## 4 Conclusions  
To analyze the defect states and their related absorption of the active region in DUV LEDs, we obtained photocurrent spectra as a function of applied bias and the sample temperature. We found several abnormal photocurrent reversal phenomena in the spectra. First, a photocurrent was generated in the 300 nm-800 nm region with a photon energy lower than the bandgap energy of AlGaN in the opposite direction to that of the normal photocurrent by QW band-edge absorption. Second, after applying a forward 4 V bias, the direction of the photocurrent was reversed in the 300 nm-800 nm range, and the direction of the reversed photocurrent recovered to its original direction after several hours at room temperature.  
To establish the cause of this phenomenon, PL measurements and temperature-dependent recovery time measurements were performed, and the absorption and emission spectra of AlGaN defects were investigated. We found that the cause of the abnormal photocurrent was carrier generation due to absorption in the 300 nm-800 nm wavelength range by the Al vacancies generated near the p-AlGaN and EBL interface. In addition, it  
Figure 8: Band diagrams around the active region and the photocurrent mechanism at (**a**) zero bias, (**b**) 4 V forward bias, and (**c**) zero bias remeasured. red arrows: long wavelength above 265 nm; blue arrows: short wavelength less 265 nm.  
was attributed to the hole traps charged by the leakage current in the forward bias at the EBL and p-AlGaN interface, resulting in charging and generating an electric field that was larger than the existing piezoelectric field.  
The activation energy of the hole trap state was obtained by the temperature-dependent recovery time with the Arrhenius equation calculation. We found the activation energy of the trap level to be 279 meV.  
As a consequence of the above, AlGaN-based DUV LEDs have a high defect density that generates an unexpected electric field, which can adversely affect the operating characteristics, such as the efficiency, threshold voltage, and the leakage current of the device. Our results suggest that it is preferably required to minimize the point defect, such as Al vacancies in the EBL layer and/or at the EBL and p-AlGaN, rather than the dislocations or deep defects in the n-AlGaN and buffer layers, in order to avoid this unexpected sheet charge generation in AlGaN-based DUV LEDs. We believe our results, the wavelength-dependent reversal of the photocurrent, can also be utilized for long-range remote optical switches, such as in free-space laser satellite communication, where the contrast and reliability of the on/off signal are important since the on/off signal can be determined by the direction of the photocurrent other than the current amplitude.  
Data curation, T.-S.K., J.K. ([PERSON]), J.K. ([PERSON]) and S.L.; project administration, S.L. and J.-H.S.; writing--original draft, J.-H.S., T.-S.K. and S.L.; writing--review and editing, M.S., H.D.K. and S.L. All authors have read and agreed to the published version of the manuscript.  
This work was funded by a National Research Foundation of Korea (NRF) grant funded by the Korean government, Ministry of Science and ICT (MSIT) (No. 2021R1A2C1095571) and an Institute for Information and Communication Technology Promotion (IITP) grant funded by the Ministry of Science and ICT, South Korea (2019-0-00008).  
The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.  
The authors declare no conflict of interest.