270 nm Pseudomorphic Ultraviolet Light-Emitting Diodes with Over 60 mW Continuous Wave Output Power

Abstract

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...

Status of Growth of Group III-Nitride Heterostructures for Deep Ultraviolet Light-Emitting Diodes

“…[3,10,84] A high IQE value of ~80% at a photo-generated carrier density of 10 18 cm -3 was achieved in UVC MQW structures emitting at ~258 nm, which were pseudomorphically grown on bulk AlN substrates that contain less than 10 3 cm -2 dislocations, [43] as revealed by synchrotron white-beam X-ray topography. [90] In an encapsulated pseudomorphic LED structure with improved LEE, through thinned AlN substrate, an output power of over 60mW and an EQE of 4.9% have been realized in continuous wave operation.…”

“…[2] Improvement of the extraction efficiency by die-thinning and encapsulation has led to enhancement of the EQE to 4.9% for 271nm LEDs under CW operation. [3] In 2012, a maximum EQE of 10.4% at 20 mA CW current with an output power up to 9.3 mW for 278 nm DUV LEDs was achieved through migration-enhanced metal organic chemical vapor deposition (MEMOCVD) to reduce the threading dislocation density (TDD) and by using transparent p-type cladding and contact layers, UV reflecting ohmic contacts, and optimized chip encapsulation. [4] In 2015, a 265 nm DUV LED with an output power density above 90 mW/cm 2 based on extraction enhancement has been reported.…”

“…Reported external quantum efficiencies (EQE) for group III-nitrides-based near and deep UV LEDs. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] In part courtesy of Prof. M. Kneissl of Technische Universität Berlin.…”

“…To minimize this absorption, the AlN substrates were thinned down to 20 μm. [3] The origin of 4.7 eV band was reportedly related to the carbon contaminations. [48] Density functional theory (DFT) calculations have predicted that substitutional carbon on the nitrogen site (CN) introduces absorption at this energy.…”

Enhanced light extraction in tunnel junction-enabled top emitting UV LEDs

“…As a solution, light is most efficiently extracted from the substrate side of the devices using flip chip bonding. [2][3][4][5][6][7] To reduce internal reflection, micro-structure patterning has to be done on the back side of the devices, and device encapsulation and packaging is necessary. Despite these techniques, the achieved light extraction efficiency is only 25% 8 , which is much lower than the visible LEDs (> 80%) 9 .…”