Narrow-Band AlGaN-Based UVB Light-Emitting Diodes

Abstract

This paper reports an AlGaN-based narrow-band ultraviolet-B (NB-UVB) light-emitting diode (LED) exhibiting a single electroluminescence peak with a full-width at half-maximum (FWHM) of less than 10 nm at forward currents (I f ) from 10 to 200 mA, broadening to 11.6 nm when the forward current reached 350 mA. We attribute the narrow FWHM to effectively decreasing the degrees of piezoelectric polarization in the MQWs, and the excess electron overflow from the MQW to the p-layer was avoided. The maximum external quantum efficiency (EQE) of 2.16% and wall plug efficiency of 1.74% occurred when the forward current was 10 mA; the EQE dropped by 8.6% when the forward current increased from 10 to 60 mA. Furthermore, the light output power decreased to 85.4 and 82.5% of its initial value after 620 and 3500 h, respectively, under conditions of 60 mA dc aging. The characteristics of this NB-UVB LED suggest great potential for its application in phototherapy.

Very Low-Efficiency Droop in 293 nm AlGaN-Based Light-Emitting Diodes Featuring a Subtly Designed p-Type Layer

“…Figure 4 b–d present the time-dependence of the forward voltage, the EL peak wavelength, and the EL FWHM, respectively, all measured during the aging tests performed at a value of J f of 63.3 A cm –2 . The forward voltage decreased dramatically at the onset of operation but remained stable during operation between 168 and 500 h. This behavior is very similar to that reported previously [ 40 , 43 ]. Interestingly, while the LOPs and forward voltages decreased simultaneously upon increasing the aging time to 500 h, no significant changes occurred in the peak wavelengths or in the FWHMs.…”

“…Rapid degradation of the LOP, to 89.4% of its initial value, occurred in the first 48 h of the aging operation; it then decreased to 86.5% of its initial value when the aging time reaching 168 h. After an aging time of 500 h, the LOP decreased to 74.3% of its initial value. Such behavior has been observed many times previously [ 38 , 39 , 40 , 41 , 42 , 43 ]. The LOP decreased more slowly after 500 h of aging operations, in accordance with previous reports [ 40 , 43 ].…”

“…Such behavior has been observed many times previously [ 38 , 39 , 40 , 41 , 42 , 43 ]. The LOP decreased more slowly after 500 h of aging operations, in accordance with previous reports [ 40 , 43 ]. This performance is close to the specifications of commercial solid-state lighting products, with the estimated 70% lifetime (L70) being comparable with those reported for a 310 nm UVB LED operated at current densities of 28.5 and 33.5 A cm –2 [ 38 , 43 ].…”

“…To overcome the efficiency droop at a higher current density (>100 A cm –2 ), it is necessary to a subtly design the p-layer to improve the hole injection efficiency and the uniformity of the hole concentration within the well region of the MQWs, thereby avoiding Auger recombination as a result of a mismatch in the hole and electron concentrations. Nevertheless, only a few recent reports have appeared that have simultaneously discussed the phenomena of T-droop, J-droop, and current-induced degradation in AlGaN-based 293 nm UVB-LEDs [ 38 , 39 , 40 , 41 , 42 , 43 ]. Thus, in this study, we fabricated an AlGaN-based 293 nm UVB-LED and examined its electroluminescence (EL) characteristics over the temperature range from 298 to 358 K and at forward current densities ( J f ) in the range of 12.7–113.9 A cm –2 to obtain insight into the behavior of the EL emission and the efficiency droop.…”

Observation of Highly Durable Silicone Resin for Encapsulating AlGaN-Based UVB Light-Emitting Diodes

“…Thereafter, the forward voltages remained more stable during operation between 48 and 3500 h, whether silicone was encapsulated or not. To separate the influence of the encapsulated silicone, two possible mechanisms have been suggested as being responsible for the degradation processes occurring in UVB-LED chips [30][31][32][33]. A possible scenario is that the number of non-radiative centers increased at the onset of operation due to an increased junction temperature, causing more rapid decay at the early stage; thereafter, a slow proliferation rate was gradually achieved, after saturating the number of non-radiative centers, through the other mechanism in response to thermal stress inside the UVB-LED chip.…”