非掺半绝缘GaAs单晶的垂直梯度凝固生长

研究了非掺ＧａＡｓ半绝缘单晶的液封垂直梯度凝固法（ＶＧＦ）生长技术,解决了Ｓｉ沾污和Ｃ浓度的控制问题,得到了直径２英寸非掺半绝缘低位错单晶。测试表明：该单晶的位错密度较ＬＥＣ单晶下降近一个数量级,电学参数与ＬＥＣ单晶类似,接近国个ＶＧＦ单晶的的参数指标。     

四步法制备高质量硅基砷化镓薄膜

为了提高在硅基上外延砷化镓薄膜的质量和实验的可重复性, 我们提出了一种叫做四步生长法的新方法, 该方法是通过在低温成核层和高温外延层中间先后插入低温缓冲层和高温缓冲层实现的。通过此方法, 可以制备出表面具有单畴结构、在强白光下依然光亮如镜、粗糙度低且缺陷少的高质量砷化镓薄膜, 而且此方法的重复性很好。即便没有任何生长后的退火处理, 外延出的1 μm厚砷化镓薄膜在5 μm×5 μm扫描区域内的表面粗糙度只有2.1 nm, 且由X射线双晶衍射测试出的砷化镓(004)峰的半高宽只有210.6 arcsec。     

Direct imaging of single Au atoms within GaAs nanowires

Incorporation of catalyst atoms during the growth process of semiconductor nanowires reduces the electron mean free path and degrades their electronic properties. Aberration-corrected scanning transmission electron microscopy (STEM) is now capable of directly imaging single Au atoms within the dense matrix of a GaAs crystal, by slightly tilting the GaAs lattice planes with respect to the incident electron beam. Au doping values in the order of 10(17-18) cm(3) were measured, making ballistic transport through the nanowires practically inaccessible.     

MOVPE of structures with aluminum nanocluster layers in a GaAs matrix

Metalorganic vapor phase epitaxy (MOVPE) was used to grow semiconductor structures comprising a GaAs single crystal matrix with incorporated layers of aluminum nanoclusters (Al-NCs). A new regime of GaAs overgrowth on Al-NC layers is proposed, which ensures planarization of the semiconductor layer surface at a thickness comparable with the height of Al-NCs.     

Dependence of the Contrast Ratio on Crystal Thickness in an Electroabsorptive Spatial Light Modulator That Uses GaAs

The contrast ratio of an optically addressed spatial light modulator that uses electroabsorption in a GaAs single crystal is discussed experimentally and theoretically. The modulator has the same structure as a Pockels readout optical modulator. The contrast ratio depends strongly on the change in the absorption coefficient and on the thickness of the GaAs crystal. From the experimental results and from theoretical investigations of the Franz-Keldysh effect, the change in the absorption coefficient is estimated by use of the quadratic equation of an applied electric field that is not excessively strong. Under this condition, an optimum thickness of the GaAs crystal plate that will yield the maximum contrast ratio can be determined.     

Acoustostimulated change in the optical absorption coefficient of gallium arsenide in the 0.81–1.77 μm wavelength range

The effect of ultrasonic waves on the spectral absorption coefficient of gallium arsenide (GaAs) in the 0.81–1.77 μm wavelength range. The treatment of GaAs single crystals with ultrasonic waves leads to a change in their coefficients of absorption of electromagnetic radiation. This phenomenon is related to the fact that an acoustic wave, being an energy carrier, modified a defect system of the crystal and produces a redistribution of impurities in the crystal lattice. The interaction of photons with acoustogenerated defects changes the optical absorption coefficient near the edge of the fundamental absorption band of GaAs.     

Modeling of difference frequency radiation energy redistribution process in a spectrum of laser pulse of a few optical cycles propagating in a quasi-phase-matching GaAs crystal

We present the results of a numerical study of the generation process of difference frequency radiation (DFR) arising via the interaction of mutually orthogonal linearly- polarized few-cycle laser pulses propagating in a quasi-phase-matching (QPM) GaAs crystal. Considered the interaction of pulses having the central wavelengths of 1.98 µm, duration of 30 fs with the electric field amplitude 200 MV/m, propagating along the normal to the ?110? plane. The period Λ of the QPM GaAs crystal in numerical simulations varied from 20.89 µm to 53.23 µm. It is shown that by changing the grating period of the QPM GaAs crystal from 23.02 µm to 37.29 µm it is possible to improve the efficiency of QPM generation of DFR in the 5.48 µm – 10.12 µm spectral range at least by 8 dB in comparison with the generation of DFR in bulk crystal GaAs in the same spectral range.     

Elastic stress relaxation in buffer layers based on porous strained InGaAs/GaAs superlattices

Potential advantages of using relaxed porous InGaAs/GaAs superlattices as buffer layers are considered. The X-ray diffraction patterns and the photoluminescence spectra of multilayer epitaxial InGaAs/GaAs heterostructures upon electrochemical etching are indicative of a partial relaxation of elastic stresses in the component epilayers. The stress relaxation in porous superlattices, used as buffer layers on both porous GaAs and periodic single crystal heterostructures, provides for a small but still significant positive effect related to a decrease in elastic energy accumulated in the growing structure.     

砷化镓单晶中微晶和非晶的形成机制

利用高分辨电子显微镜对0.0049N和0.049N荷载Vickers压痕锈发砷化镓单晶的相转变进行了观察和研究,结果表明,在大小压痕作用下分别发生了单晶向和微晶的转变,微晶的结构由小于10nm,取向各异的纳米晶和非晶组成,在完全非晶化的结构中存在少量由几个原子组成的原子簇,在非晶和晶体的交界区能观察到许多晶体缺陷以及沿这些缺陷产生的晶格扭曲和非晶相岛,对这种非晶化现象提出了两种可能的诱发机制,高压力诱导非晶化和剪切诱导非晶化。     

A hydrogen-sensitive structure based on semi-insulating gallium arsenide

The response characteristics of a simple hydrogen-sensitive structure based on semi-insulating single crystal gallium arsenide with planar palladium electrodes deposited onto the oxidized substrate surface are studied. It is demonstrated that such structures exhibit a fast response to hydrogen present in the gas phase. The sensitivity can be increased by growing, prior to the electrode formation, an intermediate epitaxial GaAs film with built-in strained quantum-confined layers of InGaAs and InAs onto the semi-insulating GaAs substrate.     

A micro X-ray fluorescence analysis method using polycapillary X-ray optics and grazing exit geometry

Grazing exit X-ray fluorescence (GE-XRF), which has unique advantages in surface and film analysis, is a development of XRF related to total reflection XRF. The combination of polycapillary X-ray optics with total reflection geometry in the detection path allows micro analysis in thin layer characterization. This technique was applied to analyze a series of titanium and iron layers which were deposited on GaAs single crystal by metal vapor vacuum arc ion sources. Thickness and density of the layers result from fitting the experimental data to model calculations, and the information of layer uniformity can be acquired by two-dimensional scan analysis. The GE-XRF method has application for complete layer characterization and process control during the layer deposition.     

Initial stage and reconstruction of GaAs/Si heterostructures

GaAs epitaxial layers on Si(1 0 0) substrates having a single or a double domain 2×1 have been grown by molecular beam epitaxy using the two-step growth mode and thermal regrowth techniques. The initial stage and the reconstruction of the GaAs/Si heterostructures have been investigated in situ by Auger electron spectroscopy and reflection high-energy electron diffraction. GaAs layers grown by both methods show the reconstruction of a single domain, and models for the process of GaAs growth have been presented to explain the self-annihilation of the antiphase boundary.     

混凝土损伤自愈的机理

研究了不同龄期受损混凝土经过相同养护期后的自然愈合现象．混凝土受损后的自愈合实质上是损伤部位未水化或水化不充分的胶凝材料加速水化或进一步水化生成新的水化产物弥合裂缝的过程．以超声波速的变化表征混凝土受压开裂后的损伤程度,建立了混凝土损伤量与愈合状况之间的关系．结果表明,混凝土材料存在一个损伤阈值：当混凝土的损伤低于损伤阈值时,自愈合率随着损伤量的增大而增大;当混凝土损伤超过损伤阈值时,自愈合率随着损伤量的增大而降低．     

Properties of GaAs nanoclusters deposited by a femtosecond laser

The properties of femtosecond pulsed laser deposited GaAs nanoclusters were investigated. Nanoclusters of GaAs were produced by laser ablating a single crystal GaAs target in vacuum or Ar gas. Atomic force and transmission electron microscopies showed that most of the clusters were spherical and ranged in diameter from 1 nm to 50 nm, with a peak size distribution between 5 nm and 9 nm, depending on the Ar gas pressure or laser fluence. X-ray diffraction, solid-state nuclear magnetic resonance, Auger electron spectroscopy, electron energy loss spectroscopy, and high-resolution transmission electron microscopy revealed that these nanoclusters were randomly oriented GaAs crystallites. An oxide outer shell of 2 nm developed subsequently on the surfaces of the nanocrystals as a result of transportation in air. Unpassivated GaAs nanoclusters exhibited no detectable photoluminescence. After surface passivation, these nanoclusters displayed photoluminescence energies less than that of bulk GaAs from which they were made. Our photoluminescence experiments suggest an abundance of sub-band gap surface states in these GaAs nanocrystals.     

Theoretical study of the formation of deformation twins in GaAs crystals grown by the vertical gradient freeze method

The objective of this paper is to predict the twin formation in the Gallium Arsenide (GaAs) crystals grown by vertical gradient freeze (VGF) method at different growth parameters. The deformation twins are formed in the GaAs crystal during its growth processes from the melt. The thermal stresses generated by the temperature profile during the crystal growth can be the primary cause of deformation twin formation. Temperature gradients are depend on the geometrical and physical crystal growth parameters, such as crystal diameter and imposed temperature gradients on the surface of the solidifying crystal in VGF. A quantitative thermal stress model is developed here for predicting the twin formation in GaAs grown by VGF at different growth parameters. This investigation is expected to further the understanding of twin formation. This understanding will provide valuable information to crystal growers to study the influence of growth parameters on twin formation for growing low defect GaAs crystal.     

Micro structuration of gaas surface by wet etching: towards a specific surface behavior

Resonant microelectromechanical systems are promising devices for real time and highly sensitive measurements. The sensitivity of such sensors to additional mass loadings which can be increased thanks to the miniaturisation of devices is of prime importance for biological applications. The miniaturisation of structures passes through a photolithographic process and wet chemical etching. So, this paper presents new results on the anisotropic chemical etching of the gallium arsenide (GaAs) crystal used for this application, in several solutions. This paper focuses on the micro/nanostructuration of the sensing surface to increase the sensor sensitivity. Indeed, this active surface will be biofunctionalized to operate in biological liquid media in view of biomolecules detection. Several experimental conditions of etching bath composition, concentration and temperature were examined to obtain a large variety of geometrical surfaces topographies and roughness. According to the orientation dependence of the chemical etching process, the experiments were also performed on various GaAs crystal plates. The bath 1 H3PO4:9 H2O2:1 H2O appeared to be particularly adapted to the fabrication of the GaAs microstructured membrane: indeed, the bath is highly stable, anisotropic, and, as a function of temperature, it allows the production of a large variety of GaAs surface topographies.     

Influence of microcrack self-healing behavior on the permeability of Engineered Cementitious Composites

Engineered Cementitious Composite (ECC) is a family of high performance fiber reinforced cementitious composites featuring strain-hardening behavior and high tensile ductility (with tensile strain capacity of 3–5%). ECC achieves high ductility by forming multiple microcracks with crack width less than 60 μm under tension. The tight crack width of ECC naturally lends itself to low permeability even in the cracked stage. Such properties are of particular interest to hydraulic structure applications. In addition to the tight crack width, self-healing of microcracks further lowers the permeability of cracked ECC, enhancing the durability and safety of hydraulic structures. In this paper, the permeability of ECC composites under the influence of self-healing was experimentally studied. Single crack permeability tests were also conducted to directly correlate the permeability and self-healing behavior of a single crack with a given initial crack width. Additionally, an analytical model capable of predicting the permeability of ECC composites that undergo self-healing process is proposed and verified with experimental data. The research findings in the present paper can be used to accurately estimate the permeability of ECC and are expected to provide support for future design and application of ECC for hydraulic structures.     

砷化镓单晶的等效微重力生长

论述了砷化镓晶体的等效微重力生长的原理和所采用的方法,并讨论了主要结果。     

The laser energy window for ion-implanted GaAs

A detailed study has been carried out of the energy window within which low resistivity, defect free, single crystal GaAs can be obtained following high dose implants of tin and selenium ions and laser annealing. A good correlation between electrical measurements, Rutherford backscattering and transmission electron microscopy has been obtained. The results indicate that there are four distinct bands of energy density corresponding to (i) an amorphous to polycrystalline transition accompanied by grain growth, (ii) the production of defective single crystal GaAs, (iii) the energy window and (iv) gross decomposition and laser induced damage.     

The structural characteristics of radiation damage produced by high energy (2.7 MeV) ion implantation in GaAs

The radiation damage induced by the implantation of 2.7 MeV P⁺ and N⁺ ions with a dose of 6.4 × 10¹⁶ ions cm^–3 into GaAs at room temperature has been studied by transmission electron microscopy. The as-implanted material was found to consist of a buried amorphous layer which was sandwiched between a heavily damaged but crystalline cover layer exhibiting a high density of black dot defects, microtwins and dislocation loops and a less damaged substrate region. Post-implantation annealing of the specimens at 250° C for 6 h resulted in the recrystallization of the amorphous and cover layers by random nucleation of grains producing a polycrystalline region on the single crystal substrate. However, a second stage annealing of these samples at 400° C for 2 h caused an epitaxial regrowth of the implanted layer on the undamaged substrate producing single crystal regions which were heavily twinned on all {111} planes. The results of the present microstructural analyses have been compared with the previous infra-red reflectivity studies on identically implanted GaAs samples to determine the effects of structural changes on the dielectric properties. The two studies are found to be in reasonable agreement. The present results are also compared with those from previous lower energy-lower dose implantations.