bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of reflectivity inversion, α- and β-phase transitions and nanostructure formation in hydrogen activated thin Pd films on silicon based substrates
Authors: Kalli K. , Othonos A. , Christofides C.
Published: July 2002

Optically thin palladium metal films evaporated on different silicon based substrates are investigated following exposure to different concentrations of hydrogen gas in air. Laser modulated reflectance off the palladium surface of silicon oxide and silicon nitride substrates is used to recover information regarding the reflectivity inversion and α/β-phases of the palladium complex after both first and multiple gas cycling. Atomic force microscopy confirms the formation of metal nanostructures following exposure to hydrogen of the optically thin palladium films.
Ultrahigh-density nanotransistors by using selectively grown vertical carbon nanotubes
Authors: Choi Won Bong , Chu Jae Uk , Jeong Kwang Seok , Bae Eun Ju , Lee Jo-Won , Kim Ju-Jin , Lee Jeong-O
Published: July 2002

A type of carbon nanotube transistors, which would be suitable for large-scale integration, has been fabricated from vertically aligned carbon nanotubes. We fabricated highly ordered carbon nanotubes, which are selectively grown on the patterned aluminum oxide nanotemplates. Each device element is formed on a vertical carbon nanotube attached to a bottom (source) and upper (drain) electrodes and a gate electrode, which is electrostatically switchable. The transistors can be integrated in large arrays with the potential for tera-level density (2×1011/cm2). The vertical carbon nanotube transistor shows ON/OFF switching operation at 30 K.
Millisecond phosphorescence of free electrons in pure GaAs
Authors: Gilinsky A. M. , Zhuravlev K. S.
Published: July 2002

A very long decay of transient free-to-bound photoluminescence (PL) in pure GaAs at low temperatures has been experimentally found. It is shown that, in undoped or dilutely doped GaAs, the decay of band-to-acceptor PL after a transient excitation follows an essentially nonexponential dependence approximated by a power law with the exponent as low as 0.3, and is observed for as long as 2.5 ms. The long decays of free-electron PL point to the domination of repeated trappings and subsequent releases of free electrons by some shallow traps, which are suggested to be the shallow donors.
Patterning organic–inorganic thin-film transistors using microcontact printed templates
Authors: Kagan C. R. , Breen T. L. , Kosbar L. L.
Published: July 2002

We report the simple, low-cost, and parallel fabrication of patterned organic–inorganic thin-film transistors (TFTs) by microcontact printing a molecular template on the substrate surface prior to film deposition. We printed molecules with hydrophobic tail groups on the gate oxide surfaces of TFTs to chemically, differentiate the substrate surface and confine the self-assembly of thin films, deposited from solutions flooded across the entire surface, to the transistor channels. TFTs are fabricated with good device characteristics and no current leakage. This process is more general to the patterning of other solution-deposited thin-film materials.
Rovibrational calculations for CH3+–Rg (Rg=He,Ne): The prototype disk-and-ball dimer
Authors: Dopfer Otto , Luckhaus David
Published: July 2002

Rovibrational calculations in the intramolecular ground vibrational states of the CH3+–Rg dimers, Rg=He and Ne, are carried out on intermolecular ab initio potential energy surfaces (PESs) calculated at the MP2 level of theory using a basis set of aug-cc-pVTZ quality. The internal CH3+ coordinates in the dimer are kept frozen at the optimal monomer coordinates (D3h symmetry, rigid monomer approximation). The three-dimensional (3D) intermolecular PESs of both dimers feature pronounced global minima at p-bound equilibrium structures: the Rg atom is attached to one side of the 2pz orbital of the central C atom along the C3 symmetry axis (C3v symmetry). The intermolecular C–He and C–Ne bonds are characterized by separations of Re=1.93 and 2.21 Å and dissociation energies of De=672 and 935 cm−1, respectively. The PESs of these prototype disk-and-ball dimers reveal substantial angular–radial coupling in the region of the global minimum which leads to significant differences between the equilibrium and vibrationally averaged separations, Re and R0. The 3D rovibrational calculations on the rigid monomer PESs yield R0=2.54 and 2.43 Å and D0=193 and 474 cm−1 for CH3+–He and CH3+–Ne, respectively. In general, the spectroscopic constants derived for the ground vibrational states of both complexes are in good agreement with recent spectroscopic data obtained by infrared photodissociation spectroscopy.
Design and performance analysis of deep-etch air/nitride distributed Bragg reflector gratings for AlInGaN laser diodes
Authors: Marinelli C. , Bordovsky M. , Sargent L. J. , Gioannini M. , Rorison J. M. , Penty R. V. , White I. H. , Heard P. J. , Benyoucef M. , Kuball M. , Hasnain G. , Takeuchi T. , Schneider R. P.
Published: July 2002

The key parameters in the fabrication of deep-etch high-order λ/4 Bragg gratings for short-wavelength nitride-based lasers are investigated. Calculations indicate that, for an air-gap thickness of 1.73 μm and single-spot Gaussian beam profile, the reduction in grating reflectivity due to light diffraction in the air gaps is only 17% with respect to a first-order structure with 0.1 μm air gaps. Scanning electron microscopy and microphotoluminescence characterizations confirm the validity of the numerical predictions and show that the 28%–38% reflectivity obtained from prototype focused-ion-beam-etched air/nitride gratings is mainly limited by imperfections and material disorder due to etching. Improving the etching technique would, therefore, allow standard lithographic fabrication of reduced-threshold GaN lasers.
Shielding polarizabilities calculated at the coupled-cluster singles and doubles level augmented by a perturbative treatment of triple excitations
Authors: Rizzo Antonio , Gauss Jürgen
Published: July 2002

The nuclear magnetic shielding polarizabilities of N2, F2, CO, HF, HCl, HCN, HNC, and C2H2 have been computed at the coupled-cluster singles and doubles (CCSD) level augmented by a perturbative treatment of triple excitations [CCSD(T)] using gauge-including atomic orbitals and a mixed analytical-numerical approach based on the numerical differentiation of analytically evaluated shieldings. Extended basis sets have been employed to ensure results close to the basis set limit, and convergence of the electron-correlation treatment is investigated.
Long-wavelength (λ≈16 μm), room-temperature, single-frequency quantum-cascade lasers based on a bound-to-continuum transition
Authors: Rochat Michel , Hofstetter Daniel , Beck Mattias , Faist Jérôme
Published: July 2002

Room-temperature operation of long-wavelength, Fabry–Perot and single-mode quantum-cascade lasers at λ≈16 μm is reported. Multimode emission with pulsed peak power up to 400 mW at −40 °C and 220 mW at 30 °C is demonstrated. Single-mode emission up to 60 mW peak power has been achieved at 30 °C.
Synthesis of superconducting Mg/MgB2 composites
Authors: Dunand David C.
Published: July 2002

Composites consisting of Mg and MgB2 phases were fabricated by liquid-metal infiltration following two routes. First, a preform of MgB2 powders was pressure infiltrated at 800 °C with liquid Mg, which was subsequently solidified. Second, a preform of B powders was pressure infiltrated with liquid Mg at 700 °C, resulting in partial, in situ formation of the MgB2 phase. Subsequent annealing at 950 °C increased the content of MgB2 through reaction between B and Mg. All composites exhibited a pore-free structure with no significant additional phases and were superconducting near 39 K.
Strong isotopic labeling effects on the pressure dependent thermodynamics of polydimethylsiloxane/polyethylmethylsiloxane blends
Authors: Beiner Mario , Fytas George , Meier Gerhard , Kumar Sanat
Published: July 2002

Using photon correlation spectroscopy we show that isotopic labeling can strongly alter the pressure dependent thermodynamics of polydimethylsiloxane/polyethylmethylsiloxane blends, while each blend enhances its miscibility upon pressurization up to 2 kbar. The pressure dependence of the Flory interaction parameter, χ, changes by a factor of 4 when the deuteration of side groups is switched from one polymer to the other, but its pressure dependence is always negative. These results may be understood as being driven by the very different negative excess volumes of mixing for these different isotopic blends. We have attempted to unify our understanding of the role of pressure on blend thermodynamics into a single master plot by examining our data on volume changes on mixing vs the interaction parameter for the polysiloxane blends, and those from neutron scattering experiments on polyolefin blends and on a blend of a polyolefin and a polysiloxane. We find no universal trends when examined on this basis, and instead observe that variations in the chemical identity of the polymers in question and their chain lengths lead to very different plots. Further, since both neutron scattering and light scattering provide the same qualitative results, we argue that these results are not artefacts of either experimental technique. Our results strongly argue that the role of pressure on blend thermodynamics are much more complex than previously anticipated, and stress the need for improved theories for this important class of experiments.
Sputtered nanodots: A costless method for inducing effective pinning centers in superconducting thin films
Authors: Crisan A. , Fujiwara S. , Nie J. C. , Sundaresan A. , Ihara H.
Published: July 2002

A straightforward and cheap method for creating extended defects, strong pinning centers, in superconducting thin films is proposed. Clearly, by very short time (3–5 s) rf sputtering at suitable substrate temperatures, we deposited Ag nanodots on SrTiO3 substrates prior to the growth of superconducting thin films. The nanodots were studied by atomic force microscopy. Due to the lattice mismatch and/or chemical poisoning, on top of the nanodots the superconducting phase does not form, creating in this way extended and effective pinning centers which increase the critical current density of the film. The method was applied to (Cu, Tl)BaSrCa2Cu3Oy films grown by amorphous phase epitaxy. Thin films grown in similar conditions, with and without nanodots, were characterized by x-ray diffraction and ac susceptibility. The results show that the nanodots increased the critical current density more than one order of magnitude.
Hydrogen-assisted pulsed-laser deposition of epitaxial CeO2 films on (001)InP
Authors: Norton D. P. , Pearton S. J. , Christen H. M. , Budai J. D.
Published: July 2002

We report on the growth of epitaxial CeO2 on (001) InP using hydrogen-assisted pulsed-laser deposition. Epitaxy is achieved via laser ablation of a CeO2 target in the presence of molecular hydrogen that is introduced during nucleation to reduce native In2O3 from the InP surface. X-ray diffraction scans confirm a cube-on-cube epitaxial relationship between the oxide film and the InP substrate. Rapid heating to the deposition temperature proved important in avoiding significant decomposition of the InP surface prior to film growth. This result should enable the integration of electronic oxide functionality with InP-based semiconductor technologies, and provide a means to explore InP metal–oxide–semiconductor field-effect transistor structures.
Electron holographic characterization of electrostatic potential distributions in a transistor sample fabricated by focused ion beam
Authors: Wang Zhouguang , Hirayama Tsukasa , Sasaki Katsuhiro , Saka Hiroyasu , Kato Naoko
Published: July 2002

A cross-sectional sample of a silicon-metal-oxide semiconductor field-effect transistor, which was directly cut from an integrated circuit wafer, has been prepared carefully using a focused-ion-beam technique and examined by means of off-axis electron holography. In the reconstructed phase image, heavily doped source, and drain regions are revealed clearly as bright contrast, from which an n-channel transistor is identified. In addition, two-dimensional phase distributions around both source and drain regions show a core area with relatively high phase in the heavily doped region, which may be attributed to the effect of doping atoms and residual defects and strains remaining after implantation. The electrostatic potentials across the core area and depletion layer are estimated and discussed. This work demonstrates the feasibility of using a focused-ion-beam technique to prepare electron holographic sections of a wide range of semiconductor devices.
Kinetic model for Eley–Rideal and hot atom reactions between H atoms on metal surfaces
Authors: Jackson Bret , Sha Xianwei , Guvenc Ziya B.
Published: July 2002

A simple kinetic model is used to describe the interaction of H and D atomic beams with H- and D-covered metal surfaces. The atoms incident from the gas phase can have a direct Eley–Rideal reaction with an adsorbate, reflect, penetrate into the bulk, knock an adsorbate out of its binding site, or trap to form a hot atom. These hot mobile atoms can go on to react with other adsorbates, or eventually relax and stick. A coarse-graining approach, which takes advantage of the large difference between the time scales for the kinetics experiments and the reaction dynamics, allows us to derive relatively simple kinetic equations for reaction rates and coverages. The approach is similar to a kinetic random walk model developed by Küppers and co-workers [J. Phys. Chem. 109, 4071 (1998)] except that our equations can be used to derive analytical expressions for saturation coverages, rates, and yields. The model is applied to the case of H atom reactions on a Ni(100) surface, and a detailed comparison is made with both experimental and quasiclassical studies.
β-
Ga
2
O
3
nanowires synthesized from milled GaN powders
Authors: Kim B. C. , Sun K. T. , Park K. S. , Im K. J. , Noh T. , Sung M. Y. , Kim S. , Nahm S. , Choi Y. N. , Park S. S.
Published: July 2002

White-colored materials synthesized by a thermal annealing of milled GaN powders at 930 °C in a nitrogen atmosphere were identified to be monoclinic β-Ga2O3 nanowires by x-ray diffraction and scanning electron microscopy. High-resolution transmission electron microscopy revealed that these nanowires are single nanocrystals, and energy dispersive x-ray indicated that these nanomaterials are free of any metals. In addition, bundles of these crystalline nanowires in the rectangular-pole shape are a few centimeters in length.
Effect of Si lattice strain on the reliability characteristics of ultrathin SiO2 on a 4° tilted wafer
Authors: Chang Hyo Sik , Choi Sangmu , Yang Hyundoek , Min Kyung-youl , Moon Dae Won , Lee Hyung-Ik , Hwang Hyunsang
Published: July 2002

The electrical and structural characteristics of an ultrathin gate dielectric, thermally grown on 4° tilted wafer has been investigated. Compared with a control wafer, a relaxation of the Si lattice strain at the SiO2/Si(001) interface was observed for the 4° tilted wafer, which was confirmed by medium energy ion scattering spectroscopy. A significant improvement in the reliability characteristics of a metal–oxide–semiconductor (MOS) capacitor, with a 2.5-nm-thick gate oxide, grown on a tilt wafer was observed. This improvement in reliability can be explained by the relaxation of strain at the SiO2/Si interface. An ultrathin gate dielectric grown on a tilt wafer represents a promising alternative for gate dielectric applications in future MOS devices.
Nanowear in scanning force microscopy: Information on deposits formed in and downstream of a hexane plasma
Authors: Dinelli F. , Leggett G. J. , Alexander M. R.
Published: July 2002

Contact mode scanning force microscopy (SFM) has been employed to induce wear in a controlled fashion on thin plasma polymerized hexane (ppHex) films. Such thin films are not readily amenable to characterization techniques such as nanoindentation, whereas the information from the outer surface that SFM provides in wear mode is ideally suited to the examination of these thin films. Nanowear of viscous ppHex films takes the form of grooves at the edges and accumulation of material in the center of the scanned area, whereas films with higher cross link density show damage uniformly across the worn area. A parameter termed the critical load is developed to enable a comparative investigation of deposits in and downstream of the plasma. Here, the critical load is defined as the load necessary to introduce a change in the topography of the scanned area in comparison to the surroundings. Films collected downstream of the plasma show viscous behavior and have a relatively low critical load independent of the deposition time. In contrast, the critical load for deposits made at the same power “in plasma” is found to greatly increase with increasing deposition time. This change in wear resistance with time is attributed to an increase in cross link density of the films deposited in the plasma region.
Near-field photocurrent measurements on boron-implanted silicon
Authors: Marocchi V. , Cricenti A. , Perfetti P. , Chiaradia P. , Raineri V. , Spinella C.
Published: July 2002

We report near-field photocurrent (NPC) measurements performed on three boron-implanted silicon samples characterized by different implantation doses. The images were acquired at λ=1330 nm corresponding to a photon energy of 0.93 eV which is smaller than the silicon energy gap (Egap=1.12 eV), representing incident radiation to which silicon is virtually transparent. The NPC images reveal the presence of boron clusters which are a consequence of B implantation and rapid thermal annealing at 1100 °C for 30 s. Boron clusters behave as metal clusters embedded into the silicon matrix and introduce gap states which give rise to the observed photocurrent.
Andreev reflection in narrow ferromagnet/superconductor point contacts
Authors: Imamura Hiroshi , Kikuchi Keiichirou , Takahashi Saburo , Maekawa Sadamichi
Published: July 2002

We theoretically study the conductance quantization and Andreev reflection of narrow ferromagnet/superconductor point contacts. The conductance quantization is shown to depend on whether the contact region is superconducting or ferromagnetic as well as on the strength of the exchange field in the ferromagnet. The Andreev reflection is more suppressed than that of the superconducting contact. We also find that the conductance–voltage curve shows a bump at zero bias voltage if there is no interfacial scattering. On the contrary, the conductance–voltage curve shows a dip if the contact has an interfacial scattering.
Magnetization tunneling in Mn12 and Mn4 single-molecule magnets
Authors: Yoo J. , Rumberger E. M. , Hendrickson D. N. , Yamaguchi A. , Ishimoto H. , Brechin E. K. , Christou G.
Published: July 2002

The quantum mechanical tunneling of the direction of magnetization is discussed for several examples of single-molecule magnets (SMMs). SMMs are molecules that function as nanomagnets. Magnetization tunneling is described for two crystallographically different forms of [Mn12O12(O2CC6H4-p-Me)16(H2O)4]⋅solvate. The two Mn12 complexes are isomers, differing both in the positioning of the H2O and carboxylate ligands and also in the orientations of the Jahn-Teller elongation at the MnIII ions. The magnetization vs magnetic field hysteresis loops are quite different for the two isomeric Mn12 complexes. Frequency-dependent ac magnetic susceptibility and dc magnetization decay data are presented to characterize the magnetization relaxation rate vs temperature responses of two mixed-valence Mn4 complexes. In both cases the Arrhenius plot of the logarithm of the magnetization relaxation rate vs the inverse absolute temperature shows a temperature-dependent region as well as a temperature-independent region.
Microstructure and electric properties of lead lanthanum titanate thin film under transverse electric fields
Authors: Song Z. T. , Chan H. L. W. , Ding Y. P. , Chong N. , Choy C. L.
Published: July 2002

Polycrystalline lead lanthanum titanate thin film having perovskite structure was fabricated by metalorganic deposition (MOD) on a ZrO2/SiO2/Si substrate at 600 °C for 1 h in O2 atmosphere. Columnar structured ZrO2 buffer layer was also prepared by a MOD process under the same condition. Electrical measurements were conducted on interdigitated electrodes. The crystalline structure and growth behavior of the films have been studied by x-ray diffraction and scanning electron microscopy. It is observed that dielectric response of the film is effected by the cable length used in the measurement and by the values of the ac voltage. Long cable gives rise to an additional resonance peak at high frequency caused by the stray inductance of the contacts and cables. The capacitance and loss tangent over low frequency range shows significant variations due to the trapped charges and space charges in the film. These variations are very dependent on the values of the ac voltage and the length of cable. Meanwhile, the trapped charges and space charges lead to abnormal P–E loops, in which the measured remanent polarization and coercive field increase with increasing frequency.
Penetration of the electromagnetic waves through doped lanthanum manganites
Authors: Rinkevich A. , Nossov A. , Ustinov V. , Vassiliev V. , Petukhov S.
Published: July 2002

Penetration of the electromagnetic field through plate-shaped bulk polycrystalline sintered Pb and Y–Ba doped manganite samples is studied in the frequency range from 20 kHz to 30 MHz. Strong magnetic-field dependence of the transmission coefficient is observed. The penetration data are compared with the other magnetic-field dependent parameters: the magnetic permeability, magnetoresistance, and magnetoimpedance. Unusual frequency dependence of the penetration depth is observed.
Magnetic properties of Nd–Fe–Co(Cu)–Al–B amorphous alloys prepared by nonequilibrium techniques
Authors: Kumar G. , Eckert J. , Roth S. , Löser W. , Ram S. , Schultz L.
Published: July 2002

The amorphous alloys Nd40Fe40Co5Al8B7, Nd57Fe20Co5Al10B8, and Nd57Fe20Cu5Al10B8 were prepared by copper mold casting, melt spinning, and mechanical alloying. Despite their similar x-ray diffraction patterns, samples display different magnetic and thermal behavior correlated with the method of preparation. The fully amorphous melt-spun ribbons exhibit relatively soft magnetic properties with coercivities ≈40 kA/m at room temperature and a Curie temperature (TC)≈474 K. Apparently only the mold-cast cylinders of 3 mm diameter show hard magnetic behavior with a coercivity in the range of 258–270 kA/m (depending on composition) and have approximately the same TC as that of the melt-spun ribbons. An additional magnetic transition at 585 K due to the presence of Nd2Fe14B phase in the case of Nd40Fe40Co5Al8B7 cast rod has been observed. Heat treatment above crystallization temperature in as-cast Nd57Fe20Co5Al10B8 and Nd57Fe20Cu5Al10B8 samples destroys the hard magnetic properties. In contrast, mechanically alloyed amorphous samples are soft magnetic with maximum coercivity up to 11 kA/m but show an entirely different TC≈680–740 K, which is rather characteristic of an Fe solid solution. The magnetic properties are discussed in terms of different local atomic environment and cluster sizes in amorphous samples prepared by different methods.
Pulsed low-energy ion-assisted growth of epitaxial aluminum nitride layer on 6H-silicon carbide by reactive magnetron sputtering
Authors: Tungasmita S. , Persson P. O. Å. , Hultman L. , Birch J.
Published: July 2002

Epitaxial aluminum nitride thin films have been grown on silicon carbide (6H-SiC) substrates by pulsed low-energy ion-assisted reactive magnetron sputter deposition (+5/−20 V of bias pulses), with ion-assisted energy (Ei)≅22 eV, under ultrahigh-vacuum conditions. Surface ion interactions during the negative bias pulse gave rise to enhanced surface mobility of adatoms with beneficial effects, which extended over the limit of ion repelling in the positive pulse as the film thickness increased. High-resolution electron microscopy shows that a large (>90 nm) AlN domain width can form on the substrate. Domain-boundary annihilation and domain suppression during film growth have been observed. The growth rate also increased by a factor of ∼4 compared to growth conditions with no ion assistance (Ei=2 eV) and by a factor of 2 from dc ion-assisted growth. This indicates that the supply of nitrogen is a limiting factor for AlN formation and that the reactivity of nitrogen is increased on the growing AlN film surface for pulse ion-assisted deposition. High-resolution x-ray diffraction shows a reduction in the full width at half maximum of the rocking curve from 1490 to 1180 arcsec when pulsed ions are used. The cathodoluminescence shows high intensity of near-band edge emissions at wavelengths of 206 (6.02 eV) and 212 nm (5.84 eV) at a measured temperature of 5 K, with relatively low defect and oxygen and carbon impurity related emission, which is indicative of a high quality electronic material.
In situ observation of magnetic domain pattern evolution in applied fields by spin-polarized low energy electron microscopy
Authors: Poppa H. , Tober E. D. , Schmid A. K.
Published: July 2002

To track the influence of applied magnetic fields on the magnetic microstructure of ultrathin metal films in high spatial resolution, the specimen holder of a spin-polarized low energy electron microscope (SPLEEM) was fitted with an in situ field coil. Using this technique, domain structure changes can be followed with data acquisition times of currently few seconds per image. Measurements of the dependence of magnetic domain patterns in Fe/Cu(100) layers on applied fields illustrate the performance of this novel experimental approach.
Elucidation of propulsive force of microrobot using magnetic fluid
Authors: Saga Norihiko , Nakamura Taro
Published: July 2002

Using the pattern of the peristaltic movement of an earthworm, a microrobot was developed for traveling in a tube using a magnetic fluid. In this microrobot, a cell equivalent to a segment of the earthworm is composed of a natural rubber tube into which a water-based magnetic fluid is sealed up. The cells are connected with rod-like elastic bodies of natural rubber. It was confirmed that this robot can travel in an acrylic tube (inner diameter: 12 mm, outer diameter: 14 mm) by providing a shifting magnetic field from the exterior. This article will describe how our microrobot is propelled, the mechanism of its propulsion, and the analytical results of the propulsive force.
Exact nucleation modes in arrays of magnetic particles
Authors: Skomski R.
Published: July 2002

Magnetization reversal in arrays of magnetic nanoparticles with perpendicular anisotropy is investigated. Aside from domain-wall propagation effects, the reversal involves two main aspects: the nucleation behavior of individual particles and interparticle interactions. Due to magnetostatic self-interaction effects, the interparticle interaction cannot be reduced to a stray-field correction to the external field. Exact nucleation fields and explicit stray-field and self-interaction contributions are obtained for rings of equidistant dots. An exact treatment of self-interactions in various structurally inhomogeneous but rotationally symmetric wire, sphere, and thin-film nanostructures leads to renormalization of the uniaxial anisotropy. Finally, an approximate method to calculate nucleation fields is discussed.
Effects of Dy doping on magnetic and reversal properties of recording media
Authors: Ren H. B. , Wang J. P. , Shan Z. S. , Malhotra S. , Bertero G. , Ng V.
Published: July 2002

In this article, the large anisotropy and strong coupling between rare-earth (RE=Sm, Dy, Tb etc.) metal and transition metal (TM=Co, Fe, etc.) are proposed to be utilized to improve the thermal stability (KuV*/kBT) and coercivity (Hc) of high-density recording media. The effects of Dy doping, which was added to the Cr underlayer and Cr overcoat layer, on magnetic and reversal properties are presented. It was found that Dy doping in the Cr1−xDyx underlayer and overcoat layer resulted in an increase of the KuV*/kBT and coercivity (Hc). The appreciable increases in KuV*/kBT and Hc indicate that Dy doping into the underlayer and intermediate layer is promising for the preparation of high-density recording media.
X-ray spectrometry investigation of electrical isolation in GaN
Authors: Kucheyev S. O. , Toth M. , Phillips M. R. , Williams J. S. , Jagadish C. , Li G.
Published: July 2002

Electrical isolation of n-type GaN epilayers bombarded with MeV light ions is studied by energy dispersive x-ray spectrometry (EDS). We show that the maximum bremsstrahlung x-ray energy (the Duane–Hunt limit) can be used to monitor the isolation process in GaN. This method allows the dose region above the threshold dose for isolation to be conveniently studied, whereas the application of conventional (low-voltage) electrical techniques in this dose range with large sheet resistances of the material (≳1011 Ω/sq) is often impossible due to comparable parasitic resistances of the experimental setup. A correlation of EDS and resistance measurements of GaN strongly suggests that the magnitude of sample charging scales with the number of ion-beam-produced deep electron traps which are empty at equilibrium. The results presented demonstrate the utility of EDS as a powerful and simple technique to study electrical isolation in wide band-gap semiconductors.
Periodic oxide breakdown during oxidation of AlN/Sapphire(0001) films
Authors: Kang H. C. , Seo S. H. , Kim J. W. , Noh D. Y.
Published: July 2002

We present an in situ synchrotron x-ray scattering study on the thermal oxidation of epitaxial AlN/Sapphire(0001) films. During annealing to 700 °C, an epitaxial AlN film transforms progressively into a planar epitaxial γ-Al2O3 layer. The oxidation proceeds through the γ-Al2O3/AlN interfacial motion that is observed directly from the intensity fringes near the AlN(0002) Bragg reflection. The oxidation rate, deduced from the interfacial motion, exhibits repeating transitions from a linear to a parabolic oxidation behavior. This suggests that the oxide break down periodically. During the oxidation the strain in the remaining AlN reverts to the value measured at the same thickness during the growth of the AlN film.
Effect of thermal fluctuations on switching field of deep submicron sized soft magnetic thin film
Authors: Lee K. J. , Lee T. D.
Published: July 2002

In this article, the effect of thermal fluctuations on switching field at short and long time scales have been investigated by micromagnetic simulation with the Langevin and the time-temperature scaling methods. When the film thickness and/or the aspect ratio are reduced, the switching field decreases substantially owing to the decrease of shape anisotropy and the increase of thermal fluctuation. In the soft magnetic film with a dimension of 0.4 μm×0.2 μm×1 nm, the calculated switching field at a pulse width of 10 ns is 32 Oe, but estimated retention time is less than 1000 s. In such a thermally unstable film, thermally activated switching of magnetization occurs by complex behaviors with creations and annihilations of small and slightly tilted magnetic clusters.
Synthesis of spherical FePd and CoPt nanoparticles
Authors: Chen Min , Nikles David E.
Published: July 2002

Co 48 Pt 52 nanoparticles were prepared by the simultaneous chemical reduction of platinum acetylacetonate and the thermal decomposition of cobalt tricarbonyl nitrosyl. Similarly, Fe50Pd50 nanoparticles were prepared by the simultaneous reduction of palladium acetylacetonate and the thermal decomposition of iron pentacarbonyl. The average diameter for the Co48Pt52 particles was 7 nm, while the average diameter for the Fe50Pd50 particles was 11 nm. Films of the particles were cast onto silicon wafers from hydrocarbon solution. Only after annealing at 700 °C for three hours did the Co48Pt52 films transform to the L10 phase. However there was no evidence for the tetragonal phase for the films containing Fe50Pd50 particles after annealing at 700 °C for three hours.
Tip motion in amplitude modulation (tapping-mode) atomic-force microscopy: Comparison between continuous and point-mass models
Authors: Rodrı́guez Tomás R. , Garcı́a Ricardo
Published: July 2002

We discuss the influence of high-order frequency components in the operation of an amplitude modulation atomic-force microscope (AFM). A comparative study of point-mass and continuous models is performed to describe the tip motion. The tip–surface interaction force excites high-order frequency components whenever a higher harmonic of the excitation force is close to an eigenmode of the cantilever beam. The strength of those components depends on the set point amplitude and the fundamental resonance frequency of the cantilever. However, for standard operating conditions with quality factors in the 102–103 range, higher-order components are about three orders of magnitude smaller than the component at the excitation frequency. We conclude that point-mass models are suitable to describe the operation of a tapping-mode AFM in air environments.
Fabrication of a low-threading-dislocation-density AlxGa1−xN buffer on SiC using highly Si-doped AlxGa1−xN superlattices
Authors: Hirayama Hideki , Ainoya Makoto , Kinoshita Atsuhiro , Hirata Akira , Aoyagi Yoshinobu
Published: July 2002

An in situ technique to reduce the threading dislocation density (TDD) of an AlxGa1−xN buffer within submicron-thick growth is demonstrated using metalorganic vapor-phase epitaxy. We achieved a marked reduction in the TDD of the AlGaN buffer on a SiC substrate by inserting superlattices (SLs) consisting of highly Si-doped AlGaN and undoped AlGaN layers. The TDD of AlGaN decreased from 2×1010 to 7×107 cm−2 by inserting SLs with total growth thickness of 0.8 μm. The Si incorporation in the highly Si-doped AlGaN layers of the SLs was estimated to be approximately 1.2×1020 cm−3 (0.24%). This is strictly an in situ technique that does not require complicated fabrication processes, and the surface is kept flat throughout the entire growth. This method is especially useful on SiC wafers to prevent cracks in a thin growth layer.
Thermodynamic model of low temperature metalorganic chemical vapor deposition of GaN
Authors: Elyukhin V. A. , Garcı́a-Salgado G. , Peña-Sierra R.
Published: July 2002

A thermodynamic model is developed to explain low temperature metalorganic chemical vapor deposition of GaN. The two stage epitaxial processes taking into account the physisorbed states on (0001) A and B planes are considered. The physisorbed Ga coverage of both GaN planes is presented as the function of the pressures, temperature, and efficiency of ammonia decomposition. The thermodynamic affinities of each stage of the growth runs are described. The formation conditions of the GaN wurtzite and zinc-blende modifications are discussed. The model developed was applied to estimate the thermodynamic characteristics of low temperature metalorganic chemical vapor deposition of GaN. It is quantitatively shown that the considered growth conditions allows formation of the stable and metastable GaN modifications in the same epitaxial run.
High-intensity interminiband terahertz emission from chirped superlattices
Authors: Köhler Rüdeger , Tredicucci Alessandro , Beltram Fabio , Beere Harvey E. , Linfield Edmund H. , Giles Davies A. , Ritchie David A.
Published: July 2002

Electroluminescence at λ∼69 μm (4.3 THz) is reported from interminiband transitions in quantum-cascade structures with superlattice active regions. Spontaneous emission gives a low-temperature linewidth of 2 meV (0.48 THz) with linear light–current characteristics observed up to high-current densities (625 A/cm2), resulting in record output powers of 500 pW. Devices operate up to above liquid-nitrogen temperature, with both emission wavelength and current–voltage characteristics in good agreement with theoretical predictions.
Collision-induced alignment of NO+ drifting in argon: Calculated distribution functions and microscopic quadrupole alignment parameters
Authors: Chen Xin , Araghi R. , Baranowski R. , Thachuk M.
Published: July 2002

Molecular dynamics simulations of NO+ drifting in argon are presented. In this system, the ion–bath gas mass ratio is close to but less than unity, thus increasing the importance of inelastic collisions. Strong velocity–angular momentum coupling is found, and in particular the quadrupolar alignment parameter as a function of the velocity parallel with the field takes on, with decreasing velocities, values that start as negative, become positive, and subsequently decay toward negative. To the best of our knowledge, this is the first report of the decay of this alignment towards negative values at the low end of the velocity distribution. The accuracy of an approximate distribution function [R. Baranowski and M. Thachuk, Phys. Rev. A 63, 032503 (2001)] is also examined in detail. It is found to give results that are generally qualitatively correct, and for many properties, semiquantitatively correct as well. The agreement, however, is not as good as that found for systems with ion–bath gas ratios greatly exceeding unity.
Hybridized boron–carbon nitride fibrous nanostructures on Ni substrates
Authors: Yap Yoke Khin , Yoshimura Masashi , Mori Yusuke , Sasaki Takatomo
Published: July 2002

Stoichiometric BC2N films can be deposited on Si (100) at 800 °C, however, they are phase separated as pure carbon and BN phases. Likewise, hybridized boron–carbon nitride (BCN) films can be synthesized on Ni substrates. On Ni, the carbon and BN phases are hybridized through carbon nitride and boron carbide bonds. These films appeared as fibrous nanostructures. Evidence indicates that the Ni substrate acts as a sink for the carbon and forces the carbon composites to grow on top of the B and N atoms. However, as these films are grown thicker, phase separation occurs again. These results indicate that hybridized BCN phases should now be regarded as semiconducting or superhard nanostructures. High-temperature deposition on Ni substrates might be a solution to the obstacle of preparing hybridized BCN phases.
Quantum theory of vector correlations in vibrationally mediated photodissociation
Authors: Dagdigian Paul J.
Published: July 2002

The quantum treatment of the photodissociation of aligned, vibrationally excited molecules prepared by polarized laser excitation is presented. A formal expression for the angle-dependent multipole moments of a photofragment is derived. As a specific example, the projection of the laboratory angular distribution (zero-order moment) along a probe direction is considered, and Doppler-shift-dependent fluxes for various arrangements of the polarization directions of the vibrational excitation and photolysis lasers are computed. The profiles depend upon the photodissociation dynamics only through the conventional recoil anisotropy parameter β, as in one-photon dissociation. More generally, the dynamical information obtainable from measurement of vector correlations in VMP is the same as that which can be extracted in a one-photon dissociation experiment. Nevertheless, measurement of vector correlations for vibrationally mediated photodissociation can provide new information on the dissociation dynamics since different regions of the excited potential energy surface(s) are accessed from those accessed in one-photon dissociation.
Moments of single-molecule spectra in low-temperature glasses: Measurements and model calculations
Authors: Naumov A. V. , Vainer Yu. G. , Bauer M. , Kador L.
Published: July 2002

Single-molecule spectroscopy of isolated chromophores in solids yields detailed information about the matrix on a microscopic level. In most studies so far, single-molecule (SM) spectra have been characterized by their linewidths. We demonstrate that in a doped amorphous polymer, in which SM excitation spectra generally have a complicated shape, the shapes of SM lines are more fully characterized by their moments than by the usually used linewidth. We present the measured and simulated distributions of the first four moments and of the linewidth for low-temperature spectra of single tetra-tert-butylterrylene molecules embedded in an amorphous poly(isobutylene) matrix. The simulations are based on the standard tunneling model of glasses with appropriate modifications and on a theory of SM line shapes in a disordered host, which was recently derived by Geva and Skinner [J. Phys. Chem. B 101, 8920 (1997)]. The comparative analysis of the measured and simulated distributions allowed us to evaluate the minimal distance of two-level-system–chromphore interactions and the main parameters of the distribution of coupling constants.
Quantum cascade lasers with double metal-semiconductor waveguide resonators
Authors: Unterrainer Karl , Colombelli Raffaele , Gmachl Claire , Capasso Federico , Hwang Harold Y. , Sergent A. Michael , Sivco Deborah L. , Cho Alfred Y.
Published: July 2002

Quantum cascade (QC) lasers with double metal-semiconductor waveguide resonators are reported for operating wavelengths of 19, 21, and 24 μm. The waveguides are based on surface-plasmon modes confined at the metal–semiconductor interfaces on both sides of the active region/injector stack and are not restricted by a cutoff wavelength for the TM polarized intersubband radiation. The double metal-semiconductor resonator devices are fabricated using an epilayer transfer process. Optical confinement factors close to 1 are obtained, with low waveguide losses. The performance of the devices is compared with that of QC lasers based on single-sided surface-plasmon waveguides. The concept of QC laser with double metal-semiconductor waveguide is applicable to a much wider wavelength range.
Nanoelectronic devices with reactively fabricated semiconductor
Authors: Kim Jooho , Akinaga Hiro , Atoda Nobufumi , Tominaga Junji
Published: July 2002

The economic fabrication of complete circuits at the nanometer level remains challenging because of the difficulty of connecting nanodevices to one another. Here, we report the electronic nanodevices fabrication method based on the Hall effect including the connection through a thermally activated reaction. By combining incident current and external perpendicular magnetic field in the reactively fabricated memory cell, electronic signal is put in storage. It is suggested that 2.5×109–2.5×1011 (2.5 G–250 G) bits/in.2 capacity flash memory or dynamic random access memory can be fabricated by means of the nanosecond pulse laser or electron-beam-induced reaction.
Mg and O codoping in p-type GaN and AlxGa1−xN (0<x<0.08)
Authors: Kipshidze G. , Kuryatkov V. , Borisov B. , Kudryavtsev Yu. , Asomoza R. , Nikishin S. , Temkin H.
Published: July 2002

We describe Mg and O codoping experiments in gas-source molecular-beam epitaxy of GaN and AlGaN that produce high levels of Mg incorporation and activation. In order to obtain the highest level of Mg incorporation the surface stoichiometry was optimized by adjusting the NH3/Ga and NH3/(Ga+Al) flux ratios. The lowest acceptor activation energy and the highest hole concentration, p=2×1018 cm−3, were measured in samples of p-GaN and p-AlxGa1−xN with well-defined Mg/O ratios determined by secondary ion mass spectrometry. Measurements of the temperature dependence of diffusion current in p–n junctions formed in Al0.08Ga0.92N and GaN show acceptor activation energy of 195±10 and 145±15 meV, respectively. Low activation energies are attributed to successful codoping.
Effects of polymer sidebranching in double- and single-layer polyfluorene light-emitting diodes
Authors: Nakazawa Y. K. , Carter S. A. , Nothofer H.-G. , Scherf U. , Lee V. Y. , Miller R. D. , Scott J. C.
Published: July 2002

We study how changes in sidebranching in electroluminescent polymers affect the performance of polyfluorene-based light-emitting diodes with and without additional hole transport materials. We observe that light emission and device efficiency are determined more by the position of the exciton recombination zone than by changes in the polymer morphology induced by sidebranching. Consequently, we find that sidebranching mainly controls the relative emission between vibrational energy levels and has a minimal effect on polymer charge transport properties. Light outputs of 10 000 cd/m2 and device efficiencies of 0.85 and 1.8 cd/A are obtained for single- and double-layer devices, respectively.
Unusual transition phenomenon in Zr-based bulk metallic glass upon heating at high pressure
Authors: Sun Liling , Kikegawa Takumi , Wu Qi , Zhai Zhanji , Wang Wenkui
Published: July 2002

Structural changes of the bulk metallic glass (BMG) Zr41.2Ti13.8Cu12.5Ni10Be22.5 were investigated at high pressure and high temperature with synchrotron radiation x-ray diffraction (SR-XRD). In situ SR-XRD measurements confirm that heating the BMG at a pressure of 10 GPa leads to an unusual sequence of transitions. The initial amorphous solid transforms into a crystalline phase at 582 K, and then the crystalline phase transforms back to an amorphous phase at 698 K. This crystalline-to-amorphous phase transition in a BMG is driven by increasing temperature at higher density produced by high pressure.
Suppressed boron penetration in p+ polycrystalline-Si/Al2O3/Si metal–oxide–semiconductor structures
Authors: Cho Heung-Jae , Park Dae-Gyu , Lim Kwan-Yong , Ko Jung-Kyu , Yeo In-Seok , Park Jin Won , Roh Jae-Sung
Published: July 2002

We demonstrate a suppressed boron penetration in p+ polycrystalline-Si (poly-Si)/Al2O3/n-Si metal–oxide–semiconductor (MOS) capacitors using a remote plasma nitridation (RPN) of Al2O3 surface. The B penetration was sufficiently suppressed for temperature to 850 °C in N2 for 30 min as manifested by the negligible flat band shift (ΔVFB) and insignificant B diffusion. The nitrogen (N) incorporation in Al2O3 surface appears to effectively impede the B diffusion into the Si channel. Increased gate leakage current for the n+ poly-Si/RPN-Al2O3/p-Si n-type MOS capacitors was observed and attributed to the reduced band gap energy of RPN-Al2O3 due to the formation of AlN and bulk defects due to RPN. Optimization of N concentration is required for the suppressed B penetration and leakage reduction.
Simple one-electron quantum capping potentials for use in hybrid QM/MM studies of biological molecules
Authors: DiLabio Gino A. , Hurley Margaret M. , Christiansen Phillip A.
Published: July 2002

Calculations demonstrate that with a minor modification conventional ab initio effective potentials can be employed in place of link atoms to truncate quantum regions in hybrid quantum mechanics/molecular mechanics calculations. Simple quantum capping potentials are formed by replacing excess valence electrons in conventional effective potentials by spherical shielding and Pauli terms chosen to duplicate all-electron molecular structures and charge distributions. Tests involving truncated histidine show errors in charge and protonation energy to be reduced as compared to the link atom approach. Because of the use of conventional effective potential expansions, this approach can be implemented with minimal or no program modifications. Indeed, in its simplest form it requires the addition of only a single Gaussian and adjustable parameter to a conventional effective potential expansion. The parametrization requires little effective potential expertise or effort.