Asymmetric quantum wells (AQWs) utilizing interband transitions enhance second-order susceptibility over a wide wavelength range compared to natural crystals. The nonlinear susceptibility is further enhanced in AQWs with type-II band alignment as compared to type-I band alignment, a result of the larger interband charge shift. This enhancement is demonstrated in this work by analyzing three type-I and type-II AQW designs based on the lattice-matched InP/AlGaInAs materials systems using the envelope wavefunction approximation. The calculated interband second-order susceptibility tensor elements in type-II structures range between 20 and 1.60 × 103 pm/V for nearly resonant optical rectification and difference frequency generation applications at near-infrared and terahertz wavelengths, an improvement of nearly 1 order of magnitude over the type-I structures and 1–2 orders of magnitude over natural crystals such as LiNbO3, KTiOPO4 (KTP), or GaAs. A factor of 2–3 further enhancement of the tensor elements is achieved by optimizing the well widths and band offsets of the type-II asymmetric quantum wells. The type-II structure can be implemented in other material systems spanning the longwave infrared to visible wavelengths, enhancing nonlinear susceptibility for various applications, including photonic integrated circuits.

Monocrystalline 1.7 eV Mg0.13Cd0.87Te/MgxCd1−xTe (x > 0.13) double heterostructure (DH) solar cells with varying Mg compositions in the barrier layers are grown by molecular beam epitaxy. A Mg0.13Cd0.87Te/Mg0.37Cd0.63Te DH solar cell featuring abrupt interfaces between barriers and absorber and the addition of a SiO2 anti-reflective coating demonstrate open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF), and device active-area efficiencies up to 1.129 V, 17.3 mA/cm2, 77.7%, and 15.2%, respectively. The VOC and FF vary oppositely with the MgxCd1−xTe barrier height, indicating an optimal design of the MgCdTe DHs as a trade-off between carrier confinement and carrier transport. Temperature-dependent VOC measurements reveal that the majority of carrier recombination in the devices occurs outside the DHs, in the a-Si:H hole-contact layer, and at the interface between the a-Si:H layer and the MgxCd1−xTe top barrier at room temperature. Simulation results for the device with the highest efficiency show that the p-type a-Si:H layer and the Mg0.37Cd0.63Te top barrier contribute 1.3 and 2.4 mA/cm2 JSC loss, respectively.

Gallium is incorporated into the strain-balanced In(Ga)As/InAsSb superlattice system to achieve the same mid-wave infrared cutoff tunability as conventional Ga-free InAs/InAsSb type-II superlattices, but with an additional degree of design freedom to enable optimization of absorption and transport properties. Time-resolved photoluminescence measurements of InGaAs/InAsSb superlattice characterization- and doped device structures are reported from 77 to 300 K and compared to InAs/InAsSb. The low-injection photoluminescence decay yields the minority carrier lifetime, which is analyzed with a recombination rate model, enabling the determination of the temperature-dependent Shockley–Read–Hall, radiative, and Auger recombination lifetimes and extraction of defect energy levels and capture cross section defect concentration products. The Shockley–Read–Hall-limited lifetime of undoped InGaAs/InAsSb is marginally reduced from 2.3 to 1.4 μs due to the inclusion of Ga; however, given that Ga improves the vertical hole mobility by a factor of >10×, a diffusion-limited InGaAs/InAsSb superlattice nBn could expect a lower bound of 2.5× improvement in diffusion length with significant impact on photodetector quantum efficiency and radiation hardness. At temperatures below 120 K, the doped device structures are Shockley–Read–Hall limited at 0.5 μs, which shows promise for detector applications.

Three InAsSbBi samples are grown by molecular beam epitaxy at 400 °C on GaSb substrates with three different offcuts: (100) on-axis, (100) offcut 1° toward [011], and (100) offcut 4° toward [011]. The samples are investigated using X-ray diffraction, Nomarski optical microscopy, atomic force microscopy, transmission electron microscopy, and photoluminescence spectroscopy. The InAsSbBi layers are 210 nm thick, coherently strained, and show no observable defects. The substrate offcut is not observed to influence the structural and interface quality of the samples. Each sample exhibits small lateral variations in the Bi mole fraction, with the largest variation observed in the on-axis growth. Bismuth rich surface droplet features are observed on all samples. The surface droplets are isotropic on the on-axis sample and elongated along the [011¯] step edges on the 1° and 4° offcut samples. No significant change in optical quality with offcut angle is observed.

The growth of Bi-containing III-V alloys requires careful control over temperature and group-V fluxes due to the low equilibrium solubility of Bi and its tendency to surface segregate into Bi-rich droplet features. A model for molecular beam epitaxy growth based on the kinetics of atomic desorption, incorporation, surface accumulation, and droplet formation is applied to the bismide alloy InAsSbBi grown on GaSb substrates. A steady-state solution is derived for the Bi, Sb, and As mole fractions and surface layer coverages based on the Bi, Sb, and As fluxes. A nonlinear least-squares algorithm is used to fit the growth model parameters to experimentally measured Bi mole fractions in bulk and quantum well InAsSbBi samples grown at 400 °C and 420 °C. The Bi mole fraction ranges from 0.12% to 1.86% among 17 samples examined. The results indicate that as the growth temperature increases, the rate of Bi incorporation decreases and the rate of Bi self-desorption increases. A strong interaction is observed between Bi and As that plays a role in the desorption of excess Bi from the growth surface, thus reducing the likelihood of Bi-rich droplet formation when an excess As flux is present. Significantly, the model predicts that the incorporation of Bi is limited to mole fractions of 1.43% at 400 °C and 0.30% at 420 °C in lattice-matched bulk InAsSbBi grown on GaSb substrates.

The conditions for molecular beam epitaxy growth of InAsBi are explored for optoelectronic applications. X-ray diffraction measurements of thick InAsBi on GaSb samples show a diffraction sideband near the main (004) diffraction peak, indicating lateral variation of Bi mole fraction in the layer. By modeling the main and sideband diffraction peaks, the average InAsBi unstrained lattice constant is determined. By comparing these results with the Bi mole fraction for each sample determined using random Rutherford backscattering, the lattice constant of zinc blende InBi is determined to be 6.6107 Å. The bandgap of InAsBi is expressed as a function of the Bi mole fraction using the band anticrossing model and a characteristic coupling strength of 1.529 eV between Bi impurity state and the InAs valence band. A software tool is programmed to identify optimal InAs/InAsBi and GaSb/InAsBi superlattice designs with maximum electron-hole wavefunction overlap as a function of transition energy.

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Strain-balanced InAs/InAsSb superlattices can be tuned to absorb and emit across the mid- to long-wave infrared, and exhibit appropriate minority carrier lifetimes for high performance infrared photodetectors. The optical quality of this material has been shown to improve with the use of Bi as a surfactant. Specifically, InAs/InAsSb superlattices grown at 425 °C and 430 °C exhibit improved photoluminescence intensity for Bi/In flux ratios up to 1.0%, and optical quality improves further with increasing growth temperature and increasing Bi/In flux ratios up to 5.0%. The identification of optimal growth conditions for InAs/InAsSb superlattices with Bi surfactant, as well as further exploration of the impact of Bi surfactant is an important component to further developing and optimizing this infrared material system. Several strain-balanced InAs/InAsSb superlattices are grown using molecular beam epitaxy at temperatures ranging from 425 °C to 475 °C using Bi/In flux ratios ranging from 0.0% to 10.0%. The structural and optical properties of the samples are evaluated using X-ray diffraction, secondary ion mass spectrometry, and photoluminescence spectroscopy. Analysis of the mass spectrometry data indicates that surfactant Bi incorporates into the InAs/InAsSb material system with a sticking coefficient of 0.3% at 450 °C, yielding dopant-level concentrations for typical Bi/In surfactant flux ratios. Analysis of the integrated photoluminescence intensity indicates that photoluminescence efficiency is greatest with a 1.0% Bi/In flux ratio for growth at 425-430 °C, and a 5.0% Bi/In flux ratio for growth at 450-475 °C. The improvement in photoluminescence efficiency is associated with a longer Shockley-Read-Hall lifetime in the superlattices grown with Bi surfactant.

The structural and optical properties of pseudomorphic InAsSbBi grown on GaSb are examined using reflection high-energy electron diffraction, X-ray diffraction, Rutherford backscattering spectrometry, and spectroscopic ellipsometry. The layer studied is 210 nm thick and was grown by molecular beam epitaxy at 280 °C under a (2 × 3) surface reconstruction using near-stoichiometric fluxes. The material is homogeneous and single crystal with no observable defects or surface Bi droplets. The group-V mole fractions are determined using Rutherford backscattering measurements of the Bi mole fraction and X-ray diffraction measurements of the lattice tetragonal distortion. The bandgap energy is determined from the room temperature optical constants measured using spectroscopic ellipsometry. These and measurements from pseudomorphic InAsSb and InAsBi on GaSb are utilized to describe the bandgap energy of InAsSbBi as a function of mole fraction using a bandgap bowing model.