bluuebunny/crossref_metadata_2025 huggingface dataset:
The dependence of low-temperature ion mixing of Y/Si bilayers on nuclear energy deposition
Authors: Alford T. L. , Bo/rgesen P. , Lilienfeld D. A.
Published: July 2002

Bilayers of yttrium and amorphous silicon were irradiated with 600-keV Ar++, Kr++, and Xe++ ions at temperatures between 80 and 372 K. The mixing rates were considerably larger than rates predicted by a model based on overlapping thermal spikes, as well as predictions generated by an extension of the model to the case of nonoverlapping thermal spikes. However, the experimental mixing rates did exhibit a linear dependence on the nuclear energy deposited at the interface. This agrees qualitatively with the premise of nonoverlapping thermal spikes. We compare our results with other ion-mixed medium-Z, metal/metal systems which also show this tendency.
The effect of volume on magnetization in RxY1−xCo2 pseudobinaries (abstract)
Authors: Hathaway K. B. , Cullen J. R.
Published: July 2002

There is experimental1 and theoretical2 evidence that the cubic Laves phase compound YCo2, paramagnetic at its equilibrium volume, will become ferromagnetic with only a slight increase in volume. This volume sensitivity of Co-containing Laves phase compounds must therefore be considered when addressing the problem of the origin of the Co magnetic moment in pseudobinaries RxY1−xCo2, where R is any rare earth element with a net spin moment. In this paper we augment the itinerant picture describing the effect of the rare earth spin on the Co magnetization3 by taking into account the changes in volume brought about by the substitution of the rare earth for yttrium. We obtained the coefficients of the Landau expansion of the free energy by fitting the expansion to the calculated energy versus moment curves of YCo2 for various volumes.2 (These energies were calculated in Ref. 2 using the fixed-spin-moment method, within the local-spin-density approximation). Thus, for example, the substitution of Gd forces the formation of Co moments not only because of the Gd spin, but because of the increase in latttice constant, whereas the rare earths heavier than Tb have smaller lattice constants and are less likely to induce a moment on the Co sublattice. Results are presented for the critical fields necessary for magnetic ordering and for the Co magnetic moment as a function of the rare earth substitution.
Two-dimensional modulated ferrimagnetic systems (abstract)
Authors: Miceli P. F. , Tarascon J. M. , Hull G. W. , Remschnig K. , McKinnon W. R. , Telpis E. , LePage Y. , McAlister S. P. , Pleizier G. , Rhyne J. J. , Neumann D. A.
Published: July 2002

Bi2Sr2MnOy, isomorphic to the high-Tc compound containing Cu, exhibits a displacement modulation driven by the bending of Bi double layers, thus creating a modulated environment for the magnetic MnO2 layers which are 12 Å apart. Neutron scattering results will be presented, showing antiferromagnetic order in zero field and a field-induced transition to weak ferromagnetism. This behavior is a direct consequence of the structural modulation which breaks the symmetry of two otherwise equivalent ordering directions of (010) and (100)−the latter being associated with weak ferromagnetism. Magnetic scattering has been observed at superlattice positions, suggesting that the displacement modulation affects the valence of the magnetic ions. Subtle differences in magnetic behavior are observed in similar compounds where Pb is substituted for Bi and Ca for Sr. These results will be discussed in terms of a modulated magnetic model.
Preparation and magnetic studies of acicular Fe-Co alloy particles
Authors: Hsu Jen-Hwa , Kuo P. C. , Lin M. Y.
Published: July 2002

Acicular α-FeOOH particles with particle length of about 0.2 μm and axial ratio of about 8 were used as the starting material to prepare acicular Fe-Co alloy particles. They were made by adsorbing Co(OH)2 on the surface of α-FeOOH particles, followed by dehydration, annealing, and reduction. In order to prevent sintering at high temperatures, the Co-α-FeOOH particles were first dehydrated at 300 °C and then coated with a thin layer of silica before high-temperature treatments. Furthermore, after reduction, the metallic powder was immersed in toluene to avoid oxidation. The particle structure of Fe-Co was determined from x-ray diffraction. It has been found that, up to 50 at. % Co, all the Fe-Co particles have the same bcc structure and similar lattice constant. The magnetic properties of Fe-Co alloy particles with different Co concentrations were investigated using a vibrating sample magnetometer. The specific saturation magnetizations σs are about 30% below those of bulk materials. The maximum value of σs at room temperature is obtained with 42 at. % Co. The coercivity that mainly originates from shape anisotropy is as high as 1250 Oe for the particles with 33 at. % Co. Furthermore, σs drops sharply with Si content while Hc peaks at 0.6% Si. The coercivities of the particles also depend on the crystallite size. Hc shows a maximum when the crystallite size is about 300 Å.
In situ YBa2Cu3O7−x superconductor films on GaAs/AlAs superlattices
Authors: Chang L. D. , Tseng M. Z. , Samoska L. A. , O’Shea J. J. , Li Y. J. , Caine E. J. , Hu E. L. , Petroff P. M. , Kroemer H.
Published: July 2002

Superconducting YBa2Cu3O7−x films have been deposited in situ onto III-V superlattice substrates. The substrates were GaAs/AlAs superlattices grown by molecular beam epitaxy onto GaAs substrates. For 5000-Å -thick YBCO films grown at 615 °C substrate temperature, we have obtained Tc of 73 K. For thinner films the Tc’s are lower, indicating poor interfaces. However, the onset of the superconducting transition is 90 K in all cases. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy (TEM) show these films to be polycrystalline. TEM shows an interaction layer of about 1000 Å at the interface. Low-temperature cathodoluminescence measurements of the substrate show that atomic interdiffusion has intruded at least 5000 Å below the interface.
Evaluation of pulsed radiation effects in buried oxides by fast C-V measurements
Authors: Simons M. , Campisi G. , Hughes H. L.
Published: July 2002

Fast C-V measurement techniques are utilized to simultaneously probe both upper and lower interfaces of silicon-on-insulator/separation by implantation of oxygen buried oxide capacitors following exposure to pulsed ionizing irradiation. In addition to the relatively stable radiation-induced positive charge, reverse annealing is observed from both Si-SiO2 interfaces over the 200 μs to 300 s post exposure interval; this behavior is consistent with electron detrapping within the oxide. The dependence of electron detrapping on dc annealing bias is attributed to field-enhanced emission effects.
Electrical properties of boron-doped hydrogenated amorphous silicon and n-type GaAs heterojunction
Authors: Nagpal Alka , Gupta R. S. , Srivastava G. P.
Published: July 2002

Electrical properties of (p)aSi:H-(n)GaAs heterojunction were investigated by measuring current-voltage and capacitance-voltage characteristics. The experimental results are interpreted in accordance with a generalized a-c junction model. The built-in potential of the heterojunction and the gap-state density in the chemical vapor deposition (p)aSi:H were obtained from the capacitance-voltage characteristics.
Laser action of Nd:YAlO3 at 1300 nm
Authors: Shen H. Y. , Zhou Y. P. , Zeng R. R. , Yu G. F. , Huang C. H. , Zeng Z. D. , Zhang W. J. , Lin W. X. , Ye Q. J.
Published: July 2002

From our measured value of the stimulated emission cross section for 4F3/2–4I13/2 transition of Nd3+ ion in YAlO3 crystal as well as the results of continuous wave (cw) and pulsed 1341.4 nm Nd:YAP lasers, it can be seen that Nd:YAP is an excellent 1300 nm laser crystal. In our laboratory cw output power of 195.8 W at 1341.4 nm with an overall efficiency of 1.43% and pulsed output energy of 5.1 J at 1341.4 nm with an overall efficiency of 2.02% have been achieved by using this kind of crystal.
Raman spectroscopy studies of progressively annealed amorphous Si/Ge superlattices
Authors: Kumar Sunil , Trodahl H. J.
Published: July 2002

The thermal stability of a-Si/a-Ge superlattices with periods ranging from 2.5 to 41.2 nm has been studied using Raman spectroscopy. Atomic relaxation and rearrangement at the Si/Ge interfaces occur up to an anneal temperature of 300 °C. Further annealing of the superlattices reveals enhanced diffusion and a retarded crystallization, and these effects are more pronounced in short period superlattices.
Characterization of epitaxially grown films of vanadium oxides
Authors: Rogers K. D. , Coath J. A. , Lovell M. C.
Published: July 2002

The growth of VO2 and V2O3 thin films by reactive sputtering has been investigated. Previously reported studies of such thin films have often presented ambiguous results concerning the precise nature of the layers produced. A thorough and comprehensive characterization program including x-ray diffraction, scanning electron microscopy, Rutherford-backscattering spectroscopy, and electrical conductivity measurements has been undertaken to ensure that the films produced were of a true epitaxial nature.
Comments on electron relaxation time measurements from transient reflectances of metals
Authors: Lörincz A. , Bozóki Z. , Miklós A.
Published: July 2002

The measurement of nonequilibrium electron and lattice temperatures via transient reflectance (TR) technique is considered for metals. It is shown that TR is a function of the electronic relaxation time and the optical properties of the material. It is argued, that the photon energy of the transient reflectivity measurement should be chosen in the free electron domain and then the electron relaxation time may be determined quantitatively with the help of the temperature coefficients of the real and imaginary parts of the dielectric constant.
Photoluminescence of indium-alloyed semi-insulating GaAs subjected to bulk heat treatments
Authors: Yu P. W. , Kuwamoto H.
Published: July 2002

A photoluminescence study has been made, at temperatures 2–300 K, on indium-alloyed semi-insulating GaAs subjected to bulk heat treatments. Heat treatments were made by slow and fast cooling following a 15 h, 950 °C soak. Two intrinsic acceptors at Ev + 0.069 eV and Ev + 0.17 eV are produced after heat treatments. The acceptor at Ev + 0.069 eV is attributed to the intrinsic pair defect GaAs−VGa and is produced more in the fast-cooled materials than in the slow-cooled materials. The deficiency of intermediate deep donors causes no drastic variation of the electron concentration after the heat treatments.
The expansion of the tritide TaT0.37 at 10 K due to single interstitial 3He atoms
Authors: Schober T. , Trinkaus H.
Published: July 2002

The expansion of the tritide TaT0.37 at 10 K was measured using strain gauge dilatometry. The volume increase as expressed by ΔvHe/Ω = 0.26 may be attributed to a random distribution of single interstitial 3He atoms. To the knowledge of the authors this is the first direct measurement of the volume requirement of interstitial 3He in a metal. For purposes of comparison swelling of the same sample was also measured at room temperature. Here, however, 3He atoms cluster and form bubbles leading to ΔvHe/Ω ≊ 0.38, which represents the volume requirement of 3He in a bubble. Conversion of this value to a pressure normalized to the shear modulus μ yields in p/μ=0.22 which is in excellent agreement with other recent experimental and theoretical work on the pressure required for dislocation loop punching.
Deep levels in Te-doped GaAs prepared by annealing under controlled arsenic vapor pressure
Authors: Oyama Yutaka , Nishizawa Jun-ichi , Dezaki Kazushi
Published: July 2002

This paper reports the photocapacitance (PHCAP) results of the deep levels associated closely with donor impurity atoms and arsenic vacancies in heavily Te-doped GaAs. The present PHCAP measurements detect two kinds of deep levels at 0.62–0.64 eV and 0.83 eV above the valence band preferentially formed under low arsenic vapor pressure, and show that the level density decreases monotonically with increasing arsenic vapor pressure. By varying the wavelength of irradiation light, the neutral level position of 0.62–0.64 eV ionized level is determined to be 1.38 eV below the conduction band. From the spectroscopic observation, it is concluded that the 0.62–0.64 eV+Ev level disappears in the absence of As vacancies and appears in the presence of the donor dopant Te. This level shows such a large value of Frank–Condon shift (dFC) as 0.25 eV.
Reply to ‘‘Comment on ‘Negative capacitance at metal-semiconductor interfaces’ ’’ [J. Appl. Phys. 70, 1090 (1991)]
Authors: Wu X. , Evans H. L. , Yang E. S.
Published: July 2002

The origin of the excess admittance at a forward-biased Schottky diode invokes a controversy among research workers. Werner commented on our papers [J. Appl. Phys. 70, 1090 (1991)], in which he believes that the excess admittance is caused by minority-carrier extraction at defective back contacts rather than charge capture and emission at interface states. This reply answers the questions raised by Werner et al. [Phys. Rev. Lett. 60, 53 (1988)] and points out that the minority-carrier effect cannot account for the experimental observations.
Electrical microcharacterization of Si-TaSi2 eutectic composite transistors by three-terminal electron-beam-induced conductivity
Authors: Levinson M. , Rossoni P. G. , Ditchek B. M.
Published: July 2002

A new three-terminal electron-beam-induced conductivity technique, which images microscopic current characteristics, has been used to examine the effects of device and material microstructure on macroscopic performance of Si-TaSi2 eutectic composite transistors. Image contrast arising from current-limiting electron velocity saturation regions shows that current control is not microscopically uniform due to the somewhat irregular distribution of active gate elements. Macroscopically, these nonuniformities result in delayed saturation in the I-V characteristics and excess leakage current when the device is pinched off. Furthermore, the effects of the uncontacted floating gate elements, which are responsible for the unique high-voltage capability of these transistors, are graphically observed.
Thermal analysis for improved performance of transferred-electron oscillators
Authors: Batchelor A. R. , Postoyalko V.
Published: July 2002

Novel analytical solutions to the heat-flow equation are presented for GaAs transferred-electron devices to be operated in microwave oscillators. Uniform and linearly varying power dissipation density in the active region is considered. The results show that, to reduce operating temperatures, devices should be operated in the heat-sink anode configuration rather than the conventional top-cap anode configuration. This will ensure improved microwave power output from transferred-electron oscillators.
Photoelectronic process of photodoping in the Ag/As-S system
Authors: Tanaka Keiji
Published: July 2002

The photodoping process in Ag/As30S70 films has been studied as a function of the chalcogenide layer thickness varying from 250 Å to 32 μm. When the sample is illuminated from the Ag side, the photodoping rate becomes faster and then saturated with an increase in the chalcogenide thickness. When illuminated from the chalcogenide side, the rate first increases and then decreases with characteristic functional forms. These dependencies are analyzed on the basis of a model in which the diffusion of photogenerated holes is assumed to govern the Ag+ migration process.
In situ preparation and transport properties of YBa2Cu3O7 films on sapphire with Zr(Y)O2 buffer layers
Authors: Adrian G. , Grabe G. , Wilkens W. , Adrian H. , Huth M. , Walkenhorst A.
Published: July 2002

YBa2Cu3O7 films on sapphire substrates prepared by laser ablation show c-axis-oriented growth and critical temperatures as high as Tc(R=0)=90 K. Due to diffusion processes and the formation of a nonsuperconducting intermediate layer the critical current density Jc is comparatively low. To overcome these difficulties the sapphire substrates were covered by Zr(Y)O2 films acting as diffusion barriers. The preparation of these films was performed by laser ablation using a multitarget system. The structure and the surfaces of the films were characterized by x-ray diffraction and scanning electron microscopy, respectively. The transport properties of the superconductor films are similar to those on suited substrates. Critical current densities up to Jc (77 K)=5×105 A/cm2 were achieved. In order to gain information about the pinning mechanisms measurements of the dependencies of Jc on the orientation between the crystal c axis and the external magnetic fields were carried out using samples with different buffer layer thicknesses. From the received data no indication of pinning by point defects in addition to the intrinsic planar pinning mechanism was detected.
Real-space spin-polarized first-principles calculations of an Fe impurity in a Cu host
Authors: Duarte Jaime , Frota-Pessôa Sonia
Published: July 2002

A scheme, based on the linear muffin-tin orbital (LMTO) formalism in the atomic sphere approximation (ASA) and on the recursion method, has been developed which allows first-principles, spin-polarized, self-consistent, density-functional calculations to be performed in real space. The scheme has been tested with success in ferromagnetic FeNi3 and in antiferromagnetic FeMn. Here a substitutional impurity of Fe in a Cu host is considered. The first-principles LMTO-ASA real-space approach is used to obtain the electronic structure and local magnetic moments of atoms in a region including four shells of Cu atoms around the impurity. The results for the magnetic moment and local density of states at the Fe site agree well with ab initio KKR–Green’s-function results in the literature. The first-principles real-space approach is extremely flexible and can be applied to investigate interactions between impurities in a nonmagnetic host or between magnetic layers separated by nonmagnetic regions. It can also handle, without extra effort, lattice relaxation around the impurities.
Epoxy-bonded Nd–Fe–B magnetic rollers: Experimental and theoretical analysis
Authors: Lee J. Kelly , Lewis Timothy S.
Published: July 2002

This paper describes the fabrication, testing, and analysis of epoxy-bonded neodymium–iron–boron (Nd–Fe–B) magnetic rollers used in electrophotographic copiers. The theoretical analysis gives a closed-form approximation for the magnetic field that agrees well with experimental measurements and is useful for design purposes.
Magnetic interface walls under applied magnetic fields
Authors: Mergel Dieter
Published: July 2002

In experiments with exchange-coupled double layers (ECDL) consisting of two ferrimagnetic layers of different composition of the general system GdTb–FeCo, it is observed that the interface wall changes its position under external fields. Furthermore, it was found that the interface wall energy σw determined on the same ECDL can be markedly different, depending on which of the two layers’ reversal curves it was determined. It is hypothesized that this effect is due to the different external fields at which the experiments have been performed. In order to check this hypothesis, a series of bilayers with different thicknesses of the ‘‘soft’’ layer was prepared, so that the reversal loop from which σw is determined occurs at different fields. The theoretical dependence of the interface-wall energy on the external field as obtained from computer simulations is found to be weaker than the experimental one.
High-resolution transmission electron microscopy observations on textured rapidly quenched NdFeB permanent magnets
Authors: Li Lin , Luzzi D. E. , Graham C. D.
Published: July 2002

High-resolution transmission electron microscopy (HRTEM) has been used to examine the microstructures of NdFeB magnets made by rapid solidification. The samples were General Motors MQ-3 magnets, and also MQ-3 magnets that had been recompressed perpendicular to the original compression axis. A major finding is that the Nd-rich grain-boundary phase is not uniformly distributed on all grain boundaries of the Nd2Fe14B phase. Some boundaries are completely clean, and where the Nd-rich phase does appear, its thickness varies widely. The grain-boundary phase (whose structure has been variously identified) is partially ordered at room temperature after slow cooling. Ordered domains and antiphase boundaries have been observed in this phase by HRTEM. Using the Nd2Fe14B fringe spacing as calibration, the lattice parameter of the grain-boundary phase is found to be 5.58 Å. In the completely ordered state, this phase is neither bcc nor fcc; it is simple cubic with a lower-symmetry space group Pm3m.
Excimer laser processing of magneto-optic garnet films
Authors: Belt Roger F. , Ings John B. , Ferranti David C. , Wigen Philip E.
Published: July 2002

A 248-nm KrF excimer laser was used to introduce defects into single-crystal GGG substrates and magneto-optic bismuth garnet films. The number, size, and distribution of defects can be controlled accurately in the 1–100-μm range. Defect introduction raises the coercivity of the films, controls their switching behavior, and modifies the hysteresis loop. Present theories do not explain the coercivity data as a function of defect density.
Magnetization and magnetic susceptibility of the diluted magnetic semiconductors Zn1−xCoxS and Zn1−xCoxSe
Authors: Chen Chen-jia , Gao Wei , Qin Zhi-feng , Hu Wei , Qu Ming , Giriat W.
Published: July 2002

Magnetization measurements have been done at 1.5 K and magnetic fields up to 7 T by using extraction method on the samples with x=0.0055, 0.048, 0.063 for Zn1−xCoxS and x=0.0097, 0.030, 0.037, 0.042 for Zn1−xCoxSe. In both Co-based systems, the magnetization increases with x. A modified Brillouin function fits the data and the fitting parameters T0( ≳ 0) and Seff are obtained. These results reflect that there exists a strong antiferromagnetic interaction among Co++ ions and the antiferromagnetic interaction in Zn1−xCoxSe is stronger than that in Zn1−xCoxS. Magnetic susceptibility was measured in the temperature range 1.5 K≤T≤300 K by using a vibrating-sample magnetometer. The susceptibility displays a high-temperature Curie–Weiss behavior. From quantitative analysis we obtain the nearest-neighbor Co++–Co++ exchange integral constant J1/kB for sulfides and selenides to be −51±6 K and −57±8 K, respectively. This value is at least three times as large as that in their Mn-based counterparts, and we confirm that the antiferromagnetic interaction is stronger in Zn1−xCoxSe than in Zn1−xCoxS. By using our results in conjunction with a direct measurement of the nearest-neighbor exchange JNN from recent inelastic neutron scattering for Zn1−xCoxS, we obtain an estimate of the next-nearest-neighbor exchange constant J2/kB = −8 K.
Domain wall observation and eddy current loss of (110)[001] oriented 17–22 μm thick Si-Fe tape-wound cores up to 5 kHz
Authors: Arai K. I. , Kim Y. H. , Yamaguchi M.
Published: July 2002

Dynamic magnetic domain observation and eddy current loss of tape-wound cores are discussed up to 5 kHz. The cores are made of 17–22 μm thick (110)[001] oriented silicon steel strips. Dynamic domain patterns were observed by using SEM on the surfaces of the tape-wound cores. In order to distinguish the moving domain walls from the standing ones, the drive frequency of the core should not be integral multiples of the scan frequency of the SEM(50.0 Hz). The model which includes dynamic wall number, wall bowing, and velocity of each domain wall agreed comparatively well with the measured eddy current losses.
Recrystallization and magnetic properties of purified 3% silicon steels
Authors: Ishiyama K. , Arai K. I. , Honda T.
Published: July 2002

The effect of impurities on the tertiary recrystallization and magnetic properties of the thin (10 to 100 μm) silicon steels was investigated. For purification, the conventional grain oriented silicon steels, which were used as starting materials with a thickness of 0.30 mm were pre-annealed at 1200 °C in a vacuum of 1×10−3 Pa before cold rolling. In sheets without pre-annealing, the tertiary recrystallization was observed after annealing for at least 3 h over 1200 °C in a vacuum. On the other hand, in pre-annealed sheets, the tertiary recrystallization was completed within 10 min at an annealing temperature of 1150 °C in a vacuum. Through recrystallization, B8 increased to 1.95 T. Even in sheets annealed at 1050 °C, the B8 increased with increasing the annealing time, and became 1.95 T after anneals of 2 h. The pre-annealed sheets were also annealed in hydrogen atmosphere. In this case, at annealing temperatures over 1100 °C, the tertiary recrystallization was completed in less than 1 h. Using the purified sheets, silicon steels with B8 over 1.9 T can be obtained by short time and low temperature annealing in either a vacuum or a hydrogen atmosphere.
The perpendicular anisotropy of multilayered Fe/Tb films (abstract)
Authors: Hoffmann H. , Endl G.
Published: July 2002

Multilayer Fe/Tb films were produced by rf sputtering. The thickness ratio of the Fe and Tb single layers was chosen in such kind to give an overall concentration of about 20 at. % Tb. Homogenous amorphous films of such a composition are well known for their perpendicular anisotropy. The investigations in the magnetic properties revealed that the multilayer films showed perpendicular anisotropy much stronger than the homogenous amorphous films. The maximum of the perpendicular anisotropy was found for multilayers with thickness combination of the single layers tFe=1.0 nm, tTb=1.5 nm. The perpendicular anisotropy increases strongly with increasing substrate temperature during rf sputtering. From the results of VSM measurements of the magnetic moment of the samples we must conclude that the Tb single layers are magnetically ordered even at room temperature. The magnetic moment of the Tb film is antiparallel coupled to the moment of the Fe film. The range of the coupling is at least 2 nm. As long as the multilayer films with various thicknesses of the single layers show perpendicular anisotropy, this anisotropy is rather independent of the thickness of the Tb layers but shows a linear dependence of the reciprocal of the thickness of the Fe layers. The magnetic investigations were accompanied by structural investigations. X-ray diffraction shows very clearly the superlattice of the multilayer films. Observations of cross sections of the multilayers by high-resolution electron microscopy revealed very sharp interfaces between Fe and Tb. The Tb films are crystallographically well ordered, whilst the Fe films showed only a nanocrystalline order. Mösbauer investigations are carried out to give an insight in the influence of the Tb films on the hyperfine field at the Fe atoms.
Dipolar ferromagnets and glasses (invited)
Authors: Rosenbaum T. F. , Wu W. , Ellman B. , Yang J. , Aeppli G. , Reich D. H.
Published: July 2002

What is the ground state and what are the dynamics of 1023 randomly distributed Ising spins? We have attempted to answer these questions through magnetic susceptibility, calorimetric, and neutron scattering studies of the randomly diluted dipolar-coupled Ising magnet LiHoxY1−xF4. The material is ferromagnetic for dipole concentrations at least as low as x=0.46, with a Curie temperature obeying mean-field scaling relative to that of pure LiHoF4. In the dilute spin limit, an x=0.045 crystal shows very unusual glassy properties characterized by decreasing barriers to relaxation as T→0. Its properties are consistent with a single low degeneracy ground state with a large gap for excitations. A slightly more concentrated x=0.167 sample, however, supports a complex ground state with no appreciable gap, in accordance with prevailing theories of spin glasses. The underlying causes of such disparate behavior are discussed in terms of random clusters as probed by neutron studies of the x=0.167 sample. In addition to tracing the evolution of the glassy and ferromagnetic states with dipole concentration, we investigate the effects of a transverse magnetic field on the Ising spin glass, LiHo0.167Y0.833F4. The transverse field mixes the eigenfunctions of the ground-state Ising doublet with the otherwise inaccessible excited-state levels. We observe a rapid decrease in the characteristic relaxation times, large changes in the spectral form of the relaxation, and a depression of the spin-glass transition temperature with the addition of quantum fluctuations.
Spin-valve magnetoresistance of uncoupled Fe-Cu-Co sandwiches
Authors: Chaiken A. , Lubitz P. , Krebs J. J. , Prinz G. A. , Harford M. Z.
Published: July 2002

The coercive field of thin Co films is strongly dependent on substrate temperature and film thickness, but the magnetic properties of Fe films are only weakly affected by growth conditions. Fe-Cu-Co sandwiches deposited on glass and silicon substrates via electron-beam evaporation exhibit plateaus in their magnetization curves when the coercive field of the Co is made larger than that of the Fe. These plateaus correspond to an applied field region in which the Co and Fe moments are antialigned and the resistance is enhanced. The isotropic or spin-valve part of the magnetoresistance is as large as 3.3% in some sandwiches. The spin-valve magnetoresistance has a broad peak centered at a ferromagnetic layer thickness of 60 Å.
Effect of twin boundaries on flux pinning in YBa2Cu3O7−x at low and intermediate magnetic fields
Authors: Kaiser D. L. , Gayle F. W. , Swartzendruber L. J. , Bennett L. H. , McMichael R. D.
Published: July 2002

Magnetic measurements on a single crystal of YBa2Cu3O7−x in the twinned and detwinned states have demonstrated that flux pinning by twin boundaries is strongly dependent upon temperature and the magnitude and orientation of the applied field with respect to the crystallographic orientation. For applied magnetic fields perpendicular to the c axis, no effect of twin boundaries on flux pinning was found at any temperature. In contrast, for fields up to 400 Oe aligned parallel to the c axis, the hysteresis loops at 77 K clearly showed increased pinning due to twin boundaries. At 40 K and low fields (H<500 Oe) the flux pinning was nearly the same for the twinned and detwinned states, whereas at higher fields (up to 40 kOe), pinning was higher in the twinned state. At this temperature, a tendency toward flux depinning occurred in the detwinned state at about 10 kOe, and both the twinned and detwinned states exhibited a more rigid flux structure at 50 kOe than at 10 kOe.
Grating-assisted surface acoustic wave directional couplers
Authors: Golan G. , Griffel G. , Seidman A. , Croitoru N.
Published: July 2002

Physical properties of novel grating-assisted Y directional couplers are examined using the coupled-mode theory. A general formalism for the analysis of the lateral perturbed directional coupler properties is presented. Explicit expressions for waveguide key parameters such as coupling length, grating period, and other structural characterizations, are obtained. The influence of other physical properties such as time and frequency response or cutoff conditions are also analyzed. A plane grating-assisted directional coupler is presented and examined as a basic component in the integrated acoustic technology.
Effect of aging on T
c of YBa2Cu3O7−y irradiated by electron beam
Authors: Nishi Yoshitake , Takagi Seiichiro , Yasuda Kohji , Itoh Keisuke
Published: July 2002

Effects of aging on Tc are investigated for YBa2Cu3O7−y irradiated by electron beam (EB). The aging increases Tc. The maximum T0c (zero resistivity, maxT0c) depends on the T0c before irradiation. The higher the T0c, the higher the maxT0c becomes. The effects are explained by x-ray diffraction peaks.
Temperature dependence of degenerate four wave mixing in azo dye doped polymer films
Authors: Tomov I. V. , Dutton T. E. , VanWonterghem B. , Rentzepis P. M.
Published: July 2002

Experimental results of the temperature dependence of the nonlinear optical response of disperse red 1 and methyl red doped in polystyrene film in the range 90–290 K are reported. It is found that the phase conjugate signal intensity resulting from degenerate four wave mixing is decreased by an order of magnitude at low temperature. The decay time of the recorded grating at fixed pump intensity also decreases with decreasing temperature.
On the thermoelectric power in n-channel inversion layers of ternary chalcopyrite semiconductors under magnetic quantization
Authors: Ghatak K. P. , Biswas S. N.
Published: July 2002

An attempt is made to investigate the thermoelectric power of the electrons under strong magnetic quantization in n-channel inversion layers of ternary chalcopyrite semiconductors at low temperatures, taking n-channel inversion layers on CdGeAs2 as examples, under both the weak and strong electric field limits, respectively. We have formulated the magneto-thermo power on the basis of newly derived two-dimensional electron energy spectra for both the limits by considering various types of anisotropies of the band parameters within the frame work of k■p formalism. It has been observed that, the magneto-thermo power decreases with increasing surface electric field and decreasing quantizing magnetic field in an oscillatory manner for both the limits. The crystal field parameter enhances the numerical magnitudes and the corresponding results for n-channel inversion layers of parabolic semiconductors have also been obtained as special cases from the generalized expressions under certain limiting conditions.
Dynamics of current filaments in p-type germanium under the influence of a transverse magnetic field
Authors: Clauss W. , Rau U. , Peinke J. , Parisi J. , Kittel A. , Bayerbach M. , Huebener R. P.
Published: July 2002

An explanation for the self-generated formation of spontaneous current oscillations developing during low-temperature impact ionization breakdown of slightly doped p-type germanium is presented for the first time, taking advantage of a model experiment. Upon applying a relatively small transverse magnetic field, the spatially inhomogeneous current distribution manifest in the form of individual high-conducting-current filaments undergoes a distinct traveling dynamics that is oriented perpendicular to the direction of the electric and the magnetic field (i.e., not coincident with the direction of the current flow). The resulting magnetic-field-induced oscillatory behavior can be described qualitatively by simple model considerations.
Testing the accuracy of calculated equilibrium carrier concentrations in the presence of surface fields
Authors: Geist Jon , Lowney Jeremiah R. , James C. R. , Robinson A. M.
Published: July 2002

Simple analytic expressions for the one-dimensional, majority-carrier concentration in a uniformly doped, semi-infinite semiconductor with a charge-accumulated front surface are derived. These expressions are based on the assumption that the effective intrinsic carrier concentration depends only upon the concentration of the majority dopant. Within the framework of this assumption, the expressions derived here are rigorously accurate for intrinsic material, and are accurate to well within 100 parts per million (ppm) for doping concentrations above 1012 cm−3. The results of calculations of the majority-carrier concentration carried out using these expressions and using a widely available one-dimensional semiconductor device modeling program are compared to illustrate how these expressions are used for testing the accuracy of iterative solutions of the drift-diffusion equations in the presence of surface fields.
Phase separation in ion bombarded FeNi Invar alloys
Authors: Gallas M. R. , Amaral L. , da Jornada J. A. H.
Published: July 2002

Studies of the effects of ion bombardment on the phase equilibria and ordering of FeNi alloys with 30.6 and 39.5 at. % Ni were performed. The FeNi samples were bombarded with Ne, Ar, and Kr at different doses and temperatures, and analyzed using the conversion electron Mössbauer spectroscopy (CEMS) technique with 57Fe. The changes in the CEMS spectra of Fe69.4Ni30.6 and Fe60.5Ni39.5 alloys, bombarded with Ne and Ar, indicate a phase separation, but no evidence of ordering. The Kr bombardment does not produce any change, however samples of 39.5 at. % Ni already showing phase separation returned to the single phase state after Kr irradiation. Our results give strong evidences of a prevalence of displacement mixing for bombardment with heavier ions, and of radiation enhanced diffusion for bombardment with lighter ions.
Surface properties of p-Hg1−xCdxTe
Authors: Höschl P. , Moravec P. , Franc J. , Grill R. , Belas E.
Published: July 2002

Electrical conductivity σ and Hall coefficient RH have been measured in p-Hg1−xCdxTe crystals with x≊0.2 in the temperature region 4.2–300 K. The influence of a crystal surface on galvanomagnetic properties is observed only in the temperature region T<20 K. Both positive and negative charges are formed on the surface under the influence of a natural air atmosphere after etching in BrMeOH. The formation of an accumulation layer on p-HgCdTe has been observed for the first time. The growth of an inversion layer was studied over one year after etching. The concentration of surface electrons reached the value 3.5 × 1012 cm−2. In the accumulation layer, the hole surface concentration rapidly increased within two months from 1013 cm−2 to 1014 cm−2. The bending of the bands has been calculated, first, by a classical method of solving the Poisson equation and second, by a quantum method using the Zawadski model of a triangular asymmetrical potential well. Preliminary study shows that the type of layer created, inversion or accumulation, can be connected with the orientation of the surface.
Study of surface acoustic waves on the {110} plane of gallium arsenide
Authors: Bright Victor M. , Kim Yoonkee , Hunt William D.
Published: July 2002

Piezoelectric semiconductors such as GaAs are of interest for signal processing devices due to the possibility of monolithic integration of surface-acoustic-wave (SAW) -based devices and active electronic circuits on the same substrate. The common crystal orientation of GaAs for these applications has been the {100} cut. In this paper the characteristics of surface wave propagation in the {110} plane of GaAs, which supports both the Rayleigh and the Bleustein–Gulyaev surface wave modes, are investigated. Several test devices have been fabricated and studied using laser probe and S-parameter measurement techniques. The SAW velocity, attenuation, piezoelectric coupling constant, slowness surface, beam profile, and the transducer characteristics have been measured, and the experimental results agree well with the theoretical calculations for both types of the surface wave modes.
Soft x-ray photoemission characterization of the H2S exposed surface of p-InP
Authors: Nelson Art J. , Frigo Sean , Rosenberg Richard
Published: July 2002

Synchrotron radiation soft x-ray photoemission spectroscopy was used to characterize the surface chemistry of p-InP before and after exposure to H2S gas at ambient temperature. The effect of the H2S dosing was determined by in situ photoemission measurements which were acquired after each gas exposure in order to observe changes in the valence band electronic structure as well as changes in the In 4d, P 2p, and S2p core lines. The results were used to correlate the surface chemistry to the electronic properties. These measurements indicate that the H2S exposure type converts the p-type InP surface to an n-type surface and that the magnitude of the band bending is 0.6 eV resulting in a homojunction interface.
Laser-induced chemical-vapor deposition of copper with a cw CO2 laser
Authors: Mao Dun-Min , Jin Zhong-Kao , Qin Qi-Zong
Published: July 2002

The kinetics of the thermal deposition of copper film by decomposing bis-(pivaloyl- trifluoroacetonate) copper(II) [Cu(pta)2] is reported. A cw CO2 laser is used to heat a quartz substrate, on which chemical-vapor deposition of copper takes place. The deposition rate is measured in situ by monitoring the transmittance of the copper film with a He-Ne laser. The deposition rates are determined as a function of CO2 laser intensity and a threshold of 7.9 W has been obtained. Above the threshold the deposition rate increases with laser intensity in the low-intensity region and becomes constant in the high-intensity region. Surface reaction is likely the rate-controlling step of the deposition process at low laser intensity, while diffusion of gaseous precursor molecules to the surface plays an important role at high intensity. Resistivity and thickness of the copper film have also been measured.
Effect of parasitic capacitance and inductance on the dynamics and noise of dc superconducting quantum interference devices
Authors: Ryhänen Tapani , Seppä Heikki , Cantor Robin
Published: July 2002

Practical thin-film dc superconducting quantum interference devices (SQUIDs) are often built with an integrated input coil which leads to the formation of various parasitic elements. We have designed and fabricated several square washer-shaped dc SQUID test devices with various amounts of parasitic capacitance formed by covering the washer slits with superconducting coverplates of various widths. In the simplest approximation, the parasitic capacitance Cp appears entirely across the Josephson junctions; in a better approximation, Cp divides the total inductance into two parts, an effective SQUID loop inductance L and a small parasitic inductance Lp. For both models, noise-free and noise-rounded current-voltage and voltage-flux characteristics are calculated for various values of the ratios L/Lp and Cp/C, where C is the Josephson junction capacitance. The SQUID dynamics are significantly affected by Cp and Lp. The energy resolution ε at first deteriorates with increasing Cp for Cp/C ≲ 2, but as Cp/C becomes large, ε improves to less than twice the value for Cp/C ≊ 0. The optimal points of operation for large Cp, however, are found at voltages above the low-frequency resonance determined by L and Cp where the flux-to-voltage transfer function is low. Furthermore, for devices with Cp/C ≫ 1 and Lp≠0, the double-loop geometry leads to an additional resonance determined by Lp and C that appears at high frequencies. The energy resolution inside this high-frequency resonance is nearly independent of Cp and comparable to the energy resolution of a device having the same total inductance but with Cp/C ≪ 1. Extensive noise mappings on several different test devices have been carried out using a computer-controlled measurement system. The measured characteristics and noise performance are shown to be in good agreement with the simulations.
Nanometer scale pattern generation in deposited SiO2 with electron beam irradiation
Authors: Pan Xiaodan , Broers Alec
Published: July 2002

SiO2 films deposited by low-temperature plasma-enhanced chemical vapor deposition (LPCVD) have been successfully used as an electron beam resist with nanometer scale resolution for the first time. In this paper we demonstrate nanometer trenches with feature sizes as small as 10 nm and periods down to 15 nm fabricated in LPCVD SiO2 films by electron beam lithography. These feature sizes and periods are comparable to those of the previously discovered lithographic process in thermally grown SiO2 [D. R. Allee and A. N. Broers, Appl. Phys. Lett. 57, 2271 (1990)]. Because the application of deposited oxide is not uniquely limited to silicon as is thermally grown oxide, this newly discovered process should allow the fabrication of a broader range of ultrasmall devices.
Well-width dependence of photoinduced intersubband absorption in GaAs/AlGaAs multi quantum wells
Authors: Elbert D. , Ehrenfreund E. , Bajaj J. , Sullivan G. J. , Lind D.
Published: July 2002

The photoinduced intersubband absorption of undoped GaAs/Al0.33Ga0.67 As multi quantum wells with different well widths is reported. Two different absorption bands are observed in each of the samples. One is polarized along the growth direction, is linear with the pump intensity, and is identified with the intersubband transition of free excitons. The other is not polarized, depends sublinearly on the pump intensity, and is assigned to an intersubband transition which involves interface trapped carriers.
Quantum well thickness effect on the deep–shallow duality of Si in AlGaAs/InGaAs heterostructures
Authors: Shen J. , Tehrani S. , Goronkin H. , Droopad R. , Maracas G.
Published: July 2002

Temperature-dependent Hall measurements show that in a narrow AlxGa1−xAs/InyGa1−yAs quantum well heterostructure at low temperature, electrons trapped in Si deep levels (DX centers) are photoexcited and directly transferred to the shallow levels. When the temperature rises, these shallow-level-bound electrons are thermally ionized. On the other hand, in the wide quantum well heterostructure, the photoexcited electrons are not bound to the shallow levels even at temperatures as low as T=20 K. The relative positions between the Fermi level and the bound Si shallow level are found to be important in determining whether electrons can or cannot be trapped by the shallow levels. A simple model supports our conclusion.
Selenium residual doping of (AlGa)Sb/GaSb epilayers grown by molecular beam epitaxy
Authors: Gauneau M. , Chaplain R. , Toudic Y. , Rupert A. , Grandpierre G. , Salvi M.
Published: July 2002

Some (AlGa)Sb layers grown by molecular beam epitaxy were found unexpectedly to be n type. Characterizing these layers by secondary ion mass spectrometry it was observed that they contained selenium (Se). Systematic analyses showed that selenium was present in all our antimonide layers, even those which were p type, at concentrations between 6×1014 and 3×1017 cm−3. The thermodynamical study of this contaminant incorporation led us to conclude that it comes from the solid antimony used for growths. That was confirmed by spark source mass spectrometry investigations. During growths, selenium behaves as other elements of group VI, sulphur for example, and this behavior is described by a simple kinetic model. It incorporates more easily at low substrate temperatures and saturates at levels depending on the antimony (Sb4) flux. At higher substrate temperatures, the incorporation is balanced by desorption according to an activation energy of 3.2 eV. As a consequence, selenium constitutes an accurate thermal probe to follow substrate temperature variations during growths. As could be expected, sulphur was also found to contaminate (AlGa)Sb films at levels of the order of a few 1015 cm−3. However, oxygen was not detected, probably being lower than 1016 cm−3 the detection limit of the analytical technique. This residual doping of antimonides by chalcogens presumably compensates their natural p-type doping, but relationships with mobilities have not been evidenced yet.
Estimation of the core cut-off parameter for misfit dislocations at a SixGe1−x/Si interface
Authors: Rajan Krishna
Published: July 2002

The misfit dislocation structure at a partially relaxed Si/SixGe1−x interface has been investigated using weak-beam electron microscopy in order to probe features near the dislocation core. The dislocations were found to be dissociated, allowing for the direct measurement of the stacking fault energy at the epitaxial interface. This has been supplemented with the measurement of the width of symmetrical threefold stacking fault nodes at the same interface. These independent measurements have been combined to estimate the dislocation core cut-off parameter using isotropic approximations of anisotropic elasticity theories. The size of the core of the misfit dislocation is consistent with both geometric and atomistic models predicting core delocalization at coherent interfaces.