Electron effective mass determination in asymmetric modulation-doped field-effect transistor heterostructures using InxGa1 − xAs quantum well and InAs–GaAs superlattice channels

Cyclotron resonance and photoluminescence measurements have been performed on two types of modulation-doped field-effect transistor heterostructures having their bidimensional channel based, respectively, on an In_xGa_1 − xAs quantum well and an InAs–GaAs short-period superlattice. A linear dependence of the electron effective mass as a function of indium content of the channel was obtained from cyclotron resonance measurements. For a given average value of the indium content, the effective mass in the InAs–GaAs short-period superlattice channel is found to be systematically higher than that obtained in structures with an alloy-based channel. This is attributed to larger nonparabolic effects in the former case. In our theoretical model, the electron and heavy hole energy levels and the electron wavefunction are determined self-consistently and used to estimate the nonparabolic corrections that apply to the effective mass deduced from cyclotron resonance measurements.     

Synthesis of silver nanowires via electroplating technology and its surface enhanced Raman scattering effect

This very paper is focusing on the preparation of silver nanostructures and the surface enhanced Raman scattering effect of the silver nanostructures produced. Via electroplating technology, silver nanowires and nanoparticles were prepared on silicon wafers. Characterization was performed by X-ray diffraction, scanning electron microscope, transmission electron microscope equipped with X-ray energy dispersion spectroscope and selected area electron diffraction, which reveals that the formation of silver nanostructures depends on the over-potential. The produced silver nanowires are of crystalline FCC structure and grow in 〈0 1 1〉 direction. The growth mechanism has been further discussed. The surface enhanced Raman scattering effect is achieved with the silver nanostructures produced.     

Development of a miniature STM holder for study of electronic conductance of metal nanowires in UHV-TEM

A new miniature scanning tunneling microscope (STM) holder was developed in order to simultaneously investigate electronic conductance and structure of nanowires in an ultra high-vacuum electron microscope (UHV-TEM). A thin gold wire held between the STM tip and substrate stage of the specimen holder was stretched to form a suspended gold nanowire. The new TEM-STM system allowed us to measure electronic conductance at intervals of 20 ms, and to record high-resolution TEM images on videotape at 30 fps. Suspended gold nanowires formed from [1 1 0] oriented electrodes were well-elongated. In contrast, [1 0 0] and [1 1 1] electrodes produced nanowires with short necks. Electronic conductance was found to change as nanowire structure changed, with conductance quantization in units of 2e²/h, where e is the electron charge and h is Planck’s constant, only being exhibited for well-elongated nanowires.     

Uniformity study of wafer-scale InP-to-silicon hybrid integration

β-SiC nanowires were synthesized by a simple carbothermal reduction of carbonaceous silica xerogel. The morphology and structure of the nanowires were investigated by X-ray diffraction, scanning electron microscope and transmission electron microscopy. The results showed that the nanowires were hexagonal prism-shaped hierarchical nanostructures. The typical stacking faults and twin defects of SiC nanowires were also observed. Band-gap characterization and photoluminescence properties of SiC nanowires were investigated by UV-vis absorption spectroscopy and fluorescence photometry, respectively. The results showed the SiC nanowire was an indirect transition semiconductor and the band gap energy for the SiC nanowires was 2.85 eV. The photoluminescence peak value at 470 nm (2.64 eV) originating from the SiC nanowires was a little higher than the value of band-gap energy.     

Structural and electrical studies of template synthesized copper nanowires

The template strategy combined with electrodeposition technique has been used to produce copper nanowires in the cylindrical pores of track-etched polycarbonate membranes. X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and UV–visible spectroscopy have been used to characterize as-prepared copper nanowires. XRD study shows the face centered cubic crystal structure of copper nanowires. Williamson–Hall (WH) analysis has been used to determine the crystallite size and microstrain induced due to lattice deformation. FESEM results reveal that copper nanowires are continuous, well aligned with uniform diameter and having high aspect ratio. The optical absorption spectra exhibit a strong peak at 568 nm attributed to the surface plasmon resonance. The current–voltage (I–V) characteristics show an ohmic behavior of the fabricated copper nanowires. The increase in resistivity of nanowires than that of bulk counterpart has been attributed to the surface and size effects in nanowires and explained in the light of Fuchs–Sondeimer and Mayadas–Shatzkes models.     

Simultaneous measurement of static and kinetic friction of ZnO nanowires in situ with a scanning electron microscope

A novel method for in situ measurement of the static and kinetic friction is developed and demonstrated for zinc oxide nanowires (NWs) on oxidised silicon wafers. The experiments are performed inside a scanning electron microscope (SEM) equipped with a nanomanipulator with an atomic force microscope tip as a probe. NWs are pushed by the tip from one end until complete displacement is achieved, while NW bending is monitored by the SEM. The elastic bending profile of a NW during the manipulation process is used to calculate the static and kinetic friction forces.     

The uniaxial stress tunable optical gain of InAs nanowires

We calculate the electronic structures and optical gain of InAs nanowires via the eight-band effective-mass $\mathbf{k}?\mathbf{p}$ theory. It is found that there is one peak in TM gain spectra, and the peak value of TM gain spectra is much larger than that of TE gain spectra when the radius R of the nanowires is 3 nm. Meanwhile, as the radius increases, the peak value of TE gain spectra approaches that of TM gain spectra, and more peaks appear in both TM and TE gain spectra. We therefore predict that InAs nanowires with R smaller than 3 nm can be used to emit TM linearly polarized light. We find, however, with R being 6 nm or 10 nm, a tensile or compressive uniaxial stress along the c axis can be used to tune the optical gain of InAs nanowires, and we can obtain TE or TM linearly polarized light. Our calculation points out the possibility for the application of InAs nanowires in the field of infrared linearly polarized lasers.     

Abnormal blueshift of UV emission in single-crystalline ZnO nanowires

Single-crystalline ZnO nanowires (NWs) were synthesized by a facile vapor transport method. The good orientation and high crystal quality were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and high-resolution transmission electron microscope (HRTEM) measurements. Excitation-power-dependence photoluminescence spectra of ZnO NWs show that the UV emission displayed an evident blueshift with increasing excitation power and the corresponding energy shift might be as large as 10 meV. This anomalous phenomenon correlates to the band bending level caused by the surface built-in electric field due to the existence of substantial oxygen vacancies. By increasing the excitation power, the enhanced neutralization effect near the surface will reduce the built-in electric field and lead to a reduction of band bending which triggers the blueshift of the UV emission.     

基于微芯片的透射电子显微镜的低温纳米精度电子束刻蚀与原位电学输运性质测量

利用微芯片制备技术制备了带有电极的原位电学薄膜芯片,并结合自制的原位透射电镜样品台,实现了低温下透射电子显微镜聚焦电子束对InAs纳米线的精细刻蚀以及不同温度下的原位电学性能测量.研究发现,随着刻蚀区域截面积的减小,纳米线的电导率也随之减小.当纳米线的截面积从大于10000 nm2刻蚀至约800 nm2时,纳米线电导的减小速率与截面积的减小具有线性关系.同时利用低温聚焦电子束刻蚀,在InAs纳米线上原位制备了一个10 nm的纳米点,并在77与300 K下对该纳米点进行了电学性能测量.通过测量发现在77 K时出现库仑阻塞效应,发生了电子隧穿现象;而300 K时,热扰动提供的能量使这种现象消失.     

BiOCl nanowire with hierarchical structure and its Raman features

BiOCl is a promising V-VI-VII-compound semiconductor with excellent optical and electrical properties, and has great potential applications in photo-catalysis, photoelectric, etc. We successfully synthesize BiOCl nanowire with a hierarchical structure by combining wet etch (top-down) with liquid phase crystal growth (bottom-up) process, opening a novel method to construct ordered bismuth-based nanostructures. The morphology and lattice structures of Bi nanowires, β-Bi₂O₃ nanowires and BiOCl nanowires with the hierarchical structure are investigated by scanning electron microscope (SEM) and transition electron microscope (TEM). The formation mechanism of such ordered BiOCl hierarchical structure is considered to mainly originate from the highly preferred growth, which is governed by the lattice match between (1 1 0) facet of BiOCl and (2 2 0) or (0 0 2) facet of β-Bi₂O₃. A schematic model is also illustrated to depict the formation process of the ordered BiOCl hierarchical structure. In addition, Raman properties of the BiOCl nanowire with the hierarchical structure are investigated deeply.     

Synthesis of β-Ga2O3 nanowires through microwave plasma chemical vapor deposition

In this study, we demonstrate the large-scale synthesis of beta gallium oxide (β-Ga₂O₃) nanowires through microwave plasma chemical vapor deposition (MPCVD) of a Ga droplet in the H₂O and Ar atmosphere at 600 W. Unlike the commonly used MPCVD method, the H₂O, not mixture of gas, was employed to synthesize the nanowires. The ultra-long β-Ga₂O₃ nanowires with diameters of about 20-30 nm were several tens of micrometers long. The morphology and structure of products were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution transmission electron microscope (HRTEM). The growth of β-Ga₂O₃ nanowires was controlled by vapor-solid (VS) crystal growth mechanism.     

Preparation of nanosized ZnO using α brass

Nanosized ZnOs were synthesized on the surface of α brass coated a film of nickel catalyst at 500-700 °C under atmosphere of O₂ and CH₄ gases. The nanosized ZnOs have shapes including pillar, leaf, sheet and rod, which were determined by the synthesis temperature and the flow rates of O₂ and CH₄ gases. The nanosized ZnOs were characterized by electron microscopy including transmission electron microscope for crystal structure, morphology and high resolution images, both field emission scanning electron microscope and scanning electron microscope for morphology, and energy dispersive X-ray spectroscope equipped in electron microscope for chemical composition. A mechanism was proposed for the growth of nanosized ZnO obtained in this work.     

InAs/GaAs和InAs/InxGa1-xAs/GaAs纳米线异质结构的生长研究

利用金辅助金属有机化学气相沉淀法(MOCVD)在GaAs(111)B衬底上分别制备了InAs/GaAs和InAs/In _{x Ga1-xAs/GaAs(0≤x≤1)纳米线异质结构.实验结果显示,直接生长在GaAs纳米线上的InAs纳米线生长方向杂乱或者沿着GaAs纳米线侧壁向衬底方向生长,生长的含有In x Ga1-xAs组分渐变缓冲段的InAs/In x Ga1-x关键词： 纳米线异质结构 xGa1-xAs')" href="#">InxGa1-xAs 组分渐变缓冲层 金属有机化学气相沉淀法}     

X-ray investigation of the interface structure of free standing InAs nanowires grown on GaAs [\bar{1}\bar {1}\bar{1}]_{\mathrm{B}}

The heteroepitaxial growth process of InAs nanowires (NW) on GaAs $[\bar{1}\bar{1}\bar{1}]_{\mathrm{B}}$ substrate was investigated by X-ray grazing-incidence diffraction using synchrotron radiation. For crystal growth we applied the vapor–liquid–solid (VLS) growth mechanism via gold seeds. The general sample structure was extracted from various electron microscopic and X-ray diffraction experiments. We found a closed Ga _x In_1?x As graduated alloy layer at the substrate to NW interface which was formed in the initial stage of VLS growth with a Au–Ga–In liquid alloy. With ongoing growth time a transition from this VLS layer growth to the conventional VLS NW growth was observed. The structural properties of both VLS grown crystal types were examined. Furthermore, we discuss the VLS layer growth process.     

Catalyst-free growth of well-aligned arsenic-doped ZnO nanowires by chemical vapor deposition method

ZnO nanowires with different arsenic concentration were grown on Si (1 0 0) substrates by chemical vapor deposition method without using catalyst. Zn/GaAs mixed powders were used as Zn and As source, respectively. Oxygen was used as oxidant. The images of scanning electron microscope show that the arsenic-doped ZnO nanowires with preferred c-axial orientation were obtained, which is in well accordance with the X-ray diffraction analysis. The arsenic related acceptor emission was observed in the photoluminescence spectra at 11 K for all arsenic-doped ZnO samples. This method for the preparation of arsenic-doped ZnO nanowires may open the way to realize the ZnO nanowires based light-emitting diode and laser diode.     

Saturation spectroscopy of electronic states in a magnetic field in InAs/AlxGa1−xSb single quantum wells

We have carried out saturation spectroscopy of cyclotron resonance in a semiconducting InAs/Al_0.5Ga_0.5Sb single quantum well using the UCSB free electron laser and have extracted an effective Landau level lifetime using an n-level rate equation model. The effective lifetime shows strong oscillations (>an order of magnitude) with frequency. Minima are shifted to higher frequencies than those given by the simple parabolic magnetophonon resonance condition due to large nonparabolicity in the InAs conduction band. We have also used this technique to investigate the origins of two lines: the X-line and cyclotron resonance in a “semimetallic” InAs/Al_0.1Ga_0.9Sb single quantum-well structure. Results show that the two lines are of different origin.     

Ferromagnetism in epitaxial layers of gallium and indium antimonides and indium arsenide supersaturated by manganese impurity

We report on laser synthesis of thin 30–200 nm epitaxial layers with mosaic structure of diluted magnetic semiconductors GaSb:Mn and InSb:Mn with the Curie temperature T_C above 500 K and of InAs:Mn with T_C no less than 77 K. The concentration of Mn was ranged from 0.02 to 0.15. In the case of InSb:Mn and InAs:Mn films, the additional pulse laser annealing was needed to achieve ferromagnetic behavior. We used Kerr and Hall effects methods as well as ferromagnetic resonance (FMR) spectroscopy to study magnetic properties of the samples. The anisotropy FMR was observed for both layers of GaSb:Mn and InSb:Mn up to 500 K but it takes place with different temperature dependencies of absorption spectra peaks. The resonance field value and amplitude of FMR signal on the temperature is monotonically decreased with the temperature increase for InSb:Mn. In the case of GaSb:Mn, this dependence is not monotonic.     

Melting and undercooling of bismuth nanocrystals by solvothermal synthesis

The nanocrystals of bismuth with nanowire and sphere in shape were synthesized by solvothermal process, and it was found that the amount of nanowires would be reduced by the proper choice of the reaction solvents. The crystal structure of the as-prepared nanocrystals was investigated with X-ray diffraction (XRD). The morphology of the nanocrystals was observed with the field emission scanning electron microscope (FE-SEM). Moreover, the melting and solidification processes of the bulk bismuth and as-prepared nanocrystals were comparatively studied with differential scanning calorimetry (DSC). During the solidification process, the nanocrystals showed a larger undercooling, the value of which was about 95 and 31 °C higher than that of the bulk bismuth.     

In-plane and perpendicular tunneling through InAs quantum dots

A Schottky diode with InAs dots in the intrinsic GaAs region was used to investigate perpendicular tunneling (in growth direction) through InAs quantum dots (QDs). At forward bias conditions electrons tunnel from the ohmic back contact into the metal Schottky gate. Peaks appear in the differential conductance when a QD level comes into resonance with the Fermi-level of the n-doped region. The observed tunneling features are attributed to electron transport through the s- and p-shell of the InAs islands. In our in-plane tunneling experiments the islands were embedded in the channel region of an n-doped GaAs/AlGaAs HEMT-structure. In order to study tunneling through single InAs islands, a quantum point contact was defined by lithography with an atomic force microscope and subsequent wet-chemical etching. In contrast to unpatterned devices sharp peaks appear in the I–V characteristic of our samples reflecting the transport of electrons through the p-shell of a single InAs QD.     

Accurate measurements of crystal structure factors using a FEG electron microscope

An experimental procedure for the accurate measurement of crystal structure factors is described. This procedure is based on the use of a field emission gun electron microscope equipped with a Gatan Imaging Filter (GIF) system. The slow-scan CCD camera of the GIF system is first characterized and a constrained least squares restoration scheme is used for the deconvolution of the experimentally recorded raw elastic CBED patterns. The procedure has been applied for the accurate measurement of the (111) and (222) structure factors of silicon single crystal. A residual χ² value of 2.87 is achieved and the determined structure factors agree well with previous measurements using X-ray and electron diffraction techniques.