Freestanding graphene writing on a silicon carbide wafer

Freestanding graphene on a trench has been fabricated extensively using a transfer process of chemical vapor deposition grown graphene. Here, we demonstrate that freestanding graphene can be grown directly on a trench without a transfer process. A shallow trench was made on a 6H–SiC(0001) wafer using a focused ion beam lithography. The shallow trench was heated to a high temperature under Ar atmosphere. The heat treatment made the shallow trench become deeper and wider. Subsequently, epitaxial graphene was floating on the trench, resulting in freestanding graphene, where underlying bulk SiC was self-etched after the growth of epitaxial graphene. The freestanding graphene on a trench was characterized using Raman spectroscopy and atomic force microscopy. Such freestanding graphene writing can be applied to semiconductor fabrication process of freestanding graphene devices without a transfer process.     

Photoluminescence of nanostructured cubic-lattice silicon carbide films grown on a silicon wafer

The light-emitting properties of cubic-lattice silicon carbide SiC films grown on Si(100) and Si(111) substrates with VPE at low temperatures (T _gr ∼ 700°C) are discussed. Investigations of the grown films reveal a homogeneous nanocrystalline structure involving only the 3C-SiC phase. When the electron subsystem of the structure is excited by a He-Cd laser emitting at λ_exit = 325 nm, the photoluminescence (PL) spectra contain a rather strong emission band shifted by about 3 eV toward a short-wave spectral region. At low temperatures, the PL integral curve is split into a set of Lorentz components. The relation between these components and the peculiarities of the energy spectrum of electrons in the nanocrystalline grains of the silicon carbide layers is discussed.     

Molecular dynamics investigations on polishing of a silicon wafer with a diamond abrasive

During the final stages of polishing silicon wafers, much of the interactions between silicon and diamond abrasive takes place at the silicon asperities. These interactions, leading to material removal, were investigated in a MD simulation of polishing of a silicon wafer with a diamond abrasive under dry conditions. Simulations were conducted with silicon asperities of different geometries, different abrasive configurations, and polishing speeds. Under the conditions of polishing, the silicon atoms from the asperities were found to bond chemically to the surface of the diamond abrasive. Continued transverse motion of the diamond abrasive (relative to the silicon asperity) leads to tensile pulling, necking, and ultimate separation of the silicon asperity material instead of conventional material removal in polishing (chip formation) involving cutting/ploughing, which takes place in the absence of chemical bonding between the abrasive and the asperity material. This phenomenon has not been reported previously in the literature. The thrust and cutting forces initially increase due to the increase in the number of asperity atoms affected finally reaching a maximum. This is followed by a decrease of these forces due to tensile pulling and formation of individual strings followed by ultimate separation or breakage of the final string. The ratio of thrust force (F _z ) to the cutting force (F _x ), i.e. |(F _z /F _x )| was found to increase continuously to a maximum of ～0.8 followed by continuous decrease to ～0.25. This is in contrast to a more or less constant value of ～2 in the case of tools with rounded radii or tools with large negative rake angles, where material is removed in the form of chips ahead of the tool. Three regions of the asperity have been identified that are useful in the development of a phenomenological model for polishing that enables computation of material removal rates: (1) the region directly in front of the abrasive for which the probability of the removal of an asperity atom is close to unity, (2) the distant region where this probability is nearly zero, and (3) an intermediate region from which the probability of removal is close to half.     

Photoluminescence characterization of silicon nanostructures embedded in silicon oxide

This paper describes a simple method to analyze the photoluminescent characteristics of materials based on embedded light-emitting nanoclusters. Photoluminescence spectra of deposited silicon sub-oxide layers with the same composition and different thicknesses have been obtained. A saturation of the total luminescence intensity is observed with increase in thickness. By analyzing the photoluminescence spectra several optical and structural parameters can be evaluated. We thus propose a model in which the absorption of light from a nanostructure layer implies the possibility of subsequent luminescence and affects the underlying layers as well. By fitting the data to the developed model, two fundamental parameters are extracted: nanostructures absorption probability, which is independent of the emission energy and the spectra of emission probability of an excited nanostructure which fits a Gaussian shape.     

HPM效应实验中区间删失数据的处理与统计分析

在HPM效应实验中经常可以获得区间删失数据,为了能够合理利用这类数据对HPM效应进行有效分析,需要对它进行处理。根据电子器件的微波失效机理和实验现象,基于插值方法的思想,在充分利用删失数据信息情况下建立了不同阶插值精度的数据处理方法。理论分析可知,高阶精度处理方法要优于低阶精度方法。此外,根据构建的区间删失数据,通过统计分析可知,处理后数据与原始数据在统计意义上没有显著差异,可用于HPM效应研究,为数据的可靠分析提供了有利支撑。     

HPM效应实验中区间删失数据的处理与统计分析

在HPM效应实验中经常可以获得区间删失数据,为了能够合理利用这类数据对HPM效应进行有效分析,需要对它进行处理。根据电子器件的微波失效机理和实验现象,基于插值方法的思想,在充分利用删失数据信息情况下建立了不同阶插值精度的数据处理方法。理论分析可知,高阶精度处理方法要优于低阶精度方法。此外,根据构建的区间删失数据,通过统计分析可知,处理后数据与原始数据在统计意义上没有显著差异,可用于HPM效应研究,为数据的可靠分析提供了有利支撑。     

Vibrational spectroscopy characterization of magnetron sputtered silicon oxide and silicon oxynitride films

Vibrational (infrared and Raman) spectroscopy has been used to characterize SiO_xN_y and SiO_x films prepared by magnetron sputtering on steel and silicon substrates. Interference bands in the infrared reflectivity measurements provided the film thickness and the dielectric function of the films. Vibrational modes bands were obtained both from infrared and Raman spectra providing useful information on the bonding structure and the microstructure (formation of nano-voids in some coatings) for these amorphous (or nanocrystalline) coatings. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) analysis have also been carried out to determine the composition and texture of the films, and to correlate these data with the vibrational spectroscopy studies. The angular dependence of the reflectivity spectra provides the dispersion of vibrational and interference polaritons modes, what allows to separate these two types of bands especially in the frequency regions where overlaps/resonances occurred. Finally the attenuated total reflection Fourier transform infrared measurements have been also carried out demonstrating the feasibility and high sensitivity of the technique. Comparison of the spectra of the SiO_xN_y films prepared in various conditions demonstrates how films can be prepared from pure silicon oxide to silicon oxynitride with reduced oxygen content.     

Removal of nanoparticles on silicon wafer using a self-channeled plasma filament

The effective removal of nanoparticles from a silicon wafer surface was demonstrated using the self-channeled plasma filament excited by a femtosecond (130?fs) Ti:sapphire laser (?? _p=790?nm). The photoinduced self-channeled plasma filament in air reached a length of approximately 110?C130?mm from the first focal spot with diameters ranging from 40 to 50???m at input intensities of more than 1.0×10¹⁴?W/cm². By the scan of wafer using the X?CY?CZ stage during self-channeled plasma filament, the removal variation of nanoparticles on surface was observed in situ before and after the plasma filament occurred. The cleaning efficiency was strongly dependent on the gap distance between the plasma filament and the surface. The removal efficiency of nanoparticles reached 96?% with no damage to the surface when the gap was 150???m.     

Kinetic study of antibody adhesion on a silicon wafer by laser reflectometry

Antibody adhesion kinetic in real time has been studied by laser reflectometry technique. An ellipsometer is used to measure the light intensity reflected by a silicon wafer. Light intensity reflected by the wafer presents a minimum at the pseudo-Brewster angle. Then, the reflectance increases as the antibodies (monoclonal anti-AB) adhere on interface. Mathematical analysis of reflectance curves versus time verifies that the antibody adhesion at the interface follows Langmuir kinetics (Prog. Biomed. Opt. Imaging 1(5) (2000) 19) for low antibody concentrations. Parameters obtained allow to carry out a detailed study of the antibody adsorption and the antigen–antibody interaction. This conduces to development of an optical immunosensor for detection and quantification of soluble antigens, and a novel method for commercial antiserum quality control. This technique does not require labeled antibodies, being also independent of cellular factors. Also, this technique is quicker and sensible than the conventional immunohematology methods.     

Effect of the doping level on temperature bistability in a silicon wafer

The influence of the doping level on the effect of the temperature bistability in a silicon wafer upon radiative heat transfer between the wafer and the elements of the heating system is studied. Theoretical transfer characteristics are constructed for a silicon wafer doped with donor and acceptor impurities. These characteristics are compared with the transfer characteristics obtained during heating and cooling of wafers with the hole conduction (with dopant concentrations of 10¹⁵, 2 × 10¹⁶, and 3 × 10¹⁷ cm^?3) and electron conduction (with impurity concentrations of 10¹⁵ and 8 × 10¹⁸ cm^?3) in a thermal reactor of the rapid thermal annealing setup. It is found that the width and height of the hysteresis loop decrease with increasing dopant concentration and are almost independent of the type of conduction of the silicon wafer. The critical value of the impurity concentration of both types is 1.4 × 10¹⁷ cm^?3. For this concentration, the loop width vanishes, and the height corresponds to the minimal value of the temperature jump (～200 K). The mechanism of temperature bistability in the silicon wafer upon radiative heat transfer is discussed.     

Formation of dicarboxylic acid-terminated monolayers on silicon wafer surface

Dicarboxylic acid-terminated monolayers on hydroxylated silicon wafer were prepared via the chemisorption of 3-glycidoxypropyldimethylethoxysilane (GPDMES) molecules and subsequent reaction of the epoxy groups with iminodiacetic acid (IDA). The structure and surface composition of the monolayers were characterized by the means of contact-angle measurement, ellipsometric thickness measurement, reflectance FTIR spectroscopy, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Moreover, we found that the dicarboxylic acid-terminated monolayers on silicon wafer exhibit well-defined contact angle titration curve from which the surface acid dissociation constants were determined. The results were compared with the pK_a values reported in the literature for IDA in aqueous solution. Small difference in the surface pK_a values was attributed to variations of the microenvironment of the acid moieties. These experimental findings provide fundamental knowledge at the molecular level for the preparation of bioactive surfaces of controlled reactivity on silicon substrates.     

Raman analysis of oxide cladded silicon core nanowires grown with solid silicon feed stock

Solid–liquid–solid (SLS) combined with Vapor-liquid–solid (VLS) growth mechanism has been used for synthesizing Core-clad silicon nanowires (SiNWs) by thermal annealing onto two different catalyst substrates (Au/Si and Ni/Ti/Si). It provides a novel method to synthesize SiNWs which is cost-effective, large-area-compatible and may give a higher degree of control of the end product, facilitated by the simple experimental process for further device applications. The first-order Raman peaks of the SiNWs were found to shift and to broaden asymmetrically in comparison to the c-Si Raman peak. Using a phonon confinement model, the average diameter of the wires can be estimated from the Raman spectra but are consistently lower than the diameters measured using high-resolution transmission electron microscopy. We interpret this as due to the confining contribution of the oxide clad. Due to the simplicity of the method, it could be adapted in industry for large scale synthesis of SiNWs with oxide clad for device fabrication, e.g., surround-gate field effect transistors.     

Growth and characterization of crystalline gadolinium oxide on silicon carbide for high- application

We have investigated the growth and electrical properties of crystalline Gd₂O₃ grown on 6H-SiC(0001) substrates by molecular beam epitaxy. Initially, Gd₂O₃ islands with hexagonal structure were formed. Further growth resulted in the formation of flat layers in a mixture of [111]-oriented cubic bixbyite and monoclinic structure. The fabricated capacitors with 14 nm Gd₂O₃ exhibited suitable dielectric properties at room temperature; such as a dielectric constant of ε=22, a leakage current of 10⁻⁸ A/cm²@1 V and breakdown fields >4.3 MV/cm.     

Parametric and non-parametric statistical analysis of DT-MRI data

In this work parametric and non-parametric statistical methods are proposed to analyze Diffusion Tensor Magnetic Resonance Imaging (DT-MRI) data. A Multivariate Normal Distribution is proposed as a parametric statistical model of diffusion tensor data when magnitude MR images contain no artifacts other than Johnson noise. We test this model using Monte Carlo (MC) simulations of DT-MRI experiments. The non-parametric approach proposed here is an implementation of bootstrap methodology that we call the DT-MRI bootstrap. It is used to estimate an empirical probability distribution of experimental DT-MRI data, and to perform hypothesis tests on them. The DT-MRI bootstrap is also used to obtain various statistics of DT-MRI parameters within a single voxel, and within a region of interest (ROI); we also use the bootstrap to study the intrinsic variability of these parameters in the ROI, independent of background noise. We evaluate the DT-MRI bootstrap using MC simulations and apply it to DT-MRI data acquired on human brain in vivo, and on a phantom with uniform diffusion properties.     

Imaging of electrically detected magnetic resonance of a silicon wafer.

An imaging technique of electrically detected magnetic resonance (EDMR) was newly developed. Because the EDMR signal is obtained from paramagnetic recombination centers, one may expect the image to represent the distribution of defect and/or impurity sites in the sample. We successfully obtained EDMR images of a light-illuminated silicon plate 8 mm in width and 15 mm in length, which was cut from a silicon wafer (n-type, 100 Omega cm), under ESR irradiation at a frequency of 890 MHz (wavelength, 340 mm). The reproducibility of the EDMR image obtained from a sample was amply satisfactory. When the oxidized surface of the silicon was removed, the EDMR signal disappeared. Although the EDMR signal reappeared when the surface of the sample became reoxidized, the EDMR image obtained was slightly different from the earlier one. This finding shows that the EDMR image obtained from the sample shows the distribution of defects at the Si/SiO(2) interface.     

Micro-machining of silicon wafer in air and under water

Laser ablation micro-machining tests are conducted on silicon wafer, both in air and under flowing water stream, with the use of 355 nm-X AVIA laser. Effects of laser pulse frequency, power level, scan velocity and focal plane position on the associated laser spatter deposition (in air), irradiated areas (under flowing water film) and taper are investigated. It shows that low frequency, i.e. 30–40 kHz, and high peak power result in smaller spatter and irradiated areas, and the hole taper decreases with increase in pulse frequency. Increase in the laser fluence broadens both the areas and increases the hole taper. Both areas enlarge with the increase of scanning velocity of more than 3 mm s^?1. The scan velocity has no effect on hole taper in air environment but inconsistent hole taper is obtained under flowing water stream. Furthermore, moving the focal plane position below the workpiece surface contributes relatively smaller areas of spatter deposition, irradiation and taper in comparison to zero focal plane position. Finally, the differences between laser ablation in air and under water are identified. The reduction in the spatter deposition and irradiated areas around the perimeter of the ablated hole and a smaller taper with the use of laser trepan drilling method in air and under water machining are investigated in this paper.     

Extended homogeneous nanoripple formation during interaction of high-intensity few-cycle pulses with a moving silicon wafer

We report the study of extended nanoripple structures formed during the interaction of high-intensity 3.5 fs pulses with a moving silicon wafer. The optimization of laser intensity (8×10¹³ W?cm^?2) and sample moving velocity (1 mm?s^?1) allowed the formation of long strips (～5 mm) of homogeneous nanoripples along the whole area of laser ablation. The comparison of nanoripples produced on the silicon surfaces by few- and multi-cycle pulses is presented. We find that few-cycle pulses produce sharp and more homogenous structures than multi-cycle pulses.     

Fabrication of silicon wafer with ultra low reflectance by chemical etching method

A pyramid and nanowire binary structure of monocrystalline silicon wafer was fabricated by chemical etching. Much lower reflectance of silicon wafer with this structure was obtained compared with that of single pyramid or nanowaire arrays. The morphology, reflectivity and etching thickness of this structure were studied, as well as the influence on them caused by etching time and thickness of silver film. An average reflectance of 0.9% was obtained under optimized condition. The formation mechanism of silicon nanowires was explained by experimental evidence.     

Silicon heterojunction solar cell with amorphous silicon oxide buffer and microcrystalline silicon oxide contact layers

This Letter reports on the fabrication and characterization of silicon heterojunction solar cells with silicon oxide based buffer (intrinsic amorphous silicon oxide) and contact layers (doped microcrystalline silicon oxide) on flat p‐type wafers. The critical dependency of the cell performance on the front and rear buffer layer thickness reveals a trade‐off between the open circuit voltage V_oc and the fill factor FF. At the optimum, the highest efficiency of 18.5% (active area = 0.67 cm²) was achieved with V_oc = 664 mV, short circuit current J_sc = 35.7 mA/cm² and FF = 78.0%. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)