An electron microscopy investigation of the structure of porous silicon by oxide replication

The morphology of porous silicon is studied by scanning electron microscopy (SEM) by making an oxide replica of the pore structure. Highly branched n-type porous silicon samples were prepared and a replica was formed by oxidation of the pores followed by selective removal of the silicon substrate to expose the oxide pores. Scanning and transmission electron microscopy images confirmed many previously held assumptions about porous silicon formation, including the fractal structure and crystallographic propagation; they also provided a clearer understanding of the details of pore formation. The replica procedure also provides a platform for a more facile and comprehensive analysis of the porous silicon morphology.     

X-ray photoelectron spectroscopy of Iuminescent porous silicon

Abstract

X-ray photoelectron spectroscopy has been performed in photoluminescent porous Si layers after different treatments, namely, anodisation only in a HF-ethanol solution, rinsing in deionised water, ethanol immersion, and exposure to ambient conditions. The spectra recorded just after formation in the HF solution are the most representative of real porous Si layer structures. However, the pretreatment in water, exposure to ambient conditions for a long period, and immersion in ethanol degrades the porous Si layer surface to a greater or lesser extent. The growth of different oxide phases and the amorphisation of the structures after the pretreatments are also discussed.     

Calibration and X-ray spectroscopy with silicon CCDs

It is shown that the novel mode of operation of charge coupled devices (CCDs) allows a new method of measuring the X-ray energy-to-charge conversion factor, without requiring externally calibrated circuits etc., thereby reducing possible contributions to systematic uncertainties in this measurement. In addition, the low noise operation of CCDs is shown to lead to possibilities for measuring the Fano factor in silicon with improved precision. Advances in CCD X-ray detection performance are described, including energy resolutions of 80 eV FWHM at a temperature of 180 K, and detection efficiencies of greater than 90%. Such improvements are shown to have potential benefit for various X-ray spectroscopic applications.     

Electronic structure of carbon nitride thin films studied by X-ray spectroscopy techniques

Magnetron-sputtered carbon nitride thin films with different structures and compositions were analyzed by X-ray and ultraviolet photoelectron spectroscopy (XPS and UPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), as well as X-ray emission spectroscopy (XES). In all techniques, the carbon spectra are broad and featureless with little variation depending on growth conditions. The nitrogen spectra, on the other hand, show more distinct features, providing a powerful tool for structural characterization. By comparing the experimental spectra with calculations on different model systems, we are able to identify three major bonding structures of the nitrogen—N1: nitrile (CN) bonds; N2: Pyridine-like N, i.e., N bonded to two C atoms; and N3: graphite-like N, i.e., N bonded to three C atoms as if substituted in a graphitic network, however, possibly positioned in a pentagon and/or with sp³ carbon neighbors. The presence of N2 and N3 are best detected by XPS, while N1 is better detected by NEXAFS. The calculated XES spectra also give good indications how valence band spectra should be interpreted. Films grown at the higher temperatures (≥350 °C) show a pronounced angular dependence of the incoming photon beam in NEXAFS measurements, which suggests a textured microstructure with standing graphitic basal planes, while amorphous films grown at low temperatures show isotropic properties.     

Complex investigation of electronic structure transformations in Lead Sulphide nanoparticles using a set of electron spectroscopy techniques

It has been reported recently that kinetic energy of photoelectrons emitted from core levels decreases with decreasing of the nanocrystal size. This phenomenon is called the size shift. The size shift value is the same for donor and acceptor in the compound. The present work is aimed on the explanation of this phenomenon. Crystals of lead sulfide PbS with different size from 50 to 350 nm were grown by chemical bath deposition (CBD) technique from alkaline solution onto Si and GaAs substrates. The morphology and size of crystals were analyzed by high resolution scanning electron microscopy (HRSEM). Complex electron spectroscopy investigations of electronic structure were carried out. In recent experiments X-ray photoelectron spectroscopy (XPS) was used for determination of Pb 4f, and S 2p electronic level positions and their size shifts. To explain the observed dependences in this work, we applied the following methods: analysis of PbS valence band (VB) and Pb 5d electronic level structure in the range ∼0-30 eV by XPS, high resolution electron energy losses spectroscopy (HREELS) for analysis of band gap transformations and work function measurements by Kelvin probe microscopy for the contact potential difference (CPD). The influence of work function increasing, widening of the band gap, transformations in VB and inter-level energy distances with decreasing of nanocrystal size on the size shift function ΔE(R) is discussed.     

超高真空电子束蒸发在多孔硅上外延单晶硅

采用超高越空电子束蒸发的方法在用阳极氧化制备的多孔硅衬底上外延单晶硅,研究了不同多孔硅衬底对外延质量的影响。采用高能电子衍射表征外延层的晶体结构,截面透射电镜上材料的微结构,原子力镜表征外延层表面的粗糙度,卢瑟福背散射／沟道表征外延层晶体质量,扩展电阻表征材料的电学性能。一系列的测试结果表明对在5mA／cm^2电流密度下阳极氧化10min形成的多孔硅衬底,可用超高真空电子束蒸发的方法外延出质量良好的单晶硅。     

Mechanical properties of porous silicon by depth-sensing nanoindentation techniques

Porous silicon (PS) was prepared using the electrochemical corrosion method. Thermal oxidation of the as-prepared PS samples was performed at different temperatures for tuning their mechanical properties. The mechanical properties of as-prepared and oxidized PS were thoroughly investigated by depth-sensing nanoindentation techniques with the continuous stiffness measurements option. The morphology of as-prepared and oxidized PS was characterized by field emission scanning electron microscope and the effect of observed microstructure changes on the mechanical properties was discussed. It is shown that the hardness and Young's elastic modulus of as-prepared PS exhibit a strong dependence on the preparing conditions and decrease with increasing current density. In particular, the mechanical properties of oxidized PS are improved greatly compared with that of as-prepared ones and increase with increasing thermal oxidation temperature. The mechanism responsible for the mechanical property enhancement is possibly the formation of SiO₂ cladding layers encapsulating on the inner surface of the incompact sponge PS to decrease the porosity and strengthen the interconnected microstructure.     

Investigation of composite structure of magnetically ordered material-semiconductor composite based on cobalt and porous silicon

Preliminary data are reported concerning the character of magnetically ordered material particles formed in porous silicon matrix using a special chemical incorporation technology. According to this, the internal volume of a porous silicon matrix is filled with cobalt chloride, which is subsequently reduced to a magnetic phase by means of a chemical reaction with sodium tetraborohydride. The obtained composite samples have been studied by the methods of electron microscopy and magnetometry.     

Preparation of Co-passivated porous silicon by stain etching

Co-passivated porous silicon (CPS) was prepared by stain etching. CPS samples prepared at different etching stages have different morphologies. All these morphologies are significantly different from those of conventional porous silicon (PS) etched in none cobalt-etching solution. The experimental results indicate that Co atoms only exist in a very thin layer on CPS surface, where Co atoms are well-distributed and Co atoms have hardly diffused into the substrate. Compared with the formation mechanism of conventional PS, there are two routes to generate holes in the formation of CPS, and there is NO₂ gas evolved from etching solution in a certain condition during the etching.     

Deposition of thin cobalt films onto silicon by galvanostatic and potentiostatic techniques

In this study, thin cobalt films were electrodeposited directly onto n-Si (100) using two different electrodeposition techniques: galvanostatic and potentiostatic. The morphological difference between galvanostatic and potentiostatic deposits was observed by atomic force microscopy (AFM) and X-ray diffraction (XRD). Analysis of the deposits by an alternating gradient field magnetometer (AGFM) showed the influence of the electrodeposition process on the magnetic properties of the film.     

X-ray photoelectron spectroscopy characterization of CoO/SiO2 nanocomposites and their optical transmittance change by nitrogen oxide

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Auger electron spectroscopy and electron energy loss spectroscopy studies of the formation of silicon nitride by implanting low energy nitrogen ions into silicon

Thin (d ? 10 nm) films of almost stoichiometric silicon nitride were prepared by implanting nitrogen ions with energies below 5 keV into silicon with subsequent thermal annealing. A combination of Auger electron spectroscopy (AES) and electron energy loss spectroscopy (ELS) was used to determine the chemical composition and to investigate the chemical bonding states of the thin films. The spectra are in good agreement with those obtained for Si₃N₄ produced by chemical vapour deposition. The use of AES and electron ELS allowed the stepwise formation of the SiN bonds to be investigated for the first time, and hence further information about the electronic structure of Si₃N₄ was obtained. Depth-concentration profiles for nitrogen are presented.     

In situ Auger electron spectroscopy investigation of the chemical bonding of ion-beam-deposited silicon nitride

The necessity of knowing the atomic ratio of nitrogen to silicon and the contamination of silicon nitride to obtain layers with reproducible physical, chemical and electrical properties is reviewed. Ion-beam-sputtered thin films of various nitrogen-to-silicon ratios with an oxygen content of less than 1% are analysed by in situ Auger electron spectroscopy. By deliberately allowing a pure silicon-rich nitride layer to become oxidized, the different signals contributing to the line shape of the derivative Si LVV peak are unambiguously identified. Assuming the peak-to-background ratio to be an intrinsic measurement, we propose a method to evaluate the composition of non-stoichiometric silicon nitride layers from the study of the Si LVV peak recorded in the EN(E) mode. This peak is reconstituted by using a linear combination of experimental Si LVV peaks from pure silicon and completely nitrided silicon. A realistic background is deduced from a step-by-step deposition of Si₃N₄ onto carbon. The results are compared with Rutherford backscattering spectrometry measurements and discussed.     

Core and valence electron excitations of amorphous silicon oxide and silicon nitride studied by low energy electron loss spectroscopy

Valence and core electron excitations were measured for thermal SiO₂ films and chemical-vapour-deposited Si₃N₄ films using low energy electron loss spectroscopy (ELS). A reasonable interpretation of the spectra can be obtained using electron energy level diagrams derived from optical and X-ray experiments and from theoretical calculations using literature values. The electron levels of these amorphous materials can then be described by localized molecular states. The ELS spectra show the formation of basic [SiO₄] and [SiN₄] tetrahedra and the existence of Si-Si bonds in mixed tetrahedra, suggesting a deviation from ideal stoichiometry and the presence of broken Si-O and Si-N bonds. Auger spectra have also been measured for elemental silicon, for SiO₂ and for Si₃N₄ and are discussed in terms of chemical shifts and line shape.     

Preparation of cobalt oxide nanoparticles and cobalt powders by solvothermal process and their characterization

Co₃O₄ nanoparticles and cobalt (fcc-Co) powders were successfully synthesized by solvothermal process from a single precursor. The reaction of Co(Ac)₂ with sodium dodecylbenzenesulfonate (SDBS) shows evident-dependent temperature effect. At 180 °C, Co(Ac)₂ reacts with SDBS to produce precursor CoCO₃ plate structures, which are assembled by small nanoparticles. At the temperature of 250 °C, the precursor CoCO₃ can be gradually decomposed to form Co₃O₄ nanoparticles with diameter of ca. 70 nm. While, at 250 °C, the reaction of Co(Ac)₂ with SDBS also produce precursor CoCO₃ nanoparticles/plates, but the CoCO₃ nanoparticles/plates would only decompose to give metal Co. In this process, SDBS acts as not only a surfactant but also a reagent. Magnetic measurements reveal that the as-prepared Co₃O₄ nanoparticles exhibit weak ferromagnetic properties and Co powders show ferromagnetic properties. In addition, a possible formation mechanism was elaborately discussed.     

Oxidation kinetics of silicon carbide whiskers studied by X-ray photoelectron spectroscopy

Silicon carbide whisker surfaces were characterized by X-ray photoelectron spectroscopy (XPS) to determine changes in the surface oxide film which occurred as a result of heating in air at temperatures from 600 to 800 °C. Equations were derived for the calculation of surface oxide film thickness from the SiC to SiO₂ 2p intensity ratios. Oxidation was found to follow a linear rate law in this temperature range for the first 10 nm of oxide growth. An activation energy of 17.2 ± 2.8 kcal mol^–1 (72 ± 12 kJ mol^–1) was measured.     

Preparation of aligned silicon carbide whiskers from porous carbon foam by silicon thermal evaporation

Aligned silicon carbide whiskers were prepared from porous carbon foams by thermal evaporation of silicon. High-density silicon carbide whiskers were vertically deposited on the surface of siliconizing carbon foam. The whiskers were straight and hexagon-shaped with diameter of 1-2 μm and length of about 40 μm. They consisted of a single-crystalline zinc blende structure crystal in the [111] growth direction. The pore structure of carbon foam played an important role in determining distribution of the whiskers on the surface of siliconizing carbon foam. When carbon foam with higher porosity and larger pore size was employed, distributions of the whiskers were more ordered and more intensive. The whiskers were grown by the vapor-solid (VS) mechanism.