bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of nitrogen compositions in the as-grown GaNxAs1−x on GaAs measured by high-resolution x-ray diffraction and secondary-ion mass spectroscopy
Authors: Fan W. J. , Yoon S. F. , Ng T. K. , Wang S. Z. , Loke W. K. , Liu R. , Wee A.
Published: July 2002

High-resolution x-ray diffraction (HRXRD) and secondary-ion mass spectroscopy were used to measure the N compositions of a series of as-grown GaNAs samples grown by solid-source molecular-beam epitaxy. We found that N compositions measured by the two methods agree well at lower N compositions (x<3%), and deviate at larger N compositions (x>3%). The HRXRD measurement by using Vegard’s law to extract the lattice constant of GaNAs, underestimates N composition at larger N compositions. We found that the underestimation is up to 14.3% at the x=4.2%. In order to explain the deviation, a model for analyzing the correlation between lattice parameters and point defects in the epilayer was carried out.
Characteristics of SrS:Cu thin-film electroluminescent device fabricated by pulsed-laser deposition
Authors: Choe Jae Young , Blomquist S. M. , Morton D. C.
Published: July 2002

SrS:Cu film was deposited by a pulsed-laser deposition method to fabricate the phosphor film for thin-film electroluminescent (EL) devices. The devices were annealed in dry nitrogen gas to give the characteristic blue emission with a chromaticity coordinate of (0.17, 0.24). The luminance at 100 Hz driving frequency reached 6 cd/cm2 with corresponding transferred charge of 4.1 μC/cm2. The excellent stoichiometry replication of the film from the target being sputtered, pulsed-laser deposition can serve as a convenient method for fabricating high-quality thin-film EL devices of different composition by changing target concentrations.
Nonlinear noise reduction for electrocardiograms
Authors: Schreiber Thomas , Kaplan Daniel T.
Published: July 2002

The electrical activity of the heart usually shows dynamical behavior which is neither periodic nor deterministically chaotic: The interbeat intervals seem to contain a random component. Although long term predictions are thus impossible, good predictions can be made for times smaller than one heart cycle. This fact is used in order to suppress measurement errors by a local geometric projection method which was originally developed for chaotic signals. The result constitutes evidence that techniques of time series analysis based on chaos theory can be useful despite the fact that very few natural phenomena have been actually established to be deterministically chaotic.
Distributed three-dimensional finite-difference modeling of wave propagation in acoustic media
Authors: Villarreal Alberto , Scales John A.
Published: July 2002

Finite-difference modeling of wave propagation in heterogeneous media is a useful technique in a number of disciplines, including earthquake and oil exploration seismology, laboratory ultrasonics, ocean acoustics, radar imaging, nondestructive evaluation, and others. However, the size of the models that can be treated by finite-difference methods in three spatial dimensions has limited their application to supercomputers. We describe a finite-difference domain-decomposition method for the three-dimensional acoustic wave equation which is well suited to distributed parallelization. We have implemented this algorithm using the PVM message-passing library, and show here benchmarks on two different distributed memory architectures, the IBM SP2 and a network of low-cost PCs running the Linux operating system. We present performance measurements of this algorithm on both the low-bandwidth PC network (10-Mbits/s Ethernet) and the high-bandwidth SP2 cluster (40-Mbits/s switch). These results demonstrate the feasibility of doing distributed finite-difference acoustic modeling on networks of workstations, but point to the substantial efficiencies that can be expected as higher bandwidth networks become available. © 1997 American Institute of Physics.
Dynamic uniaxial stress experiments on alumina with in-material Manganin gauges
Authors: Rosenberg Z.
Published: July 2002

Rigid wall impact experiments were conducted on alumina tiles, instrumented with in-material Manganin gauges, in order to determine the magnitude of the dynamic yield strength of these ceramics. We found that the amplitude of the wave is very near the static yield strength. Thus, we conclude that these materials are not rate sensitive and that their increased strength in planar impact experiments is due to their pressure dependence rather than rate effects.
Frequency dependence of ion bombardment of grounded surfaces in rf argon glow discharges in a planar system
Authors: Köhler K. , Horne D. E. , Coburn J. W.
Published: July 2002

The energy distribution of positive ions incident on a grounded surface in a low-pressure argon planar rf glow discharge system has been measured as a function of excitation frequency from 70 kHz to 13.56 MHz for both capacitive and direct coupling of the rf power to the excitation electrode. The results are interpreted by taking into consideration both the transit time for the ion to traverse the sheath relative to the period of the rf excitation voltage, and the resistive or capacitive characteristics of the sheaths. The importance of system geometry and of the dc potential of the excitation electrode (as determined by external circuitry) on the maximum energy of ions incident on grounded surfaces is shown.
Binding energy of the screened exciton in two-dimensional systems
Authors: Lee Johnson , Spector Harold N. , Melman Paul
Published: July 2002

Recent optical nonlinearities in GaAs/Ga1−xAlxAs quantum well structures have been attributed to the screening of the electron-hole interaction in such structures by the free carriers generated. In this paper, we perform a variational calculation of the binding energy of the exciton confined in a two-dimensional structure, taking into account the screening of the Coulomb interaction between the electron-hole pair due to the free carriers. The screened potential used is that obtained by Stern and Howard for hydrogenic impurities in semiconducting inversion layers. We find that, although the binding energy of the exciton is reduced because of the presence of screening, the exciton remains bound even for fairly strong screening. This is in contrast to the 3-D case which occurs in bulk semiconductors where the presence of screening can prevent the electron-hole pair from forming an exciton. We have found that the optical absorption at the exciton peak also decreases as the screening increases.
Potential barrier model incorporating localized states explaining tunnel anomalies
Authors: Halbritter J.
Published: July 2002

Most tunnel barriers contain localized electronic states nl(Δx, ε) in large amounts decreasing with distance Δx from the metal. The localized states hybridize with conduction electrons forming interface states with a decay width Δl∝exp(−2Δxκ) and a correlation energy ΔU* ∝ 1/εrΔx. For ΔU*>Δl these states are localized, which yields a strong coupling to surface plasmons, phonons, and spins. These states cause diffuse surface scattering and enhance exponentially [∝ Δ−1l  ∝exp(+2Δxκ)] the tunnel matrix element by resonant tunneling  jR as compared to tunneling jφ̄ through the whole potential barrier φ̄. Consequently at voltages ‖eU‖ <φ̄, jR(U,T) is identified by its stronger U and T dependencies and can even dominate over jφ̄. The enhanced interaction of the localized electrons with surface plasmons, phonons, and spins yield strong U, T, and time dependencies in the tunnel current which produce giant zero-bias anomaly and spin-flip zero-bias anomaly; capacitance changes; inelastic processes, noise, and barrier reduction with increasing temperature; and pair weakening, leakage current, and reduction of the Josephson current.
Metal film deposition by gas-phase laser pyrolysis of nickel tetracarbonyl
Authors: Jervis T. R.
Published: July 2002

A new technique for the deposition of nickel metal films by gas-phase pyrolysis of nickel tetracarbonyl gas is described. A pulsed CO2 laser is used to form a reaction zone adjacent to a cold substrate, resulting in a rapidly quenched film. The technique relies on dielectric breakdown of a mixture of source and carrier gases and therefore lends itself to a variety of source gases and chemistries. Adherence data and compositional and structural analyses are presented.
Laser recording in tellurium suboxide thin films
Authors: Tyan Y.-S. , Preuss D. R. , Vazan F. , Marino S. J.
Published: July 2002

Tellurium suboxide (TeOx) films prepared by coevaporation consist of tiny grains of tellurium embedded in a TeO2 matrix. The size of the tellurium grains depends on the preparation conditions. Except for samples with exceedingly small grains, crystalline diffraction patterns can be observed by using selected-area electron diffraction even though these samples show amorphous x-ray diffraction patterns. Heat treatment causes a transition in optical properties and the appearance of peaks for crystalline tellurium in x-ray diffraction. Thermal writing with a focused diode laser on both heat-treated and as-deposited samples gave recorded marks with high sensitivity and good contrast, although an incubation period of many seconds is required for the full development of the marks. The observed laser recording behavior cannot be explained by a simple amorphous-to-crystalline transition, and a new writing mechanism is proposed.
The study of charge carrier kinetics in semiconductors by microwave conductivity measurements
Authors: Kunst M. , Beck G.
Published: July 2002

The study of the excess conductivity induced in a material by pulsed optical excitation yields information on the optoelectronic properties of the material and is receiving increasing attention. As conventional conductivity techniques are hampered by the need to apply electrical contacts, we have investigated the reliability and the possibilities of microwave conductivity measurements. This paper first presents the general background for excess conductivity measurements in the microwave range, and then derives the quantitative relationship between the reflected microwave signal and the change in conductivity for a wafer of single-crystalline Si. For this sample, the theory of excess charge carrier kinetics is also developed. After a short description of our apparatus, kinetic measurements on a nano- and microsecond timescale are compared to theory.
High-pressure 57Fe Mössbauer study of Fe3O4 at 298 K
Authors: Evans B. J. , Pan Lu-San
Published: July 2002

At 298 K, 57Fe Mössbauer spectra of Fe3O4 exhibit an increase in complexity with increasing pressure in the range 1×105 to 5×109 Pa. There is an incipient splitting of the B-site pattern into two components with relative intensities of 3 : 1. For the more intense pattern, Hn is 462 kOe and 470 kOe at 1×105 and 5×109 Pa, respectively; the corresponding values for the less intense patterns are 454 and 448 kOe. The B-site electric quadrupole interactions also exhibit appreciable changes. The A-site pattern exhibits no qualitative changes in line profiles and only modest changes in the hyperfine interaction parameters are observed. For example, at 1×105 and 5×109 Pa, Hn is 492 and 490 kOe, respectively; the corresponding isomer-shift values are 0.258 and 0.233 mm s−1. These results are consistent with the broadening of the lines of the B-site patterns at ambient temperature and pressure arising from the different relative orientations of the direction of magnetization and of the axes of the electric field gradient tensor.
Hyperfine fields and spin orientations in (Fe57/Ag) superlattices from conversion electron Mössbauer studies (invited)
Authors: Volkening F. A. , Jonker B. T. , Krebs J. J. , Koon N. C. , Prinz G. A.
Published: July 2002

Conversion electron Mössbauer studies have been carried out as a function of temperature on a series of single-crystal [(100)57Fe/(100)Ag] superlattices grown by MBE on GaAs substrates. The 57Fe layer thickness was varied from approximately 1 to 5.5 ML. From the relative intensity of the Mössbauer lines we show conclusively that the orientation of the magnetic moment for films 2.4 ML and thinner in zero applied field is perpendicular to the film plane at low temperatures. For a 5.5-ML film the spin alignment at room temperature is in the film plane and partially out of the plane at low temperatures. The low-temperature magnetic hyperfine fields are enhanced relative to bulk Fe.
Optical microphone for photoacoustic spectroscopy
Authors: de Paula M. H. , de Carvalho A. A. , Vinha C. A. , Cella N. , Vargas H.
Published: July 2002

An optical microphone configuration is experimentally demonstrated. It is based on the deflection of a He-Ne laser beam from a thin reflective pellicle bound on one edge over the duct of a conventional photoacoustic cell. Possible applications of this kind of photoacoustic detection are discussed.
Raman scattering and x-ray photoemission spectroscopy studies of carbon films sputtered in a triode system by Ne, Ar, and Xe
Authors: Parmigiani F. , Kay E. , Seki H.
Published: July 2002

Sputter-grown amorphous films using Ne, Ar, and Xe exhibit different Raman scattering spectra. The observed differences suggest that, as the mass of the sputtering gas increases, the disorder in the film increases. Related modifications occur if the ion bombardment of the film is achieved during film growth. Bombardment with a small light ion, i.e., Ne+, induces some crystallite formation but no additional disorder in the film structure, whereas large heavy ions, i.e., Ar+ and Xe+, induce considerable disorder in the material. X-ray photoemission spectroscopy measurements suggest that gas atoms trapped in the carbon lattice are very likely the origin of the disorder. Finally, it is shown that laser annealing of our films gives rise to changes in the Raman spectra very similar to those produced by thermal annealing.
Magnetic properties of Fe-rich rare-earth intermetallic compounds with a ThMn12 structure
Authors: Ohashi K. , Tawara Y. , Osugi R. , Shimao M.
Published: July 2002

Sm(Fe1−xMx)12 ternary compounds based on the tetragonal ThMn12 structure where M is Ti, Si, V, Cr, and Mo were investigated. M atoms have a preference for site occupation. Ti atoms occupy the 8i or 8j site and Cr atoms occupy the 8i site. Curie temperatures on Sm(M,Fe)12 compounds are around 590 K except for the SmMo2Fe10 compound (Tc=483 K). The SmTiFe11 and SmSi2Fe10 compounds have a high saturation magnetization and magnetic anisotropy.
Percolation theory for the plastic yield transition
Authors: Tonks D. L.
Published: July 2002

It will be shown how the percolation threshold in a standard continuum percolation model corresponds to the plastic yield transition in a dislocation model with infinitely strong point obstacles and with no dislocation cancellation allowed. The cluster properties of areas defined to be ‘‘open’’ to dislocation loop expansion will be simulated using a lattice model and Monte Carlo methods. The critical exponents ν, γ, and β for these areas will be given. Only the value of β, the exponent corresponding to the infinite cluster, is the same as that for the standard continuum percolation model.
Microtwin formation in gallium arsenide by iron ion implantation and amorphization by annealing
Authors: Taniwaki Masafumi , Yoshiie Toshimasa , Koide Hideto , Ichihashi Motomi , Yoshimoto Naoto , Yoshida Hiroyuki , Hayashi Yoshihiko
Published: July 2002

Cross-sectional views of iron ion-implanted gallium arsenide to doses of 4×1014 cm−2 and 1×1015 cm−2 were observed by transmission electron microscopy before and after annealing at 673 K. Microtwins were formed below the surface by implantation of 4×1014 ions cm−2. In the sample implanted to the dose of 1×1015 cm−2 , the region below the surface was amorphized and a thin microtwin layer was observed between the amorphous layer and the crystalline matrix. The electron diffraction pattern and the high-resolution multibeam lattice image showed that the structure of the microtwin is an atomic stacking of ABCA″CB″ABC along the 〈111〉 direction. In both samples, the microtwin layer transformed to the amorphous structure by annealing at 673 K for 30 min. It is suggested that the behavior in gallium arsenide implanted with iron, which is much different from that implanted with the other ion species, may be attributed to the coordination number of the covalent bonding iron.
Time-resolved optical emission during laser ablation of Cu, CuO, and high-T
c superconductors: Bi1.7Sr1.3Ca2Cu3Ox and Y1Ba1.7Cu2.7Oy
Authors: Saenger K. L.
Published: July 2002

Time-resolved measurements of optical emission from the ablation plume of ArF excimer-laser-irradiated Cu, CuO, Bi1.7Sr1.3Ca2Cu3Ox, and Y1Ba1.7Cu2.7Oy are reported. Data were collected on several emission lines for a variety of neutral and ionic species, including Cu+, Cu, Sr+, Sr, Ca+, Ca, and Y. The time dependence of the emission intensity is analyzed to obtain effective particle velocities. The results indicate most probable velocities of (3–10)×105 cm/s for the neutral species and (1–2)×106 cm/s for the ions, in qualitative agreement with previous work. However, the time dependence of the optical emission intensity can be influenced by radiation trapping and the detailed mechanisms of species excitation. These effects are shown to complicate the quantitative relationship between the emission intensity versus time and the particle velocity.
Low-resistance noble metal contacts to high-temperature superconductors
Authors: Selim R. , Caton R. , Buoncristiani A. M. , Byvik C. E. , Edahl R. A. , Wise S.
Published: July 2002

We have extended our studies of a melting technique for making low-resistance contacts to high-temperature superconductors. We have made contacts to both YBa2Cu3O7−x and Bi2BaSr2Cu2O8, and to related superconducting compounds by melting gold or silver pads onto the samples before the final oxygen treatment. Scanning electron microscope studies show that both gold and silver do not diffuse far from the contact area. The surface contact resistivity of the best contacts made by the melting technique has an upper limit value in the 10−8 Ω cm2 range at 77 K. This contact resistivity shows no significant change in its value over a period of 17 months. Furthermore, an electron radiation dose of 5.7×1017 electron/cm2 only doubled the contact resistivity. This method of making low-resistance contacts to high-Tc materials can be integrated into the final oxygen treatment of many prospective superconducting elements or devices.
The distribution of radiation-induced charged defects and neutral electron traps in SiO2, and the threshold voltage shift dependence on oxide thickness
Authors: Walters M. , Reisman A.
Published: July 2002

As insulated gate field effect transistor dimensions continue to decrease, and fabrication sequences rely increasingly on processes that involve ionizing radiation, it becomes essential to understand the radiation-induced threshold voltage shift (ΔVT) dependence on gate-insulator thickness (tox), since threshold voltage tolerances are required to scale with device dimensions. In the present study, n-channel insulated gate field effect transistor devices were fabricated with gate-insulator thicknesses ranging from 6–50 nm, and were then exposed in an unbiased state to AlKα x-ray radiation to simulate process-induced ionizing radiation exposure. Gate-oxide Coulombic defects and neutral electron traps were measured before and after irradiation using optically assisted electron injection. Following irradiation and injection, the measured voltage shifts indicated that the ‘‘extrinsic’’ defects are localized near, but not at, the Si/SiO2 interface. For oxide thicknesses where the top gate electrode lies in the region above the extrinsic defect volume ΔVT is found to be linear in tox; at thicknesses where the top gate electrode encroaches upon the defect region, ΔVT is found to be quadratic in tox, and for very thin oxides, ΔVT is observed to approach zero. A defect distribution model that applies to process-induced radiation exposure is formulated to explain this behavior. This model also provides a unique and simple method for determining the defect centroid in gate insulators that have been exposed under such irradiation conditions.
Nucleation and growth of current filaments in semiconductors
Authors: Mayer K. M. , Huebener R. P. , Rau U.
Published: July 2002

The nucleation and disappearance of current filaments in homogeneously slightly doped semiconductors during avalanche breakdown at low temperatures is quantitatively evaluated in terms of a minimum and maximum value of the filament current. Both values arise from a model based on the power balance in the system. Experimental results obtained by the two-dimensional imaging of the filament configurations in n-GaAs and p-Ge agree quantitatively with the predictions of our power balance model.
Interfacial-band discontinuities for strained layers of InxGa1−xAs grown on (100) GaAs
Authors: Marie X. , Barrau J. , Brousseau B. , Amand Th. , Brousseau M. , Rao E. V. K. , Alexandre F.
Published: July 2002

A study of excitonic transitions in pseudomorphic single quantum wells of InxGa1−xAs grown on a GaAs substrate is presented. The experimental data are obtained by the in-plane photocurrent spectroscopy technique. The interpretation, which is based on the deformation potential and elastic theories, includes valence-band mixing with no adjustable parameters. The present experimental results, and those compiled from recently published data, are well interpreted from a unique set of measured parameters. From calculations and experiments it emerges that the light holes confine in the strained ternary for all In fractions below 0.83.
Electrical transients during interfacial debonding and pullout of a metal rod from an epoxy matrix
Authors: Zimmerman K. A. , Langford S. C. , Dickinson J. T.
Published: July 2002

We examine electrical transients which are produced during the propagation of a debonding crack along the interface of an axially loaded stainless-steel rod embedded in epoxy. Time-resolved correlations with fracto-emission signals and tensile strain in the rod are made to assist interpretation. We show that on microsecond time scales, the motion of the debonding crack as well as the initial pullout motion of the rod following debonding can be detected and followed. A model is developed which consistently describes the sign and amplitude of the detected electrical signals. These experimental data provide time-resolved information on the sequence of events leading to interfacial fracture and initial pullout of the rod from the epoxy.
Mass spectroscopic study of CH3 radicals produced in a hollow cathode discharge cell
Authors: Sanz M. M. , Abad L. , Herrero V. J. , Tanarro I.
Published: July 2002

Neutral CH3 radicals in a hollow cathode discharge cell containing methane have been detected with a quadrupole mass spectrometer, using the threshold ionization technique. With the experimental conditions employed in this work the measured efficiency in CH3 production has been 0.2%, which is higher than that shown for other types of discharges in the bibliography. On the other hand a high depletion of the CH4 density has been observed and the mass spectrum of the discharge shows the appearance of compounds at m=25, 26, and 27, proceeding from recombination processes.
Maximum entropy method in inverse black body radiation problem
Authors: Dou Lixin , Hodgson R. J. W.
Published: July 2002

The maximum entropy method is applied to the inverse black body problem, as an alternative procedure for the determination of the temperature distribution of a black body from the measured power spectrum. Starting with the classical moment problem and the generalized moment problem, the maximum entropy method is developed to solve the Fredholm equation of the first kind by introducing a set of transformation functions. This procedure is then used to solve the inverse black body radiation problem by formulating it as a minimization problem. A few examples are studied where exact analytical results are known. The results are very good, and the procedure offers advantages over previous approaches.
Lattice vibrations of CuInSe2 and CuGaSe2 by Raman microspectrometry
Authors: Rincón C. , Ramírez F. J.
Published: July 2002

The vibrational spectra of the chalcopyrite compounds CuInSe2 and CuGaSe2 have been studied by Raman microspectrometry. This technique is very useful in the present case where large single crystals are not generally available. The results have been used to resolve discrepancies in the reported data on the vibrational spectrum of CuInSe2.
Low temperature photoluminescence characteristics of carbon doped GaAs
Authors: Kim Seong-Il , Kim Moo-Sung , Kim Yong , Eom Kyung Sook , Min Suk-Ki , Lee Choochon
Published: July 2002

We have analyzed low temperature (12 K) photoluminescence (PL) characteristics of carbon(C) doped GaAs epilayers. No traces of donor levels were observed in the PL spectra. This suggest that well-behaved carbon is incorporated as an acceptor into the GaAs lattice. The measured peak energy of the PL intensity distribution shifts to lower energy and the full width at half maximum (FWHM) increases with increasing hole concentration. We have obtained empirical relations for FWHM of PL intensity distribution in two distinct hole concentration regions. These relations are considered to provide a useful tool to determine free hole concentration in C doped GaAs by low temperature PL measurements. As the hole concentration is increased above 2×1019 cm−3, a shoulder separated from the PL peak was observed in the PL spectra at Eg+EF, where  Eg is the band gap and EF is the Fermi energy. The shoulder became very prominent at 9.2×1019 cm−3.
An analytic solution for transverse-magnetic scattering from a rectangular channel in a conducting plane
Authors: Park Tah J. , Eom Hyo J. , Yoshitomi Kuniaki
Published: July 2002

The problem of transverse-magnetic plane-wave scattering from a rectangular channel in a perfect conducting plane is investigated. A Fourier-transform technique is employed to express the scattered field in the spectral domain in terms of parallel-plate waveguide modes. An approximate series solution for scattering is presented in a closed form that is valid for a high-frequency scattering regime. The presented formulation is computationally much more efficient than other existing numerical solutions.
Impact ionization, trap creation, degradation, and breakdown in silicon dioxide films on silicon
Authors: DiMaria D. J. , Cartier E. , Arnold D.
Published: July 2002

Degradation of silicon dioxide films is shown to occur primarily near interfaces with contacting metals or semiconductors. This deterioration is shown to be accountable through two mechanisms triggered by electron heating in the oxide conduction band. These mechanisms are trap creation and band-gap ionization by carriers with energies exceeding 2 and 9 eV with respect to the bottom of the oxide conduction band, respectively. The relationship of band-gap ionization to defect production and subsequent degradation is emphasized. The dependence of the generated sites on electric field, oxide thickness, temperature, voltage polarity, and processing for each mechanism is discussed. A procedure for separating and studying these two generation modes is also discussed. A unified model from simple kinetic relationships is developed and compared to the experimental results. Destructive breakdown of the oxide is shown to be correlated with ‘‘effective’’ interface softening due to the total defect generation caused by both mechanisms.
Evaporation rate of water in a vessel
Authors: Hisatake Kazuo , Tanaka Satoko , Aizawa Youko
Published: July 2002

The evaporation rate of water in a vessel was studied as a function of of temperature, humidity, air velocity, and diameter of the vessel. The atmospheric temperature was varied between 14 and 29 °C, the humidity between 24% and 75%, the air velocity between 0.09 and 7.9 m/s, and the diameter between 5.6 and 12.4 cm. The experimental results are compared with a theory that is based on the idea that the evaporation rate is equal to the diffusion rate of water vapor through the boundary layer covering the water surface in a vessel. In order to compare the experimental evaporation rate with the theory, the temperature of the water surface was carefully measured with a thin thermocouple. The agreement between the present experiments and the theory is generally good.
The stability of zinc oxide electrodes fabricated by dual ion beam sputtering
Authors: Valentini Antonio , Quaranta Fabio , Penza Michele , Rizzi Federica R.
Published: July 2002

The stability of the electrical and optical properties of dual ion beam sputtered zinc oxide films, with resistivities of 10−3 Ω cm, were investigated. ZnO films were deposited at room temperature by argon ion beam sputtering of a zinc oxide target. Argon or hydrogen/argon mixtures of positive ions were irradiated by the second source on the growing film. After heat treatment in air, no significant changes in the electrical and optical properties are observed for films irradiated by hydrogen/argon ions. ZnO films prepared by this method would be useful in the production of transparent and conductive electrodes for practical use at high temperature.
Impression creep of a thin film by vacancy diffusion. II. Cylindrical punch
Authors: Yang Fuqian , Li J. C. M.
Published: July 2002

The diffusion problem in the impression creep of a thin film by a cylindrical punch is analyzed. It is found that at low stresses the impression velocity is proportional to the punching stress. For the same punching stress and the same punch radius the impression velocity decreases for a rigid substrate, and increases without the substrate as the thickness of film decreases. Two limiting cases for the rigid substrate agree with the previous work, namely, the impression velocity is inversely proportional to the punch radius for impression creep of a half-space, and inversely proportional to the square of the punch radius for impression creep of a thin film. Without the substrate the impression velocity for impression creep of a thin film is inversely proportional to the thickness of the film and independent of the punch radius.
Initial elementary processes in tetrafluoroethylene plasma: An ab initio molecular orbital study
Authors: Sato Kota , Komatsu Toru , Iwabuchi Susumu
Published: July 2002

Initial elementary processes in tetrafluoroethylene plasma are studied by using an ab initio molecular orbital method. The energy-surfaces at excited states are obtained by the Hartree–Fock method with a double zeta basis set, plus Rydberg orbitals. A π-π* transition is low-lying both at singlet and triplet excited states. Vinyl-polymerization-type reactions are expected in the presence of some radical species via these transitions. The C=C bond cleaves via a triplet π-σ* transition to form CF2. The predicted elementary processes via these states are compatible with experimental results that C2F4* and CF2 are primary precursors. No excited states that bring about a C—F bond cleavage are obtained within 10 eV of the ground state. As a path for a C—F bond cleavage, a dissociative electron attachment process is found in a low energy region. This process is considered to be important for producing fluorine anions.
Magnetostriction in polycrystalline sputter-deposited TbDyFe films
Authors: Williams P. I. , Lord D. G. , Grundy P. J.
Published: July 2002

Magnetic and microstructural investigations of some polycrystalline rare-earth—transition-metal (RE-TM) alloy thin films are reported. Emphasis was placed on pseudobinary alloys in the (TbxDy1−x)Fe2 and (TbxDy1−x)3Fe4 systems. The films were prepared by sputter deposition onto both room-temperature and high-temperature substrates. Films prepared on room-temperature substrates were amorphous and were crystallized by postdeposition annealing. Transmission electron microscopy and x-ray diffraction showed and indicated a complex microstructure consisting of small grains of free iron, REFe compounds, and some RE oxide phases. Magnetization, coercivity, and magnetostriction measurements showed the films to be essentially magnetized in plane with a trend across the composition range which reflected the different influences of Tb and Dy and also the compensation condition. Magnetostriction values greater than 0.75×10−3 were found near the Terfenol composition Tb0.27Dy0.73Fe2 and for x≳0.3.
Photoluminescence mechanisms of porous Si oxidized by dry oxygen
Authors: Takazawa Akira , Tamura Tetsuro , Yamada Masao
Published: July 2002

Photoluminescence mechanisms in porous oxidized Si were investigated. We observed marked enhancement in the photoluminescence intensity of porous Si when it was oxidized at high temperatures from 800 to 900 °C in dry oxygen. The photoluminescence decay of both as-prepared and dry-oxidized porous Si was intrinsically nonexponential. As reported by other groups, the photoluminescence lifetime, defined as 1/e times, decreased as the emission energy increased for as-prepared samples, and were from 60 to 200 μs. The spread in lifetimes is usually interpreted in terms of the size distribution of Si microcrystals, and the long lifetime on a microsecond time scale is explained by a carrier’s tunneling model. The photoluminescence lifetime for dry-oxidized porous Si, however, did not depend on the emission energy and was about 100 μs. The result clearly shows the presence of radiative recombination processes via luminescence centers, especially in dry-oxidized porous Si.
Theoretical analysis of differential gain of 1.55 μm InGaAsP/InP compressive-strained multiple-quantum-well lasers
Authors: Seki Shunji , Yamanaka Takayuki , Lui Wayne , Yokoyama Kiyoyuki
Published: July 2002

Basic design principles are formulated for increasing the differential gain of 1.55 μm InGaAsP/InP compressive-strained multiple-quantum-well (MQW) lasers. An InGaAsP quaternary active layer can provide more freedom of design compared to an InGaAs ternary active layer since the amount of strain and the quantum-well thickness can be independently determined in an InGaAsP material system for a given emission wavelength. Compressive strain ranging from 1% to 1.5% is large enough to reduce the density of states below one-third of that of unstrained wells. No further reduction is expected even if more compressive strain is applied. When the well thickness is, in turn, determined, it is essential to incorporate a trade-off between the conduction and valence subband energy spacings and the squared optical matrix elements. The extra enhancement of differential gain in MQW structures with a modulation p doping is also studied. By designing MQW structures according to these principles, differential gain can be increased to over 2×10−15 cm2, which corresponds to a relaxation oscillation frequency of more than 30 GHz at an output power of 20 mW.
Trapping levels in Bi12SiO20 crystals
Authors: Petre D. , Pintilie I. , Botila T. , Ciurea M. L.
Published: July 2002

Localized levels play a major role in the electro-optic properties of Bi12SiO20 (BSO) crystals. The activation energy of trapping levels was studied by different laboratories using various methods (e.g., thermally stimulated currents and photoinduced current transient spectroscopy). A more sensitive investigation of traps in undoped BSO single crystals has been performed by optical charging spectroscopy. The presence of traps in the energy range 0.2–1.1 eV was found, and the results are in good agreement with previous studies. On the other hand, this method led us to suggest that the trapping levels observed can be both electron traps and hole traps. For deep trapping levels at higher temperatures, a strong temperature dependence of the cross section was observed.
Determination of ionization energies of the nitrogen donors in 6H-SiC by admittance spectroscopy
Authors: Raynaud C. , Ducroquet F. , Guillot G. , Porter L. M. , Davis R. F.
Published: July 2002

The nitrogen donor levels have been studied by admittance spectroscopy between 20 and 200 K in Schottky barriers made on lightly n-type epitaxial 6H-SiC layers. Measurements at different frequencies yield different freezeout temperatures which in turn are used to determine the donor level energies. Two electron traps at Ec−0.082 eV and at Ec−0.140 eV were detected. These levels are associated with nitrogen, respectively, at the hexagonal sites for the former and at the cubic sites for the latter level.
Multiband coupling effects on electron quantum well intersubband transitions
Authors: Peng L. H. , Fonstad C. G.
Published: July 2002

We discuss multiband coupling effects on electronic intersubband single particle excitations (SPE) of infrared (IR) and Raman processes in type I quantum wells (QWs). Interband couplings between Γc1-Γv15, Γc1-Γc15, and Γc15-Γv15 bulk band edges are treated within 14-band k⋅p first-order perturbation theory. Enhanced band-mixing effects are observed at Brillouin zone center (k∥=0) because of quantization effects (kz≠0). For zinc-blende-type QWs, Γc15-Γv15 band-mixing effects at high-lying Γc1 conduction subbands contribute (x,y)-polarized IR and z(x,x)z̄- and z(x,y)z̄-polarized Raman intersubband SPE. Intersubband polarization splittings occur when the QWs are under biaxial strains.
Study of rf-sputtered yttrium oxide films on silicon by capacitance measurements
Authors: Ling C. H. , Bhaskaran J. , Choi W. K. , Ah L. K.
Published: July 2002

Yttrium oxide Y2O3 films are prepared by rf sputtering on to silicon substrates and characterized by current-voltage I-V, capacitance-voltage C-V and capacitance-time C-t measurements. Relative permittivity increases with substrate heating from room temperature to 600 °C. As-deposited films show large flat-band voltage and interface trap density, which improve upon annealing in oxygen or hydrogen ambient up to 600 °C. However, film resistivity degrades by about 1 order of magnitude. Large hysteresis is observed in the C-V curves, particularly for films deposited at high substrate temperature. This observation is attributed to the interfacial polarization at the Y2O3/SiO2 (native) interface. By monitoring the device capacitance over time for a given applied voltage, the resultant curves can be fitted to an exponential time dependent relation: C(t)=α0−α1exp(−t/τ). The time constant is further found to exhibit a dependence on the applied voltage of the form τ=τ0exp(−V0/V). Comparison with electron-beam deposited films is also made.
Effects of excess oxygen on the properties of reactively sputtered RuOx thin films
Authors: Lee Jeong-gun , Kim Young Tae , Min Suk-ki , Choh Sung Ho
Published: July 2002

Conducting RuOx thin films are fabricated by reactive sputtering with Ar+O2 plasma for an application as the bottom electrode in a ferroelectric thin film capacitor. The structure and composition of RuOx should be controlled to promote the required characteristics in the deposited ferroelectric film. The stoichiometry and resistivity of RuOx are easily controlled by varying the relative O2 partial pressure in the sputtering ambient between 10% and 50%, where the oxygen composition is varied from 2.0 to 2.4 and the resistivity of 100-nm-thick RuO2 film is changed from 145 to 280 μΩ cm corresponding to the variation of oxygen concentration. The films deposited in O2 pressures lower than 30% show preferential (110) RuO2 grain growth, whereas they show (101) in higher O2 pressures. The films deposited at high O2 pressures show some decomposition of the film during a high temperature annealing in N2 ambient.
Superlattice applications to thermoelectricity
Authors: Whitlow L. W. , Hirano T.
Published: July 2002

We present a theoretical framework for estimating the effects of a superlattice (SL) on the thermoelectric (TE) transport in semiconductors. We consider both transverse and parallel conduction through the SL, and in the latter case, the possibility of modulation doping. Our calculations of electron and phonon transport are based on a two-band single-valley model for heavily doped n-type Si80Ge20, with modifications to account for the effects of parallel and transverse transport through a SL. For parallel transport, we find modest improvement in the TE figure-of-merit, only for the narrowest SL (5.4 Å). For transverse conduction, we find that if the SL can be designed to inhibit the conduction of low-energy carriers, then large improvements in the TE figure-of-merit are possible, as large as +100% over the bulk.
Electron density fluctuations in a dusty Ar/SiH4 rf discharge
Authors: Stoffels W. W. , Stoffels E. , Kroesen G. M. W. , de Hoog F. J.
Published: July 2002

The average electron density and electron density fluctuations in a dusty Ar/SiH4 rf discharge have been studied using a microwave resonance technique. The average electron density increases with rf input power and it has a maximum as a function of pressure at about 30 mTorr. Within the first second of plasma operation the electron density decreases with a factor of ten. This is caused by submicroscopic particles, formed in the discharge, which rapidly absorb electrons. When the particles reach a critical size they are expelled from the plasma. This process is governed by a balance between the Coulomb force, trapping the particles in the positive plasma glow and the neutral drag force, flushing them out. The periodic growth and expulsion of particles, monitored by light scattering, results in an oscillatory behavior of the electron density. From the measured oscillation period (τ), which is in the order of seconds to minutes, and its dependence on the gas flow rate (ℱ) and on the fraction α of SiH4 in the plasma (τ[s]≊4.5×102α−1ℱ−2 [sccm], at 10 W rf power input), the trapping force (FC) on particles can be calculated: FC[N]≊4×10−18r [nm], where r is the radius of a particle.
An attempt towards quantitative photothermal microscopy
Authors: Seidel U. , Haupt K. , Walther H. G. , Burt J. A. , Munidasa M.
Published: July 2002

Based on the description of photothermal image formation as a convolution between a defect distribution and the photothermal point spread function, a technique is presented by which subsurface thermal inhomogeneities can be quantitatively reconstructed from photothermal images. Numerical simulations demonstrate the performance and the limits of the suggested approach. Experimental results obtained from a low thermal contrast sample are presented and compared with the theory.
The influence of CH4/H2/Ar plasma etching on the conductivity of n-type gallium nitride
Authors: Molnar B. , Eddy C. R. , Doverspike K.
Published: July 2002

The influence of plasma etching on the electrical properties of n-type gallium nitride (GaN) thin films has been investigated. Electron-cyclotron-resonance microwave plasma reactive ion etching in CH4/H2/Ar, in CH4/H2, and in H2 results in an increase in the GaN layer’s sheet carrier concentration and a decrease in the effective Hall mobility. Neither wet chemical etching nor etching in Cl2 or BCl3 plasmas introduces similar changes. Therefore, the observed damage is considered to be related to the CH4/H2/Ar plasma chemistry. In particular, it suggests hydrogen influence on the defect generation. Subsequent annealing of the affected GaN layers at 800 °C removes the plasma damage.
Photoinduced synthesis of porous silicon without anodization
Authors: Andersen O. K. , Frello T. , Veje E.
Published: July 2002

An easy and very reproducible method to produce porous silicon by light-stimulated etching of crystalline silicon in HF is described, and the formation mechanism is discussed in terms of carrier diffusion and band bending near the surface. The method avoids the use of electrodes and electrochemical etching. The photosynthesized porous silicon shows bright photoluminescence. Spectra recorded at room temperature for n-type as well as p-type silicon are presented.
First experimental observation of the ignition of a ‘‘superdense glow’’ before the glow-to-arc transition in a pseudospark discharge
Authors: Stetter M. , Felsner P. , Christiansen J. , Frank K. , Mehr T. , Stenzenberger J. , Tkotz R.
Published: July 2002

The presented results are the first experimental indication for the existence of a homogeneous superdense glow in the bore hole of the cathode in a pseudospark discharge. The start of this mechanism is leading to the low-impedance high-current phase and starts before the glow-to-arc transition in the pseudospark discharge. It is assumed that the high-current density is carried mainly by metal ions which originate from a self-sustained self-sputtering mechanism, or by a large number of, on a macroscopic scale, homogeneously distributed Schottky emitters. It is shown that the geometric features of the bore hole cause an enhancement of this effect. These results and the theoretical assumptions can also explain very effectively the current quenching of the pseudospark discharge. Current quenching is only occurring during the superdense glow when no cathode spots are ignited.
Magnetic and crystallographic properties of CoCrPt thin films formed on Cr–Ti single crystalline underlayers
Authors: Inaba N. , Nakamura A. , Yamamoto T. , Hosoe Y. , Futamoto M.
Published: July 2002

By using an epitaxial growth technique, Co73Cr15Pt12(112̄0) bi-crystalline thin films formed on Cr–Tix(001)[x=0–15 at. %] single crystalline underlayers were prepared to investigate the relationship between the magnetic properties and the lattice distortion in the magnetic layer. X-ray diffraction measurement shows that the best lattice matching between the c axis of the CoCrPt and the [110] direction of the Cr–Ti underlayer is realized and the CoCrPt layer shows an ideal symmetrical hcp. structure when the Ti concentration is 10 at. %, where the maximum magneto-crystalline anisotropy energy and coercivity values are observed. Distortion in the hcp. CoCrPt magnetic crystal grains decreases the uniaxial magneto-crystalline anisotropy energy, thus lowering the attainable medium coercivity.
A generalized approach to surface photovoltage
Authors: Liu Q. , Ruda H. E. , Chen G. M. , Simard-Normandin M.
Published: July 2002

A detailed majority and minority carrier current balance model for surface photovoltage (SPV) is presented. A rigorous treatment of carrier junction generation and surface recombination is shown to be essential. We show that under weak above band gap photoexcitation, standard constant open-circuit voltage and constant photon flux SPV measurements can both reveal the bulk minority carrier diffusion length, provided the initial band bending Vbi is sufficiently high. However, when Vbi is small, approximate SPV expressions are inapplicable and both standard SPV manifestations will then yield incorrect diffusion length values. We also show that for cases of low surface recombination velocity, the SPV technique can be used to measure the width of the space charge region and Vbi; however, when the surface recombination velocity is large, this approach cannot be used as majority carrier backdiffusion can play a dominant role under high photon energy excitation conditions.
The bottleneck in pseudospark discharges
Authors: Mehr T. , Tkotz R. , Stenzenberger J. , Hintz G. , Christiansen J. , Felsner P. , Frank K. , Stetter M.
Published: July 2002

Physical mechanisms during early phases of pseudospark discharges are still under discussion. Experiments at low rates of current rise (dI/dt<109 A/s) were performed to investigate these phenomena, using filtered fast shutter photography of the discharge plasma in the visible wavelength region. Additionally temporally resolved measurements of line emission of the filling gas (hydrogen and helium) and cathode material (copper) were performed. These experiments revealed a discharge phase where the cathode surface involved in the discharge is restricted mainly to the inside of the cathode aperture. The voltage drop during that phase is about 200–400 V and the current density exceeds 1 kA/cm2. A possible explanation of the discharge mechanisms during that discharge phase is self-sputtering of cathode material from the cathode surface inside the cathode aperture. The described mechanisms might be fundamental for the transition from the transient hollow cathode phase to the high current phase in pseudospark discharges.