bluuebunny/crossref_metadata_2025 huggingface dataset:
A simple theory of the Kondo hole
Authors: Sollie R. , Schlottmann P.
Published: July 2002

The Kondo hole is an isolated nonmagnetic impurity in a heavy-fermion lattice. Experimentally, the Kondo hole can be realized by La or Th atoms replacing a Ce ion in a stoichiometric Ce compound or by Th impurities in a U–heavy-fermion system. Adding impurities to a Kondo lattice gradually destroys the coherence of the heavy-fermion ground state. Perturbations of the coherent state are conveniently studied in the f-electron density of states. We consider a dispersionless orbitally nondegenerate f band locally hybridized with one nearest-neighbor hopping tight-binding conduction band on a simple cubic lattice. A hybridization gap opens in the symmetric case, and the Kondo hole introduces a bound state in the gap. We study the effects of the scattering on the local density of f states in the neighborhood of the nonmagnetic impurity. The effects of the correlations within the f band are introduced via a self-energy, evaluated to second-order perturbation in U, using the d=∞ treatment of Schweitzer and Czycholl [Solid State Commun. 74, 735 (1990)].
Free-volume and pore size distributions determined by numerical Laplace inversion of positron annihilation lifetime data
Authors: Gregory Roger B.
Published: July 2002

Positron annihilation lifetime spectroscopy is a powerful method for characterizing free volumes in a variety of materials. Correlations between positron annihilation rates and the size of free-volume regions in which o-Ps localizes are well described. Unfortunately, difficulties in the analysis of positron annihilation lifetime data have limited the approach to the determination of average lifetimes and average free volumes. Recent advances in the development of numerical integral transform methods now make it possible to extract continuous distributions of positron annihilation rates. The application of these methods to the determination of free-volume distributions is described. The variable transformations required to convert positron annihilation rate probability density functions (PDF) to the corresponding lifetime, radius, and free-volume PDFs are given and the approach is illustrated by application to positron annihilation lifetime data for amorphous polytetrafluoroethylene.
Effects of dc bias on the kinetics and electrical properties of silicon dioxide grown in an electron cyclotron resonance plasma
Authors: Carl D. A. , Hess D. W. , Lieberman M. A. , Nguyen T. D. , Gronsky R.
Published: July 2002

Thin (3–300-nm) oxides were grown on single-crystal silicon substrates at temperatures from 523 to 673 K in a low-pressure electron cyclotron resonance (ECR) oxygen plasma. Oxides were grown under floating, anodic or cathodic bias conditions, although only the oxides grown under floating or anodic bias conditions are acceptable for use as gate dielectrics in metal-oxide-semiconductor technology. Oxide thickness uniformity as measured by ellipsometry decreased with increasing oxidation time for all bias conditions. Oxidation kinetics under anodic conditions can be explained by negatively charged atomic oxygen, O−, transport limited growth. Constant current anodizations yielded three regions of growth: (1) a concentration gradient dominated regime for oxides thinner than 10 nm, (2) a field dominated regime with ohmic charged oxidant transport for oxide thickness in the range of 10 nm to approximately 100 nm, and (3) a space-charge limited regime for films thicker than approximately 100 nm. The relationship between oxide thickness (xox), overall potential drop (Vox) and ion current (ji) in the space-charge limited transport region was of the form: ji ∝ V2ox/x3ox. Transmission electron microscopy analysis of 5–60-nm-thick anodized films indicated that the silicon-silicon dioxide interface was indistinguishable from that of thermal oxides grown at 1123 K.

High-frequency capacitance-voltage (C-V) and ramped bias current-voltage (I-V) studies performed on 5.4–30-nm gate thickness capacitors indicated that the as-grown ECR films had high levels of fixed oxide charge (≳1011 cm−2) and interface traps (≳1012 cm−2 eV−1). The fixed charge level could be reduced to ≊4×1010 cm−2 by a 20 min polysilicon gate activation anneal at 1123 K in nitrogen; the interface trap density at mid-band gap decreased to ≊(1–2)×1011 cm−2 eV−1 after this process. The mean breakdown strength for anodic oxides grown under optimum conditions was 10.87±0.83 MV cm−1. Electrical properties of the 5.4–8-nm gates compared well with thicker films and control dry thermal oxides of similar thicknesses.
Comment on ‘‘Negative capacitance at metal-semiconductor interfaces’’ [J. Appl. Phys. 68, 2845 (1990)]
Authors: Werner Jürgen H.
Published: July 2002

Recently, Wu, Yang, and Evans [J. Appl. Phys. 68, 2845 (1990)] ascribed excess negative and positive capacitances at abrupt Schottky diodes to interface charges and a ‘‘waterfall’’ of electrons ‘‘falling off the Schottky barrier cliff.’’ This comment points out that such strange admittances are not related to interface charge at the front Schottky contact but to defective back contacts, as previously demonstrated [Phys. Rev. Lett. 60, 53 (1988) and Mater. Res. Soc. Symp. Proc. 91, 433 (1987)].
Piezoelectric properties of thin films of aromatic polyurea prepared by vapor deposition polymerization
Authors: Takahashi Yoshikazu , Ukishima Sadayuki , Iijima Masayuki , Fukada Eiichi
Published: July 2002

Aromatic polyurea films of a few hundred nanometer thickness were prepared by vapor deposition polymerization of the monomers 4,4′-diaminodiphenylmethane and 4,4′-diphenylmethane-diisocyanate. Piezoelectric activity was observed after poling the films under an electric field of 100 MV/m at 210 °C for 10 min. The piezoelectric stress constant (polarization/strain) remains constant at about 15 mC/m2 over the temperature range from −50 to 200 °C. The multilayer film produced by the alternate deposition of polyurea layers and aluminum electrodes exhibited a response proportional to the number of layers. The residual polarization caused by alignment of the dipoles of the urea bond is believed to be the origin of the effect.
Model of EL2 formation in GaAs
Authors: Morrow Richard A.
Published: July 2002

It is demonstrated that existing thermodynamic data on the native deep donor, EL2, in melt-grown and epitaxially grown GaAs are consistent with that defect having the atomic structure AsGaVGa. In melt-grown GaAs at high temperatures (∼1200 °C) arsenic antisite defects appear as the complex AsGaVAsVGa. As the temperature drops toward 1000 °C and the equilibrium concentration of divacancies decreases this complex dissociates, the divacancies outdiffusing and the antisites capturing gallium vacancies to form EL2. In GaAs grown by organometallic vapor-phase epitaxy it is suggested that the arsenic interstitial is the dominant native defect produced in equilibrium with the vapor and that it dictates the deviation from stoichiometry of the epilayer. Below the growth interface these interstitials rapidly react with indiffusing divacancies to form primarily arsenic antisites. Other divacancies then react with the antisites to briefly form the complexes AsGaVAsVGa which, in the nonuniform temperature regime of the epilayer, dissociate into EL2 and arsenic vacancies. The model predicts [EL2]∝(As/Ga)1/2 in agreement with selected data and predicts that the EL2 concentration will increase under a nonuniform thermal anneal. It also accounts for the formation of EL2 in GaAs grown by molecular-beam epitaxy when subsequently annealed at ∼800 °C in a nonuniform temperature environment.
Thermal quenching mechanism of Yb intra-4f-shell luminescence in InP
Authors: Taguchi Akihito , Nakagome Hiroshi , Takahei Kenichiro
Published: July 2002

Thermal quenching mechanism of Yb intra-4f-shell luminescence were clarified by studying the temperature dependences of electrical and optical properties of Yb-doped InP samples. The quenching mechanism which depends on shallow donor concentration was found by comparing temperature dependences of Yb 4f-shell luminescence and free-carrier concentration. This mechanism is a localized Auger effect and is efficient below about 70 K in samples having a larger donor concentration than Yb concentration. At higher temperatures, another quenching mechanism was found to be efficient which does not depend on donor concentration. This quenching of Yb intra-4f-shell luminescence is accompanied by some increase of band-edge related luminescence. This phenomenon is explained by the energy back-transfer mechanism from the excited Yb 4f-shell to the InP host. Although an importance of the free-carrier Auger effect has been suggested in conducting materials, we propose that above two mechanisms dominate the quenching of the Yb 4f-shell luminescence in InP.
Electrical characteristics of Zn in heavily doped InP grown by the liquid-encapsulated Czochralski technique
Authors: Hirano R. , Kanazawa T. , Inoue T.
Published: July 2002

The electrical activity of Zn in heavily doped InP crystals grown by the liquid-encapsulated Czochralski technique has been studied through annealing experiments. When the wafers were annealed at 650 °C, the saturated hole concentration increased as the cooling rate after the heat treatment increased. Annealing at 400 °C led to a reduction in the saturated hole concentration, which was not dependent on the cooling rate after the heat treatment. Some defect reactions at 400 °C would account for this phenomenon. The experimental results of the saturation of the hole concentration may be explained in terms of a defect state whose nature depends on the Fermi level position, as has been suggested by Walukiewicz [Appl. Phys. Lett. 54, 2094 (1989)].
Crystallization of amorphous Co-Si alloys
Authors: Hong Q. Z. , Barmak K. , Clevenger L. A.
Published: July 2002

The crystallization of thin-film, amorphous Co-Si alloys has been studied in the composition range of 42–80 at. % Si. Structural relaxation of the amorphous alloys preceded the crystallization. Crystallization temperature decreased slowly with increasing Si concentration, but showed a sharp decrease near the stoichiometric silicide compositions of CoSi and CoSi2. The crystallization of alloys close in composition to the disilicide (Co0.33Si0.67 and Co0.29Si0.71) was a one-step process with an activation energy of about 1.3 eV and an Avrami exponent of 3. In situ kinetic studies revealed that for the stoichiometric Co0.33Si0.67 alloy, the Avrami exponent of 3 arose from three-dimensional crystal growth from nuclei whose density saturated at the early stages of the transformation. The heat release and the volume contraction during crystallization of the Co0.33Si0.67 alloy were measured to be 0.118 eV/atom and 0.6%, respectively. In the case of a Co0.2Si0.8 alloy, which is considerably richer in Si than the disilicide composition, complete crystallization was achieved in a two-step process with a higher activation energy of 2.1 eV for the primary crystallization.
Activation volume for the interdiffusion of Ag-Au multilayers
Authors: Wonnell S. K. , Delaye J. M. , Bibolé M. , Limoge Y.
Published: July 2002

The activation volume ΔV for chemical diffusion in Ag50Au50 alloy has been determined from the pressure dependence of the rate of decrease in amplitude of composition-modulated thin films. Based on resistivity measurements, it is argued that the technique yields the bulk chemical diffusion coefficient, in close agreement with the experimental results. Eight measurements of the diffusion coefficient were made, four as a function of temperature (237–309 °C) at atmospheric pressure and four as a function of pressure (0–0.85 GPa) at constant temperature (269.5 °C). The samples, 2000–3000-Å-thick multilayers of alternating silver and gold, with periods between 72 and 160 Å, were made with a newly built UHV sputter-deposition apparatus. The bulk chemical interdiffusion activation energy (1.5 eV) and activation volume (0.72 Ω) agree with those expected from other interdiffusion and tracer measurements.
Raman scattering in polycrystalline silicon doped with boron
Authors: Nakano Noboru , Marville Louis , Reif Rafael
Published: July 2002

The Raman spectra of polycrystalline silicon doped with implanted boron were measured before and after thermal annealing. In addition to the peak of the silicon phonon mode at 521 cm−1, a shoulder at about 500 cm−1 was observed. For samples without annealing, this shoulder increased with the increase of the dopant dose. The origin of this shoulder was attributed to the small correlation length due to lattice damage. After thermal annealing, the peak of the silicon phonon mode was greatly broadened. This was best explained by a theoretical model which included both a finite size effect and a Fano effect. The asymmetry factors associated with the Fano effect were obtained as a function of the dopant dose.
Anomalous charge transport phenomena in molecular-based magnet V(TCNE)x⋅y(solvent)
Authors: Du G. , Joo J. , Epstein A. J. , Miller Joel S.
Published: July 2002

We present charge transport studies on recently developed molecular-based magnets V(TCNE)x⋅y(solvent) (TCNE=tetracyanoethylene) which show local magnetic ordering at temperatures, T, as high as 400 K. V(TCNE)x⋅y(solvent) prepared from the solvent CH2Cl2 has σ(300 K)∼10−3 S/cm, and that prepared from the solvent CH3CN has σ(300 K)∼10−5 S/cm. The σ(T) of both materials follows the Mott 3D variable range hopping [log(T1/2σ)∝T−1/4] behavior. An anomalously strong T-dependent ac conductivity is likely caused by the short-range ferrimagnetic correlations. Cole–Cole analysis of the ac complex dielectric constant suggests there are two relaxation mechanisms, with an Arrhenius relation fitted to both. The microwave frequency response is consistent with audio frequency data and yields a localization length of ∼ 5 Å, comparable with the dimensions of a [TCNE]−. group.
A photoemission investigation of the SnO2/CdS interface: A front contact interface study of CdS/CdTe solar cells
Authors: Niles David W. , Rioux Dennis , Höchst Hartmut
Published: July 2002

A frequently used front contact in CdS/CdTe heterojunction solar cells is SnO2. We have performed a soft x-ray synchrotron radiation photoemission investigation of the formation and thermal stability of the SnO2/CdS interface in an attempt to understand how device processing influences this interface. The most important results are that (1) the CdS and SnO2 do not interact chemically, even after annealing to 400 °C, (3) the first ∼16 Å CdS deposited on polycrystalline SnO2 grows in a layer-by-layer mode, (3) subsequent CdS layers agglomerate, forming a topologically rough surface, (4) diffusion of Sn, Cd, and S across the interface does not occur, (5) annealing a CdS thin film grown at room temperature on SnO2 to 400 °C enhances the agglomeration, and (6) the near coincidence of the conduction-band minimum across the interface facilitates ohmic contact between n-type CdS and n-type SnO2.
The thermoelectric properties and crystallography of Bi-Sb-Te-Se thin films grown by ion beam sputtering
Authors: Noro H. , Sato K. , Kagechika H.
Published: July 2002

The effects of the substrate temperature on the composition and microstructure of Bi-Sb-Te-Se thin films are discussed and related to the Seebeck coefficient and resistivity. The films were prepared on silica substrates by argon ion beam sputtering. The effect of subsequent heat treatment is also discussed. The composition becomes stoichiometric at the substrate temperature around 500 K at which the Seebeck coefficient has a maximum and the resistivity has a minimum. The Seebeck coefficient changes its sign at the substrate temperature around 340 K. The crystal structure of the films deposited at room temperature is identified as the rock-salt type, and the change of the thermoelectric properties as a function of the substrate temperature can be mainly attributed to the volume fraction of the Bi2Te3 and rock-salt structures. The subsequent heat treatment is effective in improving the power factor of the stoichiometric thin films.
Relation between optical property and crystallinity of ZnO thin films prepared by rf magnetron sputtering
Authors: Takada Syuichi
Published: July 2002

ZnO (zinc oxide) thin films have been prepared by using rf magnetron sputtering on a SiO2/Si substrate. The crystallographic properties and optical properties were investigated. It is shown that the ZnO films exhibit strong c-axis orientation. The standard deviation of the c axis was obtained less than 1°. The optical propagation loss was less than 2 dB/cm (TE0 mode) in a 0.6-μm-thick film. To achieve the ZnO films with low light propagation loss, it was found that we must take into account not only c-axis orientation of ZnO films but also the density of ZnO films.
Dual-ion-beam sputter deposition of ZnO films
Authors: Quaranta Fabio , Valentini Antonio , Rizzi Federica R. , Casamassima Giuseppe
Published: July 2002

Physical properties of zinc oxide films deposited by dual-ion-beam sputtering are analyzed to point out the performance of this technique for the deposition of this material. The films are deposited by sputtering a zinc oxide target with an argon-ion beam, while a second low-energy beam, the assistance ion beam, impinges directly on the growing films. Results are presented for ZnO films deposited at room temperature with different oxygen/argon ratios in assistance ion beam and different sputtering ion-beam currents. Elemental, structural, and electrical analyses have been performed on films. All the films show the typical crystallographic orientation, with the c axis perpendicular to the substrate. The oxygen percentage in the assistance ion beam plays an important role in controlling the electrical resistivity of the films.
Band-band impact ionization and solar cell efficiency
Authors: Landsberg P. T. , Nussbaumer H. , Willeke G.
Published: July 2002

The effect of impact ionization has been taken into account in the calculation of the maximum solar cell efficiency in the thermodynamic limit. A red shift of the optimum band gap is observed with respect to the Shockley–Queisser result. A maximum solar cell efficiency of 60.3% at Eg=0.8 eV is predicted, which compares with 43.9% at Eg=1.1 eV for the usual case. In order to obtain a significant increase in η, an impact ionization probability of at least a few percent is required. A most pronounced effect is observed for bandgap energies below about 1 eV.
Emissivity of silicon at elevated temperatures
Authors: Timans P. J.
Published: July 2002

The temperature dependences of the spectral and total hemispherical emissivities of silicon have been experimentally determined, by using a technique which combines isothermal electron beam heating with in situ optical measurements. Emission spectra were used to deduce the absorption coefficient for phosphorus-doped silicon samples for wavelengths between 1.1 and 1.6 μm, in the temperature range from 330 to 800 °C. For lightly doped samples, the data show good agreement with a model which includes the effects of the various phonon-assisted processes involved in interband transitions in silicon, as well as the free-carrier absorption. For heavily doped samples the agreement was less satisfactory, possibly because of inadequacies in the model for free-carrier absorption. It was shown that reflection spectra can also be used to determine the absorption spectrum, for the range where the absorption coefficient lies between 1 and ∼70 cm−1. By fitting the theoretical model to the absorption coefficients derived from the reflection spectrum, it is possible to deduce the temperature of a sample, which is especially useful for temperatures less than 300 °C, where the thermal emission is very weak. The total hemispherical emissivity of the specimens was determined from the input electron-beam power densities and the measured temperatures. The total emissivity of a 390-μm-thick specimen of lightly doped silicon rises from 0.12 at 280 °C to a limiting value of 0.7 at 650 °C. This behavior is a consequence of the increase in the free-carrier concentration with the temperature. For heavily doped specimens the total emissivity remains approximately constant at ∼0.7 between 200 and 800 °C because the carrier concentration is high even at room temperature, and the additional thermal generation of carriers produces an insignificant change in the total emissivity.
Nitriding of melt-spun Nd-Fe-Mo alloys
Authors: Pinkerton F. E. , Fuerst C. D. , Herbst J. F.
Published: July 2002

The hard magnetic properties of nitrided melt-spun Nd-Fe-Mo ribbons have been investigated as functions of composition, quench rate, and nitriding time. Ingots having the compositions Nd1Fe10Mo2, Nd1.15Fe10Mo2, Nd1.3Fe10Mo2, and Nd1.45Fe10Mo2 were melt spun over a range of quench rates specified by substrate wheel velocities vs in the 5 m/s≤vs≤40 m/s interval. Ribbons prepared with vs≤17.5 m/s from the Nd1.15Fe10Mo2 alloy consist only of stoichiometric NdFe10Mo2 with the ThMn12 crystal structure; at higher quench rates the ThMn12 structure is suppressed. It is found that all ribbons prior to nitriding are characterized by intrinsic coercivity Hci<0.4 kOe. For ribbons direct quenched at wheel speeds between 12.5 and 17.5 m/s, nitriding produces technologically significant coercivity (Hci∼6 kOe). The maxima in the technical magnetic properties as functions of vs resemble those observed in direct-quenched Nd-Fe-B ribbons.
Magnetic properties of carbon-coated rare-earth carbide nanocrystallites produced by a carbon arc method
Authors: Diggs B. , Zhou A. , Silva C. , Kirkpatrick S. , Nuhfer N. T. , McHenry M. E. , Petasis D. , Majetich S. A. , Brunett B. , Artman J. O. , Staley S. W.
Published: July 2002

Carbon-coated gadolinum and holmium carbide nanocrystallites have been generated using a modification of the Huffman-Krätschmer carbon arc process. Bulk amounts of these particles were isolated from the other by-products using a magnetic field gradient. Transmission electron microscopy revealed the presence of 10–50 nm diameter crystallites coated with numerous graphitic layers. The nanocrystallite phases were identified as Gd2C3 and Ho2C3, respectively, by x-ray and electron diffraction. Magnetization measurements were performed using a superconducting quantum interference device magnetometer between ±5 T at temperatures ranging from 4 to 200 K. The magnetization curves were shown to scale as a function of H/T. The RE3+ sites in RE2C3 have C3 site symmetry. For Gd2C3 the universal curve was fit with a Brillouin function consistent with the Gd3+ free-ion ground-state values of J=7/2 and g=2. The 5I8 Ho3+ free-ion ground state is split, presumably due to a C3 symmetry crystal field. Consequently, for Ho2C3 the Ho3+ free-ion parameters could not be used to fit the experimental data. Empirical fits to the Brillouin function yield a reduced moment of 7.5μB, compared to the free-ion value of 10.6μB. A similarly reduced moment was observed in holmium-containing endohedral fullerenes.
Magnetic viscosity in high-density recording
Authors: Lu Pu-Ling , Charap Stanley H.
Published: July 2002

For future ultrahigh-density magnetic recording, the magnetic viscosity in thin-film media will become an issue due to the drastic reduction in grain size. An algorithm combining a Monte Carlo method and molecular dynamics was employed to study the thermal effects in thin-film media. The component of the field perpendicular to the plane defined by the axes of shape anisotropy and uniaxial crystalline anisotropy makes it necessary to use the three-dimensional energy surface to find the minimum energy barrier. This barrier is used to sample the reversal rate and the elapsed time. Hysteresis loops for various KuV/kT ratios and sweep times are simulated. Isolated and di-bit transitions are written, taking into account thermally assisted switching. After the head field is turned off, the subsequent thermal decay is computed for time spans as long as 6 months. Significant aftereffect is found for grain volumes about twice that for ordinary superparamagnetism.
Measured radial and angular distributions of sputtered atoms in a planar magnetron discharge
Authors: Turner G. M. , Rossnagel S. M. , Cuomo J. J.
Published: July 2002

The effect of current, voltage, and magnetic field on the radial profile of ions sputtering a flat cathode has been studied in a hollow cathode enhanced magnetron discharge. The measurements of sputtering rate were made using a directional quartz crystal microbalance, translated parallel to the cathode. We find the radial sputtering profiles are determined by the plasma sheath thickness, which can be calculated using the Child–Langmuir law for collisionless space-charge limited current flow. A hollow cathode electron source was used to control the sheath thickness by increasing the current to the magnetron cathode at a given voltage and constant pressure (0.07 Pa). Uniformity of the radial profiles increases with decreasing sheath thickness (i.e., at lower voltages and higher current densities). The angular distribution of sputtered atoms was measured by changing the polar angle of the microbalance. The angular distributions are asymmetric on either side of the radial position where the ion current density to the cathode is maximum. We believe this is due to ions which impact the cathode at angles up to ≊10° from the normal, having been radially accelerated away from the position of maximum plasma density.
Electrical resistivity of bulk nanocrystalline nickel
Authors: Aus M. J. , Szpunar B. , Erb U. , El-Sherik A. M. , Palumbo G. , Aust K. T.
Published: July 2002

The effect of grain boundaries on the electrical resistivity of bulk nickel with a nanocrystalline structure has been investigated. For this study, nanocrystalline nickel was produced using a pulse plating technique that yields equiaxed nanostructures with negligible porosity. Using the four probe technique, resistivity measurements were performed at various temperatures ranging from 77 K to room temperature. The values of the resistivity were found to increase with decreasing grain size. This is mainly due to the increased volume fraction of interfaces at smaller grain sizes and the associated electron scattering events at the grain boundaries. The temperature coefficient of resistivity was found to decrease with decreasing grain size.
Microstructural development in nanophase TiO2 during annealing
Authors: Eastman J. A.
Published: July 2002

In situ x-ray-diffraction studies of nanophase TiO2 during annealing reveal a strong angular-dependent decrease in diffuse background intensities and a corresponding increase in rutile and anatase phase peak intensities for samples annealed above 623 K. This behavior is consistent with the crystallization of an amorphous phase present in unannealed samples in addition to the two previously observed crystalline phases. The rutile lattice is significantly strained in unannealed samples, with a0 increased and c0 decreased compared to the values for coarse-grained material. Normal bulk lattice parameters are obtained after annealing samples at 1073 K. Assuming that strain contributions to the measured low-angle peak breadths are negligible, measurements of the time and temperature dependencies of grain growth in nanophase rutile during annealing indicate a grain growth exponent of 0.64±0.06 and a grain growth activation enthalpy of 83±40 kJ/mol.
Die-upset PrCo5-type magnets: Enhanced coercivities
Authors: Fuerst C. D. , Brewer E. G.
Published: July 2002

Rare-earth–cobalt alloys have been melt spun, yielding ribbons whose major phase was the hexagonal PrCo5 phase. Fully dense hot-pressed magnets with essentially isotropic magnetic properties have been formed by consolidating the ribbons at high temperatures and pressures. The hot-pressed precursor was then subjected to hot deformation or die upsetting, thereby inducing partial alignment of the c axis along the press direction. The resulting magnets had remanences approaching ∼75% of the saturation magnetization of the hexagonal PrCo5 phase. Three approaches were used to optimize the coercivity of the die-upset magnets: (i) replacing small amounts of cobalt with carbon; (ii) partially substituting samarium for the praseodymium; and (iii) high-temperature heat treatment followed by a thermal quench. Coercivities near ∼20 kOe were achieved with thermally quenched die-upset (Pr0.6Sm0.4)18Co81C magnets, while maintaining remanences above 8 kG.
Computer simulations of pore growth in silicon
Authors: Erlebacher J. , Sieradzki K. , Searson P. C.
Published: July 2002

The dynamics of pore formation in silicon was studied by computer simulation. Porous structures were generated by a Monte Carlo algorithm that controlled the motion of an ensemble of electronic holes on a two-dimensional square lattice. Hole motion was biased to the nearest pore tip in order to simulate local electric-field effects corresponding to the depletion layer setup in n-type silicon. Several morphological characteristics seen in porous silicon were seen in the simulation: highly directional pores, steady-state pore spacing, and a smooth pore front. The pore spacing and the degree of sidebranching depended on the concentration of holes and the magnitude of the bias. The simulation trends are analogous to the pore morphologies seen in n-type silicon.
Ballistic metal-oxide-semiconductor field effect transistor
Authors: Natori Kenji
Published: July 2002

Experiments on ultra-small metal-oxide-semiconductor field effect transistors (MOSFETs) less than 100 nm have been widely reported recently. The frequency of carrier scattering events in these ultra-small devices is diminished, so that further suppression of carrier scattering may bring these devices close to the regime of ballistic transport. Carrier scattering is suppressed by constructing their channel regions with intrinsic Si and also by low temperature operation. This article proposes the ballistic transport of carriers in MOSFETs, and presents the current-voltage characteristics of the ballistic n-channel MOSFET. The current is expressed with the elementary parameters without depending on the carrier mobility. It is independent of the channel length and is proportional to the channel width. The current value saturates as the drain voltage is increased and the triode and the pentode operation are specified as in the conventional MOSFET. Similar current-voltage characteristics in the ballistic transport regime are also investigated for the p-channel MOSFET, the dual gate ultra-thin silicon on insulator MOSFET, and the high electron mobility transistor device. The obtained current gives the maximum current limitation of each field effect transistor geometry. The current control mechanism of ballistic MOSFETs is discussed. The current value is governed by the product of the carrier density near the source edge in the channel, and the velocity with which carriers are injected from the source into the channel.

Influence of optical phonon emission to the transport is discussed. It is suggested that if the device is operated with relatively low carrier density at low temperatures, and if the scattering processes other than the optical phonon emission are suppressed so as to attain the ballistic transport, the optical phonon emission is also suppressed and ballistic transport is sustained. A convenient figure of merit to show the ballisticity of carrier transport in an experimental MOSFET is proposed. Its value is estimated for some examples of the recent ultra-small MOSFET experiment. The proposed current voltage characteristics are evaluated for a dual gate silicon on insulator MOSFET geometry. The result is compared with the recently reported elaborate Monte Carlo simulation with satisfactory agreement.
Surface diffusion processes in molecular beam epitaxial growth of GaAs and AlAs as studied on GaAs (001)-(111)B facet structures
Authors: Koshiba S. , Nakamura Y. , Tsuchiya M. , Noge H. , Kano H. , Nagamune Y. , Noda T. , Sakaki H.
Published: July 2002

Mechanisms of molecular beam epitaxy have been investigated for GaAs and AlAs by growing and analyzing the shapes of facet structures consisting of an (001) top surface and two (111)B side surfaces. It is found that all of the Ga flux on the three facet planes is incorporated into the film, but the growth rates on (111)B and (001) depend strongly on the As flux and are mainly determined by the diffusion of Ga ad-atoms between the two planes. In contrast, the diffusion of Al is found to be almost negligible, irrespective of the As flux. By analyzing the shape of the facet, the diffusion length, λ, of Ga on a (001) surface is estimated to be about 1 μm at 580 °C, while that of Al is about 0.02 μm. On (111)B, λ of Ga is found to be several μms. The reflectivity of diffusing Ga atoms is found to be far less than 1 for the (001)-(111)B boundary, and almost unity at facet boundaries where the (111)B side surfaces are bound by the (11̄0) side walls.
Ferroelectric properties and fatigue of PbZr0.51Ti0.49O3 thin films of varying thickness: Blocking layer model
Authors: Larsen P. K. , Dormans G. J. M. , Taylor D. J. , van Veldhoven P. J.
Published: July 2002

Ferroelectric capacitors having Pt bottom and top electrodes and a ferroelectric film of composition PbZr0.51Ti0.49O3 (PZT) were fabricated and investigated. The PZT films of thicknesses varying from 0.12 to 0.69 μm were prepared by organometallic chemical-vapor deposition. Annealed capacitors were investigated by capacitance, hysteresis, and pulse switching measurements. It is found that the thickness dependence of the reciprocal capacitance, the coercive voltage, and the polarization measured by pulse switching can all be explained by a blocking layer model, in which a dielectric layer of thickness dbl and relative permittivity εbl is situated between the PZT film and an electrode. It is shown that (i) the coercive field is independent of thickness having a value of 2.4 V/μm; (ii) the ratio εbl/dbl is in the range 20–28 nm−1; (iii) the voltage across the blocking layer is proportional to the polarization, Vbl=cP, where c=4.1±0.5 V m2/C; and (iv) the polarization depends on the electric field in the PZT layer, independent of thickness. Pulse switching endurance measurements showed that in the saturation range the fatigue for these ferroelectric capacitors is determined by the pulse voltage and is independent of the thickness.
Soliton switching in ferroelectric liquid crystals and their transient electro-optic response
Authors: Abdulhalim I. , Moddel G. , Clark N. A.
Published: July 2002

The switching behavior of ferroelectric liquid crystals (FLCs) is studied both experimentally and theoretically. Experimentally the electro-optic response of multiwavelength-thick FLC cells revealed transient peak and dip characteristics. This behavior could not be explained at all according to the standard picture of a switchable birefringent plate, nor according to transient light scattering associated with ferroelectric domain reversal. Solutions of a numerical model of the FLC electric and elastic torques, which results in the sine-Gordon equation, showed the propagation of kink-antikink pair solitons. The nonuniform director structures corresponding to these solitons are sufficient to explain the experimental behavior of the transient electro-optic response with the wavelength, the field, and the polarizer-cell-analyzer orientation. The electro-optic response is found to be highly sensitive to the fixed boundary dipole orientations.
Electrical and metallurgical behavior of Au/Zn contacts to p-type indium phosphide
Authors: Fatemi Navid S. , Weizer Victor G.
Published: July 2002

The metallurgical and electrical behavior of Au/Zn contacting metallization on p-type InP is investigated as a function of the zinc content in the metallization. It was found that ohmic behavior can be achieved with Zn concentrations as small as 0.05 at. % Zn. For Zn concentrations between 0.1 and 36 at. % the contact resistivity was found to be independent of the Zn content. For low Zn concentrations the realization of ohmic behavior was found to require the growth of the compound Au2P3 at the metal-InP interface. The magnitude of the specific contact resistivity is shown to be very sensitive to the growth rate of the interfacial Au2P3 layer. The possibility of exploiting this sensitivity to provide low-resistance contacts while avoiding the semiconductor structural damage that is normally attendant to contact formation is discussed. Evidence is presented, finally, that the presence of Zn in the Au-Zn/p-InP contact system lowers the contact resistivity by effecting an increase in the work function of the Au2P3 interfacial layer.
Shapiro steps in large-area metallic-barrier Josephson junctions
Authors: Kautz R. L.
Published: July 2002

The current amplitudes of Shapiro steps in large-area metallic-barrier Josephson junctions, both with and without a ground plane, are investigated with the goal of optimizing junction parameters for programmable voltage standards. Using the resistively shunted junction model without capacitance, we calculate maximum step amplitudes as a function of reduced frequency and junction dimension for both one- and two-dimensional junctions. For junctions without a ground plane, we conclude that step amplitudes of order 10 mA are practical, but significantly larger amplitudes require excessive microwave power.
Standing-wave acoustic trap for nonintrusive positioning of microparticles
Authors: Hertz H. M.
Published: July 2002

A nonintrusive three-dimensional trap for microscopic particles in a liquid is described. The trap is based on acoustic radiation forces in an ultrasonic confocal standing-wave cavity. Experiments at 11 MHz demonstrate the concept and verify the theoretically calculated forces. Theoretical calculations for higher-frequency systems indicate a significant potential for low-rms-displacement trapping of submicrometer particles, making the trap suitable for nonintrusive scanning near-field optical microscopy.
Enhancement of material ablation using 248, 308, 532, 1064 nm laser pulse with a water film on the treated surface
Authors: Dupont A. , Caminat P. , Bournot P. , Gauchon J. P.
Published: July 2002

Many industrial laser processes, such as surface cleaning, require the removal of small thicknesses of matter, often on large samples. An experimental study has been performed in order to characterize and enhance the ablation of materials by means of the interaction between a pulsed laser beam and matter using common industrial laser sources, particularly at 248, 308, 532, and 1064 nm. Ablation was achieved on a static sample with one or several successive pulses and for different energy densities. These parameters enabled us to control the depth of the ablation in the micrometer range. Experiments have been carried out in ambient air, under gaseous cover, and then under a flowing water film on the material surface at atmospheric pressure. The material was a stainless-steel alloy. The experiments made it possible to determine the ablation fluence threshold for each wavelength and the alteration of the surface by the successive laser pulses. In order to avoid oxidation during the process, an argon gaseous medium was used, but peripheral oxide deposits always appeared and the efficiency of the ablation did not seem to increase. However, when the water flow on the surface was employed, the efficiency of the ablation increased by a factor of 2–15 for a certain incident power density. Peripheral oxide redeposition was absent using this method. Experiments have demonstrated that the confinement of the plasma within a transparent layer such as water at the given laser wavelength is an effective method for increasing ablation yield. This technique prevents the expansion of the plasma upstream of the target and the mechanical impulse communicated to the material increases. Calculation of ablated mass confirms that ablation yield could be increased by 15 for higher power densities. Similar results have been found using other materials such as alumina or silica. This means that this ablation technique can be employed in various types of industrial laser applications, for example, for cleaning oxidized surfaces or removing paint coatings.
Reactive ion etching mechanism of plasma enhanced chemically vapor deposited aluminum oxide film in CF4/O2 plasma
Authors: Kim Jae-Whan , Kim Yong-Chun , Lee Won-Jong
Published: July 2002

Aluminum oxide film prepared by plasma enhanced chemical vapor deposition (PECVD) is one of the promising candidates for an etch mask or an etch barrier material in very large scale integrated fabrication. We have investigated the reactive ion etching mechanism of the PECVD aluminum oxide films in the CF4/O2 plasma. The dependences of the aluminum oxide etch rate on the atomic fluorine concentration and the incident particle bombardment energy are studied at various etching conditions. The etch products and their depth distribution are also determined by analyzing the surface layer of the etched aluminum oxide films with Auger electron spectroscopy. The CF4/O2 plasma fluorinates the aluminum oxide surface layer through the particle bombardment activated reaction, producing etch products in the form of AlF3 or AlOXFY, which is nonvolatile but has a higher sputtering yield than aluminum oxide. The reactive ion etching of aluminum oxide proceeds by the formation and the sputter removal of the etch product. The etch rate strongly depends on the particle bombardment energy because it determines not only the formation rate but also the removal rate of the etch products. However, the atomic fluorine concentration has little effect on the etch rate because the consumption rate of fluorine radicals is so low that they are always superfluous under any experimental conditions.
Orthotropic elastic constants of a boron-aluminum fiber-reinforced composite: An acoustic-resonance-spectroscopy study
Authors: Ledbetter Hassel , Fortunko Christopher , Heyliger Paul
Published: July 2002

Acoustic-resonance spectroscopy was used to determine the complete elastic constants of a uniaxial-fiber-reinforced metal-matrix composite: boron-aluminum. This material exhibits orthotropic macroscopic symmetry and, therefore, nine independent elastic stiffnesses Cij. The off-diagonal elastic constants (C12,C13,C23), which contain large errors when measured by conventional methods, were especially focused on. Good agreement emerged among present results, a previous pulse-echo study, and theoretical predictions using a plane-scattered-wave ensemble-average model. Attempts to measure the internal-friction ‘‘tensor’’ failed.
Reduced temperature growth of crystalline 3C-SiC films on 6H-SiC by chemical vapor deposition from silacyclobutane
Authors: Yuan C. , Steckl A. J. , Chaudhuri J. , Thokala R. , Loboda M. J.
Published: July 2002

3C- on 6H-SiC (0001) epitaxial growth from the single-source organosilane precursor silacylobutane (c-C3H6SiH2) has been investigated over the temperature range of 800–1100 °C. Spectrophotometry was used to determine an optical absorption edge of ∼2.27 eV for the films grown at 900 °C, corresponding approximately to the energy band gap of 3C-SiC. The crystallinity, structure, strain, and dislocation density in the 3C-SiC thin films were determined using double crystal x-ray diffractometry (DCXRD). The films grown at 800–1000 °C were found to be exclusively 3C-SiC. The films grown at 1100 °C were a mixture of 3C, 4H, and 6H polytypes of SiC. All films shown an excellent surface morphology. The optimum films are obtained at 900 °C, exhibiting structural properties nearly equal to those of the substrate: narrow DCXRD peak width (∼17 arcsec) and low dislocation density (∼3×106 cm2).
Characterization of heavily B-doped polycrystalline diamond films using Raman spectroscopy and electron spin resonance
Authors: Gonon P. , Gheeraert E. , Deneuville A. , Fontaine F. , Abello L. , Lucazeau G.
Published: July 2002

Heavily B-doped polycrystalline diamond films ([B]≳1019 cm−3) are studied by Raman spectroscopy and electron spin resonance. The formation of an impurity band is accompanied by a Fano-type interference for the one-phonon scattering. Bands at 1200 and 500 cm−1 are observed in Raman spectroscopy for concentrations above 1020 cm−3. They are related to maxima in the phonon density of states, and are ascribed to disordered regions or crystalline regions of very small size. The concentration of defects associated with the paramagnetic signal observed around g=2.0030 increases drastically above 1021 B cm−3. The Mott insulator-metal transition is accompanied by the presence of a new paramagnetic signal (g=2.0007 for 2×1020 B cm−3, g=1.9990 for 1021 B cm−3) ascribed to free holes in the impurity band.
Formation of Ti–Zr–Cu–Ni bulk metallic glasses
Authors: Lin X. H. , Johnson W. L.
Published: July 2002

Formation of bulk metallic glass in quaternary Ti–Zr–Cu–Ni alloys by relatively slow cooling from the melt is reported. Thick strips of metallic glass were obtained by the method of metal mold casting. The glass forming ability of the quaternary alloys exceeds that of binary or ternary alloys containing the same elements due to the complexity of the system. The best glass forming alloys such as Ti34Zr11Cu47Ni8 can be cast to at least 4-mm-thick amorphous strips. The critical cooling rate for glass formation is of the order of 250 K/s or less, at least two orders of magnitude lower than that of the best ternary alloys. The glass transition, crystallization, and melting behavior of the alloys were studied by differential scanning calorimetry. The amorphous alloys exhibit a significant undercooled liquid region between the glass transition and first crystallization event. The glass forming ability of these alloys, as determined by the critical cooling rate, exceeds what is expected based on the reduced glass transition temperature. It is also found that the glass forming ability for alloys of similar reduced glass transition temperature can differ by two orders of magnitude as defined by critical cooling rates. The origins of the difference in glass forming ability of the alloys are discussed. It is found that when large composition redistribution accompanies crystallization, glass formation is enhanced. The excellent glass forming ability of alloys such as Ti34Zr11Cu47Ni8 is a result of simultaneously minimizing the nucleation rate of the competing crystalline phases. The ternary/quaternary Laves phase (MgZn2 type) shows the greatest ease of nucleation and plays a key role in determining the optimum compositions for glass formation.
Second-harmonic generation in nonbirefringent semiconductor optical microcavities
Authors: Rosencher E. , Vinter B. , Berger V.
Published: July 2002

We analyze the second-harmonic generation process in a microcavity of optical nonlinear material. We show that, in the case when the cavity thickness is much smaller than the material coherence length and for sufficiently high finesses, no phase match procedures are necessary to obtain a quite large optical conversion yield. It is thus possible to capitalize on the large optical nonlinear susceptibilities of III-V or II-VI materials though these compounds are not birefringent. It is shown that by adjusting mirror reflectivity phases it is possible to obtain doubly resonant cavities in which there is no phase mismatch between pump and harmonic waves.
A double metal structure Pt/Al/n-InP diode
Authors: Huang Wen-Chang , Lei Tan-Fu , Lee Chung-Len
Published: July 2002

A high performance double metal structure, Pt/Al/n-InP diode is discussed. The diode exhibits an effective barrier height of 0.74 eV, forward characteristics with an ideality factor n=1.11 over five decades, and a reverse leakage current of 1.34×10−4 A/cm2 at −3 V. A detailed study of the electrical characteristics and secondary ion mass spectroscopy analysis of the sample suggest that the high barrier height is due to formation of an interfacial layer at the contact interface. The diode exhibits good thermal stability, maintaining a barrier height of 0.70 eV after annealing at 300 °C for 10 h.