Showing papers in "Journal of Applied Physics in 1971"

Ion‐Beam Deposition of Thin Films of Diamondlike Carbon

Abstract: Ion beam deposition of insulating carbon thin films on room temperature substrates, considering transparency, index of refraction, insulating capacity, glass scratching ability, etc

High‐Pressure Equation of State for NaCl, KCl, and CsCl

Abstract: A calculation of the equation of state for NaCl from a Mie‐Gruneisen equation was repeated using more accurate values of the zero‐pressure compressibility. It was also extended to KCl and CsCl. An analysis of this approach to pressure calibration indicates that it will yield pressures with about the same accuracy as can be presently achieved by experimental measurements above 25 kbar, and thus furnishes a temporary practical pressure scale.

Stimulated Emission of Bremsstrahlung in a Periodic Magnetic Field

Abstract: The Weizsacker‐Williams method is used to calculate the gain due to the induced emission of radiation into a single electromagnetic mode parallel to the motion of a relativistic electron through a periodic transverse dc magnetic field. Finite gain is available from the far‐infrared through the visible region raising the possibility of continuously tunable amplifiers and oscillators at these frequencies with the further possibility of partially coherent radiation sources in the ultraviolet and x‐ray regions to beyond 10 keV. Several numerical examples are considered.

Determination of Kilovolt Electron Energy Dissipation vs Penetration Distance in Solid Materials

Abstract: A universal curve of energy‐dissipation range vs normalized electron energy is proposed, which includes the average atomic number Z of the material being bombarded in the energy normalization factor. Range‐energy expressions of the form R=kEBα, derived from the Bohr‐Bethe energy‐loss relation, are valid over limited energy ranges, but the exponent α differs for materials of greatly different atomic numbers over the same energy range. For the aluminum‐silicon dioxide‐silicon system used here, RG = 4.0 EB(keV)1.75 μg/cm2 has been found accurate for 5

Temperature Dependence of the Elastic, Piezoelectric, and Dielectric Constants of Lithium Tantalate and Lithium Niobate

Abstract: The elastic, piezoelectric, and dielectric constants, and their temperature dependence within the range 0°–110°C, have been determined for single crystals of lithium tantalate and lithium niobate. For each material, the constants were extracted from a combination of many ultrasonic phase‐velocity measurements, the measured resonant and antiresonant frequencies of a single length‐extensional bar resonator, and the measured low‐frequency capacitances of two thin flat‐plate specimens. For lithium tantalate, the normalized first temperature derivatives of the elastic constants range from −0.4 to −6.7×10−4/°C, while the derivatives for the piezoelectric stress constants range from −1.3 to +1.5×10−4/°C. In the case of lithium niobate, the elastic constant temperature derivatives are all on the order of −2×10−4/°C, while the piezoelectric temperature derivatives are all positive in the range 0.8–8.9×10−4/°C. The dielectric or permittivity temperature derivatives are positive for both materials.

Method for Measuring Electrical Resistivity of Anisotropic Materials

Abstract: A rectangular prism with edges in principal crystal directions is prepared with electrodes on the corners of one face. Voltage‐current ratios for opposite pairs of electrodes permit calculation of components of the resistivity tensor. The method can use small samples, and is best suited to materials describable by two or three tensor components. Examples are given of measurements of V2O3–Cr and oriented amorphous graphite.

Ultralow Gas Concentration Infrared Absorption Spectroscopy

Abstract: A new technique is presented for obtaining the absorption spectra of small samples and low concentrations of gases. The technique makes use of currently available sources of wavelength‐tunable intense coherent light such as the optical parametric oscillator, dye laser, or tunable diode laser. The absorbed power is detected by the heating and resultant pressure rise in the absorbing gas. An initial experiment with a 15‐mW He–Ne laser operating at 3.39 μ has shown a sensitivity adequate to measure the absorption of a concentration of 10−8 of methane in nitrogen. It is expected that, with higher‐power sources of tunable ir radiation, it may be possible in the future to detect concentrations of impurities as low as 10−13.

Influence of Particle Size on the Electrical Resistivity of Compacted Mixtures of Polymeric and Metallic Powders

Abstract: Powder samples of high‐density polyethylene and nickel of particle sizes Rp and Rm , respectively, were mixed and compacted at room temperature under a pressure of 1000 kg/cm2. Microscopic examination of polished sections of the compact, by reflected light, showed that the metallic particles did not penetrate the polymeric particles and that this resulted in a segregated distribution of metallic particles at high ratios of Rp/Rm . The electrical resistivity of the compacts had a value of 1016 Ω cm unless the composition of metal reached a critical value, beyond which the resistivity decreased markedly by as much as twenty orders‐of‐magnitude. This critical composition was found to decrease with an increase in the ratio Rp/Rm throughout the range studied of from 1 to 16. The general features of the dependence of electrical resistivity on composition of metal could be rationalized by reference to a model according to which small particles of nickel form a monolayer on the large particles of polymer in the mixture of powders. This arrangement was supposed to be but little changed during compaction and to result in a segregated distribution of metal which can be visualized as approximating to the accommodation of metallic particles on three mutually perpendicular sets of lattice planes. The critical composition for a sudden decrease in electrical resistivity was assumed to correspond to the first nonzero probability for infinitely long chains of contiguously occupied lattice sites.

Tunneling through Fatty Acid Salt Monolayers

Abstract: Sandwiches of monolayers of Cd salts of fatty acids CH3(CH2)n−2COOH of different chain lengths between metal electrodes show the exponential decrease of conductivity vs thickness predicted by the tunnel theory. The electron work function from metal to dielectric is obtained. The current‐on‐voltage dependence is predicted and it is found to be in good agreement with experiment. The effects of changing the electrode material are quantitatively predicted from the differences between the vacuum work functions of the metals used (Hg, Al, Pb, and Au). The good agreement between theory and experiment can be taken as an extremely sensitive test for the uniformity of the monolayers obtained by the monolayer assembling technique used in this investigation.

Hall Mobility in Chemically Deposited Polycrystalline Silicon

Abstract: Hall‐mobility measurements have been performed on polycrystalline silicon films deposited on a silicon oxide surface by the thermal decomposition of silane. Samples with doping impurities added during deposition or by diffusion from a doped vapor‐deposited oxide showed similar behavior. For both n‐type and p‐type samples approximately 5 μ thick, the mobility reached a maximum value of about 40 cm2/V sec at a free carrier concentration of about 1018 cm−3 and decreased for both higher and lower carrier concentrations. The observed Hall mobility was generally higher in p‐type samples than in n‐type samples. The decrease in observed mobility with decreasing carrier concentration is attributed to the effects of high resistivity space‐charge regions surrounding grain boundaries in the polycrystalline material. The mobility was seen to increase as the film thickness increased for samples with similar doping, indicating a more ordered structure in thicker films.

Electrochemical Charging of Thermal SiO2 Films by Injected Electron Currents

Abstract: When electron currents flow in thermal SiO2 films which have been exposed to water, a buildup of negative charge occurs in the oxide. This paper describes a series of experiments designed to characterize this charging effect. It is found that if water is diffused into a SiO2 film, water related centers are formed which act like electron traps with capture cross section of approximately 1.5 × 10−17 cm2. Experiments are described which show that when one of these centers captures an electron, atomic hydrogen is released which diffuses away and escapes or reacts and a stable negative charge is left behind. Electrochemical charging effects of this type have not previously been considered, although they may play a very important role in some semiconductor device failure effects.

Lattice Trapping of Fracture Cracks

Abstract: Continuum treatments of lattice defects such as dislocations and fracture cracks do not predict the resistance to the defect mobility which is due to the Peierls energy in the case of the dislocation. The discrete character of the lattice in the case of the fracture crack leads to a stress stability range for the crack above and below the Griffith stress over which the crack is stable or ``lattice trapped''. We develop here two essentially qualitative theoretical treatments of the lattice structure of cracks. In the first, we carry out a lattice sum of the bond energies of atoms facing each other across the crack plane under certain assumptions regarding the crack shape and atomic force laws. In the second, we introduce a one‐dimensional lattice model of a crack which can be solved exactly. In both of these treatments the range of stress over which the crack is lattice trapped appears to be of the order of magnitude of the Griffith stress itself. As in the case of dislocations the lattice trapping is a strong function of the ``width'' of the elastic singularity at the tip, and our prediction is that one should expect materials to exist in which lattice trapping is not only observable, but is an important effect. We also find that because of lattice trapping, the true surface energy is a lower bound to the mechanical surface energy as expressed in the Griffith‐stress relation.

Electrical Properties of High‐Quality Stannic Oxide Crystals

Abstract: Single crystals of the wide bandgap semiconductor stannic oxide, SnO2, have been grown and studied electrically. A chemical vapor deposition technique using chlorine transport, no inert carrier gases, and low pressure has been used to grow stannic oxide crystals of higher purity and with almost an order‐of‐magnitude higher low‐temperature Hall mobility, 8800 cm2/V sec at 80°K, than have previously been available. Measurements of Hall mobility, carrier concentration, and resistivity have been made between 20 and 625°K on crystals with room‐temperature carrier concentrations between 8×1015 and 2×1018 cm−3. The effects of the crystal anisotropy on these measurements have been investigated and found to be small (all results reported are for the a direction). A donor level ∼35‐meV deep due to antimony and another level ascribed to oxygen vacancies at ∼140 meV have been observed. Polar optical mode scattering with a dominant characteristic temperature of 1080°C is the main carrier scattering mechanism above 250°K. Below 250°K acoustic deformation potential scattering dominates. Ionized impurity scattering is eventually important in all samples as the temperature is lowered. A polaron effective mass of 0.39 me has been found consistently in the analyses of the data. A technique of fabricating good Schottky barriers on SnO2 has also been developed and used to measure the net donor concentration in samples. The agreement found between these measurements and Nd from Hall measurements indicates that shallow trapping is not a problem in these crystals.

Nonstoichiometry and Crystal Growth of Lithium Niobate

Abstract: Deviations from stoichiometry occurring during the crystal growth of lithium niobate have been studied by 93Nb NMR and by shifts in the ferroelectric transition temperature. The high‐temperature phase equilibria of nonstoichiometric lithium niobate were accurately determined by these techniques together with DTA and crystal‐growth experiments. The congruently melting composition is located at 48.6 mole% Li2O and the solid‐solution region at room temperature extends from 48% to 50% Li2O. The implications of growth at noncongruently melting compositions on the compositional uniformity of lithium niobate are discussed in detail.

Double Extraction of Uniformly Generated Electron‐Hole Pairs from Insulators with Noninjecting Contacts

Abstract: A simple physical model has been used to derive an approximate theory of the double extraction of uniformly generated electron‐hole pairs from a photoconductor layer with noninjecting contacts. The theory predicts 4 regimes of current versus applied voltage behavior:(1) At low voltage, I∝V.(2) At higher voltage, a transition region between I∝V and I∝V1/2.(3) At still higher voltage, I∝V1/2.(4) At very high voltage (above a saturation value), I=constant.The model relates the extent of these regimes to the physical parameters of the system, viz., μpτp (mobility × lifetime product for holes), μnτn (mobility × lifetime product for electrons), and the layer thickness. Good agreement is obtained between the theory and measurements on lead oxide vidicons. The theory also provides some insight into the nature of the transient phenomena ``red fade'' and ``after image'' sometimes observed in the operation of lead oxide vidicons. At high light excitation levels, space‐charge‐limited currents are expected. In this ca...

GaAs Epitaxy by a Molecular Beam Method: Observations of Surface Structure on the (001) Face

Abstract: The epitaxial growth of GaAs by a molecular beam deposition method on bromine‐methanol etched (001) GaAs substrate has been studied in situ in an ultrahigh vacuum high‐energy electron diffraction system. GaAs (001)−C(2×8) and (001)−C(8×2) surface structures were observed during growth: one is arsenic stabilized, the other is gallium stabilized, and they are rotated 90° about the [001] axis from each other. Which surface structure is present during growth is a function of the ratio of the arsenic to gallium intensity in the molecular beam and the substrate temperature. For a given As2/Ga ratio, there is a transition from growth with the arsenic‐stabilized (001)−C(2×8) to the gallium‐stabilized (001)−C(8×2) with an increase in substrate temperature. In the case of a constant substrate temperature, the same transition can be achieved by varying the As2/Ga ratio. Without the molecular beam impinging on the surface, a gallium‐stabilized (001)−C(8×2) surface structure quenches to a (001)−6 structure while the a...

Photodecomposition of Amorphous As2Se3 and As2S3

Abstract: Electromagnetic radiation approximately equal to band‐gap energy has been established as responsible for the dissociation of amorphous As2Se3 and As2S3. The dissociation is accompanied by an optical densification observable as a ``photographic'' effect in thin films of these materials. The dissociation can be expressed by the following equations: As2Se3→ lim hvxAs+As2−xSe3, 0

Pressure‐Volume‐Temperature Properties and Transitions of Amorphous Polymers; Polystyrene and Poly (orthomethylstyrene)

Abstract: An apparatus based on Bridgman's bellows method, originally devised for fluids, has been constructed for use with solid samples. Mercury is employed as the confining liquid. The operating range is 0≤t≤200 °C and 1≤P≤2000 bar. Calibration with benzene and mercury shows an accuracy of ±2×10−4 cm3/g in the measurement of the specific volume change. A detailed investigation of two systems, polystyrene and poly(orthomethylstyrene) in the glassy and liquid states is undertaken. The P‐V‐T results in both states can be well represented by the Tait equation, with the single parameter B as an exponentially decreasing function of T. Similar to the earlier result for polystyrene, a β‐relaxation region is observed in terms of the isothermal compressibility of the methylated polymer with Tβ/Tg≃0.70 for both. The pressure coefficients for the two transition temperatures are discussed. For the glass transition, the Ehrenfest‐type equation appears to be obeyed by the low‐pressure glasses (formed by cooling the liquid at a...

Cone Formation on Metal Targets during Sputtering

Abstract: When a Cu surface is sputtered by ion bombardment under the condition that Mo atoms arrive at the Cu surface during sputtering an unexpected phenomenon can arise: The surface of the Cu target becomes covered with microscopic cones. The cone density increases with increasing flux density of arriving Mo atoms. When the cones are closely spaced they give the target a velvet‐like black appearance. The result of dense cone coverage is a lower sputtering yield and a more oblique ejection of sputtered material. The cone tops seem to consist of Mo nuclei which are constantly replenished via surface migration and protect the underlying Cu from being sputtered.

Organic Dye Laser Threshold

Abstract: This paper describes the results of an investigation of the interrelated physical parameters that determine laser threshold for three organic dyes: rhodamine B, rhodamine 6G, and fluorescein. Not only are these dyes (rhodamine 6G in particular) among the most widely used and important laser dyes, but insights into the threshold condition derived from our studies of these particular dyes should be generally applicable to future dye laser research. Our detailed characterization of threshold for these dyes and our novel insights result from the use of triplet state spectral data in a thorough and quantitative way for the first time, coupled with the physically realistic approximation that the triplet population is proportional to the singlet excited state population. For the case of self‐tuning of the laser emission wavelength, solutions to the threshold equations are presented that establish relations between all the parameters affecting lasing: critical inversion, emission wavelength, extrinsic losses, dye concentration, length of active medium, and triplet to excited singlet population ratio. For reasonably high extrinsic cavity losses, the critical inversion is a simple power law function of extrinsic loss, and the emission wavelength is self‐tuned in such a manner that the ratio of extrinsic losses to intrinsic singlet absorption losses is substantially constant. There exists a region of low, but still physically realizable, cavity losses, where laser action is determined exclusively by intrinsic characteristics of the dyes (triplet absorption, singlet emission, and absorption). Asymptotic limits on the maximum wavelength of emission and on the minimum critical inversion appear in this region, beyond which there is no point in reducing extrinsic cavity losses. The fraction of triplet state molecules also determines long wavelength cutoffs for an externally tuned laser. Methods are outlined for measuring the ratio of triplet state to excited singlet state populations for dyes with known triplet absorption spectra and for determining semiquantitatively the triplet effects in dyes where this information is not known. A final interesting conclusion is that molecular modifications of rhodamine 6G cannot be expected to improve its threshold characteristics as a laser dye by much more than a factor of 4.

Photoinjection into SiO2: Electron Scattering in the Image Force Potential Well

Abstract: The voltage dependence of photoinjected currents in SiO2 films for electric fields less than about 106 V/cm exhibits anomalous behavior It is shown that the physical mechanism responsible for this behavior is the scattering of photoinjected electrons in the SiO2 image force potential well between the emitter and the potential maximum A previously published model attributing the effect to electron trapping in SiO2 is shown to be inconsistent with the experimental results A theoretical model is presented and the voltage dependence of photocurrents is derived including the effects of scattering and barrier lowering Experimental results from MIS structures using thermally grown SiO2 are found to be in excellent agreement with theoretical predictions when a 34 A mean free path for scattering in SiO2 is used in the model

Magnetic Domain Structures in Thin Uniaxial Plates with Perpendicular Easy Axis

Abstract: We present a convenient formulation of the single‐domain energy in the wall‐energy model and minimize the total energy of a lattice of cylindrical domains and of a parallel‐stripe array in an infinite plate. The latter structure is shown to have the lower energy for external transverse fields Hex

GaAs–AlxGa1−xAs Double Heterostructure Injection Lasers

Abstract: Double heterostructure GaAs–AlxGa1−xAs junction lasers which have very low thresholds and which have been operated continuously at and above room temperature have been fabricated by liquid phase epitaxial growth The threshold current density of these lasers decreases approximately linearly with the thickness of the active region from 3 to at least 05 μm This is interpreted as the result of near perfect carrier and optical confinement as the result of large steps in the energy gap and index of refraction at the heterojunctions in these diodes The gain in these lasers is very high and its dependence upon current density is superlinear Loss is very low and almost that expected from free carriers Complete polarization of the lasing mode was observed This latter is interpreted to be the result of an increased reflection coefficient for the T E mode

Void Formation and Growth During Electromigration in Thin Films

Abstract: One of the modes of electromigration damage is shown to be related to irregularities in the grain‐boundary structure of films. Based on the restriction of electromigration to grain boundaries, calculations are performed to determine the magnitude and distribution of excess vacancy concentrations at sites where electromigration flux divergencies may be present. The attainment of an equilibrium vacancy supersaturation (Δc/c0) is found to be practically instantaneous. The maximum supersaturations are less than 1 under all conditions used. This is found to be inadequate for void formation by vacancy condensation unless heterogeneous nucleating sites are present (e. g., oxide particles, boundary‐surface oxide intersection). The rate of observable hole formation is accounted for by either void growth at a grain‐boundary triple‐point junction or by grain‐boundary grooving, each being related to the electromigration flux. Grooving seems most likely in pure films with all boundaries having similar character (e. g....

Optical Distortion by Heated Windows in High-Power Laser Systems,

Abstract: In a typical long‐focal‐length laser system, the distortion of the laser beam by the heated exit window can significantly reduce the intensity at the target at power levels well below those required to melt or fracture the window. The spatially inhomogeneous incident laser intensity causes a temperature gradient which changes the thickness and the index of refraction n of the window, causing it to become a lens having, in general, aberration and birefringence in addition to a finite focal length. Expressions are derived for the thermally induced optical distortion in terms of measurable parameters such as n, dn/dT, the strain‐optical coefficients p11 and p12, and the thermal‐expansion coefficient α. Since the values of all these parameters are not known for most materials of interest, alternate expressions are derived for use in obtaining rough estimates of the amount of distortion. The temperature differential ΔTcrit across the radius of the window which causes a factor of 1/q reduction in target intensity is derived and tabulated (for the case of q=2). Figures of merit for rating materials are derived and tabulated for various materials. The distortion is smaller in general for ionic crystals than for covalent crystals. The briefringence is expected to be larger for the ionic materials than for the covalent ones. In typical systems, rather large increases, say an order of magnitude, in the value of ΔTcrit can be obtained by changing the focal length of the optical system. Other methods of reducing the optical distortion are discussed briefly. Since the optical distortion is expected to be the factor which determines the limit of the target intensity in long‐focal‐length systems, a large heat capacity and small values of α, dn/dT, p11, p12, and n are important, while the melting point, tensile strength, and Young's modulus are less important, within obvious limits. Experiments are suggested for studying the thermally induced optical distortion.

Alloy Sputtering Studies with in situ Auger Electron Spectroscopy

Abstract: Triode sputtering and Auger electron spectroscopy were combined in an uhv apparatus for in situ studies of the composition of ion‐bombarded surfaces of alloy targets and of sputter‐deposited films. The changes in surface composition of a 55 Cu–45 Ni alloy (Constantan) target were studied under Ar+‐ion bombardment as a function of bombarding energy. Below 150 eV ion energy, the Cu/Ni sputtering ratio increases steeply with decreasing ion energy. This causes the nickel to become much enriched in the target surface. By ion bombarding a substrate during film deposition (biasing) one can produce a film which has an altered composition throughout. By varying in our geometry the bias voltage we were able to change the film composition from that of the target to one in which the Cu concentration was only 5% that of the Ni. The depleted species in this case appears as enrichment in films at other less biased surfaces (such as at the anode in triode systems).

Polaronic Hopping Conduction in Vanadium Phosphate Glasses

Abstract: Conductivity and dielectric properties of semiconducting glass of composition 80% V2O5:20% P2O5 have been measured as a function of frequency from dc to 3.6 GHz over a temperature range from 77°K to 420°K. A fit of the dc results to a model of polaronic hopping conduction leads to an activation energy due to disorder ΔW=0.09 eV at 77°K and an optical phonon energy of 0.053 eV compared to a calculated Miller‐Abrahams energy of approximately 0.12 eV. The real and imaginary parts of the ac conductivity are shown to increase with frequency according to power law σ=Aωs, where s=0.85 to 0.95, and it is found that the dielectric behavior can be described by a Debye‐type relaxation behavior only if a broad distribution of relaxation times exists. The results are shown to be consistent with a model of ac conduction normally applied to impurity‐doped broad‐band semiconductors, and differences between the temperature dependence of the dc and ac conductivity are attributed to a distribution of site energies in the gl...

Refractive Index of AlAs

Abstract: The refractive index of AlAs at 300 °K has been measured in the 0.5‐ to 2.2‐eV range of photon energy. The long‐wavelength dielectric constant with lattice vibrations neglected e∞ is determined to be 8.16 ± 0.02, and the resulting static dielectric constant e0 is 10.06 ± 0.04.

Dynamic Yield Strengths of B4C, BeO, and Al2O3 Ceramics

Abstract: The dynamic yield strengths for several ceramics were measured under shock‐wave compression. The Hugoniot elastic limit (HEL), taken as the amplitude of the observed elastic precursor traveling ahead of the compressional wave, was 154 and 82 kbar for B4C and BeO ceramics, respectively, and ranged from 61 to 134 kbar for several Al2O3 ceramics. These values apply to 6‐mm‐thick samples; in some cases there were variations with sample thickness. The offset of the Hugoniot from the isotropic compression curve in the stress‐volume plane indicates a nonzero yield strength above the HEL for Al2O3. This yield strength is somewhat less than the yield strength at the HEL. The compaction of porous ceramics is found to follow a quadratic variation with stress, full compaction being reached at about four times the HEL.