Low-temperature Grown GaAs Research Articles

Coupled distributed feedback laser system at 820 nm for THz beat-frequency generation.

We present a coupled distributed feedback (DFB) laser system, based on AlGaAs/GaAs epitaxially grown compound semiconductor, with electroluminescence near 820 nm. This DFB laser system supports two lateral modes sharing a Bragg grating, thereby enabling simultaneous lasing operation at two different frequencies. We recorded a dual-mode operation with a 4.2 nm wavelength spacing, corresponding to a 1.86 THz beat frequency, and an output power of 14.7 mW at an injection current of 195 mA. Compared to previous works on dual-mode DFB lasers, this design simplifies the fabrication process, potentially enables tunability of the beat frequency, and offers greater compatibility with low temperature grown GaAs (LT-GaAs) high-frequency photodetectors.

Continuous wave terahertz detection using 1550nm pumped nonlinear photoconductive GaAs metasurfaces.

Terahertz (THz) continuous wave (CW) spectroscopy systems can offer extremely high spectral resolution over the THz band by photo-mixing high-performance telecommunications-band (1530-1565 nm) lasers. However, typical THz CW detectors in these systems use narrow band-gap photoconductors, which require elaborate material growth and generate relatively large detector noise. Here we demonstrate that two-step photon absorption in a nano-structured low-temperature grown GaAs (LT-GaAs) metasurface which enables switching of photoconductivity within approximately one picosecond. We show that LT-GaAs can be used as an ultrafast photoconductor in CW THz detectors despite having a bandgap twice as large as the telecommunications laser photon energy. The metasurface design harnesses Mie modes in LT GaAs resonators, whereas metallic electrodes of THz detectors can be designed to support an additional photonic mode, which further increases photoconductivity at a desired wavelength.

Efficient THz generation from low-temperature-grown GaAs photoconductive antennas driven by Yb-doped fiber amplifier at 200 kHz repetition rate

We demonstrate the generation of terahertz (THz) pulses with electric field strength reaching 34 kV/cm from low-temperature-grown GaAs (LT-GaAs) interdigitated photoconductive antennas driven by 1030 nm optical pulses delivered by a commercial ytterbium-doped fiber laser operating at a repetition rate of 200 kHz. By probing the Urbach absorption in LT-GaAs layers, we show that the THz generation mechanism predominantly relies on the photoexcitation of electrons from the valence band to shallow defect states arising from the incorporation of excess As during the growth process. Our THz source opens the route toward nonlinear time-resolved study of low-energy excitations in matter with high signal-to-noise ratios.

Fabrication and Characterization of a Leaky-Lens Photoconductive Antenna on Low-Temperature Grown GaAs Membranes

State of the art THz pulsed commercial systems operating over large bandwidth suffer from high dispersion or low radiation efficiency due to the poor coupling between the transmitter and receiver photoconductive antennas (PCAs). In this work, we present the fabrication and characterization of a leaky lens photoconductive antenna (PCA) that has the potential to solve this problem. The presented PCA is based on a low temperature grown gallium arsenide (LT-GaAs) membrane with a 1:15 bandwidth coverage (0.1 THz – 1.5 THz), where the frequency response if constant. In order to fabricate the PCA on a LT-GaAs membrane, a novel fabrication process is developed. This process is dramatically faster than previously used processes (∼1.5 hours instead of ∼20 hours). Furthermore, an experimental validation of the radiated power together with the comparison to a standard bow-tie based PCA fabricated on the same LT-GaAs wafer is shown in this paper. We show that the PCA source on the LT-GaAs membrane is more efficient due to the enhanced leaky wave radiation. The leaky lens PCA stands out as a great candidate to improve the coupling efficiency in THz pulsed commercial systems, where the maximum laser power that can be used is limited by the dispersion in the optic fiber.

Ultra‐High Speed, High‐Sensitivity Spin‐Cast MXene‐Semiconductor‐MXene Photodetectors

AbstractA simple room‐temperature process of depositing MXene on a III‐V structure with embedded 2D electron gas (2DEG) is used, which results in a large area, , photodetector (PD) device that greatly outperforms vacuum deposited Ti/Au metal‐semiconductor‐metal (MSM) PD's. By co‐optimizing properties of 2D MXene contacts and the III‐V material heterojunctions, this device sets new operating records with responsivity up to 1.04 A W‐1 at low optical powers, corresponding to >230% internal quantum efficiency, dark current of 50 , >105.6‐dB dynamic range, and 25–150 ps response time, which improves the previous MXene‐Semiconductor‐MXene responsivity by >2.7 times and is 7 × 103 –−106 times faster compared to other MXene‐based PDs. This is achieved by enhancing the Schottky barrier height by forming a Van der Waals (vdW) heterojunction between a wide bandgap AlGaAs surface layer and spin coated Ti3C2Tz electrodes. A layered architecture transports the optically generated electrons to a 2DEG channel at the GaAs/AlGaAs heterointerface, where they are rapidly collected. The landscaped electric field pushes the slow holes to an underlying low temperature‐grown GaAs (LT‐GaAs) region where they recombine. The proposed Schottky‐2DEG Photoconductor‐Schottky model for device operation shows how this device circumvents the canonical limitations of gain‐bandwidth product, and carrier transit time, while replacing the need for vacuum deposition of gold or other precious metals.

Spintronic emitters for super-resolution in THz-spectral imaging

We investigate local THz field generation using spintronic THz emitters to enhance the resolution for micrometer-sized imaging. Far-field imaging with wavelengths above 100 μm limits the resolution to this order of magnitude. By using optical laser pulses as a pump, THz field generation can be confined to the area of laser beam focusing. The divergence of the generated THz beam due to laser beam focusing requires the imaged object to be close to the generation spot at a distance below the THz field wavelength. We generate THz-radiation by fs-laser pulses in CoFeB/Pt heterostructures, based on spin currents, and detect them by commercial low-temperature grown-GaAs (LT-GaAs) Auston switches. The spatial resolution of THz radiation is determined by applying a 2D scanning technique with motorized stages allowing step sizes in the sub-micrometer range. Within the near-field limit, we achieve spatial resolution in the dimensions of the laser spot size on the micrometer scale. For this purpose, a gold test pattern is evaporated on the spintronic emitter separated by a 300 nm SiO2 spacer layer. Moving these structures with respect to the femtosecond laser spot, which generates THz radiation, allows for resolution determination. The knife-edge method yields a full-width half-maximum beam diameter of 4.9±0.4 μm at 1 THz. The possibility to deposit spintronic emitter heterostructures on simple glass substrates makes them attractive candidates for near-field imaging in many imaging applications.

Operation of quantum dot based terahertz photoconductive antennas under extreme pumping conditions

Photoconductive antennas deposited onto GaAs substrates that incorporate InAs quantum dots have been recently shown to efficiently generate both pulsed and CW terahertz radiation. In this Letter, we determine the operational limits of these antennas and demonstrate their extreme thermal breakdown tolerance. Implanted quantum dots serve as free carrier capture sites, thus acting as lifetime shorteners, similar to defects in low-temperature grown substrates. However, unlike the latter, defect-free quantum-dot structures possess perfect lattice quality, thus not compromising high carrier mobility and pump intensity stealth. Single gap design quantum dot based photoconductive antennas are shown to operate under up to 1 W of average pump power (∼1.6 mJ cm−2 energy density), which is more than 20 times higher than the pumping limit of low-temperature grown GaAs based substrates. Conversion efficiency of the quantum dot based photoconductive antennas does not saturate up to 0.75 W of pump power (∼1.1 mJ cm−2 energy density). Such a thermal tolerance suggests a glowy prospect for the proposed antennas as a perspective candidate for intracavity optical-to-terahertz converters.

Effect of heteroepitaxial growth on LT-GaAs: ultrafast optical properties

Epitaxial low temperature grown GaAs (LT-GaAs) on silicon (LT-GaAs/Si) has the potential for terahertz (THz) photoconductive antenna applications. However, crystalline, optical and electrical properties of heteroepitaxial grown LT-GaAs/Si can be very different from those grown on semi-insulating GaAs substrates (‘reference’). In this study, we investigate optical properties of an epitaxial grown LT-GaAs/Si sample, compared to a reference grown under the same substrate temperature, and with the same layer thickness. Anti-phase domains and some crystal misorientation are present in the LT-GaAs/Si. From coherent phonon spectroscopy, the intrinsic carrier densities are estimated to be 1015 cm−3 for either sample. Strong plasmon damping is also observed. Carrier dynamics, measured by time-resolved THz spectroscopy at high excitation fluence, reveals markedly different responses between samples. Below saturation, both samples exhibit the desired fast response. Under optical fluences ⩾54 μJ cm−2, the reference LT-GaAs layer shows saturation of electron trapping states leading to non-exponential behavior, but the LT-GaAs/Si maintains a double exponential decay. The difference is attributed to the formation of As–As and Ga–Ga bonds during the heteroepitaxial growth of LT-GaAs/Si, effectively leading to a much lower density of As-related electron traps.

Enhancement effect on photoelectric conversion efficiency of plasmon-induced terahertz photoconductive antenna

The laser electric field distribution, carrier distribution and photoconductive current densities under different electric field intensities in low temperature grown GaAs (LT-GaAs) layer of photoconductive antenna (PCA) have been simulated by finite element method. When a plasmon-induced PCA is pumped by an 800 nm laser beam, the plasmon on the interface of air and LT-GaAs aids the conversion of more laser photons into photo-generated carriers and thus increases the density of the photo-current leading to a higher-power terahertz pulse. A quadratic function fitting relationship between the laser electric field intensity and the maximum density of photoconductive current has been obtained for the first time. On this basis, a plasmon-induced terahertz PCA has been designed and packaged using electron beam exposure technology. The enhancement effect on photo-electric conversion efficiency of plasmon in PCA is verified by comparing terahertz radiation spectra from conventional and plasmon-induced PCAs. The terahertz radiation properties of two kinds of PCAs fabricated on an identical LT-GaAs layer were evaluated by means of terahertz time-domain spectroscopy.

A modulation-doped heterostructure-based terahertz photoconductive antenna emitter with recessed metal contacts

We present the implementation of an efficient terahertz (THz) photoconductive antenna (PCA) emitter design that utilizes high mobility carriers in the two-dimensional electron gas (2DEG) of a modulation-doped heterostructure (MDH). The PCA design is fabricated with recessed metal electrodes in direct contact with the 2DEG region of the MDH. We compare the performance of the MDH PCA having recessed contacts with a PCA fabricated on bulk semi-insulating GaAs, on low temperature-grown GaAs, and a MDH PCA with the contacts fabricated on the surface. By recessing the contacts, the applied bias can effectively accelerate the high-mobility carriers within the 2DEG, which increases the THz power emission by at least an order of magnitude compared to those with conventional structures. The dynamic range (62 dB) and bandwidth characteristics (3.2 THz) in the power spectrum are shown to be comparable with the reference samples. Drude-Lorentz simulations corroborate the results that the higher-mobility carriers in the MDH, increase the THz emission. The saturation characteristics were also measured via optical fluence dependence, revealing a lower saturation value compared to the reference samples. The high THz conversion efficiency of the MDH-PCA with recessed contacts at low optical power makes it an attractive candidate for THz-time domain spectroscopy systems powered by low power fiber lasers.

Low-temperature GaAs-based plasmonic photoconductive terahertz detector with Au nano-islands

We have fabricated low-temperature grown GaAs (LT-GaAs)-based plasmonic photoconductive antennas by RF sputtering of Au nanoparticles and have evaluated their terahertz detection properties. Localized surface plasmon resonance enhances the electric fields near the surface and increases the optical absorption of nanoparticles. The resonance frequency depends on the density of electrons, the effective electron mass, and the size and shape of the nanoparticles. Therefore, we tried to develop a high-sensitivity LT-GaAs photoconductive detector (PCD), which is effective over a wide range of wavelengths, by RF sputtering of Au nano-islands with a variety of aspect ratios from 1.2 to 5.1 on the dipole gap region of the PCD. As a result, we succeeded in increasing the sensitivity by 29% and 40% in the amplitude of observed terahertz pulse for 800 nm and 1560 nm femtosecond laser excitations, respectively.

Trilayer low-temperature-grown GaAs terahertz emitter and detector device with doped buffer

A low-temperature-grown GaAs (LTG-GaAs) terahertz (THz) photoconductive antenna device with layer structure consisting of doped buffer demonstrated enhanced emission intensity and detection sensitivity. As THz emitter, a 116% increase is exhibited by the LTG-GaAs with doped buffer relative to its undoped counterpart at 9 V bias and 0.8 mW pump power with spectral distribution extending to ∼3 THz. As a THz detector, a dynamic range of 55 dB is obtained at 0.5 THz, which is 5 dB higher than its undoped counterpart. The device proved effective as a THz emitter at low operating bias and pump power while its detection characteristics are acceptable even at low incident THz powers.

Detection properties of photoconductive antennas fabricated on low-temperature-grown GaAs and ErAs:GaAs at subterahertz band

Terahertz (THz) spectroscopy with high sensitivity is essential for biological application considering the strong absorption and scattering effects therein. As the most commonly used THz detector, the photoconductive antenna’s (PCA) response greatly relies on the properties of the substrate’s material. THz detection properties of the PCAs fabricated on low-temperature-grown GaAs (LT-GaAs) and ErAs:GaAs superlattices were compared at the sub-THz band. The detection efficiency of the PCAs with regard to incident laser power was characterized. In addition, using the PCAs as detectors, the signal-to-noise ratio (SNR) and dynamic range (DR) of a terahertz time-domain spectroscopy were quantified. The result indicates that the PCA detector with LT-GaAs has higher efficiency than the one with ErAs:GaAs. Consequently, the corresponding THz spectrometer has better SNR and DR. This result is contrary to the previous report, in which enhanced detection efficiency was observed with ErAs:GaAs-based PCA, which is probably due to the different structures of ErAs:GaAs superlattices used in the experiment.

Atomically-resolved interface imaging and terahertz emission measurements of gallium arsenide epilayers

Semiconductor interfaces are the backbone of modern optoelectronic devices. In terahertz (THz) science, the narrow region of an interface is crucial in the emission process. However, reports on the direct correlation of THz emission with local interface properties remain scarce owing to the inherent difficulty of using the same sample for nanoscale and macroscale studies. In this study, we combined scanning tunneling microscopy/spectroscopy (STM/STS) and THz emission spectroscopy to study the interface between a highly n+-doped and undoped gallium arsenide (GaAs). Using STS, we identify a carrier density of 1×1015 cm−3 in the low-temperature-grown GaAs (LT-GaAs) layer, which we used to visualize the energy band diagram at the interface and the surface of LT-GaAs. THz emission intensity is higher in the LT-GaAs/n+-GaAs structures relative to semi-insulating GaAs owing to the high electric field at the interface regardless of the LT-GaAs layer thickness. Pump fluence dependence of THz showed that the thinner LT-GaAs layers saturate at lower pump fluence compared to thicker LT-GaAs and SI-GaAs. This behavior is explained by the built-in field screening by the photogenerated carriers and the free carriers from the n+-GaAs to the LT-GaAs. Our results demonstrate the utility of STM/STS to the design of semiconductor-based THz emitters.

Enhanced terahertz radiation of photoconductive antenna fabricated on GaAs-on-sapphire

The terahertz (THz) radiation properties of a photoconductive antenna (PCA) fabricated on a GaAs-on-sapphire (GoS) substrate are reported at sub-THz band. The GaAs layer with a thickness of approximately 1 µm was directly deposited on a sapphire wafer by means of molecular beam epitaxy. A butterfly-shaped antenna structure was then fabricated on the GoS substrate by photolithography, and the device was tested as the emitter of an in-house built THz time-domain spectrometer. The performance of this antenna was compared with a commercial one, which had an identical antenna structure but was fabricated on low-temperature-grown GaAs (LT-GaAs). The results showed that the GoS-based PCA radiated an enhanced THz field, which could be as much as 1.9 times that of the LT-GaAs-based PCA, indicating that GoS could be a promising photoconductive material. In addition, the optical transparency of the sapphire substrate allows the device to be illuminated from the backside, which is crucial for THz near-field imaging applications where the sample is usually in close proximity to the front surface of the PCA device.

Metallic AsSb nanoinclusions strongly enriched by Sb in AlGaAsSb metamaterial

We have elucidated the microstructure of Al0.28Ga0.72As0.972Sb0.028 metamaterial containing a developed array of AsSb nanoinclusions. The AlGaAsSb films were grown by low-temperature molecular-beam epitaxy followed by high-temperature postgrowth thermal treatment. The process resulted in an array of self-organized AsSb nanoinclusions with an average diameter of 15 nm (after annealing at 750 °C), which occupied about 0.003 of the volume of the crystalline AlGaAsSb matrix. The nanoinclusions were found to have an A7-type rhombohedral microstructure and contained a very high concentration of antimony that exceeded 90%, whereas the Sb content in the AlGaAsSb matrix was 2.8 at. %. The strong enrichment of the inclusion with Sb resulted from the local thermodynamic equilibrium between the solid AlGaAsSb matrix and the As0.1Sb0.9 inclusions that became liquid at an annealing temperature of 750 °C. The inclusion-matrix orientation relationship was established to be (003)p || {111}m and [2¯110]p || 〈220〉m, similar to that for pure As precipitates in low-temperature grown GaAs despite a rather large difference in particle lattice constants and the c/a ratio. The increased inclusion-matrix lattice mismatch resulted in the generation of (001) prismatic dislocation loops associated with the inclusions bigger than 8.5 nm in diameter. The Burgers vector of the dislocation was determined as b = amatrix[001]. The established microstructural and compositional features create an opportunity to specify plasmonic resonance conditions in the low-temperature grown AlGaAsSb metamaterial.

High-sensitivity photoconductive detectors with wide dipole electrodes for low frequency THz wave detection

Photoconductive detectors (PCDs) with wide dipole electrodes were fabricated on low temperature-grown GaAs for observation of low frequency terahertz electromagnetic waves. The screening effect in PCDs caused by the space charge of photoexcited electron-hole pairs brings poor sensitivity in detection. This can be effectively reduced by widening the dipole electrodes in PCDs. The width dependence of terahertz (THz) wave detection properties of PCDs showed that not only the sensitivity but also the signal-to-noise ratio can be fairly improved by increasing the width, especially at frequencies lower than 1.2 THz.

Lens-less fiber coupling of a 1550-nm mode-locked fiber laser light on a low-temperature-grown GaAs photoconductive antenna

A fiber-coupled photoconductive antenna (PCA) is a powerful tool for portable terahertz (THz) systems using a compact 1550-nm mode-locked Er:fiber laser with a fiber output port. However, a low-temperature-grown GaAs (LTG-GaAs) PCA could not be used for this purpose due to the need for wavelength conversion of the 1550-nm light, regardless of the good characteristics for PCA. In this article, we achieved the fiber coupling of the 1550-nm mode-locked fiber laser light on a bowtie-shaped LTG-GaAs PCA detector without the need for wavelength conversion. While the two-step photo-absorption mediated by midgap states in the LTG-GaAs PCA makes it possible to use the 1550-nm light, the similarity of the size between the PCA gap spacing and the fiber core diameter enables the direct contact coupling between the fiber output tip and the PCA gap without any optical components. The developed lens-less fiber-coupled LTG-GaAs PCA detector was effectively applied for the absolute frequency measurement of continuous-wave THz radiation based on the photo-carrier THz frequency comb. The combination of the lens-less fiber-coupled LTG-GaAs PCA with the compact 1550-nm fiber laser will be useful for the portable apparatus for the absolute frequency measurement of practical CW-THz sources and other applications.

New Structure for Photoconductive Antennas Based on {LTG-GaAs/GaAs:Si} Superlattice on GaAs(111)A Substrate

The structural characteristics of a new structure for photoconductive antennas have been investigated. This structure is a multilayered epitaxial film grown on a GaAs(111)A substrate; it consists of alternating undoped low-temperature grown GaAs (LTG-GaAs) layers and GaAs layers synthesized in the standard high-temperature regime and doped with silicon (GaAs:Si). The As4/Ga flow ratio γ is chosen such as to produce p-type GaAs:Si layers. LTG-GaAs layers were grown at an enlarged γ value. The samples grown on GaAs(100) substrates were single-crystal, whereas the single-crystal growth on GaAs(111)A substrates changed to polycrystalline when the film thickness reached 320–340 nm. The sizes of As precipitates in annealed samples, their distribution over the film thickness, and specific features of their crystal structure have been analyzed.

Terahertz radiation of a butterfly-shaped photoconductive antenna(invited)

The terahertz(THz) far-field radiation properties of a butterfly-shaped photoconductive antenna (PCA) were experimentally studied using a home-built THz time-domain spectroscopy(THz-TDS) setup. To distinguish the contribution of in-gap photocurrent and antenna structure to far-field radiation, polarization-dependent THz field was measured and quantified as the illuminating laser beam moved along the bias field within the gap region of electrodes. The result suggests that, although the far-field THz radiation originates from the in-gap photocurrent, the antenna structure of butterfly-shaped PCA dominates the overall THz radiation. In addition, to explore the impact of photoconductive material, radiation properties of butterfly-shaped PCAs fabricated on both low-temperature-grown GaAs(LT-GaAs) and semi-insulating GaAs(Si-GaAs) were characterized and compared. Consistent with previous experiments, it is observed that while Si-GaAs-based PCA can emit higher THz field than LT-GaAs-based PCA at low laser power, it would saturate more severely as laser power increased and eventually be surpassed by LT-GaAs-based PCA. Beyond that, it is found the severe saturation effect of Si-GaAs was due to the longer carrier lifetime and higher carrier mobility, which was confirmed by the numerical simulation.