Showing papers in "Journal of Applied Physics in 1981"

Polymorphic transitions in single crystals: A new molecular dynamics method

Abstract: A new Lagrangian formulation is introduced. It can be used to make molecular dynamics (MD) calculations on systems under the most general, externally applied, conditions of stress. In this formulation the MD cell shape and size can change according to dynamical equations given by this Lagrangian. This new MD technique is well suited to the study of structural transformations in solids under external stress and at finite temperature. As an example of the use of this technique we show how a single crystal of Ni behaves under uniform uniaxial compressive and tensile loads. This work confirms some of the results of static (i.e., zero temperature) calculations reported in the literature. We also show that some results regarding the stress‐strain relation obtained by static calculations are invalid at finite temperature. We find that, under compressive loading, our model of Ni shows a bifurcation in its stress‐strain relation; this bifurcation provides a link in configuration space between cubic and hexagonal close packing. It is suggested that such a transformation could perhaps be observed experimentally under extreme conditions of shock.

A theory crack healing in polymers

Abstract: A theory of crack healing in polymers is presented in terms of the stages of crack healing, namely, (a) surface rearrangement, (b) surface approach, (c) wetting, (d) diffusion, and (e) randomization. The recovery ratio R of mechanical properties with time was determined as a convolution product, R = Rh (t)*φ(t), where Rh (t) is an intrinsic healing function, and φ(t) is a wetting distribution function for the crack interface or plane in the material. The reptation model of a chain in a tube was used to describe self‐diffusion of interpenetrating random coil chains which formed a basis for Rh (t). Applications of the theory are described, including crack healing in amorphous polymers and melt processing of polymer resins by injection or compression molding. Relations are developed for fracture stress σ, strain e, and energy E as a function of time t, temperature T, pressure P, and molecular weight M. Results include (i) during healing or processing at t

Interface states and electron spin resonance centers in thermally oxidized (111) and (100) silicon wafers

Abstract: Interface states and electron spin resonance centers have been observed and compared in thermally oxidized (111) and (100) silicon wafers subjected to various processing treatments. The ESR Pb signal, previously assigned to interface ⋅Si≡Si3 defects on (111) wafers, was found to have two components on (100): an ⋅Si≡Si3 center oriented in accord with (100) face structure, and an unidentified center consistent with ⋅Si≡Si2O. The quantitative proportionality of Pb spin concentration to midgap interface trap density Dit is maintained on (100), and both are lower by a factor of about 3 compared to (111). This correlation persists over the range of oxidation temperatures 800–1200°C, for both n‐ and p‐doped silicon, cooled by fast pull in oxygen, and cooled or annealed in nitrogen or argon. The correlation is independent of doping level. In samples with different oxide thickness, neither Pb nor Dit varied significantly over the range 100–2000 A, but Pb was smaller at 50 A. In general, ESR is judged to offer prom...

Shock compression of aluminum, copper, and tantalum

Abstract: Hugoniot curves for Al (alloy 11000), Cu (type oxygen‐free high‐conductivity), and Ta have been measured in the shock pressure range 30–430 GPa (0.3–4.3 Mbar) with a two‐stage light‐gas gun. Impactor velocities were measured to 0.1% by flash radiography. Shock velocities were measured to 0.5–1.2% with an electronic detection system with subnanosecond time resolution. Our data and those of other workers were fitted to a linear relation between shock and mass velocities. The fractional standard deviations of the data from the fits range from 0.6 to 0.9% for the three metals. Methods of data analysis and error analysis for individual data points and for the least‐squares fitting to the data sets are presented. Bands of uncertainty about the fits, arising from experimental uncertainties in the data, are presented and are used to calculate the systematic error introduced by the method of shock‐impedance matching. The accuracy of the data and of the fits qualifies these metals as equation‐of‐state standards for shock‐wave experiments.

Radiative cooling to low temperatures: General considerations and application to selectively emitting SiO films

Abstract: Radiative cooling occurs because the atmospheric emittance is low in the wavelength interval 8–13 μm particularly if the air is dry. We derive expressions which specify the optical properties demanded for a surface capable of being cooled to low temperatures. The key factor is infrared selectivity with low reflectance in the 8–13 μm ’’window’’ but high reflectance elsewhere. Considering only radiation balance, ideal surfaces of this type can yield temperature differences of ∼50 °C while the cooling power at near‐ambient temperatures is ∼100 W/m2. However, nonradiative exchange limits the practically achievable temperature difference. SiO films on Al were investigated as an example of an infrared‐selective surface. The infrared optical properties of SiO were determined by a novel and accurate technique. These data were used to compute the spectral radiative properties of Al coated with SiO films of different thicknesses. The spectral selectivity was largest for 1.0‐μm‐thick films. This kind of surface was produced by evaporation of SiO onto smooth Al. The measured reflectance agreed with computations. Practical tests of radiative cooling were performed using a SiO‐coated Al plate placed under transparent polyethylene films in a polystyrene box. An identical panel containing a blackbody radiator was used for comparison. The performance of the panels was tested during clear nights. It was in good qualitative agreement with theoretical expectations.

Dynamic fracture and spallation in ductile solids

Abstract: A mathematical model of ductile hole growth under the application of a mean tensile stress is developed and applied to the problem of spallation in solids. The object is to describe dynamic ductile fracture under a wide range of tensile loading conditions. The mathematical model presented here describes both plate‐impact spallation (as observed by postshot examination and time‐resolved pressure measurements) and explosively produced spallation (as observed by dynamic x‐radiographic techniques) in copper. It is found to be inapplicable to ductile fracture of expanding rings, even in the absence of possible adiabatic shear banding and classical necking instabilities, because of the fact that the mean tensile stress (void growth) and the deviatoric stress (homogeneous plastic shear strain) are not independent. A phenomenological model of void growth under uniaxial stress conditions is developed independently and applied to the numerical finite‐difference solution of fracture in an expanding ring. The initial...

Bounds on the complex permittivity of a two‐component composite material

Abstract: A generalization of the bounds obtained by Wiener, Hashin and Shtrikman, and others is derived for complex permittivities. Provided the scale of inhomogeneities in the composite is sufficiently small compared with the wavelength of the applied radiation, the permittivity of the composite is found to lie within a simply constructed region of complex plane. The appropriate region depends on what is known about the composite material. We show that in many cases the region is the most restrictive which can be given, using limited information about the structure of the composite.

Breakdown and prebreakdown phenomena in liquids

Abstract: In this paper we present a comprehensive account of our results on streamer propagation in dielectric fluids in point‐plane geometries. Propagation velocities for both positive and negative streamers have been determined as a function of the following parameters: temperature, pressure, density, viscosity, composition, and conductivity. Effects of voltage and interelectrode spacing were examined. Current and light emission during streamer growth were measured. The relation between shock wave and streamer velocities was investigated. Small concentrations of low‐ionization potential additives markedly accelerated the positive streamers, while electron scavengers accelerated the negative streamers. Mechanisms to account for these observations are discussed.

The reaction of fluorine atoms with silicon

Abstract: Fluorine atoms etch silicon with a rate, RF(Si) = 2.91±0.20×10−12T1/2nFe−0.108 eV/kT A/min, where nF (cm−3) is the atom concentration. This etching is accompanied by a chemiluminescent continuum in the gas phase which exhibits the same activation energy. These phenomena are described by the kinetics: (1) F(g)+Sisurf→SiF2(g), (2) SiF2(g) +F(g) →SiF*3(g), (3) SiF2(g) +F2(g) →SiF*3(g) +F(g), (4) SiF*3(g) →SiF3(g) +hνcontinuum where formation of SiF2 is the rate‐limiting step. A detailed model of silicon gasification is presented which accounts for the low atomic fluorine reaction probability (0.00168 at room temperature) and formation of SiF2 as a direct product. Previously reported etch rates of SiO2 by atomic fluorine are high by a constant factor. The etch rate of SiO2 is RF(SiO2) = (6.14±0.49)×10−13nF T1/2e−0.163/kT A/min and the ratio of Si to SiO2 etching by F atoms is (4.74±0.49)e−0.055/kT.

Spectral dependence of the change in refractive index due to carrier injection in GaAs lasers

Abstract: The refractive index change caused by changes in the absorption edge of GaAs is determined by analysis of the spontaneous emission spectrum of a buried heterostructure window laser. The spontaneous emission spectrum is converted to a gain spectrum from which changes in the imaginary part of the refractive index can be calculated as the laser is excited from low current up to threshold. The real change in refractive index is then determined by a Kramers‐Kronig transformation. The change in refractive index n′ of the GaAs active layer is slightly sublinear with minority carrier density nc. At the laser line, including the calculated contribution of free carriers, Δn′ = −0.025±0.005 and dn′/dnc = −(1.8±0.4)×10−20 cm3 in lasers with carrier densities at threshold estimated as 1.02×1018 cm−3. Near threshold, the ratio of the change in the real index to the change in the imaginary index is about 6.2.

The characterization of x‐ray photocathodes in the 0.1–10‐keV photon energy region

Abstract: A method and an instrument are described for the measurement of the absolute quantum yield for front‐surface and transmission photocathodes in the 0.1–10‐keV photon energy region. The total and the secondary electron photoemission yields have been measured for the Al, Au, CuI, and CsI photocathodes as required for the absolute calibration of the x‐ray diode detectors and for the x‐ray streak cameras. The relative secondary electron yields have also been measured for the same photocathodes by high resolution electron spectroscopy of the secondary electron energy distributions, which are in good agreement with the absolute yield measurements. The secondary electron yield of CsI is ten to one‐hundred times higher than that for Au in the 0.1–10‐keV region and with a secondary energy distribution that is appreciably sharper. For these reasons, CsI should be an effective photocathode for sensitive, time‐resolved spectroscopy into the picosecond region. It is verified experimentally that the secondary electron quantum yield varies approximately as Em(E), with E as the photon energy and m(E) as the photoionization cross section, and that the primary (fast) electron quantum yield is a small fraction of the total yield and varies approximately as E2m(E). A simple model for x‐ray photoemission is described which leads to semiempirical equations for front‐ and back‐surface secondary electron photoemission as based upon an escape depth parameter that may be obtained from yield‐versus‐photocathode thickness data. The model predictions are in good agreement with experiment.

Analysis of gain suppression in undoped injection lasers

Abstract: This paper gives an analysis and discussion of gain suppression in injection lasers which have an undoped active region and an index guiding structure. In previous papers, we used a semiclassical density‐matrix analysis to show that an injection laser with an updoped active region has a nearly, but not perfectly, homogeneous (or uniform) gain property under operating conditions due to the mode coupling effects by phase synchronization of electrons to the lasing field. The gain of adjacent modes is well suppressed by the oscillating mode, and single‐longitudinal‐mode operation is obtained in undoped injection lasers. However, such suppression depends closely on the spacial distribution of the resonating field and injected carrier density. In this paper, the suppression effect is examined theoretically considering electronic intraband relaxation, effects from the standing wave of the lasing field, spatial diffusion of carriers, etc. When the relaxation time is larger than 3×10−13 sec, the gain shows ’’hole burning,’’ namely, strong nonuniformity across the spectral or energy distributions, and, at the same time, the gain of some resonating modes is increased. Single‐longitudinal‐mode operation is not obtained in such a strongly inhomogeneous laser. When the relaxation time is smaller than 2×10−13 sec, the gain can be seen to be nearly homogeneous, and the gain of nonoscillating modes is sufficiently suppressed, that is, lower than that at threshold, because of the strong‐mode‐coupling effect. The relaxation time of GaAs is expected to be approximately 1×10−13 sec, implying 0.1% of excess suppression. The spatial distribution of the resonating field and induced ’’spatial hole burning’’ of carriers tends to increase the gain of higher transverse modes, but only weakly affects the fundamental transverse modes when the oscillating mode is the fundamental mode. It is then necessary to design the laser so that such higher transverse modes are cut off. The results of this analysis explain the experimental data of well‐designed lasers which have an undoped active region and an index guiding structure. Such a gain suppression effect is crucial for the operation of injection lasers in a single‐longitudinal mode.

Sputtering from elastic‐collision spikes in heavy‐ion‐bombarded metals

Abstract: Estimates are presented for the sputtering yield as well as the specturm of sputtered atoms for metals bombarded by heavy atomic or molecular ions, under the assumption of evaporation from an elastic‐collision spike of high initial temperature. The calculational is based on a cylindrical spike and a thermal diffusivity proportional to the square root of the temperature. Good qualitative agreement is obtained between calculated and measured sputter yields for antimony incident on silver.

Spectroscopic diagnostics of CF4‐O2 plasmas during Si and SiO2 etching processes

Abstract: The emission intensities in the range of 2000–8000 A of CF, CF2, O, F, CO, CO+, and CO2 produced in CF4/O2 radio‐frequency discharges, operated at 1 Torr of pressure and 50 W of power input, have been used to follow the etching process of Si and SiO2, as a function of the CF4/O2 feed composition. The addition of small amounts of nitrogen or argon to the plasma mixtures has permitted the determination of the effect of the oxygen addition to the gas feed on the electron densities of the plasma for a wide range of electron energies. The relative concentration profiles of F and O, as well as of CO and CO2, have been determined with this technique, as a function of the oxygen content in the feed. The important role played by atomic F as active etchant for both Si and SiO2 substrates has been confirmed.

Bounds on the transport and optical properties of a two‐component composite material

Abstract: An infinite set of bounds on the effective permittivity ee of two‐component composite materials is derived. All the bounds can be expressed in terms of a single function g. Analogous bounds apply to the other transport properties of the composite, such as the thermal and electrical conductivities and the magnetic permeability. The work also applies to the optical properties of the composite, provided the wavelength is sufficiently large compared with the structure of the composite. In all cases we find ee is confined to a region of the complex plane bounded by arcs of circles. The appropriate region is determined by what is known about the composite and as more information is known the region becomes progressively smaller. We show that in many cases the region is the most restrictive which can be found using only the known information about the composite material.

Oxidation kinetics of TiN thin films

Abstract: We have investigated the oxidation kinetics of TiN thin films in dry O2 in view of a possible application of TiN as material for gate electrodes and interconnections in large‐scale integrated circuits. We found that in the temperature range of 500 to 650 °C the oxidation is thermally activated with an activation energy of 2.05±0.05 eV. Thereby the diffusion of oxygen through the oxide is the rate‐limiting process. Analysis with x rays indicates that dry oxidation transforms TiN to the rutile form of TiO2. Films of TiO2 formed in such a manner are found to be semi‐insulating with resistivities in the order of 106 Ω cm and at higher applied electric fields the injection of space‐charge–limited currents is observed.

Plasma etching of Si and SiO2 in SF6–O2 mixtures

Abstract: The products of reaction and etch rates of Si and SiO2 in SF6‐O2 plasmas have been studied as a function of feed composition in an alumina tube reactor at 27 mHz, 45 W, and 1 Torr. There is a broad chemical analogy with CF4‐02 plasmas. As in CF4‐02 mixtures, the rate of Si etching and 703.7‐nm emission from electronically excited F atoms each exhibit distinct maxima as a function of feed gas composition; these data support a model in which fluorine atoms, the etching species, compete with oxygen atoms for chemisorption on the Si surface. Without oxygen in the feed or Si in the reactor, no stable products could be detected. With an SF6‐O2 mixture in the absence of silicon, the final reaction products are F2, SOF4, and SO2F2. The product distribution was unaffected by small SiO2 substrates. When Si is etched, SiF4 is the only stable silicon‐containing etch product and SOF2 is formed in oxygen‐poor mixtures. Rapid etch rates (≳104 A/min for Si) can be obtained with a high selectivity in favor of silicon (Si:...

Properties of zinc oxide films prepared by the oxidation of diethyl zinc

Abstract: Polycrystalline transparent semiconducting zinc oxide films have been deposited by the oxidation of diethyl zinc. The film growth rate is controlled by a complex multistep oxidation process which is dominated by radical reactions. The effect of substrate temperature and gas pressures have been studied. Samples deposited between 280 and 350 °C have a conductivity varying from 10−2 to 50 Ω−1 cm−1. The electrical properties of the films which are typical of polycrystalline material with small crystallites are shown to depend very closely on the film growth conditions. A study of oxygen chemisorption at grain boundaries confirms the importance of grain boundary effects in ZnO polycrystalline films.

Optical properties of Nd3+ in tellurite and phosphotellurite glasses

Abstract: Optical‐absorption and emission spectra and fluorescence lifetimes were measured for Nd3+ in tellurite glasses containing various alkali and higher valence state cations and in a series of new phosphotellurite glasses. Judd‐Ofelt intensity parameters were determined and used to calculate radiative lifetimes and stimulated emission cross sections for 4F3/2→4I11/2 and 4F3/2→4I13/2 transitions. Cross sections for several of the glasses are the largest obtained for any pure oxide glass. The dependence of the spectroscopic properties on composition and the application of tellurite glasses for lasers are discussed.

Stoichiometric disturbances in ion implanted compound semiconductors

Abstract: Disturbances in the stoichiometry of compound semiconductors which result from ion implantation are calculated using a Boltzmann transport equation approach. Results for 50‐keV boron, 150‐keV silicon, and 400‐keV selenium implanted into silicon carbide, indium phosphide, and gallium arsenide are presented. Possible complications in the annealing of such implants are discussed.

An Analytical Formula and Important Parameters for Low-Energy Ion Sputtering: Erratum of the Publication in Journal of Applied Physics Vol. 51, S. 2861 ff, (1980)

Abstract: Sputtering yields for different ions and materials at low ion energies have a similar energy dependence. Due to this similarity, yield data can be characterized by a normalized energy function and two parameters for each ion target combination. One of these parameters is the threshold energy. An energy scaling can be based on this parameter. The other parameter is a multiplication factor. Both parameters depend mainly on the ion and target mass M1 and M2 and on the surface binding energy EB. An analytic expression for the normalized functions and both the parameters is given. This empirical relation also allows an estimate of unknown sputtering data, if M1, M2, and EB are noted. A physical interpretation of the empirical relation is given for the case M1≪M2, as in this case special collision processes which dominate the sputtering can be identified.

Magnetic structure of small NiFe2O4 particles

Abstract: The magnetic structure of small NiFe2O4 particles has been investigated. Samples (in the few hundred angstrom size range and up) were prepared by chemical precipitation followed by a heat treatment at relatively low temperatures. Mossbauer spectra of the 57Fe nuclei, obtained with a longitudinal magnetic field applied, unambiguously establish that a non‐collinear structure exists. Further, the magnetic moment at low temperatures is appreciably lower than the value reported for bulk material. A model is proposed in which the NiFe2O4 particles consist of a core with the usual spin arrangement and a surface layer with atomic moments inclined to the direction of the net magnetization. The temperature dependence of this structure is also reported.

Criteria for automatic x‐ray absorption fine structure background removal

Abstract: Criteria for terminating smoothing to remove a cubic spline background from the x‐ray absorption coefficient are stated in terms of three parameters obtained from the k3 weighted Fourier transform of the resulting x‐ray absorption fine structure data. The three parameters are HR, the average value of the transform magnitude between 0 and 0.25 A, HM, the maximum value in the transform magnitude between 1 and 5 A, and HN, the average value of the transform magnitude between 9 and 10 A. The termination criteria are HR−HN ⩾0.05HM; or if HN ≳0.1HM, then HR ⩾0.1HM. The incorporation of the criteria into a computer program to facilitate automatic background removal is discussed. Examples of application of the technique to copper, β‐PtO2, and ferritin samples are presented.

A finite material temperature model for ion energy deposition in ion‐driven inertial confinement fusion targets

Abstract: We have developed a model for use in ion‐driven inertial confinement fusion (ICF) target design to describe the deposition of energy by an arbitrary ion traversing a material of arbitrary composition, density, and temperature. This model particularly emphasizes the deposition physics of light ions having specific energies of 3 MeV/amu or less. However, the model is also applicable to heavy ion fusion problems where there are specific energies in excess of 10 MeV/amu. We have found that an accurate description of the cold material stopping power must include both shell corrections to the Bethe theory as well as the alternative LSS (Linhard‐Scharff‐Schio/tt) model at low energies. We have incorporated finite temperature effects by scaling the relevant bound electron parameters with the degree of material ionization as well as by including the free‐electron stopping power. We discuss both the phenomenon of range shortening and range relengthening in heated material. Our preliminary calculations indicate that...

Thermalization of sputtered atoms

Abstract: We have calculated the energy distributions of sputtered Nb and Cu atoms ejected from amorphous targets under low‐energy Ar bombardment. A formula based on elementary kinetic gas theory is used to calculate the subsequent energy loss of the ejected atoms due to collisions in the sputtering gas. The energy distributions of the sputtered atoms arriving at the substrate is compared with the distributions obtained using thermal evaporation techniques. This comparison indicates that the preparation of epitaxial metallic films, such as Layered Ultrathin Coherent Structures using sputtering techniques may have fundamental advantages over thermal evaporation.

Diffusion of zinc in gallium arsenide: A new model

Abstract: Experimental results on the diffusion of Zn in GaAs which could not be satisfactorily explained in terms of a Frank–Turnbull mechanism involving vacancies can be understood with a ’’kick‐out model’’ in which the equilibrium between interstitial and substitutional Zn is established via gallium interstitials.

Theory of continuously distributed trap states at Si‐SiO2 interfaces

Abstract: A calculation method to treat the electronic structures of crystalline Si‐amorphous SiO2 interfaces with or without microstructural defects is developed based on semiempirical tight‐binding Hamiltonians and the Green’s function formulation, and applied for calculation of the energy level of the trap states between amorphous SiO2 and the Si substrate with (111) orientation. The major results are (i) the perfect interface does not have any states in the forbidden gap of Si although the Si‐O‐Si bonding angle at the interface is varied in the range between 120° and 180°, and neither does the interface with oxygen dangling bonds have any; (ii) trap states due to a Si dangling bond appear at about the middle of the Si band gap; and (iii) O‐vacancy and Si‐Si weak bonds at the interface produce trap states at the energy range higher than the midgap, whereas Si‐O weak bonds at the interface produces trap states at the energy range lower than the midgap. The energy level of these trap states varies with changing bo...

Electric field enhanced emission from non‐Coulombic traps in semiconductors

Abstract: Electric field enhancement of emission from three non‐Coulombic traps has been calculated: the shielded Coulombic potential, the polarization potential, and the dipole potential. Both the Poole‐Frenkel effect and phonon‐assisted tunneling have been included, and both were found to be important. The field effect can be used to distinguish between these potentials on the basis of their long range character. This effect is most important in interpreting the results of capacitance transient studies of deep levels.

The effects of water on oxide and interface trapped charge generation in thermal SiO2 films

Abstract: Water was diffused into very dry thermal SiO2 films under conditions such that the penetration of water related electron trapping centers was of the order of the oxide thickness. In both dry oxides and water diffused oxides, production of negative bulk oxide charge Qot and positive interface charge Qit by an avalanche‐injected electron flux was observed. The efficiencies of both processes were enhanced by water indiffusion. Analysis of the kinetics of charge generation indicated that production of trapped electron centers (Qot ) was required for subsequent production of interface states and charge (Qit ). Models for both processes are discussed. We suggest that inelastic collisions of conduction electrons with the trapped electron centers releases mobile hydrogen atoms or excitons. The mobile species migrate to the Si–SiO2 interface and form states and fixed charge.

Structural investigation of thin films of diamondlike carbon

Abstract: Diamondlike carbon films produced both by ion‐beam technique and by radio‐frequency (rf) plasma decomposition of hydrocarbon gases (C4H10, C2H6, C3H8, and CH4) have been examined using the technique of transmission electron microscopy. Although these examinations indicate that these films are predominantly amorphous, both single‐crystal and polycrystalline diffraction patterns have been obtained from films of both types that indicate formation of several different phases. Some of these phases appear to be cubic and could be new forms of carbon. The results of secondary ion mass spectrometric analysis of carbon films produced by rf plasma decomposition of hydrocarbon gases are also discussed.