Index of content:
Volume 78, Issue 1, 01 July 1995
Investigation of microwave π transitions in cesium beam clocks operated with U‐shaped H plane waveguide cavities78(1995); http://dx.doi.org/10.1063/1.360652View Description Hide Description
In this article, we present the characteristics of the ΔF=±1, Δm F =±1, π microwave transitions which appear in the response of cesium beam resonators operating with U‐shaped waveguide Ramsey cavities bent in the plane of the H field. Such resonances are due to the presence in the cavity of microwavemagnetic induction perpendicular to the static field direction. It is shown that the π resonance feature is derived from a pure two‐level atomic system interacting with four spatially separated oscillating fields. Good agreement is found between experimental data and theoretical predictions when we take into account the actual transverse microwave field profile in the cavity obtained with tridimensional electromagnetic field computations.
78(1995); http://dx.doi.org/10.1063/1.360587View Description Hide Description
Electro‐optic properties of thin‐film materials grown on opaque substrates are determined utilizing the principles of reflection differential ellipsometry. The scheme of the measurement involves the detection of the field‐induced phase shift in a probing light beam reflected from thin‐film samples. To quantitatively determine the field‐induced indices change or birefringence and the field‐induced strain it is essential to model the differential ellipsometric process in the stratified structure. The modeling reveals that the field‐induced changes of the ordinary and the extraordinary indices contribute to the measured phase shift with different incident‐angle dependences. The field‐induced strain gives yet another unique incident‐angle distribution. By means of an incident‐angle‐varying technique, therefore, the field‐induced changes of ordinary index and extraordinary index in a thin‐film material may be determined separately. Detailed descriptions of the measuring technique and the modeling work are presented.
Effects of azimuthal and radial spreads of canonical momentum on electron‐beam focusing characteristics in the presence of space‐charge forces78(1995); http://dx.doi.org/10.1063/1.360647View Description Hide Description
A theoretical and numerical investigation of the effects of azimuthal and radial spreads of canonical momentum on an electron beam focused by a magnetic lens in the presence of space‐charge forces is presented. The particles are inserted with an initial Gaussian distribution in the transverse space and in the momentum coordinates or with a uniform initial current distribution. The particle trajectory equation is derived for parameters of an arbitrary applied fields configuration with cylindrical symmetry, and a nonvanishing initial canonical momentum. In the absence of an initial momentum spread particles launched above a critical radial distance from the axis exhibit a phase‐space tearing effect in the electron distribution. The inclusion of initial canonical momentum spread in the model allows for skewed trajectories with strong centrifugal force which prevents the appearance and overshadows the effect of strong space‐charge forces near the axis, which are responsible for the phase‐space tearing effect.
78(1995); http://dx.doi.org/10.1063/1.360667View Description Hide Description
Numerical simulations of quantum tunneling with time‐dependent barriers show that there is a resonance, with a marked increase in the tunneling current. For square barriers the resonance occurs when the tunneling particles absorb modulation quanta and the length of the barrier is a multiple of one‐half de Broglie wavelengths. The resonance has a similar mechanism with triangular barriers. However, the relationship is more complex because the absorption and emission of modulation quanta takes place throughout the full length of the barrier, whereas this exchange only occurs at the ends of a square barrier.
Effects of Penning reactions and excitation rate on the pulsed transverse‐discharge neon laser for photodynamic therapy78(1995); http://dx.doi.org/10.1063/1.360677View Description Hide Description
Theoretical analysis of the output performance of a transverse discharge pumped neon Penning laser (585.3 nm) using a mixture of Ne/H2 is described. The validity of the kinetic model is confirmed by comparing the results to the experimental discharge and laser performance. It is theoretically shown that the optimum mixing ratio of the Ne/H2 mixture is 1:2.5, and the optimum operating pressure is about 56 Torr. The model also predicts that the intrinsic efficiency reaches a peak of 8.5×10−6 at an excitation rate of 0.5 MW/cm3 under the optimum mixing ratio and operating pressure conditions. At excitation rates in excess of 0.5 MW/cm3 the laser output power is slowly increasing and then saturates due to electron collisional quenching of the upper laser level. The laser power extraction is increased by laser injection seeding in order to rapidly build up the lasing. The improved intrinsic efficiency is about two times higher than without the injection seeding. The improved specific laser output is 8 W/cm3, therefore, a discharge volume of 125 cm3 will be able to generate the peak laser power reaching 1 kW. This power value is sufficient to obtain the same treatment effect as the gold vapor laser used in photodynamic therapy. Moreover, by fitting this model to the experimental results of the laser output energy with a Ne/D2 mixture, it is shown that the Penning ionization rate constant of H2 is larger than that of D2.
78(1995); http://dx.doi.org/10.1063/1.360615View Description Hide Description
In this article, we address the nonlinear properties of bulk acoustic waves in anisotropic solids. While ordinary driving waves are obtained by means of the quasiwave mode concept in the discussion on propagation problems, we focus our analysis on the driving wave satisfying the resonant condition which exhibits the characteristics of three‐dimensional spatial accumulation. Then a theory as the basis for solving nonlinear boundary value problems is developed. The novel analysis is presented for nonlinear reflection of bulk acoustic waves at a free surface of a generally anisotropic solid, which reveals that the surface may generate freely propagating second‐harmonic waves, and that the reflective second‐harmonic waves increase two dimensionally in the bulk. This technique can also be applied to wave reflection problems at other kinds of planar boundaries. Some numerical calculation is carried out for the case in an isotropic solid.
On the role of atomic oxygen in the etching of organic polymers in a radio‐frequency oxygen discharge78(1995); http://dx.doi.org/10.1063/1.360630View Description Hide Description
The kinetics of rise and decay of ground‐state atomic oxygen was investigated in a low‐pressure O2 radio‐frequency discharge. The ground state of O was monitored using laser‐induced‐fluorescence spectroscopy. The discharge was operated in two modes: a continuous mode and a pulsed mode. Typical discharge settings are rf power of 100 W, a pressure of 10 Pa, and a gas flow of 10 sccm. Using a pulsed mode the behavior of the O density was determined in a pure O2discharge with no reaction products, and in a discharge during etching of organic polymers. Production and loss mechanisms for O in a pure O2discharge are discussed. For an O2discharge during etching of resist it is found that under the given experimental conditions atomic oxygen does not play a significant role in the chemical etch mechanisms involved.
78(1995); http://dx.doi.org/10.1063/1.360640View Description Hide Description
A collisionless dynamic sheathmodel for multispecies plasmas has been derived for the application of plasma immersion ion implantation(PIII) processes with multispecies dopant gases. In this dynamic sheathmodel, a general form of an effective massM, which includes species compositions and ion masses, is used to determine the sheathexpansion during the pulse. The total ion current, total implant dose, and the corresponding dose components for different species can also be determined with this effective mass. This offers a method for easily evaluating PIII processing and obtaining the correct implant doses for different species. A plasma simulation is used to compare this model.
78(1995); http://dx.doi.org/10.1063/1.360582View Description Hide Description
Visible photoluminescence at room temperature has been observed in amorphous hydrogenated silicon particulates during their formation in a silane radio‐frequency plasma. Oxygen injection along with mass spectrometrymeasurements demonstrate that oxygen has no influence on the photoluminescence. The appearance of visible photoluminescence coincides with a particle agglomeration phase as shown by laser light scattering experiments, and electron microscopy shows siliconnanocrystals within these particulates. These observations of visible photoluminescence are consistent with the model of quantum confinement in the siliconnanocrystals.
78(1995); http://dx.doi.org/10.1063/1.360583View Description Hide Description
A theoretical model for the microsecond‐conduction‐time plasma opening switch (POS) based on magnetohydrodynamics is presented. The operating processes are associated with the magnetic field transport in the switch. The concept of the freezing‐in of magnetic field in fluids is used to analyse the dynamics of this field in the plasma. One‐dimensional and two‐dimensional numerical simulations lead to the identification of two operating regimes, related to the Hall and the snowplow effects. The transition between these two regimes is determined by the plasma density. Scaling relations are developed from this model. In particular the electrical chargeQ, conducted by the POS during the conduction time, provides a useful tool for comparisons between theory and experiments; it is shown to be the essential parameter for the switch design.
78(1995); http://dx.doi.org/10.1063/1.360584View Description Hide Description
Triggered breakdown in hollow cathode discharges in geometries similar to those used for pseudospark switches and pseudospark pulsed electron beams has been investigated experimentally and with a two‐dimensional model previously developed. A systematic study of the influence of the discharge conditions (applied voltage and pressure), geometry, and trigger conditions (trigger intensity and position) on the time to breakdown in helium is presented, and some data are also shown for argon. Excellent qualitative agreement is found between the model predictions and the experimental results. The relation between the time to breakdown and the geometrical distribution of injected charge is discussed, and the understanding gained from these model results is used to suggest guidelines for trigger optimization. Conditions wherein significant oscillations in the current—a ‘‘current quenching’’ effect—are observed in the prebreakdown current wave form are discussed.
Measurements of electromagnetic fields in a planar radio‐frequency inductively coupled plasma source78(1995); http://dx.doi.org/10.1063/1.360585View Description Hide Description
Electromagnetic fields in a planar radio‐frequency inductively coupled plasmasource were measured using an inductive loop (B‐dot) probe. The probe was oriented to measure the time derivative of the axial component of the magnetic field (Ḃ z ). Using these measurements and Faraday’s law, taking advantage of cylindrical symmetry, the time varying azimuthal electric field (E φ) was calculated directly. Contour plots of Ḃ z and E φ in the r‐z plane show that the radio‐frequency electromagnetic fields penetrate further into the plasma at lower gas pressure and lower rf power, corresponding to less effective shielding of the fields at lower plasma density. Estimates of skin depth from the axial decay of the field amplitudes near the axis of the discharge are consistent with values calculated from plasma parameters measured with Langmuir probes, confirming that near the axis the degree of shielding is most strongly dependent on the local plasma density. Near the conducting walls of the chamber, the skin depth calculations from the Langmuir probe data diverge from the B‐dot probe data. B‐dot probe measurements taken in the absence of plasma show that near the walls of the chamber the axial decay of the field amplitude is partly a geometrical effect in addition to a plasma shielding effect.
78(1995); http://dx.doi.org/10.1063/1.360586View Description Hide Description
The damage produced at the interfaces in a sample of GaAs/Al0.6Ga0.4As/GaAs that has been subjected to ion implantation at 77 and 293 K with 1 MeV Kr+ ions has been investigated using a combination of ion channeling and transmission electron microscopy(TEM) techniques. Low temperature ion‐channeling spectra obtained from samples implanted at 77 K, to an ion dose of 1014 ions cm−2, were similar to the random spectrum, indicating that the GaAs and Al0.6Ga0.4As layers had sustained a considerable degree of damage. An asymmetric signal developed in the He+ ion‐channeling spectrum as the sample warmed to room temperature. The backscattering yield corresponding to the bottom interface (i.e., Al0.6Ga0.4As grown on GaAs) resembled that of the random yield, whereas that from the top interface (GaAs grown on Al0.6Ga0.4As) decreased, shifting toward the unirradiated channeled spectrum. This observation suggests that the damage produced near the top of the Al0.6Ga0.4As layer is thermally unstable. Cross‐sectional TEM images reveal a greater amount of damage in the form of extended defects and amorphous regions at the bottom interface than at the top one. This difference is sufficient to account for the observed asymmetry in the channeling spectra. Increasing the ion dose to 1015 ions cm−2 produced a damage state throughout the Al0.6Ga0.4As layer that was stable at both 77 and 293 K. TEM examination revealed that at this ion dose the GaAs and Al0.6Ga0.4As layers were both amorphous. Room‐temperature implantation to a dose of 1×1016 ions cm−2 was also performed. Planar defects were observed at both interfaces, although their density appeared to be greater near the bottom interface.
In addition, the bottom interface was rougher than the top. The difference in the damage states at the bottom and top interfaces can be attributed to a variation in the number of displacement cascade events as a function of depth through the Al0.6Ga0.4As layer. This variation in the number of cascades results in different amounts of ion mixing at the top and bottom interfaces.
78(1995); http://dx.doi.org/10.1063/1.360658View Description Hide Description
A highly unusual electric response of the dislocations and the impurity dipoles in KNbO3single crystals has been found. The etch pits representing these defects move in the structure carrying these defects with them. This shows that the defects have associated with them structural units that are orders of magnitude larger in physical size. Naturally these units are also associated with stress fields and electric effects on account of the ferroelectric nature of the crystal. The nature of the stress fields could be determined by studying the movement of the etch pits in response to the externally applied electric fields. It is found that the impurity dipoles in the observed pseudocubic (001) surface have stress fields similar to those of screw dislocations, and the dipoles in the front and side faces have stress fields similar to those of edge dislocations. The external dc field causes rotation of the dislocation loops and, hence, changes the domain structure. It may order or disorder the dislocations, the effect being reversible. Similar effects have been observed for impurity dipoles. Several consequences of the stress fields have been considered theoretically and observed experimentally. For example, the stress fields of impurity dipoles and dislocation loops lead to their mutual exclusion. Hence, at a given place in the crystal the domain structure is nucleated either by the dislocation loops or by the impurity dipoles. The conjoint domain nucleation, sometimes possible by an impurity dipole and a dislocation loop worked out on theoretical considerations, agrees with experimental observation. Large distance mutual interaction, bulk ordering, dependence of bulk ordering on the density of defects, and domain nucleation have all been explained on the basis of the structural units associated with the impurity dipoles and the dislocation loops.
Study of a〈001〉 and a〈110〉 screw dislocations viewed end‐on in crystal Ba(NO3)2 by birefringence topography78(1995); http://dx.doi.org/10.1063/1.360659View Description Hide Description
The screw dislocations with Burgers vectora〈001〉 and a〈110〉 in Ba(NO3)2 crystals have been studied by birefringencetopography. In combination with the results obtained previously [C.‐Z. Ge, N.‐B. Ming, K. Tsukamoto, K. Maiwa, and I. Sunagawa, J. Appl. Phys. 69, 7556 (1991); C.‐Z. Ge, H.‐W. Wang, and N.‐B. Ming, ibid. 74, 139 (1993)], it has been shown that not only a〈111〉, but also a〈110〉 screw dislocations viewed end‐on are visible; however, a〈100〉 screw dislocation viewed end‐on are always invisible in barium nitrate crystals. The intensity distributions and extinction rules of birefringence images of a〈110〉 screw dislocation viewed end‐on are determined both experimentally and theoretically.
78(1995); http://dx.doi.org/10.1063/1.360660View Description Hide Description
By combining the complementary techniques of x‐ray diffraction and transmission electron microscopy we have been able to accurately determine the structure of imperfect GeSi superlattices (SL’s). The samples were epitaxially grown on Ge(001) substrates using Si2H6 and GeH4 source gases. In this report, details of the x‐ray experiment and analysis are emphasized. In particular, a model is presented for calculating the diffracted intensity from a SL containing gradients in composition and thickness. Applying this model to the data it is found that between the first and last layers of each SL there exists a roughly 10% increase in the growth rate, without a corresponding change in the alloy composition. This is attributed to a slow increase in the substrate temperature, T sub, of just a few degrees. A sample grown with a corresponding gradual decrease in the control temperature, T C , was found to be much more uniform.
78(1995); http://dx.doi.org/10.1063/1.360661View Description Hide Description
An effective interatomic potential suitable for all body‐centered‐cubic (bcc) metals is developed based on the embedded atom method. The potential predicts all major anomalies displayed in the phonon spectra of the bcc transition metals, as well as the large relaxation of the (100) surface of V, Nb, and Ta.
78(1995); http://dx.doi.org/10.1063/1.360662View Description Hide Description
In this article, recent results employing scanning tunneling microscopy‐based techniques for the generation of nanometer‐scale patterns on hydrofluoric acid treated silicon(100) and YBa2Cu3O x superconducting thin films are presented. Furthermore, we were able to extract silicon (Si) atoms from Si(100)−1×1 surfaces, thereby producing siliconvacancies in the surface. These results thus demonstrate a possible approach for the construction of an atomic scale data memory as well as fabrication of artificial nucleation sites. The emission mechanism is believed to be field assisted evaporation due to the close proximity of the surface and the probe of the microscope.
Study of the mechanical nanocrystallization process of amorphous Fe78B13Si9 alloy using the pair function method78(1995); http://dx.doi.org/10.1063/1.360663View Description Hide Description
An amorphous Fe78B13Si9 alloy was subjected to high‐energy ball milling. Studies were performed for several increasing milling times by means of differential scanning calorimetry and x‐ray diffraction. The initial alloy and the milled samples were studied using the pair function method. The analysis of the results has shown that in an amorphous phase defects are created during the first stage of milling, leading to a modified crystallization mechanism. Bulk nucleation is the next stage. The process of the nucleus growth is very limited.
Helium reemission, desorption and microstructure evolution of graphites under helium ion implantation78(1995); http://dx.doi.org/10.1063/1.360664View Description Hide Description
Helium reemission, trapping, and thermal desorption from highly ordered pyrolytic graphite (HOPG and HPG) of different orientation, polycrystallinegraphite (EK98), and titaniumdopedgraphite (RG‐Ti‐91) have been measured at irradiation temperatures of 300 K and 800 K. The implantation was performed with a 40 keV 4He ion beam. Detailed transmission electron microscopy(TEM) investigation of the microstructure evolution was made on the implanted specimens. He reemission from basal oriented (BO) highly oriented pyrolytic graphite is accompanied by blistering and flaking leading to repetitive gas bursts. On edge oriented (EO) pyrolytic graphite three reemission peaks are observed during room temperatureimplantation, the first and smallest one being assigned to He release from intrinsic lenticular cavities, the second one occurs during early bubble formation when a small amount of implanted gas still escapes accumulation, and the third and largest peak being due to He release by bubble coalescence. The He reemission rate grows very slowly at room temperature and does not reach 100% up to the highest implanted fluence of 3.5⋅1018 He/cm2. At 800 K the He reemission rate from EO pyrolytic graphite reaches 100% immediately after starting implantation due to the high diffusive mobility of He.
EK98 and RG‐Ti‐91 show similar reemission behaviour. No gas bursts due to blistering are observed. The initial reemission rate at 300 K is higher than in EO pyrolytic graphite due to release of He via a network of intergranular channels. At 800 K reemission is rather similar to that from EO pyrolytic graphite. No thermal desorption of He from BO HOPG up to 1200 K is observed for implanted fluences ≤1016 He/cm2. At higher fluences the onset temperature of desorption decreases from 750 K at 2⋅1016 He/cm2 to 380 K at 1017 He/cm2 caused by thermal flaking due to pressure increase of He in submicroscopic cracks. In the other materials two desorption peaks are observed, the first one being related to He release from ‘‘solid solution,’’ while the second is attributed to gas escape from He filled bubbles. In contrast to the conclusions of Niwase et al. we find from the reemission kinetics and from selected area electron diffraction patterns (SADP) that graphiteimplanted at 300 K with He up to a damage of 10 dpa and more cannot be regarded as amorphous and shows a distorted turbostratic structure. The c parameter increases to (3.6..3.7)Å. Radiation effects in graphiteimplanted at 800 K are less pronounced up to damage levels of 200 dpa but may decrease the He diffusion coefficient.