bluuebunny/crossref_metadata_2025 huggingface dataset:
Imaging and modeling of pulse laser induced evaporation of metal films
Authors: Niedrig R. , Bostanjoglo O.
Published: July 2002

Evaporation triggered with nanosecond laser pulses in self-supporting aluminum films was imaged by high-speed transmission electron microscopy. This unconventional method provides up to three images from a single transient process with exposure times ⩾5 ns and at intervals ⩾25 ns with a spatial resolution of 100 nm. The chronological order of ablation was observed on the nanosecond time scale by successive shifting of the moment of exposure. Information was obtained about the mode and duration of evaporation. The ablation reveals a threshold behavior. Above a laser energy density of 5–6 J/cm2 the irradiated film region is completely evaporated during the laser pulse. Below this threshold evaporation is marginal and the film disintegrates mainly by liquid flow. The experimental results are compared with a new model comprising two different evaporation mechanisms, surface and volume evaporation. The ablation as observed cannot be explained by surface evaporation only. The strong rise of the evaporation rates above the threshold is associated with the onset of volume evaporation. It is also shown that the vaporization enthalpy must be considered as a function of temperature for a correct modeling of evaporation by short laser pulses.
Optical studies of highly strained InGaAs/GaAs quantum wells grown on vicinal surfaces
Authors: López C. , Mayoral R. , Meseguer F. , Porto J. A. , Sánchez-Dehesa J. , Leroux M. , Grandjean N. , Deparis C. , Massies J.
Published: July 2002

We use photoluminescence and photoluminescence excitation experiments with and without magnetic field to study the electronic properties of InxGa1-xAs/GaAs quantum wells grown on vicinal (001) substrates. We analyze samples of a wide range of In contents (from 17% to 35%) and various misorientation angles (up to 6°). The optical quality of the samples increases with the tilt angle and is explained as mainly controlled by alloy disorder. A fit of the electron-heavy-hole transitions is performed by means of a method which consists of the resolution of a two-dimensional Schrödinger equation and which includes two adjustable parameters: the In surface segregation energy Es and the length ξ in which the hydrostatic pressure becomes biaxial as defined by the Nagai’s model [J. Appl. Phys. 45, 3789 (1974)]. For a given angle and In content the differences between the PL peaks of vicinal and nominal samples present a maximum as a function of the well width, a fact which is well explained by our theoretical model. A study of the exciton dimensionality has been also carried out using models that take dimensionality into account in different manners.
Trailing edge light emission from ZnS:Mn and ZnS:Tb, F in a thick dielectric electroluminescent display
Authors: Bailey P. , Carkner D. , Wu X.
Published: July 2002

The study of a thick dielectric electroluminescent device has identified the presence of a light emission from ZnS:Mn and ZnS:Tb, F phosphors coincident with the trailing edge of the driving voltage pulse. The trailing edge emission appears to be related to some of the unique electro-optical properties of this EL technology. Both leading and trailing edge current measurements, together with capacitance–voltage data, have been employed to characterize the device. A doped phosphor probe layer was used to clarify the location of the source for the most intense trailing edge luminance emission. A mechanism for the trailing edge luminance is proposed, similar to that which is responsible for the luminescence peak observed at the leading edge of the driving pulse as seen in displays using only thin-film dielectric layers.
A predictive model for transient enhanced diffusion based on evolution of {311} defects
Authors: Gencer Alp H. , Dunham Scott T.
Published: July 2002

It has been observed that {311} defects form, grow, and eventually dissolve during annealing of Si-implanted silicon wafers. The fact that for subamorphizing silicon implants {311} defects initially contain the full net dose of excess interstitials, and that the time scale for dissolution of these defects is about the same as the time scale of transient enhanced diffusion (TED) leads to the conclusion that {311} defects are a primary source of interstitials under TED conditions. We describe a comprehensive model which accounts for the evolution of these defects during ion implant annealing, and in combination with point defect parameters from previous work also correctly predicts TED behavior.
Study of nitrogen implanted amorphous hydrogenated carbon thin films by variable-energy positron annihilation spectroscopy
Authors: Freire F. L. , Franceschini D. F. , Brusa R. S. , Karwasz G. R. , Mariotto G. , Zecca A. , Achete C. A.
Published: July 2002

Hard amorphous hydrogenated carbon (a-C:H) films deposited by self-bias glow discharge were implanted at room temperature with 70 keV nitrogen ions at fluences between 2.0 and 9.0×1016 N/cm2. The implanted samples were analyzed by positron Doppler broadening annihilation spectroscopy to determine the voids distribution. For samples implanted with 2.0×1016 N/cm2 the defect distribution is broader than the vacancies depth profile predicted by Monte Carlo simulation. For higher fluences we observed a reduction of the defect density. These results are discussed in terms of a competition between two processes: ion induced defects and structural modifications induced in the films due to ion implantation.
The role of substrate quality on misfit dislocation formation in pseudomorphic high electron mobility transistor structures
Authors: Meshkinpour M. , Goorsky M. S. , Jenichen B. , Streit D. C. , Block T. R.
Published: July 2002

We examined the role of substrate quality on the epitaxial microstructure and performance of pseudomorphic InGaAs/AlGaAs/GaAs high electron mobility transistors (HEMTs). High resolution x-ray diffraction, high resolution x-ray topography, and transmission electron microscopy indicate that, for a given channel layer thickness, the misfit dislocation density is always lower for HEMT structures grown on substrates having lower threading dislocation densities. Furthermore, the onset of misfit dislocation formation occurs at higher channel layer thicknesses for HEMTs grown on substrates having a lower threading dislocation density when compared to those grown on substrates with a higher threading dislocation density. However, the ratio of the density of misfit dislocations to threading dislocations is greater than one on the low dislocation density substrates, which demonstrates that other misfit dislocation nucleation sources (i.e., surface particulates) are significant when there is an insufficient density of threading dislocations. The Hall conductivity measurements show that the performance of HEMT structures improves with higher substrate quality, showing that the range of epitaxial layer metastability increases with improved substrate quality.
Magnetic properties of sputtered Fe1−xO and Fe+Fe3O4 thin films
Authors: Chin T. S. , Chiang N. C.
Published: July 2002

An attempt was made to increase saturation magnetization of Fe3O4 films by incorporating α-Fe phase through the decomposition of as-deposited Fe1−xO films, which were dc-sputtered onto Si(100) substrate by controlling the oxygen flow rates to a thickness of 70–400 nm. The as-prepared Fe1−xO films, being ferrimagnetic, show a thickness dependent microstructure and properties. The formation region of the single phase Fe1−xO was worked out as functions of oxygen flow rate and thickness. After vacuum-annealing, the Fe1−xO phase decomposed to Fe+Fe3O4 phases leading to an increase in coercivity from 15 Oe to 520–990 Oe, and an increase in saturation magnetization from 100 to 290–410 emu/cm3 depending on film thickness. The nanostructure of as-prepared and annealed films were examined by a high resolution transmission electron microscope, the former does not show well defined grain boundaries, while the later has well defined grains, 20–50 nm in size.
Magnetization, coercivity, and remanence in amorphous terbium–iron metalloid alloys
Authors: Jerems F. , Greenough R. D. , Ahlers H.
Published: July 2002

Crystalline terbium–iron compounds generate large magnetostrictive strains but for practical application require relatively large fields to overcome magnetocrystalline anisotropy. Their amorphous counterparts are magnetically softer and yet potentially useful magnetostrictive strains are still exhibited due to the presence of the Tb ions. As part of our investigation into the origins and magnitudes of the magnetostriction in amorphous rare earth–iron alloys, their magnetic properties have been subjected to close examination. Melt-spun ribbons of TbFe2 containing between 3 and 14 at. % boron were prepared and shown to be amorphous (>6 at. % B). The field and temperature dependences of magnetization have been measured in the region 4.5 K<T<300 K with fields up to 5 T using a superconducting quantum interference device magnetometer. The temperature dependences of coercivity and remanence have been determined. Zero field and field cooled magnetization curves indicate a spin freezing temperature ∼240 K. Analyzing the demagnetization curves in terms of the random anisotropy model proposed by Harris, Plischke, and Zuckermann (HPZ) [R. Harris, M. Plischke, and M. J. Zuckermann, Phys. Rev. Lett. 31, 160 (1974)] shows that the anisotropy and exchange energy in (TbFe2)1−xBx are of comparable magnitudes. Coupled with the sharp decrease of the anisotropy with increasing temperature this leads to an interesting thermal behavior of the remanence with a local minimum at ∼140 K. Different concentrations of boron seem to have no significant effect on the magnetic properties. Boron acts purely as a “glass former” necessary for producing amorphous TbFe2 by rapid cooling.
Three dimensional alignment of magnetic particles
Authors: Zhang Y. D. , Budnick J. I. , Hines W. A.
Published: July 2002

In this article, novel magnetic particle alignment methods, which we refer to as dynamic alignment, are presented. In the process of a dynamic alignment, the magnetic field direction is periodically changed while the particle orientation is captured in epoxy resin. Depending on the manner of the variation of the magnetic field, particles can be aligned in various patterns. By employing the dynamic alignment methods described in this article, the alignment of magnetic particles can be achieved such that particles are aligned along all three crystalline axes.
Low-field colossal magnetoresistance in bandwidth-controlled manganites
Authors: Kuwahara H. , Moritomo Y. , Tomioka Y. , Asamitsu A. , Kasai M. , Tokura Y.
Published: July 2002

We have investigated magnetoresistance (MR) phenomena relevant to the charge ordering (CO), namely, the real space ordering of both Mn3+ and Mn4+, in single crystals of (Nd1−ySmy)1/2Sr1/2MnO3, in which the one-electron bandwidth (W) is systematically controlled by varying the average ionic radius of the A site. The low-field colossal MR is observed for the small-W region of y⩾0.5; e.g., ρ(0)/ρ(H)>103 in a field of 0.4 T at 115 K for the y=0.94 crystal. This is viewed as a first-order insulator-to-metal phase transition induced by a magnetic field, which accompanies a lattice-structural change. In the small-W region, the CO instability accompanying the antiferromagnetic spin correlations subsists even above the ferromagnetic transition temperature Tc. The competition between the ferromagnetic double-exchange and antiferromagnetic CO interactions gives rise to such a lattice-coupled first-order phase transition induced by a relatively low magnetic field.
Magnetic properties of Pr2Co17−xAl
x
compounds
Authors: Zhang D. , Zhang Z. D. , Buschow K. H. J. , Brück E. , de Boer F. R.
Published: July 2002

The magnetic properties of Pr2Co17−xAlx compounds with 0⩽x⩽4 were studied by means of magnetic measurements. From these measurements it is derived that there is an almost linear decrease of the Curie temperature with Al concentration accompanied by a somewhat less strong decrease of the saturation moment at 5 K. For the compounds with 2⩽x⩽4, we have observed spin-reorientation transitions at temperatures that decrease strongly with increasing Al concentration. These spin-reorientations are interpreted as originating from a competition between the Pr- and Co-sublattice anisotropies.
Photoluminescence study of heavy doping effects in Te-doped GaSb
Authors: Bignazzi A. , Bosacchi A. , Magnanini R.
Published: July 2002

The photoluminescence (PL) of heavily Te-doped GaSb has been investigated for different free carrier concentrations. A careful line shape analysis of the dominant free-to-bound transition has been performed using nonparabolic bands and taking into account the band tailing through the Kane model. The Fermi level and the band edge position have been determined from the fit of the PL band. Our results show that the energy gap value is significantly lower than in lightly doped and undoped material. This band-gap narrowing can be well understood taking into account both manybody interaction (exchange) and the random impurity distribution, that induces a rigid shift of the bands toward each other and tail states into the forbidden gap, respectively. Measurements have been performed at different temperatures and excitation power densities to evidence the role of the acceptor fluctuation and of the minority carrier distribution in determining the optical emission energy. Simple relations connecting the band-gap narrowing and the PL linewidth to the free carrier concentration are proposed in GaSb.
77 K single electron transistors fabricated with 0.1 μm technology
Authors: Altmeyer S. , Hamidi A. , Spangenberg B. , Kurz H.
Published: July 2002

Metal based single electron transistors are fabricated by the step edge cut off process. Titanium metal lines with a width of 0.1 μm are deposited on prepatterned silicon substrates, that serve as the dielectric barriers for the tunnel junctions. In structures with multiple tunnel junctions, clear Coulomb blockade and Coulomb oscillation features can be observed at temperatures up to 77 K.
Ti underlayer effect on the recording characteristics of Co–Cr perpendicular media with weak and strong interparticle interactions
Authors: Sonobe Y. , Ikeda Y. , Uchida H. , Toyooka T.
Published: July 2002

We analyzed the effect of a Ti underlayer on the magnetic properties and read-write characteristics of perpendicular Co–Cr media, using a merged magnetoresistive head to investigate the latter. The output at 100 kfci for Co78Cr22 was decreased by using a Ti underlayer, whereas that for Co72Cr28 was increased. Such phenomena cannot be clearly explained only by the improvement in the crystalline orientation, the increase in the anisotropy field Hk⊥, and the reduction in the thickness of the initially grown layer. The maximum gradient in the curve showing the dependence of ΔM⊥ on the external field for the Co72Cr28 was about four times larger than that for the Co78Cr22. This result indicates that Co72Cr28 has strong interparticle interaction that aligns the magnetization direction of the particles coherently, and that the magnetization in the initial layer has a strong effect on the magnetization near the top surface through the exchange force. Consequently, the use of a Ti underlayer increases the reproduced output of perpendicular media by 60% at 100 kfci. Co72Cr28 media with a Ti underlayer have a D50 of 150 kfci at a magnetic spacing of 62 nm and a medium thickness of 200 nm.
Effects of carbon underlayer and substrate temperature on the structural and magnetic properties of Co86Cr12Ta2 thin-film media
Authors: Yang Cheng , Sivertsen John M. , Judy Jack H.
Published: July 2002

A thin carbon underlayer can promote the growth of the c axis of Co86Cr12Ta2 film perpendicular to the film surface at a much thinner stage. A smaller lattice constant c of the hcp structure was observed for Co86Cr12Ta2 thin-film media on a sputtered carbon underlayer. High substrate temperature increases the perpendicular coercivity of the Co86Cr12Ta2 film significantly. This increase is due to the decrease in the c/a ratio of Co hcp lattices.
The temperature dependence of the transient current in ferroelectric Pb(ZrxTi1−x)O3 thin films for memory devices applications
Authors: Chen Hong-ming , Lee Joseph Ya-min
Published: July 2002

The temperature dependence of the transient current in ferroelectric Pb(Zr0.53Ti0.47)O3 (PZT) thin film capacitors is studied by examining the current density versus time (J-t) characteristics in the temperature range from room temperature to 140 °C. The transient current consists of two components, the conduction current and the polarization current. The conduction current is related to traps inside the films. The trapped holes cause an increase of the local electric field and hence the conduction current. The polarization current decreases and saturates as time increases. The dependence of the transient current on temperature therefore relies on the relative magnitudes of the two components. The transient current of PZT at room temperature is found to be dominated by trap-related conduction current at field higher than 83 kV/cm and by polarization current at field lower than 83 kV/cm. The increase of temperature enhances the emission rate of captured holes and the conduction current component decreases. The discharging current in the PZT capacitor is found to follow the tunneling front model in the temperature range below 100 °C, whereas a different model is required above 100 °C.
Roughness effects on the electrostatic-image potential near a dielectric interface
Authors: Palasantzas George
Published: July 2002

In this work, we investigate roughness effects on the electrostatic-image potential of a point charge situated in the vicinity of a rough dielectric–vacuum interface. The roughness is modeled as a self-affine structure with root mean square roughness σ, correlation length ξ, and roughness exponent 0<H<1. The roughness contribution on the electrostatic-image potential could be significantly large (>10%) with respect to that of a flat interface, and its effect outside the dielectric medium is dominated mainly by the long-wavelength roughness parameters σ and ξ. However, inside the medium, the contribution of the roughness exponent H could become comparable to that of σ and ξ.
Titanium oxide thin films for NH3 monitoring: Structural and physical characterizations
Authors: Manno D. , Micocci G. , Rella R. , Serra A. , Taurino A. , Tepore A.
Published: July 2002

Titanium oxide thin films have been deposited by thermal evaporation in vacuum and then have been analyzed before and after a suitable thermal annealing in order to test their application in NH3 gas-sensing technology. In particular, spectrophotometric and conductivity measurements have been performed in order to determine the optical and electrical properties of titanium oxide thin films. The structure and the morphology of such material have been investigated by high resolution electron microscopy and small area electron diffraction.
Mn site doping induced insulator to metal transition in Pr0.6Ca0.4MnO3
Authors: Damay F. , Maignan A. , Martin C. , Raveau B.
Published: July 2002

The doping of a manganese site with various M elements M=Fe3+, Al3+, Ga3+, Ti4+, Sn4+, and Mg2+ was studied in insulating charge ordered manganite Pr0.6Ca0.4MnO3 that exhibits a canted CE-type magnetic structure. Whatever M, the charge ordering disappears for low x levels (x⩾0.02) in doped manganite Pr0.6Ca0.4Mn1−xMxO3. But the most important feature deals with the fact that a transition to a ferromagnetic metallic state is induced for M=Fe3+, Al3+, Ga3+, and Mg2+ in spite of the small size of the interpolated cation (Pr,Ca). In contrast, doping with a tetravalent cation does not drastically modify the magnetic and transport properties of the phase.
Characterization of 3C-SiC films grown on monocrystalline Si by reactive hydrogen plasma sputtering
Authors: Sun Yong , Miyasato Tatsuro , Wigmore J. Keith , Sonoda Nobuo , Watari Yoshihiko
Published: July 2002

Detailed characterization using x-ray diffractometry, scanning electron microscopy, transmission electron microscopy, x-ray photoelectron spectroscopy, and Auger infrared and focused ion-beam spectroscopy, was carried out on cubic SiC films grown on single-crystal (100) Si substrates by reactive hydrogen plasma sputtering over a range of growth temperatures between 700 and 1000 °C. It was found that the first few deposited atomic layers were always amorphous. The subsequent SiC films showed well-defined (111) growth at the lowest temperatures, becoming randomly oriented by 1000 °C. The measured C:Si ratio was always >1, and varied with depth inside a film and also with temperature. At higher temperatures, the presence of “hollow voids” was observed, our data being consistent with their formation by outdiffusion of Si atoms from the substrate through the SiC layer. Associated with the hollow voids we observed the presence of a porous, highly C-rich region at the Si–SiC interface. We propose that this was due to diffusion of C from the SiC film into the voids themselves.
Photoreflectance study of pseudomorphic high electron mobility transistors
Authors: Han A. C. , Wojtowicz M. , Pascua D. , Block T. R. , Streit D. C.
Published: July 2002

Room temperature photoreflectance (PR) measurements have been carried out on a series of pseudomorphic AlGaAs/InGaAs/AlGaAs high electron mobility transistor (HEMT) structures with different doping profiles grown by molecular beam epitaxy. The GaAs and AlGaAs features in the PR spectrum were studied and their origins were determined by a sequential etching method. It was found that the GaAs buffer layer and AlGaAs barrier layer dominate the GaAs and AlGaAs features in the PR spectrum, respectively. The electric fields for Franz-Keldish oscillations (FKOs) associated with the E0 transition of both GaAs and AlGaAs are affected by the doping on both sides of the InGaAs channel, but are not directly related to the channel carrier concentration. The aluminum composition in the AlxGa1−xAs layers was determined and compared with the results of FKO energy and the critical-point energy methods.
Phase and microstructure investigations of boron nitride thin films by spectroscopic ellipsometry in the visible and infrared spectral range
Authors: Franke Eva , Schubert Mathias , Neumann Horst , Tiwald Thomas E. , Thompson Daniel W. , Woollam John A. , Hahn Jens , Richter Frank
Published: July 2002

Spectroscopic ellipsometry over the spectral range from 700 to 3000 cm−1 and from 1.5 to 3.5 eV is used to simultaneously determine phase and microstructure of polycrystalline hexagonal and cubic boron nitride thin films deposited by magnetron sputtering on (100) silicon. The results are obtained from a single microstructure-dependent model for both infrared and visible-light thin-film anisotropic dielectric functions. The optical behavior of high c-BN content thin films is described by an effective medium approximation. We obtain the amount of h-BN within high c-BN content thin films. A thin oriented nucleation layer between the silicon substrate and the high c-BN content layer is demonstrated. The preferential arrangement of the grain c axes within the h-BN thin films are found to be dependent on the growth parameters. The results from the infrared and visible spectral range ellipsometry model are compared to each other and found to be highly consistent.
Giant magneto radio frequency absorption in magneto-resistive materials La0.7(Sr,Ca)0.3MnO3
Authors: Owens Frank J.
Published: July 2002

The dc magnetic field, frequency, and temperature dependence of electromagnetic absorption at rf frequencies of 335 kHz and 10 MHz are measured in the as sintered samples of colossal magneto resistive materials La0.7Sr0.3MnO3 and La0.7Ca0.3MnO3 by measuring the penetration depth using the method of Schawlow and Devlin. Very large magneto-electromagnetic absorption effects, much larger than the magneto resistive effects, are observed. A dc magnetic field of 1000 G changes the penetration depth at 350 kHz by 53% at room temperature in La0.7Sr0.3MnO3, whereas the resistance of the sample only decreases by 0.75% for the same field. The effects are primarily a result of the decrease of the permeability of the sample with dc magnetic field in the ferromagnetic state. A simple dc magnetometer employing the as sintered La0.7Sr0.3MnO3 contained in the coil of an LC oscillator as the sensing element is described.
Magnetic and structural properties of high remanence Nd8(FeCo)88B4 prepared by mechanical milling
Authors: Miao W. F. , Ding J. , McCormick P. G. , Street R.
Published: July 2002

The effect of Co substitution for Fe on the structure and magnetic properties of mechanically milled Nd8Fe88B4 has been investigated. As-milled Nd8(Fe1−xCox)88B4 (x=0–0.6) alloys exhibited a two-phase mixture consisting of an amorphous phase and a bcc FeCo phase. Optimal annealing of the as-milled samples led to the formation of a nanocrystalline mixture of Nd2(FeCo)14B and a bcc FeCo phase. The optimally annealed samples exhibited single-phase magnetic behavior and remanence enhancement. Replacement of up to 20% of Fe by Co increased both remanence and coercivity. Over substitution of Co and heat treatment at temperatures exceeding 600 °C caused the deterioration of both remanence and coercivity.
Spectroscopic response of photoinduced currents in a laser-assisted scanning tunneling microscope
Authors: Bragas A. V. , Landi S. M. , Coy J. A. , Martı́nez O. E.
Published: July 2002

Thermal expansion of the sample or tip in a laser-assisted scanning tunneling microscope (STM) junction can obstruct the study of other phenomena related with STM–light interactions, when photoinduced currents are analyzed. In this article, we show that the thermal contribution is proportional to the average tunneling current and that this can be used to distinguish it from other contributions. Simultaneous tunneling current–voltage and photoinduced current–voltage curves are recorded for highly oriented pyrolitic graphite and gold samples with a Pt tip. We have done the measurements for two different polarizations of the incident beam. We show that the I–V curves can be used to discriminate between different mechanisms that appear, producing photoinduced currents.
Microstructure and magneto-optical properties of Pr–Ni substituted Ba hexaferrite films prepared by sputtering
Authors: Gomi M. , Cho J. , Abe M.
Published: July 2002

Hexaferrite thin films of Ba1−xRxFe12−xNixO19 (R=Pr, La) were grown on nonmagnetic garnet substrates by rf sputtering. When deposited at a substrate temperature of 550 °C at rf power density (PDrf) larger than 19 W/cm2, the films were completely crystallized, with the c axis preferentially oriented normal to the film plane. Transmission electron microscopy revealed that the films deposited at low PDrf were amorphous but locally contained microcrystallites several nm in size. On the other hand, the films deposited at PDrf larger than 20 W/cm2 were polycrystalline with a crystallite size as large as 300 nm. Faraday rotation measurements showed that the Ni substitution induced a large negative rotation in the photon energy range of 2.1–2.6 eV. This Ni contribution was predominantly attributed to the crystal-field transition of octahedrally coordinated Ni2+ ions lying in the 2 eV range. No contribution by Pr3+ ions to the Faraday rotation was observed within the photon energy range measured.
Microstructure and atomic effects on the electroluminescent efficiency of SrS:Ce thin film devices
Authors: Warren W. L. , Seager C. H. , Sun S.-S. , Naman A. , Holloway P. H. , Jones K. S. , Soininen E.
Published: July 2002

Transmission electron microscopy and x-ray diffraction data show that rapid thermal anneals of SrS:Ce thin films enhance grain size and reduce crystalline defects. Electron paramagnetic resonance results suggest that these anneals lead to less variance in the crystal field environments at the nearly cubic Ce3+ sites along with the formation of another type of Ce3+ site believed to involve a nearby Sr vacancy. We suggest that the association of Ce3+ sites with VSr shifts the electroluminescence towards larger wavelengths as the symmetry of the activator site is lowered.
Step structure and ordering in GaInP
Authors: Lee S. H. , Stringfellow G. B.
Published: July 2002

Steps are known to affect the ordering phenomenon in GaInP; [110] steps assist ordering and [1̄10] steps retard ordering. However, the mechanism leading to this behavior has not been determined. In this paper, the step spacing is used as a semiquantitative indicator of the attachment of adatoms at the step edge for organometallic vapor phase epitaxial growth on singular (001) GaAs substrates. The step spacing and degree of order in the epitaxial layers have been studied as a function of temperature and the flow rate of the P precursor for both tertiarybutylphosphine (TBP) and phosphine. As the flow rate of the P precursor is lowered beyond a certain critical point, both the degree of order and the step spacing are seen to decrease together. The behavior is the same for TBP and for PH3. Similarly, as the growth temperature is increased above a certain value, the step spacing and order parameter decrease together. This suggests a relationship between adatom attachment at steps and the ordering mechanism. Possible explanations for these phenomena are explored. At low temperatures, the degree of order is reduced and the step spacing decreases exponentially. The data suggest that the reduction in the surface diffusion coefficient with decreasing temperature is the main factor acting to reduce both the step spacing and the degree of order. However, the increasing adatom sticking coefficient at both step edges expected at low temperatures will affect both phenomena as well.
Cathodoluminescence studies of the deep level emission bands of AlxGa1−xN films deposited on 6H–SiC(0001)
Authors: Perry William G. , Bremser M. B. , Davis R. F.
Published: July 2002

A comprehensive study of the sub band-edge emission bands of AlxGa1−xN (0.06⩽x⩽1) thin films deposited on vicinal and on-axis 6H–SiC(0001) substrates is presented. At 4.2 K strong band-edge emission, ascribed to donor-bound excitons, shallow donor-shallow-acceptor pair emission, and a deep emission band associated with the “yellow” band of GaN, were observed via cathodoluminescence. The energy shift of the shallow donor-shallow-acceptor pair band with respect to the peak of the donor-bound excitons peak exhibited a less than one-to-one correspondence with increasing Al mole fraction due the increasing localization of either the shallow donor and/or shallow acceptor. The yellow band was observed for all compositions and exhibited a similar energy shift with respect to both the donor-bound excitons and the shallow donor-shallow-acceptor pair bands as the Al mole fraction increased, except for a brief decrease at x≈0.5. This decrease was attributed to a donor oxygen level which entered the band gap at approximately this composition. A strong, broad emission band observed at 3.25 eV at 295 K in AlN and commonly associated with oxygen impurities was shown to be closely related to the yellow band of GaN.
Synthesis and characterization of Nd2Fe14B-25 vol % Fe nanocomposite magnets
Authors: Raviprasad K. , Funakoshi M. , Umemoto M.
Published: July 2002

Synthesis of nanocomposite two-phase permanent magnet materials by mechanical milling requires optimization of the volume fractions of the two phases, milling time, heat treatment temperature, and time. In the present study, optimization of milling time, heat treatment temperature, and time have been carried out by fixing, respectively, the volume fractions of hard and soft phases at 75 and 25. Prior to the synthesis of nanocomposite magnets, the effect of milling on the magnetic properties of the hard phase was studied. The results on milling of hard phase have shown the lower and upper limits of milling time to be 108 and 144 ks, respectively. Milling for 144 ks and heat treatment at 823 K for 1.8 ks was found to result in nanocomposite magnets with the best properties. The microstructural characterization has shown clearly that when the heat treatment temperature is higher, grain growth takes place and consequently the magnetic properties of the nanocomposites deteriorate. Compositional analysis using transmission electron microscopy has shown grains richer in Fe and Nd. The rationalization of magnetic properties shows that to improve the energy product, coercivity needs to be improved. Alloying addition of Cr and Cu was found to improve coercivity and consequently the energy product. Finally, it was shown that by using finer Fe particles one can reduce the time required for milling and avoid many adverse effects resulting from long time milling.
X-ray diffraction and channeling-Rutherford backscattering spectrometry studies of ion implantation damage in AlxGa1−xAs
Authors: Partyka P. , Averback R. S. , Forbes D. V. , Coleman J. J. , Ehrhart P.
Published: July 2002

X-ray diffraction and channeling-Rutherford backscattering spectrometry (RBS) were employed to investigate damage accumulation in AlxGa1−xAs (x=0.50, 0.75, 0.85, and 1.0) irradiated at 80 K with MeV ions. The x-ray measurements, performed both before and after warming the samples, showed a transition in the strain accumulation behavior as the Al content increased. For samples with low Al content, x=0.50, the strain increased monotonically with fluence until the sample amorphized, a behavior similar to GaAs. For samples with x⩾0.75, the strain initially increased, then plateaued, and finally diminished at high fluences. The RBS data, obtained at both 80 K and room temperature, revealed a similar dependence of the amorphization behavior upon Al content. For pure AlAs films, amorphization in the bulk was not observed even after a fluence of 2×1017 cm2 of 1.7 MeV Ar+. For films with low Al content, however, the AlxGa1−xAs layer readily amorphized with a fluence of only 6.8×1014 cm2 of 1.7 MeV Ar+. From these data, along with previously published information provided by transmission electron microscopy studies, a model for damage accumulation in ion irradiated AlxGa1−xAs is proposed.
Temperature dependence of the Faraday effect in Rh4+-substituted magnetic garnets
Authors: Kurimoto M. , Matsubayashi S. , Ando K. , Shinagawa K. , Saito T. , Tsushima T.
Published: July 2002

The temperature dependence of the Faraday rotation (FR) spectra is measured at temperatures between 60 and 300 K in the wavelength range of 0.7–1.6 μm for single crystalline (Y3+, Ca2+)3(Fe3+, Rh4+)5O12 films grown by a liquid phase epitaxy method onto (111)-oriented gadolinium gallium garnet substrates. In addition, the FR spectra at some temperatures are calculated by a molecular field approximation, and the calculated spectra are reproduced well by assuming the molecular field 155⋅〈SZFe〉(cm−1), where 〈SZFe〉 is a thermal average of an iron spin SZFe in magnetic garnets.
Transport and structural modification during nitrogen implantation of hard amorphous carbon films
Authors: Grigull S. , Jacob W. , Henke D. , Spaeth C. , Sümmchen L. , Sigle W.
Published: July 2002

Hard amorphous carbon (ta-C) films were implanted with 20 keV N+ ions with different fluences up to 6×1017/cm2 at different substrate temperatures. The nitrogen content of the films was monitored in situ using elastic recoil detection analysis. A characteristic temperature dependence is observed for the maximum achievable [N]/[C] composition ratio, with a drop of the saturation level from the room-temperature value of 0.35 to 0.17–0.12 above 150 °C. It is shown that the higher nitrogen retention at room temperature is correlated with the formation of N2-containing gas bubbles which are not present in samples implanted with high fluences at elevated temperatures. From residual-gas analyses it is found that nitrogen is reemitted from the films mainly as N2 when saturation occurs. Double-implantation experiments with spatially separated N14 and N15 implanted regions, respectively, indicate that the N–N molecule recombination observed at large implantation fluences occurs inside the films and not at the surface. Significant changes of the microstructure of the films are found with increasing implantation fluences. Inside the implanted near-surface region of several 10 nm thickness the density of the material decreases from 3.0 to about 1.7 g/cm−3. Graphitic clusters are identified in samples implanted up to saturation at 400 °C, using cross-section transmission electron microscopy. A basic approach to modeling the nitrogen saturation and release at large fluences is presented. Both nitrogen release and structural modification processes are interpreted as a tendency towards thermodynamic equilibrium which may constitute a strong driving force against the synthesis of nitrogen-rich hard C:N materials, compared to other nitride phases.
Generation of second-order cumulative waves accompanying the fundamental modes in a two-dimensional hard-walled waveguide
Authors: Deng Mingxi
Published: July 2002

Generation of second-order cumulative waves (SOCWs) accompanying the fundamental modes in a two-dimensional hard-walled waveguide of semi-infinite length is examined by use of second-order perturbation. With appropriate boundary and initial conditions of excitation, SOCW analytical expressions have been derived. In this manner, a physical process of SOCW generation is clearly displayed. The solution shows that the SOCWs grow linearly with the propagation distance, and that the SOCW fields in the waveguide are symmetrical. Based on the formula, some numerical calculations are performed. The numerical results clearly exhibit the SOCW field distortion.
Exploring magnetic roughness in CoFe thin films
Authors: Freeland J. W. , Chakarian V. , Bussmann K. , Idzerda Y. U. , Wende H. , Kao C.-C.
Published: July 2002

The behavior of chemical and magnetic interfaces is explored using diffuse x-ray resonant magnetic scattering (XRMS) for CoFe thin films with varying interfacial roughnesses. A comparison of the chemical versus magnetic interfaces shows distinct differences in the behavior of these two related interfaces as the chemical roughness is increased. Such changes appear to be correlated with the behavior of the magnetic hysteresis of the interface, measured by tracking the diffuse XRMS intensity as a function of applied magnetic field.
Magnetic properties and spin dynamics in hole-doped S=1 AF chain: Y89 NMR and susceptibility in Y2−xCaxBaNiO5
Authors: Tedoldi F. , Rigamonti A. , Brugna C. , Corti M. , Lascialfari A. , Capsoni D. , Massarotti V.
Published: July 2002

Hole-doping effects in Haldane chain have been studied by means of magnetic susceptibility and Y89 nuclear magnetic resonance in Y2−xCaxBaNiO5 for x=0, x=0.06 and x=0.18. In nominally pure YBNO detailed information on this prototype of Haldane systems is extracted. In the charge-doped compounds the Y89 relaxation rates indicate that holes induce low-energy excitations with an effective spectral density having a structure characterized by a narrow central peak.
Magnetic domains of single-crystal Nd2Fe14B imaged by unmodified scanning electron microscopy
Authors: Lewis L. H. , Wang J.-Y. , Canfield P.
Published: July 2002

The stray flux manifestations of surface magnetic domains found in as-grown Nd2Fe14B single crystals were observed, with a resolution in the range of 1 μm, using conventional scanning electron microscopy (SEM) without instrumental modifications. A modified image-distortion mode was applied to image the three-dimensional stray flux emanating from the sample. The simplicity of the technique and the ready adaptability of the SEM to such modifications as in situ current and magnetic field application suggest that the results of this study may be extended to investigations of other materials of technological interest, such as perpendicular media disks.
Measurements of biomagnetic fields using a high-resolution dc superconducting quantum interference device magnetometer
Authors: Iramina K. , Hong B. , Uchida S. , Goto K. , Ueno S. , Nakayama S.
Published: July 2002

We have developed a multichannel high-resolution superconducting quantum interference device magnetometer for measuring biomagnetic fields produced by small animals. We measured the magnetocardiogram produced by a rat. Topographies of the P wave, QRS wave, and T wave of the rat magnetocardiogram were obtained. We also measured the magnetoretinogram, visually evoked magnetic fields, and auditory evoked magnetic fields of the rat. The signals of the magnetoretinogram appeared at 60 ms latency and the field intensities were about 0.6 pT. It was not possible to obtain the clear visually evoked fields because those signals were hidden behind the magnetoretinogram. It was possible to obtain auditory evoked magnetic fields of the rat. The significant differences of the wave forms were observed in both sides of the right ear, which are separated by 15 mm. Our system has adequate spatial resolution for measurement of the magnetocardiogram and auditory evoked magnetic fields produced by small animals.
Epitaxial Fe100−xCox/Ag(001) alloy films: Structure and element-specific magnetic moments from magnetic linear dichroism
Authors: Kisker E. , Faust A. , Schellenberg R. , Fanelsa A. , Hillebrecht F. U.
Published: July 2002

The structure and element-specific local magnetic moments in Fe100−xCox/Ag(001) (16<x<70) epitaxial ultrathin alloy films have been determined by photoelectron diffraction (PED) and related magnetic linear dichroism in angle-resolved core-level photoemission, using a laboratory x-ray source. The PED peaks of the Fe and Co 2p core levels for any of the alloy compositions are at emission angles as expected for a bcc lattice. The dichroism increases for the Fe 2p3/2 core level with increasing x, whereas that of the Co 2p3/2 dichroism is virtually independent on the alloy composition. The dichroism of both the Fe and the Co core levels scales well with the calculated dependence of the local magnetic moment on the alloy composition rather than with the average saturation magnetization.
Spin-polarized tunneling by spin-polarized scanning tunneling microscopy
Authors: Kodama Hiroyoshi , Uzumaki Takuya , Oshiki Mitsumasa , Sueoka Kazuhisa , Mukasa Kōichi
Published: July 2002

A spin-polarized scanning tunneling microscope (STM) using a GaAs tip has been studied to observe the magnetization with nanoscale resolution for a sample magnetized in the plane of the film. A STM image of Si(111) 7×7 surface reconstruction using the GaAs tip was observed clearly under ultra-high-vacuum conditions. We found that the cleaved GaAs tip has a capability of high atomic resolution. Next, circularly polarized light irradiated the GaAs tip, and the I–V characteristics were measured for magnetic Ni80Fe20 and nonmagnetic Au samples. Changes in the tunneling current, which is estimated to be approximately 7%, were observed only for the NiFe sample under the circularly polarized light irradiation. It suggests that the change in the tunneling current is due to the spin-polarized tunneling from the GaAs tip to the NiFe.
A thermionic-emission-diffusion model for graded base Pnp heterojunction bipolar transistors
Authors: Datta S. , Roenker K. P. , Cahay M. M.
Published: July 2002

An analytical model which matches thermionic-emission-diffusion of holes across the emitter-base heterojunction with drift-diffusion transport across a graded base has been developed and used to examine the performance capabilities of InP-based Pnp heterojunction bipolar transistors (HBTs). Hole drift-diffusion across the emitter-base space charge region is shown to be of comparable importance to thermionic emission in controlling hole injection into the base. The effects of compositional base grading on the recombination currents is also taken into account. Compositional grading of the base is shown to enhance the device’s current gain by as much as a factor of 10 by reducing recombination in the quasi-neutral base. More importantly, compositional base grading significantly reduces the base transit time which improves the device’s peak cutoff frequency by as much as a factor of 1.5. A cutoff frequency as high as 35 GHz is found to be possible. The analysis indicates that composition grading of the base can be useful in developing high performance Pnp InP-based HBTs.
Two-dimensional tight-binding model of ac conductivity in porous silicon
Authors: Cruz H. , Luis D. , Capuj N. E. , Pavesi L.
Published: July 2002

A time-dependent tight-binding model has been developed to study the ac conductivity in porous silicon. Assuming that carriers are allowed to hop between isolated pairs of Si nanocrystals embedded in a SiO2 matrix, the tunneling times have been calculated according to different geometries. The geometrical structure of porous silicon has been modeled with simple percolationlike clusters. By using the tunneling times, the ac conductivity behavior in the high-frequency regime has been calculated in the pair approximation. The conductivity increases with the frequency according to a power law with an exponent lower than unity. It is found that there is a strong dependence of the ac conductivity on the thickness of the SiO2 interconnecting layer.
A matrix formulation of magnetoresistance for an arbitrary J-fold multicarrier semiconductor system via the reduced-conductivity-tensor scheme in the nonquantizing regime
Authors: Kim J. S.
Published: July 2002

In this article, a matrix formulation of magnetoresistance in semiconductors is presented for an arbitrary J-fold multicarrier system which is based on the reduced-conductivity-tensor scheme and applicable in the nonquantizing regime where Landau orbital quantization is negligible. In the formalism, a unique expression of the magnetoresistance is deduced in terms of two vectors which depend on the carrier densities and mobilities, and three matrices which represent various inter-carrier couplings under the applied magnetic field. In particular, the mobility-difference matrix plays a key role, and its simple form strongly suggests a two-carrier model of magnetoresistance for a narrow continuum distribution. Explicit closed-form formulas of magnetoresistance are derived for the two-carrier (J=2) and three-carrier (J=3) systems as special cases of the general formalism. The field dependence and asymptotic behavior of the magnetoresistance are also discussed, and a two-carrier model of magnetoresistance is formally proposed.
Refined modeling of optical confinement in thin silicon layers with a Lambertian back reflector for solar cell applications
Authors: Ghannam M. Y. , Abouelsaood A. A. , Mertens R. P.
Published: July 2002

An attempt is made to assess the accuracy of the simplifying assumption of total transmission (total loss) of the light inside the “loss” or “escape” cone which is made in many models of optical confinement in thin-layer silicon solar cells. A closed form expression is derived for the absorption enhancement factor as a function of the refractive index in the low-absorption limit for a thin layer with a Lambertian back-reflector and is compared with the expression derived previously on the basis of the “total loss” assumption. The generation profile, needed for the evaluation of solar cell performance parameters, is also obtained and studied. More general problems involving realistic light trapping schemes and/or actual material properties are treated numerically, sometimes by means of a suitably modified ray-tracing computer program. It turns out that the error in making the above-mentioned approximation is relatively minor if the layer has a Lambertian back-reflector, whether its front surface is textured or not, but becomes more appreciable for a flat back-reflector particularly as its reflectance approaches unity.