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Journal of Applied Physics

Issue 3 • Date Feb 2004

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Displaying Results 1 - 25 of 128
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Calibration of magnetic force microscopy tips by using nanoscale current-carrying parallel wires

    Page(s): 775 - 792
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    Experimental results on the characterization of commercially available magnetic force microscopy (MFM) thin film tips as a function of an external magnetic field are presented. Magnetic stray fields with a definitive z-component (perpendicular to the substrate) and a magnetic field strength of up to Hz=±45 Oe are produced with current carrying parallel nanowires with a thickness of t=60 nm, which are fabricated by electron-beam lithography. The magnetic fields are generated by electrical dc-currents of up to ±6 mA which are directed antiparallel through the nanowires. The geometry and the dimensions of the nanowires are systematically varied by choosing different wire widths w as well as separations b between the parallel wires for two different sets of samples. On the one hand, the wire width w is varied within 380 nm≪w≪2460 nm while the separation b≈450 nm between the wires is kept constant. On the other hand the separation b between the parallel wires is varied within 120 nm≪b≪5100 nm, while the wire width w=960 nm is kept constant. For all the geometrical configurations of parallel wires the resulting magnetic contrast is imaged by MFM at various tip lift-heights. By treating the MFM tip as a point probe, the analysis of the image contrast as a function of both the magnetic field strength and the tip lift height allows one to quantitatively determine the effective magnetic dipole and monopole moments of the tip as well as their imaginary locations within the real physical tip. Our systematic study quantitatively relates the above point-probe parameters to (i) the dimensions of the parallel wires and (ii) to - the characteristic decay length of the z-component of the magnetic field of parallel wires. From this the effective tip-volume of the real thin film tip is determined which is relevant in MFM-imaging. Our results confirm the reliability of earlier tip calibration schemes for which nanofabricated current carrying rings were used instead of parallel wires, thereby proving that the tip calibration equations depend on the underlying stray field geometry. Finally, we propose an experimental approach which allows one to measure the magnetization of nanoscale ferromagnetic elements with an in-plane orientation of the magnetization, quantitatively, by using a calibrated MFM-tip. © 2004 American Institute of Physics. View full abstract»

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  • Light diffraction from a bilayer lattice of microspheres enhanced by specular resonance

    Page(s): 793 - 805
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    Anomalously strong diffraction from a bilayer lattice of dielectric microspheres, previously reported by the authors, has been attributed to the enhancement of diffraction by specular resonance in constituent bispheres. On the basis of rigorous calculations and experiments, specular resonance from bispheres is found to be dominant even in the scattering from a cluster of several microspheres. As a consequence, a diffraction model, in which a bilayer lattice of microspheres is viewed as a two-dimensional array of bispheres, is constructed. By incorporating the specular resonance into a diffraction theory of two-dimensional lattices as a structure factor, observed diffraction behavior is explained. In the diffraction from a bilayer lattice of microspheres, the specular resonance functions as a blazing mechanism. Possible configurations and suitable parameters for efficient gratings are discussed, and a self-assembled bilayer lattice of dielectric microspheres with a diameter 1.6–3.2 times as large as the wavelength is found to be promising as a low-cost and highly efficient transmission grating. The spheres can be replaced with cylinders or lenses. Tunable diffraction devices with efficiency exceeding that of conventional gratings would be realized by microelectromechanical systems comprised of two optimally designed microlens arrays. © 2004 American Institute of Physics. View full abstract»

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  • Ab initio tight-binding approach to photonic-crystal based coupled cavity waveguides

    Page(s): 806 - 809
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    We develop a model to study coupled cavity waveguides (CCWs) in photonic crystals, composed of linear chains of coupled cavities. By extending previous work on single mode cavities, we have developed a model applicable to any finite number of quasi-degenerate cavity modes. Our model, which uses a multimode tight-binding approach, allows us to study the interaction between the cavity modes as well as the coupling between the CCW bands. In contrast to previous work the coupling parameters are calculated ab initio from the single cavity field profile. The calculated dispersion relations are shown to be in good agreement with those obtained from the plane wave expansion method. © 2004 American Institute of Physics. View full abstract»

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  • Polarization switch using thick holographic polymer-dispersed liquid crystal grating

    Page(s): 810 - 815
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    Theoretical study of the optical diffraction properties of holographic polymer-dispersed liquid crystal (HPDLC)-based thick volume grating was presented. It is found that the HPDLC grating can selectively diffract the s-polarized light only, p-polarized light only, or both s and p beams by selecting suitable grating parameters. When an electric field is applied, the s- and p-polarized lights exhibit different diffraction behaviors and, thus, the tunable dual-state or three-state polarization switching could be realized. © 2004 American Institute of Physics. View full abstract»

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  • Laser–plasma interactions in fused silica cavities

    Page(s): 816 - 822
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    The effect of laser energy on formation of a plasma inside a cavity was investigated. The temperature and electron number density of laser-induced plasmas in a fused silica cavity were determined using spectroscopic methods, and compared with laser ablation on a flat surface. Plasma temperature and electron number density during laser ablation in a cavity with aspect ratio of 4 increased faster with irradiance after the laser irradiance reached a threshold of 5 GW/cm2. The threshold irradiance of particulate ejection was lower for laser ablation in a cavity compared with on a flat surface; the greater the cavity aspect ratio, the lower the threshold irradiance. The ionization of silicon becomes saturated and the crater depths were increased approximately by 1 order of magnitude after the irradiance reached the threshold. Phase explosion was discussed to explain the large change of both plasma characteristics and mass removal when irradiance increased beyond a threshold value. Self-focusing of the laser beam was discussed as being responsible for the decrease of the threshold in cavities. © 2004 American Institute of Physics. View full abstract»

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  • Spark-eroded particles: Influence of processing parameters

    Page(s): 823 - 829
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    Ni particles were prepared by spark erosion in a fixed-gap apparatus, and in the usual “shaker-pot” assembly, in an investigation of the influence of various processing parameters on the particles’ properties. The sizes of the particles were studied as functions of spark energies ranging from 10 μJ to 1 J, and a scaling relation derived from a simple model was verified. Several different static and rotating electrode configurations were compared with respect to their suitability for producing significant yields of small particles. The advantages of stirring the dielectric with the fixed-gap apparatus and of rotating the electrodes were demonstrated. Water, kerosene, and liquid argon and nitrogen were used as dielectric liquids. When compounds were formed, the reaction with the dielectric proceeded inversely with particle size. Spark erosion in kerosene at low spark energies, followed by annealing, proved to be an effective method to produce fine nickel particles. © 2004 American Institute of Physics. View full abstract»

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  • Sheath-lens probe for negative ion detection in reactive plasmas

    Page(s): 830 - 833
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    A method that allows easy and inexpensive detection of negative ions is introduced. The method is based upon the electrostatic lens effect of the sheath layer evolving to a positively biased planar probe that focuses the negative charges to distinct regions on the surface. Trajectories of negative ions inside the sheath are obtained after computing the potential and electric field distribution by solving in three dimensions the nonlinear Poisson equation. The negative ions’ flux to square and disk probes is developed in Ar/SF6 and O2 plasmas. The method allows negative ion detection with sensitivity higher than that of Langmuir probes. © 2004 American Institute of Physics. View full abstract»

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  • Modeling of magnetically enhanced capacitively coupled plasma sources: Ar/C4F8/O2 discharges

    Page(s): 834 - 845
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    Magnetically enhanced, capacitively coupled radio frequency plasma sources are finding continued use for etching of materials for microelectronics fabrication. MERIE (magnetically enhanced reactive ion etching) sources typically use magnetic fields of tens to hundreds of gauss parallel to the substrate to either increase the plasma density at a given pressure or to lower the operating pressure. The use of MERIEs for etching of dielectric materials, such as SiO2, often involves the use of complex gas mixtures, such as Ar/C4F8/O2/CO. In this paper results from a two-dimensional hybrid-fluid computational investigation of MERIE reactors operating in such mixtures are discussed. Fluxes and energy distributions for ions incident on the wafer are discussed for an industrially relevant geometry. The reduction in transverse electron mobility as the magnetic field increases produces a decrease in the sheath electric fields and a decrease in the dc bias (becoming more positive) at large magnetic fields thereby decreasing ion energies and increasing the angular spread of ions. These trends affect heavier ions more acutely than lighter ions. Subtle variations in the electrical geometrical layout of the reactor significantly affect the spatial uniformity of ion energy distributions. © 2004 American Institute of Physics. View full abstract»

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  • Modeling of microdischarge devices: Pyramidal structures

    Page(s): 846 - 859
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    Microdischarge (MD) devices are plasma sources typically operating at 100s Torr to atmospheric pressure with dimensions of 10s–100s μm. Their design in based on pd (pressure×characteristic dimension) scaling; smaller dimensions are enabled by higher operating pressures with typical devices operating with pd=1–10 Torrcm. MD devices have exhibited behavior that resemble both Townsend and hollow-cathode discharges, with bulk and beam electrons providing the dominant excitation, respectively. In this article, results from a two-dimensional computational study of MD devices operating in neon using a pyramidal cathode structure are discussed. Pressures of 400–1000 Torr and device dimensions of 15–40 μm are investigated. The onset of behavior resembling negative glow discharges with decreasing pressure correlates with an extension of cathode fall accelerated beam electrons into the bulk plasma. For constant applied voltage, peak electron densities increase with increasing pressure as the beam electrons are slowed in more confined regions. The MD devices typically require higher applied voltages to operate at lower pressures, and so resemble discharges obeying Paschen’s curve for breakdown. MD devices having similar magnitudes and spatial distributions of plasma and excited state densities can be obtained to dimensions of ≪15 μm by keeping pd and current density constant, and having a cathode fall thickness small compared to the characteristic dimension. © 2004 American Institute of Physics. View full abstract»

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  • Collisional sheath dynamics in the intermediate radio-frequency regime

    Page(s): 860 - 869
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    A sheath model is proposed for the case when the radio-frequency ω is comparable to or larger than the ion plasma frequency of the bulk plasma ωpi and the ion collisionality in the sheath is significant. In this case, the ion momentum equation can be solved easily. We find that the ion velocity in the sheath varies with time and the resulting ion energy distribution is bimodal even though the radio frequency is much larger than the ion plasma frequency in the sheath. The results of the model are compared with the numerical solutions of the fluid equations. Both are in very good agreement. © 2004 American Institute of Physics. View full abstract»

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  • Functional separation in two frequency operation of an inductively coupled plasma

    Page(s): 870 - 876
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    Measurements of densities of excited atoms and metastables were performed in pure Ar and in mixtures of Ar and CF4 in inductively coupled plasma sustained by a high frequency (13.56 MHz) source and biased by a low frequency (500 kHz) voltage applied to the wafer supporting electrode. The measurements are made in front of the biased electrode with a goal to understand the effects of different parameters on the plasma profile and to test whether functional separation between plasma sustaining and biasing voltage is achieved. We find a very efficient separation with small or no observable effects of biasing voltage both in pure argon and in mixtures. These results have been achieved at all pressures (5–50 mTorr) and were confirmed by additional microwave measurements of electron density. The effect of flow rate, pressure, power, and distance from the biased electrode was studied from the spatial profiles of short lives excited states and metastable states of argon. We have also compared the profiles close to the biasing electrode, close to the coil and in extended processing chamber, and found a slight increase of metastable density close to the biasing electrode due to reduced electron quenching far from plasma source. © 2004 American Institute of Physics. View full abstract»

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  • Amorphization/recrystallization of buried amorphous silicon layer induced by oxygen ion implantation

    Page(s): 877 - 880
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    In this paper we discuss the structural modifications observed in a buried amorphous Si (a-Si) layer containing high oxygen concentration level (up to ∼3 at. %) after being implanted at elevated temperature with 16O+ ions. For implants conducted at temperatures lower than 150 °C, the a-Si layer expands via layer by layer amorphization at the front and back amorphous–crystalline (a–c) interfaces. When performed at temperatures above 150 °C, the implants lead to the narrowing of the buried a-Si layer through ion beam-induced epitaxial crystallization at both a–c interfaces. Cross section transmission electron microscopy analysis of samples implanted at 400 °C revealed an array of microtwins and a dislocation network band in the recrystallized material. In samples implanted at 550 °C, only a buried dislocation network band is observed. © 2004 American Institute of Physics. View full abstract»

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  • Anisotropic optical properties and molecular orientation in vacuum-deposited ter(9,9-diarylfluorene)s thin films using spectroscopic ellipsometry

    Page(s): 881 - 886
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    This article reports on the investigation of anisotropic optical properties of vacuum-deposited thin films of high-efficiency blue-emitting ter(9,9-diarylfluorene)s using variable-angle spectroscopic ellipsometry. Under deposition conditions typical for thin-film organic devices, both real and imaginary parts of refractive indices of vacuum-deposited ter(9,9-diarylfluorene) films exhibit rather significant uniaxial anisotropy with the optical axis along the surface normal. In particular, for the absorption associated with the π–π* transition of the terfluorene backbone, they show substantially larger in-plane extinction coefficients than the out-of-plane extinction coefficients. It is thus inferred that the vacuum-deposited ter(9,9-diarylfluorene) molecules tend to align their molecular axes and π–π* transition dipole moments along the substrate surface as observed previously in spin-coated films of alkyl-substituted polyfluorenes or oligofluorenes, even though the present ter(9,9-diarylfluorene)s have rigid and bulky aryl substituents on C9, relatively shorter oligomer chains, and very different deposition conditions. © 2004 American Institute of Physics. View full abstract»

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  • Transport and exchange of hydrogen isotopes in silicon-device-related stacks

    Page(s): 887 - 895
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    Thermally driven transport and exchange of hydrogen and deuterium in silicon-based metal-oxide-semiconductor (MOS) device-related structures were experimentally investigated using elastic recoil detection analysis. The samples were planar stacks of different materials on crystalline silicon. The materials studied included silicon oxide prepared by thermal growth, polycrystalline silicon silicon nitride, silicon oxynitride, and borophosphosilicate glass (BPSG) prepared by chemical vapor deposition (CVD). CVD was performed using either standard (hydrogen-containing) or deuterated precursors. Thermal annealing was carried out at 350–800 °C for 10–300 min in argon or in forming gas, either standard (90 vol. % N2,10 vol. % H2) or deuterated. All materials except silicon nitride were permeable to hydrogen and deuterium in the temperature range studied. Isotope exchange in the polycrystalline Si/SiO2 structure was observed above 450 °C. BPSG showed very little relative isotope exchange. Implications to MOS device processing are discussed. © 2004 American Institute of Physics. View full abstract»

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  • Extended Cauchy equations for the refractive indices of liquid crystals

    Page(s): 896 - 901
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    The extended Cauchy equations are derived based on the Vuks equation for describing the wavelength- and temperature-dependent refractive indices of liquid crystal compounds and mixtures. This model fits experimental data well in the off-resonance spectral region. Correlations between the Cauchy coefficients for the nematic and isotropic phases are derived and validated by experiments. © 2004 American Institute of Physics. View full abstract»

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  • Nonlinear refraction and absorption of Mg doped stoichiometric and congruent LiNbO3

    Page(s): 902 - 908
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    The light induced change of refraction is studied in pure and Mg doped LiNbO3 with congruent and stoichiometric compositions by the Z-scan method using all-lines visible argonion laser, up to MW/cm2 intensity level. In Mg-doped congruent and stoichiometric crystals with Mg concentrations above threshold a positive change in the refractive index was found, in contrast to all other cases where beam fanning and negative change of the refractive index were observed. The beam distortion in the samples doped above threshold was related to thermal lensing, while below it to the photorefractive effects. It was also shown that for thermal lensing nonlinear absorption plays a dominant role. The Z-scan method was found to be an alternative technique to decide whether the Mg dopant level is above or below the photorefractive threshold. The damage resistance of the Mg doped samples above threshold was higher for the stoichiometric crystal than for the congruent one and increased with the amount of the built-in Mg concentration. © 2004 American Institute of Physics. View full abstract»

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  • Interface structure and adhesion of wafer-bonded GaN/GaN and GaN/AlGaN semiconductors

    Page(s): 909 - 912
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    Material integrations of GaN/GaN and Al0.25Ga0.75N/GaN semiconductors through wafer bonding technology were reported in this work. The wafer surface and interface microstructures were characterized by scanning electron microscopy and energy dispersive x-ray spectroscopy. The interface adhesion (bonding strength) was estimated based upon the interface fracture energy γo measured by double-cantilever beam technique. The interface adhesion properties of several different wafer-bonded III-V semiconductors were also compared. By comparing the atomic chemical bond energy Eo with the measured interface fracture energy γo, the bondability of a few major III-V semiconductors was analyzed. © 2004 American Institute of Physics. View full abstract»

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  • Vacancy-type defects in electroplated Cu films probed by using a monoenergetic positron beam

    Page(s): 913 - 918
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    Positron annihilation was used to probe vacancy-type defects in electroplated Cu films. Doppler broadening spectra of the annihilation radiation for Cu films deposited on samples with a Ta(20 nm)/SiO2(100 nm)/Si structure were measured with a monoenergetic positron beam. For an as-deposited Cu film, the line-shape parameter S measured 20 days after deposition was larger than that measured 1 day after deposition. The observed increase in the value of S was attributed to grain growth at room temperature and the corresponding increase in the fraction of positrons trapped by vacancy clusters in the grains. In isochronal annealing experiments, the value of S for an electroplated Cu film increased for annealing below 200 °C, suggesting agglomeration of vacancy-type defects (vacancy clusters). A decrease in the S value was observed for annealing above 300 °C, and this was attributed mainly to the decrease in the concentration of vacancy clusters. The annealing stages of the defects in electroplated Cu were found to agree with those for irradiated pure Cu. © 2004 American Institute of Physics. View full abstract»

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  • 1.48 and 1.84 μm thulium emissions in monoclinic KGd(WO4)2 single crystals

    Page(s): 919 - 923
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    By exciting at 788 nm, we have characterized the near infrared emissions of trivalent thulium ions in monoclinic KGd(WO4)2 single crystals at 1.48 and 1.84 μm as a function of dopant concentration from 0.1% to 10% and temperature from 10 K to room temperature. We used the reciprocity method to calculate the maximum emission cross-section of 3.0×10-20cm2 at 1.838 μm for the polarization parallel to the Nm principal optical direction. These results agrees well with the experimental data. Experimental decay times of the 3H43F4 and 3F43H6 transitions have been measured as a function of thulium concentration. © 2004 American Institute of Physics. View full abstract»

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  • In situ transmission electron microscopy study of the crystallization of Ge2Sb2Te5

    Page(s): 924 - 932
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    Crystallization of amorphous Ge2Sb2Te5 films (10, 40, and 70 nm thick) was studied by in situ heating in a transmission electron microscope (TEM). Electron irradiation-induced crystallization is possible at room temperature using a 400 kV electron beam where the reciprocal of the incubation time for crystallization scales linearly with the current density during electron irradiation. Without electron-beam exposure, crystallization starts at 130 °C. Using a 200 kV beam, crystallization also occurred in the temperature interval between 70 and 130 °C. In principle, electron irradiation always affects the crystallization kinetics, strongly promoting nucleation and probably not hampering growth. At 130 °C without electron-beam exposure, 400 nm diameter colonies of 10–20 nm grains develop in the 40 and 70 nm thick films showing clear symmetric bending contour contrast. These spherulites prefer to have in their center the <111> zone axis of the Fmm structure perpendicular to the surface of the film and show a typical tilt variation of ±10°. At 340 °C, the transition from the metastable to the stable trigonal (Pm1) crystal structure takes place. Fast and excessive grain growth occurs with the [0001] axis perpendicular to the film surface of the film. Also shown is that oxidation of the Ge2Sb2Te5 film strongly influences its crystallization; its critical temperature decreases from 130 to 35 °C. © 2004 American Institute of Physics. View full abstract»

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  • Photoluminescence from self-assembled long-wavelength InAs/GaAs quantum dots under pressure

    Page(s): 933 - 938
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    The photoluminescence from self-assembled long-wavelength InAs/GaAs quantum dots was investigated at 15 K under hydrostatic pressure up to 9 GPa. Photoemission from both the ground and the first excited states in large InAs dots was observed. The pressure coefficients of the two emissions were 69 and 72 meV/GPa, respectively. A nonlinear elasticity theory was used to interpret the significantly small pressure coefficients of the large dots. The sequential quenching of the ground and the excited state emissions with increasing pressure suggests that the excited state emissions originate from the optical transitions between the first excited electron states and the first excited hole states. © 2004 American Institute of Physics. View full abstract»

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  • Bandgap energies and refractive indices of Pb1-xSrxSe

    Page(s): 939 - 942
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    Optical transmission measurements were carried out on Pb1-xSrxSe samples, grown by molecular beam epitaxy, with different Sr compositions (x) ranging from 0 to 1. Refractive indices were calculated for all the samples at room temperature and at 77 K by fitting the transmittance data. Bandgap energies of all compositions were calculated by fitting the absorption coefficients to theoretical models of direct and indirect transitions. A distinct bandgap inversion from the direct to the indirect was observed at a Sr composition of approximately x=0.20. The direct and indirect bandgaps of SrSe calculated from the experimental results were found to be 3.81 and 1.82 eV, respectively. © 2004 American Institute of Physics. View full abstract»

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  • Ce/GaN(0001) interfacial formation and electronic properties

    Page(s): 943 - 947
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    X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and low-energy electron diffraction have been used to study the growth, interfacial reaction, and Fermi level movement of Ce on a n-type GaN(0001)-(1×1) surface. The results demonstrate that Ce grows in a layer-by-layer-like mode and reacts with the substrate at the interface, leading to formation of metallic Ga at room temperature. With increasing Ce coverage, a downward Fermi level movement is observed, and the resultant Schottky barrier height is 1.3 eV. Annealing promotes further diffusion and thus interfacial Ce–Ga exchange reaction. © 2004 American Institute of Physics. View full abstract»

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  • Phosphorescence from iridium complexes doped into polymer blends

    Page(s): 948 - 953
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    Energy transfer from the polymer blends, poly(vinylcarbazole) (PVK) with 2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazol (PBD), to an organometallic emitter, tris[9,9-dihexyl-2-(phenyl-4-(-pyridin-2-yl))fluorene] iridium (III) [Ir(DPPF)3], is investigated by steady-state and time-resolved photoluminescence (PL) spectroscopy. A redshifted PL and slow fluorescence decay are due to the formation of an exciplex in PVK-PBD blends. A decrease in intensity in polymer blends observed at 425 nm with increasing concentrations of Ir(DPPF)3 and an evident rising feature observed in films with 1 wt % Ir(DPPF)3 in the range of 578 to 615 nm within a 200 ns timescale indicate that efficient Förster energy transfer from exciplex to Ir(DPPF)3 occurs. The electrophosphorescent light-emitting diodes fabricated with PVK-PBD doped with Ir(DPPF)3 have external quantum efficiency of 8% ph/el, luminous efficiency of 29 cd/A and brightness greater than 3500 cd/m2 at 1 wt % Ir(DPPF)3. The devices exhibited no electroluminescence (EL) emission from PVK or PBD even at a low concentration of Ir(DPPF)3 (0.1 wt %), which indicates that the dominant mechanism in EL is charge trapping rather than energy transfer. © 2004 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