[21a-146-1~12] 15.4 III-V-group nitride crystals

▲ [21a-146-9] Over 20 mW operation of 303 nm AlGaN UVB LED with p-AlGaN transparent contact layer

〇Muhammad Ajmal Khan^1,2, Noritoshi Maeda^1,2, Masafumi Jo^1,2, Hideki Hirayama^1,2 (1.RIKEN, 2-1, Hirosawa, Wako, Saitama 351-0198, Japan, 2.RIKEN Center for Advanced Photonics (RAP), 2-1, Hirosawa, Wako, Saitama 351-0198, Japan)

Keywords:AlGaN, 303nm-band UVB LED, LP-MOVPE

Environmentally safe, smart and high-power UVB LED light sources are demanded for both medical and agricultural applications, including vitamin D3 production in the human body, immunotherapy, and enriching phytochemicals in the plants. Previousely we demonstrated and reported an output power of 11.1 mW for a 310 nm UVB LED under a wafer level measurement after conducting some optimization of growth conditions both for n-AlGaN and p-AlGaN in the LP-MOVPE. We used a graded n-AlGaN buffer layer and n-AlGaN current spreading layer to increase the output power of the UVB LEDs. Previousely we reported about EQE of 3.8 % under a dc drive current of 20 mA with maximum output power of 14 mW, which were obtained by using the structure with a transparent p-AlGaN contact layer and a conventional Ni/Au p-electrode. In this work, we performed further optimizations for a UVB LED and obtained more than 20 mW output power. The LED layer structure was almost the same as previous one consisting of an n-AlGaN buffer layer grown on AlN/sapphire, a 3-layer AlGaN MQW, a 2-layer p-AlGaN multi quantum barrier (MQB) electron blocking layer (EBL) and a p-AlGaN contact layer. We demonstrated high IQE of 47 % for an AlGaN MQW emitting at 303 nm measured by the excitation power density and temperature dependencies of PL. We also tuned the barrier thickness and potential height of the final barrier (between MQW and MQB) to improve the electron injection efficiency (EIE), which determines the energy barrier height of EBL.The light-extraction efficiency (LEE) was improved both by introducing a highly-reflective Ni/Al p-electrode (82% reflectance) and highly transparent p-AlGaN contact (97%). Subsequently, a record EQE of 5.8 % and maximum output power of 23 mW under a dc of 20 mA on bar wafer level measurment were obtained for a 303 nm wavelength emission UVB LED. This result has a great potential for the development of UVB laser diodes (LDs) on AlN substrate or AlN template on sapphire substrate.