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The GaAs1–xBix 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 CuPtB-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 CuPtB 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. [ABSTRACT FROM AUTHOR]

Next-generation solar cells employ multiple junctions to push power conversion efficiency beyond the Shockley–Queisser limit. As the tandem devices based on c-Si and wide band gap absorbers are showing impressive performances, the introduction of a third junction can boost the efficiency even higher. For a three-junction solar cell with c-Si as the middle one, the optimum band gaps for the top and bottom sub-cells are 1.7 and 0.7 eV, respectively. While there are numerous wide band gap compounds being explored and studied for the top sub-cells, there is a lack of suitable materials for bottom sub-cells. In this work, we explore a novel Sn(Zr1−xTix)Se3 alloy system and evaluate its fundamental optoelectronic properties. Using a solid state reaction, we synthesized Sn(Zr1−xTix)Se3 materials with various Ti/Zr ratios in both powder and single crystal forms. Structural, optical, and electrical properties were measured as a function of the Ti/Zr ratio. We found that Sn(Zr1−xTix)Se3 crystallizes in the needle-like phase and is stable up to x = 0.44. With an increase in the Ti/Zr ratio, the absorption edge of Sn(Zr1−xTix)Se3 shifted towards lower energy and the lowest band gap achieved at x = 0.42 was 0.78 ± 0.01 eV. Electrical measurements revealed that the resistivity of Sn(Zr1−xTix)Se3 with respect to the Ti/Zr ratio was non-linear and had three distinct regions. Finally, we determined that Sn(Zr1−xTix)Se3 with Ti-rich composition is a direct semiconductor, has a very high absorption coefficient and the band gap is located near the optimal region for the bottom sub-cell absorber, making it a promising material candidate for multijunction solar cell applications. [ABSTRACT FROM AUTHOR]

Group III–V semiconductor multi-junction solar cells are widely used in concentrated-sun and space photovoltaic applications due to their unsurpassed power conversion efficiency and radiation hardness. To further increase the efficiency, new device architectures rely on better bandgap combinations over the mature GaInP/InGaAs/Ge technology, with Ge preferably replaced by a 1.0 eV subcell. Herein, we present a thin-film triple-junction solar cell AlGaAs/GaAs/GaAsBi with 1.0 eV dilute bismide. A compositionally step-graded InGaAs buffer layer is used to integrate high crystalline quality GaAsBi absorber. The solar cells, grown by molecular-beam epitaxy, achieve 19.1% efficiency at AM1.5G spectrum, 2.51 V open-circuit voltage, and 9.86 mA/cm2 short-circuit current density. Device analysis identifies several routes to significantly improve the performance of the GaAsBi subcell and of the overall solar cell. This study is the first to report on multi-junctions incorporating GaAsBi and is an addition to the research on the use of bismuth-containing III–V alloys in photonic device applications. [ABSTRACT FROM AUTHOR]

InAsBi layers with different bismuth content were grown on InAs substrates by solid source MBE. The amount of bismuth incorporated in the layers was estimated using X-ray diffraction measurements. The relaxation degree of the grown crystalline layers was evaluated using reciprocal space map analysis. The intervalley energy separation in the conduction band of InAsBi was studied by Terahertz Excitation Spectroscopy. It has been found that this separation slightly decreases with increasing Bi content. In the studied samples with Bi content varying from 2.7% to 4.5% the Γ- L separation shifts down to about 0.9 eV. [ABSTRACT FROM AUTHOR]

Surfaces of semiconducting materials excited by femtosecond laser pulses emit electromagnetic waves in the terahertz (THz) frequency range, which by definition is the 0.1–10 THz region. The nature of terahertz radiation pulses is, in the majority of cases, explained by the appearance of ultrafast photocurrents. THz pulse duration is comparable with the photocarrier momentum relaxation time, thus such hot-carrier effects as the velocity overshoot, ballistic carrier motion, and optical carrier alignment must be taken into consideration when explaining experimental observations of terahertz emission. Novel commercially available tools such as optical parametric amplifiers that are capable of generating femtosecond optical pulses within a wide spectral range allow performing new unique experiments. By exciting semiconductor surfaces with various photon energies, it is possible to look into the ultrafast processes taking place at different electron energy levels of the investigated materials. The experimental technique known as the THz excitation spectroscopy (TES) can be used as a contactless method to study the band structure and investigate the ultrafast processes of various technologically important materials. A recent decade of investigations with the THz excitation spectroscopy method is reviewed in this article. TES experiments performed on the common bulk A3B5 compounds such as the wide-gap GaAs, and narrow-gap InAs and InSb, as well as Ge, Te, GaSe and other bulk semiconductors are reviewed. Finally, the results obtained by this non-contact technique on low-dimensional materials such as ultrathin mono-elemental Bi films, InAs, InGaAs, and GaAs nanowires are also presented. [ABSTRACT FROM AUTHOR]

Bismuth films with thicknesses between 6 and ∼30 nm were grown on Si (111) substrate by molecular beam epitaxy (MBE). Two main phases of bismuth — α -Bi and β -Bi — were identified from high-resolution X-ray diffraction (XRD) measurements. The crystal structure dependencies on the layer thicknesses of these films were analyzed. β -Bi layers were epitaxial and homogenous in lateral regions that are greater than 200 nm despite the layer thickness. Further, an increase in in-plane 2 θ values showed the biaxial compressive strain. For comparison, α -Bi layers are misoriented in six in-plane directions and have β -Bi inserts in thicker layers. That leads to smaller (about 60 nm) lateral crystallites which are compressively strained in all three directions. Raman measurement confirmed the XRD results. The blue-sift of Raman signals compared with bulk Bi crystals occurs due to the phonon confinement effect, which is larger in the thinnest α -Bi layers due to higher compression. [ABSTRACT FROM AUTHOR]

The distribution of alloyed atoms in semiconductors often deviates from a random distribution which can have significant effects on the properties of the materials. In this study, scanning transmission electron microscopy techniques are employed to analyze the distribution of Bi in several distinctly MBE grown GaAs1−xBix alloys. Statistical quantification of atomic-resolution HAADF images, as well as numerical simulations, are employed to interpret the contrast from Bi-containing columns at atomically abrupt (001) GaAs-GaAsBi interface and the onset of CuPt-type ordering. Using monochromated EELS mapping, bulk plasmon energy red-shifts are examined in a sample exhibiting phase-separated domains. This suggests a simple method to investigate local GaAsBi unit-cell volume expansions and to complement standard X-ray-based lattice-strain measurements. Also, a single-variant CuPt-ordered GaAsBi sample grown on an offcut substrate is characterized with atomic scale compositional EDX mappings, and the order parameter is estimated. Finally, a GaAsBi alloy with a vertical Bi composition modulation is synthesized using a low substrate rotation rate. Atomically, resolved EDX and HAADF imaging shows that the usual CuPt-type ordering is further modulated along the [001] growth axis with a period of three lattice constants. These distinct GaAsBi samples exemplify the variety of Bi distributions that can be achieved in this alloy, shedding light on the incorporation mechanisms of Bi atoms and ways to further develop Bi-containing III-V semiconductors. [ABSTRACT FROM AUTHOR]

Formation of bismuth nanocrystals in GaAsBi layers grown by molecular beam epitaxy at 330 °C substrate temperature and post-growth annealed at 750 °C is reported. Superlattices containing alternating 10 nm-thick GaAsBi and AlAs layers were grown on semi-insulating GaAs substrate. AlAs layers have served as diffusion barriers for Bi atoms, and the size of the nanoclusters which nucleated after sample annealing was correlating with the thickness of the bismide layers. Energy-dispersive spectroscopy and Raman scattering measurements have evidenced that the nanoparticles predominantly constituted from Bi atoms. Strong photoluminescence signal with photon wavelengths ranging from 1.3 to 1.7 μm was observed after annealing; its amplitude was scaling-up with the increased number of the GaAsBi layers. The observed photoluminescence band can be due to emission from Bi nanocrystals. The carried out theoretical estimates support the assumption. They show that due to the quantum size effect, the Bi nanoparticles experience a transition to the direct-bandgap semiconducting state. [ABSTRACT FROM AUTHOR]

Compact and tunable semiconductor terahertz sources providing direct electrical control, efficient operation at room temperatures and device integration opportunities are of great interest at the present time. One of the most well-established techniques for terahertz generation utilises photoconductive antennas driven by ultrafast pulsed or dual-wavelength continuous wave laser systems, though some limitations, such as confined optical wavelength pumping range and thermal breakdown, still exist. The use of quantum dot-based semiconductor materials, having unique carrier dynamics and material properties, can help to overcome limitations and enable efficient optical-to-terahertz signal conversion at room temperatures. Here we discuss the construction of novel and versatile terahertz transceiver systems based on quantum dot semiconductor devices. Configurable, energy-dependent optical and electronic characteristics of quantum-dot-based semiconductors are described, and the resonant response to optical pump wavelength is revealed. Terahertz signal generation and detection at energies that resonantly excite only the implanted quantum dots opens the potential for using compact quantum dot-based semiconductor lasers as pump sources. Proof-of-concept experiments are demonstrated here that show quantum dot-based samples to have higher optical pump damage thresholds and reduced carrier lifetime with increasing pump power. [ABSTRACT FROM AUTHOR]

We report on the performance of photoconductive terahertz detectors made from low temperature grown GaAs (LTG GaAs) and Ga0.7In0.3As (LTG GaInAs) in the wavelength range from 620 to 1550 nm. These THz detectors exhibited a significant sub-bandgap photoconductivity due to the electron transitions from the defect band. A simple model evaluating picosecond photoconductivity was proposed to explain the experimental data. By comparing the modeling results with the experimental data, the defect band position was found to be at 0.77 and at 0.4 eV below the conduction band minimum for LTG GaAs and LTG GaInAs, respectively. [ABSTRACT FROM AUTHOR]

Terahertz radiation pulses emitted after exciting semiconductor heterostructures by femtosecond optical pulses were used to determine the electron energy band offsets between different constituent materials. It has been shown that when the photon energy is sufficient enough to excite electrons in the narrower bandgap layer with an energy greater than the conduction band offset, the terahertz pulse changes its polarity. Theoretical analysis performed both analytically and by numerical Monte Carlo simulation has shown that the polarity inversion is caused by the electrons that are excited in the narrow bandgap layer with energies sufficient to surmount the band offset with the wide bandgap substrate. This effect is used to evaluate the energy band offsets in GaInAs/InP and GaInAsBi/InP heterostructures. [ABSTRACT FROM AUTHOR]

Electron dynamics in the polycrystalline bismuth films were investigated by measuring emitted terahertz (THz) radiation pulses after their photoexcitation by tunable wavelength femtosecond duration optical pulses. Bi films were grown on metallic Au, Pt, and Ag substrates by the electrodeposition method with the Triton X-100 electrolyte additive, which allowed us to obtain more uniform films with consistent grain sizes on any substrate. It was shown that THz pulses are generated due to the spatial separation of photoexcited electrons and holes diffusing from the illuminated surface at different rates. The THz photoconductivity spectra analysis has led to a conclusion that the thermalization of more mobile carriers (electrons) is dominated by the carrier–carrier scattering rather than by their interaction with the lattice. [ABSTRACT FROM AUTHOR]

Terahertz time-domain spectroscopy (TDS) system based on femtosecond Yb:KGW laser, photoconductive emitters and detectors made using low-temperature-grown (LTG) GaAs layers annealed at moderate temperatures (∼400 °C) was demonstrated. Measured photoexcited charge carriers concentration of these layers increased lineary with an optical power, showing that the transitions from defect band to conduction band are dominant. The largest amplitude THz pulse was emitted by the device made from the LTG-GaAs layer annealed at 420 °C temperature with useful signal bandwidth reaching 3 THz. On the other hand, detectors made from this material were more than an order of magnitude less sensitive than conventional GaBiAs detectors (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

The influence of thermal annealing on optical properties and carrier lifetime of dilute GaBixAs1-x layers was investigated. Thick (0.5-1.5 μm) relaxed and thin (30-100 nm) strained bismide layers were grown onto semi-insulating (100)-oriented GaAs substrates by molecular beam epitaxy (MBE) using As4 and As2, respectively. The post-growth annealing was carried out in the rapid thermal annealing (RTA) oven at the temperatures ranging from 550 °C to 750 °C under nitrogen atmosphere. The high resolution X-ray diffraction (HRXRD), photoluminescence (PL) and optical pump-THz probe measurements revealed two different GaBixAs1-x anneal mechanisms. The annealing at temperatures higher than 600 °C caused a significant reduction of carrier lifetime of relaxed bismide layers with x > 0.05. Meanwhile, thick layers with a smaller content of Bi (x < 0.05) and thin strained layers annealed up to 600 °C still exhibited long carrier lifetimes. On the other hand, the significant photoluminescence signal increase at room and liquid nitrogen temperatures was observed for the GaBiAs layers grown using As2 (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Experiments were carried out to elucidate aspects of the preparation process, which enables glass fibers to efficiently frequency-double infrared laser light. A picture is described which is consistent with most of the observations. [ABSTRACT FROM AUTHOR]

We demonstrate that the rectifying field effect transistor, biased to the subthreshold regime, in a large signal regime exhibits a super-linear response to the incident terahertz (THz) power. This phenomenon can be exploited in a variety of experiments which exploit a nonlinear response, such as nonlinear autocorrelation measurements, for direct assessment of intrinsic response time using a pump-probe configuration or for indirect calibration of the oscillating voltage amplitude, which is delivered to the device. For these purposes, we employ a broadband bow-tie antenna coupled Si CMOS field-effect-transistor-based THz detector (TeraFET) in a nonlinear autocorrelation experiment performed with picoseconds-scale pulsed THz radiation. We have found that, in a wide range of gate bias (above the threshold voltage V th = 445 mV), the detected signal follows linearly to the emitted THz power. For gate bias below the threshold voltage (at 350 mV and below), the detected signal increases in a super-linear manner. A combination of these response regimes allows for performing nonlinear autocorrelation measurements with a single device and avoiding cryogenic cooling. [ABSTRACT FROM AUTHOR]

We report the terahertz frequency radiation detection using photoconductive antennas fabricated from molecular-beam-epitaxy-grown GaAsBi. We have estimated that the detector has the highest sensitivity when illuminated with 1.1- to 1.3-µm-wavelength femtosecond pulses. Electron scattering to higher conduction valleys takes place at shorter wavelengths; thus, we have determined the intervalley separation in the conduction band (0.26–0.29 eV). Though the sensitivity of GaAsBi detector at 1.55 µm decreases by about 40% compared with its maximum value, we have demonstrated a terahertz time domain spectroscopy system based on the femtosecond Er-doped fiber laser using this detector. [ABSTRACT FROM AUTHOR]

InyGa1-yAs1-xBix layers on InP:Fe (100) substrates have been grown by molecular beam epitaxy. Up to 7% of Bi incorporation has been confirmed by optical and structural analyses of grown samples. Photoluminescence signals at wavelengths up to 3 µm have been observed, implying InyGa1-yAs1-xBix to be a prospective material for mid-infrared applications. A weak band-gap temperature sensitivity of Bi-containing InGaAs has been evaluated from optical absorption measurements. [ABSTRACT FROM AUTHOR]

Uni-Travelling-Carrier Photo-Diodes (UTC-PD) with GaAs1-xBix absorber layer have been fabricated and characterized. Two UTC-PD structures with and without a cliff layer between the absorber and collector were investigated. The spectral sensitivity dependences are similar for both UTC-PDs and are reaching the wavelength of 1.3 μm. Photosensitivity of structure with a cliff was one order of magnitude larger than that of the device without it. [ABSTRACT FROM AUTHOR]