bluuebunny/crossref_metadata_2025 huggingface dataset:
Dark J-V characteristic of p-i-n a-Si:H solar cells
Authors: Mittiga A. , Fiorini P. , Falconieri M. , Evangelisti F.
Published: July 2002

The J-V characteristics of p-i-n solar cells in the dark have been simulated by solving numerically the full set of transport equations. We have analyzed the dependence of dark current on the density of gap states in the intrinsic layer and on the presence of interface states between the p and i layers. The dependence on the thickness of the intrinsic layer has also been investigated and we have found that the agreement with the experimental data is possible only in the presence of interface states. A simplified model which qualitatively explains the results of the complete simulation has been developed. We have also investigated the changes of J-V characteristics with measurement temperature obtaining results in good agreement with the experimental data without introducing any tunneling mechanism.
Thermal stability of the Cu/Ta/PtSi structures
Authors: Chang Chin-An
Published: July 2002

Cu/Ta/PtSi structures are heated between 200 and 700 °C, with Ta as a barrier for improving the thermal stability of Cu/PtSi. A small amount of Cu silicides is observed after a 30-min anneal in N2-H2 at 300−400 °C. This is accompanied by an extensive mixing among the components present, and increasing sheet resistances. By comparing with the reactions of the Cu/Ta/Si, Al/Ta/PtSi, and Al/Ta/Si structures, the mechanisms suggested earlier are supported, with the high affinity of Cu toward Si playing a major role for the low thermal stability of the Cu/PtSi structures with and without various barrier layers.
Amorphization and solid-phase epitaxial growth in tin-ion-implanted gallium arsenide
Authors: Taniwaki Masafumi , Koide Hideto , Yoshimoto Naoto , Yoshiie Toshimasa , Ohnuki Somei , Maeda Masao , Sassa Koichi
Published: July 2002

The amorphization and recrystallization of tin-ion-implanted gallium arsenide were studied by cross-sectional transmission electron microscopy. Amorphization occurred in the sample implanted at a dose of 1014 ions/cm2. The interface between the amorphous region and the crystalline matrix is not flat. The amorphous region recrystallizes epitaxially with microtwin formation at 673 K. The amorphous-crystalline interface in the sample implanted at a dose of 1016 ions/cm2 is flat. In the deep region of this sample a solid-phase epitaxial growth without microtwin formation is observed after annealing at 673 K. These structural changes were compared with the nuclear energy loss (damage energy) distribution simulated by the trim code. It is concluded that the amorphization of the sample implanted at a dose of 1014 ions/cm2 is induced by the accumulation of damage energy; on the other hand, the amorphization of the sample implanted at a dose of 1016 ions/cm2 cannot be explained only by this process. The contribution of stress at the amorphous-crystalline interface is suggested.
Development of high-power chemical oxygen-iodine laser for industrial application
Authors: Fujii Hiroo , Yoshida Sanichiro , Iizuka Masahiro , Atsuta Toshio
Published: July 2002

A scaling up of kW-class chemical oxygen-iodine laser (COIL) is achieved. The key issues for the system design of COIL for industrial application is discussed. The scaled up COIL system of 1-kW output power is now under trial operations. In these operations, a maximum output power of 1100 W, and a continuous operation time of more than 2 h with a power of over 500 W is recorded.
Ion extraction characteristics by an electric field on laser-produced barium plasma
Authors: Yamada Kimio , Tetsuka Tsutomu , Deguchi Yoshihiro
Published: July 2002

Time evolution of a two-dimensional ion-density distribution in a barium plasma produced by simultaneously illuminating a tuned dye laser light and an excimer laser light has been measured by means of laser-induced fluorescence technique to investigate the ion extraction characteristics by an electric field. When the plasma is produced between two parallel plates with a potential difference, the ion behavior is governed by three factors: bulk plasma flow, plasma diffusion, and ion extraction by the electric field. The flow velocity of the bulk plasma is equal to the mean velocity of the atoms emanating from a hot source. The expansion velocity of the plasma boundary into a vacuum agrees well with the ambipolar expansion velocity corresponding to that of an ion acoustic wave, while the diffusion coefficient obtained from the density decay rate is smaller by a factor of about 8 than the ambipolar expansion coefficient for the steady state because of low collision frequency between ions. Enhancement of the ion extraction rate by the electric field is supplied by retrograde motion of the plasma edge which contributes to reduction of the ion extraction time.
Ferromagnetic resonance of single-crystal YIG/gadolinium gallium garnet/YIG layers
Authors: Sun Kunquan , Vittoria Carmine , Glass H. L. , De Gasperis P. , Marcelli R.
Published: July 2002

We report on the microwave properties of single-crystal YIG/gadolinium gallium garnet/YIG grown in (110) layers, where YIG is yttrium iron garnet. The YIG layers were sufficiently thin so that single domain ferromagnetic resonance (FMR) was observed. FMR fields were measured as a function of field direction and magnitude and frequency. Bulk magnetic parameters deduced from FMR and vibrating-sample magnetometer measurements agree with published values. In addition, FMR measurements at low fields show typical ‘‘butterfly’’ variations of the FMR fields with a frequency for H, the magnetic field, parallel to either the 〈100〉 or 〈110〉 axis. However, a new butterfly pattern was also observed for the magnetic field parallel to the 〈111〉 axis. This result is intrinsic to layered structures and not to single layer excitations. We attribute this new result to nonuniform distribution of strain in the two YIG layers, and as a consequence, we predict that magnetostatic fringe fields from edges are sufficient to induce spin-flop magnetic configurations in single-crystal YIG double layers.
Ultrasonic motor operation using the unidirectional surface displacement of a cylindrical piezoelectric vibrator
Authors: Toda Kohji , Kamo Akihiko
Published: July 2002

A method for exciting unidirectional elliptical motions on the side surface of a cylindrical piezoelectric vibrator with a high electromechanical coupling coefficient is described. One of two circular electrodes of the vibrator column has a partially inactive region for the electrode operation. Longitudinal and radial vibration modes effectively constitute a unidirectional elliptical motion when the ratio of length to radius is nearly equal to unity. Practical application to an ultrasonic motor is demonstrated using two vibrators.
Plasma nitridation process for the fabrication of all-refractory Josephson junctions
Authors: Cantor R. , Drung D. , Peters M. , Koch H.
Published: July 2002

A process for the fabrication of high-quality, all-refractory Josephson junctions of the type Nb/Six Ny /Nb is described in detail. The junctions have been fabricated using the selective niobium anodization process with a tunnel barrier consisting of an amorphous Si film converted to its nitride by the application of a rf plasma of nitrogen. An attractive feature of these junctions is their comparably low specific capacitance C/A=3.9±0.1 μF/cm2 (for Jc =6 A/cm2 ). Junctions of this type having critical current densities up to Jc =2000 A/cm2 and Vm =16 mV have been fabricated. Excellent magnetic field threshold curves have also been observed for these junctions, indicating that the critical current density is very uniform.
Porous silicon oxynitrides formed by ammonia heat treatment
Authors: Tsao S. S. , Guilinger T. R. , Kelly M. J. , Stein H. J. , Barbour J. C. , Knapp J. A.
Published: July 2002

Porous silicon and its oxide can be converted into porous silicon oxynitrides by ammonia heat treatment. For example, ammonia treatment at 1000 °C for 1 h following 850 °C, 30-min steam oxidation of porous silicon can result in up to 40 at. % nitrogen in the porous oxynitrides. These porous silicon oxynitrides are compositionally more uniform than ammonia-nitrided thermal oxides which exhibit nitrogen buildup at the oxide layer interfaces. However, the order of the oxidation and nitridation treatment matters: nitrided oxidized porous silicon exhibits higher electrical breakdown strength than nitrided porous silicon or oxidized nitrided porous silicon.
Investigation of laser blow-off atomic beams by electron impact excitation
Authors: Bakos J. S. , Földes I. B. , Ignácz P. N. , Kocsis G.
Published: July 2002

Electron impact excitation in the edge region of the MT-1 tokamak was used to measure the velocity distribution of a sodium laser blow-off beam in a single shot. The different dependencies of the propagation velocity of the blow-off neutral beam on the laser fluence at different sodium layer thicknesses reveal different regimes of the blow-off process. In the case of thick targets laser ablation is the main accelerating mechanism giving a v∼I1/3 dependence. Thin targets, however, burn through during the laser pulse, and simple thermal heating process results in v∼I1/2 in good agreement with the experiments.
A model kinetics for nucleation at a solid surface with application to diamond deposition from the gas phase
Authors: Tomellini M. , Polini R. , Sessa V.
Published: July 2002

A model kinetics has been developed which describes the time evolution of the surface densities of nucleation centers, germs, and nuclei of the new phase as might be occurring during chemical vapor deposition (CVD). The solutions of the system of three differential equations have been obtained for the case of small fractional surface coverage by the new phase and for the case when capture of both nucleation sites and germs should be included. The model, which refers to conditions of well-separated particles, has been applied to nucleation data obtained for diamond deposition by the hot-filament CVD method and allows for a satisfactory description of experimental data.
Laser-driven metal photocathodes for picosecond electron and x-ray pulse generation
Authors: Anderson Todd , Tomov Ivan V. , Rentzepis P. M.
Published: July 2002

Linear photoemission from pure metal surfaces has been studied using the fourth (263 nm) and fifth (211 nm) harmonic of a Nd:YLF laser-regenerative amplifier system for the purpose of generating short (<70 ps), intense electron pulses. Capacitive charge limitations for the pulsed mode of operation of a laser-driven photodiode are used to explain the observed current-voltage characteristics. Quantum efficiencies at the two different wavelengths for eleven photocathode materials are presented.
Thermal conductivity of thermoelectric Si0.8-Ge0.2 alloys
Authors: White D. P. , Klemens P. G.
Published: July 2002

The thermal conductivity of heavily doped, n-type Si-Ge alloys has been studied from 300 to 1200 K. The scattering rate of several phonon scattering mechanisms has been calculated, including intrinsic scattering, mass defect and distortion scattering, phonon-electron scattering, and scattering by inclusions. These rates were then used to calculate the lattice thermal conductivity. The electronic component of the thermal conductivity was calculated from the calculated Lorenz ratio and measured values of the electrical conductivity. The total thermal conductivity was then compared to measured values for a specimen studied by Vining et al. [J. Appl. Phys. 69, 15 (1991)].
Luminescence and structural study of porous silicon films
Authors: Xie Y. H. , Wilson W. L. , Ross F. M. , Mucha J. A. , Fitzgerald E. A. , Macaulay J. M. , Harris T. D.
Published: July 2002

The luminescence properties of 3 μm thick, strongly emitting, and highly porous silicon films were studied using a combination of photoluminescence, transmission electron microscopy, and Fourier transform infrared spectroscopy. Transmission electron micrographs indicate that these samples have structures of predominantly 6–7 nm size clusters (instead of the postulated columns). In the as-prepared films, there is a significant concentration of Si—H bonds which is gradually replaced by Si—O bonds during prolonged aging in air. Upon optical excitation these films exhibit strong visible emission peaking at ≊690 nm. The excitation edge is shown to be emission wavelength dependent, revealing the inhomogeneous nature of both the initially photoexcited and luminescing species. The photoluminescence decay profiles observed are highly nonexponential and decrease with increasing emission energy. The 1/e times observed typically range from 1 to 50 μs. The correlation of the spectral and structural information suggests that the source of the large blue shift of the visible emission compared to the bulk Si band gap energy is likely to be due to quantum confinement in the nanometer size Si clusters. The electron-hole recombination process, on the other hand, remains unclear.
A comparative study of the electron trapping and thermal detrapping in SiO2 prepared by plasma and thermal oxidation
Authors: Zhang J. F. , Taylor S. , Eccleston W.
Published: July 2002

Preparation of silicon dioxide (SiO2) thin films by plasma anodization is a promising low temperature (≤600 °C) silicon oxidation technique. This paper investigates the electron trapping and thermal detrapping properties of the plasma grown oxide and compares them with those of the conventional thermal oxide. The avalanche electron injection measurement shows that the electron trapping in the plasma oxide is more severe. The two electron capture cross sections detected in the plasma oxide are of the order of 10−15 and 10−17 cm2, respectively. The former is absent in the thermal oxide and is a feature of oxides prepared at low temperature. The latter is also found in thermal oxide, but its chemical nature is different. Both of them are located near SiO2/silicon interface, indicating that they are related to the intermediate layer. The electron detrapping process in the plasma oxide is different from that in the thermal oxide and has a linear dependence on the logarithmic time. The detrapping is thermally activated with an activation energy of 0.63–0.75 eV. The above traps in plasma oxide can be removed by a high temperature (e.g., 960 °C) post-oxidation annealing (POA) in an inert ambient. After such a POA, the electron trapping in plasma oxide is similar to that in thermal oxides. Some speculation on the trap formation processes is included.
Intrinsic ferrimagnetic resonance linewidth of barium ferrite due to spin-wave scattering by trigonal site single-particle excitations
Authors: Sokoloff J. B. , Marshall S. P. , Vittoria C.
Published: July 2002

Time-dependent two-magnon scattering was previously proposed as a mechanism to explain the large magnitude of the ferrimagnetic resonance (FMR) linewidth of barium ferrite as a function of frequency. In the present work, it is shown that a quantum mechanical mechanism like the Kasuya–Le Craw process (KL)1 but with the phonon excitation replaced by a single-particle excitation of a trigonal site iron ion, which moves in an anharmonic potential well, gives a linewidth contribution of less than a tenth of an Oersted and proportional to the frequency, as in the KL mechanism. We conclude, based on this work and our previous work on the KL mechanism, that neither of these mechanisms can explain the observed FMR linewidths in barium ferrite at any frequency.
Intensity effects in final focusing for heavy-ion inertial fusion
Authors: Hofmann I. , Hasse R. W. , Reiser M.
Published: July 2002

Focusing of heavy-ion beams on an indirectly driven inertial fusion target can be achieved, in principle, under vacuum conditions in the reactor chamber. Due to performance limitations in the storage rings of the driver accelerator, a relatively large number of small emittance beams is required. We suggest that the effect of space charge can be described in terms of three distinct mechanisms: an increase of the entrance divergence angle; a current sensitivity of the focal spot; and the effect of a nonuniform distribution. The first two can be calculated by means of an envelope model. The resulting current sensitivity formula is confirmed by a computer simulation taking into account the correct Coulomb forces between interacting beamlets. We find that for propagation in vacuum the current sensitivity is considerable, in particular for charge states Z≥1, since the spot size blow-up scales like Z4/A2. The effect depends on the total current in a bundle—and not the number of beamlets in it—and thus determines how many bundles have to be used. Possible cures are briefly discussed.
Effect of substrate preparation on smectic liquid crystal alignment: A structural study
Authors: Smela Elisabeth , Martínez-Miranda Luz J.
Published: July 2002

Octylcyanobiphenyl, a bilayer smectic A liquid crystal at room temperature, was deposited on gratings, grids, and flat surfaces, and was studied using high-resolution x-ray scattering at shallow angles of incidence. One surface of the film was in contact with air and the other was in contact with the glass or silicon substrate, which had been treated with one of two different silane solutions. At the air interface, surface tension forces caused the liquid crystal molecules to align perpendicularly with respect to the plane of the substrate. Competing with the liquid crystal–air interface, which is a strong aligner, a grating at the liquid crystal–substrate interface can produce distortions in the smectic layering which results in excess elastic energy and favors alignment parallel to the substrate and the grooves. Our results show that in films less than ∼30 μm thick, the homeotropic orientation was maintained throughout the film due to the constraint of perpendicular alignment at the air interface. However, for thicker films on gratings, Bragg scattering from molecules lying parallel to the grooves was observed. The free-air surface was thus the strongest aligning force, followed by the surface topology, while surface anchoring was not found to play a role.
Compound-lens injector for a pulsed 13-TW electron beam
Authors: Sanford T. W. L. , Poukey J. W. , Halbleib J. A. , Mock R. C.
Published: July 2002

A robust injector capable of controlling the radius and angle of incidence of the pulsed, intense, annular electron beam extracted from the HERMES III accelerator is described. The injector, called the compound lens, uses a tapered anode to control the beam electrostatically, followed by a gas cell with externally applied current to control the beam magnetically. Adjustment of the anode-cathode gap and external current of the injector permits the radius and angle to be defined independently of one another. Measurements of these quantities confirm model expectations.
Optical doping of soda-lime-silicate glass with erbium by ion implantation
Authors: Snoeks E. , van den Hoven G. N. , Polman A.
Published: July 2002

Soda-lime-silicate glass has been implanted with 500 keV Er ions at fluences between 8.6×1014 and 1.8×1016/cm2 with the aim to optically dope the material in the near surface region. The ion range was 100 nm, and Er concentrations in the range 0.09—1.9 at. % were obtained. The characteristic photoluminescence (PL) of Er3+ around 1.54 μm is observed at room temperature in as-implanted glass. The PL intensity increases by an order of magnitude after annealing above 500 °C, as a result of annihilation of implantation-induced defects. Annealing causes an increase in PL lifetime. As a function of Er fluence, the PL intensity first increases, but levels off above ∼6×1015 Er/cm2 (0.6 at. % Er peak concentration). The PL lifetime decreases from 13 to 1.5 ms for increasing Er concentration. The decrease in PL efficiency with concentration is attributed to concentration quenching caused by Er-Er interactions. The optimal combination of PL intensity and lifetime is reached at ≊0.4 at. % peak concentration, for which the lifetime is 6 ms. For high Er concentrations and high pump intensities (∼3 kW/cm2) an additional, intensity dependent quenching mechanism (possibly cooperative upconversion) is observed.
Phase transitions in the classical X-Y and some related models (invited)
Authors: Landau D. P.
Published: July 2002

Although classical spin systems in two dimensions cannot show usual long range order, they may exhibit unusual phase transitions of several different types. We present an overview of the work which has been done to understand both the statics and dynamics of these systems and offer some challenges for the future.
Optical properties of the layered transition-metal-dichalcogenide ReS2: Anisotropy in the van der Waals plane
Authors: Friemelt K. , Lux-Steiner M.-Ch. , Bucher E.
Published: July 2002

The spectral quantum efficiency in ReS2 heterodiodes was measured in the wavelength region between 600 and 850 nm. Anisotropy effects in the van der Waals plane were observed by means of linearly polarized light at normal incidence to the (001) plane of ReS2 single crystals. The anisotropy in quantum efficiency reaches its maximum at λ=627 nm and therefore, ReS2 devices are well suited for detecting the angle of polarization in experiments using a HeNe laser. At wavelengths above 750 nm the sign in polarization quantum efficiency changes due to various optical transitions.
Effect of carbon on the structural and magnetic properties of intermetallic compounds with the ThMn12 structure
Authors: Yang Ying-Chang , Zhang Xiao-Dong , Dong Sheng-Zhi , Pan Qi
Published: July 2002

Carbon was introduced by arc melting to RTiFe11, where R=Sm, Gd, Th, Dy, Ho, Er, and Y, and its effects on the crystallographic and magnetic properties of these compounds have been investigated by x-ray and magnetic measurements. The experimental results show that the carbon atoms occupy 8i sites in ThMn12 structure. It is found that the substitution does not have an effect on the Curie temperature, but instead increases both saturation magnetization and magnetocrystalline anisotropy. The intrinsic magnetic properties of SmTiFe11 are improved with the addition of the carbon atoms, indicating that SmTiFe11−xCx is a better candidate for permanent magnetic applications than SmTiFe11.
Study of electrical properties of pulsed excimer laser deposited strontium titanate films
Authors: Rao G. Mohan , Krupanidhi S. B.
Published: July 2002

Polycrystalline SrTiO3 films were prepared by pulsed excimer laser ablation on Si and Pt coated Si substrates. Several growth parameters were varied including ablation fluence, pressure, and substrate temperature. The structural studies indicated the presence of 〈100〉 and 〈110〉 oriented growth after annealing by rapid thermal annealing at 600 °C for 60 s. Deposition at either lower pressures or at higher energy densities encouraged film growth with slightly preferred orientation. The scanning electron microscopy studies showed the absence of any significant particulates on the film surface. Dielectric studies indicated a dielectric constant of 225, a capacitance density of 3.2 fF/μm2, and a charge density of 40 fC/μm2 for films of 1000 nm thick. The dc conductivity studies on these films suggested a bulk limited space charge conduction in the high field regime, while the low electric fields induced an ohmic conduction. Brief time dependent dielectric breakdown studies on these films, under a field of 250 kV/cm for 2 h, did not exhibit any breakdown, indicating good dielectric strength.
Pressure of phosphorus in equilibrium with solid InP at different temperatures
Authors: Fornari R. , Brinciotti A. , Sentiri A. , Görög T. , Curti M. , Zuccalli G.
Published: July 2002

The phosphorus pressure in equilibrium with solid InP heated at temperatures of up to 980 °C was measured using a new-concept apparatus appositely designed and manufactured. With this apparatus, all the difficulties inherent to handling of phosphorus vapor and high temperatures were overcome without making use of complicated devices such as quartz membranes or helix. This has allowed a reproducible series of measurements of the phosphorus equilibrium pressure to be made. This pressure was found to vary exponentially between 2 and 550 mbar as the InP temperature grows from 800 to 980 °C. The results are of practical interest, particularly if one considers the recent technological efforts for growing InP single crystals under P atmosphere and for obtaining semi-insulating undoped InP by thermal annealing under P counter pressure.
Hydrostatic pressure effect on switching phenomena in copper-containing plasma thin films
Authors: Elallam T. , Akinnifesi J. , Despax B. , Thê-Giam Hoang , Saïdi M. , Bendaoud M.
Published: July 2002

Copper-containing polymer thin films sandwiched between metal electrodes were prepared by plasma polymerization of metal compounds in a radio-frequency glow discharge. Current-voltage (I-V) characteristics reveal the switching phenomenon, characterized by an abrupt impedance variation from the off state (about 108–1010 Ω) to the on state (about 10–102 Ω). The I-V characteristics exhibit an ohmic behavior in the on state. The pressure dependence of the switching threshold voltage (Vth) in the range of 0.1–80 MPa was investigated. It was assumed that Vth variations were due to the sample compression effect by the pressure. Results are analyzed by using Abeles’ model.
Red-to-violet and near-infrared-to-green energy upconversion in LaF3:Er3+
Authors: Reddy B. R. , Nash-Stevenson S. K.
Published: July 2002

When the 4F9/2 state was resonantly excited, emission was detected from the higher states 4S3/2(2H11/2), 4G11/2, and 2P3/2 in addition to the resonant emission. Two- and three-photon processes were found to be responsible in populating the 4S3/2 and the 2P3/2 states, respectively. Energy upconversion efficiencies into the 4S3/2 and the 2P3/2 states were found to be 7.2×10−3 and 1.4×10−4, respectively. When the 4I9/2 state was resonantly excited we detected green emission from the 4S3/2(2H11/2). The energy upconversion efficiency of this process was found to be 1.4×10−3.
Eddy-current inversion in the thin-skin limit: Determination of depth and opening for a long crack
Authors: Burke S. K.
Published: July 2002

A method for crack size determination using eddy-current nondestructive evaluation is presented for the case of a plate containing an infinitely long crack of uniform depth and uniform crack opening. The approach is based on the approximate solution to Maxwell’s equations for nonmagnetic conductors in the limit of small skin depth and relies on least-squares polynomial fits to a normalized coil impedance function as a function of skin depth. The method is straightforward to implement and is relatively insensitive to both systematic and random errors. The procedure requires the computation of two functions: a normalizing function, which depends both on the coil parameters and the skin depth, and a crack-depth function which depends only on the coil parameters in addition to the crack depth. The practical performance of the method was tested using a set of simulated cracks in the form of electro-discharge machined slots in aluminum alloy plates. The crack depths and crack opening deduced from the eddy-current measurements agree with the actual crack dimensions to within 10% or better. Recommendations concerning the optimum conditions for crack sizing are also made.
The influence of the elasto-plastic properties of piezoresistance gauges on their loading-unloading characteristics as lateral shock stress transducers
Authors: Rosenberg Z. , Brar N. S.
Published: July 2002

The elasto-plastic properties of piezoresistance gauges influence their response as stress transducers in shock-wave experiments. In the present article, we discuss these relations for lateral stress gauges upon shock loading and unloading. It is shown that piezoresistance response in the elastic range of the gauge material depends on the properties of the matrix material. On the other hand, these gauges have a unique calibration curve, both for loading and unloading, within their plastic range.
Thermal wave propagation in thin films on substrates
Authors: Maznev A. A. , Hartmann J. , Reichling M.
Published: July 2002

A simple dispersion equation for surface thermal waves propagating along a solid surface covered with a thin film of higher thermal conductivity is presented. It is shown to describe well phase measurements with a photothermal microscope carried out on metal films on glass substrates.
Gain switching of an erbium-doped silica-based planar waveguide laser
Authors: Hattori Kuninori , Kitagawa Takeshi , Ohmori Yasuji
Published: July 2002

The gain switching of an erbium-doped waveguide laser is investigated both theoretically and experimentally. We analyze the lasing characteristics of gain-switched laser based on coupled rate equations, taking ground state absorption and the saturation effect into account. The analyzed theory indicates that the repetition rate, pulse width, and peak power vary when the pump conditions are changed. Gain-switched optical pulses are experimentally generated in the erbium-doped waveguide laser with intensity modulated optical pumping at 651 nm using a dye laser and an acousto-optic modulator. The pulses are controlled with widths of 660–2600 ns and peak powers of 1.0–6.5 mW at repetition rates of 18–60 kHz by changing the average pump rate between 2.0 and 7.0. These experimental results agree with theoretical calculations of the pulse width, peak power, and repetition rate against the pump rate.
Detection and size determination of Ag nanoclusters in ion-exchanged soda-lime glasses by waveguided Raman spectroscopy
Authors: Ferrari M. , Gonella F. , Montagna M. , Tosello C.
Published: July 2002

Waveguided Raman measurements have been performed in order to detect the presence of Ag nanoclusters in soda-lime glass waveguides obtained by ion exchange. By subtracting a strong broadband luminescence and the Raman contribution of the glass matrix, a low frequency Raman peak is resolved. The peak is ascribed to surface acoustic vibrations of silver clusters. The involved modes are the quadrupolar spheroidal ones. The size distribution of the Ag clusters is deduced from the energy and the linewidth of the peak. Silver particles have small dimensions with an upper size limit of about 1.5 nm. This result is in good agreement with recent transmission electron microscopy measurements.
A comparison of annealing kinetics in crystalline and amorphous InP
Authors: Cliche L. , Roorda S. , Kajrys G. E. , Masut R. A.
Published: July 2002

Relaxation of ion-induced strain in crystalline InP is compared with structural relaxation of amorphous InP. Crystalline InP was bombarded with Se ions at low fluence to produce a damaged surface layer. The room temperature evolution of strain in this layer was determined by high-resolution x-ray diffraction and compared to the evolution of the defect-activated viscosity of amorphous InP during similar room temperature structural relaxation. Both processes can be described by double exponential decay functions with characteristic times of a few hours to a few days.
Structure of epitaxial PbSe grown on Si(111) and Si(100) without a fluoride buffer layer
Authors: Müller P. , Fach A. , John J. , Tiwari A. N. , Zogg H. , Kostorz G.
Published: July 2002

Epitaxial growth of PbSe on (111)- and (100)-oriented Si substrates without an intermediate buffer layer is studied. It is found that on Si(111) the orientation of the IV-VI layer can by varied from (100) at 200 °C to (111) at 400 °C substrate temperature. On Si(100), only (100)-oriented layers were obtained for the whole temperature range. (100)-oriented layers with thicknesses above 0.5 μm were cracked due to thermally induced mechanical strain on cooldown to room temperature. This strain cannot be relaxed by dislocation glide in the first glide systems as it is the case for (111)-oriented layers. The structural quality of (100)-oriented PbSe layers on Si(100) and Si(111) is inferior compared to layers grown with an intermediate BaF2/CaF2 or CaF2 buffer layer. This implies that the covalent/ionic PbSe/Si interface seems to impede high-quality epitaxy, contrary to the well known ionic/ionic IV-VI/IIa-fluoride interface.
Induced and local anisotropies in amorphous CoZr–rare earth thin films containing Pr, Nd, and Tb
Authors: Calleja J. F. , Corrales J. A. , Rivas M. , Iglesias I. , Contreras M. C. , Suran G.
Published: July 2002

The induced in-plane and local magnetic anisotropies of rf sputtered (Co93Zr7)100−xREx thin films were investigated as a function of RE content for RE=Pr, Nd, and Tb by transverse biased initial susceptibility measurements at both film/air and glass/film interfaces. The deposition was performed in a magnetic field. The films exhibit a very well defined in-plane anisotropy with negligible long-range fluctuations and a small coercive field Hc, in accordance with the experimental data. The overall variations of the transverse susceptibility are related to the ripple. The variations of the local anisotropy with composition were measured. Both the induced anisotropy Ku and the local anisotropy Kloc increase with the increasing amount of RE in the layers. A clear relationship between Ku and Kloc could be established. The results are discussed in terms of the single-ion anisotropy of the rare earth. While the value of Ku is the same at both interfaces, the ripple constants were found to be slightly different. This last result is believed to be related to some local defects, the origin of which is also suggested. Also the coercive force is discussed and explained in terms of a theoretical model.
Neutron diffraction and magnetic studies of RFe12−xTxCy (R=Y,Er; T=V,Ti,Mo) alloys
Authors: Hu Z. , Yelon W. B. , Zhang X. , James W. J.
Published: July 2002

RFe12−xTxCy (R=Y,Er; T=V,Ti,Mo) alloys were prepared by rf induction melting and analyzed using neutron powder diffraction and superconducting quantum interference device (SQUID) measurements. Rietveld analysis of the neutron diffraction data indicates that V, Ti, and Mo atoms all prefer the 8i sites. The refined amount of carbon atoms found in the interstitial sites from neutron diffraction data is significantly less than the nominal carbon content. All samples have the easy direction along the c axis. The Er sublattice couples to the Fe sublattice antiferromagnetically. The average Fe site moments range from 1.3 to 2.8 μB. The anisotropies of the crystal structures are found to relate to both the rare earth atoms and the stabilizing transition metal atoms. The SQUID measurements show that all samples have a Curie temperature near 600 K.
Controlled micro oxidation of CdTe surface by laser irradiation: A micro-spectroscopic study
Authors: Rai Brajesh K. , Bist H. D. , Katiyar R. S. , Chen K.-T. , Burger A.
Published: July 2002

A small area of CdTe single crystal within a spot of ≤2 μm diameter has been oxygenated under ambient conditions using 514.5 nm radiation from Ar+ laser. The oxygenation could be monitored using laser Raman spectroscopy, and it has been established that the asymmetric TeO2−3 ion is formed on the surface. The significance of the process for basic material science and technological applications in electronic and optoelectronic devices has been emphasized.
Electron spin resonance studies in β-FeSi2 crystals
Authors: Aksenov I. , Katsumata H. , Makita Y. , Kimura Y. , Shinzato T. , Sato K.
Published: July 2002

The electron spin resonance studies have been carried out in the temperature range 130–300 K on semiconducting β-FeSi2 single crystals grown by a chemical vapor transport technique. Two anisotropic doublets with apparent g factors in the range 2.025–2.05 and 1.98–2.03, as well as one complex signal having an isotropic g factor of 2.0195 and exhibiting a five-line hyperfine structure, have been detected. The doublet signals are believed to arise from spin triplet (S=1) states of, presumably, substitutional Ni2+ transition ions, whereas the signal exhibiting the hyperfine structure has been attributed to the spin of a hole, captured by silicon vacancy and interacting with nuclear spins of four iron atoms in the first shell surrounding of the silicon vacancy.
Effect of stress and composition on the Raman spectra of etch-stop SiGeB layers
Authors: Pérez-Rodríguez A. , Romano-Rodríguez A. , Cabezas R. , Morante J. R. , Jawhari T. , Hunt C. E.
Published: July 2002

Si1−(x+y)GexBy strained layers on Si (x≤3.4%, y≤0.4%) have been analyzed by Raman spectroscopy. Stress in the layers has not been observed to affect the Fano interaction parameters of the first-order Si–Si Raman line. These parameters have been determined in the range of B concentrations from 5×1019 to 2×1020 cm−3 and Ge fractions from 1% to 3.4%. The observed shift in the spectra has been found to depend linearly on both the germanium and boron contents. These data have been correlated with the stress measured in the layers by mechanical wafer bow measurements. The dependence of the Raman shift on the germanium content and strain agrees with that previously reported for strained SiGe layers. According to these data, Raman spectroscopy appears as an interesting tool for the nondestructive assessment of stress and composition of these layers.
Controllable laser-induced periodic structures at silicon–dioxide/silicon interface by excimer laser irradiation
Authors: Lu Y. F. , Choi W. K. , Aoyagi Y. , Kinomura A. , Fujii K.
Published: July 2002

Laser-induced periodical microstructure in a Si substrate covered with a thin layer of silicon dioxide has been studied using KrF excimer laser irradiation for controlling the periodicity. It was found that KrF excimer laser irradiation can produce periodical microstructures in SiO2/Si samples by a single pulse if the laser fluence is large enough when the SiO2 thickness is small. When the SiO2 layer is thick and more than one laser pulse is required, circular patterns can be observed due to the interface defects. The periodicity of the ripple structure linearly depends on the SiO2 thickness. The formation of microstructure does not change the thickness of the SiO2 layer and the crystallinity in the Si substrate. The ripple structure formation in the SiO2/Si structure is related to the thermally generated surface waves. The existence of a SiO2 layer on Si substrate can change the surface tension during the melting of the Si interface and hence control the periodicity of the ripple formation. The lateral periodicity and vertical roughness of the ripple structures are within the range required for laser microtexturing of magnetic recording media.
Theory of two species of electron–hole with shallow electron trap transport in photorefractive media
Authors: Li Yanqiu , Liu Shutian , Hong Jing , Xu Kebin
Published: July 2002

A new model of two species of electron–hole with shallow electron trap is presented. The model predicts that if the shallow trap accumulates large density of charge carriers, the photoconductivity can be a sublinear (or superlinear) function of light intensity, and the space-charge field increases (or decreases) and reaches a saturate value as intensity increases depending on the sign of dominant carrier. Comparison this model with previous ones shows that this model provides more common descriptions about photoconductivity and space-charge-field dependence of intensity.
Photoinduced admittance spectroscopy to detect the shallow electron traps in nitrogen-doped highly compensated ZnSe
Authors: Lu Fang , Wang Shouqi , Jung Hyundon , Zhu Ziqiang , Yao Takafumi
Published: July 2002

This article describes a technique to detect shallow levels in highly compensated ZnSe by photoinduced admittance spectroscopy (PIAS). Nitrogen doped ZnSe with a Schottky barrier on top has been investigated, and the photovoltage measurements show that the electric charge in the depletion layer becomes positive under high intensity illumination with photon energies larger than the energy gap of ZnSe. An electron trap with an activation energy of 50 meV was obtained from the PIAS measurement. The accuracy of this technique is confirmed by reconstruction of the temperature dependence of the capacitance and conductance by a computer simulation.
Tunable band offsets in ZnSe/GaAs heterovalent heterostructures grown by metalorganic vapor phase epitaxy
Authors: Funato Mitsuru , Aoki Satoshi , Fujita Shizuo , Fujita Shigeo
Published: July 2002

The tunability of band offsets in ZnSe/GaAs(001) heterovalent heterostructures is investigated. The interface composition, Ga/As, is controlled by means of Zn or Se treatment or by thermal etching of the GaAs surfaces before the growth of ZnSe. Consequently, it is revealed by X-ray photoemission spectroscopy that artificial control of Ga/As from 1.0 to 2.8 leads to variation of the valence band offsets from 0.6 to 1.1 eV. Based on the electron counting model and the layer-attenuation model, a structural model which is responsible for the Ga-rich interface and for the increase of valence band offset is proposed, in which the As plane just below the interface consists of As, anti-site Ga and As vacancies. The electronic properties of the n-ZnSe/p+-GaAs heterojunction diodes (HDs) provide further evidence of the tunability of the band offsets at the interface, that is, the diffusion potentials in the HDs are modified according to the interface compositions.
Thermal deposition and characterization of Se-Sn mixed oxide thin films for NO gas sensing applications
Authors: Manno D. , Micocci G. , Serra A. , Tepore A.
Published: July 2002

Thin films made by a mixing of selenium and tin oxides have been obtained by high vacuum thermal evaporation. A detailed optical, electrical and structural characterization has been performed on all deposited films. Electron microscopy observations show that the films consist of nanosized grains of SeO2 and SnO2 homogeneously arranged. The NO gas sensing properties of this material have been tested in controlled atmosphere. In addition, the influence of oxygen in the test gas mixture was analyzed.
Structure-property relationships in lead zinc niobate based ferroelectric ceramics
Authors: Fan Huiqing , Kong Lingbing , Zhang Liangying , Yao Xi
Published: July 2002

The region of the (0.9-x)Pb(Zn1/3Nb2/3)O3-0.1BaTiO3-xPbTiO3 (0.0⩽x⩽0.2) phase diagram was established through analysis of the structure and dielectric behavior associated with changes in temperature and composition. The zero-field spontaneous and field-induced relaxor-normal ferroelectric transitions are reported in this ferroelectric system, and this transition corresponds to the rhombohedral-tetragonal first-order phase switching. From careful measurement of the hystersis loop, it is suggested that the microscopic mechanism of spontaneous polarization of the ferroelectric microregion was further correlated with the phase structure. Thus, the thermal hysteresis of micro–macrodomain transition of relaxor ferroelectrics can be explained very well.
Field solutions for bidirectional high-gain laser amplifiers and oscillators
Authors: Azadeh Mohammad , Casperson Lee W.
Published: July 2002

General analytical solutions are obtained for the amplitude, phase, and intensity of the electromagnetic waves in bidirectional homogeneously broadened high-gain laser amplifiers and oscillators. These solutions are important as increasingly high-gain lasers are being employed in practical systems. Expressions are derived relating the output power to the input, including the effects of arbitrary mirror reflectivities and frequency detunings from the line center. For negligible reflectivities, these regenerative amplifier results reduce to earlier expressions for single-pass high-gain amplifiers. Multivalued outputs also occur, and in the limit of low gain per pass the results are consistent with earlier studies of single-frequency laser oscillators.
Magnetic behavior of NiFe/NiO bilayers
Authors: Qian Zhenghong , Sivertsen John M. , Judy Jack H.
Published: July 2002

In this study, the magnetic behavior of NiFe/NiO bilayers has been investigated. The relationship between the magnetic properties and interfacial interaction has been briefly discussed. Since magnetic properties of the bilayers are mainly determined by interfacial magnetic interaction, in this article, not only unidirectional interfacial anisotropy has been used to explain exchange field, Hex; also, for the first time, uniaxial interfacial anisotropy is introduced to describe interfacial interaction affecting the magnetic properties, especially coercivity Hc of the bilayers. In addition, a model is proposed to explain the magnetic behavior of the bilayers. The model has been tested by experiment. The experimental results are consistent with the model prediction, and this also confirms the assumption that the magnetization of the bilayer reverses by magnetic rotation.
Origin of magnetic anomalies in the spin glass system, La0.85Sr0.15CoO3
Authors: Anil Kumar P. S. , Joy P. A. , Date S. K.
Published: July 2002

The composition with x=0.15 in the spin glass regime (0⩽x⩽0.18) of the perovskite system, La1−xSrxCoO3, is investigated in detail to understand the unusual magnetic properties reported in the literature. AC susceptibility and field-cooled and zero-field-cooled magnetization measurements give sufficient evidence for compositional inhomogeneity as the origin of the magnetic anomalies. A well-defined spin glass behavior is obtained only for a highly homogeneous sample.
Extended x-ray absorption fine structure measurements of nonsuperconducting PrBa2Cu3O6.9: Evidence against Ba site Pr substitution
Authors: Harris V. G. , Fatemi D. J. , Browning V. M. , Osofsky M. S. , Vanderah T. A.
Published: July 2002

Extended x-ray absorption fine structure (EXAFS) measurements on nonsuperconducting PrBa2Cu3O6.9 are reported. Theoretical Pr LII EXAFS data, calculated for the case of Y-site and Ba-site occupation, were least-squares fit to the experimental Pr LII EXAFS. Results indicate that the Pr ions overwhelmingly occupy the Y sites. These results present a serious challenge to models which rely on Ba-site Pr substitution as an explanation for the lack of superconductivity in PrBa2Cu3O6.9.
Anisotropic thermal conductivity of c-axis aligned Bi2Sr2CaCu2Ox superconductor in high magnetic fields
Authors: Nakamae S. C. , Crow J. , Sarrao J. , Schwartz J.
Published: July 2002

The anomalous behavior of the thermal conductivity, κ, of high temperature superconductors (HTS) has been attributed to two conflicting theories (phonon domination and electron domination) since the discovery of HTS. Change in κ due to a magnetic field is a direct consequence of additional scattering processes between the phonons, the charge carriers, and the flux lines. The existing theories predict different scattering effects for the phonons and the electrons. Therefore, knowing magnetothermal conductivity can provide useful information for the nature of heat carriers in HTS and the understanding of the vortex dynamics. We have measured the in-plane thermal conductivity of a c-axis aligned Bi2Sr 2CaCu2Ox superconductor in magnetic fields up to 17 T. The magnetic field was applied both parallel and perpendicular to the c axis and always perpendicular to the direction of heat flow. The results are compared to existing electronic and phononic models. It has been discovered that the electronic thermal conductivity, κe, calculated from the Wiedemann–Franz law, does not predict the measured behavior of the thermal conductivity. Rather, the phonon thermal conductivity, κph, calculated from the extended BRT (Bardeen, Richayzen, and Tewordt) theory, reproduces the experimental results accurately. The anisotropy of the in-plane magnetothermal conductivity is also investigated.