AlGaN deep ultraviolet light-emitting diodes (DUV-LEDs) are attracting much attention for a wide variety of applications, however, the efficiency of DUV-LED is still low suppressed by low light-extraction efficiency (LEE). Transparent contact layer is considered to be necessary in order to obtain high LEE in AlGaN DUV LEDs. In this work, we demonstrate over 10% external quantum efficiency (EQE) in an AlGaN DUV-LED by using transparent p-AlGaN contact layer and highly reflective p-type electrode.
We fabricated AlGaN quantum well (QW) DUV LEDs with transparent p-AlGaN contact layers on AlN/sapphire templates. EQEs were compared between LEDs with Ni/Al highly reflective electrode and with conventional Ni/Au electrode. The transparency of the p-AlGaN contact layer was confirmed to be more than 97 %. The maximum EQE for 261 nm LEDs with Ni/Al and Ni/Au electrodes were approximately 2 and 3.3%, respectively. We confirmed that the LEE was increased by about 1.7 times. We also fabricated flip-chip (FC) UVC LED module with transparent p-AlGaN contact layer and reflective electrode. The FC LED module was encapsulated to increase LEE. The emission wavelengths were 276 nm. The EQE value under the forward current of 120 mA was increased from 2.7 to 8.6% by increasing an LEE. The output power of approximately 60 mW was obtained under the forward current of 150 mA. The EQE value was maximally increased up to 10.8%. LEE was estimated to be increased from 8.6 % to 25.5 % by introducing LEE enhancement structure.
Deep-ultraviolet (DUV) light-emitting diodes (LEDs) have a wide range of potential applications, such as sterilization, water purification, and medicine. In recent years, the external quantum efficiency (EQE) and the performance of AlGaNbased DUV LEDs on sapphire substrates have increased markedly due to improvements in the crystalline-quality of high Al-content AlGaN layers, and the optimization of LED structures. On the other hand, DUV LEDs fabricated on Si substrates are very promising as a low-cost DUV light-source in the near future. However, AlN layers on Si have suffered from cracking induced by the large mismatch in lattice constants and thermal expansion coefficients between AlN and Si. In this paper, DUV LEDs on Si were realized by a combination of a reduction in the number of cracks and of the threading dislocation density (TDD) of AlN templates by using the epitaxial lateral overgrowth (ELO) method. The ELO-AlN templates were successfully coalesced on trench-patterned substrates, with the stripes running along the <1-100> direction of AlN. The density of cracks was greatly reduced in 4- μm-thick ELO-AlN templates, because voids formed by the ELO process relaxed the tensile stress in the AlN layer. Furthermore, the AlN templates showed low-TDD. The full-width-at-half-maximum values of the (0002) and (10-12) X-ray rocking curves were 780 and 980 arcsec, respectively. DUV LEDs fabricated on these high-quality ELO-AlN/Si substrates showed single peak emission at 256- 278 nm in electroluminescence measurements. It is expected that we will be able to realize low-cost DUV LEDs on Si substrates by using ELO-AlN templates.