Metal-induced reconstructions of the silicon(111) surface

We have used scanning tunnelling microscopy (STM) to study changes in the structure of the Si(111) surface induced by deposition of the group III metals In and Ga. For both metals, several different ordered reconstructions are seen as a function of coverage. The STM images provide new structural information on each of these reconstructions. With In metal deposition, we have seen the surface reconstructions √3×, √3, √31× √31, √7×√3 and 4×1 as the coverage increases. In the case of Ga on Si(111), we have studied structures that exist up to 0·7 ML. At 1/3 ML, there is a √3×√3 structure identical to that of In. Above 0·3 ML there is a different phase that may correspond to the (6·3times6·3) RHEED pattern reported in this coverage range. This surface tends to grow as triangular islands at higher coverages.     

Atmospheric effects of friction,friction noise and wear with silicon and diamond. Part II. SEM tribometry of silicon in vacuum and hydrogen

Scanning electron microscope (SEM) tribometric data on polycrystalline silicon (poly-Si) vs. poly-Si, Si(100) vs. Si(100) and Si(111) vs. Si(111) interfaces, obtained in Torr and in 0.2 Torr partial pressure of hydrogen gas ( ) from room temperature to 850^°C, were performed under standard and much slower thermal ramping rates. The friction data were analyzed per the methodology described in part I of this paper series. The results indicate a highly beneficial friction- and wear-reducing regime within a relatively narrow thermal region. This desirable region coincides with some chemisorption of excited species of molecular hydrogen just before the mass thermal desorption of surface hydrides. These data represent the tribochemical equivalent of a method routinely used in electronics, whereby deep electron traps (dangling Si bonds) are passivated by baking in molecular hydrogen. The also exerts a moderating influence on the size of the friction noise at all test temperatures. However, the general level of friction beyond the beneficial thermal region is high. In parallel, the general wear rate of Si representative of the entire range of standard thermal ramping in both atmospheric environments is in the extremely high 10^-12m³/(N m) range. Operating strictly in the beneficial, low-friction thermal regime resulted in a several orders-of-magnitude reduction in the wear rate over those measured under standard thermal ramping conditions. Although the results confirm previous findings that Si is not a good material of construction for miniaturized moving mechanical assemblies (e.g., microbearings and gears), there seems to be some limited possibility of gas-phase lubrication of Si micromechanisms with rarefied hydrogen at surface temperatures between 100 and 300^°C. This revised version was published online in July 2006 with corrections to the Cover Date.     

The microstructure of silicon carbide (Nicalon) fibre reinforced silicon carbide ceramic-matrix composites heat treated in vacuo and in pure oxygen

Silicon carbide (Nicalon) fibre reinforced SiC composites have been heat treated in vacuo and in pure oxygen environments at 1400°C for 100 h. The response of the microstructure and, in particular, of the interface between fibre, carbon interlayer and SiC matrix components has been studied. Microstructural modifications were observed by transmission electron microscopy, using imaging, electron energy-loss spectroscopy and electron diffraction techniques, and fibre stoichiometries were determined using a scanning Auger microprobe. Recrystallization of Nicalon fibres within composites heat treated in vacuo was found to result from decomposition of the metastable silicon oxy-carbide phase found in the fibres. No significant changes to the pyrolytic carbon interlayer were observed. Fibre recrystallization was considered to embrittle the composite. Samples heat treated in oxygen showed no appreciable fibre recrystallization. Study of interlayers in such aged samples often revealed decohesions, holes and narrow silica layers. In the most extreme cases, complete displacement of carbon by SiO₂ was found and such interfaces were identified as silica and α-cristobalite. Interfacial modifications were considered to be responsible for the retention of the small β-SiC grain size in Nicalon fibres and were also considered to be deleterious to the mechanical properties.     

SIPM在脉冲光检测系统中的应用研究

为了实现硅光电倍增管(silicon photomultiplier,SIPM)对超出光子计数极限的微弱脉冲光信号的测量,建立了基于SIPM积分工作模式的脉冲光检测系统。测试了SIPM在同一光信号照射下,偏置电压与增益以及信噪比之间的关系,并测试了同一增益条件下,SIPM对不同光信号的响应特性。结果表明:SIPM在积分工作模式下,其增益可以达到104以上,并随着偏置电压的增加而指数增长;其信噪比也随着电压的增加而增加,在光强比较微弱的情况下,SIPM对光强是线性响应的。所设计的系统可以在一定程度上替代光电倍增管进行微弱脉冲光信号的测量。     

Mean free paths for inelastic electron scattering in silicon and poly(styrene) nanospheres

The mean free paths for inelastic electron scattering, lambda(in), in silicon [Si] and poly(styrene) [PS] have been measured using off-axis electron holography in a field-emission transmission electron microscope (FEG TEM). The holographic imaging method determines both quantitative wave phase information as well as elastic energy-filtered wave amplitude information. Using the energy-filtered amplitude data, two-dimensional t/lambda(in) images are reconstructed. The present work uses spherical nanoparticles as samples, so the sample thickness at any point in a two-dimensional image can be calculated knowing the center and radius of the projected nanosphere. The thickness contribution to t/lambda(in) is removed to obtain quantitative lambda(in) values. This work finds values of lambda(i)Si = 53.8 +/- 5.5 and 88.6 +/- 6.9 nm, and lambda(PS) = 78.1 +/- 3.4 and 113.0 +/- 5.9 nm for 120 and 200 keV incident electron energies, respectively.     

Heteroepitaxy of Nd(0.67)Sr(0.33)MnO3 on silicon for bolometric x-ray detector application

We have recently reported the design concept and sensor fabrication for a novel bolometric x-ray detector based on a rare earth manganite material for application as a total energy monitor for the Linac Coherent Light Source (LCLS) free electron laser at the Stanford Linear Accelerator Center (SLAC). The detector employs epitaxial thin films of Nd(0.67)Sr(0.33)MnO(3) grown on Si by pulsed laser deposition. In this paper we report details of the fabrication of the actual detector, its response characteristics under photon illumination from LCLS, and improvements in the growth scheme of the sensor material on Si using a buffer/template layer scheme that employs yttria-stabilized zirconia, cerium oxide (CeO(2)), and bismuth titanate (Bi(4)Ti(3)O(12)). The thermal sensor response changes linearly with the energy of an optical calibration laser as expected, and the signals from optical and x-ray pulses at LCLS are very similar, thereby validating the design concept. To the best of our knowledge, the LCLS detector application reported here is the first practical use of colossal magnetoresistive manganite bolometers.     

Transmission electron microscopy studies in relation to production of solar cells from polycrystalline silicon sheets (R.A.D. process)

Ribbon against drop (R.A.D.) ribbons exhibit a polycrystalline texture similar to that of most melt-grown ribbons; we review here the defects of as-grown materials as well as after the solar cells' production.     

Microtribology of silicon,oxide, and carbide surfaces

This paper presents an overview of the microtribological properties of silicon oxide, sapphire, and titanium carbide surfaces as well as a self‐assembled monolayer with respect to their application in microsystems. Testing was performed with a reciprocating microtribometer with normal loads in the micronewton to millinewton range. Silicon and titanium carbide balls were used as counterbodies. For silicon oxide, sapphire and a perfluorodecyltrichlorosilane self‐assembled monolayer (FDTS), the microfriction corresponds to the water contact angle when the smoother titanium carbide ball or the relatively rougher silicon ball was used as a counterbody. Microfriction measurements performed on tribopairs of the same material, but having different roughnesses, showed that the friction of the rougher tribopairs is lower than that of the smoother ones. Interestingly, in the microforce regime, reduction in friction was significant and almost as much as when hydrophobic self‐assembled monolayers are applied on smooth surfaces. This investigation showed that comparative microtribological investigations between different material systems can be very challenging due to the fact that comparable roughness values on samples and countersamples are difficult to realize. Copyright © 2006 John Wiley & Sons, Ltd.     

Molecular dynamics simulation on the tribology properties of two hard nanoparticles (diamond and silicon dioxide) confined by two iron blocks

The tribology behaviors of diamond and silicon dioxide (SiO₂) nanoparticles were examined via molecular dynamics simulations; four cases were simulated. At low velocity and low load, the nanoparticles separated the two blocks from each other and acted as ball-bearings. The plastic deformation, temperature distribution, and friction force were all improved due to the action of the nanoparticles. However, the crushing of the SiO₂ nanoparticles was accompanied by deformation-induced loss of the rolling effect, when the load was increased. Without nanoparticles, a transfer layer formed at high velocity and low load. The two nanoparticles provided support for a certain duration. However, at high velocity and high load, the support effect of these nanoparticles was lost in a short sliding time.     

Investigation of intrinsic crystalline Si(111) and amorphous silicon surfaces in air with STM

Crystalline (111) and amorphous silicon surfaces have been studied by scanning tunnelling microscopy. An orientated, reproducible, corrugated structure has been observed on Si(111) surfaces. The voltage dependence of the corrugation amplitude may be attributable to surface states. The surfaces of amorphous silicon thin films show some reproducible structure in the range of a few tens of ångströms, observable only when the applied voltage between the tip and sample is between ?1·3 and +0·4V.     

Morphology and histology of P. argentinus (Crustacea, Decapoda, Caridea) digestive tract.

This work describes the morphology and histology of the P. argentinus digestive tract. The foregut comprises the mouth, oesophagus, and stomach and is lined by a simple cylindrical epithelium overlain by cuticle. There are tegumental glands in the oral region and in the first portion of the oesophagus and of the hindgut. The cardiac stomach is an oval dorsal sac in the cephalothorax and has no calcified structures. The pyloric stomach comprises an upper chamber and a lower gland filter. The filter consists of an outer row of elongated setae and an inner row of dorsally curved setae forming longitudinal channels 16-18 microm wide. The midgut runs from the dorsal chamber of the pyloric stomach to the sixth abdominal somite without caeca. The hindgut runs from the sixth abdominal somite to the ventral anus. The mid-gut epithelium comprises dominant cylindrical cells and small undifferentiated cells in the first portion. The hindgut wall presents longitudinal folds, conspicuous muscular bundles, and a folded cuticle. The digestive tract of P. argentinus is basically similar to that of most of decapods. The absence of calcified structures in the stomach and the width of the longitudinal channels in the filter are related to the predominantly detritivorous diet.     

Coating of tips for electrochemical scanning tunneling microscopy by means of silicon, magnesium, and tungsten oxides

Different combinations of metal tips and oxide coatings have been tested for possible operation in electrochemical scanning tunneling microscopy. Silicon and magnesium oxides have been thermally evaporated onto gold and platinum-iridium tips, respectively. Two different thickness values have been explored for both materials, namely, 40 and 120 nm for silicon oxide and 20 and 60 nm for magnesium oxide. Alternatively, tungsten oxide has been grown on tungsten tips via electrochemical anodization. In the latter case, to seek optimal results we have varied the pH of the anodizing electrolyte between one and four. The oxide coated tips have been first inspected by means of scanning electron microscopy equipped with microanalysis to determine the morphological results of the coating. Second, the coated tips have been electrically characterized ex situ for stability in time by means of cyclic voltammetry in 1 M aqueous KCl supporting electrolyte, both bare and supplemented with K(3)[Fe(CN)(6)] complex at 10 mM concentration in milliQ water as an analyte. Only the tungsten oxide coated tungsten tips have shown stable electrical behavior in the electrolyte. For these tips, the uncoated metal area has been estimated from the electrical current levels, and they have been successfully tested by imaging a gold grating in situ, which provided stable results for several hours. The successful tungsten oxide coating obtained at pH=4 has been assigned to the WO(3) form.