Improved operating voltage in InGaN-capped AlGaN-based DUV LEDs on bulk AlN substrates
Better wall plug efficiency of deep-ultraviolet light emitting diodes (DUV-LEDs) requires simultaneous low resistivity p-type and n-type contacts, which is a challenging problem. In this study, the co-optimization of p-InGaN and n- AlGaN contacts for DUV LEDs are investigated. We find that using a thin 7%InGaN cap is effective in achieving ohmic p-contacts with specific contact resistivity of 3.10x10^{-5} Ohm.cm^2. Upon monolithic integration of p- and n- contacts for DUV LEDs, we find that the high temperature annealing of 800C required for the formation of low resistance contacts to n-AlGaN severely degrades the p-InGaN layer, thereby reducing the hole concentration and increasing the specific contact resistivity to 9.72x10^{-4} Ohm.cm^2. Depositing a SiO2 cap by plasma-enhanced atomic layer deposition (PE-ALD) prior to high temperature n-contact annealing restores the low p-contact resistivity, enabling simultaneous low-resistance p- and n-contacts. DUV-LEDs emitting at 268 nm fabricated with the SiO2 capping technique exhibit a 3.5 V reduction in operating voltage at a current level of 400 A/cm^2 and a decrease in differential ON-resistance from 6.4 mOhm.cm^2 to 4.5 mOhm.cm^2. This study highlights a scalable route to high-performance, high-Al-content bipolar AlGaN devices.