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Journal of Applied Physics

Issue 4 • Date Feb 1998

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Displaying Results 1 - 25 of 86
  • Issue Cover

    Page(s): c1
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    Freely Available from IEEE
  • Issue Table of Contents

    Page(s): toc1
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  • Probing ultrafast carrier and phonon dynamics in semiconductors

    Page(s): 1789 - 1830
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    Over the past 2 decades there has been tremendous advancements in the field of ultrafast carrier dynamics in semiconductors. The driving force behind this movement other than the basic fundamental interest is the direct application of semiconductor devices and the endless need for faster response and faster processing of information. To improve and develop microelectronics devices and address these needs, there must be a basic understanding of the various dynamical processes in the semiconductors which have to be studied in detail. Therefore, the excitation of semiconductors out of their equilibrium and the subsequent relaxation processes with various rates has become a key area of semiconductor research. With the development of lasers that can generate pulses as short as a few femtoseconds the excitation and subsequent probing of semiconductors on an ultrashort timescale have become routine. Processes such as carrier momentum randomization, carrier thermalization, and energy relaxation have been studied in detail using excite-and-probe novel techniques. This article reviews the status of ultrafast carrier and phonon dynamics in semiconductors. Experimental techniques such as excite-and-probe transmission, time-resolved up-conversion luminescence, and pump-probe Raman scattering along with some of the significant experimental findings from probing semiconductors are discussed. Finally, a selfconsistent theoretical model, which correlates the carrier and phonon dynamics in germanium on an ultrashort time scale, is described in detail. © 1998 American Institute of Physics. View full abstract»

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  • The role of internal dissipation and process average temperature in chiller performance and diagnostics

    Page(s): 1831 - 1836
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    The thermodynamic behavior of conventional chillers (generalized air conditioning and refrigeration systems) is acutely sensitive to internal dissipation. Previous chiller analyses have excluded entropy production in the evaporator and condenser heat exchangers, on the assumption of its being negligible. With experimental measurements from a commercial chiller, we demonstrate that heat exchanger internal dissipation is not inconsequential, and that ignoring its contribution can lead to substantial errors in chiller diagnostics and in the prediction of chiller performance curves. To evaluate the impact of this dissipation on chiller efficiency, one needs to define a proper process average temperature (PAT). In addition to discussing the fundamental significance of the correct PAT, we will show that earlier conventional definitions of PAT, where internal irreversibilities in chiller heat exchangers have been overlooked, result in inaccurate and sometimes unphysical predictions. © 1998 American Institute of Physics. View full abstract»

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  • Polarization effects in reflection-mode scanning near-field optical microscopy

    Page(s): 1837 - 1843
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    It is shown that the polarization state of light emitted from an aperture-type scanning near-field optical microscopy (SNOM) tip may be significantly altered on reflection from a metal surface in close proximity (5 nm) to the tip, while remaining unchanged at distances of several microns. Proximity to dielectric surfaces produces no discernible change. This effect is discussed and explained theoretically. We demonstrate that optical image contrast of metal samples may be enhanced by using this effect. The mechanism of the enhancement is based on selectively detecting the light emitted from the SNOM aperture and filtering out the light emitted through the sidewalls of the probe. It is also shown that images of a metal grating pattern on glass show strong polarization contrast, which may be understood in terms of simple dipole–image dipole schemes and transmission-line concepts. © 1998 American Institute of Physics. View full abstract»

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  • Nanoscale oxidation of GaAs-based semiconductors using atomic force microscope

    Page(s): 1844 - 1847
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    Fundamental results obtained in an atomic force microscope (AFM) direct nanoscale oxidation process are presented, which is regarded as a simple method for fabricating nm-scale devices such as single electron tunneling transistors and quantum effect electronic devices. Using Au-coated Si cantilevers, we have succeeded in drawing nm-scale oxide lines in GaAs-based semiconductor surfaces; n+-GaAs(100) and self-assembled InGaAs quantum dots grown by molecular beam epitaxy on GaAs(100) substrates. The effects of AFM drawing parameters such as tip bias voltage and writing speed on oxide linewidth and height have been explored. GaAs oxide lines as narrow as ∼40 nm have been patterned by this technique. © 1998 American Institute of Physics. View full abstract»

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  • Analysis of a Mach-Zehnder interferometry measurement of the Pockels coefficients in a poled polymer film with a reflection configuration

    Page(s): 1848 - 1853
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    Mach-Zehnder interferometry is employed to measure the Pockels coefficients in a poled thin polymer film, which serves as a reflection mirror in the sample arm of the interferometer. As a complete optical characterization of the electro-optic polymer film, the modulated light intensity of the Mach-Zehnder interferometer is investigated as a function of the optical bias in the reference arm, the modulation voltage applied to the film, the polarization angle of the incident light, and the angle of incidence on the film. The Mach-Zehnder interferometry measurement of the Pockels coefficients in the reflection configuration has an advantage over single-beam polarization interferometry in permitting the independent determination of the Pockels tensor components, r13 and r33. Particularly, in a reflection configuration two-beam interferometric measurement, a proper consideration of the optical path change due to the refractive angle change is found to be critical in determining the absolute values of the Pockels coefficients. © 1998 American Institute of Physics. View full abstract»

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  • Optical characterization of the in-plane switching effect utilizing multidomain structures

    Page(s): 1854 - 1862
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    Color shift and gray scale reversal for very large viewing angles are analyzed theoretically for in-plane switching. It follows that the color shift depends on the change of the effective birefringence dΔn, whereas gray scale reversal depends on the effective angle between the polarizers and the liquid-crystal director angle. Experimental results confirmed the theory that small gray levels result in a larger color shift. Likewise, the viewing angles in which a gray scale reversal occurs, correlate with the theory. To counter these drawbacks, two multidomain structures are proposed, neither of which requires additional orientation processes. With these multidomain structures, color shift was suppressed and no gray scale reversal was observed. However, the multidomain structures reduce maximum transmission, the extent of which depends on the type and design of the structure. © 1998 American Institute of Physics. View full abstract»

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  • Optimization of double drive pulse pumping in Ne-like Ge x-ray lasers

    Page(s): 1863 - 1868
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    Pumping of the Ne-like Ge x-ray laser with two 100 ps duration pulses (a prepulse and main pulse) is investigated using a fluid and atomic physics code coupled to a 3D ray tracing postprocessor code. The modeling predicts the optimum ratio of the irradiance of the two pulses for the maximum x-ray laser output resulting from the balance between the relative lower electron density gradients and wider gain region which is produced with a larger prepulse and the higher peak gain coefficients produced with a small prepulse. With a longer pulse interval between prepulse and main pulse, a relatively lower optimum pulse ratio is found. The threshold irradiance of the main driving pulse with a prepulse required to make an order of magnitude enhancement of laser output compared to irradiation without a prepulse is also found at 3–4×1013W/cm2 for Ne-like Ge. © 1998 American Institute of Physics. View full abstract»

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  • Ensemble Monte Carlo analysis of self-heating effects in graded heterojunction bipolar transistors

    Page(s): 1869 - 1877
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    Self-heating effects in graded AlGaAs/GaAs heterojunction bipolar transistors are analyzed using an ensemble Monte Carlo description of electron transport from emitter to subcollector including space-charge effects through a self-consistent solution of Poisson’s equation. For a given collector current density, the temperature rise in the intrinsic portion of the device is calculated using a simple analytical treatment. The temperature dependence of the scattering rates and various device parameters is included in the Monte Carlo simulations. The extent of velocity overshoot in the collector and its dependence on the device temperature is analyzed. The collector delay time is most affected by self-heating effects leading to a degradation of the unity current gain cut-off frequency of graded heterojunction bipolar transistors. For the biasing conditions considered here, the unity current gain cut-off frequency is found to be as much as 18% below its value calculated while neglecting self-heating effects. © 1998 American Institute of Physics. View full abstract»

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  • Real-time depth profile reconstruction of the thermal conductivity of inhomogeneous solids

    Page(s): 1878 - 1883
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    A new method for deriving the one-dimensional temperature distribution in a highly absorbing solid with continuously varying parameters is proposed. This method is based on a Riccati first-order ordinary differential equation and assumes sinusoidally modulated surface heating. Numerical solutions for several thermal conductivity profiles are compared with data obtained from models that approximate the original profiles with steplike variation of the thermal conductivity. A fast real-time inversion procedure is proposed to reconstruct the thermal conductivity profile from surface temperature data. It is based on the step-by-step reconstruction of this profile by sweeping the modulation frequency of the heating beam. The algorithm uses data from previous, higher frequencies to find the next optimum frequency. The measurements and the profile reconstruction are performed simultaneously. This procedure is tested for different profiles, including some with complex behavior and is found to be stable and reliable. Some limitations concerning the profile of the thermal conductivity are discussed. © 1998 American Institute of Physics. View full abstract»

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  • Two-dimensional simulation of an alternating current matrix plasma display cell: Cross-talk and other geometric effects

    Page(s): 1884 - 1897
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    A two-dimensional, user-friendly model of the discharge occurring in a plasma display panel cell was developed. This model was used to study the transient discharges in an alternating current plasma display with a matrix electrode configuration. The space and time variations of the charge particle densities, excitation rates, electric potential, and surface charge densities are described. The model is also used to study the conditions of existence of electrical interaction between adjacent cells and the effects of electrode misalignment. © 1998 American Institute of Physics. View full abstract»

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  • Transient response of the radio frequency inductively coupled plasma to a sudden change in power

    Page(s): 1898 - 1908
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    A two-dimensional, axisymmetric model was developed to study the response of a radio-frequency inductively coupled plasma to a sudden change in its active power. The time-dependent equations for the conservation of mass, momentum, and energy, along with Maxwell’s equations were solved numerically. Results were obtained for a pressure range of 200–760 Torr, a frequency range of 1–3 MHz; torch diameters between 40 and 75 mm; and, argon/hydrogen flow rates of 40–75 slpm. Initially, the plasma was assumed to be under steady-state condition at 20 kW. The plasma power was then reduced to 10 kW for 35 ms and, the response of the plasma fields and the coil current were predicted numerically. When power was reduced, the coil current reduced significantly in 2 ms. It then increased to a maximum before smoothly decreasing to its new steady-state value. The response of the plasma depended, to different degrees, on all the parameters considered here. Depending on the position within the torch, it could vary from 2 ms to several tens of millisecond. The plasma response was fastest within the skin-depth region where power was dissipated. The response time was most strongly affected by the changes in discharge pressure; and was least affected by the induction frequency. The response time increased with pressure and/or torch diameter, and decreased with frequency and/or flow rate. We also found that, depending on the magnitude of induction frequency, an increase in plasma power may cause an oscillatory plasma behavior. © 1998 American Institute of Physics. View full abstract»

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  • A study of electron energy distributions in an inductively coupled plasma by laser Thomson scattering

    Page(s): 1909 - 1916
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    The electron energy distribution function (eedf) in an inductively coupled plasma was studied using the method of laser Thomson scattering. eedfs were measured for various plasma conditions, at different gas pressures, and at different input rf powers. In high electron density plasmas, the eedf was observed to be Maxwellian, while in low electron density plasmas, a non-Maxwellian eedf was observed. The transition between Maxwellian and non-Maxwellian eedfs was attributed to the thermalization of the electron population in higher density plasmas. In order to completely characterize the eedf, spatial and temporal dependencies were measured and measurements were also made in the afterglow period of a pulsed discharge. Discussions are made of the electron behavior and the shape of the eedf. © 1998 American Institute of Physics. View full abstract»

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  • Comparison of electron cyclotron resonance and radio-frequency inductively coupled plasmas of Ar and N2: Neutral kinetic energies and source gas cracking

    Page(s): 1917 - 1923
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    Neutral mean kinetic energies and the amount of neutral source gas cracking of particles emanating from an electron cyclotron resonance (ECR) plasma source and a radio-frequency inductively coupled plasma (ICP) source are compared as a function of applied power by modulated beam time-of-flight analysis for Ar and N2 gases. For both sources, the source chamber is maintained at 0.07 Pa by a constant flow of either Ar or N2, while the applied power is varied from 50 to 250 W and from 50 to 550 W for the ECR and the ICP sources, respectively. For the Ar plasmas, the neutral Ar mean kinetic energies are similar and range between 0.07 and 0.15 eV, with the ICP being somewhat higher. In the case of the N2 plasmas, the ICP mean energies are also somewhat higher than those found for the ECR source. The mean energies range between 0.02 and 0.26 eV and 0.02 and 0.40 eV for the N2 and N species, respectively. The N:N2 flux ratio for the ICP source increases to a plateau of approximately 0.021±0.002 after an applied power of 250 W is reached, while for the ECR plasmas, the N:N2 flux ratio varies in a more complex fashion with a much higher value of 0.17±0.04. © 1998 American Institute of Physics. View full abstract»

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  • In situ measurement of radiation induced conductivity in oxide insulators during neutron irradiation

    Page(s): 1924 - 1930
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    An experimental investigation of the in situ electrical conductivity of Wesgo Al995 polycrystalline alumina at approximately 450 °C has been performed at the high flux beam reactor at Brookhaven National Laboratory. The measured radiation induced conductivity (RIC) was about 10-8 S/m at an ionizing dose rate of 6000 Gy/s. No evidence for permanent radiation induced electrical degradation was observed for an applied electric field of 147 V/mm up to a dose level of 1.8 displacements per atom. The effect of neutron irradiation on the electrical properties of two mineral insulated cables was also investigated. The RIC in the MgO insulation of a coaxial and a triaxial cable was measured to be in the range of 6–20 ×10-8 S/m at an ionizing dose rate of 6000 Gy/s. View full abstract»

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  • Effect of the surface upon misfit dislocation velocities during the growth and annealing of SiGe/Si (001) heterostructures

    Page(s): 1931 - 1937
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    We have measured the velocity of misfit dislocation threading segments in real time during ultrahigh vacuum (UHV) chemical vapor deposition heteroepitaxial growth of thin SiGe epilayers on Si (001) using ultrahigh vacuum transmission electron microscopy. We observe no measurable difference in dislocation velocities during growth and during post-growth annealing of samples with an atomically clean surface, in contrast to previous observations in the InGaAs/GaAs (001) system. However, dislocations are seen to move approximately three times slower during growth and post-growth UHV annealing than during annealing of samples which have a native oxide present on the surface. We have used post-growth depositions of arsenic and oxygen to investigate the effect of surface condition on dislocation velocities, and discuss possible causes for the increase in dislocation velocities in the presence of a native oxide. These systematic studies suggest a hitherto unappreciated interaction between moving dislocations and the surface in this system. © 1998 American Institute of Physics. View full abstract»

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  • Effects of cell structure on the reflection of cholesteric liquid crystal displays

    Page(s): 1938 - 1944
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    Cholesteric liquid crystal displays employ cells consisting of parallel glass plates which have indium-tin-oxide coating as the electrode and polyimide as the alignment layer. We have performed a systematic study of the reflections from the interfaces and the liquid crystal. We measured the reflection spectra using various incident light, various polarizations, and detection polarizations. Using the Berreman 4×4 method, we simulated the reflection spectra without fitting parameters; the results agreed well with the experimental results. © 1998 American Institute of Physics. View full abstract»

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  • Transmission electron microscopy and cathodoluminescence studies of extended defects in electron-beam-pumped Zn1-xCdxSe/ZnSe blue-green lasers

    Page(s): 1945 - 1952
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    We report on studies of extended defects in electron-beam-pumped Zn1-xCdxSe/ZnSe blue and blue-green laser structures. To establish a direct correlation between the local luminescence properties and the presence of structural defects, the same thin foil samples were sequentially examined by transmission electron microscopy (TEM) and cathodoluminescence (CL) microscopy. The majority of the non-radiative defects were found to have one or more threading dislocations in their vicinity. Stacking faults, as a rule, did not give rise per se to non-radiative recombination centers. In several instances we observed nonradiative defects by CL with no extended defect counterparts in TEM. © 1998 American Institute of Physics. View full abstract»

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  • Electronically enhanced kink motion on 30° partial dislocations in Ge directly observed by plan-view high resolution electron microscopy

    Page(s): 1953 - 1957
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    In the high resolution electron microscopic (HREM) images of dislocations in Ge obtained with the electron beam incident normal to the slip plane, 30° partial dislocation lines were extracted by image processing, and kinks in them were identified at atomic resolution. Analysis of sequential images taken from the same dislocation lines revealed that (1) geometrical kinks of different signs, 30° (left) and 90° (right) kinks, are both mobile even at room temperature (2) generation of kink pairs occurs, albeit less frequently, in the course of HREM observations. These results are reasonably understood when considering that the migration of kinks, at least one of the two types, and the formation of the smallest double kinks are enhanced by the electronic excitation that is caused by the electron beam used for the HREM observations. © 1998 American Institute of Physics. View full abstract»

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  • Optical absorption study of Si grown in a hydrogen ambient

    Page(s): 1958 - 1961
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    We studied optical absorption spectra of Si crystals grown in a hydrogen ambient. Specimens were grown by the floating-zone growth method in a hydrogen ambient of 1 atm. Optical absorption spectra of those specimens were measured at 6 K. Many optical absorption lines were observed in the range of 1900–2200 cm-1. They were observed only when the concentrations of carbon or boron were high. To understand these results, we referred to a report according to which a so-called A-type swirl, an interstitial-type dislocation loop, was observed when impurities of smaller covalent radii than that of Si were doped. Hence, we concluded that the above optical absorption lines were due to localized vibration of hydrogen atoms bound to interstitial Si atoms. © 1998 American Institute of Physics. View full abstract»

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  • Surface modification of n-GaAs by 50 MeV silicon ions

    Page(s): 1962 - 1966
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    An effective passivation of the surface states in n-GaAs has been achieved by high energy (50 MeV) silicon ion irradiation at a fluence of 1×1013ions cm-2. The effect of passivation on the nature of defects has been studied by the technique of thermally stimulated exoelectron emission (TSEE) measurements. Consequently an enhancement in the intensity of the band edge, photoluminescence (PL) has also been noticed. The results of TSEE and PL measurements are augmented by surface analysis using x-ray photoelectron spectroscopy. An effective migration of silicon was observed from the buried layer towards the surface by radiation enhanced outdiffusion process. A stable passivating layer of silicon dioxide was found on GaAs surface which has been accounted for the enhancement in the PL intensity. © 1998 American Institute of Physics. View full abstract»

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  • Wide supercooled liquid region and soft magnetic properties of Fe56Co7Ni7Zr0–10Nb (or Ta)0–10B20 amorphous alloys

    Page(s): 1967 - 1974
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    An amorphous phase with a wide supercooled liquid region before crystallization was formed in Fe56Co7Ni7Zr10-xMxB20 (M=Nb or Ta, x=0–10 at. %) alloys by melt spinning. The glass transition temperature (Tg) and crystallization temperature (Tx) increase by the dissolution of 2% M and the degree of the increase is larger for Tx, leading to maximum ΔTx(=Tx-Tg) of 85 K at 2% Nb and 87 K at 2% Ta which are larger by about 20 K than the largest value for newly developed Fe–(Al, Ga)–(P,C,B,Si) amorphous alloys. The crystallization of the Nb-containing alloys occurs through two stages of amorphous (Am)→Am+α-Fe+γ-Fe+Fe76Nb6B18 →α-Fe+γ-Fe+Fe76Nb6B18+Fe2Zr in the range less than about 6% Nb and Am→Am+γ-Fe→γ-Fe+Co3Nb2B5+Ni8Nb in the range above 8% Nb. The change in the crystallization process with Nb content seems to reflect the ea- sy precipitation of γ-Fe by the increase in the number of Fe–Nb pairs with weaker bonding nature as compared with the Fe–Zr pairs. The best soft magnetic properties were obtained at 2% Nb or 8% Ta. The saturation magnetization, coercive force, effective permeability at 1 kHz, and saturated magnetostriction in the annealed state for 300 s at 800 K are, respectively, 0.96 T, 2.0 A/m, 19 100, and 10×10-6 for the 10% Zr alloy, 0.75 T, 1.1 A/m, 25 000, and 12×10-6 for the 2% Nb alloy, and 0.85 T, 1.5 A/m 17 400, and 14×10-6 for the 8% Ta alloy. The Curie temperature is 531 K for the 2% Nb alloy and 538 K for the 8% Ta alloy. The success in synthesizing the new amorphous alloys with the wide supercooled liquid region and good soft magnetic properties is promising for future development as soft magnetic bulk amorphous alloys. © 1998 American Institute of Physics. View full abstract»

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  • First principles calculations of structural properties of β-Si3-nCnN4 (n=0, 1, 2, 3)

    Page(s): 1975 - 1978
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    First principles calculations were performed for the structural properties of superhard materials, hexagonal β-Si3N4, β-C3N4, and two model structures of β-Si2CN4 and β-SiC2N4. Bulk modulus and lattice constants a and c have been obtained by fitting the total energy calculations to the Birch equation of state. As more C atoms are substituted for Si in β-Si3N4, the bulk modulus is observed to progressively increase up to 4.44 Mbar, comparable to that of diamond (4.43 Mbar), and both lattice constants a and c are reduced. For β-Si3N4 and β-C3N4, the lowest energy geometries have been searched by optimizing the internal structural parameters at the theoretical equilibrium lattice constants. The results are discussed and compared with available experiments. © 1998 American Institute of Physics. View full abstract»

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  • Thermal expansion of LaAlO3 and (La,Sr)(Al,Ta)O3, substrate materials for superconducting thin-film device applications

    Page(s): 1979 - 1982
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    The thermal expansion for the perovskite (La,Sr)(Al,Ta)O3, i.e., LSAT, grown from the formulation 0.29(LaAlO3):0.35(Sr2AlTaO6), was determined by Rietveld refinement of neutron powder diffraction data over the temperature range of 15–1200 K. In comparison to LaAlO3 the relative volume thermal expansion is the same, although the cell volume of LSAT is slightly larger. Site occupation refinement for LSAT gives a structural formula of (La0.29(5)Sr0.71(5))A site(Al0.65(1)Ta0.35(1))B siteO3. At and below 150 K, LSAT shows a small distortion from cubic symmetry. Unlike the cubic-to-rhombohedral transition (800 K) observed in LaAlO3, the low temperature structural phase transition in LSAT appears to be cubic-to-tetragonal or cubic-to-orthorhombic. The rms displacement of the A site in LSAT is significantly larger than that for LaAlO3, and about half of the difference can be accounted for by a static displacement component. © 1998 American Institute of Physics. 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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P. James Viccaro
Argonne National Laboratory