bluuebunny/crossref_metadata_2025 huggingface dataset:
Observation of downstream energetic electron/ion flow and accompanied high-power microwave radiation during the opening of a submicrosecond plasma opening switch
Authors: Krasik Ya. E. , Dunaevsky A. , Felsteiner J. , Goyer J. R.
Published: July 2002

In this article, results of experimental investigations of high-energy electron beam generation and transportation in the region downstream of a submicrosecond plasma opening switch (POS) (Ipos ⩽35 kA, τ1/4≅300 ns) are presented. This electron beam is accompanied by collectively accelerated ions having energies several times higher than the electron energies. Experiments were carried out with different loads: short-circuit, high-impedance, and planar electron diode. It is shown that the electron/ion beam appearance in the region downstream of the POS load occurs almost simultaneously with the beginning of the inductive voltage, independently of the type of load. In addition, generation of high-power microwave radiation (fmw ⩽10 GHz, Pmw⩽200 MW) was observed for all types of loads. We believe that the generation of the microwave radiation is related to the injection of a noncomplete charge-neutralized electron beam into the downstream drift region. These results and a qualitative explanation of the generation of the inductive voltage and microwave radiation are discussed.
Diffusion of Au in ZnSe and its dependence on crystal quality
Authors: Slotte J. , Salonen R. , Ahlgren T. , Rauhala E. , Keinonen J. , Räisänen J. , Uusimaa P. , Salokatve A. , Pessa M. , Laakso A.
Published: July 2002

Diffusion of gold in zinc selenide has been studied by using a C12 and He12 ion backscattering technique. The samples were thin films grown by molecular beam epitaxy on bulk GaAs (100) substrates and on GaAs (100) epitaxial layers followed by evaporation of gold and annealing in the temperature range 400–800 °C. The surface properties of the samples were studied with scanning electron microscopy and atomic force microscopy. The crystal quality of the samples was studied with He4 ion channeling. The gold diffusion was found to depend significantly on the crystal quality of the ZnSe. An empirical model for calculating the diffusion coefficient for different crystal quality ZnSe is presented.
Higher-order optical nonlinearity observed in host–guest liquid crystals
Authors: Ono Hiroshi , Harato Yoshiro
Published: July 2002

Optical nonlinearity in host–guest liquid crystals was characterized by systematically investigating both the experimental diffraction beam pattern and the theoretical one originating in photothermal self-phase modulation. The experimental diffraction patterns were in very good agreement with the calculated results when the refractive index changes (Δn) were assumed to be described by the equation Δn=n2I+n4I2, where n2 and n4 are nonlinear coefficients, and I is the light intensity. These higher-order nonlinear coefficients were strongly dependent on the nematic–isotropic transition temperatures of the host–guest liquid crystals, dye concentration, and sample thickness.
Experimental evidence of surface conduction in AlSb–InAs tunneling diodes
Authors: Nomoto K. , Taira K. , Suzuki T. , Hase I.
Published: July 2002

The peak-to-valley ratio of AlSb–InAs resonant tunneling diodes decreases as the diameter of the diode decreases due to the surface current. To clarify the origin of the surface current, we studied AlSb–InAs single-barrier diodes with various diameters and barrier thicknesses at various temperatures. We conclude from experimentally obtained results that bulk current is caused by tunneling through an AlSb barrier influenced by the band structure and surface current is caused by an electron emission from band-gap surface states at the AlSb barrier based on the Poole–Frenkel mechanism with ionization energy of 0.24 eV.
Admittance spectroscopy analysis of the conduction band offsets in Si/Si1−x−yGexCy and Si/Si1−yCy heterostructures
Authors: Singh D. V. , Rim K. , Mitchell T. O. , Hoyt J. L. , Gibbons J. F.
Published: July 2002

Schottky diodes fabricated on in situ doped n-type Si/Si1−x−yGexCy/Si heterostructures grown by chemical vapor deposition were used for admittance spectroscopy in order to study the impact of carbon on the conduction band offsets. Samples with a nominal Ge concentration of 20 at. % and carbon fractions up to 1.3 at. % were studied. In these experiments, the measurement frequency was swept continuously from 1 kHz to 5 MHz, and the temperature was scanned in small increments from 20 to 300 K. Admittance signals in these samples were found to originate from three sources, namely doping freeze-out, band offsets, and traps. Signals arising from the band offsets indicate a conduction band edge lowering for Si/Si1−x−yGexCy of ∼33±22 meV/at. % C. A trap-related admittance signal at an energy of 228±25 meV below the Si conduction band was observed in the Si1−x−yGexCy sample with the highest C fraction (1.3 at. %). The trap energy measured by admittance spectroscopy is in close agreement with the activation energy of 230 meV, which has been reported in the literature for a complex involving interstitial carbon. The conduction band offset in a Si/Si1−yCy sample with 0.95 at. % C was also measured by both admittance spectroscopy and Schottky capacitance–voltage profiling. The two techniques yield excellent agreement, with Si/Si0.9905C0.0095 conduction band offsets of 48±10 and 55±25 meV, respectively.
Modelling the ultraviolet irradiation effect on the effective minority carrier recombination lifetime of silicon wafers
Authors: Lee W. P. , Khong Y. L. , Seow W. S.
Published: July 2002

A mathematical model is presented to explain the effect of ultraviolet (UV) irradiation on the minority carrier recombination lifetime (LT) and surface barrier observed in thermally oxidized, RCA cleaned, and contaminated silicon wafers. The proposed model assumes that UV photons (4.9 eV) modify the density of two interface traps and the oxide charge density. It considers the conversion between the two species of traps with energy levels E1 and E2, relative to the intrinsic Fermi level (Ei). These modifications directly affect the surface recombination velocity and hence the effective LT. The changes in the effective lifetime and the surface barrier as a result of UV irradiation as simulated by the model was found to be in good agreement with experimental data.
Electrical resistivity and enhanced magnetoresistance in (La0.6Pb0.4MnO3/La0.85MnO3−δ)n superlattices
Authors: Sahana M. , Hegde M. S. , Prasad V. , Subramanyam S. V.
Published: July 2002

Resistivity and magnetoresistance studies have been carried out on (La0.6Pb0.4MnO3/La0.85MnO3−δ)n (n=30 periods) ferromagnetic/paramagnetic oxide multilayers fabricated by pulsed laser deposition. An epitaxial film of La0.6Pb0.4MnO3 is ferromagnetic and metallic below 320 K, whereas that of La0.85MnO3−δ is a paramagnetic (and antiferromagnetic at low temperatures) insulator. The multilayers showed an insulator-to-metal transition at 285±5 K. The magnetoresistances of these multilayers were about 60%–75% at the transition temperature in 6 T magnetic field. The magnetoresistance is also shown to be dependent on the thickness of the ferromagnetic oxide layer. A maximum magnetoresistance of 75% was observed for a [La0.6Pb0.4MnO3(50 Å)/La0.85MnO3−δ(20 Å)]30 superlattice as against 25% in a single La0.6Pb0.4MnO3 film at 285 K.
Time of flight mass spectroscopy of recoiled ions studies of surface kinetics and growth peculiarities during gas source molecular beam epitaxy of GaN
Authors: Kim Esther , Berishev I. , Bensaoula A. , Rusakova I. , Waters K. , Schultz J. A.
Published: July 2002

High growth rate GaN thin films were successfully grown by gas source molecular beam epitaxy and studied in situ by time of flight mass spectroscopy of recoiled ions (TOF-MSRI) and reflection high energy electron diffraction (RHEED). We show that TOF-MSRI allows for in situ monitoring and control of sapphire surface chemistry and its nitridation. In the latter case, TOF-MSRI is more sensitive to the surface changes during nitridation than RHEED. Using both RHEED and TOF-MSRI, growth of low-temperature GaN buffer layers was monitored, and their recrystallization and island-like nature were demonstrated. A model describing the probable growth mechanism for gas source molecular beam epitaxy of GaN is suggested. The model explains both the chemical dissociation of ammonia at low temperature and the origin of Ga to N TOF-MSRI peak ratio changes for various Ga and ammonia fluxes. High-resolution transmission electron microscopy studies confirm that GaN films grown with a buffer layer have excellent structural quality without any evidence of interfacial defects. Those without a buffer layer are highly defective.
Lower pressure limit of diamond growth in inductively coupled plasma
Authors: Teii Kungen , Yoshida Toyonobu
Published: July 2002

A study of diamond growth at pressures below 20 mTorr by using an inductively coupled radio frequency plasma is presented. Emissive and Langmuir probes, and optical emission spectroscopy were used to examine the plasma, and deposits were obtained on single crystalline silicon and diamond substrates with controlling the bombarding ion energy by the sheath potential (Vsheath). A higher threshold of Vsheath that allowed diamond growth was found in the range of 11–19 V above 20 mTorr, while a shift down of the Vsheath threshold was observed below 10 mTorr, as confirmed by Raman spectroscopy and electron diffraction. The growth at 10 mTorr was successful only when Vsheath was reduced to 2 V, however, the growth at 5 mTorr was no longer possible even when Vsheath was reduced to 2 V. Effects of the pressure decrease on the suppression of diamond growth below 10 mTorr were interpreted in terms of an increase in ion flux relative to radical flux as well as low radical density corresponding to the plasma density of approximately 2×1010 cm−3.
Electron swarm parameters in carbon tetrafluoride
Authors: Vasenkov A. V.
Published: July 2002

This communication reports the first rigorous study of electron swarm parameters in carbon tetrafluoride (CF4). To describe the available differential cross sections for electrons in CF4, we developed the analytical fits to these data, which were useful for Monte Carlo calculations. An agreement between calculated swarm parameters and recommended data of Christophorou, Olthoff, and Rao [J. Phys. Chem. Ref. Data 25, 1341 (1996)] was obtained. It was shown that errors in swarm parameters resulting from the isotropic scattering assumption were high for low and middle values of electric field to gas density ratios, wherein the isotropic electron scattering was considered traditionally.
Epitaxial Si1−yCy alloys: The role of surface and gas phase reactions
Authors: Mocuta A. C. , Greve D. W.
Published: July 2002

We have studied substitutional carbon incorporation in Si1−yCy alloys grown on Si (100) by ultrahigh vacuum chemical vapor deposition using silane and methylsilane as silicon and carbon precursors. Carbon incorporation has been studied in the growth temperature range of 550–650 °C. The total carbon content of the alloys increases linearly with the methylsilane partial pressure and a methylsilane sticking coefficient approximately two times higher than that of silane was determined from the data. Substitutional carbon content in the alloys tends to saturate at higher methylsilane flows. Our results, together with other previous reports, provide new insight into the roles of gas phase and surface reactions in determining growth rates and carbon incorporation efficiency.
Effective sine-Gordon model for the static properties of narrow window junctions
Authors: Caputo Jean-Guy , Flytzanis Nikos , Kurin Vladislav , Lazarides Nikos , Vavalis Emmanuel
Published: July 2002

We show that the maximum tunneling current for a long thin window Josephson junction in the presence of an external magnetic field is given to a good approximation by the solution of an effective sine-Gordon model with appropriate rescalings. This model is obtained via a reduction of the two-dimensional system to a one-dimensional integro-differential equation (nonlocal sine-Gordon). The passive region introduces an effective Josephson characteristic length. Using numerical simulations we show that for a narrow junction the cases of inline and overlap current feed are well described by the effective model. We discuss the influence of the width of the junction and find that the model gives a good approximation for the behavior of large area windows in the inline geometry, while it deviates for the overlap.
Theoretical analysis of electromigration-induced failure of metallic thin films due to transgranular void propagation
Authors: Gungor M. Rauf , Maroudas Dimitrios
Published: July 2002

Failure of metallic thin films driven by electromigration is among the most challenging materials reliability problems in microelectronics toward ultra-large-scale integration. One of the most serious failure mechanisms in thin films with bamboo grain structure is the propagation of transgranular voids, which may lead to open-circuit failure. In this article, a comprehensive theoretical analysis is presented of the complex nonlinear dynamics of transgranular voids in metallic thin films as determined by capillarity-driven surface diffusion coupled with drift induced by electromigration. Our analysis is based on self-consistent dynamical simulations of void morphological evolution and it is aided by the conclusions of an approximate linear stability theory. Our simulations emphasize that the strong dependence of surface diffusivity on void surface orientation, the strength of the applied electric field, and the void size play important roles in the dynamics of the voids. The simulations predict void faceting, formation of wedge-shaped voids due to facet selection, propagation of slit-like features emanating from void surfaces, open-circuit failure due to slit propagation, as well as appearance and disappearance of soliton-like features on void surfaces prior to failure. These predictions are in very good agreement with recent experimental observations during accelerated electromigration testing of unpassivated metallic films. The simulation results are used to establish conditions for the formation of various void morphological features and discuss their serious implications for interconnect reliability.
Microstructure and nanomechanical properties of nitrogenated amorphous carbon thin films synthesized by reactive radio frequency sputtering
Authors: Lu W. , Komvopoulos K.
Published: July 2002

Thin films of nitrogenated amorphous carbon (a-CNx) were deposited on Si(100) substrates by reactive radio frequency sputtering using a gas mixture of Ar and N2 at a total working pressure of 3 mTorr. X-ray photoelectron spectroscopy (XPS) showed that the films consisted of amorphous carbon (a-C) containing a β-C3N4-like phase with N atoms bonded to C atoms in tetrahedral coordination (sp3) and a graphite-like phase with N atoms bonded to C atoms in trigonal coordination (sp2). Analysis of the XPS spectra revealed a strong effect of the N2 partial pressure on the concentration and composition of each constituent. The thickness and nanohardness of the a-CNx films were in the ranges of 7–35 nm and 12.5–40 GPa, respectively, depending on the fraction of N2 in the sputtering gas. Conversely to the chemical composition, the growth rate (thickness), microstructure, and nanomechanical properties of the a-CNx films were found to depend on the total mass flow rate. While the N/C atomic ratio increased with the N2 partial pressure, the film nanohardness and in-plane elastic modulus decreased due to the reduced energetic ion bombardment on the growing film surface and the incorporation of soft phases in the sputtered films. Nanoindentation and XPS results are presented to elucidate the deposition kinetics and to interpret the dependence of the resulting film microstructure and nanomechanical properties on the plasma conditions.
Magnetic properties of two-dimensional arrays of epitaxial Fe (001) submicron particles
Authors: Hanson M. , Johansson C. , Nilsson B. , Isberg P. , Wäppling R.
Published: July 2002

The magnetic properties of two-dimensional arrays of Fe particles with well-defined geometry, prepared by electron lithography from epitaxial Fe (001) films of thickness of 50 nm, have been studied. Circular particles with diameters of 0.6 and 0.4 μm and rectangular particles 0.9 μm×0.3 μm and 0.7 μm×0.2 μm, were positioned in square and rectangular lattices, respectively, with lattice constants about twice the particle dimensions. Samples were prepared with the lattices oriented along the 〈100〉 and 〈110〉 directions. Hysteresis curves were obtained in the field range ±2 T at temperatures between 30 and 300 K. The hysteresis curves are characteristic for single particles with a multidomain zero-field state. The magnetization and demagnetization processes are mainly governed by the geometrical shape of the particles and their orientation with respect to the crystal directions and the applied field. This implies that the high quality in the epitaxial Fe films is preserved during the patterning process. Magnetic images, obtained by magnetic force microscopy, support the conclusions drawn from the magnetization measurements. The circular particles showed a fourfold symmetry in their magnetic image, which is compatible with the expected domain structure in a disk with cubic crystalline anisotropy.
Electrostatic force microscopy studies of surface defects on GaAs/Ge films
Authors: Xu Q. , Hsu J. W. P.
Published: July 2002

We apply electrostatic force microscopy (EFM) to study defects in GaAs films grown on Ge. On a GaAs film with surface antiphase boundaries (APBs), we reproducibly measure the surface contact potential (SCP) at the APBs to be (30±5) mV higher than that of the domains, due to the surface Fermi level at APBs being pinned closer to the valence band maximum. On a thick film which contains buried APBs and wedge-shaped depressions on the surface, we find that the SCP of the wedge-shaped depressions is (25±5) mV lower than that of the GaAs surface. Hence, these wedge-shaped depressions have defect electronic states different from those of APBs. The capacitance gradient (∂C/∂z) contrasts on the two samples are also shown to arise from different origins. Factors that can affect the measured SCP and ∂C/∂z values are discussed. We demonstrate a new application of EFM to distinguish different types of defects by measuring variations in relative SCP (thus the work function or position of Fermi level) and/or ∂C/∂z on sample surfaces. The spatial resolutions of SCP and ∂C/∂z are 30 nm, limited by the tip size.
Current responsivity of semiconductor superlattice THz-photon detectors
Authors: Ignatov Anatoly A. , Jauho Antti-Pekka
Published: July 2002

The current responsivity of a semiconductor superlattice THz-photon detector is calculated using an equivalent circuit model which takes into account the finite matching efficiency between a detector antenna and the superlattice in the presence of parasitic losses. Calculations performed for currently available superlattice diodes show that both the magnitudes and the roll-off frequencies of the responsivity are strongly influenced by an excitation of hybrid plasma-Bloch oscillations which are found to be eigenmodes of the system in the THz-frequency band. The expected room temperature values of the responsivity (2–3 A/W in the 1–3 THz-frequency band) range up to several percents of the quantum efficiency e/ℏω of an ideal superconductor tunnel junction detector. Properly designed semiconductor superlattice detectors may thus demonstrate better room temperature THz-photon responsivity than conventional Schottky junction devices.
Evidence of strong effect from the interface on the electrical characteristics of ZnSe/GaAs heterojunctions
Authors: Seghier D. , Hauksson I. S. , Gislason H. P. , Prior K. A. , Cavenett B. C.
Published: July 2002

The interface properties of ZnSe/GaAs heterojunctions grown by molecular beam epitaxy have been studied by electrical methods. The current–voltage and capacitance–voltage characteristics show a hysteresis which can be related to unusually slow current and capacitance transients in response to a change in the reverse bias. We performed admittance spectroscopy measurements at various frequencies in order to investigate this phenomenon. A large frequency dispersion of the capacitance and a broad peak in conductance spectra suggest the presence of interface states at the heterojunction between ZnSe and GaAs. We ascribe the long transients to slow changes in the charge on these interface states. We also propose that interface states result in a voltage-induced lowering of the barrier at the heterointerface. The capacitance versus frequency data were analyzed in terms of Lehovec’s model from which the density of interface states of 3–4×1012 cm−2 eV−1 were calculated.
Thermal profiles and thermal runaway in microwave heated slabs
Authors: Vriezinga C. A.
Published: July 2002

The microwave heating of slabs of water bound with a gel is modeled and analyzed without any restriction to the Biot number regime. Despite the fact that the temperature distribution over the slab is not uniform at all, the phenomenon of thermal runaway is basically caused by resonance of the electromagnetic waves within the object, combined with heat loss. A plot of the steady-state temperature at any position within the slab, versus the microwave power, is an S-shaped or a multi S-shaped curve. With respect to thermal runaway there is a strong similarity between isothermal and nonisothermal slabs. Using the average temperature of the nonisothermal slab, regardless of its Biot number, yields a reasonable approximation to describe the runaway. This is caused by the specific characteristic of the dielectric loss factor of water, which decreases with increasing temperature. This results in an almost constant absorption of energy over the whole slab without disturbing the wave character of the absorption. It turned out that this smoothing of the absorbed power plays a dominant role in the calculations of the temperature profiles. Any calculation where the temperature dependence of the permittivity is omitted, will not only pass the phenomenon of thermal runaway, but its temperature profiles will differ substantially from the ones where the temperature dependence has been taken into account.
Electron transport across a Gaussian superlattice
Authors: Gómez I. , Domı́nguez-Adame F. , Diez E. , Bellani V.
Published: July 2002

We study the electron transmission probability in semiconductor superlattices where the height of the barriers is modulated by a Gaussian profile. Such structures act as efficient energy band-pass filters and, contrary to previous designs, it is expected to present a lower number of unintentional defects and, consequently, better performance. The j–V characteristic presents negative differential resistance with peak-to-valley ratios much greater than in conventional semiconductor superlattices.
Inelastic neutron scattering study of Mn12–acetate
Authors: Zhong Yicheng , Sarachik M. P. , Friedman Jonathan R. , Robinson R. A. , Kelley T. M. , Nakotte H. , Christianson A. C. , Trouw F. , Aubin S. M. J. , Hendrickson D. N.
Published: July 2002

We report zero-field inelastic neutron scattering experiments on a deuterated powder sample of Mn12–acetate consisting of a large number of nominally identical spin-10 magnetic clusters. Our resolution enables us to see a series of peaks corresponding to transitions between the anisotropy levels within the spin-10 manifold. A fit to the spin Hamiltonian H=−DSz2−μBB⋅g⋅S−ASz4+C(S+4+S−4) yields an anisotropy constant D=(0.54±0.02) K and a fourth-order diagonal anisotropy coefficient A=(1.2±0.1)×10−3 K (the other terms being negligible). Performed in the absence of a magnetic field, our experiments do not involve the g values as fitting parameters, thereby yielding particularly reliable values of D and A.
Brillouin investigation of the dynamic properties of cobalt wire arrays
Authors: Chérif S. M. , Roussigné Y. , Moch P. , Hennequin J.-F. , Labrune M.
Published: July 2002

We have investigated the long-wavelength spin waves in 400 Å thick cobalt wires with various width/period ratios: 0.5/1, 1/1.5, and 2/3 μm, using Brillouin light scattering. All of them show a new low-frequency line which is absent in the unpatterned films: it is attributed to modes originating from the uniform bulk mode of the infinite layer which give rise to a measurable Brillouin intensity only when both anisotropy and patterning are present in the sample. The variations of the frequency of the magnetostatic surface mode (the so-called Damon–Eshbach mode) were studied versus a magnetic-field H applied parallel or perpendicular to the axis of the wires. Compared with unpatterned layers, the Brillouin frequencies are higher when H lies along the wire axis; they are not found to be significantly sensitive to the width or to the separation of the wires. We interpret this result in terms of a slight reduction of the magnetic anisotropy for the wires. For H perpendicular to the wire axis, the spin wave frequencies are reduced by the shape- and orientation-dependent demagnetizing field Hd. The experimental determinations of Hd agree with our calculations of its mean value assuming a uniform magnetization and taking into account the interaction between the wires.
A study on the exchange coupling of NdFeB-type nanocomposites using Henkel plots
Authors: Chen Q. , Ma B. M. , Lu B. , Huang M. Q. , Laughlin D. E.
Published: July 2002

The phase transformation and magnetic properties of (Nd0.95La0.05)9.5FebalCo5Nb2B10.5 nanocomposite have been investigated systematically via thermomagnetic analysis, vibration sample magnetometer, x-ray diffraction, and conventional transmission electron microscopy. The Henkel plot was employed to quantify the strength of the exchange coupling between the hard and soft magnetic phases in the as-spun and the thermally treated samples. It was found that remanence Br, coercivity Hci, and maximum energy product BHmax obtained were affected by the magnetic phases present as well as the grain size of constituent phases and their distribution. The Henkel plot successfully interpreted the effect of the exchange coupling on Br, Hci, and BHmax obtained for samples treated below 750 °C. However, it became inadequate for samples treated above 750 °C. Although similar shapes of ΔM–H curves were obtained in the Henkel plot, severe degradation in Br, Hci, and BHmax was found when the thermal treatment temperature was increased from 750 to 850 °C. This degradation may be attributed to the grain growth of the main phases, from 45 to 68 nm, and the development of precipitates. In conjunction with the mass balance, the precipitated phases (presumably borides) may explain the increase in the Tc of Nd2(FeCo)14B and the decreased amount of α-Fe and Fe3B with increasing thermal processing temperature.
High magnetic field studies of 3d and 4f magnetism in (R0.7A0.3)MnO3: R=La3+, Pr3+, Nd3+, A=Ca2+, Sr2+, Ba2+, Pb2+
Authors: Thomas R-M. , Skumryev V. , Coey J. M. D. , Wirth S.
Published: July 2002

Magnetization curves up to 25 T at 4.2 K are reported for mixed-valence manganites (R0.7A0.3)MnO3. All the R=La compounds show an extrapolated ferromagnetic moment of ∼3.4 μB/f.u. The A=Sr and Ca compounds show essentially no high-field slope, dM/dB<0.001 μB/T whereas the Pb and Ba compounds show slopes of 0.0047 and 0.0025 μB/T, suggesting that electrons of both spins are present at the Fermi level. All Pr compounds show much greater slopes, dM/dB≈0.02 μB/T associated with a singlet ground state for Pr3+. All except the Ca compound have extrapolated moments of 3.6±0.3 μB/f.u. but (Pr0.7Ca0.3)MnO3 exhibits two first-order magnetization processes, at 5 and 6 T, due respectively to melting of charge order as the compound becomes ferromagnetic and a level-crossing transition for the Pr3+ ion. All the Nd compounds, except for the charge ordered, Ca containing compound, have a magnetization which is ∼1 μB greater than their La counterparts, due to ferromagnetic alignment of Nd and Mn moments.
“Bloch lines” in domain walls of yttrium orthoferrite
Authors: Didosyan Y. S. , Reider G. A. , Hauser H.
Published: July 2002

Images of Bloch lines and variations of the brightness of the domain wall images were observed by means of a dark field method in thin plates of orthoferrites cut perpendicularly to the optical axis. A connection between the brightness variations and changes in the domain wall inclinations is discussed. The domain wall structure changes with the external inhomogeneous magnetic field and depends on the preceeding domain wall state.
Noise in 6H-SiC ion implanted p–n diodes: Effect of the active area on the noise properties of these junctions
Authors: Ouacha H. , Willander M. , Wahab Q. , Ouacha A. , Holmén G.
Published: July 2002

Low frequency noise in 6H-SiC ion implanted p–n diodes has been investigated. The noise measurements were performed under forward bias over a wide temperature range from 77 to 300 K and frequency range from 10 to 100 kHz. Three diodes with different areas A1, A2, and A3, (A1<A2<A3), have been studied. The effect of the area on the noise properties of these junctions was found to be at room temperature, depending on the origin of the noise. Two relationships between the noise spectral density SI(f ) and the area (A) have been found to describe the noise behavior observed in these junctions at different current density ranges. From the noise experimental data at low temperatures, two different trap states were detected in the diode with the smallest area. This explains the higher noise level observed in this diode in the recombination-generation region.
Light-induced charge transport processes in photorefractive Ba0.77Ca0.23TiO3 doped with iron
Authors: Mazur A. , Veber C. , Schirmer O. F. , Kuper C. , Hesse H.
Published: July 2002

Photorefractive barium-calcium titanate crystals with the congruently melting composition (Ba0.77Ca0.23TiO3) doped with iron are annealed in a hydrogen atmosphere at various temperatures. The samples are studied with electron paramagnetic resonance (EPR) and optical absorption, both performed separately as well as simultaneously, before and after illumination. The reduced samples are electron conductive and the charge transport at room temperature is governed by only one level identified by photo-EPR as Fe2+/3+. The optical absorption of Fe2+ is found to be peaked at 1.9 eV.
Anisotropy of magnetoresistance in (110)La0.7Sr0.3MnO3 thin films
Authors: Suzuki Y. , Hwang H. Y.
Published: July 2002

We have performed magnetization and magnetoresistance measurements on (110)La0.7Sr0.3MnO3 thin films in order to understand the role of in-plane uniaxial magnetic anisotropy in the magnetoresistance. Transport measurements with current flowing along the magnetically easy [001] and magnetically hard [11̄0] directions exhibit a linear dependence of magnetoresistance on field at high fields due to bulk colossal magnetoresistance. The low field behavior depends not only on the angle of the magnetization with the current but also the direction of the current in the crystal.
Laser-induced modification of electron field emission from nanocrystalline diamond films
Authors: Ugarov M. V. , Ageev V. P. , Karabutov A. V. , Loubnin E. N. , Pimenov S. M. , Konov V. I. , Bensaoula A.
Published: July 2002

A significant reduction of electron field emission thresholds resulting from ArF laser irradiation of nanocrystalline diamond films in borazine (B3N3H6) and ammonia (NH3) atmospheres is reported. The change of emission characteristics is not connected with either laser surface graphitization or formation of bulk defects. X-ray photoelectron spectroscopy surface analysis and Raman spectroscopy data from these samples show that laser irradiation results in the synthesis of ultrathin layers of boron–carbon–nitrogen and in B and N atoms intergrain penetration into the film to a depth above 50 Å. The synthesis of the ternary B–C–N and the presence of sp2 bonded C–N compounds is demonstrated.
Solder reaction-assisted crystallization of electroless Ni–P under bump metallization in low cost flip chip technology
Authors: Jang J. W. , Kim P. G. , Tu K. N. , Frear D. R. , Thompson P.
Published: July 2002

Solder reaction-assisted crystallization of electroless Ni–P under bump metallization in the Si/SiO2/Al/Ni–P/63Sn–37Pb multilayer structure was analyzed using transmission electron microscopy, scanning electron microscopy, energy dispersive x-ray, and electron probe microanalyzer. The electroless Ni–P had an amorphous structure and a composition of Ni85P15 in the as-plated condition. Upon reflow, the electroless Ni–P transformed to Ni3Sn4 and Ni3P. The crystallization of electroless Ni–P to Ni3P was induced by the depletion of Ni from electroless Ni–P to form Ni3Sn4. The interface between electroless Ni–P and Ni3P layer was planar. From the Ni3P thickness-time relationship, the kinetics of crystallization was found to be diffusion controlled. Conservation of P occurs between electroless Ni–P and Ni3P, meaning that little or no P diffuses into the molten solder. Combining the growth rates of Ni3Sn4 and Ni3P, the consumption rate of electroless Ni–P was determined. Based upon microstructural and diffusion results, a grain-boundary diffusion of the Ni or an interstitial diffusion of the P in the Ni3P layer was proposed.
Hot electron effects and dynamic behavior of gold doped germanium thin films as cryogenic phonon sensors
Authors: Wang X. X. , Getaneh M. , Martoff C. J. , Kaczanowicz E.
Published: July 2002

As a part of continuing development of thin film Ge1−xAux thermistors for ultralow mass highly sensitive cryogenic phonon sensors, the ac responsivity and hot electron effects have been explored. Responsivity as high as 1.6 nV/eV was achieved at 20 mK. A derivation is presented for the power index β=6 for electron-phonon heat exchange in variable range hopping conductivity. Applying our previously developed optimization theory for bias-assisted tunneling, hot electron effects and bias-assisted tunneling were shown to account equally well for the nonlinear dc current–voltage characteristic. However, a model was developed for the thermal time constants of these devices which does predict a clear distinction between bias-assisted tunneling and the hot electron effects. Comparison of data with this model showed that bias-assisted tunneling rather than hot electron effects was the dominant finite-bias effect in the Ge1−xAux thin films.
Comment on “Mixed electron emission from lead zirconate–titanate ceramics” [J. Appl. Phys. 83, 6055 (1998)]
Authors: Dorfman S. , Dunaevsky A. , Felsteiner J. , Krasik Ya. E.
Published: July 2002

Understanding the mechanism responsible for electron emission from ferroelectrics is crucial for the design of ferroelectric cathodes. The mixed process suggested in the model of Zhang et al. [J. Appl. Phys. 83, 6055 (1998)] appears to contain some contradictions. The most problematic is the coexistence of two electron emission processes on the same surface in different electric fields.
Investigation of dose uniformity on the inner races of bearings treated by plasma immersion ion implantation
Authors: Zeng Z. M. , Kwok T. K. , Tian X. B. , Tang B. Y. , Chu P. K.
Published: July 2002

Plasma immersion ion implantation (PIII) is an effective technique for the surface modification of industrial components possessing an irregular shape. We have recently used PIII to treat a real industrial ball bearing to enhance the surface properties of the race surface on which the balls roll. The implantation dose uniformity along the groove is assessed using theoretical simulation and experiments. The two sets of results agree very well, showing larger doses near the center. However, the highest dose is not observed at the bottom or center of the groove, but rather offset toward the side close to the sample platen when the bearing is placed horizontally. The minimum dose is observed near the edge or corner of the groove and our model indicates that it is due to the more glancing ion incidence as a result of the evolution of the ion sheath near the corner. The dose nonuniformity along the groove surface is about 40% based on our experimental data.
Dielectric and electrical properties of sputter grown (Ba,Sr)TiO3 thin films
Authors: Shin Ju Cheol , Park Jaehoo , Hwang Cheol Seong , Kim Hyeong Joon
Published: July 2002

The dielectric behavior containing the film thickness dependent dielectric constant, capacitance–voltage (C–V) variation, and the electrical conduction behavior containing the film thickness dependent current density-electric field variation of rf sputter deposited (Ba,Sr)TiO3 (BST) thin films are analyzed based on a fully depleted film and a combined Schottky-tunneling conduction model. The fact that the capacitance values in C–V increase with the increasing film thickness clearly indicates that the films are fully depleted. Also, the decreasing and nonvariant dielectric constants of the BST film on the Pt and IrO2 electrodes, respectively, with decreasing film thickness are attributed to the intrinsic interfacial layers at the interfaces with electrodes which have low dielectric constants and very small thicknesses. The increased leakage current density of a thicker film under a given electric field originates from the increased interfacial field strength due to the high space charge density in the sputtered film. The leakage current density under the electric field strength smaller than 120 kV/cm is controlled by the Schottky conduction mechanism (thermionic emission), and over that field strength, the tunneling related mechanism (thermionic field emission) is predominant. The calculated leakage current density on the basis of the combined Schottky-tunneling conduction model fits well to the measured current density at various temperatures and over a wide field range. For all the calculations, the electric field dependent dielectric constant is always taken into account. The electric field dependent dielectric constant and high defect or space charge density of the BST film make the conventional analysis of the dielectric and conduction behavior based on the classical metal/semiconductor contact theory invalid. Therefore, a more comprehensive formalism is developed to take into account the field dependent dielectric properties and high charge density of the sputter deposited BST films.
Depth distribution of luminescent Si nanocrystals in Si implanted SiO2 films on Si
Authors: Brongersma M. L. , Polman A. , Min K. S. , Atwater H. A.
Published: July 2002

Depth-resolved measurements of the photoluminescence of Si implanted and annealed SiO2 films on Si have been performed to determine the depth distribution of luminescent Si nanocrystals. Si nanocrystals with diameters ranging from ∼2 to 5 nm were formed by implantation of 35 keV Si ions into a 110-nm-thick thermally grown SiO2 film on Si(100) at a fluence of 6×1016 Si/cm2, followed by a thermal anneal at 1100 °C for 10 min. The photoluminescence spectrum is broad, peaks at λ=790 nm, and contains contributions from both recombination of quantum confined excitons in the nanocrystals and ion-implantation-induced defects. By chemical etching through the SiO2 film in steps and analyzing the changes in the photoluminescence spectrum after each etch step, the depth from which each of the two luminescence features originate is determined. The etch rate of the oxide, as derived from Rutherford backscattering spectrometry data, varies from 1.3 nm/s in the regions of small excess Si to 0.6 nm/s at the peak of the concentration profile (15 at. % excess Si). It is found that the defect luminescence originates from an ∼15-nm-thick near-surface region. Large nanocrystals luminescing at long wavelengths (λ=900 nm) are mainly located in the center of the film, where the Si concentration is highest (48 at. %). This is corroborated by transmission electron microscopy that shows a high density of Si nanocrystals in the size range of 2–5 nm in the center of the film. The largest density of small luminescent nanocrystals (λ=700 nm), not detectable by electron microscopy is found near the SiO2 surface and the SiO2/Si interface. This is attributed to either the fact that the surface and the SiO2/Si interface affect the Si nanocrystal nucleation kinetics in such a way that small nanocrystals are preferentially formed in these regions, or an optical interaction between nanocrystals of different sizes that quenches the luminescence of small nanocrystals in the center of the film.
Midgap density of states in hydrogenated polymorphous silicon
Authors: Meaudre M. , Meaudre R. , Butté R. , Vignoli S. , Longeaud C. , Kleider J. P. , Roca i Cabarrocas P.
Published: July 2002

When silicon thin films are deposited by plasma enhanced chemical vapor deposition in a plasma regime close to that of the formation of powder, a new type of material, named polymorphous silicon (pm-Si:H) is obtained. pmSi:H exhibits enhanced transport properties as compared to state-of-the-art hydrogenated amorphous silicon (a-Si:H). The study of space-charge-limited current in n+-i-n+ structures along with the use of the modulated photocurrent technique, of the constant photocurrent method and of steady-state photoconductivity and dark conductivity measurements allows us to shed some light on the origin of these improved properties. It is shown that the midgap density of states in the samples studied here is at least ten times lower than in a-Si:H, and the electron capture cross section of deep gap states is also expected to be lower by a factor of 3–4 to account for photoconductivity results. An interesting field of theoretical research is now open in order to link these low densities of states and capture cross sections to the peculiar structure of this new material.
Measurement of aluminum concentration in the Ga1−xAlxSb/GaSb epitaxial system
Authors: Bocchi C. , Franchi S. , Germini F. , Baraldi A. , Magnanini R. , De Salvador D. , Berti M. , Drigo A. V.
Published: July 2002

The composition of several Ga1−xAlxSb epitaxial layers of different thicknesses grown by molecular beam epitaxy on GaSb with x ranging between 0.1 and 0.8, has been obtained independently by high resolution x-ray diffraction, Rutherford backscattering spectrometry, and reflection high-energy electron diffraction. From the comparison between the results obtained by the different experimental methods, it has been possible to point out that the lattice constant of the layer increases nonlinearly with the Al content. A comparison with theoretical models has been done. A phenomenological equation has been derived for a correct analysis of the x-ray results.
Interactions of intentionally diffused hydrogen with nitrogen acceptors and nitrogen related donor centers in molecular beam epitaxy grown ZnSe
Authors: Pelletier H. , Theys B. , Lusson A. , Tournié E. , Chevallier J. , Marfaing Y. , Faurie J. P.
Published: July 2002

Nitrogen doped ZnSe layers grown by molecular beam epitaxy have been exposed to a hydrogen or deuterium plasma. Deuterium diffusion profiles have been measured by secondary ion mass spectroscopy. The main feature of these profiles is the presence of a plateau on which the H concentration closely matches the total N content of the layers. Electrical and photoluminescence studies of the layers, before and after hydrogen plasma exposure, show that both acceptor and donor N-related centers are passivated by hydrogen. These results demonstrate that in ZnSe:N hydrogen passivates the nitrogen acceptors and the related nitrogen donors.
Effects on the resonant tunneling characteristics of a double-barrier diode of intentional and unintentional dopings in the quantum well
Authors: Sakai J. W. , Beton P. H. , Eaves L. , Main P. C. , Henini M.
Published: July 2002

Donor-assisted resonant tunneling in nominally symmetric GaAs/(AlGa)As large area double-barrier diodes is investigated. The log(I)–V characteristics are used to evaluate doping density in the quantum well and are investigated in connection with donor cluster-assisted resonant tunneling. The single-donor-related feature in the resonant-tunneling characteristics is used to detect the presence of donors in the quantum well, even at concentrations of the order of the lowest achieved so far in molecular beam epitaxy GaAs. Expected effects of the presence of donors in the quantum well on the log(I) vs V characteristics are discussed.
Mobility of 180° domain walls in congruent LiTaO3 measured using real-time electro-optic imaging microscopy
Authors: Gopalan Venkatraman , Gerstl Stephan S. A. , Itagi Amit , Mitchell T. E. , Jia Q. X. , Schlesinger T. E. , Stancil D. D.
Published: July 2002

We report the electric-field dependence of 180° domain-wall mobility in congruent LiTaO3 measured at room temperature using in situ electro-optic imaging microscopy. The measured sideways domain-wall velocity of serrated domain fronts formed upon merger of domains was an order of magnitude larger than that for independently growing domains. The wall velocities also show a strong dependence on the nature of the applied electric field, being an order of magnitude larger for steady-state voltages as compared to pulsed voltage measurements. This is shown to be due to wall stabilization between applied voltage pulses resulting in an inertial delay in moving a domain wall which has been at rest for many seconds.
Analysis of high voltage operation of gallium arsenide photoconductive switches used in high power applications
Authors: Islam N. E. , Schamiloglu E. , Fleddermann C. B. , Schoenberg J. S. H. , Joshi R. P.
Published: July 2002

An opposed-contact photoconductive semiconductor switch, with a n+ region next to the cathode electrode has been simulated. Physical conditions during the pulse charging state, prior to high power switching, are analyzed in order to explain the increased hold-off characteristic of such devices. Results show that the introduction of the n+ region near the cathode inhibits the flow of electrons at the n+/semi-insulating interface until very high fields are reached. The formation of trap-filled regions near the contacts and the resultant inhomogeneous device characteristics that lead to breakdown are thereby shifted to higher voltages. Thus, for switches with a n+ region next to the cathode, the breakdown voltage due to unstable filamentary conduction is also increased beyond those achieved previously, allowing for higher power operation.
Synthesis of spherical luminescent particulate coatings
Authors: Fitz-Gerald J. M. , Rack P. D. , Trottier T. A. , Ollinger M. , Pennycook S. J. , Gao H. , Singh R. K.
Published: July 2002

In this communication, we show the feasibility of the pulsed laser ablation technique to make very thin, uniform luminescent coatings on particulate systems. To deposit continuous particulate coatings, the laser-induced plume from the target comes in contact with an agitated bed of core particles (size 1 μm). The pressure and nature of the background gas (inert or active) controls the cluster size of the particles in the laser plume. Experiments were conducted for laser deposition of phosphor nanoparticles on SiO2 core particles by pulsed excimer laser (wavelength=248 nm and pulse duration=25 ns) irradiation of a yttria:Eu (Y2O3:Eu3+) sputtering target.