Showing papers in "Journal of Applied Physics in 1973"

Calculated elastic constants for stress problems associated with semiconductor devices

Abstract: Theoretical estimates or experimental determinations of stress fields associated with semiconductor devices are generally simplified with the aid of two elastic constants, Young's modulus E and Poisson's ratio ν. In this paper, a generalized expression for ν has been derived for arbitrary orientations of cubic semiconductor crystals, and the variation of E, ν, and E/(1‐ν) for directions within the important {111}, {100}, and {110} planes is examined. The results show that isotropic elasticity theory is exact for all directions within {111} planes and that the composite elastic constant E/(1‐ν) which frequently occurs in problems of practical interest is also invariant for all directions within {100} planes. Numerical values for the various elastic constants are tabulated for GaAs, GaP, Si, and Ge.

Scaling laws for flux pinning in hard superconductors

Abstract: For all hard high‐field superconductors examined to date, there is a maximum in the pinning force density Fp as a function of the reduced magnetic field h. Fietz and Webb first demonstrated in dilute Nb alloys that the peak in Fp scales as [Hc2(T)]2.5 if the temperature is changed; the maximum value of Fp occurred at the same value of reduced field regardless of temperature. Recent data on the temperature dependence of pinning in Nb3Sn, Nb–25% Zr and a Nb–Ti alloy, which exhibits the ``peak effect'', are analyzed to show that similar scaling laws are obeyed by these materials. All presently available evidence indicates however that the reduced field hp at which the maximum Fp occurs, as well as the height and shape of this maximum, can be altered by metallurgical treatment. Apparently weak pinning defects, or widely spaced ones, produce a small peak in Fp(h) at high h whereas strong closely spaced pins produce a large peak in Fp(h) at low h without producing much change in Fp(h) at high h. A model which p...

Hydrostatic limits in liquids and solids to 100 kbar

Abstract: The hydrostatic properties of the materials methanol, isopropyl alcohol, water, sodium chloride, silver chloride, and the binary mixtures pentane‐isopentane and methanol‐ethanol have been determined in the diamond‐anvil pressure cell up to 180 kbar by line‐broadening and line‐shift measurements of the sharp R1 ruby fluorescence line. A liquid mixture 4 : 1 by volume of methanol : ethanol remains hydrostatic to almost 100 kbar at room temperature. This mixture exceeds the hydrostatic limit of the previous generally accepted fluid, 1 : 1 pentane : isopentane which has a hydrostatic limit of about 70 kbar. Silver chloride and water (ice VII) are better than sodium chloride as pressure‐transmitting media, but do not even qualitatively approach hydrostatic conditions much above 70 kbar. The stress sensitivity level of the ruby limits the extent to which slight deviations from hydrostatic conditions can be determined in solid systems and suggests the qualitative nature of the method in characterization of quasi...

Extension of theory of growth of chain‐folded polymer crystals to large undercoolings

Abstract: The kinetic theory of the rate of growth G and the initial lamellar thickness lg* of chain‐folded crystals is extended so that it is applicable at high undercoolings. Attention is centered on the details of how the first step element and the first fold are put down on the substrate. A parameter φ that varies between zero and unity, which apportions the free energy of attachment of the step element between the forward and backward reactions, is used to denote variations in this process. Expressions for G are derived from flux equations for two limiting cases: regime I, single surface nucleation act with rapid substrate completion and regime II, numerous surface nucleation acts with very slow substrate completion. Data from the literature on G for isotactic polystyrene (regime II) and polyethylene single crystals (regime I) are analyzed to obtain surface free energies, and these are used with the revised theory for lg* to predict the lamellar thickness of these polymers. Good agreement between lg* and publi...

Epitaxially grown AlN and its optical band gap

Abstract: Single‐crystal layers of AlN have been grown on sapphire substrates between 1000 and 1100 °C by vapor‐phase reaction of aluminum chlorides with ammonia. The purity, color, crystallinity, growth morphology, and electrical resistivity of the epitaxial layers have been investigated. Infrared specular reflection measurements showed the presence of an appreciable strain at the AlN‐sapphire epitaxy interface. Optical absorption data strongly suggest the AlN is a direct band‐gap material with a value of about 6.2 eV at room temperature.

Erosion and ionization in the cathode spot regions of vacuum arcs

Abstract: The net erosion rate at the cathode spots of 100‐A vacuum arcs has been determined experimentally for Cd, Zn, Ag, Cu, Cr, Fe, Ti, C, Mo, and W electrodes. Ion currents to the metal walls surrounding each of these cathode materials have also been investigated. For each material, the dependence of the wall ion current on the electrode spacing and anode geometry is consistent with an arc model which assumes predominant vapor ionization in the cathode regions, with subsequent isotropic free flight motion from these regions. Comparison of the net erosion rate with the wall ion current indicates that, for high‐vapor‐pressure materials such as Cd and Zn, ≈ 15% of the vapor leaves the cathode regions ionized. For low‐vapor‐pressure materials such as C, Mo, and W, this fractional ionization is almost 100%. The ion current magnitudes observed at long electrode spacings are similar for each material, and lie in the range 7–10% of the arc current. Ion currents of this magnitude are also predicted for Mg, Al, and Ni u...

Depolarization fields in thin ferroelectric films

Abstract: A mechanism is proposed to explain depolarization phenomena that have been observed in thin ferroelectric films and related multilayer devices. It is shown that, for a short‐circuited electrode‐ferroelectric structure, incomplete compensation of the ferroelectric polarization charge results when the center of gravity of this charge and the free compensation charge are not coincident. Depolarization fields in the ferroelectric arising from such incomplete compensations are estimated. A simple switching calculation shows such fields to be of sufficient strength to account for the initial polarization decay rate observed in Pb0.92Bi0.07La0.01 (Fe0.405Nb0.325Zr0.27)O3 films. The results of measurements involving changes in film thickness, electron concentration in the electrodes, and contact materials will be discussed and shown to be consistent with the mechanism proposed.

Absorption and noise in x‐ray phosphors

Abstract: When the signal in an x‐ray image system is formed by integrating the scintillation pulses rather than by counting them, the signal‐to‐noise ratio is reduced by a factor which depends on the shape of the pulse‐height distribution. The signal‐to‐noise ratio cannot be related directly to either quantum absorption or energy absorption, and a new quantity called noise‐equivalent absorption is defined which bears a simple relationship to the signal‐to‐noise ratio. Quantum, energy, and noise‐equivalent absorption are calculated as a function of thickness and x‐ray energy for CsI, Gd2O2S, LaOBr, Zn0.6Cd0.4S, and CaWO4.

Generalized approach to multiphase dielectric mixture theory

Abstract: A self‐consistent solution for finding the complex dielectric constant of a multiphase mixture with confocal ellipsoidal shell inclusions is presented. Implicit in the solution are the first‐order effects of neighboring inclusions, and hence, the high inclusion density limit is approached correctly. The solution contains the special cases of spherical shells, ellipsoids, spheres, disks, and needles. Reasonable agreement of the theory with some experimental data for wet wood is found. The presence of ellipsoidal shells in a mixture gives rise to a much greater separation between the upper and lower bounds of a mixture dielectric constant as compared to the limits obtained for solid ellipsoidal or spherical inclusions.

Luminescence of Bi4 Ge3 O12 : Spectral and decay properties

Abstract: Intense broadband emission in the visible is observed from crystals of Bi4Ge3O12 under optical and x‐ray excitation. From measurements of absorption, reflection, fluorescence, and excitation spectra, the emission is assigned to 3P1 → 1S0 transitions of Bi3+. The Stokes shift is large, [inverted lazy s]14 000 cm−1. The temperature dependences of the fluorescence intensity and lifetime in the range 77–400 °K establish that nonradiative decay becomes significant at temperatures ⪞250°K. Comparison of the properties of Bi4Ge3O12 with those of Bi12GeO20 and other bismuth‐activated materials demonstrates the importance of the Stokes shift and the 1S‐3P energy difference in determining the luminescence behavior. The use of Bi4Ge3O12 as a laser host crystal for rare‐earth and iron group activator ions, and as a scintillator material is discussed briefly.

Predicted electron transport coefficients and operating characteristics of CO2–N2–He laser mixtures

Abstract: Calculations have been made of transport coefficients of electrons in gas mixtures for ratios CO2:N2:He of 1:1:8, 1:2:3, 1:7:30, and 1:0.25:3. New cross sections for CO2 derived from swarm experiments are used together with previously published cross sections for N2 and He. Curves are presented of the predicted electron drift velocity, transverse and longitudinal diffusion coefficients, and ionization and attachment coefficients for E/N values ranging from 10−18 to 1 × 10−15 V cm2; E is the electric field strength and N the gas number density. Examples are given of derived distribution functions and comparisons are made with a Maxwellian distribution function. The percentage of the input electrical power which excites vibrational processes coupled to the 001 upper laser level of CO2 is given as a function of E/N. The maximum efficiency from these calculations increases for increasing ratios of N2:CO2, because the proportion of energy used to excite the bending and stretching modes of CO2 is then reduced. ...

Defect structure and electronic donor levels in stannic oxide crystals

Abstract: By measuring the conductivity of stannic oxide crystals as a function of oxygen partial pressure at elevated temperatures, it is shown that the dominant native defect in SnO2 is a doubly ionizable oxygen vacancy. Both donor levels of this defect, the first 30 meV deep and the second 150 meV deep, are identified and a model is presented that explains previous results. The behavior in hydrogen is contrasted to that in oxygen, and preliminary results are presented indicating that hydrogen introduces a donor 50 meV deep.

Free‐volume model for ionic conductivity in polymers

Abstract: Electrical conduction in polymers under a relatively low applied electric field is considered to be ionic and is affected strongly by the structural factors of the polymers. The following equation for the electrical conductivity σ was derived in which free volume Vf, jump energy Ej, and ionic dissociation energy W were taken into consideration: σ=σ0exp{− [γ Vi*/Vf+(Ej+W/2e)(kT)−1]}, where σo is a constant, γ the numerical factor to correct the overlap of free volume, Vi* the critical volume required for transport of an ion, e the dielectric constant, k Boltzmann's constant, and T the absolute temperature. This equation describes well the conduction phenomena in polymethylmethacrylate, polystyrene, and an unsaturated polyester. Relationships between electrical conduction and free volume are discussed.

cw degradation at 300°K of GaAs double-heterostructure junction lasers. II. Electronic gain

Abstract: The rapid degradation at 300°K in the cw regenerative output of stripe‐geometry GaAs double‐heterostructure junction lasers is shown to be a result of the formation of a local optical absorber in the laser cavity. Gain measurements performed on diodes before and after degradation show that the optical loss within the cavity increases during degradation. By observing the (predominantly) spontaneous emission from the active region directly through the n‐GaAs substrate, it is confirmed that the increased loss is localized in a region where little or no spontaneous emission takes place at lasing energies. In such diodes, the internal radiative efficiency of the undegraded portion of the optical cavity shows a relatively small decrease compared to the external differential quantum efficiency. When the local absorber extends over a sufficient length of the cavity the electronic gain in the undegraded section is insufficient to overcome the loss and the device ceases to act as a regenerative optical oscillator. Net gain measurements on DH laser devices in which the active region is lightly (≈ 1017) n‐doped indicate that the optical gain increases linearly with current prior to degradation. At lasing threshold the medium exhibits net gain over a wavelength range of 100 A. After degradation the gain dependence on current can become superlinear due to the saturation of the optical absorber. Estimates on the attenuation constant in the local absorber at low currents give a value of ≈ 60 cm−1 at 8800 A. For pulsed currents close to lasing threshold the attenuation constant increases to nearly 160 cm−1 at 8760 A.

X‐ray emission in laser‐produced plasmas

Abstract: Numerical results are presented for the characteristics of heavy‐ion laser‐produced plasmas. A conversion ratio defined as x‐ray power/absorbed power is then computed. Collisional‐radiative equilibrium is assumed for the ionization model. In the case of low radiation emission, a relation between laser flux and plasma temperature is derived. The one‐dimensional geometrical model is then corrected in the expansion by assuming that the emission occurs, in fact, over a length L only. Results are given and compared with experimental ones. The influence of the laser wavelength is also discussed and conclusions on the possible applications of these laser‐produced plasmas are drawn.

Impact of cylinders on a rigid boundary

Abstract: An experimental procedure originally proposed by G. I. Taylor to determine the dynamic yield point of metals is studied, using high‐speed computer simulations. A simple method is outlined for determining the yield strength of materials that can be described by elastic‐plastic theory. Results for several metals are presented.

Theory of interdigital couplers on nonpiezoelectric substrates

Abstract: This paper is concerned with the theory of excitation of surface acoustic waves on a nonpiezoelectric material, by using an interdigital electrode structure overlaid with a piezoelectric film The impedance and capacity of such transducers have been worked out The results are given in terms of Δv/v, the perturbation in acoustic wave velocity when a perfect conductor is placed in the position of the transducer A computer program which calculates surface wave propagation on a layered medium consisting of a film on a substrate, with arbitrary elastic and piezoelectric properties for both media, has been used to determine Δv/v, and hence the coupling coefficients and radiation resistance of the interdigital transducer As a function of piezoelectric film thickness, two peaks in coupling are observed The results for the interdigital couplers are in quantitative agreement with experiments employing single‐crystal materials Data for experiments using sputtered piezoelectric films also give qualitative agreement with the theory If high‐quality crystalline layers are used, the theory suggests that interdigital couplers on nonpiezoelectric delay lines with properties comparable to or better than the present piezoelectric delay lines may be achievable

Variation of minority-carrier diffusion length with carrier concentration in GaAs liquid-phase epitaxial layers

Abstract: Minority‐carrier diffusion lengths in GaAs at 298 K were determined in Ge‐doped p‐type layers and Sn‐doped n‐type layers by short‐circuit photocurrent measurements. The layers were grown by liquid‐phase epitaxy and utilized a 1‐ to 2‐μ top layer of Al0.5Ga0.5As to eliminate surface recombination. Electron diffusion lengths Ln of 7–8 μ were obtained at concentrations less than 1×1018 holes/cm3 with a decrease to 0.8 μ at 1×1019 holes/cm3. Hole diffusion lengths Lp of 2–4μ were obtained at concentrations less than 1×1018 electrons/cm3 with a decrease to 0.3 μ at 6×1018 electrons/cm3. The relatively constant diffusion length in p‐ and n‐type layers below concentrations of 1×1018 cm−3 is determined by nonradiative recombination. At hole concentrations above 1×1018 cm−3, the radiative recombination contributes to the decrease in Ln. For the n‐type layers, the rapid decrease in Lp above concentrations of 1×1018 cm−3 results from an increase of nonradiative recombination at high donor concentrations.

Study of ion‐nitriding

Abstract: A study of ion nitriding has been conducted in a nitrogen‐hydrogen‐argon gas mixture with emphasis on the mechanism and the active plasma ingredients which cause nitriding. The study is based on mass and energy data of ions sampled through a 4‐mil hole in the cathode, metallurgical data, and gas‐absorption data. The results of the experiment demonstrate that ion nitriding is not a gas‐absorption process. Ion nitriding requires ionic bombardment. Nitrogen ions and nitrogen‐hydrogen molecular ions are the active plasma ingredients. Nitrogen‐hydrogen molecular ions are responsible for the superior nitriding properties produced by a nitrogen‐hydrogen plasma compared to the properties produced by a nitrogen or nitrogen‐noble gas mixture.

Theory of domain-wall motion in magnetic films and platelets

Abstract: We calculate dynamical properties of plane and cylindrical magnetic domain walls in a uniaxial film or platelet whose plane is perpendicular to the easy axis. First, we calculate the internal structure of a freely moving wall to determine the nonlinear velocity‐momentum relation. Using a Bloch‐line approximation to the kinetic wall energy, we find that stray magnetic fields emanating from surface poles destabilize the structure at a critical velocity Vp, above which uniform motion is not possible. For a plane wall, we have Vp=24γA/hK1/2, where γ is the gyromagnetic ratio, A is the exchange stiffness, h is the film thickness, and K is the anisotropy. This velocity is usually much less than the critical velocity for bulk wall motion derived by Walker. Combination of the velocity‐momentum relation with the conservation of momentum provides a simple method of calculating the time‐dependent velocity in the presence of a small drive field and small viscous damping. For very small drive, the terminal velocity eq...

Microstructure, growth, resistivity, and stresses in thin tungsten films deposited by rf sputtering

Abstract: The growth process and the microstructure of very thin W films (80–500 A) deposited by rf sputtering on SiO2 and Si substrates have been observed by transmission electron microscopy (TEM). The resistivity and stress in these films have been related to the film microstructure, composition, and to the deposition conditions (substrate bias and rf deposition power). Thin W films deposited on silicon dioxide substrates under zero or positive bias have been found to grow in two distinct growth stages. Stage I corresponds to the formation of a thin continuous film (80–100 A thick) of β‐W. The β‐W phase has the A‐15 crystal structure and has been identified as a faulted W3W compound. A small grain size (50–100 A) is characteristic of the β‐W film. Stage II corresponds to the transformation of the β‐W film into a pure α‐W film with the bcc crystal structure. This thermally activated phase transformation takes place in the temperature range 100–200 °C. It is characterized by the growth of α‐W nuclei until complete ...

Transport processes of photoinduced carriers in Bi12SiO20

Abstract: The optical absorption edge of undoped, lightly Al‐doped, and heavily Al‐doped Bi12SiO20 single crystals is found to be exponential and follows Urbach's rule with σ0=0.71 at room temperature. The band edge is at 3.25 eV and is broadened by excitons and perhaps by impurities or defects. At 80 °K, the band edge is found to be shifted to 3.40 eV. The broad shoulder in the optical absorption and the secondary peak in the photocurrent excitation spectrum are attributed to the presence of a silicon vacancy complex. The longitudinal photocarrier response due to pulsed uv excitation leads to a value of the electron drift mobility of μd=0.029±0.003 cm2/V sec and a value for the range of electrons (μτ)e=8.5×10−7 cm2/V. The response times of electrons and holes (or the relaxation times) are determined to be 6.5×10−3 and 4.3 ×10−3 sec, respectively. Electrons dominate the photocurrent in undoped and lightly Al‐doped crystals, while holes dominate the photocurrent in the heavily Al‐doped crystals. Thermally stimulated...

Thermal diffusion measurements using spatially periodic temperature distributions induced by laser light

Abstract: Two light waves with different directions of propagation derived from a pulsed Nd:YAG laser are superimposed in an absorbing sample and generate an interference field. Due to absorption, a spatially periodic temperature distribution occurs, producing a spatial modulation of the refractive index which can be considered a thermal phase grating. The light of an argon laser simultaneously incident on the sample is diffracted by the thermal grating. When excitation is over, the decay time of the diffracted light is measured. From this decay time the thermal diffusivity of the sample is determined. Measurements on colored methanol and glycerin as well as on ruby compare favorably with the results of other authors. The possibility of exciting and detecting temperature waves (second sound) in solids by the method of light‐induced gratings is discussed.

Optical spectra of Ce3+ and Ce3+‐sensitized fluorescence in YAlO3

Abstract: Infrared and ultraviolet absorption spectra arising from f→f and f→d transitions of Ce3+ in YAlO3 crystals are presented and used to derive the 4f and 5d energy levels. Excitation and fluorescence spectra of 5d→4f emission are also reported. The 5d fluorescence has a decay time of 16 nsec and a radiative quantum efficiency of near unity at 300 °K. Possible use of Ce3+ as a sensitizer for rare‐earth and iron‐group ion fluorescence in YAlO3 is surveyed; included is an investigation of energy transfer in a Ce, Nd, Cr‐codoped crystal.

Polycrystalline ceramic lasers

Abstract: A method of making a laser body from a high density yttria-based ceramic material is provided. The method involves forming an aqueous solution containing water soluble salts of yttrium, thorium and neodymium, coprecipitating the cations from solution with oxalic acid, recovering the oxalate precipitate in the form of a purified dried powder, calcining the powder to convert the oxalates to the oxides, particulating the oxides in a rubber-lined ball mill, pressing the powdered oxides into a "green body", sintering the "green body" in a hydrogen atmosphere and rapidly cooling the sintered body to form an improved lasing ceramic material.

Thermal conductivity of Ga1−xAlxAs alloys

Abstract: This paper presents the results of our measurement of the room‐temperature thermal conductivity of the Ga1−xAlxAs alloy system. The study was motivated by our need to characterize the thermal properties of cw heterostructure lasers, in which the heat generated in the optically active layer must flow through a substantial thickness of Ga1−xAlxAs to reach the heat sink. The measurement was made on LPE layers which were separated from their substrates and fashioned into rectangular bars. A steady‐state heat flow was established in the bars utilizing an argon laser as the heat source, and the conductivity was obtained by measuring the temperature gradient in the bars with thin films of cholesteric liquid crystals. The results indicate that the thermal conductivity of the alloy is well described by an existing theoretical treatment of high‐temperature lattice thermal conductivity of disordered semiconductor alloys. In the Ga1−xAlxAs system, the increased thermal resistivity of the alloy is shown to be a result...

Mode guidance parallel to the junction plane of double-heterostructure GaAs lasers

Abstract: We investigate the origins of mode confinement, parallel to the junction plane, of stripe‐geometry double‐heterostructure GaAs injection lasers. Based upon the kind of control of the mode extent by the stripe width, we establish a distinction between two types of lasing mode guidance; (i) well‐behaved, where a single mode substantially fills the entire active region, and (ii) filamentary, where the mode size is much smaller than the width of the active region and its location is random. For low‐order well‐behaved modes, it is shown, at least in principle, that gain alone (no refractive index effects are required) can explain not only the confinement but also the approximate waveguide dimensions for which leakage losses, connected with the penetration of the mode into the regions outside the active guide, become important. When only this postulated gain‐guiding prevails in the case of higher‐order modes, we find that their spatial character is markedly different from the Hermite‐Gaussian distributions which have been seen for GaAs lasers. The introduction of a positive incremental index, which may reasonably be associated with the gain and/or with a thermal mechanism, restores agreement with observation. If any incremental index within the active region is negative, then although there is a defocusing effect, confinement may still be maintained by the gain. In the case of filamentary lasing we analyze four focusing mechanisms; two are connected with local built‐in gain (loss) and refractive index variations, while two are current dependent and are related to the saturation of gain‐associated refractive index and the free carrier effect. While all four processes appear to be reasonable candidates for providing the necessary confinement in filamentary lasing, the latter two simultaneously violate a condition necessary for stability of a filament. We conclude that imperfections in lasers are likely to be related to filamentary lasing and that such behavior may not be intrinsic to such devices.

A new class of switching materials

Abstract: By combining several volume percent of conducting particles in a semicrystalline matrix, a highly temperature‐dependent resistivity is obtained. The resistivity changes by several orders of magnitude in a small temperature interval centered on the crystal melting temperature. Moreover, a typical resistivity changes by a factor of hundreds as the frequency changes from 102 to 104 Hz. Materials of this type show dielectric constants in excess of 103 which vary strongly with temperature and frequency.

Recombination mechanisms in 8–14‐μ HgCdTe

Abstract: Photoconductive measurements on n‐type Hg1−xCdxTe, for compositions of 0.195 < x < 0.210, have been carried out in the temperature range 65–300°K. A comparison of the experimental data with simple band‐to‐band recombination theory indicates that the lifetime in the intrinsic range of temperatures is determined by an Auger limited process. The measured variation of lifetime with temperature can be used to predict values for the Auger limited lifetime in intrinsic material, τAi, over the entire temperature range 65–300°K. Calculations, using these values of τAi, for the extrinsic temperature range (<100°K) indicate that for our purer material (ND−NA≳4×1014 cm−3;μ77≳1.5×105 cm2 V−1 sec−1), the lifetime is still determined by an Auger limited band‐to‐band process. In somewhat more compensated material (ND−NA<4×1014 cm−3;μ77≲1.5×105 cm2 V−1 sec−1), Shockley‐Read limited lifetimes are observed, although in the majority of samples measured the electron lifetime is independent of temperature. However, in some ext...

Trap‐assisted charge injection in MNOS structures

Abstract: Injection of charge carriers into the nitride in an MNOS structure by direct tunneling to traps in the nitride is studied in this paper. It is shown that this injection mechanism is very important for MNOS structures with thin oxides at low nitride fields (1–4 MV/cm). The dependence on field, temperature, and oxide thickness of this injection current are examined both theoretically and experimentally. It is furthermore proposed that this mechanism may be of considerable importance for charge transport in most insulators.