We perform first-principles calculations based on density functional theory (DFT) to study the electronic properties of HgCdTe alloys for infrared detection applications. The Heyd, Scuseria, and Ernzerhof (HSE) and the modified Becke-Johnson (mBJ) functionals are employed to predict the bandgap of the ternary alloy over the full composition range. Due to the disordered nature of ternary alloys, we compute the bandgap values by enumerating all distinct atomic arrangements of supercells up to 16 atoms. By using the alloy composition to tune the HSE and mBJ functionals, we show that both functionals successfully produce bandgap values in good agreement with the experimental data. Subsequently, we apply the developed model to study the electronic structure properties of the alloy and its binary compounds under biaxial strain. Our results show that biaxial strain leads to a reduction in the bandgap in CdTe. In contrast, HgTe transitions from a semimetal at its equilibrium geometry to an indirect gap semiconductor under the same strain conditions. For the ternary alloys, we examine alloy compositions for applications in the long and mid-wavelength infrared detection regimes. Strain was applied to 32-atoms representative supercells for Cd compositions of 21 % and 31 %, which were obtained using the Special Quasirandom Structure (SQS) method. For both compositions, all strain configurations lead to a reduction in the bandgap. However, bandgap narrowing exhibits a stronger dependence on the strain magnitude in the case of tensile strain compared to compressive strain.
Mercury cadmium telluride (HgCdTe or MCT) is the material of choice for infrared avalanche photodetectors (APDs) owing to its desirable qualities including high quantum efficiency and low excess noise factor. Recent advancements in growth techniques have allowed for bandgap engineered MCT films that further enhance the performance of MCT APDs. Monte Carlo has been a widely used method for simulating the multiplication process within avalanche photodiodes (APDs) due to its ability to accurately simulate non-equilibrium transport. In this work, we demonstrate how the gain, excess noise, and bandwidth of bandgap engineered MCT APDs can be accurately modeled in 3-D using Monte Carlo.
In this paper we report on recent advancements in the development of linear-mode photon-counting (LMPC) electron-initiated avalanche photodiodes (e-APDs) at Leonardo DRS. The Hg1-xCdxTe linear-mode e-APD fills a gap in single-photon detectors from near- to mid-infrared wavelengths and enables several new space lidar and laser communication applications. The combination of high e-APD gain and near unity excess noise factor enables robust, single-photon detection. Another important feature of the Hg1-xCdxTe e-APD is that there is no dead time or latency between successive photon detection events. Since the inception of the device, Leonardo DRS has sought to improve the performance of these e-APDs by: increasing linear gains to greater than 1000; decreasing single photon jitter; reducing ROIC glow contributions to dark counts; and decreasing intrinsic detector dark currents. To these ends, we begin by showing that ROIC glow contributions to the false-event rate (FER) can be significantly reduced using an improved, photon blocking shield. We continue by examining the performance of focal-plane arrays (FPAs) with two differing material cutoff wavelengths, demonstrating record low FERs at high photon detection efficiencies (PDEs); this improvement in performance is assisted in part to the successful integration of micro-lens arrays (MLAs) onto the detectors. We conclude our study by integrating one detector unit into a tactical, Integrated Dewar Cooler Assembly (IDCA) and comparing performance prior and following this integration.
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