Abstract
Extended short-wavelength infrared (eSWIR) avalanche photodiodes (APDs), operating in the 1700â2200 nm range, are widely used in LIDAR, sensing, and telecommunications. These applications require high-sensitivity photo detectors with a high signal-to-noise ratio (SNR). Recently, Sb-based materials such as AlInAsSb have demonstrated remarkably low excess noise factors, with k values in the range of 0.01â0.05. Using a thinner narrow-band-gap absorption layer within a separate absorption, charge, and multiplication (SACM) structure is a promising candidate for eSWIR and MWIR applications, offering both low excess noise and low dark current to enhance the SNR. My Ph.D. work focuses on low excess noise factor and low dark current AlInAsSb APD structures to improve the system SNR.
First, I investigated the physics behind the low excess noise of Sb-based avalanche photodiodes using a full band Monte Carlo model. The model simulates carrier transport at the angstrom level and calculates figures of merit such as gain and excess noise factor. I found that in Sb-based photo detectors, alloy scattering dominates over phonon scattering. Within the framework of the dead space model, I showed that carrier impact ionization becomes more deterministic in the presence of strong alloy scattering. The gain fluctuation, which directly contributes to the excess noise factor, is lower than in conventional APDs, where phonon scattering dominates.
Second, I designed, fabricated, and characterized thin-absorber AlInAsSb SACM APDs incorporating photon-trapping gratings. A 2D metal grating structure consisting of a periodic array of parallel Au pillars fabricated on top of the devices compensates for the reduction in external quantum efficiency (EQE) that comes with thinning the absorber. One practical challenge is that non-vertical sidewalls on the fabricated Au pillars limit the input coupling efficiency. I found that this can be significantly mitigated by filling the gaps between the pillars with a material of optimized refractive index, which I demonstrated successfully using BK-7 index-matching liquid (n ~ 1.5). Due to the metal grating geometry, the photo response is polarization dependent. By combining index matching with optimized input polarization, I achieved EQE values of 8.2%, 16%, 30.6%, and 35% for absorber thicknesses of 50, 100, 200, and 400 nm, respectively. Finally, I analyzed noise equivalent power (NEP) results for a 400 nm thin-absorber SACM APD at a gain of approximately 10, achieving a value as low as 26.1 fW/âHz at 200 K.
