bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic and structural properties of epitaxial fcc Co layers on Cu(001) (invited)(abstract)
Authors: Kirschner J.
Published: July 2002

We studied the magnetic properties, the electronic structure, and the growth mode of thin epitaxial cobalt films on Cu(001). The growth was studied by means of electron diffraction, Auger spectroscopy, and scanning tunneling microscopy. Cobalt grows in the layer-by-layer mode, with the lateral lattice constant determined by the copper substrate, in the face-centered tetragonal crystal structure. The tetragonal distortion is perpendicular to the surface and amounts to a few percent as found by LEED studies. The films show a strong uniaxial anisotropy leading to strong in-plane magnetization. A fourfold in-plane anisotropy was observed, with the easy axis of magnetization along [011]. The Curie temperature of the films was studied in situ by the magneto-optic Kerr effect. Tc varies strongly with film thickness from below 50 K at 1 ML to room temperature at about 2 ML. We found that the Curie temperature depends on the smoothness of the substrate as well as on the coverage by a copper layer. The coupling of cobalt double layers with copper intermediate layer was studied as a function of the thickness of the interlayer and the thickness of the top layer cobalt. We observed ferromagnetic/antiferromagnetic oscillatory exchange coupling with a rather short period (3–4 ML Cu).
A phenomenological theory for the temperature dependence of the antiferromagnetic coupling in thin-film sandwiches and multilayers (abstract)
Authors: Cullen J. R. , Hathaway K. B.
Published: July 2002

Recent magnetoresistance experiments1,2 on Fe/Cr/Fe sandwiches and superlattices show that the critical magnetic field for saturation varies linearly with temperature from liquid helium to room temperature. The drop in critical field is as large as 30%, even though the reduction in Fe magnetization is negligible over this temperature range. In order to gain some insight into the physical origin of this softening of the critical field, which is a measure of the antiferromagnetic interlayer coupling, we have studied a phenomenological model of an Fe/Cr/Fe sandwich structure which consists of two Fe films coupled at their interface by an antiferromagnetic exchange at zero temperature. The Fe films themselves are ferromagnetic and subjected to an external field and uniaxial in-plane anisotropy. The Fe spins are confined to the film plane. The temperature dependence of the critical field, i.e., the field below which the spins in the two films deviate from the direction of the field, arises from fluctuations in these spin deviations, which are present for any temperature greater than zero. Using an approximation in which these fluctuations are treated classically, the critical field decreased linearly with temperature, with a coefficient that increases logarithmically with the transverse dimension of the films. Interpreting this length to be of the order of the grain size in polycrystalline films or a coverage terrace width in epitaxially grown material, we can directly relate the temperature variation of the antiferromagnetic interaction to film quality. The larger this dimension, the greater the decrease of the coupling with temperature; hence, ‘‘better’’ films are expected to show weaker (or zero) coupling at room temperature. Conversely, the magnetization of the Fe films is not critically dependent on this dimension. Extension of the model to include spin-wave effects will be discussed.
Magnetization of a superconducting epitaxial (Y0.8Ca0.2)Ba2Cu3Ox film with O2 anneal
Authors: McGuire T. R. , La Placa S. J. , Chandrashekhar G. V. , Gupta A. , Bringley J. F. , Segmüller A.
Published: July 2002

A c-axis epitaxial film of composition (Y0.8Ca0.2)Ba2Cu3O6+x is found to be superconducting over a wide range of oxygen stoichiometry, from x≊0 to x≊1.0. The oxygen stoichiometry was varied systematically by annealing films over four decades of oxygen partial pressure, ranging from 0.0067%–100% O2. Both the superconducting transition temperature and the remanent magnetization increase with increasing x and are approximately linear with log oxygen partial pressure over three decades. These increases are attributed mainly to the change in the number of holes in the Cu–O planes caused by oxidation.
Electron spin resonance of Er3+ ions in ErzY1−zBa2Cu3O6
Authors: Huang M. X. , Barak J. , Bhagat S. M. , Gupta L. C. , Rajarajan A. K. , Vijayaraghavan R.
Published: July 2002

We report electron spin resonance measurements on ErzY1−zBa2Cu3O6 powdered samples for 0.02≤z≤1 at 36 GHz and 1.3<T<77 K. For z≥ (R18)0.6 a single highly distorted line, L1, is observed and interpreted in terms of a Kubo–Toyabe stochastic model. For lower z a partially resolved second line appears on the low field side of L1. The data are best represented as the sum of two Kubo–Toyabe lines.
Nondestructive detection of the bombardment-induced Gibbsian segregation using angle-resolved Auger-electron spectroscopy
Authors: Li Ri-Sheng , Li Chun-Fei , Liu Gu , Zhang Xun-Sheng , Bao De-Song
Published: July 2002

Using angle-resolved Auger electron spectroscopy, we show, nondestructively, a sequential variation of the composition from the outermost layer to the second layer of a Cu0.81Pt0.19 alloy and provide new evidence for ion bombardment-induced Gibbsian segregation in that alloy when it was sufficiently bombarded with 2.8-keV Ar ions at 10 and −80 °C.
p-type ZnSe homoepitaxial layers grown by molecular beam epitaxy with nitrogen radical doping
Authors: Ohkawa K. , Mitsuyu T.
Published: July 2002

We have grown high-quality homoepitaxial layers of p-type ZnSe for the first time. The layers were grown on dry-etched ZnSe substrates by molecular-beam epitaxy with nitrogen radical doping. The p-type conduction with carrier concentration of 8.9×1015 cm−3 at 300 K was confirmed by Hall measurement. Low-temperature photoluminescence from the N-doped ZnSe layers was dominated by single acceptor-bound exciton emission I1 at 2.7931 eV, which indicates a formation of a number of shallow acceptors and strain free of the homoepitaxial layers.
Enhanced polarization-dependent coupling between an optical waveguide and a laterally coupled photodetector
Authors: Kapser K. , Deimel P. P.
Published: July 2002

We have calculated the parameters for an optimal matching layer between a waveguide and a laterally coupled photodetector for TE and TM modes. We find a difference on the order of a few percent between the classical and exact calculations based on optical modes concerning the optimal matching layer thickness. Also, the thickness of the matching layer depends critically on the thickness of the absorbing layer. Comparing TE and TM modes, the absorption maxima for TE(TM) and the minima for TM(TE) coincide approximately, and the maxima and minima depend on the matching layer thickness.
Ambipolar diffusion in strained Si1−xGex(100) layers grown by molecular beam epitaxy
Authors: Grivickas V. , Netiksis V. , Noreika D. , Petrauskas M. , Willander M. , Ni W.-X. , Hasan M.-A. , Hansson G. V. , Sundgren J.-E.
Published: July 2002

Carrier dynamics in strained Si1−xGex layers and Si/Si1−xGex superlattices, grown by molecular beam epitaxy with different Ge concentrations, were investigated by a transient grating method. The ambipolar diffusion coefficient Da of carrier transport parallel to the layer plane was determined at high-density carrier excitation. An increase to Da values by a factor of up to 1.5 was observed for the strained alloy films compared to the value for moderately doped molecular beam epitaxy silicon layers. This is the first experimental evidence for enhanced performance of parallel carrier transport in Si1−xGex alloy layers.
Polarization selective widely tunable external cavity diode laser
Authors: Syvridis D. , Guekos G. , Besse P. A. , Dall’Ara R.
Published: July 2002

Polarization selectivity and wide tunability for transverse electric (TE) and transverse magnetic (TM) radiation are achieved through an external cavity diode laser with a grating. The laser output is single mode, centered around 1.47 μm and tunable over 140 nm for the TE and for 70 nm for the TM polarization.
Filament formation in semiconductor laser gain regions
Authors: Paxton Alan H. , Dente Gregory C.
Published: July 2002

We have linearized the equations for propagation of the beam of light in a semiconductor optical amplifier about an operating point and have derived the rate of growth of small sinusoidal perturbations of the phase and modulus of the complex field amplitude. The perturbations grow if the spatial frequency is below a critical value that depends on the intensity of the field at the operating point. For spatial frequencies above the critical value, the perturbations die out. The critical spatial frequency decreases as the intensity increases above a certain value. In other words, the tendency to filament becomes weaker as the intensity increases above a certain value. Computer-generated solutions of the propagation and gain equations are included to illustrate the growth of filaments as the plane-wave intensity changes in an amplifier.
Coherent continuous wave operation of 10×10×2 grating-surface-emitting diode laser array in a ring configuration
Authors: Liew S. K. , Carlson N. W. , Evans G. A. , Amantea R. , Bour D. P. , Hammer J. M. , Kirk J. B. , Reichert W. , Stolzenberger R. , Butler J. K. , DeFreez R. K. , Felisky M. , Wang C. A. , Choi H. K. , Walpole J. N.
Published: July 2002

Coherent cw operation of a 10×10×2 grating-surface-emitting diode laser array in a ring configuration is demonstrated. At near twice the threshold current, the ring array exhibits a high degree of spatial coherence (an average of 86%) between emitting grating sections and a narrow linewidth of 28 MHz. The far field fringe visibility is 80% and 88% in the lateral and longitudinal directions, respectively.
High-intensity ultraviolet laser interaction with a metallic filament
Authors: Holtmeier G. M. , Alexander D. R. , Barton J. P.
Published: July 2002

Results of research conducted on interaction of high-intensity ultraviolet pulsed KrF laser radiation with a metallic filament in atmospheric air are reported. Experimental results are presented on the time dependent material ejection dynamics associated with pulsed single shot focused excimer laser beams interacting with a small diameter wire. Copper wire 75 μm in diameter was illuminated with a KrF (λ=248 nm, pulse duration =17 ns full width at half-maximum) excimer laser at an irradiance of 80 GW/cm2. Interaction dynamics were recorded using both real-time and double-strobed ultraviolet laser imaging techniques. Temporal and spatial evolutions of both vaporized and molten material are reported. Material observed, during two-dimensional shadow imaging, on the side opposite the incident beam (shadow side) was shown to originate from the front ablating surface without spallation of material on the shadow surface. Several spatial material emission velocities are reported utilizing two different imaging techniques. A maximum velocity of 1500 m/s was obtained for material ejected in a direction toward the ablating laser.
The induced charge on a grounded conductor near a point charge
Authors: Chow Y. L. , Qian J. , Srivastava K. D.
Published: July 2002

The induced charge on a grounded arbitrary conductor, excited by a nearby point charge, is shown to be approximated by that on an equivalent conducting sphere near the same point charge. The proof is modified from that for capacitance by Payne and Weinberger [J. Math. Phys. 33, 291 (1955)]. A number of examples for different grounded blocks and plate are given. The potential field distributions from the induced charge are also given.
Influence of voids in interface zones on Lamb-mode spectra in fiber-reinforced composite laminates
Authors: Vasudeva R. Y. , Rao P. Govinda
Published: July 2002

A recently formulated porous interface layer model is employed to study the influence of bond defects particularly due to the presence of voids in interface zones on Lamb wave propagation in a laminate made of two identical fiber-reinforced laminas. The model is developed making use of the mathematical theory of linear elastic materials with voids to describe the interface zone−the adhesive with its minute separation from the adherent laminas. Theoretical Lamb-mode dispersion spectra in a symmetric bilaminate made of boron fiber-reinforced aluminum sheets are obtained and compared with those obtained assuming perfect bond condition. It is hoped that the model will facilitate ultrasonic nondestructive evaluation of bond strengths in composite laminates.
Soliton return effect in an annular Josephson junction and nonzero minimum voltage and current in the current step
Authors: Wang Qianghua , Wang Wei , Yao Xixian
Published: July 2002

The soliton return effect in an annular Josephson junction is analyzed in detail by using an analytical treatment. Some results are compared with that of direct numerically simulation. The conditions for a resonant soliton motion that corresponds to a current step in the I-V characteristic are discussed. It is shown that the minimum current and voltage of the current step is not equal to zero because of the soliton return effect, and relates to the geometric and dissipative parameters of the junction. It is found that a kind of critical situation exists in an annular junction on the lower end of a current step that separates the zero voltage states and finite voltage states. A discussion of the difference between an in-line and annular junction is also made.
Instabilities in annealed proton exchange waveguides in lithium tantalate
Authors: Matthews Paul J. , Mickelson Alan R.
Published: July 2002

The stability of annealed proton exchange waveguides and devices in LiTaO3 is examined by monitoring the output power ratio of passive directional couplers. This is shown to be an extremely sensitive technique for measuring waveguide stability. Mode propagation constants of planar waveguides and near-field mode sizes and losses of channel waveguides are also measured as a function of time. Data from these measurements indicate that the refractive index profile is unstable and continuously evolves even for relatively long anneal times. The instabilities appear to lower the local change in the refractive index either through the room-temperature migration of hydrogen or a change in the magnitude the refractive index increase due to the proton exchange. Assuming that the hydrogen concentration profile is stable, the decrease in the refractive index was estimated to be ≊10−3 over a period of 90 days. Although the instabilities were seen in all of the fabricated devices, only a small portion of the total fabrication parameter space was investigated. Therefore, it is possible that the instability may be avoided as in LiNbO3, by using dilute melts or even longer anneal times.
Magnetic properties of X-type Ba2Me2Fe28O46 (Me=Fe, Co, and Mn) hexagonal ferrites
Authors: Gu B. X.
Published: July 2002

Polycrystalline samples of X-type Ba2Me2Fe28O46 (Me2-X, Me=Fe, Co, and Mn) hexagonal ferrite have been prepared by the classic ceramic method. Low-temperature magnetic properties, Curie temperatures, and distribution of divalent cations were studied. The substitution of Fe2+ ions with Co2+ and Mn2+ ions causes a decrease of the spontaneous moment μ and the Curie temperature Tc of X-type hexaferrites. The lower μ and Tc of Co2-X hexaferrite originate from the smaller magnetic moment of Co2+ ions. In the case of Mn2-X, the lower μ may be due to the valence state of Mn and the lower Tc may be caused by the fact that the space distribution of 3d electrons of Mn is different from that of Fe. In the Co2-X and Fe2-X compounds the Co2+ and Fe2+ ions enter the octahedral sites of spinel S blocks.
Droplet diameter, flux, and total current measurements in an electrohydrodynamic spray
Authors: Dunn Patrick F. , Snarski Stephen R.
Published: July 2002

The measured diameter and flux distributions and total current of ethanol droplets generated electrohydrodynamically from a single hypodermic needle at two different spray charge densities are presented and discussed. The resultant droplet diameter and droplet flux distributions reveal a progressive production of an increasing number of smaller diameter droplets with increasing spray charge density. These are consistent with observations made of the filament breakup-to-droplet process in this region. The droplet flux distributions are used to construct droplet charge flux distributions by assuming various models of the droplet charging process. The resultant charge fluxes are logarithmically related to lateral position, regardless of the droplet charging model assumed. The integrals of the various charge flux profiles are compared to the measured total current. This approach reveals the best agreement between measured and predicted total droplet currents when an equilibrium end-state process for droplet charging is assumed.
Effect of neutral and charged particles on the line shape of the 577 and 491.6 nm mercury lines: Application to mercury pressure determination in high-pressure Hg lamps
Authors: Asselman A. , Aubès M. , Damelincourt J. J. , Salon J.
Published: July 2002

The broadening and shift of the optically thin Hg 577 and 491.6 nm lines emitted in high-pressure mercury discharges operating at 50 Hz ac frequency are studied. An investigation is carried out in order to appreciate the influence of different factors on the broadening and shift of the local line profile. Comparison between computed and experimental apparent line shapes permits the determination of a set of broadening and shift parameters which leads to a satisfactory representation of the line shapes. Furthermore, applications to mercury discharge diagnosis are proposed.
Determination of the interface charge between an epilayer and a substrate using capacitance-voltage measurements
Authors: Goren D. , Amir N. , Nemirovsky Y.
Published: July 2002

A method is presented for the measurement of the interface charge between an epitaxial layer and a substrate. Capacitance-voltage characteristics of Schottky contacts formed on the epilayer are measured and analyzed. The interface charge is determined by the modified built-in potential and capacitance. Experimental results are reported for p-type CdTe epilayers grown by metalorganic chemical vapor deposition on a p-type CdTe substrate using indium contacts.
The electronic structure of the impurity boron-vacancy complex in transition metal γ-Fe
Authors: Wang Chongyu , Chen Liqun , Li Keng , Gao Zhe , Wang Bing
Published: July 2002

By use of the self-consistent-field Xα scattered-wave (SCF-Xα-SW) method, the electronic structure was calculated for two models of γ-Fe12-B2-V cluster as well as the γ-Fe13 cluster for contrast. Results are presented for total energy, energy levels, density of states, and one-electron properties. Comparisons are made between two models of γ-Fe12-B2-V cluster, and between γ-Fe12-B2-V and γ-Fe13 clusters. The analyses are of the effect of the boron-vacancy (B-V-B) complex on the physical properties of γ-Fe. The calculations indicate that there is a strong interaction between boron and host atoms. Thus the B-V-B complex is favorable for thermal stability. Comparison of the average energy of an atom between the γ-Fe12-B2-V and α-Fe8-B2-V clusters shows that a B-V-B complex may restrain the nucleation of ferrite in austenite transformation and thereby is beneficial to the hardenability on quenching of austenitic steel. The results also indicate a strong binding between B atoms and vacancies, which is favorable for the creep-rupture life of austenitic steel.
Gas degradation and O2 production in a high repetition rate, transversely excited atmospheric CO2 laser
Authors: Tashiro Hideo , Suda Akira , Kurachi Masashi , Shibata Setsuo , Toshikuni Masami , Maki Masatoshi , Midorikawa Katsumi
Published: July 2002

Based on viewpoint that O2 is the most informative indication of gas degradation in transversely excited atmospheric (TEA) CO2 lasers, we have experimentally investigated the O2 concentration using a 100 Hz, 500 W class TEA-CO2 laser with a catalytic gas recycler. A simple theoretical model is also proposed to describe the O2 concentration and then compared with experimental data over a wide range of operating conditions. The results show that the model can be used for design of optimized gas recyclers.
Luminescence peculiarities on (AlGa)As single quantum well
Authors: Oelgart G. , Lehmann L. , Araújo D. , Ganière J. D. , Reinhart F. K.
Published: July 2002

This paper reports on single quantum well characterization by means of luminescence excited with a laser (photoluminescence) and an electron beam (cathodoluminescence) at liquid helium temperatures. Small quantum well regions with smaller confinement were observed. They seem to be associated with dislocations originated at the substrate. The observed shift of the X(e-hh) peak position versus the excitation level can be explained by luminescence generation in different lateral regions of the quantum well.
Possible low-loss Faraday rotation and phase shifts using multilayer ferrite structures
Authors: Sokoloff J. B. , Vittoria C.
Published: July 2002

It is suggested that it should be possible to make low insertion loss devices (e.g., Faraday rotators, phase shifters, and broad-band filters) to operate in the millimeter wave range using a multilayer array of thin films magnetized perpendicular to the films with waves of frequency below the magnon continuum normally incident.
Interfacial photovoltage microscopy: A new diagnostic for silicon-on-insulator materials
Authors: Blanc J. , Gittleman J. I. , Matey J. R.
Published: July 2002

The interface between silicon and various dielectric materials is of great technological importance for the operation of solid-state devices. The sign and magnitude of trapped charge at these interfaces is of particular importance, since it is often a controlling factor in device performance. Interfacial photovoltage microscopy is a technique for measuring the electric fields at such interfaces that is nondestructive, process compatible, and relatively fast. The microscope applies an intense flux of photons to generate an electron-hole plasma in the silicon. The plasma polarizes in response to the electric field generated by interfacial charge and the polarization can be sensed outside the wafer by a capacitive sensor. We infer interfacial charge as a function of position by scanning the photon beam across the sample and measuring the resulting polarization. Micrographs generated from these measurements show the sign, magnitude, and uniformity of the charge at the interface. The system has been tested by intentionally modifying the charge density at the interface using x rays. A description of the instrument, the results of experiments, and a comparison of the results with simple physical models are presented.
X-ray generation from 50-mJ, 120-ps KrF laser-produced plasmas
Authors: Broughton J. N. , Fedosejevs R.
Published: July 2002

The generation of x rays from the interaction of low-energy (10–50 mJ) 120-ps KrF laser pulses with solid targets has been experimentally investigated. Conversion efficiency as a function of laser energy and detailed keV x-ray spectra have been obtained for targets (Fe, Ni, and Cu) with potential application in high-resolution x-ray lithography. Peak x-ray conversion efficiencies of over 3% in the 800–1400-eV photon energy band have been measured for laser intensities of 1.5 × 1014 W/cm2. In addition, transmission grating spectra have been obtained to characterize the overall distribution of x-ray emission at photon energies down to 100 eV.
Magnetization behaviors in a chain of disks
Authors: Ishii Y. , Anbo H. , Nishida K. , Mizuno T.
Published: July 2002

The magnetic behavior of a chain of N disks are investigated. The critical field Hcri for the discontinuous jump of the magnetization and the coercive force Hc for the fanning mechanism, and the uniaxial magnetic anisotropy constant Ku are estimated as a function of N, the aspect ratio of the disks, and the magnetocrystalline anisotropy constant K1. By comparing the MH loops calculated and the experiments, the values of K1 and N are obtained for an assembly with random orientation of Ba ferrite particles. This value of K1 agrees well with the experimental results obtained by Fujiwara [IEEE Trans. Magn. MAG-23, 3125 (1987)].
Luminescence and relaxation processes in Er3+-doped glass fibers
Authors: Mita Y. , Yoshida T. , Yagami T. , Shionoya S.
Published: July 2002

Luminescence and related characteristics have been investigated for Er3+-doped silica glass fibers and for fibers and bulk material of calcium metaphosphate glass. Special interest has been directed toward elucidating relaxation processes leading to 1.5-μm light emission. Several emissions from higher excited states were investigated and excited-state lifetimes were also measured. Approximate radiative and nonradiative transition probabilities have been obtained for the excited levels. These results have confirmed that dominant transitions populating the lowest excited level are step-by-step nonradiative relaxations and that only a limited amount of energy is supplied via intermediate radiative transitions. It has also been shown that the 1.5-μm emission decay time constant elongates with increasing fiber length.
Luminescence kinetics of semiconductor doped glasses in the long time region
Authors: Burkitbaev S. , Bertolotti M. , Fazio E. , Ferrari A. , Liakhou G. , Sibilia C.
Published: July 2002

Photoluminescence time behavior has been studied with a photon counting technique by using long pulse excitation (1–10 μs). The presence of a long luminescence decay at λ=632 nm in Corning CS-261 glass has been revealed.
Effect of local layer-thickness deviation on x-ray diffraction of multilayers
Authors: Guo Shiping , He Xianchang , Wu Ziqin
Published: July 2002

A simulation of Cu Lα and Cu Kα diffractions from W/Si multilayers with a local layer-thickness deviation at various depths of the multilayers has been presented. The results show that the peak intensity decreases rapidly at first while increasing the depth of the local deviation, and then the peak splits into double peaks. But, afterward, the double peaks coincide again. These changes are affected by the ratios (x) of heavy-atom layer thickness to total period thickness. The intensities of the double peaks are nearly equal when x is 0.5, but are quite different when x is 0.22. The changes due to the local deviation are different from those due to random fluctuations or systematic deviations in periods. An explanation has been carried out by the amplitude-phase diagrams.
Increased surface voltage relaxation times in polyvinylidene fluoride films
Authors: Andry P. , Filion A. Y. , Perlman M. M.
Published: July 2002

An increase in surface voltage decay time has been noted in corona-charged polyvinylidene fluoride films solution cast on silicon, once these films have been charged a number of times and annealed. A re-examination of the fundamental equations that describe these surface voltage decays shows that the relaxation of the electric field inside the polymer may be considerably lengthened, when the trapped charge density is comparable to the thermal free- carrier density.
The preparation and characterization of nearly hysteresis-free metal-nitride-silicon capacitors on both p- and n-type silicon substrates
Authors: Lau W. S.
Published: July 2002

The hysteresis in metal-nitride-silicon (MNS) capacitors can be reduced to nearly zero by using nitrogen-rich silicon nitride as the gate dielectric and treating the silicon substrate by an ammonia plasma before dielectric deposition. However, the ammonia plasma treatment step also causes an increase in interface state density, especially in the middle of the silicon band gap. Without the ammonia plasma treatment, the virgin flat-band voltage V*FB is always negative. With the ammonia plasma treatment, V*FB can be shifted from a negative value to zero for MNS capacitors on n-type silicon, whereas V*FB will be shifted from a negative value to a more negative value for MNS capacitors on p-type silicon. These effects can be explained by postulating that the interface states generated by the ammonia plasma treatment step are amphoteric defects similar to Pb centers at the oxide/silicon interface in metal-oxide-silicon capacitors.
Growth of Cu films on hydrogen terminated Si(100) and Si(111) surfaces
Authors: Demczyk B. G. , Naik R. , Auner G. , Kota C. , Rao U.
Published: July 2002

We have employed reflection high energy electron diffraction (RHEED) and high resolution transmission electron microscopy (HREM) to study Cu films grown on hydrogen terminated Si(100) and Si(111) substrates by molecular beam epitaxy. X-ray diffraction and RHEED studies indicate 〈100〉Cu growth on Si(100) and 〈111〉Cu growth on Si(111). HREM reveals orientation relationships of [001]Cu∥[011]Si, (010)Cu∥(011)Si and [1̄12]Cu∥[011]Si, (220)Cu∥(111̄)Si for Si(100) and Si(111), respectively. A copper silicide layer forms on Si(100) with deposition and appears to aid in proper lattice matching. No significant interdiffused region was detected in the films deposited on Si(111), however, distinct orientational variants were observed in this case.
Preparation of diamondlike carbon films by high-intensity pulsed-ion-beam deposition
Authors: Johnston Gregory P. , Tiwari Prabhat , Rej Donald J. , Davis Harold A. , Waganaar William J. , Muenchausen Ross E. , Walter Kevin C. , Nastasi Michael , Schmidt Howard K. , Kumar Nalin , Lin Boyang , Tallant David R. , Simpson Regina L. , Williams David B. , Qiu Xiamei
Published: July 2002

Diamondlike carbon (DLC) films were prepared by high-intensity pulsed-ion-beam ablation of graphite targets. A 350 keV, 35 kA, 400 ns beam, consisting primarily of hydrogen, carbon, and oxygen ions was focused onto a graphite target at a fluence of 15–45 J/cm2. Amorphous carbon films were deposited at up to 30 nm per pulse, corresponding to an instantaneous deposition rate greater than 1 mm/s. Electrical resistivities were between 1 and 1000 Ω cm. Raman spectra indicate that diamondlike carbon is present in most of the films. Electron-energy-loss spectroscopy indicates significant amounts of sp3-bonded carbon, consistent with the presence of DLC. Scanning electron microscopy showed most films contain 100 nm features, but micron size particles were deposited as well. Initial tests revealed favorable electron field-emission behavior.
Interface structure and surface morphology of (Co, Fe, Ni)/Cu/Si(100) thin films
Authors: Demczyk B. G. , Naik V. M. , Lukaszew A. , Naik R. , Auner G. W.
Published: July 2002

We have examined bilayer Co/Cu, Fe/Cu, and Ni/Cu films deposited by molecular-beam epitaxy on hydrogen-terminated [100] silicon substrates. The magnetic metal/copper interface was examined by atomic resolution transmission electron microscopy and compared with the surface morphology as depicted by atomic force microscopy. The general orientation relationships across the magnetic metal/copper interfaces were found to be: [001]Co, Ni∥[001]Cu; (010)Co, Ni∥(010)Cu and [001]Fe∥[001]Cu; (110)Fe∥(200)Cu. The latter system is equivalent to the [11̄ 1]Fe∥[011]Cu and (110)Fe∥(100)Cu Pitsch relationship, as has been reported earlier. Furthermore, there was a general correlation between interfacial and surface roughness, indicating that the initial interface character is propagated throughout the film during growth.
Facts and artifacts in near-field optical microscopy
Authors: Hecht B. , Bielefeldt H. , Inouye Y. , Pohl D. W. , Novotny L.
Published: July 2002

Near-field optical (NFO) microscopes with an auxiliary gap width regulation (shear force, tunneling) may produce images that represent the path of the probe rather than optical properties of the sample. Experimental and theoretical evidence leads us to the conclusion that many NFO results reported in the past might have been affected or even dominated by the resulting artifact. The specifications derived from such results for the different types of NFO microscopes used therefore warrant reexamination. We show that the resolving power of aperture NFO microscopes, 30–50 nm, is of genuine NFO origin but can be heavily obscured by the artifact.
Self-assembled InAs/GaAs quantum dots under resonant excitation
Authors: Adler F. , Geiger M. , Bauknecht A. , Haase D. , Ernst P. , Dörnen A. , Scholz F. , Schweizer H.
Published: July 2002

The energy structure and the carrier relaxation in self-assembled InAs/GaAs quantum dots (SADs) is investigated by photoluminescence excitation spectroscopy (PLE) and photoluminescence (PL) at resonant excitation (below the GaAs and the wetting layer bandgap). In PLE measurements we find a clear resonance from the first excited hole state as well as resonances from a relaxation via different phonons. From a comparison of the PL-rise times in time resolved spectroscopy, we conclude on a fast electron relaxation (⩽50 ps) and a slow hole relaxation with a time constant of about 400 ps. Different relaxation paths are observed in the InAs/GaAs quantum dot system and allow us to identify the hole relaxation in the SADs as multiphonon assisted tunneling. The PL-decay time in the SADs after resonant excitation (about 600 ps) is attributed to the lifetime of the quantum dot exciton. In agreement with theoretical predictions, we find a constant lifetime of about 600 ps for temperatures below 50 K and a linear increase of the lifetime between 50 and 100 K with a slope of 26 ps/K.
X-ray excited luminescence and local structures in Tb-doped Y2O3 nanocrystals
Authors: Soo Y. L. , Huang S. W. , Ming Z. H. , Kao Y. H. , Smith G. C. , Goldburt E. , Hodel R. , Kulkarni B. , Veliadis J. V. D. , Bhargava R. N.
Published: July 2002

Pronounced structure in x-ray excited luminescence (XEL) has been observed in dilute Tb-doped Y2O3 (Y2O3:Tb) nanocrystals. This effect affords a means to assess different energy transfer mechanisms in the nanocrystals and also an opportunity for novel device applications. Sharp jumps and oscillations are found in the XEL output with the incident x-ray energy around the absorption edges of Y and Tb. When compared with a bulk Y2O3:Tb sample, these effects are attributed to some unique electronic and optical properties of doped nanocrystals related to quantum confinement of charge carriers, and the main features can be explained by a proposed model of multichannel energy transfer. Extended x-ray absorption fine structure techniques have also been employed to study the effect of size variation and chemical doping on the local structures in Y2O3 and Y2O3:Tb nanocrystals. The local environment surrounding Y and Tb in the nanocrystals is compared with that in the respective bulk material. The results indicate that Tb impurity atoms substitute for Y sites in bulk Y2O3, while doping in the nanocrystals is complicated by the large fraction of surface atoms and local disorder.
Bright blue organic light-emitting diode with improved color purity using a LiF/Al cathode
Authors: Shaheen S. E. , Jabbour G. E. , Morrell M. M. , Kawabe Y. , Kippelen B. , Peyghambarian N. , Nabor M.-F. , Schlaf R. , Mash E. A. , Armstrong N. R.
Published: July 2002

We report a two-layer, blue organic light-emitting diode with a 4,4′-bis(2,2-diphenylvinyl)-1,1′-biphenyl emission layer and a LiF/Al cathode which has an external quantum efficiency of 1.4% and a maximum luminance of 3000 cd/m2. Insertion of the thin LiF layer results in a 50-fold increase in the device efficiency compared to a device with an aluminum only cathode, and eliminates the need for an electron-transporting layer, such as tris(8-hydroxyquinoline)aluminum. This results in a device with excellent color purity with an emission peak at 476 nm and a full width at half maximum of 78 nm. Using ultraviolet photoelectron spectroscopy, we find that the effective work-function of aluminum decreases dramatically with sub-monolayer amounts of LiF deposited on the surface.
Ultimate resolution electron energy loss spectroscopy at H/Si(100) surfaces
Authors: Tautz F. S. , Schaefer J. A.
Published: July 2002

We have re-investigated the well-studied hydrogen covered Si(100) surface using high resolution electron energy loss spectroscopy (HREELS) with a spectral resolution down to 13 cm1, representing an improvement by a factor of 3–4 compared to earlier work. For the first time in HREELS, it has been possible to clearly distinguish between the stretching vibrations of H-Si-Si-H units on monohydride surfaces with a 2×1 low energy electron diffraction pattern, on the one hand, and SiH2 units on dihydride surfaces with 1×1 symmetry on the other hand, the two being shifted by 5 cm-1 with respect to each other, in good agreement with results from infrared spectroscopy. Furthermore, we find trihydride units even for relatively low exposures beyond monohydride saturation coverage from their distinct stretching frequencies, and this points towards early etching stages. The question of the scattering mechanism applicable in our experiments is discussed. Since there are no spectral limitations in HREELS, we can analyze the bending and scissor vibrations with similar accuracy as the stretching vibration. Again, we observe fine structure in our loss peaks, the umbrella mode, for example, giving once more evidence for trihydride species at the surface. The complicated line shape of all vibrational modes can thus be used to deduce detailed structural information about the surface atomic structure, opening up entirely new possibilities by employing HREELS for the structural characterization of (silicon) surfaces with atomic hydrogen as a local probe.
Magnetic hardening mechanism study in FePt thin films
Authors: Kuo C. M. , Kuo P. C. , Wu H. C. , Yao Y. D. , Lin C. H.
Published: July 2002

Fe 100−x Pt x alloy thin films with x=25–67 at. % were prepared by dc magnetron sputtering on naturally oxidized Si substrates. Effects of film composition, annealing temperature (300–650 °C), annealing time (5–120 min), and cooling rate (furnace cooling or ice water quench cooling) on the magnetic properties were investigated. Optimum conditions for saturation magnetization and coercivity of the Fe100−xPtx alloy films were found with x=50 at.%, annealed at 600 °C for 30 min and cooled by ice water quenching. Our experimental data suggests that the magnetic hardening in Fe100−xPtx alloy thin films is mainly due to the fct γ1-FePt phase and the domain wall pinning effect. The domain nucleation mechanism is dominated in samples with furnace cooling; the domain wall pinning mechanism dominates in samples cooled with ice water quenching.
Ferromagnetic resonance linewidth and anisotropy dispersions in thin Fe films
Authors: Fermin J. R. , Azevedo Antonio , de Aguiar F. M. , Li Biao , Rezende S. M.
Published: July 2002

In-plane ferromagnetic resonance (FMR) has been used to study the room-temperature linewidth ΔH of single crystal Fe films grown by dc magnetron sputtering onto MgO(100) substrates. Several samples were grown with the film thickness in the range 70 Å<tFe<250 Å. The measurements were carried out in the frequency range from 7.0 to 12.3 GHz. A phenomenological model for the FMR linewidth was developed that includes simultaneous effects due to intrinsic damping and angular dispersions of the cubic and uniaxial axes of anisotropy. These angular dispersions are found to be responsible for the relatively larger linewidths observed as a function of the in-plane field direction with fixed frequency, and as a function of frequency for the easy and hard directions as well. The behavior of the linewidth with the film thickness can be described by a sum of a constant volume term plus a term proportional to 1/tFe, representing the relaxation due to the misfit dislocations.
Strain and SiC particle formation in silicon implanted with carbon ions of medium fluence studied by synchrotron x-ray diffraction
Authors: Eichhorn F. , Schell N. , Matz W. , Kögler R.
Published: July 2002

Implantation of C ions with an energy of 195 keV into Si wafers heated up to 800 °C results in an elastic distortion of the Si host lattice and in the formation of crystalline SiC particles or their prestages depending on implantation dose and temperature. Synchrotron x-ray diffraction at the Rossendorf beamline in Grenoble was used to reveal phase formation and the correlated lattice strain changes. Only a Si lattice deformation without growth of SiC was observed if the fluence did not exceed 5×1015 C ions/ cm2. After implantation of C ions up to 4×1017 cm−2 at a temperature of 500 °C, agglomerations of Si–C and an altered state of Si lattice deformation are found. By implantation of 4×1017 ions/cm2 at 800 °C, particles of the 3C–SiC (β-SiC) phase grow, which are aligned with the Si matrix. They are aligned in such a way with the Si matrix that the cubic crystallographic axes of matrix and particles coincide with an accuracy of 3°.
Magnetotransport properties of (Ga, Mn)Sb
Authors: Matsukura F. , Abe E. , Ohno H.
Published: July 2002

The preparation of dilute alloy of GaSb and Mn, (Ga, Mn)Sb, with a few percent of Mn by molecular beam epitaxy and its magnetotransport properties are reported. Magnetotransport measurements show a pronounced anomalous Hall effect and negative magnetoresistance below 50 K. The results suggest that Mn atoms are incorporated in the GaSb host, resulting in the formation of the ferromagnetic semiconductor, (Ga, Mn)Sb.
Dopant effect on point defect incorporation into growing silicon crystal
Authors: Voronkov V. V. , Falster R.
Published: July 2002

Acceptor and donor dopants affect the critical ratio v/G for the change-over from interstitial to vacancy incorporation (v is the growth rate and G is the near-interface temperature gradient) in growing silicon crystals. The boron effect (an increase in the critical v/G) is nicely accounted for by a simple mechanism of electronic shift in the equilibrium concentrations of the charged point defects at the melting point (Tm). By fitting the theoretical curve to the experimental data, the ratio of the equilibrium concentrations of vacancy and self-interstitial is defined to be 1.3 at Tm. Given this the interstitial diffusivity and the two equilibrium concentrations at Tm can then be specified. Beside the electronic shift, alternative mechanisms (interstitial impurity component, impurity pairing to vacancy) are discussed to provide a general formula for the impurity-induced shift in the critical v/G.
Extrinsic contributions to the ferromagnetic resonance response of ultrathin films
Authors: Arias Rodrigo , Mills D. L.
Published: July 2002

We discuss the theory of extrinsic contributions to the linewidth of ultrathin ferromagnetic films, magnetized parallel to their surfaces. We invoke defect-induced two magnon scatterings, rendered active by unique aspects of dipolar contributions to spin wave dispersion in two dimensions. A two magnon shift in resonance frequency is present as well. We summarize our principal earlier conclusions, and present new results on the in-plane anisotropy of linewidth, for samples with stepped surfaces.
Effect of friction on electron transfer in biomolecules
Authors: Garg Anupam , Onuchic José Nelson , Ambegaokar Vinay
Published: July 2002

In biological and chemical electron transfer, a nuclear reaction coordinate is coupled to other nuclear and/or ‘‘solvent’’ coordinates. This coupling, or friction, if strong enough, may substantially slow down motion along the reaction coordinate, and thus vitiate the assumption of electron transfer being nonadiabatic with respect to the nuclei. Here, a simple, fully quantum mechanical model for electron transfer using a one mode treatment which incorporates this coupling is studied. Path integral methods are used to study the dependence of the reaction rate on friction, and the limits of the moderate and the high friction are analyzed in detail. The first limit will prevail if the reaction coordinate is, e.g., an underdamped nuclear vibration, whereas the second limit will prevail if it corresponds to a slow or diffusive degree of freedom. In the high-friction limit, the reaction rate is explicitly shown to vary between the nonadiabatic and adiabatic expressions as the tunneling matrix element and/or the friction are varied. Starting from a path integral expression for the time evolution of the reduced density matrix for the electron and reaction coordinate, a Fokker–Planck equation is obtained which reduces in the high-friction limit to a Smoluchowski equation similar to one solved by Zusman.
Correlation of connected transitions by two-dimensional NMR spectroscopy
Authors: Griesinger C. , So/rensen O. W. , Ernst R. R.
Published: July 2002

A novel and general concept of restricting coherence transfer in nuclear spin systems is described. It opens new possibilities for editing one- and two-dimensional NMR spectra. For example, the widely applied two-dimensional correlation experiment COSY can be modified such as to restrict coherence transfer to take place exclusively between connected transitions in the energy level diagram. Such two-dimensional spectra possess ideal features for assignment of complex scalar coupling networks and for computer assisted analysis. Experimental 1H spectra of a cyclic decapeptide are presented. Other applications of the general filtering concept are briefly discussed.
Phase diagram and dynamics of Yukawa systems
Authors: Robbins Mark. O. , Kremer Kurt , Grest Gary S.
Published: July 2002

The phase diagram and dynamical properties of systems of particles interacting through a repulsive screened Coulomb (Yukawa) potential have been calculated using molecular and lattice dynamics techniques. The phase diagram contains both a melting transition and a transition from fcc to bcc crystalline phases. These phase transitions have been studied as a function of potential shape (screening length) and compared to phenomenological criteria for transition temperatures such as those of Lindemann and of Hansen and Verlet. The transition from fcc to bcc with increasing temperature is shown to result from a higher entropy in the bcc phase because of its softer shear modes. Even when the stable solid phase below the melting temperature is fcc, bcc-like local order is found in the liquid phase. This may substantially slow crystallization. The calculated phase diagram and shear modulus are in good agreement with experiments on colloidal suspensions of polystyrene spheres. The single particle dynamics of Yukawa systems show several unusual features. There is a pronounced subdiffusive regime in liquids near and below the melting temperature. This regime reflects the existence of two time scales: a typical phonon period, and the time for a particle to feel a new environment. The second time scale becomes longer as the temperature is lowered or the range of interaction (screening length) increases.
Electronic states and potential energy surfaces of PdH2: Comparison with PtH2
Authors: Balasubramanian K. , Feng P. Y. , Liao M. Z.
Published: July 2002

Complete active space MCSCF (CASSCF) followed by multireference singles and doubles configuration interaction (MRSDCI) calculations are carried out on 13 electronic states of PdH2 and 9 electronic states of PdH+ . The bending potential energy surfaces of 1A1 and 3A1 electronic states were obtained. The 1A1 surface contains two bent minima (θe =30° and θe =62°) and a higher linear minimum separated by a saddle point. The 3A1 surface is relatively flat in the region of the saddle point. The Pd(1S0 ) atom forms a weak complex with H2 but inserts spontaneously into H2 while the Pd(3D3 ) atom has to surmount a large barrier for insertion into H2. The spin–orbit effects for the electronic states of PdH2 are estimated through relativistic configuration interaction calculations of PdH+ whose d Mulliken population is close to that of the d population of PdH2. The ground state of PdH2 is found to be a weakly bound bent 1A1 state with θe =30° and re (Pd–H)=1.67 Å. The electronic state and potential energy surfaces of PdH2 are compared with PtH2. The Pt–H bond is found to be stronger than the Pd–H bond mainly due to relativistic effects. The Pd–H bonds are more ionic than the Pt–H bonds.