Volume 41, Issue 2, 01 February 1970

Acoustic Propagation in the Presence of Drifting Carriers and an Oscillating Electromagnetic Field
View Description Hide DescriptionA theory for amplification of acoustic waves in the presence of dc and oscillating electric fields in a piezoelectricsemiconductor is developed. Acoustic gain constants for both k_{s}lâȘ1 and k_{s}lâ«1 are derived, where k_{s} is the wave vector for the acoustic wave, and l is the mean free path for the electron in the piezoelectric medium. As expected, the expressions for the gain constants reduce to those for the ordinary acoustic amplifier case when the applied oscillating electromagnetic signal is zero. The theory indicates that it is possible to transfer modulation from the electromagneticwave to the acoustic wave.

Effect of Longitudinal Magnetic Field on Acoustoelectric Amplification in nâInSb
View Description Hide DescriptionAppreciable enhancement of the acoustoelectric gain by longitudinal magnetic fields is observed in the classical regime (âÏ_{c}<kT). The enhancement increases monotonically with magnetic field, reaching saturation at fields corresponding to Ï_{ c }Ï being equal to a few units. The dependence of the magnetoacoustic enhancement on carrier concentration is analogous to that of the longitudinal magnetoresistance, both are more pronounced the lower the carrier concentration. The observed effect of the longitudinal magnetic field on the acoustic gain is similar to that reported for the acoustic attenuation in the quantum limit (âÏ_{c}â«kT), but for the regime studied (âÏ_{c}<kT) no theory has as yet been developed.

Observation of Propagating Domain in Semiconductive CdS Using the Optical Probe Method
View Description Hide DescriptionThe optical modulation has been observed in semiconductive CdSsingle crystals at room temperature by means of the optical probe method using a monochromatic light. The light transmitted through materials with a wavelength corresponding to the intrinsic absorption edge was strongly modulated. The optical modulation observed here is caused primarily by the acoustic flux rather than by the high electric field in the domain. The acoustic domain arrives at its full growth near the point 2 mm from the cathode and travels towards the anode. The aspect of the acoustic domain is subjected to the variation by the strength of the applied electric field, and the domain velocity varies from 2.2Ă10^{5} cm/sec to 3.0Ă10^{5} cm/sec according to the increasing applied electric field up to 3040 V/cm.

Linear and Nonlinear Attenuation of Acoustic Surface Waves in a Piezoelectric Coated with a Semiconducting Film
View Description Hide DescriptionThe interaction of acoustic surface waves in a piezoelectric with carrier waves in a semiconducting film on the surface is considered. Particular attention is paid to the limitation of the small signal theory due to nonlinear effects. It is found that large signal effects, which occur at moderate acoustic power levels, are important when the maximum rf charge density exceeds the available background charge density. Measurements of acoustic attenuation and acoustoelectric current in a CdS film on the surface of Yâcut quartz are reported. A fair agreement between theory and experiment is found.

Ultrasonic Attenuation in Germanium at Microwave Frequencies
View Description Hide DescriptionThe attenuation of longitudinal ultrasonic waves in singleâcrystal germanium has been measured at several frequencies between 0.15 and 2.84 GHz in the temperature range 10Â°â60Â°K. The measuredattenuation was the same for propagation in the , and crystallographic directions, and varied with temperature as T ^{7} at the highest frequencies and lowest temperatures, and as T ^{3} at the lowest frequencies near 60Â°K. The frequency dependence was less than linear at the lowest temperatures and between linear and quadratic at 60Â°K.

QuantumâMechanical RandomâNumber Generator
View Description Hide DescriptionFor generating the numbers 1, 2, âŠ M in random sequence, an electronic moduloâM counter is used, driven by the pulses from a highâfrequency pulse generator. If the pulse train is interrupted at a random time, then the counter stops at random in one of its M possible states, producing thus a randomânumber modulo M. The random time is the time at which a GeigerâMueller tube registers an electron from a ^{90}Sr source. The electronic circuitry is designed such that variations in the characteristics of the components do not impair the randomness. Due to the simplicity of the circuit, the degree of randomness obtainable can be discussed in detail. The randomness of the primarily generated ``basic sequence'' is limited by the finite value of the quotient (pulseâgenerator frequency) / (numberâgeneration frequency). An extremely high degree of randomness can be realized by ``contracting'' the basic sequence, i.e., by adding (modulo M) strings of k (k=2, 3, 4, or even higher) successive numbers of the basic sequence, to form one number of the final sequence. This contraction can be achieved very easily by interrupting the pulse train only after exactly k electrons have been recorded by the GeigerâMueller tube. The performance of a generator was tested by recording a basic sequence of generated numbers on paper tape. For probing the longâtime reliability of the generator these recordings were made over an 18 months period, and the randomness tests were designed to discover also temporary malfunctions. In accord with the theoretical expectation, these tests did not indicate any deviation from randomness.

Radiation Transport Calculationsâ``Fore and Aft'' Approximation
View Description Hide DescriptionAn approximate method for describing radiation transport is derived. Photoelectric effect and Compton effect are assumed to be the only mechanisms affecting the transport. Thus the method is restricted to the xâray and lowâenergy gammaâray region of the electromagnetic spectrum. Further, the radiation is assumed to be a collimated beam incident normal to a homogeneous semiâinfinite slab. The method described goes one step beyond the classical ``straightâahead'' approximation used in the theory of gammaâray attenuation. Because of the assumptions used in obtaining the solution, the expressions derived can most appropriately be applied for the case where Compton scattering is near isotropic (relatively low energies) and where the photoelectric cross section is larger than the Compton cross section.

Initial Processes in CO_{2} Glow Discharges
View Description Hide DescriptionInitial dissociation rates of CO_{2} in pure CO_{2}glow discharges and CO_{2} mixtures with He, N_{2}, H_{2} have been determined in the 1âTorr pressure range using a mass spectrometric sampling technique combined with plasma diagnostic methods. The observed results for the pure CO_{2} case are discussed in terms of dissociative attachment and collisional dissociation of vibrationally excited CO_{2} molecules both of which contribute to the dissociation process; most influential in mixtures is H_{2} which reduces the dissociative process. At the same time, the quasistationary values of the CO_{2}âCOâO_{2} mixture as well as that of CO_{2}âCOâO_{2} with He, N_{2}, H_{2} have been measured: [CO_{2}]/[CO][O_{2}]^{1/2} is approximately constant at pressures >500 mTorr and linearly dependent upon H_{2} concentration.

Effect of Reflecting Boundaries on the Transport of Resonance Radiation. II. Comparison with Diffusion Theory
View Description Hide DescriptionThe generalized HolsteinâBiberman transport theory developed in Part I is used to calculate the effect of reflecting walls in increasing the decay time of Doppler broadened resonance radiation in cylinders and slabs. The results are compared with the corresponding results obtained from the Cayless diffusion theory for optical thicknesses between about 20 and 3000. It is shown that the Cayless theory grossly underestimates the effect over the entire range, giving values for the fractional increase in decay time which are too small by about a factor of 3 at the smaller optical thicknesses, and too small by about a factor of 30 at the larger thicknesses. The discrepancy is shown to arise partly from the inaccurate boundary condition used in the Cayless theory and partly from the inapplicability of the mean free path concept to processes involving the transport of resonance radiation. The consequences for the behavior of lowâpressure mercury rareâgas discharges are discussed, and it is estimated that wall reflectance must begin to have a significant effect on the characteristics of such discharges at reflectances as low as 20% rather than at the 50% reflectance predicted by the Cayless theory.

Crystal Growth and Galvanomagnetic Properties of Mg_{2}Pb
View Description Hide DescriptionA method for growing singleâcrystal Mg_{2}Pb is described. A Bridgman method is used, starting with a nonstoichiometric melt (<31.3 at.% Pb). This requirement follows from the recent discovery that Mg_{2}Pb is not congruently melting but forms from a peritectic reaction. The usefulness of an xâray microprobe for the analysis of crystal growing problems is demonstrated by a study of eutectic inclusions observed in some Mg_{2}Pb ingots. Measurements are reported of the angular dependence of the magnetoresistance and the magnetic field dependence of the Hall coefficient in fields up to 5200 G and at temperatures of 4.2Â° and 77Â°K. The phenomenological coefficients, b, c, and d, determined from the angular dependence of the magnetoresistance show strong field dependence at 4.2Â°K and obey the symmetry condition for [100] oriented many valley ellipsoids at all fields and temperatures. The Hall coefficient exhibits a strong field dependence, similar to pâtype Ge, which has been fitted by a simple model consisting of two spherical bands. The mobilities and carrier densities obtained by fitting the data from several wellâannealed samples to the twoâband model are ÎŒ_{1}â„7Ă10^{4} cm^{2}/VÂ·sec, n _{1}â7Ă10^{17} holes/cm^{3}, ÎŒ_{2}â3Ă10^{3} cm^{2}/VÂ·sec, n _{2}â4Ă10^{20} holes/cm^{3} at 4.2Â° K.

Electrical Resistivity of Amorphous NiâPdâP Alloys
View Description Hide DescriptionTernary alloys containing nickel,palladium, and phosphorus have been obtained in an amorphous form by rapid quenching from the liquid state. These alloys contain from 13â73 at.% Ni, 15â20 at.% P with Pd adding up to 100%. The electrical resistivity of these alloys has been measured from 4.2Â°K to about 850Â°K, which is close to their melting point. A Kondo effect, probably due to very small concentration of iron impurities, has been observed in all the alloys below 15Â°K. Between 15Â° and about 40Â°K, the resistivity increases with temperature like T ^{2} and then linearly up to about 550Â°K. The very small temperature coefficient of resistivity (around 10^{â4}/Â°C) is attributed to the high degree of structural disorder in the amorphousalloys. Above about 570Â°K, the amorphousalloys transform progressively into crystalline phases, The variation of electrical resistivity with temperature in specimens heated at rates of about 1.5Â°C/min was correlated with structural changes observed by xâray diffraction.

Ionized Impurity Density in nâType GaAs
View Description Hide DescriptionTotal ionized impurity densities (N_{D} +N_{A} ) from 7Ă10^{13} to 3Ă10^{17} cm^{â3} are determined for epitaxial samples of nâtype GaAs by analyzingmobility and carrier concentration data as a function of temperature with the BrooksâHerring formula for ionized impurity scattering. This procedure results in the determination of a temperature range within which the effects of other scattering mechanisms are minimal and gives values of N_{D} and N_{A} which are in good agreement with impurity densities obtained from analyses of the temperature variation of the Hall constant. These results are then used to determine empirical curves relating the impurity density to the 77Â°K Hall mobility. With these data a good estimate of the total ionized impurity concentration in a sample can be determined from Hall constant and resistivity measurements at 77Â°K.

Double Injection in the Perfect Insulator: Further Analytic Results
View Description Hide DescriptionThe general problem of double injection into a perfect insulator, free of traps and of thermal free carriers, does not yield to a complete, analytical description. A complete solution is presented for the limiting cases of very large and very small Ï_{ R }, where Ï_{ R } is the electronâhole recombination cross section. The first situation corresponds to a pair of backâtoâback, oneâcarrier, spaceâchargeâlimited currents, the second to an injected plasma. The IâV characteristics obtained in these two limits agree precisely with the earlier general analytical result of Parmenter and Ruppel. For intermediate values of Ï_{ R } the Regional Approximation method is used to obtain an approximate solution. The field and density distributions, obtained for large, intermediate, and small values of Ï_{ R }, are the first available for this problem. The constantâlifetime and bimolecularârecombination injected plasmas are compared and found to have remarkably similar distributions.

Experimental Study of the ElasticâModulus Effect in the Interaction of Vacancies with Dislocations and Dislocation Ribbons in Pure Silver
View Description Hide DescriptionThe interaction of point defects with dislocation ribbons was studied experimentally by observation of vacancy clusters in deformed pure silver by means of electron microscopy. The results were in good agreement with theoretical predictions: The energy of the interaction was attractive, increased as the dislocation became more edgeâlike in character, and decreased as the width of the dislocation ribbon increased.

Energy of a bcc Iron Deformation Twin Boundary
View Description Hide DescriptionBy thermal grooving, the energy of an Î±âiron deformation twin boundary was determined and compared with the energy of an Î±âiron annealingtwin boundary. The stacking fault energy was approximated and compared with that of other bcc iron alloys.

StackingâFault Model for Stoichiometry Deviations in LiNbO_{3} and LiTaO_{3} and the Effect on the Curie Temperature
View Description Hide DescriptionSeveral arrangements are suggested to explain deviations from stoichiometry in LiNbO_{3} in a recent report by Lerner, Legras, and Dumas. These arrangements are considered to be unlikely, and an alternative mechanism in terms of partial stacking faults is proposed for LiNbO_{3} and LiTaO_{3}. A microscopic statistical theory of ferroelectricity, which is outlined, can predict the variation of the Curie temperature with changes in the bonding forces and atomic masses. Using this approach, the stackingâfault model is consistent with the observed variation of Curie temperature with composition.

Alternating Current Electrical Properties of Highly Doped Insulating Films
View Description Hide DescriptionMetalâinsulatorâmetal systems are discussed in which the insulator is highly doped and in which Schottky barriers exist at the metalâinsulator interface. An equivalent circuit for the system is proposed and the ac electrical characteristics derived. It is shown that the capacitance is extremely temperature and frequency dependent. At high frequencies or low temperatures the capacitance is thickness dependent and equal to the geometric capacitance. At low frequencies and high temperatures it is thickness independent and equal to the Schottky barriercapacitance, which is determined by the doping density. Several methods of determining the activation energy of the donor centers from experimental capacitance versus frequency and temperature curves are suggested. The parallel equivalent conductance is also shown to be extremely frequency and temperature sensitive. It is found to have a pronounced maximum in both cases, which increases in magnitude and occurs at higher temperatures the thicker the insulator.

Electrical Properties of Evaporated Molybdenum Oxide Films
View Description Hide DescriptionThin filmcapacitors of MoO_{3} are found to be extremely temperature and frequency dependent. Changes in capacitance are reported as high as 60:1 over a temperature range of 100Â°C (at constant frequency) and over a twoâdecade frequency range (at constant temperature). At lower temperatures and higher frequencies the capacitance corresponds to the geometric capacitance, but at higher temperatures and lower frequencies the capacitance is independent of the film thickness. Conductance and quality factor of the films are also observed to be extremely frequency and temperature sensitive. The results are explained in terms of Schottky barriers existing at the metalâinsulator interfaces, which arise due to the autodoping of the insulator, occurring during deposition of the films, by excess molybdenum. Excellent agreement is found to exist between the experimental data and the theory developed in the previous paper, and this correlation permits determination of the doping density (â10^{18} cm^{â3}), the donor depth (â0.27 eV), and the width of the Schottky barriers (â160 Ă ), among other properties of the films.

Model for the ResistiveâConductive Transition in Reversible ResistanceâSwitching Solids
View Description Hide DescriptionA model is given for the resistiveâconductive transition in an amorphous or crystalline bistable resistance switch. The model is based on the observation that the current path in the switch is filamentary. The hypothesis is that the switching current heats the filament, and the resistiveâconductive transition occurs only after the filament has achieved a critical temperature T_{c} . The predictions of the model are in excellent agreement with experimental observations of the resistiveâconductive transition in singleâcrystalline Cu_{2}O.

Quantization of Electromagnetic Radiation Fields in Moving Uniaxial Media
View Description Hide DescriptionRadiation fields in moving uniaxial media have been quantized without introducing auxiliary potential functions. Hamiltonian and momentum operators are diagonalized in momentum space and written in terms of annihilation and creation operators of the photon state. It is found that two types of photons corresponding to classical ordinary and extraordinary waves will exist, and they are called ordinary photons and extraordinary photons. Physical interpretations of the various results are discussed.