Low-temperature Gallium Arsenide Research Articles

Highly integrated automatic injection terahertz microfluidic biosensor based on metasurface and LT-GaAs photoconductive antenna

In this paper, a highly integrated terahertz (THz) biosensor is proposed and implemented, which pioneered the preparation of low-temperature gallium arsenide (LT-GaAs) thin film photoconductive antenna (PCA) on the sensor for direct generation and detection of THz waves, simplifying complex terahertz time-domain spectroscopy (THz-TDS) systems. A latch type metasurface is deposited in the detection region to produce a resonance absorption peak at 0.6 THz that is independent of polarisation. Microfluidics is utilised and automatic injection is incorporated to mitigate the experimental effects of hydrogen bond absorption of THz waves in aqueous-based environment. Additionally, cell damage is minimised by regulating the cell flow rate. The biosensor was utilised to detect the concentration of three distinct sizes of bacteria with successful results. The assay was executed as a proof of concept to detect two distinct types of breast cancer cells. Based on the experimental findings, it has been observed that the amplitude and blueshift of the resonance absorption peaks have the ability to identify and differentiate various cancer cell types. The findings of this study introduce a novel approach for developing microfluidic THz metasurface biosensors that possess exceptional levels of integration, sensitivity, and rapid label-free detection capabilities.

Time Domain Simulated Characterization of the Coplanar Waveguide in an On-Chip System for Millimeter Waveform Metrology

We investigate the time domain characterization of a coplanar waveguide (CPW) based on an on-chip electro-optic sampling (EOS) system for millimeter waveform metrology. The CPW is fabricated on a thin layer of low-temperature gallium arsenide (LT-GaAs), and the substrate material is GaAs. A femtosecond laser generates and detects ultrashort pulses on the CPW. The forward propagating pulses are simulated using a simplified current source for the femtosecond laser at different positions on the CPW for the first time. Then, the influences of the CPW geometry parameters on the measured pulses are discussed. The varying slot width has larger influences on the amplitude of millimeter wave pulses than the center conductor width and the pumping gap. Finally, in the frequency range of 10 GHz to 500 GHz, the transfer functions calculated by the time domain pulses are in good agreement with the transfer functions calculated by the frequency domain ports. The above results are important for improving the measurement precision of the millimeter waveform on the CPW for millimeter waveform metrology.

Time-Domain Modelling of Pulsed Photoconducting Sources—Part II: Characterization of an LT GaAs Bow-Tie Antenna

Drude’s description of the response of low-temperature gallium arsenide to optical pulse excitation is used to evaluate the components of a time-domain Norton equivalent circuit of a photoconductive antenna (PCA) source. The saturation of the terahertz (THz) radiated power occurring at large optical excitation levels was previously associated by the scientific community to radiation and charge screening of the bias. With the present circuit, we are able to model accurately the measured saturation as only due to the EM feedback from the antenna to the bias. The predicted THz radiated power is shown to match very accurately the measurements when the circuit is combined with an accurate description of the experimental conditions and the modeling of the THz quasi-optical (QO) channel.

LT-GaAs Thin film antenna based on the asynchronous optical sampling system

Using a photoconductive antenna to generate terahertz (THz) waves is a common method, and the material of the antenna is usually low-temperature gallium arsenide (LT-GaAs). A LT-GaAs epitaxial wafer with four layers structure is prepared in this paper, and then the LT-GaAs thin film is obtained by etching the aluminium arsenide (AlAs) with dilute hydrochloric acid and transferring it to the silicon wafer to make a thin film photoconductive antenna. The performance of the antenna is evaluated using an asynchronous optical sampling (AOS) terahertz time-domain spectroscopy (THz-TDS) system with variable repetition frequency. The system uses the spectral coherence method of two lasers to obtain trigger signals and generates THz signals with high signal-to-noise ratio. This also confirms that the system has the advantages of high spectral resolution and fast detection speed. It has incomparable advantages over the traditional THz-TDS system in the environment requiring rapid measurement.

A new hybrid metasurface design for performance improvement of thin film unbiased terahertz photoconductive source

In this paper, a new bias free hybrid structure is proposed to improve the performance of unbiased bimetallic terahertz photoconductive antenna (PCA). Each element of unbiased bimetallic emitters consists of an asymmetric metal-semiconductor-metal (MSM) that is made of two side by side dis-similar Schottky contacts. Engineering the surface of low temperature gallium arsenide (LT-GaAs) layer by different kind of nanostructure metasurfaces in combination with bimetallic nanostructure located underneath the LT-GaAs have been used to enhance the 800 nm femtosecond laser beam absorption inside the LT-GaAs layer and so boost the terahertz wave generation. Solving the Maxwell’s equation in combination with drift-diffusion/Poisson’s using finite element method have been applied to investigate the optical and electrical response and optimize the performance of proposed structure. According to the simulation results, using optimized metasurface on LT-GaAs decreases the reflection to less than 0.8% compared to the 29% of the conventional PCA. Moreover, proposed structure shows 83% photocurrent enhancement compared to unbiased PCA without metasurface and with Si3N4 antireflection coating.

Terahertz generation and detection of 1550-nm-excited LT-GaAs photoconductive antennas

Terahertz generation and detection is the key means to explore terahertz spectrum, one of which is the application of photoconductive antenna. In this study, a low-temperature gallium arsenide (LT-GaAs) epitaxial wafer antenna and an LT-GaAs thin-film antenna were fabricated. The LT-GaAs epitaxial wafer antenna was fabricated using an epitaxial wafer composed of LT-GaAs, AlAs,GaAs, and semi-insulating GaAs. An LT-GaAs thin film was obtained by etching the AlAs layer in the epitaxial wafer and then transferring it to a clean silicon wafer to fabricate the LT-GaAs thin-film antenna. The LT-GaAs epitaxial wafer and thin-film antennas were used as the generation and detection antennas, respectively. The two antennas were directly aligned at a distance of 2 mm from each other and placed into a self-made measurement system for detection. A THz spectrum of approximately 2.5 THz was obtained under 1550-nm laser excitation, thus verifying the antenna performance.

The Research for Approaches to Increase Power of the Compact THz Emitters Based on Low-Temperature Gallium Arsenide Heterostructures

The design and technological conditions for the manufacture of photoconductive antennas based on low-temperature gallium arsenide (LT-GaAs) have been developed. The optimized photoconductive THz antenna is made based on LT-GaAs with the flag geometry of the contacts and with the interdigitated structure including metal closing through the dielectric of each second period. LT-GaAs samples were obtained by molecular beam epitaxy at temperatures of 210 °C, 230 °C, 240 °C on GaAs substrates (100). Dark and photocurrent were measured depending on the bias voltage of the LT-GaAs heterostructure at the EP6 probe station. Full wave finite element method solver has been used to investigate the proposed plasmon PCA electrical and optical behavior by combining the Maxwell's wave equation with the drift-diffusion/Poisson equations.

Improving unbiased terahertz photoconductive antenna based on dissimilar Schottky barriers using plasmonic mode excitation

A new design for an unbiased antennaless terahertz (THz) photoconductive antenna (PCA) using an array of bimetallic nanostructures located underneath the low temperature gallium arsenide (LT-GaAs) layer is designed and proposed which is highly under focus for THz imaging applications. Each element of array consists an asymmetric metal-semiconductor-metal (MSM) structure with dissimilar Schottky barriers. Simulation results demonstrated that by irradiating this structure using 800 nm femtosecond laser, THz field as high as a conventional biased PCA can be generated. Finite element method was used to optimize the PCA’s geometrical parameters through solving the Maxwell’s equation in combination with drift-diffusion/Poisson’s equations. According to the simulation results, by optimizing the array’s periodicity, width and height of the proposed bimetallic nanostructures, plasmonic resonant cavity mode between two adjacent nanostructures enhances the optical absorption and so, the transient photocurrent. Transient photocurrent of 45 μA was obtained that is 29% higher than the one from the conventional biased PCA.

The Influence of the Annealing Regime on the Properties of Terahertz Antennas Based on Low-Temperature-Grown Gallium Arsenide

Low-temperature gallium arsenide (LT-GaAs) films were grown by the method of molecularbeam epitaxy (MBE) at a reduced temperature (230°C) on GaAs(100) substrates and subjected to postgrowth annealing in various regimes. Photoconductive antennas (PCAs) with flag geometry formed on the film surface were characterized by terahertz (THz) response power at various bias voltages. The method of THz spectroscopy was used to study the characteristics of PCAs based on LT-GaAs films annealed in various regimes and the optimum interval of postgrowth annealing temperatures (670–720°C) was established.

Significant enhancement in the efficiency of photoconductive antennas using a hybrid graphene molybdenum disulphide structure

We propose a hybrid graphene molybdenum disulphide-based photoconductive antenna to overcome the restrictions of metallic photoconductive antennas and graphene-based photoconductive antennas, simultaneously. The structure is composed of a hybrid graphene-MoS2 strip as the antenna deposited on a low-temperature gallium arsenide substrate. A full-wave electromagnetic solver, namely, high frequency structural simulator (HFSS) is used to analyze the whole structure. It is shown that the proposed photoconductive antenna provides us with not only high input impedance and reconfigurability but also high values of matching efficiency and radiation efficiency. The impact of increasing MoS2 layers on the performance of the antenna is also investigated.

Dynamic characteristics of “low-temperature” gallium arsenide for terahertz-range generators and detectors

The lifetime of free photoexcited carriers in epitaxial films of “low-temperature” gallium arsenide (LT-GaAs) is determined by the pump-probe optical reflection method. The dark resistivity of LT-GaAs layers is estimated. Emission spectra of LT-GaAs photoconductive antennas are measured in the terahertz frequency region by the Fourier transform spectroscopy.

A Study of Emission Power and Spectrum of LT-GaAs Based THz Photoconductive Antennas

Emission spectra of LT-GaAs photoconductive antennas based on epitaxial films of “low-temperature” gallium arsenide (LT-GaAs) are measured in the terahertz frequency region by the Fourier transform spectroscopy.

Electron spectroscopy of objects for nanoelectronics

An electron-spectroscopic analysis is made of layered nanostructures and clusters at the surface and in the bulk of a solid. A new method of forming metal/insulator/semiconductor (superconductor) nanostructures is proposed based on ion-stimulated metal segregation effects at the surface of low-temperature gallium arsenide and a 123 high-temperature superconductor. The geometric parameters and electronic structure of these nano-objects are studied. It is shown that their electronic properties can be controllably varied in situ by acting on the surface. The dimensional transformation of the electronic properties of metal clusters is studied for clusters in the insulator SiO2, in the superconductor LTMBE-GaAs, and on silicon and graphite surfaces. The nature of this transformation is clarified. A diagnostics for cluster ensembles is developed by which one can determine the parameters needed to describe singleelectron transport: the average number of atoms per cluster, the average distance between clusters and isolated atoms, and the chemical state of the atoms. Ensembles of silver clusters with specified parameters are obtained on a silicon surface. It is shown that these ensembles are potentially useful for developing single-electron devices.

X-ray diffraction examinations of EL2-like defect metastability in low-temperature gallium arsenide.

X-ray diffraction examinations of EL2-like defect metastability in low-temperature gallium arsenide.

High resolution x-ray diffraction analysis of annealed low-temperature gallium arsenide

High resolution x-ray diffraction methods have been used to characterize GaAs grown at low substrate temperatures by molecular beam epitaxy and to examine the effects of post-growth annealing on the structure of the layers. Double crystal rocking curves from the as-deposited epitaxial layer show well-defined interference fringes, indicating a high level of structural perfection despite the presence of excess arsenic. Annealing at temperatures from 700 to 900 °C resulted in a decrease in the perpendicular lattice mismatch between the GaAs grown at low temperature and the substrate from 0.133% to 0.016% and a decrease (but not total elimination) of the visibility of the interference fringes. Triple-crystal diffraction scans around the 004 point in reciprocal space exhibited an increase in the apparent mosaic spread of the epitaxial layer with increasing anneal temperature. The observations are explained in terms of the growth of arsenic precipitates in the epitaxial layer.

Investigation of the electronic properties of in situ annealed low-temperature gallium arsenide grown by molecular beam epitaxy

Current conduction through in situ annealed low-temperature gallium arsenide (LT GaAs) grown by molecular beam epitaxy (MBE) has been studied using Schottky diodes. The dominant transport mechanism in these films was found to be space-charge-limited current conduction in the presence of deep level states. We also examined two LT GaAs films, one with initially 4000 Å thick subsequently etched back to 2000 Å and the other sample with an initial thickness of 2400 Å, and found that the excess arsenic redistribution or clustering, carrier trap levels, and concentration depend strongly on the initial thickness of the LT GaAs film.