Porous-silicon coatings for photovoltaic devices

Summary Porous silicon was formed on multicrystalline Si substrates by stain etching in aqueous HF/HNO₃ solutions. In this work optical and electrical properties of the resulting films are discussed as a function of process parameters. Porous-Si films have been shown to be able to reduce surface reflectance to 3% in 350–700 nm wavelength range and their application in anti-reflection coating of photovoltaic solar cells has been demonstrated, obtaining (10×10) cm² multicrystalline Si solar cells with efficiency approaching 12% under standard AM 1.5 simulated sunlight. Paper presented at the III INSEL (Incontro Nazionale sul Silicio Emettitore di Luce), Torino, 12–13 October 1995.     

Effect of surfactants in the electrochemical preparation of porous silicon

Summary Porous-silicon layers were prepared by anodic oxidation of mono- and multi-crystalline Si substrates,n ⁺ andp-doped, in aqueous HF solutions containing surfactants. The resulting samples show a bright red-orange photoluminescence: PL spectra and uniformity are analysed as a function of surfactants kind and concentration. Moreover, the results of Raman spectroscopy are discussed in terms of current theories. Paper presented at the III INSEL (Incontro Nazionale sul Silicio Emettitore di Luce), Torino, 12–13 October 1995.     

Ordering effects in InGaP/GaAs and InGaAsP/GaAs heterostructures grown by LP-MOVPE

Summary The cationic ordering in InGaP epilayers grown by low-pressure vapour phase epitaxy on GaAs substrates has been investigated by X-ray diffraction. The effects of both the substrate miscut and the doping with Si and Zn have been studied. It has been found that ordering effects occur inside relatively small domains on (1–11) and (−111) planes; however, by increasing the miscut angle the domains of the first kind tend to increase their dimensions, while the second ones tend to disappear. Moreover, doping with impurities substituting cations is seen to destroy the order. Photoluminescence anomalies have been revealed and correlated to the size and ordering degree of the ordered domain.     

Amorphous silicon-carbon films prepared by reactive evaporation

Summary An ion-beam-assisted deposition (IBAD) technique, based on reactive evaporation using hydrogen/ methane gas mixture, has been used for the preparation ofa-Si_1−x C _x : H films. Measurements are reported on the composition, optical gap, infra-red vibrational absorption bands and on the electrical dark-conductivity temperature dependence to verify the reliability of this deposition method. On increasing the compositional parameterx up to 0.35, the IR results show an increasing hydrogenation and the presence of Si-CH₃ units in addition to the ones with the carbon fully coordinated with the silicon, while the optical-gap and the dark-conductivity activation energies reached the values of 2.44 eV and 0.77 eV, respectively. The authors of this paper have agreed to not receive the proofs for correction     

The binding energy of an adsorbed atom

Summary We discuss the effect of substrate vibrations on the binding of an adsorbed atom. At zero temperature, we compute the binding energyD ₀-E, whereD ₀ is the surface well depth (classical binding energy) andE is the quantum correction. For several simple models, we find thatE is surprisingly model independent. We compareD ₀-E with the binding energies to a rigid substrate,D ₀-E _rs, and to a vibrationally averaged substrate,D ₀-E _va. We prove thatE _va≥E≥E _rs and that similar relations hold at finite temperature for the free energy of binding. We find that in most casesE _rs is better thanE _va as an approximation toE. In honour of Prof. Fausto Fumi on the occasion of his retirement from teaching.     

Diode laser spectroscopy: Water vapour detection in the atmosphere

Summary Diode laser spectroscopy of water vapour has been performed in the near IR and the pressure broadening coefficients in nitrogen and in air have been measured for one rotovibrational line. Detection of water vapour in the atmosphere has been obtained and an evaluation of the sensitivity of our apparatus is reported. Discussion for further improvements is made.     

Diode laser spectroscopy of ammonia overtone transitions

Summary Diode laser spectroscopy of ammonia ro-vibrational overtone transitions has been performed in the near IR, by using direct absorption, wavelength modulation, and two-tone frequency modulation techniques. Pressure broadening coefficients in air, hydrogen, helium and self-broadening have been measured. The high sensitivity of the apparatus permitted the detection of 70 p.p.m. of NH₃ in air per meter of path at atmospheric pressure. New lines have been observed in the 719.9 nm band. The authors of this paper have agreed to not receive the proofs for correction.     

High-resolution measurements of pressure self-broadening and shift for the metyl-cyanide rotational J=1−0 line

Summary The capabilities of the travelling-wave spectrometry in the field of high-resolution measurements of low-pressure gases at microwave frequencies are discussed. Some details about the main aspects of the experimental procedure and the data management, useful for the improvement of overall performances, are specified. A pressure broadening of (72.5±1.5) MHz/Torr and a pressure shift of (4.3±0.3) MHz/Torr are measured for theJ=1−0 rotational line of CH₃C¹⁴N. These results are more accurate than those previously reported and in better agreement with the theoretical predictions.

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Riassunto Si discutono le prestazioni di uno spettrometro a microonde ad onda progressiva nel campo della spettroscopia ad alta risoluzione in gas a bassa pressione. Sono in particolare considerati alcuni aspetti delle procedure sperimentali e dell'elaborazionedei dati che hanno portato a decisi miglioramenti nella sensibilità e nelle prestazioni complessive dell'apparato sperimentale. Sono stati misurati per la riga rotazionaleJ=1–0 di CH₃C¹⁴N un coefficiente di autoallargamento per pressione di (72.5±1.5) MHz/Torr e uno shift di (4.3±0.3) MHz/Torr. Queste misure risultano piú accurate di quelle precedentemente riportate e in migliore accordo con le previsioni teoriche.

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Резюме Обсуждаются возможности использования микроволнового спектрометра со скользящей волной для измерений с высоким разрешением в газах при низких давлениях. Рассматриваются основные аспекты экспериментальной процедуры и обработки данных, которые позволяют улучшить чувствительность аппаратуры. Измерены величины самоуширения с давлением (72.5±1.5) МГц/Тор и сдвига с давлением (4.3±0.3) МГц/Тор для ротационной линииJ=1–0 для CH₃C¹⁴N. Полученные результаты являются более точными, чем ранее опубликованные, и лучше согласуются с теоретическими предесказаниями.

     

Exact treatment of interface fluctuations in a two-dimensional emulsion

Summary We present a model for a polydisperse ensemble of two-dimensional droplets wich accounts for the effects of arbitrarily large distortions of the droplet shape. Interactions within a droplet include bending rigidity and spontaneous curvature. Interactions between droplets are omitted. Even at high temperatures, the effects of the shape fluctuations on the droplet size distribution remain small, as they are dominated by the contributions from the mixing entropy. In contrast, shape fluctuations lead to a pronounced peak in thermodynamic quantities like specific heat. This peak occurs at temperatures where thermal excitations become of the order of the bending energies of the droplet surface. The fluctuation-dominated regime extends to temperatures far lower than expected from a mean-field calculation. Paper presented at the I International Conference on Scaling, Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

The pressure broadening and shift for the J=1−0 line of CH3C14N: Resolved and overlapped hyperfine components

Summary Pressure broadening and shift parameters are measured for theJ=1−0 line of CH₃C¹⁴N at 18.4 GHz, both for the three hyperfine components and for the single line which envelopes the hyperfine components, observed at higher pressures (p≥40 mTorr). In agreement with theoretical predictions, these parameters are the same for all the observed lines: Γ _p = (50 ± 5) MHz/Torr,s=(7±2) MHz/Torr. These values are compared with previous measurements, showing a large variance expecially for the shift parameter. The measured broadening parameter is 40% lower than prediction of the Anderson theory.

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Riassunto Sono stati misurati i parametri di allargamento e spostamento per pressione della rigaJ=1−0 del CH₃C¹⁴N a 18.4 GHz. Tali misure sono state eseguite, sulle tre componenti iperfini e sul loro inviluppo osservato a pressionip≥40 mTorr. In accordo con le previsioni teoriche, questi parametri risultano gli stessi per tutte le righe osservate: Γ _p = (50 ± 5) MHz/Torr,s=(7±2) MHz/Torr. Questi valori sono confrontati con quelli delle precedenti misure che mostrano una considerevole dispersione, specialmente per quanto riguarda il parametro di spostamento. L'allargamento per pressione è risultato piú basso di quello previsto dalla teoria di Anderson di circa il 40%.

     

Experimental investigation of the angular dependence of photon-induced differential cross-sections of L X-rays from U,Th and Bi at 59.5 keV

Summary The differential cross-sections for the emission of L_ρ, L_α, L_β and L_γ groups of L X-ray lines from U, Th and Bi have been measured at different angles varying from 45° to 135° at intervals of 15°. The vacancies were created by 59.5 keV γ-rays from²⁴¹Am source and L X-rays were measured using a Si(Li) detector. It is found that the L_γ and L_β groups of L X-rays are isotropic, the L_ρ and L_α groups are anisotropic in the spatial distribution. The authors of this paper have agreed to not receive the proofs for correction.     

X-ray line broadening analysis of explosively consolidated Y−Ba−Cu−O powders

Summary The residual lattice strain, the crystallite size, the components of Gaussian and Cauchy functions of the X-ray line profile, determined by the Fourier analysis, are correlated for a better understanding of microstructural characteristics of a coarse and fine Y−Ba−Cu−O powder after explosive consolidation. The results are discussed taking into account the lattice defects generation, in particular, the twin formation in different crystallographic directions after shock loading.     

Study of recovery and recrystallization of Cu after dynamic-loading and conventional deformations by XRD line broadening Fourier analysis

Summary The residual lattice strain, the crystallite size, the components of Gaussian and Gauchy functions of the line profile, determined by Fourier analysis, are correlated for a better understanding of recovery and recrystallization phenomena of Cu annealed after fast and conventional deformations. The density and distribution of the dislocations in the deformed starting material strongly influence the mechanisms of intermediate and high-temperature recovery as well as those of primary recrystallization. The authors of this paper have agreed to not receive the proofs for correction.     

Separation of enantiomers in a diol monolayer studied by fluorescence microscopy and grazing incidence X-ray diffraction

Summary Monolayers of a racemic mixture and of the pure S- and R-enantiomers of 3-hexadecyloxy-propane-1,2-diol have been investigated by fluorescence microscopy and grazing incidence X-ray diffraction. Above the transition pressureΠ _c domains with a polygonal shape appear. On compressing the monolayers shape instabilities occur. While the spirals of the S-enantiomer turn clockwise and the spirals of the R-enantiomer turn counterclockwise, the spirals from one domain of the racemate exhibit opposite handedness. This behaviour is explained by a separation of enantiomers on the molecular level. At all pressures investigated the enantiomers exhibit an oblique lattice. The tilt angle of the molecules decreases with increasing pressure. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.     

Early Universe Thermodynamics and Evolution in Nonviscous and Viscous Strong and Electroweak Epochs: Possible Analytical Solutions

Simple SummaryIn the early Universe, both QCD and EW eras play an essential role in laying seeds for nucleosynthesis and even dictating the cosmological large-scale structure. Taking advantage of recent developments in ultrarelativistic nuclear experiments and nonperturbative and perturbative lattice simulations, various thermodynamic quantities including pressure, energy density, bulk viscosity, relaxation time, and temperature have been calculated up to the TeV-scale, in which the possible influence of finite bulk viscosity is characterized for the first time and the analytical dependence of Hubble parameter on the scale factor is also introduced.AbstractBased on recent perturbative and non-perturbative lattice calculations with almost quark flavors and the thermal contributions from photons, neutrinos, leptons, electroweak particles, and scalar Higgs bosons, various thermodynamic quantities, at vanishing net-baryon densities, such as pressure, energy density, bulk viscosity, relaxation time, and temperature have been calculated up to the TeV-scale, i.e., covering hadron, QGP, and electroweak (EW) phases in the early Universe. This remarkable progress motivated the present study to determine the possible influence of the bulk viscosity in the early Universe and to understand how this would vary from epoch to epoch. We have taken into consideration first- (Eckart) and second-order (Israel–Stewart) theories for the relativistic cosmic fluid and integrated viscous equations of state in Friedmann equations. Nonlinear nonhomogeneous differential equations are obtained as analytical solutions. For Israel–Stewart, the differential equations are very sophisticated to be solved. They are outlined here as road-maps for future studies. For Eckart theory, the only possible solution is the functionality, H(a(t)), where H(t) is the Hubble parameter and a(t) is the scale factor, but none of them so far could to be directly expressed in terms of either proper or cosmic time t. For Eckart-type viscous background, especially at finite cosmological constant, non-singular H(t) and a(t) are obtained, where H(t) diverges for QCD/EW and asymptotic EoS. For non-viscous background, the dependence of H(a(t)) is monotonic. The same conclusion can be drawn for an ideal EoS. We also conclude that the rate of decreasing H(a(t)) with increasing a(t) varies from epoch to epoch, at vanishing and finite cosmological constant. These results obviously help in improving our understanding of the nucleosynthesis and the cosmological large-scale structure.     

Description of a hybrid PECVD-PVD process: Application to Zn-Si-O and Ti-Si-O composites thin films

A radio frequency hybrid process where sputtering and plasma enhanced chemical vapour deposition (PECVD) occur simultaneously is studied to describe the specificity it gains when the two techniques are merged. A model is developed to describe how the deposition rate evolves when the flow rate of the PECVD precursor increases. First, it is shown that it is constant below a critical value of the precursor flow rate because of the wind effect due to sputtering that strongly limits the transport of the precursor. Then it increases almost linearly with the precursor flow rate when PECVD and sputtering simultaneously occur. Finally, above a certain threshold in the precursor flow rate, the surface of the target is poisoned by the precursor and composite thin films can no longer grow. The previous model is deduced from results obtained in deposition of Zn-Si-O and Ti-Si-O thin films. These composites are synthesised respectively by sputtering of zinc and titanium targets in a vapour of oxygen and hexamethyldisiloxane (HMDSO-Si₂C₆H₁₈O). Limitations of the model used are also discussed.     

The effect of neutron and gamma irradiation on the dislocation relaxation peak in silver

Summary The internal-friction spectrum has been determined in extremely pure polycrystalline silver crystals after being subjected to neutron and gamma irradiation. The spectra obtained for an applied frequency of 10 MHz show that the mechanical losses due to dislocation relaxation decrease as the doses of neutron and gamma radiation increase. The effect of gamma radiation was found to be more significant than that of neutron irradiation. The relationship between damping neutron and gamma dose was found to be of the formQ _max ⁻¹ α ΛD ^η, where η is equal to −1.2 for neutron and −1.45 for gamma irradiation. The variations of the peak height and width, and temperature of the dislocation relaxation peak as functions of neutron and gamma doses are explained in terms of the pair-kink formation model.     

Lidar measurements of atmospheric transmissivity

Summary The possibility of performing atmospheric-transmissivity measurements by lidar is considered. In the present paper two distinct methods have been successfully applied. The first one is based on the detection of the Raman return from molecular nitrogen. An alternative method is based on the simultaneous detection of the elastic and N₂ Raman returns. Measurements based on both techniques are discussed in detail, the second technique producing more accurate results. Through this technique an estimation of the ?ngstr?m coefficient can also be obtained.     

An auger investigation of oxygen-enhanced tin segregation on a liquid Pb−Sn alloy

Summary Auger measurements of the surface composition have been performed on solid and molten Pb-5at%Sn alloys. They confirm the theoretical predictions that, in the absence of oxygen, liquid or solid alloys exhibit no significant surface segregation. On the contrary, surface composition measurements and simultaneous surface tension measurements clearly show that oxygen adsorption strongly affects the segregation of tin at the liquid-vapour interface. This surface enrichment is driven by the contribution of the reaction free energy and of the products surface tension.     

The bordoni peak in silver single crystals at megahertz frequencies

Summary The dislocation relaxation maximum has been investigated at frequencies of 5, 10 and 30 MHz. The three (very high-purity) silver crystals of crystallographic orientation 〈111〉, 〈110〉 and 〈100〉 which are given a resolved shear stress of 20 MPa were found to produce dislocation relaxation maximum at 107 K in all the three samples. The maximum shifts to 117 K when measurement is carried out at 10 MHz and to 127 K at 30 MHz. The results reveal that the maximum is orientation independent. The activation energy and the attempt frequency were calculated and found to be equal to 0.105 eV and 2·10⁹ Hz, respectively. Meanwhile the larger magnitude of the maximum was found on the 〈111〉 crystal and the least was on the 〈100〉 crystal. Finally, it was clearly shown that the dislocation relaxation strength decreases as the frequency increases.