Mid-wave infrared (MWIR) detectors, with a spectral range of 3?5 μm, are indispensable in a wide range of applications, including aerospace, missile early warning systems, infrared imaging, biochemical gas detection, and environmental monitoring. There is an increasing demand for high-performance MWIR focal plane arrays (FPAs) to support the development of compact, efficient, and high-resolution imaging systems. However, traditional MWIR materials like InSb and HgCdTe (MCT) face inherent challenges. InSb detectors, with their narrow bandgap tunability and low operating temperatures (~80?100 K), are limited in applicability to compact infrared systems. MCT, despite its tunable bandgap, suffers from poor material uniformity, high defect density, and limited scalability for large arrays. To address these issues, III-V semiconductor materials, particularly InAs/InAsSb type-II superlattices (T2SLs), have emerged as promising candidates due to their excellent material uniformity, larger bandgap tunability, and compatibility with low-cost substrate technologies. This study focuses on the design, fabrication, and performance characterization of InAs/InAsSb T2SL-based MWIR FPAs, with an emphasis on achieving high operating temperature (HOT) performance. We employ an nBn barrier structure and strain-balanced epitaxial growth via molecular beam epitaxy (MBE) to fabricate 640×512 array detectors and evaluate their material and optoelectronic properties under operational conditions.

The nBn structure is designed to minimize dark current and enhance carrier transport. The epitaxial layers are grown on n-type GaSb (001) substrates using solid-source MBE. Key parameters, including growth temperature, V/III beam equivalent pressure ratios, and layer thicknesses, are carefully optimized to achieve strain balance and high crystal quality. Real-time monitoring with reflection high-energy electron diffraction (RHEED) ensures precise control of the growth process. The complete nBn structure comprises a 200 nm n-doped InAs/InAsSb T2SL bottom contact layer, a 3 μm unintentionally doped T2SL absorber layer (AL), a 180 nm AlAsSb barrier layer (BL), a 200 nm n-doped T2SL top contact layer, and a 20 nm n-doped InAs capping layer. The T2SL layers consist of alternating InAs (3.75 nm) and InAsSb (1.3 nm) layers with constant Sb composition. Device fabrication involves ICP etching to define the mesa structure, followed by dielectric passivation to reduce surface leakage currents. Ti/Pt/Au metal contacts and In bump interconnects are then deposited to establish reliable electrical connections. The fabricated 640×512 FPAs are hybridized with readout integrated circuits (ROICs) via flip-chip bonding. The final devices are encapsulated in Dewar packages for performance testing at 130 K. Material characterization methods include atomic force microscopy (AFM) for evaluating surface morphology and high-resolution X-ray diffraction (HRXRD) for assessing crystal quality and lattice matching. Optoelectronic characterization involves spectral response measurements, dark current analysis, and imaging performance evaluation.

AFM results confirm the excellent surface morphology of the epitaxial layers. Root mean square (RMS) surface roughness values are measured at 0.239, 0.200, and 0.179 nm for scanning areas of 50 μm×50 μm, 5 μm×5 μm, and 1 μm×1 μm, respectively (Fig. 3). The presence of atomic steps in the smallest scanned area indicates high surface quality and precise control over the epitaxial growth process. HRXRD analysis further validates the structural quality of the layers. The (004) ω-2θ diffraction profile exhibits sharp and well-defined satellite peaks (SL is ±1, ±2, ±3), with the full width at half maximum (FWHM) of the primary SL0 peak measuring 25.1″ (Fig. 5). These results confirm the successful implementation of strain-balanced growth and precise lattice matching between the T2SL layers and the GaSb substrate. Additionally, the atomic number fraction of Sb of 0.35 in the absorber layer is consistent with the design specifications, while the barrier layer demonstrates excellent alignment with the substrate lattice. The fabricated MWIR FPAs exhibit outstanding performance under hot conditions. At 130 K, the detectors achieve an average peak detectivity of 4.81×1011 cm·Hz1^/2·W^-1, with a noise equivalent temperature difference (NETD) of 15.8 mK. The defective pixel rate is as low as 0.16%, and the responsivity non-uniformity is measured at 4.67% (Table 1). The devices exhibit a 50% cutoff wavelength of 5.18 μm and a 100% cutoff wavelength of 5.75 μm (Fig. 9), which fully meets the requirements for MWIR detection applications. Dark current analysis indicates that, at a -0.5 V bias, the device exhibits a dark current density of 4.57×10^-5 A/cm2 at 150 K, which increases to 9×10^-2 A/cm2 at 295 K [Fig. 8(a)]. The Arrhenius plot of the J-V characteristics reveals an activation energy of 206 meV for temperatures above 150 K, which closely matches the material’s bandgap energy. This strongly confirms that diffusion current is dominated in this range [Fig. 8(b)]. At lower temperatures, tunneling currents become more significant, with an activation energy of 25 meV. Imaging tests at 130 K further demonstrate the detector’s capability to capture high-resolution thermal images. The device effectively resolves fine thermal details, such as facial features, flame contours, and subtle temperature variations on textured surfaces (Fig. 10). These results confirm the detector’s high temperature resolution and imaging quality, which makes it ideal for applications that require detailed infrared imaging and target detection.

This study demonstrates the potential of InAs/InAsSb T2SL MWIR FPAs for high-performance infrared imaging under hot conditions. The optimized nBn barrier structure and strain-balanced epitaxial growth lead to superior material quality, low dark current, and high detectivity. The 640×512 array exhibits excellent uniformity, low noise, and robust imaging performance, which confirms its suitability for MWIR imaging systems. The detector’s performance highlights its potential as a viable alternative to traditional MWIR materials like InSb and MCT, particularly in applications requiring compact, high-temperature-capable systems. Future efforts will focus on scaling the array size further, optimizing device fabrication processes, and integrating advanced ROICs to enhance system-level performance. These advancements aim to broaden the technology’s applicability in portable, high-resolution, and high-dynamic-range infrared imaging systems.