摘要:

The positive column of a hydrogen glow discharge was studied under typical operating conditions: gas pressure from 0.3 up to 5.0 Torr and discharge current from 1 up to 50 mA. Optical emission spectroscopy, optical absorption spectroscopy, and laser induced fluorescence have been employed in order to determine the gas temperature(300<Tg<600 K),the density of ground state hydrogen atoms(1012<[H(1s)]<1013 cm−3),and the kinetic temperature of H atoms(336<Ta<1600 K),respectively. Langmuir probes were utilized to measure the electric field(8<E<61 V cm−1)in the positive column. A kinetic model based on the solutions to the homogeneous electron Boltzmann equation coupled to a set of rate balance equations for the vibrational levelsH2(X 1&Sgr;g+,v),H atoms, andH−ions was developed in order to predict the concentrations of these species. From a comparison between the measured and calculated concentrations of H atoms, the reassociation probability on the wall, in Pyrex glass, is estimated to be≅10−2.©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366110

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The spatial distributions of excited states in radio frequency electrical gas discharges have been observed to be dynamic functions of gas mixture, pressure, and applied voltage. Recent measurements of two-dimensional profiles of excited states in the Gaseous Electronics Conference reference cell (GECRC) [McMillin and Zachariah, J. Appl. Phys.77, 5538 (1995);79, 77 (1996)] have shown that the spatial distribution of theAr(4s)density varies considerably with operating conditions. The peak density ofAr(4s)systematically shifted in position, as well as changed in magnitude, with variations in pressure, applied voltage, and gas mixture. In this article, we present results from a two-dimensional computer simulation of Ar,Ar/O2,andAr/CF4discharges sustained in the GECRC with the intent of investigating the experimental trends. The simulations, performed with the Hybrid Plasma Equipment Model, agree well with experiments. They show that the shift inAr(4s)densities is largely explained by the reduction in the electron mean free path, and local perturbations in the ambipolar electric field resulting from electrode structures. Additions of small amounts ofO2andCF4decrease theAr(4s)density due to quenching, and change its profile due to a transition to an electronegative plasma. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366111

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The characteristics of pulse-modulated inductively coupled plasmas in argon and chlorine have been experimentally investigated. Measurements were performed for peak rf powers between 150 and 400 W at 13.56 MHz, duty cycles between 10 and 70&percent;, and pulse repetition frequencies between 3 and 20 kHz. Over this parameter space, measurements were performed of the time dependent forward and reflected rf powers into the matching network, coil voltage, rf variation of the plasma potential, electron density, andCl−density. These measurements indicated that for the first 5–30 rf cycles of each pulse, the discharges probably were operating in a capacitively coupled discharge mode with rf variations in the plasma potential of several hundreds of volts and relatively low electron density. Measurements of the electron density in pulse-modulated chlorine discharges indicated that the plateau electron density was a function of the duty cycle; the plateau electron density was lower for higher duty cycles. This may indicate that the ratio of Cl toCl2was changing with duty cycle. In addition, a microwave radiometer was used to provide an indication of the time-dependent electron temperature. Large spikes in the microwave radiation temperature were noted at the turn-on of the rf power pulses and, in some cases, at the transition from a capacitively coupled to an inductively coupled plasma. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366277

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The enhancement of energy loss of short pulses of electrons in plasma has been experimentally observed. An enhancement of up to3.5×104over single-particle losses was observed when a 15 ps electron bunch was injected into a preformed target plasma with an electron density of approximately1013 cm−3.This matches the theoretical prediction that the energy loss should be enhanced when the temporal duration of the electron bunch is approximately equal to&pgr;/&ohgr;pe,where&ohgr;peis the plasma frequency of the target plasma. 2D numerical simulations are in good agreement with the observations. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366112

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Generation of a high amplitude shock wave by laser plasma in a water confinement regime has been investigated for an incident 25–30 ns/40 J/&lgr;=1.064 &mgr;mpulsed laser beam. Experimental measurements of temporal and spatial profiles of induced shock waves for this regime of laser shock processing of materials were performed using a velocimetry interferometer system for any reflector system. Above a 10GW/cm2laser intensity threshold, a saturation of the peak pressure is shown to occur while the pressure pulse duration is reduced by parasitic plasma occurring in the confining water. The observation of the interaction zone with a fast camera system shows that this breakdown plasma, which mainly occurs at the very surface of the water rather than within the water volume, limits the efficiency of the process. This plasma absorbs the incident laser energy, and the power density reaching the target gradually decreases with increasing power densities while the shock-wave duration is correspondingly reduced. Both pressure measurements and plasma observations allow explaining the current limit of high amplitude shock-waves generation by laser plasma in the water-confinement mode and open new research areas for the understanding of breakdown plasma effects at the surface of the confining water. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366113

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Elastic constants for zinc-blende and wurtzite AlN, GaN, and InN are obtained from density-functional-theory calculations utilizingab initiopseudopotentials and plane-wave expansions. Detailed comparisons are made with the available measured values and with results obtained in previous theoretical studies. These comparisons reveal clear discrepancies between the different sets of elastic constants which are further highlighted by examining derived quantities such as the perpendicular strain in a lattice-mismatched epitaxial film and the change in the wurtzitec/aratio under hydrostatic pressure. Trends among results for the three compounds are also examined as well as differences between results for the zinc-blende and wurtzite phases. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366114

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Elemental powder mixes were subjected to plane-wave shock processing which reduced the initial porosity to essentially zero. Two powder mixes in a 5:3 Ti:Si atomic ratio were used: −325 mesh Ti and Si (<45 &mgr;m), and −100 mesh Ti and Si (<150 &mgr;m) with shock pressures up to 7.3 GPa and shock energies up to 671 J/g. Shock pressures were calculated using hugoniot parameters for porous elemental powder mixtures and shock energies were taken to be the work done by the shock(P&Dgr;V/2).Shock energy thresholds for complete reaction of the elemental powders were found which depend upon powder particle size and the initial porosity of the powder. The threshold energy for the larger powder mix was found to be ∼80&percent; larger than that for the smaller powder. A decrease in initial porosity from 0.49 to 0.40 caused an increase in threshold shock energy of about 75&percent; for both powders. At shock energies slightly below the threshold energy, evidence for the reaction of solid Ti and liquid Si was observed in small isolated regions. These regions contained spherical micronodules with the composition ofTiSi2in Si. The results are compared to those of previous studies reported in the literature, and mechanisms for reaction initiation and the observed threshold values are proposed. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366115

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The ability to quantify the complete stress state in solids subjected to shock wave, uniaxial strain loading, is an important need. Toward this end, the dynamic behavior of lateral piezoresistance gauges embedded in shocked ceramics was examined using two-dimensional, numerical calculations to understand the relationship between the mechanical and piezoresistance response of manganin foils and the sample lateral stresses. The results show that the dynamic mechanical states within and near a lateral gauge deviate significantly from uniaxial strain due to perturbations caused by gauge emplacement including the presence of epoxy bonds. The dynamic response of the gauge represents a coupling of the sample material response and gauge emplacement details. In general, direct time-resolved inference of the sample lateral stress from the lateral gauge data, using simple assumptions about the gauge mechanical state, is not meaningful. A rigorous interpretation of the lateral gauge data requires the use of two- or three-dimensional computations. However, the present results show that for an assumed time-independent sample response, the equilibrium gauge response (corresponding to a constant state) is a good measure of the far-field, lateral stress in the shocked sample. Hence, lateral gauge data can be analyzed to provide sample lateral stresses with reasonable accuracy in particular situations. Given the importance of determining lateral stresses in shocked solids, various issues related to the use of lateral piezoresistance gauges including the limitations in extrapolating the present results are discussed. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366116

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The presence of a supersaturation of Si self-interstitials in ion implanted silicon has been shown to be the origin of several physical phenomena such as transient enhanced diffusion (TED) of boron, the formation of extended defects at the projected range of implanted atoms at doses below the amorphization threshold, and the formation of end-of-range (EOR) defects in the case of a preamorphization stage. In this article, we discuss the relation between boron anomalous diffusion and end-of-range defects. Modeling of the behavior of these defects upon annealing allows one to understand why and how they affect dopant diffusion. This is possible through the development of the Ostwald ripening theory applied to extrinsic dislocation loops. This theory is shown to give access to the variations of the mean supersaturation of Si self-interstitial atoms between the loops and also to be responsible for anomalous diffusion. This initial supersaturation is, before annealing, at least five decades larger than the equilibrium value and exponentially decays with time upon annealing with activation energies that are the same as the ones observed for TED. It is shown that this time decay is precisely at the origin of the transient enhancement of boron diffusivity through the interstitial component of boron diffusion. Side experiments shed light on the effect of the proximity of a free surface on the thermal behavior of EOR defects and allow us to quantitatively describe the space and time evolutions of boron diffusivity upon annealing of preamorphized Si layers. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366117

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Hydrogen evolution transients were measured for hydrogenated amorphous silicon prepared by Si implantation of crystalline silicon and subsequent hydrogen implantation. The evolution curves are found to be similar for different H concentrations but with entirely different atomic and nanoscale structures, as was evidenced by small-angle x-ray scattering and infrared absorption investigations [Phys. Rev. B53, 4415 (1996)]. This behavior is explained by a hydrogen-diffusion controlled effusion with a limited density of sites in the amorphous material that can be occupied by hydrogen. The experimental effusion curves are modeled by using diffusion coefficients in the implanted layers that were determined by secondary-ion mass spectrometry. Diffusion through a highly disordered material of low H content is found to have an activation energy of 2.26 eV. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.366118

出版商:AIP    

年代:1997

数据来源: AIP