bluuebunny/crossref_metadata_2025 huggingface dataset:
Diagnostics of inductively amplified magnetron discharges by optical emission, absorption spectroscopy, and Langmuir probe measurements
Authors: Vanderbecq A.-C. , Wautelet M. , Dauchot J. P. , Hecq M. , Pointu A.-M. , Ricard A.
Published: July 2002

A direct current (dc) magnetron discharge is coupled to an inductively induced radio frequency (rf) one. The rf part is obtained by means of an induction coil, placed parallel to the cathode. We study the effects of the rf induction coil on the properties of an argon discharge. The discharge is characterized experimentally by optical absorption and emission spectroscopy, Langmuir probe measurements and quartz microbalance. The results are analyzed in order to obtain information on the electrons (density, temperature), ions (density), and excited neutrals (density) as a function of the pressure, the dc and rf powers. It is found that the density of the cathode species is proportional to the dc power while the excitation or ionization degree of the species impinging the substrate is dependent on the rf power.
Negative peaks in photocurrent spectra of thick barrier GaAs/AlAs multiple quantum wells
Authors: Kawasaki K. , Imazawa M. , Kawashima K. , Fujiwara K. , Hosoda M. , Tominaga K.
Published: July 2002

We have experimentally studied photocurrent (PC) spectral features of a relatively thick barrier multiple quantum well (MQW) p-i-n diode at 18 and 80 K as a function of axial electric field. It is found that PC spectra do not always reflect the photoabsorption spectral line shape under the low field condition and show negative peaks at the exciton resonance wavelengths. These PC dips are qualitatively explained by considering the distribution of photogenerated carriers within the intrinsic region and competition between carrier transit and recombination times. Furthermore, the dominance of electron-tunneling transport for the low temperature PC mechanism explains enhanced negative peaks in the PC spectra at 80 K when the tunneling assisted drift is reduced.
Efficient photo and electroluminescence of regioregular poly(alkylthiophene)s
Authors: Barta Piotr , Cacialli Franco , Friend Richard H. , Zagórska Małgorzata
Published: July 2002

We report absolute photoluminescence (PL) and electroluminescence quantum efficiencies for thin films and polymer light-emitting diodes prepared with regioregular (that is, exclusively head-to-head/tail-to-tail conformation and/or exclusively head-to-tail conformation) poly(alkylthiophenes). In particular, we find a maximum PL quantum efficiency of ∼11% for the head-to-head/tail-to-tail conformers, an order of magnitude higher than for the head-to-tail conformers. The results show that chemical conformation plays a crucial role in determining the material electronic structure and, hence, the relevant optical properties. The results are significant to the wide field of conjugated, synthetic, light-emitting materials, and hence to the development of large-area organic displays.
Transport properties of gated resonant tunneling diodes in the single electron regime
Authors: Griebel M. , Indlekofer K. M. , Förster A. , Lüth H.
Published: July 2002

To investigate the transport properties of resonant tunneling diodes with dimensions in the submicron range, small area mesa diodes with surrounding Schottky gates have been processed. The gate turns out to provide excellent current control, which makes a resonant tunneling transistor operation mode feasible for our devices. In the single electron regime very distinct staircase-like features are observed in the current voltage characteristics. An accurate analysis of this staircase characteristic by means of magnetotransport measurements shows that tunneling through defect states can be ruled out as a reason for these current steps. Moreover, we show that the current steps are exclusively due to quantization effects of the gate potential. At high magnetic fields a saturation-like behavior of the step onset voltages occurs as a function of a magnetic field applied parallel to the direction of transport. This effect can be explained by boundary conditions for the electron number and the Fermi level in the electron supply layer next to the double barrier structure.
Magneto-optical properties of MnBi doped with Cr
Authors: Sabiryanov R. F. , Jaswal S. S.
Published: July 2002

In their recent Kerr rotation studies Bandaru et al. [P. Bandaru et al., Appl. Phys. Lett. 72, 1 (1998)] were able to lower the Curie temperature of MnBi by doping it with 5–20 at. % Cr thereby decoupling the magnetic phase transition from the structural phase transition at 630 K. This development makes much-studied MnBi a promising candidate for magneto-optic applications. Here we report first-principle electronic structure calculations to study the magnetic and magneto-optical properties of MnBiCr alloys as functions of the Cr concentration. Both interstitial and substitutional occupancies of Cr in MnBi are investigated. The exchange interaction parameters calculated show that Cr substitution for Mn results in antiferromagnetic coupling between Mn and Cr along the c axis but ferromagnetic coupling in the a–b plane. This effectively decreases the Curie temperature of MnBi. The calculated Kerr rotation and ellipticity are in good agreement with the experimental data for Mn1−xCrxBi. The Kerr rotation of MnBi decreases with the substitution of Cr. The decrease is stronger for the “red” peak at 1.96 eV (633 nm) than for the “blue” peak at 3.35 eV (370 nm), in qualitative agreement with the experiment. Interstitial Cr also lowers the Kerr rotation but the occupancy of interstitial sites is expected to be much smaller than that of substitutional sites.
Mössbauer study on the coupling interaction of Fe-based nanocrystalline alloys
Authors: Yang X. L. , Yang J. X. , Jiang K. Y. , Chen G. , Jin H. J. , Zhang Y. Z.
Published: July 2002

A two-phase system of Fe-based nanocrystalline alloys is here investigated. The nanocrystalline Fe69.5Cu0.5Cr4V5Si13B8 alloys, which consist of two magnetic phases: α-FeSi grains and an amorphous matrix, were prepared by annealing the amorphous ribbons. Mössbauer spectroscopy and magnetic measurements at elevated temperatures were carried out. The results show that the exchange coupling interaction exists between grains through the amorphous matrix, and obviously affect the soft magnetic properties of the nanocrystalline alloys. A phenomenological coupling model is applied to estimate the coupling intensity for samples annealed at different temperatures and the data can reasonably explain why the nanocrystalline alloys annealed at 540 °C hold the best soft magnetic properties.
Isotropic hysteresis modeling of Fe–Co–B alloys
Authors: Hauser Hans , Grössinger Roland
Published: July 2002

The energetic model of ferromagnetic hysteresis calculates the magnetic state of materials by minimizing the total energy function for statistical domain behavior. The physical constants of this model are derived from anisotropy energy constants, initial susceptibility, coercivity, and saturation magnetization. The approach shows a good agreement to the magnetization curves of FeCoB strips, also in dependence of applied stress.
Magnetization and magnetocrystalline anisotropy of R2Fe17 intermetallics under pressure
Authors: Kamarád J. , Mikulina O. , Arnold Z. , Garcı́a-Landa B. , Ibarra M. R.
Published: July 2002

The measurements of magnetization and magnetic anisotropy of R2Fe17 (R=Y, Er, Ho, and Gd) single crystals were performed under hydrostatic pressure up to 1 GPa, in temperature range 5–300 K and at magnetic fields up to 5 T, using the SQUID magnetometer with a miniature pressure CuBe cell. A pronounced decrease of spontaneous magnetization M of the R2Fe17 intermetallics has been observed under pressure. The values of dM/dp vary in an interval from −0.32μB/f.u. per kbar (Y2Fe17) up to −0.1μB/f.u. per kbar (Ho2Fe17). The R elements in the R2Fe17 intermetallics have a strong influence on the d(ln mFe)/dp values at 5 K. A remarkable decrease of the anisotropy constant with pressure, d ln K1/dp=−1.7×10−2 kbar−1 at 5 K, was observed on the Y2Fe17 compound. In the Gd2Fe17 compound, |K1(T)| increases with increasing pressure below 150 K and decreases with increasing pressure above this temperature value. The critical field HC(T) of the FOMP in the Er2Fe17 compound exhibits a similar behavior, it increases under pressure at temperatures below 60 K and decreases at higher temperatures.
Metastable states and information propagation in a one-dimensional array of locally coupled bistable cells
Authors: Anantram M. P. , Roychowdhury Vwani P.
Published: July 2002

We study the effect of metastable states on the relaxation process (and hence information propagation) in locally coupled and boundary-driven structures. We first give a general argument to show that metastable states are inevitable even in the simplest of structures, a wire. At finite temperatures, the relaxation mechanism is a thermally assisted random walk. The time required to reach the ground state and its lifetime are determined by the coupling parameters. These time scales are studied in a model based on an array of quantum dots.
Structure and voltage dependence of ferroelectric properties of SrBi2Ta2O9 thin films
Authors: Zhang Z. G. , Liu J. S. , Wang Y. N. , Zhu J. S. , Liu J. L. , Su D. , Shen H. M.
Published: July 2002

Polycrystalline SrBi2Ta2O9 (SBT) ferroelectric thin films were synthesized on Pt/Ti/SiO2/Si substrates by the metalorganic decomposition technique. The SBT crystalline phase appeared at 550 °C, completely crystallized at 650 °C, and little pyrochlore phase was observed in the x-ray diffraction patterns. The grains, which are rodlike, increased from about 50 to 200 nm in diameter with increasing annealing temperature from 550 to 800 °C. Typically, the SBT thin film annealed at 750 °C had Pr=8.8 μC/cm2 and Ec=29.3 kV/cm at applied voltage of 5 V, and the hysteresis loops become saturated with an increase of the maximum applied voltage. The fatigue and retention characteristics of SBT thin films dependence on applied voltage and frequency have also been investigated. It revealed that the fatigue endurance at a higher frequency and a higher applied voltage could be better than that at a lower frequency and a lower applied voltage. The retention properties of the SBT thin films are quite good over a range of 1–10 000 S, and the influence of applied voltage and bias voltage is not obvious.
Direct measurement of surface scattering in giant magnetoresistance spin valves
Authors: Bailey William E. , Fery Christophe , Yamada Kenichiro , Wang Shan X.
Published: July 2002

We present a technique to measure directly the extent of surface scattering in giant magnetoresistance (GMR) spin valves. By monitoring both resistance and magnetoresistance during deposition, we may quantify any discontinuous changes in electron scattering associated with the formation of a given surface. Our technique is applied to the case of noble metal overlayer deposition (Ag and Cu) on NiO/Co/Cu/Co spin valves. We find no evidence to support the establishment of a specularly reflecting surface.
Studies on structural properties of amorphous nitrogenated carbon films from electron energy loss, ellipsometry, Auger electron spectroscopy, and electron-spin resonance
Authors: Bhattacharyya S. , Vallée C. , Cardinaud C. , Chauvet O. , Turban G.
Published: July 2002

We report the variation of the structure and the electronic properties of amorphous nitrogenated carbon films (a-CH:Nx) prepared in dual electron cyclotron resonance-radio-frequency plasma from different mixtures of methane and nitrogen. Electron energy-loss spectroscopy, Auger electron spectroscopy, spectroscopic ellipsometry, and electron-spin-resonance spectroscopy are used to characterize the films. Unlike previous reports, addition of a low percentage (2.3%) of nitrogen in the films induces a strong change in their structure. The variation of electronic properties is rather small for a high concentration of nitrogen. From these experimental studies it seems that the efficiency of nitrogen doping depends on the nitrogen concentration. Modification of structure of the carbon network by nondoping and doping configurations of carbon nitrogen bonds is also emphasized. Our analyses establish an inter-relationship between the structure and electronic properties of nitrogenated carbon films, which helps to understand the structural change occurring in the carbon films with the incorporation of a low amount of nitrogen.
Characteristics of vacuum ultraviolet emission from a surface discharge type alternating current plasma display panel cell
Authors: Jeong Heui Seob , Seo Jeong Hyun , Yoon Cha Keun , Kim Joong Kyun , Whang Ki-Woong
Published: July 2002

We have measured the time variation of 147 nm vacuum ultraviolet (VUV) resonance emission as well as the continuum such as 152 and 173 nm VUV and the two-dimensional images of 147 nm VUV from the He–Xe discharge in a surface discharge type single alternating current (ac) plasma display panel (PDP) cell using an image intensified charge-coupled device camera. A detailed measurement of the resonance radiation exhibits a fast rising to its peak and decay with multiple time constants. The changes in decay time constants, relative intensities of resonance, and dimer radiation change with the total gas pressure as well as the Xe partial pressure implies that collisional excitation, deexcitation, and interlevel transition play important roles in determining the VUV emission characteristics. The two-dimensional images show that a weak, broad VUV emission comes out from the cathode area with a strong, narrow emission from the anode area. We have solved the He–Xe discharge reaction equations together with the two-dimensional multifluid equations for electrons and ions to explain the experimental results and elucidate the dominant kinetic pathway which leads to the VUV emission in the ac PDP cells. The time variations of the averaged Xe*(3P1) resonance atom and Xe2*(0u+,v≫0), and Xe2*(1u,v=0) dimer densities which emit 147, 152, and 173 nm VUV were compared with the measured time variation of each spectral intensities and showed very close qualitative agreements.
Strain dependence of the magnetic properties of nm Fe films on W(100)
Authors: Enders A. , Sander D. , Kirschner J.
Published: July 2002

The thickness dependence of the magneto-elastic coupling B1, the intrinsic film stress, and the magnetic in-plane anisotropy K4 of Fe films on W(100) are measured with an in situ combination of a highly sensitive optical deflection technique with magneto-optical Kerr-effect measurements. We find that both B1 and K4 depend strongly on the Fe film thickness. The thickness dependence of B1 can be described by considering a second order magneto-elastic coupling constant D=1 GJ/m3 as a strain dependent correction of B1. We tentatively ascribe the deviation of K4 from its bulk value to the tetragonal lattice distortion caused by an effective tensile in-plane strain of 5.3% in the pseudomorphic region and of 0.2% in thicker films.
Grain boundaries and growth kinetics of polycrystalline ferrimagnetic oxides with chemical additives
Authors: Cho Yong S. , Hoelzer David T. , Burdick Vernon L. , Amarakoon Vasantha R. W.
Published: July 2002

Grain boundaries and grain growth kinetics of two ferrimagnetic oxides, Y2Gd1Fe5O12 and Li0.3Zn0.4Fe2.3O4, modified by a sol–gel particulate coating process were investigated on the basis of their microstructural characteristics. Interestingly, the addition of small amounts of MnO2 and SiO2 using the sol–gel coating process led to different results for the two magnetic materials. In the case of Y2Gd1Fe5O12, the additives had a role as a grain growth inhibitor because Si-rich precipitates were segregated along the grain boundary and exerted a drag force against grain boundary movement. On the other hand, the same additives acted as an accelerator for grain growth by forming a glassy phase at the grain boundaries for Li0.3Zn0.4Fe2.3O4. The results were analyzed quantitatively by calculating activation energies for grain growth and grain boundary mobilities. For example, the calculated grain boundary mobilities, i.e., 5.6×10−15 m3/N s at 1400 °C for Y2Gd1Fe5O12 and 22.3×10−15 m3/N s at 1150 °C for Li0.3Zn0.4Fe2.3O4, supported the microstructural observation.
Contactless measurement of giant magnetoresistance in CoAg granular films using infrared transmission spectroscopy
Authors: Gester M. , Schlapka A. , Pickford R. A. , Thompson S. M. , Camplin J. P. , Eve J. K. , McCash E. M.
Published: July 2002

A new technique is presented for studying spin dependent transport properties in mesoscopic magnetic structures which exploits the magnetorefractive effect (MRE). A Fourier transform infrared spectrometer was used to measure the MRE of CoAg granular films deposited on thinned Si(001) substrates. Infrared transmission spectra were recorded over a wavelength range from 2.5 to 18.2 μm in an applied magnetic field up to 1.5 kOe. The resulting relative transmission curves have a minimum at approximately 7 μm which deepens if the applied magnetic field is increased. This behavior can be described by model calculations in the self-averaging limit from which the scattering rates can be extracted when fitted to the experimental spectra. The field dependent behavior of the MRE reproduces the magnetoresistance behavior measured using a conventional four-point probe demonstrating the capability of the MRE to study magnetotransport without making electrical contacts.
Domain formation near the reorientation transition in perpendicularly magnetized, ultrathin Fe/Ni bilayer films (invited)
Authors: Arnold C. S. , Pappas D. P. , Venus D.
Published: July 2002

Ultrathin films with perpendicular magnetization convert from a single domain state into a multidomain structure as the reorientation phase transition to an in-plane magnetization is approached. Reorientation transitions in magnetic ultrathin films result from the interplay of interfacial magnetic anisotropy, the dipolar interaction, and two-dimensional thermodynamics. These transitions can be driven by changing either the film thickness or temperature. Experimental and theoretical studies of this effect are briefly discussed in the context of the thickness–temperature phase diagram of the reorientation transition. We then describe magnetic susceptibility experiments on ultrathin Fe/Ni(111) bilayers. Our experiments indicate an exponential increase in domain density of a multidomain structure with temperature and identify the region of the thickness–temperature reorientation transition phase diagram where this condensation is most pronounced. The temperature dependence of the domain density agrees quantitatively with theoretical predictions. Films that are slightly too thin to exhibit the reorientation transition with temperature are a special case. They undergo a ferromagnetic-to-paramagnetic transition from the perpendicularly magnetized state and exhibit domain-like behavior many tens of Kelvin above estimates of the Curie temperature. This surprising observation is interpreted using the two-dimensional dipolar Ising model.
Activation entropy, activation energy, and magnetic viscosity
Authors: Skomski R. , Kirby R. D. , Sellmyer D. J.
Published: July 2002

Starting from an exact quantum-statistical description, the influence of the shape of the energy landscape on the magnetic viscosity is investigated. Magnetic phase-space analysis based on Kramers’ escape-rate theory of chemical reaction kinetics theory shows that the activation entropy associated with thermally activated hopping modifies the magnetic viscosity by reducing the attempt-frequency prefactor compared to an earlier prediction by Brown [W. F. Brown, Phys. Rev. 130, 1677 (1963)]. Energetic contributions are analyzed in terms of a model applicable to a range of coherent and noncoherent magnetization processes, and in the long-time limit deviations from the linear logarithmic magnetic-viscosity law are found.
Controlling chaos of nonlinear domain-wall motion
Authors: Okuno H. , Sakata T. , Takeda H.
Published: July 2002

The two-step Ott–Grebogi–Yorke (OGY) method and the prediction OGY method for controlling chaos of magnetic domain-wall motion are proposed to improve the long settling time in the original OGY method. In the two-step OGY method, a magnetic domain wall is first moved on a periodic orbit and the OGY method is used when the orbit approaches a saddle point. In the prediction OGY method, the motion of the domain wall is predicted before the OGY method is applied. An attractor in the state space can be reconstructed by using the time series of the domain-wall motion. The near future can be predicted even in the chaotic system, because the short time developments of the neighborhood system of a predictee in the attractor are not so different from each other. The settling time of the improved OGY methods is 1/5–1/30 times as long as that of the original OGY method.
Magnetic and magneto-optical properties of TbFeCo/(Pt, Pd) multilayers optimized for short wavelength recording
Authors: Itoh Y. , Suzuki T. , Birukawa M. , Weissenrieder J.
Published: July 2002

Magnetic and magneto-optical properties of TbFeCo/Pt and TbFeCo/Pd multilayer films prepared by sputter deposition are discussed in conjunction with the multilayer structure. The magnetic behaviors in TbFeCo/Pt and TbFeCo/Pd multilayers are very similar to each other, and the saturation magnetization Ms at 298 K increases with thickness t of Pt and Pd up to tPt, tPd=12 Å. In TbFeCo/Pt multilayers, the contribution of Pt to the polar Kerr spectra is significant at ultraviolet photon energies. The Kerr rotation θK and ellipticity ηK are enhanced with Pt layer thickness and become saturated at tPt=12 Å. On the other hand, little contribution from Pd atoms is found in the TbFeCo/Pd multilayer.
Disorder effects in the one-dimensional Anderson lattice model
Authors: Chen Feng , Kioussis Nicholas
Published: July 2002

We have studied the one-dimensional periodic, symmetric Anderson model at half filling in the presence of disorder using finite-temperature quantum Monte Carlo techniques. We have considered the disorder of both the hybridization between the local f-orbitals and the conduction electrons and the local f-site energy, using a uniform distribution of width Δ. The f-orbital local magnetic moment, the static magnetic susceptibility, the charge susceptibility, and the nearest-neighber magnetic correlation function have been calculated as a function of the disorder distribution width Δ. We find that the hybridization disorder has a dramatical effect on the low-temperature magnetic properties giving rise to a non-Fermi liquid behavior, and that the magnetic susceptibility can be scaled by a power law with an exponent which is in excellent agreement with experiment. On the other hand, disorder of the local f-orbital energy level does not show a non-Fermi liquid behavior.
Magnetic and Mössbauer spectral study of mixed valent La0.2Sr0.8Cu0.2Fe0.8O2.68 and related perovskites
Authors: Long Gary J. , Hautot Dimitri , Mohan Amitabh , Kaus Ingeborg , Anderson Harlan U. , Grandjean F.
Published: July 2002

The La0.2Sr0.8Cu1−xFexO3−δ perovskites are important because they are mixed electron and oxygen anion conductors and thus are able to serve as dense oxygen separation membranes. In order to better understand the mechanism for this separation a Mössbauer spectral study has been carried out to determine the ratio of iron(III) to iron(IV) in these mixed valent compounds. The Mössbauer spectra of La0.2Sr0.8Cu0.2Fe0.8O3−δ obtained between 4.2 and 78 K reveal that its Néel temperature is ∼75 K and the two well-resolved magnetic sextets observed between 4.2 and 45 K indicate the presence of trapped valence iron(III) and iron(IV). At 4.2 K the respective hyperfine fields are 454 and 299 kOe and the isomer shifts are 0.388 and 0.057 mm/s. The iron(III) to iron(IV) ratio is 1.20, a ratio which yields a δ value of 0.32 and, if one assumes the presence of only copper(II), a corresponding stoichiometry of La0.2Sr0.8Cu0.2Fe0.8O2.68.
Test of unified picture of spin-dependent transport in perpendicular (CPP) giant magnetoresistance and bulk alloys
Authors: Park W. , Loloee R. , Caballero J. A. , Pratt W. P. , Schroeder P. A. , Bass J. , Fert A. , Vouille C.
Published: July 2002

Measurements of current perpendicular (CPP) magnetoresistance (MR) on hybrid and exchange-biased spin valves allow determination of the layer anisotropy parameter β for ferromagnetic alloys, thus allowing testing of whether this CPP β is similar to values obtained from direct measurements on, and theoretical estimates for, dilute bulk ferromagnetic alloys. Of special interest are alloys where β is expected to be negative. In this article, we derive a value of β for a Ni97Cr3 alloy using such spin valves based upon permalloy. We confirm prior CPP-MR results that this β is negative, but our inferred value, β=−0.35±0.1, is closer to bulk and theoretical estimates. Our data thus provide new support for a unified picture of β in the CPP-MR and bulk alloys. We also derive a short spin-diffusion length in NiCr, lsf=3±1 nm.
Structure and composition of GaN(0001) A and B surfaces
Authors: Held R. , Nowak G. , Ishaug B. E. , Seutter S. M. , Parkhomovsky A. , Dabiran A. M. , Cohen P. I. , Grzegory I. , Porowski S.
Published: July 2002

Homoepitaxial, GaN films on both c-plane surfaces of bulk GaN crystals were examined using reflection high-energy electron diffraction (RHEED). Differences in the RHEED pattern, time development of the RHEED intensity, and surface reconstructions were observed. The substrate surfaces were prepared either by mechanical polishing [GaN(0001)A] or by chemo-mechanically polishing [GaN(0001̄)B]. Then films were grown by molecular beam epitaxy; Ga was provide by a Knudsen cell and nitrogen from NH3. On the B surface, the Ga rich reconstructions reported by Smith and co-workers [Phys. Rev. Lett. 79, 3934 (1997)] were observed. On the A surface, a (2×2) reconstruction was observed. Both reconstructions were much sharper than those seen on GaN films grown on sapphire. RHEED measurements of the specular intensity vs time showed that two different surface terminations could be maintained on the B surface, one of which is a stable, gallided surface, while the other is a nitrided surface, which is unstable in vacuum. If the nitrided surface is heated in vacuum it changes to the gallided surface in several minutes at 800 °C. Only one termination was detected on the A surface. The results are complemented by desorption mass spectroscopy measurements, and the resulting surfaces were then investigated using atomic force microscopy and scanning tunneling microscopy. We were able to distinguish the two surface terminations on the B surface, and a unique annealing process under NH3 will be documented. Preliminary investigation of the A surface revealed decorated step edges. The results were compared to films grown on sapphire with different nucleation layers, which can be grown to yield either polarity.
Evaluation of the energy transfer rate for the Yb3+:Pr3+ system in lead fluoroindogallate glasses
Authors: Pecoraro E. , de Sousa D. F. , Lebullenger R. , Hernandes A. C. , Nunes L. A. O.
Published: July 2002

In this article, we report on spectroscopic studies of Pr3+:Yb3+ codoped lead fluoroindogallate glasses. By pumping with a diode laser in 0.98 μm, it was verified that the intensity of infrared luminescence at 1.3 μm of Pr3+ (1G4→3H5) is highly increased (approximately 40 times) by the presence of Yb3+. Anti-Stokes luminescence (upconversion), which represents a loss mechanism for the infrared emission, was also observed. These features were attributed to an efficient energy transfer process from Yb3+ to Pr3+. Calculation of the energy transfer constant (Cs−a) and energy transfer rate (Ps−a) showed that the process from Yb3+ to Pr3+ is nearly one order of magnitude greater than in the opposite direction. The energy transfer efficiency obtained was about 92%. The best molar ratio Pr3+:Yb3+ is 1:4 for both infrared and upconverted luminescence signals.
Improvement of electrical properties of epitaxial SrTiO3 films on Si(001)-2×1 by in situ annealing
Authors: Tambo T. , Maeda K. , Shimizu A. , Tatsuyama C.
Published: July 2002

We have studied the formation of a high-quality SrTiO3 (STO) film on a Si surface which is an appropriate buffer film for fabricating high-Tc superconductor devices on Si by molecular beam epitaxy. The STO films with thicknesses of 1300–6700 Å are grown on a SrO buffer layer with a thickness of 100 Å on Si(001)-2×1 in ∼10−8 Torr. The growth temperatures for the STO films and SrO layer are 500 and 400 °C, respectively. The as-grown STO films are insufficiently oxidized regarding the Ti–O bond which is confirmed by the observation of x-ray photoemission spectroscopy, although in situ reflection high-energy electron diffraction spectroscopy and ex situ x-ray diffraction (XRD) reveal a high-quality crystalline structure. The in situ postannealing for the as-grown STO films is performed at 500–900 °C for 60 min in an oxygen atmosphere of 1×10−6 Torr. The heat treatment promotes the oxidation of STO films and results in a high resistivity of 109–1011 Ω cm and a dielectric constant of 130 at 100 kHz at room temperature. The films consist of large grains with 150–200 nm diameter on the surface in the image of atomic force microscopy. The heat treatment does not prominently affect the crystallinity of STO films in the XRD patterns.
Band gap state formation in InP (110) induced by 10 and 100 eV argon ion bombardment
Authors: Deng Z. W. , Kwok R. W. M. , Lau W. M. , Cao L. L.
Published: July 2002

The dynamical process of surface band bending induced by ion bombardment as a function of ion fluence and energy has been studied with a special low energy ion beam system and an x-ray photoelectron spectrometer. It was found that 10 and 100 eV Ar+ bombardment of n- and p-InP (110) samples, which were prepared in ultrahigh vacuum by in situ cleavage of InP (100) bars, moved their surface Fermi levels to a common pinning position 0.95 eV above the valence-band maximum. The observed surface band bending was attributed to the displacement damage in the near-surface region induced by the ion bombardment. A quantitative analysis of the band bending data as a function of ion fluence showed that for the 10 eV Ar+ bombardment the formation of ionized surface states at a density of about 1×1012/cm2 in the band gap of InP was induced by an ion fluence of 5×1016/cm2. As expected, an increase of bombardment energy from 10 to 100 eV greatly raised the formation probability of surface defects. The results imply that very brief plasma exposure is sufficient to cause surface damage to a semiconductor that is severe enough to affect the device performance of the semiconductor.
Time dependent interface stability during rapid solidification
Authors: Greven K. , Ludwig A. , Sahm P. R.
Published: July 2002

The destabilization of a planar solid/liquid interface during resolidification of the molten surface layer of Si–Sn alloys is investigated. Experimental data are compared with the results of a transient numerical stability analysis. In that analysis the time evolution of an infinitesimal sinusoidal perturbation of a planar interface is investigated. For this the concentration profile in the melt as well as the temperature distributions in the melt and solid are divided into a base state and a state due to the perturbation. The highly nonuniform Sn profiles implanted in (100) silicon samples serve as initial conditions for the numerical calculations. Deviations from thermal equilibrium at the solid/liquid interface are considered. The critical concentration for breakdown is calculated as a function of solidification velocity and compared with the results of the analytical analysis of Huntley and Davis [Acta Metall. Mater. 41, 2025 (1993)]. It is further investigated whether an analytical steady-state stability analysis can be used to approximately describe the experiments considered.
Charge distribution function of plasma dust particles with secondary electron emission
Authors: Gordiets B. F. , Ferreira C. M.
Published: July 2002

The charge distribution function (CDF) of dust particles in low temperature plasmas is obtained as a function of both negative and positive discrete charges Z=∓1,∓2;,…, and various plasma parameters. Positively charged grains can be formed when secondary electron emission, that is, ionization of the grains by electron impact, is sufficiently important to change their charge. An expression for the CDF is derived from steady-state master equations for the densities of monodispersed, negatively or positively charged dust particles, with Z=∓1,∓2,… discrete elementary charges and radius R, taking into account single and double elementary charge changes in collisions of the particle with plasma electrons or ions.
Gettering of Cu by microcavities in bonded/ion-cut silicon-on-insulator and separation by implantation of oxygen
Authors: Zhang Miao , Zeng Xuchu , Chu Paul K. , Scholz R. , Lin Chenglu
Published: July 2002

Microcavities formed by H+ and He+ implantation and subsequent annealing are effective gettering sites for transition metal impurities in silicon. However, gettering in silicon-on-insulator (SOI) materials is quite different from that in silicon. In this work, we investigate the gettering of Cu to these microcavities in silicon, separation by implantation of oxygen (SIMOX) and bonded/ion-cut SOI wafers. Our data indicate that He+ implantation in the high dose regime (0.2–1×1017 cm−2) creates a wide band of microcavities near the projected range without causing blistering on the sample surface. On the other hand, the implantation dose of H+ needed for stable microcavity formation is relatively narrow (3–4×1016 cm−2), and this value is related to the projected range. The different behavior of H and He in silicon is discussed and He implantation is more desirable with regard to impurity gettering. Cu is implanted into the surface region of the Si and SOI samples, followed by annealing at 700 and 1000 °C. Our results indicate that the microcavities can effectively getter a high dose of Cu (2.5×1015 cm−2) at 700 °C in bulk Si wafer, but higher temperature annealing is needed for the effective gettering in SIMOX. Gettering of Cu by the intrinsic defects at or beneath the buried oxide interface of the SIMOX is observed at 700 °C, but no trapped impurities are observed after 1000 °C annealing in the samples in the presence of microcavities. Almost all of the 1×1014 cm−2 Cu implanted into the Si overlayer of the bonded/ion-cut SOI diffuse through the thermally grown oxide layer and are captured by the cavities in the substrate after annealing at 1000 °C.
Investigation of nanostructures in ordinary Portland cement through positron annihilation lifetime spectroscopy
Authors: Consolati G. , Dotelli G. , Quasso F.
Published: July 2002

Early hydration (up to four weeks) of five ordinary Portland cement pastes with different water-to-cement ratios (0.3, 0.4, 0.5, 0.6, and 1.0 wt) was investigated through positron annihilation lifetime spectroscopy. Measurements were performed at four different hydration times (1 day and 1, 2, and 4 weeks) and compared with results obtained from thermogravimetric analyses. Deconvolution of positron spectra allowed us to separate orthopositronium annihilations in the evacuated gel pores from those in the pores containing water, and therefore, to monitor the pores’ evolution with aging time. It is found that the concentration of gel pores increases with aging time, as well as by increasing the content of water in the paste; on the other hand, the typical sizes of the pores do not show significant variations, either with aging time or with the water-to-cement ratio.
Characterization of the local mechanical stress induced during the Ti and Co/Ti salicidation in sub-0.25 μm technologies
Authors: Steegen An , De Wolf Ingrid , Maex Karen
Published: July 2002

In this article, micro-Raman spectroscopy measurements of the local mechanical stress in the silicon substrate induced by the Ti and Co/Ti salicidation were combined with simulations by finite element modeling down to 0.1 μm technologies. It is shown that the scaling possibilities of TiSi2 and of CoSi2 will both be hampered by the stress induced during the salicidation and will result in the generation of 60° dislocations in the silicon substrate. These experiments prove that, because of the difference in material properties between TiSi2 and CoSi2, a higher stress is generated in the silicon substrate during the Ti salicidation than during the low stress Co/Ti-salicidation process. Therefore, the mechanical characteristics related to the silicide formation become a critical parameter in the optimization of the silicide process.
Raman study of the topology of InAs/GaAs self-assembled quantum dots
Authors: Zanelatto G. , Pusep Yu. A. , Moshegov N. T. , Toropov A. I. , Basmaji P. , Galzerani J.C.
Published: July 2002

The topology of self-assembled InAs/GaAs quantum dots was studied by resonant Raman scattering caused by the interface modes localized near the edges of the dots. Evidences were found that on both sides of the InAs layer containing the dots, their topologies show some resemblances. In addition, in the multilayered systems the evidence of the coalescence of the dots (which form vertical columns) in neighbor layers separated by the distance smaller than 25 monolayers was obtained.
Asymmetry in the hysteresis loop of Pb(Zr0.53Ti0.47)O3/SiO2/Si structures
Authors: Lin Y. , Zhao B. R. , Peng H. B. , Hao Z. , Xu B. , Zhao Z. X. , Chen J. S.
Published: July 2002

The integrated ferroelectric/semiconductor systems, Pb(Zr0.53Ti0.47)O3 (PZT)/SiO2/Si have been prepared and investigated. It was found that an asymmetry related to the conduction type of the substrate (Si) and the polarization direction of PZT exists in the polarization–voltage loops of the integrated system. According to the configuration and the characteristic of the integrated structure, we suggest that the space charge layer in Si and the tunneling process between Si and PZT during the polarization are the main causes for this asymmetry.
Inhomogeneity in local magnetic field due to susceptibility contrast
Authors: Sen Pabitra N. , Axelrod Scott
Published: July 2002

A method for computing local magnetic field for the case of small permeability or susceptibility contrast is described. The method is illustrated by using an example of a periodic array of cylinders which affords the simplest yet a representative case for visualizing the local fields. The cylinders represent material of one susceptibility such as solid grains in a rock matrix (or tissues in biological systems) and the space outside represents material of another susceptibility such as fluid which fills the pore space (or fluids outside tissues). We calculate the position dependent correction from the continuum to the local field inhomogeneity and go beyond the standard uniform field term. A simple (separate) illustration when the local field diverges is given.
Mechanical modulation of sample magnetization in magnetic resonance force microscopy
Authors: Marohn John A. , Fainchtein Raúl , Smith Doran D.
Published: July 2002

We have developed a mechanical method for modulating sample magnetization in magnetic resonance force microscopy experiments. A small-amplitude, low-frequency dithering of a magnetic field gradient source relative to a sample is used to create an oscillating magnetic field at the sample, which leads to a modulation of magnetization in rf-irradiated samples by magnetic resonance. Using this technique in concert with modulating rf power, we have achieved a 10 000× decrease in a sensitivity-limiting spurious response of microcantilevers to rf fields in sample-on-cantilever magnetic resonance force microscopy experiments. This mechanical implementation of “anharmonic modulation” is especially well suited for low power and vacuum applications, and is unique in its ability to modulate sample magnetization in magnet-on-cantilever experiments where the cantilever frequency is found to be a strong function of magnetic field.
Thermoelectroelastic moduli of textured piezoelectric polycrystals: Exact solutions and bounds for film textures
Authors: Li Jiang Yu , Dunn Martin L. , Ledbetter Hassel
Published: July 2002

Conditions are given for the existence of exact solutions for the effective thermal and electroelastic moduli of polycrystals exhibiting fiber texture. The exact solutions are then developed through the consideration of a class of uniform fields in a polycrystal acted upon by various external thermal and electroelastic loadings. These exact results are verified by direct orientational averaging that shows the coincidence of the Voigt and Reuss bounds on the overall moduli.
Analytical solutions of film planarization for periodic features
Authors: Wu Pei-Ying , Chou Fu-Chu , Gong Shih-Chin
Published: July 2002

Analytical solutions of film profiles and degree of planarization for periodic features during the spin coating process are presented. This article shows that the degree of planarization is independent of Ω2 and changes slightly with the dimensionless line spacing α for Ω2>300. The governing dimensionless parameter Ω2 represents the ratio of centrifugal force to surface tension force during spin coating. In the cases of low Ω2 (Ω2<1), the degree of planarization decreases with increasing α for a fixed Ω2.
Sb mediated C60 film growth on mica, Si, and glass substrates
Authors: Xu Wentao , Hou J. G.
Published: July 2002

The structure and growth morphology of Sb mediated C60 films, prepared by predepositing a layer of surfactant Sb over mica(001), Si(111), and glass substrates with the vacuum vapor deposition method, were studied along with pristine C60 films by an atomic force microscope. It was found that the growth behavior of C60 films was greatly affected by the substrate temperature. On the mica substrate, the average size of C60 crystal grains in Sb mediated C60 films was about two times larger than that in pristine C60 films. On Si and glass substrates, the predeposited Sb had no significant effect on the size of the C60 crystal grains, but made C60 films grow more easily especially at high substrate temperatures. Possible growth mechanisms of the Sb mediated C60 films are proposed.
Influence of ion-beam energy and substrate temperature on the synthesis of carbon nitride thin films by nitrogen-ion-assisted pulsed laser deposition
Authors: Lu Y. F. , Ren Z. M. , Chong T. C. , Cheong B. A. , Pang S. I. , Wang J. P. , Li K.
Published: July 2002

Carbon nitride thin films were deposited on silicon wafers by pulsed KrF excimer laser (wavelength 248 nm, duration 23 ns) ablation of graphite with assistance of nitrogen ion beam bombardment. X-ray photoelectron spectroscopy, Raman spectroscopy, and ellipsometry were used to identify the binding structure, nitrogen content, and optical properties of the deposited thin films. The influence of the nitrogen ion beam energy on the compositional, electronic, and optical properties of the deposited thin films was investigated. The thin films deposited with nitrogen ion bombardment had an N/C ratio of 0.43. Raman spectroscopy measurements indicated the formation of CN triple bonds in the deposited thin films. The optical band gap Eopt was observed to decrease with the nitrogen ion energy. A nitrogen ion energy between 50 and 100 eV was deduced to be the optimal condition for depositing the carbon nitride thin films.
Effects of the microstructure on the magnetotransport properties of polycrystalline manganite films grown by metalorganic chemical vapor deposition
Authors: Dubourdieu C. , Audier M. , Sénateur J. P. , Pierre J.
Published: July 2002

The effects of the microstructure on the magnetoresistance of polycrystalline manganite (La1−xSrxMnO3) thin films have been investigated. The films have been grown by metalorganic chemical vapor deposition on silicon (001) substrates covered with a native silica layer. Different microstructures were obtained by varying the substrate temperature in the range (550–750 °C). The films exhibit a ferromagnetic/paramagnetic transition around 325 K. The grain size is in the range 20–100 nm and, for deposition temperature lower than 650 °C, weak links appear in the films as the grains are mostly noncohesive. The low-field contribution to the magnetoresistance is independent of the grain size in the studied range. A factor of ∼1.8 is gained in this contribution as the substrate temperature is lowered below 650 °C, which can be related to the occurrence of noncohesive grains in the films. The largest low-field magnetoresistance obtained is about 20% at 22 K (under 0.2 T). The high-field contribution to the total magnetoresistance appears to be independent of the microstructure of the films. Our results are discussed in the framework of spin-dependent transport mechanisms (tunneling or scattering). Finally, the low-temperature (T⩽20 K) resistivity upturn observed for all the polycrystalline films is discussed. It can be attributed to activated transport phenomena (Coulomb blockade tunneling or tunneling through impurities, with activation energies on the order of 0.05 meV) or to localized states in the material.
A scalar variational method for resonant modes of oxide-apertured vertical-cavity surface-emitting lasers
Authors: Chang K. Y. , Woodhead J. , Roberts J. , Robson P. N.
Published: July 2002

A scalar variational method for calculating the mode sizes and resonant wavelengths of the eigenmodes in oxide-apertured vertical-cavity surface-emitting lasers is presented. This method uses Hermite–Gaussian (TEMmp) functions to approximate the transverse-field variations of the eigenmodes. It is applicable to devices with circular, square, or rectangular apertures. Calculated results for the four lowest-order modes show that aperture size, thickness, and axial position are important factors in determining the blueshift in resonant wavelengths, wavelength separation between eigenmodes, and mode sizes. When the geometric symmetry is broken, as in rectangular-apertured devices, the degeneracy between the TEM01 and TEM10 is also lifted. For validation of the model presented, three vertical-cavity surface-emitting laser structures with different oxide thicknesses were grown and fabricated for this study. The resonant wavelength characteristics of near-square and rectangular devices were assessed. The theoretical results are found to agree well with the experimental observations.
Structural characterization of oxidized allotaxially grown CoSi2 layers by x-ray scattering
Authors: Kaendler I. D. , Seeck O. H. , Schlomka J.-P. , Tolan M. , Press W. , Stettner J. , Kappius L. , Dieker C. , Mantl S.
Published: July 2002

A series of buried CoSi2 layers prepared by a modified molecular beam epitaxy process (allotaxy) and a subsequent wet-oxidation process was investigated by x-ray scattering. The oxidation time which determines the depth in which the CoSi2 layers are located within the Si substrates has been varied during the preparation. The electron density profiles and the structure of the interfaces were extracted from specular reflectivity and diffuse scattering measurements. Crystal truncation rod investigations yielded the structure on an atomic level (crystalline quality). It turns out that the roughness of the CoSi2 layers increases drastically with increasing oxidation time, i.e., with increasing depth of the buried layers. Furthermore, the x-ray data reveal that the oxidation growth process is diffusion limited.