bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of the deformation potentials of GaAs0.80P0.20
Authors: González Y. , Armelles G. , González L.
Published: July 2002

Deformation potentials can be determined by measuring the variation of the energy of the electronic transitions with strain. In this work, the hydrostatic and shear potentials of the band-gap electronic transition (E0) and the transitions along the 〈111〉 direction (E1) of GaAs1−xPx, x≊0.20, have been determined by electroreflectance characterization of GaAs1−xPx layers with different levels of strain.
Photoelectronic properties of porous silicon
Authors: Ozaki Tsuyoshi , Araki Minoru , Yoshimura Shotaro , Koyama Hideki , Koshida Nobuyoshi
Published: July 2002

It is shown that the introduction of a porous structure into single-crystal Si produces substantial changes in the photoelectronic properties. The porous Si (PS) layer is formed by anodization of p-type Si wafers in a HF solution. The photoconduction cells used in this study consists of a semitransparent thin Au film, PS, Si substrate, and Al ohmic contact. The photoconductive behavior of PS is characterized by an extremely high dark resistivity, a definite photosensitivity for visible light, and an intrinsic bias voltage dependence of the spectral response. These properties, which are interpreted to be the result of a band-gap widening in PS, provide further support of the assumption that the visible luminescence of PS is explained by the band scheme.
Thermochromism and optical absorption in Langmuir–Blodgett films of alkyl-substituted polythiophenes
Authors: Ahlskog M. , Paloheimo J. , Stubb H. , Dyreklev P. , Fahlman M. , Inganäs O. , Andersson M. R.
Published: July 2002

Thermochromism and optical absorption in mono- and multilayers of Langmuir–Blodgett films of poly(3-alkylthiophenes), poly(3-octyl-2,2′-bithiophene), and poly(3′-octyl-2,2′;5′,2″-terthiophene) were studied. In sparsely alkylated polythiophenes the magnitude of the thermochromic shift was smaller than in poly(3-alkylthiophenes) and roughly proportional to the sidechain concentration. Results of Valence Effective Hamiltonian calculations were compared with the experimental results of thermochromism. A vibronic structure was found in the absorption spectra of Langmuir–Blodgett films at room temperature. The vibronic splitting in poly(3-hexylthiophene) was approximately 0.18 eV as previously has been observed in poly(3-alkylthiophenes) but in poly(3′-octyl-2,2′;5′,2″-terthiophene) it was 0.20–0.25 eV. The vibronic peaks stay approximately at constant energies and vanish at elevated temperatures.
Nucleation and step motion in chemical vapor deposition of SiC on 6H-SiC{0001} faces
Authors: Kimoto Tsunenobu , Matsunami Hiroyuki
Published: July 2002

Growth kinetics of SiC on 6H-SiC {0001} surfaces by a chemical vapor deposition method at 1200–1600 °C were studied. Through short-time growth experiments, nucleation, and lateral growth rate of atomic steps on the surface were investigated. On well-oriented 6H-SiC {0001} faces, the nucleus density was much higher on (0001̄) C faces than on (0001) Si faces. The polytypes of nuclei are discussed based on Raman scattering measurements. The lateral growth rate was about three orders of magnitude higher than the vertical growth rate at temperatures higher than 1400 °C. Anisotropy in lateral growth rates, higher in the 〈112̄0〉 and lower in the 〈11̄00〉 directions, was observed. With the decrease in growth temperature, the lateral growth rates significantly decreased, and the anisotropy was enhanced. On off-oriented {0001} faces, no nucleation took place at 1500 °C. Based on these results, surface diffusion of adatoms and step dynamics in SiC growth are discussed.
Giant magnetoresistance in sputtered Cr-Fe heterogeneous alloy films
Authors: Takanashi K. , Sugawara T. , Hono K. , Fujimori H.
Published: July 2002

We have observed large negative magnetoresistance (MR) in Cr-Fe heterogeneous alloy films sputter deposited on heated substrates. The largest MR, 37.3% at 4.2 K and 14 T, appears around the Fe concentration of 20 at. %. While a large substrate temperature dependence of MR is observed when the Fe concentration is lower than 20 at. %, MR does not vary noticeably with changes in the substrate temperature when Fe concentration exceeds 20 at. %.
Magnetic wire and box arrays (invited)
Authors: Maeda Atsushi , Kume Minoru , Ogura Takashi , Kuroki Kazuhiko , Yamada Takashi , Nishikawa Madoka , Harada Yasoo
Published: July 2002

Ferromagnetic wire and box arrays with widths and spacings in the range of 2–0.6 μm were successfully fabricated utilizing high-resolution electron-beam lithography and lift-off techniques. The box arrays possessed a unique magnetic switching mechanism. As the magnetic field decreased, the magnetic coherency between the boxes which are in a row first disappeared. Domain wall motion in each box occurred in the next step. The demagnetizing fields observed in the wire arrays with short spacings were different from those calculated by a model, in which the wires are isolated from each other. Ferromagnetic resonance measurement implied the appearance of interwire dipole-dipole interaction. Multilayered wire arrays were also prepared to study the magnetoresistive characteristics.
Itinerant electron metamagnetism and related phenomena in Co-based intermetallic compounds (invited)
Authors: Goto T. , Katori H. Aruga , Sakakibara T. , Mitamura H. , Fukamichi K. , Murata K.
Published: July 2002

Susceptibility and high field magnetization have been measured for Y(Co1−xMx)2 (M=Al, Fe, and Ni). YCo2 exhibits a metamagnetic transition at 69 T. Ni doping in YCo2 increases the critical field (Bc), while Fe doping decreases it. These changes can be elucidated with the band picture of metamagnetism. Bc of YCo2 exhibits a positive shift proportional to the square of temperature. In the paramagnetic region of Y(Co1−xAlx)2 with x≤0.11, the susceptibility is enhanced with Al and a sharp metamagnetic transition with lower Bc is observed. The susceptibility becomes maximum at a finite temperature Tmax. Bc in the ground state is proportional to Tmax. These experimental results are discussed with a new theory based on a spin fluctuation model. The susceptibility and the metamagnetic transition are found to be very sensitive to pressure.
Structural and magnetic characterization of Co particles coated with Ag
Authors: Rivas J. , Sánchez R. D. , Fondado A. , Izco C. , García-Bastida A. J. , García-Otero J. , Mira J. , Baldomir D. , González A. , Lado I. , López Quintela M. A. , Oseroff S. B.
Published: July 2002

Co fine particles coated with Ag have been synthesized through the microemulsion method in an inert atmosphere. The size of the particles is controlled by the water droplets of the microemulsions. Fine particles prepared by this method, consist of a magnetic core of Co covered by a layer of Ag. Samples containing from 3.3 to 40.5 vol % Co have been prepared. The average size of the particles obtained is in the nanometer range. The magnetic properties were studied by dc magnetization at 77 K and room temperature. The data show a strong dependence of the magnetic properties on the annealing temperature.
Investigations of the interplay between crystalline and magnetic ordering in Fe3O4/NiO superlattices
Authors: Lind D. M. , Borchers J. A. , Erwin R. W. , Ankner J. F. , Lochner E. , Shaw K. A. , DiBari R. C. , Portwine W. , Stoyonov P. , Berry S. D.
Published: July 2002

Using SQUID magnetometry and both x-ray- and neutron-diffraction techniques, we have studied the structural and magnetic ordering of a series of Fe3O4/NiO superlattices grown by MBE. X-ray diffraction reveals that the superlattices are coherent, single phase crystals with narrow interfaces. Symmetry differences between the Fe3O4 spinel and NiO rocksalt structures lead to interfacial stacking faults, manifested in some diffraction intensities. Analysis of the neutron-diffraction spectra show that the NiO antiferromagnetic ordering is coherent through several superlattice bilayers, while the Fe3O4 magnetic ordering is confined to individual interlayers by stacking faults in all superlattices but those with thinnest (≤10 Å) NiO interlayers. Neutron diffraction and SQUID magnetometry have been used to study the Fe3O4 Verwey phase transition in thin-layered superlattices. The charge ordering in superlattices such as [Fe3O4 (75 Å)‖NiO (9 Å)]500, below the Verwey transition, directly observable in (4, 0, 1/2) neutron intensities, indicates a shift to higher temperature of the charge ordering transition from the bulk Fe3O4 TVerwey at 123 K. We also describe ongoing efforts to extract the moment distribution in these superlattices from field dependent high-angle neutron diffraction.
Properties of carbon nitride films with composition ratio C/N=0.5–3.0 prepared by the ion and vapor deposition method
Authors: Ogata Kiyoshi , Chubaci José Fernando Diniz , Fujimoto Fuminori
Published: July 2002

Carbon nitride films with the composition ratio CR(C/N)=0.5–3.0 were prepared by the ion and vapor deposition method, where carbon was evaporated on various substrates while being simultaneously bombarded with 0.5–10.0 keV nitrogen ions. The properties of the films were studied by x-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrometry (FT-IR), ultraviolet transmission spectroscopy, x-ray diffraction, and hardness measurements. The films formed at energies lower than 0.8 keV and CR(C/N)=0.6–0.7 on tungsten carbide showed the highest Knoop harness of 6400 kgf/mm2 on films with 1 μm thickness and a maximum optical band gap of 2.7 eV. X-ray diffraction measurements demonstrated that all films have an amorphous structure. The XPS and FT-IR studies indicated that the peak newly observed at 286.3 eV in the C-1s1/2 XPS spectra arises from triple bonding C≡N.
Optical gain evaluation in GaInAsP quantum-well lasers: A comparison of the different growth techniques
Authors: Barrau J. , Issanchou O. , Brousseau M. , Mircea A. , Ougazzaden A.
Published: July 2002

GaInAsP quantum-well lasers are usually built by keeping constant the anion (CY lasers) or the cation (CX lasers) concentration into wells and barriers. We interpret the higher performances of CY-laser structures comparatively with CX structures by the significant increase of the effective nonradiative lifetime coming from the reduced overlap between the electron and hole gases.
Minority- and majority-carrier trapping in strain-relaxed Ge0.3Si0.7/Si heterostructure diodes grown by rapid thermal chemical-vapor deposition
Authors: Grillot P. N. , Ringel S. A. , Fitzgerald E. A. , Watson G. P. , Xie Y. H.
Published: July 2002

Strain-relaxed Ge0.3Si0.7/Si, grown by rapid thermal chemical-vapor deposition, has been investigated with deep-level transient spectroscopy (DLTS) and bias-dependent electron-beam-induced current (EBIC). A single electron trap and several hole traps have been detected in these samples. The apparent electron capture cross section is found to be ∼2×10−13 cm2, which is several orders of magnitude larger than the apparent hole capture cross sections (∼10−17 cm2), and is responsible for the detection of the minority-carrier electron trap even under reverse-bias majority-carrier capture conditions. All observed traps which were investigated as a function of filling pulse time exhibit logarithmic capture kinetics, as expected for extended defects, and the bias-dependent DLTS peak height and EBIC relative defect contrast are consistent with the spatially varying dislocation density. Moreover, the trap concentration, as determined by DLTS, is correlated to the dislocation density, as determined by EBIC measurements. Based on a comparison of Arrhenius plots, the observed logarithmic capture kinetics, the correlation of trap density to dislocation density, and the observed bias dependence, the electron trap appears to be related to dislocation core states, while two of the hole traps appear to be related to either dislocation core states or Cottrell atmospheres.
Optical properties of Zn1−xMgxSySe1−y epitaxial layers for blue-green laser applications
Authors: Lunz U. , Jobst B. , Einfeldt S. , Becker C. R. , Hommel D. , Landwehr G.
Published: July 2002

The refractive index and the E0 energy gap has been determined for a number of Zn1−xMgxSySe1−y alloys with x≤0.3 and y≤0.3 at temperatures between 5 and 300 K. It has been shown that the refractive index can be empirically expressed as a function of the E0 energy gap. Furthermore we have determined the temperature dependence of the E0 energy gap and hence the temperature dependence of the refractive index for these alloys.
Empirical temperature dependence of the refractive index of semiconductors
Authors: Hervé P. J. L. , Vandamme L. K. J.
Published: July 2002

Values of the temperature coefficient of the refractive index were obtained from the derivation of a simple relation between energy band-gap and refractive index in semiconductors. These values, (dn/dT)/n, were compared to the experimental data found in literature. Our model, with only one fitting parameter dB/dT=2.5×10−5 K−1 for all semiconductors, results in the best agreement with experimental data.
Grain orientation in thick laser-deposited Y1Ba2Cu3O7−δ films: Adjustment of c-axis orientation
Authors: Sievers S. , Mattheis F. , Krebs H. U. , Freyhardt H. C.
Published: July 2002

Y1Ba2Cu3O7−δ films with a thickness between 0.5 and 5 μm were grown on Si covered with an amorphous SiO2 layer, on Zr foils, and on a single-crystalline MgO substrates by KrF laser ablation. The influence of film thickness and substrate temperature on the structure, texture, and microstructure of the as-grown films was investigated by x-ray diffraction and scanning electron microscopy. At an appropriate, substrate-dependent temperature, on all three substrate materials, the films grow c-axis oriented up to a thickness of about 2 μm (critical thickness), followed by a sharp transition to a-axis-oriented growth occurring within about 100 nm. Similar changes could be observed by lowering the substrate temperature by 120 °C. Therefore, the hypothesis was propounded that the thickness dependence of the growth orientation of the film is due to a decrease of the surface temperature. To prove this the influence of raising the substrate temperature during the growth process was investigated. It could be shown that a linear increase of the substrate temperature leads to completely c-axis-oriented films up to thicknesses of 5 μm. A change of the thermal emissivity of the film surface as a possible cooling mechanism is discussed.
Magnetic field distribution measurements in a low-pressure inductive discharge
Authors: Piejak R. , Godyak V. , Alexandrovich B.
Published: July 2002

The magnitude and relative phase of the rf magnetic flux density has been measured as a function of radial position using a magnetic (B-dot) probe in an inductively coupled low pressure argon discharge driven at 13.56 MHz. The spatial variation of the rf electric field and the discharge current density were determined from the probe measurements and are compared at gas pressures of 3, 30, and 300 mTorr for a fixed discharge power of 50 W and at discharge powers of 21, 50, and 103 W for a fixed gas pressure of 30 mTorr. In the active zone near the induction coil where the rf field is strong, the electron rf drift velocity derived from the B-dot data and Langmuir probe measurement was found to be considerably less than the electron thermal velocity even at the lowest argon pressure. For an axially symmetric inductive discharge, as is found in many applications, a two-dimensional magnetic probe measurement with space and phase resolution is necessary to correctly infer the rf electric field and the discharge current density distribution.
Simultaneous measurements of thermoluminescence and thermally stimulated currents in poly(N-vinylcarbazole)/polycarbonate blends
Authors: Gl/owacki I. , Ulański J.
Published: July 2002

Photoconducting model systems: poly(N-vinylcarbazole)/polycarbonate (PVK/PC) blends were investigated by simultaneous measurements of thermally stimulated currents (TSC) and thermoluminescence (TL). It was found that with increasing applied electric field, the TSC maximum increases and shifts towards lower temperatures, but the TL maximum decreases and its position on temperature scale remains unchanged. Moreover, the TSC maximum shifts also towards lower temperatures with increasing PVK content and with decreasing sample thickness and its position depends on the applied electric field polarity in a case when the sample is excited at low temperatures with a strongly absorbed light. The position of the TL maximum is unaffected by these factors. It is concluded that the field dependent TSC maximum cannot be interpreted in terms of the Poole-Frenkel effect, but it is the so-called ‘‘transport peak,’’ determined by transport and trapping phenomena, and can be regarded as thermally stimulated time-of-flight signal. Spectral analysis of TL indicates that it is characteristic of PVK phosphorescence. It results from recombination of charge carriers thermally liberated from neutral traps under the influence of a charged recombination center, and it can be described by a model based on the Onsager theory of geminate recombination. The detrapping is probably stimulated by the so called ‘‘wet dog’’ effect, related to molecular γ-relaxation in PVK.
Nanometer-scale oxidation of Si(100) surfaces by tapping mode atomic force microscopy
Authors: Pérez-Murano F. , Abadal G. , Barniol N. , Aymerich X. , Servat J. , Gorostiza P. , Sanz F.
Published: July 2002

The nanometer-scale oxidation of Si(100) surfaces in air is performed with an atomic force microscope working in tapping mode. Applying a positive voltage to the sample with respect to the tip, two kinds of modifications are induced on the sample: grown silicon oxide mounds less than 5 nm high and mounds higher than 10 nm (which are assumed to be gold depositions). The threshold voltage necessary to produce the modification is studied as a function of the average tip-to-sample distance.
Diamond nucleation on stressed substrates
Authors: Hirakuri Kenji K. , Mutsukura Nobuki , Machi Yoshio
Published: July 2002

The type of substrate and the condition of the substrate surface have a pronounced influence on the nucleation density and growth direction of chemical-vapor-deposition (CVD) diamond particles. Nucleation and the subsequent growth of CVD diamond particles on several kinds of stressed substrates were investigated. The substrate stress was controlled by a mechanical system composed of a metal spring and metal rod, and had an obvious influence on the diamond nucleation density of CVD diamond particles. Diamond nucleation density on the stressed substrate was estimated to be greater than that for a normal substrate. It was found that the nucleation density radially decreased with distance from the point of stress on the substrate. The stressed Si and W substrates were examined by x-ray-diffraction analysis and a scanning electron microscope. After growth of the diamond particles, the analysis indicated that the full width at half-maximum of the substrate Si (400) peak decreased as the magnitude of the stress increased, and that the lattice constant of the Si substrate was decreased by the stress. At the close vicinity of the stress point, the silicon carbide layer was fabricated in a short time as compared with the normal Si substrate.
Nonexponentiality in photoinduced current transients in undoped semi-insulating gallium arsenide
Authors: Giri P. K. , Mohapatra Y. N.
Published: July 2002

An isothermal spectroscopic technique called time-analyzed transient spectroscopy (TATS) has been used to study photoinduced current transients in undoped semi-insulating GaAs. It is demonstrated that this has many advantages over conventionally used photoinduced transient spectroscopy (PITS). Specifically, TATS provides both quantitative measure and qualitative insights to the nonexponentiality of current transients commonly encountered in these materials. Using this spectroscopy, features related to enigmatic negative peaks resulting from rising current transients in these materials are reported. A simple kinetic model has been proposed to explain the essential features of rising transients leading to negative peaks in both TATS and PITS spectra.
Magnetic anisotropy of Co on Cu(1 1 17)
Authors: Wulfhekel W. , Knappmann S. , Oepen H. P.
Published: July 2002

The in-plane magnetic anisotropy of ultra-thin Co films, epitaxially grown on Cu(1 1 17), was determined in situ by means of the magneto-optic Kerr effect down to thicknesses as low as 2 monolayers. Uniaxial and biaxial anisotropy contributions were observed. At room temperature, the uniaxial component is dominant and the easy axis of magnetization is parallel to the step edges. Above 4 monolayers the magnetic anisotropy exhibits a thickness dependence which can be described by volume and interface contributions. For thinner films a pronounced deviation from that behavior is found. The anisotropy drops abruptly by one order of magnitude below 3 monolayers. Thickness dependent relaxations are proposed as driving forces for that behavior.
Crystallographic texture in cubic boron nitride thin films
Authors: Medlin D. L. , Friedmann T. A. , Mirkarimi P. B. , Cardinale G. F. , McCarty K. F.
Published: July 2002

We examine the crystallographic texture exhibited by cubic boron nitride (cBN) in thin films grown by ion-assisted deposition. Our analysis indicates that the cBN is preferentially oriented such that individual crystallites have at least one [111] direction lying in the plane of the film but are otherwise randomly oriented about (1) the substrate normal and (2) the in-plane cBN [111] axis. This preferential orientation is consistent with an alignment between the cBN {111} planes and the basal planes of the layer of highly oriented graphitic boron nitride that forms in the initial stages of film growth.
Ion-beam synthesis of amorphous SiC films: Structural analysis and recrystallization
Authors: Serre C. , Calvo-Barrio L. , Pérez-Rodríguez A. , Romano-Rodríguez A. , Morante J. R. , Pacaud Y. , Kögler R. , Heera V. , Skorupa W.
Published: July 2002

The analysis of SiC films obtained by carbon ion implantation into amorphous Si (preamorphized by Ge ion implantation) has been performed by infrared and Raman scattering spectroscopies, transmission electron microscopy, Rutherford backscattering, and x-ray photoelectron spectroscopy (XPS). The data obtained show the formation of an amorphous Si1−xCx layer on top of the amorphous Si one by successive Ge and C implantations. The fitting of the XPS spectra indicates the presence of about 70% of Si–C bonds in addition to the Si–Si and C–C ones in the implanted region, with a composition in the range 0.35<x<0.6. This points out the existence of a partial chemical order in the layer, in between the cases of perfect mixing and complete chemical order. Recrystallization of the layers has been achieved by ion-beam induced epitaxial crystallization (IBIEC), which gives rise to a nanocrystalline SiC layer. However, recrystallization is not complete, observing still the presence of Si–Si and C–C bonds in an amorphous phase. Moreover, the distribution of the different bonds in the IBIEC processed samples is similar to that from the as-implanted ones. This suggests that during IBIEC homopolar bonds are not broken, and only regions with dominant Si–C heteropolar bonds recrystallize.
Preparation of acicular iron nanoparticles by the reduction of ferrous salt in the presence of tubular lecithin assemblies
Authors: Cain Jason L. , Nikles David E.
Published: July 2002

Ferromagnetic α-Fe particles were prepared by reduction of Fe2+ in the presence of tubular lecithin assemblies found in lecithin/cyclohexane/water association colloids. The particles consisted of both large single- and multi-domain acicular rods, 0.25–5 μm in length with a length to width ratio of at least 6:1, encapsulated in lecithin. The coercivity depended on the reaction medium and ranged from 7 to 131 Oe. The highest coercivity particles were obtained in the cyclohexane-rich region of the ternary phase diagram.
Domain wall jaggedness induced by the random anisotropy orientation in magneto-optic materials: A computer simulation study
Authors: Han Seung Kee , Yu Seong-Cho , Rao K. V.
Published: July 2002

Computer simulations of magnetic domain structure in thin films of amorphous rare-earth transition metal magneto-optic recording media are performed. In these simulations, the recording media are two-dimensional lattice of 64×64 magnetic moments with a lattice distance of 10 Å. The motion of the magnetic moments follows the dynamic Landau–Lifshitz–Gilbert (LLG) equation under the influence of an effective field arising from the local anisotropy, nearnest-neighbor exchange, classical dipole–dipole interaction, and an external applied field. Within the constraints of the LLG equation, we have investigated the process of domain wall formation for a medium with random anisotropy orientation. Even for very small randomness of anisotropy orientation, it is found that the 2π-Bloch wall line appears at the boundaries of magnetic domains and the domain wall is jagged. Finally, the effects of exchange stiffness coefficient Ax and anisotropy coefficient Ku on the wall width and the jaggedness which measured the straightness of domain wall are studied.
Structure and magnetism in fcc magnetic transition metals on (001) diamond (abstract)
Authors: Wolf J. A. , Krebs J. J. , Idzerda Y. U. , Prinz G. A. , Kemner K. M.
Published: July 2002

We have prepared single crystal face centered cubic (fcc) magnetic transition metal films (Co,Ni,Fe) on (001) diamonds, the thickness of the films varying between a few tenths of a nanometer to over 100 nm. The crystalline quality and fourfold symmetry of these layers was monitored in situ during the film growth with RHEED and a chemical analysis was performed using Auger spectroscopy. In addition, the structure of the samples was investigated ex situ using X-ray diffraction and EXAFS, demonstrating the single crystal, fcc (001) structure throughout each entire film. The magnetic characterization was performed with Ferromagnetic Resonance (FMR) and Superconducting Quantum Interference Device (SQUID) hysteresis loops. The saturation magnetization of the Co films is only slightly lower than the literature values. The coercive fields are very small (∼25 Oe) and the magnetization reversal very sharp. The FMR yielded a fourfold anisotropy comparable to literature values. The observation of the first standing spinwaves underlines the good quality. The Ni films are tetragonally distorted due to the 1.2% mismatch. The FMR data indicate a significant perpendicular anisotropy slightly smaller than 4πM, the fourfold in-plane anisotropy being comparable to the bulk value. The saturation magnetization is reduced by 30% compared to bulk values, probably due to nickel-carbide which was observed in the XRD data from some samples. Neither the in-plane 〈110〉 nor the in-plane 〈100〉 axis show an easy axis behavior and both require fields in excess of 6000 Oe to saturate, the coercive fields being about 200 Oe. The RHEED patterns of the Fe films show single crystal growth with a lattice constant comparable to diamond, indicating an fcc structure. A similar structural and magnetic characterization of these films will also be presented.
The role of stress-induced anisotropy in longitudinal thin film magnetic recording media
Authors: Ross C. A. , Schabes M. E. , Ranjan R. , Bertero G. , Chen T.
Published: July 2002

Measurements of stress and magnetostriction have been made in longitudinal CoCrTa magnetic recording media. The stress in Cr and CoCrTa/Cr films deposited onto grooved substrates is higher parallel to the grooves compared to perpendicular to the grooves by up to a few hundred MPa. Combined with a magnetostriction coefficient of −3×10−5 in CoCr10Ta6/Cr, magnetostriction makes a contribution to the in-plane anisotropy observed in such media.
Alternating current (ac) susceptibility studies in Ni-SiO2 granular film
Authors: Zhao B. , Chow Jeff Y. , Yan X.
Published: July 2002

Alternating current (ac) magnetic susceptibility was measured as a function of frequencies (1 Hz–1 kHz) and temperatures (5–300 K) in nominal Ni-SiO2 granular films for zero field cooled samples. When temperature is well below and well above the blocking temperature Tb, the real part of the susceptibility, χ′, remains independent of frequency, while the imaginary part of the susceptibility, χ″, is essentially zero within the experimental error. Near Tb, however, both χ′ and χ″ were found to be greatly enhanced, and χ′ decreases with increasing frequency following log(f ) dependence. A scaling method was applied for χ′(f,T) to determine the distribution of energy barriers and that of particle size.
Optical absorption in alloys of Si, Ge, C, and Sn
Authors: Orner B. A. , Hits D. , Kolodzey J. , Guarin F. J. , Powell A. R. , Iyer S. S.
Published: July 2002

Group IV semiconductor alloy systems offer promise as variable band gap alloys compatible with Si technology. Binary, ternary, and quaternary group IV alloys were grown by molecular beam epitaxy on Si substrates. The fundamental absorption edge was measured by Fourier transform infrared spectroscopy to obtain the optical band gap of the alloys, and the position of the fundamental absorption edge was observed to depend on the experimentally measured alloy composition. Our results indicate a variety of Si-rich group IV alloys with various band gaps are experimentally producible.
Interface properties of (In,Ga)Sb/InAs heterostructures
Authors: Zborowski J. T. , Vigliante A. , Moss S. C. , Golding T. D.
Published: July 2002

The interfaces between (In,Ga)Sb and InAs(100) grown by molecular-beam epitaxy have been investigated by x-ray diffraction, x-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy. The InAs on (In,Ga)Sb interface has been found to be significantly broader than the reverse one and the asymmetry is the result of mixing between arsenic and antimony. The studies of the growth surfaces have shown a persistent presence of antimony on an InAs surface suggesting a lower, antimony-rich, surface free energy. This energy imbalance indicates a driving mechanism behind the mixing of the group-V elements as the growth of InAs on (Ga, In)Sb is commenced. The band offset of the InAs on (Ga,In)Sb has been determined by XPS. The In 4d and Ga 3d to valence-band maximum binding energy differences for bulk InAs and GaSb were obtained by fitting the experimental valence-band density of states (VBDOS) to the experimentally broadened, theoretical VBDOS. The core-level separation between In 4d and Ga 3d peaks from the InAs/GaSb structure was determined by fitting Gaussian–Lorentzian functions to the peaks. The band offset was determined to be 0.62±0.1 eV.
A model of electroluminescence in organic double-layer light-emitting diodes
Authors: Khramtchenkov D. V. , Bässler H. , Arkhipov V. I.
Published: July 2002

A model has been developed for charge recombination in double-layer organic light-emitting diodes (LEDs) in which charge transport across the interface between the anodic and cathodic cell compartments is impeded by energy barriers. Current flow is assumed to be controlled by the interplay between the field-assisted injection of majority carriers (holes) and minority carriers (electrons) at the contacts and field-assisted barrier crossing, both obeying Fowler–Nordheim-type relations. Charge recombination at the internal interface is considered as the dominant source for electroluminescence. Accumulation of majority carriers at that interface causes an enhancement of the cathodic electric field giving rise to enhanced electron injection. This effect tends to compensate for imbalanced injection due to different energy barriers at the contacts and causes an increase of the luminescence yield as compared to single-layer LEDs. The model is able to predict (i) the redistribution of the electric field inside the LED, (ii) the field dependence of the cell current, (iii) the dependence of the steady state luminescence intensity, (iv) the luminescence yield as a function of the cell current, and (v) the characteristic rise time of the light output, each parametric in the cathodic and the interfacial energy barriers normalized to the energy barrier for hole injection.
Critical points of Si1−yCy and Si1−x−yGexCy layers strained pseudomorphically on Si(001)
Authors: Kissinger W. , Osten H. J. , Weidner M. , Eichler M.
Published: July 2002

We investigate the influence of carbon on the optical transitions of Si1−yCy and Si1−x−yGexCy layers grown pseudomorphically on a Si(001) substrate also including full strain compensation. The layers were investigated by spectroscopic ellipsometry and electroreflectance spectroscopy for carbon fractions y≤1.2 at. % and germanium fractions up to x=16 at. %. The spectra were analyzed by measuring and fitting electroreflectance spectra at 80 K and ellipsometry data at room temperature, resulting for both techniques in a weak and nearly linear dependence on the carbon fraction at all transitions. The results of both techniques are compared and discussed. The strong line broadening for increasing carbon fractions can be caused by a high scattering efficiency of the carbon. Our results indicate that the interpretation of optical spectra of carbon-containing alloys cannot be performed straightforwardly by simple interpolating between the appropriate band structures of silicon, germanium, and carbon. An analysis based on strain-induced contributions only also does not describe the experimental results correctly. For a complete description of the observed energy shifts detailed band structure calculations and further experimental data are necessary.
Fluorescence properties of Sm2+ ions in silicate glasses
Authors: Nogami Masayuki , Abe Yoshihiro
Published: July 2002

The fluorescence properties of Sm2+ ions incorporated in silicate glasses were investigated using steady-state and laser-induced line narrowing techniques. Sm2+ ions were incorporated in the glass matrix by heating the Sm3+ ion-containing glasses in H2 gas. The reaction of Sm3+ into Sm2+ was accelerated with increasing temperature. The fluorescence intensity of the Sm2+ ions increased with heating to 800 °C. Further heating above 900 °C precipitated mullite crystals. Accordingly, the fluorescence intensity of the Sm2+ ions abruptly decreased and the fluorescence of Sm3+ ions again appeared. Sm2+ ions are preferentially coordinated with Al–O polyhedrons. The crystallization of mullite induces the decrease in Al2O3 content in the remaining glassy phase, resulting in the redox equilibrium producing the high valence samarium ion. Three Stark splitting fluorescence peaks of the 5D0→7F1 transition were analyzed to determine the parameters of the crystal field surrounding the Sm2+ ion. The decrease in the number of Al–O bonds surrounding Sm2+ causes the Sm2+ ions’ coordination number to decrease with the short distance of the Sm–O bonds.
Nanometer scale optical studies of twin domains and defects in lanthanum aluminate crystals
Authors: McDaniel Eric B. , Hsu J. W. P.
Published: July 2002

A near-field scanning optical microscope (NSOM) in transmission geometry was used to study optical characteristics of twin domains, domain boundaries, and other defects in lanthanum aluminate (LaAlO3) single crystals. Large optical contrast between domains was observed using linearly polarized NSOM light. This domain contrast can be changed and even reversed by varying the direction of the polarization axis with respect to the local crystallographic axis. We further demonstrate that the contrast depends critically on the numerical aperture of the collection objective lens, enabling us to differentiate between possible contrast mechanisms. In addition, microscopic stress birefringence associated with the domain boundaries was observed under crossed-polar conditions. Submicron circular optical features on LaAlO3 are attributed to surface terminations of dislocations.
Electrical characterization and physical analysis of epitaxial CoSi2 grown from the Si〈100〉/Ti/Co system
Authors: Hatzikonstantinidou Sofia , Wikman Peter , Zhang Shi-Li , Sture Petersson C.
Published: July 2002

The electrical and structural properties of epitaxial CoSi2 layers grown on Si by solid-state interaction between Ti/Co bimetallic layers and Si〈100〉 substrates have been investigated. The Schottky barrier height (SBH) of the CoSi2–Si contact determined by current–voltage characteristics at room temperature varies between 0.64 and 0.71 eV on the n-type substrates and between 0.47 and 0.43 eV on the p-type substrates. The variation of the SBH is found to be related to the interfacial properties at the CoSi2–Si contact which is in turn determined by the heat treatment used for the CoSi2 formation. The formation of polycrystalline CoSi2 is found to be responsible for the low SBH deviated from 0.71 eV on the n-type substrates and the high SBH deviated from 0.43 eV on the p-type substrates. The formation of a ternary compound, identified as Co16Ti6Si7, within the epitaxial CoSi2 does not seem to affect the SBH on the n-type substrates. A permeable base transistor (PBT) was fabricated using the epitaxial CoSi2 for self-aligned contact metallization. The current–voltage characteristics of the PBTs are presented.
Excitonic properties in Zn1−xCdxSe/ZnSe multi-quantum well structures by one- and two-photon spectroscopy
Authors: Netti M. C. , Lepore M. , Adinolfi A. , Tommasi R. , Catalano I. M. , Vanzetti L. , Sorba L. , Franciosi A.
Published: July 2002

A detailed investigation of excitonic states by means of one- and two-photon absorption photoluminescence excitation spectroscopy in Zn1−xCdxSe/ZnSe multi-quantum wells is reported. Ground state and excited heavy- and light-hole excitons associated with the n=1,2 subbands have been selectively probed exploiting the different selection rules governing one- and two-photon absorption processes. Experimental exciton transition energies are found to be in good agreement with theoretical predictions when strain and confinement effects are included. The comparison between experiments and theory allowed us to single out a well defined set of band parameters and to gauge the band-offsets in these heterostructures. Furthermore, the absorption band edge clearly evident in the two-photon absorption photoluminescence excitation spectra allowed us to directly measure excitonic binding energies with good accuracy.
Depth inhomogeneity of porous silicon layers
Authors: Thönissen M. , Billat S. , Krüger M. , Lüth H. , Berger M. G. , Frotscher U. , Rossow U.
Published: July 2002

The two possible causes of depth inhomogeneities of the microstructure of porous silicon are changes in the HF concentration with depth and a varying chemical etching rate of the porous silicon layer. During anodization chemical etching will become important for microporous silicon — e.g. p-porous silicon — due to the large internal surface area, especially at long etching times. On the other hand, a considerable decrease of the HF concentration will occur during etching with high current densities to produce p+-porous silicon with high porosities. We have investigated the depth inhomogeneity of porous silicon layers by spectroscopic ellipsometry, Raman spectroscopy and photoluminescence measurements. From a line shape analysis of the Raman signal a size distribution of nanocrystals is deduced. For p-porous silicon smaller nanocrystals are found near the surface of the layer; for p+-porous silicon etched with high current densities smaller nanocrystals are found near the porous silicon/substrate interface.
Growth and photoluminescence of high quality SiGeC random alloys on silicon substrates
Authors: Liu C. W. , St. Amour A. , Sturm J. C. , Lacroix Y. R. J. , Thewalt M. L. W. , Magee C. W. , Eaglesham D.
Published: July 2002

We report chemical vapor deposition growth of SiGeC layers on 〈100〉 Si substrates. At the growth temperature of 550 °C, the C concentration as high as 2% can be incorporated into SiGe (Ge content ∼ 25%) to form single crystalline random alloys by using low flow of methylsilane (0.25 sccm) as a C precursor added in a dichlorosilane and germane mixture. For intermediate methylsilane flow (0.5 sccm – 1.5 sccm), the Fourier transform infrared spectroscopy (FTIR) absorption spectra indicate the growth of amorphous layers. For the layers with high flow of methylsilane (12 sccm), there are silicon-carbide-like peaks in the FTIR spectra, indicating silicon carbide precipitation. The films were also characterized by x-ray diffraction, high resolution transmission electron microscopy, secondary ion mass spectroscopy, and Rutherford backscattering spectroscopy to confirm crystallinity and constituent fractions. The defect-free band-edge photoluminescence at both 30 K and 77 K was observed in Si/SiGeC/Si quantum wells, even at power densities as low as 0.5 W/ cm2 and 1 W/cm2, respectively. Deep photoluminescence around 0.8 eV and luminescence due to D3 dislocations at 0.94 eV were not observed under any excitation conditions.
Ion damage buildup and amorphization processes in GaAs–AlxGa1−xAs multilayers
Authors: Tan H. H. , Jagadish C. , Williams J. S. , Zou J. , Cockayne D. J. H.
Published: July 2002

The nature of ion damage buildup and amorphization in GaAs–AlxGa1−xAs multilayers at liquid-nitrogen temperature is investigated for a variety of compositions and structures using Rutherford backscattering-channeling and cross-sectional transmission electron microscopy techniques. In this multilayer system, damage accumulates preferentially in the GaAs layers; however, the presence of AlGaAs enhances the dynamic annealing process in adjacent GaAs regions and thus amorphization is retarded close to the GaAs–AlGaAs interfaces even when such regions suffer maximum collisional displacements. This dynamic annealing in AlGaAs and at GaAs–AlGaAs interfaces is more efficient with increasing Al content; however, the dynamic annealing process is not perfect and an amorphous phase may be formed at the interface above a critical defect level or ion dose. Once an amorphous phase is nucleated, amorphization proceeds rapidly into the adjacent AlGaAs. This is explained in terms of the interplay between defect migration and defect trapping at an amorphous–crystalline or GaAs–AlGaAs interface. In addition, enhanced recrystallization of the amorphous GaAs at the interface may occur during heating if an amorphous phase is not formed in the adjacent AlGaAs layer. This is most likely the result of mobile defects injected from the AlGaAs layer during heating.
The early stages of solid-state reactions in Ni/Al multilayer films
Authors: Michaelsen C. , Lucadamo G. , Barmak K.
Published: July 2002

Ni/Al multilayer films with pair thicknesses of 10 and 20 nm and with overall compositions in the range 48–88 at. % Al were prepared by sputtering. For comparison, Ni-Al alloy films in the same concentration range were prepared by co-deposition of the elements. The films were studied by x-ray diffraction, electron diffraction, and differential scanning calorimetry. It was found that the B2 NiAl phase with a metastable concentration of approximately 63 at. % Al was the first phase to grow upon annealing of the multilayer films. The growth of this phase could be described by Johnson–Mehl–Avrami kinetics with an activation energy of 0.8 eV and an Avrami exponent of 0.5. This low activation energy was consistent with the observation that the phase had formed during deposition and continued to grow upon annealing at low temperatures to thicknesses of a few nanometers. If the reactant phases were not fully consumed by the B2 phase growth, the subsequent reaction was the formation of NiAl3, previously thought to be the first product phase in the Ni-Al system. The reduction of driving force by the preceding B2 phase growth explains why the formation of NiAl3 takes place by a nucleation-and-growth process, an observation that has been discussed controversially in the recent literature. The nucleation and growth of NiAl3 had an activation energy of 1.5 eV in agreement with previous studies.
Influence of surface coverage on the effective optical properties of porous silicon modeled as a Si-wire array
Authors: Lugo J. E. , del Rio J. A. , Tagüeña-Martı́nez J.
Published: July 2002

The effective dielectric function, effective absorption coefficient and effective refractive index for a model of porous silicon (PS) are calculated using the volume and surface averaging method. The model consists of periodic Si wires with different surface coverages. This approach allows to obtain analytical results within certain approximations. The method uses experimental parameters to characterize the bulk and the surface. We choose the bulk c-Si, and cover it with three different possible surface skins: siloxanes, a-Si:H and SiO2. The results are compared with PS experimental data and other theoretical approaches for silicon wires. We obtain good agreement for certain coatings. Our results emphasize the important role of surface coatings in the effective response of porous silicon.
Optical measurement of the ambipolar diffusion length in a ZnCdSe–ZnSe single quantum well
Authors: Logue F. P. , Fewer D. T. , Hewlett S. J. , Heffernan J. F. , Jordan C. , Rees P. , Donegan J. F. , McCabe E. M. , Hegarty J. , Taniguchi S. , Hino T. , Nakano K. , Ishibashi A.
Published: July 2002

We describe a straightforward technique for the measurement of carrier diffusion in semiconductors. Using an optical microscope we can spatially image luminescence with a resolution of ∼500 nm. We measured the ambipolar diffusion length in a Zn0.75Cd0.25Se–ZnSe single quantum well by fitting the spatially resolved luminescence profile with the solution of the two-dimensional diffusion equation. The ambipolar diffusion length was found to be 498 nm at a carrier density of ∼1×1018 cm−3 and we deduce an ambipolar diffusion constant of 1.7 cm2 s−1.
Effect of surface anisotropy on the exchange resonance modes
Authors: Aharoni Amikam
Published: July 2002

Exchange resonance modes in small ferromagnetic spheres were first predicted for the case of a cylindrical symmetry of the magnetization configurations. It is shown here that when the constraint of this symmetry is removed, other modes are also possible, but their lowest resonance frequency is very nearly the same as that of one of the cylindrically symmetric modes. Therefore, new resonances may only be sought at higher frequencies than those observed in the most recent experiment. It is also shown that the dependence on the particle size observed in that experiment may be accounted for by assuming a rather strong surface anisotropy in the measured spheres. The original prediction of an R−2 dependence, typical for the curling mode, may still be reached if either the particle size or the surface anisotropy is considerably reduced.
Correlations for damage in diffused-junction InP solar cells induced by electron and proton irradiation
Authors: Yamaguchi Masafumi , Takamoto Tatsuya , Taylor Stephen J. , Walters Robert J. , Summers Geoffrey P. , Flood Dennis J. , Ohmori Masamichi
Published: July 2002

The damage to diffused-junction n+-p InP solar cells induced by electron and proton irradiations over a wide range of energy from 0.5 to 3 MeV and 0.015 to 20 MeV, respectively, has been examined. The experimental electron and proton damage coefficients have been analyzed in terms of displacement damage dose, which is the product of the particle fluence and the calculated nonionizing energy loss [G. P. Summers, E. A. Burke, R. Shapiro, S. R. Messenger, and R. J. Walters, IEEE Trans. Nucl. Sci. 40, 1300 (1993).] Degradation of InP cells due to irradiation with electrons and protons with energies of more than 0.5 MeV show a single curve as a function of displacement damage dose. Based on the deep-level transient spectroscopy analysis, damage equivalence between electron and proton irradiation is discussed. InP solar cells are confirmed to be substantially more radiation resistant than Si and GaAs-on-Ge cells.