摘要:

Self‐consistent particle‐fluid hybrid simulations have been used to study the structure of hydrogen dc discharges between parallel plates. A Monte Carlo simulation is used to describe individual energetic electrons in the cathode sheath, while the electrons and ions in the low‐field region of the discharge are modeled as a fluid. Simulation results for pure H2discharges at conditions typical in diamond growth (approximately 20–30 Torr at 1000 K, 100–200 A m−2) are in reasonable agreement with optical emission and Langmuir probe measurements. The space potential in these discharges is negative with respect to the anode, unlike comparable discharges at lower pressures. Anode glows are predicted by the simulation and are observed experimentally. Dissociation of H2in the anode region contributes significantly to the flux of atomic hydrogen to the anode, where diamond is typically grown.

ISSN:0021-8979    

DOI:10.1063/1.350575

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We have prepared polycrystalline Co films on epitaxially oxidized Si(111) surfaces and studied their oxidation kinetics under atmospheric conditions using the fact that metallic cobalt is a ferromagnet but CoO is an antiferromagnet and therefore practically nonmagnetic. As a consequence, oxidation is associated with a loss in magnetism. Results show that all samples with an initial thickness ofdi≳ 5 nm oxidize practically instantaneously, whereby a constant amount of 2.5 nm of metal is transformed into oxide. Fordi< 5 nm the time constant for oxidation increases considerably and follows an approximately linear dependence with decreasing film thickness, reaching an extrapolated value of &tgr;=190 days fordi→ 0. This increasing time constant let all samples withdi< 2.5 nm appear ferromagnetic within the course of this study due to a nonoxidized metallic rest. Auger electron spectroscopy analyses revealed that the main oxidation product is in fact CoO, especially near to the metal interface, but that other compounds are also likely to be formed summing up to a total thickness of 6 nm including adsorbates. A second point of interest was the unidirectional magnetic anisotropy which the oxide imposes to the metal due to an exchange interaction at the interface. This anisotropy shows a sharp onset at a CoO thickness as small as ∼0.25 nm from which an anisotropy constant of ∼4.5×106J/m3can be estimated for ultrathin CoO layers.

ISSN:0021-8979    

DOI:10.1063/1.351378

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Boron‐doped microcrystalline hydrogenated silicon (&mgr;c‐Si:H) films were grown from a gas mixture of silane, diborane, and hydrogen employing mercury sensitized photochemical vapor deposition. At a low diborane doping ratio (1.1×10−3) hydrogen dilution of source gases resulted in films exhibiting a maximum conductivity of 7.4 S cm−1. For a higher doping ratio (10−2) the value of conductivity remained almost unchanged (10−5–10−6S cm−1) with hydrogen dilution indicating nonexistence of microcrystallinity under such conditions. Transmission electron microscopy of B‐doped &mgr;c‐Si:H films revealed formation of crystallites possessing different crystallographic orientations, e.g., (111), (220), and (311) along with other planes. X‐ray spectra confirmed a large number of crystallites with (220) orientations.

ISSN:0021-8979    

DOI:10.1063/1.350576

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Several etchants have been applied to polar (111)Cd and (111)Te surfaces of CdTe. Induced surface layers were analyzed by x‐ray photoelectron spectroscopy and grazing‐incidence x‐ray diffraction for studying compositions, chemical states, and crystalline phases of respective layers. A bromine/methanol etch led to a layer composed of mainly tellurium oxides on both (111) surfaces. Oxidation depth, however, was larger at the (111)Te side. Etching with HCl subsequently removed the oxides. A film made of crystalline tellurium was, however, produced with the thickness being larger on the (111)Te surface than on the other surface. A fairly thick layer consisting of TeO2and crystalline Te for etching with N solution (H2O2:H2O:HF=2:2:3 v/v), formed on both surfaces with the cadmium having been severely depleted, especially on the (111)Te surface.

ISSN:0021-8979    

DOI:10.1063/1.350577

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Ni–P–B ultrafine amorphous alloy particles have been prepared by chemical reaction of nickel chloride, sodium hypophosphite, and potassium borohydride in aqueous solution. Two samples prepared at different initial pH values (pH=5 and 9) have been characterized by different experimental methods. X‐ray and electron diffraction indicated that both samples were amorphous in nature, and transmission electron microscopy gave average diameters of 50 and 30 nm for particles prepared at pH=5 and 9, respectively. X‐ray photoelectron spectroscopy revealed that nickel and phosphorus existed exclusively in their elemental states, while boron existed mainly in its elemental state but with some trace of its oxide on the surface. The crystallization process of the amorphous samples was investigated by differential scanning calorimetry and by x‐ray diffraction after annealing the samples at various temperatures. The results are correlated with the microstructure of the as‐prepared samples. It is found that when the initial pH of the reaction mixture is adjusted to 9, the pH value can be maintained automatically throughout the reaction and the sample thus formed has a fairly homogeneous microstructure.

ISSN:0021-8979    

DOI:10.1063/1.350578

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

In the directional solidification of a binary liquid, salt‐finger and plume convection are the two major types of fluid motion that occur when the lighter species is rejected from the solid. Fingering convection is usually observed prior to the initiation of plume formation. When the nominally planar liquid‐solid interface is morphologically unstable, solidification is dendritic. Consequently, the melt is separated from the solid by a mushy zone consisting of solid dendrites and interdendritic liquid. This can be considered to be a porous layer with anisotropic and inhomogeneous permeability and, possibly, thermal and solutal diffusivities. A linear stability analysis is empoyed to study salt‐finger instability in a fluid layer bounded above by a rigid wall and below by an anisotropic and inhomogeneous porous medium saturated by the same fluid. The effects of anisotropy and inhomogeneity on the stability characteristics are discussed for a variety of porous media. Results are also presented for a porous medium for which the anisotropy and inhomogeneity of the permeability and thermal and solutal diffusivities have been obtained experimentally during the directional solidification of aqueous ammonium chloride solutions. The nature of the instability modes relevant to the present problem are also discussed. Based on the present results, a relationship is proposed between salt‐finger and plume convection during directional solidification. Methods for inhibiting salt‐finger convection are also suggested, and the feasibility of their implementation is discussed.

ISSN:0021-8979    

DOI:10.1063/1.350579

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

The effective shading of metal‐plated solar‐cell fingers after cell encapsulation is substantially less than would be expected from simple geometric calculations. The reason for this is that much of the light reflected from the fingers is either directed downwards toward the silicon or is totally internally reflected at the air/glass interface. Experimental data is presented to support calculations of this effect. Calculations of the impact of this effect on solar‐cell performance are also included.

ISSN:0021-8979    

DOI:10.1063/1.350580

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

A model for the effect of the presence of photogenerated electron‐hole pairs on the optically induced mechanical moment in a silicon cantilever beam has been developed. This effect is twofold and is the consequence of electronic strain on one hand and of the recombination heat of the photogenerated electron‐hole pairs on the other. It manifests itself by a deviant phase‐ frequency relation of the vibrating beam in comparison with a pure thermally excited beam. Experiments have been performed, and the results show excellent agreement with the model.

ISSN:0021-8979    

DOI:10.1063/1.350581

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

Two quite different models have indicated their ability to describe the experimental data for the kinetics of light‐induced defect creation in hydrogenated amorphous silicon at or quite near room temperature. These have been called the ‘‘t1/3’’ model and the ‘‘stretched exponential’’ model. When measurements are made at different temperatures, however, the experimental data can still be described completely by the stretched exponential model with a change in the value of the stretch parameter, which may reasonably be a function of temperature, but they no longer follow at1/3dependence. Since thet1/3dependence is based on a particular physical mechanism for defect creation, it is concluded that this mechanism is not applicable to hydrogenated amorphous silicon.

ISSN:0021-8979    

DOI:10.1063/1.350582

出版商:AIP    

年代:1992

数据来源: AIP

摘要:

We demonstrate that high quality thin films of fullerenes (C60) and fluorinated fullerenes can be prepared from solution by the spin coating technique on float glass or silicon wafers. The films were characterized by x‐ray reflectivity and diffuse x‐ray scattering. A systematic study of films of different thicknesses allows estimation of the distance between the fullerenes spheres to be 10.9±1 A˚ in our C60films. The C60film‐air surface is very smooth. Annealing in air above 90 °C leads to an irreversible increase of the film thickness, which is attributed, at present, to oxidation.

ISSN:0021-8979    

DOI:10.1063/1.350583

出版商:AIP    

年代:1992

数据来源: AIP