Molecular hydrogen and oxygen interactions with armchair Si nanotubes

Molecular hydrogen and oxygen adsorptions on a (6, 6) armchair silicon nanotube have been studied by optimizing the distances of the admolecules from both inside and outside the tube. Full geometry and spin optimizations have been performed without any symmetry constraints with an all electron 3-21G* basis set and the B3LYP functional. The molecule is originally placed perpendicular or parallel to the tube axis. Hydrogen adsorption with the molecular axis aligned parallel to the surface of the nanotube is less favorable. Hydrogen molecule does not dissociate while oxygen molecule dissociates after optimization. The on-top site is the only preferred site for hydrogen molecule with an adsorption energy of 3.71 eV and an optimized distance of 3.31 for external adsorption whereas the on-top site is the most preferred site with adsorption energy of 3.69 eV for internal adsorption. For oxygen, the molecule dissociates and the most preferred sites are the two bridge sites with an adsorption energy of 9.64 eV, the optimized distance being 1.65/1.68 Å when it is adsorbed from the outside of the tube. When oxygen molecule is originally placed at on-top site it will hold as a molecule after adsorption with a slightly increased bond length. For the internal adsorption of oxygen, the molecules also dissociate in most cases and the zigzag bridge site is the most preferred site. After molecular adsorption for both hydrogen and oxygen, the buckling of the nanotubes increased. Frustration effects in the nanotube due to molecular adsorption are also noted.     

Dipole interactions and diffusion of μ meson in copper

The dipole relaxation rate and the diffusion coefficient of μ⁺-meson in copper are measured at different temperatures. The activation energy of μ⁺-meson in a crystal of copper is determined.     

The dispersive diffusion of hydrogen in undoped a—Si:H

We have measured the dispersion parameter as a function of the annealing temperature in three samples deposited under different conditions. increases with the temperature and the initial hydrogen concentration. The rate of variation of with the annealing temperature seems to depend strongly on the deposition conditions. More data will be taken to elucidate this behaviour.     

First-principles study of diffusion and interactions of vacancies and hydrogen in hcp-titanium

We present a study of the stability of n-vacancies (V (n)) and hydrogens in the hexagonal close-packed titanium system computed by means of first-principles calculations. In this work, performed by using the generalized gradient approximation of density functional theory, we focused on the formation energies and the processes of migration of these defects. In the first part, the calculated formation energy of the monovacancy presents a disagreement with experimental data, as already mentioned in the literature. The activation energy is underestimated by almost 20%. The stability of compact divacancies was then studied. We show that a divacancy is more stable than a monovacancy if their migration energies are of the same order of magnitude. We also predict that the migration process in the basal plane of the divacancy is controlled by an intermediate state corresponding to a body-centered triangle (BO site). The case of the trivacancies is finally considered from an energetic point of view. In the second part, the insertion of hydrogen and the processes of its migration are discussed. We obtain a satisfactory agreement with experimental measurements. The chemical nature of the interactions between hydrogen and titanium are discussed, and we show that the H-atom presents an anionic behavior in the metal. The trapping energy of hydrogen in a monovacancy as a function of the number of hydrogen atoms is finally presented.     

Energetics of atomic hydrogen diffusion on Si(100)

We present first principles calculations of the potential energy surface for the diffusion of a single hydrogen atom on Si(100)2 × 1. Surface relaxation is found to be very important for the energetics of diffusion. A strong anisotropy is predicted for hydrogen motion: H should diffuse mainly along dimer rows, where activation energies are ~ 1.3 eV, while the barrier for row-to-row hopping is ~ 0.5 eV higher. Our results indicate that diffusion can be considered a fast process compared to H₂ recombinative desorption.     

Influence of hydrostatic pressure on the diffusion of hydrogen in n-GaAs : Si

Hydrogen diffusion experiments have been performed in buried silicon doped GaAs epilayers under hydrostatic pressure. The deuterium diffusion profile in n-GaAs : Si depends on the hydrostatic pressure: a plateau followed by a steep decrease progressively appears as the pressure is increased. This has been interpreted as being due to the increasing importance of the trapping-detrapping process of H⁻ on Si⁺ donors during the hydrogen diffusion. This increase has been attributed to a deepening of the hydrogen acceptor level with respect to the bottom of the Γ conduction band of GaAs as the hydrostatic pressure increases.     

Bound exciton diffusion in Si : B

Photoluminescence measurements as a function of excitation intensity and temperature are presented for moderately doped Si : B. Some evidence is obtained for hopping motion of the bound excitons around the impurity centers with trapping at the clusters of impurities. Kinetic parameters of the excitation transfer are estimated.     

Reaction of cyanide ions with copper on Si surfaces and its use for Si cleaning

A Si cleaning method has been developed by use of potassium cyanide (KCN) dissolved in methanol. When silicon dioxide (SiO₂)/Si(1 0 0) specimens with 10¹⁴ atom/cm² order copper (Cu) contaminants are immersed in 0.1 M KCN solutions of methanol at 25 °C, the Cu concentration is reduced to below the detection limit of total X-ray fluorescence spectrometer of ∼3 × 10⁹ atoms/cm². X-ray photoelectron spectra show that the thickness of the SiO₂ layers is unchanged after cleaning with the KCN solutions. 10¹⁴ cm⁻² order Cu contaminants on the Si surface can also be removed below ∼3 × 10⁹ atoms/cm², without causing contamination by potassium ions. UV spectra show that Cu-cyano complex ions are formed in the KCN solutions after the cleaning. The main Cu species in the KCN solutions is ions with the concentration of []:[Cu⁺] = 1:1.6 × 10²³. Even when the KCN solutions are contaminated with 64 ppm Cu²⁺ ions in the solutions, which form ions, the cleaning ability does not decrease, showing that ions are not re-adsorbed. The KCN solutions can also passivate defect states such as Si/SiO₂ interface states, leading to the improvement of characteristics of Si devices.     

Thermal diffusion of hydrogen in zirconium with allowance for thermal stresses

The thermal diffusion of hydrogen atoms in zirconium is studied with allowance for thermal stresses. As a mathematical model, a steady-state temperature in a hollow cylinder is considered. The logarithmic coordinate dependence of this temperature allows an exact analytical solution to be obtained for a diffusion kinetics problem. The results of a theoretical analysis can serve as a test example for studying mass transfer in systems with a more complex coordinate dependence of temperature.     

Sites and diffusion for muons and hydrogen in titanium hydrides

Low temperature sites for muons implanted in TiHx have been found to be a mixture of interstitial and substitutional sites, with substitutional occupancy determined by the probability that a muon in an interstitial site will have a vacant nearest neighbor substitutional site. As with ZrHx, activation from the interstitial site is observed below 300 K. From the depolarization rate in the substitutional site, the muon likely displaces the neighboring H atoms by about 0.1 A. Diffusion for the substitutional muons occurs above room temperature with an activation of about 0.38 eV, which is less than the 0.505 eV for hydrogen vacancy motion observed by NMR. To explain this the muon transition rate to a vacancy must be less than that of hydrogen.     

Muon diffusion in nanocrystalline copper

A systematic μSR study on nano‐Cu has demonstrated that the diffusion of μ⁺ in nanocrystalline metals is influenced by both features of the nanostructure, i.e., by the very small grain size and by the comparatively large fraction of grain boundaries. The former feature yields a size effect of the phonon‐assisted muon tunneling, but only at particle diameters below 20 nm. The latter feature, in samples with crystallite sizes above 20 nm diameter, i.e., with bulk diffusional behaviour, establishes a connection between μ⁺ diffusion coefficient and particle size: if one of these quantities is known, the other could be evaluated. This revised version was published online in August 2006 with corrections to the Cover Date.     

Nonlinear diffusion limit for a system with nearest neighbor interactions

We consider a system of interacting diffusions. The variables are to be thought of as charges at sites indexed by a periodic one-dimensional lattice. The diffusion preserves the total charge and the interaction is of nearest neighbor type. With the appropriate scaling of lattice spacing and time, a nonlinear diffusion equation is derived for the time evolution of the macroscopic charge density.Work supported by the National Science Foundation under grants no. DMS 8600233 and DMS 8701895     

Muon diffusion and level crossing in copper

We have studied the quantum diffusion of positive muons in pure copper over the temperature range 12 mK≤T≤150 K using weak longitudinal field μSR. Below 150 K, this technique has proved to be the most sensitive to the muon hop rate. Our final results for the behaviour of the muon hop rate are well explained within the framework of theories for the quantum diffusion of light interstitials in metals of Kondo, Yamada and others. In addition, the use of level-crossing resonance has allowed us to measure the electric quadrupole interaction strength (and sign) of the copper nuclei, ω_Q= −3.314(7) μS⁻¹. These results have enabled us to show that the muon occupies the same octahedral site at all the temperatures studied, ruling out the possibility of metastable muon sites contributing to any significant portion of the muon polarization.     

Copper diffusion in copper sulfide: a systematic study

Copper sulfide Cu_2−xS is a model mineral for chalcogenides because of the existence of a non-stoichiometric compounds series in the range Cu₂S - CuS in which properties change with x. For this reason, we have studied the influence of the mineral composition on the diffusion in this solid. Electrochemical Impedance Spectroscopy (EIS) applied to Cu_2-xS/cupric sulfate electrolyte was the main investigation technique. It enabled us to work at the equilibrium potential at which the composition is fixed and known. Changing the composition by electrochemically removing (or adding) a known amount of Cu, we were able to determine the chemical diffusion coefficient of copper in the composition range (from x=0 to 0.066). In this work, we present the results obtained in the chalcocite and djurleite phases. These results were compared to other values reported in the literature. From this systematic study we discuss various diffusion mechanisms. Our observations support that in chalcocite and djurleite Cu diffuses via a vacancy mechanism. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Dopant diffusion and segregation in semiconductor heterostructures: Part III, diffusion of Si into GaAs

We have mentioned previously that in the third part of the present series of papers, a variety of n-doping associated phenomena will be treated. Instead, we have decided that this paper, in which the subject treated is diffusion of Si into GaAs, shall be the third paper of the series. This choice is arrived at because this subject is a most relevent heterostructure problem, and also because of space and timing considerations. The main n-type dopant Si in GaAs is amphoteric which may be incorporated as shallow donor species Si_Ga ⁺ and as shallow acceptor species Si_As ^-. The solubility of Si_As ^- is much lower than that of Si_Ga ⁺ except at very high Si concentration levels. Hence, a severe electrical self-compensation occurs at very high Si concentrations. In this study we have modeled the Si distribution process in GaAs by assuming that the diffusing species is Si_Ga ⁺ which will convert into Si_As ^- in accordance with their solubilities and that the point defect species governing the diffusion of Si_Ga ⁺ are triply-negatively-charged Ga vacancies V_Ga ^3-. The outstanding features of the Si indiffusion profiles near the Si/GaAs interface have been quantitatively explained for the first time. Deposited on the GaAs crystal surface, the Si source material is a polycrystalline Si layer which may be undoped or n⁺-doped using As or P. Without the use of an As vapor phase in the ambient, the As- and P-doped source materials effectively render the GaAs crystals into an As-rich composition, which leads to a much more efficient Si indiffusion process than for the case of using undoped source materials which maintains the GaAs crystals in a relatively As-poor condition. The source material and the GaAs crystal together form a heterostructure with its junction influencing the electron distribution in the region, which, in turn, affects the Si indiffusion process prominently. Received: 19 April 1999 / Accepted: 3 May 1999 / Published online: 4 August 1999     

Thermal stability and diffusion processes in Mo x Si y /Si multilayers studied with high-resolution RBS

Mo _x Si _y /Si multilayers with a period thickness of ∼7.5 nm and bilayers Mo _x Si _y /Si have been fabricated by e⁻-beam evaporation in UHV at a deposition temperature of 150°C [1]. The composition of the as-deposited layer systems and changes in the composition after baking the samples have been studied with high-resolution RBS. For a multilayer with a mixing ratioy/x≃2, no interdiffusion is observed up to a baking temperature of 830°C. For samples with a mixing ratioy/x≃1, diffusion is observed up to a baking temperature of 630°C, resulting in a mixing ratio close toy/x≃2. This mixing ratio remains almost stable up to ∼830°C, and considerable interdiffusion is only observed in those systems where regions with a mixing ratio smaller than 2 still exist. Possible reasons for the high thermal stability of the samples are the lack of a concentration gradient for Si in the system and/or the crystallization of MoSi₂.