bluuebunny/crossref_metadata_2025 huggingface dataset:
Electronic and vibrational predissociation in ArI2 photodissociation dynamics
Authors: Lepetit Bruno , Roncero Octavio , Buchachenko Alexei A. , Halberstadt Nadine
Published: July 2002

Quantum dynamical calculations on the photodissociation process: ArI2(X)+hν→Ar+I2(B) or Ar+I+I have been performed using diatomics-in-molecule semiempirical potential energy surfaces in the spectral region of the I2(B,v=15–25)←I2(X,v=0) transition. The B state responsible for vibrational predissociation producing Ar+I2(B) is coupled to four dissociative states inducing electronic predissociation to Ar+I(2P3/2)+I(2P3/2). These dissociative states correlate to the a(1g), a′(0g+), B″(1u), 1(2g) electronic states of I2. Both linear and perpendicular initial ArI2(X) isomers are considered. For the linear isomer, only the a′ state has non-negligible effect on photodissociation dynamics, although total photon absorption cross sections are not significantly modified when coupling to a′ is taken into account, partial cross sections corresponding to vibrational predissociation are smaller. For the perpendicular isomer, resonance decay rates are increased, mainly by the coupling to a′(0g+), 1(2g), and a(1g) states. Decay rates oscillate as a function of the vibrational excitation of I2(B) but the main source of oscillation is the intramolecular vibrational energy redistribution which occurs in vibrational predissociation, rather than Franck–Condon oscillations in electronic predissociation.
Spectroscopic determination of the water dimer intermolecular potential-energy surface
Authors: Goldman N. , Fellers R. S. , Brown M. G. , Braly L. B. , Keoshian C. J. , Leforestier C. , Saykally R. J.
Published: July 2002

Two polarizable six-dimensional water dimer intermolecular potential surfaces have been determined by fitting the distributed multipole ASP (anisotropic site potential) potential form to microwave, terahertz, and midinfrared cavity ringdown (D2O)2 spectra via a rigorous calculation of the water dimer eigenstates with the PSSH (pseudo-spectral split Hamiltonian) method. The fitted potentials accurately reproduce most ground-state vibration-rotation-tunneling spectra and yield excellent second virial coefficients for both H2O and D2O. The calculated dimer structure and dipole moment are close to those determined from microwave spectroscopy and high level ab initio calculations, except that the O–O distance (2.952 Å) is significantly shorter than the currently accepted experimental value. The dimer binding energy (4.85 kcal/mol) is considerably smaller than the accepted experimental result, but in excellent agreement with recent theoretical results, as are the acceptor switching and donor–acceptor interchange tunneling barriers and the cyclic water trimer and tetramer structures and binding energies.
Near-field second-harmonic generation at a metal tip apex
Authors: Takahashi Satoshi , Zayats Anatoly V.
Published: July 2002

Second-harmonic generation at a gold tip apex has been studied within the context of applications in apertureless scanning near-field microscopy. Polarization and distance dependencies of second-harmonic generation have been investigated and compared to respective dependencies for scattered fundamental light. The results shows that, in general, different topological features of a tip are responsible for scattering and second-harmonic generation in the far and near field. The near-field interaction between a probe tip and a surface significantly modifies enhancement of the electromagnetic field at the tip and results in different distance dependencies of near-field scattering and second-harmonic generation. The polarization contrast obtained with the second-harmonic signal significantly exceeds the polarization contrast of linear scattering from the tip.
Giant magnetoimpedance and colossal magnetoresistance in La0.75Sr0.25MnO3 at room temperature
Authors: Qin Hongwei , Hu Jifan , Chen Juan , Wang Yizhong , Wang Zhenxi
Published: July 2002

The impedance as well as the resistance and reactance of the La0.75Sr0.25MnO3 manganite increase with increasing ac frequency due to the skin effect of the metal state below Curie temperature. The manganite La0.75Sr0.25MnO3 shows not only the colossal magnetoresistance effect but also the giant magnetoimpedance effect. For the giant magnetoimpedance, the magnetic field influences the impedance by affecting the penetration depth via the transverse permeability, whereas for the colossal magnetoresistance the magnetic field tends to align the local spins, leading to the decrease of the dc resistance via the interplay between a strong electron–phonon coupling and the double exchange effect of spin alignment on electron kinetic energy.
Two-dimensional microscopic uniformity of resistivity in semi-insulating GaAs
Authors: Matsumura Takao , Obokata Takeshi , Fukuda Tsuguo
Published: July 2002

The two-dimensional microscopic uniformity of resistivity has been studied using the three-electrode guard method with 70-μm resolution as a new microscopic characterization technique for semi-insulating GaAs. A clear microscopic correlation between resistivity and crystal defects is revealed for the first time by this method. In undoped GaAs, a significant decrease of resistivity greater than one order of magnitude is observed at the vicinity of the walls of dislocation clustered networks and slip lines. Resistivity fluctuation corresponding to rotational growth striation is also observed in undoped GaAs.
Changes in the electrical properties of GaAs due to electron irradiation
Authors: Yamaguchi Masafumi , Uemura Chikao
Published: July 2002

The carrier removal rates and the minority carrier diffusion length changes due to 1-MeV electron irradiation have been measured for n- and p-type GaAs single crystals. Initial carrier concentration and conductivity-type effects on the electrical properties in the irradiated GaAs are clarified. These results can be well explained by taking into account the electron irradiation-induced electron and hole traps identified in previous work.
Laser triggering of a 500-kV rail-gap switch
Authors: Endoh Akira , Watanabe Shuntaro
Published: July 2002

Laser triggering of a SF6-filled, 500-kV rail gap is described. A softly focused KrF laser beam was employed to irradiate insulator SF6 gas through the rail gap along a 60-cm-long, high-voltage electrode with the power level of 5×107 W/cm2. The rail gap was operated as the main switch of a 2.8-Ω, 500-kV Blumlein circuit, two of which drive the electron beam diodes to pump a 200-J, 70-ns KrF laser from both sides. Multichannel operation of the rail gap was obtained by the application of laser triggering with the result of reduced voltage fall time of 20 ns (10–90%) compared with 50 ns in the self-breakdown mode. The jitter was also decreased to less than 2 ns in the trigger operation of the gap.
Monocrystal elastic constants of NbC
Authors: Ledbetter H. M. , Chevacharoenkul S. , Davis R. F.
Published: July 2002

Using ultrasonic methods at ambient temperatures, for niobium carbide we determined the monocrystalline elastic stiffnesses: C11, C12, and C44 in Voigt’s contracted notation. From these, we calculated the quasi-isotropic (polycrystalline) elastic constants and the elastic Debye characteristic temperature. Results derived from a Blackman diagram suggest that ionic forces contribute significantly to the elastic constants and to interatomic bonding. This conclusion applies not only to NbC but also to other MX carbides with an NaCl-type crystal structure.
On the effective mass in modeling nonstationary transport
Authors: Ye Liangxiu , Stewart R. A. , Churchill J. N.
Published: July 2002

Two new formulas for obtaining the effective mass in a multiple-valley semiconductor are derived. A formula for calculating the average effective mass from a steady-state Monte Carlo simulation is first introduced. It is shown that a simpler averaging formula, used by others, predicts an effective mass that is too large. A second formula, derived analytically from first principles, gives an expression for effective mass in a nonparabolic band. It is shown that the effective mass depends on the wave vector as well as on energy. A simpler formula, used by others, is shown to be a special case of the more general result and leads to an overestimation of the effective mass.
The mechanism of polymer alignment of liquid-crystal materials
Authors: Geary J. M. , Goodby J. W. , Kmetz A. R. , Patel J. S.
Published: July 2002

Smetic and nematic liquid-crystal materials can be homogeneously aligned by buffed thin films of appropriate polymers. We propose that the buffing process orients the polymer’s molecular chains in a manner similar to cold drawing of bulk polymer samples. Experimental verification of this theory is obtained by measuring buffing-induced birefringence in thin films of various polymers coated on glass. Further experiments establish that the oriented state of the polymer chains, and not scratching or grooving of the surface, is necessary to produce alignment. Alignment is found to occur when the polymer is both oriented and crystalline. A picture of alignment is presented in which the formation of a liquid-crystal phase on the crystalline,oriented polymer surface is analogous to the epitaxial growth of conventional solid crystals.
The elastic field associated with a square dislocation loop in a two-phase medium
Authors: Jagannadham K. , Narayan J.
Published: July 2002

The surface dislocation analysis of the three-dimensional distortion associated with a prismatic square dislocation loop in a two-phase medium is presented. Closed-form expressions for the stresses and displacements due to the dislocation loop are obtained. These results are used to calculate the image force acting on the dislocation loop either due to a free surface or a second phase with a different shear modulus and Poisson’s ratio. Further, the application of the results to the formation of denuded zones free from dislocation loops in ion-implanted silicon upon thermal annealing are illustrated in terms of the glide movement due to the attractive image force exerted by the free surface.
Galvanomagnetic luminescence and the quantum efficiency of radiative recombination of InSb
Authors: Berdahl Paul
Published: July 2002

The galvanomagnetic luminescence (GML) technique is used to determine carrier lifetimes in pure and doped InSb samples near room temperature. For this purpose the theory of the GML effect is derived. The Onsager reciprocity relation between the GML effect and the photoelectromagnetic effect is also derived. (Furthermore, a related connection is obtained between the thermophotovoltaic effect and light emission by diodes.) The experimental data yield lifetimes in the range of 70 ps to 30 ns. Based on these lifetimes the quantum efficiency ηq for an excess electron-hole pair to produce a photon is determined. For intrinsic samples, ηq=3% at 20 °C and 1.5% at 70 °C. Some p-type samples have values of ηq as high as 5%. These results show that p-type doping can suppress the dominant Auger recombination mechanism in intrinsic InSb.
Dipolar magnetic surface anisotropy in ferromagnetic thin films with interfacial roughness
Authors: Bruno Patrick
Published: July 2002

The demagnetizing field and magnetostatic energy of a thin film with surface roughness has been calculated. It is shown that the surface roughness gives rise to an effective perpendicular anisotropy whose order of magnitude is evaluated as a function of the parameters characterizing the roughness. The results are discussed in connection with experimental situations.
Raman and Brillouin scattering of LiClO4 complexed in poly(propylene-glycol)
Authors: Schantz S. , Torell L. M. , Stevens J. R.
Published: July 2002

Raman spectra of LiClO4 complexed in poly(propylene-glycol) (PPG) have been obtained for concentrations of the monomer to salt ratio (ether oxygen):Li in the range 30:1–5:1. Splitting of the symmetric stretching mode of the ClO4− anion was observed with an intensity profile that varied with salt concentration. This phenomenon indicates a changing environment about the anion. A two-component band analysis leads to the identification of dissociated ions on one hand and solvent-separated ion pairs on the other. The concentration of ion pairs is relatively low compared to that of the dissociated ions, which are predominant for all concentrations. Despite the observed increase in the absolute number of dissociated ions at higher salt concentration, the electrical conductivity is reported to decrease in the same range. This indicates that the number of ‘‘free’’ charge carriers is of less importance for the conductivity than the mobility, which is damped in this concentration range. Frequency shifts of the disordered longitudinal-acoustic mode and increased hypersonic velocities, measured with Raman and Brillouin scattering techniques, respectively, indicate increased stiffness of the polymer matrix for increasing salt concentration, which probably results in decreased ion mobility.
Effects of subgrain structures on the optical homogeneity of Tl3AsSe3 single crystals
Authors: Yoo K. C. , Henningsen T. , Grimmett-Dawson K. D. , Singh N. B. , Hopkins R. H.
Published: July 2002

Studies of subgrain structures in Tl3AsSe3 have been performed by x-ray topography. Birefringent interference patterns show that a direct correlation exists between the subgrain structures and the optical homogeneity of the crystals. A quantitative measurement of lattice misorientation was performed from the interference patterns and compared with x-ray rocking measurements. The origin of the subgrains is also discussed in detail.
Magnetic order in RBa2Cu2O6+x (invited)
Authors: Lynn J. W. , Li W-H.
Published: July 2002

Neutron scattering results on the RBa2 Cu3 O6+x (R=rare earth) class of systems are reviewed. For the oxygen-deficient systems (x<0.5), which are semiconducting, the Cu ions are observed to order with Néel temperatures as high as 500 K. The magnetic structure consists of strong nearest-neighbor antiferromagnetic couplings of the Cu spins within the Cu-O2 layers, while nearest-neighbor spins in adjacent layers are also aligned antiferromagnetically. The moments are constrained in the tetragonal plane. At lower temperatures the Cu moments in the oxygen-deficient ‘‘chain’’ layers also order antiferromagnetically, with a moment that can be quite substantial (∼ 1/2  μB). Both transition temperatures are very sensitive to the oxygen concentration. The magnetic heavy rare-earth compounds, on the other hand, order antiferromagnetically at very low temperatures, analogous to the ‘‘conventional’’ magnetic-superconductor systems such as RMo6 S8 and RRh4 B4. The ordering is not very sensitive to the oxygen concentration, and the antiferromagnetic state coexists with superconductivity. Differences in the ordering behavior are ascribed to crystal-field effects, which are briefly discussed. Finally, small-angle neutron scattering results in the vicinity of the superconducting transition temperature are also discussed.
Magnetic properties and structure change from tetragonal to hexagonal for the rapidly quenched SmTiFe11 alloy ribbons
Authors: Saito H. , Takahashi M. , Wakiyama T.
Published: July 2002

Structure and magnetic properties of the rapidly quenched SmTiFe11 alloy ribbons fabricated under various quenching rates were systematically examined. Phase transformation from an equilibrium ThMn12-type structure (tetragonal) to a metastable TbCu7-type disordered structure (hexagonal) was found for the ribbons fabricated around the roll velocity, VR≂30 m/s. Magnetocrystalline anisotropy changed from uniaxial (Ku1=5.0×107 erg/cm3) to in-plane anistropy in accordance with the structure change from ThMn12- to TbCu7-type structure. Ms and Tc varied from 125 emu/g and 320 °C (ThMn12 type) to 114 emu/g and 242 °C (TbCu7 type), respectively. The maximum hard magnetic properties for SmTiFe11 alloy ribbons, iHc=3 kOe, (BH)max=3 MGOe, were obtained for the ribbons with a ThMn12-type structure.
The resistivity and microstructure of heavily drawn Cu-Nb alloys
Authors: Verhoeven J. D. , Downing H. L. , Chumbley L. S. , Gibson E. D.
Published: July 2002

A combined resistivity transmission electron microscopy (TEM) study has been done on heavily drawn Cu-20 vol % Nb alloys (so-called in situ alloys). The results show that electron scattering at Cu-Nb interfaces makes the major contribution to resistivity in heavily drawn wire. The dislocation contribution is small and constant at deformation strains greater than around 4, apparently as a result of dynamic recovery/recrystallization of the Cu matrix which occurs during room-temperature drawing. Results of this study and other recent TEM dislocation studies indicate that the dislocation density in heavily drawn Cu-20 vol % Nb material does not exceed 1011 cm−2. It is demonstrated here that the 1013- cm−2 dislocation density predicted by the resistivity study of Karasek and Bevk [J. Appl. Phys. 52, 1370 (1981)] is high because the interface scattering contribution is more strongly reduced by coarsening than they assumed. It is shown that resistivity measurements provide a means of evaluating an average Cu channel diameter in the aligned composite alloys formed at large deformation strains.
Picosecond pump-probe technique to measure deep-level, free-carrier, and two photon cross sections in GaAs
Authors: Valley George C. , Boggess Thomas F. , Dubard J. , Smirl Arthur L.
Published: July 2002

Optical transmission of a weak probe pulse in the presence of a strong cross-polarized pump pulse is measured as a function of pump-probe delay. We use pulses of duration 35 ps at a wavelength of 1.06 μm and two samples of undoped, semi-insulating GaAs cut from two dissimilar boules. Numerical solutions to the rate equations coupled with the nonlinear wave equation for propagation in GaAs are fit to the experimental results in order to determine rate coefficients. We obtain a two-photon absorption coefficient of 27±5 cm/GW, a free-carrier cross section (5.5±2)×10−18 cm2, and the ratio of the hole cross section to the electron cross section for the EL2 defect 0.76±0.1.
Influence of dislocations on electrical properties of large grained polycrystalline silicon cells. I. Model
Authors: El Ghitani H. , Martinuzzi S.
Published: July 2002

The influence of the density (Ndis) and of the recombination activity (Sd) of dislocations on the photocurrent (Jsc), the spectral dependence of Jsc and the effective electron diffusion length (Leff) of a P+-N junction solar cell are computed by means of a model which makes use of the Green’s function method. In this model Sd is the surface recombination velocity of the space-charge cylinder surrounding the dislocation line and the dislocations are assumed to be perpendicular to the illuminated surface of the cells and homogeneously recombining. The results obtained indicate that the values of Jsc and their spectral variations, as well as values of Leff, are dependent on both Ndis and Sd, especially when Ndis and Sd are larger than 103 cm−2 and 104 cm s−1, respectively. The base thickness (d) of the cells and the value of electron diffusion length (Ln) in the undislocated regions of the wafers are also introduced in the model as independent parameters.
Broadband nuclear magnetic resonance pulses with two-dimensional spatial selectivity
Authors: Hardy Christopher J. , Cline Harvey E.
Published: July 2002

Two-dimensional spatial localization can be achieved using a single nuclear magnetic resonance (NMR) pulse if the pulse is applied in the presence of a magnetic field gradient which is reorienting through two dimensions. The application of these pulses to whole-body NMR imaging systems is hampered, however, by amplitude and slew-rate constraints on the magnetic field gradients, resulting in longer pulses with relatively limited bandwidth. We describe here a variable rate method for shortening these pulses by as much as 36% for pulses limited by slew rate and 55% for pulses limited by gradient amplitude, without changing their spatial excitation profiles on resonance.
Characterization of p-type ZnSe
Authors: Haase M. A. , Cheng H. , DePuydt J. M. , Potts J. E.
Published: July 2002

Lithium-doped ZnSe has been grown on (100) GaAs by molecular beam epitaxy. The epitaxial layers are p-type with net acceptor concentrations (NA−ND) as high as 8×1016 cm−3— the highest ever reported for molecular beam epitaxial ZnSe. Room temperature ac measurements show resistivities as low as 2.9 Ω cm. Higher Li concentrations give rise to self-compensation and a decrease in NA−ND. The details of the electrical and optical characterization of these layers are presented. Rudimentary blue light emitting pn junction diodes have been fabricated. While these devices show dominant blue emission (463 nm) at room temperature, large turn-on voltages indicate that the p-ZnSe/p-GaAs interface presents a large barrier to hole transport. Moreover, we find that difficulty in making device-quality ohmic contacts to p-ZnSe is the next major obstacle to the fabrication of efficient blue light emitting diodes.
Photothermal reflectance investigation of processed silicon. II. Signal generation and lattice temperature dependence in ion-implanted and amorphous thin layers
Authors: Vitkin I. Alex , Christofides Constantinos , Mandelis Andreas
Published: July 2002

Photothermal reflectance investigations as a function of temperature of various forms of silicon (crystalline, ion-implanted, amorphous) are reported largely in the thermal wave regime (low-modulation frequencies and large laser beam spot sizes). The observed results have been explained by a thermal wave model that has been extended to include the effect of lattice temperature. The influence of substrate on the observed signal has been examined in light of the temperature and frequency dependence of the thermal diffusion length. The possible application of the technique to depth-profiling studies, and to the investigation of ion-implanted semiconductors, is discussed.
Thermal conductivity of GaSb and InSb in solid and liquid states
Authors: Nakamura Shin , Hibiya Taketoshi , Yamamoto Fumio
Published: July 2002

Thermal conductivities of InSb and GaSb in solid and liquid states were measured with a ceramic probe. Thermal conductivities at melting points were about 17.7 (W/mK) at 803 K for molten InSb and 21.7 (W/mK) at 993 K for molten GaSb. Thermal conductivities were slightly increased with increasing temperature in liquid state. Lorenz numbers in liquid state were 2.34×10−8 (WΩ/K2) at 803 K for InSb and 2.07×10−8 (WΩ/K2) at 993 K for GaSb.
Cu0, Cu+, and Cu2 from excimer-ablated copper
Authors: Dreyfus R. W.
Published: July 2002

Bulk copper is laser etched with 193- and 351-nm excimer radiation. The transition from the thermal to the plasma etch region is studied by measuring the densities and kinetic energies of three copper species (Cu0, Cu+, and Cu2 ) in the etch plume. A unique laser-induced fluorescence experiment allows these three species to be followed essentially simultaneously as a function of fluence. Three separate types of etching behavior are clearly evident (even within the small fluence range of ∼1–12.5 J/cm2 ); i.e., thermal vaporization of Cu, multiphoton ionization of the Cu vapor, and electron-atom collision-induced ionization (breakdown) and dissociation (of Cu2 ).
Properties of the ablation process for excimer laser ablation of Y1Ba2Cu3O7
Authors: Neifeld R. A. , Potenziani E. , Sinclair W. Robert , Hill III W. T. , Turner B. , Pinkas A.
Published: July 2002

The process of excimer laser ablation has been studied while varying the laser fluence from 0.237 to 19.1 J/cm2. Ion time-of-flight, total charge, target etch depth per pulse, and etch volume per pulse have been measured. Results indicate a maximum ablation volume and minimum ionization fraction occur near 5 J/cm2. Several of the parameters measured vary rapidly in the 1–5 J/cm2 range. Variation in these parameters strongly influences the properties of films grown by this technique.
Deposition of high quality YBa2Cu3O7−δ thin films over large areas by pulsed laser ablation with substrate scanning
Authors: Davis M. F. , Wosik J. , Forster K. , Deshmukh S. C. , Rampersad H. R. , Shah S. , Siemsen P. , Wolfe J. C. , Economou D. J.
Published: July 2002

We describe the transport and structural properties of YBa2Cu3O7−δ thin films deposited by pulsed laser ablation with computer-controlled substrate scanning. Films were deposited on LaAlO3 and SrTiO3 substrates covering a 2×3 cm area with thicknesses of 90 and 160 nm. The 90-nm thick films exhibited a thickness variation of ±8%, Tco = 90.7 ± 0.5 K, Jc = 4.8 ± 0.2 × 106 A/cm2 at 77 K, and a surface resistance (corrected for finite thickness) at 75 GHz of 10–12 mΩ at 77 K. For the 160-nm thick films, the thickness variation was <5%, Tco = 91.0 ± 0.3 K, Jc=5.4±0.4×106 A/cm2, and corrected surface resistance was 6–10 mΩ. X-ray diffraction showed that the c-axis mosaic in the films is closely related to that of the substrates and that the only in-plane defects are due to the expected twinning in the a-b plane of the film. The c-axis lattice constants were 1.1688±0.0004 nm. The above properties showed a high degree of uniformity across the substrate area and between films from different deposition cycles. The surface resistance values add significantly to the body of results which show that the temperature-scaled values for niobium can be equaled and perhaps surpassed by YBa2Cu3O7−δ.
Electro-spin resonance study of positive holes in J-aggregates of a cyanine dye on AgBr microcrystals: Effect of aggregate size
Authors: Tani Tadaaki , Sano Yasuhisa
Published: July 2002

The J-aggregates with variation of size of 3,3′-disulfopropyl-5,5′-dichloro-9-ethyl-thiacarbocyanine (Dye 1) were formed on the surface of octahedral AgBr microcrystals in a photographic emulsion, and their size was evaluated by a spectroscopic method. Light-induced electron transfer from the aggregates to the microcrystals left positive holes in the aggregates, which gave rise to an electron-spin resonance (ESR) signal. The intensity of the ESR signal increased, and the rate of its decay decreased, as the aggregate size increased. The decay of the signal was caused by the recombination of positive holes with photolytic silver clusters after repetition of detrapping and trapping of positive holes by the aggregates. The enthalpy of activation of the decay, for which the detrapping of positive holes was responsible, was independent of the aggregate size. The analysis of the saturation of the signal intensity with increasing the power of microwave has indicated that the increase in the aggregate size could enhance the migration and thus increase the entropy of positive holes in the aggregates. It is therefore considered that the decrease in the rate of the decay of positive holes with increasing the aggregate size resulted from the change, not in the enthalpy, but in the entropy of positive holes in the aggregates. The above-stated behavior of positive holes in the aggregates gave an explanation for the insufficiency in photographic sensitivity caused by the growth of J-aggregates of Dye 1.
H
c
enhancement in partially reduced γ-Fe2O3 via surface treatment with sodium polyphosphate (revisited)
Authors: Spada F. E. , Berkowitz A. E. , Prokey N. T.
Published: July 2002

The coercive force of acicular, nonstoichiometric (partially reduced) γ-Fe2O3 particles is increased after surface treatment with solutions of sodium polyphosphate. The coercive force of 0.6×0.1 μm particles having Fe+2/Fe+3=0.134 increases from 450 to 670 Oe when treated with a 0.15 g/ml aqueous solution of (NaPO3)n≊14⋅Na2O. The magnitude of the change in the coercive force is dependent upon the relative amounts of polyphosphate and iron oxide present during treatment, but is independent of the polyphosphate solution concentration within the range 0.05–0.15 g/ml and the solution pH within the range 7–9. The enhanced coercivity values decrease slowly with time when the treated particles are exposed to air. In contrast to the partially reduced oxide, enhanced coercive forces are not observed in stoichiometric γ-Fe2O3 after similar treatment. These results are qualitatively consistent with earlier reports that Hc can be increased in nonstoichiometric γ-Fe2O3 via surface treatment with chainlike sodium polyphosphates. Possible mechanisms for this behavior are discussed.
Analysis of the effects of constant-current Fowler–Nordheim-tunneling injection with charge trapping inside the potential barrier
Authors: Lopez-Villanueva J. A. , Jimenez-Tejada J. A. , Cartujo P. , Bausells J. , Carceller J. E.
Published: July 2002

Charge trapping and the generation of interface traps in thermally grown SiO2 and its interface with silicon, produced by Fowler–Nordheim tunneling injection at low temperatures from highly doped Si substrates, have been investigated. The results that can be obtained with the constant-current-injection method, when a moderate amount of charge is trapped inside the potential barrier, have been analyzed. This has afforded information about the position of the charge trapped in the oxide. No increase in the interface-trap density has been produced immediately after injection at 77 K, but, as the temperature is raised after injection, the growing of a peak of interface states has been observed. This phenomenon had been reported to be produced as a consequence of a previous hole trapping but, in this case, this intermediate stage of positive-charge building has not been observed. This effect is discussed, taking into account published models.
Laser-plasma x-ray generation using an injection-mode-locked XeCl excimer laser
Authors: Xenakis D. , Hutchinson M. H. R. , O’Neill F. , Turcu I. C. E.
Published: July 2002

A discharge-excited XeCl excimer laser equipped with unstable resonator optics was injection locked to a low-power picosecond pulse train resulting in an efficient high-power output consisting of a train of ∼150-ps pulses separated by ∼2 ns and lasting for the duration of the excimer gain. The laser had a total energy of approximately 250 mJ and when focused to an irradiance of ∼5×1013 W cm−2 on metal targets produced keV x rays with a conversion efficiency of ≊2.5%.
Nondestructive determination of damage depth profiles in ion-implanted semiconductors by multiple-angle-of-incidence single-wavelength ellipsometry using different optical models
Authors: Fried M. , Lohner T. , Jároli E. , Khanh N. Q. , Hajdu C. , Gyulai J.
Published: July 2002

Several-parameter fitting of multiple-angle-of-incidence ellipsometry data is developed to characterize near-surface layers on semiconductors damaged by implantation. The damage depth profiles were described by either rectangular, trapezoid-type, or coupled half-Gaussian (realistic) optical models. The rectangular model has three parameters: the average damage level and effective thickness of the implanted layer, plus the thickness of the native oxide. The trapezoid-type model is enhanced with a fourth parameter, the width of the back (a/c) interface. The realistic optical model consists of a stack of layers with fixed and equal thicknesses and damage levels determined by a depth profile function (presently the coupled half-Gaussians). Five parameters were used: the center, the height, and two standard deviations of the profile, plus the thickness of the native oxide. The complex refractive index of each layer is calculated from the actual damage level by the Bruggeman effective medium approximation. The method was tested on Ge-implanted silicon layers (at a wavelength of 632.8 nm) and was cross checked with high depth resolution Rutherford backscattering spectrometry and channeling.
Crystal field in nitrogenated rare-earth intermetallics
Authors: Skomski R. , Kuz’min M. D. , Coey J. M. D.
Published: July 2002

The crystal-field in Sm2Fe17N3−δ and Sm(Fe11Ti)N1−δ due to interstitial nitrogen has been investigated. Intrinsic parametrization in the superposition model allows separation of the crystal field created by a neighboring nitrogen atom from a purely geometrical factor, which is different for Sm2Fe17N3−δ and Sm(Fe11Ti)N1−δ. Using published magnetic data, values for the intrinsic parameter Ã2 per nitrogen atom of Ã2=200±60 Ka0−2 and Ã2=270±60 Ka0−2 for Sm2Fe17N3−δ and Sm(Fe11Ti)N1−δ, respectively, are obtained. Because of charge penetration, which is discussed in the form of an explicit crystal-field weight function, it is not possible to interpret A20 or Ã2 as crystal-field parameters independent of the 4f ion.
AlGaAs/GaInP heterojunction tunnel diode for cascade solar cell application
Authors: Jung D. , Parker C. A. , Ramdani J. , Bedair S. M.
Published: July 2002

A p+-AlGaAs/n+-GaInP heterojunction tunnel diode with band gap Eg≊1.9 eV was fabricated by the atomic layer epitaxy growth. Doping levels of 1×1020 cm−3 and 5×1019 cm−3 were achieved in the p and n side of the diode using carbon and selenium, respectively. The diode can be used to interconnect the high and low band-gap cells in the AlGaAs/GaAs cascade solar cell structure. For forward current of 20 A/cm2, which is the expected current density at 1000 suns operation, there is only ∼20 mV voltage drop across the tunnel junction. When annealed at 650 and 750 °C to simulate the growth of the top cell, the diode was still suitable for 1000 suns operation. This is the first reported tunnel diode fabricated in high band-gap material systems that can be used as the connecting junction in the cascade solar cell structure operating at 1000 suns.
Elastic energy release due to domain formation in the strained epitaxy of ferroelectric and ferroelastic films
Authors: Pompe W. , Gong X. , Suo Z. , Speck J. S.
Published: July 2002

Twin related domain formation is examined as a strain relaxation mechanism for a heteroepitaxial tetragonal film on a cubic substrate. Elastic relaxations are calculated for a single twin band in which the c axis of the tetragonal domains is either related by a 90° rotation about an axis in the plane of the film or by a 90° rotation about the surface normal. In all cases, the strain energy change is evaluated for both the film and the substrate. A domain pattern map is developed that predicts single domain and multiple domain fields depending on the relative misfit strains and domain wall energy. The concept of a critical thickness, hc, for domain formation is developed. For cases in which the c axis is rotated 90° about an axis in the plane of the film, the critical thickness depends only on the relative coherency strain between the substrate and film and the ratio of the domain wall energy to the stored elastic energy. For the case of a pattern consisting of energetically equivalent domains with the c axis in plane, the equilibrium distance of multiple domains is derived. For such multiple domains, a minimum wall separation distance exists that depends nonlinearly on the film thickness.
Raman scattering study of the recovery process in Ga ion implanted GaSb
Authors: Kim S. G. , Asahi H. , Seta M. , Takizawa J. , Emura S. , Soni R. K. , Gonda S. , Tanoue H.
Published: July 2002

Recovery process in GaSb crystals with Ga ion implantation and subsequent annealing by rapid thermal annealing (RTA) or furnace annealing (FA) methods are studied by Raman scattering. The intensity of the GaSb LO phonon mode decreases with increasing ion implantation fluence. It is found that the threshold fluence to the amorphization for the Ga ion implanted GaSb is 5×1013 cm−2. It is much lower than that for InP (1×1014 cm−2). In the face-to-face FA, the recovery processes in the Ga ion implanted GaSb are very different above and below the fluence of 5×1014 cm−2. For the 5×1014 cm−2 Ga ion implantation, no recovery is observed. Below 5×1014 cm−2 implantation, the GaSb LO mode intensity increases with increasing annealing temperature and with time up to 400 °C and 15 min, respectively. However, the recovery of damage is very poor compared with that of GaAs, InP, and GaP. On the other hand, in the Si3N4 caped RTA the recovery is observed even for the 5×1014 cm−2 implantation. New modes are observed at around 114 and 150 cm−1 in the implanted and annealed GaSb samples. These two modes are related to Eg and A1g modes of Sb—Sb bond vibrations, respectively, and are produced due to the outdiffusion of Sb atoms. It is found that the face-to-face annealing enhances the outdiffusion of Sb, and that the Si3N4 caped RTA process is superior to the face-to-face FA process for the healing of the damaged layers. These anomalous behaviors are closely related to the weak bond strength of Sb containing materials. The degree of the recovery as a function of annealing temperature, annealing time, and ion implantation fluence is also investigated in both RTA and FA methods.
A vibrational study of the adsorption and desorption of hydrogen on polycrystalline diamond
Authors: Thoms Brian D. , Pehrsson Pehr E. , Butler James E.
Published: July 2002

The adsorption and desorption of hydrogen from diamond films were studied in ultrahigh vacuum using high resolution electron energy loss spectroscopy as a probe of surface vibrations. Auger electron and energy loss spectroscopies were also used to characterize the diamond surface. The samples studied were boron-doped polycrystalline diamond films with chiefly (111) oriented facets. We attribute the observed spectral features to a monohydride species and local sp3 bonding on the diamond surface exposed to atomic hydrogen. A significant fraction of the hydrogen desorbs from the surface between 950 and 1000 °C, although some hydrogen persists even after heating to 1050 °C.
Intrinsic upper limits of the carrier lifetime in silicon
Authors: Häcker Rolf , Hangleiter Andreas
Published: July 2002

We have measured the carrier lifetime in ultrapure n- and p-type silicon with carrier densities between 1015 cm−3 and 1019 cm−3. At high carrier densities the dominant recombination mechanism is known to be band-to-band Auger recombination. At carrier densities below some 5×1018 cm−3 experimentally determined carrier lifetimes in n- and p-type silicon were always found to be smaller than expected for band-to-band Auger recombination. Our results are explained by taking into account electron–hole correlations in the Auger process. Since Auger recombination is an unavoidable material property this represents a new intrinsic upper limit of the carrier lifetime in silicon.
Spectroscopic measurement of atomic hydrogen level populations and hydrogen dissociation degree in expanding cascaded arc plasmas
Authors: Otorbaev D. K. , Buuron A. J. M. , Guerassimov N. T. , van de Sanden M. C. M. , Schram D. C.
Published: July 2002

Optical absorption spectroscopy has been applied to measure the absolute population densities of the first excited levels of atomic hydrogen H*(n=2) and argon Ar*(4s) in an expanding cascaded arc plasma in hydrogen-argon mixture. It is demonstrated that the method allows us to determine both H*(n=2) and Ar*(4s) absolute density radial profiles for H2 admixtures in Ar ranging from 0.7% to 10% with good accuracy. The measured H*(n=2) densities are in the 1014–1016 m−3 range, and Ar*(4s) densities are in the range of 1015–1018 m−3. It has been shown, that the density of hydrogen excited atoms H*(n=2) serves as an indicator of the presence of argon ions and hydrogen molecules in the expanding plasma. A kinetic model is used to understand evolution of H*(n=2) density in the expansion, and to estimate the total atomic hydrogen population density and hydrogen dissociation degree in sub- and supersonic regions of the plasma.
Plasma modification of polymethylmethacrylate and polyethyleneterephthalate surfaces
Authors: Gröning P. , Collaud M. , Dietler G. , Schlapbach L.
Published: July 2002

Noble gas (He,Ar,Xe) and reactive gas (O2,N2) plasma treatments of polymethylmethacrylate (PMMA) and polyethyleneterephthalate (PET) surfaces were performed in an electron-cyclotron-resonance plasma. In situ surface analysis by x-ray photoelectron spectroscopy reveals well-defined surface compositions. From these measurements it is concluded that, independently of the plasma gas, the plasma ions easily decompose the ester group in PMMA in its constituents by an ion-electron recombination process, while in PET the ester decomposition is less pronounced. The difference is ascribed to the presence in PET of a phenyl ring, which protects the ester group by various mechanisms. The study of O2 plasma treatments shows that the equilibrium between the depletion of oxygen and the incorporation of the reactive species in the polymer surface is solely determined by the ion current. The plasma-polymer interactions are qualitatively explained by simple rules of intermolecular forces and ion-electron recombination phenomena.
A local vibrational mode investigation of p-type Si-doped GaAs
Authors: Ashwin M. J. , Fahy M. R. , Newman R. C. , Wagner J. , Robbie D. A. , Sangster M. J. L. , Silier I. , Bauser E. , Braun W. , Ploog K.
Published: July 2002

Infrared absorption (IR) and Raman scattering measurements have been made of the localized vibration modes (LVM) due to defects incorporating silicon impurities in p-type Si-doped GaAs grown by liquid phase epitaxy (LPE) on (001) planes and by molecular beam epitaxy (MBE) on (111)A and (311)A planes. Analysis of a closely compensated LPE sample indicated that an existing calibration factor for the SiAs LVM (399 cm−1) relating the integrated absorption coefficient (IA) to the concentration [SiAs] should be increased by 40%, so that IA=1 cm−2 corresponds to [SiAs]=7×1016 cm−3. The SiAs LVM appeared as a Fano dip in the hole absorption continuum at ∼395 cm−1 in the highly doped p-type material, some 4 cm−1 lower in frequency than its normal position in compensated GaAs. Electron irradiation of samples led to the progressive removal of the Fano dip and a shift with the emergence of the expected SiAs LVM absorption line at 399 cm−1. In MBE samples the irradiation also generated SiGa donors, but the site switching was not detected in LPE material. By contrast, Raman spectra of as-grown p-type samples exhibited a symmetrical peak at 395 cm−1, which also shifted towards 399 cm−1 as the free carriers were removed. MBE (111)A GaAs:Si compensated by SnGa donors revealed the SiAs LVM at its normal position. After hydrogenation of MBE and LPE samples, only stretch modes due to H-SiAs were observed. Passivated MBE GaAs (111)A codoped with Si and Be showed stretch modes due to both shallow acceptors. It was thereby concluded that only one type of acceptor (SiAs) was present in p-type Si-doped GaAs, contrary to previous proposals. There was no evidence for the presence of SiAs pairs or larger clusters.
Electrical properties of BC2N thin films prepared by chemical vapor deposition
Authors: Watanabe M. O. , Itoh S. , Mizushima K. , Sasaki T.
Published: July 2002

The electrical properties of BC2N thin films have been investigated in terms of the temperature dependence of the resistivity and Hall effect measurements. The BC2N thin films were prepared by chemical vapor deposition from acetonitrile and boron trichloride on polycrystalline Ni and quartz substrates. The experimental results indicated that the BC2N films were p-type semiconductors on both substrates, with acceptor levels between 7.5 and 23 meV relative to the valence band. The hole mobility on Ni substrates was one order of magnitude higher than that on the quartz substrates, suggesting that the thin film quality is better on Ni substrates than on quartz substrates.
The structure and crystallization characteristics of phase change optical disk material Ge1Sb2Te4
Authors: Mao Z. L. , Chen H. , Jung Ai-lien
Published: July 2002

The crystallization characteristics of amorphous Ge1Sb2Te4 thin films were studied by means of time-resolved transition measurements. It was found that a metastable phase appeared at the first stage of the crystallization process and then the metastable phase was transformed into a stable crystalline phase at higher annealing temperatures. The x-ray diffraction and transmission electron microscopy results indicated the metastable phase was identified as a face-centered-cubic structure and the stable crystalline phase corresponded to a hexagonal structure. Our experimental results show that the Ge1Sb2Te4 materials are applicable for phase change erasable optical storage.
Resonant cavity enhanced photonic devices
Authors: Ünlü M. Selim , Strite Samuel
Published: July 2002

We review the family of optoelectronic devices whose performance is enhanced by placing the active device structure inside a Fabry-Perot resonant microcavity. Such resonant cavity enhanced (RCE) devices benefit from the wavelength selectivity and the large increase of the resonant optical field introduced by the cavity. The increased optical field allows RCE photodetector structures to be thinner and therefore faster, while simultaneously increasing the quantum efficiency at the resonant wavelengths. Off-resonance wavelengths are rejected by the cavity making RCE photodetectors promising for low crosstalk wavelength division multiplexing (WDM) applications. RCE optical modulators require fewer quantum wells so are capable of reduced voltage operation. The spontaneous emission spectrum of RCE light emitting diodes (LED) is drastically altered, improving the spectral purity and directivity. RCE devices are also highly suitable for integrated detectors and emitters with applications as in optical logic and in communication networks. This review attempts an encyclopedic overview of RCE photonic devices and systems. Considerable attention is devoted to the theoretical formulation and calculation of important RCE device parameters. Materials criteria are outlined and the suitability of common heteroepitaxial systems for RCE devices is examined. Arguments for the improved bandwidth in RCE detectors are presented intuitively, and results from advanced numerical simulations confirming the simple model are provided. An overview of experimental results on discrete RCE photodiodes, phototransistors, modulators, and LEDs is given. Work aimed at integrated RCE devices, optical logic and WDM systems is also covered. We conclude by speculating what remains to be accomplished to implement a practical RCE WDM system.
Imaging of optical and topographical distributions by simultaneous near field scanning optical/atomic force microscopy with a microfabricated photocantilever
Authors: Fukuzawa Kenji , Tanaka Yuriko , Akamine Shinya , Kuwano Hiroki , Yamada Hirofumi
Published: July 2002

Simultaneous near field scanning optical and atomic force microscopy with a microfabricated photocantilever reveal both optical and topographical distributions. The cantilever tip changes the evanescent field into scattering light, and this scattering light is detected with a photodiode fabricated in the tip of the cantilever. The cantilever deflection signal leads to atomic force images. The resolution for imaging the evanescent field variation was 20 nm (λ/30). The near field optical and atomic force images indicate that the same point of the cantilever tip generates both optical and atomic force signals. This method is a new approach to optical and topographical microscopy with nanometer resolution.
Plane wave propagation in finite 2-2 composites
Authors: Cao Wenwu , Qi Wenkang
Published: July 2002

A common practice in the study of wave propagation in stratified structures is to use the Floquet (or Bloch) condition to derive the dispersion relation, leading to the passband and stopband structures. However, the Floquet condition is valid only for an infinite system while a real system always has finite dimensions. We report a study on wave propagation in a finite 2-2 composite by using the transfer (T) matrix technique. Through introducing a new definition for the dispersion relation using the T matrix, the passbands and stopbands are calculated for a finite system without the Floquet condition. The formation of stopbands and passbands with the increase of composite size can now be clearly seen. The spatial profile of the vibration pattern inside a finite composite can also be calculated using this technique, which reveals strong edge effects. The effects of randomization on the wave localization in a 2-2 composite are also studied.