Showing papers in "Applied physics in 1975"

Sputtering—a review of some recent experimental and theoretical aspects

Abstract: After a brief outline of the present sputtering theory for a random solid, recent results of the sputtering yieldS for polycrystalline targets are discussed, in particular in view of the influence of the projectile mass and the bombarding angle. The angle dependence ofS at low bombarding energies, and results on the angular distribution of sputtered particles for oblique ion incidence point out necessary modifications of present sputtering theories with respect to the anisotropy of the collision cascades in the solid and the influence of the target surface. The energy distribution of the neutral particles ejected along the target normals is related to the theoretically predictedE −2-distribution of low energy recoils in the Recent mass spectrometric studies of postionized sputtered neutrals are discussed in view of the formation of sputtered molecules and the application of sputtered neutral mass spectroscopy for surface analysis. Finally, the paper deals with ion-induced surface effects on non-elementary sputtering targets, and the protracted removal of foreign atoms from a matrix.

A one-dimensional SiGe superlattice grown by UHV epitaxy

Abstract: One-dimensional SiGe superlattices with periods ranging from 100 to 800 A have been deposited on Si substrates by periodically varying the Ge content of a mixed Si1-x Ge x multilayer structure fromx=0 tox=0.15. The deposition was successful, employing and UHV evaporation technique at a substrate temperature of 750°C fulfilling the four conditions: Single crystal growth, no interdiffusion, two-dimensional growth, and pseudomorphic growth. It is shown that mismatch above 8 · 10−3 favours growth by three-dimensional nucleation. The experimentally determined spacing of misfit dislocations is compared with theoretical results obtained by van der Merwe. The pseudomorphic growth behaviour of layers thinner than a critical thickness could be confirmed.

The effect of randomness in the distribution of impurity atoms on FET thresholds

Abstract: Significant regions of the depletion layer of a field effect device may contain only hundreds of dopant atoms. The randomness of the distribution of impurity atoms means that the average doping in the depletion layer varies from place to place in the plane of the surface. The cube approximation introduced by Shockley is adopted and used to divide the channel region of an FET into cubes whose edge is equal to the thickness of the depletion layer. The probability distribution of the threshold voltages of the cubes can then be calculated by using the Poisson distribution of impurity numbers. The conductivity of the array of cubes is treated by a modification of percolation theory. The arrays of importance in the theory of the FET are not very large, containing only tens or hundreds of elements, and the differences between nominally identical arrays is of interest to the average behavior of a large system. Random number experiments are used to develop a quantitative description of the probability of conductivity thresholds in the two-dimensional site problem with a finite number of elements. The finite percolation theory is combined with the cube threshold probability distribution to yield the probability distribution of threshold voltages of a field effect transistor in equilibrium.

A high-sensitivity laser microprobe mass analyzer

Abstract: A laser microprobe mass analyzer has been developed. It is intended for application in biomedical and physiological research. A frequency-doubled ruby laser is focussed through an incident light microscope to a spot of minimally 0.5 μm in diameter on a thin section specimen of 0.1–1.5 μm thickness. The microplasma generated from the irradiated volume is analyzed in a time-of-flight mass spectrometer recording the complete spectrum for each shot. From lithium doped epoxy resin (5 ppm by weight), used as an organic standard, 1.4×10−19g or 1.4×104 atoms of the6Li isotope have been detected. This sensitivity corresponds to that of ion microprobes but is at least an order of magnitude higher than obtained with electron probe X-ray microanalyzers.

Photorefractive effects for reversible holographic storage of information

Abstract: Optical storage of information offers great potential for high capacity and speed. A very promising approach to the embodiment of an optical memory is based on volume storage in the form of phase holograms. Attractive storage materials for such a system are electrooptic crystals. The storage mechanism in these materials is based on light induced permanent changes of the refractive index-the photorefractive effect. In this article the physical processes underlying photorefractive hologram recording are outlined, and some of the advantages and limitations of this method are discussed.

Formation and nature of swirl defects in silicon

Abstract: Point defect agglomerates in dislocation-free silicon crystals, usually called “swirls”, have been investigated by means of high-voltage electron microscopy. It was found that a single swirl defect consists of a dislocation loop or a cluster of dislocation loops. By contrast experiments it could be shown that these loops are formed by agglomeration of self-interstitial atoms. Generally the loops have a/2〈110〉 Burgers vectors, but in specimens with high concentrations of carbon (∼1017 cm−3) and oxygen (∼1016 cm−3) also dislocation loops including a stacking fault were observed. In crystals grown at growth rates higher thanv=4 mm/min no swirls are observed; lower growth rates do not markedly affect the size and shape of the dislocation loops. With decreasing impurity content (particulary of oxygen and carbon) the swirl density decreases, whereas the dislocation loop clusters become larger and more complex. A model is presented which describes the formation of swirls in terms of agglomeration of silicon self-interstitials and impurity atoms.

Monovacancy formation energy in copper, silver, and gold by positron annihilation

Abstract: Monovacancy formation energies in copper, silver, and gold have been deduced from the temperature variation of the peak counting rate in the angular correlation curve of positron annihilation radiation from these metals. The counting rate was temperature dependent over the entire temperature range, including temperatures so low that no trapping of positrons at vacancies is effective. At these temperatures the increase in counting rate results from thermal expansion of the lattice. By separating this thermal expansion effect from the vacancy trapping effect at higher temperatures, we obtained values for the monovacancy formation energyE 1v for copper, silver, and gold to 1.29±0.02 eV, 1.16±0.02 eV, and 0.97±0.01 eV, respectively.

Computer simulation of atomic mixing during ion bombardment

Abstract: The atomic mixing in the target under ion bombardment is assumed to result from cascades of atomic collision events. Computer simulations have been applied to collision cascades to estimate the depth resolution of surface analysis with an ion probe. The Monte Carlo method based on a single scattering model has been used mainly in the calculation under the assumptions of random collision process, no diffusion and no target saturation processes. High-energy collisions are characterized by a Lenz-Jensen or a Thomas-Fermi potential, while a Born-Mayer potential is used in the low energy region. The simulations have been performed for the bombardment of Ar ions withE 0=5 keV and 10 keV at angles of incidence θ=0° and 60° on Si targets. The depth resolutions [the definition of which is explained by (15) in the text] are about 140A for the Lenz-Jensen cross section and about 80A for the Thomas-Fermi one for θ=0° atE 0=5 keV, and decrease by 20–40% at θ=60° and increase by 70–90% forE 0=10 keV.

A study of electron traps in vapour-phase epitaxial GaAs

Abstract: The occurence and physical properties of electron traps in device-quality VPEn-GaAs are studied using transient capacitance and DLTS (deep level transient spectroscopy). Four traps labeled A, B, C, and F are seen. Trap A is identified to be the same as the often reported one, commonly attributed to oxygen in substrate material; this trap is also dominant in the VPE layers. The other traps are (to our knowledge) reported here for the first time. No correlation appears to exist between the concentrations of the various traps. The emission rate vs. temperature dependence, a characteristic physical property, is obtained for each of these traps.

Depth-Profiling of Cu-Ni Sandwich Samples by Secondary Ion Mass Spectrometry

Abstract: SIMS depth profiles of copper-nickel thin film targets were measured with argon and nitrogen primary ions. While pronounced cone formation is observed in case of argon irradiation the erosion is much more uniform with nitrogen projectiles, probably due to formation of nitride surface layers.

Improved perturbation analysis of dielectric gratings

Abstract: An improved perturbation procedure that was previously presented for TE modes guided by dielectric gratings is extended here to TM modes. In both cases, the fields are described in terms of a transverse transmission-line network, which lends considerable physical insight into the behavior of dielectric gratings. However, TM modes require equivalent sources of both the voltage and current varieties, in contrast to TE modes which involve only current sources. By comparing with an exact solution, the numerical results obtained by the present perturbation procedure are found to be very accurate.

Self-Trapping of Positively Charged Particles in Metals

Abstract: It is shown that the existence of a metastable state in which positrons in metals are “self-trapped” by strong interaction with the lattice gives rise to an anomalous temperature dependence in positron annihilation properties. The “intermediate” temperature variation of the shape of the annihilation photon line discovered by MacKenzieet al. is well accounted for by this mechanism; alternative interpretations in terms of thermal expansion effects may be refuted. This result calls for considerable revision of some of the published monovacancy formation energies obtained from positron annihilation measurements. Approximate criteria for the existence and the metastability of a selftrapped state of positively charged particles in metals are given. It is found that metastable self-trapping may occur for positrons; hydrogen isotopes and positive muons should be self-trapped in configurations that are always stable relative to the Bloch-wave states of these particles.

Losses introduced by tilting intracavity etalons

Abstract: An approximate analytical expression and numerical results for the on-resonance losses caused by tilting an intracavity etalon are given, when the laser operates in the zero-order Gaussian mode.

Density Dependence of Electron Drift Velocities in Helium and Hydrogen at 77.6 K

Abstract: Electron drift velocities have been measured in helium and hydrogen at 77.6 K and gas density of 6.6×1021 cm−3 (approximately 80 atm). At these high densities the electron drift velocities do not depend only on the ratio of the electric field to gas density (E/N). At constantE/N the electron drift velocity decreases with increasing gas density. In helium a decrease was found down to 6.4% of the value at low density, in hydrogen down to 0.52%. The results are discussed in terms of theories of multiple scattering. Legler's theory fits our data in the lower density range, but at the highest densities predicts too small an effect. The percolation theory by Eggarter and Cohen gives no agreement with the experiment. Up to the highest densities we did not find bubbles; slow negative charge carriers could be identified as oxygen ions.

Influence of the cation distribution on the magnetization of Y-type hexagonal ferrites

Abstract: Mossbauer absorption and magnetization measurements have been made in Mg2−Y and Co2−Y ferrites. The magnetizations of the various iron sublattices did not show any marked difference in Mg2−Y, whilst in the case of Co2−Y three different behaviours of the sublattice magnetizations have been detected. Low-temperature magnetization measurements in Mg2Y gave evidence of a noncollinear magnetic order. The different magnetic properties of these compounds have been explained on the ground of a preferential occupation by Mg2+ and Co2+ ions of the inner octahedral sites of the T crystallographic block. The presence in these sites of a non-magnetic ion such as Mg2+ is responsible for the peculiar magnetic order observed in Mg2−Y.

Analysis of optical propagation in thick holographic gratings

Abstract: A method of analyzing optical propagation in thick holographic gratings by decomposition of the thick material into thin gratings is discussed. The method is readily applicable to study propagation in multiple gratings of arbitrary spatial frequency and orientation recorded in the same thick emulsion. Applied to the double grating case, the method predicts strong cross-coupling between the two gratings for proper relative slope of the gratings. Results are given.

Theory of collision effects on atomic and molecular line shapes

Abstract: A review of recent developments in the theory of the effects of binary collisions on the spectral profiles associated with atomic and molecular systems is presented. To consistently account for collisional perturbations of both the internal energy levels and the velocity of active (emitting or absorbing) atoms or molecules, one must use a theory in which the center-of-mass motion of the active atoms has been quantized. Following this procedure general equations for absorption or emission line shapes are obtained. The line shapes may exhibit narrowing or broadening with increasing perturber pressure, depending upon the nature of the collision interaction. The physical significance of the collision mechanisms giving rise to such behavior is discussed, as is the experimental evidence in support of the theory. Various applications of the theory are presented.

Infrared stimulated Raman generation: Effects of gain focussing on threshold and tuning behaviour

Abstract: It is shown that for generation of infrared radiation by stimulated Raman scattering, the diffraction spread of the Stokes wave can have a significant effect on the threshold. Compared with an analysis in which gain focussing is neglected, the threshold powers may be much higher with a corresponding reduction in tuning range. The design of a Raman oscillator is considerably influenced by these diffraction effects, and also it is found that the Stokes wave is subject to frequency-pulling which is dependent on the pump power.

Momentum density measurements by positron annihilation in metals and alloys. Recent experiments with a multicounter two-dimensional angular correlation apparatus

Abstract: The technique of positron annihilation as applied to the study of momentum densities and Fermi surfaces is reviewed. The angular correlation of the two annihilation photons is directly related to the momentum distribution of the positron-electron system; breaks in this distribution reveal the size and shape of the Fermi surface. After a general introduction to the theory and the experimental techniques used, the results yielding various features of the fermi surface in high concentration disordered alloys are reviewed, and are compared with theoretical predictions. A new multicounter two-dimensional correlation apparatus is described and results in several solids are presented.

Nuclear magnetic relaxation by self-diffusion in solid lithium:T 1-frequency dependence

Abstract: We report on measurements of the7Li nuclear spin relaxation timeT 1 in solid lithium as a function of temperature (−170°C≦T≦+180°C) and Larmor frequency (450kHz≦v Li≦31.5 MHz). Using a relaxation model developed by Wolf and Cavelius and combining it with Seeger's diffusion formalism, the diffusion parameters for mono-and divacancy migration were evaluated by a least squares fit to the newly obtainedT 1 data as well as to previousT 1ϱ measurements. The result for the self-diffusion coefficientD SD is given byD SD=D 10·exp(−Q 1/RT)·[1+D 21·exp(−Q 21/RT)], withD 10=0.038 cm2s−1,Q 1=12.0 kcal mol−1,D 21=250,Q 21=4 kcal mol−1 andR=1.985·10−3 kcal mol−1 degree−1. Due to the flexibility of Seeger's formula, which contrasts with the standard Arrhenius interpretation of diffusion, discrepancies between earlier high- and low-frequency NMR investigations were eliminated. Furthermore, an excellent agreement with available results from tracer experiments was achieved by taking into account the theoretical predictions of the isotope effect and the vacancy correlation factor.

A new equilibrium effect in positron trapping in metals

Abstract: The equilibrium lineshape for positrons annihilating in Cd shows an anomalous temperature dependence which complicates analysis for vacancy formation energies. The phenomenon, tentatively identified as trapping by transient dilatations, probably occurs in all metals.

Experiments on the space and wavelength dependence of speckle

Abstract: A comprehensive series of speckle experiments is described for opal glass diffusers ranging in thickness from 10 to 500 μm. Wavelength decorrelation measurements are made using a cw dye laser source; and accurate square-law detection is provided by the use of an optic-fiber, photomultiplier combination rather than film. For this diffuser set intensity distribution functions, polarized and unpolarized, are plotted. Second-order correlation functions are presented for different apertures, relating diffuser motion and probe motion. Remote texture determination using contrast ratio and frequency decorrelation is illustrated.

Stimulated and spontaneous emission from laser dyes in the vapor phase

Abstract: Vapor phase dye lasers pumped by a nitrogen laser are described using the dyes POPOP, α-NPO, BBO, and 1.2.-di-(5.-methyl-2.-benzoxazolyl)ethylene. Absorption spectra and spontaneous fluorescence spectra and vapor pressure curves are given for several dyes.

High power cw dye lasers

Abstract: The dependance of the output power of cw dye lasers on the pump power and the system-and stream parameters is calculated and experimentally tested It was found that the pump power for a given system is limited to a critical value due to the influence of thermal inhomogeneities This critical pump power is predominantly dependant on the dye solvent, the flow velocity, the focussing of the pump laser and the stability range of the dye laser system Possibilities for the reduction of the thermal inhomogeneities are discussed and attainable output powers are calculated

Laser diagnostic in molecular beams

Abstract: The output from a stabilized, tunable single-mode laser is split into two beams which cross the molecular beam perpendicularly and at an angle of β to the beam axis. Tuning the laser frequency across the unshifted and Doppler-shifted absorption profile of the molecular transition yileds the velocity distribution of the molecules in the beam for individual quantum states. The internal state distribution among vibrational and rotational levels in the beam can be determined by comparing the fluorescence simultaneously induced in the beam and in a cell with molecules in thermal equilibrium.

Dynamic stark effect of an optical line observed by cross-saturated absorption

Abstract: Cross-saturated absorption at the neon line 2s 2–2p 4 caused by the saturating line 3s 2–2p 4 has been applied to the observation of the dynamic Stark effect by 1) power broadening, due to triple-quantum and higher multiple-quantum transitions, of the light shift contribution to the susceptibility, and 2) a direct splitting in the electronic Raman-Stokes contribution which occurs because of the partial compensation of the Doppler broadening.

Positron annihilation in germanium

Abstract: High precision long-slit and cross-slit geometry angular distributions of annihilation radiation from oriented germanium are presented The momentum distributions from the long-slit measurements are compared with recent Compton profile data to test the importance of the positron wavefunction and positron-electron correlations Evidence for Umklapp annihilation is discussed No observable differences are found between distributions from heavily doped and intrinsic germanium samples Differences in the angular distributions between 5 K and 300 K are attributed to positron thermal motion, and the results are analyzed by a new method to obtain an estimate of the positron effective mass

On the cation distribution in the spinel systemyMg2TiO4−(1−y)MgFe2O4

Abstract: The cation distribution and the oxygen parameter for the system Mg1+y Fe2(1−y) Ti y O4 have been determined using the intensities of X-ray reflections. Results are given for slowly cooled and quanched samples. An explanation is proposed to account for the behaviour of the lattice parameter and the oxygen parameter.

Effect of an external electric field on the positronium formation in the positron spur

Abstract: The decrease of positronium (Ps) formation in condensed matter caused by the presence of an external electric field is discussed in terms of the spur reaction model of Ps formation. The rather few experimental results available are shown to be in good agreement with the predictions of the model. Many new results are predicted by use of the available radiation-chemistry results. The work is also a reply to Brandt, who estimated that the spur process of Ps formation is insignificant in hydrocarbons. It is shown that Brandt's estimate disagrees with well established results of radiation chemistry.

Molecular symmetry and exciton interaction in photosynthetic primary events

Abstract: In this paper, the molecular details of the recently proposed energy upconversion theory of photosynthesis are reviewed. The primary light reactions are explained in terms of aC2 symmetrical structure of the reaction center involving a (Chl−H2O)2 adduct. It is shown that exciton interaction within the (Chl−H2O)2 complex leads to an antisymmetric triplet state which may act as an energy trap. The presence of the energy trap in the reaction center suggests that the trigger step for the photoionization of active chlorophylls may involve the summation of two red excitation photons. Under normal conditions, the steadystate one-photon-per-electron quantum requirement is obtained. The functional properties of the various molecular constituents of the Chl-a molecule, such as the Ring V β-ketoester group, the phytyl tail, the central Mg atom, and the π-system of the macrocycle are explained within the present theoretical framework. A detailed analysis is given of the postulates and the consequences of the proposed model. The ramifications of the theory are probed, and their biological consequences are suggested for future study.