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Journal of Applied Physics

Issue 2 • Date Jan 2005

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Displaying Results 1 - 25 of 117
  • Issue Cover

    Page(s): c1
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  • Issue Table of Contents

    Page(s): toc1
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  • Correlated photon pairs from single (In,Ga)As/GaAs quantum dots in pillar microcavities

    Page(s): 023101 - 023101-4
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    We demonstrate the triggered generation of photon pairs by the cascaded biexciton-exciton emission from a single (In,Ga)As/GaAs quantum dot (QD) in a pillar microcavity. Photon cross-correlation measurements between the biexciton and exciton decay reveal highly asymmetric features under continuous wave excitation and a bunching effect under pulsed excitation due to the cascaded nature of the process. An enhancement of the QD photoluminescence (PL) intensity by a factor of 40 was found for pillar microcavities (0.6 μm diameter) compared to the excitonic PL intensity in bulk semiconductors, thus reflecting the enhanced photon collection effect out of the cavity structure. The observed pillar mode structure is well understood on the basis of an extended transfer-matrix method. View full abstract»

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  • Internal laser probing: A theoretical study of wave propagation effects and the physical limits of measurement accuracy

    Page(s): 023102 - 023102-9
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    The transient distribution of charge carriers and temperature in semiconductors has become accessible to experiments by internal laser probing techniques that exploit the electro-optical and the thermo-optical effect. In this article, two representative methods, namely free carrier absorption and laser deflection measurements, are theoretically investigated. The study makes use of analytical calculations and the recently introduced concept of virtual experiments, which is based on a physically rigorous simulation of the entire measurement process. Wave propagation phenomena and parasitic effects introduced by preparing the sample or performing the actual measurement are thoroughly analyzed to identify major sources of error and to reveal the theoretical limits of measurement accuracy and resolution. We derive a consistent evaluation rule for extracting carrier and temperature gradients from the detector signals of deflection measurements. It is quantitatively demonstrated how to optimize the setup with respect to a desired purpose, e.g., a large measurement range or a large sensitivity. View full abstract»

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  • Enhanced emission from liquid-crystal lasers

    Page(s): 023103 - 023103-9
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    The performance of a photonic band-edge laser fabricated from a low molar mass dye-doped chiral nematic liquid crystal is found to have a strong thermal dependence. At each temperature the performance of the laser has been characterized by the slope efficiency which was calculated from a plot of the emission energy as a function of excitation energy. This slope efficiency was found to increase by 36% when the dye-doped chiral nematic liquid crystal was cooled from 53 to 43 °C. The increase in slope efficiency is considered to be due to a change in the lasing conditions, in particular, changes in the emission efficiency of the dye and possibly the quality factor of the liquid-crystal resonator, which is dependent upon the linewidth of the resonant mode. The wavelength dependency of the spontaneous emission intensity and the quantum efficiency of the dye were not found to influence the lasing conditions in this case. The order parameters relating to the dye-doped chiral nematic liquid crystal were considered to be the principle factors responsible for the increase in performance at the lower operating temperature of 43 °C. View full abstract»

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  • Resonator-based all-optical Kerr-nonlinear phase shifting: Design and limitations

    Page(s): 023104 - 023104-8
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    The design of all-optical phase shifting by means of Kerr-nonlinear resonators is investigated using a one-dimensional analytical model. Dependence on different device parameters and design optimization are discussed. In particular, it is shown that a tradeoff in terms of optical input power and device length is required, which is limited by the signal bandwidth. View full abstract»

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  • Temperature dependence of polarized absorption bands in p-type CdGeAs2

    Page(s): 023105 - 023105-6
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    The temperature and polarization behaviors of four absorption bands in p-type CdGeAs2 bulk crystals have been studied from 5 to 300 K. One band, peaking near 0.22 eV (5.5 μm), is the V2V1 intervalence band transition, and its intensity taken with E||c is about 2.8 times larger than that taken with E⊥c. Three additional absorption bands can be separately resolved below 200 K. A broad band peaking near 0.38 eV is present only with E||c. A second broad band peaking near 0.52 eV is detected using E⊥c. These two bands are assigned to transitions from the top two valence bands to a deep acceptor. Infrared photoluminescence studies of a 0.35 eV emission confirm the presence of the deep acceptor. A third absorption band peaking near 0.56 eV is detected using E||c. This band is assigned to transitions from a shallow 120 meV acceptor to shallow donor states and/or conduction band states. At room temperature, the normally observed broad absorption feature extending from the band edge to beyond 0.2 eV is a result of the superposition of the four bands. These absorption bands presently limit the use of p-type CdGeAs2 crystals as nonlinear optical materials in infrared high-power laser systems. View full abstract»

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  • Effect of oblique incidence on the spectral response of quantum well infrared photodetectors

    Page(s): 023106 - 023106-6
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    A full understanding of the effect of oblique incidence on the response of the pixels of a focal plane array is required for calibrated radiometric imaging. Measuring the angular response of infrared detectors requires an extremely complex experimental setup. This paper presents two complementary experimental setups that enable us to study the effect of the oblique incidence of light on the response of cooled infrared phototdetectors based on different technologies. The angular response measurement setup provides spectrally integrated measurements over a wide angular range (-80°–+80°) with a low background level and a low stray light. In contrast, the spectral response measurement setup provides the spectrally resolved response for incidence angles lower than 30°. Both experimental setups were used to characterize single-quantum well infrared photodetectors with crossed gratings. Relatively large pixel arrays (100×100 μm2) were used to reduce finite-size effects. However, the comparison of our experimental spectral responses with calculations based on the modal expansion method, which assumes a grating of infinite extension, reveals discrepancies at high wavelengths. In order to determine if these discrepancies are due to finite pixel size effect, a more complicated model based on a finite element code is used. These calculations are in far better agreement with the experimental results and allow us to identify the effect of finite pixel size. View full abstract»

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  • Numerical simulation of de-NOx performance by repetitive pulsed discharge when added with hydrocarbons such as ethylene

    Page(s): 023301 - 023301-8
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    Emission regulations are gradually being tightened recently to prevent further air pollution. Cost-effective and efficient technologies must be developed to process the NOx generated in the combustion of fossil fuels. One of the candidate technologies to process NOx is the denitrification of flue gas by pulsed corona discharge, which has been demonstrated experimentally to show high de-NOx performance. However, the optimization of operation conditions and the appropriate understanding of the de-NOx process still remain to be clarified. Therefore, following our previous study on ammonia injection, we have simulated in the present study the de-NOx process to which hydrocarbons such as ethylene have been added to provide guidelines on its proper operation conditions and its main reaction paths to remove NOx. The simulated results show that the removal efficiency in a case of ethylene addition becomes lower than for ammonia addition, but the de-NOx energy consumption rate becomes lower than for ammonia injection. However, with ethylene injection the production of the pollutant, formaldehyde, limits the allowable amount of injected ethylene. The de-NOx performance is better with propylene than ethylene injection because propylene reacts with the OH radical more than ethylene to oxide NOx. However, formaldehyde is also produced in the case of propylene injection, limiting the allowable amount of injected propylene. The de-NOx performance is also assessed in a case where HNO2 is considered as NxOy. View full abstract»

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  • Dynamics of dust in the sheath of weakly electronegative plasmas

    Page(s): 023302 - 023302-5
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    The dynamics of dust in the sheath of weakly electronegative plasmas are investigated with the single dust model as well as the self-consistently variable dust charge. It is shown that when the dust particles enter the sheath region from the sheath edge with different initial velocities they may display different motion states: levitation in the sheath, returning from the sheath edge, and traversing the sheath region, under action of electrostatic, gravitational, ion-drag, and neutral collision forces. Furthermore, the electronegativity also plays an important role in the dust particle motion states in the sheath besides affecting the distributions of the spatial potential and the charging of the dust particles. View full abstract»

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  • Underwater electrical explosion of a Cu wire

    Page(s): 023303 - 023303-6
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    Results and analysis of a microsecond time scale underwater electrical wire explosion are presented. Experiments were carried out with a Cu wire exploded by a current pulse ≤100 kA with microsecond time duration. The analysis is based on shadow and spectrally resolved streak photography which were used to monitor the evolution of the discharge channel and the shock wave. The obtained data were used for hydrodynamic calculation of the generated water flow parameters, such as pressure and flow velocity distribution between the discharge channel and the shock wave. In particular, the pressure at the discharge channel boundary and the energy transferred to the water were estimated. The results of the calculation have been verified by comparing the measured and calculated trajectories of the shock wave. Based on the results of the calculation the energy transferred to the water was estimated. In addition, the analysis shows that the energy initially deposited in the discharge channel continues to produce mechanical work after the deposition of the electrical energy has ended. View full abstract»

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  • Comparing the short and ultrashort pulsed laser ablation of LiF

    Page(s): 023304 - 023304-9
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    Pulsed laser ablation of LiF was studied using both nanosecond (ns) and femtosecond (fs) pulses at 248 nm. Optical emission from electronically excited Li and F atoms in the plume of ejected material was investigated by wavelength, time and spatially resolved imaging methods. Careful analysis of images of species selected optical emission yielded estimates of the mean velocities of the Li+ ions arising in both excitation schemes (∼11 and ∼13 km/s, respectively), and highlighted the dramatic effects of radiation trapping, most notably by the reabsorption of Li(2p→2s) emission by ground state Li atoms in the ns ablation studies. Plumes formed by fs excitation are found to contain a higher fraction of energetic/electrically excited components, including excited F atoms and ions, indicative of an explosive boiling mechanism, whereas the ablation plume resulting from ns ablation is deduced to arise primarily from thermal evaporation of the transiently heated target surface. The amount of target material removed per shot is significantly less in the case of fs excitation. The density (and size) of unwanted droplets in films grown by fs ablation is much smaller than in the case of ns ablation, especially on substrates mounted in an off-axis ablation geometry, implying that hydrodynamic sputtering is much reduced by the use of short pulses and that fs ablation must be the preferred route to forming very thin LiF coatings. View full abstract»

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  • Electron drift and the loss balance of charged particles in planar-unbalanced dc magnetron discharge

    Page(s): 023305 - 023305-10
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    The electron drift phenomenon is investigated in the downstream region of an unbalanced dc magnetron argon discharge. The spatially resolved measurements of the electron velocity distribution function (EVDF) using a planar probe reveal the existence of a strong on-axis electron drift parallel to magnetic field in spite of a very small axial variation less than 1 V in the plasma potential. The average drift velocities calculated from the asymmetry of the measured EVDFs show that there exists a significant electron drift from cathode to substrate with a maximum speed of about 1×106 m/s, which is comparable to the bulk electron temperature. The magnetic mirror force which is driven by the axial gradient of the magnetic field (i.e., the parallel ∇B force) is suggested as a possible source for the parallel electron drift. Carrying out a scaling of current densities with the measured data, it is found that the parallel ∇B force can produce the electron current enough to balance the discharge current, implying that the electron transport in the downstream region is determined not by the classical diffusion model in which electron motion toward the anode is diffusion and mobility dominated but by the modified diffusion model in which electron motion is drift dominated. View full abstract»

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  • Mechanisms of the α and γ modes in radio-frequency atmospheric glow discharges

    Page(s): 023306 - 023306-6
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    Large-volume and uniform atmospheric glow discharges are finding a vast range of processing applications, many of which have been traditionally addressed with the vacuum plasma technology. When excited at kilohertz or above, these atmospheric plasmas operate typically at low current densities below 30 mA/cm2 and often they are perceived to have very similar properties regardless of their operation conditions. Recently a radio-frequency (rf) atmospheric glow discharge was observed at high current density of up to 1 A/cm2, thus suggesting a previously overlooked and potentially different operation regime. Through a computational study of rf atmospheric glow discharges over a wide range of current density, this paper presents evidence of at least two glow modes, namely, the α mode and the γ mode. It is shown that gas ionization in the α mode is volumetric occurring throughout the electrode gap whereas in the γ mode it is dominated by localized events near the boundary between the sheath and the plasma bulk. Secondary electron emission strongly influences gas ionization in the γ mode yet matters little in the α mode. These findings suggest a wider operation range of atmospheric glow plasmas than previously believed. The contrasting dynamic behaviors of the two glow modes highlight both the potential to preferentially match the operation regime of atmospheric glow discharges to the specific requirements of their intended applications and the importance to develop diagnostics strategies appropriate for their operation regimes. View full abstract»

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  • Etching of porous and solid SiO2 in Ar/c-C4F8, O2/c-C4F8 and Ar/O2/c-C4F8 plasmas

    Page(s): 023307 - 023307-10
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    C-C4F8-based plasmas are used for selective etching of high aspect ratio (HAR) trenches in SiO2 and other dielectrics for microelectronics fabrication. Additives such as Ar and O2 are often used to optimize the process. Understanding the fundamentals of these processes is critical to extending technologies developed for solid SiO2 to porous SiO2, as used in low-dielectric constant insulators. To investigate these issues, reaction mechanisms developed for etching of solid and porous SiO2 in fluorocarbon plasmas and for etching of organic polymers in O2 plasmas have been incorporated into a feature profile model capable of addressing two-phase porous materials. The reaction mechanism was validated by comparison to experiments for blanket etching of solid and porous SiO2 in Ar/c-C4F8 and O2/c-C4F8 plasmas using inductively coupled plasma reactors. We found that the blanket etch rates of both solid and porous SiO2 had maxima as a function of Ar and O2 addition to c-C4F8 at mole fractions corresponding to an optimum thickness of the overlying polymer layer. Larger Ar and O2 additions were required to optimize the etch rate- for porous SiO2. Whereas etch stops occurred during etching of HAR features in solid and porous SiO2 using pure c-C4F8 plasmas, Ar and O2 addition facilitated etching by reducing the polymer thickness, though with some loss of critical dimensions. Mixtures of Ar/O2/c-C4F8 can be used to manage this tradeoff. View full abstract»

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  • Modeling of microcrystalline silicon film deposition in a capacitively coupled radio-frequency plasma reactor

    Page(s): 023308 - 023308-14
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    We present a numerical model of plasma-enhanced chemical-vapor deposition of hydrogenated microcrystalline silicon c-Si:H) film from SiH4 and H2 gas mixtures in a capacitively coupled radio-frequency plasma reactor. The model takes into account electron-impact, gas-phase, and surface reactions within a well-mixed reactor model. Plasma parameters such as the electron density, the electron temperature, and the electron-impact reaction rates are determined through a discharge model and used as inputs for the reactor model. The gas-phase reactions include electron-impact and neutral–neutral reactions. Some of the surface reaction rates are determined using quantum chemical calculations and transition state theory. In the reactor model, concentrations of each chemical species are calculated at steady state using mass conservation equation uniformed throughout the reactor. Numerical results of the deposition rate as a function of the plasma reactor operating parameters show good agreement with experiments. Based on the model, the correlation between μc-Si:H properties, such as the crystal grain orientation and the hydrogen content, and deposition operating parameters has been studied using a design of experiment. Finally, optimal operating parameters are investigated using optimization techniques. View full abstract»

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  • Second order nonlinear optical properties of zinc oxide films deposited by low temperature dual ion beam sputtering

    Page(s): 023501 - 023501-6
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    We investigated second order optical nonlinearity of zinc oxide thin films, grown on glass substrates by the dual ion beam sputtering technique under different deposition conditions. Linear optical characterization of the films was carried out by spectrophotometric optical transmittance and reflectance measurements, giving the complex refractive index dispersion. Resistivity of the films was determined using the four-point probe sheet resistance method. Second harmonic generation measurements were performed by means of the Maker fringes technique where the fundamental beam was originated by nanosecond laser at λ=1064 nm. We found a relatively high nonlinear optical response, and evidence of a dependence of the nonlinear coefficient on the deposition parameters for each sample. Moreover, the crystalline properties of the films were investigated by x-ray diffraction measurements and correlation with second order nonlinearity were analyzed. Finally, we investigated the influence of the oxygen flow rate during the deposition process on both the second order nonlinearity and the structural properties of the samples. View full abstract»

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  • A study of InxGa1-xN growth by reflection high-energy electron diffraction

    Page(s): 023502 - 023502-5
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    Epitaxial growth of InxGa1-xN alloys on GaN(0001) by plasma-assisted molecular-beam epitaxy is investigated using the in situ reflection high-energy electron-diffraction (RHEED) technique. Based on RHEED pattern changes over time, the transition of growth mode from two-dimensional (2D) nucleation to three-dimensional islanding is studied for various indium compositions. RHEED specular-beam intensity oscillations are recorded during the 2D wetting-layer growth, and the dependences of the oscillation period/frequency on the substrate temperature and source flux are established. By measuring the spacing between diffraction spots in RHEED, we also estimated indium composition, x, in alloys grown under different flux combinations. Incorporation coefficients of both gallium and indium are derived. Possible surface segregation of indium atoms is finally examined. View full abstract»

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  • Correlation between Y2O3 concentration and photoluminescence excitation spectra of Tb3+ impurity in Y2O3-stabilized ZrO2

    Page(s): 023503 - 023503-4
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    Room temperature photoluminescence (PL) spectra from Tb ions included as unintentional impurities in yttria-stabilized zirconia (YSZ) were measured at various concentrations of Y2O3. Two PL excitation bands were observed from the Tb at around 380 nm for all of the YSZ samples. These two bands can be attributed to the 5D3-7F6 transition of Tb3+. We found that the energy separation between the two Tb bands decreased as the concentration of Y2O3 increased. This relationship can be explained by considering the oxygen vacancy concentration and the effective charge in the YSZ, and we found that the effective charge in YSZ can be evaluated from the energy separation. The relative strength of the crystal field in the YSZ samples could also be estimated from the energy separation. Furthermore, the phase boundary of the YSZ could be determined from the change in slope of the plots of energy separation against Y2O3 concentration. View full abstract»

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  • Cathodoluminescence study of the radiative recombination properties of Se-doped GaSb crystals

    Page(s): 023504 - 023504-4
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    The radiative recombination properties of Se-doped GaSb crystals grown by the Bridgman method have been investigated by cathodoluminescence (CL) microscopy and spectroscopy in the scanning electron microscope. A CL band centered at about 765 meV, not previously observed in undoped GaSb, is generally the dominant emission. CL spectra recorded under different excitation conditions suggest that this band can be attributed to a Se-related level–to–band transition. The spatial distribution of the 765 meV emission, as observed in the CL images, indicates an inhomogeneous Se distribution in the material. View full abstract»

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  • Nickel solubility in intrinsic and doped silicon

    Page(s): 023505 - 023505-6
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    Solubility of nickel in intrinsic, moderately, and heavily doped n-type and p-type silicon was determined using instrumental neutron activation analysis. The solubility data for intrinsic silicon were found to be in good agreement with the literature data. In heavily doped p-type silicon the enhancement of nickel solubility, if present, was close to the error margins of the experiment, indicating that interstitial nickel is predominantly neutral in silicon and suggesting that its donor level lies close to the valence band edge, if not within the valence band itself. No dependence of nickel solubility on doping level of the samples was observed in n-type silicon. This is consistent with the model reported in the literature of two acceptor levels of substitutional nickel located in the upper half of the band gap, one of the levels close to the conduction band edge. Consequently, unlike copper or iron, nickel does not segregate in heavily p-type or n-type doped areas of silicon wafers. View full abstract»

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  • Compression studies of TiB2 using synchrotron x-ray diffraction and ultrasonic techniques

    Page(s): 023506 - 023506-6
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    The compressibility of TiB2 has been determined separately by synchrotron x-ray diffraction and ultrasonic measurements on samples loaded in diamond-anvil cells and in multianvil high-pressure apparatus up to pressures of 65.9 GPa and 13.9 GPa, respectively, at ambient temperature. The high-pressure x-ray diffraction measurements, carried out on a pure polycrystalline sample, show a monotonic decrease of the lattice parameters with pressure, with the c/a ratio variation suggesting an increase in the structural rigidity along the c direction. No structural transition is observed even after laser heating of the sample up to 2200 K at 40 GPa, indicating a phase stability of TiB2 in this pressure range. The ultrasonic measurements of compressional Vp and shear Vs velocities on a cylindrical specimen show linear pressure dependencies. Using the third-order Eulerian finite strain equation to fit the pressure-volume x-ray data, we obtain an isothermal bulk modulus of KTo=232.9±4.2 GPa and its pressure derivative KTo=3.86±0.23. Similarly, the parameters derived from fitting the finite strain equation to the ultrasonic data give an adiabatic bulk modulus of KSo=232.9±1.6 GPa and KSo=3.02±0.24, in good agreement with the x-ray diffraction measure- ments. The shear modulus and its pressure derivative derived from the ultrasonic measurements are GSo=259.8±0.4 and GSo=2.52±0.01, respectively. View full abstract»

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  • Short range interaction in tunneling near-field optical microscopy

    Page(s): 023507 - 023507-4
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    We use a modulation technique to detect a short-range interaction between a probe and a sample of a tunneling near-field optical microscope. This interaction might explain the resolution enhancement of the apertureless near-field techniques compared with conventional aperture near-field ones. To detect this interaction we measure approach curves of the modulating probe towards the sample and fit the experimental data with a simulation of the intensity, where a coupled dipoles interaction is added to the conventional near-field interaction. While the effect of the coupled dipoles interaction is negligible in conventional near-field microscopes, it dominates the modulated signal at scanning distances. Thus better resolution can be achieved, even in an aperture near-field microscope, by vibrating the tip vertically over the sample. View full abstract»

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  • 18O tracer diffusion in Pb(Zr,Ti)O3 thin films: A probe of local oxygen vacancy concentration

    Page(s): 023508 - 023508-8
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    A series of 18O tracer in-diffusion experiments have been conducted on state-of-the-art metal organic chemical vapor deposition–grown Pb(Zr,Ti)O3 (PZT) films. Prior to 18O incorporation, samples were preequilibrated in 16O2 at the same temperature and total pressure used for the following 18O2 isotope exchange anneal. The final 18O depth profiles were obtained using secondary ion mass spectroscopy (SIMS), which provides a depth resolution of 3–5 Å. The SIMS results indicate that the 18O tracer diffusivity varies as a function of depth from the PZT film surface in a way that is inconsistent with a combined grain boundary and lattice diffusion mechanism. Based on a previously-developed model for point defect equilibrium in metal titanate thin films, simulations of 18O tracer diffusion into PZT thin films that are consistent with the experimental results are described. View full abstract»

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  • Imaging of defect-mediated surface morphology of GaN(0001 ) grown on sapphire by molecular beam epitaxy

    Page(s): 023509 - 023509-8
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    In situ scanning tunneling microscopy and ex situ atomic force microscopy are used to study the surface morphology of GaN(0001 ) grown on sapphire substrates by plasma-assisted molecular beam epitaxy. Several types of defect-mediated surface morphologies are characterized, including surface terminations of screw and mixed dislocations, defect-induced grooves, and impurity-induced step bunching. In addition to the early theory of dislocation-enhanced crystal growth developed by Frank [F. C. Frank, Acta Crystallogr 4, 497 (1951)] step structures are also found to be critical to understand the formation of spiral growth hillocks around surface terminations of different types of threading dislocations. Growth experiments are also performed under different conditions, e.g., by varying III/V flux ratio and growth temperature to further investigate the relationship between the final surface morphology and the early stages of growth, such as sapphire nitridation and the transition stage from the three-dimensional buffer layer growth under nitrogen-rich conditions to the two-dimensional active layer growth under slightly Ga-rich conditions. 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