bluuebunny/crossref_metadata_2025 huggingface dataset:
Formation of iron or chromium doped epitaxial sapphire thin films on sapphire substrates
Authors: Yu Ning , Wen Qingzhe , Clarke David R. , McIntyre Paul C. , Kung Harriet , Nastasi Michael , Simpson Todd W. , Mitchell Ian V. , Li DeQuan
Published: July 2002

This work summarizes results of a simple procedure to incorporate dopants into the near surface region of single-crystal sapphire. We demonstrate the formation of iron-doped and chromium-doped sapphire thin films by solid-phase epitaxial growth. Amorphous alumina films of about 200–350 nm thickness were deposited onto single-crystal sapphire substrates. Fe or Cr ions were introduced into the films during deposition. A post-deposition thermal process was performed in oxidizing ambients at 800–1400 °C to induce epitaxial growth and to incorporate dopants. The epitaxial relationship of the grown film with the substrate was confirmed by both ion channeling and cross-sectional transmission electron microscopy. The growth kinetics were determined by time-resolved reflectivity measurements for different dopant concentrations. Ion channeling angular scans revealed that the Fe or Cr ions are incorporated onto octahedral sites (Al3+ sites) in the corundum structure as expected in equilibrium. External optical transmittance measurements exhibited absorption in the near ultraviolet range associated with the Fe3+ state. The substitution of Cr for Al3+ was also confirmed by the observation of R1 and R2 luminescence lines characteristic of ruby. The doping procedure has potential applications in the fabrication of thin film planar optical waveguides and thin film stress sensors.
Effect of growth conditions on the structural properties of ion beam sputter deposited SiGe epilayers
Authors: Lyakas M. , Arazi T. , Eizenberg M. , Demuth V. , Strunk H. P. , Mosleh N. , Meyer F. , Schwebel C.
Published: July 2002

Structural properties of Si1−xGex layers epitaxially grown on Si(100) by Ion Beam Sputter Deposition were studied as a function of growth temperature and film thickness. It was shown that the structure of defects strongly depends on the growth temperature, Tg. The dislocations cross grid which is observed at the SiGe/Si interface for layers grown at high (700 °C) Tg is missing in layers grown at low (≲550 °C) Tg, while a new type of defects parallel to {001} and {113} lattice planes appear at these temperatures. The optimal Tg for a Ge content of 20-25 at. % was found to be close to 550-625 °C. Surface roughness for all the growth temperatures was found to be less than that for such a ‘‘smooth’’ technique as MBE. Photoluminescence studies revealed, to the best of our knowledge for the first time, two peaks on the low energy side in the neighborhood of the Si(TO) peak of the epilayers. The evolution of the intensity of these peaks is strongly correlated with the dynamics of strain relaxation and can be attributed to a set of dislocations at the SiGe/Si interface extending both to the epilayer and to the bulk Si.
Interfacial reactions in the Zr–Si system studied by in situ transmission electron microscopy
Authors: Tanaka Hiroyuki , Konno Toyohiko J. , Sinclair Robert , Hirashita Norio
Published: July 2002

The interfacial reactions in the Zr–Si system have been studied by in situ cross-section transmission electron microscopy (TEM) including high-resolution-mode energy-dispersive spectroscopy (EDS) and nanobeam electron diffraction (nanodiffraction). The as-deposited Zr film has a columnar structure and an amorphous interlayer is observed at the Zr/Si interface. The amorphous layer is found to grow during annealing at 400 °C. The growth of the amorphous layer consists of three stages: a rapid increase in the early stage, a gradual increase in the intermediate metastable stage, and saturation in the final stage. The kinetics at each stage are discussed with in situ TEM observation and ex situ EDS analysis. Annealing at 500 °C creates a ZrSi2 layer at the amorphous layer/Si interface. The phase and orientation relationship are determined from the nanodiffraction patterns. The ZrSi2 is found to grow layer by layer into the Si substrate via a ledge mechanism.
Electrical properties of heavily doped μc-Si:H
Authors: Di Nocera Anna , Mittiga Alberto , Rubino Alfredo
Published: July 2002

We have investigated the conduction in heavily doped μc-Si:H samples grown by glow discharge by measuring the conductivity and the Hall coefficient in a wide temperature range. We show that the conductivity of these materials cannot be satisfactorily interpreted using the models already present in the literature. We put forward a model in which the material is considered composed of small crystalline grains embedded in an amorphous tissue and in which potential barriers exist between adjacent grains due to the band discontinuities. A comparison between detailed calculations and transport data shows that the tunneling (probably aided by the presence of localized states) is the fundamental conduction mechanism.
Complex impedance spectroscopy for metal-semiconductor field-effect-transistor surface characterization
Authors: Charache G. W. , Maby E. W.
Published: July 2002

A new metal-semiconductor field-effect-transistor surface characterization technique is presented. The complex impedance (magnitude and phase) between the source and drain contacts is measured as a function of frequency and temperature. It is shown that the phase data exhibit peaks in the frequency spectrum that correspond to characteristic emission rates for surface-state traps. Measurements at different temperatures provide the energetic position and capture cross section of dominant traps. The technique provides a relatively rapid surface characterization tool in comparison to deep-level transient spectroscopy.
Magnetization of thin films with in-plane uniaxial anisotropy studied by microwave absorption
Authors: Rivoire M. , Suran G.
Published: July 2002

The resonance spectra of a thin film, which possesses an in-plane uniaxial anisotropy field Hk, exhibit one or two resonance modes when the external dc field is applied along the hard axis, depending upon the respective values of the signal frequency and the natural resonance frequency f0=γ√Hk(Hk+4πMs)/2π. When f≳f0 the low-field side mode is no longer resolved and instead a peak is detected on the absorption spectrum. It is shown that the location and the shape of this peak are determined by the magnetization process occuring along the hard axis. The theoretical computation is based upon the hypothesis that the magnetization arises by a pure rotational mechanism, and therefore follows the Landau–Lifshitz equation of motion. The experiments were performed on CoZr–rare-earth-substituted amorphous thin films. The full experimental spectra can be fitted numerically with the measured magnetic parameters if one takes the effect of the large-angle magnetization ripples in the vicinity of H≂Hk into account by a phenomenological expression. This computation also makes it possible to determine an effective fluctuating field δHeff, the magnitude of which correlates nicely with the local anisotropy Kloc deduced from the transverse bias initial susceptibility measurements.
Valence subband structure of 〈011〉-oriented quantum wells
Authors: Shechter G. , Shvartsman L. D. , Golub J. E.
Published: July 2002

We present semianalytical calculations of the valence subband structure for 〈011〉-oriented quantum wells of cubic semiconductors. Working in the Kohn–Luttinger formalism [Phys. Rev. 97, 869 (1955)], we present analytic expressions for the effective mass tensor and show the full subband dispersion for GaAs and InAs. In addition to the usual extremum at k∥=0, 〈011〉-oriented wells support saddle point spectra. In this case, the effective mass changes sign as a function of in-plane crystallographic direction.
Thermal expansion of textured polycrystalline aggregates
Authors: Dunn Martin L. , Ledbetter Hassel
Published: July 2002

We examine thermal expansion of textured polycrystalline aggregates and focus on polycrystals containing hexagonal monocrystals. Our analysis is based on the coupling of the Voigt–Reuss–Hill averaging procedures for elastic constants, Roe’s [R. J. Roe, J. Appl. Phys. 36, 2024 (1965)] framework for the mathematical description of the crystallite orientation-distribution function (ODF), and the exact connections of Hashin [Z. Hashin, J. Mech. Phys. Solids 32, 149 (1984)] and Schulgasser [K. Schulgasser, J. Mech. Phys. Solids 35, 35 (1987)] between the thermal-expansion coefficients of a polycrystalline aggregate and the corresponding polycrystal elastic constants, monocrystal elastic constants, and monocrystal thermal-expansion coefficients. The results are simple expressions for the polycrystal thermal-expansion coefficient in terms of the monocrystal elastic constants, thermal-expansion coefficients, and the coefficients in the expansion of the crystallite ODF in a series of generalized spherical harmonics. We present numerical results for zinc and zirconium polycrystals with textures typical of rolling and extrusion processes.
An analytic approximation with a wide range of applicability for electron initiated Auger transitions in narrow-gap semiconductors
Authors: Beattie A. R. , White A. M.
Published: July 2002

Theoretical calculations of Auger transition rates are complicated by the interconnection of energy and momentum conservation. The use of a flat valence band where the heavy holes have infinite mass decouples energy and momentum conservation and greatly simplifies the calculation. This flat valence band model has been used to obtain a simple analytic approximation for Auger transition rates. It requires just two parameters to cover a wide range of temperature and carrier Fermi levels (both degenerate and nondegenerate) and their values may be found either by comparison with an accurate calculation or from Auger lifetimes determined experimentally. The results have been applied to InSb and Cd0.2188Hg0.7812Te where good agreement with accurate theoretical values using realistic band structures has been attained. The same model has also been used to provide a simple analytic approximation for impact ionization probability rates, again agreeing well with accurately determined values. Such approximations will prove to be useful in modeling semiconductor transport effects and devices.
Structural and electrical characterization of heteroepitaxial lead zirconate titanate thin films
Authors: de Keijser M. , Cillessen J. F. M. , Janssen R. B. F. , De Veirman A. E. M. , de Leeuw D. M.
Published: July 2002

PbZrxTi1−xO3 films have been grown heteroepitaxially onto (001)SrTiO3 and SrRuO3/(001)SrTiO3 by organometallic chemical vapor deposition. As a start, the microstructure of PbZrxTi1−xO3 films on (001)SrTiO3 was studied as a function of the zirconium fraction, x. Rutherford backscattering spectrometry, including channeling experiments, and transmission electron microscopy have shown that the microstructure is dominated by the crystal structure of the PbZrxTi1−xO3. In the case of tetragonal PbZrxTi1−xO3 the films may contain a-axis oriented regions. These regions have not been observed for films with a composition giving a rhombohedral unit cell. Despite the rather large mismatch of rhombohedral PbZrxTi1−xO3 with the (001)SrTiO3, values as low as 4% for the minimum channeling yield have been obtained. For a rhombohedral film the ferroelectric properties have been measured. To this end a single crystalline PbZr0.8Ti0.2O3 film was grown onto (001)SrTiO3 provided with a heteroepitaxial SrRuO3 electrode grown by pulsed-laser deposition. A heteroepitaxial top electrode was grown onto the PbZrxTi1−xO3 using the same technique. The channeling minimum yield of the heteroepitaxial stack was 11%. The hysteresis loop saturates already at 1 V. Endurance up to 1012 cycles was observed without severe degradation of the ferroelectric properties.
Effect of dry etching and subsequent annealing of Si/SiGe/Si heterostructure
Authors: Swain P. K. , Misra D. , Thompson P. E.
Published: July 2002

Exposure to dry etching can deleteriously affect strained SiGe films by initiation of misfit strain discontinuity in the film as well as at the interface. In this paper annealing behavior of defects generated as a consequence of dry etching of Si/SiGe/Si heterostructure metal–oxide–semiconductor capacitors have been reported. Some samples were wet etched for comparison. Samples were annealed at four different temperatures after being etched, namely, 650, 700, 750, and 800 °C. Results of the investigations using deep-level transient spectroscopy reveal defect levels at 0.62, 0.57, 0.56, and 0.44 eV for the dry-etched samples and at 0.31, 0.43, 0.56, and 0.44 eV for the wet-etched samples annealed at the aforetold temperatures. The results could be explained on the basis of appearance of point defects condensing into dislocation loops which eventually shrink and disappear as the annealing temperature is increased. The estimated size of the dislocation loops agrees quite well with the experimentally measured values obtained for identically processed samples.
Magneto-optical recording on patterned substrates (invited)
Authors: Gadetsky S. , Erwin J. K. , Mansuripur M. , Suzuki T.
Published: July 2002

Patterning of glass or plastic substrates in the form of shallow square patches is a promising method of increasing the storage density for magneto-optical disks. The sidewalls of the patches pin the reverse-magnetized domains that develop in these samples. Confinement of domains within the patch boundaries during thermomagnetic recording has also been demonstrated. We have measured polarization conversion of the incident light on the sidewalls of the patches; a method to reduce the amount of such polarization conversion is proposed in this article.
First-principles calculation of the electronic and magnetic properties of Nd2Fe17−xMx (M=Si, Ga) solid solutions
Authors: Huang Ming-Zhu , Ching W. Y.
Published: July 2002

We have studied the electronic and magnetic properties of the Si and Ga substituted Nd2Fe17 compounds by means of first-principles calculations. The results show that for the Nd2Fe17−xSix (x=1,2,3, and 4) series, the total Fe moments per cell decrease as x increases but the average local Fe moments actually increase up to x=3, except for the 18h site which is the most preferable site for substitution. For Nd2Fe15Ga2, the moments on every Fe site increase. These results are also compared with the earlier calculation on Nd2Fe17−xAlx (x=1,2, and 4). From the spin-polarized density of states (DOS) and local DOS for Si, Al, and Ga, it is shown that different substituting elements can change the electronic structure of Nd2Fe17 crystal substantially because of the slightly different interactions with the Fe atoms, and the change in the lattice constants. Estimation of Curie temperatures based on the spin-fluctuation model shows qualitative agreement with experiments.
Simultaneous occurrence of multiphases in the interfacial reactions of ultrahigh-vacuum-deposited V and Zr thin films on (111)Si
Authors: Lin J. H. , Hsieh W. Y. , Chen L. J.
Published: July 2002

Simultaneous occurrence of multiphases was observed in the interfacial reactions of ultrahigh-vacuum (UHV) -deposited V and Zr thin films on (111) Si by high-resolution transmission electron microscopy in conjunction with fast Fourier transform diffraction analysis and image simulation. For the V/Si system, an amorphous interlayer (a-interlayer), V3Si and V5Si3, were found to form simultaneously in samples annealed at 500 °C for 60 min. For the Zr/Si system, an a-interlayer, Zr5Si3 as well as FeB and CrB types of ZrSi, were observed to form in samples annealed at 440 °C for 40 min. The formation of multiphases appeared to be quite general in the initial stages of interfacial reactions of UHV-deposited refractory metal and rare-earth–metal thin films on silicon. The observation of the prevalence of the formation of multiphases in the initial stages of thin film reactions called for a reexamination of generally accepted ‘‘difference’’ in reaction sequence between bulk and thin-film couples.
Modeling voltage distribution and current limit in Ag/Bi2Sr2Can−1CunO2n+4
Authors: Fang Y. , Danyluk S. , Cha Y. S. , Lanagan M. T.
Published: July 2002

Current and voltage distributions in Ag/Bi2Sr2Can−1CunO2n+4 (BSCCO) composites are calculated from an analytical model that is based on interfacial resistivity and geometric parameters. The model was verified by measuring the voltage distribution along Ag/Bi2Sr2Ca2Cu3O10 bars that were fabricated by sinter forging between 400–845 °C and 5–10 MPa. The results show that the solutions depend on a single dimensionless parameter, λL, where L is the length of the interface and λ is associated with resistivity of the Ag (ρs), interfacial resistivity (ρi) between the Ag and the BSCCO, and thickness of the Ag (ds). The voltage drop across the interface is proportional to (ρiρs/ds)1/2. The model was extended for powder-in-tube tapes to examine the effects of cracking on critical current density.
Hydrogen determination in Si-rich oxide thin films
Authors: Monelli A. , Corni F. , Tonini R. , Ferrari C. , Ottaviani G. , Zanotti L. , Queirolo G.
Published: July 2002

Quantitative in-depth distribution of the elements contained in silicon-rich oxide thin films deposited on single-crystal silicon by low temperature plasma-assisted deposition has been performed by a combination of various MeV ion beam techniques. The quantity of oxygen and nitrogen has been measured by nuclear reactions, the silicon content has been determined by Rutherford backscattering, and elastic recoil detection was used for hydrogen. All the samples contain not only Si and O, but also N and H, which are residuals from the reactions involved in the deposition process. We did find that the MeV beam used in the nuclear techniques can induce a process of hydrogen desorption, which causes the measured H content to be a function of the He dose received by the sample. This phenomenon, not previously reported, must be taken into account to give the correct H content. The study of the kinetics of the He-induced hydrogen desorption has been used to correct the experimental data and to determine the original hydrogen content. The correction factor is in most of the cases close to 2 and outside any experimental error. Moreover the studies of the kinetics give information on the kinds of hydrogen complexes contained in the films. The results suggest that, on the basis of the strength of the binding energies, hydrogen is present in at least two different configurations, weakly and strongly bonded. In the first configuration hydrogen is easily desorbed either under the action of the ion beam or of the heat treatment at 600 °C, in the second, hydrogen is lost only after treatment at 900 °C.
Ordering of Tl and Bi in Bi-doped Tl-1223 high T
c superconductors
Authors: Lee E. , Wu S. , Zhou W. , Liang W. Y.
Published: July 2002

High Tc superconducting materials with nominal compositions of Tl1-xBix(Sr, Ba)2Ca2Cu3O9 (0≤x≤0.5) have been investigated using x-ray powder diffractometry and electron microscopy. It was found that Tl and Bi cations were in an ordered arrangement in the (Tl, Bi)–O atomic layer. Although the proposed supercell is 4a×4b×4c derived from the basic unit cell of the undoped TlBa2Ca2Cu3O9 (Tl-1223), the basic two-dimensional superlattice in the (Tl, Bi)–O layer is 2a×4b or 4a×2b with Bi cations at the center and the corner of the rectangle, giving an ideal and preferred composition with the Tl:Bi ratio of 3:1. Accordingly, the maximum Bi doping for Tl in the solid solution is 25%. The proposed model of the superstructure was supported by computer simulation of electron diffraction patterns. The mechanism of formation of the superstructure is discussed.
Determination of beam affected area and degradation of cathodoluminescence in GaAs induced by focused ion beam irradiation
Authors: Furuya Kazuo , Saito Tetsuya
Published: July 2002

The beam affected area of n-type GaAs, induced by the focused ion beam (FIB) irradiation, and the corresponding degradation in cathodoluminescence (CL) were directly determined in a FIB-interfaced 200 keV transmission electron microscope (TEM). Square-areas, 2–10 μm on a side, were microfabricated by a 25 keV Ga-FIB on both thick and thin portions of a GaAs foil, for CL and TEM observations, respectively. Clear boundaries between FIB-irradiated and unirradiated areas in TEM and scanning electron microscopy (SEM) indicated a sharp transition from the single crystal to amorphous and/or polycrystals, and less effect of the beam broadening of the FIB in the scale of the present experiment. The formation of Ga enriched precipitates due to the selective etching of As was seen on the surface. A room-temperature CL image obtained from the fabricated area showed a decrease in the peak intensity at 866 nm as the exposure time of FIB increased. The diffuse CL image of the boundary of the FIB fabrication implied the importance of the strain induced by the FIB.
Electron density measurement in a rf helium plasma by laser-collision induced fluorescence method
Authors: Dzierżȩga K. , Musiol/ K. , Benck E. C. , Roberts J. R.
Published: July 2002

Laser-collision induced fluorescence (LCIF) is the emission of light from states that have been populated by laser excitation and a subsequent collision. By simultaneously measuring the LCIF from two different states, it is possible to determine both the electron density and temperature of the low energy bulk electrons within a plasma. This method is described in detail and has been applied in the determination of the total, temporally averaged, and spatially resolved electron density in a rf (13.56 MHz) helium discharge in the Gaseous Electronics Conference reference cell. The rf discharge was operated at pressures P=33.3–133.3 Pa (0.25–1.0 Torr) and peak-to-peak voltages of Vpp=75–300 V were applied. We found the total electron density varied from 1.8×108 cm−3 at P=33.3 Pa and Vpp=75 V to 4.0×1010 cm−3 at P=133.3 Pa and Vpp=300 V. A comparison of results from different experiments has been made.
Transient voltage behavior of free-standing porous silicon layers
Authors: Pennelli Giovanni
Published: July 2002

The transport in free-standing porous silicon layers has been investigated by applying a current step to samples with a four contact geometry and by measuring the voltage time behavior across the voltage contacts. The voltage response showed a transient waveform typical of a network with two time constants and a continuous regime for very long times. The values of the time constants τ1 and τ2 differ noticeably, τ1 and τ2 being in the range 0.1–10 milliseconds and 10–150 milliseconds, respectively. The dependence of the time constants on the current step amplitude and on the temperature has been also investigated. Both τ1 and τ2 showed a temperature activated behavior, which can be related to the resistance behavior vs absolute temperature.
Influence of lower-valent A-site modifications on the structure-property relations of lead zirconate titanate
Authors: Tan Qi , Xu Z. , Li Jie-Fang , Viehland Dwight
Published: July 2002

Studies of the structure-property relationships have been performed for lead zirconate titanate as a function of concentration of a lower-valent A-site substitution. Investigations focused on the system (Pb1−yKy)(Zr0.65Ti0.35)O3−y/2 for 0<y<0.20. These studies revealed modest changes in the dielectric response with increasing K1+ concentration. However, no evidence of relaxor ferroelectric characteristics was found. Transmission electron microscopy (TEM) studies were also performed. These investigations revealed strong changes in the ferroelectric domain structures with moderate increases in K1+ concentration. For K content ≳4%, fine ferroelectric domains were found, which possessed a strong degree of wavy character. Increasing K1+ concentration resulted in further decreases in domain size. Sawyer–Tower polarization studies then demonstrated a strong reduction in the switchable polarization with increasing K1+ concentration, indicating that the polarization becomes pinned. Hot-stage TEM investigations were then performed. These studies revealed evidence for a pinning of the polarization into fine wavy domains at temperatures significantly below that of the ferroelectric transformation. These results are then compared to recent investigations of the influence of La3+ on the same base composition. We believe that the differences between lower- (K1+) and higher- (La3+) valent A-site modifications can be explained on the basis of the influence of quenched (which become mobile on heating in the ferroelectric phase) versus randomly quenched (above the ferroelectric transformation) defect complexes on the evolution of polar order.
Growth of metal oxide thin films by laser-induced metalorganic chemical vapor deposition
Authors: Tokita Koji , Okada Fumio
Published: July 2002

The growth of metal oxide thin films by laser-induced metalorganic chemical vapor deposition was investigated by changing wavelength, power, repetition rate, and irradiation angle of the excimer laser. When O2 was used as an oxidizing gas with 193 or 248 nm irradiation, amorphous TiO2 and crystalline PbO films were obtained in the laser-irradiated area of Si substrates from the parent metalorganic compounds, Ti(O–iC3H7)4 and (C2H5)3PbOCH2C(CH3)3, respectively. In contrast, no ZrO2 film could be formed from Zr(O–tC4H9)4. One-photon formation of TiO2 films was confirmed from laser power dependence measurements. The maximum growth rate of 0.05 Å per laser pulse was compared with that estimated by a simple surface reaction model, according to which the slow growth rate is due to the small absorption cross section of Ti(O–iC3H7)4 and mild fluence of laser irradiation. In experiments of ozone gas excitation by KrF laser, a SiO2 film was obtained by gas-phase reactions of the oxygen radical, O(1D), with Si(O–C2H5)4. The direct patterning of TiO2 and PbO films as well as the possibility of producing patterned PbTiO3 film was demonstrated. The growth of the patterned SiO2 film was prevented by gas-phase diffusion of intermediates.
A simple three-dimensional free electron laser code
Authors: Dattoli G. , Dipace A. , Sabia E. , Torre A. , Voykov G. K. , Carpanese M.
Published: July 2002

We present a simple and inexpensive computer code for free electron laser (FEL) operation that includes high gain and inhomogeneous broadening effects. The code is based on analytical formulae, obtained within the context of the theory of FEL approximants, requires negligible CPU time, and may run on small computers.
Efficient second-harmonic generation in novel Cerenkov type nonlinear-optical polymer waveguides
Authors: Schmitt K. , Benecke C. , Schadt M.
Published: July 2002

New cinnamic acid type nonlinear-optical (NLO) photopolymers and their use in frequency doubling Cerenkov waveguides are reported. Cerenkov configurations are shown to be particularly appropriate for efficient second-harmonic generation in NLO polymers. Analysis of Cerenkov waveguides in three-layer configurations allows optimization of their performance with respect to polymer layer thickness and substrate parameters. The NLO efficiencies predicted from the model and from independently determined NLO material parameters are qualitatively in agreement with the experimentally observed large Cerenkov NLO efficiency of 0.2% W−1 cm−1. We show further that improved device performance results from novel four-layer waveguide configurations.
Tunneling and impact ionization at high electric fields in abrupt GaAs p-i-n structures
Authors: Benz C. , Claassen M. , Liebig D.
Published: July 2002

GaAs p-i-n structures with very abrupt doping transitions and different lengths of the quasi-intrinsic zone have been fabricated by molecular beam epitaxy technology. For structures with ultra-thin intrinsic zones, tunnel currents were determined experimentally up to electric fields of 1.9 MV/cm, in good agreement with a modified Kane model. The difference of calculated tunnel current and measured total current in structures which also perform impact ionization is used to fit a Monte Carlo simulation program at high electric fields. Resulting idealized homogeneous-field ionization rates are given which are experimentally verified up to fields of 1.3 MV/cm.
Thermoelectronic breakdown with pressure and space charge effects in polyethylene
Authors: Zebouchi N. , Hoang T. G. , Ai Bui
Published: July 2002

The electrical breakdown of low density polyethylene films 100 μm thick was investigated experimentally and theoretically under high hydrostatic pressure (1 to 500 bar) at temperatures of 20, 70, and 90 °C. The theoretical analysis was based on combining the two basic mechanisms of thermal and electronic breakdowns by introducing a new parameter, pressure. This model mainly takes into account the transient current using the hopping transport phenomenon and local electric field distortion due to the presence of space charges in the material. The results of simulation show good agreement with experiments compared to our previous works on the modeling of electrical breakdown in which the effect of space charges was neglected.
Small-signal gain measurements in a discharge-pumped F2 laser
Authors: Kitamura Tahei , Arita Yoshihiko , Maeda Keisuke , Takasaki Masayuki , Nakamura Kenshi , Fujiwara Yoshinao , Horiguchi Shiro
Published: July 2002

In a discharge-pumped F2 laser, time-resolved net-small-signal gain has been measured to study the population inversion by employing an oscillator-amplifier configuration. Characteristics of the gain and the fluorescence are studied under various conditions in two cases of He and Ne buffer gases. The peak of the net-small-signal gain in He/F2 and Ne/F2 show values of about 17%/cm and 9%/cm, respectively. The formation processes of the upper laser level F2(D′) are discussed on the basis of these experimental results.
Controlled growth of SiO2 tunnel barrier and crystalline Si quantum wells for Si resonant tunneling diodes
Authors: Wei Yi , Wallace Robert M. , Seabaugh Alan C.
Published: July 2002

Two methods for producing Si-oxide barriers upon which crystalline Si layers can be grown are presented. One method entails oxide island nucleation on a clean vicinal Si(001) surface. The second method makes use of void formation in ultrathin oxides on the Si(100) surface at elevated temperatures. Either method results in an oxide barrier which is porous and the exposed Si within these pores can serve as a way to seed c-Si overgrowth. We demonstrate that it is feasible to grow crystalline Si overlayers on top of such porous oxide barriers, while on the continuous Si-oxide surface, only amorphous or nanocrystalline Si layer overgrowth can be achieved. The controlled oxide growth and Si overgrowth on the oxide can find possible applications in Si-based resonant tunneling devices, optoelectronics, and other Si-based nanoelectronics.
Study of magnetic properties of magnetic force microscopy probes using micronscale current rings
Authors: Kong Linshu , Chou Stephen Y.
Published: July 2002

Metal rings with inner diameters of 1 and 5 μm, fabricated using electron-beam lithography, were used to calibrate magnetic force microscopy (MFM). A MFM tip’s effective magnetic charge, q, and effective magnetic moment along the tip long axis, mz, can be determined by the current flowing in the ring. The magnetic moments in the directions transverse to the tip’s long axis were estimated by a straight current wire. It was found that for a silicon tip coated with 65 nm thick cobalt on the side, q=2.8×10−6 emu/cm, mz=3.8×10−9 emu, and mx=my=10−13 emu, which are negligible compared with mz. Furthermore, the tip’s sensitivity to the second derivative of the magnetic field was found to be about 0.1 Oe/nm2.
Ferromagnetic resonance of micrometer-sized samples
Authors: Zhang S. , Oliver S. A. , Israeloff N. E. , Widom A. , Vittoria C.
Published: July 2002

Submicrometer-sized slotline and coplanar waveguide devices were used for ferromagnetic resonance (FMR) measurement on submicrometer-sized magnetic fine particles. FMR measurements were taken on individual magnetic disks, having diameters ranging from 50 to 1 μm, using either a slotline–coplanar waveguide (CPW) junction device or a coplanar waveguide transmission line junction device. The magnetic samples were etched from a 105 nm thick amorphous Fe80B15Si5 film on a quartz substrate. Absorption modes, which were measured with the submicrometer-sized slotline CPW, followed the Kittel magnetic resonance model for frequencies from 1 to 20 GHz for the 1 μm diam disk. By detecting the second harmonic of the lock-in amplifier’s reference signal, we have measured disk samples as small as 1 μm in diameter. We estimated that the sensitivity is of the order of 3×109 spins, almost four orders of magnitude better than the sensitivity of a standard electron paramagnetic resonance system.
Fast current limitation by conducting polymer composites
Authors: Strümpler R. , Maidorn G. , Rhyner J.
Published: July 2002

The transition of materials from low resistivity to comparatively high resistivity can be utilized for current limitation, enabling permanent fuses that do not have to be replaced after an overload or short-circuit operation. An interesting class of materials for this purpose are particulate filled polymer composites with a strong positive temperature coefficient (PTC) of resistivity. If an applied current becomes too high, the PTC element is heated to its critical temperature and trips from the conducting into the insulating state. The dynamic heating of the composite upon current flow is described by a one-dimensional model. It is predicted that the heating of a composite depends on the size of the filler particles. Smaller filler particles should allow a faster heating and, hence, a better limitation of the current. Experimental verification is performed using composite of TiB2 particles in a polyethylene matrix. Commercial TiB2 powders with different particle-size distributions between 1 and 200 μm were used. The specific resistivity of the composites is small, in the range of 0.01–0.02 Ω cm. Around the melting temperature of the polymer, the resistivity increases within only 20 °C by seven orders of magnitude. In order to verify the expected dependence of the switching dynamic on the filler particle size, the tested elements had comparable electrical characteristics. Samples were prepared having, to a certain degree, the same specific resistivity, cross section, and total resistance. Free parameters were the length, and for some samples, the filler content. Short-circuit experiments show that for decreasing particle size the time until the material trips into the high-resistive state becomes shorter. The best current limitation occurs for composites containing particles in the range of 1–45 μm. Current limitation starts already after 150 μs, and a current density of up to 10 kA/cm2 can be switched off within a further 200 μs. The experiments are in excellent agreement with the predictions from theory. Due to the low resistance in the cold state and the very fast limitation of electrical currents, PTC elements based on conducting polymers can be highly attractive for power applications.
Heterogeneous free-surface profile of B4C polycrystal under shock compression
Authors: Mashimo Tsutomu , Uchino Masakazu
Published: July 2002

Observations of the free-surface behavior under shock compression by the gapped-flat mirror method were performed on B4C and Si3N4 ceramics to study their shock-yielding properties. Jagged profiles of the moving free-surface in the plastic region, with a special scale of about one mm and a maximum local displacement of a few 10s of μm, were observed for B4C polycrystals. Similar profiles for Si3N4 polycrystals were smooth. Such profiles for B4C polycrystals were also observed in the elastic region. It is suggested that these observations reflect the heterogeneous nature of shock compression in solids, and further indicate that a macroscopic slip system plays an important role in the elastoplastic transition of B4C material under shock compression and decompression.
Photothermal study of subsurface cylindrical structures. I. Theory
Authors: Ocariz A. , Sanchez-Lavega A. , Salazar A.
Published: July 2002

A study of the photothermal signal generated by subsurface cylindrical structures is presented in two parts: theoretical modelization (Part I), and experimental measurements on calibrated samples (Part II). In this first part we develop a theoretical study of the scattering of planar and spherical thermal waves by a buried single infinite cylinder, parallel to the sample surface, as a function of its size and depth, and the thermal characteristics (thermal conductivity and diffusivity) of both the sample and cylinder. Temperature field solutions to the heat conduction equation are explored in two ways: by means of the Green’s function method and by a series expansion. We discuss the limits and accuracy provided by both types of calculations, and we compare the resulting temperature field for extended and pointlike illumination. Our study shows that photothermal methods allow one to locate and characterize the geometrical and thermal properties of the buried cylinder.
Wet thermal oxidation of AlAsSb alloys lattice matched to InP
Authors: Legay P. , Petit P. , Le Roux G. , Kohl A. , Dias I. F. L. , Juhel M. , Quillec M.
Published: July 2002

Wet thermal oxidation of AlAsSb was investigated. The oxidation kinetics was studied as a function of temperature and oxidation duration. An expression to allow accurate determination of the oxide depth for any temperature and time was established. Secondary ion mass spectrometry profiles and x-ray diffraction were used to demonstrate that an interfacial layer, composed of pure Sb and As and textured on InP, is formed during the oxidation process.
Switching of negative and positive dielectro-anisotropic liquid crystals by in-plane electric fields
Authors: Oh-e Masahito , Yoneya Makoto , Kondo Katsumi
Published: July 2002

This article compares switching behaviors between negative (Nn) and positive (Np) dielectro-anisotropic nematic liquid crystals driven by an in-plane electric field which is generated with interdigital electrodes. Even for Np type liquid crystals, excellent viewing angle characteristics were obtained as expected. Theoretical descriptions of the switching principle, i.e., threshold behavior and response mechanism, could be applied to both the Nn and Np type liquid crystals. However, the orientational deformation of the Np type liquid crystals caused by a distorted electric field which occurred near the edges of electrodes was not the same as that of the Nn type liquid crystals. The switching of the Np type liquid crystals near the edges of the electrodes by this field was followed by the in-plane switching of the liquid crystals between the electrodes. This remarkably distinctive dynamical behavior implied a difference in the response of the longitudinal axes for Nn and Np type liquid crystal molecules. The longitudinal axes of the latter seemed to be sensitive to the electric field component perpendicular to the substrates when applying the in-plane electric field. Furthermore, these experimentally obtained results were supported by computer simulations which analyzed the liquid crystal director distribution and transmittance pattern from edge-to-edge of a pair of electrodes when applying the in-plane electric field.
Supercooling and structural relaxation in amorphous Ge films under pulsed laser irradiation
Authors: Solis J. , Siegel J. , Afonso C. N. , Jimenez J. , Garcı́a C.
Published: July 2002

Melting and rapid solidification has been induced in amorphous Ge films by irradiation with nano- and picosecond laser pulses. The degree of structural relaxation of the rapidly solidified amorphous material has been investigated both by determining the minimum fluence required for surface melting and by means of Raman spectroscopy. The results evidence that the degree of relaxation of the rapidly solidified material is controlled by several parameters such as the duration and fluence of the laser pulse, and the thermal conductivity of the substrate, all of them affecting the supercooling prior to solidification. It is demonstrated that both relaxation and derelaxation can be induced if the above mentioned parameters are properly selected. The degree of relaxation is observed to decrease as the supercooling increases.
Photorefractive effect in LiNbO3 crystals enhanced by stoichiometry control
Authors: Kitamura K. , Furukawa Y. , Ji Y. , Zgonik M. , Medrano C. , Montemezzani G. , Günter P.
Published: July 2002

LiNbO 3 single crystals with a composition close to stoichiometry are grown by using a double crucible Czochralski method equipped with an automatic powder supply system. The photorefractive properties are compared with those of LiNbO3 crystals grown from the congruent composition melt. Two beam coupling experiments show that both nondoped and Fe-doped stoichiometric crystals exhibit stronger photorefractive effect than the Fe-doped congruent crystals.
Shifts and splitting of energy bands in elastically strained InGaP/GaAs(111)B epitaxial films
Authors: Alperovich V. L. , Bolkhovityanov Yu. B. , Jaroshevich A. S. , Katkov A. V. , Revenko M. A. , Scheibler H. E. , Terekhov A. S.
Published: July 2002

Strain-induced shifts and splitting of energy bands are studied by optical techniques in compressively strained pseudomorphic InxGa1−xP films grown by liquid phase epitaxy on lattice-mismatched GaAs(111)B substrates. The elastic strains are measured by the x-ray diffraction technique and reach the value of 0.92%. The splitting of the valence band is revealed as a doublet in the derivative of a photocurrent spectrum which is precisely measured on the semiconductor-electrolyte interface near the fundamental absorption edge. The maximum splitting reaches 45 meV. The sublinear behavior of the valence band splitting versus elastic strain is clearly observed. This nonlinearity is explained by the interaction between the strain-split subband with J=3/2, mJ=±1/2 and the spin-orbit split subband (J=1/2, mJ=±1/2). The experimentally measured dependences of shifts and splitting on the magnitude of strain are well described by the theoretical calculations.
Investigation of hydrogen induced changes in SrBi2Ta2O9 ferroelectric films
Authors: Zafar Sufi , Kaushik Vidya , Laberge Paul , Chu Peir , Jones Robert E. , Hance Robert L. , Zurcher Peter , White Bruce E. , Taylor Deborah , Melnick Bradley , Gillespie Sherry
Published: July 2002

The effect of hydrogen on strontium bismuth tantalate (SrBi2Ta2O9; SBT) ferroelectric capacitors is investigated. Using several analytical techniques such as x-ray diffraction, electron diffraction, Auger electron, scanning and transmission electron microscopies, the structural and compositional changes in the ferroelectric film are studied as a function of annealing gas and temperature. The mechanism for hydrogen induced damage to the capacitor is identified. Measurements show that the hydrogen induces both structural and compositional changes in the ferroelectric film. Hydrogen reacts with the bismuth oxide to form bismuth and the reduced bismuth diffuses out of the SBT film causing the electrodes to peel.
Theoretical analysis of disorder effects on electronic and optical properties in InGaAsP quaternary alloy
Authors: Bouhafs B. , Aourag H. , Ferhat M. , Zaoui A. , Certier M.
Published: July 2002

The effects of structural and chemical disorder on electronic and optical properties of InGaAsP quaternary alloy are studied on the basis of a modified virtual crystal approximation calculated within a simple tight-binding sp3s* theory, which incorporates compositional disorder as an effective potential. Using a minimal set of fitting parameters, we show that such an approach provides analytical results for calculating energy gaps and bowing parameters. We show that the calculated bowing parameter agrees reasonably well with experimental data. The essential features of structure and disorder-induced changes in electronic and optical structure are exhibited in the sp3s* results by two characterization parameters: the subband energy spacings, and the density of states. The changes in each of them are found to depend on the interrelated trends of structure and disorder effects.
Microwave generation in a high voltage triggered pseudospark discharge experiment
Authors: Ramaswamy K. , Destler W. W. , Rodgers J.
Published: July 2002

A set of five experiments to study the microwave emission accompanying the high voltage pseudospark discharge are reported. The generation of the microwaves in all cases but one is attributed to electron beam-plasma interaction. In the other case, Cherenkov radiation was also considered as a possible mechanism of generation. Initial simulation studies were carried out which predict a background peak plasma density on the order of 1–2×1013cm−3.
Ultrasonic damping in pure aluminum at elevated temperatures
Authors: Johnson Ward
Published: July 2002

Ultrasonic resonant damping was measured in polycrystalline 99.999% pure aluminum spheres between 22 and 465 °C at frequencies between 0.4 and 2.0 MHz. In fully annealed samples, the damping increases monotonically with temperature in a manner similar to that reported in numerous lower-frequency studies of a variety of materials. However, the activation energy of this increase is significantly less than values reported for aluminum at lower frequencies, and the frequency dependence is much different; these facts suggest that a different physical mechanism dominates the damping in the higher frequency range. The dependences on temperature, frequency, and vibrational amplitude are consistent with an anelastic dislocation mechanism. During recovery and recrystallization of cold-worked material, the damping drops irreversibly with a time dependence matching that expected for the dislocation density.
Dynamical studies of gratings formed in polymer-dispersed liquid crystal films
Authors: Fuh Andy Y.-G. , Ko T.-C. , Tsai M.-S. , Huang C.-Y. , Chien L.-C.
Published: July 2002

The use of polymer-dispersed liquid crystal films to fabricate gratings was demonstrated. The written gratings are permanent, but are electrically switchable. In this article, we report the results obtained from the dynamical studies of gratings formed in polymer-dispersed liquid crystal films having various liquid crystal-polymer mixing ratios. The results showed that during the initial phases of formation, a thermal grating effect dominated. Later, it was offset or quenched by the photopolymerization effect which eventually dominated and determined the characteristics of the final grating.
Critical thickness and strain relaxation in lattice mismatched II–VI semiconductor layers
Authors: Pinardi K. , Jain Uma , Jain S. C. , Maes H. E. , Van Overstraeten R. , Willander M.
Published: July 2002

Critical thickness hc has been calculated for capped and uncapped lattice mismatched II–VI semiconductor epilayers. Both the old equilibrium theory and the improved theory have been used. The calculated values are compared with the experimental data on epilayers of several II–VI semiconductors and alloys. The observed values of hc are larger than the calculated values, a result similar to that observed with GeSi and InGaAs strained layers. The discrepancy is attributed to the difficulty in nucleating the dislocations. Strain relaxation in layers with thickness h>hc is also calculated. Observed strain relaxation in ZnSe layers grown on (100) GaAs shows good agreement with the equilibrium theory. In other cases, the observed relaxation is sluggish and the residual strain is larger than the calculated value. Many authors have observed that strain near the surface of the II–VI epilayers is small and increases as the depth increases. We describe an improved model to explain this observation. The agreement between the prediction of our model and the observed strain distribution is excellent. A new model based on continuum elasticity theory is described to explain strain oscillations during the initial stages of growth of highly mismatched layers. In highly mismatched layers, the dislocations are distributed uniformly. A model to interpret this observation is suggested.