Si Emission Research Articles

ChandraACIS andXMM‐NewtonEPIC Observations of the X‐Ray–Luminous SN 1978K in NGC 1313

We describe an observation of the X-ray-luminous SN 1978K in NGC 1313 using the ACIS detector on board Chandra and an archival XMM-Newton EPIC observation. The models that provided good fits to the ASCA SIS and GIS and the ROSAT PSPC spectra no longer do so for the ACIS and EPIC spectra. The best-fit models to the ACIS and EPIC spectra are dual hot plasma models (VMEKAL); one component is soft (T = 0.61 keV, 90% errors), and the other is harder (T = 3.16 keV). For the varying abundances permitted within the model, only the Si abundance of the soft component differs from solar, with a value n/n = 3.20 (90% errors). From a ratio of the low- and high-T model fits to the Chandra and XMM-Newton spectra, we infer an exponent n of the ejecta density distribution, ρejecta ∝ r-n, of ~5.2, adopting a circumstellar matter distribution exponent of s = 2 (ρcs ∝ r-s). The 0.5-2 keV light curve shows essentially no decline; the 2-10 keV light curve, comprised of only the ASCA, XMM-Newton, and Chandra observations, shows a drop of 1.5 from the ASCA epoch. The hard-band decline, together with the apparently enhanced Si emission, signal the start of the X-ray decline of SN 1978K.

Mid- to far-infrared spectroscopy of Sharpless 171

We have collected one-dimensional raster-scan observations of the active star-forming region Sharpless 171 (S171), a typical H  region-molecular cloud complex, with the three spectrometers (LWS, SWS, and PHT-S) on board ISO. We have detected 8 far-infrared fine-structure lines, (O  )5 2µm, (N  )5 7µm, (O  )6 3µm, (O  )8 8µm, (N ) 122 µm, (O ) 146 µm, (C ) 158 µm, and (Si  )3 5µm together with the far-infrared continuum and the H2 pure rotation transition (J = 5-3) line at 9.66 µm. The physical properties of each of the three phases detected, highly-ionized, lowly-ionized and neutral, are investi- gated through the far-infrared line and continuum emission. Toward the molecular region, strong (O ) 146 µm emission was observed and the (O  )6 3µm to 146 µm line ratio was found to be too small (∼5) compared to the values predicted by current photodissociation region (PDR) models. We examine possible mechanisms to account for the small line ratio and conclude that the absorption of the (O  )6 3µ ma nd the (C) 158 µm emission by overlapping PDRs along the line of sight can account for the observations and that the (O ) 146 µm emission is the best diagnostic line for PDRs. We propose a method to estimate the effect of overlapping clouds using the far-infrared continuum intensity and derive the physical properties of the PDR. The (Si  )3 5µm emission is quite strong at almost all the observed positions. The correlation with (N ) 122 µm suggests that the (Si ) emission originates mostly from the ionized gas. The (Si  )3 5µ mt o (N) 122 µm ratio indicates that silicon of 30% of the solar abundance must be in the diffuse ionized gas, suggesting that efficient dust destruction is undergoing in the ionized region.

Conditional data processing for single-shot spectral analysis by use of laser-induced breakdown spectroscopy.

Schemes of conditional data processing are evaluated based on either the peak-to-base ratio or the signal-to-noise ratio as a metric for analyte detection in single-shot laser-induced breakdown spectra. The analyte signal investigated is the 288.1-nm Si I emission line provided by an aerosol stream of monodisperse 2.5-microm-sized silica microspheres. Both the Si emission line and a spectral region corresponding to continuum emission are used to evaluate the statistical distribution of spectral noise. The probability of false hits is determined by evaluating various conditional processing thresholds. As the detection threshold increases, the rate of detected silica particle hits decreases along with the expected fraction of false-particle hits (i.e., spectral noise). For all threshold values the signal-to-noise ratio is found to provide a more robust metric for single-shot analyte detection compared with the peak-to-base ratio.

Specific heat capacity and hemispherical total emissivity of liquid Si measured in electrostatic levitation

Containerless solidification of Si was carried out via electrostatic levitation. Typical undercooling up to 370 K, which is 0.22 of its melting temperature, was obtained. A t(T) function in a time–temperature profile was analytically determined from radiative cooling curves measured during containerless solidification. With the analytical t(T) function, the ratio of constant pressure specific heat capacity to hemispherical total emissivity cP(T)/εT(T) was estimated. cP(T) and εT(T) were further derived to describe the temperature-dependent cooling behavior of liquid Si during containerless solidification in electrostatic levitation, which can be expressed as cPdrv(T)=3R+4.8×10−4⋅T+4.157×105⋅T−2−1.002×10−7⋅T2 (J/mol K) and εTdrv(T)=0.267−5.615×10−5⋅T+9.133×10−9⋅T2, respectively.

On the origin of the 417 and 437 nm double-peak emission in porous Si with crystallite sizes smaller than 3.0 nm

Exploration of strong blue emission from Si nanostructures has been a subject of enormous experimental studies since the discovery of porous Si in 1990. It is revealed in this work that a strong blue double-peak (417 and 437 nm) photoluminescence (PL) can be obtained from fresh and thermally annealed porous Si with crystallite sizes smaller than 3.0 nm, when excited at the 370 nm line of a Xe lamp. The blue double-peak PL is stable in air and shows no crystallite size dependence. Its intensity varies with thermal annealing time, but its peak position remains unchanged. Spectral analyses suggest that the double-peak PL is closely related to the existence of small Si nanocrystallites, but cannot be explained by the quantum confinement effect. A theoretical calculation about electronic states caused by the vacancy defects in the gap of small Si balls is in good agreement with our experiments.

A novel approach for the growth of μc-Si at a high rate over 3 nm/s

High-rate growth of microcrystalline-silicon (μc-Si) using very-high-frequency plasma-enhanced chemical vapor deposition in combination with a kind of triode method was investigated. In this method, a mesh electrode is placed near substrates, and discharge spots are generated on the mesh and moving around. It was found that the moving spots are generated under limited conditions of deposition pressure and flow rates of silane and hydrogen, and they are required to obtain high-rate and high-quality materials. Under such conditions, optical emission of Si* significantly increases near the mesh and it increases more by increasing hydrogen-dilution-ratio. It is assumed that the spots are generated by hollow-cathode-like discharges in the cavities of charged mesh, and such discharge promotes the decomposition of silane to realize high-rate growth of μc-Si.

Optical properties of structures with ultradense arrays of Ge QDs in an Si matrix

The structural and optical properties of ultrathin Ge insertions in an Si matrix were studied. Transmission electron microscopy revealed the spontaneous formation of arrays of disk-shaped quantum dots (QDs) with a small lateral size (3–10 nm) at a nominal Ge insertion thicknesses, from submonolayer to nearly critical, for the transition to 3D growth by the Stranski-Krastanow mechanism. Optical study revealed type-I band alignment in these structures, which results from the strong contribution of the electron-hole Coulomb interaction overpowering the repulsion potential for an electron existing in the Ge conduction band. The small lateral size of QDs lifts the selection rule prohibiting indirect recombination in the inverse k space. At the same time, the high surface density of QDs (1012–1013 cm−2) and the possibility of their stacking with the use of ultrathin Si spacers allows the obtainment of an ultrahigh volume density of QDs (up to 1019 cm−3), which is necessary to achieve stimulated emission in Si. A sample with stacked QDs formed by 0.7-nm-thick Ge insertions exhibited a superlinear increase of the photoluminescence (PL) intensity, accompanied by narrowing of the PL line. The doping of Ge-Si structures with donors allows for a drastic increase in the PL intensity at high temperatures, which prevents depletion of the active region in weakly localized electrons.

Blue-light emission from sputtered Si:SiO2 films without annealing

We observed a photoluminescence (PL) spectrum at room temperature that had a peak with full width of half maximum (FWHM) of 0.38 eV near the band gap energy of 3.2 eV from a Si:SiO2 sputtered film without annealing. Blue-light emission could be seen by the naked eye. A low-intensity PL peak with FWHM of 0.20 eV was also observed at around 1.6 eV. We have already demonstrated that our method automatically forms Si clusters contributing to visible emission. Our results did not contradict the well-founded conjecture that there were two mechanisms of emission from Si clusters: emission at 1.6–1.7 eV due to the surface state of the oxidized Si nanocrystals and emission at the band gap energy originating from the quantum confinement effect.

Features of polyatomic ion emission under sputtering of a silicon single crystal by Au m− cluster ions

Comparative studies of the emission of secondary cluster Si n + ions ( n=1–11) and polyatomic Si n X l Y k + ions (X, Y are Au, B, C, N), as well as doubly charged Si 2+ ions under bombardment of single crystalline silicon by cluster Au m − ( m=1–5) ions with energy E 0=4–18 keV have been carried out. High non-additivity enhancement of the yield of the Si n + ions and most polyatomic ones has been observed with an increase of the number of atoms in the projectiles. For Si 2+ ions the negative non-additive effect has been observed. The increase in the yield of impurity-containing cluster Si n X + ions allows for an increase by a factor of 100–1000 for the sensitivity of the SIMS analysis of the Au, B, C, N impurities in Si with the use of cluster ions as primary and secondary ones.

Interfacial Silicon Emission in Dry Oxidation-the Effect of H and Cl

The effect of H and Cl on the interfacial silicon emission has been investigated by the simulation of dry oxidation with the addition of small amounts (≤ 10%) of water (H2O) and chlorine (Cl2). The simulation results for the oxide thickness indicate that the interfacial silicon emission rate is decreased and the equilibrium concentration of the oxygen in the oxide is increased by the addition of these species, as expected from the interfacial Si emission model. From these results, we show that the model is effective for describing oxidation processes.

Microscopic mechanism of thermal silicon oxide growth

The atomistic mechanism of silicon oxidation is investigated by a combination of first-principles calculations and macroscopic approaches. The results of first-principles calculations show that oxidation-induced strain accumulates at the interface as oxidation proceeds, which indicates the emission of a large number of Si atoms to release the accumulated strain. Further calculations to investigate the favorable location of the emitted Si atoms indicate that most of them diffuse into the oxide layer and are oxidized there. Based on these results, we propose a model that the emitted Si atoms in the oxide govern the oxidation rate due to their high concentration. Next, based on the model, we construct the macroscopic diffusion equations, which include Si diffusion species in addition to oxidant species, and we successfully simulate the whole range of oxide thickness, including the thin film regime, in a wide range of oxidation conditions. In addition, combined with the elastic continuum theory, we show that the strain release by the Si emission can explain the recent observations of Si layer-by-layer oxidation together with the formation of many small oxide islands.

Field emission, structure, cathodoluminescence and formation studies of carbon and Si–C–N nanotubes

Catalyst growth carbon and Si–C–N nanotubes have been synthesized successfully by microwave plasma chemical vapor deposition (MPCVD) method using CH4+H2 or CH4+N2 as gas sources, Si columns as additional Si solid sources, and Fe, Fe–Y, Co–Ni as catalysts. Nanotubes consisting of Si, C and N were made under process gases of CH4+N2, and with or without additional Si columns. Well-aligned and nested nanotubes were observed dependent on the catalyst materials. Besides, Si–C–N nanotubes were observed as bamboo-like structure. The as-grown nanotubes were purified in an air furnace to investigate their CL signal shift for potential application involving blue light emission. The field emission results indicate that the emission current densities can be above 10 mA/cm2 at 10 V/μm, and aligned nanotubes belong to better current stability at a constant electric field than nested nanotubes. Nanotubes with a low ID/IG ratio (=0.23) via Raman analysis are achieved. The mechanisms of formation for carbon nanotubes and Si–C–N nanotubes are also discussed.

Stimulated emission in blue-emitting Si+-implanted SiO2 films?

We investigate the blue photoluminescence of Si+-implanted SiO2 films under picosecond UV excitation. The emission intensity exhibits a nonlinear increase with increasing excitation intensities, accompanied by pulse shortening. The photoluminescence decays nonmonoexponentially in time. However, the nonlinearities are not associated with significant spectral narrowing. To explain the results, we propose and numerically investigate a kinetic model based on competition between radiative (both spontaneous and stimulated) and nonradiative recombination in isolated luminescence centers in the SiO2 matrix. Good agreement between theoretical and experimental data seems to confirm the existence of stimulated emission in the films, however, under extremely high excitation densities only (approximately 100 MW/cm2).

Time-resolved investigation of the mid-infrared-induced enhancement of Er 3+ emission in Si

Previously, we reported on the enhancement of Er photoluminescence (PL) in silicon upon mid-infrared (MIR) laser pulse fired at certain delay with respect to Nd : YAG pulsed primary excitation. This effect was explained as the ionization of traps by the MIR beam. The shallow nature of the traps was concluded from its temperature and wavelength dependence. In this contribution, we have studied temporal characteristics of the enhancement effect as a function of the delay time between the two pulses. Decay kinetics of erbium PL measured at 4 K was found to show two different time constants of 1 ms (fast) and 26 ms (slow). A correlation between the enhancement effect and the amplitude of the slow component of the Er PL has been observed. We also have noticed that the enhancement of Er emission is accompanied by subsequent quenching of the slow component, indicating redistribution of energy in the system. Based on the study, we conclude that MIR laser ionizes non-equilibrium traps whose thermalization is responsible for the slow component of the Er PL.

Coexisting photoluminescence of Si and Ge nanocrystals in Ge/Si thin film

Thin films of composite germanium/silicon (Ge/Si) were prepared by pulsed laser ablation alternately on Ge and Si materials on a rotary target, followed by vacuum deposition of the ablated materials on an ultraclean glass substrate. X-ray diffraction and atomic force microscopy phase analysis confirmed that the film structure consisted of a mixture of Si and Ge nanoparticles which could exist in two possible phases. Most of the particles are of less than 30 nm in diameter even after the sample was annealed at 500 °C for 6 h. With different excitation light of wavelengths 280 and 380 nm the composite film yielded independent photoluminescence emissions corresponding, respectively, to the Si and Ge nanoparticles which did not interfere with each other. These results demonstrate that there was very little interaction between the Si and Ge emissions arising from their coexisting mixture in the thin film, even after high temperature annealing of the film in the atmosphere. It opens up the possibility for application of the Si/Ge composite film in multiple function optoelectronic devices.

Theory of thermal Si oxide growth rate taking into account interfacial Si emission effects

A novel theory for the thermal silicon oxide growth rate is constructed based on a new picture of the oxidation process and its efficiency is theoretically discussed on the basis of both analytical and numerical approaches. In the picture, silicons are massively emitted from the oxide/silicon interface into the oxide during the growth process to release the large strain caused by a volume expansion from silicon to oxide at the interface. The flow of the emitted silicons controls the oxidation reaction rate at the interface as well as the flow of the oxidant. Our picture can consistently explain faults in the classical Deal–Grove picture, such as the failure to explain the initial enhanced oxidation.

Enhancement of Etch Rate by the Addition of O2 and Ar in Chemical Dry Etching of Si Using a Discharge Flow of Ar/CF4 and CF4/O2 Gas Mixtures

The chemical dry etching of Si in a fast discharge flow was studied using a low-power (80 W) microwave discharge of Ar/CF4/O2 mixtures. The variation of etch rate was measured as a function of the O2 or Ar flow rate in order to determine the effects of the addition of O2 and Ar. The maximum etch rate was about 3600 Å/min at the Ar, CF4 and O2 flow rates of 2500, 100 and 10 sccm, respectively. This etch rate was larger than those obtained without the addition of O2 by a factor of 8 and without the addition of Ar by a factor of 23. Auger and XPS spectra of Si substrates and emission spectra of discharges were measured in order to examine the effects of the addition of O2 on the Si surface and discharge. The marked enhancement of the etch rate at low O2/CF4 flow ratios below 10% was explained by an increase in the F concentration and a decrease in the concentrations of carbons and CFn (n=1,2). The decrease in the etch rate at high O2/CF4 flow ratios above 10% was attributed to the formation of SiO2 on the substrate. The marked enhancement of the etch rate by the addition of Ar was explained by the generation of active Ar species which enhance [F] and [O] in the discharge flow.

Stimulated blue emission in reconstituted films of ultrasmall silicon nanoparticles

We dispersed electrochemical etched Si into a colloid of ultrabright blue luminescent nanoparticles (1 nm in diameter) and reconstituted it into films or microcrystallites. When the film is excited by a near-infrared two-photon process at 780 nm, the emission exhibits a sharp threshold near 106 W/cm2, rising by many orders of magnitude, beyond which a low power dependence sets in. Under some conditions, spontaneous recrystallization forms crystals of smooth shape from which we observe collimated beam emission, pointing to very large gain coefficients. The results are discussed in terms of population inversion, produced by quantum tunneling or/and thermal activation, and stimulated emission in the quantum confinement-engineered Si–Si phase found only on ultrasmall Si nanoparticles. The Si–Si phase model provides gain coefficients as large as 103–105 cm−1.

Electrically stimulated movement of edge dislocations in silicon in the temperature range 300–450 K

The movement of edge dislocations and the related acoustic emission of Si (111) carrying a direct current of density 0.5−5×105 A/m2 in the [110] direction are studied in the temperature range T=300–450 K. It is shown that the basic mechanism of dislocation movement is the electric wind determining the magnitude of the effective charge (per atom of the dislocation line) Zeff=0.06 (n-Si) and −0.01 (p-Si). Matching theory with experimental data has made it possible to determine the main contribution of edge dislocations to the acoustic-emission response of the silicon samples under investigation. The characteristic transition frequencies of dislocations in n-and p-Si from one metastable state into another are found to be fmax=0.1–0.5 Hz. The numerical values of the diffusion coefficient for atoms in the dislocation impurity atmosphere are estimated as 3.2×10−18 m2/s (n-Si) and 1.5×10−18 m2/s (p-Si).

FUV emissions on Io: Role of Galileo‐observed field‐aligned energetic electrons

The excitation of Io's atmosphere by Galileo‐observed magnetically field‐aligned energetic (0.1–100 keV) electrons have been studied using a Monte Carlo model. The calculated OI and SI FUV emissions are compared with HST‐STIS observations. It is found that Galileo EPD‐observed 15–100 keV electron beams are an insignificant source of OI and SI emissions on Io since they are too energetic to be degraded in Io's atmosphere. We show that Galileo PLS‐observed field‐aligned 0.1–5 keV electron fluxes can explain the HST‐observed S and O FUV brightness provided a fraction of these electrons precipitate into Io's atmosphere. The absorption of FUV emissions by SO2 is found to be less than 10%. From the qualitative scenario of a correlation between Jovian FUV emissions from IFT footprint and Io's FUV emissions we suggest a dual role of field‐aligned IFT electrons in producing emissions at Io and Jupiter.