bluuebunny/crossref_metadata_2025 huggingface dataset:
Imaging InGaAsP quantum-well lasers using near-field scanning optical microscopy
Authors: Buratto S. K. , Hsu J. W. P. , Trautman J. K. , Betzig E. , Bylsma R. B. , Bahr C. C. , Cardillo M. J.
Published: July 2002

The application of near-field scanning optical microscopy (NSOM) to the characterization of InGaAsP multiquantum-well lasers is reported. Collection mode images are collected at varying drive currents from well below to well above the threshold current. The high resolution of NSOM (∼λ/20) provides a detailed mapping of the laser output from the active region as well as additional output from the surrounding mesa. Spectral analysis of the image shows that the extra emission is due to InP electroluminescence. In addition to the emission characteristics of the laser it is also possible to detect local heating of the laser facet via thermal expansion. Topographical images are achieved simultaneously with NSOM images by digitizing the feedback signal which maintains a constant tip-surface gap. It is shown that these data have direct implications on device performance and problems associated with carrier leakage and nonradiative defects.
Temperature dependence of photoluminescence in modulation-doped pseudomorphic high electron mobility transistor AlxGa1−xAs/ InyGa1−yAs/GaAs structures
Authors: Yu P. W. , Jogai B. , Rogers T. J. , Martin P. A. , Ballingall J. M.
Published: July 2002

We report the temperature-dependent characteristics of photoluminescence emission in modulation δ-doped AlxGa1−xAs/InyGa1−yAs/GaAs structures. Transition energies are analyzed using a self-consistent solution of the coupled k⋅P Hamiltonian–Poisson equation. At low temperatures, dominant emissions are due to the transitions from the first electron subband to the first heavy-hole subband and from the second electron subband to the first heavy-hole subband irrespective of the location of modulation doping. The second hole subband related transitions associated with the first electron subband or the second electron subband emerges with increasing temperature depending on the location of doping. The relative intensities of the transitions from the first and second electron subbands to the first heavy-hole subband transitions are analyzed as a function of the Fermi energy position. An excellent agreement is found between the measurements and calculations.
Properties and measurement of scanning tunneling microscope fabricated ferromagnetic particle arrays (invited)
Authors: Kent A. D. , Molnár S. von , Gider S. , Awschalom D. D.
Published: July 2002

The low temperature magnetic properties of arrays of scanning tunneling microscope (STM) fabricated ferromagnetic particles have been studied as a function of their dimension using a novel high sensitivity Hall magnetometer. Iron deposits with controlled shape and nanometer scale diameters (∼25 nm) are formed using a STM to decompose a metalorganic precursor [Fe(CO)5] in the active area of the measurement device. The hysteresis loops change significantly in going from nearly isotropic to oriented high aspect ratio (6:1 length to diameter) filamentary particles. In particles of intermediate aspect ratio and diameter the largest coercive field of 2.7 kOe is observed. This behavior as well as the characteristics of the Hall magnetometer (spin sensitivity of 10−14 emu/ Hz1/2) are described.
Electrical characteristics of (100), (111), and randomly aligned lead zirconate titanate thin films
Authors: Kim Chang Jung , Yoon Dae Sung , Lee Joon Sung , Choi Chaun Gi , Lee Wong Jong , No Kwangsoo
Published: July 2002

(100), (111), and randomly aligned lead zirconate titanate thin films on Pt/Ti/Corning 7059 glass substrates were prepared using a sol-gel method. The thin films, having different alignments, were fabricated by different drying conditions for pyrolysis. The hysteresis loop and the capacitance-voltage characteristics were investigated using a standardized ferroelectric test system. The dielectric constant and the current-voltage characteristics of the films were investigated using an impedance analyzer and a pA meter, respectively. The microstructure was investigated using scanning electron microscopy. The (100) aligned film showed a relatively larger dielectric constant than the (111) and the randomly aligned films. The films aligned in particular directions showed higher hysteresis parameters than the randomly aligned film. The leakage current densities of the films aligned in particular directions were lower than that of the randomly aligned film.
Oxygen loss during thermal donor formation in Czochralski silicon: New insights into oxygen diffusion mechanisms
Authors: McQuaid S. A. , Binns M. J. , Londos C. A. , Tucker J. H. , Brown A. R. , Newman R. C.
Published: July 2002

As-grown Czochralski silicon samples with different oxygen concentrations have been heated at temperatures in the range 350–500 °C. Oxygen loss during anneals at low temperatures (T≤400 °C) is shown to follow second-order kinetics and measurements led to values of oxygen diffusivity that were larger than normal by a factor of ∼3, assuming the capture radius for dimer formation was 5 Å. Variations in the rate of [Oi] loss during more extended anneals could be explained if oxygen diffusion was initially enhanced but tended to its normal value as the anneals progressed. Much greater initial enhancements were derived from similar measurements for samples which had been hydrogenated by a heat treatment in H2 gas at 1300 °C for 30 min followed by a rapid quench to room temperature, and the enhancements were consistent with values derived from measurements of the relaxation of stress-induced dichroism. At higher temperatures (T≥450 °C) the measured rates of [Oi] loss were less than the expected rate of Oi-Oi interaction and tended to vary with increasingly high powers of [Oi]. Modeling of the clustering process demonstrated that the reductions could be explained if the oxygen dimers were present in a quasiequilibrium concentration throughout the anneals. The establishment of this equilibrium appears to require that oxygen dimers diffuse much more rapidly than isolated Oi atoms. The kinetics of oxygen loss over the whole range of temperatures can then be explained if dimer clustering leads mainly to increases in concentrations of agglomorates containing large numbers (≥8) of oxygen atoms. It is therefore possible to account for thermal donor (TD) formation based on the formation of different sizes of oxygen clusters, although the possibility that self-interstitials are involved in TD formation is not excluded.
Plasma expansion dynamics in reactive and inert atmospheres during laser ablation of Bi(2)Sr(2)Ca(1)Cu(2)O(7−y)
Authors: Gonzalo J. , Vega F. , Afonso C. N.
Published: July 2002

The dynamics of the species ejected by excimer laser ablation of a BiSrCaCuO target in different gas environments is studied by spatially resolved, real-time optical emission spectroscopy. The evolution of the velocity and the emission intensity of the excited species versus the distance and the pressure of the foreign gas present a similar behavior both in reactive (oxygen) and inert (argon) environments. Furthermore, the results show that the plume expansion process is dominated by the interaction of the ejected species and the gas background atoms or molecules through collisional interactions rather than by reactions in the gas phase. The dynamics of the plume expansion is analyzed in the frame of the shock wave and drag models, the latter leading to a very good agreement with the experimental results and the dependence of the slowing coefficient with the gas pressure is established. Finally, the influence of the excitation energy of the considered transition on the observed emission features is discussed.
Lattice matched GaAs/Sc0.3Er0.7As/GaAs heterostructures grown on various substrate orientations
Authors: Zhu Jane G. , Palmstro/m Chris J. , Carter C. Barry
Published: July 2002

Lattice-matched heterostructures of GaAs/Sc0.3Er0.7As/GaAs have been grown on GaAs substrates with various orientations by molecular beam epitaxy and characterized by transmission electron microscopy. The Sc0.3Er0.7As layer usually has good epitaxy on GaAs; the purpose of this study is to investigate the overgrowth of GaAs on Sc0.3Er0.7As. In comparison with the samples grown on nominal (100) substrates, the epitactic growth of GaAs on Sc0.3Er0.7As is slightly improved on a vicinal substrate oriented (100) 6° off towards (111)A. {311} and {211} are shown to be preferred orientations for the epitactic growth of GaAs/Sc0.3Er0.7As/GaAs heterostructures. The epilayers grown on {311} and {211} substrates are epitactically well aligned, and the density of planar defects (stacking faults and microtwins) in the overgrown GaAs layer is significantly reduced. It is suggested that the mixed {111} and {100} character surfaces assist the nucleation of GaAs on Sc0.3Er0.7As. Stacking faults and microtwins are the major defects for the epilayers grown on (110) substrates. For samples grown on (111)B-oriented substrates, twinning at the interfaces occurs frequently even when selftwinning inside each material is eliminated.
Pulsed laser deposition of diamond-like amorphous carbon films from graphite and polycarbonate targets
Authors: Voevodin A. A. , Laube S. J. P. , Walck S. D. , Solomon J. S. , Donley M. S. , Zabinski J. S.
Published: July 2002

Amorphous carbon (a-C) and amorphous hydrogenated carbon (a-C:H) films are produced by 248 nm pulsed laser ablation of graphite and polycarbonate targets in high vacuum conditions. Plasma plumes generated by target irradiation with different laser fluences are investigated with laser induced fluorescence spectroscopy and an electrostatic probe. Ions of C2+ with kinetic energies of several hundred eV are detected in the leading edge of the plasma plumes from both targets. These energetic species are proposed to be responsible for the formation of film structures corresponding to diamond-like carbon (DLC), as it is found from electron-energy-loss spectroscopy (EELS) and Raman investigations of 0.5 μm films deposited onto steel substrates. The validation of a laser wavelength/fluence region for DLC formation found earlier for graphite targets is discussed and expanded to polycarbonate targets. An increase in laser fluence leads to higher percentages of sp3 bonds in the a-C and a-C:H films. For the a-C:H films, the incorporation of large molecular conglomerates ejected from polycarbonate targets results in the formation of unique heterogeneous structures revealed from scanning electron microscopy (SEM) studies. The embedded conglomerates cause a decrease in the a-C:H film hardness to 15 GPa, in comparison to 60 GPa for the a-C films.
Transport and photoluminescence of hydrogenated amorphous silicon–carbon alloys
Authors: Chu V. , Conde J. P. , Jarego J. , Brogueira P. , Rodriguez J. , Barradas N. , Soares J. C.
Published: July 2002

The optoelectronic properties of hydrogenated amorphous silicon–carbon alloys (a-SiC:H) are studied over the entire compositional range of carbon content. The films are prepared using radio-frequency glow discharge and optimization was made with respect to deposition power and pressure, hydrogen dilution, and methane (or ethylene) -to-silane gas phase ratio. Regardless of the deposition conditions and source gases used, the optical, structural, and transport properties of the a-SiC:H alloys followed simple universal dependencies related to changes in the density of states associated with their structural disorder. The Urbach tail energy Eu and the B factor of the Tauc plot correlate with E04 (defined as the energy at which the absorption coefficient is equal to 104 cm−1) taken from photothermal deflection spectroscopy measurements. Up to E04pds≊2.6 eV, Eu increases monotonically from 50 up to ≊200 meV, while the B factor decreases from ≊800 down to ≊200 cm−1/2 eV−1/2. Above E04pds≊2.6 eV, both Eu and B remain approximately constant. The photoconductivity decreases exponentially with E04pds and is below 10−10 Ω−1 cm−1 for E04pds≥2.6 eV. Room-temperature photoluminescence is observed when E04pds≥2.6 eV. The photoluminescence peak position lies an average of 0.6 eV below the value of E04pds and increases linearly with decreasing value of the B factor of the Tauc plot.
1.54 μm photoluminescence of in situ erbium-oxygen co-doped silicon films grown by ion-beam epitaxy
Authors: Matsuoka Morito , Tohno Shun-ichi
Published: July 2002

Erbium-doped silicon films are grown by ion-beam epitaxy using an electric-mirror sputtering-type metal ion source in an ultrahigh vacuum. In situ erbium doping with concentrations ranging from 1×1016 to 6×1020 cm−3 is achieved by sputtering the erbium metal pellet with ions extracted from the silicon metal ion source. The oxygen concentration in the films, which is closely related to the effective luminescence of erbium in silicon, is also controlled in situ over the range from below 1×1018 to 2×1020 cm−3 by using argon gases containing 1 ppb–100 ppm of oxygen impurities. The erbium incorporation efficiency drastically increases (by two or more orders of magnitude) when oxygen is contained in the argon gas during film growth. Erbium segregation is well suppressed by the oxidation. Photoluminescence with a wavelength of 1.54 μm is clearly observed in as-deposited films grown typically at 500 °C with argon gas containing 5 ppm of oxygen. The maximum luminescence intensity is obtained at an erbium concentration of about 2×1018, above which the intensity drastically decreases. This decrease is probably caused by degradation of the crystallinity of host silicon film or the formation of erbium precipitation.
A two-dimensional particle-in-cell simulation of an electron-cyclotron-resonance etching tool
Authors: Ashtiani K. A. , Shohet J. L. , Hitchon W. N. G. , Kim G.-H. , Hershkowitz N.
Published: July 2002

A particle-in-cell (PIC) simulation of an axisymmetric electron-cyclotron-resonance (ECR) etching tool is developed in which up to 2×106 particles per species are loaded in a two-dimensional spatial computational mesh (r,z), along with three velocity components (vr,vθ,vz). An ECR heating scheme based on single-particle trajectories in the resonance zone generates the simulated plasma. Electron- and ion-neutral elastic and inelastic collisions are treated by a null Monte Carlo collision method. The code generates the electron and ion-velocity distributions, plasma potentials, and densities in a CF+3/CF4 etching plasma. In addition, a novel scaling technique which bridges the gap between the ion and electron-time scales and accelerates the rate of convergence of the code is introduced for a PIC code. The predictions of the code show that microwaves are completely absorbed before reaching the exact location of resonance.
Calculation of the magnetic susceptibility and the Verdet constant in neodymium trifluoride
Authors: Liu Gongqiang , Yuan Bin , Zhang Ninggao , Gong Xiaoyan
Published: July 2002

This paper first considers the effects of multielectron interaction, L−S interaction and the weak crystal-field on the 4f3 ground state of Nd3+ ion in the paramagnetic medium NdF3; then, further takes account of the splitting of the crystal-field ground levels caused by both the effective superexchange field Hv and the applied field He, and calculates quantitatively the temperature dependence of the magnetic susceptibility χ in NdF3. For the splitting of 4f25d excited state of Nd3+ ion in NdF3, the paper investigates the effect of the strong crystal-field on the 5d electrons. Using the model of three-level transition, the specific Faraday rotation θF, the Verdet constant V, and their temperature dependence, which originate from the electronic transitions between the electron configurations 4f3 and 4f25d, are calculated quantitatively. The theoretical calculations show that the superexchange interaction between Nd3+ ions has an important effect on the magneto-optical properties in NdF3, and both V−1 and χ−1 are linearly dependent on T in the temperature range 70 K<T<300 K. The theory is in good agreement with the experimental results.
Estimation of optimum electron temperature for maximum x-ray laser gain from 3p–3s transitions of neonlike ions in laser plasmas
Authors: Gupta G. P. , Sinha B. K.
Published: July 2002

The fraction of neonlike ions and the average ionic charge state in laser plasmas formed at the surface of the solid targets of germanium, selenium, and krypton have been obtained at several electron temperatures using the steady-state collisional radiative ionization model. Dielectronic recombination has been considered. The effect of consideration of dielectronic recombination of ions of one or more charge states on these parameters has been studied. It is observed that the values of electron temperature corresponding to the maximum abundance of neonlike ions in different cases of dielectronic recombination are significantly different from those obtained without consideration of dielectronic recombination. The optimum electron temperature of the plasma is estimated from the maximization of the x-ray laser gain through temperature-dependent variables of the gain coefficient and is observed to be the electron temperature at which the abundance of neonlike ions is maximum.
Zero-dimensional hybrid model for analysis of discharge excited XeCl lasers
Authors: Lamrous O. , Gaouar A. , Yousfi M.
Published: July 2002

A powerful zero-dimensional hybrid model to study the positive column of a glow discharge used as an excitation medium for XeCl lasers is presented. This model was employed using a numerical code including three strongly coupled parts: electric circuit equations (electric model), electron Boltzmann equation (particle model), and kinetics equations (chemical kinetics model). From this hybrid model, kinetics and electrical parameters of Ne–Xe–HCl laser discharge mixtures have been discussed and analyzed. Calculated discharge current and voltage are also compared with available theoretical and experimental results. The good qualitative agreement observed shows the validity of the present model which can used as an efficient tool for the investigation of the homogeneous excimer laser discharge.
Axial distributions of metastable atoms and charged particles in an ultrahigh frequency argon plasma column at moderate pressures
Authors: Benova E. , Petrova Ts. , Blagoev A. , Zhelyazkov I.
Published: July 2002

The axial distributions of electrons, atomic and molecular ions as well as metastable atoms in an argon ultrahigh frequency (UHF) discharge at a moderate pressure sustained by a traveling electromagnetic wave have been theoretically obtained. The dependence between the absorbed wave power and the electron density necessary for plasma modeling has been numerically derived by taking into account both direct and step-wise ionization processes. The inclusion of step-wise ionization process leads to a lower electron temperature comparing to that calculated under the assumption that only the direct ionization takes place. The results have been compared with the experimental data for an UHF argon plasma at 1.8 Torr. A better agreement is achieved when the ionization by metastable–metastable collisions is taken into account together with the step-wise ionization.
Role of epitaxy and polycrystallinity in the magnetoresistance and magnetization of La0.8Sr0.2MnO3 thin films
Authors: Krishnan Kannan M. , Modak A. R. , Lucas C. A. , Michel R. , Cherry H. B.
Published: July 2002

La0.8Sr0.2MnO3 thin films were simultaneously deposited by pulsed laser ablation on silicon (Si) and LaAlO3 (LAO) substrates. Films on Si were polycrystalline while those on LAO were (100) epitaxial with an in-plane correlation length of ≊10 nm. The magnetization and magnetoresistance behavior of these two films were significantly different. Both films exhibit antiferromagnetic–ferromagnetic transitions—at different temperatures [180 K (LAO); 230 K (Si)]—and their magnetic moments at 10 K were significantly different (Si—0.0035 emu; LAO—0.0022 emu). However, both films showed significant high field slope in magnetization at 10 K. Significant fractions of both films remain antiferromagnetic at low temperatures and hence net susceptibilities, dependent on the direction of the applied magnetic field, are different for the epitaxial (LAO) and randomly oriented polycrystalline (Si) films. The magnetoresistance peak, corresponding to the semiconductor–metal transition is observed at 170 and 130 K for the epitaxial (LAO) and polycrystalline (Si) films, respectively. Moreover, their resistance values are two orders of magnitude different (Si—MO/hms; LAO—KOhms). These properties can be interpreted in terms of the major role of grain boundaries in determining the scattering as well as possible differences in O2 stoichiometry.
Metastable optical gratings in compound semiconductors
Authors: Nolte D. D.
Published: July 2002

Spatially nonuniform optical illumination of semiconductors containing metastable defects, such as GaAs:EL2 and DX-related defects in III–V and II–VI semiconductors, induces metastable photorefractive and free-carrier gratings. Detailed calculations of the space-charge gratings in these materials are made as functions of fringe spacing and optical exposure. In both defect cases a unique nonequilibrium Fermi level, obtained from a charge balance approximation, can be defined that describes the spatial charge transfer and the resulting modulation of the optical properties of the semiconductor, without resorting to multiple quasi-Fermi levels. The process of writing holographic gratings with low intensities is analogous to adiabatic modulation doping. The transfer of charge from photoquenched regions to nonphotoquenched regions determines the highest spatial resolution that can be supported in the materials. In the case of EL2, the modulation doping produces a p-i-p-i doped superlattice. The DX defects support smaller Fermi level modulation, but have larger optical effects based on persistent photoconductivity and free-carrier gratings.
Intrawell and interwell intersubband transitions in multiple quantum wells for far-infrared sources
Authors: Smet Jurgen H. , Fonstad Clifton G. , Hu Qing
Published: July 2002

A theoretical study of electrically pumped unipolar lasers exploiting intrawell or interwell intersubband radiative transitions in multiple quantum-well heterostructures for the generation of IR and FIR radiation is presented. The feasibility of these coherent sources critically depends on the non-radiative intersubband transition rates. Numerical simulations of acoustical phonon, optical-phonon, and electron-electron scattering were implemented, including their temperature dependence. For far-infrared coherent sources, electron-electron scattering emerges as the dominating non-radiative relaxation mechanism. Interwell schemes offer distinctive advantages such as simplicity in design, greater tolerance in design and fabrication errors, field tunability of the emission frequency, improved internal quantum efficiency and aid in establishing population inversion. Design rules are put forward for such long wavelength sources.
Equilibrium concentrations of defects in pure and B-doped silicon
Authors: Rasband Paul B. , Clancy Paulette , Thompson Michael O.
Published: July 2002

Empirical tight-binding (ETB) calculations have been used in extensive searches for new point defect structures in pure silicon as well as silicon doped with boron. In general, these searches, which use a steepest-descents energy minimization from random starting structures, have produced the same set of simple defects in pure silicon (tetrahedral interstitials, split interstitials, and simple vacancies) which have been widely studied. However, a variety of boron interstitials, and several new di-interstitials (with and without boron) have been discovered. Similarities between these defects and defects found in ab initio and classical studies are discussed, as well as the accuracy of the theoretical results in general. A Stillinger–Weber (SW) model for Si–B interactions has been developed in order to obtain vibrational entropies for simple point defects. Using the SW potential, concentration prefactors have been obtained, and traditional Arrhenius plots for concentration have been produced. The theoretical equilibrium concentrations of self-interstitials are consistent with results obtained from Pt and Au In-diffusion experiments, and contrast with oxidation-enhanced diffusion/oxidation-retarded diffusion (OED/ORD)-derived results. The theoretical results for point defect concentrations and binding energies are used to examine the assumptions of several diffusion models.
Continuous evolution of the in-plane magnetic anisotropies with thickness in epitaxial Fe films
Authors: Gester M. , Daboo C. , Hicken R. J. , Gray S. J. , Ercole A. , Bland J. A. C
Published: July 2002

We have studied the evolution of the magnetic in-plane anisotropy in epitaxial Fe/GaAs films of both (001) and (1̄10) orientation as a function of the Fe layer thickness using the longitudinal magneto-optic Kerr effect and Brillouin light scattering. Magnetization curves which are recorded in situ during film growth reveal a continuous change of the net anisotropy axes with increasing film thickness. This behavior can be understood to arise from the combination of a uniaxial and a cubic in-plane magnetic anisotropy which are both thickness dependent. Structural analysis of the substrate and Fe film surfaces provides insight into the contribution of atomic steps at the interfaces to the magnetic anisotropy. Changing the degree of crystalline order at the Fe–GaAs interface allows us to conclude that the magnetic anisotropies are determined by atomic scale order.
Nanoneedle formation on silicon (111) surface using a scanning tunneling microscope tip
Authors: Heike Seiji , Hashizume Tomihiro , Wada Yasuo
Published: July 2002

Nanoneedles are formed on the Si(111) surface when negative ramp voltages are applied to a scanning tunneling microscope (STM) tip. These nanoneedles allow the direct imaging of the STM tip, because of their extreme sharpness with an estimated diameter of ∼2 nm and a maximum height of 20 nm. In this article, voltage, time, and current dependences of nanoneedle growth are examined. Based on the experimental results, we propose that the nanoneedle formation mechanism is: Si atom extraction from the Si surface to the tip due to the applied high voltage, migration of the atoms to the tip apex, and redeposition from the tip apex to the sample surface. A nanoneedle can also be formed on the tip apex by applying a positive voltage and the growth process is observed by a nanoneedle on the sample surface.
Effects of anodic oxide induced intermixing on the structural and optical properties of quantum wire structure grown on nonplanar GaAs substrate
Authors: Kim Yong , Yuan Shu , Leon R. , Jagadish C. , Gal M. , Johnston M. B. , Phillips M. R. , Stevens Kalceff M. A. , Zou J. , Cockayne D. J. H.
Published: July 2002

Effects of anodic oxide induced intermixing on the structural and optical properties of stacked GaAs quantum wire (QWR) structures grown on a sawtooth-type nonplanar GaAs substrate are investigated. Cross-sectional transmission electron microscope (XTEM) observation, temperature dependent photoluminescence (PL) and cathodoluminescence (CL) imaging were used. Intermixing was achieved by pulsed anodic oxidation of the GaAs cap layer and subsequent rapid thermal annealing, was verified by XTEM analysis. A significant enhancement of QWR PL is observed accompanied by a notable blueshift of the sidewall quantum well (SQWL) PL due to the intermixing. Furthermore, an extended necking region is observed after the intermixing by spatially resolved CL. The temperature dependence of the PL intensities of both SQWL and QWR show maxima at approximately T∼110 K indicating the role of the extended necking region in feeding carriers to SQWL and QWR.
Ion contribution to the deposition of silicon dioxide in oxygen/silane helicon diffusion plasmas
Authors: Charles C. , Boswell R. W.
Published: July 2002

A very high silane dissociation rate (⩾95%) has been measured using mass spectrometric analysis of neutral species in a low pressure (a few mTorr) high density (⩾1011 cm−3 for a radio frequency source power of 800 W) oxygen/silane (O2/SiH4) helicon deposition reactor. Energy selective mass spectrometric measurements of positive ions have been performed for O2/SiH4 flow rate ratios varying from 1 to 10 and for a constant power of 800 W. A simple model of the ion-induced deposition rate has been developed and the results have been compared to the measured deposition rate. It appears that 20%–50% of the silicon atoms in the near-stoichiometric deposited oxides could result from the flux of silicon-containing ions (essentially Si+ and SiOH+) to the substrate during deposition. An oxidation process via O2+ ions and an etching process via H3+ ions could possibly be involved in the ion-induced deposition mechanism.
Chemical potential pinning due to equilibrium electron transfer at metal/C60-doped polymer interfaces
Authors: Heller C. M. , Campbell I. H. , Smith D. L. , Barashkov N. N. , Ferraris J. P.
Published: July 2002

We report electroabsorption measurements of the built-in electrostatic potential in metal/C60-doped polymer/metal structures to investigate chemical potential pinning due to equilibrium electron transfer from a metal contact to the electron acceptor energy level of C60 molecules in the polymer film. The built-in potentials of a series of structures employing thin films of both undoped and C60-doped poly[2-methoxy, 5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) were measured. For undoped MEH-PPV, which has an energy gap of about 2.4 eV, the maximum built-in potential is about 2.1 eV, whereas for C60-doped MEH-PPV the maximum built-in potential decreases to 1.5 eV. Electron transfer to the C60 molecules close to the metal interface pins the chemical potential of the metal contact near the electron acceptor energy level of C60 and decreases the built-in potential of the structure. From the systematic dependence of the built-in potential on the metal work function we find that the electron acceptor energy level of C60 in MEH-PPV is about 1.7 eV above the hole polaron energy level of MEH-PPV.
Optimization of phosphor screens for charge coupled device based detectors and 7–34 keV x-rays
Authors: Wang P.-C. , Cargill G. S.
Published: July 2002

Phosphor screens which convert x-ray images to visible light images are key components in two-dimensional charge coupled device (CCD) based detector systems used for x-ray diffraction. Some experimental and theoretical aspects of phosphor screen performance are described in this article. The efficiencies of x-ray-to-light conversion were measured using a CCD camera for transmission phosphor screens fabricated from two different phosphor powders, Y2O2S:Eu (P22R) and Gd2O2S:Tb (P43), for screen mass thicknesses of 3–50 mg/cm2 and for x-ray energies of 7–34 keV. A model was developed and evaluated for the dependence of the emitted light brightness on screen thickness and x-ray energy. Inputs to the model are x-ray absorption coefficients available from published compilations, and light attenuation versus thickness data, which were determined experimentally for the phosphors and found to be dominated by scattering rather than absorption. The angular distribution of emitted light, measured for one of the phosphor screens, was found to be nearly Lambertian. Broadening of image features in the x-ray-to-visible-light conversion by phosphors for 19.6 keV x-rays was found to increase approximately linearly with phosphor screen thicknesses in the range of 30–160 μm, but with a minimum width of 110 μm for P22R phosphor and 70 μm for P43 phosphor. In the range of 7–15 keV, maximum brightness was obtained for P43 phosphor screens of about 10 mg/cm2 mass thickness (60 μm). For P22R screens, the thickness for maximum brightness increased from about 8 mg/cm2 (50 μm) for 7 keV to more than 46 mg/cm2 (210 μm) for 15 keV. For 7 keV the maximum brightnesses for P22R and P43 phosphors were about the same. For 10 keV the maximum brightness for P43 phosphor was about 60% greater than the maximum brightness for P22R phosphor samples tested. For 15 keV the maximum brightness for P43 phosphor was again about 60% greater than that for the P22R samples tested. In the range of 20–34 keV, maximum brightness would occur for thicknesses greater than 46 mg/cm2 (210 μm) for P22R phosphor and greater than 40 mg/cm2 (160 μm) for P43 phosphor. Comparing the brightness for 90 μm thickness for the two phosphors, P43 was about 30% brighter for 20 keV, 20% brighter for 24 keV, and 10% brighter for both 29 and 34 keV.
Geometrically enhanced magnetoresistance in ferromagnet–insulator–ferromagnet tunnel junctions (abstract)
Authors: Moodera J. S. , Kinder L. R. , Nowak J.
Published: July 2002

Ferromagnetic–insulator–ferromagnetic trilayer tunnel junctions show mangetoresistance (JMR) effects of about 16% or greater at room temperature.1 Much larger values of the JMR (100% or more) are observed in low-resistance junctions when the actual tunneling resistance (RT) is comparable to electrode film resistance (RL) over the junction area in our cross-geometry junction measurements.2 For the fixed junction area of 6×10−4 cm2, the measured junction resistance RJ was changed from less than an ohm to several kilohms, by controlling the duration of glow discharge in partial pressure of N2 or O2 to create AlN or Al2O3 tunnel barriers. The ferromagnetic films were CoFe base layer and Co or Ni0.8Fe0.2 top layer. These trilayer junctions show nonvolatile memory effects, are stable and reproducible. The geometrically enhanced large JMR can be qualitatively explained by the nonuniform current flow over the junction area when RT is comparable to RL, in the crossed geometry junction structure. These junctions can be effectively used as magnetic sensors and memory elements.
Correlated band magnetism of cerium and actinide materials
Authors: Cooper Bernard R. , Lin Yeong-Lieh , Sheng Q. G.
Published: July 2002

We discuss (1) the effects to be expected by the introduction into the electronic structure of locally-based two-electron correlations between the f electrons and bonding electrons of p and d atomic origin centered off-site as well as f-f correlations, (2) the expected observable consequences of these two-electron correlations, and (3) how to perform electronic structure calculations including the two-electron correlations. We first review certain general features of the physics associated with capturing the dual energetically localized-delocalized nature of the f electron spectral density; and review model calculations involving a single on-site f electron and a single ligand p/d electron of off-site parentage which lead to the possibility of a narrow singlet and triplet (magnetic) band picture explaining heavy fermion phenomenology. We then show that the same singlet/magnetic state picture arises when we include two-electron f-l and f-f correlations for actinides, which have atomic fn configurations with n>1; and we describe a practical electronic structure scheme for real materials based on a sequence in which a conventional one-electron linearized combination of muffin-tin orbitals (LMTO) LDA+U calculation is followed by a calculation for the lattice with a helium like two-electron Hamiltonian at the f atom sites, i.e., two-electron atoms where initially for the core two electrons worth of charge are removed from the LMTO f-site atom. This procedure will reconstruct the LMTO bands to include two-electron texturing.
Ru-induced loss of long-range magnetic order in a-Fe90−xRuxZr10
Authors: Ryan D. H. , Cadogan J. M. , Tun Zin
Published: July 2002

Mösbauer spectroscopy, magnetization, susceptibility, and neutron depolarization have been used to investigate the loss of long-range ferromagnetic order in a-Fe90−xRuxZr10. Our depolarization results show a rapid reduction in domain size from 1.6 to 0.4 μm as x increases from 0 to 2, with no evidence for long-range order in either depolarization or magnetization data for x⩾3. Furthermore, for x=1 and 2, we observe a clear break in the temperature dependence of 〈Bhf〉 which may be associated with ordering of transverse spin components in these partially frustrated alloys.
Charge carrier recombination in organic bilayer electroluminescent diodes. I. Theory
Authors: Khramtchenkov D. V. , Arkhipov V. I. , Bässler H.
Published: July 2002

A previously developed analytic model for charge carrier recombination in bilayer organic light emitting diodes [D. V. Khramtchenkov, V. I. Arkhipov, and H. Bässler, J. Appl. Phys. 79, 9283 (1996)] in which charge transport across the interface between anodic and cathodic cell compartments is impeded by energy barriers is extended to cells of arbitrary thickness of the constituent layers and variable energy barriers. The results indicate that the recombination yield is the result of a complicated interplay between redistribution of the electric field affecting the injection at the electrodes, internal charge accumulation, and field assisted barrier crossing. Unit charge carrier recombination efficiency is predicted to occur at moderate cell voltages and electron injection barriers less than ≈0.4 eV. At higher voltages, leakage process across the interface becomes progressively important. With increasing electron-injection barrier, that injection process becomes rate limiting.
Electrical alternative to pulsed fiber-delivered lasers in microsurgery
Authors: Palanker Daniel , Turovets Igor , Lewis Aaron
Published: July 2002

An electrical system based on a tapered microelectrode has been developed for generation of high voltage sub-microsecond discharge in physiological medium. Different types of the resulting pulses of current are investigated as well as the dynamics of the associated cavitation bubbles. A highly localized zone of power dissipation—about 20 μm in size—results in a low threshold energy of cavitation bubble generation—about 3 μJ—in comparison to laser-based intraocular microsurgical instrumentation with fiber delivery systems. Cavitation bubble dynamics resulting from the electric discharge is similar to that observed with ns-pulsed fiber-delivered lasers in strongly absorbing liquid medium. Efficiency of the pulse energy conversion to the bubble energy is about 12%, which is lower than the best results obtained with lasers. In spite of that, due to the low threshold energy, cavitation bubbles required for effective cutting of soft tissue can be generated at energies lower than that used in laser instrumentation with fiber-based delivery systems. The proposed device has a potential to become a convenient and a cost-effective alternative to such lasers in vitreoretinal microsurgery.
Optoelectronic properties, structure and composition of a-SiC:H films grown in undiluted and H2 diluted silane-methane plasma
Authors: Desalvo A. , Giorgis F. , Pirri C. F. , Tresso E. , Rava P. , Galloni R. , Rizzoli R. , Summonte C.
Published: July 2002

a -SiC:H films with energy gap in the range 2.00–2.65 eV have been grown by plasma enhanced chemical vapor deposition in undiluted and H2 diluted SiH4+CH4 gas mixtures, by making use of optimized deposition conditions. A complete picture of structural, compositional, optoelectronic, and defective properties for high quality films has been drawn for the first time. We show that the addition of H2 to the gas mixture leads to a different chemical composition of the deposited films; in particular, carbon incorporation is enhanced and a carbon fraction in the solid matrix up to C/(C+Si)≈0.45 can be obtained. These films have a higher mass density, a reduced microvoid and carbon cluster concentration, a better structural connectivity, and improved optoelectronic properties. For samples with optical gap below 2.4 eV, the reduced defect concentration of H2 diluted films results in an increase of the photoconductivity gain and the steady-state (ημτ)ss values up to two orders of magnitude.
High field magnetic properties of HoIG:Sc single crystals
Authors: Ostoréro J. , Guillot M.
Published: July 2002

Magnetic properties of garnet single crystals Ho3Fe5−zSczO12 (z=0.3, 0.5, 0.7 and 0.9) are reported under high magnetic field up to 240 kOe in the 4.2–300 K temperature range. To interpret the influence of the diamagnetic Sc3+ substitution in octahedral Fe3+ sublattice, it is shown that the Néel–Dionne model has to be improved; from the analysis of the temperature dependences of the spontaneous magnetizations, it is concluded that the molecular field constant Ndc related to the Fe-Ho superexchange interactions has to be, as Ndd and Nad, a function of z. Furthermore from the M(H,T,z) variations, the “fictive” canting angle of the Dionne model is found to be strongly H, T, and z dependent with two different behaviors below and above the compensation temperatures. Finally in high magnetic field and at low temperatures, unusual field-induced transitions are observed for the higher substituted samples, whereas the onset of the usual canted magnetic phase takes place in higher applied field for all the crystals.
High resistive nanocrystalline Fe-M-O (M=Hf, Zr, rare-earth metals) soft magnetic films for high-frequency applications (invited)
Authors: Hayakawa Y. , Makino A. , Fujimori H. , Inoue A.
Published: July 2002

Microstructure, soft magnetic properties, and applications of high resistive Fe-M-O (M=Hf, Zr, rare-earth metals) were studied. The Fe-M-O films are composed of bcc nanograins and amorphous phases with larger amounts of M and O elements which chemically combine each other. Consequently, the amorphous phases have high electrical resistivity. The compositional dependence of magnetic properties, electrical resistivity, and structure have been almost clarified. For example, the high magnetization of 1.3 T, high permeability of 1400 at 100 MHz and the high electrical resistivity of 4.1 μΩ m are simultaneously obtained for as-deposited Fe62Hf11O27 nanostructured film fabricated by rf reactive sputtering in a static magnetic field. Furthermore, Co addition to Fe-M-O films improves the frequency characteristics mainly by the increase in the crystalline anisotropy of the nanograins. The Co44.3Fe19.1Hf14.5O22.1 film exhibits the quality factor (Q=μ′/μ′′) of 61 and the μ′ of 170 at 100 MHz as well as the high Is of 1.1 T. This frequency characteristics is considered to be superior to the other films already reported. The films also exhibit high corrosion resistance in an isotonic sodium chloride solution. Therefore, these films enable us to realize the high-frequency magnetic devices, such as thin-film inductors and transformers for microswitching converters and ultrahigh-density recording heads.
Investigation of mechanisms of vacancy generation in silicon in the presence of a TiSi2 film
Authors: Herner S. B. , Jones K. S. , Gossmann H.-J. , Tung R. T. , Poate J. M. , Luftman H. S.
Published: July 2002

We have determined the perturbance in the silicon vacancy concentration induced by the presence of TiSi2 films. Antimony in silicon doping superlattices was employed as a vacancy detector. Under all conditions studied (deposited titanium thickness 4–312 nm, 800–850 °C, 15–600 min), we always observe a relative vacancy supersaturation on the order of 1.5. Two mechanisms of vacancy injection during titanium silicidation were studied: (1) stress compensation; by varying the thickness of the deposited films and annealing for 60 min at 850 °C, a range of stresses was induced in the substrate via the coefficient of thermal expansion mismatch between the film and substrate. The observed vacancy supersaturation was independent of film thickness, indicating that stress compensation is not a mechanism of vacancy generation for titanium disilicide; (2) volume contraction; annealing for 15, 60, and 600 min at 800 °C after identical 30-nm-thick titanium films were deposited allowed the time variation of the vacancy supersaturation to be studied. While the vacancy supersaturation decayed slightly with time, its time dependence is incompatible with a large “pulse” of vacancies injected during the silicidation reaction. This indicates that volume contraction at the growing film interface is not a mechanism for vacancy generation. The thicker TiSi2 films (>22 nm) and those annealed for ⩽60 min were continuous in their coverage of the substrate as observed by transmission electron microscopy, while the thinner films and those annealed for longer times had islanded. However, there was no relationship between film coverage and vacancy behavior in the substrate, or was there any relation between deposition method (evaporation versus sputtering) and vacancy behavior.
A cross-sectional atomic force microscopy study of nanocrystalline Ge precipitates in SiO2 formed from metastable Si1−xGexO2
Authors: Caragianis-Broadbridge C. , Blaser J. M. , Paine D. C.
Published: July 2002

In recent years, many investigators have reported visible photoluminescence from structures that consist of Ge or Si nanocrystals embedded in a SiO2 matrix deposited or grown on various substrates. We have developed a rapid technique for studying the through-thickness microstructure of this class of materials via atomic force microscopy (AFM) and, using this technique, we report on the precipitation and growth of Ge crystallites formed via a two-step process of hydrothermal oxidation of Si1−xGexO2 (x=0.15) at 450–500 °C and subsequent chemical reduction in forming gas (85/15: N2/H2; 800 °C). The Ge-particle distributions obtained with this AFM cross-sectional technique are consistent with those previously reported using other techniques. The utility of cross-sectional AFM for the evaluation of nanoscale features in the thickness of a thin film is evaluated.
Short-pulse laser-induced crystallization of intrinsic and hydrogenated amorphous germanium thin films
Authors: Mulato M. , Toet D. , Aichmayr G. , Santos P. V. , Chambouleyron I.
Published: July 2002

We report on the laser crystallization of intrinsic (a-Ge) and hydrogenated (a-Ge:H) amorphous germanium thin films using short, i.e., ns range, laser pulses. The influence of hydrogen on the phase transitions was investigated by monitoring the reflectance of the sample during laser irradiation. We determined the thresholds for melting (36 mJ/cm2) and for surface damage (66 mJ/cm2) of the a-Ge film. In a-Ge:H, hydrogen effuses on a short time scale (10 ns) upon laser irradiation. The effusion leads to the formation of a lifted-off (100 nm thick) crystalline Ge membrane, leaving behind a rough and incompletely crystallized surface. In a-Ge, on the other hand, no surface disruption is observed. The Raman spectra of hydrogenated samples are dominated by stress effects, while those corresponding to non-hydrogenated samples are dominated by crystallite size distribution effects. We also conclude that laser-induced annealing, carried out by applying several pulses with increasing intensity, can be used as a tool for the crystallization of a-Ge:H samples without hydrogen-induced surface damage.
Charge transport in strained Si1−yCy and Si1−x−yGexCy alloys on Si(001)
Authors: Osten H. J. , Gaworzewski P.
Published: July 2002

We have investigated the temperature dependencies of charge carrier densities and Hall mobilities in tensile strained Si1−yCy and in compressively strained Si1−x−yGexCy layers. In both cases, the measured charge carrier densities at room temperature are not affected substantially by the addition of a small concentration of carbon (<1%) under identical growth conditions and dopant fluxes. The measured Hall mobilities monotonically decrease with increasing carbon content for electrons in Si1−yCy, and for holes in Si1−x−yGexCy, respectively. Our results indicate that electrically active defects are formed with the addition of carbon. These defects are presumably connected with carbon/Si interstitials or other C-related complexes. It seems to be difficult to attribute the formation of those electrically active defects solely to contaminations originating from the used carbon evaporation source. We observed that donor- and acceptor-like defects are formed in Si1−yCy as well as in Si1−x−yGexCy layers with roughly a constant ratio, independent of source temperature.