Volume 43, Issue 7, 01 July 1972

Surface‐Charge Decay in Insulators with Nonconstant Mobility and with Deep Trapping
View Description Hide DescriptionThe motion of space charge initially located at the free surface of an insulator is studied as a function of time. Analytic results are given for the following cases: (a) mobility proportional to a power of the field; (b) mobility proportional to a power of the free‐carrier concentration; and (c) fast deep trapping which goes to completion. After one transit time, the surface voltage becomes a unique function of time, independent of its initial value. The available experimental data for polyethylene do not fit these simple models, and the discrepancies are discussed. It is also shown that the observation of thermally stimulated currents in thin‐film electrets implies that they are electrically inhomogeneous.

Interaction between Dislocation Pile Ups
View Description Hide DescriptionThe interaction between two pile ups containing equal number of dislocations in four different configurations is calculated by an approximate analytical method. The calculated lengths of the interacting pile ups are compared with those obtained numerically, and shown to be in good agreement. Both analytical and numerical results show that the interaction between the pile ups is appreciable when the interplanar spacing of the pile up is less than the length of an isolated pile up. The analytical method presented here could prove useful in the study of strain hardening of solids.

Ultrasonic Harmonic Generation in Piezoelectric Semiconductors
View Description Hide DescriptionSecond‐harmonic generation of ultrasound in piezoelectricsemiconductors is investigated using a quantum treatment which is valid at high frequencies and in strong magnetic fields. The effects of collisions are neglected so that our treatment is valid only for ql≫1. It is shown that the amplitude of the second harmonic can be expressed in terms of the fundamental using the linear and nonlinear conductivity tensors. The latter quantities are related to the current densities which are linear and nonlinear in the piezoelectric fieldsgenerated by the ultrasound. The linear and nonlinear conductivities are calculated using the parabolic and nonparabolic models for the energy bands of a semiconductor. It is found that only those sound waves which induce longitudinal electric fields will contribute appreciably to second‐harmonic generation. For waves propagating parallel to a magnetic field, we find that the amplitude of the second harmonic is independent of the magnetic field for the parabolic model. It is also found that the acoustic flux in the second harmonic will peak at sound wave vectors of the order of the Debye wave vector. In contrast, for the nonparabolic model it is found that the amplitude of the second harmonic depends upon magnetic field. The results of the calculation are applied to n‐type InSb, and the magnitude of the second‐harmonic generation is estimated for ultrasonic frequencies in the microwave region. It is found that the magnitude of the second harmonic is considerably enhanced using the nonparabolic model over what would be predicted using the parabolic model.

Elastic Moduli of Diamond as a Function of Pressure and Temperature
View Description Hide DescriptionValues of wave velocities and elastic moduli at 25°C were measured for hydrostatic pressures to 20 000 psi (excess over 1 atm). Variations of velocities and moduli at 1 atm were obtained over a temperature range of +50°C to −195.8°C. Adiabatic stiffness moduli (units of 10^{12} dyn/cm^{2}), their pressure derivatives, and their temperature coefficients (units of 10^{−5}/C), are shown below for 1 atm and 25°C.

Burgers Vector of Dislocations Generated by Small Stresses in Copper Crystals
View Description Hide DescriptionIn an x‐ray topography study of the deformation of copper, the dislocations which were generated during the beginning stage of deformation were determined by the external geometry of the crystal in addition to the usual requirements of the maximum resolved shear stress.

Epitaxy at Low Temperatures by Vapor Deposition of Pd, Pt, Rh, and Ir Observed by Field Ion Microscopy
View Description Hide DescriptionThe minimum substrate temperature required to form epitaxial layers by vapor deposition of Pd, Pt, Rh, and Ir on either Ir or Rh is determined to be 50, 100, 180 and 300°K, respectively, using a field ion microscope. This shows that the minimum temperature increases with the binding energy of the deposited metal. It is suggested that a very few jumps before an incident atom settles in a site, plus the possible displacement of an adsorbed atom by an incident one, will give epitaxy in these fcc metals. Any involvement of foreign adsorbed atoms in the formation of epitaxy is ruled out by the good (<10^{−9} Torr) vacuum used in these experiments.

Ion Machining of Diamond
View Description Hide DescriptionIt is demonstrated that argon‐ion beams are effective in drilling holes in diamonds. Truncated conical holes were made with basal diameters as small as ∼0.375 mm and tapering to ∼0.05‐mm diameter at the top. Larger holes with diameters at the two surfaces of 1.45 and 2.26 mm were milled through a 1.58‐mm‐thick diamond.Milling rates of up to 2 μ/min were obtained. The inside surfaces of the holes are essentially polished, with adjacent regions appearing to be damage free. The technique also has the capability of milling holes of arbitrary shape and of finely polishing the inside surfaces.

Oscillation Modes of Nematic Liquid Crystals
View Description Hide DescriptionA novel self‐contained two‐dimensional contiuum theory for thin nematic liquid crystal layers has been developed. The theory, which is quasistatic in nature, predicts spatially varying oscillations of the molecular axes in modal striated patterns in response to an acelectric field. An oscillation threshold voltage is derived which increases with increased frequency of excitation. Also, the theory predicts that the spatial frequency of threshold striations increases similarly with increased frequency of excitation. These experimentally observed threshold effects are predicted without the introduction of shear torques, which have been of importance in earlier one‐dimensional treatments of the problem, including hydrodynamic effects. The present theory includes torques arising from dielectric and conductivity anisotropy, curvature elastic torques, and dissipative torques determined by a curvature viscosity.

Cesium Adsorption on W: Ellipsometry, Auger Spectroscopy, and Surface Potential Difference Studies
View Description Hide DescriptionA study has been made of cesium adsorption of W and Ti using the combined measurements of ellipsometry,Auger spectroscopy, and surface potential difference measurements. The single and polycrystals of W have been cleaned in ultrahigh vacuum (∼2×10^{−10} Torr) and the cesium deposited as an ion beam at low voltages (5–30 V). The deposition of Cs^{+} did not occur unless some oxygen was detected on the sample by Auger spectroscopy. The sticking coefficient for Cs/W proved to be directly proportional to the concentration of oxygen on the W in the range 0–0.1 fraction of a monolayer oxygen coverage. The optical constants of bulk Cs can be used to interpret ellipsometric results for the adsorption of Cs on W and Ti in the region above, at and below monolayer coverage. The coverage obtained for the work‐function minimum is consistent with earlier work by Taylor and Langmuir.

Photon Appearance Potential Spectroscopy of Thorium
View Description Hide DescriptionEvaporated films of thorium on a tungsten substrate have been examined by photon appearance potential spectroscopy (PAPS) with incident electron energies in the range 80–1000 eV (O _{5} to N _{3} levels). For low coverage, the peaks corresponding to the O _{5}, O _{4} and N _{7}, N _{6} levels were immediately observed. As coverage was increased, these peaks increased slightly and the other O and N peaks appeared suddenly; the N _{3} was the last to appear. Peak shape and position were independent of coverage. After exposure to oxygen, the major peaks were not significantly shifted but new satellites of the O _{4}, N _{7}, and N _{6} peaks were observed.

Determination of Lattice Disorder Profiles in Crystals by Nuclear Backscattering
View Description Hide DescriptionA detailed analysis is presented for the determination of lattice disorder in crystals by nuclear backscattering. By means of models and numerical integration, calculations are performed to determine, as a function of depth, the energy loss of projectiles in crystals, their critical angle for dechanneling, their nuclear elasticscattering cross section, and the effects of lattice disorder on the specific energy loss of channeled ions. These calculations are more detailed than those previously used. The efficacy of the analysis is tested with experiments at 1500 and 2500 keV, and it is shown that these detailed calculations are necessary to extract accurate lattice disorder profiles. The effects of misalignment of the target during channeling experiments are determined. The error introduced by misalignment can be made small.

Dislocation Mobility in Aluminum
View Description Hide DescriptionDislocation velocities in aluminum were measured, using the etch‐pit technique of Gilman and Johnston, as a function of stress in the temperature region between liquid‐helium temperature and room temperature. The dislocation‐damping constant B was calculated at various temperatures from the linear portion of the velocity‐vs‐stress curve. The value of B decreases with decreasing temperature in the region from room temperature to 30°K. This result agrees with theoretical predictions of the dislocation damping produced by phonon‐dislocation interactions. Below 30°K the damping constant B increases with decreasing temperature. This latter result disagrees with the accepted theoretical predictions of dislocation damping produced by electron‐dislocation interactions.

Piezoresistance and Hole Transport in Beryllium‐Doped Silicon
View Description Hide DescriptionThe resistivity and piezoresistance of p‐type silicondoped with beryllium have been studied as a function of temperature, crystal orientation, and berylliumdoping concentration. It is shown that the temperature coefficient of resistance can be varied and reduced to zero near room temperature by varying the berylliumdoping level. Similarly, the magnitude of the piezoresistance gauge factor for beryllium‐doped silicon is slightly larger than for silicondoped with a shallow acceptor impurity such as boron, while the temperature coefficient of piezoresistance is about the same for material containing these two dopants. These results are discussed in terms of a model for the piezoresistance of compensated p‐type silicon.

Population Inversion in Laser Plasmas
View Description Hide DescriptionWe define the conditions for which a population inversion with respect to the ground state, due to recombination at high electron densities, can appear in cold laser plasmas. We point out the interest of giant and short laser pulses for producing such plasmas.

Anomalous Skin Effect in Thin Films
View Description Hide DescriptionNumerical calculations are presented for the impedance per square of metallic films. We consider in detail the case in which the mean free path of the conduction electrons is comparable to or greater than the film thickness (thin films at low temperatures), with particular attention to the transition between the dc limit and the anomalous skin‐effect limit.

Relativistic Electron Beam Propagation in Low‐Pressure Gases
View Description Hide DescriptionStudies of the propagation of pulsed relativistic electron beams through initially un‐ionized background gases are reported. At very low pressures, pronounced beam front erosion occurs owing to the slow buildup of radial force neutralization of the beamelectrons. Numerical simulation and an approximate analytical model are used to describe this beam‐loss process. The results of experiments with two different electron accelerators (ν/γ ∼ 0.05 and 1) are shown to be in good quantitative agreement with the mathematical descriptions over the pressure range of ∼ 1 μ to ∼ 1 Torr in a large variety of gases. Propagation characteristics are found to scale in pressure from gas to gas inversely with the high‐energy ionization cross sections of the gases. The study shows that direct ionization alone is sufficient to explain the experimental results. The theoretical calculations indicate that the low‐pressure propagation characteristics of the lower ν/γ beam are sensitive indicators of the beam's transverse energy. The calculations also indicate that this is not the case for the higher ν/γ beam.

Relativistic Electron Beam Propagation in High‐Pressure Gases
View Description Hide DescriptionExperimental results are reported for the transport characteristics of relativistic electron beams in high‐pressure atomic and molecular gases. Experimental results obtained with two different accelerators (ν/γ ∼ 0.05 and 1) are reported. It was found that the highest pressure at which efficient transport occurred was determined by two different physical processes for the two different intensity electron beams. The lower‐intensity beam propagated efficiently throughout a large pressure range, and was attenuated at high pressure by Coulomb scattering of the beam electrons in the screened Coulomb field of the nuclei of the background gas atoms. The more intense electron beam was attenuated at a gas pressure which varied with different gases approximately inversely with the high‐energy ionization cross section. A radial breakdown mechanism is proposed which may explain the loss process of the intense electron beam.

Interaction of Relativistic Particles and Free Electromagnetic Waves in the Presence of a Static Helical Magnet
View Description Hide DescriptionIt is shown that a particle passing along the axis of a helical magnet (in which the field is perpendicular to the axis and rotating as a function of position along the magnet) can be continuously accelerated by its interaction with circularly polarized radiation passing in the same direction. An example is given in which an electron is accelerated to 10 GeV, using a laser of 10^{14} W. A second example shows how pions and kaons might be separated at momenta over 1000 GeV. It is further shown that bunched charged particles passing down the helical magnet will radiate coherent circularly polarized electromagnetic waves, and it is speculated that the required bunching may under some circumstances be self‐generating. An example is shown in which a 10‐A current of 15‐MeV electrons is used to generate a 75‐MW beam of 10‐μ radiation.

Predicted Effect of Exponential Charging Profiles on Photoinjected Currents in Silicon Dioxide
View Description Hide DescriptionPhotocurrents produced by electron photoinjection over a contact interface barrier into an insulator are significantly affected by any charging which occurs in the insulator. The photoinjection is most seriously affected by negative charging near the maximum of the interface barrier potential. In this paper normalized photocurrent solutions are derived for negative exponential charging distributions at the injecting interface. With the aid of the normalized solutions the photocurrent I‐vs‐V measurements can be used to directly determine (or approximate to) the peak charge density at the interface (N _{0}) and the exponential decay distance (x_{c} ). A plot of the ratio of the barrier distance with charging (x _{0}) to the barrier distance without charging (x _{00}) vs voltage has a maximum for x _{0} equal to 2x_{c} . The N _{0} value may then be calculated from the quadratic dependence of N _{0} on the magnitude of the maximum ratio of x _{0}/x _{00}. Total electron charge densities (N _{0} x_{c} ) as low as 10^{9} cm^{−2} can be measured.

Voltage Pulse Profile Characteristics with Space Charge of a Loaded Pulsed Ionization Chamber
View Description Hide DescriptionAn analytical model describing the voltage pulse profile of a pulsed ionization chamber and its relationship to the electron density in a field drift dominated plasma is formulated. The differential equation derived from the equations of motion and conservation of electron density combined with Poisson's equation for the electric space‐charge field in the system is solved analytically for the cylindrical‐electrode geometry with an external RC circuit. The numerical analysis for the given initial and boundary conditions yields the anode voltage‐signal pulse profiles for the period of electron collection as a function of the initial electron density, the gas pressure, and ion‐chamber radii and length. Thus, the theory permits the determination of the electron density from the observed voltage signal.