## 3 Results and Discussion  
We measured the photocurrent spectrum from the DUV LED to analyze the absorption in the active region. Figure 4a is the result of measuring the photocurrent spectrum of the DUV LED under zero bias. As mentioned in Section 2.2, the photocurrent spectrum is expected to have a peak in the reverse direction. In Figure 4a, the photocurrent spectrum exhibits a dip between 250 and 270 nm, as expected. However, the photocurrent was also observed at wavelengths above 300 nm, which was unexpected, and the photocurrent was a _forward current_ in this case. This result implies that light with wavelengths above 300 nm is also absorbed near the active region in the DUV LED, and the electric field associated with this absorption is opposite to the built-in electric field in the MQW region. This phenomenon has not been observed before in other LEDs, such as InGaAs-based red and InGaN blue LEDs [16; 17; 18]. In addition, when a forward voltage of 4 V was applied to the DUV LED, as shown in Figure 4b, and returned to the zero bias again, the direction of the photocurrent at wavelengths above 300 nm was reversed, as shown in Figure 4c. Figure 3: A schematic diagram of the photocurrent spectroscopy measurement. In order to confirm the reversal of the photocurrent observed at wavelengths above 300 nm, the wavelength of excitation light was fixed at 480 nm and a temporal evolution of this photocurrent was measured after the applied bias was returned to the zero bias. The 4 V forward bias was applied for 15 min and then turned off to the zero bias. These results are shown in Figure 5. As seen in the figure, the photocurrent reversed by the 4 V forward bias became smaller and was restored to the positive value after about 6 h.  
To summarize the experimental results  
1. A photocurrent flowed at a wavelength above 300 nm at zero bias;
2. A photocurrent flowing at a wavelength above 300 nm is a forward current;
3. When the 4 V forward voltage was applied and then returned to the zero bias, the photocurrent generated at wavelengths above 300 nm was converted into a reverse current;
4. The photocurrent reversal phenomenon was restored over approximately 6 h.  
Since a photocurrent occurs at wavelengths above 300 nm, it implies there is a region that absorbs these longer wavelengths. In addition, the absorption band is wide, about 300 to 800 nm. We attribute this wide absorption band to the high point defect densities in these DUV LEDs. If the density of the point defects is high enough, defect states may result in forming the absorption band. In order to check the validity of the assumption, the photoluminescence (PL) spectrum of this DUV LED was measured using a 325 nm He-Cd laser as the excitation source, as shown in Figure 6. We note here that the 325 nm line has a lower photon energy than the  
Figure 4: The results of photocurrent spectroscopy divided by lamp power, measured at (**a**) zero bias, (**b**) 4 V forward bias, and (**c**) zero bias remeasured after applying the 4 V forward bias. Figure 5: Photocurrent change with time in a 480 nm excitation light. The biases applied were as follows: 0–15 min: zero bias; 15–30 min: 4 V forward bias; 30 min: returned to zero bias. bandgap energy of the QWs. As seen in Figure 6, a broad strong PL signal was observed with two broad peaks at 400 nm and 545 nm. It was reported that the Al vacancies in the AlN have deep-level defects that can cause emission at 387 nm and 445 nm and absorption at 375-590 nm [19; 20; 21]. The observed PL peaks had a longer wavelength than the previously reported emission wavelength of Al vacancies in AlN. However, it should be noted that this result was observed in AlGaN-related DUV LEDs with Al composition ratios of 0.53 to 0.7. Since the bandgap energy of AlGaN is smaller than that of the AlN, the deep-level energy of the Al-vacancies may also be small.