## 3 Results and Discussion
Our results imply that the defect-related below-gap photoabsorption should occur in the EBL or the EBL/p-AlGaN interface to account for the electric field reversal, which means the related defects should be spatially localized in these very thin layers. Therefore, we believe the photo-absorbing defect band is likely generated by the point defects, not by extended defects such as threading dislocations. In AlGaN, Al vacancy, N vacancy, substitutional oxygen, and the Al vacancy-oxygen complex are known to generate deep-level states [19; 20; 21]. Considering the central wavelength of the PL peak, it is assumed that Al vacancy or the Al-vacancy-oxygen complex is the dominant cause for these point defects [19; 20; 21], while further study is needed to specify the exact origin of these defects. Since a forward current was observed at a wavelength above 300 nm, absorption occurred in a region other than the active region where the built-in electric field generated by the junction is located. This implies that an electric field in the opposite direction to the built-in electric field exists in the absorption layer. Since nitride compound semiconductors have a Wurtzite structure, a strong piezoelectric field (\(\sim\) 1 MV/cm) exists when the structure deforms due to a difference in the lattice constant between the growth layers [22; 23]. The EBL has a high Al content to block electron overflow and, therefore, has a smaller lattice constant than p-AlGaN. p-AlGaN, which has a relatively large lattice constant, can be subjected to compressive strain, and the direction of the formed piezoelectric field is in the opposite direction to the built-in electric field generated at the PN junction. Simulation results show that the piezoelectric field is generated by the strain between the QW and barrier or the EBL and p-AlGaN layers in AlGaN-based DUV LEDs [24]. It was shown that the direction of the piezoelectric field generated by the EBL in the p-AlGaN layer was in the opposite direction to that of the built-in electric field [24]. Therefore, we conclude that the photocurrent above 300 nm is generated in the Al vacancy present in p-AlGaN and flows as a forward current due to the piezoelectric field. We investigated the temperature dependence of the recovery time to find the origin of the photocurrent's reversal at wavelengths over 300 nm in more detail. Measurements were made at four different temperatures: 26 \({}^{\circ}\)C (room temperature), 50 \({}^{\circ}\)C, 100 \({}^{\circ}\)C, and 150 \({}^{\circ}\)C. After the 4 V forward bias was applied at 15 min, as shown in Figure 5, the photocurrent was measured over time in the excitation light of 480 nm at zero bias. The results are shown in Figure 7. These show that the photocurrent recovery exhibited an exponential-like  
Figure 6: The resulting spectrum of PL spectroscopy using a 325 nm He-Cd laser in a DUV LED. decay, and the recovery time decreased with increasing temperatures. This photocurrent reversal phenomenon by forward bias disappeared at 150 \({}^{\circ}\)C and above. This means that all the trapped holes were thermally activated above 150 \({}^{\circ}\)C. The recovery time constant was calculated by fitting the photocurrent recovery to the double exponential decay, as in Equation (1), and the result is shown in Table 1.  
\[I(t)=A_{1}e^{-\frac{t}{\tau_{1}}}+A_{2}e^{-\frac{t}{\tau_{2}}}+l_{0} \tag{1}\]  
The double exponential decay implies the existence of two trap levels. As a result, the time constants decreased as the temperature increased and went to 0 when \(\tau_{1}\) and \(\tau_{2}\) equaled 100 \({}^{\circ}\)C and 150 \({}^{\circ}\)C, respectively. The exponential temperature dependence of the recovery time implies that the cause of the electric field reversal is associated with the charge traps. The trap states can be charged by the leakage current at 4 V of forward voltage, which is smaller than the turn-on voltage of the device. In this device, a significant leakage current was observed at 4 V of forward voltage.