Low Temperature Grown Gallium Arsenide Research Articles

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.

Terahertz Quasi-Time Domain Spectroscopy using a 808nm multimode diode laser

We report on a terahertz quasi-time domain spectroscopy (QTDS) system based on a low-cost continuous wave multimode diode laser. Commercially available low-temperature grown gallium arsenide (LT-GaAs) based photoconductive antennas (PCAs) with spiral and dipole configurations were used as emitter and detector, respectively. Terahertz pulses spaced at approximately 55 ps with a bandwidth of 400 GHz were obtained. Parametric measurements of the terahertz peak-to-peak intensity were performed by varying the injection current and temperature while maintaining incident laser power. The highest peak-to-peak intensity was obtained at 170mA injection current and 20° C temperature settings. The change in the THz peak-to-peak intensity is attributed to the mode hopping characteristics of the device which in turn, is dependent on injection current and temperature.

Colloidal upconversion nanocrystals enable low-temperature-grown GaAs photoconductive switch operating at λ = 1.55 μm

Microwave photoconductive switches, allowing an optical control on the magnitude and phase of the microwave signals to be transmitted, are important components for many optoelectronic applications. In recent years, there are significant demands to develop photoconductive switches functional in the short-wave-infrared spectrum window (e.g. λ = 1.3–1.55 μm) but most state-of-the-art semiconductors for photoconductive switches cannot achieve this goal. In this work, we propose a novel approach, by the use of solution-processed colloidal upconversion nanocrystals deposited directly onto low-temperature-grown gallium arsenide (LT-GaAs), to achieve microwave photoconductive switches functional at λ = 1.55 μm illumination. Hybrid upconversion Er3+-doped NaYF4 nanocrystal/LT-GaAs photoconductive switch was fabricated. Under a continuous wave λ = 1.55 μm laser illumination (power density ∼ 12.9 mW μm−2), thanks to the upconversion energy transfer from the nanocrystals, a more than 2-fold larger value in decibel was measured for the ON/OFF ratio on the hybrid nanocrystal/LT-GaAs device by comparison to the control device without upconversion nanoparticles. A maximum ON/OFF ratio reaching 20.6 dB was measured on the nanocrystal/LT-GaAs hybrid device at an input signal frequency of 20 MHz.

Femtosecond Broadband Frequency Switch of Terahertz Three-Dimensional Meta-Atoms

We present the experimental demonstration of a subpicosecond all-optical THz switch based on three-dimensional (3D) terahertz meta-atoms. Combining a special design of 3D meta-devices and the ultrafast dynamics of low temperature grown gallium arsenide, we can modulate the reflectance of the THz microcavities within 2.2 ps. The device enables a 280 GHz switch in resonance frequency within less than 200 fs. The switch back to the original resonance takes 800 fs. Experimental results show that the speed values are strongly convoluted by the THz probing field and, thus, the real switching times are even shorter, in the few 100 fs range.

Terahertz generation and detection of LT-GaAs thin film photoconductive antennas excited by lasers of different wavelengths

A new method of generating and detecting terahertz waves is proposed. Low-temperature-grown gallium arsenide (LT-GaAs) thin films are prepared by etching a sacrificial layer (AlAs) in a four-layer epitaxial structure constituted with LT-GaAs, AlAs, GaAs, and semi-insulating gallium arsenide (SI-GaAs). The thin films are then transferred to clean silicon for fabricating the LT-GaAs thin film antennas. The quality and transmission characteristics of the films are analyzed by an 800-nm asynchronous ultrafast time domain spectroscopy system, and the degree of bonding between the film and silicon wafer is determined. Two LT-GaAs thin film antennas for generating and detecting the terahertz waves are tested with a 1550-nm femtosecond laser. The terahertz signal is successfully detected, proving the feasibility of this home-made LT-GaAs photoconductive antennas. This work lays a foundation for studying the mechanism of terahertz wave generation in GaAs photoconductive antennas below the semiconductor band gap.

Comparison of full-wave models of terahertz photoconductive antenna based on ordinary differential equation and Monte Carlo method

The investigation and comparison of two full-wave models of photoconductive terahertz antenna, made from low-temperature grown gallium arsenide, is performed. One model solves simple approximation of drift-diffusion equations another uses Monte Carlo simulation for estimation of the electrical current in the active region of antenna. Simulation results revealed that the simple model can be useful in the cases when the duration of photoexcitation is relatively long (FWHM $$\ge $$ 250 fs). In a case of shorter laser pulses (FWHM $$\ge $$ 130 fs), electron recombination (or trapping) time should be in the order of 0.1 ps, which is characteristic of highly compensated semiconductors. In other cases, transient dynamics of electron drift velocity at sub-picosecond timescales makes significant impact to the growth speed of the photocurrent. This leads to the overestimation of electric field amplitude in the high-frequency range when the simple model is used. Full-wave simulation shows good agreement with experimental results when the detectors’ response is included in the calculation. The calculated results were confirmed experimentally which increases the reliability of the full-wave model presented in the paper.

Complete optical transmission through nano-scale periodic metallic structure with three-stepped slits

Metallic structures with periodic array of subwavelength slits are well-known to lead to extraordinary optical transmission (EOT). Excellent power transmission originates from surface waves excited by incident transverse magnetic wave. Here we show that the metallic structure with array of three-stepped slits can transmit almost the entire power of incident wave into the substrate at desired optical frequency for low-temperature-grown gallium arsenide (LT-GaAs) substrate. Transmitted power of the proposed structure is studied both by an analytical model as well as numerical method by finite element (FEM). It is found that existence of three corners in the proposed structure can favorably confine generated carriers in the vicinity of the electrodes as this structure is used in photoconductive antenna (PCA); this feature contributes to increase terahertz (THz) current and subsequently THz radiation power of PCAs.

Photorefractive effect of low-temperature-grown aluminum gallium arsenide

The ternary compound aluminum gallium arsenide is an important material that can be used in all-optical solid-state ultrafast diagnostic technology. The low-temperature-epitaxially-grown AlGaAs (LT-AlGaAs) not only has the characteristics of ultra-short carrier lifetime of low-temperature-grown gallium arsenide (LT-GaAs), but also possesses the advantage of adjustability of band gap, which will provide great flexibility for the design of ultra-fast diagnostic systems. We use low-temperature epitaxial growth technology to grow AlGaAs on a GaAs substrate. The low-temperature-grown AlGaAs can effectively absorb 400 nm pump light to generate excess carrier. Therefore, we use a femtosecond laser with a wavelength of 800 nm and a pulse width of 200 fs as a light source to generate 400-nm pump light after passing through the BBO crystal, and 800 nm light without frequency doubling as the probe light. Using such a light source, we build a pump probe experimental platform to test the LT-AlGaAs. We normalize the experimental results and deconvolute it with the normalized laser pulses to obtain the response function of the semiconductor to the pump light. Therefore, we know that the nonequilibrium carrier relaxation time is less than 300 fs, and the nonequilibrium carrier recombination time is 2.08 ps. Due to the special passivation process, the effect of surface recombination on the carrier decay process is greatly reduced. The As clusters introduced by low-temperature epitaxial growth form deep level defects are the main factor for accelerating carrier recombination. In order to understand the complex process of photogenerated nonequilibrium carriers in depth, we use the indirect recombination theory of single recombination center to calculate the carrier recombination process, and establish an LT-AlGaAs carrier evolution model. Thus we obtain the key physical parameter related to the recombination rate, which is the carrier trapping area. We also use a theoretical model of carrier-regulated refractive index to calculate the effect of carrier concentration on the amount of change in refractive index. Combining our AlGaAs carrier evolution model, we simulate the refractive index change process of LT-AlGaAs after being illuminated by pump light. The simulation results are in good agreement with the experimental results. The method can be used for the quantitative analysis of carrier evolution characteristics of semiconductor materials, and it can conduce to the optimization and improvement of ultra-fast response semiconductor materials.

Generation of Terahertz Pulsed Radiation with Photoconductive Antennas Based of Low-Temperature-Grown Gallium Arsenide and Its Applications

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An Experimental Study on LT-GaAs Photoconductive Antenna Breakdown Mechanism

In this paper, an experimental study on the low-temperature-grown gallium arsenide (LT-GaAs) photoconductive antenna (PCA) breakdown mechanism is presented. A terahertz (THz) PCA is an important device for the THz wave generation. Due to that the typical LT-GaAs PCA breakdown scenarios, different trigger positions and different bias voltages, have been studied. Breakdown mechanism analysis has shown that when the trigger position was on the antenna electrode edge, the PCA breakdown was more probable to happen, because the electrode edge electric field was nearly 3 times greater than the electric field of the electrode center. Moreover, it has been shown that the thermal effect was the main cause of the LT-GaAs PCA breakdown. Last, the PCA lifetime has been calculated using the current waveform and the thermal effect analysis.

Low temperature-grown GaAs carrier lifetime evaluation by double optical pump terahertz time-domain emission spectroscopy.

We present the use of a "double optical pump" technique in terahertz time-domain emission spectroscopy as an alternative method to investigate the lifetime of photo-excited carriers in semiconductors. Compared to the commonly employed optical pump-probe transient photo-reflectance, this non-contact and room temperature characterization technique allows relative ease in achieving optical alignment. The technique was implemented to evaluate the carrier lifetime in low temperature-grown gallium arsenide (LT-GaAs). The carrier lifetime values deduced from "double optical pump" THz emission decay curves show good agreement with data obtained from standard transient photo-reflectance measurements on the same LT-GaAs samples grown at 250 °C and 310 °C.

High dynamic range single channel sampling of wideband RF signals using ultra‐fast nanoscale photoconductive switching

This Letter reports on new experimental high dynamic range sub-sampling performances obtained from nanoscale microwave photoconductive switches operating under a 0.8 µm wavelength pulsed illumination. Experimental results have been performed on different samples benefiting of a low temperature grown Gallium Arsenide absorbing layer on GaAs substrate, coplanar microwave electrode profile optimisation, and the use of an ultra-low noise jitter optical source delivering optical pulses of 1 to10 ps pulse width at a 2 GHz repetition frequency at 0.8 µm optical wavelength. Thanks to carrier lifetime tunability achievement from 1 to 20 ps, in association with a carrier mobility of 3900 cm²/V.s, experimental demonstrations of 54 and 45 dB signal-to-noise ratio microwave signal sampling for signals at 5,76 and 20 GHz frequency bandwidth respectively confirms submicron devices functionality as a new class of single channel photonic analogue to digital converters.

A bow-tie photoconductive antenna using a low-temperature-grown GaAs thin-film on a silicon substrate for terahertz wave generation and detection

This paper presents heterogeneously integrated bow-tie emitter–detector photoconductive antennas (PCAs) based on low-temperature grown-gallium arsenide (LTG-GaAs) thin-film devices on silicon-dioxide/silicon (SiO2/Si) host substrates for integrated terahertz (THz) systems. The LTG-GaAs thin-film devices are fabricated with standard photolithography and thermal evaporation of metal-contact layers of chromium (Cr), nickel (Ni) and gold (Au). They are etched selectively and separated from their growth GaAs substrate. The LTG-GaAs thin-film devices are then heterogeneously integrated on bow-tie antenna electrodes patterned on the surface of a SiO2/Si host substrate for THz emitters and THz detectors. Cost-effective and selective integration of LTG-GaAs thin-film devices on a Si platform is demonstrated. THz radiation from the fabricated THz PCAs is successfully measured using a pump–probe THz time-domain configuration. The THz temporal duration was measured at full width half maximum of 0.36 ps. Its frequency spectrum exhibits a broadband response with a peak resonant frequency of about 0.31 THz. The demonstration illustrates the feasibility of creating heterogeneously integrated THz systems using separately optimized LTG-GaAs devices and Si based electronics.

A Method to Maximize the Amplitude of Generated Terahertz Pulse From LT GaAs Photoconductive Semiconductor Switch

The substrate characteristics of GaAs photoconductive switch has been studied in order to determine the conditions required to generate maximum terahertz pulse amplitude in the device. In particular, the substrate material characteristics, such as trap density, capture cross sections, trap occupancy that determine fields, recombination rate, and photogeneration that influence the pulse rise-time in a low temperature grown gallium arsenide semiinsulating material, are studied. Results show that maximum pulse amplitude occurs when the carriers of both polarities contribute to the terminal currents, and the voltage at this point is referred to as the dual injection voltage (DIV). DIV for a given device is directly dependent on the substrate properties as well as the proximity of the electrodes.

On-chip picosecond pulse detection and generation using graphene photoconductive switches.

We report on the use of graphene for room temperature on-chip detection and generation of pulsed terahertz (THz) frequency radiation, exploiting the fast carrier dynamics of light-generated hot carriers, and compare our results with conventional low-temperature-grown gallium arsenide (LT-GaAs) photoconductive (PC) switches. Coupling of picosecond-duration pulses from a biased graphene PC switch into Goubau line waveguides is also demonstrated. A Drude transport model based on the transient photoconductance of graphene is used to describe the mechanism for both detection and generation of THz radiation.

A PHOTOMIXER DRIVEN TERAHERTZ DIPOLE ANTENNA WITH HIGH INPUT RESISTANCE AND GAIN

A terahertz (THz) antenna is proposed that offers high input resistance and gain in the presence of an electrically thick GaAs substrate. The antenna is centrally fed using two vertical probes connected to a photomixer on a thin low temperature grown gallium arsenide (LTG-GaAs) film which is supported by the GaAs substrate. An input impedance of ∼ 3.3 kΩ has been achieved using a dipole antenna that is printed on a thin dielectric slab, and isolated from the supporting substrate using a metal ground plane. A square aperture has been introduced to facilitate the illumination of the photomixer with two laser beams. Furthermore, a frequency selective surface (FSS) has been incorporated in the configuration, which results in a broadside gain of ∼ 19 dBi at a resonance frequency of 0.97 THz.

Influence of Substrate Material on Radiation Characteristics of THz Photoconductive Emitters

We present in this paper spectral and spatial characteristics of terahertz emission from standard dipole antenna structures used as emitters depending on the substrate material. All antenna structures were lithographically fabricated on low-temperature (LT) grown, few-micrometers-thick gallium arsenide (GaAs) layers. To investigate the effect of the substrate material on the radiation pattern of terahertz beams, either semi-insulating gallium arsenide or high-resistivity silicon substrate wafers have been used. As detector a standard 40 µm long dipole antenna on a semi-insulating GaAs substrate with a low-temperature grown gallium arsenide layer on it has been employed; this configuration allows for broadband detection and is still efficient enough for the characterization purpose. Strong dependence of the radiation pattern on the substrate used for the terahertz source is demonstrated. The measured patterns and differences between the two cases of substrates are well explained by means of classical diffraction.

Generation of widely tunable Fourier-transform-limited terahertz pulses using narrowband near-infrared laser radiation

Widely tunable, Fourier-transform-limited pulses of terahertz (THz) radiation have been generated using (i) crystals of the highly nonlinear organic salt 4- N, N-dimethylamino-4′- N′-methyl stilbazolium tosylate (DAST), (ii) zinc telluride (ZnTe) crystals, (iii) gallium phosphide (GaP) crystals, and (iv) low-temperature-grown gallium arsenide (LTG-GaAs) photomixers with THz spiral antennas. Outputs from two narrowband (Δ ν < 1 MHz, λ ∼ 800 nm) cw titanium-doped sapphire (Ti:Sa) ring lasers with a well-controlled frequency difference were shaped into pulses using acousto-optic modulators (AOM), coupled into an optical fiber, pulse amplified in Nd:YAG-pumped Ti:Sa crystals and used as optical sources to pump the THz emitters. The THz radiation was detected over a broad frequency range and its bandwidth was determined to be ∼10 MHz. The spectroscopic potential of the THz source is illustrated by the absorption spectrum of a pure rotational transition of OCS.

High field electron-spin transport and observation of the Dyakonov–Perel spin relaxation of drifting electrons in low temperature-grown gallium arsenide

High field electron-spin transport in low temperature-grown gallium arsenide is studied. We generate electron spins in the samples by optical pumping. During transport, we observe the Dyakonov–Perel (DP) [M.I. Dyakonov, V.I. Perel, Zh. Eksp. Teor. Fiz. 60 (1971) 1954] spin relaxation of the drifting electrons. The results are discussed and are compared with those obtained in calculations of the DP spin relaxation frequency of the hot electrons. A good agreement is obtained.

A New Approach to Terahertz Local Spectroscopy Using Microfabricated Scanning Near-Field Probe

In this paper, a new approach to local terahertz time-domain spectroscopy (THz-TDS) using a microfabricated probe is reported. A microfabricated scanning near-field optical microscopy (SNOM) probe was assembled with a low-temperature-grown gallium arsenide (LT-GaAs) photoconductive antenna. The assembly was used as the THz emitter and local THz source. Another LT-GaAs antenna situated on the opposite side was used as the detector. A THz-TDS measurement using the microfabricated SNOM probe and photoconductive antenna was performed.