摘要:

This is the first report of the fact that absorption of a cw laser beam by a high‐pressure arc may result in a fluorescence signal from the arc that has a negative sign. The laser‐induced fluorescence (LIF) is detected at a number of optical sodium and mercury lines, while the wavelength of the laser is tuned in the wings of the Na‐Dlines. The negative fluorescence signal is found if the laser beam is chopped at frequencies below about 100 Hz. The amplitude and the sign of the fluorescence signal appear to depend strongly on the relative spatial positions of the laser beam and the arc volume from which the fluorescence is detected. At higher chopper frequencies the generally expected positive LIF is regained. The frequency region where the transition occurs depends on the total gas pressure in the arc. A model is presented which explains the negative fluorescence signal. In this model the absorbed laser power affects the electrical conductivity of the plasma. This causes a distortion of the electrical current distribution, leading to a change in the electrical heating of the gas in the arc. Based on this model, results are calculated which qualitatively agree with the experimental results. We discuss the consequences of the discovery of the negative fluorescence for the application of LIF in arc diagnostics.

ISSN:0021-8979    

DOI:10.1063/1.342100

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Plasma sheaths are often assumed to be collision free; however, high‐voltage cathode sheaths are typically thicker than the mean free path for charge transfer collisions at pressures encountered in glow discharge processing equipment (greater than 10 mTorr). In this paper, the potential distribution in a plasma sheath is determined by solving Poisson’s equation self‐consistently using a kinetic theory nonlocal ion transport model for charge transfer collisions. The relationship between the potential distribution, ion flux, and thickness of a plasma sheath is presented for arbitrary values of the sheath thickness relative to the mean free path for charge transfer. The results may be used to estimate the ion flux from measurements of the sheath thickness and potential drop across the sheath. Ion energy distribution functions and a one‐parameter approximation to the numerically determined potential distribution are also presented. These results apply to rf discharges in a time‐averaged sense when the ion sheath transit time is much longer than the rf cycle time, and they apply to high‐voltage cathode sheaths in ‘‘abnormal’’ dc and low‐frequency rf discharges. The present model is compared to earlier self‐consistent sheath models, including the collision‐free approximation, the local mobility model, and a nonlocal fluid approximation known as the viscous drag model.

ISSN:0021-8979    

DOI:10.1063/1.342077

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Sources of root‐mean‐square (RMS) emittance growth are described for negative ion extractors in both volume and surface negative ion sources and plasma low‐energy beam transport systems. For surface negative ion sources of the Lawrence Berkeley Laboratory or Los Alamos National Laboratory [on Los Alamos Meson Particle Facility (LAMPF)], attention is paid to the nonlinear transverse emittance growth mechanism of the beam/warm plasma interaction. In some cases this is a large effect. In addition, non‐normal sheath fields at a convertor are examined as a source of emittance growth. For volume sources, attention is paid to aberration production due to field penetration from the extractor, RMS emittance growth enhancement by a negatively biased plasma extraction electrode, and emittance growth caused by transverse extraction across a magnetic field. For both volume and surface sources, RMS emittance growth due to nonlinear aberration at the plasma extraction electrode is analyzed. Time‐dependent contributions to emittance growth are also examined. Nonlinear saturation of ion acoustic waves caused by ion extraction from a warm plasma and beam transversal through a plasma are studied.

ISSN:0021-8979    

DOI:10.1063/1.342078

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Elements from groups IB, IIIB, VB, and VIIB have been added to crystals of Hg(1−x)Cd(x)Te grown using the accelerated crucible rotation technique. Assessment on both as‐grown and annealed (in Hg vapor) samples was carried out using Hall effect measurements to establish the electrical activity and stability of each dopant. Chemical analysis by atomic absorption spectroscopy and laser scan mass spectrometry determined the total levels of dopants in each crystal. The behavior of dopants in these crystals is compared to that found in Bridgman and epitaxial material and discussed in terms of the stoichiometry of melts and crystals at the growth temperature. It is concluded that group VB and VIIB elements are prevented from occupying Te sites when there is a high Hg vacancy concentration present during growth. Gold, which is inactive in Bridgman material, is found to be an active acceptor in a crystal grown using the accelerated crucible rotation technique. Group IB and IIIB elements are acceptors and donors, respectively, on metal sites irrespective of stoichiometry.

ISSN:0021-8979    

DOI:10.1063/1.342079

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The orientation structure of ferroelectric liquid‐crystal cells in layer‐normal geometry was studied based on a one‐dimensional elastic continuum model which included three nonequal elastic constants. By minimizing the distortion energy and the surface energies, the critical cell thickness for the uniform, splayed, and helical states has been obtained as functions of the cone angle, the helical pitch, the elastic constants, and the surface interaction coefficients. By a proper choice of the handedness of the liquid crystal helicoid and the material constants, the critical cell thickness for the uniform state can be made very large, and the constraint on small polar surface‐interaction coefficient for the stabilization of uniform state can be relaxed to a great extent. It is also shown that bistable states can only be stabilized in cells with no polarity difference between the substrate surfaces.

ISSN:0021-8979    

DOI:10.1063/1.342080

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Pure Fe is implanted with B+in a target temperature range of −20–300 °C. The effects of the target temperature on the crystallinity and hardness of the B+‐implanted Fe are investigated. It is found that Fe2B is formed by using direct B+implantation into pure Fe at target temperatures ranging from −20 to 300 °C without subsequent annealing. ‘‘Cold‐implantation,’’ i.e., implantation at a lower target temperature, reduces crystallinity and increases the hardness of the B+‐implanted Fe. By contrast, increased target temperature produces B+‐implanted Fe with high crystallinity and relatively low hardness. Discussion focuses on how the target temperature during implantation affects the crystallinity of the B+‐implanted Fe from the standpoint of the recrystallization during implantation.

ISSN:0021-8979    

DOI:10.1063/1.342081

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Using the intermediate neglect of differential overlap approximation, the total energies of a number of clusters modelled to represent the SiC/Al interface have been calculated as a function of distance separating the two surfaces of SiC and Al. The adhesive energy of the interface is calculated by minimizing the total energies corresponding to various crystallographic orientations of the SiC and Al surfaces. The results are used to provide semiquantitative understanding of the bonding mechanism and the effect of reconstruction at the SiC/Al interface.

ISSN:0021-8979    

DOI:10.1063/1.342082

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Numerous studies have shown that the threshold voltage (Vth) of field‐effect transistors fabricated on Czochralski‐grown semi‐insulating GaAs substrates using silicon ion implantation is sometimes dependent and sometimes independent of proximity to dislocations, according to the processing method used. We present a comprehensive model that reconciles these diverse observations and semiquantitatively describes the dependence ofVthon EL2 concentration, boron concentration, silicon fluence, proximity to a dislocation, and post‐implant annealing method. Under conditions holding in most of the cited studies, silicon fluence between 2×1012cm−2and 5×1012cm−2at 60 keV followed by an anneal with a SiN cap, we find thatVthdepends on the substrate concentration of EL2 and on the concentration of the boron‐silicon acceptor complex BGaSiAs. We suggest that a nonuniform stress field across a wafer reduces the binding energy of BGaSiAscomplexes within 50 &mgr;m of a dislocation resulting in a local increase in silicon activation and a more negativeVth. When the post‐implant anneal is done under a SiO2cap we suggest that the infusion ofVGadue to the out‐diffusion of Ga eliminates the stress field nonuniformities and results inVthshowing no dislocation‐proximity effect. Under these latter conditions we find that any variation in local substrate EL2 concentration across the wafer is reflected in a variation ofVth.

ISSN:0021-8979    

DOI:10.1063/1.342083

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

The minority‐carrier lifetime inn‐type GaAs‐on‐sapphire structures has been measured by transient laser‐induced photoluminescence. Single‐ and double‐heterojunction structures grown by organometallic vapor phase epitaxy have been examined and yield values of lifetime between 0.1 and 0.4 ns. Transmission and scanning electron microscopic analyses of layers show that the best surfaces and lowest defect densities resulted from isothermal growth at 690 °C. Threading dislocations in the range of 5×107to 6×108cm−2are observed in GaAs grown on (0001)‐oriented sapphire substrates.

ISSN:0021-8979    

DOI:10.1063/1.342084

出版商:AIP    

年代:1988

数据来源: AIP

摘要:

Acoustic emission (AE) signals generated by microcrack formation in front of a propagating fatigue crack in a thin‐plate compact tension specimen of 7075‐T6 aluminum alloy were analyzed to investigate the dynamics of microcrack formation. The dominant wave motion was in‐plane so that the dynamic elastic wave field could be analyzed by generalized plane stress theory. The acoustic signals were detected by an array of four miniature piezoelectric transducers which were mounted on the edge of a specimen ahead of the advancing crack. A second‐rank force moment tensor was used to model the microcrack formation and a time‐domain double‐iterative signal deconvolution method was used to process the detected acoustic signals to remove the effects of the structure and to recover the temporal and spatial characteristics of the AE source. It was found that a force moment tensor model can be used to represent the growth of the microcrack.

ISSN:0021-8979    

DOI:10.1063/1.342085

出版商:AIP    

年代:1988

数据来源: AIP