bluuebunny/crossref_metadata_2025 huggingface dataset:
Enhanced magnetic moment and magnetic ordering in MnNi and MnCu surface alloys
Authors: O’Brien W. L. , Tonner B. P.
Published: July 2002

Two-dimensional, ordered surface alloys of MnCu and MnNi were grown on Cu(001) and Ni(001) substrates. These surface alloys, which have structures that are not present in the bulk phase diagram, are highly corrugated with a c(2×2) periodicity. The stability of these surfaces has been predicted theoretically to be due to a gain in magnetic energy of the Mn atoms. Using a combination of soft x-ray absorption spectroscopy and x-ray magnetic circular dichroism we find that Mn is in a high-spin state and is ferromagnetically ordered in the MnNi surface alloy. MnNi surface alloys have been grown with an easy axis of magnetization perpendicular to the surface.
Observation of x-ray magnetic circular dichroism at the Rh M2,3 edge in Co-Rh alloys
Authors: Harp G. R. , Parkin S. S. P. , O’Brien W. L. , Tonner B. P.
Published: July 2002

The M2,3 edge of Rh in Co-Rh alloys shows measurable x-ray magnetic circular dichroism. Such dichroism is not present, however, for analogous alloys of Co-Ru. The induced Ru magnetic moment, if any, is thus demonstrated to be significantly smaller than the induced Rh moment in otherwise similar Co alloys.
Influence of crystal structure on the magnetoresistance of Co/Cr multilayers
Authors: Liou Y. , Huang J. C. A. , Yao Y. D. , Lee C. H. , Wu K. T. , Lu C. L. , Liao S. Y. , Chen Y. Y. , Liang N. T. , Yang W. T. , Chen C. Y. , Hu B. C.
Published: July 2002

Epitaxial Co/Cr multilayers, and single-crystal Co thin films etc. have been grown on MgO and Al2O3 substrates with Cr and Mo as buffer layers by molecular beam epitaxy technique. From the structure and magnetoresistance studies, we have found that the ferromagnetic anisotropy of resistance (AMR) is strongly influenced by the buffer layer, but with negligible effect due to the variation of the structure of Co films. The AMR of Co film on Cr buffer layer is quite small (0.1%); however, the MR of Co/Cr multilayers is almost one order larger than the AMR of Co film on Cr buffer layer. An enhancement factor of 4 for the MR in Co/Cr multilayers by the interface roughness has been observed. This suggests that the effect due to the spin dependent scattering at the interfacial regions of the superlattice is larger than that due to the spin dependent scattering in the ferromagnetic layers for the MR in the Co/Cr multilayer system.
Lorentz electron microscopy studies of magnetization reversal processes in epitaxial Fe(001) films
Authors: Gu E. , Bland J. A. C. , Daboo C. , Gester M. , Brown L. M. , Ploessl R. , Chapman J. N.
Published: July 2002

The magnetic domain structure and microscopic magnetization reversal processes in epitaxial Fe/GaAs(001) films with cubic anisotropy and in-plane easy axes have been investigated by a Lorentz microscope equipped with a magnetizing stage. For the films of a few hundred angstroms thickness we observe the single domain remanent state predicted for a two-dimensional film but find that domains play a crucial role in the magnetic reversal process. For reversal along the in-plane 〈110〉 directions (hard axes), magnetization reversal proceeds via a combination of coherent rotation and displacements of weakly pinned 90° domain walls at critical fields. For magnetization reversal along the in-plane 〈100〉 directions (easy axes), an irregular checkerboard domain structure develops at the critical field and both 180° and 90° domain walls coexist. The reversal of the domains with magnetization vector opposite to the applied field direction takes place by a combination of two 90° reorientations. We discuss how these processes are related to the magnetic anisotropies present in the film and the macroscopic M-H hysteresis curves.
Critical behavior of the random Potts model
Authors: Khasanov B. M. , Belov S. I. , Tayurskii D. A.
Published: July 2002

The phase transition in the continual random n-component Potts model is studied by the renormalization group method. It is shown that for the three-dimensional model and n=3 the phase transition is to be of the first order. In the case n=2 which corresponds to the random Ising model the stable fixed point exists as early as in the one-loop approximation of renormalization group equations.
Mössbauer measurements of spin correlations in a-(Fe,Ni)90Zr9Sn
Authors: Wiarda D. , Ryan D. H.
Published: July 2002

The local spin structure of the partially frustrated amorphous Fe90−xNixZr9Sn system was investigated by Mössbauer spectroscopy for x=1,3. The magnetic probe atom 57Fe and the nonmagnetic 119Sn were used to monitor the local correlation of the spins. The additional Ni renders the system less frustrated. We measured the temperature dependence of the hyperfine fields of the two probe atoms. The ratio 〈Bhf(119Sn)〉/〈Bhf(57Fe)〉 rises with increasing temperature. This is in contrast to the temperature independence found for this ratio in Fe92Zr7Sn. A similar rise was found in the crystalline spin-glass AuFe where the slope was much larger and was related to a loss of longitudinal correlations.
Heavy fermion behavior of U2T2X compounds
Authors: Havela L. , Sechovský V. , Svoboda P. , Diviš M. , Nakotte H. , Prokeš K. , de Boer F. R. , Purwanto A. , Robinson R. A. , Seret A. , Winand J. M. , Rebizant J. , Spirlet J. C. , Richter M. , Eschrig H.
Published: July 2002

Magnetic and specific-heat studies of U2T2X compounds show a frequent occurrence of the γ enhancement in conjunction with the onset of antiferromagnetic ordering. The largest value of 830 mJ/mol K2 was observed in U2Pt2In, which is nonmagnetic down to 1.2 K. Variations of electronic structure are documented by optimized relativistic LCAO calculation.
Magnetic and magnetocaloric properties of melt-spun GdxAg100−x alloys
Authors: Fuerst C. D. , Herbst J. F. , Mishra R. K. , McMichael R. D.
Published: July 2002

With the object of assessing their magnetocaloric properties, we have prepared GdxAg100−x alloys (x=50, 70, 75, 77.5, 80, 100) by melt spinning. The microstructures of the melt-spun ribbons were investigated by x-ray diffraction and transmission electron microscopy. Data from field- and temperature-dependent magnetization measurements were used to calculate the field-induced entropy change ΔS. The Gd-only ribbons are characterized by a single ΔS peak near 280 K. For the GdxAg100−x (70≤x≤80) materials ΔS contains another peak in the 100–120 K range which we ascribe to an amorphous, ferromagnetic Gd-Ag component. We find melt-spun Gd50Ag50 ribbons to consist primarily of crystalline GdAg (cubic CsCl structure), which is antiferromagnetic.
Magnetoresistance of CuNiCo ternary alloys (abstract)
Authors: Beach R. S. , Rao D. , Berkowitz A. E.
Published: July 2002

Magnetoresistance (MR) measurements on four film samples of the ternary alloy CuNiCo were performed at room temperature in fields H≤20 kOe. Following growth by sputtering onto thermally oxidized Si substrates, the films (100–200 nm thick) were annealed between 1 and 6 h at temperatures TA=200, 350, 500, and 700 °C. The samples display anisotropic magnetoresistances (AMR) for H<100 Oe of up to 3% at room temperature for Cu20Ni53Co27 (TA=350 °C). Despite the fact that in the bulk these alloys tend to phase separate into Co rich and Co poor regions, we find evidence for giant magnetoresistance (i.e., an isotopic negative component to the MR) in only one sample, Cu51Ni17Co31, after a 6 h, 700 °C anneal. In the as-deposited condition, samples Cu20Ni53Co27 and Cu13N41Co46 display a pronounced asymmetry between the resistance decrease for H applied perpendicular to the current I and the corresponding increase for H parallel to I which substantially exceeds the 1:2 ratio in bulk materials or the 1:1 ratio expected for a thin film. The large observed values of AMR (more evident in samples with low Cu concentrations) are likely linked to AMR in binary CuCo alloys, which are known to exhibit large AMR. The disparity between the MR for H parallel and perpendicular to the current we attribute to magnetic anisotropy induced during fabrication.
Mixed valence in a generalized Hubbard model
Authors: Kocharian A. N. , Reich G. R.
Published: July 2002

A generalized Hubbard model involving two kinds of spinless fermions with different masses is proposed to explain the properties of mixed-valence compounds. An equivalence between the proposed model and an effective anisotropic antiferromagnetic Heisenberg model with external field is established in the strong-interaction limit. The ground-state energy and partition function are obtained analytically using generalized mean field theory which, for bipartite lattices, allows the system to be reduced to an equivalent two-site problem. The analytic behavior of the valence and compressibility under variation of pressure and the phase diagram in the ground state and at finite temperature are investigated. The conditions for a first-order transition depending on the position of the f band are obtained, taking into account the effect of local hybridization between the s and f states. The known anomalies in the behavior of nf and χ in mixed-valence systems are interpreted in analogy with the magnetization and susceptibility in the corresponding pseudospin model.
Comment on ‘‘Experimental investigation and modeling of the role of extended defects during thermal oxidation’’ [J. Appl. Phys. 74, 5821 (1993)]
Authors: Huang R. Y. S. , Dutton R. W.
Published: July 2002

In our recent article, we stated that Eq. (11) accounts for both the diffusion of interstitials to stacking faults as well as the reaction energy barrier to fault growth. It was pointed out to us by S. M. Hu that the equation that is shown only accounts for the reaction of interstitials at the dislocation loop boundary. By using this equation, we in fact assumed some reaction-limited component of dislocation loop growth. suprem−iv was used to solve the diffusion of interstitials throughout the sample, CI(depth), and in so doing, solved for the interstitial concentration that was used in Eq. (11).
SiO2/amorphous As/GaAs passivation systems prepared at room temperature by 50 Hz plasma-enhanced chemical-vapor deposition
Authors: Hashizume Tamotsu , Yoshino Masaki , Shimozuma Mitsuo
Published: July 2002

Chemical properties and thermal stability of the SiO2/amorphous As/GaAs passivation systems were investigated for the postannealing of GaAs. The 50 Hz plasma-enhanced chemical-vapor-deposition technique allowed the deposition of high-quality SiO2 films onto GaAs at room temperature without destroying the surface structure of amorphous-As-covered GaAs. No interfacial reactions took place in the present passivation system during the rapid thermal annealing (RTA) process. In addition, this system gave abrupt carrier profiles without extended tailing for the Si-implanted and RTA-processed GaAs.
Conductivity relaxation in germanate glasses
Authors: Saha S. K. , Maity A. K. , Chakravorty D.
Published: July 2002

An inhomogeneous laminar conductor model developed earlier has been applied to glasses in the system Na2O-GeO2. Detailed modulus spectra have been measured for glasses containing 10–20 mol % Na2O. The experimental data are in satisfactory agreement with the theoretical values as obtained by this model.
A broad photoluminescence band in heavily Ge-doped GaAs grown by the liquid-encapsulated Czochralski technique
Authors: Kang Junyong , Huang Qisheng
Published: July 2002

Heavily Ge-doped GaAs single crystals were grown by the liquid-encapsulated Czochralski technique in the absence and the presence of a magnetic field of 4000 G. Three photoluminescence emission bands A, B, and C were observed in the as-grown crystals. Bands A and B were explained by a deep acceptor and self-activated center emissions, respectively. The central energy and half width of band C increased with increasing Ge-doped concentration. The photoluminescence excitation spectrum of band C was observed to be broad. The temperature dependence of the full width at half maximum of band C was analyzed. The results were considered to be caused by distributed states with different energy positions and electron-lattice couplings due to the impurity concentration effect on the unknown Ge-related complexes responsible for band C. The emission intensity of band C was markedly weaker in the magnetic-field growth crystals where both donor and acceptor concentrations were higher than those in traditional growth crystals, which showed that the formation of the unknown Ge-related complex can be reduced and the Ge doping efficiency can be improved during crystal growth by applying a magnetic field to suppress the temperature fluctuations in molten GaAs.
Instabilities in the resistivity of intrinsic polysilicon resistors during hydrogenation: Role of sodium and cooling kinetics
Authors: Moore Chad B. , Ast Dieter G.
Published: July 2002

The influence of the hydrogenation procedure on the occasional collapse of the resistivity of intrinsic polysilicon during hydrogenation was studied. In specimens showing the effect, the resistivity was found to decrease in less than 10 s by 4 orders in magnitude during hydrogenation. This collapse in resistivity was traced to the formation of a highly conductive electron channel confined to the top 0.5 μm of the polysilicon surface. The creation of this conductive channel strongly depended on the hydrogenation procedure used. Slow cooling of the specimen in the hydrogen plasma always yielded a highly resistive state even in samples intentionally contaminated with sodium. On the other hand, the resistivity of such samples invariably collapsed when they were quenched to room temperature. Neutron activation analysis showed over 50% less sodium in the polysilicon surface for the slowly cooled samples when compared to those that were quenched. A model to explain these observations will be presented.
Neutron diffraction and magnetic studies of Nd2Fe17−x−zAlxSiz
Authors: Hu Z. , Yelon W. B.
Published: July 2002

The Al and Si double-substituted 2:17 phase, Nd2Fe17−x−zAlxSiz, was prepared and analyzed using neutron powder diffraction and SQUID magnetization measurements. Rietveld analysis of the neutron diffraction data indicates that the lattice parameters are close to a linear combination of the corresponding single-substitution compounds. The unit cell could be expanded or contracted, depending on the Al/Si ratio. The aluminum and silicon fractional occupancy on the different crystallographic sites in the double-substitution compounds are related to those of single-substitution compounds. The SQUID measurements show that all samples in this study have Curie points higher than that of the unsubstituted compound. For a Si/Al ratio of 2:1, the Curie point, 492 K, was found at a total substituent content less than in the singly substituted Si compound with the same Tc.
Molecular-beam-epitaxy doping kinetics: A rate equation model
Authors: Bendi Shridhar , Venkatsubramanian R. , Dorsey Donald L.
Published: July 2002

A rate equation model based on the master equation approach is developed for the study of molecular-beam-epitaxy doping kinetics. The model includes elementary surface processes such as adsorption, evaporation, and migration of atoms. The model is applied to the study of the surface segregation phenomenon during In doping of Si. The doping studies were performed for the following growth conditions: T in the range 500–750 °C; a growth rate of 1 μm/h; and a flux ratio JIn/JSi equal to 2.0×10−4. The predicted sticking coefficient of In versus 1/T shows excellent agreement with experiments. The sticking coefficient decreases with T due to surface segregation aided evaporation of In at higher temperature. The predicted dopant depth profile also shows excellent qualitative agreement with experiments. The surface segregation of In occurs due to a strong repulsive interaction between In and the host lattice. The results of this study show that there is a dopant-depleted zone (DDZ) where the In concentration is lower than both the bulk and the top surface layer. The observed DDZ qualitatively matches that observed in experiments. The time and growth rate dependencies of the phenomenon are studied and found to be in good agreement with experiments. The model was used to study δ doping of dopants in the range of 673 to 973 K. The results are in qualitative agreement with experimental results. With an increase in temperature, the dopant profiles become sharper. This is caused by a smoother growing surface at higher temperatures.
Torsion modulus and internal friction of a fiber-reinforced composite
Authors: Ledbetter Hassel , Lei Ming
Published: July 2002

By a kilohertz-frequency resonance method, we determined the torsion modulus and internal friction of a uniaxially fiber-reinforced composite. The composite was composed of glass fibers in an epoxy-resin matrix. We studied three fiber contents: 0, 41, and 49 vol %. The internal friction failed to fit a classical free-damped-oscillator model where one assumes a linear rule-of-mixture for three quantities: oscillator mass, force constant, and mechanical-resistance constant. The torsion modulus approximately fits a plane-wave-scattering ensemble-average model. The microstructure showed strong fiber-distribution nonhomogeneity. Considering this nonhomogeneity yielded a better agreement between model and observation. Thus, torsion-modulus measurements provide a method to detect and quantify fiber-distribution nonhomogeneity.
Electron trapping in oxynitride layers in metal-oxide-semiconductor structures
Authors: Rahat Ido , Shappir Joseph
Published: July 2002

This paper discusses electron injected induced trapping–detrapping behavior of nitrided gate insulators and its consistency with the dynamic balance model which characterizes standard silicon dioxide insulators. The experimental results show that the dynamic trapping model is also applicable for the novel nitrided insulators; trapping–detrapping characteristics are field dependent, the magnitude of the trapped negative charge is reduced when the electric field is increased, trapping–detrapping reversibility is evident, steady state trapping level is determined by the insulator field, and the trapping and detrapping transients are dominated by the density of the injected electrons. The occupation probability function is determined to be almost identical for both insulator types. The main difference between nitrided insulators and control oxides is lower trapping cross sections for nitrided insulators, and lower concentration of generated traps leading to improved immunity to degradation during charge injection. The novel dielectric layer is of potential use for the fabrication of reliable ultrathin gate oxide films for standard complementary metal-oxide-semiconductor technology.
Antiferromagnetic versus ferromagnetic coupling in Fe/Cr(107) and Cr/Fe(107)
Authors: Vega A. , Dreyssé H. , Demangeat C. , Chouairi A. , Balbás L. C.
Published: July 2002

We have calculated the local magnetic moments and magnetic order for a Fe(Cr) monolayer adsorbed on a stepped Cr(Fe)(107) substrate. The electronic structure at T=0 K has been self-consistently determined within the unrestricted Hartree–Fock approximation of the Hubbard Hamiltonian in the framework of a real-space tight-binding method. In the Cr/Fe(107) system, two magnetic arrangements have been obtained, the more stable being the less frustrated as obtained in the case of V overlayers on vicinal substrates of Fe. An analysis of both solutions in terms of the total energy calculation and the different degree of frustration is presented. For Fe/Cr(107), a two-step periodicity is obtained. The sign of the magnetization at the Fe overlayer changes from step to step. This spin-flop transition is consistent with the two-layer period oscillation recently observed in Fe/Cr/Fe wedge structures, and with the total magnetization determined from in situ magnetometer measurements during growth of ultrathin Fe films on Cr(001).
Correlation between strain fields on the facet and along the cavity in semiconductor diode lasers
Authors: Yang Jian , Cassidy Daniel T.
Published: July 2002

Strain fields on the facet and along the cavity for both ridge waveguide and oxide stripe geometry lasers were measured. The strain fields were found to be structure dependent across the stripe and essentially uniform along the stripe. A correlation between the strain in the plane of the facet and the strain along the cavity was found. This means that the strain field observed on the facet is representative of the average strain along the device and that facet measurements provide useful information on material properties along the length of the device.
Magnetic properties of Fe/Pd multilayers grown by electron-beam evaporation
Authors: Li M. , Ma X. D. , Peng C. B. , Zhao J. G. , Mei L. M. , Liu Y. H. , Gu Y. S. , Chai W. P. , Mai Z. H. , Shen B. G. , Dai D. S.
Published: July 2002

The magnetic properties of [Fe(20 Å)/Pd(x Å)]25 multilayers prepared by ultrahigh vacuum electron-beam evaporation are presented. It is found that the crystal structure of Fe layers change from bcc to fcc when the thickness of Pd layers dPd≥36 Å. The saturation magnetization per unit Fe volume at 5 K is enhanced and oscillates with the increment of the thickness of Pd layers, due to the polarization of Pd atoms. The magnetic hysteresis loops of samples indicate low coercive forces, and ferromagnetic coupling between the Fe layers for all Pd thicknesses (6–60 Å). The conversion electron Mössbauer spectra measurements proved that the magnetic moment of fcc Fe is the same as that of the bcc Fe. It is also found that the magnetic anisotropy dependence on dPd is similar to that of the saturation magnetization, and relates to the Fe layer structure transition. The Curie temperature of Fe/Pd multilayers decreases monotonously with the increasing of dPd. The low temperature magnetization measurement of Fe/Pd multilayers suggests that the interlayer coupling between Fe layers and polarization of Pd layers influences the temperature dependence of saturation magnetization. No evidence of antiferromagnetic coupling between Fe layers and giant magnetoresistance effect is found. The relationship among structure, polarization of Pd layers, and magnetic coupling is discussed.
Kinetic of the growth of chemically etched porous silicon
Authors: Di Francia G. , Citarella A.
Published: July 2002

In this communication we report the results of a systematic study on the formation of chemically etched porous silicon. Samples have been fabricated in HF/HNO3-based solutions for various etching times, using different molar concentrations of nitric acid. The growth of the porous layer has been studied by correlating film porosity and thickness to the etching time. Pore formation is shown to be more related to the surface morphology than to the etch characteristics. Preliminary results on photoluminescence measurements show that substrate morphology also play a major role in the luminescent behavior of chemically etched porous silicon.
Scheme for stationary continuous-wave vacuum- and extreme-ultraviolet lasing based on a plasma jet
Authors: Pöckl Manfred , Störi Herbert , Aumayr Friedrich
Published: July 2002

A scheme to achieve stationary continuous-wave (cw) lasing in the vacuum- and extreme-ultraviolet spectral region is proposed. It is based on recombination pumping in a freely expanding plasma produced in a high pressure, high temperature plasma jet. Stationary cw-lasing at 1640 Å or 729 Å can be expected by inverting the He II (2←3) or Li III (2←3) Balmer-α transition following three-body recombination of bare He2+ or Li3+ ions, respectively. The expected gain coefficients are calculated using a time dependent collisional-radiative model and estimates of the required pumping power as well as considerations on the technical feasibility of the concept are presented.
Tapered GaAs quantum wells and selectively contactable two-dimensional electron gases grown by shadow masked molecular-beam epitaxy
Authors: Lorke Axel , English John H. , Gossard Arthur C. , Petroff Pierre M.
Published: July 2002

We report on the use of shadow masks for the growth of GaAs quantum wells with spatially varying thicknesses. Using cathodoluminescence we observe a clear shift of the luminescence energy as a function of the lateral position. Combining masked and directional epitaxy, the quantum wells can be both laterally and vertically patterned. This way, stacked, two-dimensional electron gases are realized which can be selectively contacted without the use of complicated in-situ or post-growth patterning techniques. The quality of the epitaxial material grown through the openings of the masks is investigated by optical and electrical characterization.
Composition profiles at initial stage of altered layer formation on Cu-Pt alloy surfaces under Ar+ ion bombardment
Authors: Li Chunfei , Asahata T. , Shimizu R.
Published: July 2002

The dynamic process of subsurface altered layer formation in a Cu-Pt alloy under Ar+ ion bombardment has been studied by sequential Auger electron spectroscopy (AES) and ion scattering spectroscopy (ISS) measurements. For this study, we recently developed a compact coevaporator, which enables Cu and Pt to be deposited at a constant composition ratio (within ±3 at. %) onto a Si substrate cooled to liquid-nitrogen temperature. The average surface composition of the alloy film was monitored by AES throughout the deposition procedure, and the composition of the topmost atomic layer was measured by ISS right after the end of the film deposition. The data show that the topmost atomic layer was considerably Cu-rich while subsurface atomic layers were of constant composition. During Ar+ ion bombardment, however, the surface composition of Cu, measured by sequential AES and ISS, gradually decreased with time toward a steady-state value. The temporal evolution of the composition profile is described. The surface-segregation constant and radiation-enhanced diffusion coefficient for the Cu-Pt alloy under Ar+ ion bombardment are inferred from the measured composition profile.
Use of the DX center as a probe to study the profile of Si impurities in planar-doped GaAs
Authors: Bezerra J. C. , de Oliveira A. G. , Mazzoni M. S. C. , Chacham H.
Published: July 2002

Photo Hall concentration and mobility were measured for two molecular beam epitaxy-grown samples having a silicon planar-doped structure in the GaAs layer of a GaAs/AlGaAs heterojunction. The nominal silicon concentration for both samples was 1.5×1013 cm−2 and the distance between the ideal localization of the doped plane and the interface was adjusted to be 15 Å. The difference between the two samples is the growth direction. The Hall measurements were carried out at 77 K both in darkness and under illumination using an infrared light emitting diode as light source. Photoexcited effects indicate the presence of silicon atoms inside the undoped AlGaAs layer and that the silicon profile spreads mainly in the growth direction. Self-consistent electronic structure calculations, in the effective-mass approximations, were performed assuming doping profiles that simulate both samples. The calculations show that parallel conduction occurs when the growth direction is from GaAs to AlGaAs. This is consistent with the Hall measurements.
Ab initio studies on the electronic structures of strained-layer superlattices (InAs)n(InP)n(001), (n=1–5)
Authors: Ke San-huang , Wang Ren-zhi , Huang Mei-chun
Published: July 2002

We report ab initio studies on the electronic structures of strained-layer superlattices (SLSs) (InAs)n(InP)n(001), (n=1–5) under three different strain conditions via a linearized muffin-tin-orbital method with two treatments for d states. The effects of In 4d orbitals on the band structure and the valence band offset (VBO) are investigated. The hydrostatic pressure behavior and the Γ–X mixture in these SLS systems are also discussed. It is found that the band gap of the InP/InAs system is insensitive to its layer thickness. The VBO values at the SLS’s under the three strain conditions are determined by a frozen potential approach. Our results demonstrate that for the InP/InAs system the bulklike property is almost recovered in the molecular layer next to the interface. This behavior may be due to the common cation on both sides of the interface. This makes the frozen-potential approach converge very fast with the layer thickness. Our results of band structure for the (1+1) SLS under a free-standing mode are consistent with those given by a more elaborate full-potential linearized augmented-plane-wave (FLAPW) calculation. The present results of VBO are about 0.07–0.09 eV larger than the data from an x-ray photoelectron spectroscopy measurement and the FLAPW method, while in good agreement with the results given by the model solid theory.
Multi-fluxon zero-field modes in long Josephson tunnel junctions
Authors: Lachenmann S. G. , Filatrella G. , Ustinov A. V. , Doderer T. , Kirchmann N. , Quenter D. , Huebener R. P. , Niemeyer J. , Pöpel R.
Published: July 2002

The higher order zero field steps of long inline and overlap Josephson junctions have been investigated experimentally by means of low temperature scanning electron microscopy. The results indicate that several stable states are possible when more than one fluxon is present in the junction, corresponding to a different spacing among the fluxons during their propagation. These dynamic states should differ by the spectrum of the emitted radiation. Numerical simulations show that the fluxon interaction with plasma oscillations appears to be responsible for the observed behavior.
Magnetoresistivity below T
c in silver-clad Bi1.8Pb0.4Sr2Ca2Cu3Oy tape
Authors: Han G. C. , Han H. M. , Wang Z. H. , Wang S. X. , Liu X. N. , Wang F. T. , Chen J. L.
Published: July 2002

We have measured the magnetoresistance curves of a silver-clad Bi (2223) tape below critical temperature Tc over a magnetic field range up to 8 T. At temperatures well below Tc the magnetoresistive transition can be well described by a thermally activated flux flow (TAFF) process. The activation energies inferred from such dissipation measurements are found to have the form U=U0+U′ (H,T), where U0 is magnetic field and temperature independent, U′ (H,T) is proportional to (1−T/Tc)n/H. At temperatures very close to Tc the TAFF dissipation can no longer be found and the magnetoresistance seems to be determined by the combination of viscous flux flow and thermal fluctuation.
Magnetic flux pinning in Y-Ba-Cu-O superconductor containing thermally diffused impurities
Authors: Tenya Kenichi , Miyajima Hideki , Otani Yoshichika , Ishikawa Yuuichi , Yoshizawa Shuji
Published: July 2002

Fine powders of SnO and Fe2O3 were thermally diffused into Y-Ba-Cu-O systems composed of YBa2Cu3O7−δ matrix with oriented c axes and Y2BaCuO5 precipitates. The magnetic hysteresis loops of the system were measured at 77 K as a function of an applied magnetic field direction defined as the angle between the field and the c axis. The amplitude of the magnetization hysteresis ΔM, which is proportional to the critical current density Jc, was increased by nonmagnetic impurities SnO. On the other hand, ΔM was decreased drastically by magnetic impurities Fe2O3 although the critical temperature Tc was hardly decreased. In all the samples, the angular variation of ΔM showed a maximum at θ=0° and a minimum around θ=90°. However a local maximum at θ=90°, which is due to the intrinsic pinning, was observed only in the sample with diffused SnO. The intrinsic pinning of the sample with diffused Fe2O3 was significantly suppressed and the two-dimensional pinning property apparently became more pronounced.
Interfacial energy calculation at interconnect-metal/barrier-metal interfaces for grain orientation control
Authors: Nakasaki Yasushi , Minamihaba Gaku , Suguro Kyoichi , Itow Hitoshi
Published: July 2002

The interfacial energy of interconnect-metal/barrier-metal interfaces in ultralarge-scale integrations have been investigated numerically by assuming a simple additive interaction potential within a rigid-lattice approximation. The calculation gave some noteworthy results: Al prefers the (111) orientation on a TiN(111) plane, but Cu did not have such a strong preference. Al(111) can be expected to show strong epitaxial growth on VNx(111), as did Cu on Nb(110). On the basis of the calculations, a Cu/Nb bilayered structure was sputter deposited sequentially by dc magnetron sputtering and examined for crystalline orientation by x-ray-diffraction analysis. Cu exhibited strong (111) orientation preference, and a narrow full width at half-maximum in (111) rocking angle around the substrate normal.
Calculations of femtosecond differential optical transmission in germanium
Authors: Bailey D. W. , Stanton C. J.
Published: July 2002

Nonequilibrium electron and hole carrier dynamics are calculated for femtosecond photoexcitation in Ge using an ensemble Monte Carlo method. From the carrier distributions and k⋅p band-structure calculations, the time-dependent differential optical transmission that corresponds to pump-probe experiments is determined. It is found that electrons quickly scatter out of the optically coupled region and that the primary electron relaxation channel is from the Γ valley to the X valleys, then to the L valleys. Holes dominate the nonlinear absorption spectra in Ge. It is also found that light holes make a substantial contribution to the initial transient of the differential transmission, particularly for high-energy photoexcitation.
Optical constants and film density of TiNxOy solar selective absorbers
Authors: Lazarov M. , Raths P. , Metzger H. , Spirkl W.
Published: July 2002

Thin TiNxOy films were deposited on glass substrates in an activated reactive evaporation process. The influence of the reactive gas composition and pressure during deposition on the film density ρMass was investigated by grazing incidence x-ray-diffraction measurements. The density increases from 3.8 to 4.8 g/cm3 when lowering the total gas pressure ptotal from 10−3 h Pa down to 2.5×10−4 h Pa, whereas the ratio of the partial gas pressures of nitrogen and oxygen has no measurable effect on the film density. Data on the refractive index are presented, obtained from reflectance and transmittance measurements in the optical wavelength range (λ=0.3–2.5 μm). Bruggeman effective medium theory was applied to explain the optical behavior ñ(λ) as a result of the low density of the coatings. The high porosity of the films suppresses the metallic character of the phases TiN and TiO contained in the TiNxOy film. Its dielectriclike behavior in the infrared region is essential for low thermal emittances when deposited on copper to form a solar selective absorber.
Photorefractive property and self-pumped phase conjugation of Mn-doped (K0.5Na0.5)0.2(Sr0.75 Ba0.25)0.9Nb2O6 crystal
Authors: Lu Xinliang , Wu Yaocai , Jiang Quanzhong , Chen Lei , Chen Huanchu , Dou Shuoxing , Ye Peixuan
Published: July 2002

High quality Mn-doped (K0.5Na0.5)0.2(Sr0.75 Ba0.25)0.9Nb2O6 crystal was grown and its two-wave mixing and self-pumped phase conjugation properties were studied in detail. The two-wave coupling gain is as high as 37.60 cm−1 and the phase conjugation reflectivity is higher than 40%. It is shown that Mn dopants effectively increase photorefractive property of the crystal, and the possible mechanism is analyzed.
Response to ‘‘Comment on ‘Generation phenomena of localized interface states induced by irradiation and post-irradiation annealing at the Si/SiO2’ ’’ [J. Appl. Phys. 77, 2223 (1995)]
Authors: Kimura M.
Published: July 2002

The accuracy of our measurements of the interface state [J. Appl. Phys. 73, 4388 (1993)] is reassessed. The oxide thickness is found to be better than 1%, using techniques not only by high-frequency C-V and quasistatic C-V but also by ellipsometry.
Mirage effect: A theoretical and experimental study of anisotropic media in rear configuration
Authors: Quélin X. , Perrin B. , Louis G.
Published: July 2002

In this article, we describe a method based on mirage effect in rear configuration, i.e., a modulated pump beam illuminates one side of the sample and the temperature gradient is probed on the opposite side. We show that a detailed investigation of the amplitude and phase of the photothermal deflection angle, versus the modulation frequency, connected with a numerical analysis, provides a determination of the thermal conductivity coefficient in the normal direction to the sample surface. The method, suitable for thin slabs of low thermal diffusivity materials, is calibrated with a series of known samples and then applied to a polydiacetylene single crystal.
Comprehensive Monte Carlo simulation of the nonradiative carrier capture process by impurities in semiconductors
Authors: Palma A. , Jiménez-Tejada J. A. , Melchor I. , López-Villanueva J. A. , Carceller J. E.
Published: July 2002

The main nonradiative capture mechanisms, cascade and multiphonon emission, have been numerically simulated by the Monte Carlo method. To do so, both mechanisms were included in the frame of a previous numerical procedure to which the nonacoustic-phonon contribution was also added. Different centers were studied. Capture by shallow donors (P, As, and Sb) in n-type silicon were interpreted considering only the cascade process. Capture by acceptors levels of platinum, gold, and titanium in silicon, and one level of Cr, EL2, and EL3 in gallium arsenide, were analyzed considering only multiphonon emission, and calculating the values of Huang–Rhys factor when it is not available. In the study of capture by attractive deep centers, such as single ionized donor centers of sulfur and selenium in silicon, both cascade and multiphonon mechanisms must be combined. In this case the importance of the nonacoustic phonon has been shown in the cascade process.
Determination of free-carrier-enhanced emission rate of carriers in depletion-edge region by constant capacitance technique
Authors: Tomokage Hajime , Ueta Takeshi , Furuta Hirosuke , Miyamoto Tokuo
Published: July 2002

The rate of majority-carrier emission at the trap level located in the edge of the depletion layer is enhanced by the influence of a free-carrier tail. This causes a nonexponential change in transient capacitance with time after the application of a filling pulse. The emission rates of the midgap level in GaAs and the Au acceptor level in Si are measured by the constant capacitance technique which eliminates the additional nonexponential factor due to the high trap concentration. The apparent emission rate determined from the initial slope of transient voltage increases with decreasing the difference of the depletion widths in the emission and capture processes. The enhanced emission rate due to the free-carrier tail is observed even on the sample of the low carrier concentration. Fitting the calculated emission rate to the experimental results enables the estimation of the capture cross section of majority carriers in the trap level as well as the emission rate.
ac-susceptibility studies on Zr1−xHoxCo2 [0≤x≤1] system and their hydrides
Authors: Ramesh R. , Ramachandra Rao M. S. , Rama Rao K. V. S.
Published: July 2002

The ac-susceptibility studies were carried out on the intermetallic compounds Zr1−xHoxCo2 [0≤x≤1] in the temperature range 13–300 K. The magnetic transition is found to be of the first-order type for HoCo2 (x=1.0) and second-order type for x=0.4–0.9. No magnetic ordering is observed for x=0.2 down to 13 K and x=0.0 is found to be Pauli paramagnetic. The magnetic ordering temperature (TC) increases from about 13 K for x=0.4 to 77 K for x=1.0. In addition, spin reorientation transitions are observed for x=0.9 and 1.0. On hydrogenation, TC is found to decrease considerably in all compounds. Hydrogen absorption in these compounds is found to weaken the exchange interactions which can be attributed to charge transfer effects.
Optical-absorption study of synthetic pyrite FeS2 single crystals
Authors: Yang Tzuen-Rong , Yu Jiang-Tsu , Huang Jeng-Kuang , Chen Shinn-Horng , Tsay Ming-Yih , Huang Ying-Sheng
Published: July 2002

The optical-absorption study of synthetic pyrite FeS2 single crystals prepared by the chemical-vapor-transport method is reported over a temperature range from 4.2 to 300 K. At lower temperatures several features which occur below the absorption edge are superposed on the absorption curve. The origin of the sharpest peak is identified as the transition between the top of valence band and Cr3+ trapping center through comparison of the results with that of the low-temperature photoconductivity and the photoelectron paramagnetic resonance measurements. The nature of other weaker features is discussed. The existence of these features prevents the determination of the band gap of FeS2 by fitting the optical-absorption data to the form of (αhν)∝(hν−E0)n. Therefore, the energy gap of FeS2 at low temperatures is estimated by adding the thermal ionization energy of the shallow acceptor states to the photoionization energy between the (SCl)2− states and the bottom of the conduction band. The band gap of various temperatures is determined and its temperature dependence is analyzed by an empirical expression proposed by O’Donnell and Chen [Appl. Phys. Lett. 58, 2924 (1991)]. The parameters that describe the temperature dependence of Eg of the material are evaluated and discussed.
Influence of hydrogenated defects and voids on the thermal conductivity of polycrystalline diamond
Authors: McNamara Rutledge K. M. , Scruggs B. E. , Gleason K. K.
Published: July 2002

Hydrogenated defects in microwave plasma chemical-vapor-deposited (CVD) diamond films have been examined by proton nuclear magnetic resonance and infrared spectroscopy and correlated with the thermal conductivity of these samples. The measured thermal conductivities of 1–22 W cm−1 K−1 are consistent with an upper limit predicted based on the assumption that the scattering rate from hydrogenated defects is proportional to the as-deposited hydrogen contents of ≤0.5 at. % H. This hydrogen is predominantly clustered, consistent with hydrogen passivated diamond surfaces. Thus, the as-deposited hydrogen content may be a measure of the internal surface area of the polycrystalline diamond film which is hydrogen passivated. Part of this surface area may be due to elongated voids, several microns in dimension, which are observed by scanning electron microscopy of polished diamond film cross sections. These voids may arise from the columnar film growth process and thus be a common feature of CVD diamond films. Phonon scattering from these voids which are oriented along the film growth direction could account for film-to-film variation in and the anisotropy of the thermal conductivity of CVD diamond films.
Role of electron traps in the excitation and de-excitation mechanism of Yb3+ in InP
Authors: Tsimperidis I. , Gregorkiewicz T. , Ammerlaan C. A. J. , Godlewski M. , Scholz F. , Lambert B.
Published: July 2002

An experimental study concerning the excitation mechanism of the Yb impurity in n- and p-type InP crystals was performed by the method of optically detected microwave-induced impact ionization. Based on the results it is argued that the Yb3+ core excitation is intermediated by a nonradiative recombination of a bound exciton. A fingerprint of the existence of such an excitonic state is given. Also, the nonradiative decay channel is discussed and shown to involve an Auger process with the energy transfer to a locally bound electron. Experimental evidence is presented that by the impact ionization of the bound electron the nonradiative recombination channel may be removed, leading to an increase of the characteristic Yb3+ luminescence. An unprecedented microwave-induced 5% increase of the Yb3+ intrashell emission has been recorded.
Electrophotographic discharge of CdS explained by a surface-depletion discharge model
Authors: van Swaaij R. A. C. M. M. , Willems W. P. M. , Lokker J. P. , Bezemer J. , van der Weg W. F.
Published: July 2002

We report on a new model which explains the dark decay of the surface potential of a charged thin-film photoconductor on a grounded substrate. The model, called here the surface-depletion discharge model, has been developed to explain the dark discharge of cadmium sulfide films with thicknesses varying from 0.17 to 1.59 μm. The model is also applicable to thin hydrogenated amorphous silicon carbide films. During the discharge the bulk charge density remains constant. However, a fundamental change occurs in the discharge of a film with thickness L when at the depletion time td the surface charge density σ(td) is counterbalanced by the bulk charge density ρ(td), thus, σ(td)=ρ(td)L. From that moment onward, the bottom of the bulk charge region shifts to the surface of the photoconductor. In the model proposed here, the discharge is governed by surface charge injection. In addition, we show that the bulk space charge depends on the surface charge deposited by corona charging. Furthermore, the dark discharge rate is found to increase if the amount of surface charge deposited increases.
Changing photoluminescence intensity from GaAs/Al0.3Ga0.7As heterostructures upon chemisorption of SO2
Authors: Geisz J. F. , Kuech T. F. , Ellis A. B.
Published: July 2002

Chemisorption of SO2 gas molecules onto photowashed GaAs surfaces results in a change in the density of charge at the surface, giving rise to a measurable change in the charge distribution within the semiconductor. We consider the development of a novel semiconductor chemical sensor based on this phenomenon. The intensity of photoluminescence (PL) from metalorganic vapor phase epitaxy grown GaAs/Al0.3Ga0.7As structures at 300 K is used to detect this change in charge distribution. The doping densities and layer thicknesses within these heterostructures are shown to have a profound influence on the relative magnitude of PL intensity and its sensitivity to changes in the density of surface charge brought about by chemisorption of SO2. New analytical and finite element method (FEM) numerical models are derived to quantitatively predict the effect of changing surface charge on the PL intensity from finite thickness heterostructure layers. The PL response from these heterostructures is shown to follow the trends predicted by these models and suggests that the adsorption of SO2 from a 0.6 mol % mixture of SO2 in N2 results in a reduction in the negative surface charge density by 9×109 to 2×1010 cm−2. Simultaneous changes in surface recombination rates due to adsorption are shown (using the FEM model) to have no effect on the PL intensity from the structures studied here. The results are applied to the improvement of signal characteristics from a potential chemical sensor device.
Photoemission from Mg irradiated by short pulse ultraviolet and visible lasers
Authors: Srinivasan-Rao T. , Fischer J. , Tsang T.
Published: July 2002

Quantum efficiency of diamond turned magnesium was measured with low intensity, 266 nm, 12 ps, p-polarized radiation for various incident angles. The optimum quantum efficiency of 3×10−4 was obtained with p-polarized light incident close to Brewster’s angle where the absorption is maximum. Investigations of the field dependence of the yield indicate that application of a high surface field is expected to lower the surface barrier by Schottky effect and increase the quantum efficiency further. Measurements of the electron yield with 632 nm radiation of 300 fs pulse duration indicate that at intensities exceeding 20 MW/cm2, the electric field due to the laser may become important and start contributing to the electron emission. Autocorrelation measurements at low intensities with this wavelength indicate that the electron emission is prompt.