Photoconductivity of Si/Ge/SiO<Subscript>x</Subscript> and Si/Ge/Si structures with germanium quantum dots

A nonmonotonic dependence of the lateral photoconductivity (PC) on the interband light intensity is observed in Si/Ge/Si and Si/Ge/SiO_x structures with self-organized germanium quantum dots (QDs): in addition to a stepped increase in PC, a stepped decrease in PC is also observed. The effect of temperature and drive field on these features of the PC for both types of structures with a maximum nominal thickness of the Ge layer (N_Ge) is studied. The results obtained are discussed in the context of percolation theory for nonequilibrium carriers localized in different regions of the structure: electrons in the silicon matrix and holes in QDs.     

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> 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     

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.     

Shape controlled quantum dot (QD)-decorated-mesoporous SiO<Subscript>2</Subscript> (m-SiO<Subscript>2</Subscript>/QD) hollow particles for photodynamic therapy

This work reported a novel kind of CdTe quantum dot (QD) decorated mesoporous SiO₂ (m-SiO₂/QD) hybrid hollow nanoparticles for carrying photodynamic therapy (PDT) reagent. Both rod-like and spherical nanoparticles were prepared by using different shaped templates. Due to the porous shell and hollow interior, the hybrid m-SiO₂/QD hollow nanorod with 360 nm long and 120 nm in diameter was selected for carrying zinc(II) phthalocyanine (ZnPc) photosensitizing molecules (61 mg/g) since the generated reactive ¹O₂ could be easily delivered out of the hollow particles through the porous shell (BET area 251 cm²/g). It was found that the m-SiO₂/QD-ZnPc hollow nanorods exhibited a good PDT activity and showed effective photocytotoxicity for the cancer cells. Because of the porous nature, fluorescence characteristic, and excellent storage ability, the m-SiO₂/QD hybrid hollow particles possessed broad potential in the fluorescent labeled PDT.

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Graphic abstract m-SiO₂/QD hybrid hollow particles with different morphologies could be successfully synthesized by using the templating method and they could be used as carriers for photodynamic therapy reagents.

     

Electroluminescence in SiO<Subscript>2</Subscript> layers in various structures

Electroluminescence (EL) in an electrolyte-insulator-semiconductor system is used to study oxide layers in Si-SiO₂-Si₃N₄ and Si-SiO₂ structures prepared using various techniques. The EL spectra of all the structures contained the 2.7-eV emission band characteristic of the radiative relaxation of excited sililene centers. A comparative study of the conditions conducive to the formation of such luminescence centers in various structures containing SiO₂ layers was made, and their nature was refined.     

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.     

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     

Mössbauer studies of magnetic Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> nanocomposites

A large variety of glass and glass ceramics may be obtained by sol-gel process from hydrolysis of tetraethoxysilane. The transformation involves hydrolysis and polycondensation reactions leading to the growth of clusters that eventually collide together to form a gel. The structure and properties of the final product have been found to be strongly dependent on the initial conditions of preparation. Silica nanocomposites based on Fe₂O₃/SiO₂ were prepared with the help of ultrasonic activation and subsequent annealing in nitrogen atmosphere or air with concentrations of iron oxide of about 20 to 30wt.%.     

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     

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.     

Casimir friction force between a SiO<Subscript>2</Subscript> probe and a graphene-coated SiO<Subscript>2</Subscript> substrate

The possibility of mechanical detection of Casimir friction with the use of a noncontact atomic force microscope is discussed. A SiO₂ probe tip located above a graphene-coated SiO₂ substrate is subjected to the frictional force caused by a fluctuating electromagnetic field produced by a current in graphene. This frictional force will create the bend of a cantilever, which can be measured by a modern noncontact atomic force microscope. Both the quantum and thermal contributions to the Casimir frictional force can be measured using this experimental setup. This result can also be used to mechanically detect Casimir friction in micro- and nanoelectromechanical systems.     

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.     

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.     

Photoluminescence of germanium quantum dots grown in silicon on a SiO<Subscript>2</Subscript> submonolayer

The photoluminescence of quantum dots in Si/Ge/SiO₂/Si and Si/Ge/Si structures is investigated as a function of temperature. The low activation energies for the temperature quenching of photoluminescence of germanium quantum dots in both structures are explained in terms of the thermally stimulated capture of holes from quantum dots to the energy levels of defects localized in their vicinity.     

Facile growth of monolayer MoS<Subscript>2</Subscript> film areas on SiO<Subscript>2</Subscript>

Areas of single-layer MoS₂ film can be prepared in a tube furnace without the need for temperature control. The films were characterized by means of Raman spectroscopy, photoluminescence, low-energy electron diffraction and microscopy, and X-ray photoelectron spectroscopy and mapping. Transport measurements show n-doped material with a mobility of 0.26 cm² V^-1 s^-1.     

RETRACTED ARTICLE: Refractive-index tailoring and morphological evolutions in Gd<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript> and ZrO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> composite thin films

Refractive-index tailoring and morphological evolutions in two different thin film composite systems of gadolinia–silica (Gd₂O₃:SiO₂) and zirconia–silica (ZrO₂:SiO₂) deposited through reactive electron-beam codeposition processes are discussed in this research paper. For Gd₂O₃:SiO₂ the refractive-index tuning has been achieved from 1.45 to 2.18, whereas in the case of ZrO₂:SiO₂ the achieved tunable range is from 1.45 to 2.45 in the ultraviolet region. Under certain compositional mixings with lower silica fractions both the systems demonstrated relative microstructural and morphological densifications. Such evolutions were very successfully derived through phase-modulated ellipsometry and atomic force microscopy. The composition-dependent refractive-index tailoring and microstructural densifications have been investigated by adopting Tauc–Lorentz and single-effective-oscillator models. The morphological correlation functions have also very aptly supported such evolutions in these composite films. These experimental results indicate their favourable properties and applicability down to the extreme ultraviolet wavelength region of the electromagnetic spectrum. PACS 42.79.Wc; 78.66.-w; 78.20.Ci; 61.16.Ch; 42.70.-a;68.55.-a; 68.35.Bs; 81.15.Ef     

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.     

Friction and wear properties of ZrO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript> composite nanoparticles

In this article, the lubrication properties of ZrO₂/SiO₂ composite nanoparticles modified with aluminum zirconium coupling agent as additives in lubricating oil under variable applied load and concentration fraction were reported. It was demonstrated that the modified nanoparticles as additives in lubrication can effectively improve the lubricating properties. Under an optimized concentration of 0.1 wt%, the average friction coefficient was reduced by 16.24%. This was because the nanoparticles go into the friction zone with the flow of lubricant, and then the sliding friction changed to rolling friction with a result of the reduction of the friction coefficient.