Advances in AlGaN-based Deep UV LEDs

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 ×10¹⁰ cm ^-2 range to the 6-9 × 10⁹cm^-2 range results in an improvement of electrical conductivity and Al _0.90Ga_0.10N 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 × 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 × 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 × 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.