S1-S4 decreased by 4.2, 2.7, 4.7, and 4.4%, respectively, from their initial values.