bluuebunny/crossref_metadata_2025 huggingface dataset:
Noise and coupling in magnetic super-resolution media for magneto-optical readout
Authors: Peng Chubing , Mansuripur M.
Published: July 2002

Interfacial magnetic coupling, transient thermal response, and carrier and noise levels are investigated for two central aperture detection magnetic super-resolution disks. In one of the disks, the two magnetic layers are exchange-coupled, while in the other the coupling is of magneto-static nature. For the exchange-coupled disk, the coupling between the two magnetic layers is fairly strong, and the Kerr loop of the readout layer does not have a square shape. For the magneto-statically coupled disk, the strength of coupling depends on the nonuniformity of the magnetization of the storage layer. The readout layer has a square Kerr loop, but its perpendicular magnetization in the hot region under the focused spot has random orientation if the stray field from the storage layer is weak. This random orientation of magnetization within the readout layer gives rise to a high level of noise during readout.
Simultaneous analysis of current–voltage and capacitance–voltage characteristics of metal–insulator–semiconductor diodes with a high mid-gap trap density
Authors: Cova P. , Singh A. , Masut R. A.
Published: July 2002

We present an improved method to analyze simultaneously the current–voltage and capacitance–voltage characteristics of metal–insulator–semiconductor (MIS) diodes. We use the method to study the effect of Zn doping concentration on the current transport in Au MIS contacts fabricated on In0.21Ga0.79As layers grown by metalorganic vapor phase epitaxy on highly doped GaAs substrates. At room temperature and for low reverse bias voltage, the generation/recombination process via mid-gap traps is the only dominant mechanism in these MIS diodes. For high reverse bias, both this mechanism and thermionic-field emission control current transport. The generation/recombination current observed is due to donor type mid-gap traps whose density shows an almost linear dependence with Zn concentration. The value of the barrier height at zero bias and at room temperature (φb0=0.73 V±12%) is independent of the Zn concentration. For the procedure used to prepare the In0.21Ga0.79As:Zn surfaces, the thickness of the oxide layer and the transmission coefficient of holes across this layer depend on the Zn doping concentration in the range 7×1014⩽NA⩽5×1018 cm−3. Zn doping seems to inhibit the formation of the unintentional native oxide on the surface of In0.21Ga0.79As epilayers.
Analysis of the scattering of electrons by special vibrational modes in a GaAs quantum well with a thin AlAs middle barrier: Mobility modulated by a transversal electric field
Authors: Wang X. F. , da Cunha Lima I. C. , Troper A. , Lei X. L.
Published: July 2002

The scattering rates for the intra- and intersubband transitions of electrons in a GaAs/AlAs quantum well with a central AlAs thin layer assisted by the emission and absorption of optical phonon modes are calculated in the case where a transversal electric field is applied through the structure in order to modulate the electronic density inside the well. The role of the confined and interface phonon modes in reducing the electron mobility is analyzed separately to permit the identification of the effect of the transversal electric field on this mobility. The electron behavior indicates that electrons of different subbands can be separated in real space by the applied transversal electric field.
Structure, magnetic properties, and magnetostriction of Sm0.88Dy0.12(Fe1−xMx)2 (M=Mn, Al) (0⩽x⩽0.2)
Authors: Guo Z. J. , Busbridge S. C. , Wang B. W. , Zhang Z. D. , Zhao X. G.
Published: July 2002

The structure, magnetic properties, and magnetostriction of arc-melted polycrystalline Sm0.88Dy0.12(Fe1−xMx)2 (M=Mn, Al, 0⩽x⩽0.2) alloys were investigated by microscopy, x-ray diffraction, A.C. initial susceptibility, vibrating sample magnetometry, and standard strain gauge techniques. It was found that the matrix consists almost entirely of the MgCu2-type cubic Laves phase, with a small amount of rare-earth rich phase. The lattice parameter increases approximately linearly with increasing x from 0.740 to 0.750 nm for M=Al. For M=Mn the lattice parameter increases by 0.002 8 nm over the same increase in x. The Curie temperature decreases with increasing x from 670 to 410 and 550 K for M=Al and Mn, respectively. The spontaneous magnetostriction, λ111, and polycrystalline magnetostriction, λs, both decrease with increasing Al or Mn substitution for Fe from their initial values of −1452 and −1010 ppm, respectively. Premature disappearance of the splitting of the (440) line and changes in the shape of the magnetostriction curves suggest the substitution of Mn for Fe may produce a change in the anisotropy. No such effects were observed for M=Al. The saturation magnetization, MS, decreases approximately linearly with increasing substitution of Al or Mn. The changes in the magnetization and magnetostriction can be understood on the basis of a decreasing 3d sublattice moment.
Fracture criteria for piezoelectric materials containing multiple cracks
Authors: Chao Lu-Ping , Huang Jin H.
Published: July 2002

This study presents a failure criteria for multiple permeable cracks embedded in an infinite piezoelectric solid, which is separately subjected to a set of uniform-electromechanical loads. Based on the equivalent inclusion method, permeable cracks are initially treated as elliptical inclusions with fictitious eigenstrains and eigenelectric fields, in which their elastic moduli and piezoelectric coefficients are taken as zero while dielectric constants remain finite. Consequently the interaction energy between the cracks and the applied electromechanical loading can be simply expressed in terms of the applied electromechanical loads and the equivalent eigenfields. With this energy function, the energy release rates and the critical loads for fracture are separately acquired in a closed form for a simple tension, in-plane and out-of-plane shears, as well as normal electric flux density applied. The closed forms for energy release rate and critical electromechanical loading reveal that these forms are a function of the aspect ratio of the permeable elliptical cracks, type of the electromechanical loading, volume fraction of the cracks, and piezoelectric properties. Analysis results indicate that the different mechanical loading propagates the expansion of the elliptical cracks. Meanwhile, the distinct electric fields can retard the dilation of the elliptical cracks, particularly for an in-plane electric field incited perpendicularly with cracked faces.
A theoretical model for neutral velocity distributions at a planar target in plasma source ion implantation
Authors: Wang Dezhen
Published: July 2002

A theoretical model is developed to study the velocity distributions of neutrals striking a planar target in plasma source ion implantation (PSII) for the case in which the pressure of the gas is large enough that the fast neutrals can be produced in the sheath by ion-neutral charge exchange collisions. An analytic expression for the neutral velocity distribution at the target is provided. The theoretic results agree with Monte Carlo simulations.
Magnetic properties of sputtered strontium ferrite films prepared from sintered target with Fe/Sr composition ratio of 8–14
Authors: Morisako A. , Saitoh M. , Matsumoto M.
Published: July 2002

Hexagonal strontium-ferrite (SrM) thin films were deposited by sputtering with the target composition ratio N (Fe/Sr) in the range of 8–14. The discrepancy of the composition ratio between the films and the target was found and the deposited films were in Fe-rich. In the x-ray diffraction diagram, the observed diffraction lines varied with the composition of the target and preferential c-axis orientation can be observed in the film prepared with the target, where the composition N is 10. The grain shape as well as size was found to be independent of N. The magnetic easy axis was normal to the plane. The maximum saturation magnetization and coercivity in the perpendicular direction were about 300 emu/cc and 3.5 kOe, respectively, when the film was prepared with the target with N of 8.
Modification of the twist angle in chiral nematic polymer films by photoisomerization of the chiral dopant
Authors: van de Witte Peter , Neuteboom Edda E. , Brehmer Martin , Lub Johan
Published: July 2002

A method for the production of polarization sensitive recordings in liquid crystalline polymers is presented. The system is based on local modification of the twist angle of chiral nematic polymer films. The twist angle of the polymer film is varied by modifying the chemical structure of the chiral dopant. Here a photoisomerizable menthone derivative is used that has a fivefold difference in helical twisting power between the E and the Z isomer. The twist angle of the film can be varied between −90° and 0° by introducing a nonphotosensitive chiral dopant with opposite twisting sense. Complex pictures with gray scales can be recorded in the films with black and white contrasts higher than 20.
Orange-green emission from porous Si coated with Ge films: The role of Ge-related defects
Authors: Wu X. L. , Gu Y. , Siu G. G. , Fu Eric , Tang N. , Gao T. , Bao X. M.
Published: July 2002

Photoluminescence (PL) spectra of porous Si (PS) coated with Ge films were examined using the 514.5 nm line of Ar+ laser. A new orange-green PL band, centered at 2.25 eV, was observed with full-width at half-maximum of ∼0.1 eV, in addition to the reported PL bands at 3.1, 2.0, and 1.72 eV. With increasing the thickness of Ge layer coated, the new PL band remains unchanged in peak energy but drops abruptly in intensity. Spectral analysis and some experimental results from Raman scattering and x-ray diffraction indicate that Ge-related defects at the interfaces between PS and the Ge nanocrystals embedded in the pores are responsible for the orange-green PL band.
Differential gain, differential index, and linewidth enhancement factor for a 4 μm superlattice laser active layer
Authors: Anson S. A. , Olesberg J. T. , Flatté Michael E. , Hasenberg T. C. , Boggess Thomas F.
Published: July 2002

We describe temporally and spectrally resolved measurements of the material differential gain, differential refractive index, and linewidth enhancement factor for a multilayer superlattice intended for use in midwave-infrared semiconductor lasers. We find good agreement between measured quantities and theoretical predictions based on a superlattice K⋅p formalism. The superlattice was designed for suppression of Auger recombination and intersubband absorption, and we find that the strategies employed in this process result in other characteristics that are desirable in a semiconductor laser gain medium. Specifically, for carrier densities and wavelengths appropriate to threshold in an optimized cavity configuration, this structure has a differential gain of approximately 1.5×10−15 cm2, a value comparable to that reported for near-infrared strained quantum wells. The peak gain and peak differential gain are nearly spectrally coincident, leading to a small value for the differential index. The large differential gain and small differential index result in a linewidth enhancement factor of less than one. This indicates that filamentation in high-power lasers based on this superlattice should be suppressed and that this structure is attractive for use in midwave-infrared lasers designed for spectrally pure operation.
Linearity of sensitive YBa2Cu3O7−x dc superconducting quantum interference device magnetometers
Authors: Beyer J. , Drung D. , Ludwig F. , Schurig Th.
Published: July 2002

The linearity of sensitive superconducting quantum interference device (SQUID) magnetometers fabricated from high critical temperature (high-Tc) superconductors and operated in a direct-coupled flux-locked loop with bias reversal was investigated. The system nonlinearity was determined by applying a sinusoidal test signal and measuring the output signal harmonics using a setup which ensures the elimination of nonlinearities arising from the test signal source and the spectrum analyzer. The experimental setup enables the simultaneous measurement of the harmonic distortion of a single magnetometer and of a first-order electronic gradiometer formed by two individual magnetometers. Test signal amplitudes and frequencies were chosen to simulate typical laboratory interferences. An analysis of the results was performed using analytical equations in which the SQUID’s inherent nonlinear character is taken into account. At signal frequencies above about 0.1% of the system bandwidth, the system nonlinearity was found to be mainly caused by the nonlinear behavior of the SQUID magnetometers, whereas at low frequencies nonlinearities arising from read-out electronics components predominate. For signal peak-to-peak amplitudes up to 1 μT and frequencies up to 0.5 kHz, total harmonic distortions below about −105 dB for a single sensor channel and −100 dB for an electronic gradiometer were obtained.
A discharge lamp based on MgH radiation
Authors: Ukegawa Shin , Gallagher Alan
Published: July 2002

We report the efficacy and spectrum of an inductively-coupled, radio-frequency discharge in mixtures of magnesium (Mg), hydrogen (H2), and argon vapor. The predominantly green emission is dominated by the A–X band of MgH, plus some Mg lines. The discharge operates in a frit-sealed, ∼600 °C alumina cell with a sapphire window, at pressures of 200–600 Pa. The plasma impedance is established from the impedance of the matching network parameters, and the electron temperature is determined from line intensity ratios. Assuming isotropic emission, a maximum energy efficiency (η) of ∼ 10% (efficacy ∼ 90 lm per watt) is achieved, where η is the ratio of visible emission to power delivered to the matching network.
Defects in Ge+-preamorphized silicon
Authors: Chen Peng-Shiu , Hsieh T. E. , Hwang Yih-Chyang , Chu Chih-Hsun
Published: July 2002

This work studied the morphology and annealing behaviors of extended defects in Si subjected to various Ge+ preamorphization and BF2+ implantation conditions. The extended defects formed were near the specimen surface when Ge+ implantation energy and dose amount were low. During subsequent annealing, the end-of-range (EOR) loops were enlarged and then moved out of the specimen. High energy/low dose Ge+ implantation generated a damaged layer which initially transformed into a wide zone containing dislocation loops and rodlike defects in the annealed specimen. As the annealing proceeded, the width of defective zone gradually shrunk so that most of the extended defects could be annihilated by defect rejection/recombination process. In addition to the category II defects found in previous investigations, hairpin dislocations emerged in high energy/high dose Ge+-implanted specimens. In this specimen, rodlike defects and hairpin dislocations could be removed by annealing, while the EOR loops became relatively inert so that their removal would require high temperatures and/or long annealing times. Microwave plasma surface treatment was also carried out to form a nitride layer on specimen surface. Experimental results indicate that in addition to effectively reducing the size of EOR loops, surface nitridation might serve as a vacancy source injecting vacancies into Si to annihilate the interstitials bounded by dislocation loops. Reduction in the defect size was pronounced when bias voltage was added to the plasma process. However, radiation damage might occur with too high of a bias voltage.
Theoretical and experimental evidence for “true” atomic resolution under non-vacuum conditions
Authors: Sokolov I. Yu. , Henderson G. S. , Wicks F. J.
Published: July 2002

Imaging of atomic scale features (Angstrom resolution) such as individual atoms using an atomic force microscope (AFM) remains highly controversial. Arguments that such resolution is not achievable revolve around two points: (1) that atomic scale defects are not observed and (2) often the experimental images are not reproduced theoretically. Here we show that the AFM is capable of imaging atomic features by presenting images of atomic scale defects at ∼1 Å resolution. Dislocation defects and monatomic growth steps on the {001} surface of the mineral anhydrite (CaSO4) are observed in unfiltered AFM images. Furthermore, the atomic features observed in the experimental AFM images are reproduced by numerical simulation; conclusively addressing the two points above. The combination of experimental images and theoretical simulations enables the comprehensive interpretation of the images. The images of the {001} surface indicate that the AFM observes both the calcium and oxygen atoms at the cleavage plane. In addition, two configurations of the oxygen atoms are observed; a dumbbell shaped feature due to two oxygens (from two different SO4 tetrahedra) sitting just below the cleavage plane and a doughnut shaped feature composed of oxygens from four different SO4 tetrahedra (the two above and two standing above the cleavage plane). The latter are partly due to tip deformation of the surface and are the most common features in experimental images.
Classical coarsening theory in heteroepitaxial systems
Authors: Iwamatsu Masao , Okabe Yutaka
Published: July 2002

Classical coarsening theory, originally developed by Lifshitz, Slyozov, and Wagner (LSW), describes the growth of grains or islands and predicts time-independent self-similar island-size distribution functions. This dynamic scaling is reconsidered here to include island–island elastic interactions, line tension of the islands, and diffusion barriers for adatoms due to the strained substrate with a simple model that preserves self-similarity. These effects significantly modify the size distribution of islands. In particular, the size distribution becomes narrower than originally predicted by LSW theory as derived from the Gibbs–Thomson formula. However, the time dependence of the average island size is described by usual power law formula.
The growth and electrical properties of silicon on GaAs(110) studied by scanning tunneling microscopy and scanning tunneling spectroscopy
Authors: Dunstan P. R. , Wilks S. P. , Teng K. S. , Pritchard M. A. , Williams R. H.
Published: July 2002

A scanning tunneling microscope (STM) has been used to study the structural formation of silicon overlayers deposited at room temperature on GaAs(110). In addition spectroscopic measurements were obtained simultaneously to reveal electrical properties associated with the interface and overlayer formation. The Si coverage varied in thickness from submonolayer growth up to ∼16 monolayers. The height variations of the STM images indicate that silicon islands did not exceed ∼3 Å at either submonolayer growth or thicker coverages. The lack of atomic resolution of the adsorbed silicon suggests that the silicon overlayers are amorphous in nature, confirming soft x-ray photoelectron spectroscopy (SXPS) measurements. The scanning tunneling spectroscopy measurements clearly indicate a Fermi shift of 0.8 eV when silicon was deposited on the clean cleaved surface of p+ GaAs(110). This shift is also observed on regions of the substrate which remained exposed after the silicon deposition. The Fermi shifts and resultant band bending are attributed to the formation of the Si–GaAs heterojunction and correlation with earlier SXPS measurements prove favorable. The possibilities of measuring the band offsets are also considered.
Effect of a dimensional crossover on the upper critical field of practical Nb–Ti alloy superconductors
Authors: Cooley L. D. , Hawes C. D.
Published: July 2002

Multilayers of a Nb0.37Ti0.63 alloy, a chief material of superconducting magnet technology, and a Cu0.95Sn0.05 or Cu0.70Ni0.30 alloy exhibit a dimensional crossover with a decreasing bilayer period Λ=dN+dS, where dS=3dN. Cusps of Hc2(θ) and square root Hc2∥(T) develop when Λ⩽40 nm, which indicate a crossover to 2D behavior from 3D behavior seen at Λ=60 nm. Full proximity coupling of Cu–Sn layers for Λ=13 nm restores isotropic angular dependence, but with sharply lower Hc2 values. By contrast, proximity coupling was suppressed by magnetic Cu–Ni layers, and 2D behavior was retained while Tc fell below 4 K for Λ<20 nm. The data are consistent with numeric results obtained by Takahashi and Tachiki [Phys. Rev. B 33, 4620 (1986)] when the variation of the Bardeen–Cooper–Schrieffer pairing potential is the primary cause of the dimensional crossover. Since practical Nb–Ti conductors have a layered nanostructure, this result suggests that a dimensional crossover should also be found in wires. However, the 3D–2D crossover occurs when Λ is much greater than the separation of the flux lines at high field (10–20 nm) and above the range where optimum flux pinning is found. This implies that a 2D state (for insulating or magnetic layers) or a 2D strongly coupled state (for normal metals) exists when flux pinning is strongest. These implications are discussed in the context of practical Nb–Ti wires used in superconducting magnet technology.
Inspection of inhomogeneous samples by combined laterally scanned and frequency resolved photothermal measurements
Authors: Walther H. G. , Aleshin V.
Published: July 2002

We proposed an approach for the estimation of the depth profile of thermally inhomogeneous samples from combined laterally and frequency resolved photothermal measurements. The mathematical procedure of data inversion makes use of the quasianalytical solution of the forward problem. Introducing an appropriately chosen grid of depth coordinates the Hankel transform of the surface temperature can be expressed by a continuous fraction formula. This enables the usage of the effective conjugated gradient technique to retrieve the thermal depth profiles by minimization of the objective function. Making use of the a priori information about the inhomogeneous sample we chose an appropriate Tikhonov‘s stabilizer function and by this way remarkably improved the iteration procedure. The success of this method consisted both in a drastically reduced computation time and in a better approximation of the searched profiles. This was demonstrated by numerical simulations modeling the case of surface hardened steel.
Thermoelastic and stress-optical behavior of aromatic-aliphatic polyester networks
Authors: Reis-Nunes R. C. , Riande E. , Guzmán J. , Chavez N. A.
Published: July 2002

Thermoelastic networks were prepared by crosslinking hydroxyl terminated poly(pentamethylene glycol isophthalate) with tri(p-isocyanate-phenyl)-thiophosphate. The thermoelastic and stress-optical behavior of the networks were studied at several temperatures in the range of 10–70 °C. The values of the temperature coefficient of the unperturbed dimensions, d ln〈r2〉/dT, and the optical configuration parameter Δa were −4.5×10−3 K−1 and 22.7×10−24 cm3, respectively. Theoretical calculations carried out with the rotational isomeric state model gives values of d ln〈r2〉/dT in very good agreement with the experimental results. However, the theoretical values of Δa are nearly one order of magnitude lower than the experimental ones. No reasonable modification of conformational energies or contributions to the anisotropic part of the polarizability tensor would achieve agreement between theory and experiments. The discrepancy between theoretical and experimental results may be qualitatively explained by intermolecular interactions.
Electron swarm parameters in SF6/Ne mixtures determined by swarm experiment
Authors: Xiao D. M. , Liu H. L. , Chen Y. Z.
Published: July 2002

The electron swarm growth processes in SF6/Ne gas mixtures have been studied by a pulsed Townsend method over the range 8⩽E/P⩽140 V mm−1 kPa−1, where E is the electric field and P is the gas pressure. The variation patterns as a function of the pressure reduced electric field of the effective ionization coefficient ᾱ, electron drift velocity, Ve, and longitudinal diffusion coefficient DL in SF6/Ne gas mixtures have been given. The dielectric strength of SF6/Ne gas mixtures has also been determined, which varies linearly with SF6 concentration in the gas mixtures.
Dielectric relaxation in silica glass
Authors: Kakiuchida H. , Saito K. , Ikushima A. J.
Published: July 2002

Dielectric dispersion in silica glasses with various OH concentrations were investigated from 20 Hz to 1 MHz in the temperature range from 30 to 1000 °C. Dielectric relaxation, which could be attributed to the elementary process of structural secondary relaxation caused by OH motion, has the activation energy between 2.3 and 2.6 eV. ac and dc electrical conductivities and diffusion coefficient of OH have been deduced from the imaginary part of the dielectric constant. The ac electrical conductivity shows the characteristic feature as is usually observed in amorphous solids. The dc electrical conductivity and diffusion coefficient of OH derived therefrom obey the Arrhenius law with the activation energy of 1.0±0.2 eV in the temperature range from 350 to 1000 °C.
Effect of density of Ge nanocrystals on violet–blue photoluminescence of Ge+-implanted SiO2 film
Authors: Zhang Jia-Yu , Ye Yong-Hong , Tan Xi-Lin , Bao Xi-Mao
Published: July 2002

Luminescent SiO2 films containing Ge nanocrystals (nc-Ge) are fabricated by Ge+ ion implantation and thermal annealing. The nc-Ge density is controlled by the implanted Ge+ dose. Under ultraviolet excitation, the films exhibit two photoluminescent (PL) bands simultaneously in the violet–blue region. With increasing annealing temperature (Tan) in the range of 1100 °C, the relative ratio of the intensities of the two PL bands increases slightly from 2.3 to 3 for the films with a dose of 1×1016 cm−2, whereas it increases remarkably from 2.8 to 4.5 for films with a dose of 1×1017 cm−2. On the other hand, the PL integrated intensities of the two kinds of films reach their maxima at Tan of 900 and 700 °C, respectively. It is proposed that the two PL bands are due to germanium-related oxygen-deficient centers. The annealing characteristics are explained well.
Cross modulation of two lasers in a semiconductor
Authors: Bharuthram R. , Sharma A. K.
Published: July 2002

A high power amplitude modulated laser of carrier frequency ω1 and modulation frequency Ω propagating through a semiconductor exerts a ponderomotive force on electrons at Ω. When Ω∼ωp/εL (ωp being the plasma frequency and εL the lattice permittivity), the ponderomotive force produces a large amplitude plasma wave. The free carrier density fluctuations associated with the plasma wave are also large, particularly when the laser has forward and backward propagating components. Another laser of frequency ω2 propagating through this region couples with the free carrier density oscillations to produce sidebands at ω2±Ω. Thus the information contained in the first laser is transferred to the second laser. The scheme is effective when Ω is larger than the collision frequency.
Resistance of a domain wall in La0.7Ca0.3MnO3
Authors: Mathur N. D. , Littlewood P. B. , Todd N. K. , Isaac S. P. , Teo B.-S. , Kang D.-J. , Tarte E. J. , Barber Z. H. , Evetts J. E. , Blamire M. G.
Published: July 2002

Although colossal magnetoresistance (CMR) materials exhibit large changes in electrical resistance (up to 106%), large magnetic fields (several tesla) must be applied. To obtain a sizeable low-field effect (<102% in several millitesla), it is necessary to incorporate structural discontinuities such as grain boundaries, or other types of interfaces. The potential for applications, however, remains limited because structural discontinuities increase electrical resistance by several orders of magnitude and hence create noise. Moreover, it has proven to be difficult to fabricate structural discontinuities reproducibly. We have attempted to investigate discontinuities that are purely magnetic via transport measurements through a precisely controlled number of magnetic domain walls of known area in thin film devices of the ferromagnetic CMR perovskite La0.7Ca0.3MnO3. A sharp low-field switching seen below ∼110 K is ascribed to the formation of a precise number of magnetic domain walls, each with resistance-area product 8×10−14 Ω m2 at 77 K. This is four orders of magnitude larger than expected, suggesting that the domain walls contain an additional structure. Our findings demonstrate that CMR devices are capable of low-noise low-field switching, and suggest the possibility of exploiting a hitherto unexpected intrinsic effect reproducibly and therefore commercially.
Undesirable contaminants possibly introduced in LiNbO3 electro-optic devices
Authors: Nagata Hirotoshi , Mitsugi Naoki , Sakamoto Toshihiro , Shima Kaori , Tamai Masumi , Haga Eungi Min
Published: July 2002

Inorganic contaminants in the fabrication of LiNbO3 optical waveguide devices are examined with regard to their effect on the ultimate device performance and quality. We find a possibility that some chemicals such as a photoresist developer, etc., leave contaminants including silicon and alkalis on the LiNbO3 wafer, and result in an increase of light propagation loss, a pealing of the SiO2 layer from the wafer, and a large dc drift.
A comprehensive study of AlGaAs/GaAs beryllium- and carbon-doped base heterojunction bipolar transistor structures subjected to rapid thermal processing
Authors: Wang Hong , Ing Ng Geok , Zheng Haiqun , Zhang Penghua
Published: July 2002

AlGaAs/GaAs single heterojunction bipolar transistor (HBT) structures with Be- and C-doped bases have been annealed at different temperatures using rapid thermal processing (RTP). Both electrical and low-temperature photoluminescence measurements were used to investigate their thermal stability. We found that the conventional AlGaAs/GaAs abrupt HBT structures could undergo significant degradation at temperatures commonly encountered in typical RTP for device fabrication. The decrease of current gain was observed in both molecular beam epitaxy-grown HBTs with a Be-doped base and metalorganic chemical vapor deposition-grown HBTs with a C-doped base after RTP at temperatures greater than 600 °C. Our studies show that high-temperature RTP could induce undesirable degradation in AlGaAs/GaAs HBTs. Different degradation mechanisms, which are similar to those for the degradation of the Be- and C-doped base HBTs under current-induced stress, are responsible for the degradation of the Be- and C-doped HBTs subjected to RTP. The degradation of Be-doped HBTs is believed to be due to the outdiffusion of Be from the highly doped base, whereas the decrease of current gain for C-doped HBTs is closely related to the unintentionally incorporated hydrogen during material growth.
Femtosecond laser micromachining of TiO2 crystal surface for robust optical catalyst
Authors: Furusawa Kentaro , Takahashi Kousuke , Cho Sung-Hak , Kumagai Hiroshi , Midorikawa Katsumi , Obara Minoru
Published: July 2002

The two-photon laser ablation of TiO2 photocatalyst using intense ultrashort titanium sapphire laser was investigated experimentally and theoretically aiming at the enhancement of photocatalytic reaction. The black surfacing of the TiO2 photocatalyst crystal was successfully achieved by drilling a large number of conical microholes with two-photon laser ablation. The ablated surface has a roughness of submicrometer order, and no heat-affected zone was observed. The simple equation is developed to explain two-photon ablation process of the TiO2 photocatalyst and the dependence of the ablation characteristic on the pulse duration. A maximization procedure of the ablated surface area is achieved and the optimization of laser parameters is shown for dense integration of ablated conical holes to increase the surface area.
Annihilation of thermal double donors in silicon
Authors: Kamiura Yoichi , Takeuchi Yoshinori , Yamashita Yoshifumi
Published: July 2002

We performed systematic experiments on the annihilation of six species of thermal double donors, or TDDs (TDD1–TDD6) under various conditions in both carbon-lean and carbon-rich Si crystals, by means of low-temperature infrared spectroscopy. We found that two kinds of TDD annihilation occurred in two different time regions. The first annihilation occurred typically within 1000 min at 500 °C, and is ascribed to the dissociation of oxygen clusters responsible for TDDs. We analyzed TDD annihilation on the basis of the model of successive dissociation of oxygen clusters, and obtained an activation energy of about 4 eV and a pre-exponential factor of the order of 1022 s−1, regardless of TDD species and carbon density. We discuss the origin of such a high activation energy and a large pre-exponential factor. The second annihilation occurred typically after 1000 min at 500 °C, and is ascribed to the neutralization of TDDs. The neutralization behavior strongly depends on the amount of carbon in the silicon crystals. The C-rich crystal shows the usual thermal activation process with an activation energy of 1.7 eV and a pre-exponential factor of about 106 s−1, independently of TDD species. On the other hand, the C-lean crystal shows no distinct temperature dependence on neutralization rate, suggesting an athermal process. These results strongly suggest two different mechanisms of TDD neutralization, one of which is related to carbon and the other independent of carbon. We propose models to explain the features of these two kinds of TDD neutralization.
Cobalt and tin substituted barium ferrites: the K and R phases
Authors: Sole R. , Ruiz X. , Gavaldà Jna. , Massons J. , Sandiumenge F. , Aguiló M. , Diaz F.
Published: July 2002

Crystals of different barium ferrite phases (M, K, and R phases) were grown by slow cooling and the top seeded solution growth technique, using the (0.4Na2O–0.6B2O3)–BaO–(4.5Fe2O3+1.5CoSnO3) system. It has been demonstrated that different ferrite phases, e.g., the K and R phases, may coexist in the same crystal with an interface that is free from observable misfit dislocations and amorphous zones. The magnetization of these two phases, measured with an applied magnetic field parallel to the crystallographic c axis, had a paramagnetic behavior.
Rutherford backscattering study of sputtered CdTe/CdS bilayers
Authors: Grecu D. , Compaan A. D.
Published: July 2002

Rutherford backscattering (RBS) and x-ray photoelectron spectroscopy (XPS) were used to investigate interdiffusion and surface properties of CdS/CdTe bilayers. The films were grown by radio-frequency sputtering and received postdeposition heat treatments similar to the ones employed in CdTe solar cell fabrication. It is found that a CdCl2 anneal strongly enhances both the diffusion of S into the CdTe layer and the surface oxidation. The diffusion of S in CdTe in this process can be described by a constant surface source model with the diffusivity given by D=3.2×10−5 exp(−1.2 eV/kT) cm2 s−1 in the temperature range studied. The change in the chemical composition of the surface following the CdCl2 anneal was analyzed by XPS showing that Te oxides and residual compounds containing Cl were present which could account for the straggling in the RBS spectrum. An HCl etch completely removes oxides and residues leaving a smooth surface.
Study of the band alignment in (Zn, Cd)Se/ZnSe quantum wells by means of photoluminescence excitation spectroscopy
Authors: Guénaud C. , Deleporte E. , Filoramo A. , Lelong Ph. , Delalande C. , Morhain C. , Tournié E. , Faurie J. P.
Published: July 2002

We report on photoluminescence excitation spectroscopy performed on several (Zn, Cd)Se/ZnSe quantum wells at low temperature. Experimental results are compared with a calculation of the heavy- and light-hole excitonic transition energies. It is shown that the energy difference between the first heavy- and light-hole transitions E1H1 and E1L1 can be very sensitive to the valence-band offset under some circumstances, making a determination of this parameter then possible. The strain-free relative valence-band offset is found qv0=13%±2%. The heavy-hole excitons are type-I excitons, whereas the light-hole excitons are type-II excitons.
Characterization of AlxGa1−xAs/GaAs heterojunction bipolar transistor structures using cross-sectional scanning force microscopy
Authors: Rosenthal P. A. , Yu E. T. , Pierson R. L. , Zampardi P. J.
Published: July 2002

We have characterized base-layer width and dopant distributions on cleaved cross-sections of AlxGa1−xAs/GaAs heterojunction bipolar transistor (HBT) structures using a variation of electrostatic force microscopy. The contrast observed is sensitive to the local dopant concentration through variations in the depletion layer depth extending into the sample surface, and enables delineation of individual device regions within the epitaxial layer structure with nanoscale spatial resolution. In two epitaxially grown HBT structures, one with 50 nm base width and the other with 120 nm base width, we are able to delineate clearly the emitter, base, collector, and subcollector regions, and to distinguish regions within the collector differing in dopant concentration by a factor of two. We have also distinguished clearly between the base widths in these samples and have precisely measured the difference to be 63±3 nm, in excellent agreement with the nominal difference of 70±7 nm.
Energy filters using modulated superlattices
Authors: Yang Q. K. , Li A. Z.
Published: July 2002

In this article, we demonstrate that a randomly chosen “Λ” shape modulated superlattice can serve as a better energy filter than an unmodulated superlattice. The oscillation of transmission probability in the minibands of the Λ shape modulated superlattice is shown to be greatly suppressed. We also demonstrate that precise control of the potential shape is not crucial for energy filters. The results should have prospective application in quantum cascade lasers, in which the improved energy filter can serve as the injection/relaxation region between active regions.
Time-dependent O2 mass balance change and target surface oxidation during mode transition in Ti–O2 reactive sputtering
Authors: Kusano Eiji , Kinbara Akira
Published: July 2002

Time-dependent O2 mass balance change among the amounts injected into the chamber, pumped out from the chamber, gettered by sputtered Ti metal, residing in the chamber, and consumed to oxidize the target surface has been investigated as a function of time elapsed after the discharge ignition in Ti–O2 reactive sputtering. From the mass balance results obtained, target surface coverage has been estimated. In the period of up to 10 s after discharge ignition, the gettering of O2 by sputtered Ti dominated the process change. In this period, the target surface oxidation rate was low. In the period of 20–50 s, the amount of O2 consumed to target surface oxidation surpassed the amount of O2 gettered by deposited Ti, resulting in a drastic increase in the target coverage. After the target surface oxidation was completed, the process became stable. In this period, the amount of O2 pumped out without causing any process changes increased and a very small amount of O2 was consumed to oxidize the target surface. The equilibrium in the balance between the formation and sputter etching of the oxide layer on the target surface resulted in the stable condition. Calculated target coverage achieved more than 250 monolayers at 600 s after the glow discharge ignition. This value equated with the oxide thickness of about 100 nm. It is concluded that the nonlinear target surface oxidation process causes a nonlinear process change occurring during the mode transition.
Ablation rate and threshold flux of cartilage irradiated by a XeCl excimer laser
Authors: Efthimiopoulos T. , Kritsotakis E. , Naomidis L. , Kiagias H. , Helidonis E.
Published: July 2002

Results are presented related to the ablation of animal cartilage by a XeCl laser beam (308 nm). In particular the minimum energy flux for ablation and the ablation rate as a function of the energy flux were measured. The effect of the pulse repetition rate on the cartilage temperature and the influence of a flowing cooling gas were also investigated. A new method, based on the observation of the acoustic wave produced during the ablation process and the delay of arrival of the wave to the back of the sample with respect to the laser pulse, was introduced to measure the ablation threshold and the on line ablation rate. This method was compared with other methods that use either a profilometer or the perforation of the sample to measure the ablation rate. It is shown that the acoustic method can provide accurate, on line information of the ablation rate and threshold. It is also shown that the ablation rate is reduced considerably along the depth of the sample.
Swelling of CFx and CFx(Au) films
Authors: D’Addabbo A. , Valentini A. , Convertino A.
Published: July 2002

The article reports on the swelling phenomenon, observed in the presence of acetone vapors in undoped and Au doped Teflon-like films. The Au presence enhances the degree of the swelling of the Teflon. Moreover, the metal grains embedded in the insulating matrix are used to detect swelling with an electrical method. A conductance model of metal embedded dielectric materials is used to describe the conductance variations with the polymer volume change.
The coupling of microwave radiation to surface plasmon polaritons and guided modes via dielectric gratings
Authors: Hibbins A. P. , Sambles J. R. , Lawrence C. R.
Published: July 2002

It is shown that an absorbing dielectric layer, sinusoidally modulated in height, on top of a planar metal substrate, may be used to provide coupling between both s- and p-polarized incident microwave photons and surface plasmon polaritons, which propagate along the metal–dielectric interface. The study is carried out using paraffin wax as the dielectric material on an aluminum-alloy plate and the wax is sufficiently thick such that it may also support a guided mode. Energy reradiated from these excited modes into diffracted orders is recorded by monitoring the specular beam reflectivity as a function of wavelength (7.5<λ0<11.3 mm) and azimuthal angle of incidence (0°<φ<90°). The azimuthal-angle-dependent reflectivity scans are fitted using a multilayer, multishape differential formalism to model conical diffraction with a single set of parameters describing the grating profile, and the permittivity and thickness of the wax layer.
Threshold effect for photorefractive light-induced scattering and signal beam amplification in doped LiNbO3 crystals
Authors: Kamber Nouel Y. , Xu Jingjun , Mikha Sonia M. , Zhang Guoquan , Zhang Xinzheng , Liu Simin , Zhang Guangyin
Published: July 2002

In this article, we have studied experimentally the threshold effect of incident light intensity for the photorefractive light-induced scattering and signal beam amplification in doped LiNbO3 crystals. Our results show that the signal beam can be most effectively amplified for a specific pump light intensity when the threshold effect of incident light intensity for photorefractive light-induced scattering is taken into account in doped LiNbO3 crystals. Doping the LiNbO3 crystal with suitable concentration of Fe and damage-resistant dopants will provide an effective way to control the photorefractive properties of LiNbO3 crystal. We demonstrated the advantage of the fanning-noise-free double-doped photorefractive LiNbO3 crystals for three-dimensional storage. This method to suppress the fanning noise is very simple and convenient to practice.
Retarded C54 transformation and suppressed agglomeration by precipitates in TiSi2 films
Authors: Suh You-Seok , Park Dae-Gyu , Jang Se-Aug , Lee Sang-Hyeob , Kim Tae-Kyun , Yeo In-Seok , Kim Sam-Dong , Kim Chung-Tae
Published: July 2002

We report the effects of excess Si phase on the C54 transformation and the thermal stability of TiSi2 thin films as a function of Si/Ti (x) ratio. The resistivity and x-ray diffraction data of TiSix(x=2.1–2.4) films showed a retardation of C54–TiSi2 transformation with incremental molar ratio x; the TiSix (x=2.1–2.3) films were completely transformed to C54-TiSi2 with rapid thermal annealing (RTA) of 750 °C, while the TiSix (x=2.4) films were transmuted to C54–TiSi2 over the RTA of 800 °C. Transmission electron microscopy study revealed that the growth of Si precipitates in the Si-rich TiSix films competes with the grain growth during the transformation of C49 to C54 TiSi2 phase, resulting in the increase of C54 transformation temperature. An excellent sheet resistance (Rs) and its standard deviation in concert with a reliable gate oxide integrity were attained from the Si-rich TiSi2/polycrystalline-Si (poly-Si) structure up to the solid state annealing of 850 °C for 60 min. These attributes are due to the reduced solid state reaction at TiSi2/Si interface and the suppressed TiSi2 agglomeration by Si precipitates. The average Rs of the 0.12-μm-wide TiSi2(800 Å)/poly-Si gate lines is lower than 4.5 Ω/□, demonstrating a robust gate electrode structure for gigabit scale device application.
The use of strain gauges in the measurement of the B1/B2 phase transformation in shock-loaded potassium chloride
Authors: Millett J. C. F. , Bourne N. K. , Galbraith S. D. , Rosenberg Z.
Published: July 2002

The shock induced phase transformation in potassium chloride (from the NaCl B1 structure to the CsCl B2 structure) has been investigated using constantan strain gauges. These gauges have been mounted in pressed, polycrystalline samples in an orientation that renders them sensitive to the uniaxial strain that characterizes the plate impact geometry. At the phase transformation stress (of ∼2 GPa), measured strains of ∼ −0.1 were observed, slightly higher than the values quoted in the literature. We suggested that residual porosity within the microstructure is a possible explanation. Over impact stresses of 2.5–4.0 GPa, total measured strains agree with the work of others at the lower end of our measured range, but are significantly lower at higher stresses. We suggest that this might be due to the nonequilibrium state of the material at the time of measurement.
High-spatial-resolution quantum-well intermixing process in GaInAs/GaInAsP laser structure using pulsed-photoabsorption-induced disordering
Authors: Ong T. K. , Gunawan O. , Ooi B. S. , Lam Y. L. , Chan Y. C. , Zhou Y. , Helmy A. Saher , Marsh J. H.
Published: July 2002

Raman spectroscopy was used to study the spatial resolution of pulsed-photoabsorption-induced quantum-well intermixing in a GaInAs/GaInAsP laser structure. A differential band gap shift of up to 60 meV has been obtained from a sample masked with SixNy/Au and exposed to the laser irradiation. Intermixing was detected in the irradiated regions through the shift of GaAs-like modes to lower frequencies. In addition, the intermixing induced GaInP longitudinal optical modes in the irradiated regions, which is evidence of the intermixing between the upper GaInAs cap and the GaInAsP layer. The spatial resolution of this process, which was obtained from micro-Raman spectra when scanned across the interface of the intermixing mask, was found to be better than 2.5 μm.
Sequential formation of quasicrystalline and amorphous phases in the Fe–Nb and Fe–Ta multilayers upon ion irradiation
Authors: Lin C. , Yang G. W. , Liu B. X.
Published: July 2002

An unusual phase formation sequence of first quasicrystalline and then amorphous phases was observed in the Fe–Nb and Fe–Ta multilayers upon room temperature 200 keV xenon ion irradiation. The sequential structural evolution was attributed to the gradually destroying of order in the original crystalline lattices with increasing ion dose.