Improvements of the internal quantum efficiency by reduction of the threading dislocation density and of the light extraction by using UV transparent p-type cladding and contact layers, UV reflecting ohmic contact, and chip encapsulation with optimized shape and refractive index allowed us to obtain the external quantum efficiency of 10.4% at 20 mA CW current with the output power up to 9.3 mW at 278 nm for AlGaN-based deep-ultraviolet light-emitting diodes grown on sapphire substrates.

In this letter, the achievement of over 60 mW output power from pseudomorphic ultraviolet light-emitting diodes in continuous wave operation is reported. Die thinning and encapsulation improved the photon extraction efficiency to over 15%. Improved thermal management and a high characteristic temperature resulted in a low thermal rolloff up to 300 mA injection current with an output power of 67 mW, an external quantum efficiency (EQE) of 4.9%, and a wall plug efficiency (WPE) of 2.5% for a single-chip device emitting at 271 nm in continuous wave operation. # 2013 The Japan Society of Applied Physics M id-ultraviolet (MUV) light-emitting diodes (LEDs), which emit in the wavelength range of 250 to 275 nm, have demonstrated performance suitable for disinfection applications. [1][2][3][4][5] Their demonstrated advantages, such as small size, robustness, and instant turn-on have allowed them to replace mercury lamps in niche applications that do not require large power outputs. However, improvements in the output power and efficiency will enable the MUV LEDs to compete with mercury lamps in many more applications.The external quantum efficiency (EQE) is the product of the internal efficiency (IE) and the extraction efficiency, where the IE is defined as the ratio of the number of photons created per unit time to the current in the device divided by the electronic charge e, and the photon extraction efficiency is defined as the percent of created photons that are able to escape the chip. As previously reported, 1) it has been possible to produce pseudomorphic ultraviolet (PUV) LEDs on highquality AlN substrates where the low defect density in the pseudomorphic active layers is instrumental in achieving internal efficiencies near 70% which is comparable to values achieved in visible LEDs. However, the EQE of the PUVLEDs in the previous report was much lower than that of visible LEDs due to the low light extraction efficiency which was estimated to be less than 4%. Three main issues cause this low extraction efficiency. The first is absorption in the p-GaN contact layer, resulting in the inability to collect any photons directed downwards toward the p-contact in a flip-chipped device. The second is absorption in the AlN substrate. Although the 6.1 eV band gap of AlN suggests that it should be transparent to radiation with a wavelength longer than 205 nm, AlN substrates can have absorption in the mid-UV due to point defects. 6-9) For a typical substrate prepared at Crystal IS with a thickness of 200 m and an absorption coefficient of 35 cm À1 , approximately 50% of the light is absorbed prior to reaching the surface. Finally, the high refractive index contrast between AlN and air leads to a small critical angle for light extraction. Although visible LEDs are able to use high-refractive-index encapsulants to extract photons with angles greater than the critical angle, 10) these materials typically suffer from a low transparency and/or a low stability under mid-ultraviolet radiation.In the present paper, improvements in the...

This letter reports on the improved performance of a pseudomorphic ultraviolet light-emitting diode (LED). At 100 mA input current, 9.2 mW of quasi-CW output power was measured in a calibrated integrating sphere. The addition of a heat sink, required for CW and higher power operation, introduced a numerical aperture of 0.86, and 72 mW was measured in pulsed mode at 1.7 A, indicating that the total output power exceeds 100 mW when corrected by the coupling factor. The high characteristic temperature of 983 K was instrumental in achieving these record output powers for an LED with wavelength shorter than 265 nm. #

Native cation vacancies in Si-doped AlGaN studied by monoenergetic positron beamsIdentification of vacancy-oxygen complexes in oxygen-implanted silicon probed with slow positrons Vacancy-type defects in AlN grown by metal-organic vapor phase epitaxy ͑MOVPE͒ and lateral epitaxial overgrowth ͑LEO͒ using halide vapor phase epitaxy were probed by a monoenergetic positron beam. Doppler broadening spectra of the annihilation radiation were measured and compared to the spectra calculated using the projector augmented-wave method. For MOVPE-AlN, the concentration of vacancy-type defects was high near the interface between AlN and the GaN buffer layer, and the defect-rich region expanded from the interface toward the surface when the NH 3 flow rate increased. For the sample grown on the AlN buffer layer, however, the introduction of such defects was suppressed. For LEO-AlN, distinct deep emission peaks at 3-6 eV were observed in cathodoluminescence spectra. From a comparison between Doppler broadening spectra measured for LEO-AlN and computer simulated ones, an origin of the peaks was identified as complexes of Al vacancy ͑V Al ͒ and oxygen atoms substituting nitrogen sites such as V Al ͑O N ͒ n ͑n = 3 and 4͒.

Lifetime measurements on single-chip, packaged 285 nm light-emitting diodes (LEDs) performed under constant current injection at 20 and 75 mA, were compared to the performance of unbiased LEDs baked at the equivalent operating junction temperatures. The thermally stressed devices showed a lesser degradation than those electrically stressed, indicating that elevated temperature alone does not cause degradation. Despite a decay to less than half of the initial power under current injection, time-resolved photoluminescence of the active region exhibits little change, while capacitance-voltage measurements imply that the reduced efficiency and power decay originate from the generation of point defects near the p-side of the p-n junction.