Investigation on reliability of red micro-light emitting diodes with atomic layer deposition passivation layers

In this study, AlGaInP red light emitting diodes with sizes ranging from 5 to 50 micrometers were fabricated and characterized. The atomic layer deposition technology is applied to coat a layer of silicon dioxide for passivation and protection. The top emission area is covered by ITO layer to maximize the optical output. From the optical measurement, the linewidth and emission peaks shift very little among different current levels (from 30 to 150 A/cm²). High current level lifetests are performed and a 15 µm ALD device can last 27 hours of continuous operation at 100 A/cm² before their diode junction failed. A much shorter lifetime of 5.32 hours was obtained when the driving current is raised to 400 A/cm². When the same condition was applied to 15 µm PECVD devices, 25 hours and 4.33 hours are registered for 100 A/cm² and 400 A/cm² tests, respectively. The cross-sectional SEM reveals the voids, defects, and dark lines developed during the aging tests, and most of them are caused by top contact failure. The surface layers of ITO and SiO₂ were melted and the dark lines which were originated from the top surface propagated through the device and led to the eventual failure of the diode. The optical intensity degradation slopes of different sizes of devices indicate a large device can last longer in this accelerated aging test. The efficiencies of the devices are also evaluated by the ABC model and the fitted bimolecular coefficient ranges from 1.35 to 3.40×10^-10 cm³/s.