bluuebunny/crossref_metadata_2025 huggingface dataset:
Stable anionic site on hydrogenated (111) surface of diamond resulting from hydrogen atom removal under chemical vapor deposition conditions
Authors: Komatsu Shojiro
Published: July 2002

The stability of a deprotonated site on the hydrogenated (111) surface of diamond under chemical vapor deposition (CVD) conditions was studied by using ab initio and semiempirical molecular orbital (MO) methods. The formation of this sort of anionic vacant site (AVS) was expected to occur when a surface hydrogen atom is removed and the resultant radical site captures an electron; this yields a closed shell structure of the anionic carbon atom. The effects of model cluster size as well as basis set selection in the MO studies were also examined. The stabilization energy approached about 1.5–1.9 eV at the limit of the calculation; this surface localized state lies within the band gap of diamond as was expected. The spatial distribution of the highest occupied molecular orbital confirmed that the captured electron was localized around the AVS. The AVS formed on the hydrogenated (111) surface of diamond is worth taking into account in studying the CVD growth mechanism of diamond.
Stacking InAs islands and GaAs layers: Strongly modulated one-dimensional electronic systems
Authors: Miller Mark S. , Malm Jan-Olle , Pistol Mats-Erik , Jeppesen So/ren , Kowalski Bernhard , Georgsson Kristina , Samuelson Lars
Published: July 2002

With chemical beam epitaxy we stacked small InAs islands, separated by thin GaAs layers. Reflection electron diffraction during growth showed that after a seed-layer growth, subsequent depositions require less InAs to form the islands. At 5 K the stacks have narrower luminescence peaks at lower energies than single island layers, and the stacks luminesce at room temperature. For 4-nm-high pyramidal islands with 20-nm-wide bases, we observed vertical periods down to 5.4 nm, small enough to couple quantum mechanically. The electronic structures possible for this class of objects should be sufficient for designing and observing room temperature quantum mechanical phenomena.
Determination of the quasi-Fermi-level separation in single-quantum-well p-i-n diodes
Authors: Tsui Ernest , Nelson Jenny , Barnham Keith , Button Chris , Roberts John S.
Published: July 2002

The radiative behavior of quantum-well (QW) devices depends upon the quasi-Fermi-level separation ΔEf induced in the quantum well. We present a method of obtaining ΔEf in absolute units from the emission spectra of optically or electrically biased QWs. Emission spectra are calibrated by comparison with measurements of the limiting photocurrent. A theoretical model is then used to separate out the effects of carrier generation rate and field-dependent QW absorption. We apply the method to the low-temperature photoluminescence spectra of a set of single QW p-i-n photodiodes at different electric fields. We show that modeled emission spectra agree closely with measured spectra in flatband conditions. We also observe a field-dependent loss in emission intensity—leading to a reduction in ΔEf of several meV—which we attribute mainly to carrier escape from the QW. The derived values for field-dependent nonradiative efficiency are consistent with independent measurements of low-temperature dc photocurrent, and with a simple model for thermally assisted carrier escape. We show how the method can be applied to electroluminescence spectra in order to investigate the dependence of ΔEf on applied bias.
Low-loss polycrystalline silicon waveguides for silicon photonics
Authors: Agarwal Anuradha M. , Liao Ling , Foresi James S. , Black Marcie R. , Duan Xiaoman , Kimerling L. C.
Published: July 2002

Photonic integrated circuits in silicon require waveguiding through a material compatible with silicon very large scale integrated circuit technology. Polycrystalline silicon (poly-Si), with a high index of refraction compared to SiO2 and air, is an ideal candidate for use in silicon optical interconnect technology. In spite of its advantages, the biggest hurdle to overcome in this technology is that losses of 350 dB/cm have been measured in as-deposited bulk poly-Si structures, as against 1 dB/cm losses measured in waveguides fabricated in crystalline silicon. We report methods for reducing scattering and absorption, which are the main sources of losses in this system. To reduce surface scattering losses we fabricate waveguides in smooth recrystallized amorphous silicon and chemomechanically polished poly-Si, both of which reduce losses by about 40 dB/cm. Atomic force microscopy and spectrophotometry studies are used to monitor surface roughness, which was reduced from an rms value of 19–20 nm down to about 4–6 nm. Bulk absorption/scattering losses can depend on size, structure, and quality of grains and grain boundaries which we investigate by means of transmission electron microscopy. Although the lowest temperature deposition has twice as large a grain size as the highest temperature deposition, the losses appear to not be greatly dependent on grain size in the 0.1–0.4 μm range. Additionally, absorption/scattering at dangling bonds is investigated before and after a low temperature electron-cyclotron resonance hydrogenation step. After hydrogenation, we obtain the lowest reported poly-Si loss values at λ=1.54 μm of about 15 dB/cm.
Superconductor–normal–superconductor step-edge junctions with Au barriers
Authors: Bode M. , Grove M. , Siegel M. , Braginski A. I.
Published: July 2002

YBa2Cu3O7−δ superconductor–normal–superconductor junctions in step-edge geometry are fabricated by an in situ process using off-axis sputter deposition. Gold is used as the normal metal interlayer. A special inhibiting layer is introduced to ensure proper separation of the superconducting electrodes. The temperature dependence of the critical current Ic is nearly linear. The junctions show high normal resistances up to RN=10 Ω and high IcRN products up to 8.8 mV at 4.2 K. I–V characteristics exhibit a large excess current Iex over the whole temperature range, even for electrically narrow junctions. The power dependence of Shapiro step heights shows characteristic deviations from the resistively and capacitively shunted junction model. Calculations using the time-dependent Ginzburg–Landau equations are carried out to model the measured I–V characteristics with and without microwave irradiation. A point-contact-like superconductor–constriction–superconductor model is suggested to explain the excess current and the measured temperature dependence of Ic and Iex. Time-dependent Ginzburg–Landau equations are used to account for the observed dynamic characteristics.
Ferromagnetic resonance of gadolinium doped calcium vanadium garnets
Authors: Srivastava A. K. , Patni M. J.
Published: July 2002

The ferromagnetic resonance (FMR) line shapes of Gd-substituted calcium vanadium garnets Y1.6−zCa1.4GdzFe4V0.4Zr0.6O12 (0.8⩽z⩽1.4) have been studied as the function of the Gd content. The samples had been synthesized by two different routes, with porosities varying from 0.23% to 8.9%. The FMR linewidth, ΔH, in low porosity samples is small (15–20 Oe) and becomes seven to eight times higher in high porosity samples. The linewidth can be explained on the basis of Schloemann’s theory of anisotropy and porosity broadening in polycrystalline materials. The line shape is Lorentzian for low porosity and Gaussian for high porosity samples. This variation of the FMR line shape with porosity is explained on the basis of the stochastic theory.
Transferred charge analysis of evaporated ZnS:Mn alternating-current thin-film electroluminescent devices
Authors: Myers R. , Wager J. F.
Published: July 2002

Evaporated ZnS:Mn alternating-current thin-film electroluminescent (ACTFEL) devices are assessed via frequency- and temperature-dependent transferred charge analysis. The frequency-dependent trends involve the threshold voltage and the slope of the transferred charge immediately above threshold, both of which increase with increasing frequency. At ∼15–20 V above threshold, the slope of the transferred charge curve is relatively independent of frequency and is approximately equal to the physical insulator capacitance. The temperature-dependent trends indicate that the phosphor capacitance increases and the slope of the transferred charge immediately above threshold decreases with increasing temperature. These frequency- and temperature-dependent trends are interpreted as arising from metastable hole trapping in which holes created in the phosphor by band-to-band impact ionization remain trapped in metastable traps at the cathode interface instead of being annihilated by electrons trapped at interface states.
Growth of strained InGaAs layers on InP substrates
Authors: Okada T. , Weatherly G. C. , McComb D. W.
Published: July 2002

A series of InGaAs films, compressively or tensilely strained, were grown on (001)InP substrates at 490 °C by gas source molecular beam epitaxy. Compressively strained (−0.5%) (and lattice matched) layers were morphologically stable, but layers grown under tension (+0.5 to +0.6% strain) developed facets on (113)A or (114)A planes. In the first stages of growth of films under tension, and throughout all stages of growth for the compressively strained films, a fine scale (10 nm wavelength) composition modulation was found in the [110] direction. In the later stages of growth of films under tension, the regions of composition segregation were confined to the peaks and valleys of the faceted surface. Regions of high and low stress concentration (the valleys and the peaks) exhibit In/Ga ratios higher or lower, respectively, than the flat faceted surfaces. The elastic strain energy built into the film, associated with the [110] composition modulation, depends on the ratio of the modulation wavelength to film thickness. In films grown under tension, facet coarsening provides a means for the system to reduce the strain energy associated with segregation. Faceting (and facet coarsening) leads to a reduction in the misfit strain energy stored in the film. However, an analysis of the first stages of faceting shows that faceting cannot be explained as a roughening transition.
A full-band Monte Carlo model for hole transport in silicon
Authors: Jallepalli S. , Rashed M. , Shih W.-K. , Maziar C. M. , Tasch A. F.
Published: July 2002

Hole transport in bulk silicon is explored using an efficient and accurate Monte Carlo (MC) tool based on the local pseudopotential band structure. Acoustic and optical phonon scattering, ionized impurity scattering, and impact ionization are the dominant scattering mechanisms that have been included. In the interest of computational efficiency, momentum relaxation times have been used to describe ionized impurity scattering and self-scattering rates have been computed in a dynamic fashion. The temperature and doping dependence of low-field hole mobility is obtained and good agreement with experimental data has been observed. MC extracted impact ionization coefficients are also shown to agree well with published experimental data. Momentum and energy relaxation times are obtained as a function of the average hole energy for use in moment based hydrodynamic simulators. The MC model is suitable for studying both low-field and high-field hole transport in silicon.
Dielectric dispersion and piezoelectric resonance in an organic electro-optic crystal: (−)−2−α-methylbenzylamino-5-nitropyridine
Authors: Shankar M. V. , Varma K. B. R. , Bailey R. T. , Cruickshank F. R. , Pugh D. , Sherwood J. N.
Published: July 2002

The frequency response of the dielectric constant (εr), dielectric loss tangent (tan δ) and impedance (Z) of an organic electro-optic crystal, (−)−2−α- methylbenzylamino-5-nitropyridine (MBA-NP) monitored along the polar axis, is characterized by the presence of strong peaks in the kHz region. The observed resonance behavior, which is strongly dependent on the geometric size and shape of the sample, is attributed to its piezoelectric nature. The resonance peaks were found to shift steadily towards lower frequencies with increasing temperature and towards higher frequencies with increasing pressure. The temperature dependence of the stiffness coefficient (C) of the crystal has been defined using the resonance data. The temperature dependence of the damping of these piezoelectric vibrations has been monitored by the resonance–anti-resonance method. The influence of the application of pressure and the variation of temperature on εr, monitored along the polar axis of the crystal, has been studied.
Influence of defects on the electrical and optical characteristics of blue light-emitting diodes based on III–V nitrides
Authors: Mártil I. , Redondo E. , Ojeda A.
Published: July 2002

We have measured the electrical and optical properties of blue light-emitting diodes (LEDs) based on III–V nitrides. The current–voltage characteristic is described by means of the relation I=I0 exp(αV). In this equation α is temperature independent, suggesting a process of conduction by tunneling, as was recently reported also for blue-green LEDs based on III–V nitrides [Appl. Phys. Lett. 68, 2867 (1996)]. We explain the differences between blue and blue-green devices taking into account the tunneling process across semiconductor interfaces, in which a great number of defects is present. The light output intensity of the LED as a function of junction–voltage data reveals a dependence on the junction–voltage of the type L=L0 exp(qV/1.4 KT), indicating that the radiative recombination path is via deep levels located at the forbidden gap. Furthermore, we find that the light output–current characteristic follows a power law like L∝Ip. From the analysis of data it appears that, contrary to expectations, the nonradiative centers are saturated at very low current values that are comparable to the values at which this saturation takes place in LEDs based on III–V arsenides with a low content of defects.
Bistable Si growth conditions on Ge(100) in synchrotron-radiation-excited atomic layer epitaxy from SiH2Cl2
Authors: Akazawa Housei
Published: July 2002

When Si layers were grown on Ge(100) by alternating SiH2Cl2 gas exposure and irradiation with synchrotron radiation, bistable growth occurred at temperatures (i) below 400 °C (0.21 Å/cycle) and (ii) between 470 and 530 °C (0.42 Å/cycle). The growth rate saturated for both SiH2Cl2 exposures and synchrotron-radiation (SR) irradiation time. In regime (i) chemisorption and photon-stimulated desorption of H and Cl atoms on the Si overlayer resulted in Stranski–Krastanov-type growth. In regime (ii) chemisorption on a bare Ge surface, removal of the ligands by photothermal desorption, and enhanced migration of Si adatoms resulted in Volmer–Weber-type growth. Island size increased with increasing SR irradiation time in the exposure-irradiation cycle.
Magnetic structure and Fe moment distribution in Nd3Fe28Ti and Nd3Fe29 compounds
Authors: Ching W. Y. , Huang Ming-Zhu , Hu Z. , Yelon W. B.
Published: July 2002

We have carried out spin-polarized calculations for the Nd3Fe28Ti and the hypothetical Nd3Fe29 (3–29 phase) compounds using the first-principles OLCAO method. Because the 3–29 phase has eleven distinctively different Fe sites of varying local coordination and nearest-neighbor distances, it is an ideal case to study the distribution and correlation of the Fe moments in relation to their local environments. We find the Ti moment to be oppositely polarized to the Fe moment, and its substitution of Fe in the pure 3–29 phase actually increases the moment per Fe site from 1.72 μB to 1.78 μB. In both cases, the Fe moments have a wide range of distribution. For Nd3Fe28Ti, the largest Fe moment of 2.77 μB is found at the Fe6 (4i) site and the smallest Fe moment of 1.40 μB is at the Fe2 (4e) site. Our results are in reasonable agreement with neutron diffraction data.
Critical behavior of magnetoresistivity in La2/3Ca1/3MnO3
Authors: Sousa J. B. , Amaral V. S. , Pogorelov Yu. G. , Guimarães A. L. , Araújo J. P. , Braga M. E. , Mendes J. A. , Ibarra M. P. , Algarabel P. , Blasco J. , Garcı́a J. , Marquina C. , del Moral A. , De Teresa J. M.
Published: July 2002

The electrical resistivity has been measured in ceramic La2/3Ca1/3MnO3 as a function of temperature and magnetic field, including the vicinity of the Curie temperature TC≅267 K and down to low fields. We found that the field dependence of the magnetoresistance Δρ/ρ reveals peculiar features close to TC, unknown in conventional ferromagnets. Particularly, a finite linear response in field, besides the usual quadratic term, is found above TC. We propose that this behavior is due to the coexistence of two different types of magnetic fluctuations, related to itinerant and localized charge carriers.
Charge-ordered stripes in La1−xCaxMnO3 with x>0.5 (invited)
Authors: Chen C. H. , Cheong S-W. , Hwang H. Y.
Published: July 2002

Charge-ordered stripes in La1−xCaxMnO3 with x>0.5 were studied by electron diffraction and electron microscopy. Charge-ordered domains were observed in dark-field images and the stripes within the domain were revealed by high resolution lattice images. The modulation wave vector δ of the charge ordering follows closely the relationship of δ=1−x and shows a slight variation from area to area. The incommensurability of the charge ordering to the underlying fundamental lattice gives rise to the presence of discommensurations in the incommensurate phase. Configurations of discommensurations in this system do not follow the general rules observed in other incommensurate systems.
Role of interfaces in the exchange coupling of Fe/Cr/Fe(001) systems
Authors: Heinrich B. , Cochran J. F. , Monchesky T. , Myrtle K.
Published: July 2002

Exchange coupling has been studied in Fe whisker/Cr/Fe(001) systems that were grown in a perfect layer by layer mode. The exchange coupling through Cr was found to be very sensitive to alloying at the Fe whisker/Cr(001) interface. It will be shown that the observed reversed phase of the short wavelength oscillations compared to those predicted by ab initio calculations can be caused by alloying at the Fe whisker/Cr(001) interface. In order to test this point, we have grown samples with the Cr/Fe(001) interface intentionally alloyed by codepositing the Cr and Fe atoms during the formation of the last Cr atomic layer. The strength of the exchange coupling has also been investigated in systems fabricated with heterogeneous spacers using bcc Cu(001) and fcc Ag(001). Cu and Ag layers have been inserted between the Cr spacer and the Fe(001) film. The strength of the antiferromagnetic coupling was found to be substantially increased due to the presence of Cu at the Cr/Fe(001) interface. It will be argued that the observed increase in the exchange coupling is caused by an increased asymmetry in spin dependent reflectivity at the Cr/Cu/Fe interface.
The effects of intrinsic coercivity on the thermal stability of Sm(Co0.70Fe0.21Cu0.06Zr0.03)7.6 sintered magnets up to 450 °C
Authors: Ma B. M. , Liang Y. L. , Bounds C. O.
Published: July 2002

Sm(Co0.70Fe0.21Cu0.06Zr0.03)7.6 sintered magnets with various Hci have been prepared by the conventional powder metallurgy method. Five magnets with Hci ranging from 15 to 40 kOe (at 25 °C) were selected to examine the impact of Hci on the irreversible loss and reversible temperature coefficient of induction (α). Magnets with higher Hci and Hk were found to exhibit lower irreversible losses when heated to 450 °C. Unlike the irreversible loss, α was found to be nearly independent of the Hci or Hk. These trends suggest that the irreversible loss is microstructure related and can be manipulated by process control, while α is more composition related and is dependent upon the overall chemical composition of the magnet. Br of 8.3 kG, Hc of 2.7 kOe, Hci of 3.5 kOe and BHmax of 6.7 MGOe at 450 °C were obtained on a Sm(Co0.70Fe0.21Cu0.06Zr0.03)7.6 magnet with a Hci of approximately 40 kOe at room temperature. Additional efforts are required to produce magnets which can be utilized at 450 °C.
The electric field and current density in a low-pressure inductive discharge measured with different B-dot probes
Authors: Piejak R. , Godyak V. , Alexandrovich B.
Published: July 2002

The magnitude and relative phase of the time varying magnetic field in an inductively coupled discharge have been measured with two dB/dt (B-dot) probes: One B-dot probe was enclosed by a dielectric tube (as is commonly used in dB/dt measurements) while the other, a thin wire probe, was immersed directly into the plasma. Each probe was used to measure the radial and axial component of dB/dt. A comparison of rf electric fields and currents obtained by the two probes showed essentially different results. The disagreement is interpreted to be due to a large local disturbance of the plasma density and current caused by the dielectric tube.
Stress-induced magnetic anisotropy in thick oriented NiZn–ferrite films on (100) MgO substrates
Authors: Dorsey P. C. , Rappoli B. J. , Grabowski K. S. , Lubitz P. , Chrisey D. B. , Horwitz J. S.
Published: July 2002

Thick films (10–12 μm) of NiZn–ferrite (Ni0.6Zn0.4Fe2O4) were grown on single-crystal (100) magnesium oxide substrates using pulsed laser deposition (PLD). The morphology, phase, orientation, strain, and magnetic properties of the as-deposited films were investigated as a function of substrate temperature (400–700 °C) and O2 background pressure (50–200 mTorr). Compositional analysis shows that the PLD NiZn–ferrite films are about 45% Zn deficient when grown using a standard polycrystalline single phase Ni0.4Zn0.6Fe2O4 target regardless of substrate temperature or O2 pressure. However, Zn-rich targets were successfully used to compensate for the Zn deficiency in the NiZn–ferrite films. PLD NiZn–ferrite films grown at 700 °C exhibit the highest degree of crystalline quality and nearly bulk saturation magnetization values (i.e., 5000 G). At low O2 pressures (<75 mTorr) the films, which were grown at 700 °C, are under a significant compressive stress. The stress decreases when the PLD NiZn–ferrite films are grown in higher O2 pressures but the crystalline quality and surface morphology deteriorate. The compressive stress produces a planar anisotropy field of about 1000–3500 Oe depending on the O2 pressure, which is consistent with the stress results from x-ray diffraction measurements on the NiZn–ferrite films. It is hypothesized that the film stress is largely the result of oxygen loss from the films during deposition.
Effect of high energy ion irradiation on electrical and optical properties of para-hydroxy acetophenone
Authors: Sreeramana Aithal P. , Nagaraja H. S. , Mohan Rao P. , Avasthi D. K. , Sarma Asati
Published: July 2002

Organic nonlinear optical crystals have several advantages over inorganic crystals for frequency conversion and electro-optic modulation. In the present work, the effect of 100 MeV Ag14+ ion irradiation on the electrical and optical properties of organic nonlinear crystal para-hydroxy acetophenone (HAP) is investigated. It is observed that second harmonic generation efficiency is marginally affected. However, changes in refractive index and dielectric constants (13 times higher) are noticed in the irradiated HAP.
Optical anisotropies of InP(001) surfaces
Authors: Goletti C. , Esser N. , Resch-Esser U. , Wagner V. , Foeller J. , Pristovsek M. , Richter W.
Published: July 2002

Metalorganic vapor phase epitaxy grown InP samples capped with a protective As/P double layer were used to study clean (001) surfaces in ultrahigh vacuum. By the thermal desorption of As and P cap layers, InP surfaces producing sharp 2×4 low energy electron diffraction (LEED) patterns were prepared. The reconstructed surface and intermediate preparation stages were studied by using reflectance anisotropy spectroscopy (RAS), LEED, and Auger electron spectroscopy. Features in the optical anisotropy related to the contribution of P–P and In–In surface bonds are identified and discussed. The results show that the surface-related optical anisotropy of the 2×4 In-rich reconstruction of InP(001) is due to In–In bonds along the [110] direction that produce a large optical anisotropy below 2 eV. Furthermore, at intermediate annealing stages, information by RAS on a P-rich 1×1 reconstructed phase was obtained. The contribution of P dimers to surface reflectance anisotropy above 3 eV is discussed.
Built-in electric field and surface Fermi level in InP surface-intrinsic n+ structures by modulation spectroscopy
Authors: Hwang J. S. , Chou W. Y. , Hung M. C. , Wang J. S. , Lin H. H.
Published: July 2002

The techniques of the photoreflectance and electroreflectance (ER) were used to study the built-in electric fields and the surface Fermi levels of InP surface-intrinsic-n+ (SIN+) structures. The substrates of SIN+ structures are either Fe-doped semi-insulated InP or Sn-doped N+ InP with the same doping concentrations as its buffer layer. The built-in electric field and the Fermi level were calculated from the Franz–Keldysh oscillations of the photoreflectance spectra. Our studies found that for the samples with the same doping concentration in the buffer layer and substrate, the built-in electric field increases as their top layer thickness decreases. The surface Fermi level, on the other hand, remains approximately constant. For samples with a semi-insulated substrate, the photoreflectance spectra indicate the simultaneous existence of two built-in electric fields, one in the top layer and the other at the interface region between the buffer layer and substrate. ER spectra were measured with the application of a modulation electric field across the top layer. The built-in electric field across the top layer obtained from the ER spectra increases as the top layer thickness decreases while the surface Fermi level, again, remains approximately constant.
Second-harmonic generation accompanying nonlinear reflection of shear waves in an isotropic layered structure
Authors: Xi Deng Ming
Published: July 2002

A theory is proposed for solving second-harmonic generation accompanying nonlinear reflection of shear (SH) waves. An analysis is presented for the resonant second-harmonic generation in a layered structure consisting of an isotropic layer and two semi-infinite isotropic solids. The result reveals that the effect of second-harmonic cumulative growth will occur once the component of incident SH-wave vector along the boundary equals the magnitude of the longitudinal wave vector of the elastic medium of the solid layer. Moreover, we have formally obtained the analytical expressions for the resonant second-harmonic generation in each region of the layered structure which yields physical insight into the process of the second-harmonic generation not previously available.
Particle accumulation in a flowing silane discharge
Authors: Jelenković B. M. , Gallagher Alan
Published: July 2002

Particle trapping in different areas of a parallel-plate, radio frequency silane discharge, and its effect on plasma optical emission of SiH and Hα, has been studied under high gas-flow and low power-density conditions, as used for “device-quality” hydrogenated amorphous silicon (a-Si:H) film deposition. The largest density of particles occurs between the electrodes, near the downstream corners of the rectangular electrodes. Particles are trapped in these positions by sheath fields, until reaching sufficient size to escape with the flow. The region of strong particle trapping has an increased intensity of optical emission, with Hα increased nearly fourfold. Slow oscillatory behavior of particle scattering and discharge emission was observed for pressures near 30 Pa. Power deposited in the discharge has also been measured; for a constant rf voltage and gas-flow speed it changes weakly with pressure, with the maximum at ∼40 Pa. Combined with film growth-rate measurements, this yields a discharge energy deposition of ∼17 eV per deposited Si atom.
Electrical properties of the Ti(SiGe)2/Si0.89Ge0.11/Si(001) contact system
Authors: Lyakas M. , Beregovsky M. , Eizenberg M. , Meyer F.
Published: July 2002

The electrical properties of a thin (350 Å) layer deposited on a molecular beam epitaxial grown Si0.89Ge0.11/Si(001) heterostructure and subsequently annealed at Ta=550–800 °C were studied in a wide (80–325 K) temperature range. Annealing at 800 °C produces a single reaction product, the C54 phase of Ti(SiGe)2, while lower temperature anneals result in the coexistence of a few intermetallic compounds. It was found that while for annealing temperatures lower than 800 °C, the Fermi level is pinned with respect to the conduction band, annealing at 800 °C results in Fermi level partial pinning with respect to the valence band. The current flow in this case is controlled mainly by thermionic emission in the presence of interface states. Two kinds of traps were observed by deep level transient spectroscopy in the barrier region after the 800 °C annealing. Acceptor-like traps with an activation energy of ≈0.46–0.5 eV, a capture cross-section σa=1.3×10−12 cm2, and a density Dt≈3×1013 eV−1cm−2, which most likely originate from the strain relaxation in the SiGe epilayer, were found to be responsible for the partial Fermi level pinning at the interface. Electron traps with an activation energy of ≈0.17 eV and a capture cross-section σd=7.7×10−16 cm2 were also identified and attributed to the SiGe epilayer; they are assumed to originate from a well-known vacancy-oxygen center.
Ground-state depletion mechanisms in pulsed barium vapor lasers
Authors: Mildren Richard P. , Brown Daniel J. W. , Piper James A.
Published: July 2002

Depletion of the metal vapor density from the central region of metal vapor laser tubes has recently been identified as an important factor limiting laser output. We have studied the mechanisms that deplete the ground-state metal density profile by measuring radially resolved population histories of neutral and ionic species during a “burst” (i.e., a pulse train of 20–30 excitation pulses) in a barium vapor laser (BVL). The observed spatiotemporal density behavior during the afterglow of the first shot in the burst agrees well with a simple model for diffusion and recombination, which we have used to show that the primary depletion mechanism during the establishment of steady-state conditions arises from the ambipolar diffusion of ions to the tube wall, and that gas-heating effects are secondary. Analysis of the steady-state (i.e., late-burst) afterglow behavior further reveals that the depletion is significantly greater than that expected from the measured barium ionization, particularly when operating at low barium densities at the wall; this we attribute to a large radial ambipolar field induced by the presence of ionized buffer–gas atoms on axis. The results show that it is important to reduce the time-averaged ionization in order to minimize ground-state depletion. The implications for power scaling of the BVL and other metal vapor lasers are discussed.
On noise sources in hot electron-degraded bipolar junction transistors
Authors: Llinares P. , Ghibaudo G. , Chroboczek J. A.
Published: July 2002

The effects of electrical stress on static characteristics and power spectral density, SIb, of base current, Ib, fluctuations at low frequencies, f<1 kHz, have been studied in quasiself-aligned bipolar n-p-n junction. In as-fabricated devices SIb∝1/AE, where AE is the transistor emitter area, whereas in strongly degraded transistors Sib∝1/PE, where PE is the transistor perimeter. The latter demonstrates directly that hot carrier-induced noise sources are generated at the periphery of the transistors, in agreement with former work on hot electron-induced aging of bipolar junction transistors.
Magnetic properties of the FexMn0.70−xAl0.30 (0.40⩽x⩽0.58) alloy series
Authors: Zamora Ligia E. , Pérez-Alcázar G. A. , Bohórquez A. , Marco J. F. , González J. M.
Published: July 2002

The magnetic properties of the FexMn0.7−xAl0.30 (0.40⩽x⩽0.58) alloy series have been investigated by means of Fe57 Mössbauer spectroscopy and ac magnetic susceptibility measurements. From our data, we propose a magnetic phase diagram for the system which is adequately reproduced by a diluted and random-bond Ising model which was analyzed on the basis of the mean-field renormalization-group method.
Microfabrication of microtip on photocantilever for near-field scanning microscopy and investigation of effect of microtip shape on spatial resolution
Authors: Tanaka Yuriko , Fukuzawa Kenji , Kuwano Hiroki
Published: July 2002

We experimentally investigated the dependence of spatial resolution on the shape of a microtip on our photocantilever, in order to improve the spatial resolution of near-field scanning optical microscopy. Two different cone angles of silicon-dioxide microtips were microfabricated by a new fabrication process. The experimental results, which indicate there is a relationship between the spatial resolution and cone angle of the microtip, were interpreted by calculations based on a simple theoretical model.
Stacking effects on dielectric properties of sol-gel derived Pb(Zr0.53Ti0.47)O3/PbTiO3 thin films
Authors: Yoon Ki Hyun , Shin Ji Hwan , Park Jeong Hwan , Kang Dong Heon
Published: July 2002

A thin film multilayer structure consisting of Pb(Zr0.53Ti0.47)O3(PZT) and PbTiO3(PT) were fabricated by a sol-gel process. The effects of the number of PZT/PT layers upon microstructure and dielectric characteristics were investigated. For a pure PZT thin layer annealed at 600 °C, the microstructure observed was a rosette type, whereas the insertion of PT interlayers yielded thin films with homogeneous grain distribution regardless of the number of PZT/PT layers. With increasing number of PZT/PT layers, the leakage current density and coercive field effectively decreased, while the dielectric constant increased. Loss tangent and fixed charge were found to be independent of the number of PZT/PT layers. These results are possibly explained by the enhanced crystallization resulting from the introduction of large number of nucleation sites in the multilayer film, and by the stacking of stable and dense PZT/PT layers. The thin film composed of three PZT/PT layers with a thickness of ∼450 nm exhibited dielectric constant of 1000, remnant polarization of 20 μC/cm2, coercive field of 40 kV/cm, and tan δ of 0.03. The relaxorlike ferroelectric behavior was observed with an increasing number of PZT/PT layers.
Optical and structural properties of epitaxially lifted-off GaAs films
Authors: Fan J. C. , Lee C. P. , Tsai C. M. , Wang S. Y. , Tsang J. S.
Published: July 2002

The influence of the layer thickness on the optical and structural properties of the epitaxially lifted-off (ELO) thin films has been studied. The ELO films bonded to Si, InP, and GaAs substrates have also been compared. The structure was characterized by high-resolution double-crystal x-ray diffraction and the optical properties were measured by the temperature-dependent photoluminescence spectroscopy. A biaxial compressive strain was observed for the samples bonded to Si with a buffer layer thinner than 1000 nm. Due to different thermal expansion coefficient between the grafted thin film and the host substrate, the emission spectra of the quantum wells of the lifted-off thin films are redshifted compared to the as-grown sample. The amount of the redshift is larger for thinner films.
Calculation of critical layer thickness considering thermal strain in Si1−xGex/Si strained-layer heterostructures
Authors: Huang Jingyun , Ye Zhizhen , Lu Huanming , Que Duanlin
Published: July 2002

In this article, interactions of thermal strain in growth of Si1−xGex strained-layers on Si are analyzed. A formula of critical layer thickness (hc) is obtained based on energy balance considering thermal strain under the assumption that the screw dislocation energy density equals to the sum of the areal strain energy density and thermal strain energy density. The relationship between the thermal expansion coefficient associated with thermal strain and Ge content x is linear. Our calculated values for hc of Si1−xGex strained layers on Si substrates versus mismatch, considering thermal strain, are better agreement with measurements of hc by People and Bean.
Band-gap separation in InGaN epilayers grown by metalorganic chemical vapor deposition
Authors: Chichibu S. , Arita M. , Nakanishi H. , Nishio J. , Sugiura L. , Kokubun Y. , Itaya K.
Published: July 2002

Two energetically-separated resonance structures were found in photoreflectance (PR) spectra of three-dimensional InxGa1−xN epilayers (0.05⩽x⩽0.2) grown by metalorganic chemical vapor deposition. Energy difference between the two structures was nearly constant about 50 meV for all x examined. The broadening parameter of each structure was nearly independent of x (∼50 meV), indicating that each separated region has rather homogeneous net potential fluctuation. The photoluminescence (PL) peak energy agreed with the resonance energy of the lower-energy PR structure, showing nearly zero Stokes-like shift at room temperature. Observation of a single PL peak indicated that photoexcited carriers were effectively corrected into the separated regions having lower PR resonance energy.
Cavity-locked ring-down spectroscopy
Authors: Paldus B. A. , Harb C. C. , Spence T. G. , Wilke B. , Xie J. , Harris J. S. , Zare R. N.
Published: July 2002

We have performed cavity ring-down spectroscopy by locking a high-finesse resonator to the probe laser. We have obtained combination overtone spectra of water vapor in the ambient environment with a baseline noise of 5×10−9 cm−1 for decay constants (R=99.93% reflectors) of 1 μs. This cavity-locked approach ensures single transverse mode excitation, reduces shot-to-shot fluctuations in the decay constant to 4×10−3, and eliminates oscillations in spectral backgrounds. This approach also allows ring-down decay acquisition rates limited only by the ring-down and buildup constants of the resonator, and holds the promise of offering truly shot-noise-limited cavity ring-down spectroscopy measurements.
Improved Born approximation for scattering of radiation in a granular medium
Authors: Mätzler Christian
Published: July 2002

An improved Born approximation for volume scattering and absorption of electromagnetic radiation by a nonmagnetic, granular medium of small grains with large dielectric contrast is formulated. In this low-frequency approximation it is assumed that the permittivity of the medium is given by the effective-medium theory of Polder and van Santen which also provides an expression for the internal field. The model is used to derive formulas for the scattering and absorption coefficients of freely arranged spheres and spherical shells. In addition, measured dielectric properties are used to derive relevant model data of dry snow. In contrast to the conventional Born approximation, the results reproduce Rayleigh scattering for a single particle, and for a dense medium the scattering coefficient shows a nonlinear behavior with increasing density, comparable to the dense-medium radiative transfer theory. Characteristic differences of the spectra of the scattering coefficients result for the different particle types.
Magnetoelastic behavior of the Heusler Ni2MnGa alloy
Authors: Obradó Eduard , Gonzàlez-Comas Alfons , Mañosa Lluís , Planes Antoni
Published: July 2002

In this work the effect of the interplay between magnetic and structural degrees of freedom in the structural transitions undergone by Ni2MnGa alloy is investigated. Elastic constant and magnetic susceptibility measurements in a magnetic field are presented. A simple phenomenological model is proposed to account for the experimental observations.
Magnetic properties of (Pr(Ca, Sr))MnO3 studied by nuclear magnetic resonance
Authors: Tomka G. J. , Riedi P. C. , Kapusta Cz. , Balakrishnan G. , Paul D. McK. , Lees M. R. , Barratt J.
Published: July 2002

The valence state of Mn in magnetoresistive perovskites has been widely related to the level of alkaline metal substitution of the rare earth, by assuming that the Mn exists in 3+ or 4+ valence states. Measurements have been made on a series of samples in the system Pr1−(x+y)CaxSryMnO3. Mn55 nuclear magnetic resonance (NMR) zero field spin echo measurements on a single crystal with x=0.1 and y=0 show distinct resonances around 310 MHz (Mn4+) and 400 MHz (Mn3+) but also at 590 MHz (Mn2+). An increase in the Mn2+ resonance frequency in an applied field implies that the Mn2+ moment is antiferromagnetically coupled to the net magnetization. Spectra from powder samples are much broader. The resonance frequencies increase as the Pr content is reduced. For systems with x+y=0.4, there is little change in the NMR spectrum as the proportions of Ca and Sr are altered.
Magnetic properties, resistivity, and heat capacity of EuMnO3 and Eu0.7A0.3MnO3 (A=Ca, Sr) compounds
Authors: Mukovskii Y. M. , Hilscher G. , Michor H. , Ionov A. M.
Published: July 2002

We present magnetic-, resistivity-, and specific-heat measurements of the title compounds and found that in Eu0.7Sr0.3MnO3 an insulator-metal transition can be induced by an external field below 60 K while Eu0.7Ca0.3MnO3 remains insulating. EuMnO3 exhibits two magnetic transitions at 35 and 47 K which are associated with the appearance of a weak ferromagnetic component and antiferromagnetic ordering. With respect to the parent compound, Ca and Sr substitution enhance the ferromagnetic component significantly, however, for low fields and after zero-field cooling the antiferromagnetic ground state is preserved to a large extent.
A mean field theory of magnets with competing double exchange and superexchange interactions
Authors: Golosov D. I. , Norman M. R. , Levin K.
Published: July 2002

We study the competition between ferromagnetic double exchange (DE) and direct nearest-neighbor antiferromagnetic exchange JAF in two-dimensional colossal magnetoresistance materials. Towards this end, a single site mean field theory is proposed which incorporates the hopping-mediated nature of the DE contribution. The canted state appears only under highly restricted conditions. Instead, in the regime where the antiferromagnetic and DE ferromagnetic interactions are of comparable scale (and in the classical limit), the system exhibits ferro- or antiferromagnetic order with incomplete saturation of the lattice or sublattice magnetization. We interpret these results as an indication that spin fluctuations remain strong even at low temperatures and discuss the associated experimental implications.
Composite polaron mechanism for colossal magnetoresistance in perovskite manganites
Authors: Zou Liang-Jian , Lin H. Q. , Zheng Q.-Q.
Published: July 2002

The mechanism of carriers accompanied by spin-wave cloud and Jahn–Teller phononic cloud, called composite polaron, is proposed to explain the unusual transport and thermodynamic properties in perovskite manganites over all the temperature and magnetic field range. It is shown that the energy bandwidth of composite polarons is strongly renormalized with the increases of temperature and magnetic field, the bandwidth narrows with elevated temperature, while it broadens with increasing magnetic field. The resistivity of composite polarons increases with elevated temperature strongly, reaches a maximum near the Curie temperature TC, and decreases with the increase of magnetic field. The specific heat of composite polarons also exhibits a maximum near TC. These results agree with experiments in a broad temperature and magnetic field range, suggesting that composite polarons contribute to the unusual properties in manganites.
Two-step dopant diffusion study performed in two dimensions by scanning capacitance microscopy and TSUPREM IV
Authors: Kim J. , McMurray J. S. , Williams C. C. , Slinkman J.
Published: July 2002

We report the results of a two-step two-dimensional (2D) diffusion study by scanning capacitance microscopy (SCM) and 2D SUPREM IV process simulation. A quantitative 2D dopant profile of a gate-like structure is measured with the SCM on a cross-sectioned polished silicon wafer. The gate-like structures consist of heavily implanted n+ regions separated by a lighter doped n-type region underneath 0.56 μm gates. The SCM is operated in the constant-change-in-capacitance mode. The 2D SCM data are converted to dopant density through a physical model of the SCM/silicon interaction. This profile has been directly compared with 2D SUPREM IV process simulation and used to calibrate the simulation parameters. The sample is then further subjected to an additional diffusion in a furnace for 80 min at 1000 °C. The SCM measurement is repeated on the diffused sample. This final 2D dopant profile is compared with a SUPREM IV process simulation tuned to fit the earlier profile with no change in the parameters except the temperature and time for the additional diffusion. Our results indicate that there is still a significant disagreement between the two profiles in the lateral direction. SUPREM IV simulation considerably underestimates the diffusion under the gate region.
Slow photoconductivity decay in 3C-SiC on Si substrates
Authors: Ichimura Masaya , Yamada Noboru , Tajiri Hirotaka , Arai Eisuke
Published: July 2002

N-type 3C-SiC layers grown on p-type (001) Si substrates were characterized by the conventional photoconductivity decay method. A N2 laser (337 nm wavelength) was used as the excitation source. A very slow component with a time constant larger than 1 ms was observed in the photoconductivity decay curves. A numerical simulation considering a trap with a very small capture cross section for electrons (<1×10−21 cm2) was able to reproduce main qualitative features of the experimental results. From comparison of the experimental decay curves with the theoretical ones, the following conclusions were drawn about the trap in 3C-SiC. (1) The trap level Et is close to the conduction band edge Ec(Ec−Et=0.1–0.15 eV). (2) The concentration is considered to decrease with increasing donor concentration.
Optical and x-ray characterization of HxLi1−xNbO3 phases in proton-exchanged LiNbO3 optical waveguides
Authors: Korkishko Yu. N. , Fedorov V. A. , Kostritskii S. M.
Published: July 2002

Infrared reflection, attenuated total reflection, prism coupling, and x-ray diffraction methods were used to characterize seven crystallographic HxLi1−xNbO3 phases in proton-exchanged Z-cut LiNbO3 waveguides. Drastic changes of the crystal lattice parameters and the lattice vibrational spectrum at the phase boundaries were observed. The LO–TO splitting, which is proportional to the width of the main band ranging from 580 to 900 cm−1, was demonstrated to vary significantly from phase to phase. Strong decrease of the LO–TO splitting for the polar mode attributed to NbO6 octahedra vibrations (800–900 cm−1) was detected. It was shown that the HxLi1−xNbO3 phases are organized as individual layers.
Discrete meso-dynamic simulation of thermal explosion in shear bands
Authors: Tamura S. , Horie Y.
Published: July 2002

A two-dimensional discrete element code (DM2), was used to model a complex interplay of deformation and chemical reaction in a region of localized shear at the particle level. Two exothermic mixtures of Nb–Si and Ni–Al particles having dimensions of 5 μm×25 μm were considered. Computational experiments showed that the mixtures exhibit a classical phenomenon of thermal explosion under high rates of shearing. The threshold shear rates were found to be approximately 1.2×108/s and 8.0×107/s for the Nb–Si and Ni–Al mixtures, respectively. The ignition conditions were sensitive to the thermal boundary conditions and the computationally observed values are considered to be the upper limits. The thermal explosion results from the interaction of mechanical mass mixing and heating that were primarily caused by thinning (plastic deformation) and fragmentation. The modeling showed an interesting result: that there is a ratio of fracture strengths that maximizes the mass mixing.