Si Pairs Research Articles

Enhanced luminescence from Si quantum dots/SiO2 multilayers by hydrogen annealing

Si quantum dots/SiO2 multilayers were prepared by annealing a-Si:H/SiO2 stacked structures at 1100°C . Photo- and electro-luminescence band around 750nm can be observed from Si QDs/SiO2 multilayers due to the recombination of electron-hole pairs in Si QDs/SiO2 interfaces. The electro-luminescence intensity was obviously enhanced after post hydrogen annealing at 400°C. Electron spin resonance measurements were used to characterize the change of the defect states after hydrogen annealing. It is found that there exists a-centers (g value=2.006), which is related to the Si dangling bonds in Si QDs in our samples. Hydrogen annealing can significantly reduce non-luminescent a-centers and enhance the electro-luminescence intensity consequently.

Glass transition kinetics and radial distribution function to investigate the role of Cr 2O 3 on the structure of Li 2O–Al 2O 3–SiO 2 photonic glass

Thermal kinetic parameters of Cr 2O 3 doped lithium-aluminum-silicate (LAS) photonic glass are calculated using differential thermal analysis (DTA). The dependence of kinetic parameters together with the glass forming ability of the investigated glass on the Cr 2O 3 doping level showed an anomalous behavior around 0.4 g. Correlation between the obtained thermal properties and the materials electronegativity and band gap is given. An energy band diagram is proposed explaining the density of energy states of Cr 3+ and its transformation to Cr 6+. The non-bridging oxygen bonds generated by Cr 2O 3 produce a deep donor level responsible for the red shift of the absorption edge. The local order structure is studied for the LAS glass network using the radial distribution function of XRD. The atomic pairs of Si–O, Li–O and Si–Si are clearly revealed in both short range (SRO) and medium range (MRO) orders, respectively. The addition of Cr 2O 3 into the glass network has changed appreciably both the atomic pair distances and the coordination numbers in the MRO due to the multivalent nature of Cr ions. The observed decrease in the fourth shell coordination number, N 4, reflects the atomic size decrease in the given glass network which means an increase in glass compactness.

Three carbon pairs in Si

Carbon impurities in Si are common in floating-zone and cast-Si materials. The simplest and most discussed carbon complex is the interstitial–substitutional CiCs pair, which readily forms when self-interstitials are present in the material. This pair has three possible configurations, each of which is electrically active. The less common CsCs pair has been studied in irradiated material but has also recently been seen in as-grown C-rich cast-Si, which is commonly used to fabricate solar cells. The third pair consists of two interstitial C atoms: CiCi. Although its formation probability is low for several reasons, the CiCi pair is very stable and electrically inactive. In this contribution, we report preliminary results of first-principles calculations of these three C pairs in Si. The structures, binding energies, vibrational spectra, and electrical activity are predicted.

Study on tribological properties of ceramics/stainless steel rubbing pairs in hydrogen peroxide solutions

► The properties of ZrO 2 , Al 2 O 3 , Si 3 N 4 /1Cr18Ni9Ti were investigated in H 2 O 2 solutions. ► There were significant differences on wear-resistant for three types of ceramics. ► As a whole, Si 3 N 4 /1Cr18Ni9Ti had the best tribological properties in H 2 O 2 solutions. Rubbing pairs of ceramics and stainless steel are widely used under the hydrogen peroxide (H 2 O 2 ) strong oxidizing environment. It is crucial to study the wear behavior and failure modes of the rubbing pairs. The tribological properties of three types of ceramics (ZrO 2 , Al 2 O 3 , Si 3 N 4 ceramics) and 1Cr18Ni9Ti stainless steel rubbing pairs were investigated using a tribo-tester under different concentrations of H 2 O 2 solution. The comparative analysis of the friction coefficients, wear volumes, worn surface topographies and corrosion potentials was conducted under different concentrations of H 2 O 2 solutions. There were significant differences in the tribological and electrochemical properties of the rubbing pairs in different H 2 O 2 solutions. As a whole, the rubbing pairs of Si 3 N 4 ceramics/1Cr18Ni9Ti stainless steel have the best tribological properties in H 2 O 2 solutions. It is believed that the knowledge obtained in this study will provide a guide for the selection of the rubbing pairs in the H 2 O 2 solutions.

Ion channeling study of epitaxy of iron based Heusler alloy films on Ge(111)

We have investigated perfection of atomic rows on iron-based Heusler alloy films on Ge(111) planes by using ion channeling technique in order to find the dominant factors for the perfection. Fe 3Si/Ge(111) and Fe 2CoSi/Ge(111) have a high quality of atomic rows at the heterointerface like that of perfect crystals. Fe 3−xMn xSi/Ge(111) ( x = 0.84, 0.72 and 0.36) interfaces have imperfection of atomic rows which may be controlled by both the lattice mismatch with the Ge substrate and the Mn–Si pairs due to the site disorder in the film with the Mn content x > 0.75. Analysis of axial channeling parameters employed in this study is very useful for quantitative evaluation of perfection of atomic rows at the heterointerface.

Enhanced photoluminescence from condensed electron-hole pairs in trenched Si

In this paper, we report the characteristics of low-temperature photoluminescence (PL) from condensed electron-hole (e-h) pairs in p-type silicon substrates with and without high-aspect-ratio trench structures. The average hole diameters of trenches were 140 and 170 nm, and the depth of trenches were about 5 μm. An Al2O3 surface passivation layer with thickness of 10 nm was grown by atomic layer deposition on surfaces of all samples to reduce nonradiative recombination and provide nucleation centers of condensed e-h pairs at Si/Al2O3 interface. Significant enhancement of PL intensity from e-h liquid/droplet (EHL/EHD) was observed in the trenched silicon at temperature below 30 K. Dominant PL from EHL/EHD at 14 K in the trenched silicon with larger hole diameters may be attributed to stronger spatial confinement of condensed e-h pairs in a smaller volume.

Stability of Frenkel pairs in Si(1 0 0) surface in the presence of germanium and oxygen atoms

A first-principles pseudo-potential study of Frenkel pair generation close to the Si(1 0 0) surface in the presence of germanium and oxygen atoms is reported. The energies and structures of the defect structures (i.e. vacancy and relaxed tetrahedral Si interstitial) are calculated using supercell with up to 88 atoms. We present results obtained using the generalized gradient approximation (GGA) for the exchange–correlation energy. We examine the effect of the presence of germanium and oxygen atoms on the stability of Frenkel pairs generated near the Si(1 0 0) surface by comparing a number of individual cases, starting from vacancy interstitial pairs situated at various positions. The general tendency of the created interstitials is to climb towards the surface, but they generally remain in subsurface layers, ready to migrate into the layer. This tendency is enhanced by the presence of the Ge and/or O atoms. We show that the formation energy is lower and Si interstitials can be created with energies as low as 1.5 eV.

Structural changes in femtosecond laser modified regions inside fused silica

Structural characterization of photomodified microvolumes formed by tightly focusedfemtosecond laser pulses inside silica glass was carried out using synchrotron x-raydiffraction. The observed distinct separation between the O–O and Si–Si pair correlationpeaks can be interpreted as a phase separation induced by microexplosions at the focalvolume. The mechanisms of structural transitions induced by femtosecond laser pulsesinside dielectrics are discussed.

Stress-induced magnetic and structural anisotropy of nanocrystalline Fe-based alloys

The structural anisotropy of Fe–Si–B–Nb–Cu nanocrystalline alloys annealed under tensile stress was studied by x-ray diffraction techniques with transmission geometry. A clear difference was observed in the peak positions of the Fe–Si crystals under two different conditions: with the diffraction vector parallel or perpendicular to the applied stress. The strains calculated from the anisotropy of the peak positions show a linear response to the applied stress, independent of Si content, indicating that the observed structural anisotropy is due to a quenching of the elastic strain, not in the directional ordering of the Fe–Si pair. The induced magnetic anisotropy energy is well explained by the residual strains and their magnetostrictions.

Enabling enelike reactions on Si(111)-7×7 through tuning organic molecular structures

We previously demonstrated that acetonitrile (N[triple bond]C-CH(3)) binds to the adjacent adatom-rest atom pair of Si(111)-7x7 through a [2+2]-like cycloaddition reaction, forming a (Si)N=C(Si)-CH(3)-like surface species [Tao et al., J. Phys. Chem. B 106, 3890 (2002)]. Current investigation clearly showed that chloroacetonitrile (N[triple bond]C-CH(2)Cl), propargyl chloride (HC[triple bond]C-CH(2)Cl), and 3-chloropropionitrile (N[triple bond]C-CH(2)-CH(2)Cl) react with the surface via enelike reactions, concurrently involving N[triple bond]C/C[triple bond]C as well as the breakage of the C-Cl/C-H bond. Further separation of the unsaturated bond (C[triple bond]C) from the C-Cl bond using CH(2) spacers in 5-chloro-1-pentyne (CH[triple bond]C-CH(2)CH(2)CH(2)Cl) would direct the reaction to a [2+2]-like cycloaddition. These experimental results clearly suggest the possibility of controlling the surface reaction pathways by tuning the organic molecular structures. This strategy can be useful in designing and fabricating functional molecular templates on Si(111)-7x7.

Structure determination of the zeolite IM-5 using electron crystallography

Abstract The structure of the complex zeolite IM-5 (Cmcm, a = 14.33(4) Å, b = 56.9(2) Å, c = 20.32(7) Å) was determined by combining selected area electron diffraction (SAED), 3D reconstruction of high resolution transmission electron microscopy (HRTEM) images from different zone axes and distance least squares (DLS) refinement. The unit cell parameters were determined from SAED. The space group was determined from extinctions in the SAED patterns and projection symmetries of HRTEM images. Using the structure factor amplitudes and phases of 144 independent reflections obtained from HRTEM images along the [100], [010] and [001] directions, a 3D electrostatic potential map was calculated by inverse Fourier transformation. From this 3D potential map, all 24 unique Si positions could be determined. Oxygen atoms were added between each Si–Si pair and further refined together with the Si positions by distance-least-squares. The final structure model deviates on average 0.16 Å for Si and 0.31 Å for O from the structure refined using X-ray powder diffraction data. This method is general and offers a new possibility for determining the structures of zeolites and other materials with complex structures.

Short-range order in Fe1−x Si x (x=0.05−0.08) alloys with induced magnetic anisotropy

Previously, iron—silicon alloys were investigated using X-ray diffraction and Mossbauer spectroscopy. It was demonstrated that, at low silicon concentrations, the alloys undergo a local separation into regions of the α iron phase depleted in silicon and silicon-rich clusters with a B2 ordering. The structure of locally ordered regions of the B2 phase is characterized by a pair ordering of silicon atoms: the Si—Si pairs are formed by next-nearest neighbors, and the axes of pairs are oriented along the 〈100〉 directions, which are the easy-magnetization axes. The thermomagnetic treatment in a constant magnetic field applied along the 〈100〉 axis induces an axial magnetic anisotropy, results in the formation of an anisotropic distribution of the B2 phase, and leads to a slight decrease in the volume fraction of the coordination 6: 2 with two silicon atoms in the first coordination shell of the iron atom. Therefore, the formation of an anisotropic local order of pairs of silicon atoms occurs as a result of their reorientation.

Molecular dynamics simulation of silicon oxidization

We compare here the different oxidization protocols that can be used to generate an SiO 2/Si interface. All these protocols are based on molecular dynamics at high temperature but differ by the way the oxygen atoms are incorporated one-by-one. When they are inserted between two neighbouring Si atoms, forming one of the Si–Si pair closest to the surface, the silicon oxide grows layer-by-layer and it is structured as a random network of SiO 4 entities connected by vertices with only a small amount of SiO 3 and SiO 5 defects. On the contrary, when they are incorporated into the longest Si–Si bond instead of the highest one, a dendritic-like SiO 2 oxide spreads into the substrate in contradiction with experimental observations, which definitely rules out such protocols as realistic ones.

Structure, chemical bonding, and 45Sc solid state NMR of Sc 2RuSi 2

The silicide Sc 2RuSi 2 was synthesized from the elements by arc-melting. The structure was refined on the basis of single crystal X-ray diffractometer data: Zr 2CoSi 2 type, C2/ m, a = 1004.7 (2), b = 406.8 (1), c = 946.6 (2) pm, β = 117.95 (2), w R2 = 0.0230, 743 F 2 values, and 32 variables. The structure consists of a rigid three-dimensional [RuSi 2] network in which the two crystallographically independent scandium atoms fill larger cages of coordination numbers 16 and 15, respectively. The [RuSi 2] network shows short Ru–Si distances (234–247 pm) and two different Si 2 pairs: Si1–Si1 at 247 and Si2–Si2 at 243 pm. Each silicon atom has trigonal prismatic Sc 6 (for Si2) or Sc 4Ru 2 (for Si1) coordination. These building units are condensed via common edges and faces. The various Sc–Sc distances between the prisms range from 327 to 361 pm. From electronic structure investigation within DFT, chemical bonding shows a major role of Ru–Si bonding and the presence of strong electron localization around Si–Si pairs pointing to a polyanionic silicide network [RuSi 2] δ −. The 45Sc MAS-NMR spectra recorded at 11.7 and 9.4 T clearly resolve the two distinct scandium sites. The large electric field gradients present at both scandium sites result in typical line shapes arising from second-order quadrupole perturbation effects.

X-ray Optics with Small Vertical Divergence and Horizontal Focusing Foran X-ray Standing-wave Measurement

An x-ray optics was proposed for dynamical diffraction measurements of a non-ideal crystal. It was composed of a pair of channel-cut Si (004) crystals and a one-dimensional focusing lens system. Vertical angular divergence values were measured as FWHM ’s of ca. 1.3 arc secs for an incident photon energy of 12.4 keV during a rocking scan around the analyzer sapphire 0006 reflection when the second channel-cut crystal was fixed at five deviation angles. The experimental angular resolution deconvoluted is about 1.3 arc secs at the deviation angle of ca. 0.7 arc sec. A typical photon flux was around 10 photons /s as a peak value of the rocking scan for an incident slit of 0.2 x 0.1 mm. The horizontal beam size measured at the focal distance of ca. 200 mm was 4.3 μm.

Evidence for a vacancy–phosphorus–oxygen complex in silicon

Low-energy (∼0.5 MeV)electrons arising from 60Co γ-irradiation were used to create phosphorus–vacancy (PV) pairs andoxygen–vacancy pairs in Czochralski-grown Si. Positron annihilation data showthat PV pairs anneal in two stages: the commonly observed stage around125 °C, where one third of the pairs disappear with an activation energy of0.8 ± 0.2 eV,and a new stage where none disappear, but form PV–oxygen complexes with an activation energy of2.0 ± 0.2 eV.

Ge-vacancy pair in Ge-doped Czochralski silicon

The potential configurations of Ge-vacancy pairs in a Ge-doped Czochralski Si (GCz-Si) crystal have been identified through first-principles theory using a total-energy pseudopotential method. The Ge atoms in the GCz-Si lattice are suggested to aggregate with the vacancy/vacancies to generate the Ge-related complexes. The total energy of the GCz-Si lattice configuration containing multivacancies decreases with the reduction of bond lengths among the vacancies and decreases with the reduction of bond lengths between the Ge atom and the vacancies. It is suggested that the Ge atom introduced in the GCz-Si crystal tends to accumulate with the vacancy and then seeds for the Ge-vacancy pairs.

Electroluminescence from nano-crystalline Si/ SiO 2 structures embedded in pn junctions

Phosphorous-doped and boron-doped amorphous Si thin films as well as amorphous SiO 2/Si/ SiO 2 sandwiched structures were prepared in a plasma enhanced chemical vapor deposition system. Then, the p–i–n structures containing nano-crystalline Si/ SiO 2 sandwiched structures as the intrinsic layer were prepared in situ followed by thermal annealing. Electroluminescence spectra were measured at room temperature under forward bias, and it is found that the electroluminescence intensity is strongly influenced by the types of substrate. The turn-on voltages can be reduced to 3 V for samples prepared on heavily doped p-type Si (p +-Si) substrates and the corresponding electroluminescence intensity is more than two orders of magnitude stronger than that on lightly doped p-type Si (p-Si) and ITO glass substrates. The improvements of light emission can be ascribed to enhanced hole injection and the consequent recombination of electron–hole pairs in the luminescent nanocrystalline Si/ SiO 2 system.

Hydrogen-related radiation defects in SiO2-based glasses

Spectroscopic properties of hydrogen atom trapped in an oxygen vacancy in SiO2 glass were studied. Samples were loaded with D2 and H2 gases to convert O vacancies to pairs of Si–D and Si–H groups, and subsequently irradiated by F2 laser in order to destroy some of these groups. Electron paramagnetic resonance, infrared absorption and visible/UV absorption spectra were measured. Proton hyperfine doublet with splitting of 1.05mT was found in all H2-treated/irradiated samples. UV-bleaching treatment showed that this signal is independent of the other, well-known hydrogen-related signals in silica. The size of the hyperfine splitting corresponds to twice the 1H nuclear Zeeman splitting in applied magnetic field. The observed 1.05mT doublet is tentatively attributed to the forbidden nuclear spin-flip transitions of proton trapped in an oxygen vacancy in silica. The allowed EPR transitions of this center are not resolved, possibly reflecting a variation in Si⋯H distance in oxygen vacancy with a trapped H atom.

Large anomalous Hall resistance of pair δ-doped GaAs structures grown by molecular-beam epitaxy

Beryllium/silicon pair δ-doped GaAs structures grown by molecular-beam epitaxy exhibit a Hall resistance which has a nonlinear dependence on the applied magnetic field and which is strongly correlated to the negative magnetoresistance observed under the applied magnetic field parallel to the δ-doped layers. Dependence of the occurrence of the nonlinear Hall resistance on the growth condition is investigated. A significantly large increase in both the magnitude and the nonlinearity of the Hall resistance is observed from samples whose GaAs buffer layers are grown under the condition of a low As∕Ga flux ratio. Reflection high energy electron diffraction and electron microscope observations show that a faceted surface develops with the growth and postgrowth annealing of a GaAs buffer layer under the condition of a low As flux. From samples which have only Si δ-doped layers and exhibit the n-type conduction, such nonlinear Hall resistance is not observed. The nonlinearity of the Hall resistance of Be∕Si pair δ-doped structures depends on the single parameter B∕T, where B and T are the applied magnetic field and the temperature, respectively. Based on these results, it is suggested that the nonlinear Hall resistance of Be∕Si pair δ-doped structures is the anomalous Hall effect caused by localized spins in δ-doped layers.