Molecular beam epitaxy (MBE) is a highly controlled thin-film deposition technique widely used for growing high-quality infrared (IR) and terahertz (THz) materials such as InSb and HgCdTe. These materials are critical for advanced optoelectronic applications, including thermal imaging, spectroscopy, and high-speed communication systems. The precision of MBE allows for tailored composition grading, minimized defect densities, and optimized lattice matching with substrates, all of which are essential for achieving superior device performance.

One of the key advantages of MBE is its ability to precisely control material composition at the atomic level. For HgCdTe, the alloy composition (Hg₁₋ₓCdₓTe) determines the cutoff wavelength of the IR detector, which can range from short-wave infrared (SWIR) to very long-wave infrared (VLWIR). By adjusting the flux ratios of Hg, Cd, and Te during growth, the Cd fraction (x) can be graded to create heterostructures with tailored bandgap profiles. This is particularly useful for avalanche photodiodes (APDs) and dual-band detectors, where composition grading minimizes carrier recombination and enhances quantum efficiency.

InSb, another important IR material, is grown by MBE with strict stoichiometric control to avoid antisite defects and vacancies. The narrow bandgap of InSb (0.17 eV at 300 K) makes it ideal for mid-wave infrared (MWIR) detection. However, achieving high electron mobility requires minimizing defects such as In vacancies and Sb precipitates. MBE growth under Sb-rich conditions, coupled with optimized substrate temperatures (typically 350–400°C), reduces defect densities to below 10¹⁵ cm⁻³, enabling high-performance photodetectors.

Defect minimization is critical for both HgCdTe and InSb due to their susceptibility to Shockley-Read-Hall recombination, which degrades carrier lifetimes. In HgCdTe, the primary defects are Hg vacancies and Te precipitates. Post-growth annealing in a Hg-rich atmosphere can reduce Hg vacancy concentrations below 10¹⁴ cm⁻³. For InSb, dislocation densities must be kept low (< 10⁴ cm⁻²) to prevent carrier scattering. This is achieved by using lattice-matched substrates and low-growth-rate MBE (typically 0.5–1.0 µm/h).

Substrate selection and lattice matching are crucial for minimizing strain-induced defects. HgCdTe is commonly grown on CdZnTe substrates due to their close lattice match (< 0.1% mismatch for x ≈ 0.3). However, CdZnTe substrates are expensive and limited in size. Alternative substrates such as GaAs and Si require buffer layers to accommodate the large lattice mismatch (~14% for GaAs). Graded CdTe/ZnTe buffers can reduce threading dislocation densities to < 10⁶ cm⁻², enabling viable HgCdTe growth on non-native substrates.

InSb is typically grown on InSb or GaAs substrates. While homoepitaxial growth on InSb ensures perfect lattice matching, GaAs substrates are more cost-effective despite a 14.6% mismatch. A metamorphic AlSb or AlInSb buffer layer can bridge the lattice mismatch, reducing dislocation densities to < 10⁶ cm⁻². The buffer layer must be thick enough (≥ 1 µm) to prevent strain propagation into the active InSb layer.

Applications of MBE-grown IR and THz materials span imaging and communication systems. HgCdTe detectors dominate high-performance thermal imaging due to their high quantum efficiency and tunable spectral response. Focal plane arrays (FPAs) with cutoff wavelengths up to 12 µm are used in military, astronomical, and environmental monitoring applications. Dual-band HgCdTe FPAs enable simultaneous detection in MWIR and LWIR bands, enhancing target discrimination in complex environments.

InSb photodiodes are widely used in MWIR imaging systems, particularly for missile guidance and surveillance. Their high detectivity (D* > 10¹¹ cm·Hz¹/²/W) and fast response (< 1 ns) make them suitable for high-speed applications. Recent advances in InSb APDs have extended their utility to photon-counting lidar systems, where low-noise amplification is critical.

THz applications leverage the unique properties of narrow-gap semiconductors. InSb-based devices, such as THz emitters and detectors, exploit impact ionization and intervalley scattering to generate and detect THz radiation (0.1–10 THz). These devices are used in security screening, biomedical imaging, and high-bandwidth wireless communication. HgCdTe heterodyne detectors are also employed in THz astronomy for studying cosmic background radiation.

The future of MBE-grown IR and THz materials lies in monolithic integration with silicon readout circuits and advanced heterostructure designs. Strain-engineered superlattices, such as InAs/InSb type-II systems, offer tunable bandgaps and enhanced carrier transport for next-generation detectors. Additionally, the development of large-area MBE systems will enable cost-effective production of HgCdTe and InSb wafers for commercial applications.

In summary, MBE provides unparalleled control over the growth of IR and THz materials, enabling precise composition grading, defect reduction, and lattice matching. These capabilities are essential for high-performance imaging and communication systems, driving advancements in both military and civilian technologies. Continued innovation in MBE techniques will further expand the applications of these critical semiconductor materials.