Advances in AlGaN-based Deep UV LEDs
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Advances in AlGaN-based Deep UV LEDs M. H. Crawford1, A. A. Allerman2, A. J. Fischer1, K. H. A. Bogart2, S. R. Lee1, W. W. Chow1, S. Wieczorek1, R. J. Kaplar1 and S. R. Kurtz1 1 Semiconductor Material and Device Sciences Department, Sandia National Laboratories, Albuquerque, NM 87185, USA 2 Advanced Material Sciences Department, Sandia National Laboratories, Albuquerque, NM 87185, USA ABSTRACT Materials studies of high Al-content (> 30%) AlGaN epilayers and the performance of AlGaN-based LEDs with emission wavelengths shorter than 300 nm are reported. N-type AlGaN films with Al compositions greater than 30% reveal a reduction in conductivity with increasing Al composition. The reduction of threading dislocation density from the 1-5 x1010 cm-2 range to the 6-9 x 109cm-2 range results in an improvement of electrical conductivity and Al0.90Ga0.10N films with n= 1.6e17 cm-3 and µ=20 cm2/Vs have been achieved. The design, fabrication and packaging of flip-chip bonded deep UV LEDs is described. Large area (1 mm x 1 mm) LED structures with interdigitated contacts demonstrate output powers of 2.25 mW at 297 nm and 1.3 mW at 276 nm when operated under DC current. 300 µm x 300 µm LEDs emitting at 295 nm and operated at 20 mA DC have demonstrated less than 50% drop in output power after more than 2400 hours of operation. Optimization of the electron block layer in 274 nm LED structures has enabled a significant reduction in deep level emission bands, and a peak quantum well to deep level ratio of 700:1 has been achieved for 300 µm x 300 µm LEDs operated at 100 mA DC. Shorter wavelength LED designs are described, and LEDs emitting at 260 nm, 254nm and 237 nm are reported. INTRODUCTION A new and expanding area of research is the exploration of nitride-based materials and device structures for electroluminescence (EL) at wavelengths shorter than 300 nm. Interest in this wavelength range is motivated by the large number of applications that would benefit from a compact, robust, wavelength tailorable, milliwatt-level deep-UV source that could replace mercury lamps and similar UV sources. These applications include fluorescence-based biological agent detection [1], water purification, sterilization and decontamination, non-line-ofsight communications [2] and thin film curing. While most near-UV (380-400 nm) light emitting diodes (LEDs) employ InGaN quantum well structures with GaN barriers, reaching deep-UV wavelengths requires AlGaN alloys with aluminum concentrations of 50% and higher. These wide bandgap alloys suffer from a number of materials issues which can dramatically reduce LED performance, including high dopant ionization energies and a tendency for dopant compensation [3,4], high densities of threading dislocations (typically greater than 5x109 cm-2), and large internal fields due to spontaneous polarization and piezoelectric effects. Despite these challenges, notable advances have recently been made in the performance of deep UV LEDs through the efforts of several groups [5-10]. In this paper, we present materia
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