bluuebunny/crossref_metadata_2025 huggingface dataset:
Origin of F spots and stress sensitivity in lanthanum lead zirconate titanate
Authors: Viehland D. , Xu Z. , Payne D. A.
Published: July 2002

In this study a systematic microstructural characterization of La-modified lead zirconate titanate was carried out by hot- and cold-stage transmission electron microscopy and selected area electron diffraction. The existence of 1/2[111] superlattice reflections (F spots) became evident near 300 °C on cooling. The intensity of the F spots was found to increase with decreasing temperature, approaching a near constant intensity below 100 °C. In conjunction with compositional studies, it was proposed that the F spots arise from a doubling of the unit cell associated with a tilting of oxygen octahedra. The presence of F spots at temperatures approaching the onset of local polarization was then interpreted as the existence of a hierarchy of domain states. This hierarchy was proposed to consist of an average-cubic macroscopic structure containing rhombohedral micropolar regions on the scale of 50–100 Å that contain ferroelastic microtwins on the scale of several tens of angstroms.
Coercivity of Sm-Fe-N ferromagnets produced by the mechanical alloying technique
Authors: Kou X. C. , Qiang W. J. , Kronmüller H. , Schultz L.
Published: July 2002

Sm-Fe-N permanent magnets have been prepared by mechanical alloying of Sm and Fe powder and subsequently treating the material in a two-step process. Measurements of the minor hysteresis loops and of the initial magnetization curve were performed at room temperature. The reversible susceptibility (χrev), the total susceptibility (χtot) of the initial magnetization curve and of the demagnetization curve, and the temperature dependence of the coercive field (Hc) of a magnetically isotropic nanocrystalline Sm-Fe-N magnet have been measured up to a field strength of 6.4 MA/m. The anisotropy constants, K1 and K2, of Sm2Fe17Nx were deduced from the crystalline electric field calculation in the temperature range from 0 to 500 K. The critical diameter of single domain Sm-Fe-N particles was calculated to be 320 nm at room temperature. This leads to the conclusion that mechanically alloyed Sm-Fe-N is composed of single-domain particles. The temperature dependence of the coercive field of Sm-Fe-N has been analyzed within the framework of the micromagnetic models. It is concluded from these results that the nucleation process controls the coercivity mechanism of mechanically alloyed Sm-Fe-N material. The thermostability of the Sm-Fe-N magnet has also been tested.
Structural properties of InN films grown on sapphire substrates by microwave-excited metalorganic vapor-phase epitaxy
Authors: Guo Qi-Xin , Yamamura Toshimi , Yoshida Akira , Itoh Nobuo
Published: July 2002

The structural properties of InN films grown on (0001) sapphire (α-Al2O3) substrates are investigated by reflection high-energy electron-diffraction and x-ray-diffraction methods. The epitaxial relationship between the InN layer and the α-Al2O3 substrate is revealed to be (0001)InN //(0001)α-Al2O3 with [101̄0]InN//[101̄0] α-Al2O3, which is different from that of AlN and GaN epitaxial films. The x-ray rocking curve measurements prove that the crystalline quality of as-grown InN films is not so good, while it can be significantly improved by thermal annealing at temperatures of 450–500 °C within 10 min in nitrogen ambient. The improvement of quality is ascribed to the rearrangement of mosaic crystallites in the films through the movement of their boundaries during a short period of time.
Orientational dependence of the exchange biasing in molecular-beam-epitaxy-grown Ni80Fe20/Fe50Mn50 bilayers (invited)
Authors: Jungblut R. , Coehoorn R. , Johnson M. T. , aan de Stegge J. , Reinders A.
Published: July 2002

The exchange biasing field (Heb) and coercive field (Hc) of molecular-beam-epitaxy-grown Cu/Ni80Fe20/Fe50Mn50 samples in [111], [001], and [110] orientations have been investigated by longitudinal Kerr effect measurements. Ni80Fe20 and Fe50Mn50 were deposited as orthogonal wedge-shaped layers on single-crystal Cu substrates in a magnetic field, enabling the study of the thickness dependence of Heb and Hc on a single sample for each orientation. A strong dependence of Heb and Hc on the growth orientation is observed. The results are interpreted in terms of the observed noncollinear spin structure of the antiferromagnet and a comparison is given with the predictions from recent theoretical models.
Optical quenching of bound excitons in CdTe and Cd1−xZnxTe alloys: A technique to measure copper concentration
Authors: Lee Jaesun , Myers T. H. , Giles N. C. , Dean B. E. , Johnson C. J.
Published: July 2002

A noncontact, nondestructive optical technique with potential to measure the acceptor impurity concentrations in bulk CdTe and Cd1−xZnxTe alloys is described here. The technique, called optical quenching, uses an infrared light source in conjunction with the above-band-gap excitation beam used in standard photoluminescence spectroscopy. The quenching, or decrease, of acceptor-bound exciton emission is related to the incident infrared beam flux. The rate at which the infrared photons ionize neutral acceptors can be used to determine relative concentrations of the substitutional centers. We have applied this technique to the copper acceptor-bound exciton in CdTe and Cd1−xZnxTe bulk samples. The 1.064 μm output from a Nd:YAG laser provided near-resonant absorption with the copper acceptor level. Relative copper concentrations in the range from 4×1014 to 1×1016 cm−3 were obtained. These values agree reasonably well with sample comparisons based on atomic absorption data.
Optimization of two dimensional gratings for very long wavelength quantum well infrared photodetectors
Authors: Sarusi G. , Levine B. F. , Pearton S. J. , Bandara K. M. S. , Leibenguth R. E. , Andersson J. Y.
Published: July 2002

We have performed a detailed study of two-dimensional grating coupling for quantum well infrared photodetectors in the very long wavelength spectral region λ∼16–17 μm. Using calculations based on the modal expansion method we quantitatively explain the double peaked responsivity spectrum. By optimizing the grating parameters we achieve a normal incidence responsivity and detectivity which are three times larger than the 45° angle of incidence geometry.
Heteroepitaxy and characterization of CuGaSe2 layers grown by low-pressure metalorganic chemical-vapor deposition
Authors: Chichibu S. , Harada Y. , Uchida M. , Wakiyama T. , Matsumoto S. , Shirakata S. , Isomura S. , Higuchi H.
Published: July 2002

CuGaSe2 chalcopyrite compounds were grown heteroepitaxially on both GaAs and GaP substrates by means of the low-pressure metalorganic chemical-vapor deposition method. Optical and structural properties were characterized comprehensively by photoreflectance (PR), photoluminescence (PL), x-ray diffraction, transmission electron microscopy, transmission electron diffraction, and electron-probe microanalysis. The CuGaSe2 epilayers had c(001) surface on GaAs(001) substrates and a(100) surface on GaP(001) substrates, respectively, the results being similar to the case of CuAlSe2. Energies of A, B, and C excitons associated with uppermost valence bands were determined from analysis of PR spectra, and the energies of good-quality epilayers are close to those of the bulk crystal. The slight increase of the crystal-field splitting in the valence bands were discussed in terms of the lattice strain in the epilayer caused by the lattice mismatch. Low-temperature PL spectra exhibited an intense peak at 1.71 eV, the energy being in good agreement with the A-exciton energy. A weak peak due to a free-to-acceptor transition was also observed at 1.66 eV. A broad PL peak at 1.76 eV was observed together with the intense band-edge PL at 1.67 eV, and the peak was assigned to relate to the B-exciton transition.
Low-temperature synthesis of silicon oxide, oxynitride, and nitride films by pulsed excimer laser ablation
Authors: Fogarassy E. , Fuchs C. , Slaoui A. , de Unamuno S. , Stoquert J. P. , Marine W. , Lang B.
Published: July 2002

Silicon oxide, oxynitride, and nitride films are deposited, at low temperature (≤450 °C) by pulsed ArF excimer laser ablation from silicon, silicon monoxide, fused silica, and silicon nitride targets, performed under vacuum and in an oxygen atmosphere. The specific influence of laser fluence, target materials, substrate temperature, and oxygen pressure on the composition and final properties of SiOxNy grown layers is investigated using various complementary experiments such as infrared optical absorption, Rutherford backscattering, Auger electron spectroscopy, ellipsometry, and scanning electron microscopy. The process conditions are optimized in order to deposit good quality silicon oxide and silicon nitride thin films.
Diminution of the surface states on GaAs by a sulfur treatment
Authors: Oh Y. T. , Byun S. C. , Lee B. R. , Kang T. W. , Hong C. Y. , Park S. B. , Lee H. K. , Kim T. W.
Published: July 2002

The surface chemical properties of a GaAs layer grown by molecular beam epitaxy were investigated by photoluminescence (PL) and photoreflectance (PR) measurements. While the intensity of the PL spectra for the sulfur-treated GaAs, using a (NH4)2Sx solution, increased 75 times compared to that for the as-grown GaAs, the peaks for the as-grown GaAs measured by PR vanished after a sulfur treatment. These results indicate that the surface state acting as the nonradiative recombination centers was passivated by the sulfur. The chemical adsorption behavior resulting from the sulfur is discussed.
Design, fabrication, switching, and optical characteristics of new magneto-optic spatial light modulator
Authors: Cho Jaekyong , Santhanam Suresh , Le Tan , Mountfield Keith , Lambeth David N. , Stancil Daniel , Ross William E. , Lucas John
Published: July 2002

A new reflected mode magneto-optic spatial light modulator (R-MOSLM) has been developed for miniature optical correlators and computers. A factor of 4 improvement in pixel switching sensitivity, compared to the conventional transmission mode magneto-optic spatial light modulator, has been achieved by the use of narrower drive lines, and burying the conductor into the film. A factor of 3 higher resolution and a factor of 2 higher optical efficiency have also been achieved by the use of smaller pixels and narrower pixel gaps. The smaller pixels and improved switching sensitivity permit an order of magnitude reduction in optical path length and increase in frame rate, respectively. The progress that has been made in the design of the R-MOSLM, issues concerning its fabrication, a comparison by finite element analysis of field modeling to experimentally determined current requirements to drive individual lines, and some optical characteristics are discussed.
Spatial characteristics of electron- and photon-induced secondary electron cascades in CsI
Authors: Gibrekhterman A. , Akkerman A. , Breskin A. , Chechik R.
Published: July 2002

Our microscopic model for electron transport in alkali halides was used for the calculation of the spatial characteristics of secondary electron cascades induced by x rays and electrons in an infinite CsI volume, in the energy range of 5–50 keV. The results show that the shape of the cascade cloud preserves the features of the primary interactions only at the core, where the cascade has an elongated, forward peaked shape for incident electrons and is spherically symmetric for photons. At the periphery the cloud is practically spherical, and of very low electron density. The maximal cascade dimensions do not exceed 10 μm at the highest considered energy. The impact of these characteristics on secondary electron emission from finite thickness CsI layers is discussed.
In situ observations of As atoms at step sites of vicinal Si(100) surfaces by coaxial impact-collision ion scattering spectroscopy
Authors: Hashimoto A. , Saitoh T. , Tamura M. , Itoh T.
Published: July 2002

In situ observations of As atoms at step sites of vicinal Si (100) surfaces have been performed by coaxial impact-collision ion scattering spectroscopy. It is found that some As atoms remain at Si step sites even at a high substrate temperature of 780 °C under an As residual pressure, in spite of evaporation of As atoms from terrace sites. This result indicates that As atoms at step sites are energetically more stable than the As dimers on the terrace. Moreover, the angular profiles of the scattering intensity from As atoms at step sites suggest that there is atomic displacement of As atoms towards the Si substrate at the step sites. An atomic model of the As/Si system is proposed from the results of computer simulation for the scattering intensity profiles.
Photoemission studies of the ZnO/CdS interface
Authors: Ruckh M. , Schmid D. , Schock H. W.
Published: July 2002

The highest conversion efficiencies of solar cells based on Cu(In,Ga)Se2 have been achieved using multilayer CdS/ZnO front contacts. The formation of the heterojunction interface between polycrystalline ZnO and CdS has been studied with x-ray and ultraviolet photoemission spectroscopy. The valence band offset between ZnO and CdS has been determined to be 1.2 eV. No chemical reactions at the interface between ZnO and CdS have been detected up to 200 °C. In order to obtain a standard reference for the band discontinuities the valence band offsets of ZnO and CdS relative to Ge have been measured.
Maximum power and thermal efficiency of an irreversible power cycle
Authors: Ait-Ali Mohand A.
Published: July 2002

A simple Carnot-like irreversible power cycle is modeled with two isothermal and two adiabatic, irreversible processes. The generic source of internal irreversibility, deduced from the Clausius inequality, produces entropy at a rate proportional to the external heat conductance and the engine temperature ratio. This cycle is optimized for maximum power and maximum efficiency, and its performances compared to those of the endoreversible cycle, based on typical heat source and heat sink temperatures. Both cycles produce maximum power at the same engine temperature ratio, but the irreversible cycle prediction of maximum efficiency and heat conductance allocation between steam boiler and condenser, appear to be not only more realistic, but also more relevant to actual design considerations of power plants.
Dark current-voltage characteristics of transverse asymmetric hydrogenated amorphous silicon diodes
Authors: Martins R. , Fortunato E.
Published: July 2002

The aim of this work is to provide the basis for the interpretation, under steady state and in the low-voltage regime of the dark current-density–voltage (J–V) characteristics of transverse asymmetric amorphous silicon (a-Si:H) p-i-n and n-i-p diodes. The transverse asymmetric a-Si:H diodes present ratios between the metal contact and the underneath doped layer areas larger than five, leading to the inclusion, in the diode equation, of a lateral leakage current, responsible for the high saturation current density and the forward shape of the J–V curves recorded. The leakage current depends on the lateral spatial potential developed with which varies following a power-law dependence. The experimental J–V curves in diodes with the doped layer around the metal contact unetched and etched prove the role and origin of this lateral leakage current and, thus, the proposed model.
Symmetry consideration of Raman modes in Nd-doped lead zirconate titanate thin films for structure characterization
Authors: Frantti J. , Lantto V. , Lappalainen J.
Published: July 2002

A short group-theoretical discussion is given of phonon symmetries in different phases of the Pb(ZrxTi1−x)O3 (PZT) perovskite together with a description of the degeneracy splitting in consequence of the long-range Coulomb fields both in the paraelectric cubic phase and ferroelectric tetragonal and trigonal phases. Phonon symmetries together with the frequency splitting are used for the consideration of Raman modes, measured from Nd-doped PZT thin films with Zr concentrations below or near to the morphotropic phase boundary between the ferroelectric tetragonal and trigonal phases. Raman spectroscopy was used in addition to x-ray diffraction experiments for the characterization of the phase structure in the films. Raman spectra were measured at room temperature and also at some higher temperatures up to 318 °C. A XeCl-excimer laser was used for the pulsed laser ablation of thin films from a Pb0.97Nd0.02(Zr0.55Ti0.45)O3 target on sapphire and MgO substrates without substrate heating. After ablation, the amorphous films were annealed at temperatures between 700 and 900 °C. Energy dispersive spectroscopy of x rays was used for the composition analysis of the films. Different film compositions were obtained by using different laser-beam fluences during the ablation process. The Zr/(Zr+Ti) ratio in the films varied between 0.32 and 0.50.
Photovoltage generation of Si/C60 heterojunction
Authors: Kita K. , Wen C. , Ihara M. , Yamada K.
Published: July 2002

Photovoltage generation was observed for C60/Si heterojunction fabricated by the deposition of C60 thin film on Si substrate. Four types of Si substrates (two for p-type and two for n-type) were used. All junctions showed rectifying behaviors and photovoltage generation under illumination of Ar-ion laser. The highest value of the photovoltage was 0.40 V. From the saturated photovoltage values for four junctions, the Fermi level of C60 thin film was estimated to be located about 4.7 eV below the vacuum level.
Classical and quantum magnetism in synthetic ferritin proteins
Authors: Gider S. , Awschalom D. D. , Douglas T. , Wong K. , Mann S. , Cain G.
Published: July 2002

The magnetic properties of nanometer-scale particles are studied using the protein-complex ferritin as a vesicle for either an antiferromagnet or a ferrimagnet. For antiferromagnetic ferritin particles, the anisotropy energy is found to depend linearly on the particle volume, suggesting that bulk anisotropy dominates over surface anisotropy. Effects due to the bulk and surface spins are discerned at high magnetic fields (27 T). At very low magnetic fields (1 nT) and temperatures (20 mK), the tunneling frequency of the Néel vector is observed to scale exponentially with the particle volume, consistent with the linear dependence of the anisotropy barrier on volume and with theories of macroscopic quantum coherence. In the ferrimagnetic particles, the anisotropy barrier decreases for smaller particles while simultaneously displaying a slight increase in coercivity and a dramatic decrease in the remanence over three orders of magnitude.
Nonlinear partial erasure and its correlation with transition noise in longitudinal thin-film media
Authors: Zhu Jian-Gang , Lam Terence , Luo Yansheng , Ye Xiao-Guang
Published: July 2002

Nonlinear partial erasure and the supralinear noise enhancement at high recording densities are the two critical factors limiting linear recording density in longitudinal thin-film disk media. In this paper, via spin stand measurements, nonlinear partial erasure was studied in terms of medium magnetic parameters, such as MrT, Hc and orientation ratio, and recording conditions, such as fly height and write current optimization. It is found that the nonlinear partial erasure and the supralinear noise increase always occur at the same recording density, independent of media magnetic properties and recording condition.
Influence of the crystal structure on the magnetic property of Co/Cr superlattices
Authors: Liou Y. , Huang J. C. A. , Yao Y. D. , Yang W. T. , Liao S. Y. , Chang C. P.
Published: July 2002

Co/Cr superlattices with Co layer thickness of 15 Å and Cr layer thickness of 10 Å have been successfully grown on different substrates, such as MgO, Si, and quartz, by molecular-beam-epitaxy. The crystal structure of all samples has been characterized by in situ reflection high-energy electron diffraction. Both magnetoresistance (MR) and magneto-optical Kerr effect (MOKE) measurements were used to analyze the magnetic properties of these samples. Excellent epitaxial growth of hcp-Co(11̄00)/bcc-Cr(211) superlattices grown on MgO(110) was observed. Co/Cr superlattices were polycrystalline when grown on Si and amorphous when grown on quartz. Isotropic MRs of less than 1% were observed for all polycrystalline and amorphous Co/Cr superlattices. For Co(11̄00)/Cr(211) superlattices grown on MgO(110), a transverse MR of 14.2% and longitudinal MR of 0.67% were observed. MOKE hysteresis loops show perpendicular magnetization for polycrystalline Co/Cr superlattices grown on Si, and in-plane isotropic magnetization for amorphous Co/Cr superlattices grown on quartz. MOKE hysteresis loops for Co/Cr superlattices grown on MgO(110) show strong anisotropic behavior between the hard and easy directions. The magnetic properties of Co/Cr superlattices are strongly influenced by the crystal structure of Co and Cr which is controlled by the substrate.
Neutron depolarization study of magnetic order in a-FexZr100−x (x=90–93), a-Fe90Sc10 and their hydrides
Authors: Ryan D. H. , Cadogan J. M. , Kennedy S. J.
Published: July 2002

Neutron depolarization measurements have been used to show that ferromagnetic domains exist in a-FexZr100−x alloys for 90≤x≤93, and that these domains are not affected by the ordering of transverse spin components at Txy. Domain sizes decrease from 1.1 μm at x=90, to 0.08 μm by x=93. Measurements in an applied field set an upper limit of ∼50 nm for correlations in the transverse components. By contrast, a-Fe90Sc10 shows no evidence of ferromagnetic order and is confirmed to be a spin glass. Measurements on deuterium-loaded samples show that all of the a-Fe–Zr and a-Fe–Sc alloys studied here are ferromagnetic with Tc’s in the range 380–400 K, and domains ∼1 μm in size.
Finite-size effect and its temperature dependence of giant magnetoresistance in magnetic granular materials
Authors: Wang Jian-Qing , Xiao Gang
Published: July 2002

In magnetic granular alloys such as Co-Ag, a large finite-size effect in giant magnetoresistance (GMR) has been observed as the sample thickness (t) varies. The value of GMR monotonically decreases and eventually vanishes as t approaches zero. The t range, over which the GMR value drops rapidly to zero, has been found to be weakly dependent on temperature. Our results are consistent with a picture that a strong spin-orbit interaction at surfaces is responsible for randomizing spins in different spin channels, thus reducing GMR.
Polarities of ferroelectric 180° domains observed using scanning electron microscopy and a liquid toner method in an uncoated triglycine sulfate crystal
Authors: Ozaki Tōru , Fujii Kentarou , Aoyagi Sadao
Published: July 2002

Both scanning electron microscopy and a liquid toner method have been carried out on an identical uncoated b plate of triglycine sulfate (TGS), to clarify the correspondence between the polarities of ferroelectric 180° domains and the contrast of their secondary electron images (SEI). It has been found that the contrast between the ends of adjacent 180° domains is reversed in a toner pattern, as well as in an SEI, between two temperature regions above and below the aging temperature Ta. This is attributed to the change in the sign of the total surface charge density σt at Ta, which consists of spontaneous polarization Ps and true charge density σn on the ends of adjacent domains. The magnitude of σn, which exactly neutralizes Ps at Ta, remains almost unchanged while the magnitude of Ps increases with decreasing temperature. Negatively charged toner particles are adsorbed on the end with σt≳0 of each domain, which corresponds to the (+) domain of Ps≳0 below Ta and the (−) domain of Ps<0 above Ta. It has been determined, on the basis of the above sign of Ps, that the end with σt<0 of each domain is observed to be bright in the SEI, which corresponds to the (−) domain below Ta and the (+) domain above Ta. Electric fields generated both inside and outside the TGS plate can contribute to the secondary electron emission from the end with σt<0 of each 180° domain.
Electron acoustic imaging of BaTiO3 single crystals
Authors: Zhang B. Y. , Jiang F. M. , Yang Y. , Yin Q. R. , Kojima S.
Published: July 2002

Scanning electron acoustic microscopy (SEAM) has been used in the observation of ferroelectric domain configurations in barium titanate single crystals. The contribution to the image contrasts is related to signal generation through a piezoelectric coupling mechanism. The results prove that SEAM really has the ability to show not only the surface shape and domain structures but also the subsurface domain structures. Furthermore, SEAM is a convenient, fast and nondestructive way of observing the ferroelectric domains compared with other methods.
Young’s modulus of amorphous Terfenol-D thin films
Authors: Su Q. , Morillo J. , Wen Y. , Wuttig M.
Published: July 2002

Young’s modulus of amorphous Terfenol-D films has been determined utilizing the resonances of Terfenol-D/Si composite cantilevers. Its value, 120 GPa about equals that of magnetically saturated Terfenol-D but displays no ΔE effect.
Effect of fluorine on dielectric properties of SiOF films
Authors: Lee Seoghyeong , Park Jong-Wan
Published: July 2002

The effect of fluorine addition on silicon oxide film properties as a function of SiF4/O2 gas flow ratio was investigated. The films were deposited by using electron cyclotron resonance plasma chemical vapor deposition with SiF4 and O2 as source gases diluted in Ar. Characterization of films was carried out in terms of various gas flow ratios (SiF4/O2=0.2–1.0). The microwave power and substrate temperature during deposition were fixed at 700 W and 300 °C, respectively. The chemical bonding structure of the films was evaluated by Fourier transform infrared spectroscopy (FTIR), fluorine concentration by x-ray photoelectron spectroscopy, and refractive index by ellipsometry. The dielectric constant was determined from C–V measurements at 1 MHz. FTIR spectra show that as the fluorine concentration increased, the intensities of the Si–F bonding peak and the shoulder peak (at around 1160 cm−1) of the Si–O stretching mode both increased. Refractive index and deposited film density decreased with increasing SiF4/O2 gas flow ratio. The SiOF film deposited at a SiF4/O2 gas flow ratio of 1.0 exhibited a fluorine content of 11.8 at. % and a dielectric constant of 3.14.
Activation modeling of Si implanted GaAs
Authors: Apiwatwaja R. , Gwilliam R. , Wilson R. , Sealy B. J.
Published: July 2002

The total concentration of ionized impurities in silicon implanted GaAs was estimated from carrier concentration and mobility values obtained by Hall effect measurements together with published compensations. We have demonstrated that the calculated profiles (ND++NA−) are in good agreement with that of the silicon atomic distributions obtained by secondary-ion-mass spectroscopy. We have observed that a large concentration of gallium vacancies are injected into the sample during a 900 °C anneal for 1000 s using a Si3N4 cap.
Direct observation of Cr magnetic order in CoCrTa and CoCrPt thin films
Authors: Kemner K. M. , Idzerda Y. U. , Harris V. G. , Chakarian V. , Elam W. T. , Kao C.-C. , Johnson E. , Feng Y. C. , Laughlin D. E. , Chen C.-T. , Lee K.-B. , Lodder J. C.
Published: July 2002

Magnetic circular dichroism measurements of room temperature, sputter deposited Co86Cr12Ta2 and Co86Cr12Pt2 films were performed to investigate the local magnetic ordering of the Co and Cr atoms. The results demonstrate that the Cr has a net magnetic moment and that a small fraction of the Cr is magnetically oriented opposite to the Co moment.
Magnetic anisotropy, coupling, and transport in epitaxial Co/Cr superlattices on MgO(100) and (110) substrates
Authors: Picconatto J. Johanna , Pechan Michael J. , Fullerton Eric E.
Published: July 2002

Superlattices of Co/Cr have been epitaxially sputtered onto MgO(100) and (110) substrates coated with epitaxial Cr(100) and (211) buffer layers. The Co thickness is fixed at 20 Å and the Cr thickness varied from 7 to 22 Å. On the MgO(110)/Cr(211) substrates, coherent hcp-Co(11̄00)/bcc-Cr(211) superlattice structures are formed. On MgO(100)/Cr(100), x-ray-diffraction results suggest strained hcp-Co(112̄)/bcc-Cr(100) superlattices. Magnetization measurements show fourfold magnetic in-plane anisotropy for the MgO(100) orientation and twofold for the MgO(110). By utilizing a simple model based upon perpendicular uniaxial anisotropies, we have concluded that the fourfold anisotropy has its origin in the second-order uniaxial Co anisotropy energy. The antiferromagnetic interlayer coupling strength exhibits a maximum value of 0.15 erg/cm2 at a Cr thickness of 13 Å in the MgO(110) orientation. The MgO(100) orientation exhibits its strongest coupling of 0.55 erg/cm3 at 10 Å Cr thickness. Modest giant magnetoresistance values no larger than 3% are observed and we find no evidence of enhanced anisotropic magnetoresistance effects recently reported for Co(11̄00)/Cr(211) superlattices.
Role of attractive forces in tapping tip force microscopy
Authors: Kühle Anders , So/rensen Alexis H. , Bohr Jakob
Published: July 2002

We present experimental and numerical results demonstrating the drastic influence of attractive forces on the behaviour of the atomic force microscope when operated in the resonant tapping tip mode in an ambient environment. It is often assumed that tapping is related to repulsive interaction. In contrast, we find that in general the attractive forces are the most dominant interaction in this mode of operation. We show that attractive forces in combination with the repulsive elastic type of forces cause points of instability in the parameter space constituted by: the cantilever swing amplitude, the frequency bias point, and the distance between the fixed end of the cantilever and the sample. These points of instability can result in disturbances during image acquisition on hard elastic surfaces.
Synthesis and magnetic properties of novel compounds R3(Fe, T)29 (R=Y, Ce, Nd, Sm, Gd, Tb, and Dy; T=V and Cr)
Authors: Han Xiu-Feng , Yang F. M. , Pan H. G. , Wang Y. G. , Wang J. L. , Liu H. L. , Tang N. , Zhao R. W. , Li H. S.
Published: July 2002

A systematic investigation of structure and intrinsic magnetic properties of the novel compounds R3(Fe, T)29 (R=Y, Ce, Nd, Sm, Gd, Tb, and Dy; T=V and Cr) has been performed. The lattice constants and unit cell volume decrease with increasing the rare-earth atomic number from R=Nd to Dy, except for Ce, reflecting the lanthanide contraction. The Curie temperature increases from R=Ce to Gd and decreases from Gd to Dy, respectively, with increasing atomic number and Gd3Fe29−xTx has the highest Curie temperature for each series of R3Fe29−xTx (T=V or Cr) compounds. The saturation magnetization of R3Fe29−xTx at 4.2 K decreases gradually from R=Nd to Dy with increasing atomic number, except for Ce, in each series of R3Fe29−xTx. It is suggested that the Ce ion in Ce3Fe29−xTx is valence fluctuated which leads to the unusual magnetic properties. The spin reorientations of the easy magnetization direction of R3Fe29−xTx are observed at around 230, 230, and 160 K for R=Nd, Sm, and Tb when T=V, and at around 230 and 180 K for R=Nd and Tb when T=Cr, respectively. First order magnetization processes are observed around 5.7 T for Sm3Fe26.7V2.3 and 4 T for Sm3Fe24.0Cr5.0 at 4.2 K, 2.0 T for Tb3Fe28.0V1.0, and 2.3 T for Tb3Fe28.0Cr1.0 at room temperature. A phenomenological analysis shows that the saturation magnetization of R3Fe29−xTx compounds with a low T concentration can be roughly calculated based on a combination of those of the 2:17R and 1:12 units in a ratio of 1:1.
Refractive index measurements of MgZnCdSe II–VI compound semiconductors grown on InP substrates and fabrications of 500–600 nm range MgZnCdSe distributed Bragg reflectors
Authors: Morita Toshihiro , Shinbo Hiroyuki , Nagano Takeshi , Nomura Ichirou , Kikuchi Akihiko , Kishino Katsumi
Published: July 2002

Refractive indices of Mgx(Zn0.48Cd0.52)1−xSe compounds grown on InP substrates were systematically investigated as a function of Mg composition (x). The refractive indices with various Mg compositions were estimated from the reflectance measurements. By approximating the refractive indices by the modified single effective oscillator method, the direct band gap energy EΓ=2.03+1.45x, the dispersion energy Ed=24.5−15.2x, and the oscillator energy E0=5.13−1.03x were obtained. MgZnCdSe multilayer distributed Bragg reflectors (DBRs) designed by using the refractive indices obtained in this study were fabricated by a molecular beam epitaxy. As a result, high reflectance values over 98% at 595 nm were experimentally obtained for the 30 pairs Mg0.1(Zn0.48Cd0.52)0.9Se/Mg0.6(Zn0.48Cd0.52)0.4Se DBR, and the reflectance spectrum agreed with the theoretical values. Furthermore, good agreements of the experimental and the theoretical maximum reflectance of the DBRs as a function of the layer pair number are obtained. From theoretical investigations of 500–600 nm wavelength range DBRs, reflectance values of 99.9% are calculated for the layer pair numbers from 30 to 40.
Observation of the evolution of anisotropic magnetoresistance in thin magnetic films
Authors: Zhao H. W. , Lu M. , Du J. , Zhai H. R.
Published: July 2002

The evolutionary process of the anisotropic magnetoresistance from the positive to the negative by changing the field direction was observed in magnetic thin films of Ni-Co, Ni-Fe and Co. Various peculiar magnetoresistance-field curves occur, that can be understood by a model based on the superposition of the positive and negative magnetoresistance with different amplitudes and phases.
Magnetism and nanostructure of Fe93−x−yZr7BxCuy alloys
Authors: Kopcewicz M. , Grabias A. , Williamson D. L.
Published: July 2002

A set of Fe-based amorphous alloys, Fe93−x−yZr7BxCuy, with x=4, 6, 8, or 12, and y=0 or 2 has been systematically characterized in their ability to form nanocrystalline, magnetically soft material via annealing in the range of 430–600 °C. Conventional Mössbauer spectroscopy is used to follow the degree of bcc-Fe formation as well as changes in the hyperfine field distribution of the amorphous phase as a function of anneal temperature. Copper plays a strong role in the bcc-Fe formation for x=12 but less of a role for x=8 and 6. Unconventional Mössbauer studies utilizing radio frequency (rf) fields provide information on the soft magnetic nature of the alloys by observing the degree of rf-induced collapse of the hyperfine fields. The Mössbauer experiment in which the rf collapse and rf sideband effects are used allows the soft nanocrystalline bcc phase to be distinguished from magnetically harder microcrystalline α-Fe. The rf Mössbauer technique, being particularly sensitive to the magnetic anisotropy, provides information on the anisotropy fields and hence on the grain size distribution. X-ray diffraction (XRD) is used to estimate the bcc-Fe grain size based on the diffraction peak linewidths. Average grain sizes of 5–14 nm are found for 500–550 °C annealed specimens where smaller grain sizes are always observed for y=2 compared to y=0 for fixed x. Small-angle x-ray scattering is also used to study the grain size and this method yields sizes in the range from 3 to 7 nm, consistently almost a factor of 2 smaller than those from the XRD line broadening. This discrepancy is attributed to the difference in the regions of the 20-μm-thick ribbons probed by the two methods.
Electrical and structural properties of zirconium germanosilicide formed by a bilayer solid state reaction of Zr with strained Si1−xGex alloys
Authors: Wang Z. , Aldrich D. B. , Nemanich R. J. , Sayers D. E.
Published: July 2002

The effects of alloy composition on the electrical and structural properties of zirconium germanosilicide (Zr–Si–Ge) films formed during the Zr/Si1−xGex solid state reaction were investigated. Thin films of Zr(Si1−yGey) and C49 Zr(Si1−yGey)2 were formed from the solid phase reaction of Zr and Si1−xGex bilayer structures. The thicknesses of the Zr and Si1−xGex layers were 100 and 500 Å, respectively. It was observed that Zr reacts uniformly with the Si1−xGex alloy and that C49 Zr(Si1−yGey)2 with y=x is the final phase of the Zr/Si1−xGex solid phase reaction for all compositions examined. The sheet resistance of the Zr(Si1−yGey)2 thin films was higher than the sheet resistance of similarly prepared ZrSi2 films. The stability of Zr(Si1−yGey)2 in contact with Si1−xGex was investigated and compared to the stability of Ti(Si1−yGey)2 in contact with Si1−xGex. The Ti(Si1−yGey)2/Si1−xGex structure is unstable when annealed for 10 min at 700 °C, with Ge segregating from Ti(Si1−yGey)2 and forming Ge-rich Si1−zGez precipitates at grain boundaries. In contrast, no Ge segregation was detected in the Zr(Si1−yGey)2/Si1−xGex structures. We attribute the stability of the Zr-based structure to a smaller thermodynamic driving force for germanium segregation and stronger atomic bonding in C49 Zr(Si1−yGey)2. Classical thermodynamics were used to calculate Zr(Si1−yGey)2–Si1−xGex tie lines in the Zr–Si–Ge ternary phase diagram. The calculations were compared with previously calculated Ti(Si1−yGey)2–Si1−xGex tie lines.
Zn incorporation and band gap shrinkage in p-type GaAs
Authors: Hudait Mantu Kumar , Modak Prasanta , Hardikar Shyam , Krupanidhi S. B.
Published: July 2002

Dimethylzinc (DMZn) was used as a p-type dopant in GaAs grown by low pressure metalorganic vapor phase epitaxy using trimethylgallium and arsine (AsH3) as source materials. The hole carrier concentrations and zinc (Zn) incorporation efficiency are studied by using the Hall effect, electrochemical capacitance voltage profiler and photoluminescence (PL) spectroscopy. The influence of growth parameters such as DMZn mole fraction, growth temperature, and AsH3 mole fraction on the Zn incorporation have been studied. The hole concentration increases with increasing DMZn and AsH3 mole fraction and decreases with increasing growth temperature. This can be explained by vacancy control model. The PL experiments were carried out as a function of hole concentration (1017–1.5×1020 cm−3). The main peak shifted to lower energy and the full width at half maximum (FWHM) increases with increasing hole concentrations. We have obtained an empirical relation for FWHM of PL, ΔE(p)(eV)=1.15×10−8p1/3. We also obtained an empirical relation for the band gap shrinkage, ΔEg in Zn doped GaAs as a function of hole concentration. The value of ΔEg(eV)=−2.75×10−8p1/3, indicates a significant band gap shrinkage at high doping levels. These relations are considered to provide a useful tool to determine the hole concentration in Zn doped GaAs by low temperature PL measurement. The hole concentration increases with increasing AsH3 mole fraction and the main peak is shifted to a lower energy side. This can be explained also by the vacancy control model. As the hole concentration is increased above 3.8×1018 cm−3, a shoulder peak separated from the main peak was observed in the PL spectra and disappears at higher concentrations.
Crystallographic and magnetic properties of nitride R3Fe29−xCrxN4 (R=Y, Ce, Nd, Sm, Gd, Tb, and Dy)
Authors: Han Xiu-Feng , Yang F. M. , Pan H. G. , Wang Y. G. , Wang Y. Z.
Published: July 2002

A systematic investigation of crystallographic and magnetic properties of nitride R3Fe29−xCrxN4 (R=Y, Ce, Nd, Sm, Gd, Tb, and Dy) has been performed. The lattice constants and unit cell volume decrease with increasing rare earth atomic number from Nd to Dy, reflecting the lanthanide contraction. After nitrogenation the relative volume expansion of each nitride is around between 5% and 7%. The nitrogenation results in a good improvement in the Curie temperature, the saturation magnetization and anisotropy fields at 4.2 K, and room temperature for R3Fe29−xCrxN4. Magnetohistory effects of R3Fe29−xCrxN4 and R3Fe29−xCrx (R=Nd and Sm) are observed in a low field of 0.04 T. First order magnetization process occurs in Sm3Fe24.0Cr5.0N4 in magnetic fields of 2.8 T at 4.2 K. After nitrogenation, the easy magnetization direction of Sm3Fe24.0Cr5.0 is changed from the easy-cone structure to the uniaxial. The good intrinsic magnetic properties of Sm3Fe24.0Cr5.0N4 make this compound a hopeful candidate for new high-performance hard magnets.
Effect of low Fe doping in La0.8Sr0.2MnO3
Authors: Ajan Antony , Venkataramani N. , Prasad Shiva , Shringi S. N. , Nigam A. K. , Pinto R.
Published: July 2002

The effect of low Fe substitution in Mn sites of La0.8Sr0.2MnO3 is presented. We report the electrical and magnetic properties of these materials in the Fe concentration range x=0 to x=0.1. The metal-insulator transition temperature is found to vary significantly with very small Fe doping. The metal to insulator transition temperature increases slightly when a very small amount of Fe is doped (<1%) and then it decreases significantly with further doping of Fe. The magnetic moment of La0.8Sr0.2MnO3 decreases almost linearly with Fe doping at room temperature.
Preferred orientation and piezoelectricity in sputtered ZnO films
Authors: Gardeniers J. G. E. , Rittersma Z. M. , Burger G. J.
Published: July 2002

Thin ZnO films were deposited on various types of substrates with rf magnetron reactive sputtering of a Zn target in pure O2 atmosphere. The layers were characterized by x-ray diffraction and electrical measurements. The piezoelectric strain constant d31 of the films was determined via optical interferometric measurements on the piezoelectrically forced vibration of silicon cantilevers. Because of the high resistivity of the ZnO samples (higher than 109 Ω cm), piezoelectric excitation down to frequencies of 100 Hz was found to be feasible. The relation between the piezoelectric strain constant and the c-axis orientation distribution of the film [obtained from the full width at half maximum of the x-ray rocking curves at the ZnO (002) diffraction] was determined both experimentally and theoretically. It was found that the experimental effective piezoelectric strain constants are, at most, 60% of the value predicted by the theoretical calculations. It is thought that this discrepancy is due to the cancellation of piezoelectric activity by ZnO grains of opposing polarity. The latter effect and other structural properties of the films are discussed in relation to nucleation and growth of sputtered ZnO films.
Stark splitting scheme and an induced emission cross section of TbAlO3
Authors: Sekita Masami , Miyazawa Yasuto , Ishii Motohiko
Published: July 2002

Detailed absorption and emission spectra on TbAlO3 are measured at room temperature and liquid-nitrogen temperature. By decomposition of the spectra, the Stark splitting scheme is established for F76, F75, and D54 multiplets of Tb3+ ions in TbAlO3 together with the peak energies in the Fourier transform infrared spectrum at liquid-nitrogen temperature. The multiplets are split into their maximum number of levels, which shows that the point symmetry is low for the Tb3+ ion site in TbAlO3. An induced emission cross section is estimated using the results of decomposition at liquid-nitrogen temperature to be 3.55×10−19 cm2. This value is comparable to the cross sections reported for Nd:Y3Al5O12, which is a well known solid-state laser material. The population inversion threshold is also estimated for TbAlO3 in comparison with Nd:Y3Al5O12. It is found that the threshold value for TbAlO3 is only about three times larger than that for Nd:Y3Al5O12. This TbAlO3 may have the potential to be a laser material with a visible wavelength at least at low temperature.
Bridging the pressure gap for palladium metal-insulator-semiconductor hydrogen sensors in oxygen containing environments
Authors: Johansson M. , Lundström I. , Ekedahl L.-G.
Published: July 2002

A comparison has been made between the steady state response obtained from palladium metal-insulator-semiconductor (Pd-MIS) structures exposed to hydrogen in presence of oxygen under atmospheric conditions and the response calculated from a model valid under ultrahigh vacuum conditions. It is shown that the model gives a good description of the steady state response as a function of hydrogen and oxygen pressure. This is of interest not only for the understanding of the sensing mechanism of Pd-MIS hydrogen sensors but would also imply that the used model for the water forming reaction on Pd gives realistic estimates for the hydrogen coverage on the Pd surface over a vast pressure range.
Fast Fourier transformation of piezoreflectance in δ-doped GaAs
Authors: Hsu T. M. , Chang W.-H. , Liao D. H. , Lee W. C.
Published: July 2002

Franz–Keldysh oscillations in Si-δ-doped GaAs have been studied by the application of fast Fourier transformations to the piezoreflectance spectra. In such studies, we find that the Franz–Keldysh oscillations of the piezoreflectance and its associated Fourier transformation can be described by a model calculation which considers the energy gap modulation in a uniform electric field. Owing to the character of nonelectromodulation, the Franz–Keldysh oscillation lineshape in the piezoreflectance has no modulation electric field distortion such as that which occurs in the photoreflectance and electroreflectance experiments. We demonstrate that the piezoreflectance accuracy is better than the photoreflectance or electroreflectance for the measurement of a uniform built-in electric field. The fast Fourier transformation of piezoreflectance spectra exhibits two separate heavy- and light-hole frequencies at temperatures from 20 to 425 K. Using these separated peaks, the ratio between the heavy- and light-hole effective reduced mass, the surface Fermi level, and the broadening parameters of the Franz–Keldysh oscillations are measured and discussed.
Simple Ru electrode scheme for ferroelectric (Pb,La)(Zr,Ti)O3 capacitors directly on silicon
Authors: Bandaru Jordana , Sands Timothy , Tsakalakos Loucas
Published: July 2002

A two-step process consisting of low temperature growth followed by rapid thermal annealing is demonstrated for Ru/(Pb,La)(Zr,Ti)O3 (PLZT)/Ru ferroelectric capacitors directly on silicon. PLZT is a promising material for Gbit-scale ferroelectric memories, but its growth on silicon has proved challenging. The two-step process is designed to enable nucleation of perovskite phase PLZT while limiting diffusion which often leads to device failure. Minimization of stress and interdiffusion during film growth were necessary to optimize the remanent polarization. (Pb-4%La)(Zr0.3Ti0.7)O3 capacitors with remanent polarizations up to 17 μC/cm2 were grown on Ru/Si at 400 °C and 50 mTorr O2 using pulsed laser deposition. Direct high temperature growth (>600 °C) was not possible due to interface reactions, and film cracking was observed at low temperatures as well as at high and low oxygen pressures.
Aluminum-implantation-induced deep levels in n-type 6H–SiC
Authors: Fung S. , Gong M. , Beling C. D. , Brauer G. , Wirth H. , Skorupa W.
Published: July 2002

Deep-level defect centers on the n-side of p+n junction diodes formed by low and elevated temperature aluminum-ion implantation into n-type 6H–SiC have been studied using deep-level transient spectroscopy. Two shallow Al-acceptor levels have been observed in the n region just beyond the implantation depth through their minority-carrier emission signatures. The dominant level is situated at 0.26 eV above the valence band and is accompanied by a shallower level of small intensity. Comparison with photoluminescence results suggests the dominant level (labeled Ak) and the shallower level (labeled Ah), are associated with the cubic and hexagonal lattice sites, respectively. Unlike previously reported results, which show many different implantation-induced donors within the implantation region, only one deep donor level at EC−0.44 eV is found to occur in the postimplantation region, indicating that the various crystal damage sites occur with different spatial distributions.