Beryllium and silicon δ-doped GaAs∕AlAs multiple quantum wells (MQWs) were designed and fabricated for selective sensing of terahertz radiation. Optical characterization of the structures by photoreflectance spectroscopy has indicated a presence of built-in electric fields–from 18 up to 49kV∕cm depending on the structure design–located mainly in the cap and buffer layers, but do not penetrating into the MQWs region. Terahertz sensing under normal incidence via impurity bound-to-unbound transitions is demonstrated in photocurrent experiments within 0.6–4.2THz in silicon-doped MQWs and 3.5–7.3THz range in beryllium-doped MQWs at low temperatures.

InGaAs-based bow-tie diodes for the terahertz (THz) range are found to be well suited for development of compact THz imaging systems. To further optimize design for sensitive and broadband THz detection, one of the major challenges remains: to understand the noise origin, influence of growth conditions and role of defects for device operation. We present a detailed study of photoreflectance, low-frequency noise characteristics and THz sensitivity of InGaAs bow-tie diodes. The diodes are fabricated from InGaAs wafers grown by molecular beam epitaxy (MBE) on semi-insulating InP substrate under different technological conditions. Photoreflectance spectra indicated the presence of strong built-in electric fields reaching up to 49 kV/cm. It was demonstrated that the spectral density of voltage fluctuations at room temperature was found to be proportional to 1/f, while at lower temperatures, 77–200 K, Lorentzian-type spectra dominate due to random telegraph signals caused by individual capture defects. Furthermore, varying bias voltage, we considered optimal conditions for device room temperature operation in the THz range with respect to signal-to-noise ratio. The THz detectors grown with beam equivalent pressure In/Ga ratio equal to 2.04 exhibit the minimal level of the low-frequency noise, while InGaAs layers grown with beam equivalent pressure In/Ga ratio equal to 2.06 are found to be well suited for fabrication of room temperature bow-tie THz detectors enabling sensitivity of 13 V/W and noise equivalent power (NEP) of 200 pW/√Hz at 0.6 THz due to strong built-in electric field effects.

The molecular beam epitaxy-grown epitaxial, partially relaxed, GaAs 1 − x Bi _x bismide layers of ∼ 1 μ m thickness and x ≈ 0.04 composition are examined. The atomic-structure analysis by x-ray diffraction and transmission electron microscopy shows the bismides to be CuPt B -type atomic-ordered in both B + and B − subvariants. The ordering induces an optical anisotropy, which manifests at normal incidence light-beam propagation. The anisotropy is revealed by various optical spectroscopy techniques—polarized photoluminescence, photo-modulated transmittance and reflectance, polarized transmittance, and spectroscopic ellipsometry. The ordering-induced valence band splitting, determined from modulation spectroscopy measurements, is of about 60 meV . The splitting is comparable to that in CuPt-ordered conventional III–V semiconductor alloys even though the maximal ordering parameter in investigated bismides is one order of magnitude lower.

The GaAs_1–xBi_x semiconductor alloy allows one to achieve large bandgap reduction and enhanced spin–orbit splitting energy at dilute Bi quantities. The bismide is currently being developed for near- to mid-infrared lasers, multi-junction solar cells, and photodetectors. In this structure–property relationship study of GaAsBi alloys, we report polarization dependent photoluminescence that reaches a polarization ratio up to 2.4 at room temperature. Polarization dependence is also presented using transmittance spectra, birefringence, and linear dichroism. The optical anisotropy observations agree with the predictions of point symmetry reduction in the CuPt_B-type ordered GaAsBi phase. The structural ordering is investigated experimentally from the atomic scale in molecular-beam epitaxy (MBE) grown samples on exact and miscut (001) GaAs substrates, as well as on (001) Ge. The latter sample is composed of anti-phase domains in which the ordering axes are rotated by 90° angles. Since the conditions stabilizing the CuPt_B ordered phase fall within the typical MBE growth regime of dilute bismides, the optical anisotropy in GaAsBi alloys is expected to be ubiquitous. These findings are important for the future development of GaAsBi-based optoelectronics and also provide new means to analyze structurally complex bismide alloys.

Terahertz excitation spectroscopy was used for the determination of energy separation between the main (Γ) and subsidiary ( L and X ) conduction band valleys of GaAs _{1− x} Bi _x . The samples used in this study were 1 µ m–1.5 µ m thick bismide layers grown by Molecular Beam Epitaxy on GaAs substrates. They contained up to 8% of bismuth as determined by high resolution x-ray diffraction (HR-XRD) and reciprocal space mapping (RSM), taking into account the layer relaxation. It was found that both subsidiary conduction band valleys at L and X points of the Brillouin zone move away from the conduction band minimum at rates of 18 meV/%Bi and 25 meV/%Bi, respectively, with increasing Bi content in the alloy.