Si/SiO<Subscript>2</Subscript> superlattice based optical planar microcavity

Si/SiO₂ Fabry–Pérot microcavities with a silicon nanocrystal (Si-nc) active spacer have been realized using a novel process based on a reactive magnetron sputtering of a pure silica target. Spectral, spatial and temporal behaviours of the quantum dots confined inside the resonator are detailed. Compared with a reference sample, the spectral and spatial emission distributions are significantly narrowed and the forward emission intensity is enhanced. Time resolved photoluminescence measurements also revealed an increase of the spontaneous emission rate. PACS 42.70.Qs; 78.55.-m; 78.66.-w     

Energy band structure and X-ray spectra of phenakite Be<Subscript>2</Subscript>SiO<Subscript>4</Subscript>

The electronic structure of crystalline phenakite Be₂SiO₄ is investigated using x-ray emission spectroscopy (XES) (Be K _α XES, Si L _{2, 3} XES, O K _α XES) and x-ray absorption spectroscopy (XAS) (Be 1s XAS, Si 2p XAS, O 1s XAS). The energy band structure is calculated by the ab initio full-potential linearized augmented-plane-wave (FLAPW) method. The total and partial densities of states and the dispersion curves for the Be₂SiO₄ compound are presented. It is shown that the top of the valence band and the bottom of the conduction band of the Be₂SiO₄ compound are predominantly formed by the oxygen 2p states. According to the results obtained, the electron transition with the lowest energy supposedly can occur at the center of the Brillouin zone. The effective masses of electrons (0.5m _e ) and holes (3.0m _e ) for the Be₂SiO₄) compound are estimated.     

Comparison between Si/SiO<Subscript>2</Subscript> and InP/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> based MOSFETs

The discrete breathers in graphane in thermodynamic equilibrium in the temperature range 50–600 K are studied by molecular dynamics simulation. A discrete breather is a hydrogen atom vibrating along the normal to a sheet of graphane at a high amplitude. As was found earlier, the lifetime of a discrete breather at zero temperature corresponds to several tens of thousands of vibrations. The effect of temperature on the decay time of discrete breathers and the probability of their detachment from a sheet of graphane are studied in this work. It is shown that closely spaced breathers can exchange energy with each other at zero temperature. The data obtained suggest that thermally activated discrete breathers can be involved in the dehydrogenation of graphane, which is important for hydrogen energetics.     

Laser-induced thermal effects on Si/SiO<Subscript>2</Subscript> free-standing superlattices

The thermal effects produced by continuous-wave laser radiation on free-standing Si/SiO₂ superlattices are studied. We compare two samples with different SiO₂ layer thicknesses (2 and 6 nm) and the same Si layer thickness (2 nm). The as-prepared free-standing superlattices contain some amount of Si nanocrystals (Si-nc). Intense laser irradiation at 488 nm of the as-prepared samples enhances the Raman scattering of Si-nc by two orders of magnitude. This laser-induced crystallization originates from melting of Si nanostructures in silica, which makes Si-nc better ordered and better isolated from the oxide surrounding. Continuous-wave laser control of Si-nc stress was achieved in these samples. In the proposed model, intense laser radiation melts Si-nc, and Si crystallization upon cooling down from the liquid phase in a silica matrix leads to compressive stress. The Si-nc stress can be tuned in the ∼3 GPa range using laser annealing below the Si melting temperature. The high laser-induced temperatures were verified with Raman spectroscopy. The laser-induced heat leads to a strongly nonlinear rise of light emission. The light emission is also observed in the anti-Stokes region, and its temperature dependence is practically the same for the two studied samples. The laser-induced temperature is essentially controlled by the absorbed laser power. PACS 78.55.-m; 78.20.-e; 68.55.-a; 78.30.-j     

Si/SiO<Subscript>2</Subscript> core shell clusters probed by Raman spectroscopy

Using a pulsed microplasma source, clusters were produced through the ablation of a Si cathode and successive supersonic expansion. The Si cluster beam was deposited onto different substrates and the partial oxidation of the cluster surface avoided the growth of large agglomerates, preserving their nanocrystalline morphology. Micro-Raman spectroscopy was used for an accurate size diagnosis of the deposited nanoparticles. The size of the Si dots ranges between 2 and about 15 nm. The Si dots appear to have a Si oxide shell, as confirmed also by structural and compositional analysis through transmission electron microscopy and atomic force microscopy. Double Raman peaks were attributed to small Si agglomerates having a thin substoichiometric Si-O interface.     

Acoustogyration diffraction of optical waves: case of SiO<Subscript>2</Subscript> and TeO<Subscript>2</Subscript> crystals

Interaction of acoustic and light waves with accounting for elastooptic and elastogyration effects is theoretically described. Collinear acoustogyration diffraction in quartz and paratellurite crystals is experimentally investigated and thoroughly analyzed. Piezogyration effect is experimentally studied for TeO₂ crystals. The acoustogyration efficiency and the acoustogyration figure of merit calculated for a number of crystals (GaAs, TeO₂ and SiO₂) are shown to be too small for experimental detection. On the other hand, we demonstrate that the light diffraction at periodical distribution of the imaginary part of dielectric permittivity related to the piezogyration effect should, in principle, be observed for the case of interaction of optical waves with enantiomorphous ferroelastic domain structure and in cholesteric liquid crystals.     

Theoretical studies on intervalley splittings in Si/SiO<Subscript>2</Subscript> quantum dot structures

Using the coupled cluster method we investigatespin-s J ₁-J′ ₂ Heisenberg antiferromagnets (HAFs) on an infinite, anisotropic, two-dimensional triangular lattice for the two cases where the spin quantum number s = 1 and s = $\frac{3} {2}$\frac{3} {2}. With respect to an underlying square-lattice geometry the model has antiferromagnetic (J ₁ > 0) bonds between nearest neighbours and competing (J′ ₂ > 0) bonds between next-nearest neighbours across only one of the diagonals of each square plaquette, the same diagonal in each square. In a topologically equivalent triangular-lattice geometry, the model has two types of nearest-neighbour bonds: namely the J′ ₂ ≡ κJ ₁ bonds along parallel chains and the J ₁ bonds producing an interchain coupling. The model thus interpolates between an isotropic HAF on the square lattice at one limit (κ = 0) and a set of decoupled chains at the other limit (κ → ∞), with the isotropic HAF on the triangular lattice in between at κ = 1. For both the spin-1 model and the spin-$\frac{3} {2}$\frac{3} {2} model we find a second-order type of quantum phase transition at κ _c = 0.615 ± 0.010 and κ _c = 0.575 ± 0.005 respectively, between a Néel antiferromagnetic state and a helically ordered state. In both cases the ground-state energy E and its first derivative dE/dκ are continuous at κ = κ _c , while the order parameter for the transition (viz., the average ground-state on-site magnetization) does not go to zero there on either side of the transition. The phase transition at κ = κ _c between the Néel antiferromagnetic phase and the helical phase for both the s = 1 and s = $\frac{3} {2}$\frac{3} {2} cases is analogous to that also observed in our previous work for the s = $\frac{1} {2}$\frac{1} {2} case at a value κ _c = 0.80 ± 0.01. However, for the higher spin values the transition appears to be of continuous (second-order) type, exactly as in the classical case, whereas for the s = $\frac{1} {2}$\frac{1} {2} case it appears to be weakly first-order in nature (although a second-order transition could not be ruled out entirely).     

Influence of the Si/SiO<Subscript>2</Subscript> interface on the charge carrier density of Si nanowires

The electrical properties of Si nanowires covered by a SiO₂ shell are influenced by the properties of the Si/SiO₂ interface. This interface can be characterized by the fixed oxide charge density Q_f and the interface trap level density D_it. We derive expressions for the effective charge carrier density in silicon nanowires as a function of Q_f, D_it, the nanowire radius, and the dopant density. It is found that a nanowire is fully depleted when its radius is smaller than a critical radius a_crit. An analytic expression for a_crit is derived. PACS 68.65.-k; 61.46.+w; 81.10.Bk     

Growth of Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> films on the Si(111) surface covered by a thin SiO<Subscript>2</Subscript> layer

Magnetite polycrystalline films are grown by variously oxidizing a Fe film on the Si(111) surface covered by a thin (1.5 nm) SiO₂ layer. It is found that defects in the SiO₂ layer influence silicidation under heating of the Fe film. The high-temperature oxidation of the Fe film results in the formation of both Fe₃O₄ and iron monosilicide. However, the high-temperature deposition of Fe in an oxygen atmosphere leads to the growth of a compositionally uniform Fe₃O₄ film on the SiO₂ surface. It is found that such a synthesis method causes [311] texture to arise in the magnetite film, with the texture axis normal to the surface. The influence of the synthesis method on the magnetic properties of grown Fe₃O₄ films is studied. A high coercive force of Fe₃O₃ films grown by Fe film oxidation is related to their specific morphology and compositional nonuniformity.     

Properties of an Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Si interface

The phase chemical composition of an Al₂O₃/Si interface formed upon molecular deposition of a 100-nm-thick Al₂O₃ layer on the Si(100) (c-Si) surface is investigated by depth-resolved ultrasoft x-ray emission spectroscopy. Analysis is performed using Al and Si L_{2, 3} emission bands. It is found that the thickness of the interface separating the c-Si substrate and the Al₂O₃ layer is approximately equal to 60 nm and the interface has a complex structure. The upper layer of the interface contains Al₂O₃ molecules and Al atoms, whose coordination is characteristic of metallic aluminum (most likely, these atoms form sufficiently large-sized Al clusters). The shape of the Si bands indicates that the interface layer (no more than 10-nm thick) adjacent to the substrate involves Si atoms in an unusual chemical state. This state is not typical of amorphous Si, c-Si, SiO₂, or SiO_x (it is assumed that these Si atoms form small-sized Si clusters). It is revealed that SiO₂ is contained in the vicinity of the substrate. The properties of thicker coatings are similar to those of the 100-nm-thick Al₂O₃ layer and differ significantly from the properties of the interfaces of Al₂O₃ thin layers.     

Impedance of Si/SiO<Subscript>2</Subscript> composites in the vicinity of the percolation threshold

The Si/SiO₂ composites, in which the concentration of the conducting silicon phase is close to the percolation threshold, have been prepared using the ceramic technology and studied at an alternating current. It has been found that an increase in the potential difference in a direct-current electric field leads to a decrease in the dispersion of time constants of dielectric spacers in the “Si grain-SiO₂ spacer-Si grain” structures forming a conducting cluster in the composite.     

Nanostructures of Si/SiO<Subscript>2</Subscript>/metal systems with tracks of fast heavy ions

Structures based on the SiO₂/n-Si and SiO₂/p-Si systems, with nanopores in silicon dioxide layers filled with Cu and Ni nanoparticles, have been prepared and investigated using the fast heavy ion technology, which includes irradiation with ¹⁹⁷Au²⁶⁺ ions, chemical etching of ion tracks, and subpotential electrochemical deposition. The selectivity of filling nanopores with metals and cluster character of their formation in tracks is shown.     

Electronic structure of SiO<Subscript>2</Subscript>: An X-ray emission spectroscopic and density functional theoretical study

The electronic structure of SiO₂ is investigated by means of valence to core X-ray emission spectroscopy and quantum-mechanical calculations in the density functional theory approximation. Analysis of a complete set of SiK_α1, SiL_{2, 3}, and OKα X-ray emission and XPS spectra along with the calculated data provides comprehensive information on chemical interactions that occur in SiO₂.     

Features of high-power ion beam irradiation of nanoporous Si and SiO<Subscript>2</Subscript>

Nanoporous Si and SiO₂ melting observed under high-power ion beam irradiation of nanosecond duration was investigated. The sizes of ellipsoidal particles formed in Si and those of holes formed in SiO₂ under irradiation were determined. The possible origin of these morphology features was discussed.     

Structure of Ultrathin Polycrystalline Iron Films Grown on SiO<Subscript>2</Subscript>/Si(001)

The structure of polycrystalline Fe films grown on an oxidized Si(001) surface at room temperature has been studied by the technique of high-energy electron diffraction. It has been found that the grain orientation in the films depends of the amount of deposited iron. In Fe films less than 5 nm thick, grains have been found to be randomly oriented. Fe films more than 5 nm in thickness exhibit the (111) texture with an axis coinciding with the surface normal. The angular dispersion of the [111] direction in the Fe lattice from the surface normal is ±25°. It has been found that as the Fe films become thicker, the (111) texture changes to the (110) texture.     

Influence of storage time on laser cleaning of SiO<Subscript>2</Subscript> on Si

The influence of the ‘storage time’ τ_s on the threshold fluence φ_cl and the efficiency in dry laser cleaning is investigated. τ_s denotes the time between the deposition of particles and the cleaning. As a model system we employed silica spheres with diameters of 500 nm and 1500 nm on commercial silicon wafers and single-pulse KrF excimer laser radiation (τ_FWHM=28 ns). For the 1500-nm silica spheres, φ_cl was found to increase from about 65 mJ/cm² to 125 mJ/cm² for storage times of 4 h and 362 h, respectively. For 500-nm silica spheres the increase in the threshold fluence was less than 20% for storage times up to 386 h. Received: 12 July 2002 / Accepted: 12 July 2002 / Published online: 29 January 2003 RID="*" ID="*"Corresponding author. E-mail:dieter.baeurle@jku.at     

Iron nanoparticles in amorphous SiO<Subscript>2</Subscript>: X-ray emission and absorption spectra

The local structure of the chemical bond of iron ions implanted into SiO₂ glasses (implantation energy, 100 keV; fluence, 1 × 10¹⁶ cm^?2) is investigated using x-ray emission and absorption spectroscopy. The Fe L x-ray emission and absorption spectra are analyzed by comparing them with the corresponding spectra of reference samples. It is established that iron nanoparticles implanted into the SiO₂ vitreous matrix are in an oxidized state. The assumption is made that the most probable mechanism of transformation of iron nanoparticles into an oxidized state during implantation involves the breaking of Si-O-Si bonds with the formation of Si-Si and Fe-O bonds.     

Synthesis,characterization, and thermal stability of SiO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript>/CR-Ag multilayered nanostructures

The controllable synthesis and characterization of novel thermally stable silver-based particles are described. The experimental approach involves the design of thermally stable nanostructures by the deposition of an interfacial thick, active titania layer between the primary substrate (SiO₂ particles) and the metal nanoparticles (Ag NPs), as well as the doping of Ag nanoparticles with an organic molecule (Congo Red, CR). The nanostructured particles were composed of a 330-nm silica core capped by a granular titania layer (10 to 13 nm in thickness), along with monodisperse 5 to 30 nm CR-Ag NPs deposited on top. The titania-coated support (SiO₂/TiO₂ particles) was shown to be chemically and thermally stable and promoted the nucleation and anchoring of CR-Ag NPs, which prevented the sintering of CR-Ag NPs when the structure was exposed to high temperatures. The thermal stability of the silver composites was examined by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). Larger than 10 nm CR-Ag NPs were thermally stable up to 300 °C. Such temperature was high enough to destabilize the CR-Ag NPs due to the melting point of the CR. On the other hand, smaller than 10 nm Ag NPs were stable at temperatures up to 500 °C because of the strong metal-metal oxide binding energy. Energy dispersion X-ray spectroscopy (EDS) was carried out to qualitatively analyze the chemical stability of the structure at different temperatures which confirmed the stability of the structure and the existence of silver NPs at temperatures up to 500 °C.     

Magnetic Core/Shell Nanocomposites MgFe<Subscript>2</Subscript>O<Subscript>4</Subscript>/SiO<Subscript>2</Subscript> for Biomedical Application: Synthesis and Properties

Magnetic core/shell (CS) nanocomposites (MNCs) are synthesized using a simple method, in which a magnesium ferrite nanoparticle (MgFe₂O₄) is a core, and an amorphous silicon dioxide (silica SiO₂) layer is a shell. The composition, morphology, and structure of synthesized particles are studied using X-ray diffraction, field emission electron microscopy, transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), scattering electrophoretic photometer, thermogravimetric analysis (TGA), and Mössbauer spectroscopy. It is found that the MgFe₂O₄/SiO₂ MNC has the core/shell structure formed by the Fe?O–Si chemical bond. After coating with silica, the MgFe₂O₄/SiO₂ MNC saturation magnetization significantly decreases in comparison with MgFe₂O₄ particles without a SiO₂ shell. Spherical particles agglomerated from MgFe₂O₄ nanocrystallites ～9.6 and ～11.5 nm in size function as cores coated with SiO₂ shells ～30 and ～50 nm thick, respectively. The total size of obtained CS MNCs is ～200 and 300 nm, respectively. Synthesized CS MgFe₂O₄/SiO₂ MNCs are very promising for biomedical applications, due to the biological compatibility of silicon dioxide, its sizes, and the fact that the Curie temperature is in the region required for hyperthermal therapy, 320 K.