Magnetic effects in the oxidation of silicon

The mass spectrometry study has indicated that the magnetic field accelerates the oxidation of the surface of silicon crystals. The oxidation rate also depends on the nuclear spin of silicon: the oxidation rate of atoms with magnetic nuclei (²⁹Si) is almost twice as high as that of atoms with spinless, unmagnetized nuclei (²⁸Si and ³⁰Si). Both effects—magnetic field and magnetic isotope—reliably prove that the oxidation of silicon is a spin-selective reaction involving radicals and radical pairs as intermediate paramagnetic particles. A spin-selective magnetic sensitive oxidation mechanism is discussed.     

Nuclear quadrupole-quadrupole interaction in the inelastic scattering of aligned deuterons from deformed nuclei

The 2 ₁ ⁺ -excitation of deformed nuclei by tensor polarized deuterons provides an alignment of both nuclei and thus a means to study specifically the quadrupole-quadrupole interaction between both nuclei. The tensor analyzing powerA _xz (θ) has been measured for the elastic and inelastic scattering on²⁴Mg and²⁸Si. The coupled channel analysis including a deformed tensor potential reveals a clear signature of the quadrupole-quadrupole part of the nuclear projectile-target interaction.     

99Ru,61Ni,57Fe,and119Sn Mössbauer studies of Heusler alloys containing ruthenium

⁹⁹Ru,⁶¹Ni,⁵⁷Fe and¹¹⁹Sn Mössbauer spectroscopic studies were made on ternary intermetallic compounds containing ruthenium, Ru_xY_3?xZ (Y=Fe, Ni; Z=Si, Sn). In the system of Ru_xFe_3?xSi, two different hyperfine magnetic fields were observed at the⁹⁹Ru nuclei (H _hf[Ru]) in the range ofx≤1.0 and the magnitude of eachH _hf[Ru] was found to decrease with an increase in the ruthenium concentrationx. Both the⁹⁹Ru and¹¹⁹Sn Mössbauer spectra of Ru₂FeSn could be analyzed with two sets of magnetically split lines. The⁶¹Ni Mössbauer spectra of Ru₂NiSn were obtained at 5 and 77 K.     

Single crystals of mesoporous tungstenosilicate W-MCM-48 molecular sieves for the conversion of methylcyclopentane (MCP)

Highly ordered W-MCM-48 mesoporous materials containing isolated W atoms in tetrahedral framework positions were successfully synthesized following the S⁺I⁻ pathway, up to a Si/W of 40. When tungsten content was increased up to a Si/W of 20, the ordered cubic structure was only partially maintained, and for a Si/W of 10 an amorphous phase was obtained. Highly isolated tetrahedral framework tungsten atoms in the W-MCM-48 with a Si/W of 40, have been identified by UV-vis band at 225 nm, IR-TF band at 970 cm⁻¹ and XRD. The W 4f XPS results suggest that the tungsten atoms exist in two oxidation states, W⁴⁺ and W⁵⁺. The morphology of the samples varies as a function of tungsten content. The W-MCM-48 samples with a Si/W ratio of 40 existed as crystals with a unique crystalline morphology consisting of cubes truncated rhombic dodecahedrons belonging to the cubic Ia3d space group, while the samples with a Si/W ratio of 20 exhibited a different morphology consisting of spheres and cubes truncated by rhombic dodecahedrons. A comparison of samples with Si/W of ∞, Si/W of 40 and Si/W of 20 was performed using the conversion of MCP carried out at 450 °C under H₂.     

Statistical binary decay of35Cl+24Mg at ≈ 8 MeV/nucleon

High spin rotational bands in ¹⁶² Lu and ¹⁶⁴ Lu have been studied through the ¹³⁹ La( ²⁸ Si,5n) and ¹³⁹ La( ²⁹⁽³⁰⁾ Si,4(5)n) reactions respectively. For both nuclei the yrast sequence which is associated with the πh _11/2 ?νi _13/2 configuration shows the signature inversion feature.     

Study of hyperfine interactions in the intermetallic compound CePd2Si2 using PAC technique with 111Cd as probe nuclei

Perturbed γ???γ angular correlation spectroscopy (PAC) has been used to investigate the hyperfine interactions in the intermetallic compound CePd₂Si₂ using ¹¹¹In→¹¹¹Cd probe nuclei. Samples of CePd₂Si₂ were prepared by melting constituent elements in an arc furnace under pure argon atmosphere. Carrier-free ¹¹¹In nuclei were introduced into the samples by thermal diffusion at 800°C in vacuum during 12 h. The measurements were performed in the temperature range of 4.2–300 K. Above the magnetic transition temperature (T _N ?=?10 K), the results show two distinct and well defined quadrupole interactions that were assigned to probe nuclei occupying Ce and Si sites in the compound. The quadrupole frequencies were found to decrease linearly with increasing temperature. The PAC spectra taken below 10 K were analyzed with a model including combined electric quadrupole plus magnetic dipole interactions, from which the hyperfine magnetic field was determined.     

The CuPL defect and the Cus1Cui3 complex

The Cu_PL defect is characterized by intense photoluminescence (PL) emission with a 1014 meV zero-phonon line and has since long been called the ‘copper-pair’ defect. However, recent PL studies in isotopically pure ²⁸Si samples show clear evidence for four Cu atoms in this defect. This defect is one of a family of complexes that contain four (sometimes five) metal impurities. No complex containing two or three metal impurities (at least one of which is a transition metal) has been reported. We have performed systematic calculations of a priori possible Cu₄ complexes, including the Cu_s1Cu_i3 complex proposed by Shirai et al. [18]. This complex has the lowest formation energy of all the Cu₄ defects. We have studied its properties using first-principle theory. In addition to the structures, binding energies, and vibrational spectra, we studied the formation pathway in an attempt to figure out why smaller (or larger) complexes do not form. We find that Cu_s1Cu_in with n=0, 1 or 2 continue to trap Cu_i⁺ if the Fermi level is midgap, and that the process stops when n=3. © 2011 Elsevier Science. All rights reserved     

Stress development and impurity segregation during oxidation of the Si(1 0 0) surface

We have studied the segregation of P and B impurities during oxidation of the Si(1 0 0) surface by means of combined static and dynamical first-principles simulations based on density functional theory. In the bare surface, dopants segregate to chemically stable surface sites or to locally compressed subsurface sites. Surface oxidation is accompanied by development of tensile surface stress up to 2.9 Nm⁻¹ at a coverage of 1.5 monolayers of oxygen and by formation of oxidised Si species with charges increasing approximately linearly with the number of neighbouring oxygen atoms. Substitutional P and B defects are energetically unstable within the native oxide layer, and are preferentially located at or beneath the Si/SiO_x interface. Consistently, first-principles molecular dynamics simulations of native oxide formation on doped surfaces reveal that dopants avoid the formation of P-O and B-O bonds, suggesting a surface oxidation mechanism whereby impurities remain trapped at the Si/SiO_x interface. This seems to preclude a direct influence of impurities on the surface electrostatics and, hence, on the interactions with an external environment.     

New results from GASP in the A=130 mass region

The GASP array has been used to study the high spin structures of the ¹³⁴Nd and ¹³⁶Nd nuclei populated in the ¹¹⁰Pd(²⁸Si,4n) and ¹¹⁰Pd(³⁰Si,4n) reactions at 130 MeV and 125 MeV, respectively. Several deexcitation pathways have been established for the yrast and excited highly deformed (HD) bands in 134Nd. The decay out process in this nucleus has been understood in terms of mixing between the normal deformed and HD states, which is triggered by the crossing between the vi _13/2 and vd _5/2 orbitals. The study of the high spin excitations in ¹³⁴Nd and ¹³⁶Nd revealed two different mechanisms for generating high angular momentum: the stepwise occupation of the N = 6 i _13/2 intruders in the more rigid nucleus ¹³⁴Nd and the alignment of successive pairs of h _11/2 protons and neutrons coupled to a stable small-deformation triaxial core in the γ-soft nucleus ¹³⁶Nd.     

The (p,nγ) reaction on Al and Si

The reactions ²⁷Al(p, n γ) ²⁷Si and ²⁸Si(p, n γ)²⁸P have been studied at E_p = 16 and 23 MeV, respectively, with a Ge(Li) detector in coincidence with a neutron detector. In ²⁷Si, two new γ-tran- sitions were detected and accurate excitation energies were determined. In ²⁸P, the γ-decay scheme was studied for the first time and new levels were located at 879 and 1602 keV excitation energy. For both nuclei the γ-ray spectra were supplemented by neutron time-of-flight spectra. The excitation energies are compared with those of the analogue nuclei and with the predictions from Coulomb displacement calculations.     

Practical separation of silicon isotopes by IRMPD of Si2F6

Large-scale silicon isotope separation based on the IRMPD of natural Si₂F₆ has been carried out using a commercially available high power CO₂ TEA laser and a flow reaction system. The decomposition product SiF₄ containing 19–33% of ³⁰Si was obtained at a production rate of 1.5×10^–2–2.6×10^–2 mol·h^–1, depending on experimental parameters such as laser wavelength, laser fluence, pressure, and flow rate. SiF₄ containing 12% of ²⁹Si was obtained under slightly different conditions, i.e., at a shorter wavelength than that for ³⁰Si. When 39% of Si₂F₆ was decomposed at a slow flow rate, residual Si₂F₆ was found to have 99.7% of ²⁸Si. The production rate was 4.2×10^–2 mol·h^–1.     

Vanadium phosphates on mesoporous supports: model catalysts for solid-state NMR studies of the selective oxidation of n-butane

SBA-15 was utilized as mesoporous support for the dispersion of vanadium phosphate (VPO) compounds. Loading of SBA-15 with VPO compounds was found to be accompanied by decreasing ²⁹Si MAS NMR signals of Q² (Si(2Si,2OH)) and Q³ (Si(3Si,1OH)) silicon species, which indicates coverage of the mesoporous support by the guest compounds. The ⁵¹V MAS NNR spectra of the activated VPO/SBA-15 catalysts consist of patterns typical for the α_II- and β-phases of vanadyl orthophosphate. In the ³¹P MAS NMR spectra of the activated VPO/SBA-15 catalysts, signals of β-, δ-, and α_II-VOPO₄ phases could be identified. Upon conversion of n-butane-¹³C₄, a strong decrease of the ³¹P MAS NMR signals characteristic for the δ-VOPO₄ phase occurred, while by ¹³C MAS NMR spectroscopy the formation of maleic anhydride, carbon monoxide, and carbon dioxide was observed. This finding supports the active role of the δ-VOPO₄ phase in the selective oxidation of n-butane on VPO/SBA-15 catalysts.     

ESR observation of phosphorus pile-up at the Si/SiO2 interface in boron-doped Si

It is shown that dry thermal oxidation of B-doped (p-type) CZ-grown Si wafers may create P-rich near-interfacial Si layers. Thermal treatments were mostly carried out at T_ox=950±20°C for times 3 ≲ t_ox ≲ 7 h in either a 0.11 MPa high-purity O₂ ambient or strongly-reduced O₂ pressure (p_o2 ≲ 13 mPa). The existence of such layers was discovered by sensitive low T ESR observations, revealing the typical P³¹-in-(n-type) Si spectrum. These measurements in combination with selective etching experiments and number-of-spins determinations showed these surface layers to be actually semiconductor-type reversed. It is argued that the P atoms collected at the Si/SiO₂ interface during thermal treatments are inherently present as bulk impurities in the as-received Si.     

Modifications of He implantation induced cavities in silicon by MeV silicon implantation

Doppler broadening spectroscopy (DBS) coupled to a slow positron beam has been used to investigate the formation of He-cavities in the presence of high vacancy concentrations in Cz-Si (1 1 1). Si samples were first implanted with MeV Si ions in order to create a damaged Si layer. DBS measurements show the presence of divacancy (SV₂/SSi lattice=1.052,WV₂/WSi lattice=0.83) from the surface up to 4.2 μm depth with a concentration higher than 10¹⁸ cm⁻³. The thickness of this damaged layer was confirmed by spreading resistance measurements. In the second step, samples were implanted with 50 keV ³He with fluence of 10¹⁶ cm⁻². DBS results show that the apparent divancancy concentration decreases at ³He implantation depth ∼435 nm due to ³He passivation of vacancies that occurs during the implantation process. After 900 °C annealing, large defects are detected at depth up to 2 μm and (S, W) values suggest the detection of cavities at the implantation depth. We also report the possible presence of impurity complexes. The formation of these complexes is attributed to the gettering of metallic impurities present in the Si sample.     

Effect of rapid thermal oxidation on structure and photoelectronic properties of silicon oxide in monocrystalline silicon solar cells

This paper concerns the topic of surface passivation properties of rapid thermal oxidation on p-type monocrystalline silicon wafer for use in screen-printed silicon solar cells. It shows that inline thermal oxidation is a very promising alternative to the use of conventional batch type quartz tube furnaces for the surface passivation of industrial phosphorus-diffused emitters. Five minutes was the most favorable holding time for the rapid thermal oxidation growth of the solar cell sample, in which the average carrier lifetime was increased 19.4 μs. The Fourier transform infrared spectrum of the rapid thermal oxidation sample, whose structure was Al/Al-BSF/p-type Si/n-type SiP/SiO₂/SiN_x/Ag solar cell with an active area of 15.6 cm², contained an absorption peak at 1085 cm⁻¹, which was associated with the Si–O bonds in silicon oxide. The lowest average reflectance of this sample is 0.87%. Furthermore, for this sample, its average of internal quantum efficiency and conversion efficiency are respectively increased by 8% and 0.23%, compared with the sample without rapid thermal oxidation processing.     

Phase transition and relaxation processes by 87Rb and 39K nuclear magnetic measurements of RbKSO4 single crystals

⁸⁷Rb and ³⁹K nuclear magnetic resonance (NMR) spectra of RbKSO₄ single crystals were measured at room temperature. ⁸⁷Rb central line has the angular dependences of second-order quadrupolar shifts. From these results, the quadrupole coupling constant and the asymmetry parameter were determined at room temperature. In addition, the spin–lattice relaxation rate, 1/T₁, and the spin–spin relaxation rate, 1/T₂, were measured as a function of temperature. The values of 1/T₁ for the ⁸⁷Rb and ³⁹K nuclei were found to increase with increasing temperature, and 1/T₁ was determined to be proportional to Tⁿ. Therefore, for the ⁸⁷Rb and ³⁹K nuclei, Raman processes with n=2 are more significantly in nuclear quadrupole relaxation than direct processes.     

Reaction cross sections in Si of light proton-halo candidates 12N and 17Ne

Total reaction cross sections, σ_R, on Si were measured near 40 A MeV for the proton-halo candidate ¹²N and the two-proton-halo candidate ¹⁷Ne, and were compared with σ_R for other light proton-rich nuclei. The A-dependence shows enhanced σ_R's for ¹²N and ¹⁷Ne, relative to their neighbors, but the effect is smaller than for ⁸B which has been argued to have a proton halo. In general, nuclei with loosely bound last protons (S_p ⩽ 1.5 MeV) have significantly larger σ_R's than their neighbors. Cross sections for charge-removal from ¹²N and ¹⁷Ne also were obtained.     

Synthesis and characterization of conducting polypyrrole-containing iron complexes

Electrochemical or chemical oxidation of pyrrole-containing complex anions of iron cyanide or iron chloride results in the formation of films or powders of conducting polypyrroles. Freshly prepared films exhibit an additional IR band at ca 1630–1640 cm⁻¹, slowly disappearing in air and not observed in previously studies polypyrrole-based systems. It is possible that this new band is associated with the existence of a CN bond in dehydrogenated pyrrole rings which are transformed into regular pyrrole rings, probably due to the protonation reaction occurring in air and simultaneous bond rearrangement. The polypyrrole structure favours the presence of Fe(CN)₆⁴⁻ over Fe(CN)₆³⁻ since the former is the only iron species detected by Mössbauer spectroscopy in electrochemically prepared samples. It is also the dominant iron species in the samples oxidized chemically. The polypyrrole-containing Fe(CN)₆⁴⁻ is more ordered than those containing monovalent anions, as evidence by X-ray diffraction studies. High-spin eron complexes can be inserted into polypyrrole during electrochemical oxidation of pyrrole in non-aqueous solutions containing LiCl/FeCl₃. The inserted species exhibit Mössbauer parameters characteristic of slightly distorted FeCl₄⁻.     

An isotopic labeling study of the diffusion mechanism during oxidation of Si(100) in water vapor by successive oxidation

The diffusion mechanism during the wet oxidation of Si(100) at 1373 K was investigated by successive oxidations finally containing isotopic water. SiO₂ was first thermally grown on Si in non-labeled oxidizing ambient (dry O₂ or H₂O) followed by isotopic water (H₂¹⁸O) to trace ¹⁸O species in SiO₂. The distributions of ¹⁶O and ¹⁸O in the oxide film were analyzed by means of secondary ion mass spectroscopy (SIMS). SIMS depth profiles show that there was a wide overlap of both isotopes (¹⁸O and ¹⁶O) throughout the SiO₂ layer, no matter whether the first oxidation step was carried out in dry O₂ or H₂O, and the concentration gradient of ¹⁸O decreased with increasing oxidation time at the second oxidation step by H₂¹⁸O. The results suggest that the diffusion mechanism in SiO₂ during water vapor oxidation is exchange diffusion; H₂O related oxidizing species diffuse through the network with significant exchange with the pre-existing oxygen in it.     

Polarization in the quasielastic scattering of 1-GeV/c protons on light nuclei

Polarization measurements in the A(p, 2p)B reactions on ⁶Li, ⁷Li, and ²⁸Si nuclei at a proton-beam energy of 1 GeV were performed in a kinematically complete experiment. By using a two-arm magnetic spectrometer, two secondary protons were recorded in coincidence at asymmetric scattering angles of θ₁=15°?26° and θ₂=58.6° for residual-nucleus momenta in the range K _B=0–150 MeV/c. Either arm of the spectrometer was equipped with polarimeters based on proportional chambers. The data coming from this experiment are analyzed within the distorted-wave impulse approximation. It is shown that the polarization of recoil protons formed at angle θ₂ in the interaction featuring a proton from the P shell of the ⁷Li nucleus can be described under the assumption of an effective intranuclear-proton polarization by using the single-particle shell-model wave function of the nucleus. Our data on the polarizations of the two protons from the reaction (p, 2p) on a ²⁸Si nucleus also suggest the effective polarization of the protons in the D shell of the ²⁸Si nucleus. It is found that, for high recoil-nucleus momenta of K _B≥90 MeV/c, the effective polarization of the protons in the P shell of the ⁶Li nucleus—this polarization was discovered in studying the polarization of recoil protons in the reaction ⁶Li(p, 2p)⁵He—cannot be described within the shell model assuming LS coupling. As might have been expected, the polarization of recoil protons knocked out from the S shells of the ⁶Li and ⁷Li nuclei comply well with the predictions obtained in the impulse approximation with allowance for the depolarization effect alone.