Investigation of exciton complexes in 6H-SiC by isotope substitution

Unintentionally doped single crystals of 6H|SiC generally exhibit at low temperature a luminescence spectrum consisting of three zero phonon lines and a series of strong phonon replicas at lower energies including lines due to a localized vibrational mode. The fact that this localized mode shows no change for crystals doped with ¹⁵N, ¹⁰B or ¹⁸O is regarded as strong evidence that this spectrum is not due to the recombination of excitons at ionized nitrogen donors as has been reported by other authors, nor to recombination at centres involving the aforementioned impurities.     

Isotopic fingerprints of the Lake Żabińskie (NE Poland) hydrological system on contemporary carbonates precipitated in the lake

An isotopic monitoring was undertaken in 2012–2014 at Lake ?abińskie (Mazurian Lakeland, NE Poland). The aim was to identify the factors and processes controlling an isotopic composition of the lake water and to explore the mechanism responsible for recording the climatic signal in stable isotope composition of deposited carbonates. δ¹⁸O and δ²H in the precipitation, lake water column, inflows and outflow, δ¹⁸O and δ¹³C in the carbonate fraction of sediments trapped in the water column were recorded with monthly resolution. A relationship between δ¹⁸O and δ²H in local precipitation was used to estimate the local meteoric water line. The dataset obtained for the water enabled to identify the modification of the water’s isotopic composition due to evaporation, connected with seasonal lake water stratification and mixing patterns. Statistically significant correlation coefficients suggest that the δ¹⁸O of the carbonate fraction in the sediment traps depends on the δ¹⁸O of rainfall water and on air temperature. The fractionation coefficient α shows that in summer months the carbonate precipitation process is closest to equilibrium. As expected for an exorheic lake, no significant correlation was observed between δ¹⁸O and δ¹³C in precipitated carbonate.     

OCO: high-resolution absorption spectrum between 4000 and 9500 cm and global fitting of vibration-rotational line positions

The absorption spectrum of a ¹³C enriched carbon dioxide sample has been recorded with a Fourier-transform spectrometer in the spectral range 4000-9500 cm⁻¹. In addition to six bands observed from the spectrum of the atmosphere of Venus in this region, eight new ¹⁶O¹³C¹⁸O bands were measured. The new observations together with the data collected from the literature have been used to fit parameters of an effective Hamiltonian for the ¹⁶O¹³C¹⁸O. More than 4000 line positions in 38 bands have been used to derive 48 parameters of effective Hamiltonian. The RMS (root-mean-square of residuals) of the fit is 0.00106 cm⁻¹.     

Comparison of one and two-neutron transfer near the coulomb barrier for the27Al(18O,16O)29Al,27Al(180,17O)28Al and27Al(13C,12C)28Al reactions

Total reaction cross sections for the transfer reactions²⁷Al(¹⁸O,¹⁶O)²⁹Al,²⁷Al(¹⁸O,¹⁷O)²⁸Al and²⁷Al(¹³C,¹²C)²⁸Al are reported for center-of-mass energies between 13 and 20 MeV for¹⁸O projectiles and between 11 and 17.5 MeV for¹³C projectiles. The reaction products,²⁹Al and²⁸Al, beta decay to²⁹Si and²⁸Si, respectively, and the subsequentγ decays of²⁹Si and²⁸Si were measured. Due to the relatively long beta decay half lives, data were taken in a beam-off mode, resulting in very clean spectra. Total cross sections were calculated and compared with a theoretical model for barrier penetration proposed by C.Y. Wong. Differences between¹⁸O induced one and two-neutron total transfer reaction cross sections are discussed.     

A 13C isotopic tracing study of a-C:H thin-film growth

The growth mechanisms of amorphous carbon (a-C:H) thin films obtained by the decomposition of methane gas in a multipolar plasma set up were studied using ¹³C as a tracer. Hydrogenated carbon films first grown in a methane plasma with natural isotopic composition (¹²C) were further grown in highly enriched ¹³C methane discharge (experiments were also carried out with the reverse order). ¹³C profiles were obtained by resonant nuclear reaction depth profiling using the narrow (FWHM<100 eV) resonance at 1747.6 keV, in the ¹³C(p, )¹⁴N nuclear reaction. Plasma conditions were varied so as to vary the energy of particles arriving at the sample surface from 30 eV to 500 eV. In all cases, a system of two layers was observed: a pure ¹³C film overlaying a pure ¹²C film (or the reverse order) with a rather sharp interface between the two layers. These results suggest layer by layer amorphous hydrogenated carbon film growth with minimal mixing or interdiffusion of the isotopic layers induced by the ionic bombardment during the growth.     

Recombination luminescence in irradiated silicon-effects of thermal annealing and lithium impurity†

Abstract

Luminescence in irradiated silicon has been used to determine the thermal stability of the defects responsible for the recombination. It was found that the defect responsible for the zero-phonon line at 0.97 eV has an annealing behavior similar to that of the divacancy and the zero-phonon line at 0.79 eV anneals in a manner similar to the G-15 or K-center. Annealing at temperatures up to 500 °C generates other defects whose luminescence is distinct from that seen previously. Addition of lithium to the material produces defects with new characteristic luminescence. Of particular importance is a defect with a level at E_v ? 1.045 eV.     

Probable observation of unusual local vibrational modes in the oscillatory photoconductivity of semi-insulating GaAs

We have observed oscillatory photoconductivity with a period of 20.9 meV in semi-insulating GaAs at T = 18–23°K. These oscillations are associated with the 0.75 eV impurity level in GaAs which has often been attributed to O. It is suggested that the oscillations are due to a cascade process involving multiple emission of a local vibrational mode, of energy 20.9 meV, associated with the 0.75 eV defect.     

HREELS detection of resonance electron scattering in oriented polytetrafluoroethylene films

Resonance electron scattering from surfaces of oriented polytetrafluoroethylene (PTFE) has been investigated using high-resolution electron energy loss spectroscopy (HREELS). Oriented samples were made by friction-deposition onto gold-coated silicon wafers. A resonance enhancement of the 730 cm⁻¹, Raman-active ν (CC) stretch vibrational mode was detected near 5 eV beam energy with width of about 2 eV. The maximum intensity occurs for parallel alignment of the scattering plane with respect to the polymer chain, while no resonance is detected for perpendicular alignment. Studies were made for both specular and off-specular geometries. The origin of the resonance can be attributed to negative ion trapping into the (CC) σ¹ orbital. We have also obtained high resolutions of about 40 cm⁻¹ for the first time in polymer systems.     

Comparison of cross sections for C + O reactions in the second regime of complete fusion

Kinetic energy spectra, angular distributions, and elemental yield distributions have been measured for the ¹² C + ¹⁶ O, ¹² C + ¹⁸ O and ¹³ C + ¹⁷ O reaction products over an energy range from 2 to 7 times the Coulomb barrier energy. A careful kinematic analysis of the evaporation residues and comparisons with statistical-model calculations show that fusion proceeds with full momentum transfer followed by a statistical decay of the compound nucleus. The competition between complete fusion process and peripheral reactions in the ¹² C + ¹⁶ O system is less important than for the ¹² C + ¹⁸ O and ¹³ C + ¹⁷ O reactions. The unexpectedly high ¹² C + ¹⁶ O complete fusion cross sections are related to the possible occurrence of a superdeformation of the ²⁸ Si compound nucleus.     

The neutral vacancy in silicon and diamond: Generalized valence bond studies

Self-consistent ab initio generalized valence bond and configuration interaction calculations are presented for molecular cluster models of the electronic states of the neutral vacancy in diamond and silicon. In each case the ground state of the vacancy is found to be ¹E (T_d designation) with the ³T₁ and ⁵A₂ states slightly higher (0.18 eV and 0.60 eV for Si and 0.33 eV and 1.2 eV for C). The first excited singlest state is ¹T₂ in each case. The ¹E-¹T₂ excitation energy is foundto be 1.7 ± 0.7 eV for diamond and 1.3 ± 0.5 eV in silicon, the range being due to uncertainty in the model used for estimating the polarization effects due to the remainder of the infinite crystal.     

Compound transfer and direct transfer reactions induced by 17O on 13C

The elastic scattering, inelastic scattering, single-neutron transfer reactions ¹³C(¹⁷O, ¹⁶O) ¹⁴C, ¹³C(¹⁷O, ¹⁸O)¹²C and ¹³C(¹⁷O, ¹⁸O_{2+, 1.98})¹²C, and seven other exit channels which involve ⁷Li, ⁹Be, ¹¹B and ¹⁵N have been measured for the system ¹⁷O+¹³C at 12.9 and 14 MeV c.m. It is shown that all reactions mentioned above have significant contributions from compound nuclear decay, following fusion of projectile and target.     

Characteristic energy losses on carbon contaminated layers correlated with secondary electron,auger electron emission and contrasts in scanning microscope

The secondary electron (SE) spectrum (0 < E < 50 eV) has been analysed by means of a CMA. Samples were clean aluminum, aluminum becoming carbon contaminated, sintered graphite powder, electro chemically deposited polymer on platinum and monocrystals of silicon carbon contaminated. When the clean Al surface is becoming carbon contaminated a quick decrease of surface plasmon and bulk plasmon losses is observed whereas a main characteristic energy loss peak (ELS) at 20 eV and a secondary electron peak at 20 eV appear simultaneously. Both peaks are very sensitive general features of carbon contaminated surfaces. The main loss peak is attributed to the excitation of the carbon-carbon bounds (σ → σ¹) as already proposed in the transmission ELS. The few eV change of the loss peak energy of various carbon compounds may correspond to slightly different carbon-carbon distances. The 20 eV secondary electrons could be produced by the relaxation of the excited state (σ¹ → σ transition) via an Auger process. The cross section for molecular electronic excitation is higher than that of atomic ionization for inner level. The loss peak is as intense as the SE peak and higher by more than two orders of magnitude than the C KLL Auger peak. The modification of secondary emission under carbon contamination has been observed on a silicon sample by Scanning Electron Microscopy (SEM) in the Secondary Electron Image (SEI) mode.     

Determination of deep electron traps in GaAs by time-resolved capacitance measurement

A useful technique of determining the energy levels and the spatial density distributions of multiple electron traps in semi-conductor has been developed using the time-resolved measurement of the Schottky barrier junction capacitance, and this technique has been applied to characterize the electron traps inn-GaAs. In the present technique, the energy levels are determined from single scan of temperature, and the density distributions are calculated from a set of capacitance-voltage relationships. Four traps which lay at 0.39, 0.73, 0.79, and 0.58 eV below the conduction band edge were observed in boat grown or vapor phase epitaxially grown crystals. Many layers which were obtained by a vapor phase epitaxial growth system with N₂ carrier gas were measured and it was found that almost all of them include the 0.73 eV and the 0.79 eV trap with the density between 1×10¹³ and 2×10¹⁵ cm⁻³.     

Si在水汽中氧化传质机制的H218O/H216O同位素示踪研究

采用同位素H₂¹⁶O/H₂¹⁸O接续氧化同位素示踪方法,研究了单晶硅在1100 ℃水汽中氧化的微观传质机制.在H₂¹⁶O,H₂¹⁸O分别氧化和H₂¹⁶O/H₂¹⁸O接续氧化处理后,研究氧化产物形态和结构.并用二次离子质谱仪(SIMS)研究了同位素 关键词： 同位素示踪 2¹⁸O')" href="#">H₂¹⁸O 替位扩散 硅     

Phase shift analysis and extraction of structure functions for Coulomb distortion on (e, e'p) reactions at high electron energies

New results for the double beta decay of ⁷⁶ Ge are presented. They are extracted from data obtained with the HEIDELBERG-MOSCOW experiment, which operates five enriched ⁷⁶ Ge detectors in an extreme low-level environment in the Gran Sasso underground laboratory. The two-neutrino-accompanied double beta decay is evaluated for the first time for all five detectors with a statistical significance of 47.7 kg y resulting in a half-life of T _1/2 ^2ν = [1.55±0.01(stat)^+0.19 _-0.15(syst)]×10²¹ y. The lower limit on the half-life of the 0νββ decay obtained with pulse shape analysis is T _1/2 ^0ν > 1.9×10²⁵(3.1×10²⁵) y with 90% C.L. (68% C.L.) (with 35.5 kg y). This results in an upper limit of the effective Majorana-neutrino mass of 0.35 eV (0.27 eV) using the matrix elements of A. Staudt et al.'s work (Europhys. Lett. 13, 31 (1990)). This is the most stringent limit at present from double beta decay. No evidence for a majoron-emitting decay mode is observed. Received: 22 August 2001 / Accepted: 18 October 2001     

Postnatal development of urea synthesis in VLBW-infants as estimated by 15N-ammonium chloride urine test: Differences between AGA- and SGA-Infants

A prototype off-axis integrated cavity output spectrometer (OA-ICOS) utilizing two identical cavities together with a near-infrared (1.63 μm) external cavity tunable diode laser is described. The two-cavity design—one for a reference gas and one for a sample gas—takes advantage of classical double-beam infrared spectrometer characteristics in reducing uncertainties due to laser scan or power instabilities and major temperature variations by a factor of three or better compared with a single-cavity scheme. This is the first OA-ICOS instrument designed to determine ¹³C/¹²C and ¹⁸O/¹⁶O ratios from CO₂ rotation/vibration fine structure in three different combination bands. Preliminary results indicate that at 0.8 Hz a precision of 3.3 and 2.8 \permil\ is obtained for δ¹³C and δ¹⁸O, respectively, over a period of 10 h and a pure CO₂ gas sample at 26 hPa. By averaging 100 spectra over a subset of the data, we achieved a precision of 1.6 and 0.8 \permil\ for δ¹³C and δ¹⁸O, respectively.     

Local Vibrations of Oxygen-Containing Thermal Donors in Germanium

The vibrational absorption bands associated with some types of double thermal donors (TD) in Ge enriched with the oxygen isotopes ¹⁶O and ¹⁸O have been identified. The thermal donors were formed during heat treatment of Ge:O crystals at 300 and 350°C. Absorption spectra were measured at room temperature and at 10 K. The formation of the thermal donors was accompanied by the appearance of three absorption bands, which in the Ge:¹⁶O spectra at room temperature are located at 600, 740, and 780 cm^–1. In low-temperature measurements, the bands at 600 and 780 cm^–1 exhibited splitting into series of narrow lines (up to 9) associated with some types of thermal donors (TD1–TD9). The absorption spectra measured at 10 K after different cooling conditions display bistability of the first four types of thermal donors (TD1–TD4). In the samples cooled by illumination with light in the region of frequencies of the fundamental absorption of Ge, pairs of lines are observed that belong to the bistable thermal donors in the helium-like configuration of double donors (DD). After cooling the samples in the dark, these pairs of lines are replaced by three new bands, which belong to local vibrational modes of bistable thermal donors in the low-energy neutral configuration. Based on the isotopic shift of the local vibrational modes of the thermal donors in Ge:¹⁶O and Ge:¹⁸O, a rigorous proof of the oxygen atoms entering into the composition of thermal donors is obtained.     

Stable isotopic and geochemical variability within shallow groundwater beneath a hardwood hammock and surface water in an adjoining slough (Everglades National Park,Florida, USA)

Data from a 10-month monitoring study during 2007 in the Everglades ecosystem provide insight into the variation of δ¹⁸O, δD, and ion chemistry in surface water and shallow groundwater. Surface waters are sensitive to dilution from rainfall and input from external sources. Shallow groundwater, on the other hand, remains geochemically stable during the year. Surface water input from canals derived from draining agricultural areas to the north and east of the Everglades is evident in the ion data. δ¹⁸O and δD values in shallow groundwater remain near the mean of?2.4 and?12 ‰, respectively. ¹⁸O and D values are enriched in surface water compared with shallow groundwater and fluctuate in sync with those measured in rainfall. The local meteoric water line (LMWL) for precipitation is in close agreement with the global meteoric water line; however, the local evaporation line (LEL) for surface water and shallow groundwater is δ D=5.6 δ¹⁸O+1.5, a sign that these waters have experienced evaporation. The intercept of the LMWL and LEL indicates that the primary recharge to the Everglades is tropical cyclones or fronts. δ deuterium to δ¹⁸O excess (D_ex values) generally reveal two moisture sources for precipitation, a maritime source during the fall and winter (D _ex>10 ‰) and a continental-influenced source (D _ex<10 ‰) in the spring and summer.     

Studies of the electronic and vibrational signatures of the unusual bonding geometries in melt-quenched amorphous silicon

Tight-binding molecular dynamics simulations have been performed to investigate the effect of quenching rate of the Si melt on the resulting local structure of amorphous silicon. Different quenching rates were used to cool liquid silicon in the simulations to demonstrate that the choice of quenching rates significantly influences the resulting local structure. The calculated pair correlation functions show that the local structure is sensitive to the thermal processing of the liquid silicon melt. The use of cooling rates higher than 10^?13 K s^?1 appears to prevent the activation of the required structural re-arrangements necessary to stabilise the networks, causing unexpected bonding geometries to develop. The electronic signatures of the defects show that only the triangular defect structure contributes resonance states to the conduction band tail. Also, the vibrational signature of the triangular structure shows a high energy transverse optical mode at 95 meV, indicating that the defect is likely to be unstable at 300 K, although both defects contribute minimal states to the mid-gap level.     

On the potential of Raman‐spectroscopy‐based carbonate mass spectrometry

The potential for using Raman spectroscopy to measure stable oxygen isotope ratios (¹⁸O/¹⁶O) in carbonates is evaluated by measuring the Raman spectra and isotope ratios of a suite of 60 synthesized, ¹⁸O‐enriched calcite crystals ranging in composition from natural abundance (0.2 mole‐% ¹⁸O) to 1.2 mole‐% ¹⁸O. We determined the Raman‐inferred isotopic ratios (R_Raman) by fitting curves to the ν₁ symmetric stretching peak at 1086 cm⁻¹ and the smaller satellite peak, associated with the ν₁ stretching mode of singly substituted carbonate groups (C¹⁶O₂¹⁸O) at 1065 cm⁻¹. The ratio of the two peak areas shows a 1:1 correspondence with the ¹⁸O/¹⁶O ratios derived from standard mass spectrometry methods, confirming that the relative intensities of the ν₁ symmetric stretching peaks is a direct measure of the isotopic ratio in the carbonates. The 1‐sigma uncertainties of the R_Raman values of the individual crystals were 0.00079 (384‰ PDB) and 0.00043 (210‰ PDB) for the four‐crystal sample means. This level of uncertainty is much too high to provide significant estimates of natural variability; however, there are multiple prospects for improving the accuracy and precision of the technique. Carbon isotope ratios in carbonates cannot be measured by our approach, but our results highlight the potential of Raman‐based isotope ratio measurement for C and other elements in minerals and organic compounds. Copyright © 2012 John Wiley & Sons, Ltd.