Molecular beam epitaxy (MBE) is a highly controlled thin-film deposition technique enabling precise growth of semiconductor materials for infrared (IR) and terahertz (THz) applications. The method operates under ultra-high vacuum conditions, allowing for atomic-level control over composition, doping, and interfacial abruptness. This precision is critical for developing advanced optoelectronic devices, particularly detectors and imaging systems operating in spectral ranges from mid-wave infrared (MWIR) to terahertz frequencies.

One of the most extensively studied material systems grown via MBE for IR applications is mercury cadmium telluride (HgCdTe or MCT). The ternary alloy's bandgap can be tuned across a wide range (0 to 1.6 eV) by adjusting the cadmium composition during growth. This tunability enables detectors tailored for specific IR bands, including short-wave (SWIR, 1-3 μm), mid-wave (MWIR, 3-5 μm), long-wave (LWIR, 8-12 μm), and very-long-wave infrared (VLWIR, >12 μm). MBE growth of HgCdTe typically occurs on lattice-matched substrates such as CdZnTe, though alternative substrates like silicon and GaAs are also explored to reduce cost and improve scalability.

Bandgap engineering in HgCdTe is achieved by controlling the CdTe/HgTe flux ratio during deposition. The composition gradient can be precisely modulated to create complex heterostructures, including superlattices and multi-layer designs that enhance carrier confinement and reduce dark currents. For THz applications, narrow-bandgap HgCdTe layers (with Cd composition below 0.2) are grown to enable detection in the 1-10 THz range. These structures often incorporate doping profiles to optimize carrier transport and minimize noise.

Doping in MBE-grown HgCdTe is carefully controlled using extrinsic impurities. Indium is commonly employed as an n-type dopant due to its low diffusivity and high activation efficiency, while arsenic and antimony serve as p-type dopants when properly activated through annealing. The ability to precisely position dopant atoms within the crystal lattice is crucial for forming sharp junctions in photovoltaic detectors and reducing Shockley-Read-Hall recombination.

Another material system of interest for IR and THz applications is indium antimonide (InSb). With a narrow bandgap of 0.17 eV at room temperature, InSb is well-suited for MWIR detection (3-5 μm). MBE growth of InSb requires strict control over stoichiometry to minimize defects and ensure high electron mobility, which can exceed 70,000 cm²/V·s in high-quality epitaxial layers. Doping is typically achieved using tellurium (n-type) and beryllium or carbon (p-type). InSb-based detectors exhibit fast response times and high sensitivity, making them ideal for high-speed imaging and spectroscopy.

Device integration of MBE-grown materials involves fabricating photodiodes, focal plane arrays (FPAs), and heterodyne detectors. For HgCdTe, the n-on-p architecture is widely used, where an n-type absorber layer is coupled with a p-type wide-bandgap layer to reduce surface recombination. Alternatively, barrier-engineered structures such as the nBn design suppress dark currents by incorporating an electron-blocking barrier. InSb detectors are often configured as p-i-n photodiodes, with intrinsic regions optimized for high quantum efficiency.

Infrared FPAs based on MBE-grown materials are critical components in thermal imaging systems. HgCdTe FPAs dominate LWIR applications due to their high detectivity (D* > 10¹¹ cm·Hz¹/²/W) and ability to operate at elevated temperatures with thermoelectric cooling. InSb FPAs are preferred for MWIR imaging where high uniformity and low noise are required. Recent advances in MBE have enabled the development of dual-band and multi-spectral detectors, where stacked HgCdTe layers with different bandgaps are monolithically integrated to allow simultaneous detection in multiple IR bands.

Terahertz detection using MBE-grown materials leverages the photoconductive and plasma-wave response of narrow-bandgap semiconductors. HgCdTe-based THz detectors operate via direct detection or heterodyne mixing, with applications in security screening, astronomy, and biomedical imaging. InSb devices can also be employed for THz generation and detection through impact ionization and hot-electron effects.

Challenges in MBE growth for IR and THz materials include defect control, interfacial strain management, and scalability for large-area substrates. Dislocations and point defects can degrade detector performance, necessitating optimized growth temperatures and buffer layers. Strain compensation techniques are critical when growing on mismatched substrates to prevent cracking and maintain crystal quality.

Future developments in MBE for IR and THz applications focus on improving material uniformity, reducing growth defects, and integrating novel architectures such as type-II superlattices and quantum dot-in-well structures. These advancements will further enhance the performance and affordability of infrared and terahertz detectors, solidifying MBE's role in next-generation optoelectronic systems.

The precision of MBE enables tailored material properties essential for high-performance IR and THz devices, making it indispensable for military, medical, and industrial applications requiring advanced sensing and imaging capabilities. Continued refinement of growth techniques and device designs will push the boundaries of detection limits, operating temperatures, and spectral versatility.