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Journal of Applied Physics

Issue 8 • Date Oct 1988

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Displaying Results 1 - 25 of 80
  • Thin‐skin electromagnetic fields around surface‐breaking cracks in metals

    Page(s): 3777 - 3784
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    There has been a long history of the use of two electromagnetic techniques to measure surface‐breaking cracks in metals. Both the alternating current potential drop (ACPD) technique and the eddy current technique have given good agreement with experimental results, even though the theoretical models on which their interpretations are based use contrasting assumptions for the boundary condition on the metal surface. The model for the ACPD technique assumes that the magnetic scalar potential satisfies the 2D Laplace equation, while eddy current modeling assumes an approximation of Born type in which the surface field is unperturbed by the presence of the crack. This paper considers a general model matching the thin‐skin electromagnetic field around a surface‐breaking crack to that in the free space above and shows that the two contrasting boundary conditions are extremes of a more general one. The Laplace approximation is valid for high permeability materials such as mild steel, while the Born approximation is appropriate for materials of low permeability and high conductivity such as aluminum. Experimental investigations of the magnetic fields near semielliptical cracks in mild steel and aluminum show quantitative agreement with the theory. View full abstract»

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  • Application of computational fluid dynamics methods to a numerical study of electromagnetic wave scattering phenomena

    Page(s): 3785 - 3791
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    The scattering of electromagnetic waves by an obstacle is formulated as a boundary value problem analogous to the fluid dynamic problem of flows past the obstacle. The transformed Helmholtz equation in terms of Debye amplitude functions can be solved numerically in a manner similar to the solution of governing fluid flow equations. The far‐field radiation condition and media interface boundary condition can also be enforced in a manner similar to the enforcement of the free‐stream condition and flow‐tangency condition. Various numerical methods in computational fluid dynamics CFD can be carried over for the computation of scattering characteristics. An example problem of the scattering of plane electromagnetic waves by a perfectly conducting sphere is solved in the frequency domain by a finite difference method based on the concept of generalized scattering amplitude. Numerical results are presented for ka=2.9, where k is the wave number equaling 2π divided by the wavelength and a is the radius of the sphere. View full abstract»

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  • High‐temperature preservation of InP surface corrugations for distributed feedback lasers

    Page(s): 3792 - 3798
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    Consistent preservation of 〈01¯1¯〉‐oriented grating corrugations with 2850‐Å period and 1500‐Å height on (100) InP substrates has been achieved during thermal cycling at 660 °C for 60 min in a H2 ambient, using roughened GaAs cover slices. It is found that transport of Ga and As causes the formation of a graded 25‐Å‐thick InGaAsP alloy on the surface of the gratings, as detected by Auger and x‐ray photoelectron spectroscopic analysis. This preservation technique eliminates the need for lower‐temperature crystal growth of distributed feedback laser wafers by liquid‐phase epitaxy (LPE). Successful growth of distributed feedback laser wafers using conventional high‐temperature LPE is demonstrated. View full abstract»

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  • Heavy‐ion versus electron‐beam excitation of an excimer laser

    Page(s): 3799 - 3810
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    A model is described for the heavy‐ion pumping of an XeF(B) laser by uranium fission fragments (FF).The model is a self‐consistent accounting of the generation and transport of the FF’s through the fission foils, slowing of the fragments in the gas, evolution of the secondary‐electron‐source function and distribution, and the XeF laser plasma kinetics. By simulating the same quantities for an e‐beam‐pumped plasma, direct comparisons can be made for laser performance. We found that the secondary‐electron source generated by the e‐beam is more energetic than that for direct ionization by FF’s due to a more favorable mass ratio for momentum transfer collisions with orbital electrons. This difference in the electron‐source functions significantly affects W values and excitation fractions. The impact on laser performance, though, is not large due to the high efficiency of channeling deposited energy to the upper laser level in XeF lasers. For conditions typical of FF excitation (power deposition 1–3 kW cm-3, pulse length ≊200 μs), e‐beam excitation results in 10%–15% higher gain than heavy‐ion excitation. View full abstract»

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  • Electron‐density distribution in a laser tube with a hyperboloid of revolution boundary: Nonisothermal case

    Page(s): 3811 - 3814
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    The electron density is determined in a laser tube that is formed by a hyperboloid of revolution and spherical‐end surfaces. The electron temperature variation is accounted for by allowing the ratio of the free‐electron production factor α to the diffusion coefficient Da to be a function of the local tube radius. Calculations are made assuming different symmetrical optical resonator configurations ranging from near planar to near confocal. The tube geometry is specified in terms of the optical resonator parameters: resonator length L, mirror radius of curvature Rm, and optical spot size at the mirror surface d. The active medium is assumed to be low‐temperature, weakly ionized plasma. Schottky boundary conditions are assumed and the particle‐diffusion equation is solved using oblate spheroidal coordinates. The oblate spheroidal angular dependence (which in the cylindrical limit corresponds to the variation perpendicular to the axis of the cylinder) of the density is found to be that of a zero‐order Bessel function. The oblate spheroidal radial dependence (which in the cylindrical limit corresponds to the axial dependence) is obtained by means of numerical methods. Radial density profiles are calculated, assuming L=1.0 m, for various Rm /L ratios. The peak radial density is found to remain at the center of the tube (Z=0) for all values of Rm /L≫0.5. View full abstract»

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  • Thermal response of a laser‐irradiated metal slab

    Page(s): 3815 - 3819
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    The analytical solution of temperature profiles in complex laser‐heated structures is only possible for a few simple cases. This is partly due to the nature of heat‐source terms that preclude the use of conventional series‐type solutions. In this study we utilize a Laplace‐transform method in conjunction with Fourier integrals to obtain solutions for the temperature profiles in a laser‐irradiated rare‐earth–transition‐metal slab. The results provide a convenient qualitative tool to study the thermal response of laser‐irradiated solids. View full abstract»

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  • Transient, combined conduction and radiation in an absorbing, emitting, and isotropically scattering solid cylinder

    Page(s): 3820 - 3824
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    Transient, combined conduction and radiation is solved in an absorbing, emitting, and isotropically scattering solid cylinder initially at uniform temperature and for times t≫0 subjected to a constant temperature at the black cylindrical surface. The collocation method is used to solve the radiation part of the problem and the implicit finite difference scheme is used to solve the conduction part. The effects of the conduction to radiation parameter, the single scattering albedo, the optical thickness of the medium on the temperature distribution, and the heat flux are examined. View full abstract»

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  • Theoretical transient currents from two‐component unipolar space‐charge swarms in fields in media

    Page(s): 3825 - 3830
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    Using an arbitrary initial unipolar space‐charge distribution consisting of two species of charge carriers of different but constant mobilities in a medium, relations for the electric fields and charge‐carrier densities are derived as functions of positions and time. The highly nonlinear, one‐dimensional equations, which are derived for swarms of charge carriers between parallel plane electrodes with a fixed potential difference, include the effects of the space‐charge fields. A general method is outlined which, in principle, can be used to generate a second order differential equation whose solution predicts the time‐dependent current caused by the drifting space‐charge swarm. The general equations are applied to the special case where the initial space‐charge distributions are uniform in a solid or fluid medium. Although the resulting differential equation is complicated, the equation is in a form such that its solutions could be computer generated. View full abstract»

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  • Ultrahigh magnetic fields produced in a gas‐puff Z pinch

    Page(s): 3831 - 3844
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    Controlled, ultrahigh axial magnetic fields have been produced and measured in a gas‐puff Z pinch. A 0.5‐MA, 2‐cm‐radius annular gas‐puff Z pinch with a 3‐min repetition rate was imploded radially onto an axial seed field, causing the field to compress. Axial magnetic field compressions up to 180 and peak magnetic fields up to 1.6 MG were measured. Faraday rotation of an Argon laser (5154 Å) in a quartz fiber on‐axis was the principal magnetic field diagnostic. Other diagnostics included a nitrogen laser interferometer, x‐ray diodes, and magnetic field probes. The magnetic field compression results are consistent with simple snowplow and self‐similar analytic models, which are presented here. Even small axial fields help stabilize the pinches, some of which exhibit several stable radial bounces during a current pulse. The method of compressing axial fields in a gas‐puff Z pinch is extrapolable to the order of 100 MG. Scaling laws are presented. Potential applications of ultrahigh axial fields in Z pinches are discussed for x‐ray lasers, inertial confinement fusion, gamma‐ray generators, and atomic physics studies. View full abstract»

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  • Phase‐amplitude imaging: The fully automated analysis of megagauss magnetic field measurements in laser‐produced plasmas

    Page(s): 3845 - 3850
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    This paper describes the application of phase‐amplitude imaging to the measurement of self‐generated magnetic fields in laser‐produced plasmas. The magnitude of the magnetic field is determined by polarimetry and the plasma density profile by interferometry. Both pieces of information are recorded on a single interferogram in a form that can be analyzed automatically using computer‐based image processing. We describe a method for successfully determining the magnetic field profile in plasmas generated by 70‐ps Nd laser pulses at high intensity on planar targets when plasma motion or turbulence introduces an additional difficulty by reducing the visibility of the fringes. View full abstract»

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  • Pressure measurements correlated with electrical explosion of metals in water

    Page(s): 3851 - 3854
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    Time profiles of underwater shock wave pressure, current, and voltage were measured for exploding aluminum, aluminum‐lithium alloy, and copper wire in water when an inductive energy store was discharged through the wire. A linear relationship existed between the peak voltage across exploding wire and the underwater shock wave pressure generated by the wire. The aluminum‐lithium alloy reached a highly resistive state more quickly than pure aluminum wire at the same heating rate. The chemical reaction between the exploding wire material and the surrounding water plays a small role in the generation of detonation wave. View full abstract»

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  • Temporal intensity modulation of spectral lines in a low‐frequency discharge in argon

    Page(s): 3855 - 3862
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    The time‐dependent modulation of the emission intensities of several Ari and Arii spectral lines was measured with the time‐to‐amplitude conversion method for an argon discharge at 50 kHz occurring between the electrodes of an asymmetric parallel plate etch reactor. Under the assumptions that the life time of an emitting state is short compared to the electric field period and that the state is populated by electron impact from the ground state, the line intensity becomes proportional to the corresponding excitation rate coefficient. This time‐dependent rate coefficient is calculated from the electron energy distribution obtained by solving the quasistationary Boltzmann equation where the plasma electric field is taken to be proportional to the measured electrode potential. Good agreement between the time dependencies of measured and calculated line intensities is achieved for the selected 6d(1/2)01 →4p(1/2)1 transition in Ari if the amplitude of the plasma field is adjusted to 13 V/cm. View full abstract»

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  • Negative ion formation at a barium surface exposed to an intense positive‐hydrogen ion beam

    Page(s): 3863 - 3873
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    A fundamental study of the formation of negative hydrogen ions via surface conversion is presented. Employed is a novel type of converter, namely a pure barium metal surface. In spite of the high work function of barium compared to more conventional cesiated converters, considerable yields of negative ions were produced. Conversion efficiencies up of 4% are obtained, which is of the same order as for cesiated converters. The high negative‐ion yield is probably related to the electron density of barium, which is almost twice that of cesiuim. This is confirmed by model calculations and by UHV scattering experiments under well‐defined conditions. Furthermore, calculations showed that the hydrogen coverage of the converter increases with increasing flux of positive hydrogen ions to the surface. This behavior is confirmed experimentally. Seeding the hydrogen plasma with argon has no significant effect on the conversion efficiency. This is believed to be related to the competition between the lowering of the surface hydrogen coverage and the increase of the hydrogen desorption rate, both due to the higher sputter coefficient of argon compared to hydrogen. View full abstract»

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  • A tandem radio‐frequency plasma torch

    Page(s): 3874 - 3879
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    The flow and temperature fields in a tandem radio‐frequency (rf) plasma torch were calculated. The method employed here is based on our previous rf plasma model. The flow and temperature fields of this tandem plasma show similar features to those of a dc‐rf hybrid plasma, and the carrier gas can be injected into the high‐temperature region (∼9000 °C) of the second plasma by regulating the linear velocity of the sheath gas. As might be expected, the derived results suggest that the tandem plasma has a possibility to give higher efficiencies for practical processing than conventional rf plasmas. A new type of tandem rf plasma torch was designed based on the results of the calculations. The stability of this plasma was examined under various conditions. Thus, a stable plasma could be maintained even when carrier gas of high flow rate (∼5 l/min of H2) was injected into the top of the second plasma. These theoretical and experimental investigations showed that the tandem plasma torch is an effective reactor for the preparation of refractory compounds by plasma chemical vapor deposition processes. View full abstract»

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  • Dislocation effect on the activation efficiency profile in Si‐implanted and SiN‐capped annealed GaAs

    Page(s): 3880 - 3884
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    Depth profiles of carrier concentration and photoluminescence (PL) intensities are measured for Si‐implanted and SiN‐capped annealed GaAs. The peak carrier concentration decreases far from dislocations without a change in the effective thickness of the active layer. Moreover, the activation efficiency of the implanted Si decreases and the PL intensity of the Si on the arsenic site (SiAs) acceptor increases in the surface region of the active layer. These results suggest that the As vacancy concentration increases during the annealing process, and this enhances the effect of dislocations which creates an electrical nonuniformity of the Si‐implanted GaAs layer. View full abstract»

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  • Experimental evidence for creation of luminescence quenching localized defects by moving dislocations in gallium arsenide

    Page(s): 3885 - 3889
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    The production of luminescence quenching localized defects by moving dislocations in GaAs plastically deformed at 350 °C has been established by low‐temperature (80 K) band‐edge cathodoluminescence (CL) and chemical etch‐pit studies. Specimens were deformed to two levels, (1) measurable macroscopic plastic deformation, and (2) 20% of the stress applied in (1). It was established by CL and chemical etching that numerous dislocations were generated on the four {111} stressed slip planes of the macroscopically deformed specimen. Uniform bands of reduced CL signal were correlated to bands of enhanced etching in the lightly deformed samples. It was established from the etching and CL images that these bands are not produced by deformation‐induced dislocations but must consist of numerous localized luminescence quenching (Refs. 1–3) defects (e.g., point defects and small dislocation loops) created by the passage of dislocations. View full abstract»

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  • Crack nucleation at the surface of stressed fibers

    Page(s): 3890 - 3900
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    A model is proposed for crack initiation on a pristine surface. In this model failure starts from a flat surface and the crack develops during the strength or lifetime experiment. Both energy and kinetic arguments are used. The model is applied to lifetime calculations of pristine optical fibers. Surface energy and bulk elastic energy are calculated for a fiber with a flat and a slightly distorted surface. When a small amount of elastic energy is removed locally from the fiber, e.g., by dissolution, the distorted state is more stable. Like cracks in brittle materials, the stable distortions for pristine optical fibers have local radii of curvature at atomic dimensions. In this model the lifetime of a pristine fiber is mainly determined by the time needed to pass from a flat surface to a relatively small surface distortion. From the derived expression for the lifetime, the exponent n in the empirical power law expression for subcritical crack growth can be interpreted. It is shown that n is stress dependent. The discrepancy in n value observed for silica glass fibers (n≂20) and bulk fused silica (n≂40) is removed. View full abstract»

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  • Evaluation of cylinder‐impact test data for constitutive model constants

    Page(s): 3901 - 3910
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    This paper examines the usefulness of cylinder‐impact test data to determine constants for various computational constitutive models. The Johnson–Cook and Zerilli–Armstrong constitutive models are evaluated by comparing model predictions to tension, torsion, and cylinder‐impact test data. Then the cylinder‐impact test data are used to determine constitutive model constants for various forms of these models. Under bounded conditions of strains and strain rates, this approach can produce useful results. It can also produce very erroneous results if not used properly. View full abstract»

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  • The thermal conductivity of Li2O∙Al2O3∙nSiO2 glasses between 5 and 100 K

    Page(s): 3911 - 3914
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    The thermal conductivity of the Li2O∙Al2O3∙nSiO2 family of glasses for n=4,6,8,10, and 12 was measured between 5 and 100 K. For all the glasses in this study, including a vitreous silica sample, the thermal conductivity exhibits the traditional plateau between 10 and 20 K followed by a monotonic increase at higher temperature. The compositional dependence is discussed in terms of the elastic properties and comparisons are made to previous work. View full abstract»

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  • Diffusion of oxygen in YBa2Cu3O7-y

    Page(s): 3915 - 3917
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    The oxygen lattice diffusion in YBa2Cu3O7-y was studied with 18O as a tracer. For the composition of y=0.07, the oxygen lattice diffusion coefficient is represented by Dl,O =2.87×10-1 exp[-173(kJ/mol)/RT] cm2/s in the temperature range 250–400 °C. The oxygen lattice diffusion coefficient decreases with the increasing value of y. View full abstract»

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  • A Markov treatment of the self‐diffusion dyadic in homogeneous crystals

    Page(s): 3918 - 3927
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    Expressions are derived for the drift velocity V and the self‐diffusion dyadic D⃡ of a particle in a macroscopically homogeneous multicomponent crystal based on two related finite Markov descriptions. In the first or system state description, the Markov states, αβγ∙∙∙, correspond to configurational states of a crystal with periodic boundary conditions. To each Markov transition α→β is assigned the vector rαβ equal to the displacement, if any, of the particle during that transition. In the second or jump description, the Markov states ijk∙∙∙ correspond to the type of jump executed by the particle. To each Markov state i is assigned the vector ri equal to the displacement of the particle executing the ith type of jump. The parameters of the second description are derived in terms of those of the first. The treatment extends previous ones in the following ways: (1) It is three dimensional; (2) it gives a rigorous derivation of D⃡ when correlation and bias (i.e., nonzero V) effects are simultaneously present; (3) new expressions for D⃡ are given that are equivalent to but formally simpler than previous results; (4) the general form of the results is independent of the geometrical details of the system. View full abstract»

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  • The growth and structure of Cd0.95Fe0.05Te thin films grown by radio‐frequency sputtering

    Page(s): 3928 - 3933
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    The growth and structure properties of Cd1-xFexTe thin films prepared by rf sputtering were studied. Films with thicknesses in the range 0.75–2.45 μm were grown onto Corning glass substrates at temperatures between 50 and 300 °C in an argon atmosphere at a pressure of 3 mTorr. The deposition rate was determined by film thickness profile analysis. Transmission electron microscopy and x‐ray diffractometry techniques were used to evaluate the structure, grain size, and orientation of the films. These properties were found to be related to the substrate deposition temperature. View full abstract»

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  • Molecular‐beam epitaxial growth and microstructural characterization of GaAs on Si with a buried implanted oxide

    Page(s): 3934 - 3937
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    The direct growth of GaAs by molecular‐beam epitaxy on nominally (100)‐oriented silicon with a buried implanted oxide is demonstrated. Nomarski interference contrast optical microscopy, transmission electron microscopy, and Rutherford backscattering techniques have been employed to characterize these layers. The formation of hillocks and a uniform layer of GaAs in the intervening regions between hillocks have been observed. Microtwins and threading dislocations are the predominant defects in these layers. Furthermore, we report the absence of antiphase domain boundaries within the GaAs hillocks. View full abstract»

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  • In situ doping by As ion implantation of silicon grown by molecular‐beam epitaxy

    Page(s): 3938 - 3944
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    The incorporation and electrical activation of As, implanted in situ during molecular‐beam epitaxial growth of epilayers on Si(100), is reported. Parameters varied included growth temperature (460–700 °C), implantation energy (500–1000 eV), and concentration (1017→≫1020/cm3 ). In general, the material was excellent with 100% activation and bulk mobilities for concentrations up to the equilibrium solid solubility limit and carrier densities in excess of five times this limit in highly doped samples. View full abstract»

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  • Optimum growth condition of single‐crystalline undoped ZnS grown by the molecular‐beam‐epitaxial method using a H2S gas source

    Page(s): 3945 - 3948
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    Crystal characterization by photoluminescence and x‐ray diffraction has been performed to find out the optimum growth condition of ZnS grown by the gas‐source molecular‐beam‐epitaxial (MBE) method. In this crystal growth, important parameters are the cracking temperature of H2S gas (Tcr), the film thickness (d), both molecular‐beam flux intensities (JZn and JΣS or inlet H2S gas pressure Pin), and substrate temperature (Tsub). A single crystal having a fairly high quality, whose half width of x‐ray diffraction FWHM is about 0.119° (the reference value of the GaAs substrate used is 0.093°), is obtained under the growth conditions Pin=1.4×10-5 Torr (corresponding flux intensity JΣS=7×1015 cm-2 s-1), JZn/JΣS=1, and Tsub=325 °C in the case of Tcr=780 °C and d≂1 μm. View full abstract»

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Aims & Scope

Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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Editor
P. James Viccaro
Argonne National Laboratory