bluuebunny/crossref_metadata_2025 huggingface dataset:
A study of the structural properties of sputtered CoCrPt based on x-ray diffraction, small angle x-ray scattering, XAFS, and transmission electron microscopy measurements (abstract)
Authors: Yumoto S. , Ohshima N.
Published: July 2002

As a first step in trying to understand the relationship between magnetic properties and structural properties, the authors have conducted a systematic structural investigation of CoCrPt samples that have been sputtered at different levels of sputtering power, which affects magnetic properties. X-ray diffraction (XRD), small angle x-ray scattering (SAXS), and XAFS were used to examine, respectively, long, medium, and short range orderings. TEM was used to examine the structural morphology. CoCrPt was fabricated on Kapton films by dc magnetron sputtering at levels of 0.1, 0.3, 0.5, and 1 kW sputtering power. XRD indicated a continuous increase in the intensity of the peak corresponding to hcp (002), as sputtering power was increased. SAXS, on the other hand, in the range of 0.5–3.0 deg. at 2θ, indicated the presence of scattering for the 0.1 and 0.3 kW samples alone, not for the others. Similarly discontinuous were the spectrums produced by XANES of Co for the 0.3, 0.5, and 1.0 kW samples, which resembled the spectrum of sputtered Co. The spectrum of the 0.1 kW samples was quite different. TEM indicated a clear small-grain structure for the 0.1 kW sample, less clearly defined grains for the 0.3 kW sample, and no observable grains for the others. When conducted after chemical etching, it further identified the source of the grain structure as Pt segregation. This leads to speculation that Pt segregation may be responsible, to some degree, for the discontinuity observed in SAXS and XANES measurements. It is further speculated that the long range ordering (crystal orientation) is quite probably related to magnetic anisotropy, and that segregation affects wall motion, which means that, in order to understand the relationship between magnetic and structural properties, structural investigations should be systematic and include short, medium, and long range orderings.
Anisotropy and flux density enhancement in aligned ThMn12-type NdFe11Co1−yMoyN film samples
Authors: Navarathna A. , Hegde H. , Rani R. , Cadieu F. J.
Published: July 2002

Highly aligned sputtered film samples of ThMn12-type NdFe12−y−z CoyMozN, y+z≊1, have been synthesized. For low Co concentrations it has been possible to enhance the saturation flux densities, the coercivity, and the anisotropy field of the nitrided samples. Films which have a gradient along the substrate length in the Fe to Co ratio have been synthesized to determine the maximum coercivity and anisotropy values that can be obtained for the Nd(Fe,Co,Mo)12N nitrides. Maximum room temperature coercivities and anisotropies of 11.3 and 145 kOe were obtained for a ThMn12-type Nd8.9Fe80.3Co6.0Mo4.8N (002) textured film sample. At lower temperatures the coercivity and anistropy rose smoothly to 29.5 and ≊200 kOe, respectively by 10 K. Based on nitriding studies on similar films, all nitride sites should be occupied at ≊1 N per ThMn12 formula unit for the high coercivity films reported.
Surface roughness and magnetic properties of in situ heated and postannealed thin films of perpendicular barium ferrite
Authors: Sin Kyusik , Sivertsen John M. , Judy Jack H.
Published: July 2002

The surface roughness and magnetic properties of thin films of hexagonal barium ferrite (BaFe) deposited on SiO2/Si and Si substrates by facing target sputtering were studied using in situ heating during deposition and postannealing after deposition. The perpendicular coercivity of the BaFe films varied between 775 and 1430 Oe with substrate temperature for both in situ heated (560–620 °C) and postannealed (800–1200 °C) films. The effect of temperature on the surface roughness of BaFe/Si was greater than that of BaFe/SiO2/Si. The results of this study indicate that the mean surface roughness (Ra=11.4–27 nm) of BaFe films on Si substrates was greater than the roughness (Ra=3.9–17 nm) of BaFe films on amorphous SiO2/Si substrates. This increase of surface roughness with increasing substrate temperature is due to the evolution of grain boundaries and step formation during deposition and annealing.
Magnetic and structural modifications in Fe and Ni films prepared by ion-assisted deposition
Authors: Lewis William A. , Farle Michael , Clemens Bruce M. , White Robert L.
Published: July 2002

We summarize our observations of in-plane uniaxial magnetic anisotropy induced in 1000 Å Ni and Fe thin films by 100 eV Xe+ ion bombardment during deposition. The anisotropy was measured by means of the magneto-optic Kerr effect and full angular scan ferromagnetic resonance. The maximum in-plane anisotropy field was 150 Oe for Ni and 80 Oe for Fe. The hard direction of magnetization lies parallel to the plane of incidence for Ni and perpendicular to it for Fe. An expansion of the lattice of up to 0.6% normal to the film and an enhancement of the fiber texture are found in both cases. This out-of-plane expansion is accompanied by an in-plane compression. In addition, a small in-plane difference in lattice spacings (<0.2%) is found between directions parallel and perpendicular to the plane of incidence of the ions. The in-plane uniaxial magnetic anisotropy is attributed to the in-plane anisotropic strain using a simple magnetoelastic model.
Deep level transient spectroscopy investigation of a deep trap in float-zone Si
Authors: Londos C. A.
Published: July 2002

By means of deep-level transient spectroscopy we have investigated defect states in float-zone, boron-doped Si material subjected to electron irradiation. Attention was mainly focused on the Ev+0.34 eV peak, the emergence of which in the spectra shows a delay with temperature in relation to the beginning of the decay of the Ev+0.28 eV peak of the carbon interstitial defect. In addition, the concentration of the Ev+0.34 eV level was found to increase with the boron content of the Si material. Our results point to a carbon-related, boron-dependent structure although the role of boron is not clear from the present data.
Acoustic quality factor of niobium and vanadium at low temperatures
Authors: Duffy W. , Umstattd R.
Published: July 2002

The amplitude-independent acoustical mechanical quality factors Q of polycrystalline commercially prepared samples of 99.8% pure niobium and 99.75% pure vanadium have been measured as a function of temperature and heat treatment from 50 mK to 300 K. Data were obtained from 1 kHz torsional modes of resonators designed to minimize extraneous loss effects. Thermal annealing at temperatures above the recrystallization temperatures yielded strongly enhanced Q. Maximum Q values of 4.24×107 in Nb and 3.91×107 in V were observed in annealed material at the lowest measurement temperatures. Internal friction peaks are interpreted in terms of mechanisms identified from previous studies of predeformed and hydrogen-charged materials. Intergrain thermal diffusion is shown to account for a significant portion of the background acoustic loss from 60 to 300 K. Relative shear moduli, obtained from the square of the resonator frequency, are given from 50 mK to 300 K. High-resolution frequency measurements at the lowest temperatures allow evaluation of the superconductivity component of the shear modulus.
Raman investigation of light-emitting porous silicon layers: Estimate of characteristic crystallite dimensions
Authors: Gregora I. , Champagnon B. , Halimaoui A.
Published: July 2002

Light-emitting porous silicon layers prepared on two types of boron-doped p and p+-Si substrates were studied by Raman spectroscopy in a broad frequency range (0–1100 cm−1). Standard phonon confinement model supported by new results from low-frequency scattering permit us to estimate consistently the characteristic dimensions of nanometric crystallites in porous silicon, without invoking complex size distribution. A sizable fraction of a highly disordered (a-Si) phase in the p-type samples is detected, and there is no clear spectral evidence of other Si-based compounds.
Influence of the cold deformation procedures on the critical current distribution in Ag-(Bi,Pb)2Sr2Ca2Cu3Ox tapes
Authors: Glowacki B. A. , Jackiewicz J.
Published: July 2002

Dramatic differences are reported between the transversal and longitudinal in-plane transport critical current of the Ag-(Bi,Pb)2Sr2Ca2Cu3Ox tape form conductor for longitudinally rolled, uniaxially pressed, and transversely rolled tapes. An explanation is given of the physical origin of such differences based on the observation of modulations of the superconducting core coinciding with microcracks in the Ag-(Bi,Pb)2Sr2Ca2Cu3Ox tape form conductor. The data presented provide an explanation of why, when considering the critical current of conductors, combination of (a) pressing with conventional longitudinal rolling and (b) transversal rolling with conventional longitudinal rolling give superior results compared to conventional rolling alone.
Magnetization relaxation in YBa2Cu3O7−δ
Authors: Ji H. L. , Shi Z. X. , Jin X. , Yao X. X. , Rong X. S. , Ni Y. M. , Zhao Z. X.
Published: July 2002

The time and field sweep rate dependencies of magnetization in high temperature superconductors are derived from a combined view based on the thermally activated flux creep. The τ0 in the magnetization relaxation expression M=M0+M1 ln(1+t/τ0) is a macroscopic quantity related primarily to the sample geometry and other experimental conditions via the effective activation energy U(J0). This activation energy provides an alternative interpretation to the nearly inverse proportionality of τ0 to the field sweep rate. The values of τ0 in the magnetization relaxation of single crystal YBa2Cu3O7−δ are consistent with the calculated ones. As the nonlinear U(J) is taken into account, the curvature in the ‖M‖-ln t plot may change from negative to positive over a sufficiently long time window, and the magnetization hysteresis M(H) depends on the field sweep rate Ḣ by const+A ln Ḣ+B ln2 Ḣ, where A is slightly larger than M1 and B≳0. A discussion of the temperature dependency of the normalized flux creep rate is at the conclusion of the article.
Free electron distribution in δ-doped InGaAs/AlGaAs pseudomorphic high electron mobility transistor structures
Authors: Jogai B. , Yu P. W. , Streit D. C.
Published: July 2002

The free electron distribution in δ-doped high electron mobility transistors with pseudomorphic InGaAs wells is calculated self-consistently. The electron distribution is calculated from the single-particle wave functions obtained from a four-band k⋅p theory. The Hartree part of the Coulomb interaction is obtained from the Poisson equation and the exchange-correlation part from density functional theory within the local-density approximation. The calculated energy separations between the electron and hole subbands agree well with observed peak positions in photoluminescence data. In addition, it is found that for spacer layers thicker than about 40 Å and a δ-doping density of about 5×1012 cm−2, the δ layer can form a channel as deep as the p well and can draw electrons from the latter.
I-V characteristics of electron-cyclotron-resonance plasma-enhanced chemical-vapor-deposition silicon nitride thin films
Authors: Jeon Yoo-Chan , Lee Ho-Young , Joo Seung-Ki
Published: July 2002

Silicon nitride thin films were fabricated by electron-cyclotron-resonance plasma-enhanced chemical-vapor deposition (ECR PECVD) at room temperature and current-voltage characteristics were analyzed. A ledge in the first I-V curve always appeared in ECR PECVD silicon nitride films and then disappeared in the subsequent I-V curves. It turned out that the trapped charges caused by injection of electrons were responsible for the ledge in the I-V curves of fresh samples. A new conduction mechanism for low electric field was proposed, namely trapping current by tunneling. This model turned out to be very successful to explain the low-field I-V characteristics in ECR-PECVD silicon nitride films, such as temperature dependence of I-V curves and the reverse current phenomenon. Computer simulation suggested the trapping cross section as 1×10−16 cm2 and the trap density as 7×1018 cm−3. The calculated trapping cross section corresponds to that of the neutral trap centers, which agrees well with the experimental results.
Measurement of the refractive index of AlxGa1−xAs and the mode indices of guided modes by a grating coupling technique
Authors: Kaufman R. G. , Hulse G. R. , Vezzetti D. J. , Moretti A. L. , Stair K. A. , Devane G. P. , Bird T. E.
Published: July 2002

We have determined the index of refraction of AlxGa−xAs over the wavelength range 0.76–1.15 μm, and the composition range 0≤x<0.33, using a grating to couple light into waveguides. We find the mode indices of multimode slab waveguides from the coupling angle and grating period, then calculate the bulk indices of the core and cladding materials by a root searching technique using the analytical formula for the effective index of a guided mode. The method gives the core index within ±0.001, and the cladding index within ±0.01. We are in agreement with high precision index values for GaAs in the literature, confirming the method. We are in substantial agreement with literature values for AlGaAs but find a significant systematic difference in the composition dependence. An analysis of measurement uncertainties shows that the determination of composition is the dominant variable.
An x-ray photoemission spectroscopy study of the role of sample preparation on band bending at the interface of Al with poly(p-phenylene vinylene)
Authors: Ettedgui E. , Razafitrimo H. , Park K. T. , Gao Y. , Hsieh B. R.
Published: July 2002

We report on our recent x-ray photoemission spectroscopy investigations of the interface formation of Al with poly(p-phenylene vinylene) (PPV) prepared under various conditions. We have found that during deposition Al reacts with residual hydroxyl groups in the polymer. In addition, we have found that Schottky barrier formation and the associated band bending depend strongly on surface preparation. Samples converted in situ, containing 5% surface oxygen, show band bending that depends on the thickness of the Al overlayer, with effects arising after as little as 1 Å Al. By contrast, a sample converted ex situ, with 10% surface oxygen, is insensitive to aluminum deposition. In view of the results obtained, we feel that surface impurities and adsorbed species may delay Schottky barrier formation by acting as a buffer layer which prevents the PPV substrate from interacting with the growing layer of Al. In the in situ samples where band bending occurs, we find that it takes place after the formation of metallic Al.
Scanning tunneling microscope stimulated oxidation of silicon (100) surfaces
Authors: Fay P. , Brockenbrough R. T. , Abeln G. , Scott P. , Agarwala S. , Adesida I. , Lyding J. W.
Published: July 2002

The chemical modification of n- and p-type hydrogen-passivated Si(100) surfaces by a scanning tunneling microscope (STM) is reported. The modified areas have been examined with STM, Auger electron spectroscopy, scanning electron microscopy, and atomic force microscopy. Comparison of these characterization techniques indicates the features are both chemical and topographic in nature and are the result of local oxidation of the substrate. In addition, pattern transfer for the defined regions has been demonstrated with both thermal oxidation and HBr reactive-ion etching.
Orientationally independent antiferromagnetic coupling in epitaxial Fe/Cr (211) and (100) superlattices
Authors: Fullerton Eric E. , Conover M. J. , Mattson J. E. , Sowers C. H. , Bader S. D.
Published: July 2002

We present structural and magnetic characterization of epitaxial Fe/Cr(211) and Fe/Cr(100) superlattices grown simultaneously on MgO(110) and MgO(100) substrates by magnetron sputtering. The epitaxial orientation of the Fe/Cr(211) superlattice with the MgO(110) substrate is Fe/Cr[01̄1]//MgO[001] and the orientation of Fe/Cr(100) is Fe/Cr[001]//MgO[011]. We find that for both orientations, that the interlayer coupling oscillates in sign with a period of 18 Å, and, furthermore, that the strength and phase of the magnetic coupling are also nearly identical for these two orientations.
Phase transitions in coupled double-layer systems
Authors: Hu Xiao , Kawazoe Yoshiyuki
Published: July 2002

Magnetic systems consisting of a capping layer with uniaxial anisotropy and a recording layer with vertical anisotropy are studied by means of a variational method for a continuum model covering the exchange, anisotropy, and Zeeman energies. The phase transitions between different alignments of magnetization are observed when the thickness of the capping layer and/or the temperature is varied. For the capping layer with in-plane anisotropy, the phase transitions are in second order and the critical behaviors are characterized by a critical exponent 1/2. For the capping layer with vertical anisotropy, the phase transitions are in first order. The analytic expressions of the critical points are presented, in terms of the relevant magnetic constants.
dc magnetic force microscopy imaging of thin-film recording head
Authors: Rice Paul , Moreland John , Wadas Andrzej
Published: July 2002

Using a new form of magnetic force microscope (dc MFM) magnetic force images of a thin-film recording head have been made. Using dc MFM, atomic force microscope images are presented of the surface topography and magnetic forces taken simultaneously, allowing direct correlation of magnetic fields to the pole pieces. Magnetic force images of the head at typical head-to-disk spacings are presented. The tips used for these images had two different magnetic coatings.
Infrared electroabsorption modulation at normal incidence in asymmetrically stepped AlSb/InAs/GaSb/AlSb quantum wells
Authors: Xie H. , Wang W. I. , Meyer J. R.
Published: July 2002

A normal-incidence modulation mechanism is proposed which uses the Stark effect to induce Γ-L transitions in asymmetrically stepped AlSb/InAs/GaSb/AlSb quantum wells (QWs). A significant feature of this structure is the unusual band alignments which localize two deep wells for the Γ and L bands in adjacent layers, i.e., the Γ-valley minimum is in the InAs while the L-valley minimum is in the GaSb. In contrast to a square QW, where the Stark shifts for both Γ and L subbands are in the same direction, the two step wells for Γ and L valleys in the proposed structure are oppositely biased in the presence of an electric field. Therefore, the first Γ and L subbands move toward each other, making the Γ-L crossover occur more efficiently. Near this point, most of the Γ electrons transfer to the L valleys, where they are allowed to make intersubband transitions under normally incident radiation. As a result, the device switches from being transparent to normal-incidence light to strongly absorbing it. The calculations indicate that excellent on/off ratios can be achieved in this structure operating at T≤150 K with electric fields on the order of ∼100 kV/cm for any infrared wavelength within the range of 3–20 μm.
Interface and electrical properties of plasma deposited tungsten and tungsten nitride Schottky contacts to GaAs
Authors: Kim Yong Tae , Lee Chang Woo
Published: July 2002

Comparison of plasma enhanced chemical vapor deposited tungsten and tungsten nitride (W0.67N0.33) Schottky contacts to GaAs are carried out at the rapid thermal annealing temperature of 500–1000 °C for 30 s without arsenic overpressure and capping layer. The Rutherford backscattering measurement, cross-sectional transmission electron microscopy, and secondary ion mass spectroscopy indicate that no metallurgical interactions take place between W0.67N0.33 and GaAs. Hall measurements and I-V characteristics of W0.67N0.33 Schottky contacts exhibit higher temperature stability than W contacts to GaAs, which is ascribed to the role of interstitial nitrogen atoms blocking the outdiffusion of Ga and As.
Ambient and vacuum scanning tunneling spectroscopy of sulfur- and oxygen-terminated gallium arsenide
Authors: Silver R. M. , Dagata J. A. , Tseng W.
Published: July 2002

Tunneling spectroscopy of sulfur- and oxygen-terminated n- and p-type GaAs (110) surfaces is reported for air and ultrahigh-vacuum conditions. Simulations of the complete I-V characteristics with explicit inclusion of surface states within the planar junction theory are described and compared to experiment. These results provide a comprehensive understanding of the interplay between tip-induced and surface-state-induced band-bending effects observed in the tunneling spectra of passivated semiconductor surfaces.
Study on the properties of Sn-added YBCO prepared by the powder melting process
Authors: Feng Yong , Zhou Lian , Shi Lei , Lu Yafeng , Jin Xin , Zhang Yitong , Jin Jirong , Yao Xixian , Zhang Yuheng
Published: July 2002

Magnetization and magnetic relaxations have been measured for the YBa2Cu2.985Sn0.015Oy and YBa2Cu3Oy superconductors, which were prepared by the powder melting process method. It is found that the Sn-added sample exhibits much higher Jc value and lower flux creep rate than pure YBCO. The results indicate that the melting temperature can be reduced and the pinning potential can be enhanced by substituting Sn for Cu. The Jc value estimated by Bean’s model drops with the magnetic field according to a power law. The dependence of Jc on the reduced temperature and the flux pinning force density behavior is discussed. The dependence on temperature of the maximum demagnetization field Hp and the remanent magnetization Mrem is studied. Finally, we also investigate the microstructure characteristic.
Spontaneous emission and oscillation in a planar microcavity dye laser
Authors: Osuge Michihiro , Ujihara Kikuo
Published: July 2002

Characteristics of a planar microcavity laser using rhodamine 6G with pulsed excitation is studied. Theoretical aspects of controlled spontaneous emission and oscillation in a planar microcavity laser are discussed. The measured spectrum and the angular divergence of spontaneous emission below threshold are in good agreement with theory. The angular divergence yields the radius of the cavity quasimode. The spontaneous emission coupling ratio obtained from the measured input-output characteristics is in reasonable agreement with the theoretical value. The expression for the coupling ratio derived for a Fabry–Pérot-type microcavity is shown to be essentially equal to that of a closed cavity or guided mode cavity derived by Yamamoto, Machida, and Björk [Phys. Rev. A 44, 657 (1991)]. The observed spectral behavior near the threshold approximately follows the Schawlow–Townes formula, but for a limited range of output power. The observed spectrum and divergence above threshold indicate incoherence much worse than the Schawlow–Townes limit.
Low-temperature epitaxial growth of BaTiO3 films by radio-frequency-mode electron cyclotron resonance sputtering
Authors: Matsuoka Morito , Hoshino Koichi , Ono Ken’ichi
Published: July 2002

BaTiO3 (BTO) films have been deposited on SrTiO3 (STO) wafers by a rf-mode electron cyclotron resonance sputtering system. The substrate surface is pretreated in several ways before film deposition. The oxygen plasma irradiation is the best way of pretreating the substrate surface. Single-crystal BTO films with smooth surfaces are deposited on STO at temperatures ranging from 350 to 650 °C at total gas pressures ranging from 0.02 to 0.1 Pa. BTO films with (100) orientation are obtained at temperatures as low as 200 °C. The dielectric constant of films grown at 350 °C reaches 470 and that of films grown at 550 °C reaches 880.
Low frequency (1/f) noise in YBa2Cu3O7−δ grain boundary junction dc superconducting quantum interference devices
Authors: Chen J. , Ogawa T. , Nakamura H. , Myoren H. , Nakajima K. , Yamashita T.
Published: July 2002

We have studied low frequency (1/f) noise of YBa2Cu3O7−δ dc superconducting quantum interference devices (SQUIDs) on SrTiO3 bicrystal substrates. 1/f flux noise, either measured at different temperatures for optimized bias current or measured at 77 K for different bias currents, is almost constant. These facts imply that 1/f noise mainly comes from fluctuations of the critical current of the Josephson junction that form the SQUID. Also, we explain the critical current fluctuations in the junction by an equilibrium temperature fluctuation model.
Epitaxial (GaAs)1−x(Si2)x metastable alloys on GaAs(001) and (GaAs)1−x(Si2)x /GaAs strained-layer superlattices: Crystal growth, spinodal decomposition, and antiphase domains
Authors: Kim Y.-W. , Mei D. H. , Lubben D. , Robertson I. , Greene J. E.
Published: July 2002

The microstructure of single-crystal zincblende-structure (GaAs)1−x(Si2)x metastable semiconducting alloys with 0≤x≤0.40 has been investigated using triple-crystal x-ray diffraction (XRD), plan-view and cross-sectional transmission electron microscopy (TEM and XTEM), scanning transmission electron microscopy, and convergent-beam electron diffraction. The alloys, typically 1–3 μm thick, were grown using a hybrid sputter-deposition/evaporation technique on As-stabilized GaAs(001) and (GaAs)1−x(Si2)x/GaAs(001) strained-layer superlattices, (SLS). Alloy XRD peak widths were approximately equal to those of the GaAs substrates, 30 arcsec, and lattice constants, uncorrected for strain, obeyed Vegard’s ‘‘law’’ and decreased linearly with increasing x. TEM and XTEM examinations of (GaAs)1−x(Si2)x alloys with 0≤x≤0.20 grown on GaAs revealed no evidence of dislocations or other extended defects. Film/substrate lattice misfit strain in alloys with 0.11<x<0.20 was partially accommodated by the formation of a thin interfacial spinodal layer whose average thickness increased with x to ≂70 nm. The spinodal region, which remained epitaxial, consisted of lenticular platelets extending along the [001] direction with a compositional modulation in orthogonal directions. Films with x≥0.20 exhibited, together with the interfacial zones, inhomogeneously distributed a0/2〈110〉-type threading dislocations. Antiphase domains were observed in alloys with x≥0.23. The use of (GaAs)1−x(Si2)x/GaAs SLS buffer layers extended the composition range to x=0.3 over which dislocation-free alloys, with no evidence of interfacial spinodal decomposition, could be obtained.
Photoluminescence and electrical properties of erbium-doped indium oxide films prepared by rf sputtering
Authors: Kim Hong Koo , Li Cheng Chung , Nykolak Gerald , Becker Philippe C.
Published: July 2002

Erbium-doped indium oxide films were prepared by rf magnetron sputtering. The Er-doped oxide films are conducting (or semiconducting) with a resistivity in the range of 10−3–103 Ω cm, and are optically active, i.e., show a clear room-temperature photoluminescence at 1.54 μm, corresponding to intratransitions in Er3+ ions. Compared with the undoped indium oxide films, the erbium doping was found to have the effect of increasing the resistivity (up to two orders of magnitude) of the films, mainly via a reduction of carrier concentration. Postdeposition annealing in air ambient significantly enhances both the Er3+ luminescence and Hall mobility (up to 60 cm2/V s), and reduces the carrier concentration. Postdeposition annealing in reducing ambient (N2/H2), however, decreases the resistivity dramatically, mainly via an increase of carrier concentration (up to 1020 cm−3), and also enhances the Er3+ luminescence.
Command surface controlled liquid crystalline waveguide structures as optical information storage
Authors: Knobloch Harald , Orendi Horst , Büchel Michael , Seki Takahiro , Ito Shinzaburo , Knoll Wolfgang
Published: July 2002

We report on optical waveguide structures in which light propagates within a liquid crystal (LC) thin film. In this configuration, the orientation, and therefore the optical properties of the LC waveguide structure are controlled by two photochromic command surfaces, consisting of an ultra thin film of a polymer with azobenzene side chains [K. Ichimura, Y. Suzuki, T. Seki, A. Hosoki, and K. Aoki, Langmuir 4, 1214 (1988)]. When exposed to light of appropriate wavelength, the azobenzene side chains undergo a trans↔cis photoisomerization process inducing a commensurate change in the LC alignment and therefore, in the set of refractive indices of the LC film. Using this effect we could reversibly write information into the LC cell. The size of our test structure was in the range of 50 μm. The stored information was read out by optical waveguide microscopy [W. Hickel and W. Knoll, Appl. Phys. Lett. 57, 1286 (1990)]; the storing times were in the range of several hours.
Triple axis x-ray investigations of semiconductor surface corrugations
Authors: Darhuber A. A. , Koppensteiner E. , Straub H. , Brunthaler G. , Faschinger W. , Bauer G.
Published: July 2002

X-ray reciprocal space mapping around the symmetrical (004) Bragg reflection and a kinematical x-ray diffraction model were employed in order to determine the geometry and the structural perfection of surface corrugations or quantum wires. This method was used for the analysis of (001) Cd1−xZnxTe surface corrugations fabricated by holographic lithography and subsequently reactive ion etched with typical periods of 500 nm. Comparison of the measurement and simulation provides conclusive information on etching depth, wire period, wire width, and the inclination of the side walls. Furthermore, the analysis yields a parameter that contains information on side wall roughness, shape fluctuations and, in principle, the crystallographic damage caused by the reactive ion etching process. Due to the high resolution of triple axis diffractometry small strain gradients are observable in the damaged region.
Theory of Brillouin light scattering from spin waves in multilayers with interlayer exchange and dipole coupling
Authors: Slavin A. N. , Rojdestvenski I. V. , Cottam M. G.
Published: July 2002

An analytic Green-function theory is developed for the dipole-exchange spin waves and their Brillouin light-scattering spectra in perpendicularly magnetized magnetic multilayers and superlattices. We consider periodic structures consisting of a large number of similar ferromagnetic layers separated by nonmagnetic spacers. The dipole-dipole and exchange coupling are included between magnetic layers, as well as within the layers.
Magnetic anomaly in insulator-conductor composite materials near the percolation threshold
Authors: Panina L. V. , Antonov A. S. , Sarychev A. K. , Paramonov V. P. , Timasheva E. V. , Lagarikov A. N.
Published: July 2002

The effective magnetic permeability of composite materials containing fine iron particles of 1–2 μm size is investigated theoretically and experimentally. This permeability is considered due to both the ferromagnetic properties of iron and the generation of eddy currents by an alternating magnetic field. An analytical result shows that as the percolation threshold is approached, the skin effect in large conducting clusters dominates, suppressing the ferromagnetic behavior for any value of frequency. As a result of this, the effective permeability tends to become zero near the percolation threshold, having a ‘‘ν’’ form anomaly. The experimental data for frequencies of 6–10 GHz where the skin depth is of the order of a particle size, clearly exhibit a sharp decrease near the percolation threshold in the real part of the effective magnetic permeability. We believe this is the first observation of a possible magnetic anomaly in a percolating system.
Die-upset Nd-Fe-Co-B magnets from blends of dissimilar ribbons
Authors: Fuerst C. D. , Brewer E. G.
Published: July 2002

We prepared die-upset Nd-Fe-Co-B magnets from melt-spun ribbon powders which were a series of blends of two ribbon alloys. One alloy was always ternary Nd-Fe-B (no cobalt), and in the other cobalt replaced up to half of the iron. Differential scanning calorimeter measurements revealed that during hot working, the cobalt diffused across ribbon boundaries, effectively redistributing the transition metal concentration throughout the magnet. Instead of anomalies indicating the Curie temperatures of the two original ribbon compositions, we found a single transition consistent with the average cobalt concentration in the magnet. However, the transition was broader than expected, suggesting that the homogenization was incomplete. These results are new evidence of massive diffusion occurring between ribbons, changing the microstructure and facilitating the deformation of the otherwise rigid 2-14-1 magnet.
Comparison of magnetic methods for the determination of texture of permanent magnets
Authors: Asti G. , Cabassi R. , Bolzoni F. , Wirth S. , Eckert D. , Wendhausen P. A. P. , Müller K.-H.
Published: July 2002

The singular point detection (SPD) technique as well as a least-mean-square procedure of fitting trial texture functions to demagnetization curves (DMC) were applied in order to determine the orientation density of the tetragonal axes of the Nd2Fe14B crystallites in sintered Nd-Fe-B magnets with different degrees of grain alignment. Both methods are less complicated than calculating the texture function from x-ray-diffraction data. The orientation densities obtained by both methods agree fairly. Analytically, they can be described as a Gaussian or a sum of cosine-power functions of the angle between the local tetragonal axis and the texture axis. This agreement is a test for the DMC method which can also be used for materials showing much larger anisotropy fields than the maximum available measuring field. The SPD and DMC methods can also be applied to measure the temperature dependence of the anisotropy field HA of the magnetic main phase in noncompletely textured polycrystalline materials. By means of the DMC method additionally the spontaneous polarization Js as well as the anisotropy constants K1 and K2 can be determined. As expected, the values of HA of our sintered Nd-Fe-B samples, measured up to 140 °C, are nearly independent of the degree of texture and are in agreement with those reported in literature. This confirms that SPD as well as DMC can be used to measure intrinsic magnetic properties for materials which can hardly be prepared as single crystals, such as interstitial compounds made by gas–solid reactions.
Metal/intrinsic semiconductor/semiconductor field effect transistor fabricated from polycrystalline diamond films
Authors: Nishimura Kozo , Kumagai Kazuo , Nakamura Rie , Kobashi Koji
Published: July 2002

A metal/intrinsic semiconductor/semiconductor field effect transistor (MiSFET) was successfully fabricated for the first time using polycrystalline diamond films grown by microwave plasma chemical vapor deposition. The gate length, the gate width, and the source-to-drain spacing were 10 μm, 1 mm, and 30 μm, respectively. The FET characteristics were measured with the source electrode grounded. For the range of drain voltage VD, −10 V ≤ VD≤0, the fabricated MiSFET exhibited a reduction of the drain current as the gate bias VG was increased from 0 to 6 V at room temperature. The drain current modulation was considerably enhanced at 77 K. The transconductance at VD=−10 V and VG=2 V was 5 μS/mm. For VD<−10 V and VG≳6 V, or at temperatures above room temperature to 398 K, the drain current modulation decreased significantly due to increased gate leakage current.
Temperature dependence of ion-beam mixing in III–V semiconductors
Authors: Forbes D. V. , Coleman J. J. , Klatt J. L. , Averback R. S.
Published: July 2002

Ion-beam mixing in AlxGa1−xAs and InP matrices was measured as a function of irradiation temperature using 1 MeV Kr ion irradiation. For these III–V compound semiconductors, the mixing increased with temperature up to a critical temperature Tc at which point it precipitously dropped. Tc was identified as the amorphous-to-crystalline transition temperature in these materials under 1 MeV Kr irradiation.
Growth of an oriented graphitic layer on a TiC nanocube
Authors: Suzuki Tetsuya , Utsumi Yoshiyuki , Sasaki Kou , Shibuki Kunio
Published: July 2002

Cubic-shaped titanium carbide (TiC), 30–60 nm in size, was sprayed onto aluminum oxide powder and hot pressed at 1800 °C. The preferential evaporation of Ti atoms out of the surface resulted in the formation of graphitic layers on the {100} planes of TiC with the following crystallographic relationships: (010)TiC∥(112̄0) graphite; 〈001〉TiC∥〈0001〉graphite. The oriented graphite layers form a continuous nanocube of carbon, closely analogous to carbon nanotubes.
Electronic structure of nanometer-thickness Si(001) film
Authors: Gavrilenko V. I. , Koch F.
Published: July 2002

A tight-binding calculation of a Si film with nanometer dimensions is presented. We study the electron energy structure and the wave functions of a pristine Si-quantum film and one covered with hydrogen. A total energy minimization method, in the framework of self-consistent tight-binding theory, is used to investigate the reconstruction of the Si-surface after the adsorption of hydrogen. The dependence of the electron energy spectrum on the film thickness and the atomic structure of the surface is studied.
Generation of nanometer holes at Pt surfaces in air and a nonconducting liquid with the scanning tunneling microscope
Authors: Lin H.-N. , Chang C. S. , Tsong Tien T.
Published: July 2002

Two methods for the controllable generation of nanometer scale holes at the Pt surface in air and silicone oil with the scanning tunneling microscope are presented. Positive voltage pulses are applied to the Pt substrate with either the feedback circuit on or off. Holes with sizes down to 2 nm can be created with a proper choice of the pulse voltage and duration. By adjusting the tip–sample distance, reproducible holes can be generated only when the gap is within 10–40 Å in both media. When the gap is less than 10 Å, a different process dominates which results in different generated features.
Characteristics of the electromechanical response and polarization of electric field biased ferroelectrics
Authors: Zhang Q. M. , Zhao J. , Shrout T. , Kim N. , Cross L. E. , Amin A. , Kulwicki B. M.
Published: July 2002

The electromechanical and dielectric responses of ferroelectric materials near paraelectric–ferroelectric (PF) phase transitions are examined both experimentally and theoretically. It is shown that the type of PF transition, i.e., first order and continuous transitions, and diffused phase transition found in relaxor ferroelectrics, has marked effect on the electromechanical and dielectric response behavior for materials under electric field bias. It is demonstrated that an exceptionally large piezoelectric and electromechanical response can be achieved in materials with a first order PF transition. For example, in Ba1−xSrxTiO3 (x∼0.35) ceramics at room temperature, a piezoelectric d33 larger than 1500 pm/V with very little frequency dispersion can be obtained.
X-ray focusing optics. I. Applications of wave optics to doubly curved crystals with a point x-ray source
Authors: Chukhovskii F. N. , Chang W. Z. , Förster E.
Published: July 2002

A wave optics approach to the calculations of the intensity distribution of an x-ray point source image with a two-dimensional (2D) focusing geometry is presented. Analytical formulas are derived to calculate the intensity distribution at the focal plane. This approach has taken into account the effects of x-ray scattering within a 2D bent crystal, which includes the x-ray refraction and absorption inside the crystal, and the effects of elastically deformed crystals described by the anisotropic elasticity theory. Based upon the elastic bending model, the modified Bragg law and 2D lens equations are discussed. In addition, the x-ray extinction distance for curved crystals is found to be dependent on the size and the bending radius of bent crystals. For a monochromatic x-ray point source, calculation of the intensity distribution with a 2D bent silicon crystal is given for both perfect and misaligned sources. The spatial resolution and the size of the image are determined.
Substrate effect on the transport properties of semiconducting films
Authors: Bartels A. , Peiner E. , Schlachetzki A.
Published: July 2002

In this study, transport properties of InP/Si heteroepitaxial layers were investigated. Current-voltage characteristics, measured across the heterointerface, revealed diode behavior at 300 K only with Si of p-type conductivity. With n-type Si, a transition from Ohmic behavior at 300 K to a diode characteristic at low temperatures was observed at 250 K. Due to the efficient electrical isolation of layer and p-type substrate van der Pauw measurements with InP/p-Si could be analyzed in the conventional manner. For InP/n-type Si the applicability of the two-layer conduction model suggested by Petritz could be demonstrated for 300 K measurements. With decreasing temperature a pronounced deviation from the model occurred due to the increased effect of the heterointerface.
Evolution of morphology, crystallinity, and growth modes of thin superconducting YBa2Cu3O7−x films on SrTiO3 and NdGaO3 substrates
Authors: Ece Müzeyyen , Gonzalez Ester Garcia , Habermeier Hanns-Ulrich , Oral Baybars
Published: July 2002

The evolution of surface morphology, crystallinity, and growth modes of c-axis-oriented superconducting YBa2Cu3O7−x (YBCO) thin films deposited by pulsed laser deposition on SrTiO3(100) and NdGaO3(110) substrates have been systematically investigated with scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), x-ray energy-dispersive spectroscopy, and x-ray diffraction. SEM and AFM images of the YBCO films grown on NdGaO3 with a sequential thickness revealed that the films were rather smooth and grew in a two-dimensional manner. In contrast, the YBCO films of the same thickness on SrTiO3 were found to have spiral growth features, even in the thinnest film (10 nm or 8 unit cells thick) indicating that an island growth mechanism took place. A change of the full width at half-maximum of the rocking curves of the (005) reflection with the film thickness showed that the crystallinity of the films on NdGaO3 was much better than those grown on SrTiO3, suggesting that a lattice match is a crucial factor to obtain good crystallinity. In the early stages of growth the preferential a-axis orientation of the films on NdGaO3, as revealed by TEM, was consistent with the smooth and featureless film surfaces found, in contrast to the c-axis growth mode on SrTiO3.
Effect of oxygen on the electrical and optical properties of In0.5Ga0.5P grown by liquid-phase epitaxy
Authors: Kwon Ho Ki , Kwon S. D. , Kim In , Lee Jong Boong , Choe Byung-Doo , Lim H.
Published: July 2002

The effect of oxygen on the electrical and optical properties of In0.5Ga0.5P epitaxial layers grown on (100) GaAs by liquid-phase epitaxy has been investigated by adding Ga2O3 to the growth melt. As the amount of Ga2O3 increases, the carrier concentration at 300 K decreases from 4×1016 to 4×1015 cm−3 and the Hall mobility at 77 K increases from 2400 to 4000 cm2/V s. The photoluminescence at 17 K shows that the peak intensity of an extrinsic transition in the In0.5Ga0.5P layer is reduced when Ga2O3 is added to the growth melt. These facts indicate that the main effect of Ga2O3 is the reduction of impurity concentration in the growth melt. In the In0.5Ga0.5P layer grown from the Ga2O3-added growth melt, the same deep trap, with an activation energy of 0.29 eV, as in an undoped layer is observed but the trap density is decreased. This implies that the deep trap is not due to a simple intrinsic defect, but related to an impurity.
Deposition of epitaxially oriented films of cubic silicon carbide on silicon by laser ablation: Microstructure of the silicon–silicon- carbide interface
Authors: Rimai L. , Ager R. , Weber W. H. , Hangas J. , Samman A. , Zhu W.
Published: July 2002

Excimer laser ablation has been used to deposit epitaxial films of β-SiC on single-crystal Si wafers, in a vacuum, at substrate temperatures between 1050 and 1250 °C. Such films can be grown by ablating ceramic SiC, carbon, or alternating silicon and carbon targets at a range of growth rates. X-ray θ-2θ diffraction shows the presence of strong, sharp reflections from crystal planes parallel to the substrate, 200 and 400 for [100] substrates and 111 and 222 for [111] oriented substrates. Wrong reflections, such as 111 for [100] substrates, are extremely weak or absent, indicating alignment with the substrates. The characterization of these films by a number of techniques is discussed. In all cases the film-substrate interface shows a characteristic microstructure of cavities in the Si substrate, similar to that observed for the carbonization layer initially formed as a precursor for chemical-vapor deposition of SiC films on Si. This implies that the initial film growth, for all cases, involves chemical reaction of the Si substrate with the carbon in the plume as well as transport through the growing film.
A materials study of PtTiGePd ohmic contacts to p+-AlGaAs as a function of annealing temperature
Authors: Cole M. W. , Han W. Y. , Casas L. M. , Jones K. A.
Published: July 2002

Elemental diffusion, interfacial microstructure, and phase composition of Pt/Ti/Ge/Pd ohmic contacts to heavily C-doped Al0.26Ga0.74As were investigated at several annealing temperatures. Results of the material analyses were used to explain the previously determined specific contact resistances measured for each thermal treatment. Evidence of interdiffusion and compound formation between AlGaAs and Pd were visible in a Ga rich Pd-Ga-As reaction zone prior to heat treatment. This phase is critical for the formation of Ga vacancies, which upon heating are occupied by in-diffusing Ge. However, as the annealing temperatures are raised to 530 °C and above, As preferentially out diffuses. The As out diffusion, which is critical to the formation of good p-type ohmic contacts by creating vacancies that the amphoteric Ge can occupy, contributed to the creation and development of the two phase TiAs/Pd12Ga2Ge5 interfacial region overlying the AlGaAs substrate. In response to the enhanced As out diffusion at 600 °C, the interfacial region reached completion, that is, it became laterally continuous and compositionally uniform, and the specific contact resistance achieved its minimum value. At higher annealing temperatures ∼650 °C, the electrical measurements degraded in response to intensive chemical diffusion which resulted in the development of a broad, nonuniform multiphased interfacial region, and the Pt contacting layer ceased to be a homogeneous layer with a smooth surface. The As interfacial compounds form at higher temperatures in AlGaAs than in GaAs suggesting that As is more strongly bonded in the AlGaAs. This contributes to the greater temperature stability of the contacts to AlGaAs.
Theory of hopping transport of holes in amorphous SiO2
Authors: Deb B. M. , Chandorkar A. N.
Published: July 2002

A quantum-mechanical theory of hole transport in SiO2 is developed based on modeling of the oxide as an array of time-dependent δ-function potential well distribution. Holes undergo a variable range hopping transport which is dispersive in nature. The oxide, being an insulator, involves a maximum cutoff phonon frequency of 1014–1015 s−1. The Schrödinger equation is solved along with the Poisson equation using a piecewise linear internal field. The theory is applied to metal-oxide-semiconductor structures subjected to a short radiation pulse.