Development of ultraviolet bright light-emitting diodes using quaternary InAlGaN

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The demands for high-intensity ultraviolet(UV) laser diodes(LDs) and light-emitting diodes(LEDs) are increasing in the fields of high-density optical storage or high-efficiency lighting or biochemical and medical fields. GaN and III-nitride compound semiconductors are attracting considerable attention as candidate materials for the realization of UV-LDs and LEDs. The purpose of our work is to develop high-intensity UV-LEDs or LDs operating in the 250-350 nm wavelength range.We have demonstrated high-efficiency UV emission from quaternary InAlGaN-based quantum wells(QWs) in the wavelength range between 290-375 nm at room temperature(RT) using the In-segregation effect. Emission fluctuations in the submicron region due to In-segregation were clearly observed for quaternary InAlGaN epitaxial layers. An internal quantum efficiency as high as 15% was estimated for a quaternary InAlGaN-based single quantum well(SQW) at RT. Such high efficiency UV emission can even be obtained on high threading-dislocation density buffers. We fabricated 310 nm-band deep UV light-emitting diodes(LEDs) with quaternary InAlGaN active regions. We achieved sub-milliwatt output power under RT pulsed operation for 308-314 nm LEDs. We also demonstrated a high output power of 7.4 mW from a 352 nm quaternary InAlGaN-based LED fabricated on a GaN substrate under RT CW operation. The maximum external quantum efficiency(EQE) of the 352 nm InAlGaN-based LED was 1.1%, which is the highest EQE ever obtained for 350 nm-band UV LEDs with top-emission geometry. From these results, the advantages of the use of quaternary InAlGaN for 300-350nm-band UV emitters was revealed.