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The very long wavelength infrared (VLWIR, >14 µm) spectral band is an indispensable part of new-generation infrared remote sensing. Mercury cadmium telluride (HgCdTe or MCT) has shown excellent potential across the entire infrared band. However, the dark current, which is extremely sensitive to the technological level and small Cd composition, severely limits the performance of VLWIR HgCdTe photodiodes. In this study, cut-off wavelengths of up to 15 µm for HgCdTe devices with novel P-G-I (including wide bandgap p-type cap layer, grading layer and intrinsic absorption layer) designs have been reported. Compared with a device with a double-layer heterojunction (DLHJ) structure, the designed P-G-I structure successfully reduced dark current by suppressing the Shockley–Read–Hall process. Considering the balance of quantum efficiency and dark current, with the introduction of an approximately 0.8 µm thickness Cd composition grading layer, the device can achieve a high detectivity of up to 2.5×1011 cm Hz1/2 W−1. Experiments show that the P-G-I-T device has a lower dark current and a better SRH process suppressing ability than DLHJ devices, the measured detectivity achieved 8.7×1010 cm Hz1/2 W−1. According to additional research, the trap-assisted tunneling current is the primary component of the dark current. Controlling the trap concentration to as low as 1×1013 cm−3 will be continuous and meaningful work. The proposed study provides guidance for VLWIR HgCdTe photodetectors.

With the development of infrared optoelectronic technology, high responsivity, ultra-low dark current, and high response speed have become important factors of the next generation of infrared photodiodes. However, the minimum thickness of the absorber layer is limited to approximately one or several wavelength lengths to acquire high quantum efficiency, which results in a long transit time of photogenerated carriers. In this work, we propose a photon-trapping structure that uses the skin effect of metals to generate horizontal local modes to enhance the absorption of infrared photodiodes. The photon-trapping structure consists of an artificial grating structure covered by a metallic film. Importantly, we develop a simplified theoretical model to describe the local mode, which is then being used to design the realistic photon-trapping structure presented in this work. This design method is universal and we discuss the optical properties of the photon-trapping structure in InAs, InSb, InAs/GaSb type-II superlattices, InAs/InAsSb type-II superlattices, and HgCdTe infrared photodiodes. Both absorption of optical properties and responsivity of optoelectrical properties are numerically investigated in a systematic way. The optical simulations indicate that the absorption of the HgCdTe infrared photodiodes exceeds 80% at 8.5 ∼ 11 µm with a maximum value of 95% at 9.73 µm. The optoelectrical simulations show that the responsivity at 7 ∼ 10 µm is significantly enhanced compared to that of the plain HgCdTe infrared photodiodes without the photon-trapping structure. We further investigate the optical crosstalk in the HgCdTe pixel array employing the photon-trapping structure. The optical crosstalk significantly reduces as the pixel spacing increases. Our work provides a design method for developing small pixel, large scale, and low dark current focal plane array infrared photodiodes.

The avalanche is the foundation of the understanding and vast applications of the breakdown of semiconductors and insulators. Present numerical theories analyzing the avalanche photodetectors are mainly split into two categories: the macroscopic empirical model with fitting parameters and the microscopic process simulation with statistical estimations. Here, we present a parameter-free analytic theory of the avalanche for a narrow-band material, HgCdTe, originated from quantum mechanics, avoiding any fitting parameter or any statistical estimation while taking advantage of both categories. Distinct from classical theory, we propose a full spatial description of an avalanche with basic concepts such as transition rate and equation of motion modified. As a stochastic process, the probability density function (PDF) of impact ionization is utilized in a generalized history-dependent theory. On account of different carrier generation character of light and leakage current, we suggest that carrier generated at different positions should be considered separately, which is done by generalized history-dependent theory in our work. Further, in our calculation, the reason for the abnormal rise of excess noise factor (ENF) observed in the experiment in single-carrier avalanche photodetectors is clarified.

We proposed a metamaterial absorber composed of an array of trapezoid multilayered grating Au and InP on top of an opaque substrate, which covers two atmosphere-transparent-window bands with appropriate modulation of geometric parameters. The absorption higher than 0.8 is from 3.5 to 4.8 µm and 7 to 14.3 µm. From the effective medium theory and dispersion relation, the reason of the broad-band absorption is the first and third order slow light effect respectively, which is verified by the electromagnetic and thermal loss distribution further. This absorber may greatly promote the practical application of absorbers in double-color infrared imaging, detecting, infrared stealth and sub-ambient passive radiative cooling by thermal emitting.