bluuebunny/crossref_metadata_2025 huggingface dataset:
Carbon-nanotube-based resonant-circuit sensor for ammonia
Authors: Chopra S. , Pham A. , Gaillard J. , Parker A. , Rao A. M.
Published: July 2002

We present the design and development of highly sensitive and fast-responsive microwave resonant sensors for monitoring the presence of ammonia gas. The sensor consists of a circular disk electromagnetic resonant circuit coated with either single- or multiwalled carbon nanotubes that are highly sensitive to adsorbed gas molecules. Upon exposure to ammonia, the electrical resonant frequency of the sensor exhibits a dramatic downshift of 4.375 MHz. The recovery and response times of these sensors are nominally 10 min. This technology is suitable for designing remote sensor systems to monitor gases inside sealed opaque packages and environmental conditions that do not allow physical wire connections.
Optical properties of thin layers of SiOx
Authors: Levy Y. , Jurich M. , Swalen J. D.
Published: July 2002

Obliquely evaporated films of silicon oxide, SiOx, widely used to align the nematic liquid crystals in electro-optics devices, were investigated optically by attenuated total reflection on a pyramid with a rectangular base covered with a thin layer of gold. This configuration allowed us to make measurements in two perpendicular directions and to determine any in-plane anisotropy. Reflectivity measurements versus angles of incidence exhibited minima associated with both resonant guided modes and surface plasmon oscillations (SPO). Analyses revealed that the obliquely deposited SiOx layers were slightly birefringent and porous, the porosity being determined by the SPO shift when air was replaced by a liquid, hexane. Here the voids surrounding the columnar deposits of SiOx became filled with either air or hexane and, in spite of the heterogeneous structure of the films, effective indices of refraction for the SiOx layer could be determined. Applying the Maxwell–Garnett theory for spherical particles embedded in a host medium (air and hexane), we found that the dielectric constant of our SiOx layers were close to that of stoichiometric SiO2 with a slight optical anisotropy.
Characterization of microvoids in carbon fibers by absolute small-angle x-ray measurements on a fiber bundle
Authors: Shioya M. , Takaku A.
Published: July 2002

A method has been developed to characterize the voids in a fibrous material by using the small-angle x-ray scattering from a unidirectionally aligned fiber bundle, and the parameters evaluating the sizes, volume fraction, and number of the voids have been deduced. The distribution of cross-section size of voids has also been considered in relation to the size parameters deduced. The method developed has been applied to a commercial carbon fiber. The values of the size parameters of the carbon fiber, evaluated experimentally by the method developed here, are in good agreement with those calculated by assuming that the distribution of the cross-section size of voids obeys a Maxwellian distribution. A convenient method for determining the primary beam power by comparing the observed and theoretically calculated scattering intensities of air has also been developed in order to measure the scattering intensity of a specimen in absolute units.
A model for conductor failure considering diffusion concurrently with electromigration resulting in a current exponent of 2
Authors: Shatzkes M. , Lloyd J. R.
Published: July 2002

A model was constructed for electromigration failure where both Fickian diffusion and mass transport due to the electromigration driving force are considered concurrently. The solution to the resulting diffusion equation yields a current exponent of 2 and an activation energy consistent with grain-boundary self-diffusion. A modification of the standard median time to failure equation first proposed by Black is suggested.
Absorption edge measurements in chemically deposited pyrite FeS2 thin layers
Authors: Abass A. K. , Ahmed Z. A. , Tahir R. E.
Published: July 2002

Thin layers of pyrite FeS2 are prepared by chemical deposition using the spray pyrolysis technique. Optical absorption data for photon energies between 1 and 3.8 eV are analyzed and interpreted in terms of electronic transitions and assisting phonons. A direct forbidden band gap of 1.95 ±0.004 eV, an indirect band gap of 1.12±0.003 eV, and assisting phonons of 0.065 eV are detected.
Magnetic granular Fe-(SiO2) solids
Authors: Chien C. L. , Xiao Gang , Liou S. H. , Taylor J. N. , Levy A.
Published: July 2002

We have synthesized a series of granular Fe-SiO2 solids consisting of Fe metal granules of nanometer sizes embedded in an insulating SiO2 matrix. The ultrafine microstructures, revealed by TEM, are achieved by using a high-rate magnetron sputtering system. The magnetic properties of such solids are determined by the granular size and the metal volume fraction. Below the percolation threshold, isolated single-domain particles having large magnetic coercivity in the blocked state exhibit superparamagnetic relaxation, which has been studied by using SQUID magnetometry and Mössbauer spectroscopy. Above the percolation threshold, magnetic coercivity drops dramatically and the magnetic properties approach that of pure Fe. The anisotropy energy of the magnetic granules, much larger than that due to magnetocrystalline anisotropy, has been determined.
Metalorganic molecular-beam epitaxy of ZnSe and ZnS
Authors: Ando Hideyasu , Taike Akira , Konagai Makoto , Takahashi Kiyoshi
Published: July 2002

The metalorganic molecular-beam epitaxial growth of ZnSe and ZnS on (100) GaAs has been demonstrated and characterized. Diethylzinc and diethylselenide were used for the ZnSe growth, while for the ZnS growth both hydrogen sulfide and diethylsulfide (DES) were tested as sulfur sources. The dependence of the growth rate on various growth conditions was investigated to clarify the growth mechanism. When group VI alkyls were used as sources, pyrolysis in a cracking cell was required, unlike metalorganic chemical vapor deposition. Interesting differences in growth kinetics and in crystallinity between H2S-based and DES-based ZnS have been found. Photoluminescence spectra of ZnSe layers measured at 4.2 K showed resolved exciton emissions. However, donor-acceptor pair emission lines attributed to a Li acceptor (LiZn) were also observed. Smooth, monocrystalline ZnS layers could be grown at substrate temperatures above 250 °C by using cracked H2S and DES, and above 150 °C by using uncracked H2S. Photoluminescence spectra of ZnS layers at 74 K were dominated by unidentified blue emissions. These good results reflect the complete absence of a premature reaction.
Transient photoconductivity measurements in semi-insulating GaAs. I. An analog approach
Authors: Kremer R. E. , Arikan M. C. , Abele J. C. , Blakemore J. S.
Published: July 2002

An experimental arrangement is described by which emission coefficient behavior associated with deep-level traps in semi-insulating GaAs can be surveyed, from the photoconductive transient behavior following a trap-filling light pulse. This method, often referred to as PITS (for photoinduced transient spectroscopy) has analogies with the transient capacitance spectroscopy usable for conductive material, but some significant differences also. The necessary experimental precautions which can maximize the probability of getting reliable data are described, and illustrated by PITS results obtained with bulk semi-insulating samples of Cr-doped GaAs. Those results include data for five kinds of trap with activation energies from 0.3 to 0.8 eV, of which some are readily identifiable and others are not. The method described in this paper relies on analog measurement of the change in sample voltage during a specified time interval of the photoconductive decay.
Magnetoelastic effects on antiferromagnetic phase transitions (invited)
Authors: Wolf W. P. , Huan C. H. A.
Published: July 2002

The effect of elastic strains on antiferromagnetic phase transitions is considered. For cases in which the magnetic and chemical unit cells coincide, the combination of a strain and an applied field is found to lead to the possibility of a linear magnetoelastic (LME) coupling which may induce antiferromagnetic order, even in the normally paramagnetic phase. Such an effect can, in principle, destroy any second-order phase transition. An order of magnitude estimate shows that the effect is small but not negligible, and that it may explain a number of unusual effects observed in dysprosium aluminum garnet, including anomalous neutron scattering, magnetic hysteresis and magnetostriction. Similar strain-induced effects may be important in many other antiferromagnets, including CoF2, FeF2, MnF2, and αFe2O3, as well as in mixed crystals with the same structures. Strain gradients may produce similar effects in other antiferromagnets which are magnetoelectric, including DyPO4, DyAlO3, and Cr2O3.
High-current density, high-brightness electron beams from large-area lanthanum hexaboride cathodes
Authors: Loschialpo P. , Kapetanakos C. A.
Published: July 2002

Large (∼5 cm) diameter lanthanum hexaboride (LaB6) cathodes operated at 10 kV have produced 1–5-μs electron pulses with current density between 10 and 20 A/cm2. Normalized beam brightness, Bn =I/(πβγε)2, approximately 3×105 A/cm2 rad2 has been consistently measured. To obtain this high-current density, the LaB6 cathodes have been heated to temperatures between ∼1600 and 1800 °C. Very uniform temperature profiles are obtained by applying a carefully tailored electron bombardment heating power distribution. These measurements have been made between pressure 10−6–10−5 Torr, i.e., under much less demanding vacuum conditions than that required by conventional dispenser-type cathodes.
Optical transitions in a parabolic quantum well with an applied electric field—analytical solutions
Authors: Chuang S. L. , Ahn D.
Published: July 2002

Both interband and intersubband optical transitions in a parabolic quantum well with an applied electric field are investigated. The wave functions for the electrons and holes are those of the displaced harmonic oscillators, which are shifted in opposite directions. The dipole moment matrices, the absorption coefficients, and the changes in the refractive indices are evaluated with the intrasubband relaxations taken into account. Interesting electroabsorption and electro-optic effects are presented analytically and numerically.
Deep-level transient spectroscopy study of n-type GaAs epitaxial layers grown by close-spaced vapor transport
Authors: Massé G. , Lacroix J. M. , Lawrence M. F.
Published: July 2002

We have done a deep-level transient spectroscopy study of deep donor traps existing in n-type GaAs epitaxial layers grown by close-spaced vapor transport. All the samples exhibited a bulk trap having an important concentration (≳1014 cm−3), which we have noted ELCS1 and identified with the trap EL12, which was observed in vapor-phase epitaxy materials. Other bulk traps were observed in some samples, but in a weaker concentration (∼1013 cm−3). In other samples, in addition to ELCS1, two more traps were seen; their concentration increased towards the surface. One of them is probably localized near the surface; the other one (ΔEa=0.83 eV, σna=0.8×10−13 cm2) must be identified with the donor trap EL2; it is also present in the bulk, but with a lower concentration.
Identification of Ti3+ in potassium titanyl phosphate and its possible role in laser damage
Authors: Roelofs Mark G.
Published: July 2002

Four related Ti3+ centers in potassium titanyl phosphate (KTiOPO4 or KTP) may be created with either application of dc electric fields or with hydrogen at 800 °C. Single-crystal electron paramagnetic resonance allows determination of the g tensors of the four centers. Hyperfine coupling is observed with one47,49 Ti and two (nearly) equivalent 31P. The most abundant center is characterized by gx=1.7709(5), gy=1.8723(5), gz=1.9462(5), principal values of ‖ATi‖ of 80, 33, and 24 MHz, and axially symmetric AP’s with principal values of 16.3 and 21.8 MHz. Optical absorption centered at 13 000–18 000 cm−1 is quite broad, relatively strong, and approximately twice as intense for light whose electric vector is polarized parallel to c as it is for a or b. A charge transfer mechanism in addition to d−d transitions may account for the absorption. Ti3+ is much more difficult to produce in hydrothermally grown KTP than in flux-grown KTP, a difference ascribed to the presence of oxygen vacancies or interstitial potassium in the latter. Similarities in the energy and polarization of optical absorption suggest that coloration induced by laser damage is due also to Ti3+.
X-ray photoelectron spectroscopy of Pb-doped Bi-Sr-Ca-Cu-O superconductors
Authors: Rai D. K. , Sarkar Asok K. , Wittberg T. N. , Kumar Binod
Published: July 2002

X-ray photoelectron spectra of Bi-Sr-Ca-Cu-O superconductors with different Pb concentrations have been obtained. It is observed that Bi 4f and Sr 3d peaks show evidence of two possible bonding states. Incorporation of Pb into the structure is seen to increase the proportion of Sr and Bi atoms in the strongly bonded state. The relative atomic proportion of Sr and Ca also shows a significant change when Pb is added.
Magneto-optic waveguide hysteresis loops of ‘‘planar’’ magnetic garnet films
Authors: McGlashan-Powell M. , Wolfe R. , Dillon J. F. , Fratello V. J.
Published: July 2002

Epitaxial films of modified yttrium iron garnet which are used as optical waveguides for magneto-optic devices have nominally planar anisotropy so that their magnetization can be aligned by small in-plane fields. Films grown on (111) substrates usually have some residual cubic anisotropy that is difficult to measure in the active layer of the double- or triple-layer films used in single-mode waveguides. A new method to determine the magnetic properties of the optically active layer itself has been developed. It is based on magneto-optical hysteresis loops in which the Faraday rotation of the waveguided light is plotted against one component of a rotating in-plane magnetic field. Steps in these hysteresis loops are directly related to the sixfold symmetry of the magnetocrystalline anisotropy of these films. The influence of linear birefringence on the measured Faraday rotation, the magnetostrictive effects associated with the pressure of the prism used for optical coupling into the waveguide, and the advantages of end-fire coupling are discussed. These hysteresis loops are being used in the design of materials for magneto-optic isolators and other waveguide devices.
Magnetic hardening of Pr-Co-B sintered magnets
Authors: Christodoulou C. N. , Wallace W. E. , Massalski T. B.
Published: July 2002

We report preliminary results on Pr-Co-B sintered magnets with room-temperature intrinsic coercivity Hci equal to 5.2 kOe and energy product (BH)max equal to 10.3 MGOe. At liquid-nitrogen temperature (77 K), the Hci is greater than 18 kOe (which was the maximum available magnetic field) and (BH)max is equal to 12.5 MGOe. The Curie temperature of the magnet is 988 K (corresponding to the presence of the Pr2Co14B phase). The microstructure of the magnet with the high Hci is markedly different from that of the magnet with the low Hci. The major difference is the lower concentration of voids and the well-defined shape of the grains. The grain size is between 5 and 30 μm. The major phases present in the magnet are Pr2Co14B, PrCo4B, and a minor amount of PrCo5. The metallurgical characteristics and phase diagram features of the Pr-Co-B system seem to be substantially different from those of the Pr-Fe-B system, where the major phases present are the Pr2Pe14B, PrFe4B4, and Pr rich.
Effects of the presence/absence of HCl during gate oxidation on the electrical and structural properties of polycrystalline silicon thin-film transistors
Authors: Proano R. E. , Ast D. G.
Published: July 2002

The influence of the presence or absence of HCl during gate oxidation at 1100 and 1150 °C on the electrical and structural properties of polycrystalline silicon thin-film transistors was investigated. Devices processed without HCl exhibited a lower leakage current, a larger current switching ratio, a 25%–55% increase in carrier mobility, and a 21%–30% lower grain-boundary trap density. Materials investigation showed that the improvement was not primarily due to an increase in grain size, which was about 1000 Å irrespective of processing conditions, but to a change in crystallite orientation. X-ray diffraction measurements showed an increased {110} texture in devices processed without HCl. In addition, in the latter devices, dopant diffusion under the gate from the source and drain contacts was reduced by 7.6% to 12%. A linear relationship was observed between the average grain-boundary trap density and the average lateral dopant diffusion length. These findings are explained by considering the point defect chemistry and its relationship to grain-boundary mobility. Removal of HCl from the oxidation ambient increases the concentration of silicon self-interstitials. The increased migration of these point defects to grain boundaries enhances their nonconservative motion and therefore increases grain-boundary mobility. This in turn, allows the boundary to more completely acquire a lower-energy configuration. This lower-energy configuration of the boundary contains a minimum of broken bonds and therefore minimizes both the grain-boundary electrical activity and the diffusion along grain boundaries.
Experimental results of phase locking two virtual cathode oscillators
Authors: Hendricks Kyle J. , Adler Richard , Noggle R. Carl
Published: July 2002

Experiments have been completed on electrostatic and electromagnetic phase locking of two independent virtual cathode oscillators (VCO). The experiments were accomplished by using the Gemini electron beam accelerator. The Gemini accelerator was used to generate each VCO with a chosen time separation. The phase locking was observed by beating microwave signals generated in each anode-cathode gap and noting the time intervals during which the phase difference remained constant. The phase locking was observed during both electrostatic and electromagnetic coupling of the microwave power between the VCOs. We did find that one must prefill the coupling cavity with microwave power to enable one VCO to phase lock the other.
Heteroepitaxial growth and characterization of InP on Si substrates
Authors: Sugo Mitsuru , Takanashi Yoshifumi , Al-jassim M. M. , Yamaguchi Masafumi
Published: July 2002

Heteroepitaxy of a highly mismatched system (∼8%), InP/Si, has been studied using low-pressure organometallic vapor phase epitaxy. GaAs buffer layer effects on residual stress and defect density in InP/Si have been clarified. Using a 1-μm-thick GaAs buffer layer, residual stress in the InP layer has been reduced to as low as 2×108 dyn/cm2 compared to ∼4×108 dyn/cm2 for InP directly grown on Si. Moreover, the GaAs buffer layer has also been confirmed to be effective for improving InP/Si quality by evaluation of etch-pit density, x-ray diffraction measurement, and cross-sectional transmission electron microscopy. Electrical properties of InP layers on GaAs/Si were evaluated with the van der Pauw and deep level transient spectroscopy (DLTS) methods. The heteroepitaxial layer’s own electron trap has also been observed by DLTS measurements. For an InP/GaAs/Si structure, InP growth temperature effect on surface morphology and etch-pit density is also shown. High quality InP films with an etch-pit density of 8×106 cm−2 have been obtained on Si substrates by using thermal cycle growth and InP/GaAs/Si structure.
Electrical conductivity and thermodynamic functions of weakly nonideal plasma
Authors: Mohanti R. B. , Gilligan J. G.
Published: July 2002

The effect of partial screening due to electric microfields within the shielding cloud is examined and used as the basis for determination of electrical conductivity and certain thermodynamic functions of weakly nonideal plasma. A modified screening radius (non-Debye radius) that is greater than the Debye radius is used to take into account short-range Debye–Huckel-type interactions. The evaluation of nonideal plasma electrical conductivity includes corrections to the Spitzer model and accounts for localization of low-energy electrons in the continuum due to incomplete screening. Excellent argeement with experimental results is obtained in the weakly nonideal plasma regime. Corrections to certain thermodynamic functions, such as, chemical potential and internal energy, are evaluated using numerical methods. The effect of charge state on transport coefficients and thermodynamic functions is also analyzed.
Photoacoustic characterization of a two-layer system
Authors: Mansanares A. M. , Vargas H. , Galembeck F. , Buijs J. , Bicanic D.
Published: July 2002

In this paper the use of the so-called open photoacoustic cell for thermal characterization of two-layer systems of variable thickness is described. It is shown that the thermal diffusivity as well as the thermal conductivity are completely determined, based upon the effective sample model widely used in heat-transfer problems.
High pressure refractive index measurements of 4:1 methanol:ethanol
Authors: Eggert Jon H. , Xu Li-wen , Che Rong-zheng , Chen Liang-chen , Wang Ji-fang
Published: July 2002

A new technique for measuring the refractive index at high pressure in a diamond anvil cell is reported. This technique was applied to a 4:1 methanol:ethanol mixture at room temperature and pressures up to 16.9 GPa. A parameterization is given for the refractive index that fits the data with a residual standard error of 4×10−3 at frequencies between 12 000 and 24 000 cm−1 and pressures between 0.5 and 11.5 GPa. A sharp change in slope of the refractive index vs pressure was observed at the glass transition near 10 GPa. Modifications of the technique which should allow measurements of the refractive index to much higher pressures are discussed.
Phase control of cubic boron nitride thin films
Authors: Kester Daniel J. , Messier Russell
Published: July 2002

Ion beam assisted evaporation was used to deposit cubic and hexagonal boron nitride thin films. Boron was evaporated and bombardment was by argon and nitrogen ions. The effect of preparation conditions on the resulting phase was studied, and the relationship between the phase and the energy and momentum transferred into the film through ion bombardment was examined. It is shown that for a given temperature, the controlling factor in the resulting thin film phase is the momentum transferred into the film per depositing boron atom. At 300–400 °C a sharp threshold value of momentum-per-atom exists below which films are hexagonal and above which they are cubic. For 400 °C this threshold occurred at 200 (eV×amu)1/2 which is equal to 3.3×10−21 m kg s−1. Depositions performed using krypton and xenon instead of argon as the second bombarding gas confirmed this momentum-per-atom value. A second threshold was also observed, which was bombarding species dependent, above which either complete resputtering of the deposited material or reversion to the hexagonal phase occurred. Cubic boron nitride deposition was seen to occur in a window of momentum-per-atom values between these two thresholds. Using this information it was possible to grow cubic boron nitride using only nitrogen bombardment, although the window of momentum-per-atom values for nitrogen is very narrow. The effect of substrate temperature was studied, and it was found to be difficult to grow predominantly cubic phase films below 300–400 °C. The relationship between intrinsic stress and phase of the films is also discussed. A diagram is presented showing film phase as a function of bombardment, substrate temperature, and system chemistry. The parameter of momentum-per-atom is shown to combine into a single value the variables of ion beam assisted deposition: deposition rate, ion energy, ion flux, and ion species. It is suggested that, in general, for properties affected by ion bombardment the momentum-per-atom transferred into the film is the controlling factor. The results are shown to support momentum transfer as the dominant process in cubic boron nitride thin film formation.
Temperature dependence of the ruby luminescence method for measuring high pressures
Authors: Yen J. , Nicol M.
Published: July 2002

Pressures in diamond-anvil high-pressure cells often are determined by measuring ruby emission spectra because the R1 luminescence has high quantum efficiency and frequency of the R1 band shifts almost linearly with pressure, independent of temperature. To more than 600 K, pressures can be precisely determined by measuring luminescence spectra at ambient and high pressures at the same temperature. The temperature dependence of the frequency of the R1 line is not simple, and empirical approximations have been developed so that a pressure at a known temperature might be estimated from measurements of spectra at that P and T and at ambient pressure and one temperature; that is, without measuring the R1 line at ambient pressure at each temperature. Measurements reported here show that, between 100 and 260 K, pressures computed by using empirical approximations calibrated near ambient temperature may be in error from 0.1 to several GPa. The temperature dependence of the R1 line is accurately described by the function suggested by McCumber and Sturge [J. Appl. Phys. 34, 1682 (1963)]. Measurements at several temperatures between 100 and at least 400 K are needed to evaluate the precise parameters of this function appropriate for a particular spectrometer and ruby. Therefore, it is often practical for studies at pressures to a few GPa simply to determine the wavelength of the R1 line at a given temperature and ambient pressure by measuring it.
Using thermally stimulated currents to visualize defect clusters in neutron-irradiated silicon
Authors: Bruzzi M. , Borchi E. , Baldini A.
Published: July 2002

Silicon p+n junctions irradiated with fast-neutron fluences (5×1011–5×1013 neutrons/cm2) have been experimentally measured using the thermally stimulated current technique. When forward filling voltages are applied, a new peak is found in samples irradiated with fluences greater than 5.6×1011. A cluster model has been developed to describe the new peak and the cluster’s dimensions and defect concentrations have been evaluated.
Monte Carlo simulation of pseudomorphic InGaAs/GaAs high electron mobility transistors: Physical limitations at ultrashort gate length
Authors: Dollfus P. , Bru C. , Hesto P.
Published: July 2002

Using Monte Carlo simulations, we study pseudomorphic uniformly doped AlGaAs/InGaAs/GaAs high electron mobility transistors with very short gate lengths (150 and 50 nm). In open-channel range of operation the saturation of the rain current is ensured by the existence of a pseudodipolar domain between the gate and the drain where the increase in drain potential is dropped. We describe the short channel effects, such as the high drain conductance, that occur in the pinchoff range of operation (especially in the shortest device). In this case the overheated electrons can easily transfer to the upper large band gap layer and cannot form any domain. The potential barrier that controls the current is gradually lowered by the drain potential increase, which is favoring the electron injection from the source side. Short channel effects are also involved in the relatively weak transconductance obtained in the 50 nm gate device, gm=800 mS/mm, to be compared with 900 mS/mm reached in a 150 nm gate device. Finally, we show that the occurring of short channel effects can be predicted from very simple one-dimensional calculations along two perpendicular directions of the device, by taking into account the lateral diffusion from the source access zone to the active zone.
An in situ transmission electron microscopy study of electron-beam-induced amorphous-to-crystalline transformation of Al2O3 films on silicon
Authors: Liu J. , Barbero C. J. , Corbett J. W. , Rajan K. , Leary H.
Published: July 2002

An in situ transmission electron microscopy study of the amorphous-to-crystalline transformation of Al2O3 films on a silicon substrate has been carried out. It is found that a critical electron-beam dose rate is required for the transformation to be observed. The possible effect of the silicon substrate on the growth of the Al2O3 crystallites is also discussed.
Study of vibrational modes of gold nanostructures by picosecond ultrasonics
Authors: Lin H.-N. , Maris H. J. , Freund L. B. , Lee K. Y. , Luhn H. , Kern D. P.
Published: July 2002

We have used picosecond ultrasonics techniques to study the vibrations of several nanostructures composed of laterally patterned gold films on fused quartz substrates. The structures include arrays of stripes and dots with lateral repeat distance as small as 2000 Å. The frequencies and attenuation rates of the low-lying normal modes of these structures are measured. We compare these results with those obtained theoretically from a finite element analysis of the dynamics of the structures, and discuss the physical nature of the principal vibrational modes.
Influence of the ion energy on the growth and structure of thin hydrocarbon films
Authors: Reinke P. , Jacob W. , Möller W.
Published: July 2002

Noncrystalline hydrocarbon films were deposited from methane plasmas in an electron-cyclotron-resonance plasma reactor. The films were characterized by infrared spectroscopy and high-energy ion-beam analysis. Film properties were investigated as a function of the energy of ions impinging on the surface of the growing film. The ion energy was varied by applying a dc bias to the sample in the range from 30 to 200 eV. The ion energy was measured with a retarding field analyzer under identical experimental conditions. An abrupt change was found in the sp3/sp2 ratio in the energy interval from 80 to 120 eV. Other film properties such as the refractive index, density, and hydrogen-to-carbon ratio exhibit a more or less monotonic dependence on the ion energy. The results are compared with published data on hydrocarbon films. An analytical model was developed to describe the energy dependence of the hydrogen fraction in the films. It is based on the assumption that displacement of hydrogen atoms is the dominating mechanism that controls the hydrogen fraction in hydrocarbon films. The results are also discussed on the basis of the fully constrained network model.
Thermal and microstructural analysis for laser surface hardening of steel
Authors: Tayal M. , Mukherjee K.
Published: July 2002

The influence of parameters such as laser power, focusing conditions, and travel speed on the laser surface hardening of American Iron and Steel Institute 1045 steel has been investigated in terms of the microstructure and hardness of heat affected zones. Hardening trials were performed with a 500 W neodymium:yttrium aluminum garnet laser. Electron and optical microscopy were performed to correlate the hardness, hardness profiles, and microstructures that are achieved during laser surface hardening. A simple approach has been presented, which makes the Green’s functions application easier, to predict the temperature distribution during laser surface hardening. This approach can be used to predict the case depth, which can be achieved by using a given set of laser process parameters. The results obtained by using this approach compare well with the results obtained from a commercial finite element package, Engineering Analysis System.
Properties of diamagnetic fluid in high gradient magnetic fields
Authors: Ueno S. , Iwasaka M.
Published: July 2002

This study focuses on the properties of diamagnetic fluid in static magnetic fields up to 8 T with the gradient of 50 T/m. We used a horizontal type of superconducting magnet with a bore 100 mm in diameter and 700 mm long. We observed the phenomenon that the surface of the water was pushed back by magnetic fields of higher gradients. Two ‘‘frozen’’ cascades were formed at z=±50–80 mm; the surface of the water near the center of the magnet was parted, and the bottom of the water chamber appeared. The water level at both ends of the chamber was lifted up. In order to investigate the hydrodynamics of diamagnetic fluid in magnetic fields, we made a fluidic circuit with plastic tubing which passed through the superconducting magnet’s bore. When magnetic fields in the center of the bore were changed from 0 to 8 T, the flow velocity of distilled water decreased, and the flow was stopped at 8 T. A stress analysis of diamagnetic fluid in magnetic fields was carried out to explain the mechanism of these phenomena. The hydrodynamics of diamagnetic fluid in ∼400 T2/m is able to compare with that of ferromagnetic fluid in weak magnetic fields. Studying the role of diamagnetic fluid in gradient magnetic fields is important in understanding the mechanism of biological effects of magnetic fields.
Low-temperature behavior of thermopower in rare-earth iron borides R2Fe14B (R=Nd, Sm, Gd, Tb, Dy, Ho, Er)
Authors: Pinto R. P. , Braga M. E. , Amado M. M. , Sousa J. B. , Buschow K. H. J.
Published: July 2002

Recently it was shown that the electrical resistivity behaves anomalously both at low temperatures and near the Curie point in rare-earth iron borides due to the subtle interplay between 4f and 3d transition elements. Here a systematic experimental study on the behavior of the thermoelectric power (S) of R2Fe14B compounds with heavy (R=Gd, Tb, Dy, Ho, Er) and light (Nd, Sm) rare-earth elements is presented. Data for La2Fe14B, in the temperature range 10 K<T<350 K, is also included for comparison. The results are consistent with the spin-mixing model for electron scattering, with an intrinsic thermopower contribution related to Fe and a spin dependent impurity scattering term related to the rare-earth magnetic ions. From the interplay of both terms we can get maxima in S (for R=Td, Dy, Ho, Er samples) or minima (Gd, Nd, Sm) at low temperatures. The data indicates the ultimate dominance of impurity scattering at low temperatures, producing a linear variation in the magnetic thermopower (S). In La2Fe14B, only the intrinsic effect is observed, and S(T) does not follow a linear temperature dependence.
Plasma diagnostics in pulsed laser TiN layer deposition
Authors: Hermann J. , Thomann A. L. , Boulmer-Leborgne C. , Dubreuil B. , De Giorgi M. L. , Perrone A. , Luches A. , Mihailescu I. N.
Published: July 2002

Time- and space-resolved emission and laser-induced fluorescence spectroscopic measurements were performed to investigate vaporization and plasma formation resulting from excimer laser irradiation of titanium targets in a low-pressure nitrogen atmosphere. Measurement series have been done by varying the laser intensity from the vaporization threshold at 25 MW cm−2 up to values of about 500 MW cm−2 typically applied in pulsed laser deposition processing of titanium nitride films. Thus, the transition from thermal evaporation to the high-density plasma formation process, leading to the production of reactive species and high-energy ions, was evidenced. An interesting result for the comprehension of the reactive deposition process was the observation of a quantity of dissociated and ionized nitrogen, which is transported with the plasma front in the direction of the substrate.
Thermal and diffusion-induced stresses in crystalline solids
Authors: Belova I. V. , Murch G. E.
Published: July 2002

The stressed crystal that has been obtained during the crystallization process is considered with particular attention given to the comparative numerical analysis of composition and temperature gradients as the causes of the stressed state. The numerical calculations are referred to the directional solidification method. It is shown that the contribution of diffusion induced stresses to the total stressed state of a crystal in the growth process may be comparable to the contribution from thermostresses. In this article, as a consequence of the main thermodynamical laws the relations obtained by Larché and Cahn [Acta Metall. 21, 1051 (1973)] are generalized to the case of a coupling dependence of elastic moduli on temperature and composition. The relations provide the important connection between the elastic modulus of compound materials measured using acoustic methods (at constant entropy and chemical potentials) and those that are used in practice (at constant temperature and composition).
Kinetic modeling of hydrogen adsorption/absorption in thin films on hydrogen-sensitive field-effect devices: Observation of large hydrogen-induced dipoles at the Pd-SiO2 interface
Authors: Fogelberg J. , Eriksson M. , Dannetun H. , Petersson L.-G.
Published: July 2002

A kinetic modeling of the hydrogen interaction with a Pd-SiO2-Si (Pd-MOS) device is reported. The model is fitted to a number of experimental results, mainly from mass spectrometric desorption measurements. The new results verify an older description of the energetics of hydrogen adsorption states at the Pd/SiO2 interface in the sense that the adsorption may be described by a so-called Temkin isotherm; however, several new findings have to be incorporated into the model in order to obtain a consistent picture. The initial heat of adsorption at the interface is around 0.8 eV/hydrogen atom. The number of adsorption sites at the interface is considerably smaller than at the surface, 6×1017 m−2 versus 1.5×1019 m−2. Furthermore, the interface hydrogen atoms are strongly polarized. An average value of 2 Debye is obtained. It is the large hydrogen polarization at the Pd/SiO2 interface and not a large concentration of adsorbed hydrogen atoms per se which accounts for the very high sensitivity of a Pd-MOS device as hydrogen sensor.
Initial velocity effect on space-charge-limited currents
Authors: Liu Shengyi , Dougal Roger A.
Published: July 2002

The characteristics of a monopolar space-charge region under the effect of charge injection with a uniform initial velocity are studied. Poisson’s equation has been solved as a Dirichlet boundary-value problem, other than by conventional serial integrations, to yield exact analytical expressions for the potential, field, charge density, and current. The structure of the space-charge region is characterized by a potential well and an effective diode gap, and can be explained in terms of the initial kinetic energy of the charge. The Child–Langmuir limit, when calculated by using the effective diode gap, is a firm limit.
Growth and erosion of hydrocarbon films investigated by in situ ellipsometry
Authors: von Keudell A. , Jacob W.
Published: July 2002

The growth of hydrocarbon films (C:H films) from a methane plasma and their erosion by a hydrogen plasma are investigated by means of in situ ellipsometry. The kinetic energy of the ions impinging on the surface during deposition and erosion is varied by applying a rf bias resulting in a dc self-bias ranging from floating potential up to 100 V. In addition, the substrate temperature is varied from room temperature up to 600 K. The direct comparison between the growth and erosion indicates that the temperature dependence of the growth rate during deposition from a methane plasma is caused by the temperature-dependent erosion due to reactions with the abundant atomic hydrogen. Furthermore, the synergistic effects between hydrogen ions and atomic hydrogen on the etch rate of C:H films are investigated. The underlying surface reactions during the erosion show up in the optical response of the deposited films as measured by ellipsometry. These results are compared with findings in the literature on the elementary steps of the erosion of C:H films by atomic hydrogen.
Fabrication and optical properties of Si/CaF2(111) multi-quantum wells
Authors: Bassani F. , Vervoort L. , Mihalcescu I. , Vial J. C. , d’Avitaya F. Arnaud
Published: July 2002

We have synthesized, by molecular beam epitaxy, Si/CaF2(111) multi-quantum wells which are photoluminescent at room temperature after ageing in air. In this article, we report on the structural properties and on a detailed optical study of these heterostructures. The photoluminescence spectra for various confinements and the temperature dependence of the lifetimes as a function of emission wavelength are described in comparison with the corresponding characteristics of porous silicon and hydrogenated amorphous silicon. A model based on quantum confinement is proposed to explain the experimental data.
Mobility lifetime product—A tool for correlating a-Si:H film properties and solar cell performances
Authors: Beck N. , Wyrsch N. , Hof Ch. , Shah A.
Published: July 2002

The missing correlation between film characteristics and a-Si:H-based p-i-n solar cells is still a controversial subject. The authors present a new parameter μ0τ0, evaluated from steady-state transport measurements on a-Si:H layers, which can indeed relate film quality and cell performance as far as the latter is limited by the quality of the intrinsic 〈i〉 layer. Thereby, two specific features of the evaluated μ0τ0 product can explain its successful role as a quality parameter for a-Si:H: First, the computation of μ0τ0 takes into account the effects of the prevailing dangling bond occupation, which is very different in uniform films as compared to the occupation profile prevailing through the i layer of a p-i-n solar cell; second, the evaluated μ0τ0 product combines information about band mobility and defect density; furthermore it avoids some of the well-known pitfalls of usual deep defect density measurements such as constant photocurrent method and photothermal deflection spectroscopy. Experimental data on a series of layers and p-i-n solar cells illustrates the determination of μ0τ0 in a given practical case and its successful correlation with cell efficiency. In this context, an estimation for the ratio of charged to neutral capture cross sections σ±/σ0 of around 50 is found.
Ambipolar diffusion length and photoconductivity measurements on ‘‘midgap’’ hydrogenated microcrystalline silicon
Authors: Goerlitzer M. , Beck N. , Torres P. , Meier J. , Wyrsch N. , Shah A.
Published: July 2002

Hydrogenated microcrystalline silicon (μc-Si:H) deposited by VHF plasma-enhanced chemical vapor deposition has recently been proven to be fully stable, with respect to light-induced degradation, when adequately used in p-i-n solar cells. Stable solar cells efficiencies of 7.7% have been obtained with single-junction cells, using ‘‘midgap’’ microcrystalline i-layers, having an optical gap of around 1 eV. In the present paper, the electronic transport properties of such microcrystalline layers are determined, by the steady-state photocarrier grating method (SSPG) and steady-state photoconductivity measurements, in a coplanar configuration. The conditions for the validity of the procedure for determining the ambipolar diffusion length, Lamb, from SSPG measurements (as previously theoretically derived in the context of amorphous silicon) are carefully re-examined and found to hold in these μc-Si:H layers, taking certain additional precautions. Otherwise, e.g., the prevalence of the ‘‘lifetime’’ regime (as opposed to the ‘‘relaxation time’’ regime) becomes questionable, in sharp contrast with the case of amorphous semiconductors, where this condition is almost never a problem. For the best layers measured so far, Lamb is about twice as high and the photoconductivity σphoto four times as high in μc-Si:H, when compared to device quality a-Si:H. Until now, the highest values of Lamb found by the authors for μc-Si:H layers are around 3×10−5 cm.
A study of growth and the relaxation of elastic strain in MgO on Fe(001)
Authors: Vassent J. L. , Dynna M. , Marty A. , Gilles B. , Patrat G.
Published: July 2002

The deposition of MgO on the Fe(001) surface at room temperature and at elevated temperatures has been carried out using molecular beam epitaxy (MBE). MgO is observed to grow epitaxially with a 45° rotation between the Fe(001) and MgO(001) unit cell axes. The growth mode has been studied as a function of temperature using reflection high-energy electron diffraction (RHEED), while the chemical and structural characteristics of the MgO film have been studied using Auger electron spectroscopy and high resolution electron microscopy. The relaxation of the in-plane lattice parameter during growth at room temperature has been measured in situ using RHEED and ex situ using glancing incidence x-ray diffraction and during growth at elevated temperatures by means of RHEED. Pseudomorphic growth is observed up to a thickness of 4–5 monolayers, after which the in-plane lattice parameter starts to evolve towards the MgO bulk parameter as 1/2〈011〉 misfit dislocations are introduced at the Fe/MgO interface. The degree of relaxation as a function of epilayer thickness is compared with that expected for an equilibrium dislocation spacing in an array of dislocations of alternating orientation, and with that predicted by Freund’s criterion for the blocking of a threading segment by an orthogonal misfit dislocation [J. Appl. Phys. 68, 2073 (1990)].
Influence of the nucleation process on the azimuthal misorientation of heteroepitaxial diamond films on Si(001)
Authors: Schreck M. , Thürer K.-H. , Klarmann R. , Stritzker B.
Published: July 2002

In order to clarify the mechanisms that limit the alignment for diamond heteroepitaxy on Si(001) the influence of different process parameters during the bias enhanced nucleation step on the misorientation of diamond on Si(001) was evaluated using x-ray diffraction texture measurements. It is shown that the azimuthal width of the {220} pole density maxima in the substrate plane measured in transmission is a quantity that allows a systematic study of the parameter space for oriented nucleation. From the negligible influence of the substrate surface roughness and of the substrate temperature between 675 and 905 °C it is concluded that the migration, rotation, and rearrangement processes of nucleated diamond clusters do not control alignment in the temperature range studied. In contrast, the duration of the biasing procedure, the process pressure, and the absolute value of the bias voltage can strongly vary the azimuthal distribution between a full width at half-maximum of 3.9° and more than 15°. Low bias voltages favor narrow distributions whereas high bias voltages are accompanied by extremely low biasing times (down to 20 s). When the optimum biasing time is exceeded, the orientation is lost via two different routes. Several models for the underlying mechanisms are proposed with those that are most probable pointing to a detrimental influence by ion bombardment.
Monte Carlo calculation of velocity-field characteristics of wurtzite GaN
Authors: Bhapkar Udayan V. , Shur Michael S.
Published: July 2002

We present velocity-field simulations of n-doped, wurtzite-phase GaN for temperatures between 77 and 1000 K using an ensemble Monte Carlo technique. A three-valley model of the band structure is assumed, and the ionized impurity, polar optical phonon, acoustic phonon, piezoelectric, and intervally scattering mechanisms are considered. Electron degeneracy and heating are also accounted for. Properties of the two dimensional electron gas in AlGaN/GaN heterostructures are also estimated by performing simulations on bulk GaN in which the electron concentration exceeds the ionized donor concentration by factors of ten to one hundred. The simulations predict that peak steady-state drift velocities ranging from 3.3×107 to 2.1×107 cm/s for temperatures between 77 and 1000 K can be achieved in both the two dimensional electron gas and in the bulk material with an ionized donor concentration of 1016 cm−3. Furthermore, the simulations predict that the two-dimensional electron gas in GaN will exhibit a low-field mobility an order of magnitude greater than the bulk material, in agreement with experimental results.