Reliability and Performance of Pseudomorphic Ultraviolet Light Emitting Diodes on Bulk Aluminum Nitride Substrates

Abstract

Reliability and performance of ultraviolet light emitting diodes have suffered due to the high dislocation density of the AlN and high Al-content Al_xGa_1-xN layers when grown on foreign substrates such as sapphire. The development of pseudomorphic layers on low dislocation density AlN substrates is leading to improvements in reliability and performance of devices operating in the ultraviolet-C (UVC) range. One major improvement is the ability to operate devices at much higher current densities and input powers than devices on sapphire substrates. This is due to the better thermal properties and lower dislocation density of devices on AlN substrates. Devices with active area of 0.001 cm² emitting at ∼265 nm have been measured for their reliability and change in power output over time at input currents of 20 mA (20 A/cm²), 100 mA (100A/cm²) and 150 mA (150 A/cm²). When operating at currents of 20 mA over 3500 hours of consecutive operation has been demonstrated with typical decay of ∼27% over the 3500 hours. Extrapolating the decay with a linear fit gives a L50 (time to 50% of initial power) of ∼5000 hrs. However it is desirable to be able to model the decay to better understand the kinetics and better understand the mechanisms. In order to do this, the lifetime at 20 mA and 100 mA were modeled using an exponential decay function, square root transformation and a log transformation to both be able to fit the experimental data and predict future performance.