Growth, Characterization, and Application of High Al-content AlGaN and High Power III-Nitride Ultraviolet Emitters
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Growth, Characterization, and Application of High Al-content AlGaN and High Power III-Nitride Ultraviolet Emitters Z. Ren1, S.-R. Jeon1, M. Gherasimova1, G. Cui1, J. Han1, H. Peng2, Y. K. Song2, A. V. Nurmikko2, L. Zhou3, W. Goetz3, M. Krames3, H.-K. Cho4 1 Department of Electrical Engineering, Yale University, New Haven, CT 06520, USA 2 Division of Engineering, Brown University, Providence, RI 02912, USA 3 Lumileds Lighting, LLC, San Jose, CA 95131, USA 4 Materials Science and Engineering, Dong-A University, Saha-gu, Busan 604-714, Korea ABSTRACT A study of Si-doped and Mg-doped AlxGa1-xN up to x ∼ 50 % and the characteristics of ultraviolet (UV) light emitting diodes (LEDs) with emission wavelengths at 340 nm and 290 nm are reported. By using grading super-lattices (SLs) before n-type AlGaN growth, surface roughness is much improved. Resistivity of 2.9×10-2 Ωcm and free electron concentrations of 2.9×1018 cm-3 are achieved for n-type Al0.45Ga0.55N. The viability of effective p-type doping is defined by a minimum concentration of Mg required to offset the background impurities and, more importantly, a maximum limit above which inversion domains and structural defects start to nucleate, accompanied by a rapid degradation of electrical transport. Resistivity of 10 Ωcm and free hole concentrations above 1017 cm-3 are achieved for AlxGa1-xN up to x ∼ 50 % within an optimum window of Mg incorporation. Output powers up to 1.5 mW from small area 340 nm LEDs (< 100 µm diameter) and 110 µW from 290 nm LEDs (100 µm diameter) directly off a planar chip have been achieved under DC condition. For large area encapsulated lamp (1×1 mm2 device area and 0.52 mm2 mesa area), output power of 79 mW from 340 nm LEDs and 8.5 mW from 290 nm LEDs are achieved under pulse mode (1kHz, 2% duty factor). INTRODUCTION Ultraviolet light (UV) emitters based on wide-band gap III-nitride compound semiconductors have attracted much attention recently because of their numerous application in civilian and other environments, including efficient white lighting, high-density optical data storage, non-line-of-sight communication and chemical/biological agent detection[1][2]. AlGaN ternary compounds posses a wide direct band gap that varies from 3.4 eV for GaN to 6.1 eV for AlN, corresponding to wavelength range of 200-360 nm. This makes AlGaN a promising candidate for optoelectronic devices that cover a broad portion of UV spectrum. However, in order to achieve short-wavelength emission, a high Al mole fraction is necessary, which makes the growth and doping of the material very difficult. Specifically, high dislocation density, slow growth rates and insufficient conductivity of doped epilayers present serious obstacles[3][4]. Achieving highly conductive AlGaN, especially p-type AlGaN, constitutes a critical element for the realization of high-performance UV optoelectronic devices. There is no report, to the best of our knowledge, of electrical transport measurement on p-type AlxGa1-xN with x greater than 30 %. In this work we report an investiga
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