Semi-insulating GaAs Research Articles

Semi-insulating GaAs detectors with HDPE layer for detection of fast neutrons from D–T nuclear reaction

Bulk semi-insulating (SI) GaAs detectors optimized for fast-neutron detection were examined using mono-energetic neutrons. The detectors have an active area of 7.36 mm2 defined by a multi-pixel structure of a AuZn Schottky contact allowing a relatively high breakdown voltage (300 V) sufficient for full depletion of the detector structure. The Schottky contact is covered by a HDPE (high density polyethylene) conversion layer, where neutrons transfer their kinetic energy to hydrogen atoms through elastic nuclear collisions. The detectors were exposed to mono-energetic neutrons generated by a deuterium (D)–tritium (T) nuclear reaction at a Van de Graaff accelerator. Neutrons reached a kinetic energy of 16.8 MeV when deuterons were accelerated by 1 MV potential. The influence of the HDPE layer thickness on the detection efficiency of the fast neutrons was studied. The thickness of the conversion layer varied from 50 [Formula: see text]m to 1300 [Formula: see text]m. The increase of the HDPE layer thickness led to a higher detection efficiency due to higher conversion efficiency of the HDPE layer. The effect of the active detector thickness modified by the detector reverse bias voltage on the detection efficiency was also evaluated. By increasing the detector reverse voltage, the detector active volume expands to the depth and also to the sides, slightly increasing the neutron detection efficiency.

Temperature-Dependence of High-Gain Operation in GaAs Photoconductive Semiconductor Switch at 1.6 $\mu \text{J}$ Excitation

This letter reports the temperature-dependence of high-gain operation in semi-insulating GaAs photoconductive semiconductor switches at 1.6 μJ laser diode excitation. Under the excitation of two different spot patterns, the electric field threshold for high-gain operation is investigated by varying the temperature. In addition, the switching characteristics dependence on temperature are also studied under the perpendicular spot excitation.

Low-temperature growth of GaSb epilayers on GaAs (001) by molecular beam epitaxy

Non-intentionally doped GaSb epilayers were grown by molecular beam epitaxy (MBE) on highly mismatched semi-insulating GaAs substrate (001) with 2 offcut towards [110]. The effects of substrate temperature and the Sb/Ga flux ratio on the crystalline quality, surface morphology and electrical properties were investigated by Nomarski optical microscopy, X-ray diffraction (XRD) and Hall measurements, respectively. Besides, differential Hall was used to investigate the hole concentration behaviour along the GaSb epilayer. It is found that the crystal quality, electrical properties and surface morphology are markedly dependent on the growth temperature and the group V/III flux ratio. Under the optimized parameters, we demonstrate a low hole concentration at very low growth temperature. Unfortunately, the layers grown at low temperature are characterized by wide FWHM and low Hall mobility.

Influence of EL2 deep level on photoconduction of semi-insulating GaAs under ultrashort pulse photoinjection

To investigate the influence of EL2 deep level on photoconduction of in semi-insulating GaAs (SI-GaAs), a 3 mm-electrode-gap lateral SI-GaAs photoconductive chip was manufactured and tested by using ultrashort pulse laser with 1064 nm wavelength, 10 ns pulsewidth, 3.0 mm light spot diameter and single pulse energy mean of 3.0 mJ. Based on the experimental results and the theory of trapping effect, the photon absorption process of EL2 defects in SI-GaAs is analyzed. For the influence of EL2 deep level, the lifetime of the electron gets shorter and the persistent photoconductivity (PPC) is significant. With increasing of voltage, the decay time constant of photoconduction is reduced and the decay index gets bigger for the ultrashort pulse photoinjection.

Material research on the InGaAs-emitting-layer VECSEL grown on GaAs substrate

Wafers of InGaAs-emitting-layer vertical external cavity surface emitting semiconductor laser (VECSEL) gain chip and separate active region were grown on semi-insulator GaAs substrates by low pressure metal–organic vapor phase epitaxy (MOVPE). Photoluminescence (PL) wavelength of the active region could be adjusted linearly about 1nm for increasing 1sccm H2 flow rate through TMIn under AsH3 flow rates of 150sccm. The complicated surface-emitted PL signal of the VECSEL gain chip was strongly modulated by interferences within the multilayer and was interpreted by the aberrance of the quantum wells emission with a profile filtered by a micro-cavity resonance in the longitudinal confinement factor. Material tests of the VECSEL wafer showed the reflectivity of the DBR mirrors was in good agreement with the active region photoluminescence, and the wafer was obtained with high crystal quality.

Optical and magnetotransport properties of InGaAs/GaAsSb/GaAs structures doped with a magnetic impurity

InGaAs/GaAsSb/GaAs bilayer quantum-well structures containing a magnetic-impurity δ-layer (Mn) at the GaAs/InGaAs interface are experimentally studied for the first time. The structures are fabricated by metal organic chemical-vapor deposition (MOCVD) and laser deposition on substrates of conducting (n+) and semi-insulating GaAs in a single growth cycle. The InGaAs-layer thickness is varied from 1.5 to 5 nm. The significant effect of a decrease in the InGaAs quantum-well thickness on the optical and magnetotransport properties of the structures under study is detected. Nonlinear magnetic-field dependence of the Hall resistance and negative magnetoresistance at temperatures of ≤30–40 K, circular polarization of the electroluminescence in a magnetic field, opposite behaviors of the photoluminescence and electroluminescence emission intensities in the structures, and an increase in the contribution of indirect transitions with decreasing InGaAs thickness are observed. Simulation shows that these effects can be caused by the influence of the δ-layer of acceptor impurity (Mn) on the band structure and the hole concentration distribution in the bilayer quantum well.

Modulation of terahertz radiation using optical control of Fabry–Perot resonances

Modulation of the terahertz radiation with the modulation depth up to 95% has been demonstrated employing optically tunable Fabry–Perot resonance phenomenon in semi-insulating GaAs plate.

Hot Wall Epitaxy(HWE)법에 의한 BaIn<sub>2</sub>S<sub>4</sub> 단결정 박막 성장과 광전도 특성

A stoichiometric mixture of evaporating materials for <TEX>$BaIn_2S_4$</TEX> single crystal thin films was prepared from horizontal electric furnace. To obtain the single crystal thin films, <TEX>$BaIn_2S_4$</TEX> mixed crystal was deposited on thoroughly etched semi-insulating GaAs(100) substrate by the Hot Wall Epitaxy (HWE) system. The source and substrate temperatures were <TEX>$620^{\circ}C$</TEX> and <TEX>$420^{\circ}C$</TEX>, respectively. The crystalline structure of the single crystal thin films was investigated by double crystal X-ray diffraction (DCXD). The carrier density and mobility of <TEX>$BaIn_2S_4$</TEX> single crystal thin films measured from Hall effect by van der Pauw method are <TEX>$6.13{\times}10^{17}cm^{-3}$</TEX> and <TEX>$222cm^2/v{\cdot}s$</TEX> at 293 K, respectively. The temperature dependence of the energy band gap of the <TEX>$BaIn_2S_4$</TEX> obtained from the absorption spectra was well described by the Varshni's relation, <TEX>$E_g(T)=3.0581eV-(3.9511{\times}10^{-3}eV/K)T^2/(T+536K)$</TEX>. The crystal field and the spin-orbit splitting energies for the valence band of the <TEX>$BaIn_2S_4$</TEX> have been estimated to be 182.7 meV and 42.6 meV, respectively, by means of the photocurrent spectra and the Hopfield quasicubic model. These results indicate that the splitting of the <TEX>${\Delta}so$</TEX> definitely exists in the <TEX>${\Gamma}_5$</TEX> states of the valence band of the <TEX>$BaIn_2S_4/GaAs$</TEX> epilayer. The three photocurrent peaks observed at 10 K are ascribed to the <TEX>$A_1$</TEX>-, <TEX>$B_1$</TEX>-exciton for n = 1 and <TEX>$C_{24}$</TEX>-exciton peaks for n = 24. A stoichiometric mixture of evaporating materials for <TEX>$BaIn_2S_4$</TEX> single crystal thin films was prepared from horizontal electric furnace. To obtain the single crystal thin films, <TEX>$BaIn_2S_4$</TEX> mixed crystal was deposited on thoroughly etched semi-insulating GaAs(100) substrate by the Hot Wall Epitaxy (HWE) system. The source and substrate temperatures were <TEX>$620^{\circ}C$</TEX> and <TEX>$420^{\circ}C$</TEX>, respectively. The crystalline structure of the single crystal thin films was investigated by double crystal X-ray diffraction (DCXD). The carrier density and mobility of <TEX>$BaIn_2S_4$</TEX> single crystal thin films measured from Hall effect by van der Pauw method are <TEX>$6.13{\times}10^{17}cm^{-3}$</TEX> and <TEX>$222cm^2/v{\cdot}s$</TEX> at 293 K, respectively. The temperature dependence of the energy band gap of the <TEX>$BaIn_2S_4$</TEX> obtained from the absorption spectra was well described by the Varshni's relation, <TEX>$E_g(T)=3.0581eV-(3.9511{\times}10^{-3}eV/K)T^2/(T+536K)$</TEX>. The crystal field and the spin-orbit splitting energies for the valence band of the <TEX>$BaIn_2S_4$</TEX> have been estimated to be 182.7 meV and 42.6 meV, respectively, by means of the photocurrent spectra and the Hopfield quasicubic model. These results indicate that the splitting of the <TEX>${\Delta}so$</TEX> definitely exists in the <TEX>${\Gamma}_5$</TEX> states of the valence band of the <TEX>$BaIn_2S_4/GaAs$</TEX> epilayer. The three photocurrent peaks observed at 10 K are ascribed to the <TEX>$A_1$</TEX>-, <TEX>$B_1$</TEX>-exciton for n = 1 and <TEX>$C_{24}$</TEX>-exciton peaks for n = 24.

Surface photovoltage spectroscopy study of InAs quantum dot in quantum well multilayer structures for infrared photodetectors

Inter-band optical transitions in InAs submonolayer and Stranski-Krastanov quantum dot (QD) in quantum well (QW) nanostructures are studied by means of room temperature surface photovoltage (SPV) spectroscopy taking advantage of its high sensitivity and contactless nature. The QD optical transitions are identified by the combined analysis of SPV amplitude and phase spectra and are in agreement with photoluminescence results. The SPV spectra have further revealed the optical transitions in all other relevant layers in the structures - wetting layer, QWs, and AlGaAs barriers. The analysis of the SPV phase spectra has revealed that the carrier separation and transport in the QD structure is determined by the energy band bending, resulting from the slight residual p-type doping. The complicated interaction between the SPV signals from the nanostructure and the semi-insulating GaAs substrate is discussed and clarified. The advantages of the SPV spectroscopy for characterizing complicated nanostructures at room temperature are highlighted.

Growth and Characterization of InAs1–xSbx with Different Sb Compositions on GaAs Substrates**Supported by the Aeronautical Science Foundation of China under Grant No 20132435, the National High-Technology Research and Development Program of China under Grant No 2013AA031903, the National Natural Science Foundation of China under Grant Nos 61106013 and 61275107, and the China Postdoctoral Science Foundation under Grant No 2014M560936.

InAs1–xSbx with different compositions is grown by molecular beam epitaxy on (100)-oriented semi-insulating GaAs substrates. The increase of Sb content in the epilayer results in the deterioration of crystal quality and surface morphology. Hall measurements show that the carrier concentration increases with the composition of Sb. The electron mobility decreases initially, when Sb composition exceeds a certain value, and the mobility increases slightly. In this work, we emphasize the comparison of crystal quality, surface morphology and electrical properties of epilayers with different Sb compositions.

Continuous wave terahertz radiation from antennas fabricated on C¹²-irradiated semi-insulating GaAs.

We demonstrate continuous wave (CW) terahertz generation from antennas fabricated on C12-irradiated semi-insulating (SI) GaAs substrates. The dark current drawn by the antennas fabricated on irradiated substrates is ∼3 to 4 orders of magnitude lower compared to antennas fabricated on un-irradiated substrates, while the photocurrents decrease by only ∼1.5 orders of magnitude. This can be attributed to the strong reduction of the carrier lifetime that is 2.5 orders of magnitude, with values around τ(rec)=0.2 ps. Reduced thermal heating allows for higher bias voltages to the irradiated antenna devices resulting in higher CW terahertz power, just slightly lower than that of low-temperature grown GaAs (LT GaAs)at similar excitation conditions.

Method of separate determination of high-ohmic sample resistance and contact resistance

A method of separate determination of two-pole sample volume resistance and contact resistance is suggested. The method is applicable to high-ohmic semiconductor samples: semi-insulating gallium arsenide, detector cadmium-zinc telluride (CZT), etc. The method is based on near-contact region illumination by monochromatic radiation of variable intensity from light emitting diodes with quantum energies exceeding the band gap of the material. It is necessary to obtain sample photo-current dependence upon light emitting diode current and to find the linear portion of this dependence. Extrapolation of this linear portion to the Y-axis gives the cut-off current. As the bias voltage is known, it is easy to calculate sample volume resistance. Then, using dark current value, one can determine the total contact resistance. The method was tested for n-type semi-insulating GaAs. The contact resistance value was shown to be approximately equal to the sample volume resistance. Thus, the influence of contacts must be taken into account when electrophysical data are analyzed.

Growth and optimization of InxGayAl1−x−ySb buffer layers for electronic and optoelectronic applications

InxGayAl1−x−ySb alloys have been grown by molecular beam epitaxy for use as a buffer layer for growing semiconductors on GaAs substrates with lattice constants beyond that of AlSb. This is an extension of the use of AlxGa1−xSb alloys to accommodate for the lattice mismatch with semi-insulating GaAs substrates. The growth of In0.21Ga0.19Al0.6Sb with a 6.2 Å lattice constant on semi-insulating GaAs substrates is the focus of this work. Several measures of the quality of a 1 μm-thick In0.21Ga0.19Al0.6Sb layer improved when the growth temperature was increased from 460 to 600 °C. Atomic force microscopy root-mean-square values decreased from 2.9 to 1.8 nm and the peak-to-valley values decreased from 17.7 to 9.7 nm. In addition, double crystal x-ray diffraction ω-2Θ spectra linewidths decreased from 568 to 482 arc sec. At the lower growth temperatures, several photoluminescence (PL) peaks associated with radiative recombination from regions with different alloy compositions were found. However, on increasing the growth temperature a single PL line was observed, strongly suggesting a more uniform alloy composition.

Characteristics of GaAs PCSS triggered by 1 μJ laser diode

Various kinds of switches exhibit significant function in the field of pulse power. Among them Photoconductive Semiconductor Switch (PCSS) stands out because of its numerous advantages, such as low time jitter, fast response, high repetition rate, compact size, easy integration and so on. Laser Diode (LD) is small in size, low-cost and can be easily integrated compared with traditional laser devices. Waveguide tube is a kind of laser transmission device. By using LDs which emits laser pulse with a dominant wavelength of 905 nm and a waveguide tube, three different shapes of laser spot are obtained to trigger a semi-insulating GaAs PCSS which has vertical structure. Laser energy and laser power are fixed to 1 μJ and 33.3 W separately. As the illuminating area on the PCSS differs with different shapes of laser spot, the influences of average trigger energy per unit area to the output characteristics of the PCSS are studied. The experiment results demonstrate that not only the total trigger energy but also the energy density influences the characteristics of PCSS. This paper presents related experiment results, and some further discussions are given.

An alternative approach to investigate the origin of p-type conductivity in arsenic doped ZnO

P-type conductivity in MOCVD grown ZnO was obtained by directional thermal diffusion of arsenic from semi-insulating GaAs substrate. The films were single crystalline in nature and oriented along (002) direction. Ab initio calculations in the framework of density functional theory have been carried out with different chemical states of arsenic in ZnO. Present calculations suggested AsZn–2VZn defect is a shallow acceptor and results in ferromagnetism in ZnO. The magnetic measurements of the samples indeed showed ferromagnetic ordering at room temperature. X-ray photoelectron spectra confirmed the presence of AsZn and VZn. The core level chemical shift in binding energy of AsZn indicated the formation of AsZn–2VZn. Diffused arsenic substitutes zinc atom and creates additional zinc vacancies. The zinc vacancies, surrounding the oxygen atoms, result in unpaired O 2p electrons which in turn induce ferromagnetism in the samples.

InAs Planar Nanowire Gate-All-Around MOSFETs on GaAs Substrates by Selective Lateral Epitaxy

High indium content III–V materials are one of the most promising candidates for beyond Si CMOS technologies. We present InAs planar nanowire (NW) MOSFETs grown directly on a semi-insulating GaAs (100) substrate by the selective lateral epitaxy (SLE) method via the metal-seeded planar vapor-liquid-solid mechanism. Despite a $\sim 7$ % lattice mismatch, in-plane and self-aligned single-crystal InAs NWs are grown epitaxially on GaAs. Such heterogeneous SLE provides a potential solution for the integration of different channel materials on one substrate. Gate-all-around MOSFET devices are fabricated by releasing the NW channel from the substrate through a combination of digital etching and selective etching processes. The device with a NW width of 30 nm and gate length of 350 nm shows an $I_{\mathrm{{\scriptscriptstyle ON}}}$ / $I_{\mathrm{{\scriptscriptstyle OFF}}}$ ratio of $10^{4}$ and a peak transconductance of 220 mS/mm at $V_{\mathrm {ds}} = 0.5$ V.

In brief

PAGE 1146 A team working in Australia have achieved wideband gain enhancement of a slot antenna by placing a quarter-wavelength-thick unprinted dielectric slab above it. In addition to having a high gain with a large bandwidth, the antenna is easy to fabricate and has a low vertical profile, with a total height of 0.593λ0. PAGE 1140 The specific differential attenuation reflectivity, phase and reflectivity exhibited by large rand drops and melting hail have been computed and studied by a team from the USA. The work addresses the ‘hot spots’ in C-band polarimetric radar observations in storms. The measured peak gain is 16.95 dBi and the 3dB gain bandwidth is 16.25% PAGE 1172 Gerald Küstler, working in Germany, reports on a simultaneous inline triple levitation system using permanent magnets. The system allows for the inline levitation of three different bodies without energy input or feedback control, and could find application in seismometers or accelerometers after further study. Melting hailstones take the form of a water torus surrounding an ice core PAGE 1170 A team from the USA and Korea build on their previous work by investigating surface plasmon resonant splitting and merging due to infrared incidence through a thermal imaging lens. The team analyse the transmission of a plasmonic perforated gold film (PGF) against the incident light in both polar and azimuth angles. The future challenge is to characterise the system response behaviour PAGE 1132 A team working in China have achieved adjustable electromagnetically induced transparency (EIT) using a slotted circle ring structure. EIT can create a narrow transmission window in a wide absorption band, and such structures that create this effect can be used as angle modulators or state sensors. The sample was fabricated with a PGF on a semi-insulating GaAs substrate The proposed structure can also be used in frequency selective surface design

Inherent photoluminescence Stokes shift in GaAs

The intrinsic photoluminescence Stokes shift, i.e., the energy difference between optical band gap and emission peak, of 350 μm thick semi-insulating GaAs wafers is found to be 4 meV at room temperature. The result is based on the determination of the optical bulk band gap from the transmission trend via modified Urbach rule whose result is confirmed with the transmission derivative method. The findings reveal the detailed balance of the optically evoked transitions and disclose the intrinsic link between Stokes shift and the Urbach tail slope parameter.

Confined photocarrier transport in InAs pyramidal quantum dots via terahertz time-domain spectroscopy.

We present experimental demonstration of photocarrier dynamics in InAs quantum dots (QDs) via terahertz (THz) time-domain spectroscopy (TDS) using two excitation wavelengths and observing the magnetic field polarity characteristics of the THz signal. The InAs QDs was grown using standard Stranski-Krastanow technique on semi-insulating GaAs substrate. Excitation pump at 800 nm- and 910 nm-wavelength were used to distinguish THz emission from the InAs/GaAs matrix and InAs respectively. THz-TDS at 800 nm pump revealed intense THz emission comparable to a bulk p-InAs. For 910 nm pump, the THz emission generally weakened and upon applying external magnetic field of opposite polarities, the THz time-domain plot exhibited anomalous phase-shifting. This was attributed to the possible current-surge associated with the permanent dipole in the QD.

Self-organized patterns in successive bifurcations in planar semiconductor-gas-discharge device.

The formation of dissipative structures is investigated in a planar semiconductor-gas-discharge device at room temperature. The width of the discharge gap is about 1 mm. The gap is filled with nitrogen at a pressure that corresponds to the discharge operation at the right branch of the Paschen curve. Wafers of semi-insulating GaAs that exhibit linear transport in the whole range of voltage and current studied are used as semiconductor electrodes. In addition to the earlier investigated Andronov-Hopf bifurcation, a different mode of self-organization of the device is observed, where the transport of charge proceeds through an ensemble of pulsating current filaments. The corresponding critical current for the present bifurcation does not depend on the polarity of the bias voltage, while the spatiotemporal dynamics of a pattern differs substantially for a change in the polarity. Pulsating filaments can form a spatially ordered pattern when the GaAs electrode is under the positive potential. We also observe self-organization modes, where pulsating filaments form an irregular spatiotemporal dynamics of a pattern. The data obtained are briefly discussed in the frame of corresponding theoretical results in the field.