Passivation study for AlInAsSb PIN photodiodes

Avalanche photodiodes (APDs) based on the Al_0.7InAsSb material system are emerging as strong candidates for next-generation infrared detection due to their high avalanche gain, low excess noise, and tunable bandgap. However, the mesa-etched sidewalls of these devices are highly susceptible to surface oxidation and defect formation, which can significantly increase surface leakage currents and degrade overall device performance. Effective sidewall passivation is therefore essential to reduce dark current and maintain long-term reliability. In this work, we present a comparative study of three sidewall passivation approaches—SU-8 polymer, atomic layer deposition (ALD) of HfO₂, and Al₂O₃, focusing on their effectiveness in reducing surface leakage in Al_0.7InAsSb PIN APDs grown on InP substrates. Beyond electrical performance benchmarking, we use high-resolution scanning electron microscopy (SEM) to examine the morphological changes at the device sidewalls before and after passivation. Spatially resolved raster scans of the photocurrent, with and without passivation, clearly show suppression of the sidewall electric field. Additionally, transfer length method (TLM) measurements demonstrate improved contact resistance and reduced sheet resistance with an optimized ALD passivation process. Among all approaches tested, Al₂O₃ deposited by ALD at 150 °C consistently provides the best passivation performance, resulting in a significant reduction in dark current, improved breakdown voltage, and stable operation under thermal stress. These results provide key insights into the correlation between surface morphology, leakage current pathways, and passivation quality, guiding the design of low-noise, high-reliability APDs for advanced optoelectronic applications.