Amorphous SiO2 Layer Research Articles

Room-temperature ferromagnetism in CrSi2(core)/SiO2(shell) semiconducting nanocables

Room-temperature ferromagnetism has been observed in high density free-standing CrSi2(core)/SiO2(shell) semiconducting nanocables, which is contrast to diamagnetic properties of both CrSi2 and SiO2 in bulk. The hexagonal CrSi2 C40-type nanowires, sheathed with a thin amorphous SiO2 layer, grow along [0001] direction. The results of first-principles simulations indicate that Cr atoms around the interface are with anomalously high magnetization (about 2 μB/atom), due to distorted/dangling bonds and surrounded oxygen atoms. Evaluations can be very consistent with measurements by further considering the effects of interfacial roughness and more distribution of oxygen around the interface. These results point toward a different way to tune nanomagnetism in core/shell nanowires.

Concentration and Annealing Effects on Luminescence Properties of Ion-Implanted Silica Layers

The development of optoelectronic or even photonic devices based on silicon technology is still a great challenge. Silicon and its oxide do not possess direct optical transitions and, therefore, are not luminescent. The remaining weak light emission is based on intrinsic and extrinsic defect luminescence. Thus the investigations are extended to ion implantation into silica layers, mainly on over-stoichiometric injection or isoelectronic substitution of both the constituents silicon or oxygen, that is, by ions of the group IV (C, Si, Ge, Sn, Pb) or the group VI (O, S, Se). The samples have been used were 500 nm thick thermally grown amorphous SiO2 layers, wet oxidized at 1100°C on a crystalline Si substrate. The ion implantations were performed with different energies but all with a uniform dose of 5 × 1016 ions/cm2. Such implantations produce new luminescence bands, partially with electronic-vibronic transitions and related multimodal spectra. Special interest should be directed to lowdimension nanocluster formation in silica layers. Implantations of group IV elements show a general increase of the luminescence in the violet-blue region and implantations of group VI elements lead to an increase in the yellow-red spectral region. Comparing cathodoluminescence, photoluminescence, and electroluminescence still too small luminescence quantum yields are obtained.

Ambipolar silicon nanowire FETs with stenciled-deposited metal gate

We report on a fully CMOS compatible fabrication method for ambipolar silicon nanowire FinFETs. The low thermal budget processing, compatible with monolithic 3D device integration, makes use of low pressure chemical vapor deposition (LPCVD) of amorphous Si (a-Si) and SiO2 layers as well as metal gate patterning using stencil lithography, demonstrated for the first time. FinFETs with stenciled Al gates are successfully co-fabricated with polycrystalline silicon Ω-gated devices. Stencil lithography is envisaged as a key enabler for gate patterning on 3D structures, such as vertically stacked nanowire transistors.

Novel coaxial SiC–SiO2–BN nanocable: large-scale synthesis, formation mechanism and photoluminescence property

Novel one-dimensional heterostructures composed of single crystalline cubic SiC cores, intermediate amorphous SiO2 layers, and single crystalline hexagonal BN (h-BN) sheaths (i.e. SiC–SiO2–BN nanocable) have been successfully fabricated in large scale using SiC–SiO2 nanocables and ammonia borane as starting materials. The structure and chemical composition of the as-synthesized products are determined by powder X-ray diffraction, field emission scanning electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and energy filtered TEM based on electron energy loss spectroscopy. The nanocables are approximately 100 nm in diameter and up to 1 millimetre in length. The intermediate amorphous SiO2 layers and the outer h-BN sheaths are about 10 nm and 5 nm thick, respectively. Interestingly, an increase in the amount of ammonia borane leads to the transformation of SiC–SiO2 nanocables into BN nanotubes. Mass spectrometric analysis shows that the vapors decomposed from ammonia borane play crucial roles both in the growth of the BN sheath on the SiC–SiO2 nanocables and in the transformation to the BN nanotubes. The SiC–SiO2–BN nanocable displays similar photoluminescence (PL) characteristics with respect to the original SiC–SiO2 nanocable but with the 488.5-nm emission peak blue shifting. The synthetic route has also been extended to fabricate SiC–BN nanocables and has proved effective and universal for the synthesis of core–sheath nanostructures with BN sheaths.

High-performance Fe/SiO2 soft magnetic composites for low-loss and high-power applications

High-performance Fe/SiO2 soft magnetic composites (SMCs) by coating pure iron particles with amorphous SiO2 layers have been fabricated by controlled hydrolysation of tetraethyl orthosilicate (TEOS). Microstructural studies show that the pure iron particles are uniformly and entirely coated with the SiO2 insulating layers to form Fe/SiO2 core–shell structure. The dielectric SiO2 layers lead to the electrical insulation behaviour of the SiO2-coated iron particles, which results in low core loss of the compacted Fe/SiO2 SMCs in application of alternating current. The core loss measured at a frequency of 50 Hz and an induction level of 1 T is 3.5 W kg−1, which is much lower than that of the commercial SMCs prepared by coating iron particles with phosphate layers. The Fe/SiO2 SMCs exhibit nearly the same coercivity as that of pure iron, and saturation magnetization of 18.2 kG, a little lower than that of pure iron due to the nonmagnetic SiO2 layers. The low core loss and high saturation magnetization promise the great potential of Fe/SiO2 SMCs in high-power applications. High-frequency measurements reveal that the working frequency of the Fe/SiO2 SMCs can reach up to 20 MHz.

Preparation of SiC/SiO<sub>2</sub> Core-Shell Coaxial Nanostructure

A simple direct synthesis method was used to synthesis SiC/SiO2 core–shell coaxial nanocables by thermal evaporation of the mixture powders of silicon monoxide and active carbon at 1300°C and condensation on Si substrate without assistance of any catalyst. The SiC core typically has diameters of 10-40 nm and is covered by a uniform layer of 2-10nm thick amorphous SiO2. A double vapor–solid growth process is proposed for the formation of this novel structures based on detailed structural characterizations. The cable nanostructures may find applications as building blocks in nanomechanical or nanoelectronic devices.

Detection of Amorphous Silica in Air-Oxidized Ti<sub>3</sub>SiC<sub>2</sub> at 500–1000°C by NMR and SIMS

The use of secondary-ion mass spectrometry (SIMS), nuclear magnetic resonance (NMR) and transmission electron microscopy (TEM) to detect the existence of amorphous silica in Ti3SiC2 oxidised at 500–1000°C is described. The formation of an amorphous SiO2 layer and its growth in thickness with temperature was monitored using dynamic SIMS. Results of NMR and TEM verify for the first time the direct evidence of amorphous silica formation during the oxidation of Ti3SiC2 at 1000°C.

High-temperature oxidation of MoSi2

Oxidation behavior of MoSi2 was investigated in air over the temperature range of 1400–1700 °C. Spallation of the SiO2 scale did not occur at any temperature, and Mo5Si3 formation did not happen below 1700 °C. A change in the rate-controlling mechanism was detected within the temperature range of this study. Activation energy for oxidation of MoSi2 at high temperatures was determined to be 204 kJ/mol. This value is less than the value of activation energy for oxidation of MoSi2 controlled by diffusion of O2 through amorphous SiO2 layer reported at lower temperatures. The decrease in activation energy is attributed to the increased degree of crystallization of amorphous silica to β-cristobalite at high temperatures resulting in enhanced O2 diffusion through SiO 4 −4 tetrahedral structure.

Electronic States of P Donors in Si Nanocrystals Embedded in Amorphous SiO2 Layer Studied by Electron Spin Resonance: Hydrogen Passivation Effects

Si nanocrystals (SiNCs) embedded in formed amorphous SiO2 layer and P nanoscale doping have been investigated by electron spin resonance (ESR) measurements at low temperatures. Hydrogen atom treatment is found to be required to determine the proper doping effects and electronic states of doped donors and quasi-conduction electrons in SiNCs. The dependences of microwave power and temperature for the ESR hyperfine structure of isolated P donors indicate that the spin–lattice relaxation time T1 becomes longer with decreasing size of SiNCs. From the experimental results of donor concentration and temperature dependence, we propose a novel model that the spin property of SiNCs of approximately 5.8 nm size is strongly dependent on the number of donors doped in a SiNC, i.e., odd or even. Using this model, we also estimate the numbers of P donors included in each SiNC to be 1 to at most 6, which are less than 1/10 of the average P doping concentrations.

Electrical properties and interfacial structure of epitaxial LaAlO3 on Si (001)

The dielectric and structural properties of LaAlO3 make it an attractive epitaxial gate oxide for nanometer-scale field effect transistors. However, the growth of epitaxial LaAlO3 directly on Si has not been possible to date. In order to achieve LaAlO3 epitaxy, we use a SrTiO3 template layer whose thickness minimizes elastic strain and atomic-level buckling at the interface. We find that LaAlO3 grown on this template layer is crystalline and initially strained, but relaxes to its bulk lattice constant within 7 unit cells. Cross-sectional transmission electron microscopy and inelastic electron tunneling spectroscopy studies of the LaAlO3/SrTiO3/Si structure show no evidence of an amorphous SiO2 layer. Capacitance-voltage measurements on thin films of epitaxial LaAlO3/SrTiO3/Si with LaAlO3 thicknesses between 13 and 110 nm show a dielectric constant for the LaAlO3 layer of 24, the same value as for the bulk. After a post-deposition low temperature anneal, these oxide heterostructures show no Fermi level pinning and an interface state density of ∼8×1010 cm−2 eV−1.

A comparative study of YBa2Cu3O7−δ/YSZ bilayer films deposited on silicon-on-insulator substrates with and without HF pretreatment

Highly epitaxial YBa2Cu3O7−δ (YBCO) and yttria-stabilized zirconia (YSZ) bilayer thin films have been deposited on silicon-on-insulator (SOI) substrates by using in situ pulsed laser deposition (PLD) technique. In the experiment, the native amorphous SiO2 layers on some of the SOI substrates are removed by dipping them in a 10% HF solution for 15s. Comparing several qualities of films grown on substrates with or without HF pretreatment, such as thin film crystallinity, general surface roughness, temperature dependence of resistance, surface morphology, as well as average crack spacing and crack width, naturally leads to the conclusion that preserving the native SiO2 layer on the surface of the SOI substrate can not only simplify the experimental process but can also achieve fairly high quality YSZ and YBCO thin films.

Magnetotransport properties of c-axis oriented La0.7Sr0.3MnO3 thin films on MgO-buffered SiO2/Si substrates

c -axis oriented La0.7Sr0.3MnO3 (LSMO) films on MgO-buffered SiO2/Si substrates were prepared, and their texture, microstructure, and magnetotransport properties were studied and compared to epitaxial LSMO/MgO (001) and polycrystalline LSMO/SiO2/Si films. c-axis oriented MgO buffer layers were obtained on amorphous SiO2 layer through rf sputter deposition at low substrate temperature and consequent postannealing processes. In situ pulsed laser deposition-grown LSMO films, deposited on the MgO layer, show strong c-axis texture, but no in-plane texture. The c-axis oriented LSMO films which are magnetically softer than LSMO/SiO2/Si films exhibit relatively large low field magnetoresistance (LFMR) and sharper MR drop at lower field. The large LFMR is attributed to a spin-dependent scattering of transport current at the grain boundaries.

Growth behavior of ZnO nanoparticles formed on Zn implanted Si(001) combined with thermal oxidation

Si（001） chips were implanted by Zn ions of 40 keV with different ion dosages and are annealed in air at different temperatures. Atomic force microcopy， transmission electron microcopy， X-ray diffraction and electron probe are applied to study the microstructure， morphology and chemical composition of the chips, either as-implanted or annealed at different temperatures. It was found that the as-implanted Zn atoms aggregate into clusters scattering about 35 nm beneath the surface of the chips. During the annealing process， Zn atoms are found to migrate towards the surface of the chips and aggregate into nanoparticles at the interface between the amorphous SiO2 layer and polycrystal line Si layer. Annealing temperature was found to be the crucial factor controlling the formation of ZnO nanoparticles. ZnO nanoparticles begin to appear at about 400 ℃ and the diffraction intensity of ZnO becomes strong while the diffraction intensity of metallic Zn weakens with increasing annealing temperature. At the annealing temperature of 800 ℃， Zn2SiO4 phase was observed due to the reaction between ZnO and SiO2 or Si.

Enhancement of Sorption Processes in the Zeolite H‐ZSM5 by Postsynthetic Surface Modification

Catching molecules: When the surface of zeolite HZSM-5 (see picture, red) is modified by amorphous SiO2 layers (gray), increased surface roughness results. The mesopores in the rough SiO2 layer funnel benzene molecules into the micropores of the zeolite, thus increasing sorption rates for benzene at the acidic hydroxy groups by a factor of two. The size of the mesopores relative to the adsorbing molecules is critical for sorption-rate enhancement.

Microstructural, Magnetic and Microwave Properties of Large Area BaFe$_{12}$O$_{19}$ Thick Films ($>\!100\ \mu$m) Deposited on /a-SiO$_{2}$/Si and /a-Al$_{2}$O$_{3}$/Si Substrates

We have demonstrated the feasibility of thick BaFe12O19 ceramic films fabricated on silicon (100) substrates. We obtained large area (1-in diameter) thick films (100-200 mum) by using a screen printing technique and amorphous SiO2 or Al2O3 buffer layers. To characterize the microstructural, magnetic, and microwave properties of ferrite films, we employed X-ray diffractometry, scanning electron microscopy, energy dispersive X-ray spectroscopy, dc magnetometry, and ferromagnetic resonance (FMR). We placed our emphasis on investigating silicon diffusion in terms of a depth analysis of composition for different buffer layers. We present and discuss FMR measurements for two polycrystalline samples with different buffer layers. We find that a-Al2O3 is superior to a-SiO2 as a buffer layer in achieving a tradeoff between magnetic and mechanical properties.

Passive Oxidation Behavior of ZrB<sub>2</sub>-SiC Eutectic Composite Prepared by Arc Melting

ZrB2-SiC composites were prepared by arc melting using ZrB2 and -SiC powders as raw materials and their oxidation behavior were investigated. The eutectic composition of the ZrB2-SiC system was ZrB2-58.5mol%SiC and the melting point temperature was about 2570 K. At 1673 K mass gain was observed in the oxidation of ZrB2-SiC eutectic composition and the mass gain rate increased with decreasing Ptot. The oxide scale consisted of an amorphous SiO2 layer, a ZrO2-SiO2 eutectic-like layer and a Si-deficient layer.

Magnetic properties and microstructure of Fe∕Pt multilayer films capped with SiO2 amorphous layer for magnetic recording use

Fe ∕ Pt multilayer films capped with an amorphous SiO2 layer have been fabricated in order to study the microstructure and magnetic properties via a molecular beam epitaxy technique at 400°C. The formation of granularlike FePt films was obtained with this process due to the interpenetration of SiO2 which has a lower surface energy. Studies of angular dependent coercivity show a tendency of a domain-wall motion weaken toward rotation of reverse-domain type upon thickness of SiO2 capping layer atop the Fe∕Pt multilayer films. The intergrain interaction was confirmed from the Kelly–Henkel plot that indicated the exchange coupling between neighboring grains in the uncapped FePt films. On the other hand, negative δM value was obtained when the FePt films capped with SiO2 layer, indicating that the SiO2 capping layer can lead to the reduction of intergrain exchange coupling in SiO2/FePt films.

Oxidation Behavior of Fe–25Cr–20Ni–2.8Si During Isothermal Oxidation at 1,286 K; Life-time Prediction

The oxidation behavior of an austenitic steel, type 1.4841, with a Cr content of 25 wt% and a high-Si content of 2.8 wt% was studied during isothermal oxidation at 1,286 K in air. A thick, crystalline Cr2O3 layer, on top of a much thinner, amorphous SiO2 layer, developed on the alloy substrate. After formation of a closed Cr2O3 scale, parabolic growth kinetics prevailed as long as the associated constant, steady-state Cr concentration in the alloy at the substrate/oxide interface of about 13 ± 1 wt% was maintained. Upon prolonged oxidation, successive cracking and spallation of the thickening oxide scale eventually led to breakaway oxidation, because the “bulk-”Cr concentration in the interior of the alloy dropped below the critical value required to ‘heal’ the protective oxide layer after oxide spallation. Application of a lifetime prediction model of the alloy substrate under isothermal oxidation conditions allowed determination of the breakaway-oxidation time as a function of alloy-sheet thickness, by employing the Cr volume-diffusion coefficient in the alloy and the parabolic growth-rate constant, both determined in the present study by fitting calculated to experimental Cr-depletion profiles for various oxidation times.

Formation of Surface and Subsurface Oxides during Ferritic, Intercritical and Austenitic Annealing of CMnSi TRIP Steel

The equilibrium external oxidation of CMnSi TRIP steel at different annealing temperatures in a low dew point N2–10%H2 atmosphere of −30°C was investigated for the first time by means of high resolution transmission electron microscopy of cross-sectional samples. Annealing in the ferrite stability temperature range below the Ae1 temperature resulted in the formation of 300–600 nm size crystalline internal MnO in the matrix. In the subsurface region, 10–20 nm size SiO2 particles were detected. Large amorphous lens-shaped xMnO·SiO2 oxides, with x<0.5, were present at the surface, and thin films of crystalline xMnO·SiO2 oxides, with 1<x<2, were formed between these large lens-shaped oxides. The amorphous lens-shaped xMnO·SiO2 oxides were covered by a continuous thin layer of crystalline xMnO·SiO2 oxide, with x>2. Amorphous 15–50 nm size internal SiO2 particles covered with a thin layer of crystalline MnO·SiO2 and MnO·Al2O3 oxides were found in the subsurface matrix region and at the grain boundaries after annealing in the intercritical and the fully austenitic temperature ranges. A discontinuous amorphous SiO2 layer covered by a layer of crystalline 2MnO·SiO2+MnO·SiO2 mixed oxide was present at the surface after annealing in the intercritical Ae1–Ae3 temperature range. This changed to a continuous layer of amorphous SiO2 on the steel surface covered by a continuous layer of crystalline 2MnO·SiO2+MnO·SiO2 mixed oxide after annealing in the austenitic stability range. The results clearly show an increased tendency for the xMnO·SiO2 and SiO2 oxides to form two separate surface oxide films covering the entire steel surface during continuous annealing at the higher annealing temperatures used to process advanced high strength steels. The presence of these film-forming oxide layers will prevent the formation of the inhibition layer and its wetting by the liquid Zn, and cause galvanizing surface defects.

Crystallinity and Photoluminescence Properties of ZnO Films on Zn Buffer Layers Deposited by rf Magnetron Sputtering

It is very desirable to grow ZnO epitaxial films on Si substrates since Si wafers with a high quality is available and their prices are quite low. Nevertheless, it is not easy to grow ZnO films epitaxially on Si substrates directly because of formation of an amorphous SiO2 layer at the interface of ZnO and Si. A Zn film and an undoped ZnO film were deposited sequentially on an (100) Si substrate by rf magnetron sputtering. The sample was annealed at 700°C in a nitrogen atmosphere. X-ray diffraction (XRD), photoluminescence (PL) and atomic force microscopy (AFM) analyses were performed to investigate the cristallinity and surface morphology of the ZnO film. According to the analysis results the crystallinity of a ZnO thin film deposited by rf magnetron sputtering is substantially improved by using a Zn buffer layer. The highest ZnO film quality is obtained with a 110nm thick Zn buffer layer. The surface roughness of the ZnO thin film increases as the Zn buffer layer thickness increases.