## 3 Experimental Results  
Figure 2a shows the EL spectra for LEDs A, B and C under direct current of 40 mA, in which all the spectra were normalized to the band-to-band emission. The slightly different wavelengths of band-to-band emissions between LED A and the other devices was probably due to the flux deviation during the growth of quantum wells. A parasitic peak around 304 nm besides the band-to-band emission can be observed for all the LEDs. The parasitic peak has been clarified to be related to the electron overflow and Mg doping concentration in the p-AlGaN cladding layer [20]. As the QW number increased, the intensity of the parasitic peak decreased, which meant the electron overflow decreased. Figure 2b shows the intensity ratio of the band-to-band emissions to the parasitic peaks for all devices at different direct current injections. The ratio increased as the current increased, indicating more and more carriers were involved in the band-to-band radiative recombination. For the same injection current, the ratio increased as the QW number increased, which meant that the ratio of carrier recombination inside and outside the wells was increased.  
The relative output powers at different injections were obtained from the EL measurement. To avoid the influence of the thermal heating effect [3], the pulse injection with 1 \(\upmu\)s current pulse at 0.5% was used in the EL measurements. Figure 3 shows the pulse injection dependent relative output power for all three devices. Obviously, the relative output power increased as the current increased. For a certain injection, the relative output power increased as the QW number increased, which indicated that the carriers captured by quantum wells increased with the increase in QW number. The increment of relative output power from 5 QWs to 10 QWs was less than that from 10 QWs to 40 QWs. For example, the output power ratios of LED B to LED A and LED C to LED B were 1.6 and 3.0, respectively, at injection of 100 mA. This phenomenon was very different from what has been reported for blue and near-UV LEDs [24], in which the carriers were exhausted mostly in the first ten QWs. The results indicated that the increase of QW number was favorable for the optical properties.  
The influence of the EBL with different structures on the EL properties of DUV-LEDs has been investigated by many groups [25; 26; 27; 28]. Generally, we used 1 nm i-AlN as an EBL to suppress the electron overflow [20]. For comparison in this work, DUV-LEDs with a 1 nm AlN EBL was grown for five period QWs, named LED D. The EL spectrum and the relative output power were shown in Figures 2a and 3, respectively. Obviously, the suppression efficiency of electron overflow in LED D was better than in LED A. Furthermore, LED D showed advanced property compared to LED C. Correspondingly, the relative output power was increased significantly for LED D.  
Figure 2: (**a**) The electroluminescent (EL) spectra for LEDs A, B, C and D under direct current of 40 mA; (**b**) the intensity ratio of the band-to-band emissions to the parasitic peaks for LEDs A, B and C at different current. All the spectra were normalized to the band-to-band emission.