Recent advances in mid-ultraviolet light-emitting diodes grown pseudomorphically on bulk AlN substrates have led to improved efficiencies and lifetimes. For a 266 nm device an output power of 66 mW at 300 mA has been achieved with an external quantum efficiency of 4.5%. More importantly, the lifetimes of these devices have been increased substantially. Testing of LEDs in both surface mount design (SMD) and TO-39 packages show L<sub>50</sub> lifetimes well in excess of 1,000 hours under a variety of case temperatures and currents. Package-related catastrophic failures are eliminated through encapsulation and hermetic sealing, further reducing failure rates and extending the lifetime.
Compact ultraviolet light sources are currently of high interest for a range of applications, including solid-state lighting, short-range communication, and bio-chemical detection. We report on the design and analysis of AlGaN-based light-emitting diodes with an emission wavelength near 280 nm. Internal device physics is investigated by three-dimensional numerical simulation. The simulation incorporates a drift-diffusion model for the carrier transport, built-in polarization, the wurtzite energy band-structure of strained quantum wells, as well as radiative and nonradiative carrier recombination. Critical material parameters are identified and their impact on the simulation results is investigated. Limitations of the internal quantum efficiency by electron leakage and nonradiative recombination are analyzed. Increasing the stopper layer bandgap is predicted to improve the quantum efficiency and the light output of our LED substantially.
Ultra-violet light emitting diodes with a peak wavelength of 293 nm were grown by MOCVD on AlN on sapphire. The maximum output power was 15 μW at 100 mA DC current injection for on wafer, room temperature testing. We have shown that by forming an interdigitated multi-fingered n-contact compared to a square geometry LED, the series resistance is reduced by ~ 8 - 15 Ω at 100 mA. This results in a 2 - 4 V reduction in drive voltage at 100 mA. The quantum wells exhibit a sharp electroluminescence peak at 293 nm with a 9 nm full-width at half maximum, but deep level related emission was observed at 2.56, 2.80, 3.52, and 3.82 eV. The high energy peaks, 3.52 and 3.82 eV, saturate with increasing drive current while the low energy peaks, 2.56 and 2.80 eV, increase with drive current proportional to the quantum well emission. This indicates the recombination mechanism for the low energy and high energy peaks is fundamentally different. We have also shown that forward bias leakage current in these devices is another factor limiting the quantum efficiency.