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Journal of Applied Physics

Issue 7 • Date Apr 2003

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Displaying Results 1 - 25 of 98
  • Issue Table of Contents

    Page(s): toc1
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    Freely Available from IEEE
  • Small molecular weight organic thin-film photodetectors and solar cells

    Page(s): 3693 - 3723
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    In this review, we discuss the physics underlying the operation of single and multiple heterojunction, vacuum-deposited organic solar cells based on small molecular weight thin films. For single heterojunction cells, we find that the need for direct contact between the deposited electrode and the active organics leads to quenching of excitons. An improved device architecture, the double heterojunction, is shown to confine excitons within the active layers, allowing substantially higher internal efficiencies to be achieved. A full optical and electrical analysis of the double heterostructure architecture leads to optimal cell design as a function of the optical properties and exciton diffusion lengths of the photoactive materials. Combining the double heterostructure with novel light trapping schemes, devices with external efficiencies approaching their internal efficiency are obtained. When applied to an organic photovoltaic cell with a power conversion efficiency of 1.0%±0.1% under 1 sun AM1.5 illumination, devices with external power conversion efficiencies of 2.4%±0.3% are reported. In addition, we show that by using materials with extended exciton diffusion lengths LD, highly efficient double heterojunction photovoltaic cells are obtained, even in the absence of a light trapping geometry. Using C60 as an acceptor material, double heterostructure external power conversion efficiencies of 3.6%±0.4% under 1 sun AM1.5 illumination are obtained. Stacking of single heterojunction devices leads to thin film multiple heterojunction photovoltaic and photodetector structures. Thin bilayer photovoltaic cells can be stacked with ultrathin (∼5 Å), discontinuous Ag layers between adjacent cells serving as efficient recombination sites for electrons and holes generated in the neighboring c- ells. Such stacked cells have open circuit voltages that are n times the open circuit voltage of a single cell, where n is the number of cells in the stack. In optimized structures, the short circuit photocurrent remains approximately constant upon stacking thin cells, leading to higher achievable power conversion efficiencies, as confirmed by modelling optical interference effects and exciton migration. A 2.5%±0.3% power efficiency under 100 mW/cm2 AM1.5 illumination conditions is obtained by stacking two ∼1% efficient devices. Alternatively, when the contact layers between the stacked cells are eliminated, a multilayer structure consisting of alternating films of donor and acceptor-type materials is obtained. Since the thicknesses of the individual layers (∼5 Å) can be substantially smaller than the exciton diffusion length, nearly 100% of the photogenerated excitons are dissociated, and the resulting free charges are detected. In addition, the ultrathin organic layers facilitate electron and hole transport through the multilayer stack by tunneling. When these devices are operated as photodetectors under applied fields ≫106V/cm, the carrier collection efficiency reaches 80%, leading to external quantum efficiencies of 75%±1% across the visible spectrum in cells containing the thinnest layers. We find that due to the fast carrier tunneling process, the temporal response of these multilayer detectors is a direct measure of exciton dynamics. Response times of 720±50 ps are achieved, leading to a 3 dB bandwidth of 430±30 MHz. A summary of representative results obtained for both polymer and small molecule photovolta View full abstract»

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  • Writing optical waveguides in fused silica using 1 kHz femtosecond infrared pulses

    Page(s): 3724 - 3728
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    We have investigated the writing of waveguides in bulk pure fused silica glass with femtosecond Ti:Sapphire laser at 1 kHz repetition rate. The photoinduced tracks were characterized in terms of writing geometry (parallel and perpendicular), pulse duration (45 fs, 140 fs, and 200 fs), pulse energy (1–10 μJ), and translation speed (5–150 μm/s) of the sample. Under specific writing conditions, uniform buried waveguides with circular cross section, core diameter of 3–4 μm, and refractive-index change as large as 5×10-3 between core and cladding were achieved. © 2003 American Institute of Physics. View full abstract»

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  • I–V characteristics of the Langmuir probe in flowing afterglow plasmas

    Page(s): 3729 - 3746
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    The specific features of the probe I–V characteristics in flowing-afterglow plasmas are studied experimentally and in theory. As it was found at a probe potential equal to the plasma one, V=0, an electron concentration in a probe vicinity (and a probe current) is decreased due to a predominant outflow of the electrons into an electrical circuit of the probe from the probe vicinity. The expression allowing one to reconstruct the undisturbed-by-probe electron concentration from only experimental data is derived. The reconstructed values of the electron concentration enable one to find from the experiments the semiempiric expressions allowing to describe quantitatively the behavior of the probe I–V characteristics at the electron-attracting as well as at the ion-attracting potential, respectively. The expressions found (both for electron-attracting and ion-attracting potential) include the “separating length,” which merely is the Langmuir length with a factor equal to the square root of the electron mass over the ion mass ratio for two-component plasma. The intermediate part of the probe I–V characteristics is discovered for probes operating with afterglow plasmas. This intermediate part is described in terms of the experimental parameter L0 having a dimension of the length (presumably electron-orbital length). The value of the parameter L0 does not depend on plasma parameters to within the ranges of plasma parameter variations for experimentally investigated plasmas as it was found. The experiments were performed with two cylindrical probes of 10 and 25 μm diam and ∼3 mm lengths in the experimentally investigated ranges of the afterglow plasma parameters: 105cm-3≪n≪1010 cm-3, 0.017 eV≪kT≪0.3 eV, and 0.5 Torr≤p≤2 Torr of He gas pressure (corresponding to 950 μm≥λTe≥240 μm electron mean-free path) with (or without) Ar addition. © 2003 American Institute of Physics. View full abstract»

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  • High-accuracy determination of the dependence of the photoluminescence emission energy on alloy composition in AlxGa1-xAs films

    Page(s): 3747 - 3759
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    In an effort to improve the accuracy of photoluminescence (PL) measurements of the Al mole fraction (x) of AlxGa1-xAs alloys, the PL peak emission energy, EPL,peak, was measured at room temperature for molecular-beam epitaxy-grown AlxGa1-xAs films with 0≤x≪0.37, and correlated with independent measurements of x by in situ reflective high-energy electron diffraction (RHEED) and also by ex situ wavelength-dispersive x-ray spectroscopy in an electron microprobe analyzer (WDS/EMPA). The measurement uncertainty of EPL,peak was minimized through the following procedures: Accurate calibration of the photon energy (or wavelength) scale, correction of the measured spectra for the spectrometer response function, fitting the data with a well-chosen line shape function, and compensation for the effect of ambient temperature drift. With these procedures, the 2σ measurement uncertainty of EPL,peak was of the order 5×10-4eV for most samples. From correlation of the PL and WDS/EMPA composition data, the slope EPL,peak/∂x near room temperature was determined to be EPL,peak/∂x=(1.4017±0.0090 eV)-[(2.71±0.97)×10-4eV/K](T-298.3 K). Correlation with the RHEED data gave the same result within measurement uncertainty. Previously published measurements of- EPL,peak/∂x were reviewed and compared with the present study. The results of T. F. Kuech etal [Appl. Phys. Lett. 51, 505 (1987)], based on nuclear resonant reaction analysis of the Al mole fraction, were found to be in good agreement with the present study after the addition of a correction term to account for the sample temperature difference (T=2 K for Kuech etal, T=298 K for the present study). View full abstract»

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  • Temperature dependence of the properties of vapor-deposited polyimide

    Page(s): 3760 - 3764
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    The Young’s modulus and helium gas permeability of vapor-deposited poly(4,4-oxydiphenylenepyromellitimide) were measured at cryogenic and elevated temperatures (10–573 K). The Young’s modulus decreased with increasing temperature from 5.5 GPa at 10 K to 1.8 GPa at 573 K. The temperature dependency of the permeability followed the Arrhenius’ relationship, with different activation energy for permeation for samples imidized under different conditions. The effect of the imidization conditions on the permeation properties could be explained in terms of morphology/crystallinity as determined by x-ray diffraction techniques. Imidizing in air instead of nitrogen increased the permeability while lowering the activation energy for permeation and crystallinity. Imidizing at higher heating rates (in nitrogen) resulted in higher permeability, lower activation energy for permeation, and larger and fewer crystallites with better-aligned lattice planes. © 2003 American Institute of Physics. View full abstract»

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  • Local structures and damage processes of electron irradiated α-SiC studied with transmission electron microscopy and electron energy-loss spectroscopy

    Page(s): 3765 - 3775
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    Damaged structures of α-SiC below and above the critical temperature of amorphization (Tc) under high-energy electron irradiation were studied by means of transmission electron microscopy and electron energy-loss spectroscopy. Above Tc, crystal fragmentation takes place due to local lattice strains caused by preferential displacements, subsequent outward diffusion of carbon atoms and formation of silicon nano-clusters. On the other hand, the amorphous structure formed below Tc can be well characterized by the formation of Si–Si, Si–C, and sp3 C–C covalent bonds with the tetrahedral coordination locally retained and uniformly distributed. The primary amorphization process under electron irradiation can be interpreted by the defect-accumulation model, in which displaced atoms are frozen at interstitial sites before long-distance diffusion by reconstructing the surrounding structure to relax the local strains. Accordingly the amorphization process is controlled essentially by the mobility of displaced carbon and silicon atoms, and chemical disordering seems to play a minor role in triggering the amorphization. A key issue for irradiation induced volume swelling of amorphous SiC is also presented. © 2003 American Institute of Physics. View full abstract»

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  • Hydrogen structures and the optoelectronic properties in transition films from amorphous to microcrystalline silicon prepared by hot-wire chemical vapor deposition

    Page(s): 3776 - 3783
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    Transition films from amorphous (a-) to microcrystalline (μc-) silicon were prepared by hot-wire chemical vapor deposition using silane decomposition with either varied hydrogen-to-silane ratio, R, or with fixed R=3 but a varied substrate temperature, Ts. Raman results indicate that there is a threshold for the structural transition from a- to μc-Si:H in both cases. The onset of the structural transition is found to be R≈2 at Ts=250 °C and Ts≈200 °C at R=3. The properties of the material were studied by infrared absorption, optical absorption, photoluminescence (PL), and conductivity temperature dependence. We observed that the peak frequency of the SiH wag mode remains at 630-640 cm-1 for all the films, but the hydrogen content shows two regimes of fast and slow decreases separated by the onset of microcrystallinity. When microcrystallinity increased, we observed that (a) the SiO vibration absorption at 750 cm-1 and 1050-1200 cm-1 appeared, (b) the relative intensity of the 2090 cm-1 absorption increased, (c) the low-energy optical absorption at photon energy ≪1.4 eV increased one to two orders of magnitude, (d) the low-energy PL band at ∼1.0 eV emerged with a decrease of total PL intensity, and (e) the conductivity activation energy decreased. The aforementioned changes correlated well with the crystallinity of the material. We attribute the observations mainly- to the formation of the c-Si gain boundaries during crystallization. © 2003 American Institute of Physics. View full abstract»

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  • Electromagnetic enhancement effect in scanning tunneling microscope light emission from GaAs

    Page(s): 3784 - 3788
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    The electromagnetic enhancement effect in scanning tunneling microscope (STM) light emission from GaAs has been investigated by the finite difference time domain method. We have calculated the intensity of light emitted by the recombination of minority carriers injected from the tip and majority carriers in the sample. The results depend not only on the material and the shape of the tip but also on light polarization. When the tip is tungsten whose dielectric function has a positive real part at 1.5 eV, the p-polarized light intensity depends strongly on the size of the tip and the location of recombination. However, the s-polarized light emission depends only weakly on these parameters. If the tip is a perfect metal, the p-polarized light intensity becomes a few times stronger than that for the W tip of the same shape. On the other hand, the s-polarized light intensity becomes weaker than that for the W tip due to the electromagnetic screening effect of a perfect metal. We conclude that the combination of the W tip and s-polarized light detection is suitable for precise measurement of the tip-position dependence of STM light emission properties. A tip with a negative value of the real part of dielectric function is preferable to the W tip, if one does not analyze the tip-position dependence of STM light emission, because STM light emission is enhanced relative to the case of the W tip. © 2003 American Institute of Physics. View full abstract»

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  • Ab initio elasticity of chalcopyrites

    Page(s): 3789 - 3795
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    A critical review of the experimental data of elastic stiffness constants of AgGaX2 and CuInX2 (X=S,Se) chalcopyrites is given. Using the ab initio pseudopotential method, their elastic constants have been calculated from strain–energy and from strain–stress relationships. On the basis of the linear elasticity approximation, axial and total compressibilities as well as elastic anisotropy factors have been derived. The calculated results are compared with available experimental data and previous calculations based on phenomenological models. © 2003 American Institute of Physics. View full abstract»

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  • Correlation of stress and texture evolution during self- and thermal annealing of electroplated Cu films

    Page(s): 3796 - 3804
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    Electroplated Cu films with different plating conditions and thicknesses are characterized to study the stress and texture evolution involved in the transition of the microstructure during self-annealing. Grain growth during the process induces stress in the film in the tensile direction. Degradation of the (111) texture and enhancement of the (200) texture are observed in the films during the microstructural transition. The rate of self-annealing increases dramatically as the plating current density increases. Self-annealing is significantly retarded in thinner films and in films plated with pulse reverse current. A strong correlation between stress and texture development is demonstrated for all electroplated Cu films under self- and thermal annealing conditions. This correlation is explained by the surface/interface energy and strain energy in anisotropic metal films. Due to the mechanical anisotropy of Cu, the orientation with the lowest total energy changes from (111) to (200) as the dominant energy that determines the film texture changes from surface/interface related to strain related. The texture remains unchanged until the biaxial stress in the film exceeds a critical value. Above the level of critical stress, the (111) texture starts to degrade and the (200) starts to become enhanced, which reduces the strain energy in the film. Different amounts of stress develop in Cu films during thermal annealing depending on the barrier layers; the stress is largest with TaSiN, medium with Ta and smallest with TaN. This suggests that film stress and texture are also affected by the Cu/barrier interface bond strength. © 2003 American Institute of Physics. View full abstract»

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  • Study of the elastic and elastooptic properties of Zn1-xBexSe solid solutions by Brillouin spectroscopy

    Page(s): 3805 - 3810
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    We report experimental results concerning the elastic and elastooptic properties of Zn1-xBexSe solid solutions grown by the high pressure Bridgman technique. The elastic and elastooptic constants of Zn1-xBexSe crystals with different Be content have been determined at room temperature by the Brillouin scattering method. © 2003 American Institute of Physics. View full abstract»

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  • Optical properties of Bi3.25La0.75Ti3O12 thin films using spectroscopic ellipsometry

    Page(s): 3811 - 3815
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    Using spectroscopic ellipsometry, the optical properties of the Bi3.25La0.75Ti3O12 (BLT) thin films with different film thicknesses on platinized silicon substrates prepared by chemical solution methods have been investigated in the 400–1700 nm wavelength range. By fitting the measured ellipsometric parameter (Ψ and Δ) data with a multilayer model system and the classical dispersion relation for the BLT thin films, the optical constants and thicknesses of the thin films have been obtained. Meanwhile, some degree of inhomogeneity (in the form of low density layers or surface roughness) has been found in the BLT thin films except for the thinnest film, which was homogeneous throughout its thickness. The multilayer model system became very complicated with increasing thickness. It indicated that the microstructure of the BLT thin films varied with increasing thickness. The refractive index n of the BLT thin films increases with increasing thickness, and on the contrary, the extinction coefficient k decreases with increasing thickness. The dispersion of the refractive index can be well explained by a single-term Sellmeier relation. © 2003 American Institute of Physics. View full abstract»

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  • Structural and optical properties of molybdenum–tungsten mixed oxide thin films deposited by the sol-gel technique

    Page(s): 3816 - 3822
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    Sol-gel derived molybdenum–tungsten (Mo–W) mixed oxides were investigated by transmission electron microscopy in order to study the dependence of their structural and morphological properties on the relative percentage of the constituent compounds. Optical measurements were performed in the 200–800 nm wavelength range to correlate thin film structure and optical properties. Moreover, the values of the band gap energy were estimated as a function of the percentage of the W precursors added in the solution. © 2003 American Institute of Physics. View full abstract»

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  • Extinction ratio improvement of an electroabsorption modulator by using Be as a p-type dopant with small diffusion coefficient

    Page(s): 3823 - 3826
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    Beryllium (Be) has been applied as an acceptor in the p-type cladding layer and the contact layer of an InGaAsP-based electroabsorption modulator (EAM) fabricated by metalorganic chemical vapor deposition (MOCVD). The concentration of diffused dopants in the absorption layer of the EAM was significantly reduced by employing Be instead of zinc (Zn) which has been conventionally used as an acceptor for MOCVD-grown III–V semiconductors. Numerical simulation of the diffusion of Be and Zn in the EAM device structure showed that the diffusion of Be was dominated by interstitials with small diffusion coefficient, while the diffusion of Zn was dominated by a fast diffusion mechanism of a divacancy-related complex which was not observed in the Be-doped EAM. The measured extinction ratio of the Be-doped EAM was higher by approximately 40% than that of the Zn-doped counterpart. The improvement of the extinction ratio is due to improved uniformity of the electric field applied to the absorption layer of the EAM. © 2003 American Institute of Physics. View full abstract»

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  • Linear and nonlinear optical properties of cobalt-doped zinc aluminum glass ceramics

    Page(s): 3827 - 3831
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    Optical properties of cobalt-doped zinc aluminosilicate transparent glass ceramics in the visible – near-infrared (IR) spectral range have been studied. It was shown that the absorption and luminescence properties of these glass ceramics are defined mainly by tetrahedrally coordinated Co2+ ions located in ZnAl2O4 gahnite nanocrystals formed in the glass ceramic matrix. The luminescence in the visible and near IR was assigned to the transitions from the 4T1(4P) to the 4A2 and 4T2 energy levels of tetrahedrally coordinated Co2+ ion. Absorption saturation and bleaching relaxation under excitation of the 4A24T1(4F) transition were studied. Luminescence decay and bleaching relaxation times lie in the ranges 25–200 and 500–800 ns, respectively, depending on the Co concentration. The value of the ground-state absorption cross section of tetrahedrally coordinated Co2+ ions in zinc aluminosilicate transparent glass ceramics was estimated to be 2.0×10-19cm2 at the wavelength of 1.54 μm. © 2003 American Institute of Physics. View full abstract»

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  • Optical properties of SiGe alloys

    Page(s): 3832 - 3836
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    The optical properties of Si1-xGex have been investigated theoretically using a full-potential linear muffin-tin-orbital method. We present the density-of-states as well as the real and imaginary parts of the dielectric function. The calculated dielectric function was found to be in good agreement with the spectroscopic ellipsometry measurements by J. Bahng etal, J. Phys.: Condens. Matter 13, 777 (2001), and we obtained a static dielectric constant of Є0=12.19+2.45x in the Si rich regime (x≤0.5). © 2003 American Institute of Physics. View full abstract»

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  • Epitaxial growth of high quality ZnO:Al film on silicon with a thin γ-Al2O3 buffer layer

    Page(s): 3837 - 3843
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    ZnO:Al thin films were grown epitaxially on epi-γ-Al2O3/Si (111) substrates by rf sputtering and pulsed laser deposition. The γ-Al2O3 buffer layer was deposited on Si (111) at a low substrate temperature of 500 °C using the metalorganic chemical vapor deposition method. Reflection high energy electron diffraction and x-ray diffraction measurements indicated a near alignment of the ZnO:Al epilayer on γ-Al2O3/Si (111) as compared to those grown directly on Si (111). Atomic force microscopy results of the films ZnO:Al/γ-Al2O3/Si (111) deposited by pulsed laser deposition revealed a smoother surface in comparison with the films deposited by rf sputtering. The M band observed in the photoluminescence spectra of the films deposited by laser ablation suggests that high quality epitaxial ZnO:Al/γ-Al2O3/Si (111) films can be deposited by pulsed laser deposition. © 2003 American Institute of Physics. View full abstract»

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  • Monte Carlo simulation of x-ray spectra generated by kilo-electron-volt electrons

    Page(s): 3844 - 3851
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    We present a general algorithm for the simulation of x-ray spectra emitted from targets of arbitrary composition bombarded with kilovolt electron beams. Electron and photon transport is simulated by means of the general-purpose Monte Carlo code PENELOPE, using the standard, detailed simulation scheme. Bremsstrahlung emission is described by using a recently proposed algorithm, in which the energy of emitted photons is sampled from numerical cross-section tables, while the angular distribution of the photons is represented by an analytical expression with parameters determined by fitting benchmark shape functions obtained from partial-wave calculations. Ionization of K and L shells by electron impact is accounted for by means of ionization cross sections calculated from the distorted-wave Born approximation. The relaxation of the excited atoms following the ionization of an inner shell, which proceeds through emission of characteristic x rays and Auger electrons, is simulated until all vacancies have migrated to M and outer shells. For comparison, measurements of x-ray emission spectra generated by 20 keV electrons impinging normally on multiple bulk targets of pure elements, which span the periodic system, have been performed using an electron microprobe. Simulation results are shown to be in close agreement with these measurements. © 2003 American Institute of Physics. View full abstract»

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  • High pressure and high temperature phase transformations in LiNbO3

    Page(s): 3852 - 3858
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    A behavior of LiNbO3 under high pressure and temperature has been studied up to 90 GPa by means of high pressure in situ x-ray observations. Recovered samples were analyzed by transmission electron microscope (TEM). When the LiNbO3 was compressed at room temperature, a transformation occurred at about 25 GPa. The powder x-ray diffraction pattern of this “room-temperature and high-pressure” (RT–HP) phase was successfully explained by the NaIO3-type structure. No further transformation was observed at room temperature up to 90 GPa and reverse transition to starting phase occurred at about 10 GPa, thus this phase was unquenchable on release of pressure. When this RT–HP phase was heated at above 30 GPa, a phase appeared which can be recovered to ambient condition. X-ray diffraction and TEM analysis of this “high-temperature and high-pressure” (HT–HP) phase clarified that this phase has hexagonal symmetry with a most likely space group of P63. The quenched sample reverts to the starting phase on heating above 650 K. This HT–HP phase is opaque, suggesting the change of electronic property. The density of these RT–HP and HT–HP phases are, respectively, 21% and 23% higher compared to the starting LiNbO3 phase at ambient condition. © 2003 American Institute of Physics. View full abstract»

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  • Crack formation in GaAs heteroepitaxial films on Si and SiGe virtual substrates

    Page(s): 3859 - 3865
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    We have determined the critical cracking thickness, or the thickness beyond which crack formation is favored, in GaAs films grown on Si and SiGe virtual substrates analytically and experimentally. The analytical model predicts a critical cracking thickness proportional to the biaxial modulus and the crack resistance of the GaAs film, and inversely proportional to the square of the thermal stress and a nondimensional crack resistance number Z. This Z number is determined by the mechanical properties of the GaAs film for a system without substrate damage, and is also determined by the mechanical properties of the substrate for a system with substrate damage. The experimentally determined critical thicknesses were in general greater than the analytically derived values due to the kinetic barriers to crack nucleation, which were not taken into consideration in the models. In addition, we have observed an asymmetric crack array formation, where arrays running in the 〈110〉 substrate off-cut direction are favored. We have also performed finite element modeling of the crack systems to study the evolution of thermal stress around crack planes in the GaAs film. © 2003 American Institute of Physics. View full abstract»

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  • Model for interface formation and the resulting electrical properties for barium–strontium–titanate films on silicon

    Page(s): 3866 - 3872
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    The interface formation between sputtered barium strontium titanate (BST) films and both Si and SiO2 substrate surfaces has been followed using real-time spectroscopic ellipsometry and the mass spectrometry of recoiled ions. In both substrates an intermixed interface layer was observed and subcutaneous Si oxidation occurred. A model for the interface formation is proposed in which the interface includes an SiO2 film on Si, and an intermixed film on which is pure BST. During the deposition of BST the interfaces films were observed to change in time. Electrical characterization of the resulting metal–BST interface capacitors indicates that those samples with SiO2 on the Si surface had the best electrical characteristics. © 2003 American Institute of Physics. View full abstract»

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  • Mechanism of the Yb–Er energy transfer in fluorozirconate glass

    Page(s): 3873 - 3880
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    The mechanism of the Yb3+Er3+ energy transfer as a function of the donor and the acceptor concentration was investigated in Yb3+Er3+ codoped fluorozirconate glass. The luminescence decay curves were measured and analyzed by monitoring the Er3+(4I11/2) fluorescence induced by the Yb3+(2F5/2) excitation. The energy transfer microparameters were determined and used to estimate the Yb–Er transfer rate of an energy transfer process assisted by excitation migration among donors state (diffusion model). The experimental transfer rates were determined from the best fitting of the acceptor luminescence decay obtained using a theoretical approach analog to that one used in the Inokuti–Hirayama model for the donor luminescence decay. The obtained values of transfer parameter gamma [γ(exp)] were always higher than that predicted by the Inokuti–Hirayama model. Also, the experimental transfer rate, γ2(exp), was observed to be higher than the transfer rate predicted by the migration model. Assuming a random distribution among excited donors at the initial time (t=0) and that a fast excitation migration, which occurs in a very short time (t≪γ-2), reducing the mean distance between donor (excited) and acceptor, all the observed results could be explained. © 2003 American Institute of Physics. View full abstract»

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  • On the diffusion of lattice matched InGaAs/InP microstructures

    Page(s): 3881 - 3885
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    Photoluminescence and high-resolution x-ray diffraction (HRXRD) studies of the diffusion in lattice matched InGaAs/InP quantum wells show that at high temperatures intermixing can be modeled by Fick’s law with an identical diffusion rate for both the group III and group V sublattices. This results in materials that remain lattice matched for all compositions created by the diffusion. At lower temperatures, the photoluminescence shows that the diffusion process changes and HRXRD shows that strained layers are produced within the structure. This may be due to the presence of the miscibility gap within the InGaAsP phase diagram. © 2003 American Institute of Physics. View full abstract»

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  • Wetting of Si surfaces by Au–Si liquid alloys

    Page(s): 3886 - 3892
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    The behavior of liquid Au–Si alloys on Si surfaces covered by a monolayer of gold has been investigated by ultrahigh-vacuum scanning electron microscopy. On the (111) surface, the alloy displays a constant contact angle with the surface from the eutectic temperature up to a temperature of 650 °C and thereafter the contact angle increases linearly with temperature. As observed in previous work, the shape of the liquid droplets changes from circular at lower temperature to hexagonal at higher temperature. In contrast, on the (100) surface, the contact angle increases linearly from the eutectic temperature to high temperature. The behavior of the shape of the droplets is, however, reversed: it is polygonal (octagonal) at lower temperature and becomes round at higher temperature. This behavior is explained in terms of the relative surface energy of the two surfaces and changing line tension of the liquid–solid–vapor phase line. In addition, the behavior of Au–Si droplets on vicinal and patterned surfaces of Si has been examined. The droplets cause step bunching and modify the local surface structure. Solidification of the droplets on all surfaces leads to phase separation. © 2003 American Institute of Physics. View full abstract»

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Journal of Applied Physics is the American Institute of Physics' (AIP) archival journal for significant new results in applied physics

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P. James Viccaro
Argonne National Laboratory