bluuebunny/crossref_metadata_2025 huggingface dataset:
Zero-field time-of-flight characterization of minority-carrier transport in heavily carbon-doped GaAs
Authors: Colomb C. M. , Stockman S. A. , Gardner N. F. , Curtis A. P. , Stillman G. E. , Low T. S. , Mars D. E. , Davito D. B.
Published: July 2002

Minority-carrier electron-diffusion coefficients and lifetimes have been measured in heavily doped p-type GaAs using the zero-field time-of-flight (ZFTOF) technique. The materials studied included C-doped GaAs grown by molecular-beam epitaxy (MBE) using graphite as the dopant source, C-doped GaAs grown by metalorganic chemical-vapor deposition (MOCVD) using CCl4 as the dopant source, and Be-doped GaAs grown by MBE. Room-temperature photoluminescence intensity measurements were made on the structures and the results are compared with ZFTOF measurements of lifetime. The graphite-doped material (p∼1019 cm−3) exhibited diffusion lengths of less than 1000 Å. MOCVD-grown C-doped GaAs, which was optimized by adjusting the growth conditions to maximize the room-temperature photoluminescence intensity, had diffusion lengths comparable to those measured in Be-doped GaAs for hole concentrations of 1×1019 and 5×1019 cm−3. Comparison of photoluminescence intensities also suggests that addition of In to very heavily doped MOCVD-grown GaAs (p≳1020 cm−3) to eliminate the lattice mismatch with respect to the substrate does not result in an improvement in lifetime.
Raman spectroscopy, x-ray diffraction, and phase relationship determinations with a versatile heating cell for measurements up to 3600 K (or 2700 K in air)
Authors: Richet Pascal , Gillet Philippe , Pierre Alain , Bouhifd M. Ali , Daniel Isabelle , Fiquet Guillaume
Published: July 2002

A simple, rapid, and inexpensive heating-wire technique is used for physical observations at high temperatures. The upper limit is 2000 K in air with platinum-iridium or platinum-rhodium wires and 2700 K with iridium; temperatures up to 3600 K can be achieved under an inert atmosphere with tungsten wires. Raman spectroscopy measurements made up to 1900 K by this technique suggest that the high-temperature harmonic vibrational behavior of corundum (α-Al2O3) results from the cancellation of anharmonic effects. Powder x-ray diffraction experiments with synchrotron radiation show that perovskite (CaTiO3) changes from orthorhombic symmetry to cubic between 1330 and 1530 K, with an intermediate tetragonal phase likely, consistent with λ-type transitions recorded by recent calorimetric measurements. Finally, observations of CaAl2Si2O8 polymorphism has shown the existence of a new metastable phase.
Laser efficiency and gain of the 1.73 μm atomic xenon laser at high He/Ar buffer gas ratios
Authors: Hebner G. A. , Hays G. N.
Published: July 2002

Addition of helium to an Ar/Xe gas mixture has been shown to significantly improve the fission-fragment excited 1.73 μm atomic xenon laser performance. Using narrow band dielectric laser cavity mirrors to suppress the 2.03 μm atomic xenon transition, the 1.73 μm laser power efficiency varied between 1% and 3% for total pressures of 520–1550 Torr, He/Ar ratios of 3/1–16/1, and pump rates of 5–40 W/cm3. For a constant energy loading, the FWHM of the laser pulse with respect to the pump pulse increased by a factor of 2.5 when argon was replaced by helium. Small signal gain varied between 0.1%/cm and 1.0%/cm. The implication of helium substitution on the Ar/Xe laser kinetics is discussed.
Model of collisional sheath evolution in plasma source ion implantation
Authors: Wang Dezhen , Ma Tengcai , Deng Xinlu
Published: July 2002

A model is developed to study the temporal evolution of the sheath during a pulse of high negative voltage applied to a target immersed in a plasma, such as that present in plasma source ion implantation. This model covers the whole range from collision free to collision dominated sheaths. The sheath expansion velocity and the position of the sheath edge as a function of time in planar geometries for various pressures are obtained.
Behavior of the energy density in the Fresnel region of a conducting circular disk excited by single-cycle sinusoids
Authors: Samaddar S. N.
Published: July 2002

By comparing the radiated energy densities in the Fresnel’s region due to excitations by a cw (continuous wave) sin ω0t and a single-cycle sin ω0t, it is shown that the single-cycle pulse is more efficient in collimating (or focusing) the energy in the vicinity of the disk axis. We have also considered a single-cycle cos ω0t pulse as an exciting source, which may be more academic than realistic. Nonetheless, the results shed some light in showing clearly that the shorter the rise time of a pulse, the slower is the decay of the radiated energy, and hence, it is highly efficient in focusing the energy in front of the radiating device. It follows then that whenever the transmission of a collimated beam of energy to a large distance along the axis of the radiating aperture is desired, it can be accomplished more efficiently by using a short pulse, instead of a cw source.
Bulk and interface stresses in silver-nickel multilayered thin films
Authors: Ruud J. A. , Witvrouw A. , Spaepen F.
Published: July 2002

Stresses were measured in 〈111〉-textured Ag/Ni multilayered thin films from the substrate curvature and from lattice parameter measurements by x-ray diffraction. The difference between the total multilayer film stress and the layer deposition stresses can be attributed to a tensile interface stress of −2.27±0.67 J/m2. Interfacial phase formation is unlikely in this system as indicated by the exceptionally low mutual solubilities.
Photodeflection probing of the explosion of a liquid film in contact with a solid heated by pulsed excimer laser irradiation
Authors: Do Nhan , Klees Leander , Tam Andrew C. , Leung P. T. , Leung Wing P.
Published: July 2002

The explosion or rapid vaporization of a liquid film on an opaque surface by a pulsed laser is studied experimentally. Using a probe-beam deflection sensing (PDS) scheme, together with a previously developed transmission monitor, the distortion of the PDS signal due to the generation of shock waves by the exploding liquid is investigated. Various liquids, including alcohols and pure water in contact with substrates such as polyimide, amorphous carbon, and silicon, are studied for a wide range of excimer laser fluences. It is concluded that the present PDS technique is highly sensitive to the explosion threshold.
A model for the plasma anodization of silicon at constant voltage
Authors: Li Li , Peeters Jozef
Published: July 2002

Subsequent to previous work on modeling the plasma anodization of silicon at constant current, a model is presented here for the plasma anodization at constant voltage. The continuity equation for the charged oxidizing agent O− in the SiO2 film was constructed considering O− ion transport by field-imposed drift and a loss process for O− ions by electron detachment. The resulting gradual drop of the O− current density with distance in the SiO2 layer, together with the decrease of the total current as the film grows thicker, explains the decrease of the oxidation rate with time. Fits of the resulting three-parameter model equation to experimental data, from several sources, are excellent, with standard deviations on average <1% of the final oxide width. Parameter values obtained from different sources for similar experimental conditions are mutually consistent. Useful information can be deduced from the extracted parameters; the ratio of the O− mobility over the O− loss rate coefficient μ/k obtained for constant voltage Si anodization is in good agreement with the μ/k value derived from constant current anodization data—for which a different model equation applies—thus confirming the validity of the underlying physical concepts. Finally, a rationalization is given for the drop-off of the total current density and of the subsurface O− ion current density as the oxide layer becomes thicker.
Multiplexing superconducting quantum interface device detection coils
Authors: Leuthold A. , Wakai R. , Hohenwarter G. , Nordman J.
Published: July 2002

A simple thermally controlled, thin-film superconducting switch was developed for the purpose of multiplexing superconducting quantum interference device detection coils. Switching speeds of a few microseconds were achieved and multiplexing at low frequencies was demonstrated.
Perpendicular magnetic anisotropy in CoxPd1−x alloy films grown by molecular beam epitaxy
Authors: Childress J. R. , Duvail J. L. , Jasmin S. , Barthélémy A. , Fert A. , Schuhl A. , Durand O. , Galtier P.
Published: July 2002

We have grown face-centered cubic (FCC) (111)- and (001)-oriented CoxPd1−x alloy films (x≊0.20–0.25) by molecular beam epitaxy on (111)Si and (001)MgO substrates, respectively, with thicknesses ≊200–1000 Å. Magnetization (SQUID) measurements show that both (111) and (100) films present a perpendicular easy axis, indicating that a strong magnetic anisotropy overcomes the demagnetizing field favoring the in-plane orientation. We have studied this magnetic anisotropy by combining SQUID and torque measurements. Our experimental results cannot be accounted for by only invoking the magnetocrystalline anisotropy of a disordered solid solution of Co in FCC Pd, and rather indicate an anisotropic distribution of Co in the Pd host.
Effect of Dy substitution on the magnetocrystalline anisotropy of GdCo4B
Authors: Ito T. , Asano H. , Ido H. , Kido G.
Published: July 2002

X-ray diffraction and magnetization measurements have been performed for powdered and field-oriented samples of Gd1−xDyxCo4B with x=0, 0.05, 0.1, and 0.2. It has been found that Gd1−xDyxCo4B with x=0.2 has a planar magnetic anisotropy in the temperature region below room temperature; however, the samples with x=0.05 and x=0.1 have an axial magnetic anisotropy at room temperature and transform to a tilted state in the low-temperature region. By analyzing magnetization curves measured in a pulsed field up to 140 kOe at T=4 K and also in a static field up to 12 kOe at 78 K for the field-oriented samples with x=0.05 and x=0.1, approximate values of magnetic anisotropy constants of a Dy ion in Gd1−xDyxCo4B, K1(Dy) and K2(Dy), have been determined to be −2.1×10−14 erg/Dy and 1.3×10−14 erg/Dy, respectively. By comparing these values with those of DyCo5.2, it is found that K2(Dy) in the CeCo4B-type structure is enhanced in one order by the crystallographic change from the CaCu5- to the CeCo4B-type structure.
Ultrafast microscopy of shock waves using a shock target array with an optical nanogauge
Authors: Sandy Lee I.-Yin , Hill Jeffrey R. , Dlott Dana D.
Published: July 2002

A large area shock target array was fabricated. By moving the array through a ps pulsed laser beam, shock waves could be reproducibly generated at a high repetition rate of up to ten shocks per second. The dynamics of shock wave propagation through various layers of the array were studied using optical nanogauges. A nanogauge is a sub micron thick layer whose optical properties are affected when the shock front passes through the layer. Since shock velocities are typically a few nm/ps, nanogauges can be used to study picosecond time scale shock dynamics. Using picosecond optical microscopy on targets with different thickness aluminum layers, it was found that the shock required 0.5 ns to form and then it propagated for a few ns with a constant velocity of 8.3 km/s (8.3 nm/ps), indicating a shock pressure of 49 GPa. The arrival time jitter of many hundreds of shocks, at an aluminum/polymer interface was found to be ±50 ps. The shock propagation through a polymer, polyester, was studied by observing the arrival of the front at a 50 nm thick nanogauge embedded in the polymer. When the shock was transmitted from the aluminum to a polymer layer, its velocity was 5.5 km/s, indicating a shock pressure of 14 GPa, in good agreement with shock impedance calculations. The shock target array is a flexible method of studying picosecond time scale dynamics of materials at and just behind the shock front. The use of different optical nanogauges, such as dye-doped polymer films, which can sense the temperature, pressure, and which indicate multiphonon up pumping, is briefly discussed.
Resonant polaron effects on impurity metastable states in n-GaAs
Authors: van der Sluijs A. J. , Geerinck K. K. , Klaassen T. O. , Wenckebach W. Th. , Foxon C. T.
Published: July 2002

Resonant polaron effects on donor metastable states in n-GaAs have been studied using far infrared Fourier transform magneto-spectroscopy in the 100 –350 cm−1 region. From the observed splitting and pinning effects, absolute values for the matrix elements of the electron-phonon interaction between the ‖3,1,0〉‖0〉 and ‖4,1,0〉‖0〉 states on the one hand, and the ‖1s0〉‖1〉 and ‖2p−1〉‖1〉 states on the other hand are deduced. The energy splitting at resonance appear to be about a factor of two smaller than those for the corresponding Landau levels, and to depend less strongly on the quantum number N.
Optimization of a laser-plasma x-ray source for contact x-ray microscopy
Authors: Kondo H. , Tomie T.
Published: July 2002

Presented are the detailed characteristics of a laser-plasma x-ray source that is successfully used for producing x-ray images of biological specimens in water by contact microscopy. The conversion efficiencies of laser energy to soft x rays are measured for various target materials using a 0.53 μm, 500 ps laser pulse. Yttrium target showed the highest conversion efficiency of around 2.5%/sr to the ‘‘water window’’ region x rays at 3×1012 W/cm2. Angular distributions of x rays and ions are studied to find the best angular position of specimen for reducing the target debris. The effective x-ray spectrum contributing to making an image of a specimen in water is discussed. It is shown that an extremely small laser could be employed in high-resolution contact microscopy.
Transient nucleation and manipulation of nucleation sites in solid-state crystallization of a-Si films
Authors: Kumomi Hideya , Yonehara Takao
Published: July 2002

Solid-state nucleation of Si crystals in a-Si films is controlled by Si ion implantation prior to the isothermal annealing. The process of the nucleation includes a long transient period that causes the diminishing shape of the grain size distribution (GSD). The parameters appearing in the kinetic theory of transient nucleation are estimated from the GSD in a-Si. It is found that the ion implantation reduces the steady-state nucleation rate and elongates the time lag, while the growth rate is almost constant. The dependence of the above parameters is simulated based on the model of heterogeneous nucleation. It is qualitatively suggested that the suppression of the nucleation can be attributed to the modification of the interface to the SiO2 underlayer such as the changes in the self-diffusion or in the interfacial energy. When the nucleation rate is spatially controlled in the plane of the film, a small portion provides the artificial nucleation site. If the sites are periodically placed, the GSD shrinks to have a shape like log-normal. A model for the manipulated nucleation site explains the shape of the GSD, and how a single nucleus is selected to grow in the site.
Model for NL10 thermal donors formed in annealed oxygen-rich silicon crystals
Authors: Hara Akito , Aoki Masaki , Koizuka Masaaki , Fukuda Tetsuo
Published: July 2002

Electron spin resonance (ESR) studies have shown that NL10 thermal donors are formed in oxygen-rich silicon (Si) crystals after annealing. In initial NL10 formation stage, it has C2v symmetry with the principle g values g[100]=1.99982, g[01̄1]=1.99799, g[011]=1.99946. After annealing, however, we could detect ESR signals with g values close to those of NL10 in nitrogen-doped oxygen-rich Si crystals. The ESR signals corresponded to nitrogen-oxygen complex donors [D(N,O)s]. We studied the similarities between D(N,O)s and NL10 centers with C2v symmetry. We propose a model for NL10 thermal donors that NL10 with C2v symmetry is D(N,O). Our model clearly explains NL10 thermal donors characteristics.
The photonic band edge laser: A new approach to gain enhancement
Authors: Dowling Jonathan P. , Scalora Michael , Bloemer Mark J. , Bowden Charles M.
Published: July 2002

Near the band edge of a one-dimensional photonic band gap structure the photon group velocity approaches zero. This effect implies an exceedingly long optical path length in the structure. If an active medium is present, the optical path length increase near the photonic band edge can lead to a better than fourfold enhancement of gain. This new effect has important applications to vertical-cavity surface-emitting lasers.
Electron paramagnetic resonance of defects in β-iron disilicide ceramics
Authors: Miki T. , Matsui Y. , Matsubara K. , Kishimoto K.
Published: July 2002

The point defects in β-iron disilicide ceramics, which may influence the thermoelectric properties of the ceramics, have been investigated by electron paramagnetic resonance (EPR). The 77-K EPR spectra are dependent upon the Si/Fe ratio of the ceramics specimens. Several S=1/2 centers with g factors of about 2.0 are observed in both silicon-deficient p-type and iron-deficient n-type specimens prepared by sintering and subsequent annealing of FeSix(1.9<x<2.8) grains. These centers disappear in a specimen prepared by directly annealing a lump of FeSi2 alloy without sintering. By considering the difference of the sample-preparation procedures, we conclude that the S=1/2 centers exist in the oxidized intergrain region of the ceramics. An S=1/2 signal with an anisotropic g factor (g1=2.061, g2=2.047, g3=2.024) arises only in iron-deficient n-type specimens. This signal is attributable to unionized donor electrons. A multiplet signal with zero-field splitting arises in nearly stoichiometric p-type specimens. The multiplet signal is explained in terms of an S=3 center. Several possible models based on exchange interaction between two spins were examined to elucidate the origin of the S=3 center. The most plausible model is the pair of a high spin Fe3+ (S=5/2) and an S=1/2 center (a silicon vacancy or an acceptor hole).
Temperature dependence of spectral linewidths and lineshifts of Nd3+ ions in CaY2Mg2Ge3O12 laser crystal
Authors: Chen Xuesheng , Di Bartolo Baldassare
Published: July 2002

We have investigated the widths and shifts of the luminescence 4F3/2→4I9/2 line at 940 nm (R1→Z5) and of the 4F3/2→4I11/2 line at 1046 nm (R2→Y1) in the laser crystal CaY2Mg2Ge3O12:Nd3+,Cr3+ from 78 to 300 K. The linewidths are over 20 cm−1. The R2→Y1 line position was found to shift to a longer wavelength and the R1→Z5 to a shorter wavelength with increasing temperature. The results on the thermal line broadening and shifts are compared with the existing theory which uses a simplified ion-phonon interaction as a perturbation and assumes a Debye phonon distribution for the solid.
Origin of the modulated photocurrent in the presence of bias light in quantum efficiency measurements
Authors: Chatterjee Parsathi
Published: July 2002

It is demonstrated mathematically that in quantum efficiency measurement in the presence of a constant bias illumination, one of three possible phenomena occurs: (1) monochromatic-light-produced field redistribution has negligible impact and almost all photocarriers in the quantum efficiency measurement are from the probe monochromatic light; (2) the monochromatic light causes appreciable improvement in the electric field in some part of the device, causing a channeling of bias-light-produced photocarriers into the modulated current response. This creates the possibility of apparent quantum efficiencies greater than unity; and (3) the monochromatic light has a significant detrimental influence on the electric field in some part of the device. This may result in negative values of quantum efficiency, although the lock-in measurement must be suitably modified to detect the effect. Using our detailed computer-analysis model, two conditions are identified in which quantum efficiencies greater than unity can be achieved in a hydrogenated amorphous silicon Schottky barrier photovoltaic device.
A planar two-dimensional circular aperture metal grating coupler for quantum well infrared photodetectors
Authors: Wang Yeong-cheng , Li Sheng S.
Published: July 2002

A numerical analysis of the grating coupling for a two-dimensional circular aperture mesh metal grating coupled GaAs quantum well infrared photodetector (QWIP) has been carried out. The inner products of Floquet modes and waveguide modes were derived in a simple analytic form. Using the normalized wavelength and normalized aperture radius, two sets of universal plots were generated to display the normalized total power and absorption angle of the higher-order diffracted waves versus normalized wavelength for the grating coupler. From these universal plots, the optimum grating parameters for the GaAs QWIPs can be obtained. The results show a 2.7 times enhancement of the responsivity at 10.2 μm wavelength for the circular aperture metal grating coupler over that of the 45° edge-illuminated GaAs QWIP.
Measurements of thermal conductivity of diamond films by photothermal deflection technique
Authors: Bertolotti M. , Liakhou G. L. , Ferrari A. , Ralchenko V. G. , Smolin A. A. , Obraztsova Elena , Korotoushenko K. G. , Pimenov S. M. , Konov V. I.
Published: July 2002

The photothermal deflection technique has been used for contactless measurements of the thermal conductivity of thin polycrystalline diamond films grown in a dc arc discharge in different CH4/H2 mixtures. The thermal conductivity correlates inversely with amorphous sp2 carbon content in the films. Values of thermal conductivity in the range 2–5 W/cm K were obtained for ∼10-μm-thick diamond films deposited from gas mixtures containing methane percentages varying from 3% to 9%.
Ohmic contacts to n-GaAs with a Pt/Ge/Au contacting layer and a Ta-Si-N barrier: Electrical and metallurgical characteristics
Authors: Chen J. S. , Kolawa E. , Nicolet M.-A. , Ruiz R. P.
Published: July 2002

Pt/Ge/Au trilayers of various Pt:Ge compositions, overlaid with a Ta-Si-N barrier layer and an Au metallization layer, are investigated as ohmic contacts to n-type GaAs. After annealing in flowing argon at 450 °C for 15 min, a contact resistivity of 3.7×10−6 Ω cm2 is obtained for the sample of atomic ratio Pt/Ge=1. The contact resistivity of this sample degrades only slightly to 5.0×10−6 Ω cm2 upon aging at 450 °C for 60 h, while the surface stays smooth. Contact resistivities of samples with other Pt/Ge atomic ratios are in the range of 10−5–10−4 Ω cm2. To understand this electrical behavior, the contacts are characterized by backscattering spectrometry, x-ray diffraction, and transmission electron microscopy in conjunction with energy-dispersive analysis of x rays. The reaction products vary with the Pt:Ge compositions due to the difference of the chemical reactivity between Pt, Ge, and GaAs. The formation and distribution of a ternary PtGe:As phase are the determining factors for the contact resistivity. The outstanding thermal stability of the contact is due to the Ta-Si-N barrier layer which closes the GaAs-trilayer system and protects their chemical equilibria from being disrupted by an inflow of Au from the metallization layer. Without the barrier layer, the morphology of the contact degrades badly at 450 °C after 20 h or less.
Theory of magnetotransport in inhomogeneous magnetic structures
Authors: Camblong Horacio E. , Zhang Shufeng , Levy Peter M.
Published: July 2002

The origin of the giant magnetoresistance of magnetic multilayers and magnetic granular solids is investigated through a unified spin-dependent linear transport theory, in which the primary source of electrical resistivity is short-range scattering by impurities in the different magnetic or nonmagnetic regions and at the interfaces. Our theory predicts that magnetotransport in granular solids is similar to that for currents perpendicular to the plane of the layers in multilayers in that their magnetoresistance is independent of the average distance between adjacent magnetic regions.
Microstructure of high-remanence Nd-Fe-B alloys with low-rare-earth content
Authors: Mishra Raja K. , Panchanathan V.
Published: July 2002

The microstructure and magnetic properties of bonded and fully dense magnets produced from melt-spun ribbons of the composition RE4.5TM76Ga1B18.5 have been investigated, where RE refers to a mixture of Nd and Dy, and TM refers to a mixture of Fe and Co. Results show that annealing overquenched ribbons with about 3 wt. % Dy and 3 wt. % Co at 700 °C can produce materials with Hci≂4 kOe, Br≂11.5 kG, and (BH)max≂14.5 MGOe, the latter being comparable to those of commercial ribbons containing three times as much Nd. The microstructure of annealed ribbons consists of about 30% Nd2Fe14B grains, 65% Fe3B grains, and 5% α-Fe grains. The grains of all the phases are nearly spherical. The grain diameters are in the 30–50 nm range. Hot-pressed magnets made from overquenched ribbons are fully isotropic, with magnetic and microstructural characteristics similar to those of the annealed ribbons. The high remanence and low coercivity correlate well with the microstructure consisting of a uniform mixture of the hard Nd2Fe14B phase and soft α-Fe and Fe3B phases.
Kinetics of solid phase interaction between Al and a-Si:H
Authors: Masaki Yuichi , Ogata Toshihiro , Ogawa Hiroshi , Jones David I.
Published: July 2002

The kinetics of solid phase interaction between Al and a-Si:H have been investigated. The experiment led to the observation of low-temperature crystallization as has been reported. The crystallization temperature was found to be 300–350 °C from diffraction studies. From the x-ray photoelectron spectroscopy study, electron transfer from Al to Si was observed in the intermixing layer in samples annealed at RT and 200 °C whereas there is no evidence of the electron transfer for 300 and 350 °C annealed samples. To explain these results, a comparison is made with the interaction in the Cr/a-Si:H system previously reported and the interdiffusion model is proposed.
Interfacial and surface energetics of CoSi2
Authors: Adams D. P. , Yalisove S. M. , Eaglesham D. J.
Published: July 2002

The energetics of the CoSi2-Si interface and the CoSi2 surface have been investigated by analyzing the equilibrium shapes of isolated silicide precipitates. CoSi2 precipitates grown by heating 2 Å of Co on a clean, reconstructed Si{100} surface formed with a number of orientations that remained stable upon annealing to high temperatures. Precipitates buried by a Si capping layer were shown to form along {111} and {100} interfaces. A ratio of the CoSi2-Si interfacial free energies has been measured from the shapes of a large number of buried precipitates indicating that γ{100}/γ{111}=1.43±0.07. It is suggested that the shape of CoSi2 equilibrated within vacuum consists of {111}, {100}, and {110} facets.
Two pathways of positive oxide-charge buildup during electron tunneling into silicon dioxide film
Authors: Lu Yi , Sah Chih-Tang
Published: July 2002

Two positive oxide-charge generation pathways with low voltage or kinetic energy threshold in the Si-gate/SiO2/Si-substrate structure are correlated with experiments. They are initiated by Fowler–Nordheim electron tunneling through sub-10-nm SiO2. These tunneled electrons in the polycrystalline Si gate or crystalline Si substrate generate energetic holes by two collision mechanisms: interband impact generation and interband Auger recombination. The energetic holes are then back injected into the oxide valence band by surmounting the 4.25-eV Si/SiO2 hole barrier and captured by oxide hole traps. The calculated electron threshold energy to generate a positive oxide charge by the impact mechanism is EC-SiO2+2.24 eV or EC-Si+5.37 eV compared with 2.0 eV and 4.92±0.10 eV experimental data, and by the Auger mechanism, EC-SiO2+0.0 eV or EV-Si+4.25 eV compared with 4.25±0.26 eV experimental data.
Heavy-ion sources for radiation therapy
Authors: Sato Y. , Kitagawa A. , Ogawa H. , Yamada S.
Published: July 2002

The usefulness of particle beams for radiation therapy has been well and widely recognized. For the cure of cancer patients, many accelerator facilities have already been utilized, and some new facilities are now being put into operation, or are under construction. Considering the medical and biological requirements, light heavy ions with an energy of several hundred MeV/nucleon are regarded as being the most suitable species. A reasonable choice to this end is an accelerator complex, for an example, one comprising an ion source, an injector linac, and a synchrotron. The ion source is of great importance, since its characteristics strongly affect the overall performance of the accelerator system. A pulsed Penning source (PIGIS) has been successfully used at Lawrence Berkeley Laboratory. Recently, at the National Institute of Radiological Sciences a low-duty pulsed PIGIS for the heavy-ion medical accelerator in Chiba (HIMAC) has been developed; it has both a long lifetime and a high peak intensity. As other types of ion sources, an electron-beam ion source (EBIS) and an electron-cyclotron-resonance ion source (ECRIS) are being developed at several laboratories. An EBIS is basically a pulsed source, and is being successfully used at Saclay. By using an after-glow mode, two ECRISs have made remarkable progress at Grenoble and the Grand Accelerateur National d’Ions Lourds; similar tests are proceeding for the Schwer-Ionen Synchrotron at the Gesellschaft für Schwer-Ionenforschung, the booster at Centre d’Europeen de Recherche Nucleaire, and the HIMAC. These different types of heavy-ion sources are discussed from the viewpoint of their application to radiation therapy.
Modulated photoconductivity study of charged and neutral defects in undoped amorphous silicon
Authors: Hattori K. , Adachi Y. , Anzai M. , Okamoto H. , Hamakawa Y.
Published: July 2002

A novel spectroscopy technique based on modulated photoconductivity measurements with varying illumination level has been applied to investigate the capture coefficients and the energy distribution of defect states in undoped amorphous silicon. From the experimental data, charged and neutral defect distributions are clearly resolved according to their own capture coefficients. The carrier capture process as well as the defect formation mechanism are both quantitatively discussed.
Comparison of defect structure in N2O- and NH3-nitrided oxide dielectrics
Authors: Yount J. T. , Lenahan P. M. , Krick J. T.
Published: July 2002

Electron spin resonance spectroscopy is used to identify and compare point defects in N2O-nitrided, NH3-nitrided, and conventional SiO2 films. We detect only three types of defects in these dielectrics. Pb centers, the primary source of interface states in Si/SiO2 systems under all technologically significant circumstances, appear in all three dielectrics. Both N2O and NH3 nitridation result in higher as-processed Pb interface defect densities, but lower radiation-induced Pb defect generation. Thus N2O nitridation appears capable, as does NH3 nitridation, of providing reduced radiation-induced interface state generation. In addition, both nitridations appear capable of lowering the number of radiation-induced E′ centers, the dominant hole trap in conventional thermal oxides. NH3 nitridation, however, appears to offer greater resistance to radiation-induced generation of these traps. NH3 nitridation also results in a large number of bridging nitrogen centers, and strong evidence indicates that the bridging nitrogen centers are the dominant electron trap in NH3-nitrided and -reoxidized nitrided oxide films. These defects are absent in N2O-nitrided films, which are known to exhibit reduced levels of electron trapping.
Negative persistent photoeffect on cyclotron resonance in InAs/Al0.5Ga0.5Sb quantum wells
Authors: Cheng J.-P. , Lo Ikai , Mitchell W. C.
Published: July 2002

Far-infrared magneto-optical spectroscopy has been used to investigate the negative persistent photoconductivity (NPPC) effect in InAs/Al0.5Ga0.5Sb quantum wells at low temperatures. After an in situ cross-gap illumination, the electron density in the InAs well is reduced by about 28%, and the cyclotron effective mass decreases from (0.0342±0.0002)m0 to (0.0322±0.0002)m0. The time scale for the NPPC buildup transient determined from the results of a photon-dose experiment is on the order of 10 ms with an illumination power flux of ∼10 mW/cm2.
Time-dependent collisional-radiative model for capillary discharge plasmas
Authors: Pöckl M. , Hebenstreit M. , Neger T. , Aumayr F.
Published: July 2002

A time-dependent, zero-dimensional collisional-radiative model has been developed to study capillary discharge plasmas in view of possibilities for obtaining population inversion leading to laser action in the extreme ultraviolet and soft x-ray spectral region, by amplifying the Balmer-α (Hα) line of different hydrogenlike ions with nuclear charges from Z=3 to Z=6. The model is described in detail, and results for the case of a carbon plasma are presented. Limitations of the model are discussed and comparison is made between our calculations and recent experimental results.
Electron-spin-resonance evidence for an impurity-related E′-like hole trapping defect in thermally grown SiO2 on Si
Authors: Conley John F. , Lenahan P. M. , Evans H. L. , Lowry R. K. , Morthorst T. J.
Published: July 2002

Using electron spin resonance (ESR), a new electrically active point defect in thermally grown SiO2 films on Si has been detected. The defect has a large capture cross section for electrons when it is paramagnetic and holes when it is diamagnetic (ESR inactive). The g-tensor values, symmetry, and microwave power saturation characteristics are all similar to those of the well-known E′ family of amorphous SiO2 defect centers.
Self-consistent finite difference method for simulation and optimization of quantum well electron transfer structures
Authors: Weinert C. M. , Agrawal N.
Published: July 2002

A self-consistent finite difference method for the simulation of quantum well electron transfer structures is developed and applied to optimize InGaAsP/InP/InAlAs structures for fast optical switching devices. Simultaneous solution of Poisson’s equation, continuity equation, and Schrödinger’s equation on a discretized mesh yields a fast and accurate simulation method which may be applied to arbitrary layer structures and needs no artificial assumptions like abrupt space charge layers. Because of the exact treatment of charge distribution and leakage current the simulation gives new insight into the performance of barrier, reservoir, and quantum well electron transfer structures, which could not be found by previous approximate theories. With this method we calculate the important physical parameters of these devices, namely, the shift of the optical absorption edge, band filling, leakage current, and capacitance. In addition, each layer is investigated separately with respect to its influence on device performance and fabrication tolerances; the results are used for optimization. Moreover, the exact numerical simulation is used to derive simplified relations for the dependence of band filling, capacitance, and high speed behavior on the heterostructure design.
Magnetic ordering of Pr in Pb2Sr2PrCu3O8
Authors: Hsieh W. T. , Li W-H. , Lee K. C. , Lynn J. W. , Shieh J. H. , Ku H. C.
Published: July 2002

The magnetic ordering of the Pr ions in Pb2Sr2PrCu3O8 has been studied using neutron-diffraction and ac-suspectibility measurements. An imperfect three-dimensional magnetic ordering of the Pr spins was observed at a temperature well below its Neel temperature of TN≊7 K. The magnetic intensities observed at T=1.4 K can be explained by assuming long-range order in the ab plane with short-range correlations, of correlation length ξc=20 Å, along the c axis and a moment directed along the c-axis direction. The magnetic ordering is 2D in nature, and the basic magnetic structure consists of nearest-neighbor spins that are aligned antiparallel along all three crystallographic directions. The magnetic transition is also evident in the ac-susceptibility versus temperature measurements, where a cusp that is typical of antiferromagnetic ordering is clearly observed, which matches the TN obtained by neutron diffraction.
Effects of Ga doping on the magnetic ordering of Pr in PrBa2Cu3O7
Authors: Li W-H. , Jou C. J. , Shyr S. T. , Lee K. C. , Lynn J. W. , Tsay H. L. , Yang H. D.
Published: July 2002

Neutron-diffraction and ac-susceptibility measurements have been performed to study the effects of Ga doping on the magnetic ordering of the Pr in PrBa2(Cu1−xGax)3O7. The Ga atoms preferentially replace the Cu atoms in the CuO-chain layers, and this substitution is found to decrease the antiferromagnetic ordering temperature as the Ga concentration is increased. In addition, the spin arrangement along the c axis is found to change from nearest neighbors being antiparallel for x=0 to nearest neighbors being parallel for x=0.08 (24% Cu chain substitution); for an intermediate x of 0.04 a mixture of the two spin structures is observed. The susceptibility results exhibit Curie–Weiss behavior above TN, and departures from this behavior in the ordered state.
Ultraviolet absorption spectroscopy for the detection of CF2 in high-density plasmas
Authors: O’Neill James A. , Singh Jyothi
Published: July 2002

Ultraviolet absorption spectroscopy has been employed to measure the density of CF2 in a high-density discharge used for the selective etching of silicon dioxide relative to silicon films. In a plasma containing C2F4H2 and CF4, CF2 accounts for more than 10% of the gas in the reactor. The level of CF2 in the discharge is strongly dependent on the operating pressure and the applied power. A comparison of the intensity of optical emission from CF*2 with the ultraviolet absorption signal and microwave interferometry measurements shows that the optical emission signal is limited more by the electron density than by the availability of ground state CF2. The UV absorption signal for CF2 closely follows the selectivity of etching SiO2 to silicon. Both neutral fluorocarbon fragments and ions are believed to play a role in the deposition of fluorocarbon films which give rise to this selectivity. The ability to measure a fundamental plasma parameter which closely correlates with etch selectivity is an important step toward the real-time determination of an etch characteristic which currently can only be determined from post-process measurements.
Nonmagnetic contrast in scanning Lorentz electron microscopy of polycrystalline magnetic films
Authors: Takahashi Yoshio , Yajima Yusuke
Published: July 2002

The effect of the probe beam defocus on scanning Lorentz electron microscopy images has been investigated. Wave optical analyses show that, given a finite defocus, the variation of the transmission amplitude across the illuminated area on the specimen causes nonmagnetic contrast in the differential phase contrast (DPC) images. This nonmagnetic contrast depends on the sign and the amount of defocus. Since this contrast is virtually proportional to the gradient of the transmission amplitude, it should appear similar to the differentiated contrast of the bright-field scanning transmission electron microscope (STEM) image. Therefore, the nonmagnetic noise appearing in the defocused DPC image can be substantially reduced by subtracting a suitably weighted differentiated bright-field STEM image of the same area. This noise reduction procedure has been demonstrated by applying it to the unintentionally defocused DPC image of a recorded longitudinal magnetic storage medium.
Phonon-mediated carrier capture in quantum well lasers
Authors: Preisel Michael , Mo/rk Jesper
Published: July 2002

From the Boltzmann equation governing the electron–phonon interaction in a semiconductor, we derive an expression for the phonon-mediated carrier capture time in electrically pumped quantum well lasers. The result is used to study the influence of temperature, carrier density, and the width d of the quantum well on the carrier capture time. Contrary to the common belief that the capture time oscillates strongly with d, we find that such oscillations are heavily damped for typical structure parameters and could therefore be very difficult to resolve experimentally.
The dynamics of a high-pressure ac gas discharge between dielectric coated electrodes near breakdown threshold
Authors: Nagorny V. P. , Drallos P. J. , Williamson W.
Published: July 2002

An analytical theory of a high-pressure gas discharge between two metal electrodes covered with an insulator is presented. The theory is applicable when the voltage applied to the gap between electrodes only slightly exceeds the breakdown voltage. Analytic expressions for the electric field, electron and ion current densities, as a function of time and space, and an analysis of the stability of the discharge are given. A detailed discussion of the role of metastables in the discharge dynamics is included. The discharge in a plasma display cell is used as an example to demonstrate the utility of the theory.
Spontaneous (zero-field) relaxor–to–ferroelectric-phase transition in disordered Pb(Sc1/2Nb1/2)O3
Authors: Chu F. , Reaney I. M. , Setter N.
Published: July 2002

Recently, it has been shown that, upon cooling, disordered Pb(Sc1/2Ta1/2)O3 ceramics transform spontaneously from a relaxor state to a ferroelectric state when processed in a manner that suppresses lead vacancies. If lead vacancies are present, the spontaneous ferroelectric transition is suppressed and the ceramics exhibit the usual relaxor behavior in a wide temperature range. It is shown that disordered Pb(Sc1/2Nb1/2)O3 ceramics have a similar nature: When produced in a manner that does not eliminate lead vacancies, they exhibit normal relaxor behavior. However, if stoichiometry is tightly controlled, disordered Pb(Sc1/2Nb1/2)O3 transforms spontaneously (under zero-bias field) from a relaxor into a normal ferroelectric upon cooling.
Optimization of luminescent properties with respect to crystal growth temperatures for SiGe single quantum wells grown by molecular beam epitaxy
Authors: Zeindl H. P. , Nilsson S.
Published: July 2002

SiGe single quantum wells grown pseudomorphically by solid source molecular beam epitaxy on Si substrates were investigated by low-temperature photoluminescence measurements. The study was meant to correlate efficient radiative recombination of SiGe quantum well structures to crystal growth temperatures. As a model system, nominally 4-nm-thick quantum wells with a Ge concentration of 20% were used and the influence of growth temperatures on photon energy, on full width at half maximum, and on photoluminescence intensity was examined. At low growth temperatures (500 °C or less), only a deep broad luminescence band is observed while at higher temperatures a phonon-resolved, near-band-edge emission is predominant until it disappears completely at about 1000 °C. A systematic study of the photoluminescence characteristics over the entire range of crystal growth temperatures where luminescence is observable at all, is reported and a ‘‘growth window’’ for optimized photoluminescence features was determined.
Electrical detection of electron paramagnetic resonance: New possibilities for the study of point defects
Authors: Stich B. , Greulich-Weber S. , Spaeth J.-M.
Published: July 2002

An investigation of the possibilities to measure electron paramagnetic resonance (EPR) with electrical detection (EDEPR) by measuring the microwave or radio frequency-induced change of the photoconductivity of various bulk Si samples containing shallow and deep level defects is presented. It was found that an electron-hole recombination mechanism via a donor (D0)-acceptor (A0) pair explains the observations and may be necessary if EDEPR is to be detected. A qualitative expression for the EDEPR signal intensity is presented. EDEPR offers sensitivity several orders of magnitude better than normal EPR. As few as 107 shallow P donors could be observed. In addition, EDEPR can be measured with spatial resolution allowing defect mapping.
Relaxation processes in poled nonlinear optical polymer films
Authors: Suzuki Akiyoshi , Matsuoka Yoshihiko
Published: July 2002

Decay times, or the relaxation times after poling, of nonlinear optical host-guest systems at room temperature have been evaluated from the behavior at elevated temperatures below glass transition temperature Tg using Arrhenius plots. The factors were investigated by comparing the behaviors of the nonlinear systems to those of α and β relaxations in host polymers. The activation energies of the nonlinear systems seem to agree with those of the β relaxation processes of host polymers; however, the frequency factors were very different. The decay time of nonlinearity seemed to be affected by α relaxation processes below Tg of host polymers.
Observation of two charge states of ruthenium in PbTiO3 single crystals
Authors: Clem Paul G. , Payne David A. , Warren William L.
Published: July 2002

Two charge states are observed for Ru in doped PbTiO3 crystals using electron paramagnetic resonance (EPR). The two paramagnetic charge states are +3 and +5. The g tensors for the Ru3+ and Ru5+ defects were found to be near axial: g∥≂1.155 and g⊥≂2.188 for the Ru3+ center and ge∥≂2.0045 and ge⊥≂3.959 for the Ru5+ center. Analysis of the EPR spectra by crystal field theory indicates that the Ru3+ ions are in tetragonally distorted (elongated) octahedral sites, strongly suggesting that they substitute for the central Ti4+ ions in the perovskite lattice. Analysis of the Ru5+ EPR spectra indicate that this defect is also in an octahedral site with a very large zero field splitting parameter. The results suggest that both Ru3+ and Ru5+ can substitute for Ti4+ in the perovskite lattice with self-compensation. Last, it is found that optical illumination of the crystals with band-gap light at low temperatures decreases the density of both Ru centers.
Optical intervalence-subband transitions in strained p-type In1−xGaxAs/InP quantum wells
Authors: Stoklitsky S. A. , Zhao Q. X. , Holtz P. O. , Monemar B. , Lundström T.
Published: July 2002

The lattice mismatch (strain) effects on the infrared absorption in p-type In1−xGaxAs/InP quantum wells are investigated systematically for both tensile (x≳0.47) and compressive (x<0.47) strains. The mismatch of the valence-band parameters in the well and barrier materials is taken into account in the optical matrix element calculations. We find that normal incidence optical matrix elements substantially increase in the case of the compressive strain (the ground state is heavy hole) and decreases in the case of the tensile strain (the ground state is light hole). The peak of the normal incidence absorption in the compressively strained QW is shown to reach a considerable value of 5000–6000 cm−1 for a sheet hole concentration of 1012 cm−2. For the z-polarization of the light we found a substantial enhancement of the optical matrix elements in the case of tensile strain (i.e., for a light-hole ground state).