bluuebunny/crossref_metadata_2025 huggingface dataset:
Investigations of the electrical properties of electrodeposited CuInSe2 thin films
Authors: Guillén C. , Herrero J.
Published: July 2002

The electrical properties of as-grown and heat-treated copper indium diselenide (CuInSe2) thin films electrochemically deposited were measured in the temperature range 30 to about 300 K. In general the films showed variable range hopping conduction mechanism, i.e., σ= (σ0 T−1/2) exp[ − (T0/T)1/4] with T0 between 4 × 102 and 6 × 103 K, at the lowest temperatures, and thermally activated conductivity, i.e., σ = σ0 exp( − Ea/kT) with Ea between 240 and 80 meV, over the temperature range close to the room temperature. The activation energy, Ea, decreased and the conductivity, σ, increased with the annealing treatment. The data of these annealed films are in agreement with the Meyer–Neldel rule. Grain boundary trapping models were considered in the analysis of the results.
Analysis of dark current-voltage characteristics of Al/chlorophyll a/Ag sandwich cells
Authors: Oueriagli A. , Kassi H. , Hotchandani S. , Leblanc R. M.
Published: July 2002

The analysis of dark current-voltage (I-V) characteristics of Al/chlorophyll a/Ag cells at room temperature with respect to the elucidation of conduction mechanisms and evaluation of cell parameters is presented. It is seen that the presence of series and shunt resistances can considerably affect the I-V plots. It is therefore important to remove their effects for correct and meaningful analysis of the I-V curves. The results suggest that for Al/Chl a/Ag cells with microcrystalline Chl a∼3000 Å thick, the conduction mechanism for voltages between 0.53 and 1 V can be described by a modified Shockley equation from which the values of Rs, Rsh, n, and I0 obtained are 3.2×104 Ω, 1.7×109 Ω, 1.74, and 2.4×10−15 A, respectively. Rs is most likely due to the combined effect of bulk Chl a and the electrodes, particularly the insulating layer of Al2O3 that is formed as a result of oxidation of Al in air. For higher forward biases, i.e., between 1 and 2 V, the current transport is due to the space-charge-limited current in presence of exponentially distributed traps. The density of traps obtained is ∼1018 cm−3. Because of the breakdown of Al/Chl a/Ag cells at fields higher than 107 V m−1, the transition voltage to observe the trap-filled-limit situation was not possible. For a low-bias region, the conduction mechanisms seem to be dominated by Schottky emission over an Al/Al2O3 barrier; however, the temperature dependence of I-V characteristics is necessary to confirm this. The barrier Al/Al2O3 calculated is ∼1.17 eV.
Epitaxial growth and characterization of zinc-blende gallium nitride on (001) silicon
Authors: Lei T. , Moustakas T. D. , Graham R. J. , He Y. , Berkowitz S. J.
Published: July 2002

GaN films have been epitaxially grown onto (001) Si by electron cyclotron resonance microwave-plasma-assisted molecular-beam epitaxy, using a two-step growth process, in which a GaN buffer is grown at relatively low temperatures and the rest of the film is grown at higher temperatures. This method of film growth was shown to lead to good single-crystalline β-GaN and to promote lateral growth resulting in smooth surface morphology. The full width at half-maximum of the x-ray rocking curve in the best case was found to be 60 min. Optical-absorption measurements indicate that the band gap of β-GaN is 3.2 eV and the index of the refraction below the absorption edge is 2.5. Conductivity measurements indicate that the films may have a carrier concentration below 1017 cm−3.
Peak-to-valley current ratios as high as 50:1 at room temperature in pseudomorphic In0.53Ga0.47As/AlAs/InAs resonant tunneling diodes
Authors: Smet Jurgen H. , Broekaert Tom P. E. , Fonstad Clifton G.
Published: July 2002

Pseudomorphic In0.53Ga0.47As/AlAs/InAs resonant tunneling diodes with the highest peak-to-valley current ratios reported to date have been fabricated on InP substrates with molecular beam epitaxy. Peak-to-valley current ratios as high as 50:1 at 300 K are obtained. The majority of the devices on the sample have a peak-to-valley current ratio of 42 at 300 K and 85 at 77 K.
Optical damage resistance and crystal quality of LiNbO3 single crystals with various [Li]/[Nb] ratios
Authors: Furukawa Yasunori , Sato Masayoshi , Kitamura Kenji , Yajima Yoshiyuki , Minakata Makoto
Published: July 2002

Lithium niobate single crystals with various [Li]/[Nb] ratios were grown by the Czochralski method from melts having compositions varying between 45.5–58 mol % Li2O. Their optical damage resistance was characterized by measurement of the photoinduced birefringence change. Their crystal quality was characterized by x-ray topography and x-ray rocking curves. The photoinduced birefringence change increased with an increase in the Li content of the crystal. The Li-poor crystals, which were richer in Nb content than the crystal with the congruent composition, showed the smallest birefringence change, in spite of the fact that the x-ray rocking curve width of these Li-poor crystals was much wider than the congruent crystal. It is concluded that the optical damage resistance of undoped LiNbO3 is increased with an increase in the Nb content of the crystal, and that the crystal quality, as represented by x-ray rocking curve peak width, has no correlation with the optical damage resistance.
ac breakdown characteristics of ceramic materials
Authors: Owate Israel O. , Freer Robert
Published: July 2002

ac breakdown properties of alumina, aluminum nitride, and glass ceramic (SiO2-Al2O3-MgO-TiO2) materials have been investigated. The breakdown strength of glass ceramics (∼65 kV/mm) was found to be higher than that of alumina (∼31 kV/mm) and aluminum nitride (∼16 kV/mm). The specimens were characterized for their density, porosity, and microstructural parameters, such as grain size and distribution, as they are believed to influence the breakdown properties significantly. In all the cases, the prebreakdown and breakdown conduction were found to be associated with visible-light emission at the electrode-specimen interface. The samples that underwent breakdown were found to exhibit irregular puncture channel, terminated with craters at both ends. Several types of crystallization structures were observed at the rim of the craters and in some cases at the edges of the breakdown channels. It is suggested that the breakdown process of the ceramic materials is a combination of electronic, electromechanical, and thermal processes.
Superconducting, microstructural, and grain boundary properties of hot-pressed PbMo6S8
Authors: Selvam P. , Cattani D. , Cors J. , Decroux M. , Junod A. , Niedermann Ph. , Ritter S. , Fischer O/. , Rabiller P. , Chevrel R.
Published: July 2002

An alternative procedure is outlined for the synthesis of high quality, fine grained PbMo6S8. As the grain size plays an important role on the densification process, which in turn has an influence on the magnitude of Jc, an attempt has been made to produce dense samples from such powders by ‘‘Hot Pressing.’’ The effect of the hot-pressing temperature on the superconducting, crystallographic, microstructural, and grain boundary characteristics of the ternary compound was evaluated. Scanning electron micrographs and ac-susceptibility measurements indicate that hot pressing (1000–1200 °C) improves the grain connection as a consequence of better densification. However, at higher temperatures (1250–1400 °C) it also precipitates MoS2 as an additional phase. Calorimetric data indicate a continuous broadening, as a function of hot-pressing temperature, of the specific heat jump at Tc. Preliminary investigation on the Tc distribution of the samples shows a progressive degradation, as indicated by a smearing in Tc down at least to 8 K. The deterioration was examined using Auger electron spectroscopy and the results suggest possible compositional variations rather than oxygen defects in the phase. The origin of such behavior is examined on the basis of nonstoichiometry or chemical heterogeneity at the grain surface. In addition, grain boundary contaminants and their role on the superconducting properties are considered. Finally, the often encountered problem of transport Jc limitation in these materials is discussed in terms of interconnectivity of the grains, phases, the presence of secondary phases, impurities, inhomogeneities, and the grain boundary phases.
Self-heating in the Coulomb-blockade electrometer
Authors: Kautz R. L. , Zimmerli G. , Martinis John M.
Published: July 2002

A detailed comparison between theory and experiment is used to demonstrate the presence of self-heating in the Coulomb-blockade electrometer. When three different heating models are considered, the best fit with experimental electrometer characteristics is obtained for a model in which the electron temperature of the island electrode is determined by heat transfer to the lattice via electron-phonon coupling. In the successful model, the temperature Ti of the island electrons is related to the power Pi dissipated in the island and the temperature Tl of the phonons by Pi=ΣΩ(Ti5−Tl5), where Σ is an electron-phonon coupling parameter characteristic of the island material and Ω is the volume of the island. The best fit between theory and experiment yields a value of Σ=0.2 nW/K5/μm3 for the electron-phonon coupling in aluminum. Our calculations show that the electron temperature of the island commonly exceeds 100 mK even when the lattice remains at 35 mK.
Ultimate accuracy of single-electron dc current standards
Authors: Averin D. V. , Odintsov A. A. , Vyshenskii S. V.
Published: July 2002

Quantum corrections to dc current are calculated in two single-electron devices: turnstile and pump. These corrections set the ultimate limitation on accuracy of these devices as fundamental standards of dc current. A developed approach can be used in principle to analyze macroscopic quantum tunneling of charge in arbitrary single-electronic circuit.
Pressure effect on the superconducting transition temperature of Dy1−xPrxBa2Cu3O7−δ
Authors: Lin J. G. , Xue Y. Y. , Chu C. W. , Cao X. W. , Ho J. C.
Published: July 2002

The pressure effect on the superconducting transition temperature (dTc/dP) has been measured for Dy1−xPrxBa2Cu3O7−δ with x=0, 0.02, 0.05, 0.1, 0.15, 0.2, and 0.3. Although the Tc suppression by Pr is in a different fashion for Y1−xPrxBa2Cu3O7−δ and Dy1−xPrxBa2Cu3O7−δ, the dTc/dP is similar in these two systems, with respect to x. The measured dTc/dP is positive at small x but becomes negative for x≥0.2 which is associated with the pressure effect on the normal-state resistivity (1/R)(dR/dP). The positive (1/R)(dR/dP) at x≥0.2 indicates that the number of carriers is decreasing under pressure and it supports the picture of hole localization through the hybridization of Pr-4f and O-2p bands. The mechanical pressure has a negative effect on Tc for x≥0.2, but chemical pressure has a positive effect on Tc according to the results of chemical substitution. This suggests that the large R (=rare earth element) may possibly reduce the Pr-O distance, and increase the hybridization of Pr-4f and O-2p bands.
Dielectric properties of alumina/zirconia composites at millimeter wavelengths
Authors: Mollá J. , Heidinger R. , Ibarra A. , Link G.
Published: July 2002

Alumina-zirconia composites with ZrO2 contents up to 20 wt % and negligible porosity were investigated at millimeter (mm) wavelengths to determine the changes appearing in the dielectric properties of pure alumina ceramics when unstabilized or partially stabilized ZrO2 is added to improve the mechanical strength. It is demonstrated that it is essential to distinguish between the contributions of the monoclinic and the tetragonal phase of zirconia (m-ZrO2, t-ZrO2). Permittivity is raised with increasing content of either phase; the effective permittivity can be assessed by the rule of mixtures (Maxwell–Garnett formulation of the generalized Clausius–Mossotti relation) using permittivity values of 10 for Al2O3, 14–21 for m-ZrO2, and 40–45 for t-ZrO2. The permittivity data show only a small variation in the investigated range of 9–145 GHz. For the dielectric loss, there is evidence of a predominant contribution of m-ZrO2; in addition, the marked increase in loss with frequency becomes sharper. The t-ZrO2, which is responsible for strengthening, does not show any significant influence on losses. It is therefore concluded that ZrO2 strengthening of alumina is feasible without affecting mm-wave losses at room temperature as long as the presence of m-ZrO2 is avoided.
Electron transport through GaAlAs barriers in GaAs
Authors: Feng S. L. , Krynicki J. , Zazoui M. , Bourgoin J. C. , Bois P. , Rosencher E.
Published: July 2002

We have analyzed electronic transport through a single, 200-Å-thick, Ga0.74Al0.36As barrier embedded in GaAs. At low temperatures and high electric field, the Fowler–Nordheim regime is observed, indicating that the barrier acts as insulating layers. At higher temperatures the thermionic regime provides an apparent barrier height, decreasing with the field, which is equal to the expected band offset when extrapolated to zero field. However, for some samples, the current is dominated by the presence of electron traps located in the barrier. A careful analysis of the temperature and field behavior of this current allows to deduce that the mechanism involved is field-enhanced emission from electron traps. The defects responsible are tentatively identified as DX centers, resulting from the contamination of the barrier by donor impurities.
Dimensional metrology with scanning probe microscopes
Authors: Griffith Joseph E. , Grigg David A.
Published: July 2002

We review the application of scanning probe microscopes to dimensional measurement of topographic features. Probe microscopes show great promise as metrology tools because they produce three-dimensional data over almost all solids in a wide range of ambients. Even though these microscopes readily achieve atomic resolution, there are several aspects of their behavior that can cause them to exhibit large measurement errors. The actuators that drive the probe exhibit hysteresis and creep, so they must be independently monitored. In addition, the geometry of an extended probe moving across the surface makes probe-sample interaction intrinsically nonlinear. Forces on the probe that cause it to flex are another source of inaccuracy. Probe fabrication and characterization are, consequently, important issues. We describe present understanding of these problems and the techniques being developed to solve them.
A self-consistent model of Γ-X mixing in GaAs/AlAs/GaAs quantum well structures using the quantum transmitting boundary method
Authors: Sun J. P. , Mains R. K. , Yang K. , Haddad G. I.
Published: July 2002

We present a numerical study of the Γ-X mixing in GaAs/AlAs/GaAs quantum well structures. A Γ-X mixing model proposed by Liu [Appl. Phys. Lett. 51, 1019 (1987)] is extended to include the effects of self-consistency and nonzero transverse momentum. In the present model, the coupled Schrödinger equations for Γ and X electron envelope wave functions are solved self-consistently with Poisson’s equation to calculate the electron transmission probability and wave functions, which lead to the current-voltage (I-V) characteristics of single barrier and double barrier resonant tunneling diode structures. The quantum transmitting boundary method is employed in the model for numerical solution of the coupled Schrödinger equations, which proves to be very stable and efficient, even for large (≳2000 Å) structures. The features of Γ-X mixing, such as the resonance/antiresonance in the transmission probability and the virtual bound states, are clearly demonstrated. Additional physical features are observed in the transmission probability and the wave functions under applied bias conditions. Our work shows that inclusion of transverse momentum, variable effective mass, and the self-consistent potential is important in the realistic modeling of I-V characteristics for structures exhibiting Γ-X coupling.
Macroparticle-free thin films produced by an efficient vacuum arc deposition technique
Authors: Anders Simone , Anders André , Brown Ian
Published: July 2002

Macroparticle-free films of various metals and diamond-like carbon have been obtained by a pulsed vacuum arc deposition technique. An axial magnetic field (100–200 mT) generated by the discharge current itself was used to focus the plasma produced by the cathode spots and to guide the plasma flow through a macroparticle filter. The films were characterized by scanning electron microscopy, atomic force microscopy, and Rutherford backscattering spectrometry. They were found to be macroparticle-free. The total efficiency of the combined gun-filter plasma source is even higher than a standard nonfiltered vacuum arc plasma source in terms of deposited film thickness per discharge, because the plasma losses in the filter are overcompensated by the focused plasma injection into the filter.
Domain inversion in KTiOPO4 crystal near the Curie point
Authors: Kugel V. D. , Rosenman G. , Angert N. , Yaschin E. , Roth M.
Published: July 2002

A microscopic mechanism of the outdiffusion-induced domain inversion in KTiOPO4 crystals near the Curie point without applying an external electric field is developed. It is described in terms of a built-in electric field, for reasons of a gradient of donorlike centers. The predicted dominant role of the n-type charges is proved by experimental observations.
Cross sections and quantum yields of the 3 μm emission for Er3+ and Ho3+ dopants in crystals
Authors: Payne Stephen A. , Smith Larry K. , Krupke William F.
Published: July 2002

We have measured the lifetimes of the 2.8 μm emission band for Er3+- and Ho3+-doped fluorides including LiYF4, BaY2F8, LaF3, and KY3F10, and for numerous oxides such as Y3Al5O12, YAlO3, La2Be2O5, and other crystals. Several quantum yields and branching ratios were experimentally evaluated as well, in order to determine the radiative lifetime of the upper laser level for the 2.8 μm transition. This information was employed to assess the cross sections of the 2.8 μm infrared emission band for several of these crystals, which involves the 4I11/2→4I13/2 transition for Er, and the 5I6→5I7 for Ho.
Internal friction in free-standing thin Al films
Authors: Heinen D. , Bohn H. G. , Schilling W.
Published: July 2002

The internal friction of a 4 μm thick free-standing Al film has been investigated. For a measuring frequency of about 200 Hz a relaxation peak was found near 100 °C. Compared with a substrate-bonded film having the same columnar microstructure the peak is about three times higher while the temperature position is the same. This result confirms a model derived for substrate-bonded films which predicts that intrinsic grain boundary sliding controls the relaxation mechanism. The film/substrate-bonding only influences the peak height. It partly hinders the grain boundary sliding thereby reducing the relaxation strength.
The pulse propagation problem in free electron lasers: Gain parametrization formulae
Authors: Dattoli G. , Giannessi L. , Torre A. , Segreto A.
Published: July 2002

The pulse propagation in free electron lasers is treated analytically. A general gain formula including finite pulse effects, high gain corrections, inhomogeneous broadening contributions and the detuning parameter is derived. It is shown that it contains, as particular cases, the already known gain parametrization formulae.
Comparison of the diffusion barrier properties of chemical-vapor-deposited TaN and sputtered TaN between Cu and Si
Authors: Tsai M. H. , Sun S. C. , Tsai C. E. , Chuang S. H. , Chiu H. T.
Published: July 2002

This work investigated the barrier properties of metalorganic chemical-vapor-deposited (CVD) tantalum nitride (TaN) and physical-vapor-deposited (PVD) TaN between Cu and Si. The CVD TaN film had a preferred orientation (200) with a grain size of around 60 nm, while the PVD TaN had a (111) preferred orientation with a grain size of around 20 nm, as determined by transmission electron microscopy (TEM) and x-ray diffraction (XRD) analyses. Degradation study of the Cu/TaN/Si contact system was also performed by sheet resistance measurement, scanning electron microscopy (SEM), XRD, secondary ion mass spectroscopy (SIMS), and shallow junction diodes. These results indicated that the PVD TaN film can act as a better diffusion barrier than the CVD TaN film. The higher thermal stability of PVD TaN than CVD TaN can be accounted by their difference in microstructures. The failure mechanisms of both CVD TaN and PVD TaN films as diffusion barriers between Cu and Si were also discussed.
Large magnetic Hall effect in ferromagnetic FexPt100−x thin films
Authors: Canedy C. L. , Gong G. Q. , Wang J. Q. , Xiao Gang
Published: July 2002

We have observed a very large extraordinary Hall effect (EHE) in a series of Fe–Pt thin films with various Fe contents. The origin of this remarkable EHE is the large spin–orbit interaction in the Fe–Pt alloys. At certain Fe content, the Hall resistivity can be saturated with a magnetic field less than 2 kG. The large EHE persists to room temperature with little change in magnitude. The EHE, which to our knowledge is the largest among magnetic transition metals, may find potential applications in magnetic sensors and nonvolatile magnetic random access memories. We will present structural analysis of the Fe–Pt films.
Optical spectroscopy of plasma produced by laser ablation of Ti alloy in air
Authors: Wang X. T. , Man B. Y. , Wang G. T. , Zhao Z. , Liao Y. , Xu B. Z. , Xia Y. Y. , Mei L. M. , Hu X. Y.
Published: July 2002

Time-resolved emission spectrum from the plasma produced by 1.06 μm, 10 ns pulsed-laser irradiation of titanium alloy targets in air at a flux of 9.3×109 W cm−2 was analyzed in the wavelength range of 2000–8800 Å. From the evolutions of the specific spectrum lines of N II, Ti I, and Fe I, the velocities of N+ ions and the excited neutral Ti and Fe atoms have been obtained using a time-of-flight diagnostic method. The electron temperatures were deduced using the relative emission intensities of N II and Fe I isolated spectrum lines, and an electron number density was determined from the Stark-broadened line of the N II line at wavelength λ=3995 Å.
Growth of apatite films by laser ablation: Reduction of the droplet areal density
Authors: Guillot-Noël O. , Gomez-San Roman R. , Perrière J. , Hermann Jörg , Craciun Valentin , Boulmer-Leborgne Chantal , Barboux P.
Published: July 2002

The pulsed laser deposition of calcium phosphate (apatite) thin films using targets with different optical absorptivities has been studied. It was found that the surface morphology of the grown films greatly depends on the target optical properties, the higher the optical absorption coefficient a, the lower the droplet density on the surface of the deposited films. The temperature profiles inside the different targets during the action of a laser pulse were obtained by numerically solving the heat diffusion equation. These simulations indicated for the low a value targets a large volume heating with the formation of a thick layer of melted material, and the occurrence of a sub-surface superheating effect once the evaporation began. For higher a value targets, the temperature profiles corresponded to the surface heating case, where the evaporation process began at earlier times and was restricted to the outermost surface region. Time- and space-resolved optical emission spectroscopy investigations showed that there was a significant increase of electron density and temperature for the plasmas formed from targets having higher a values, in agreement with the temperature profile estimations.
Ultraviolet light induced changes in polyimide liquid-crystal alignment films
Authors: Lu J. , Deshpande S. V. , Gulari E. , Kanicki J. , Warren W. L.
Published: July 2002

Ultraviolet light induced changes in polyimide liquid-crystal alignment films were investigated. Infrared, UV-visible, x-ray photoelectron spectroscopy, and electron-spin-resonance measure- ments indicated that bond breaking and subsequent oxidation reactions occur in polyimide films (SE7210, OCG284, DuPont 2555 and 2540) during the broadband UV illumination in air. Mechanical rubbing has no effect on the optical and magnetic properties but it causes the removal of the UV-exposed film. Capacitance–voltage measurements indicate that there is a slight decrease in dielectric constant and creation of net negative charges in the film after UV exposure. Sur- face tension of polyimide films before and after UV illumination and changes in the pretilt angle of the polyimide surface following UV exposure have also been studied. The decrease in pretilt angle following UV illumination is attributed to an increase in surface tension. Our results indicate that a simple UV technique can be used to achieve domain divided liquid-crystal pixel electrode design with improved viewing characteristics.
Investigation of the physical mechanisms of shear-force imaging
Authors: Durkan C. , Shvets I. V.
Published: July 2002

It is shown that shear-force imaging, as is commonly used for distance regulation in scanning near-field optical microscopy, is not a reliable technique for accurate topographic measurements. This is because different materials experience different shear-force damping. Results of the shear-force damping characteristics are presented for a number of different materials, and some consequences of the different dampings for different materials are demonstrated. It is also shown that there are at least two distinct shear force damping mechanisms. Results of imaging small conducting islands on a glass substrate show that the damping characteristics depend on the islands’ size.
High-resolution grazing-incidence x-ray diffraction for characterization of defects in crystal surface layers
Authors: Kondrashkina E. A. , Stepanov S. A. , Schmidbauer M. , Opitz R. , Köhler R. , Rhan H.
Published: July 2002

The peculiarities of high-resolution measurements in grazing-incidence diffraction (GID) are studied, both theoretically and experimentally. It is shown that complete discrimination between coherent reflection and diffuse scattering due to defects in GID requires a three-dimensional mapping of reciprocal space. These measurements can be performed using a combination of analyzer crystal and position-sensitive detector for angular analysis of scattered x-rays in mutually perpendicular planes. The equations for the resolution function of GID experiments are given and applied to the interpretation of GID measurements taken from an AlAs/GaAs superlattice. The discrimination of diffuse scattering due to interfacial roughness in the superlattice is demonstrated.
Measuring local thermal conductivity in polycrystalline diamond with a high resolution photothermal microscope
Authors: Hartmann J. , Voigt P. , Reichling M.
Published: July 2002

A photothermal microscope that provides micrometer lateral and submicrometer depth resolution was designed. Thermal conductivity measurements with modulation frequencies up to 12 MHz on single grains in polycrystalline diamond demonstrate its lateral resolution power even for a highly conducting material. Measured conductivities strongly depend on the averaged volume and values up to 2200 W/mK are found in the high frequency limit where the properties inside a grain are sampled. The capability of the instrument to measure thermal parameters on thin films is demonstrated for gold films evaporated on quartz with a thickness ranging from 20 to 1500 nm. Measurements reveal a strong thickness dependence for both thin film conductivity and the contact resistance between film and substrate. Thermal conductivity decreases monotonically from 230 to 30 W/mK whereas the contact resistance rises from 2×10−7 to 8×10−6 m2K/W with decreasing film thickness.
Conductivity-type anisotropy in molecular solids
Authors: Ostrick J. R. , Dodabalapur A. , Torsi L. , Lovinger A. J. , Kwock E. W. , Miller T. M. , Galvin M. , Berggren M. , Katz H. E.
Published: July 2002

Thin polycrystalline films of perylenetetracarboxylic dianyhydride (PTCDA), an organic molecular solid, exhibits substantial anisotropies in its electronic transport properties. Only electrons transport in the directions along molecular planes, while mainly holes transport in the direction normal to molecular planes. A series of measurements on both field effect transistors with PTCDA active layers and light emitting diodes with PTCDA transport layers documents the anisotropy seen in the electronic transport in thin films of PTCDA.
Inhomogeneous strain states in sputter deposited tungsten thin films
Authors: Noyan I. C. , Shaw T. M. , Goldsmith C. C.
Published: July 2002

The results of an x-ray diffraction study of dc-magnetron sputtered tungsten thin films are reported. It is shown that the phase transformation from the β to α W can cause multilayered single-phase films where the layers have very different stress states even if the films are in the 500 nm thickness range.
Analysis of thermally stimulated currents measured on ionic conductors
Authors: Devautour S. , Vanderschueren J. , Giuntini J. C. , Henn F. , Zanchetta J. V.
Published: July 2002

We propose a new method for analyzing the experimental results obtained from thermally stimulated depolarization current experiments on ionic conductors. This method is based on a model developed to explain polarization in these materials. The fundamental assumptions about the elementary mechanisms responsible for polarization is assumed to be due to the “free” volume created by thermal fluctuations occurring in solids near the equilibrium state. The consequence of these fluctuations is a statistical distribution of the relaxation times or energies, which can be evidenced by the thermal windowing technique, and then analyzed by relaxation map analysis. Applied to two typical ionic compounds, the method described in this work allows us to determine more realistic values of energy and τ0, corresponding to the measured relaxation times.
Characterizations of crystalline structure and electrical properties of pyrolyzed polyfurfuryl alcohol
Authors: Wang Zhaoxiang , Lu Zhonghua , Huang Yuzhen , Xue Rongjian , Huang Xuejie , Chen Liquan
Published: July 2002

Polyfurfuryl alcohol (PFA) is pyrolyzed at 600 °C for different lengths of heat-treatment (HT) time (0–50 h) and at different pyrolytic temperatures (600–850 °C) for 4 h. Analysis by Raman spectroscopy shows that the crystallite size of PFA pyrolyzed at 600 °C decreases with increasing HT time. This suggestion is supported by the electrical conductivity measurements on PFA samples pyrolyzed at different HT temperatures and at 600 °C for different HT times. A trend of transition from semiconductive to metallic properties is suggested for the samples prepared above 850 °C.
Modeling the contribution of quantum confinement to luminescence from silicon nanoclusters
Authors: Trwoga P. F. , Kenyon A. J. , Pitt C. W.
Published: July 2002

We present a model for the luminescence spectrum of silicon nanoclusters. We propose that the major contribution to luminescence is from radiative recombination of confined excitons (quantum confinement). Utilizing the effective mass approximation we consider the variation in oscillator strength with cluster size and the associated change in the number of available free carriers. By varying both the mean cluster size and size distribution of silicon nanoclusters, the luminescence spectra are modeled to a good fit. We compare our model with experimental photoluminescence and electroluminescence data from this group and from others.
Growth of adhesive contacts for Maxwell viscoelastic spheres
Authors: Jagota A. , Argento C. , Mazur S.
Published: July 2002

Coalescence of a Maxwell viscoelastic sphere to a frictionless and flat rigid plane is analyzed to study the transition from initial elastic adhesion to viscous sintering. Deformation is driven by surface tractions due to the surface energy. The formulation for surface forces consistently combines direct van der Waals attraction across the gap ahead of the contact edge with curvature-based tractions normal to the sphere surface. These two contributions to the surface traction result in two different modes of contact growth. The initial elastic contact and the early stage of time-dependent contact growth are in a zipping mode of contact closure dominated by direct attractive forces. The later stage of sintering is by stretching of the contact and is dominated by curvature-based tractions. The transition from the initial elastic contact to the zipping mode of contact growth is viscoelastic. For a given sphere radius, kinetics of the zipping mode of contact growth scale with a characteristic viscous sintering time. However, this mode is not part of the existing sintering models because direct attractive tractions were neglected in previous analyses of sintering. This stage of coalescence does not have unique scaling with sphere radius. The transition from the zipping to stretching mode of contact growth occurs at a contact radius that depends on sphere radius. The stretching mode of contact growth is in good agreement with previous analyses of viscous sintering.
Brillouin light scattering investigations of exchange biased (110)-oriented NiFe/FeMn bilayers
Authors: Mathieu C. , Bauer M. , Hillebrands B. , Fassbender J. , Güntherodt G. , Jungblut R. , Kohlhepp J. , Reinders A.
Published: July 2002

All contributing magnetic anisotropies in (110)-oriented exchange biased Ni80Fe20/Fe50Mn50 double layers prepared by molecular beam epitaxy on Cu(110) single crystals have been determined by means of Brillouin light scattering. Upon covering the Ni80Fe20 films by Fe50Mn50, a unidirectional anisotropy contribution appears, which is consistent with the measured exchange bias field. The uniaxial and fourfold in-plane anisotropy contributions are largely modified by an amount, which scales with the Ni80Fe20 thickness, indicating an interface effect. The strong uniaxial anisotropy contribution shows an in-plane switching of the easy axis from [11̄0] to [001] with increasing Ni80Fe20-layer thickness. The large mode width of the spin wave excitations, which exceeds the linewidth of uncovered Ni80Fe20 films by a factor of more than six, indicates large spatial variations of the exchange coupling constant.
Microstructure and properties of silicon nitride thin films deposited by reactive bias magnetron sputtering
Authors: Kim Joo Han , Chung Ki Woong
Published: July 2002

Silicon nitride (SiNx) thin films have been deposited by radio frequency (rf) magnetron sputtering of a silicon target in reactive nitrogen-argon atmospheres without intentional substrate heating. The influence of negative substrate bias Vs on the microstructural, compositional, chemical, mechanical, and optical properties of the SiNx films was systematically investigated. An extensive analysis of the films was carried out using ellipsometry, transmission electron microscopy (TEM), atomic force microscopy (AFM), Rutherford backscattering spectrometry, secondary ion mass spectrometry (SIMS), Fourier transform infrared (FTIR) spectroscopy, ultraviolet-visible spectroscopy, stress and chemical etch rate measurements. TEM and AFM studies revealed that films produced at low bias voltages had a porous columnar structure containing large void, typical of zone 1, but that films produced at higher bias voltages had relatively smooth surfaces with a highly condensed structure, typical of zone T. Both FTIR and SIMS analyses showed that an extremely small amount of hydrogen was contained in the SiNx films deposited at Vs over −75 V, resulting from the film densification by energetic bombardment. It was also found that the amount of argon incorporated in the film increased with increasing bias voltage, whereas the oxygen content decreased. As the substrate bias voltage was increased, the mechanical internal stress in the SiNx films became increasingly compressive and saturated at a value of about 1.8×1010 dyne/cm2 at higher bias voltages. This was found to be well correlated with the increased argon content and the film densification. The lowest etch rate in buffered hydrofluoric acid, approximately 72 Å/min, was observed with the application of a substrate bias of −50 V. A further reduction in etch rate could be achieved by annealing at 900 °C for 1 h in a N2 ambient. The optical band gap of the SiNx films varied from 4.85 to 4.39 eV depending on the bias voltage.
Photoluminescence studies of excitonic transitions in GaN epitaxial layers
Authors: Viswanath Annamraju Kasi , Lee Joo In , Yu Sungkyu , Kim Dongho , Choi Yoonho , Hong Chang-hee
Published: July 2002

GaN epitaxial layers on sapphire substrates were grown by the rotating disk metal organic chemical vapor deposition technique. Excitonic transitions from conduction band to spin-orbit split valence bands were observed. At 12 K we observed donor bound exciton and a very weak acceptor bound exciton. The temperature dependence of luminescence peak positions of free-excitons A and B were fitted to the Varshni’s equation to study the variation of the band gap with temperature. The linewidth of the free exciton (A) was studied as a function of temperature and was explained by theoretical model considering the scattering of excitons with acoustic phonons and longitudinal optical phonons. In the 12 K spectrum we also observed phonon-assisted excitonic transitions. The activation energy of the free exciton (A) was found to be 26 meV, while that of the donor bound exciton was 7 meV. The binding energy of the donor was estimated as 35 meV and that of the acceptor as 250 meV. The band gap of GaN was found to be 3.505 eV at 12 K and 3.437 at room temperature. All the parameters obtained in the present investigation are compared with those reported in the literature.
Spallation model for the high strain rates range
Authors: Dekel E. , Eliezer S. , Henis Z. , Moshe E. , Ludmirsky A. , Goldberg I. B.
Published: July 2002

Measurements of the dynamic spall strength in aluminum and copper shocked by a high power laser to pressures of hundreds of kbars show a rapid increase in the spall strength with the strain rate at values of about 107 s−1. We suggest that this behavior is a result of a change in the spall mechanism. At low strain rates the spall is caused by the motion and coalescence of material’s initial flaws. At high strain rates there is not enough time for the flaws to move and the spall is produced by the formation and coalescence of additional cavities where the interatomic forces become dominant. Material under tensile stress is in a metastable condition and cavities of a critical radius are formed in it due to thermal fluctuations. These cavities grow due to the tension. The total volume of the voids grow until the material disintegrates at the spall plane. Simplified calculations based on this model, describing the metal as a viscous liquid, give results in fairly good agreement with the experimental data and predict the increase in spall strength at high strain rates.
Single crystal growth and ferroelectric properties of α(Ba1−xSrx)Nb2O6:β(Na1−yKy)NbO3 solid solutions
Authors: Jiang Yijian , Guo Ruyan , Bhalla A. S.
Published: July 2002

Single crystals of quaternary solid solution compositions of A-site filled tungsten bronze-type α(Ba1−xSrx)Nb2O6:β(Na1−yKy)NbO3 (where α:β=2:1, 9:2, x=0.75, 0.60, and y=0.50, 0.40, and 0.25) have been grown by the laser heated pedestal growth technique, both along the a axis and the c axis. The crystal growth studies, the crystallographic parameter analysis, the dielectric properties as well as the thermal expansion behavior of the as grown single crystals are reported. Compositions with low ratios of Ba and Na in the 2:1 section plane have high polarizability, low dielectric loss, and ferroelectric relaxor characteristics.
Nitrogen-doped diamond films
Authors: Baranauskas V. , Li B. B. , Peterlevitz A. , Tosin M. C. , Durrant S. F.
Published: July 2002

We found that very high concentrations (up to 20% vol) of nitrogen in the ethanol/hydrogen gas mixture do not prejudice the diamond quality as determined by Raman spectroscopy. Nitrogen addition also increases the diamond growth rate, as was previously reported at low nitrogen concentrations. We observed that after a second heating cycle in air at temperatures between 300 and 673 K the electrical resistance versus temperature curves of the as-grown films presented a bulk semiconductor behavior. This stabilization was due to the oxidation of the as-grown hydrogenated surface. The electrical ionization energy Ed was found to be in the range of 1.62–1.90 eV corresponding to films produced with 0 to 20% vol nitrogen in the feed. The room temperature photoluminescence spectra of films produced at low nitrogen concentration suggest that Ed results from pure electronic transitions in the nitrogen-vacancy neutral defects; for samples produced with nitrogen concentrations in the range 15–20% vol the Ed values may be due to, among others, GR1 “vibronic” transitions and charged nitrogen-vacancy defects.
Effective electron collision frequency and electrical conductivity of radio frequency plasmas
Authors: Godyak V. A. , Piejak R. B. , Alexandrovich B. M.
Published: July 2002

The electron transport collision frequency in a radio frequency (rf) field νen and the effective rf field frequency ωeff have been found as appropriate integrals of the electron energy distribution function measured in an argon inductively coupled plasma (ICP) at 6.78 MHz. Also, the effective electron collision frequency νeff and ωeff have been found from the plasma conductivity measured with a magnetic probe. It is shown that due to the Ramsauer effect and collisionless power absorption in the ICP, νeff and νeff/ωeff vary unexpectedly slowly with gas pressure.
Hysteresis and magnetostriction of TbxDyyHo1−x−yFe1.95 [112] dendritic rods
Authors: Wun-Fogle M. , Restorff J. B. , Clark A. E.
Published: July 2002

The magnetization and magnetostriction of a variety of 3/16-in.-diam Laves phase rods of TbxDyyHo1−x−yFe1.95 grown in the form of [112] oriented dendritic compounds were measured as a function of applied magnetic field −3000<H<3000 Oe and compressive stresses of 22 and 47 MPa at room temperature. Compared to Terfenol-D (Tb0.3Dy0.7Fe1.95), the widely used room temperature magnetostrictive material, Ho containing alloys can have substantially lower hysteresis with only slightly lower magnetostriction. The Ho concentration was kept relatively small (⩽0.3) to avoid a substantial decrease in the magnetostriction, while the ratio of x and y was chosen to examine alloys spanning the line of minimum magnetic anisotropy. Most of the compositions have twice the Ho content of the previous study. As expected, alloys with higher Ho concentrations showed narrower hysteresis curves. The data shows that at 22 MPa, the Tb0.28Dy0.57Ho0.15Fe1.95 composition has a minimal (3%) loss of magnetostriction, while the hysteresis width decreased by 15%. Between 15% and 20% Ho content, the magnetostriction drops abruptly. For alloys with a fixed Ho concentration, the strain showed a peak near the expected anisotropy minimum, but the hysteresis width always increased with increasing Tb content.
Magnetic phase diagrams of erbium
Authors: Frazer B. H. , Gebhardt J. R. , Ali N.
Published: July 2002

The magnetic phase diagrams of erbium in the magnetic field–temperature plane have been constructed for applied magnetic fields along the a and b axes. For an a-axis applied field our H–T phase diagrams determined from magnetization and magnetoresistance data are in good agreement and consistent with that of Jehan et al. for temperatures below 50 K. A splitting of the basal plane Néel temperature (TN⊥) above 3.75 T introduces two new magnetic phases. Also a transition from a fan to a canted fan phase as suggested by Jehan et al. is observed in an increasing field below TC. Our phase diagram for a b-axis applied field constructed from magnetization data is very similar to the phase diagram of Watson and Ali using magnetoresistance measurements. However, the anomaly at 42 K reported by Watson and Ali is not observed in the present study. No splitting of the TN⊥ transition is observed in either work for a field applied along the b axis.
Permeability and electromagnetic-interference characteristics of Fe–Si–Al alloy flakes–polymer composite
Authors: Yoshida Shigeyoshi , Sato Mitsuharu , Sugawara Eishu , Shimada Yutaka
Published: July 2002

A magnetic sheet made of Fe–Si–Al flakes was found to have excellent permeability and electromagnetic-interferences characteristics in a quasimicrowave band. The flakes prepared by attrition were embedded in a polymer with their planes lying parallel to the sheet plane. This sheet exhibits a peculiar dual dispersion in permeability. The Curie temperature of the flakes increases with increasing specific surface area of the flakes. This suggests the composition of the flakes changes to excess Fe by oxidation of Si and Al. This deviation from the Sendust composition brings an increase of a magnetoelastic effect and the dual frequency dispersion seems to be associated with stress caused by attrition. On the basis of these results, flakes with more desirable permeability characteristics were produced from raw powder with less Fe content. Consequently, we could fabricate a magnetic composite that possesses superior noise suppressing effect in a quasi-microwave band.
Room-temperature photoreflectance and photoluminescence characterization of the AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor structures with varied quantum well compositional profiles
Authors: Lin D. Y. , Liang S. H. , Huang Y. S. , Tiong K. K. , Pollak Fred H. , Evans K. R.
Published: July 2002

Using room-temperature photoreflectance (PR) and photoluminescence (PL) we have characterized four pseudomorphic AlGaAs/InGaAs/GaAs high electron mobility transistor structures with varied quantum well compositional profiles. Several features from the InGaAs modulation doped quantum well portion of the samples have been observed in addition to signals from the AlGaAs, GaAs, and GaAs/AlGaAs superlattice (SL) buffer layer. The PR spectra from the InGaAs quantum well channel can be accounted for by a line shape function which is the first derivative of a step-like two-dimensional density of states and a Fermi level filling factor. A detailed line shape fit makes it possible to evaluate the Fermi energy, and hence the concentration of two-dimensional electron gas in addition to the energies of the intersubband transitions. The lowest lying intersubband transition has been confirmed by a comparison of the PR and PL spectra. From the difference of intersubband transition energies, the surface segregation effects of indium atoms are demonstrated. In addition, other important parameters of the system such as built-in electric field, Al composition, as well as the properties of the GaAs/AlGaAs SL buffer layer are evaluated.
Surface carbonization and nucleation during chemical vapor deposition of diamond
Authors: Louchev Oleg A. , Dussarrat Christian , Sato Yoichiro
Published: July 2002

The model presented considers surface kinetics processes, such as adsorption/desorption of hydrocarbon radicals, thermal dehydrogenation, evaporation, and carbon insertion into silicon, together with the bulk diffusion equation. The calculations performed show that surface protrusions present preferential locations for diamond nucleation due to a superposition of diffusion fluxes of carbon that is able to decrease the carbonization time on the protrusion tips by an order of magnitude compared with the untreated surfaces. This mechanism allows us to explain a number of observations in which the diamond is found to nucleate on the tips of surface protrusions. It is suggested that the surface carbon increases adsorption energy for hydrocarbon species and their concentration in the adlayer, resulting in high supersaturation which provokes nucleation. The analysis also shows that increased methane concentration in the feed gas and lower substrate temperature provide conditions for surface nucleation without a preliminary carbonization stage. The resulting diagram, outlining the operational parameters for both nucleation modes (via or without carbonization) on a Si (1,0,0) surface, is given.