Role of neutral impurities and non-parabolic dispersive electrons in HgCdTe avalanche photodiodes

HgCdTe avalanche photodiodes (APDs) exhibit unique gain stability, making them advantageous for weak signal detection. However, research on neutral impurities (NIs) scattering in HgCdTe remains limited, and the non-parabolic electron dynamics pose significant error in current device scale models. These limitations hinder the theoretical and the industrial development of HgCdTe APD. Here, we designed Si-substrate devices and CdZnTe-substrate devices with high and low NI density, respectively, to clarify the role of NIs. Combining first-principle calculations with Monte Carlo transport simulations, we find that the avalanching electrons could be categorized into four types (I-IV below) with distinct behavior. Furthermore, we proposed a step-solving Monte Carlo (SSMC) simulation framework to resolve accurately electron trajectories with non-parabolic dispersion. We then clarify different contemporary models of impact ionization and polar optical phonon scattering with SSMC and experimental results. Notably, our findings also show that Si-substrate HgCdTe APDs show nearly the same avalanche performance as the classical CdZnTe ones, suggesting that they could play a pivotal role in high-performance infrared detection in the future.