Superconductivity in heavily boron-doped silicon carbide

The discoveries of superconductivity in heavily boron-doped diamond in 2004 and silicon in 2006 have renewed the interest in the superconducting state of semiconductors. Charge-carrier doping of wide-gap semiconductors leads to a metallic phase from which upon further doping superconductivity can emerge. Recently, we discovered superconductivity in a closely related system: heavily boron-doped silicon carbide. The sample used for that study consisted of cubic and hexagonal SiC phase fractions and hence this led to the question which of them participated in the superconductivity. Here we studied a hexagonal SiC sample, free from cubic SiC phase by means of x-ray diffraction, resistivity, and ac susceptibility.     

Superconductivity in heavily boron-doped silicon carbide

Abstract

The discoveries of superconductivity in heavily boron-doped diamond in 2004 and silicon in 2006 have renewed the interest in the superconducting state of semiconductors. Charge-carrier doping of wide-gap semiconductors leads to a metallic phase from which upon further doping superconductivity can emerge. Recently, we discovered superconductivity in a closely related system: heavily boron-doped silicon carbide. The sample used for that study consisted of cubic and hexagonal SiC phase fractions and hence this led to the question which of them participated in the superconductivity. Here we studied a hexagonal SiC sample, free from cubic SiC phase by means of x-ray diffraction, resistivity, and ac susceptibility.     

Thermal stability of undoped polycrystalline silicon layers on antimony and boron-doped substrates

The structure of as-deposited and annealed polycrystalline silicon layers has been investigated by scanning electron microscopy and x-ray diffraction. The structure of intentionally undoped layers prepared by low pressure chemical vapor deposition at a temperature of 640 °C was found to be stable upon annealing at temperatures lower than about 900 °C. On the other hand, primary recrystallization of the layers has been observed during annealing at temperatures in the range of 900 to 1150 °C. Isochronal annealing revealed the activation energy for the primary recrystallization of undoped layers as 0.6 eV. The activation energy for diffusion of silicon self-interstitials along the grain boundaries was calculated to be 2.2 eV. The difference in grain-growth process was observed for the undoped layers grown either (i) on lightly boron-doped substrate or (ii) on the substrate heavily doped with antimony. The different grain-growth mechanism was found to be a consequence of antimony diffusion into the polycrystalline layer.     

Band-gap modulation in single-crystalline Si1-xGex nanowires

We report the energy band-gap modulation of single-crystalline Si1-xGex (0 相似文献   

体硅上外延超薄Si1-xGex用于制备SGOI材料

采用减压化学气相沉积的方法在Si衬底上制备了高质量的Si0.75Ge0.25/Si/Si0.86Ge0.14叠层材料,通过TEM、光学显微镜和XRD分析表明,外延的SiGe薄膜具有完好的晶格结构,平整的表面质量,SiGe薄膜处于完全应变状态.通过与Si上外延渐变缓冲层制备的SiGe材料比较发现,使用这种超薄的全应变Si...     

Microstructure of boron-doped silicon layers prepared by low pressure chemical vapour deposition

The microstructures of silicon layers 0.55–0.60 μm thick with boron contents of about 10¹⁷ or 10²⁰ atoms cm^-3, prepared by low pressure chemical vapour deposition at 570 or 620°C on thermally oxidized silicon wafers, were characterized in the as-grown condition and after chemical thinning. The microstructural characteristics obtained from transmission electron microscopy examination on cross-sections, reflection high energy electron diffraction patterns, Raman spectrometry at 488 nm and UV absolute reflectance measurements as well as the optical and mechanical roughness of the sample surfaces were compared.The results showed the following.

• 1.(i) The layers are polycrystalline throughout their depth, with an external region of columnar character, intense twinning and 〈110〉 texture, and with a near-interface zone 50–100 nm thick of randomly oriented grains of size not greater than 10nm.
• 2.(ii) The thickness of the interfacial zone decreased as the deposition temperature increased. In the external region the texture became more developed as the level of boron doping and the deposition temperature increased, whereas the twinning, dislocation density, vacancy and/or impurity concentrations (from Raman results) and excess volume fraction (from absolute reflectance results) appear to be enhanced by increases in doping and decreased by increases in temperature.
• 3.(iii) The surface roughness was also clearly higher for layers with lower boron contents.

     

Determination by infrared absorption of the orientation of molecules in monomolecular layers

The mean orientation of molecules constituting Langmuir-Blodgett monomolecular layers was determined by infrared absorption with variable incidence transmission by polarizing the beam in the plane of incidence. The absorption calculation is completely developed for axial anisotropy layers when their thickness is small with respect to the wavelength. The experimental results show that the axis of the aliphatic chains makes an angle of 25±4° with the normal to the substratum for behenic acid layers, whereas this angle is only 8±5° for calcium behenate layers.     

Diameter dependent growth rate and interfacial abruptness in vapor-liquid-solid Si/Si1-xGex heterostructure nanowires

A strong diameter dependence is observed in the interfacial abruptness and growth rates in Si/Si 1- x Ge x axial heterostructure nanowires grown via Au-mediated low pressure CVD using silane and germane precursors. The growth of these nanowires has similarities to that of heterostructure thin films with similar compositional interfacial broadening, which increases with and is on the order with diameter. This broadening may reveal a fundamental challenge to fabrication of abrupt heterostructures via VLS growth.     

78 K infrared absorption method for measuring the oxygen content in silicon

A more accurate calibration curve at 78 K for measuring the oxygen content in silicon single crystals has been established by the infrared absorption difference method. The resulting formula is N_O = 2.6α ppmA, or N_O = 1.3 × 10¹⁷α atoms/cm³ where α represents the IR absorption coefficient.     

生长参数对Si1-xGex:C合金薄膜中元素分布的影响

用化学气相淀积方法在Si(100)衬底上外延生长Ge组分渐变的Si1-xGexC合金薄膜.本文通过能量色散谱仪乔(EDS)和扫描电子显微镜(SEM)对合金薄膜的元素深度分布和表面形貌进行了表征,分析研究了外延层的生长温度、生长时间对Si1-xGexC合金薄膜性质的影响.结果表明,Si1-xGexC外延层生长温度和生长时间一定范围内的增加加强了岛与岛之间的合并,促进了衬底Si原子向表面扩散、表面Ge原子向衬底扩散,且生长温度比生长时间对Si、Ge原子互扩散的影响大.     

Formation and characterization of semiconductor Ca2Si layers prepared on p-type silicon covered by an amorphous silicon cap

Formation of calcium silicide on three types of templates: Si(111)7 × 7, 2D Mg₂Si, and 3D Mg₂Si, was studied during Ca deposition at 120 °C in situ by Auger and electron energy loss spectroscopy, and by differential optical reflectance spectroscopy. A continuous Ca₂Si layer is formed on 2D and 3D Mg₂Si templates; but, on an atomically clean silicon surface (Si(111)7 × 7), a mixture of Ca₂Si with another Ca silicide was found. The growth of a Si cap layer over the Ca silicide layers at about 100 °C studied by in situ methods demonstrated the full embedding of Ca silicide in amorphous silicon, independent of the used template. Transmission electron microscopy, Rutherford backscattering spectrometry, atomic force microscopy, and electrical characterization of Schottky junctions revealed the Ca₂Si and Mg₂Si nanoparticles and the redistribution of Mg and Ca during the silicon cap growth and its effect on the electronic properties of the structures. Reproduction of the experiments on higher doped and better purity substrates is needed to understand better the role of Mg- and Ca-related defects, and defects of silicon generated by the growth process.     

Improved growth of GaN layers on ultra thin silicon nitride/Si (1 1 1) by RF-MBE

High-quality GaN epilayers were grown on Si (1 1 1) substrates by molecular beam epitaxy using a new growth process sequence which involved a substrate nitridation at low temperatures, annealing at high temperatures, followed by nitridation at high temperatures, deposition of a low-temperature buffer layer, and a high-temperature overgrowth. The material quality of the GaN films was also investigated as a function of nitridation time and temperature. Crystallinity and surface roughness of GaN was found to improve when the Si substrate was treated under the new growth process sequence. Micro-Raman and photoluminescence (PL) measurement results indicate that the GaN film grown by the new process sequence has less tensile stress and optically good. The surface and interface structures of an ultra thin silicon nitride film grown on the Si surface are investigated by core-level photoelectron spectroscopy and it clearly indicates that the quality of silicon nitride notably affects the properties of GaN growth.     

13. Measurement of the lattice constant of SiGe heteroepitaxial layers grown on a silicon substrate

The lattice constant of thin heteroepitaxial layers composed of single crystal SiGe solid solutions various compositions grown on silicon substracts were measured. The measurements were performed by X-ray diffraction method. The fact that the layers are thin and, therefore, there is also reflection from the silicon substrate (in addition to the reflection from the layer) is utilized for this measurement method. A review of the measurement method is given and the experimental results obtained by irradiating the samples with CuK_α are presented. Specific problems related to the measurement methods and to the layer lattice structure are also presented. There are: resolution problems between the reflections from the layer and the substrate for low Ge concentrations, deformation problems, the effects of the defects and non-uniform changes in the lattice constant at different crystallographic orientations.     

Determination of interface state density of PtSi/strained-Si1-xGex/Si Schottky diodes

PtSi/p-strained-Si_1-xGe_x (x=0.19 and x=0.29) Schottky barrier diodes have been fabricated and characterized for the determination of barrier height, ideality factor and the interface state density distribution with energy. Diodes having an epitaxial layer thickness of 20 and 52 nm were modeled with emphasis on comparison with experiment. To achieve better agreement with experimental data, an interfacial layer and associated series resistance were included in the model. The capacitance–voltage (C–V) technique has been used for the determination of the energy distribution of interface state density. The interface state density distribution for the Si_0.81Ge_0.19 and Si_0.71Ge_0.29 diodes is found to decrease with increasing energy from the valence band edge.     

Comparison between Double Crystals X-ray Diffraction and Micro-Raman Measurement on Composition Determination of High Ge Content Si1-xGex Layer Epitaxied on Si Substrate

It is important to acquire the composition of Si1-xGex layer, especially that with high Ge content, epitaxied on Si substrate. Two nondestructive examination methods, double crystals X-ray diffraction （DCXRD） and micro-Raman measurement, were introduced comparatively to determine x value in Si1-xGex layer, which show that while the two methods are consistent with each other when x is low, the results obtained from double crystals X-ray diffraction are not credible due to the large strain relaxation occurring in Si1-xGex layers when Ge content is higher than about 20%. Micro-Raman measurement is more appropriate for determining high Ge content than DCXRD.     

Investigations of the optical absorption spectra of porous silicon layers on the Silicon backing by the nondestructive photoacoustic method

This paper presents a series of experimental photoacoustic spectra of porous silicon layers on the crystalline silicon and their numerical analysis performed in the proposed two layer model. The goal of the analysis was to calculate the optical absorption coefficient spectra of porous silicon from the photoacoustic spectra of the porous silicon layer on the silicon backing. The character of the observed optical absorption band associated with the porous silicon was revealed.     

Attenuated total reflection surface-enhanced infrared absorption microspectroscopy for identification of small thin layers on material surfaces

Attenuated total reflection surface-enhanced infrared absorption microspectroscopy (micro-ATR-SEIRA) was developed for the identification of sub-mm size and nm-thick layers on material surfaces by using gold island films deposited on the surface of micro-ATR crystals. A thin layer of triphenyl phosphate (TPP) on a poly(tetrafluoroethylene) (PTFE) membrane filter was used to evaluate the enhancement of the absorption bands. Three types of crystals: diamond, silicon, and germanium, were evaluated. Diamond gave the greatest enhancement with a 12 nm thick gold island film. The enhancement factor was 200 times compared to bare diamond crystal, whereas it was 10 times for germanium crystal. This variation of enhancement factor according to crystal types was presumed to be due to the morphology of the gold films on the crystals. We also obtained an enhanced ATR map over an area of approximately 2 x 6 mm for a thin layer (approximately 1 nm thick) of di-2-ethylhexylphthalate on PTFE using gold-coated hexagonal silicon micro-ATR crystals. This micro-ATR-SEIRA technique has major potential for analyzing small and thin substances on material surfaces.     

Anodic oxidation of wastewater containing the Reactive Orange 16 Dye using heavily boron-doped diamond electrodes

Boron-doped diamond (BDD) films grown on the titanium substrate were used to study the electrochemical degradation of Reactive Orange (RO) 16 Dye. The films were produced by hot filament chemical vapor deposition (HFCVD) technique using two different boron concentrations. The growth parameters were controlled to obtain heavily doped diamond films. They were named as E1 and E2 electrodes, with acceptor concentrations of 4.0 and 8.0 × 10²¹ atoms cm⁻³, respectively. The boron levels were evaluated from Mott-Schottky plots also corroborated by Raman's spectra, which characterized the film quality as well as its physical property. Scanning Electron Microscopy showed well-defined microcrystalline grain morphologies with crystal orientation mixtures of (1 1 1) and (1 0 0). The electrode efficiencies were studied from the advanced oxidation process (AOP) to degrade electrochemically the Reactive Orange 16 azo-dye (RO16). The results were analyzed by UV/VIS spectroscopy, total organic carbon (TOC) and high-performance liquid chromatography (HPLC) techniques. From UV/VIS spectra the highest doped electrode (E2) showed the best efficiency for both, the aromaticity reduction and the azo group fracture. These tendencies were confirmed by the TOC and chromatographic measurements. Besides, the results showed a direct relationship among the BDD morphology, physical property, and its performance during the degradation process.