摘要:

A new method for obtaining spatially resolved electron energy distributions is described and applied to cylindrical bore electric discharges. The method is based on a modified two‐term spherical harmonic expansion of Boltzmann’s equation in which energy‐resolved drift and diffusion are included, as well as Joule heating or cooling by the ambipolar field. It is found that low‐energy electrons may be heated by the large ambipolar electric field gradients near the wall while higher‐energy electrons may escape the plasma. Therefore, diffusion cooling and Joule heating in the ambipolar field may simultaneously occur, but for different portions of the electron energy distribution.

ISSN:0021-8979    

DOI:10.1063/1.353295

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

New experimental results are reported on plasma initiation in front of a titanium sample irradiated by ir (&lgr;=10.6 &mgr;m) laser pulses in an ambient gas (He, Ar, and N2) at pressures ranging from several Torr up to the atmosphere. The plasma is studied by space‐ and time‐resolved emission spectroscopy, while sample vaporization is probed by laser‐induced fluorescence spectroscopy. Threshold laser intensities leading to the formation of a plasma in the vapor and in the ambient gases are determined. Experimental results support the model of a vaporization mechanism for the plasma initiation (vaporization‐initiated plasma breakdown). The plasma initiation is described by simple numerical criteria based on a two‐stage process. Theoretical predictions are found to be in a reasonable agreement with the experiment. This study provides also a clear explanation of the influence of the ambient gas on the laser beam‐metal surface energy transfer. Laser irradiation always causes an important vaporization when performed in He, while in the case of Ar or N2, the interaction is reduced in heating and vaporization of some surface defects and impurities.

ISSN:0021-8979    

DOI:10.1063/1.353271

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

The alloying behavior of copper atoms into gold clusters at temperatures lower than 300 K was examined using a 200 kV transmission electron microscope equipped with a double source evaporator in the specimen chamber. Isolated gold clusters of about 4 nm in mean diameter were prepared on a supporting film and were cooled down to 245, 215, 165, or 125 K. Copper atoms were then deposited onto the same film. Upon the deposition of copper, the gold clusters quickly changed into completely or partially alloyed clusters, depending upon the substrate temperature. The copper diffusivity estimated from such spontaneous alloying was many orders of magnitude faster than that in bulk gold. A possible mechanism behind the spontaneous alloying is discussed.

ISSN:0021-8979    

DOI:10.1063/1.353272

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Ultrathin (∼1.3 nm) epitaxial films of &bgr;‐FeSi2were grown on Si(001) by room temperature (RT) deposition of Fe followed by annealing. During the various stages of the growth process, the lattice structure, composition, and morphology of the films were investigated by medium‐energy ion scattering in conjunction with shadowing and blocking. At RT, the deposited Fe reacts with the Si(001) substrate and forms a continuous film of average composition FeSi. After annealing to 670 K, a conversion into &bgr;‐FeSi2has taken place and the film is no longer continuous. Further annealing at higher temperatures results in the formation of islands of increasing height. The &bgr;‐FeSi2films grown are composites of two azimuthal orientations with respect to the substrate: The predominantAorientation with &bgr;‐FeSi2[010]∥ Si⟨110⟩ and theBorientation with &bgr;‐FeSi2[010] ∥ Si⟨100⟩. The lattice strain in the films is partially relaxed. At the interface, the Fe atoms are found to be displaced from bulk lattice sites. These displacements are thought to be associated with the formation of atomic bonds at the interface of the dissimilar &bgr;‐FeSi2(100) and Si(001) lattices.

ISSN:0021-8979    

DOI:10.1063/1.353273

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Defects in lithium vanadate (Li3VO4) exposed to x rays were studied by electron spin resonance (ESR). 77 K irradiation produces two types of trapped‐hole centers. One is supposed to be carbonate radical CO3−, which is thermally stable even at room temperature. Another, having a hyperfine interaction with a vanadium nucleus (I=7/2, 99.76% natural abundant) is attributable to an intrinsic O−type center with one neighboring vanadium ion. The center is thermally unstable and anneals out around 200 K. In spite of the prominent detection of trapped‐hole centers, ESR signals due to trapped‐electron centers have not been clearly observed.

ISSN:0021-8979    

DOI:10.1063/1.353274

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Electron paramagnetic resonance (EPR), electron‐nuclear double resonance, optical absorption, and thermoluminescence have been used to investigate radiation‐induced point defects in a single crystal of lithium triborate (LiB3O5). Two prominent defects are observed after irradiation near liquid‐nitrogen temperature with 60 kV x rays. A four‐line EPR spectrum, with 12.2 G splittings, is assigned to a trapped‐hole center, and another four‐line EPR spectrum, with 120 G splittings, is assigned to a trapped‐electron center. In each case, the nucleus responsible for the observed hyperfine is11B. The trapped hole is localized on an oxygen ion and has a weak hyperfine interaction with one neighboring boron nucleus, whereas the trapped electron is localized primarily on a boron ion with a correspondingly larger hyperfine interaction. Both defects become thermally unstable near 125 K, and their decay (i.e., recombination) correlates with an intense thermoluminescence peak at this same temperature. An optical absorption peak at 300 nm is produced by the x rays and thermally decays at the same temperatures as the EPR spectra.

ISSN:0021-8979    

DOI:10.1063/1.353275

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Transient charging currents in nematic liquid crystals have been measured with a single‐ and a double‐voltage pulse technique to elucidate the role of impurity ions incorporated in the materials in the transient charging processes. In the single pulse experiment, a peak in the current transient is observed in the nematic phase. It is shown that occurrence of the peak is due to the alignment of the director of the liquid crystal molecules in the direction of applied electric field. In the double pulse experiment, a voltage pulse (prepulse) is applied to the liquid crystal before the measurement of the transient current. It is observed that the application of the prepulse significantly alters the shape of the current. The changes in the current can be explained by considering the impurity ion distribution, modified by the prepulse, in the nematic liquid crystals. It is concluded that the double pulse experiment is a powerful tool for the examination of the influence of impurity ions in liquid crystals on the current transients.  

ISSN:0021-8979    

DOI:10.1063/1.353276

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

High‐energy Fe and Co implantations were performed into InP:Sn at room temperature and 200 °C in the energy range 0.34–5.0 MeV. Range statistics were calculated for these ions in the above energy range. For the room‐temperature implants, implant redistribution peaks around 0.8RpandRp+&Dgr;Rp, and both in‐ and out‐diffusion of the implant are observed in the secondary‐ion‐mass‐spectroscopy profiles of the annealed samples. The implant redistribution present in the room‐temperature implants is much different than in elevated‐temperature implants. For buried (high‐energy) implants, much of the implant diffusion is eliminated if the implants are performed at 200 °C. For 200 °C implants, the yield of the Rutherford backscattering spectra on the annealed samples is close to that of a virgin sample. The MeV energy Fe and Co implantations at 200 °C are useful to obtain thermally stable, buried, and high‐resistance layers of good crystalline quality inn‐type InP and for the compensation of the tail of the buriedn‐type implant. However, due to the low solubility of Fe and Co in InP, the implants of these species are useful only to compensaten‐type carriers with concentrations below 1017cm−3.

ISSN:0021-8979    

DOI:10.1063/1.353277

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

A systematic study of O implantation into Si at MeV energies and stoichiometric doses is reported. Si substrates were implanted with 1 MeV O ions to doses of 0.73, 1.45, and 2.18×1018cm−2at implant temperatures of 150, 300, and 450 °C. The samples were then subjected to a high temperature anneal (1250 °C for 2 h). Cross‐sectional transmission electron microscopy and Rutherford backscattering combined with channeling were used to study the resultant defect structures. The microstructure after annealing was found to be highly implant temperature dependent while very little dose dependence was observed. In the Si overlayer, twinning and polycrystalline Si were observed at the lowest implant temperature while SiO2precipitation was apparent at all temperatures with precipitate size increasing with implant temperature. In the SiO2layer, Si islands were observed at the high implant temperatures. A comparison is made between MeV and conventional implant energies. At MeV implant energies and stoichiometric doses, crystallinity in the Si overlayer is retained even at implant temperatures as low as 150 °C. Such temperatures are shown to be advantageous for inhibiting SiO2precipitation in the Si overlayer.

ISSN:0021-8979    

DOI:10.1063/1.353278

出版商:AIP    

年代:1993

数据来源: AIP

摘要:

Modern semiconductor processes, in which low temperatures, short times, and ion implant or oxidation‐induced interstitials may occur, raise the possibility that the mobile species may not be in equilibrium with their immobile parent substitutional dopants. The simplest possible example of nonequilibrium diffusion is analyzed and shown to occur in three stages. In the first, where the diffusion time is short compared with the lifetime of the mobile species, the dopant profile is an error function complement. The second stage, in which the diffusion time is long compared with the lifetime of the mobile species but short compared with that of the parent species, yields an exponential profile. The third stage, for long diffusion times, corresponds to the usual equilibrium assumption, and the measured profile is again a complementary error function.

ISSN:0021-8979    

DOI:10.1063/1.353279

出版商:AIP    

年代:1993

数据来源: AIP