Showing papers in "Journal of Applied Physics in 1976"

Theory of the photoacoustic effect with solids

Abstract: When chopped light impinges on a solid in an enclosed cell, an acoustic signal is produced within the cell. This effect is the basis of a new spectroscopic technique for the study of solid and semisolid matter. A quantitative derivation is presented for the acoustic signal in a photoacoustic cell in terms of the optical, thermal, and geometric parameters of the system. The theory predicts the dependence of the signal on the absorption coefficient of the solid, thereby giving a theoretical foundation for the technique of photoacoustic spectroscopy. In particular, the theory accounts for the experimental observation that with this technique optical absorption spectra can be obtained for materials that are optically opaque.

New figure of merit for transparent conductors

Abstract: A figure of merit for transparent electrode materials has been defined by φTC=T10/Rs, where T is the optical transmission and Rs is the electrical sheet resistance. Expressions are derived to predict the transparent electrode properties of a material from its fundamental electrical and optical constants. The performance of thin metal films is compared to semiconducting oxide coatings.

Physical properties of thin‐film field emission cathodes with molybdenum cones

Abstract: Field emission cathodes fabricated using thin-film techniques and electron beam microlithography are described, together with effects obtained by varying the fabrication parameters. The emission originates from the tip of molybdenum cones that are about 1.5 micron tall with a tip radius around 500 A. Such cathodes have been produced in closely packed arrays containing 100 and 5000 cones as well as singly. Maximum currents in the range 50-150 microamp per cone can be drawn. Life tests with the 100-cone arrays drawing 2 mA total emission (or 3 A per sq cm) have proceeded in excess of 7000 hr with about a 10% drop in emission current. Studies are presented of the emission characteristics and current fluctuation phenomena. It is tentatively concluded that the emission arises from only one or a few atomic sites on the cone tips.

Ultrafine metal particles

Abstract: In this paper we present a novel and versatile t e c h n i q u e f o r t h e p r o d u c t i o n o f u l t r a f i n e m e t a l p a r t i c l e s by evaporation from a temperature‐regulated oven containing a reduced atmosphere of an inert gas. An extensive investigation of particles of oxidized Al, with diameters of 3 to 6 nm, has been performed. We have also studied ultrafine particles of Mg,Zn, and Sn produced in the same manner. A supplementing investigation has been carried out for particles of Cr, Fe, Co, Ni, Cu, and Ga, as well as larger Al particles, produced by ’’conventional’’ inert‐gas evaporation from a resistive filament. Diameter as a function of evaporation rate, inert‐gas pressure, and the kind of inert gas are reported. Crystalline particles smaller than 20 nm look almost spherical in the electron microscope, while larger ones often display pronounced crystal habit. S i z e d i s t r i b u t i o n s have been investigated in detail, and consistently the logarithm of the particle diameter has a Gaussian distribution to a high precision for the smallest sizes, whereas larger particles deviate from such a simple behavior. A statistical growth model, based on the Central Limit Theorem, has been formulated for liquidlike coalescence of particles; this theory accounts satisfactorily for all our data, as well as for most size distributions published in the literature. Applications of the model to colloids, discontinuous films, and supported catalysts are discussed. By comparing size distributions for particles produced by a variety of techniques we found a number of empirical rules for the width of the distributions, as defined by a (geometric) standard deviation σ. For crystalline inert‐gas‐ evaporated particles we obtained consistently 1.36?σ?1.60; for coalescing islands in discontinuous films we found 1.22?σ?1.34; and similar rules are applicable to colloids, supported catalysts, and to ultrafine droplets.

Electromigration in thin aluminum films on titanium nitride

Abstract: The aluminum electromigration drift velocity was measured at the temperature range 250–400 °C. A threshold current density was found inversely proportional to the stripe length. An activation energy of 0.65 eV was found for the drift velocity. The occurrence of the threshold is explained by opposing chemical gradients created by the atom pile‐up and depletion at the stripe ends. The threshold may explain several observations reported previously. The threshold is increased by decreasing the temperature or by enclosing the aluminum in silicon nitride. Virtually no electromigration is seen for very short aluminum stripes even at current densities above 106 A/cm2.

Magnetic heat pumping near room temperature

Abstract: It is shown that magnetic heat pumping can be made practical at room temperature by using a ferromagnetic material with a Curie point at or near operating temperature and an appropriate regenerative thermodynamic cycle. Measurements are performed which show that gadolinium is a resonable working material and it is found that the application of a 7-T magnetic field to gadolinium at the Curie point (293 K) causes a heat release of 4 kJ/kg under isothermal conditions or a temperature rise of 14 K under adiabatic conditions. A regeneration technique can be used to lift the load of the lattice and electronic heat capacities off the magnetic system in order to span a reasonable temperature difference and to pump as much entropy per cycle as possible

KxRb1−xTiOPO4: A new nonlinear optical material

Abstract: KxRb1−xTiOPO4, a new nonlinear optical material with coefficients comparable to BaNa2Nb5O15, has been prepared as optically perfect single crystals which are transparent from 350 to 4500 nm. We have shown that the material is phase matchable over its entire transparency range and that 1060‐nm radiation can be doubled with minimal beam walk‐off. The material is free from optical damage for power densities at least as large as 150 MW/cm2. Also, it is chemically stable and easily polished.

Grain‐size effects on dielectric properties in barium titanate ceramics

Abstract: Measurements of the dielectric constant κ in high‐purity BaTiO3 ceramics as a function of temperature show that κ is quite dependent on grain size in the ferroelectric state while it is almost independent of grain size in the paraelectric state. In the ferroelectric orthorhombic and rhombohedral phases, κ decreases rapidly with lowering temperature when grain size is reduced smaller than 3 μm. It is also observed that the Curie temperature Tc is decreased very slightly, but the lower two transition points are shifted to higher temperatures as grain size decreases.

Concentration‐dependent absorption and spontaneous emission of heavily doped GaAs

Abstract: A model for the calculation of the absorption and emission spectra for GaAs at carrier concentrations in excess of 1×1018 cm−3 is described. This model utilizes a Gaussian fit to Halperin‐Lax band tails for the concentration‐dependent density of states and also includes an energy‐dependent matrix element. The calculated absorption and emission spectra are compared to previous experimental results. All results are for 297 K. For p‐type GaAs, the agreement is very good. The concentration dependence of the effective energy gap is obtained and can be expressed as Eg (eV) =1.424−1.6×10−8 [p (cm−3)]1/3. The concentration‐dependent thermal equilibrium electron‐hole density product n0p0 and the radiative lifetime τr are calculated for p‐type GaAs. The value of n0p0 increases from the low‐concentration value of 3.2×1012 cm−6 to 1.2×1013 cm−6 at p=1.6×1019 cm−3. This value of n0p0, together with the thermal generation rate obtained from the experimental absorption coefficient, gives τr as 0.37 nsec at p=1.6×1019 cm−3.

Computational models for ductile and brittle fracture

Abstract: Computational models of dynamic ductile and brittle fracture are developed for wave propagation in one‐ and two‐dimensional geometries. The model features have been taken mainly from detailed observations of samples partially fractured during impacts, but the functional forms are consistent with theoretical results where applicable. Basic features of the models are the nucleation and growth (hence, the acronym NAG for the models) of voids or cracks, and the stress relaxation resulting from the growing damage. The results of the calculations include number and sizes of cracks, voids, or fragments as a function of position in the material. The NAG analysis presents the nucleation law, determined from experiment, and two growth laws: both growth and nucleation are functions of stress and stress duration. Procedures for treating cracks with a range of sizes and orientation are presented with the method for computing the stress relaxation that accompanies growth of damage. Brittle fracture is essentially aniso...

Image formation by nuclear magnetic resonance: The sensitive‐point method

Abstract: An image formation method, using nuclear magnetic resonance techniques, has been developed which provides direct and detailed information on the interior details of heterogeneous samples. Time‐dependent magnetic field gradients are used to limit and control the region of spatial sensitivity of a standard pulsed NMR spectrometer. A continuous string of intense phase‐alternated rf pulses is used to provide a continuous monitor of the nuclear magnetization. The image is produced by directly plotting the phase‐detected resonance signal on a xy plotter as the limited region of spectrometer sensitivity is scanned through the sample. An analysis of the method in terms of the phenomenological Bloch equations, a description of the experimental method, and a representative selection of images are presented. A spatial resolution of about 0.3 mm has been achieved.

Photoluminescence of ion‐implanted GaN

Abstract: Thirty-five elements were implanted in GaN. Their photoluminescence spectra were measured and compared to those of an unimplanted control sample. Most impurities emit a peak at about 2.15 eV. Mg, Zn, Cd, Ca, As, Hg, and Ag have more characteristic emissions. Zn provides the most efficient recombination center. A set of midgap states is generated during the damage-annealing treatment.

Polarity‐dependent memory switching and behavior of Ag dendrite in Ag‐photodoped amorphous As2S3 films

Abstract: High cyclical reproducibility in polarity‐dependent switching and memory function was obtained. Switching and memory effect seem to occur when both electrodes (Mo and Ag) are bridged by Ag dendrite which originates as the result of Ag photodoping. The combination of Ag electrode and another kind of electrode material may be responsible for the polarity‐dependent switching and memory by the help of appropriate voltage shapes. Behavior of Ag dendrite was observed in relation to the similar electrical characteristics in a modified device structure. No thermal effects may be contained in the functional processes. Temperature dependence and frequency characteristics of the device resistances are measured and found to support the existence of the Ag dendrite and its role in the function in the original device. Switching transients in addition to the durability show the availability of the device in the compatibility with the magnetic core memory.

Calculated spectral dependence of gain in excited GaAs

Abstract: The calculated dependence of the gain coefficient on photon energy and excitation level in GaAs is given for 297 and 77 K. The curves of gain versus excitation rate generally have downward curvature for photon energies near the gain peak, while the envelope of these curves generally has upward curvature except at high excitation rates. Results are also given for the calculated dependence of the radiative recombination rate coefficient and of the photon energy at the gain peak on excitation rate. The qualitative behavior of the results is in agreement with experiment.

Electromechanical properties of polarized polyvinylidene fluoride films as studied by the piezoelectric resonance method

Abstract: The piezoelectric resonance was observed in free vibrators of uniaxially oriented and biaxially oriented polyvinylidene fluoride films polarized at high electric fields. Using the piezoelectric resonance method, piezoelectric and elastic constants, and electromechanical coupling factors of such films were determined at high frequencies (20 kHz–30 MHz) in the temperature range −170–100 °C. For the uniaxially oriented films, the coupling factors k33 and k32 are independent of temperature, while k31 increases noticeably above the primary dispersion temperature. The value of k33 is about 0.2, one of the largest values ever reported for piezoelectric polymers. Similar results were obtained for the biaxially oriented films. The temperature dependence of elastic and piezoelectric constants of the uniaxially oriented films is interpreted by a model in which the crystalline and amorphous phases are combined in series along the stretching direction and in parallel along the perpendicular directions. The origin of t...

Laser drilling velocity in metals

Abstract: To describe the laser drilling process, a theoretical model that includes expulsion of liquid material is developed. The model allows the calculation of drilling velocity and drilling efficiency as a function of the absorbed intensity. The same quantities were determined experimentally, using Nd‐YAG‐laser pulses of rectangular shape. Good agreement between measurement and calculation was found in the intensity region where efficient drilling is possible, i.e., where reflection losses and vapor absorption can be neglected. For most metals this region is between 1 and 100 MW/cm2.

An Auger analysis of the SiO2‐Si interface

Abstract: Auger electron spectroscopy has been used in conjunction with argon ion sputtering in a study of the chemical structure of the interface region between thermally grown SiO2 and the Si substrate. The distorting effects of the electron and ion beams are dealt with in detail, and we show how the beam parameters can be chosen to minimize instrumental artifacts in the Auger spectra and the chemical depth profiles of the SiO2‐Si interface. We discuss the SiO2‐Si interface in terms of a morphology model which includes a natural interface roughness and inclusions of silicon in the oxide close to the interface. The width of the interface of a 1000‐A oxide grown at 1200 °C in dry O2 on Si (100) is approximately 35 A.

Simple analysis of torque measurement of magnetic thin films

Abstract: In order to analyze experimental data of torque measurements for magnetic thin films, a (L/H)2‐vs‐L plot is proposed, where L represents the torque of a film in a field of H whose direction is 45° to the film plane. The saturation magnetization M and perpendicular anisotropy energy K⊥ are obtained without any approximation from the slope A and intercept B of this plot as follows: M= (2B)1/2/V, K⊥= (B/V) (1/A−4π/V), where V is the volume of the film.

On the existence of surface‐wave solutions for anisotropic elastic half‐spaces with free surface

Abstract: A proof is developed that for a given direction of propagation on the free surface of a half‐infinite anisotropic crystal, a surface‐wave solution with a certain phase velocity vR

An accurate and sensitive method for the determination of the depth distribution of light elements in heavy materials

Abstract: A method for the determination of the depth distribution of light elements in heavy materials is described. It involves the detection of light elements recoiling under the bombardment by a 35Cl beam. A resolution of 300 A was achieved for the lithium present in a thin sample. The measures were done with layers of 1016 atoms/cm2 and it is estimated that quantities as small as 1014 atoms/cm2 can be located without much difficulty.

High‐field capture of electrons by Coulomb‐attractive centers in silicon dioxide

Abstract: Electron capture by Coulomb‐attractive oxide‐charge centers in thin SiO2 films thermally grown on silicon was studied at room temperature and at 77°K for average oxide fields ranging from 5×105 to 3×106 V/cm. The observed capture cross section varied with the average oxide field, EOX, approximately as EOX −3. Such a strong field dependence cannot be accounted for by the Frenkel‐Poole lowering of the potential % barrier alone which predicts an EOX −3/2 dependence. It is suggested that electron heating by the oxide field is likely at these high fields. Electron heating decreases the probability of capture. The observed field dependence can be explained by a combination of Frenkel‐Poole and electron‐heating effects. Although the zero‐field capture cross section is expected to increase rapidly with decreasing lattice temperature, the observed high‐field capture cross sections were about the same at room temperature and at 77°K. Such weak dependence of the high‐field capture cross section on lattice temperature is consistent with Lax’s model of cascade capture by acoustic‐phonon emission.

Electron‐irradiation‐induced divacancy in lightly doped silicon

Abstract: Two electron traps—A2 and A3—produced in n‐type silicon by 1.5‐MeV‐electron irradiation are characterized by deep level transient spectroscopy. Activation energies of trapped majority carriers and capture cross sections for majority carriers at these levels are reported. From their production rates and annealing behaviors, they have been identified as different charge states of the same defect. Detailed annealing studies show that their annealing kinetics is first order with an activation energy of 1.47 eV. It is suggested that the defect is the divacancy and that dissociation is the likely process for its removal in these devices.

Thermodynamic aspects of particle engulfment by solidifying melts

Abstract: This study confirms the predominance of surface thermodynamic effects in the process of particle engulfment by solidifying melts for small‐diameter particles and slow interface motion. Predictions of a thermodynamic model are compared with actual particle engulfment behavior. It turns out that, in complete quantitative agreement with the thermodynamic predictions, the more hydrophobic particles like Teflon are engulfed at even very slow rates of solidification of naphthalene and biphenyl melts. On the other hand, more hydrophilic particles like nylon are rejected, and hence pushed by the solidification front at low and moderate rates of solidification. Conclusive proof of the importance of surface properties in particle engulfing is offered by the investigation of small siliconed glass spheres and untreated glass spheres; again the hydrophobic siliconed glass spheres were engulfed at all solidification rates and the untreated hydrophilic glass spheres were pushed. Rate phenomena for the engulfment of hydr...

Determination of insulator bulk trapped charge densities and centroids from photocurrent‐voltage charactersitcs of MOS structures

Abstract: A new rapid technique for determining the density and centroid of trapped space charge in MOS structures is described. Photocurrent‐voltage characteristics for both the metal‐oxide and Si‐oxide interfaces are used to determine the internal fields due to bulk trapped charge and, hence, its density and centroid. An experimental example of this technique is shown for a MOWOS (metal‐SiO2‐W‐SiO2‐Si) structure where a layer of approximately 1014 W atoms/cm2 deposited 80 or 42 A from the Si‐SiO2 interface and charged by internal photoemission is investigated. This technique is compared to others in terms of its direct, rapid, minimally perturbing, low‐current and low‐field characteristics.

Bonding direction and surface‐structure orientation on GaAs (001)

Abstract: An experimental study to determine the orientation of surface structures with respect to atomic‐bond direction is presented. The As‐stabilized (2×4) and C(2×8), and Ga‐stabilized (4×2) and C(8×2) surface structures all have their twofold direction parallel to the plane containing the dangling surface bonds and perpendicular to that of the back bonds.

Analysis of stress and strain distribution in thin films and substrates

Abstract: Calculations are presented for the distribution of stress and strain in thin films and substrates from inhomogeneous plane stresses developed in the film during deposition. The results can be applied to the ’’bending‐plate method’’ of determining the unknown stresses in a film, in which the deformation of the substrate surface is measured. Thereby the local values of the stress tensor within the film plane (averaged through the film thickness) and the center of the stress distribution normal to the film plane can be obtained in principle. A more convenient evaluation based on linear approximations is applicable when the film thickness is small. Restrictions are discussed which result from neglecting the conditions of compatibility for the components of the strain tensor.

Study of electron traps in n‐GaAs grown by molecular beam epitaxy

Abstract: Electron trapping centers in n‐GaAs grown by molecular beam epitaxy (MBE) have been studied by deep‐level transient capacitance spectroscopy (DLTS). At least nine different electron traps are observed with energies ranging from 0.08 to 0.85 eV from the conduction band. Total electron trap concentrations in various samples range from the low 1014 cm−3 range to the mid 1012 cm−3 range. Studies of samples from various growth systems indicate that these traps are most likely due to chemical impurities. A comparison is made with electron traps in n‐GaAs grown by vapor‐ and liquid‐phase epitaxy as well as with the electron traps introduced into n‐GaAs by 1‐MeV electron irradiation.

Analysis of spatial distribution of charges and dipoles in electrets by a transient heating technique

Abstract: A method is discussed for measuring the separate spatial distributions of fixed charge and permanent polarization in a uniform insulator. Changes in the potential difference across the insulator are measured when a known nonuniform time‐dependent temperature perturbation is established through the sample thickness. The contributions to the measured signal from real charge and polarization charge have the same form, but are weighted differently. Deconvolution of the experimental data, therefore, yields a linear combination of the spatial distributions of real and polarization charges. These two distributions can be separated by utilizing the temperature dependence of these weighting factors.

Some optical properties of the AlxGa1−xAs alloys system

Abstract: Optical transmission data covering the Γ15V–Γ1C absorption edge are presented, together with photoluminescence (PL) results, for AlxGa1−xAs crystals of high purity [n (293 K) <1017 cm−3] for 0

Integral formalism for surface waves in piezoelectric crystals. Existence considerations

Abstract: An integral formalism for surface waves in piezoelectric half‐infinite solids valid up to the critical velocity is developed. Various boundary conditions are considered and, in particular, the problem of which boundary conditions allow surface‐wave solutions for velocities below the limiting velocity vL is discussed in detail. It is proved that (a) with a mechanically free surface and zero dielectric constant for adjoining medium, at most one solution is possible for v