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Journal of Applied Physics

Issue 12 • Date Jun 1998

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Displaying Results 1 - 25 of 98
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

    Page(s): toc1
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  • Analysis and design of an interdigital cantilever as a displacement sensor

    Page(s): 7405 - 7415
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    The interdigital (ID) cantilever with two sets of interleaving fingers is an alternative to the conventional cantilever used in the atomic force microscope (AFM). In this paper we present a detailed analysis of the interdigital cantilever and its use as a sensor for the AFM. In this study, we combine finite element analysis with diffraction theory to simulate the mechanically induced optical response of the ID. This model is used to compare this system with the optical lever detector as used in conventional instruments by analyzing the ratio of signal to noise and overall performance. We find that optical detection of the cantilever motion with interdigital fingers has two advantages. When used in conjunction with arrays of cantilevers it is far easier to align. More importantly, it is immune to laser pointing noise and thermally excited mechanical vibrations and this improves the sensitivity as compared to the optical lever. © 1998 American Institute of Physics. View full abstract»

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  • Gap dependence of the tip-sample capacitance

    Page(s): 7416 - 7423
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    The tip-sample capacitance has been studied in the nontunneling regime and the capacitance-distance characteristics and its dependence on the tip geometry have been determined for the gap distance 1≪s≪600 nm. Measurements were carried out in ultrahigh vacuum on a capacitor formed between a metal tip (W or Pt–Ir) and a clean Au(111) surface. Tips of different tip radius R=30∼4000 nm were used to investigate the influence of tip geometry on the capacitance. When the gap distance is reduced, the capacitance increases while its gap sensitivity |∂C/∂s| decreases with the gap distance. The capacitance therefore shows no 1/s divergence. The magnitude of the capacitance change is found to depend on the tip geometry: blunt tips (R≫1000 nm) show larger capacitance increase than that for sharp tips (R≤100 nm). The effective tip radius Reff estimated from the C-s characteristics agrees with the real tip radius in a limited distance range which varies with the tip geometry. At small distances (s≪30 nm), Reff≃R for sharp tips but Reff≪R for blunt tips. On the other hand at large distances (s≫200 nm), the relation is reversed, Reff≫R for sharp tips while Reff∼R for blunt tips. These results on Reff can be explained by the field concentration to the tip apex and the change of capacitance-contributing tip area with the gap distance. Capacitance calculations indicate that the capacitance of the “truncated cone + half sphere” - tip well reproduces the observed C-s characteristics and its dependence on the tip geometry. © 1998 American Institute of Physics. View full abstract»

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  • Measurements and analysis of radiation effects in polycapillary x-ray optics

    Page(s): 7424 - 7435
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    Polycapillary x-ray optics are arrays of large numbers of small hollow glass tubes which deflect x rays by successive total external reflection. These optics have growing numbers of applications in areas ranging from medical imaging to microanalysis. An accelerated radiation effects study has been performed to understand the performance limitation of these optics for medium to high intensity radiation applications, to study x-radiation damage mechanisms, and to investigate possible ways to mitigate the radiation effects on x-ray transmission efficiency. Exposures have been done in white beam bending magnet radiation with peak energies at 5 and 11 keV and focused broad band radiation centered at 1.4 keV. In situ and ex situ measurements of loss of x-ray transport efficiency have been executed at doses up to 1.8 MJ/cm2. Thin polycapillary fibers displayed noticeable bending and experienced substantial degradation of x-ray transmission. Thicker polycapillary fibers showed a linear but much slower transmission loss as a function of total dose. Annealing effectively restored the low energy (∼8 keV) transmission efficiency of the fibers. Exposure of these fibers at slightly elevated temperatures prevented any measurable loss in the low energy transmission efficiency. A variety of analytical techniques has been used to understand these results. No significant change was observed in the chemical composition of the capillary surface. Profile measurements and high energy transmission efficiency spectra, along with computer simulation studies, suggest that radiation induced bending is the primary cause of transmission efficiency degradation of the fibers. © 1998 American Institute of Physics. View full abstract»

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  • Temperature dependence of the enhanced excited state absorption in erbium-doped garnets

    Page(s): 7436 - 7441
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    Temperature dependence of an enhanced excited state absorption was investigated in erbium-doped garnet crystals using 787.3 and 787.8 nm laser diodes. The enhanced absorption was obtained at temperature ranges of 15–570 K, 15–420 K, and 15–1200 K in a yttrium aluminum garnet, a yttrium scandium gallium garnet, and a lutetium aluminum garnet, respectively. Absorption peaks corresponding to both the 4I15/2-4I9/2 and the 4I13/2-2H11/2 transitions of Er3+ in garnets were shifted from those for 300 K at high temperature. The enhanced absorption weakened at high temperature because an absorption cross section from the 4I13/2 level to the 2H11/2 level decreased due to the shift of the absorption peaks. It has been confirmed that a mechanism of the enhanced absorption can be explained by calculating the rate equations based on an analytical model for a five-level system of the Er3+ ion. It is suggested that the enhanced absorption weakened at temperatures lower than 12 K because the number of phonons decreased due to low temperature. © 1998 American Institute of Physics. View full abstract»

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  • Planar semiconductor lasers using the photoelastic effect

    Page(s): 7442 - 7447
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    Planar separate-confinement, double-heterostructure, single-quantum-well photoelastic GaAs/AlGaAs lasers have been fabricated using a novel yet practical processing technique involving thin-film surface WNi stressors for waveguiding and ion implantation for isolation. A p++-GaAs contact layer regrown by chemical beam epitaxy has been used to improve the WNi ohmic contacts to the lasers. Even without bonding on heat sinks, these planar photoelastic lasers operate at continuous wave at room temperature. The lowest threshold is 29 mA for a cavity length of 178 μm and a stressor width of 5 μm. The internal quantum efficiency above threshold is 75%. The characteristic temperature is 114 K. The main waveguiding mechanism of the photoelastic lasers is determined to be weak index guiding with the beam waist in the junction plane measured 10 μm behind the end facet. © 1998 American Institute of Physics. View full abstract»

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  • Optically pumped hydrogen fluoride laser

    Page(s): 7448 - 7452
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    We present results from a study of an optically pumped hydrogen fluoride laser. Rotation–vibration transitions within the (2,0) band near 1.3 μm are pumped, and both amplified spontaneous emission (ASE) and laser oscillation are observed on (2,1) band transitions near 2.7 μm. Longitudinal and transverse pumping schemes are discussed. We also have pumped the (3,0) band and observed ASE on both the (3,2) and (2,1) bands. This system serves as a prototype for other mid-infrared diatomic and polyatomic lasers based upon overtone pumping. © 1998 American Institute of Physics. View full abstract»

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  • Enhanced ultrasonic detection of fatigue cracks by laser-induced crack closure

    Page(s): 7453 - 7460
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    Fatigue cracks are usually initiated by small geometrical irregularities or material inhomogeneities that give rise to sharp local stress concentrations. In the early stages of fatigue, small cracks often remain hidden from conventional ultrasonic detection by stronger scattering from the very same structural imperfection that produced them in the first place. A new experimental method was developed to selectively increase the sensitivity of ultrasonic echographic techniques for such hidden fatigue cracks by exploiting one of their most characteristic features, their susceptibility for closure under compressive stress. Thermo-optical modulation by pulsed infrared laser irradiation was introduced to produce a temporary compressive thermal stress on the surface of the specimen. The resulting dynamic closure of microcracks was detected by a high-frequency ultrasonic surface wave technique. It is demonstrated that this method can be used to effectively distinguish fatigue cracks from other structural imperfections present in the material. © 1998 American Institute of Physics. View full abstract»

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  • Generation of second-order cumulative waves accompanying the fundamental modes in a two-dimensional hard-walled waveguide

    Page(s): 7461 - 7466
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    Generation of second-order cumulative waves (SOCWs) accompanying the fundamental modes in a two-dimensional hard-walled waveguide of semi-infinite length is examined by use of second-order perturbation. With appropriate boundary and initial conditions of excitation, SOCW analytical expressions have been derived. In this manner, a physical process of SOCW generation is clearly displayed. The solution shows that the SOCWs grow linearly with the propagation distance, and that the SOCW fields in the waveguide are symmetrical. Based on the formula, some numerical calculations are performed. The numerical results clearly exhibit the SOCW field distortion. © 1998 American Institute of Physics. View full abstract»

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  • Removal of laser-melted material with an assist gas

    Page(s): 7467 - 7473
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    Molten material is removed in many laser applications such as laser grooving and laser cutting. An assist gas is usually used to remove the molten material from the laser material interaction zone. The effect of assist gas pressure on the material removal rate is investigated in this article for laser cutting and grooving applications. The model for melt depth is based on the overall energy balance, and the cut depth is obtained by considering the effect of the assist gas. The model for kerf width is based on the modified Rosenthal solution taking into account the melting effect. The cut depths reach a constant value beyond a critical pressure if the kerf width is of the order of nozzle width and through cuts are assumed. Most of the molten material is removed by the assist gas at pressures below this critical pressure. The model predicts on the basis of the Prandtl or Meyer relation that the cutting speed decreases when the assist gas pressure exceeds a critical value if the kerf dimensions are smaller than the nozzle dimensions. © 1998 American Institute of Physics. View full abstract»

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  • Synthesis and characterization of SiC:H ultrafine powder generated in an argon–silane–methane low-pressure radio-frequency discharge

    Page(s): 7474 - 7481
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    The peculiarity of dusty plasma reactors offers a convenient way to obtain processed particles at submicronic levels, with successive layers of different materials grown by using pulsed gas flows, and different plasma chemistries in succession. This concept is applied to the synthesis of silicon carbide (SiC) particles. In this paper two significant situations are reported showing that particles can be synthesized with different properties by varying the process parameters (gas-flow handling, radio-frequency power level). These properties include broad or narrow size dispersion, almost crystalline or amorphous structure, and widely varying Si/C stoichiometry. Monosized particles with high specific surfaces have been obtained by a two-step growth process by using limited radio-frequency power. © 1998 American Institute of Physics. View full abstract»

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  • Absolute density and reaction kinetics of fluorine atoms in high-density c-C4F8 plasmas

    Page(s): 7482 - 7487
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    Absolute density and reaction kinetics of fluorine (F) atoms in high-density octafluorocyclobutane (c-C4F8) plasmas were examined using vacuum ultraviolet absorption spectroscopy. The F atom densities, corresponding to electron densities ranging from 1×1011 to 5×1012 cm-3, were 1×1012–5×1013 cm-3 for gas pressures of 2–7 mTorr and rf powers of 0.2–1.5 kW. The F atom density was linearly dependent on the electron density for ne≪1.5×1012 cm-3. According to lifetime measurements in the afterglow, two decay processes were found in the F atom density: exponential (first-order kinetics) and linear (zero-order kinetics) decay components. The linear-decay component became significant at high gas pressures. The time constant of the exponential-decay component ranged from 5 to 100 ms, which corresponds to surface loss probabilities of 10-1–10-3. The surface loss probability varied inversely with the F atom density. © 1998 American Institute of Physics. View full abstract»

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  • Influence of pulse duration on mechanical effects after laser-induced breakdown in water

    Page(s): 7488 - 7495
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    The influence of the pulse duration on the mechanical effects following laser-induced breakdown in water was studied at pulse durations between 100 fs and 100 ns. Breakdown was generated by focusing laser pulses into a cuvette containing distilled water. The pulse energy corresponded to 6-times breakdown threshold energy. Plasma formation and shock wave emission were studied photographically. The plasma photographs show a strong influence of self-focusing on the plasma geometry for femtosecond pulses. Streak photographic recording of the shock propagation in the immediate vicinity of the breakdown region allowed the measurement of the near-field shock pressure. At the plasma rim, shock pressures between 3 and 9 GPa were observed for most pulse durations. The shock pressure rapidly decays proportionally to r-(2...3) with increasing distance r from the optical axis. At a 6 mm distance of the shock pressure has dropped to (8.5±0.6) MPa for 76 ns and to ≪0.1 MPa for femtosecond pulses. The radius of the cavitation bubble is reduced from 2.5 mm (76 ns pulses) to less than 50 μm for femtosecond pulses. Mechanical effects such as shock wave emission and cavitation bubble expansion are greatly reduced for shorter laser pulses, because the energy required to produce breakdown decreases with decreasing pulse duration, and because a larger fraction of energy is required to overcome the heat of vaporization with femtosecond pulses. © 1998 American Institute of Physics. View full abstract»

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  • Collisional radiative models with loss and recycling

    Page(s): 7496 - 7503
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    We describe how the collisional–radiative model, which is usually used to simplify the treatment of excited states in the description of atomic processes in plasmas, must be extended if transport of the excited states is significant. We identify recycling of neutrals near the plasma edge as a circumstance where this is indeed the case. We show how the effective ionization and recombination rates appropriate to a correct ‘nonlocal’ collisional–radiative model depend on the recycling rate, as well as electron temperature and density. We are able to identify the specific shortcomings of the conventional collisional–radiative model, and show that its use leads to a spurious source of neutral particles. We compare results obtained for the atomic dynamics using both the conventional and extended collisional–radiative models with the exact solution of the appropriate nonlinear equations in which all the excited states are explicitly included, and demonstrate that whereas the conventional model fails to reproduce the exact result, the extended model does so. © 1998 American Institute of Physics. View full abstract»

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  • Validation of actinometry for estimating relative hydrogen atom densities and electron energy evolution in plasma assisted diamond deposition reactors

    Page(s): 7504 - 7521
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    The validity of the actinometry method applied to H-atom mole fraction measurements has been analyzed. First, a theoretical approach allowed us to determine boundary conditions for which the validity of actinometry may be critical. For these specific conditions, corresponding to an upper limit of electron temperature of 20 000 K and a lower limit of H-atom mole fraction of 2%–4%, spatial distributions of the ground state H-atom relative densities provided either by two photon allowed transition or by optical emission spectroscopy (OES) were compared and seen to be proportional. This proves that the H atoms excited in the level of quantum number n=3 (level used for OES experiments) are produced directly from the ground electronic state during collisions with electrons. Actinometry can then be applied under these experimental conditions. Second, the emission intensity ratio of two lines issued from excited states of argon was demonstrated to be indirectly related to the “electron temperature” of the hot electrons of the plasma. This allowed us to predict the way of evolution of the plasma electrons’ energy as a function of the operating conditions. Thus, experiments (which have been confirmed by calculations) showed that the electron energy decreases as a function of the microwave power density and remains constant as a function of the methane percentage introduced in the feed gas at least up to 6%. The consequence is that the domain of diamond deposition discharge conditions for which actinometry is valid is quite wide. Once the microwave volumetric power density is more than 9 W cm-3, and the percentage of methane less than 6%, actinometry can be applied. However, the estimation of variations of H-atom mole fractions as a function of the operating conditions implies the use of correcting factors, which are discussed. They are mainly - due to the large influence of the quenching processes under these experimental conditions. An experimental estimate of the quenching cross section of the H(n=3) atoms by ground state molecular hydrogen, which was unknown and involved in the correcting factors, is presented. Finally, relative variations of H-atom mole fraction in space and as a function of the methane percentage are shown. © 1998 American Institute of Physics. View full abstract»

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  • Ion composition of expanding microdischarges in dielectric barrier discharges

    Page(s): 7522 - 7532
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    The properties of the filamentary microdischarges found in dielectric barrier discharges depend on the manner of charging of the dielectric. The charging of the dielectric removes voltage from the gap thereby reducing E/N and producing a transition from an avalanching discharge to a recombination or attachment dominated discharge. In this article, we report on a computational investigation of these processes using a one-dimensional plasma chemistry model. We find that the expansion and ultimate stalling of the microdischarge is largely determined by charging of the dielectric at larger radii than the core of the microdischarge. The lowering of E/N in the core of the microdischarge in attaching gases can quickly consume electrons. This transition produces a discharge consisting of an expanding shell having a high electron density and an inner core dominated by negative ions. In extreme cases where the gas mixture contains thermal electron attaching gases, the core of the microdischarge is essentially a negative ion-positive ion plasma. Using square wave voltage pulses, the residual charge on the dielectric after the microdischarge, which contributes to the gap voltage on the next voltage pulse, is largely determined by the attachment rate in the core of the microdischarge. Rapid attachment reduces the plasma conductivity and leaves residual charge on the dielectric. © 1998 American Institute of Physics. View full abstract»

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  • The amorphization kinetics of GaAs irradiated with Si ions

    Page(s): 7533 - 7536
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    The critical relationship between ion flux and substrate temperature which defines the threshold conditions for the formation of amorphous layers in GaAs at constant ion fluence is measured for the first time. At elevated temperatures, amorphous layers are formed in GaAs by a collapselike process when a critical free energy value is exceeded. The threshold conditions for amorphization are shown to be thermally activated, with an activation energy of 0.9±0.1 eV. However, specific defects and/or processes giving rise to this value are not yet known. © 1998 American Institute of Physics. View full abstract»

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  • Surface roughness in InGaAs channels of high electron mobility transistors depending on the growth temperature: Strain induced or due to alloy decomposition

    Page(s): 7537 - 7541
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    InAlAs/InGaAs/InP based high electron mobility transistor devices have been structurally and electrically characterized, using transmission electron microscopy and Raman spectroscopy and measuring Hall mobilities. The InGaAs lattice matched channels, with an In molar fraction of 53%, grown at temperatures lower than 530 °C exhibit alloy decomposition driving an anisotropic InGaAs surface roughness oriented along [11¯0]. Conversely, lattice mismatched channels with an In molar fraction of 75% do not present this lateral decomposition but a strain induced roughness, with higher strength as the channel growth temperature increases beyond 490 °C. In both cases the presence of the roughness implies low and anisotropic Hall mobilities of the two dimensional electron gas. © 1998 American Institute of Physics. View full abstract»

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  • Characterization of phosphorus doped n-type 6H-silicon carbide epitaxial layers produced by nuclear transmutation doping

    Page(s): 7542 - 7546
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    Aluminum doped 6H-SiC epitaxial layers (p-type net doping: NA-ND=5.2×1015 cm-3, thickness 3 μm) on a p+ doped 6H-SiC substrate (NA-ND=1018 cm-3) were irradiated with neutrons in a nuclear reactor. The neutron fluences applied were 9.4×1019, 3.5×1020, and 6.4×1020 cm-2. The phosphorus impurity in SiC is produced by a nuclear (n,γ) reaction with the decay of 31Si to 31P. The irradiated samples were subsequently annealed at temperatures between 800 and 1850 °C. The annealing behavior was studied by low temperature photoluminescence, Fourier transform infrared spectroscopy, and Hall effect and I–V and C–V measurements. After the annealing process the 6H-SiC p-type epilayer changed to n type resulting in a pn junction within the material. The properties of the pn junction were characterized. © 1998 American Institute of Physics. View full abstract»

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  • Role of grain-boundary sliding in diffusional creep of polycrystals

    Page(s): 7547 - 7552
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    The kinetics of deformation caused by grain-boundary sliding and diffusion is discussed for a two-dimensional model polycrystal. It is shown that boundary sliding is a prerequisite to induce the bulk diffusion. The sliding must also occur concurrently with the diffusion for a polycrystal to continue diffusional creep. Rate equations for deformation of the polycrystal are also formulated when both of the bulk and boundary diffusions are operative in addition to the boundary sliding. It is shown that the two diffusional processes are exclusive in general. Only when boundaries slide extremely fast, the strain rate of the polycrystal in a steady state becomes the simple sum of the strain rate achieved by the bulk diffusion and that by the boundary diffusion. © 1998 American Institute of Physics. View full abstract»

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  • High-pressure melting curve of platinum

    Page(s): 7553 - 7559
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    The melting curve of platinum, determined to 70 GPa by spectroradiometry and visual observation through the laser-heated diamond cell, is described by Tm(P)=2057+27.2×P-0.1497×P2 K, where Tm is the melting temperature in K and P is pressure in GPa. This expression is valid to a precision of ±97 K in the pressure range 10 to 70 GPa. © 1998 American Institute of Physics. View full abstract»

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  • Lattice model calculation of the strain energy density and other properties of crystalline LiCoO2

    Page(s): 7560 - 7566
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    The strain energy densities for various crystalline planes of LiCoO2 were calculated from the stiffness tensors obtained from lattice model calculations using the program GULP. In addition to Coulomb and Buckingham potentials, it was necessary to include shell models for the oxygen and cobalt ions in order to obtain acceptable agreement between the observed and calculated structural parameters and high frequency dielectric constant. The strain energy densities u due to differential thermal expansion were calculated using the theoretical stiffness tensors and estimated values for the thermal expansion coefficients of LiCoO2. For a temperature change of 675 °C, these ranged from 0.5 to 1.3×108erg/cm3 or 5 to 13 J/m2 for 1-μm-thick films on alumina substrates. In particular, the energies for the (003), (101), and (104) planes were ordered as u(003)≫u(104)≫u(101). This suggests that the strong (101) preferred orientation of LiCoO2 films (≥1 μm thick) is due to the tendency to minimize volume strain energy that arises from differential thermal expansion between the film and the substrate. Additional properties obtained from the GULP calculations include the free energy, heat capacity, and the k=0 vibrational modes.   View full abstract»

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  • Analysis of lattice strains measured under nonhydrostatic pressure

    Page(s): 7567 - 7575
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    The equations for the lattice strains produced by nonhydrostatic compression are presented for all seven crystal systems in a form convenient for analyzing x-ray diffraction data obtained by newly developed methods. These equations have been used to analyze the data on cubic (bcc α-Fe) and hexagonal (hcp ϵ-Fe) systems. The analysis gives information on the strain produced by the hydrostatic stress component. A new method of estimating the uniaxial stress component from diffraction data is presented. Most importantly, the present analysis provides a general method of determining single crystal elastic constants to ultrahigh pressures. © 1998 American Institute of Physics.   View full abstract»

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  • Radiation enhanced diffusion in MgO

    Page(s): 7576 - 7584
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    Radiation enhanced diffusion and ion beam mixing of 18O, Ca, and Zn buried tracer layers in MgO, grown by molecular beam epitaxy, have been measured following irradiation with 2.0 MeV Kr+ and 1.0 MeV Ne+, He+, and H+ from 30 to 1500 °C. The ion beam mixing parameter varied between 1.0 and 5.0 Å5eV-1 for the different tracers at 30 °C and increased slowly with increasing temperature. These results are consistent with ballistic mixing. In the highest temperature range investigated, 1350–1500 °C, the radiation enhanced diffusion coefficient for 18O was proportional to the square root of the irradiation flux and displayed an apparent activation enthalpy of 1.2 eV. These dependencies on flux and temperature are indicative of recombination-limited kinetics, with the measured activation enthalpy representing one-half the migration enthalpy of anion vacancies. From 1150 to 1350 °C an activation enthalpy of 4.1 eV was obtained. The unexpectedly high value is attributed to the dissociation energy of small vacancy clusters. Measurements on the cation sublattice were limited to temperatures below 900 °C owing to the excessive thermal diffusion associated with extrinsic vacancies, which are present for trivalent impurity charge compensation. © 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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Editor
P. James Viccaro
Argonne National Laboratory