bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic anisotropy and spin reorientation transition in a TbFe11Ti single crystal (abstract)
Authors: Abadía C. , Algarabel P. A. , Ibarra M. R. , Marquina C. , Kudrevatykh N. V. , Markin P. E.
Published: July 2002

Very large discrepancies exist with regard to the spin reorientation transition in TbFe11Ti (see Ref. for a summary of the different results). In this work, this transition is studied by measuring the angular dependence of the parallel (M∥) and perpendicular (M⊥) components of the magnetization to the applied magnetic field in the temperature range of 4.2–330 K and applied magnetic fields ranging from 2 to 15 kOe. This method allows an accurate determination of the easy magnetization direction to be made, i.e., the maximum value of M∥ and the zero value of M⊥. The experimental results have undoubtedly established that the compound is easy plane below 260 K and easy axis above 320 K. Within this intermediate range of temperature the easy magnetization direction lies along a nonmajor symmetry direction making an angle θ with the c axis. This angle depends on the applied magnetic field and the magnetic history of the sample. This fact can be understood on the basis of the existence of a first-order spin reorientation transition at TSR≊320 K from easy axis to easy plane. Within the temperature region 206–320 K both phases (axial and planar) coexist with relative volumes depending on the applied magnetic field and the magnetic history. This gives rise to a macroscopic conical structure. Such behavior has already been observed in DyFe11Ti.
Short and long annealing of high-dose arsenic-implanted pseudomorphic GexSi1−x on Si(100)
Authors: Lie D. Y. C. , Im S. , Nicolet M.-A. , Theodore N. D.
Published: July 2002

Metastable pseudomorphic epi-GexSi1−x (x=8%, 16%) films of a common thickness of 145±10 nm were deposited on Si(100) substrates by chemical vapor deposition and then implanted at room temperature with 90 keV arsenic ions to a dose of 1.5×1015/cm2. This implantation amorphizes the top ∼125 nm of the epi-GeSi layers. Implanted as well as nonimplanted GeSi samples were subsequently annealed by (1): short (10–40 s) lamp annealing in nitrogen ambient at 600–800 °C; or (2): long (30 min) furnace annealing in vacuum (∼5×10−7 Torr) at 500–800 °C. Silicon samples were also implanted and annealed as references. The amorphized epi-GeSi recrystallizes via solid-phase epitaxy when annealed at or above 500 °C. The initial pseudomorphic strain of the epi-GeSi is thereby lost for both short and long annealing. High densities of dislocations (1010–1011/cm2) are typically present in the regrown GeSi layers, but not in the regrown Si samples. Just after the completion of solid-phase epitaxial regrowth, ∼80%–100% of the implanted arsenic ions become electrically active; further annealing decreases the activation. We conclude and generalize that metastably strained GeSi layers amorphized by a high dose of implanted dopants will not recover their original crystallinity and strain after solid-phase epitaxial regrowth, regardless of annealing procedure, although the implanted dopants are electrically activated in the process.
Conducting properties of polyaniline blends
Authors: Pinto N. J. , Torres C. M. , Kahol P. K. , McCormick B. J.
Published: July 2002

We report on our dc conductivity measurements of mixed samples of polyaniline in the conducting form blended with polyaniline in the insulating form and made conducting subsequent to processing. Emeraldine base polyaniline (PANEB) dissolved in N-methylpyrrolidinone (NMP) and emeraldine base polyaniline doped with camphor sulfonic acid (CSA) but dissolved in NMP [and also in dimethyl sulfoxide (DMSO)] were mixed in various ratios according to the symbolic notation (PANCSA-NMP)1−x(PANEB-NMP)x and (PANCSA-DMSO)1−x(PANEB-NMP)x, respectively. Films obtained were then equilibrated in 1 M HCl for a period of 40–60 h and vacuum dried overnight prior to performing conductivity measurements. It is shown that the room temperature conductivity increases with increasing values of x.
Strain and structural characterization of Zn1−xCdxSe laser structures grown on GaAs and InGaAs (001) substrates
Authors: Li J. H. , Bauer G. , Stangl J. , Vanzetti L. , Sorba L. , Franciosi A.
Published: July 2002

X-ray reciprocal space mapping has been used to investigate the strain status of microgun-pumped blue and blue-green laser structures. The devices exploit graded-index, separate confinement Zn1−xCdxSe/ZnSe heterostructures grown on InGaAs or GaAs substrates by molecular-beam epitaxy. The location of the reciprocal lattice point of the ZnSe buffer layer within a normally forbidden region of reciprocal space indicates that the ZnSe buffer layer is unusually strained, with an appreciable biaxial tensile strain despite the smaller lattice parameter of the III–V substrate relative to ZnSe. We associate such a phenomenon with the presence of the highly strained laser structure coupled with preferential strain relaxation at the II–VI/III–V heterointerface.
Polarization dependence of two-photon absorption in GaAs/Ga1−xAlxAs heterostructures
Authors: Avetissian S. K. , Melikian A. O. , Minassian H. R.
Published: July 2002

Two-photon absorption in GaAs/Ga1−xAlxAs with a realistic band structure and finite band offsets ΔEc, ΔEv is analyzed theoretically and compared with experiment. This approach allows us to explain the known experimental data and choose the most realistic value for the ratio ΔEc/ΔEv. It is shown that for thin quantum wells the number of possible two-photon optical transitions in the well decreases which strongly affects the polarization dependence of two-photon absorption in GaAs/Ga1−xAlxAs heterostructures.
Effect of oxygen partial pressure during postheat treatment on Bi2Sr2CaCu2Oy/Ag tapes
Authors: Fukumoto Y. , Moodenbaugh A. R. , Suenaga M. , Fischer D. A. , Shibutani K. , Hase T. , Hayashi S.
Published: July 2002

Effects of oxygen partial pressure PO2 during low temperature postheat treatments on the superconducting properties of silver-sheathed Bi2Sr2CaCu2Oy tapes were investigated. The critical temperature Tc of the resulting tapes changes systematically with PO2. This variation of Tc is attributed to the change of electronic carrier (hole) concentration by the observation of oxygen K-edge absorption spectra. The intragranular critical current density Jc is essentially unaffected by the heat treatment, while deterioration in grain connectivity is observed in the specimens treated in reduced-oxygen atmospheres. The change of anisotropy in Jc with respect to crystallographic orientations is also quite small for the variation of PO2.
Phase diagram of HgTe–ZnTe pseudobinary and density, heat capacity, and enthalpy of mixing of Hg1−xZnxTe pseudobinary melts
Authors: Su Ching-Hua , Sha Yi-Gao , Mazuruk K. , Lehoczky S. L.
Published: July 2002

In this article, the solidus temperatures of the Hg1−xZnxTe pseudobinary phase diagram for several compositions in the low x region were measured by differential thermal analysis and the HgTe–ZnTe pseudobinary phase diagram was constructed. The densities of two HgZnTe melts, x=0.10 and 0.16, were determined by an in situ pycnometric technique in a transparent furnace over, respectively, 110 and 50 °C ranges of temperature. The thermodynamic properties of the melts, such as the heat capacity and enthalpy of mixing, were calculated for temperatures between the liquidus and 1500 °C by assuming an associated solution model for the liquid phase.
Device structures on porous silicon studied by scanning electron microscopy in the electron-beam current mode
Authors: Balagurov L. A. , Katz E. A. , Petrova E. A. , Govorkov A. V. , Ritova N. I. , Evdokimov V. M. , Lük’yanov A. E. , Butilkina N. A.
Published: July 2002

Contacts metal/porous silicon and p/n device structures on porous silicon were studied by scanning electron microscopy in the electron-beam-induced current mode. It is shown that the drift processes are dominant in operation of porous silicon-based devices. We present the theoretical analysis of electron-beam-induced current measurements and estimate such important parameters of device structures as: the width of the space-charge region (several microns), charge state density in the space-charge region (1014–1015 cm−3), and electron drift length (up to 10−3 cm). The spatial distribution of the electric field in space-charge region was derived. The possibility of studying the influence of porous silicon/crystalline silicon interfaces on operation of porous silicon-based devices has been illustrated.
Wavelength dependent refractive effects and Stark broadening in laser-induced YBa2Cu3Ox plasma plumes
Authors: El-Astal A. H. , Morrow T.
Published: July 2002

Spectral lines observed at short delay times and close to the target during laser ablation of YBa2Cu3Ox show strongly distorted line shapes arising partly from optical refraction by the large density gradients within the expanding plume. Spatially and temporally resolved linewidths, corrected for refraction effects, are reported for the Ba(I) 553.5 nm absorption transition and Stark widths, deduced from these corrected linewidths, indicate that the maximum electron number density at 1.0 mm from target is ∼1×1019 cm−3.
Structural changes of silicon dioxide films caused by synchrotron irradiation
Authors: Park Young-Bae , Rhee Shi-Woo , Imaizumi Y. , Urisu T.
Published: July 2002

Effect of synchrotron radiation (SR) on silicon oxide films, deposited with remote plasma-enhanced chemical-vapor deposition, has been investigated. SR irradiation had an effect on the silicon dioxide structure which was confirmed by in situ infrared reflection adsorption spectroscopy. As the substrate temperature was increased to 373 K, the SiO2 structure became loose by irradiation and surface migration was observed. Surface migration of species induced by SR irradiation led to the increase of surface roughness and also the formation of hemispherical bubbles on the film surface, which was confirmed by atomic force microscope. Generation of oxygen-related paramagnetic defect was observed by electron spin resonance.
Raman spectra of Zn1−xMgxSySe1−y quaternary alloys
Authors: Wang Donghong , Huang Daming , Jin Caixia , Liu Xiaohan , Lin Zheng , Wang Jie , Wang Xun
Published: July 2002

Semiconductor quaternary alloys Zn1−xMgxSySe1−y were grown on GaAs substrates by molecular beam epitaxy. The crystal structures were determined from x-ray diffraction spectra. The Raman spectra of both perpendicular and parallel polarization geometries were measured. The following long-wavelength optical-phonon modes were identified: ZnSe-like TO and LO modes, and MgSe-like and ZnMgS-like LO modes. For the alloys with the same Mg composition x, the vibration frequencies of ZnSe-like TO and LO modes were observed to change linearly with S composition y for y<0.5. The vibration frequency of the MgSe-like LO mode was also observed to change linearly with y for 0.1<y<0.5.
Thermal vacancy formation and positron–vacancy interaction in Ti3Al at high temperatures
Authors: Würschum R. , Kümmerle E. A. , Badura-Gergen K. , Seeger A. , Herzig Ch. , Schaefer H.-E.
Published: July 2002

In order to study the formation of thermal vacancies in the Ti–Al alloy system, high-temperature positron lifetime measurements together with a modeling of defect formation in the framework of nearest-neighbor pair bonds were performed for α2Ti3Al and compared to recent results on γTiAl [U. Brossmann, R. Würschum, K. Badura, and H.-E. Schaefer, Phys. Rev. B 49, 6457 (1994)]. Substantial increases of the positron lifetime τ were observed for Ti65.6Al34.4 and Ti77.1Al22.9 in the temperature range T≳1200 K where thermal vacancy concentrations above the detection limit of positron annihilation are expected from the model calculations for the α2 phase. Within the high-temperature increase of the positron lifetime in the α2 and the β phase single-component positron lifetime spectra were observed. This behavior is in contrast to the two-component spectra observed conventionally at intermediate positron trapping rates and is attributed to a fast detrapping and retrapping of positrons at vacancies due to a low positron–vacancy binding energy. For this case, a vacancy formation enthalpy of HFV=(1.55±0.2) eV in α2Ti65.6Al34.4 and HFV=(1.8±0.2) eV in βTi77.1Al22.9 can be derived. These results are discussed in the context of recent 44Ti tracer diffusion studies.
Correlation between the kinetics of the amorphous-to-nanocrystalline transformation and the diffusion in alloys
Authors: Damson B. , Würschum R.
Published: July 2002

The amorphous-to-nanocrystalline transition was studied on various melt-spun, ball-milled, and cluster-synthesized alloys by means of differential scanning calorimetry. In the framework of a simple model the correlation found between the pre-exponential factors and activation enthalpies for the crystallization (KISSINGER analysis) can be attributed to the similar correlation which exists for the tracer diffusivities in the respective amorphous alloys. The shift of the present pre-exponential factors and activation enthalpies towards higher values compared to the diffusion coefficients in the amorphous state indicates a change of the diffusivity due to the amorphous-to-nanocrystalline transition. The crystallization kinetics of Fe73.5Si13.5B9Nb3Cu1 is discussed in the light of recent self-diffusion studies of the intermetallic compound Fe3Si.
Interplay of latent heat and time-dependent nucleation effects following pulsed-laser melting of a thin silicon film
Authors: Shneidman Vitaly A.
Published: July 2002

We propose a simple-form mathematical description that allows us to account simultaneously for the effects of time-dependent nucleation and of latent heat during rapid cooling of a thin film. The method is based on a combination of analytical description of nucleation and a numerical (or, semianalytical) description of thermal effects due to postnucleation growth of crystallites. The accuracy of the treatment is tested against numerically exact solutions of the Farkas–Becker–Döring master equation, and is applied to several realistic cooling histories consistent with experimental studies of silicon on silicon oxide films of Stiffler et al. [Phys. Rev. B 43, 9851 (1991)] and Sameshima and Usui [J. Appl. Phys. 70, 1281 (1991)], respectively. Special attention is paid to the region of high cooling rates (very thin films of less than 100 nm) where the transition to complete amorphization occurs. For such cooling rates the time-dependent nucleation effects turn out to be especially important, and their neglect would lead to significant overestimates of the critical cooling rate that separates the recrystallization and the amorphization regions.
Determination of electron traps in tin disulphide crystals by thermally stimulated current measurements
Authors: Micocci Gioacchino , Tepore Antonio
Published: July 2002

Electron traps have been studied by means of thermally stimulated current measurements in n-SnS2 crystals grown by the chemical vapor transport method. TSC measurements have been carried out in the temperature range 80–300 K, and the results have been analyzed according to two different methods which seem to be in good agreement with each other. In this way, two main trapping levels have been detected at 0.25 and 0.36 eV below the conduction band, with capture cross sections of 5.6×10−21 and 2.7×10−19 cm2, respectively. These traps are probably associated with structural defects.
Thermal diffusivity and conductivity of Hg1−xZnxTe solids and melts
Authors: Sha Yi-Gao , Su Ching-Hua , Mazuruk K. , Lehoczky S. L.
Published: July 2002

The thermal diffusivity of pseudobinary Hg1−xZnxTe solids and melts was measured by the laser flash method. The measured diffusivities for the solids of 0.10≤x≤0.30 are about 60% of that of the HgTe solid. Those for the melts rise rapidly with temperature but less so with increasing x. For x=0.30, the diffusivity of the melt is about one third of that of the HgTe melt. Using the calculated heat capacity data from the associated solution model and measured density values, the thermal conductivity for the pseudobinary Hg1−xZnxTe solids of 0.10≤x≤0.30 and for the melts of x=0.10, 0.16, and 0.30 was determined.
Current noise spectroscopy of deep energy levels in photoconductors
Authors: Carbone A. , Mazzetti P.
Published: July 2002

According to a barrier photoconductance theory recently developed, two current noise sources can be envisaged, related respectively to the fluctuation of the barrier height (photoinduced noise component) and to the trapping–detrapping processes in shallow states within the photoconducting material (g–r and 1/f noise component). It is shown that the first noise component, which can be separated by the second one, gives information on the energy gap and on the photoionization cross section of the deep energy levels of the photoconducting material. In particular for thin films it will be shown that the photoionization cross section of the deep energy levels varies inversely to the total number of photons impinging on the photoconductor and proportionally to the noise power spectral density as the photon energy changes. Such relationship suggests that the wavelength dependence of the noise power spectrum provides the correction to be taken into account if the constant photocurrent method is used and the variations of the minority carrier lifetime with the photon energy cannot be disregarded. Results concerning the energy gap and its temperature dependence, in the interval ranging from the room temperature to 200 K, are reported and compared with the results found in the literature for CdS based samples. The photoionization cross section of the deep centers as a function of the photon energy, at room temperature, is reported for the same sample. Such quantities have been obtained by measuring the spectral density of the photoinduced noise vs wavelength at constant photoconductance value. As for the other kinds of noise spectroscopy, the main advantage of the present method is to work out in the operative condition of the semiconductor device.
Thickness and temperature effects on thermoelectric properties of Pb0.6Sn0.4Te thin films
Authors: Das V. Damodara , Bahulayan C.
Published: July 2002

Electrical resistivity and thermoelectric power measurements have been carried out as a function of thickness in the temperature range 300–480 K on flash evaporated Pb0.6Sn0.4Te thin films. Electrical resistivity and thermoelectric power variation with temperature of the films suggests that they have a near-degenerate semiconductor behavior. The effective mean-free-path model of classical size effect has been used to analyze the thickness dependence of resistivity and thermoelectric power. Both are found to be near-linear functions of inverse thickness. From the analysis of the experimental data, material parameters like mean free path and Fermi energy have been evaluated. Thermoelectric figure of merit has been calculated using the measured values of thermoelectric power and electrical resistivity and by taking thermal conductivity data from the literature. Thermoelectric figure of merit is also found to be thickness and temperature dependent.
Ion energy and angular distributions in inductively coupled radio frequency discharges in argon
Authors: Woodworth Joseph R. , Riley Merle E. , Meister Dorothy C. , Aragon Ben P. , Le Minh S. , Sawin Herbert H.
Published: July 2002

We report measurements of the energies and angular distributions of positive ions in an inductively coupled argon plasma in a Gaseous Electronics Conference Reference Cell. Use of two separate ion detectors allowed measurement of ion energies and fluxes as a function of position as well as ion angular distributions on the discharge centerline. The inductive drive on our system produced high plasma densities (up to 1012/cm3 electron densities) and relatively stable plasma potentials. As a result, ion energy distributions typically consisted of a single feature well separated from zero energy. Mean ion energy was independent of rf power and varied inversely with pressure, decreasing from 29 to 12 eV as pressure increased from 2.4 to 50 mTorr. The half-widths of the ion angular distributions in these experiments varied from 5° to 9°, or equivalently, the transverse temperatures varied from 0.18 to 0.29 eV with the distributions broadening as either pressure or rf power was increased.
Fabrication of large arrays of micron-scale magnetic features by selective area organometallic chemical vapor deposition
Authors: Welipitiya D. , He Y. L. , Zhang Jiandi , Oden P. I. , Thundat T. , Warmack R. J. , Gobulukoglu Ismail , Shan Z. S. , Sellmyer D. J. , Dowben P. A.
Published: July 2002

We demonstrate that it is possible to deposit a wide range of magnetic features, using photoassisted selective area organometallic chemical vapor deposition. Large arrays of identical micron-scale Ni features were deposited on a Si(111) wafer by this method. Their magnetic properties were studied by alternating gradient force magnetometry as well as magnetic force microscopy. Our morphological and magnetic measurements show that the structures are spatially well defined, and the magnetic properties are related to the structural shapes of the features. This method can be adapted to the fabrication of smaller-scale magnetic and electronic devices.
Thermal evolution of carbon in annealed Co/C soft x-ray multilayers
Authors: Bai H. L. , Jiang E. Y. , Wang C. D. , Sun D. C.
Published: July 2002

The structures of the carbon sublayers in the annealed Co/C soft x-ray multilayers fabricated using a dual-facing-target sputtering system have been characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman spectroscopy (RS). The results suggest that the structural variations in the carbon layers can be roughly divided into three stages, i.e. ordering, crystalline and grain growth stages. In the ordering stage with annealing temperatures below 400 °C, the upward shift of D and G lines in Raman spectra indicates that the amorphous carbon layers are changing from ones with bond-angle disorder and fourfold-bonding only to ones containing threefold-bonding. In the crystalline stage, the amorphous carbon layers in the as-deposited multilayers crystallize to graphite crystallites in the annealing temperature range of 500–600 °C. The rapid increase in the intensity ratio of D line to G line and dramatic decrease in linewidth further confirm this substantial structural change. In the grain growth stage, the specimens are annealed at temperatures higher than 700 °C. The decrease in the intensity ratio implies a growth in the graphite crystallite dimensions, which is consistent with the XRD and TEM results.
Kinetics of evaporation of beaded Ag film on sapphire and on alumina scale
Authors: Erdélyi G. , Barkóczi M. , Moya E. , Giordano H. , Beszeda I. , Aufray B. , Beke D. L.
Published: July 2002

Morphological changes of beaded Ag film on sapphire and on alumina scale were investigated by means of scanning electron microscopy and Auger spectroscopy in the temperature range of 822–973 K. It was found that the decrease of the effective thickness and the shift of the boundary of the layer can be attributed to the evaporation of silver into the gas phase. Experimental results were analyzed in terms of the model developed by Kaganovskii and Beke. Our results show that the process is controlled by surface diffusion and temperature dependence of the surface mass transport coefficients is deduced from the experimental data.
Scanning transmission electron beam induced conductivity investigation of a Si/Si1−xGex/Si heterostructure
Authors: Brown P. D. , Humphreys C. J.
Published: July 2002

We demonstrate that scanning transmission electron beam induced conductivity can be used to image the recombination activity of an orthogonal 〈110〉 misfit dislocation array within a relaxed Si/Si1−xGex/Si(001) heterostructure on the submicrometer scale. Correlation with structural images suggests that an array of bunched dissociated 60° misfit dislocations is primarily responsible for the electrical activity in this instance.
Investigation of dc Josephson current distribution in double-barrier three-terminal devices with a thin middle superconducting layer
Authors: Nevirkovets I. P. , Doderer T. , Laub A. , Blamire M. G. , Evetts J. E.
Published: July 2002

Nb/Al–AlOx–Nb/Al–AlOx–Nb double-barrier three-terminal Josephson devices with a thin middle Nb/Al bilayer, which have potential for use as switching and amplifying elements in superconducting electronics, have been investigated in the stationary state by means of low-temperature scanning electron microscopy. For the devices with lateral sizes comparable with the effective Josephson penetration depth, we observed nearly homogeneous current distribution over the region common to the top and bottom junctions when the devices were biased across both the barriers in the absence of an external magnetic field. When the two junctions are biased separately, the current is concentrated at the junction edges in accordance with the behavior characteristic of distributed junctions. In an applied magnetic field, the vortex structure is confined to the area shared by both junctions when the device is biased as a whole. The experiment gives an indication that in the stationary state the spatial phase difference distribution of the two junctions coincides in the shared region at least in some interval of the external magnetic field.
Optical properties of Zn(S,Se) sawtooth superlattices grown by atomic layer epitaxy
Authors: Fujiwara Hiroyuki , Nabeta Toshiyuki , Shimizu Isamu , Yasuda Takashi
Published: July 2002

‘‘Sawtooth’’ superlattices of Zn(S,Se) were grown on GaAs substrates by a layer-by-layer atomic epitaxy growth technique. Larger scale variations in band gap were introduced by systematically varying the ratio of ZnS and ZnSe thickness over greater distance scales. These larger scale variation were themselves repeated in order to produce a superlattice in which the band gap had a sawtooth shaped profile. The structure and optical properties of these new materials were characterized by x-ray diffraction and photoluminescence measurement. The x-ray diffraction spectra showed satellite peaks corresponding to the large scale variations in structure. The strong blue photoluminescence peaks were observed and consistent with hole trapping in the sawtooth potential wells.
Li diffusion in Ti oxyfluoride films: Thermal activation energy and jump length derived from impedance spectroscopy
Authors: Stro/mme Mattsson M. , Niklasson G. A. , Granqvist C. G.
Published: July 2002

Li ions were electrochemically intercalated into sputtered Ti oxyfluoride films. The process was found to be thermally activated with an activation energy depending on the amount of intercalated Li and decreasing with increasing Li content. The chemical diffusion coefficient was thermally activated as well with an activation energy of ∼0.5 eV independent of the amount of intercalated Li. The origin of the activation energy was discussed in terms of the Anderson–Stuart model. It was found likely that strain energy is needed to open up ‘‘doorways’’ in the Ti oxyfluoride structure to allow Li ion transport. The jump length for the Li ions inside the Ti oxyfluoride was estimated to lie in the 4–8 Å interval.
A chemical state resolved x-ray photoelectron diffraction study: Initial stages in diamondlike carbon film deposition
Authors: Agostino R. G. , Küttel O. M. , Aebi P. , Fasel R. , Osterwalder J. , Schlapbach L.
Published: July 2002

The structural sensitivity of x-ray photoelectron diffraction is greatly enhanced by the acquisition of a full hemispherical diffraction pattern of chemically shifted core levels. Complex systems can be studied resolving the local order per element and per chemical environment. This technique is applied to study the earliest stages of hydrogenated diamondlike carbon film deposition on Si(001). Effects of the sample temperature and ion dose on the structure of deposited layers are discussed.
Lattice-controlled mobility in quantized surface layers at low temperature
Authors: Ghorai A. K. , Bhattacharya D. P.
Published: July 2002

The momentum relaxation time for intravalley acoustic phonon scattering of the conduction electrons in a two-dimensional electron gas has been obtained under the condition of low temperature when the phonon energy cannot be neglected in comparison to the average thermal energy of the electrons and for that matter the equipartition approximation for the phonon distribution is not valid. The results for an n-channel (100)-oriented Si inversion layer are significantly different from what follows from the traditional theory. The temperature dependence of the mobility as obtained from the relaxation time calculated here is rather different from the traditional T−1L law.
Vortex structure and cavity modes in stacked double Nb/AlOx/Nb Josephson junctions
Authors: Song S. N. , Maglic S. , Thomas C. D. , Ulmer M. , Ketterson J. B.
Published: July 2002

We have studied the static I–V characteristics and cavity modes in stacked double Nb/AlOx/Nb Josephson junctions. In junction stacks consisting of two junctions with identical critical currents Ic, the Ic vs H characteristics have been observed to deviate from the usual Fraunhofer pattern in the small junction limit; the data are consistent with a model involving a structural phase transformation to a triangular vortex lattice with increasing H. In a finite voltage state interlayer coupling leads to splitting of the Swihart mode, which manifests itself as Fiske steps with different voltage spacings. The results provide clear evidence that the two junctions in the stack do phase lock.
Stability of ion-implanted layers on MgO under ultrasonic cavitation
Authors: Romana L. J. , Rankin J. , Brewster J. R. , Boatner L. A. , Williams A. M.
Published: July 2002

The effects of ion implantation and ultrasonic cavitation on the near-surface region of MgO single crystals were investigated. For 150 keV, room-temperature implantations of Ar+ or K+ at 5×1016–1017 ions cm−2 into a (100)-oriented surface of MgO, a dilatometric swelling of the implanted layer is observed perpendicular to the implanted surface. When these strained surface layers are then subjected to an ultrasonic (∼20 kHz) cavitation treatment, uniform layers (∼100–200 nm) can be removed from the MgO surface in a controlled manner. The thickness of the removed layer is directly related to the duration of the ultrasonic irradiation, but does not exceed the depth of the implanted species. When MgO implanted with Ar+ or Kr+ at fluences of 5×1016–1017 ions cm−2 is subsequently irradiated with 2 MeV He+ ions at 1016–1017 ions cm−2, the region exposed to the He+ beam is effectively ‘‘stabilized’’ and is not removed by the ultrasonic treatment. K+ or Ar+ implantations at <5×1016 ions cm−2 produced no swelling of the layer, and subsequent ultrasonic irradiation with an energy flux of ∼120 W cm−2 did not remove the crystal surface as determined by profilometry. For MgO implanted with ≳2×1017 ions cm−2, the implanted layer is highly strained and detaches from the underlying crystal without exposure to ultrasonic-cavitation effects. These findings are discussed in terms of the stressed states of the implanted layers, and their potential applications to the mechanical ‘‘etching’’ of ceramics are considered.
Interaction effects in particulate recording media studied by ferromagnetic resonance
Authors: Sürig C. , Hempel K. A.
Published: July 2002

According to the Stoner–Wohlfarth theory, ferromagnetic resonance (FMR) experiments along minor loops of the hysteresis should deliver the same microwave absorption loss for vanishing applied field independent of the remanence state of a polycrystalline sample. In the present work, deviations from that prediction in particulate recording media are presented, and a strong connection of zero-field absorption and remanence magnetization of the sample, as well as the introduction of an experimental procedure to determine the interaction field with an application on differently substituted hexaferrites is shown. Additionally, the mean-field ansatz is verified to be a valid model in order to describe the influence of magnetostatic interaction in multi-particle systems.
Paramagnetic defect spin centers in low-pressure chemical-vapor-deposited silicon-dioxide films
Authors: Kamigaki Yoshiaki , Yokogawa Ken’etsu , Hashimoto Takashi , Uemura Toshio
Published: July 2002

We have found E′-like and amorphous-siliconlike (a-Si-like, ⋅Si≡Si3) spin centers, as well as the well-known E′ center (⋅Si≡O3) in low-pressure chemical-vapor-deposited silicon-dioxide (LPCVD-SiO2) films, by using the electron-spin-resonance method. The behavior of these paramagnetic defect centers is studied and their origins are considered. An E′-like center is terminated stably by either oxygen (O2) or hydrogen (H2) gas annealing. This E′-like center appears to be an intermediately generated defect but its bonding structure has not yet been identified. The a-Si-like center is terminated stably by high-temperature H2 gas annealing. The a-Si-like center is thought to exist in the Si clusters in LPCVD-SiO2 films. We have also confirmed that the E′ center is terminated stably by O2 gas annealing. As a result, we conclude that both O2 and H2 gas annealing are necessary to reduce the quantity of paramagnetic defect centers in LPCVD-SiO2 films.
Structural investigation of the titanium/diamond film interface
Authors: Terranova M. L. , Rossi M. , Vitali G.
Published: July 2002

Reflection high-energy electron diffraction (RHEED) technique, with the possibility to shift, rotate, and tilt the sample with respect to the electron beam, has been used to investigate the structure of the transition layers formed at the interface between titanium substrate and diamond films. The diamond films were deposited on partially masked titanium sheets by means of the chemical vapor deposition technique, using CH4/H2 mixtures activated by hot filament. Deposition experiments have been performed at 650 and 730 °C. The RHEED analysis has been carried out in selected area conditions on the boundary between coated and uncoated regions; it enabled us to detect and characterise the inhomogeneous and complex structural configuration of the diamond/titanium interface. For the samples deposited at 650 °C we identified, starting from the titanium surface: a mixed phase constituted by titanium hydride and carbide precipitates, graphitic clusters embedded into a titanium hydride/carbide phase, TiC layers, and finally diamond films. At 730 °C both growth location and formation sequence of the various structures were found to be similar to those detected at 650 °C, resulting, however, in the complete absence of the Ti hydride phase. A first indication about the time scale of the process has been determined from structural investigations of deposits grown by runs lasting from 3 up to 20 min. A schematic model is presented which describes the growth sequence of the various species inside the intermediate reaction layers.
Effective mass of two-dimensional electron gas in δ-doped Al0.48In0.52As/Ga0.47In0.53As quantum wells
Authors: Lo Ikai , Cheng J.-P. , Chen Y. F. , Mitchel W. C.
Published: July 2002

We have studied the lowest two subbands of two-dimensional electron gas in δ-doped AlInAs/GaInAs quantum wells by Shubnikov–de Haas, quantum Hall effect, and cyclotron resonance measurements. The effective masses determined by the field-dependent cyclotron resonance measurements are 0.0576m0 and 0.0483m0 at electron densities of 17.3 and 3.6×1011 cm−2 for the first and second subbands, respectively. It was found that the electron in the first subband has heavier effective mass and shorter quantum lifetime than that in the second subband. Using the band gap of 810 meV and the band-edge mass of 0.042m0 for Ga0.47In0.53As, we calculated the average distance of the two subbands from the conduction band edge to be 150 and 61 meV, respectively.
Studying topography and sub-surface structure of 8CB liquid crystal films with shear-force microscopy
Authors: Williamson R. L. , Miles M. J.
Published: July 2002

Scanning shear-force microscopy, which is usually employed to track scanning near-field optical microscopy probes across samples, was used to observe the sub-surface smectic layer structure through the thickness of a film of the alkyl cyanobiphenyl liquid crystal 8CB on a highly oriented pyrolytic graphite substrate from force-distance measurements. With the noncontact imaging shear-force microscopy supplies, it was possible to follow, in the topography, the movement of a monolayer of the 8CB molecules. This has enabled the diffusion of molecules in a precursor film of 8CB to be observed without significantly disrupting the free liquid surface.
Effects of geometry in pressure–shear and normal plate impact recovery experiments: Three-dimensional finite-element simulation and experimental observation
Authors: Machcha Ashok R. , Nemat-Nasser Sia
Published: July 2002

The minimization of the radial release effects in the recovery configurations of plate impact experiments is essential for accurate postmortem microstructural investigations. The present study evaluates the results of several three-dimensional finite-element simulations involving different plate geometries. The study examines combinations of circular, square, and star-shaped plate geometries, with no lateral momentum traps or guard rings. Experiments and simulations are conducted on brittle specimens. A simple, but fairly successful, combination for the pressure–shear recovery experiment is reported, which makes use of a single square and three circular plates. In the case of the normal impact recovery configuration, it is found that the star-shaped flyer in combination with the square target and momentum trap gives good results at locations away from the axis of the specimen. Crack patterns observed experimentally in the conventional nonrecovery pressure–shear mode, and in recovery pressure–shear and normal impact modes are discussed in relation to the simulation results.
A free boundary model for shape preserving dendritic growth at high undercooling
Authors: Mullis Andrew M.
Published: July 2002

Within the widely quoted Lipton, Kurz, and Trivedi model for the calculation of dendritic growth velocities [Acta Metall. 35, 957 (1987)], the kinetic and Gibbs–Thomson undercoolings evaluated at the dendrite tip are assumed to apply equally over the whole dendrite surface, approximating the nonisothermal dendrite as an isothermal dendrite with a reduced interface temperature. In a previous article [J. Appl. Phys. 78, 4137 (1995)] we described a finite difference model to calculate the growth velocity of a parabolic, nonisothermal dendrite growing into an undercooled melt, and showed that proper consideration of the nonisothermal interface reduced the growth velocity by ≊35%, relative to that predicted by the analytical model. We report an improved computational scheme which solves the free boundary problem for the shape preserving needle dendrite. At all undercoolings we find that the shape preserving needle dendrite is broadened with respect to the Ivantsov paraboloid with the same tip radius. Moreover, the extent of the broadening increases with undercooling. Thus, unlike the Ivantsov solutions, the form of the dimensionless, shape preserving needle dendrite is not invariant with undercooling. Growth velocities for the shape preserving solutions are found to be within 2% of those of our previous nonisothermal model.
Photoelectron spectroscopy studies of Li1+xMn2−xO4 for Li ion battery applications
Authors: Gao Yuan , Richard M. N. , Dahn J. R.
Published: July 2002

Using photoelectron spectroscopy (PES) we have studied the surfaces of Li1+xMn2−xO4 materials and related surface properties with the electrochemical performance of Li/Li1+xMn2−xO4 cells. The surface seems to be more lithium rich than the bulk as the amount of excess lithium is increased. A Li 1s component with binding energy of about 54.5 eV and a Mn 3p component with binding energy of about 47.6 eV are shown to be correlated with the undesirable 3.3 V discharge plateau in Li/Li1+xMn2−xO4 cells. Using a series of samples quenched from different temperatures we have also studied the surface composition at different temperatures. The PES results of a good performing sample, that cycles well even at a temperature as high as 55 °C are also shown. This shows that PES can be useful to qualify good spinel materials for Li-ion battery applications.
Rupture of thin liquid film under the magnetic field
Authors: Hung Chen-I. , Tsai Jung-Shun
Published: July 2002

The nonlinear rupture of a thin liquid film on a horizontal plane is studied by taking into account the effect of a transversely applied uniform magnetic field load. First, a nonlinear evolution equation is derived for film thickness, h(x,t), and then solved numerically. The results show that the time of rupture of a liquid film is increasingly delayed as the magnitude of the magnetic field is increased. The dominant wave number is, however, independent of the magnetic field.
Transmission electron microscopy study of rapid thermally annealed Pd/Ge contacts on In0.53Ga0.47As
Authors: Ressel P. , Österle W. , Urban I. , Dörfel I. , Klein A. , Vogel K. , Kräutle H.
Published: July 2002

Phase formation in rapid thermally annealed Pd/Ge contacts on In0.53Ga0.47As has been investigated by means of cross-sectional transmission electron microscopy, convergent-beam electron diffraction, and energy-dispersive x-ray analysis. Solid-phase regrowth is observed to occur similarly as in Pd/Ge contacts on GaAs or InP. The reaction starts at low temperatures with the formation of an amorphous Pd–In–Ga–As layer, which crystallizes at elevated temperatures yielding hexagonal Pd4In0.53Ga0.47As being first described in this work. At temperatures ≳250 °C, this phase decomposes due to epitaxial solid-phase regrowth of In0.53Ga0.47As and formation of Pd–Ge phases. The stable composition is reached at temperatures ≳350 °C with excess Ge diffused through top Pd–Ge to the contact interface and growing epitaxially on the semiconductor.
Quasiparticle injection-detection experiments in niobium
Authors: Gijsbertsen J. G. , Flokstra J.
Published: July 2002

We present the results of injection-detection experiments that have been performed to study diffusion and losses of quasiparticles in Nb strips employing a series array of Nb/Al junctions. The I–V curve of the detector junction was measured as a function of the spacing between the (high-quality) detector junction and the bias point of the injector. For our sample geometry, the quasiparticle gas is described in terms of a one-dimensional diffusion model. The experiment shows that quasiparticle trapping losses at the interface between the Nb base electrode and the anodized Nb2O5 dominate. In addition, a very low quasiparticle diffusion constant at 1 K, D=1.2×10−4 m2 s−1, has been found. We estimate that this corresponds to an electron mean free path of l≊6.5 nm. Due to the difference between the band gaps of the base and top electrode and to the peak in the density of states at the gap edge, the excess detector current shows a characteristic voltage dependence for bias voltages below 0.5 mV. Theoretical lines for the excess I–V curves are calculated on the basis of a microscopic proximity effect model. All the features observed in the excess I–V curves are well described.
Optical anisotropy of SiGe superlattices
Authors: Engvall Jesper , Olajos Janos , Grimmeiss Hermann G. , Presting Hartmut , Kibbel Horst
Published: July 2002

Optical and electrical properties of SiGe strain-adjusted superlattices have been studied. Diode structures were processed into waveguide geometries to investigate the role of optical confinement and the lowering of cubic symmetry with regards to the polarization properties of interband absorption and emission. The polarization anisotropy of the absorption coefficient suggests that the heavy-hole band of strain-adjusted Si6Ge4 superlattices is the top valence band.
Comment on ‘‘Evidence of enhanced epitaxial crystallization at low temperature by inelastic electronic scattering of mega-electron-volt heavy-ion-beam irradiation’’ [J. Appl. Phys. 79, 682 (1996)]
Authors: Heera V.
Published: July 2002

The results on epitaxial crystallization of Si by mega-electron-volt heavy-ion-beam irradiation recently published by Nakata [J. Appl. Phys. 79, 682 (1996)] can be understood in the framework of the ‘‘point defect diffusion model.’’ It is not necessary to consider inelastic electronic scattering effects in order to explain the decrease in the normalized crystallization rate with increasing nuclear deposited energy.
Perturbative analytical solution of two-wave coupling in photorefractive materials at large modulation depth
Authors: Zhou Zhongxiang , Li Yan , Sun Xiudong , Jiang Yongyuan , Zhao Hua , Xu Kebin , Wan Qiuyu
Published: July 2002

We apply directly a perturbative expansion to the hopping model of Feinberg. The first three harmonics of the space-charge field are presented, considering the contributions from the higher-order terms at large modulation depth. The dependence of the harmonics on spatial frequency, modulation depth and applied field strength are discussed. An analytical solution of the two-wave coupling equations is given in this case. The mixing gain γ is also discussed.
Depth dependence of ion implantation damage in AlxGa1−xAs/GaAs heterostructures
Authors: Turkot B. A. , Lagow B. W. , Robertson I. M. , Forbes D. V. , Coleman J. J. , Rehn L. E. , Baldo P. M.
Published: July 2002

To determine the influence of interface type on the accumulation of damage and ion mixing in GaAs/AlxGa1−xAs heterostructures, the damage produced by ion implantation at 77 K in single-layer (GaAs/AlxGa1−xAs/GaAs) and double-layer (GaAs/AlxGa1−xAs/GaAs/ AlxGa1−xAs/GaAs) heterostructures has been investigated by using a combination of Rutherford backscattering spectrometry and transmission electron microscopy. In the single-layer geometry, the degree of disorder increases with depth and the mixing is greater at the AlxGa1−xAs on GaAs interface than at the GaAs on AlxGa1−xAs one. The damage distribution in the sample with the double-layer geometry was different in the two layers, but overall it was similar to that in the single-layer geometry. These trends were observed in samples with x=0.6 and 0.2. These results indicate that migration of charged defects due to the presence of an implantation-induced electric field is not responsible for the asymmetry in the damage accumulation across the layer, the interface disorder and ion mixing, and the initiation of amorphization at interfaces. Instead, these effects can be better understood in terms of the depth dependence of the density of cascade events.
The diagonal and off-diagonal AC conductivity of two-dimensional electron gases with contactless Corbino geometry in the quantum Hall regime
Authors: Petersen Christian Leth , Hansen Ole Per
Published: July 2002

We have investigated the AC conductivity elements in the quantum Hall regime of two-dimensional electron gases coupled capacitively to electrodes with Corbino geometry. The samples are GaAlAs/GaAs single heterostructures, and the measurements are made at low frequencies, up to 20 kHz. The diagonal conductivity is derived from magnetocapacitance measurements. It increases with increasing frequency according to a power law at integer filling factors. The exponent of the power law depends on both temperature and filling factor. Ratios between Hall conductivities at different filling factors are obtained by inductive measurements of the circulating current. They are found to agree with quantization in multipla of e2/h at the integer filling factors.