No abstract

Solid state UV emitters have many advantages over conventional UV sources. The (Al,In,Ga)N material system is best suited to produce LEDs and laser diodes from 400 nm down to 210 nm—due to its large and tuneable direct band gap, n- and p-doping capability up to the largest bandgap material AlN and a growth and fabrication technology compatible with the current visible InGaN-based LED production. However AlGaN based UV-emitters still suffer from numerous challenges compared to their visible counterparts that become most obvious by consideration of their light output power, operation voltage and long term stability. Most of these challenges are related to the large bandgap of the materials. However, the development since the first realization of UV electroluminescence in the 1970s shows that an improvement in understanding and technology allows the performance of UV emitters to be pushed far beyond the current state. One example is the very recent realization of edge emitting laser diodes emitting in the UVC at 271.8 nm and in the UVB spectral range at 298 nm. This roadmap summarizes the current state of the art for the most important aspects of UV emitters, their challenges and provides an outlook for future developments.

We present a deep-ultraviolet semiconductor laser diode that operates under current injection at room temperature and at a very short wavelength. The laser structure was grown on the (0001) face of a single-crystal aluminum nitride substrate. The measured lasing wavelength was 271.8 nm with a pulsed duration of 50 ns and a repetition frequency of 2 kHz. A polarization-induced doping cladding layer was employed to achieve hole conductivity and injection without intentional impurity doping. Even with this undoped layer, we were still able to achieve a low operation voltage of 13.8 V at a lasing threshold current of 0.4 A.

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...