bluuebunny/crossref_metadata_2025 huggingface dataset:
Phase equilibria of the Ga–Ni–As ternary system
Authors: Ingerly D. B. , Swenson D. , Jan C.-H. , Chang Y. A.
Published: July 2002

Phase equilibria were investigated in the Ga–Ni–As ternary system, with particular emphasis on the regions of technological importance to Ni/GaAs electrical contacts. A 600 °C Gibbs isotherm was constructed using x-ray-diffraction analysis and electron probe microanalysis of annealed samples. Additionally, three isopleths (NiAs–GaAs, NiGa–NiAs, and NiGa–GaAs) and a partial liquidus projection were established using differential thermal analysis and metallography. These data were utilized to clarify some discrepancies in the literature pertaining to the constitution of the Ga–Ni–As system, particularly questions about the existence of ternary phases. It was demonstrated that at 600 °C, previously reported ternary phases were actually specific compositions of the binary phase, NiAs, which exhibits significant ternary solubility. Additional x-ray-diffraction and differential thermal analysis experiments suggested that superlattice structures based on the NiAs structure may become stable at lower temperatures. A ternary eutectic reaction was shown to occur at 810±5 °C, with eutectic point at the composition Ni0.48Ga0.30As0.22. The existence of this eutectic reaction has important ramifications for the development of Ni-based electrical contacts to GaAs because any metallization scheme with a composition within the region bounded by NiGa, NiAs, and GaAs, as well as elemental Ni, will experience at least partial liquid formation at temperatures greater than 810 °C.
Quantum chaos in a double barrier heterostructure
Authors: McNary A. Jason , Puri Ashok
Published: July 2002

A closed, quantum, double barrier, GaAs/AlGaAs heterostructure is considered, and the dynamical behavior of an electron cloud moving in the heterostructure is examined. The motion of the electron cloud is represented by a mean field approximation, thus we have a representation of a nondissipative system which mixes in some region of its phase space, showing some chaotic characteristics. With the addition of the potential term α〈Q(t)〉, the nonlinear, time-dependent Schrödinger equation is solved for the amount of charge trapped in the heterostructure. As the nonlinear parameter α increases, we find a tenfold increase in the Lyapunov exponent which provides a quantitative measure of the chaos inherent in the system. The phase space plots, autocorrelation function, and power spectrum are also discussed as a function of α.
Simulation of one-dimensional quasi-random gratings for quantum well infrared photodetectors
Authors: Xing B. , Liu H. C.
Published: July 2002

We perform a simulation of one-dimensional quasi-random gratings for quantum well infrared photodetectors. The simulation reveals the trade-off between the grating induced intersubband absorption efficiency and the resulting spectral response range for normal incident radiation. By controlling the degree of grating quasi-randomness, one can optimize the absorption for the desired spectral response range. The general features in the simulation results can be used as guidelines in designing two-dimensional quasi-random gratings for the fabrication of focal plane imaging arrays.
Charge dynamics and poling in glass waveguides
Authors: Statman David , Georges James A.
Published: July 2002

Germanosilicate glass waveguides were electrically poled using interdigitated electrodes. This poling resulted in a quasiphase-matched second-order susceptibility. Second-harmonic light was observed. Measured intensities were one to two orders of magnitude smaller than expected based on previous electric-field-induced second-harmonic (EFISH) generation studies with the same interdigitated electrodes. The dynamics of the EFISH signals were studied. Exponentially decaying signals suggest that through charge migration, via dielectric relaxation and electronic drift, a compromising field is established during the poling. This results in a weaker second-order susceptibility.
On the formation of inhomogeneities in epitaxial CoSi2 layers grown from the interaction of Co/Ti bilayers with Si 〈100〉 substrates
Authors: Cardenas J. , Zhang S.-L. , Svensson B. G. , Petersson C. S.
Published: July 2002

The redistribution of titanium during the formation of epitaxial CoSi2, grown from the reaction of Co(20 nm)/Ti(10 nm) bilayers with Si 〈100〉, has been investigated. Annealing of Co/Ti/Si structures, at temperatures between 850 and 1050 °C, is shown to be associated with the growth of an inhomogeneous CoSi2 layer having Ti-rich surface layer(s) on top. The formation of inhomogeneities in the CoSi2 layer is conclusively attributed to the presence of Ti-rich surface layer(s). It is shown that smooth and morphologically stable CoSi2 layers can be grown by removing these surface layers followed by a high-temperature treatment in nitrogen atmosphere. We propose that the underlying mechanism for the inhomogeneity formation within the CoSi2 layer is a nucleation-controlled process, induced by an anticipated reaction between the CoSi2 layer and Ti-rich phases near the surface.
Light scattering in p-type GaAs:Ge
Authors: Muñoz-Hernández R. A. , Jiménez-Sandoval S. , Torres-Delgado G. , Roch C. , Chen X. K. , Irwin J. C.
Published: July 2002

We present in this work the results of a Raman spectroscopy study on the plasmon–phonon coupling in Ge-doped p-type gallium arsenide. A series of polarized Raman scattering experiments were carried out on epitaxial films grown by liquid-phase epitaxy on (100) GaAs substrates at 20, 100, and 300 K. The films were p type with free hole densities varying in the range of 5×1017–1×1020 cm−3. Under the scattering configurations employed, the longitudinal optical (LO) mode is forbidden for crossed polarization while the transverse optical (TO) mode is forbidden for both parallel and crossed polarizations. However, all the polarized Raman spectra showed two peaks with frequencies close to the TO and LO phonons of semi-insulating GaAs. The appearance of such forbidden modes was accounted for with a theoretical model which considers phonon–plasmon coupled modes with wave vectors much larger than those given by the regular q≊0 wave vector transferred by photons. Ionized acceptor impurities provide such additional wave vector transfer through elastic scattering of the photoexcited electrons and holes. It is demonstrated that the experimental values for position and linewidth of the peaks are well described by the theoretical calculations when Fröhlich-type and deformation potential mechanisms are considered as means of interaction.
Ultrasonic spectroscopy in polymeric materials. Application of the Kramers–Kronig relations
Authors: Zellouf D. , Jayet Y. , Saint-Pierre N. , Tatibouët J. , Baboux J. C.
Published: July 2002

The propagation of ultrasonic plane waves through a material can be expressed by a transfer function related to the attenuation and the velocity dispersion. These acoustical parameters are determined from ultrasonic spectroscopy. Experiments were performed on different polymers and for the particular materials we have tested it was observed that amorphous materials can be considered as low-dispersive media, whereas in semicrystalline materials the velocity dispersion cannot be neglected. These experimental results are compared with those deduced from the Kramers–Kronig relations. These analytical expressions, assuming the causality conditions of every physical system, are verified for different materials proving the validity of our measurements. Other interests of the Kramers–Kronig relations are also discussed.
Guiding of electrical discharges under atmospheric air by ultraviolet laser-produced plasma channel
Authors: Miki Megumu , Wada Atsushi
Published: July 2002

Guiding and triggering of electrical discharges in 0.34 m air gaps by an ultraviolet laser-produced plasma have been demonstrated. An ultraviolet laser pulse (KrF laser, wavelength=248 nm, energy=750 mJ) produced the plasma channel by a focusing lens with a focal length of 1 m. This plasma channel forms a straight discharge channel along the laser beam (guiding ability) and reduces the flashover voltage (triggering ability). We measured the guiding and triggering ability of the laser-produced plasma channel and estimated the state of the plasma channel by calculating the rate equations. From these experiments and simulations, we found that the guiding ability of the ion plasma is mainly due to the formation of the leader from the plasma channel by enhancement of the local fields. The guiding ability of the electron-ion plasma is different from that of the ion plasma.
The Eδ′ and triplet-state centers in x-irradiated high-purity amorphous SiO2
Authors: Zhang Lin , Leisure R. G.
Published: July 2002

High-purity silicas synthesized by the chemical-vapor-deposited soot remelting method were studied by electron-spin-resonance techniques after being irradiated by x rays at 77 K or higher temperatures. The spectra of the Eδ′ center including its 29Si hyperfine splitting, and the triplet-state center, were measured using two different detection modes. The effects of x-ray dose, thermal annealing, hydrogen treatment, and impurities were examined; the Eδ′ and the triplet-state centers have a similar dependence on all these parameters, indicating that they share a common precursor. These centers are found only in low OH, oxygen-deficient samples. There appears to be no correlation with chlorine impurities. The measured intensity of the Eδ′ center’s 29Si hyperfine signal indicates that approximately four Si atoms are involved. A model for this center and the triplet-state center is proposed.
440 nm light generation by frequency tripling in germanosilicate glasses
Authors: de Araujo M. T. , Vermelho M. V. D. , Hickmann J. Miguel , Gouveia-Neto A. S.
Published: July 2002

Generation of 120 μW average power blue-violet light at 440 nm through efficient frequency tripling of 1.319 μm Nd:YAG laser pulses in bulk GeO2-SiO2 and Er3+-doped GeO2-SiO2 glasses is reported. Third-harmonic signal light enhancement owing to two- and three-photon resonant pumping of erbium ions in doped samples is also reported.
Giant magnetoresistance in Co/Cu multilayers fabricated by focused ion-beam direct deposition
Authors: Nagamachi Shinji , Ueda Masahiro , Sakakima Hiroshi , Satomi Mitsuo , Ishikawa Junzo
Published: July 2002

We report the direct deposition of patterned multilayers that exhibit giant magnetoresistance without any lithographic processes. We fabricated Co/Cu multilayers by the focused ion-beam direct deposition method and measured the magnetoresistance characteristics of the multilayers. The fabricated Co/Cu multilayers are 14×76 μm2 in size and consist of 12 layers of Co thin film with the thickness of 20 Å and 11 layers of Cu thin film with the thickness of 13–22 Å on the GaAs substrate. We used a 108 eV Co2+ ion beam and 54 eV Cu+ ion beam extracted from a Co–Cu–Au–Nb alloy ion source. The measured magnetoresistance ratio of giant magnetoresistance was 6.7% in the case of the Co(20 Å)/Cu(21 Å) multilayer. Experimental results show precise controllability of the thickness and the additional capability of the focused ion-beam direct deposition method.
Photovoltaic effect in coupled quantum wells under intersubband excitation
Authors: Soldatenko Yu. N. , Vasko F. T.
Published: July 2002

The photovoltaic effect in tunnel-coupled double quantum wells (DQWs) due to resonant infrared (IR) excitation of the electrons from the lower ground state to the excited state is considered. Intra- and interwell relaxation of electrons are studied self-consistently, taking into account modification of the electron states in DQWs due to the upper ground state occupation under IR pumping. The photoinduced potential is calculated for different energies of IR excitation, and the correlation between this potential and modification of the ground states in DQWs is discussed.
Large electro-optic modulation effect observed in ion-exchanged KTiOPO4 waveguides
Authors: Chiu Y. , Stancil D. D. , Schlesinger T. E.
Published: July 2002

We report the observation of an anomalously large electro-optic modulation effect in potassium titanyl phosphate (KTiOPO4) waveguides fabricated under various conditions. An interferometric method was used to measure the effective electro-optic coefficient in the waveguides by measuring the phase retardation of an optical mode when a voltage was applied across the substrate. We observed that at low modulation frequency the effective electro-optic coefficient in the waveguides can be higher than the bulk value by a factor up to about 100. A space charge model relating the mobile charge, space charge field, and observed enhancement in the electro-optic modulation is proposed to explain the mechanism of this effect. Calculation for dc steady state, small signal analysis, and large signal simulation is compared to the experimental observation. Possible device application and long term effects on system operation are also discussed.
Compositional evolution and magnetic properties of nanocrystalline Fe81.5Cu0.5Mo0.5P12C3Si2.5
Authors: Liu T. , Gao Y. F. , Xu Z. X. , Zhao Z. T. , Ma R. Z.
Published: July 2002

The amorphous alloy Fe81.5P12C3Cu0.5Mo0.5Si2.5 has been prepared and the crystallized alloy exhibits an ultrafine structure with a grain size of about 20 nm and excellent soft magnetic properties. The coercivity and the core loss as low as 6.7 A/m, 0.26 W/kg, respectively, and the maximum permeability as high as 10.2×104 at an optimal annealing temperature of about 360 °C were obtained. By means of x-ray diffraction, transmission electron microscope, and Mössbauer spectroscopy measurements, microstructures of the alloy were investigated as a function of the annealing temperature. The primary crystallization produces ultrafine grains of α-Fe(Si) solid solution with a grain size of about 20 nm precipitated in the residual amorphous matrix. The Si in the α phase was estimated near to be completely disorder ranged and the Si concentration was determined to be about 2%–5%. The Fe3P phase appears in the residual amorphous phase upon annealing at 420 °C.
Determination of the ferrimagnetic precession cone of in-plane magnetized garnet films using optical modulation technique
Authors: Bahlmann N. , Gerhardt R. , Wallenhorst M. , Dötsch H.
Published: July 2002

Magnetic garnet films of composition (A, Bi)3(Fe, B)5 O12 are grown by liquid phase epitaxy on [111] oriented substrates of Gd3 Ga5 O12, where A=yttrium or lutetium and B=aluminum or gallium. The ferrimagnetic resonance (FMR) of in-plane magnetized films is studied by analyzing the modulation of light, which passes the film parallel to the film normal. Modulation efficiencies up to 1.8% at 2.9 GHz are observed at a wavelength of 0.633 μm. From the measured modulation intensities the precession angles of the FMR are derived. The dependence of the precession cone on the excitation frequency or on the external induction shows a foldover effect which, however, is smaller than for the FMR of perpendicularly magnetized films. Maximum precession angles up to 14° are achieved and the experiments indicate that saturation occurs. Light modulation is also observed at the first harmonic frequency, but the intensity is about 300 times weaker than at the fundamental frequency.
Resistivity and its temperature dependence of nanostructured NiAl at temperatures from 77 to 300 K
Authors: Qin X. Y. , Zhang L. D. , Wu B. M. , Tian M. L. , Du Y. L. , Yang D. S. , Cao L. Z.
Published: July 2002

Direct current resistivity and its temperature dependence for nanostructured NiAl (n-NiAl) was investigated at temperatures from 77 to 300 K. The resistivity of n-NiAl (size 5–6 nm) was higher than that of polycrystalline NiAl and increased with decreasing density. For the n-NiAl with relative density D≳70%, the resistivity decreased with decreasing temperature, manifesting metallic behavior. However, its temperature coefficient of resistivity decreased monotonically with decreasing density, and changed sign from positive to negative at densities 68%–70%, below which it displays on nonmetallic behavior. These results can be well interpreted by dominant grain boundary scattering.
Structural ordering in InGaAs/GaAs superlattices
Authors: Ming Z. H. , Soo Y. L. , Huang S. , Kao Y. H. , Stair K. , Devane G. , Choi-Feng C.
Published: July 2002

Various x-ray techniques have been applied to a study of semiconductor superlattices consisting of 100-period of InxGa1−xAs (15 Å)/GaAs (100 Å) grown on GaAs(100) substrates by molecular beam epitaxy. Structural parameters pertaining to the morphology of interfaces and thickness variations were obtained. The interfaces in these superlattices are found to be highly correlated, and the layers all show a high degree of crystallinity. Splittings in the x-ray reflectivity and diffraction patterns in one of the samples provide clear evidence for pronounced thickness modulation, and direct comparison of the diffraction satellite peaks with results of high resolution transmission electron microscopy indicates that there exists a lateral structural ordering in the [110] direction during epitaxial growth.
Regrowth of the photoquenchable defect relating to the hopping conduction in arsenic-ion-implanted semi-insulating GaAs
Authors: Kuriyama K. , Kazama K. , Kato Takashi , Yamamoto S. , Aoki Y. , Naramoto H.
Published: July 2002

The quenching phenomenon in Mott-type hopping conduction below 125 K has been observed in As+-ion-implanted semi-insulating GaAs. The crystallinity of the implanted layer was evaluated using Rutherford backscattering and Raman scattering methods. The hopping conduction was observed in the as-implanted and annealed samples. The as-implanted layer was amorphous, while the annealed layers remained crystalline, consisting of a disordered structure with a displacement fraction of ∼1021 cm−3. In the samples annealed at 500 °C, it is suggested that some of the As precipitates are located on Ga sites, leading to the disappearance of the A1 g Raman active mode of the As clusters. The hopping conduction was photoquenched as the defect concentration reached ∼6×1017 cm−3, indicating the regrowth of the AsGa antisite defects as a main component of EL2.
Electrically detected magnetic resonance investigations of gallium phosphide green light-emitting diodes
Authors: Reinacher N. M. , Brandt M. S. , Stutzmann M.
Published: July 2002

Transport processes and degradation of GaP:N green light-emitting diodes have been investigated using electrically detected magnetic resonance (EDMR). An isotropic EDMR signal with a g value of g=1.996 and a linewidth of ΔHpp=68 G can be observed at low temperatures after current degradation. The signal exhibits a T−2 temperature dependence indicating a spin relaxation process faster than the relevant transport step. The microscopic origin of the EDMR signal—most probably recombination at a n-type dopant-related defect near the p–n interface—is analyzed with respect to possible degradation mechanisms.
Double critical temperature characteristics of semiconducting (Ba0.7Pb0.3)TiO3 materials prepared by microwave sintering
Authors: Chang Horng-Yi , Liu Kuo-Shung , Hu Chen-Ti , Lin Tsai-Fa , Lin I-Nan
Published: July 2002

In this work, we obtain (Ba0.7Pb0.3)TiO3 materials possessing double critical temperature Tc in resistivity–temperature (ρ–T) behavior by microwave sintering at 1050–1080 °C for 5 min. The cooling-rate control and postannealing processes modify the relative magnitudes of low- and high-Tc resistivity jumps without altering the Tc values. The donor Ed and trap Es levels of those materials are estimated to be Ed≂0.05–0.07 eV and Es≂1.07–1.32 eV. According to experimental results the voltage sensitivity and transient responsivity of the current passing through double-Tc materials are observed to be superior to those of single-Tc materials prepared by the conventional furnace sintering method. Also, the double-Tc characteristics are attributed to the dual phases with a core-shell structure. Moreover, the change in the relative proportion of the two phases accounts for the influence of post-heat treatment processes on the materials’ ρ–T behavior.
On the formation process of luminescing centers in spark-processed silicon
Authors: Ludwig M. H. , Augustin A. , Hummel R. E. , Gross Th.
Published: July 2002

Radiative and compositional properties of spark-processed silicon are studied by photoluminescence and x-ray photoelectron spectroscopy measurements. Spark processing of silicon is performed in different atmospheres composed of nitrogen and oxygen. As a result of the process, room-temperature radiative transitions occur at 2.35 eV and vary in intensity over five orders of magnitude depending on the N2/O2 ratio. After processing in pure nitrogen or pure oxygen, however, the green photoluminescence (PL) is wiped out and weak blue (2.7 eV) or orange (1.9 eV) PL bands, respectively, are discernable. The temperature-dependent features of the 2.35 eV emission are characterized by an intensity increase in conjunction with a red shift of the peak position at lowered temperatures. A cross-sectional study reveals that the green PL is mainly generated in a near-surface layer having a chemical composition close to SiO2 and a nitrogen concentration below 1 at. %. Nearly no PL was observed from a deeper SiO2 layer enriched by silicon clusters and with an increased density of nitrogen (up to 7 at. %). The findings do not support a quantum-dot-related PL mechanism in spark-processed silicon. It is proposed that nitrogen additions reduce the density of nonradiative centers introduced by silicon dangling bonds.
Modulated photocurrent study of localized-state distributions in copper phthalocyanine thin films
Authors: Naito Hiroyoshi
Published: July 2002

Localized-state distributions in copper phthalocyanines dispersed in polymer binders have been investigated with the modulated photocurrent (MPC) technique. The well-defined transport state of holes is observed in the excitation wavelength dependence of MPC. The frequency resolved MPC experiments reveal that the density of the localized states consists of the two exponential distributions having the characteristic temperatures of 500 and 1200 K with the inflection point at 0.53 eV above the transport state. These results are compared with those of evaporated copper phthalocyanine thin films, and no significant difference is found. The origin of the exponential distribution of the localized states is discussed in the light of statistical distribution of charged centers as sources of random potential.
Charge trapping induced electromechanical energy
Authors: Coudray C. , Blaise G.
Published: July 2002

The electromechanical energy stored in a dielectric subjected to the field of a trapped charge is calculated by a self-consistent method, using a point-dipole model for the polarization. The magnitude of this energy is 5–10 eV per trapped charge, depending on the structure (fcc or bcc) and the location of the charge (atomic site or intersite). It is proposed to attribute the degradation of dielectric materials to the release of this energy after the detrapping of initially trapped charges.
Surface acoustic waves on the (11n) cuts of gallium arsenide
Authors: Miškinis Rimantas , Rutkowski Pavel , Urba Emilis
Published: July 2002

Surface acoustic waves (SAW) propagating on the (111), (112), and (113) cuts of gallium arsenide in the [11̄0] direction are studied both theoretically and experimentally. The possibility of propagation of both normal and leaky surface acoustic waves (NSAW and LSAW, respectively) is predicted. The acoustoelectrical activity predicted for LSAW is more than 100 times higher than that for NSAW. Three particle displacement components of comparable magnitudes for LSAW propagating on the three cuts are obtained. The lowest attenuation (4.94×10−5 dB per wavelength) is predicted for the LSAW at the (112), [11̄0] orientation of GaAs. Theoretical characteristics for the LSAW on the (112) cut are corroborated experimentally. Experimental values of the phase velocity (3.05×103 m/s), electromechanical coupling coefficient (8.1×10−4), attenuation (5.7×10−3 dB per wavelength), and temperature delay coefficient (5.2×10−5 deg−1) are presented. Characteristics of experimental delay lines fabricated on the (112) cut and the well-known (001) cut are compared.
Influence of phosphorus diffusion on the recombination strength of dislocations in float zone silicon wafers
Authors: Simon J. J. , Périchaud I. , Burle N. , Pasquinelli M. , Martinuzzi S.
Published: July 2002

Dislocation arrays are investigated in float zone (FZ) grown silicon wafers by the light beam induced current (LBIC) mapping technique at various wavelengths and by deep level transient spectroscopy (DLTS). The LBIC technique allows us to recognize and detect these arrays and to evaluate their recombination strength. Dislocations are found to be less recombining in (100)-oriented FZ samples than in (111) oriented ones. In FZ dislocated wafers, a phosphorus diffusion strongly attenuates the LBIC contrast of dislocations, depending on the duration and temperature of the treatment. Electrical activity of the defects, which are still physically present, as verified by x-ray topography, seems to disappear. Simultaneously, the peak intensities of DLTS spectra related to dislocations are reduced and this reduction depends on the phosphorus diffusion temperature and duration.
Numerical solution of the electromigration boundary value problem under pulsed dc conditions
Authors: Gui Xiang , Dew Steven K. , Brett Michael J.
Published: July 2002

The one-dimensional electromigration boundary value problem under pulsed dc conditions is numerically investigated by utilizing the transmission-line matrix modeling method. A perfectly blocking boundary, where void formation and failure occur, is assumed at one end of an interconnection line. At the other end, two physically plausible boundary conditions are considered. From the design-rule point of view, an approach is proposed to convert conveniently the pulsed stress into an equivalent dc stress that would produce electromigration damage at a similar rate. Based on the fundamental diffusion-drift model, we show that the vacancy buildup behavior under a pulsed dc stress γp can be described accurately by the dc stress γdc scaled according to the duty factor r of the current pulse, namely, γdc=rγp. This study also represents a theoretical confirmation for the (jr)−2 dependence of the pulsed electromigration failure (where j is the current density), which has been observed in a number of experimental studies.
Optimal x-ray source development in the spectral range 4–14 Å using a Nd:YAG high power laser
Authors: Fraenkel M. , Zigler A. , Horowitz Y. , Ludmirsky A. , Maman S. , Moshe E. , Henis Z. , Eliezer S.
Published: July 2002

An experimental study was performed to find the optimal conditions for generating a few nanoseconds x-ray pulse source in the wavelength range 4–14 Å. X rays were emitted in a high-temperature plasma, which was created as the result of the interaction of a high power laser with a solid target. The experiments were done using 3–15 J, 1–10 ns, 1.06 μm pulses generated with a Nd:YAG laser. The x-ray yield and spectral distributions were studied as a function of the laser energy, temporal pulse width, pulse shape, and target material. For example, for samarium under optimal conditions, about 20% of the laser energy was converted into x rays in the wavelength range 8–12 Å.
Thermal pulse study of the electric polarization in a copolymer of vinylidene cyanide and vinyl acetate
Authors: Giacometti José A. , De Reggi Aimé S. , Davis G. Thomas , Dickens Brian , Ferreira G. F. Leal
Published: July 2002

The polarization induced by thermopoling the alternating vinylidene cyanide/vinyl acetate copolymer has been studied by the thermal pulse technique. The mean polarization over the thickness and its spatial profile were determined as functions of the poling variables: electric field, time, and temperature. Additionally, the thermal stability of the polarization was studied between ambient temperature and the glass-transition temperature of 175 °C. The mean polarization was found to be proportional to the poling field up to the maximum field used of 42 MV/m and to be very stable up to 150 °C. The polarization profiles obtained after poling were found to be highly inhomogeneous, especially for short poling times, indicating positive charge injection from the positive electrode during poling. The polarization distribution continues to evolve after the mean polarization has reached a steady state indicating separate time scales for space-charge migration and dipole reorientation.
Stress relaxation in SiGe layers grown on oxide-patterned Si substrates
Authors: Banhart F. , Gutjahr A.
Published: July 2002

Silicon-germanium layers with Ge concentrations up to 14% are grown by liquid phase epitaxy on oxide-patterned (111) Si substrates. Strains and defect formation are studied by transmission electron microscopy. Below Ge concentrations of 10% strain relaxation occurs via heterogeneous nucleation of dislocation loops at edges of the oxide mask. Cross-slip leads to pinning points from where Frank–Read sources eject dislocations into the substrate. These dislocations relax strain induced by misfit and oxidation. At Ge contents above 10% misfit dislocations are observed at the Si/SiGe interface. Only a few threading dislocations occur and are confined to the seed areas where the SiGe layer grows directly on the Si substrate. Because edges of the oxide mask act as obstacles for dislocation propagation, SiGe layers growing laterally on the oxide-covered Si substrate are almost dislocation free.
Microstructure of highly oriented, hexagonal, boron nitride thin films grown on crystalline silicon by radio frequency plasma-assisted chemical vapor deposition
Authors: Andújar J. L. , Bertran E. , Maniette Y.
Published: July 2002

We present a high-resolution electron microscopy study of the microstructure of boron nitride thin films grown on silicon (100) by radio-frequency plasma-assisted chemical vapor deposition using B2H6 (1% in H2) and NH3 gases. Well-adhered boron nitride films grown on the grounded electrode show a highly oriented hexagonal structure with the c-axis parallel to the substrate surface throughout the film, without any interfacial amorphous layer. We ascribed this textured growth to an etching effect of atomic hydrogen present in the gas discharge. In contrast, films grown on the powered electrode, with compressive stress induced by ion bombardment, show a multilayered structure as observed by other authors, composed of an amorphous layer, a hexagonal layer with the c-axis parallel to the substrate surface and another layer oriented at random.
Overdriven-detonation and sound-speed measurements in PBX-9501 and the ‘‘thermodynamic’’ Chapman–Jouguet pressure
Authors: Fritz J. N. , Hixson R. S. , Shaw M. S. , Morris C. E. , McQueen R. G.
Published: July 2002

Sound speeds, at pressure, and the overdriven Hugoniot were measured for the plastic-bonded explosive PBX-9501. The two curves intersect at the Chapman–Jouguet (CJ) state because of the sonic condition D=c+u. This permitted a novel determination of the ‘‘thermodynamic’’ CJ pressure. A value of 34.8±0.3 GPa was obtained. The data permit a direct experimental determination of the isentropic gamma, γS=−(∂lnP/∂lnV)S, and the Grüneisen parameter, γ=V(∂P/∂E)V, in the overdriven pressure range.
Study of ionic impurity mobility in quartz crystals by impedance and thermionic current measurements
Authors: Poignon C. , Jeandel G. , Morlot G.
Published: July 2002

Defects, notably interstitial impurities, are responsible for degradation of resonator performance with time. In order to study the mobility of ionic impurities, we have measured thermionic currents as a function of temperature by the thermally stimulated depolarization (TSD) method. We have observed several current maxima; each peak is due to the displacement of an impurity family from a given site and is characterized by its activation energy. Making impedance measurements to complete these results, we find energies of about 1 eV. We have studied quartz crystals doped with a given impurity in order to identify the nature of the impurities responsible for some current peaks. This study confirms that the TSD method is very sensitive.
A stochastic model for crystal-amorphous transition in low temperature molecular beam epitaxial Si (111)
Authors: Venkatasubramanian R. , Gorantla S. , Muthuvenkatraman S. , Dorsey D. L.
Published: July 2002

Molecular beam epitaxial Si (111) grown below a certain temperature result in amorphous structure due to the limited surface mobility of atoms in finding correct epitaxial sites. In spite of many experimental and theoretical studies, the mechanism of crystal-amorphous transition and its dynamics related to the growth conditions are not well understood. In this article, we present a theoretical model based on the formation of stacking fault like defects as a precursor to the amorphous transition of the layer. The model is simulated based on a stochastic model approach and the results are compared to that of experiments for temperatures in the range of 500–900 K and growth rate in the range of 0.1–3.0 Å/s. The agreement between our results and experimental observations is excellent.
Thermal desorption in nanocrystalline TiO2
Authors: Tan Ming , Wang Guozhong , Zhang Lide
Published: July 2002

Nanocrystalline TiO2 powder with a grain size of 9.8 nm was prepared by the hydrothermal precipitation method. Thermal desorption in the powder and compacted TiO2 samples has been studied by differential scanning calorimetry and thermogravimetric analysis. An endothermic peak was observed on these samples and the peak can be attributed to thermal desorption. The desorption enthalpy per gram of adsorbates in the compacted sample is larger than that in the powder one and it continues to increase with the compacting pressure. A tentative model has been proposed to explain the increase in desorption enthalpy due to compaction.
A two-dimensional moment equations-based analysis of radio frequency capacitively coupled discharges
Authors: Meyyappan M. , Govindan T. R. , Kreskovsky J. P.
Published: July 2002

A two-dimensional nonequilibrium fluid model is used to study plasma uniformity in radiofrequency capacitively coupled discharges. Results for argon and helium discharges are presented for 10 and 30 cm electrodes in terms of plasma density, electron and ion velocity vectors, and other characteristics. The plasma properties are found to be uniform over most of the 30 cm electrode except in an edge zone.
Population dynamics of the three-micron emitting level of Er3+ in YAlO3
Authors: Georgescu S. , Lupei V. , Trifan M. , Sherlock R. J. , Glynn T. J.
Published: July 2002

An analysis of the nonexponential behavior of the kinetics of the initial laser level for three-micron generation (4I11/2) in an Er3+(45 at. %):YAlO3 crystal under intense pump in the 4S3/2 level is performed. The observed nonexponential 4I11/2 decay can be satisfactorily described by the rate equation model which includes energy transfer processes inside the system of erbium ions, such as up-conversion from 4I13/2 and 4I11/2 and cross-relaxation from 4S3/2 and 4I9/2 provided only a small fraction (β21≊0.11) of the excitation from 4I11/2 level reaches the terminal laser level, 4I13/2. The low value of β21 and the observed reduction of the fluorescent lifetime of 4I11/2 with the increasing erbium doping could be related to the presence of accidental impurities.
Theoretical and experimental investigation of leakage power in microwave transmit–receive switches
Authors: Madsén K. , Andersson D. , Lisak M. , Lundgren L. , Semenov V.
Published: July 2002

A theoretical and experimental investigation is made of the leakage power in microwave transmit–receive switches. The peak leakage power is shown to be independent of incident peak power but to depend on the rise time of the incident pulse. The plateau leakage power through the switch is also estimated and shown to be independent of incident peak power. The theoretical predictions for peak leakage power and plateau leakage power are compared and found to be in good agreement with experimental results.
Beam propagation algorithms for frequency response of surface acoustic wave directional couplers
Authors: Haruvi-Busnach I. , Seidman A. , Croitoru N.
Published: July 2002

Surface acoustic wave directional couplers have long been known in both microwave and integrated acoustic applications. Such couplers are ideal for integration in acoustical signal processing devices, such as modulators or switches. There are, however, very few publications concerning the frequency response of these devices. This article represents both experimental and theoretical studies on the frequency response of directional couplers, with and without a thin strip deposited in the gap between the main channels. The beam propagation method, used in integrated optics, was adapted for acoustical directional couplers. It enabled the analysis and prediction of the frequency response of these types of couplers. Calculations were performed for the channels and interdigital transducers in the frequency range of 50–90 MHz. By using proper dimensions of the couplers’ parameters, coupling efficiency for the higher part of the frequencies, in the receiver channel, was increased. This increase enables the selection of receiver frequencies and, therefore, a frequency channelizer could be developed. A thin strip between the main channels increases the coupling efficiency between the two channels and thereby reduces by about 30% the length of the coupler required for different frequency selections or conventional switches. This reduction makes it possible to obtain more compact devices with reduced losses.
Photoluminescence of C60-doped matrixes
Authors: Wang Dazhi , Zuo Jian , Zhang Qijin , Luo Yi , Ruan Yaozhong , Wang Zheng
Published: July 2002

Four matrixes were doped with C60: nano-ZrO2; nano-Al2O3; nano-SnO2; and polystyrene. Under Ar+ laser (476.5 nm) excitation, pure C60 and the above matrixes had only weak photoluminescence (PL) in the visible region. However, when the above matrixes were doped with C60, the PL intensities increased strongly. The PL spectra of C60-doped matrixes are similar. All spectra show a wide band covering the visible region with intensities from three to ten times stronger than those of the pure matrixes, respectively. The intensities and peak positions of the spectra varied for different matrixes and different crystallite sizes. It is probable that the photoluminescence is caused by the optically activated forbidden tlu⇒hu transition of C60 due to the interaction of C60 with the matrixes.
Multiple subband population in delta-doped AlAsSb/InGaAs heterostructures
Authors: Ahoujja M. , Elhamri S. , Newrock R. S. , Mast D. B. , Mitchel W. C. , Lo Ikai , Fathimulla A.
Published: July 2002

We have measured the Shubnikov–de Haas (SdH) effect in δ-doped AlAs0.56Sb0.44/In0.53Ga0.47As heterostructures and observed a population of the second subband. Using the persistent photoconductivity effect we increased the electron density from 26.73 to 28.20×1011 cm−2 in the first subband and 6.61 to 7.20×1011 cm−2 in the second. The onset of the second subband population occurs when the first subband is filled to a density of 11.56×1011 cm−2. From the nonparabolic band approximation we calculated the effective masses in both subbands before illumination. The effective mass for the second subband was evaluated using the temperature dependence of the SdH amplitude. Its value agrees well with the values obtained from the k⋅p approximation and infrared cyclotron resonance measurements.
First order transitions by conduction calorimetry: Application to deuterated potassium dihydrogen phosphate ferroelastic crystal under uniaxial pressure
Authors: Gallardo M. C. , Jiménez J. , Koralewski M. , del Cerro J.
Published: July 2002

The specific heat c and the heat power W exchanged by a Deuterated Potassium Dihydrogen Phosphate ferroelectric-ferroelastic crystal have been measured simultaneously for both decreasing and increasing temperature at a low constant rate (0.06 K/h) between 175 and 240 K. The measurements were carried out under controlled uniaxial stresses of 0.3 and 4.5±0.1 bar applied to face (110). At Tt=207.9 K, a first order transition is produced with anomalous specific heat behavior in the interval where the transition heat appears. This anomalous behavior is explained in terms of the temperature variation of the heat power during the transition. During cooling, the transition occurs with coexistence of phases, while during heating it seems that metastable states are reached. Excluding data affected by the transition heat, the specific heat behavior agrees with the predictions of a 2-4-6 Landau potential in the range of 4–15 K below Tt while logarithmic behavior is obtained in the range from Tt to 1 K below Tt. Data obtained under 0.3 and 4.5 bar uniaxial stresses exhibit the same behavior.
Vibrational energies of adsorbates on GaAs surfaces
Authors: Pollard William
Published: July 2002

The structure and properties of adsorbates on GaAs surfaces are investigated. The vibrational states of adsorbed hydrogen and oxygen on GaAs surfaces are calculated. Specific attention is focused on the vibrational energies of surface species and their reported variation with surface stoichiometry. The symmetry and localization of the vibrational states are also discussed. Comparisons are made with experimental high resolution electron energy loss spectroscopy spectra. Our results here agree well with a variety of experimental data.
Characterization of the diamond growth process using optical emission spectroscopy
Authors: Cui Jingbiao , Fang Rongchuan
Published: July 2002

In situ optical emission spectroscopy was applied to an electron assisted hot filament diamond growth process. Emission lines from the Balmer series of atomic hydrogen, molecular hydrogen, CH, CH+, and Ar were observed in the visible range when CH4, Ar, and H2 were used as the input gases. The relative concentration of atomic hydrogen was estimated by using the emission line of Ar at 750.4 nm as an actinometer. The effects of deposition conditions, including filament temperature, substrate temperature, bias voltage, and methane concentration in the source gas, on the species distribution, electron temperature, and diamond film growth were investigated. The vertical spatially resolved measurements show that the concentration of atomic hydrogen decreases as the detection point moves far from the filament and suddenly drops near the substrate. The horizontal spatially resolved measurements show that the homogeneous region of reaction species and the electron temperature near the substrate define the homogeneous region of diamond film growth. Increase of filament temperature, substrate temperature, bias voltage, and the decrease of methane concentration in the feed gas increase the atomic hydrogen concentration and diamond deposition. A higher intensity ratio of CH+ to Ar and a lower ratio of CH to Ar are associated with a high quality of diamond film growth. Optical emission spectroscopy showed that atomic hydrogen and CH+ correlate with the formation of the diamond component, whereas the CH relates to the presence of amorphous carbon in the films. The effect of positive bias on the diamond growth results largely from the bombardment of electrons on the substrate.
An investigation of the origin of the compromising field in germanosilicates waveguide electric-field-induced second harmonic experiments
Authors: White W. Roc , Dajani I. , Kendall J. P.
Published: July 2002

A recent publication by Statman and Georges reported on the dynamics of electric-field-induced second harmonic (EFISH) in planar germanosilicate waveguides. They provided a model that examines the decay of the second harmonic signal as a result of charge migration inside the doped glass medium through dielectric relaxation and electronic drift. We present experimental results of EFISH using femtosecond laser pulses. We show here that this decay is actually due to the formation of a surface charge.
A C 1s core level x-ray photoelectron diffraction characterization of substitutional carbon in epitaxial Si1−yCy alloys grown on Si(111) and Si(001)
Authors: Simon L. , Aubel D. , Kubler L. , Bischoff J. L. , Gewinner G. , Balladore J. L.
Published: July 2002

Epitaxial strained growth of Si1−yCy alloys with rather high C concentrations (y∼1.5%) has been performed on Si(111) and Si(001) using molecular beam epitaxy (MBE) Si evaporation and thermal interaction of the growth surface with a low C2H4 pressure at 500 °C. Carbon contents, determined by secondary ion mass spectrometry, infrared (ir) spectrometry, in situ C 1s and Si 2p x-ray photoelectron spectroscopy measurements and x-ray diffraction (XRD), are being compared. Monocrystalline quality of the epilayers is checked by low energy electron diffraction and x-ray photoelectron diffraction (XPD). As indirectly ascertained by the ir local vibration mode (LVM) and a shifted partially strain induced epilayer diffraction line in the θ-2θ XRD analysis, carbon is accommodated in substitutional sites (Csub) whose local atomic order is investigated for the first time by XPD, C 1s polar angle distributions being measured in different azimuthal directions. As the data reveal, for a C emitter, next nearest neighbor bond orientations identical to those for Si atoms in a Si matrix, XPD readily provides direct evidence in favor of Csub positions. Up to now, our limited angular resolution does not allow observation of possible bond orientation changes due to local strain-induced lattice distortions around C atoms. Nevertheless, by increasing growth temperature (600–650 °C) and promoting formation of more C-rich phases (SinC or SiC), the effects of substitution of second or higher nearest Si neighbors by C atoms can be clearly evidenced. By the way, a significant C 1s binding energy difference between Csub and C in C-rich phases is observed and may be used as a signature of the C dilution in the grown epilayer: a characteristic value of 283.8 eV is obtained for the Csub site giving rise to the LVM.
Cathodoluminescence study of band filling and carrier thermalization in GaAs/AlGaAs quantum boxes
Authors: Rich D. H. , Lin H. T. , Konkar A. , Chen P. , Madhukar A.
Published: July 2002

We have examined carrier thermalization, recombination, and band filling in GaAs/AlGaAs quantum boxes with low-temperature cathodoluminescence (CL). The temperature dependence of the quantum box CL intensity for T⩽ 90 K exhibits an Arrhenius behavior, as a result of carrier thermalization between the quantum box and surrounding barrier regions. The width of the quantum box luminescence is found to increase rapidly with an increasing excitation density and reveals an enhanced phase-space and real-space filling, in comparison to the behavior observed for quantum wells.
Crystal-field splitting of Er+3 in Si
Authors: Ren Shang Yuan , Dow John D.
Published: July 2002

Two photoluminescent defects associated with Er+3-doped Si are (i) a high-temperature defect (which appears after annealing at ∼900 °C and produces five photoluminescence lines), and (ii) a low-temperature defect (which is created at lower annealing temperatures in addition to the high-temperature defect and yields four photoluminescence lines). We conclude that both defects are at Er sites of tetrahedral symmetry, one substitutional and the other interstitial, and that the crystal-field split ground-state levels of Er+3 at either site are of the symmetries (in order of increasing energy) Γ6, Γ8, Γ8, Γ7, and Γ8. We predict that the highest Γ8 level of the low-temperature defect has not yet been resolved, which is why that defect exhibits only four levels. The symmetries of the excited (initial) state levels, in order of increasing energy, are predicted to be Γ8, Γ6, Γ6, Γ8, and Γ7. We speculate that the high-temperature defect’s Er is at an interstitial site, while the low-temperature defect’s Er is substitutional.