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Journal of Applied Physics

Issue 1 • Date Jan 1999

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Displaying Results 1 - 25 of 107
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

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  • Pure nuclear Bragg reflection of a periodic 56Fe/57Fe multilayer

    Page(s): 1 - 7
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    Grazing incidence nuclear multilayer diffraction of synchrotron radiation from a periodic stack of alternating 56Fe and 57Fe layers was observed. Resonant layer fraction, substrate size, flatness, and surface roughness limits were optimized by previous simulations. The isotopic multilayer (ML) sample of float glass/57Fe(2.25 nm)/[56Fe(2.25 nm)/57Fe(2.25 nm)]×15/Al(9.0 nm) nominal composition was prepared by molecular beam epitaxy at room temperature. Purity structure and lateral homogenity of the isotopic ML film was characterized by magnetometry, Auger electron, Rutherford backscattering, and conversion electron Mössbauer spectroscopies. The isotopic ML structure was investigated by neutron and synchrotron Mössbauer reflectometry. Surface roughness of about 1 nm of the flat substrate (curvature radius ≫57 m) was measured by scanning tunneling microscopy and profilometry. A pure nuclear Bragg peak appeared in synchrotron Mössbauer reflectometry at the angle expected from neutron reflectometry while no electronic Bragg peak was found at the same position by x-ray reflectometry. The measured width of the Bragg peak is in accordance with theoretical expectations. © 1999 American Institute of Physics. View full abstract»

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  • Statistical modeling of charge collection in semiconductor gamma-ray spectrometers

    Page(s): 8 - 15
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    Charge collection efficiency and the variance of the collected charge of semiconductor gamma-ray spectrometers are analytically modeled for the case of a uniform electric field. The model is based on a rigorous statistical approach. In contrast to previously reported models for the variance of the collected charge, the present statistical model simultaneously takes into account a random point of photon absorption (i.e., a nonuniform absorption) and a random drift time for each generated carrier. Analytical expressions are obtained for the variance of the collected charge as a function of photon energy, applied bias voltage, electron and hole mobility-lifetime products, and the direction of irradiation. Since at present the performance of high-Z room-temperature spectrometers is mainly limited by the charge transport properties of the semiconductors, it is interesting to calculate the limit for the expected resolution. The statistical model presented here yields physical insight and determines quantitatively the expected spectrometer resolution as a function of the semiconductor transport parameters and device parameters, as well as the radiation parameters. Several limiting cases are calculated and discussed. The methodology presented here, which is based on conditional probabilities, can be extended to spectrometers with nonuniform fields. © 1999 American Institute of Physics.   View full abstract»

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  • Coupling between the ionic cyclotron motion and the circular ionic drift motion in a cylindrical ion cyclotron resonance cell

    Page(s): 16 - 22
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    In this work, we have first constructed a special cylindrical ion cyclotron resonance cell and investigated the ionic motion in the cell experimentally as well as theoretically. Due to the field symmetry in the cell, the ionic cyclotron motion is coupled with a well-defined circular drift motion in the plane perpendicular to the magnetic field and a trapping oscillation parallel to the magnetic field. The effective cyclotron frequency ωeff could be given as ωcdd2/2ωc which can be attributed to the influence of the Coriolis and the centrifugal accelerations caused by the circular drift motion with ωd. This is in contrast to the case for conventional ion drift cell with parallelepiped geometry where the effective cyclotron frequency is given in terms of the electric field gradients. Thus, this newly developed cylindrical cell can be used not only as an ion trap but also as a simulator for studying some fundamental processes such as the Coriolis and centrifugal couplings in classical dynamics. © 1999 American Institute of Physics. View full abstract»

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  • Intersubband terahertz lasers using four-level asymmetric quantum wells

    Page(s): 23 - 28
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    We demonstrate the potential for laser operation at far-infrared wavelengths (30–300 μm, 1–10 THz) by using intersubband emission in four-level GaAs/AlGaAs asymmetric (stepped) quantum wells. Achieving population inversion in these devices depends critically on the lifetimes of the nonradiative intersubband transitions, and so we have performed detailed calculations of electron–electron and electron–phonon scattering rates. Our four-subband structures show potential for the realization of room temperature lasing, unlike previously considered three-subband structures which did not give population inversions except at impractically low electron densities and temperatures. Auger-type electron–electron interactions involving the highly populated ground subband effectively destroyed the population inversion in three level systems, but in these four subband structures the inversion is maintained by strong phonon-mediated depopulation of the lower laser level. The largest population inversions are calculated at low temperatures (≪30 K), but for the structures with higher emission energies, room temperature (300 K) operation is also predicted. © 1999 American Institute of Physics. View full abstract»

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  • Upconversion mechanism in Er3+-doped fluorozirconate glasses under 800 nm excitation

    Page(s): 29 - 37
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    The upconversion emission intensities of Er3+ ion around 550 and 660 nm in fluorozirconate glasses were measured under 800 nm excitation. Though energy transfer processes played an important role in upconversion mechanism at high concentration of ErF3, those have not been treated quantitatively. The energy transfer rates were calculated from the optical parameters assuming some distribution of Er3+ ions. We calculated the upconversion intensities around 550 and 660 nm by using rate equations. It was found that the dependence of upconversion emission intensities on the ErF3 concentration could be reproducible and the principal upconversion mechanism could be evaluated. © 1999 American Institute of Physics. View full abstract»

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  • Dynamics of photorefractive grating erasure in polymeric composites

    Page(s): 38 - 43
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    Theoretical and experimental studies are presented of the dynamics of photorefractive grating erasure in polymeric photorefractive materials with hole traps. A bi-exponential decay for the rate of optical erasing of the space-charge field has been obtained by theoretical solution. The optically induced erasure rate for polymeric photorefractive materials is theoretically predicted to depend on optical intensity as a sublinear relation of Iα, in which α is dependent on the electric field. The characteristics of the traps in a photorefractive material play a key role in the dynamics of grating erasure. Experimentally studied was the dynamic process of erasing the photorefractive grating in a polymeric composite, poly-N-vinylcarbazole:fullerene:diethyl aminonitrostyrene:tricresyl phosphate. The theoretical analysis and experimental results reveal that shallow traps are dominant in this photorefractive material. © 1999 American Institute of Physics. View full abstract»

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  • Kramers–Kronig analysis of polymer acoustic data

    Page(s): 44 - 48
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    We briefly present the relationships between the acoustic velocity v and attenuation α in their exact form, while retaining the ambiguity in the magnitude of the response function. We show that some recent experimental measurements of v and α in polymers satisfy the Kramers–Kronig relations very well, while others are not as satisfactory. We demonstrate that the nearly local approximation obtained by dropping terms in (αv/ω)2 is quite inappropriate for these materials, and we point out the importance of these measurements for the experimental analysis of space charge in insulators. © 1999 American Institute of Physics. View full abstract»

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  • On the axial structure of a nitrogen surface wave sustained discharge: Theory and experiment

    Page(s): 49 - 62
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    A model for a surface wave sustained nitrogen discharge accounting in a self-consistent way for electron and heavy particles kinetics and discharge electrodynamics has been developed. The system under analysis is a plasma column produced by a traveling, azimuthally symmetric (m=0 mode) surface wave. The model is based on a set of coupled equations consisting of the electron Boltzmann equation and the rate balance equations for the most important excited species—vibrationally, N2(X 1Σg+, ν), and electronically excited states, N2(A 3Σu+, a Σu-, B 3Πg, C 3Πu, a 1Πg)—and charged particles (e, N2+, N4-) in the discharge. Electron collisions with nitrogen molecules of the first and the second kind and electron–electron collisions are accounted for in the Boltzmann equation. The field strength necessary for steady-state operation of the discharge is obtained from the balance between the total rates of ionization (including direct, stepwise, and associative ionization) and of electronic losses (due to diffusion to the wall and bulk recombination). The transfer of wave power to the discharge occurs through collisional processes, thus the set of equations is closed by an ordinary differential equation (stemming from basic electrodynamical relations) which associates the axial gradient of the electro- n density to the wave attenuation. As a result, a self-consistent interdependence between wave propagation and discharge characteristics is obtained over the whole plasma column. The axial profile of the gas temperature and the initial value of the electron density at the position of the wave launcher are used as input parameters. The model determines the axial structure of the discharge—axial variations of the electron energy distribution function and its moments, the vibrational distribution function of the electronic ground state, and the densities of the most important electronically excited states and positive ions—consistently with the electric field and the surface wave dispersion characteristics. A spatially resolved experimental investigation of the electron energy distribution function, the gas and the vibrational temperatures, and the population densities of some electronically excited states along with wave propagation characteristics measurements provides a verification of the model. Strong correlation between different plasma balances, governing the discharge production, and discharge electrodynamics—the basis of surface-wave discharge physics—has been demonstrated both theoretically and experimentally. © 1999 American Institute of Physics. View full abstract»

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  • Microwave reflections from a vacuum ultraviolet laser produced plasma sheet

    Page(s): 63 - 68
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    A pulsed, 193 nm excimer laser is utilized to photoionize the organic gas tetrakis- dimethylamino-ethylene (TMAE). The laser ionizes a plasma sheet with a width of 7.8 cm and an adjustable thickness of 0.7–1.4 cm. The axial scale length of the plasma density is a function of TMAE neutral pressure and is typically 50 cm. X-band (10 GHz) microwaves are incident on the plasma with the electric field polarized parallel to the laser beam axis. The power reflection coefficient and the phase of the reflected signal are studied as a function of time. A monostatic homodyne detection system with a response time of 10 ns is utilized to determine the amplitude and phase of the reflected wave. The peak plasma density is ne≈4×1013cm-3, sufficiently above the critical density (ncrit=1.2×1012cm-3) to produce reflections comparable to a conducting sheet placed in the same position as the plasma. A computer model is developed to interpret and optimize the plasma conditions which provide the highest backscatter and phase-stable reflection coefficient for the longest period of time. The presence of axial density gradients causes the reflected wave to be scattered through a wide angle. As the gradients relax, the backscatter reflection coefficient increases to a value of nearly 100%. The plasma density and two-body recombination coefficient are measured by means of microwave backscatter plasma reflectivity and Langmuir probes. © 1999 American Institute of Physics. View full abstract»

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  • Time dependent study of a multi-ionic xenon plasma

    Page(s): 69 - 73
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    The dynamics of a low pressure xenon discharge plasma was analyzed by using a collisional-radiative model. The calculations reproduce the main features of the temporal evolution of xenon spontaneous emission lines experimentally observed under the pulsed regime. The analysis includes transitions belonging to several xenon species (Xe I–Xe VIII). More than 100 measured lines were considered in this study. The pressure and discharge voltage dependence of the temporal emission distribution (time delay, shapes, and intensity maximum position), obtained from the calculation, are in agreement with the experimental results. The main excitation-deexcitation processes are discussed. © 1999 American Institute of Physics. View full abstract»

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  • Heteronuclear and homonuclear surface abstraction reactions of Cl, Br, and F

    Page(s): 74 - 86
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    Surface reactions of atomic halogen atoms play important roles in various plasma etching processes, commonly used in microlectronics manufacturing. However, relatively little is known about the surface chemistry of these key reactive intermediates. Previous measurements of the recombination coefficients of Cl, Br, and F on various surfaces in a molecular beam apparatus indicated that the recombination reaction is pseudofirst order [G. P. Kota, J. W. Coburn, and D. B. Graves, J. Vac. Sci. Technol. A 16, 270 (1998); 16, 2215 (1998)]. One mechanism that would result in pseudofirst order kinetics is a two-step process in which the first halogen atom adsorbs into a relatively strongly bound chemisorbed state, and the second atom reacts with it either through a direct reaction, or after being physisorbed onto the halogenated surface. In this article, we report experiments in which surfaces are first exposed to a molecular beam of one type of halogen atom, then the surface is exposed to a second type of halogen. During the second exposure, the heteronuclear reaction product is monitored with a mass spectrometer. Finally, the surface is sputtered and the mass spectrometer is used to detect any remaining presence of the original halogen atom. Analogous experiments were also performed with isotopically enriched mixtures of chlorine. These experiments unambiguously demonstrate that halogen atom surface recombination involves a two step adsorption-abstraction mechanism. Under all conditions studied, the surface recombination reactions proceeded at rates on the order of surface collision frequencies. The relative magnitudes of the heteronuclear rates (as a function of surface composition and halogen atom type) scaled in the same way as the homonuclear recombination probabilities measured previously. In every case examined, after the second halogen exposure, the surface retained a significant coverage of the halogen that had been originally exposed to the surface. This leads to t- he conclusion that only a fraction of the strongly bound surface sites are available for abstraction by free radical attack. Absolute calibration of the incident and evolved species fluxes allowed an estimate to be made of the reactive site densities for several surfaces. These ranged from 1012 to 1015cm-2 depending on the surface. © 1999 American Institute of Physics. View full abstract»

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  • Atomic absorption spectroscopic measurements of silicon atom concentrations in electron cyclotron resonance silicon oxide deposition plasmas

    Page(s): 87 - 93
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    The silicon atom densities in both silane/oxygen and tetraethoxysilane (TEOS)/oxygen electron cyclotron resonance (ECR) plasmas were measured as functions of microwave power, pressure, and gas flow rates. An atomic absorption spectrometer with a Si hollow-cathode lamp was constructed for these measurements. Silicon atom densities in silane/oxygen ECR discharges increase with rising plasma density, and a strong correlation was found between the Si atom gas-phase abundance and the silicon oxide film deposition rate. The measured Si concentrations [(1–7)×1010cm-3] were high enough to account for a significant part of the film growth in the silane based chemistry. In TEOS/O2 discharges Si atom concentrations were lower by an order of magnitude, so Si is probably not a major contributor to the growth rate in that case. The internal temperature of Si atoms was found to vary from 380 to 720 K with increasing microwave power (200–650 W). © 1999 American Institute of Physics. View full abstract»

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  • Comparison of Cu gettering to H+ and He+ implantation-induced cavities in separation-by-implantation-of-oxygen wafers

    Page(s): 94 - 98
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    Well-defined bands of cavities have been formed beneath the buried oxide (BOX) layer of two sets of separation-by-implantation-of-oxygen (SIMOX) wafers by H+ and He+ implantation. The gettering of Cu impurities, which were implanted into the top Si layer at different doses (5×1013, 5×1014, and 5×1015/cm2), to the cavities has been studied by secondary ion mass spectroscopy and cross-sectional transmission electron microscopy. The results indicated that the cavities induced either by H+ or He+ implantation are effective gettering centers for Cu in SIMOX wafers, and up to 4×1015/cm2 Cu has diffused through the BOX layer and been captured by the cavities. The gettering efficiency of cavities increases with the decrease of Cu implantation doses and the increase of annealing temperatures. He+ ion implantation is found to be more suitable for cavity formation and impurity gettering than H+ ion implantation. © 1999 American Institute of Physics. View full abstract»

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  • Confocal micro-Raman characterization of lattice damage in high energy aluminum implanted 6H-SiC

    Page(s): 99 - 104
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    High energy (MeV) and low dose aluminum implants were performed in p-type 6H-SiC at room temperature. The material was characterized by means of Rutherford backscattering in channeling configuration and confocal micro-Raman scattering. The damage induced changes in the optical absorption coefficient of the implanted layer can be extracted from the depth profiling of the first order Raman intensity of the undamaged portion of the sample, using a confocal microprobe setup. Optical modeling indicates the formation of two layers: an outermost and low absorbing layer with thickness proportional to the energy of the bombarding ions, and a more highly damaged and absorbing layer. Since the damage level is low, the disorder can be essentially removed by annealing at relatively low temperatures. © 1999 American Institute of Physics. View full abstract»

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  • Gallium implantation induced deep levels in n-type 6H–SIC

    Page(s): 105 - 107
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    Two Ga-acceptor levels, located at EV+0.31 eV and EV+0.37 eV, respectively, have been observed in the gallium implantation manufactured p+n diodes using deep level transient spectroscopy. The behavior of the implanted gallium is very similar to that of implanted aluminum, except that the positions of the introduced levels are different. This result strongly supports the recent model, which was used to explain the discrepant results between boron and aluminum implantation induced deep levels. Besides the two acceptor levels, a thermally stable electron trap is also observed and has been tentatively attributed to a Ga-related complex. © 1999 American Institute of Physics. View full abstract»

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  • Simulation of dislocations on the mesoscopic scale. I. Methods and examples

    Page(s): 108 - 119
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    Peach–Koehler theory is implemented to simulate the motion of arbitrarily configured interacting dislocations, located on arbitrary glide planes and having any allowed Burgers vector. The self-interaction is regularized by a modified Brown procedure, which remains stable and loses accuracy in a well-controlled manner as atomic dimensions are approached. The method is illustrated by applying it to several examples of single and interacting dislocations in an fcc slip system. The critical strain for the propagation of a dislocation in a capped layer is calculated and found to be in excellent agreement with theory. Dislocations in a layer with a free surface are studied to test simplified methods for modeling the dislocation–surface interaction. Frank–Read sources are simulated in an infinite medium and in a strained layer. The latter are seen to give rise to the characteristic pileup structures often observed experimentally. The interaction between two initially straight dislocations on intersecting glide planes is studied as a function of relative angle and initial separation. It is found that an attractive instability occurs for a well-defined range of relative angles, and that this range depends only weakly on the initial separation. While this suggests that the detailed calculation of such interactions could be replaced by a simple set of interaction rules specifying their outcome, a variety of factors limiting the usefulness of such rules can be identified. It is further determined that, when an attractive instability occurs, the configuration assumed by the dislocations as they near each other bears little resemblance to any simple starting configuration. This suggests that calculations of the type presented here could provide a useful starting point for atomistic calculations of interacting dislocations. © 1999 American Institute of Physics. View full abstract»

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  • Simulation of dislocations on the mesoscopic scale. II. Application to strained-layer relaxation

    Page(s): 120 - 129
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    Numerical simulations are used to study various dislocation interactions expected to occur when a strained epitaxial layer relaxes. Frank–Read sources located on parallel glide planes are shown to produce elaborate network patterns of dislocations in the unstrained substrate of the layer. These are strikingly similar to experimentally observed patterns in high-quality relaxed SiGe layers grown by low-temperature ultrahigh vacuum chemical vapor deposition. Frank–Read sources on intersecting glide planes, on the other hand, produce characteristic corner-pileup structures which have previously been attributed to a “modified” Frank–Read source mechanism. The blocking of threading arms by misfit dislocations is found to occur only very near the critical strain. A much more important mechanism preventing ideal layer relaxation appears to be the formation of immobile pairs of threading dislocations on parallel glide planes. © 1999 American Institute of Physics. View full abstract»

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  • Aging behavior in Cu–Al–Be shape memory alloy

    Page(s): 130 - 133
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    This article reports positron annihilation spectroscopy and calorimetric measurements of the aging behavior in a Cu–Al–Be shape memory alloy. An excess of single vacancies is retained in the alloy as a result of a quench. All vacancies in excess disappear after long aging time, and a migration energy EM=1.0±0.1 eV for this process has been found to be larger than in other Cu-based shape memory alloys. The good correlation found for the concentration of vacancies and the shift in the martensitic transition temperature demonstrates that, in Cu–Al–Be, changes in the transition after a quench are deeply related to the excess of vacancies. © 1999 American Institute of Physics. View full abstract»

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  • Noise-induced local heatings in beam irradiation

    Page(s): 134 - 147
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    In every kind of beam irradiation, a part of the incident power is eventually dissipated within the irradiated sample, leading to a heating which has to be evaluated, in order to be sure that thermal effects (such as diffusion, phase transitions, etc.) are under control. Expanding on usual thermal estimations based upon the coherent part of the incident power, we focus on the thermal effects due to the noisy (i.e., incoherent) part of the power. Starting from the autocorrelation of the beam, we show analytically that white noise induces an extra heating whose relative influence increases as the distance to the center of the beam decreases. Relying on the physical results derived for white noise, a simple method is proposed to extract the main aspects of these extra heatings from experiments where the autocorrelation function of the beam is not known a priori. Experimental data from MeV light ion microbeam are briefly presented as an example. © 1999 American Institute of Physics. View full abstract»

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  • Study of sample thickness dependence in electron-beam irradiation of self-developing inorganic materials

    Page(s): 148 - 152
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    In this study we apply a series of focused and overfocused electron probes with current densities ranging from 105 to 109 A m-2 to irradiate electron-gun deposited thin films of amorphous AlF3 (a-AlF3)and amorphous SiO2(a-SiO2). Statistical distributions of the time deemed necessary to produce a given amount of mass loss from the two beam-irradiated materials are measured as functions of beam current density and sample thickness. According to those results, a-AlF3 is damaged in parallel throughout the irradiated volume of the sample as indicated by no detectable thickness dependence, whereas a-SiO2 displays a distinct scaling of characteristic drilling times with thickness indicative of a combination of surface and volume mass loss processes. © 1999 American Institute of Physics. View full abstract»

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  • Thermophysical modeling of bump formation during CO2 laser texturing of silicate glasses

    Page(s): 153 - 159
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    The thermophysical nature of CO2 laser texturing of glass is explored via numerical simulations. Recent data suggest that laser texture bumps are the product of a local elevation in fictive temperature in the heat affected zone. The numerical model is used to investigate the change in microstructure as manifested in a density change. Using viscosity data, the model employs a dynamic calculation of the glass transition temperature as a function of time scale. The calculation shows that the glass transition temperature increases by 150–300 K over the conventional value in the laser texture process. The maximum thermal penetration depth of the glass transition temperature is numerically determined and the density change calculated. On chemically strengthened glasses, laser texture leads to a density reduction of approximately 2%–3% in the heat affected zone. On unstrengthened glass the density reduction is ∼1.3% © 1999 American Institute of Physics. View full abstract»

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  • Predicted bond length variation in wurtzite and zinc-blende InGaN and AlGaN alloys

    Page(s): 160 - 167
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    Valence force field simulations utilizing large supercells are used to investigate the bond lengths in wurtzite and zinc-blende InxGa1-xN and AlxGa1-xN random alloys. We find that (i) while the first-neighbor cation–anion shell is split into two distinct values in both wurtzite and zinc-blende alloys (RGa-N1≠RIn-N1), the second-neighbor cation–anion bonds are equal (RGa-N2=RIn-N2). (ii) The second-neighbor cation–anion bonds exhibit a crucial difference between wurtzite and zinc-blende binary structures: in wurtzite we find two bond distances which differ in length by 13% while in the zinc-blende structure there is only one bond length. This splitting is preserved in the alloy, and acts as a fingerprint, distinguishing the wurtzite from the zinc-blende structure. (iii) The small splitting of the first-neighbor cation–anion bonds in the wurtzite structure due to nonideal c/a ratio is preserved in the alloy, but is obscured by the bond length broadening. (iv) The cation–cation bond lengths exhibit three distinct values in the alloy (Ga–Ga, Ga–In, and In–In), while the anion–anion bonds are split into two values corresponding to N–Ga–N and N–In–N. (v) The cation–related splitting of the bonds and alloy broadening are considerably larger in InGaN alloy than in AlGaN alloy due to larger mismatch between the binary compounds. (vi) The calculated first-neighbor cation–anion and cation–cation bond lengths in InxGa1-xN alloy are in good agreement with the available experimental data. The remaining bond lengths are provided as predictions. In particular, the predicted splitting for the second-neighbor cation–anion bonds in the wurtzite structure awaits experimental testing. © 1999 American Institute of Physics. View full abstract»

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  • Optical and electron paramagnetic resonance study of light-emitting Si+ ion implanted silicon dioxide layers

    Page(s): 168 - 173
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    Thermally grown SiO2 on Si substrates implanted with Si+ ions with a dose of 6×1016cm-2 were studied by the techniques of photoluminescence, electron paramagnetic resonance (EPR), and low-frequency Raman scattering. Distinct oxygen-vacancy associated defects in SiO2 and nonbridging oxygen hole centers were identified by EPR. The luminescence intensity in the 620 nm range was found to correlate with the number of these defects. The low-frequency Raman scattering technique was used to estimate the average size of the Si nanocrystallites formed after the implantation and thermal annealing at T≫1100 °C, which are responsible for the photoluminescence band with a maximum at 740 nm. The intensity of this band can be significantly enhanced by an additional treatment in a low-temperature rf plasma. © 1999 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