bluuebunny/crossref_metadata_2025 huggingface dataset:
Excitonic energy range dielectric function in GaAs/Ga0.7Al0.3As single quantum wells at room temperature
Authors: Sigrist M. , Chassaing G. , Hameury A.
Published: July 2002

GaAs/Ga0.7Al0.3As single quantum-well heterostructures, grown by molecular beam epitaxy, are investigated at room temperature using reflectance spectroscopy in the energy range 1.2–3.6 eV. The dielectric function of GaAs in the wells is deduced in the excitonic transition region around the Γ point of the Brillouin zone. The optical constants are determined on the basis of reflectance data using a Kramers–Kronig analysis, followed by an iterative technique of extraction applied to a multilayer scheme for the samples. The optical transitions, accurately located at the peaks of the absorption spectra, are compared to the transition energies calculated using the finite-square-well model in the effective-mass approximation.
Fluttering fountains: Stability criteria
Authors: Casperson Lee W.
Published: July 2002

The fluttering oscillations that have long been seen in falling sheets of water have recently been subjected to detailed theoretical analysis. In this study stability criteria for these oscillations are derived and discussed. These criteria provide insight into the physical mechanisms of the instability and may be employed in the design of fluttering waterfall systems.
Effect of volume fraction and morphology of reinforcing phases in composites
Authors: Monette L. , Anderson M. P. , Grest G. S.
Published: July 2002

Computer modeling has been employed to study the effect of volume fraction and morphology of second-phase constituents on composite stiffness and strength. It is found that the efficiency of load transfer to the second-phase constituent increases with volume fraction vf for particulate composites. For aligned short-fiber composites, the efficiency of load transfer reaches a limiting value with increasing volume fraction for homogeneous fiber dispersions, while for fiber distributions which allow for fiber-rich and matrix-rich regions, the efficiency of load transfer decreases. The saturation or decrease in load transfer efficiency is due to fiber confinement, by which the interfiber matrix material is constrained by the presence of neighboring fibers. Hence, the amount of shear tractions and load transferred to a given fiber is altered by the local fiber distribution, as compared to the case of an isolated fiber (dilute limit). The strength of brittle particulate composites is reduced for most particulate volume fractions considered, while the strength of aligned short-fiber composites with a homogeneous fiber dispersion is marginally increased only for vf≳0.2. The composite strength has a downward concave shape, as a function of vf. This is accounted for by both the saturation in load transfer due to fiber confinement and the lower composite strain at failure (embrittlement) as vf is increased. The strength of viscoelastic aligned short-fiber composites with a homogeneous fiber dispersion displays a higher strength at high fiber volume fractions, as compared to a perfectly brittle matrix, which suggests that matrix toughness plays a key role in the strengthening of short-fiber composites.
Electroluminescence in porous silicon
Authors: Sabet-Dariani R. , McAlpine N. S. , Haneman D.
Published: July 2002

Electroluminescence (EL) in gold-coated porous silicon has been studied under pulse excitation. The room temperature EL is independent of ambient, unlike photoluminescence (PL). It decreases after 30 min annealing treatments, disappearing after about 480 °C, whereas the PL from either a similar or the same sample, observed through the gold contact, is strongly reduced after the same treatment at about 350 °C. The EL peak wavelength, unlike PL, shows no spectral shift on annealing. It is deduced that the origin of the EL is different from that of PL, and is consistent with silicon monohydride.
Structural investigation of the initial interface region formed by thin zirconium films on silicon (111)
Authors: Edwards A. M. , Dao Y. , Nemanich R. J. , Sayers D. E. , Kemner K. M.
Published: July 2002

A structural study of the initial interface region formed by zirconium on silicon (111) was undertaken. Thin films (100 Å) of zirconium were deposited in ultrahigh-vacuum conditions onto atomically, clean silicon (111) wafers and annealed in situ at 25 °C intervals between 300 and 425 °C, over which range Auger spectroscopy indicated silicon diffusion to the surface. Structural characterization of the evolving interface was performed primarily via extended x-ray-absorption fine-structure (EXAFS) measurements. Results indicate that a major structural rearrangement takes place between 350 and 375 °C. EXAFS fitting analysis reveals this transition to be from a disordered-intermixed phase to a more ordered state having interatomic distances closely resembling those of ZrSi, but lower coordination numbers. Ordering continues with progressively higher annealing temperatures until the interface region assumes the ZrSi structure at ∼425 °C. The results are discussed in terms of the free energy and strain of the interface film.
The polarization dependence of x-ray photoelectron yield of a Au photocathode
Authors: Shaw P. S. , Hanany S. , Liu Y. , Novick R. , La Rosa G. , Manzo G. , Santangelo A. , Bivona S.
Published: July 2002

We report a measurement of the total photoelectron yield of gold as a function of the incident x-ray polarization. Polarized x rays with energies of 2.5, 2.7, and 4.0 keV were used to excite a gold photocathode at a glancing incidence angle between 5° and 50°. Contrary to a previous report, we found no measurable polarization dependence for all three x-ray energies. We conclude that the modulation factor, i.e., the fractional change of the total yield when the polarization state is varied, is less than 4%.
Charge-injection theory of bipolar junction transistors
Authors: Rode D. L.
Published: July 2002

A physical theory of the bipolar junction transistor which provides closed-form solutions for current/voltage relations for generalized bias conditions is introduced. Included are the new concepts of emitter and collector collection efficiency. Both emitter and collector regions are treated symmetrically to allow for accurate treatment of operation in both the saturation region and the forward-active region, as well as the reverse-active region. There are six components each of emitter and collector current, resulting from inclusion of emitter injection efficiency, surface recombination at the pn junction peripheries, and bulk base recombination as well as finite minority-carrier collection efficiency. Direct comparison between theory and experiment over the entire transistor operating range from deep saturation where IC=0 continuously into the forward-active region where VCE is much greater than the thermal voltage VT shows excellent agreement.
Numerically generated resonant tunneling diode equivalent circuit parameters
Authors: Huang Keh-Ching , Carroll Michael , Starnes Glenn , Lake Roger , Janes David , Webb Kevin , Melloch Michael
Published: July 2002

We present results from a numerical solution of a resonant tunneling diode structure, using a time-independent Schrödinger equation without considering scattering. Intrinsic circuit parameters such as conductance, capacitance, and inductance are extracted from the numerical results and compared with experimental data obtained from dc and microwave characterization of GaAs/AlGaAs devices. A study of the influence of different lightly doped spacer layer thicknesses on the emitter side reveals the relationship between spacer layer thickness and the circuit parameters. From these results, we provide explanations for the behavior of the biased diode circuit parameters. The simulated data show good qualitative agreement with experimental results; possible explanations for deviations between experimental values and simulated data, including inelastic scattering and series resistance effects, are discussed.
Thermal properties of CdTe
Authors: Alvarado J. J. , Zelaya-Angel O. , Sánchez-Sinencio F. , Torres-Delgado G. , Vargas H. , González-Hernández J.
Published: July 2002

The thermal diffusivity α and conductivity k at room temperature of CdTe semiconductors having different crystalline qualities were determined using a photoacoustic technique. Thermal measurements obtained for each sample were correlated with photoluminescence spectroscopic results in order to associate the observed variation of α and k with crystalline imperfections in CdTe.
Comparison of two-dimensional Heisenberg and two-dimensional XY model behaviors in Pd(1.2 at. % Fe) films
Authors: McKinley J. D.
Published: July 2002

Multilayer Pd/Pd(1.2 at. % Fe)/Pd thin films were prepared by dc magnetron sputtering and characterized with dc SQUID magnetometry. Evidence for two-dimensional (2D) Heisenberg and 2D XY model behaviors was found in two different regions of temperature. In these temperature regimes, our data agree with the predicted form for magnetization M and susceptibility χ as functions of field and temperature. Both model’s susceptibilities were found to fit our data. M and χ exhibit discontinuities at magnetic layer thickness-dependent temperatures which separate the different model behaviors.
Capacitance-voltage measurement of charged defect concentration profile near semiconductor depletion zones
Authors: Frederickson A. R. , Karakashian A. S.
Published: July 2002

We have found a capacitance-voltage method to measure the spatial profile of majority carrier trap concentration in and near the depletion region of an abrupt p-n semiconductor junction. The method is analogous to the standard capacitance-voltage method for measuring the doping concentration profile. We develop specific equations for the hole trap concentration profile in the p material adjacent to heavily doped n material. Measurements are made on silicon solar cell samples and compared to deep level transient spectroscopy measurements on the same samples. The method is especially useful for measuring spatial gradients in defect concentration, and is derived for defect concentration not exceeding twice the doping concentration.
Dielectric spectroscopy of Ba(B1/2′B1/2″)O3 complex perovskite ceramics: Correlations between ionic parameters and microwave dielectric properties. II. Studies below the phonon eigenfrequencies (102–1012 Hz)
Authors: Zurmühlen Rudolf , Petzelt Jan , Kamba Stanislav , Kozlov Gennadii , Volkov Alexander , Gorshunov Boris , Dube Dinesh , Tagantsev Alexander , Setter Nava
Published: July 2002

Dielectric spectroscopy in the submillimeter, millimeter, microwave, and radio frequency range has been performed between 300 and 600 K (for some cases below 300 K) on nine Ba(B1/2′B1/2″)O3 complex perovskite ceramic compounds. The real part of the permittivity ε′ decreases linearly with the increasing tolerance factor t<1 approaching unity. It is insensitive to imperfections in the ceramic, such as impurities, vacancies, etc., and entirely determined by polar lattice vibrations. Its temperature dependence is influenced by the presence of a structural phase transition observed in six of the investigated compounds. It is shown that the imaginary part of the permittivity ε″ in the submillimeter range is mainly of intrinsic origin. The ε″(f) dependences were fitted applying a microscopic theory using polar-phonon parameters that have been determined in the phonon resonance region by infrared reflection spectroscopy (Part I). The theory allows the extrapolation of minimum intrinsic loss due to polar-phonon contributions down to the microwave region. The difference between the extrapolated and measured loss at 10 GHz is due to other intrinsic and extrinsic contributions gaining importance at lower frequencies. The submillimeter measurements reveal a systematic loss decrease with the tolerance factor approaching unity (optimal packing), suggesting the ionic size to be of importance for intrinsic loss. A fourth power dependence of loss on permittivity has been found which compares well with the theoretically expected dependence. The contribution of two-phonon difference absorption processes due to the nonpolar soft branch influences the microwave loss as evidenced in particular by ε″(T) measurements. In the case of Nd- and Gd-containing compounds losses related to the paramagnetic subsystem are believed to be the origin of increasing loss with decreasing temperature at 10 GHz.
Excitonic binding energies in diffused-intermixed GaAs/AlAs/AlGaAs double barrier quantum wells
Authors: Kupka R. K. , Chen Y.
Published: July 2002

We report a detailed study of the exciton properties in thermally diffused GaAs/AlAs/AlxGa1−xAs double barrier quantum wells (DBQW). The interband transition energies have been calculated with a standard transfer matrix method, while the exciton binding energy is obtained by a variational approach with an elliptic exciton envelope function. It is found that the inserted thin AlAs layer between the GaAs well and the AlGaAs barriers has a substantial effect on the exciton confinement and the intermixing properties. For thin enough AlAs barriers, the exciton binding energy increases for increasing diffusion lengths, reaches a maximum and then decreases gradually. The results show that a DBQW mixes faster than a single QW, due to the additional AlAs layers. Thick AlAs barriers enable the formation of an indirect AlGaAs intermixed well region, and the effects which stem from the indirect band line up are discussed.
The effect of plasma pretreatments on interface state electron emission in Si3N4–GaAs structures
Authors: Wang Q. H. , Bowser M. I. , Swanson J. G.
Published: July 2002

The static and dynamic properties of interface states between silicon nitride and n-GaAs have been studied. Comparisons have been made between interfaces that were untreated and ones which were plasma pretreated with Ar, N2, H2, and NH3. With the exception of hydrogen the pretreatments did not significantly alter the quiescent surface Fermi level position or the shape of the interface state distribution in the metal-insulator-semiconductor field effect transistor test structures. In all of the samples it required about 10 μs to complete the transfer of the induced charge from the bulk edge of the depletion region to the interface. Charge which had reached the interface equilibrated there within 100 ns. This charge gave rise to a relatively slow emission transient and had a thermal emission energy consistent with emission from the surface Fermi energy to the lower edge of an interface state band, in accord with the interface state band model. The extent in energy of this band depended on the surface treatment. The insensitivity of the emission time constant to both filling time and insulator electric field suggests that irrespective of the treatment the equilibrated charge remained at the interface and did not tunnel into the insulator.
Thermopower of Sr1−xLaxTiO3 ceramics
Authors: Moos Ralf , Gnudi Alain , Härdtl Karl Heinz
Published: July 2002

The thermopower η of Sr1−xLaxTiO3 ceramics was investigated up to x=0.5 and in the temperature range between 150 K and 1200 K. In addition, the carrier concentration n was determined by Hall measurements and by a chemical Ti3+-analysis. For low temperatures and high n, η depends linearly on temperature and on n−2/3, as expected from a degenerate quasi free electron gas. In the case of high temperatures and low n, the absolute value of η rises with 1.5⋅ln10⋅k/e per decade of temperature and with ln10⋅k/e per decade of carrier concentration, as expected from a classical broad-band semiconductor obeying the Boltzmann statistics. In the range of degeneration an effective mass meff of 4.2 electron masses can be deduced without the assumption of a transport factor Ae. In the classical range Ae=3 can be evaluated, requiring only a temperature and lanthanum independent meff. Thus, the thermopower of Sr1−xLaxTiO3 ceramics can be described by a constant effective mass and a constant transport factor within a wide range of temperature and lanthanum content. Furthermore, the transition from degeneration to classical behavior can be described as a function of temperature and electron density, e.g., at room temperature it takes place at about x≊0.2 (i.e., n≊3.4⋅1021/cm3).
Compositional distribution of laser-deposited films and rapid sequential pulsed laser deposition
Authors: Watanabe Yukio , Seo Y. , Tanamura M. , Asami H. , Matsumoto Y.
Published: July 2002

Compositional distributions of films laser deposited in vacuo at energy densities <1 J/cm2 were found to be partly different from previous observations and theories. Analyzing them, we have inferred that evaporation processes at low energy densities contain decomposition of the target materials and evaporation of the decomposed materials. Based on these analyses, we have concluded that pulsed laser codeposition was one of best pulsed laser deposition methods. To realize this using one laser source, a modified version of pulsed-laser-deposition rapid-sequential-pulsed-laser deposition, is introduced. The dependence of YBa2Cu3O7 films properties on deposition conditions is discussed. Elimination of particulates is demonstrated and good electrical and crystallographic properties as well as suppression of precipitates were achieved in films having the correct stoichiometric composition.
Schottky barriers on anodic-sulfide-passivated GaAs and their stability
Authors: Chen Z. , Kim W. , Salvador A. , Mohammad S. N. , Aktas O. , Morkoç H.
Published: July 2002

The effects of anodic (NH4)2S passivation of n-GaAs Schottky diodes have been investigated. When these Schottky diodes are prepared on anodically treated n-GaAs in (NH4)2S solution, the Schottky barrier height is lowered by at least 200 meV, and the interface trap density is estimated to be 5.5×1012 cm−2 which is two orders less than that of the untreated sample. It is observed that the Schottky barrier height or the position of the Fermi level at the surface is not stable for samples treated with a small current density (∼83 μA/cm2), but is significantly stable for samples treated with a large current density (∼1 mA/cm2). The stability of passivation is sensitive to the photon energy of the excitation source. Although for a longer-wavelength (λ=514 nm) illumination the passivation is stable, for a shorter-wavelength (λ=325 nm) illumination, the passivation is unstable. The photoluminescence intensity is found to rapidly decay due to photon-assisted oxidation. As compared to the (NH4)2S dip treatment, the anodic (NH4)2S treatment improves the stability of passivation.
Generation-recombination noise in submicron semiconductor layers: Influence of the edges
Authors: Kleinpenning T. G. M. , Jarrix S. , Lecoy G.
Published: July 2002

In highly doped thin semiconductor layers one often observes generation-recombination (g-r) noise with a broadened Lorentzian-like spectrum. In a theoretical analysis we have shown that such a spectrum can be ascribed to g-r processes between conduction band and monoenergetic traps in the edge of the layers.
Optical-absorption study of RuS2−xSex single crystals
Authors: Huang Jeng-Kuang , Huang Ying-Sheng , Yang Tzuen-Rong
Published: July 2002

An optical-absorption study of synthetic RuS2−xSex single crystals is reported over a temperature range from 80 to 300 K. The data are fitted to several expressions for the shape of the absorption edge, with the best fit being to that expected for indirect allowed transitions. The band gaps at any temperature varies smoothly with the Se composition x, indicating that the nature of the band edges are similar for RuS2, RuSe2, and the compounds of intermediate compositions. The indirect band gaps of various temperatures are determined and their temperature dependence are analyzed by the Varshni equation [Physica 34, 149 (1967)] and an empirical expression proposed by O’Donnell and Chen [Appl. Phys. Lett. 58, 2924 (1991)]. The parameters that describe the temperature dependence of energy gap of RuS2−xSex are evaluated and discussed.
Damage induced by 90 MeV silicon ions in crystalline silicon
Authors: Chavan S. T. , Bhave P. S. , Bhoraskar V. N. , Kanjilal D.
Published: July 2002

Samples of n-type crystalline silicon were exposed to different fluences, ranging from 1.25 to 5×1014 ions/cm2, of 90 MeV silicon ions. The induced disorder in the surface region was studied by grazing-angle x-ray-diffraction and optical reflection spectroscopy, and that in the bulk was studied by measuring the lifetime of the excess minority carriers generated by a 1 MeV pulsed electron beam. Both in the surface region, as well as in the bulk, the degree of induced lattice defects was found to increase with an increase in the ion fluence. The value of the damage coefficient, estimated from the lifetime of the minority carriers, gradually increased from 1.2×10−10 to 8.9×10−10 cm2 s−1 over the range of ion fluences. These results indicate that 90 MeV silicon ions create an appreciable number of defects in crystalline silicon, the concentration of which increases from the surface to their projected range of ∼31 μm, and at higher fluences secondary defects are also produced in the bulk. Even at the highest fluence used the energy deposited through electronic loss had no observable effect on annealing the induced defects.
Thermocapillary flow excited by focused nanosecond laser pulses in contaminated thin liquid iron films
Authors: Balandin V. Yu. , Otte D. , Bostanjoglo O.
Published: July 2002

A new method is established to determine surface tension and mechanisms of evaporation of liquid metals in a wide range of high temperatures. It is based on a combination of high-speed transmission electron microscopy imaging of flow in nanosecond laser pulse molten films and computer simulation. The technique was applied to iron films with native oxides to investigate the effects of surface active impurities in a melt with transient temperatures and gradients up to 4000 K and 5×108 K/m, respectively. Such melts show a shear flow with direction changing once or twice during 1–2 μs after a 20 ns laser pulse, which cannot be simulated using table values for the temperature coefficient of the surface tension and the vapor pressure. Instead, evaporation is negligible, and the flow of the liquid is mainly driven by a fast changing gradient of the surface tension caused by a time-varying distribution of temperature and dissolved surface active oxygen atoms. Current site coverage models, giving the surface tension as function of temperature and impurity content for static liquids, successfully can be applied to liquids moving on the nanosecond/micrometer scale, too. However, the number of active surface sites can be vastly reduced by short-lived oxide covers, e.g., in iron with native oxides down to 3.5% of the total number of surface sites.
Consistency between experimental data for ambipolar diffusion length and for photoconductivity when incorporated into the ‘‘standard’’ defect model for a-Si:H
Authors: Hubin J. , Shah A. V. , Sauvain E. , Pipoz P.
Published: July 2002

Reasonable consistency between experimental data for the ambipolar diffusion length and experimental data for the photoconductivity is demonstrated for steady-state measurements performed on a-Si:H layers. This consistency is obtained based on the ‘‘standard’’ defect model for a-Si:H. In this model the dangling bonds are taken into account, considering their amphoteric behavior and treating then as recombination centers, whereas the band tails are taken into account as simple two-valued defects acting as traps. Consistency is obtained based on (1) a particular form of the recombination function such as is considered appropriate for the dangling bonds, as well as, additionally, (2) the local charge neutrality condition. The experimental data analyzed are power laws of the ambipolar diffusion length and of the photoconductivity (versus light intensity); they are obtained for a series of slightly p and n-doped samples including the undoped case.
Electrode influence on the charge transport through SrTiO3 thin films
Authors: Dietz Guido W. , Antpöhler Wolfgang , Klee Mareike , Waser Rainer
Published: July 2002

The influence of the electrodes on the dielectric behavior and especially on the leakage behavior of SrTiO3 thin films was investigated by impedance analysis. Based on measurements on thin films with different electrode materials the work function of these materials was found to determine the leakage currents. The main conduction mechanism is thermionic emission of electrons from the cathode into the SrTiO3 thin film. The current–voltage characteristics are influenced by the Schottky effect.
Reaction paths and kinetics of aluminide formation in Al/epitaxial-W(001) model diffusion barrier systems
Authors: Bergstrom D. B. , Petrov I. , Allen L. H. , Greene J. E.
Published: July 2002

Single-crystal bcc W(001) layers, 140 nm thick, were grown on MgO(001) substrates by ultrahigh-vacuum (UHV) magnetron sputter deposition at Ts=600 °C. Al overlayers, 190 nm thick with strong (001) and (011) preferred orientation and an average grain size of 200 nm, were then deposited at Ts=100 °C without breaking vacuum. Changes in bilayer sheet resistance Rs were monitored continuously as a function of time ta and temperature Ta during UHV annealing. In addition, Rutherford backscattering spectroscopy, x-ray diffraction, transmission electron microscopy (TEM), and scanning TEM, in which cross-sectional specimens were analyzed by energy-dispersive x-ray analysis with a 1 nm resolution, were used to follow area-averaged and local interfacial reaction paths as well as microstructural changes as a function of annealing conditions. The initial reaction products were discontinuous regions of monoclinic-structure WAl4 which exhibit a crystallographic relationship with the underlying W layer. bcc WAl12 forms at a later stage and grows conformally to cover both W and WAl4. WAl4 and WAl12 continue to grow, with W being the primary mobile species, until the Al layer is completely consumed. Information from the microchemical and microstructural analyses was used to model the Rs(Ta,ta) results based upon a multielement equivalent circuit approach which accounts for the observed nonplanar nature of the reaction front. Reaction kinetics and activation energies were determined. The results show that the growth of WAl4 is diffusion limited with an activation energy Ea of 3.1 eV while the formation of WAl12 is reaction limited with Ea=3.3 eV.
Passivation of surface and bulk defects in p -GaSb by hydrogenated amorphous silicon treatment
Authors: Dutta P. S. , Sreedhar A. K. , Bhat H. L. , Dubey G. C. , Kumar Vikram , Dieguez E. , Pal U. , Piqueras J.
Published: July 2002

Passivation of point and extended defects in GaSb has been observed as a result of hydrogenated amorphous silicon (a-Si:H) treatment by the glow discharge technique. Cathodoluminescence (CL) images recorded at various depths in the samples clearly show passivation of defects on the surface as well as in the bulk region. The passivation of various recombination centers in the bulk is attributed to the formation of hydrogen-impurity complexes by diffusion of hydrogen ions from the plasma. a-Si:H acts as a protective cap layer and prevents surface degradation which is usually encountered by bare exposure to hydrogen plasma. An enhancement in luminescence intensity up to 20 times is seen due to the passivation of nonradiative recombination centers. The passivation efficiency is found to improve with an increase in a-Si:H deposition temperature. The relative passivation efficiency of donors and acceptors by hydrogen in undoped and Te-compensated p-GaSb has been evaluated by CL and by the temperature dependence of photoluminescence intensities. Most notably, effective passivation of minority dopants in tellurium compensated p-GaSb is evidenced for the first time.
Ferromagnetic resonance studies of noble metals based sandwiches
Authors: Wang Y. D. , Chen L. Y. , Sui Y. X. , Zhou S. M. , Wang Y. , Zheng Y. X. , Qian Y. H. , Jin Q. Y.
Published: July 2002

In this paper we have prepared Fe/Cu, Ag, Au/Fe sandwiches by ion beam sputtering and studied the interlayer coupling behavior by ferromagnetic resonance technique. In these sandwiches the Fe and noble metal layers are polycrystalline in the textures of (110) and (111), respectively. It is found that the in-plane resonance field and the linewidth oscillate as a function of nonmagnetic layer thickness in a period of about 1.0 nm for Fe/Cu, Au/Fe structures, and 1.4 nm for Fe/Ag/Fe system. It is suggested that the interlayer coupling strength between the ferromagnetic layers oscillates in the same period with the resonance field. An in-plane anisotropy was found in Fe/Ag/Fe system.
Magnetic studies of the metal–insulator transition in CuIr2S4−xSex (x=0, 0.1, and 4)
Authors: Somasundaram P. , Honig J. M. , Pekarek T. M. , Crooker B. C.
Published: July 2002

Magnetic properties and the metal–insulator transition in CuIr2S4−xSex with x=0.0, 0.1, and 4.0 were investigated. CuIr2S3.9Se0.1 exhibits a transition from paramagnetism (M/H=+1.3×10−7 emu/g G) at higher temperatures to diamagnetism (M/H=−1.5×10−7 emu/g G) at lower temperatures and exhibits hysteresis on heating and cooling. The transition on warming in CuIr2S3.9Se0.1 occurs at a lower temperature (200 K) than in CuIr2S4 (218 K) and is not observed in CuIr2Se4.
Investigation of glassy behavior of lead magnesium niobate relaxors
Authors: Cheng Zong-Yang , Zhang Liang-Ying , Yao Xi
Published: July 2002

The dielectric behavior of a solid solution, 10 mol % lead titanate in lead magnesium niobate, vs temperature at different frequencies is measured and analyzed. The shortcomings of the Vogel–Fulcher relationship are discussed. A more reasonable new relationship: ω=ω0 exp[−(T0/Tm)p] is introduced to characterize the relation between the frequency (ω) and the temperature (Tm) at the dielectric constant maximum of the relaxor ferroelectrics. The value of the parameter p in the relationship can be used to characterize the dielectric relaxation strength of relaxor ferroelectrics as well as the relation among the ferroelectrics, the relaxor ferroelectrics, the Debye medium and glass.
Photoluminescence of chemically deposited CdSe nanocrystallites: Effects of crystallite polydispersity
Authors: Garuthara Rohana , Levine Gregory
Published: July 2002

CdSe nanocrystalline films were prepared by chemical solution deposition. Photoluminescence (PL) spectroscopy was utilized to characterize the prepared films. We observed that the PL peak position of the films gradually shifts from ∼595 to ∼670 nm as the deposition time increases. Also, the PL spectra become broadened with the increase of deposition time. The data indicate that as the deposition time increases, the crystal size increases; but once the crystal size reaches a critical volume, crystal growth terminates. We used the effective mass approximation to analyze the effects of crystallite polydispersity upon the shape of the PL spectra.
18O isotopic tracer studies of the laser ablation of Bi2Sr2Ca1Cu2O8
Authors: Gomez-San Roman R. , Casero R. Pérez , Maréchal C. , Enard J. P. , Perrière J.
Published: July 2002

The mechanisms of oxygen incorporation into the growing film during the pulsed laser ablation of a Bi2Sr2Ca1Cu2O8 target have been investigated by the complementary use of 18O isotopic tracing techniques and nuclear microanalysis. Roughly 45% of the oxygen present in the film comes from the ambient gas and the remaining part is directly incorporated from the target. Moreover, oxygen is only incorporated in the Bi2Sr2Ca2Cu1O8 films during the growth period (i.e., laser pulse), since neither oxidation nor oxygen exchange occur during the time interval between successive laser pulses. It is also shown that the main mechanism of oxygen incorporation corresponds to the dissociation of molecular oxygen adsorbed at the surface, leading to the formation of very reactive atomic oxygen which is further injected in the films. This oxygen dissociation is the result of the interaction with the high flux of species coming from the target and reaching the film surface during each laser ablation pulse.
Low temperature catalytic formation of Si-based metal–oxide–semiconductor structure
Authors: Kobayashi Hikaru , Yuasa Toshiro , Nakato Yoshihiro , Yoneda Kenji , Todokoro Yoshihiro
Published: July 2002

Si-based metal–oxide–semiconductor structure is formed at temperatures as low as 300 °C using the catalytic activity of the platinum (Pt) layer. X-ray photoelectron spectroscopy and transmission electron micrography measurements show that heat treatments of the ∼5 nm-Pt/∼1 nm-chemical oxide/Si(100)〉 devices at 300 °C increase the thickness of the oxide layer to 4–4.5 nm and the oxide layer is present between the Pt layer and the Si substrate, but not on the Pt surface. It is found that the thin chemical oxide layer effectively prevents the Pt diffusion and the silicide formation during the heat treatments. Heat treatments in dry- and wet-oxygen result in nearly the same oxide thickness. Oxygen atoms (or oxygen ions) produced at the Pt surface are suggested to be a diffusing species through the Pt and silicon oxide layers.
Optical properties of ZnTe/Zn1−xMgxSeyTe1−y quantum wells and epilayers grown by molecular beam epitaxy
Authors: Watanabe K. , Litz M. Th. , Korn M. , Ossau W. , Waag A. , Landwehr G. , Schüssler U.
Published: July 2002

We have investigated optical properties of ZnTe epilayers, Zn1−xMgxSeyTe1−y epilayers, and ZnTe/Zn1−xMgxSeyTe1−y quantum wells (QWs) grown on (100)-InAs substrates by molecular beam epitaxy. We observed several sharp photoluminescence lines close to the excitonic position and no detectable luminescence from deep levels in ZnTe epilayers. Bright luminescence has been obtained from Zn1−xMgxSeyTe1−y epilayers which are lattice matched with InAs. The band alignment of ZnTe/Zn1−xMgxTe QWs was found to be type I. The reduction of the band gap energy of the ZnTe layer due to a tensile strain was confirmed in this structure. Nearly lattice-matched ZnTe/Zn1−xMgxSeyTe1−y QWs have been fabricated. A type II band alignment was observed for many of these QWs. We estimated bowing parameters not only of the band gap but also of the valence band for ZnSeyTe1−y.
Theoretical calculation of the miniband-to-acceptor magnetoluminescence of semiconductor superlattices
Authors: Latgé A. , Porras-Montenegro N. , de Dios-Leyva M. , Oliveira L. E.
Published: July 2002

The acceptor-related photoluminescence of a GaAs–(Ga,Al)As superlattice, under the influence of a magnetic field applied parallel to the interfaces, is theoretically studied following a variational procedure within the effective-mass approximation. Electron and hole magnetic Landau levels and envelope wave functions were obtained by an expansion in terms of sine functions, whereas for the impurity levels the envelope functions were taken as products of sine and hydrogenic-like variational functions. Impurity binding energies and wave functions are obtained for acceptors at a general position in the superlattice and for different in-plane magnetic fields. Theoretical results corresponding to transitions from the conduction subband to states of acceptors (miniband-to-acceptor e−A0 transitions) at the edge and center positions of the GaAs quantum well compare well with available experimental data by Skromme et al. [Phys. Rev. Lett. 65, 2050 (1990)] on the magnetic-field dependence of the photoluminescence peak position of conduction miniband-to-acceptor transitions for different temperatures and values of the superlattice period.
Structural investigation of thermally nitrided amorphous Ti silicide
Authors: Miura Y. , Fujieda S.
Published: July 2002

Thermal nitridation of thin amorphous Ti-silicide films is studied. The use of hydrazine gas enables the nitridation at 500 °C, well below the nucleation temperature of Ti silicide. Transmission electron microscopy reveals that these TiSiN fims have amorphous structures, which are shown to be stable up to 750 °C. X-ray photoelectron spectroscopy indicates the simultaneous formation of Ti–N and Si–N bonds. This is consistent with the Ti–Si–N ternary phase diagram. Further, the presence of this nitrided film between Al and Si retards the interfacial reaction up to an annealing temperature of 550 °C near the eutectic point (577 °C) of an Al–Si system.
Investigation of magnetic properties of NdMn2(Ge1−xSix)2
Authors: Wang Yin-gang , Yang Fuming , Chen Changpin , Tang N. , Wang Qidong
Published: July 2002

The structure and magnetic properties of NdMn2(Ge1−xSix)2 compounds (x=0–1.0) have been investigated. All the compounds crystallize in ThCr2Si2-type structure. Substitution of Si for Ge leads to a linear decrease of the lattice constants and the unit-cell volume. With increasing temperature, for the compounds with x⩽0.5, both spin reorientation and ferromagnetic– paramagnetic transition are observed as well at Tr and TC(Mn), respectively. The spin reorientation temperature Tr and the Curie temperature TC(Mn) decrease linearly with Si content from 215 and 339 K at x=0–40 and 306 K at x=0.5, respectively. In compounds with 0.3<x⩽0.5 the ferromagnetic component and the antiferromagnetic component may coexit. For compounds with x⩾0.6 the ferromagnetic–antiferromagnetic and the antiferromagnetic–paramagnetic transition are observed and the Curie temperature TC(Nd) decreases monotonically while the Néel temperature TN(Mn) increases monotonically with Si content. A tendentious spin phase diagram has been obtained. The saturation magnetization at 1.5 K decreases first, goes through a minimum at x=0.5, and then increases again.
Characteristics of InxGa1−xAs/GaAs pseudomorphic modulation doped field effect transistor
Authors: Kuan H. , Su Y. K. , Wu T. S.
Published: July 2002

High performance pseudomorphic InxGa1−xAs/GaAs modulation doped field effect transistors (MODFETs) grown by metalorganic chemical vapor deposition have been characterized at dc using modulation spectroscopy. A transconductance as high as 175 and 253 mS/mm was obtained at 30% and 40% indium content, respectively. In order to identify the origin of the MODFETs’ features we have performed a self-consistent Schrodinger–Poisson calculation of the subband and intersubband energies. The photoreflectance spectra can be divided into two parts by the photon energy; one belongs to the GaAs bulk transition and the other belongs to the InxGa1−xAs/GaAs quantum well transition including two-dimensional electron gas (2DEG) signals. The built-in electric fields are 20.6×104 V/cm and 22.6×104 V/cm. From photoreflectance spectra peaks at 1.035 and 1.06 eV show that their energies agree with line shape fitting and 2DEG theory.
Strain effects on photoluminescence properties of Ge/Si disordered superlattices
Authors: Wakahara Akihiro , Nomura Yoshihiro , Ishii Motonori , Kuramoto Kyosuke , Sasaki Akio
Published: July 2002

Strain effects on the enhancement of the luminescence capability of Ge/Si disordered superlattices (d-SLs) are investigated by comparing the photoluminescence (PL) properties of Ge/Si superlattices grown on Si substrate and on strain-free SiGe buffer layer as a substrate. The conduction valley, Δx,y or Δz, becomes the minimum energy state in the superlattices which is dependent on the strain and thus strained or strain-free substrate. In the case of d-SLs on the Si substrate, there is no significant enhancement by artificially introduced disordering on the PL properties. On the contrary, remarkably enhanced photoluminescence is observed in the d-SLs grown on the SiGe buffer layer, showing that d-SLs of SiGe material enhance the luminescence capability similar to the AlAs/GaAs and AlP/GaP d-SLs. The improvement of the PL properties by the d-SL grown on the SiGe buffer layer is discussed in the viewpoint of the directions of carrier localization and required for the carrier recombination.
Atomic hydrogen concentration in a diamond depositing dc arcjet determined by calorimetry
Authors: Juchmann W. , Luque J. , Jeffries J. B.
Published: July 2002

The fraction of hydrogen dissociated in the plume of a dc arcjet used for diamond deposition is determined by calorimetry to be 0.33±0.12. A dc arc is struck in a mixture of argon and hydrogen at 90 psi and the effluent is expanded through a converging/diverging nozzle into a reactor maintained at 25 Torr. Methane (<1%) is added to the luminous gas plume in the diverging nozzle. This supersonic jet impinges on a water cooled molybdenum substrate, and diamond thin film grows from the reactive mixture. The electrical power input of the arcjet (1.6 kW) is balanced by the power losses due to cooling of the nozzle, enthalpy change in the gas, ionization of the gas, dissociation of H2, and the directed velocity of the gas phase. The gas temperature is determined by linear laser-induced fluorescence (LIF) measurements of several rotational lines of NO seeded to the gas plume. The velocity of the gas plume is obtained via the Doppler shift between LIF signals measured simultaneously in a stationary reference cell and in the arcjet. The atomic hydrogen fraction increases as a function of the feedstock H2/Ar ratio and with increasing input power to the arcjet.
Intrinsic mechanism of phase locking in two-dimensional Josephson junction networks in presence of an external magnetic field
Authors: Krech W. , Platov K. Yu.
Published: July 2002

We present numerical simulations of the dynamics of two-dimensional Josephson junction arrays to study the mechanism of mutual phase locking. We show that in the presence of an external magnetic field two mechanisms are playing a role in phase locking: feedback through the external load and internal coupling between rows due to microwave currents induced by the field. We have found the parameter values (junction capacitance, cell loop inductance, impedance of the external load) for which the interplay of both these mechanisms leads to the in-phase solution. The case of unloaded arrays is discussed as well.
Orientation-dependent Ga surface diffusion in molecular beam epitaxy of GaAs on GaAs patterned substrates
Authors: Takebe T. , Fujii M. , Yamamoto T. , Fujita K. , Watanabe T.
Published: July 2002

Generation of extra facets on ridge-type triangles with (001)-, (110)-, and (201)-related equivalent slopes on GaAs (111) A substrates and stripes running in the [1̄10], [110], and [100] directions on (001) substrates during molecular beam epitaxy of GaAs/AlGaAs multilayers was investigated. By investigating local variation in layer thickness in the regions adjacent to extra (114)A, (110), and (1̄1̄1̄)B facets common to the (111)A and (001) patterned substrates and extra facets specific to the respective substrates and growth rates of the facets relative to the growth rate on the substrate plane, the orientation-dependent Ga surface diffusion length, λGa, was elucidated as λGa(001)≈λGa(1̄1̄3̄)B<{λGa(1̄1̄1 ̄)B,λGa(3̄3̄1̄)B,λGa(013),λGa(113)A}<λGa(159)≈λGa(114)A≈λGa(111)A<λGa(110). That is, λGa increases in the order of the (001), (1̄1̄1̄)B-related, (111)A-related, and (110) surfaces.
Gummel–Poon model for Npn heterojunction bipolar phototransistors
Authors: Frimel S. M. , Roenker K. P.
Published: July 2002

A Gummel–Poon model for abrupt, single heterojunction Npn bipolar phototransistors is described including the effects of the dc base bias on the current and optical gains. Initially, the excess electron concentration at the emitter end of the quasineutral base is determined by matching the thermionic field emission across the emitter–base heterojunction with the diffusion current at the emitter end of the base and including the effects of optical generation. The result is used in determining the electron profile in the base from which the base charge and the electron component to the emitter and collector currents are calculated following the Gummel–Poon model. The photocurrent’s components due to optical absorption in the quasineutral base, the base–collector space charge region, and the collector region are determined taking into account the nonuniform optical generation assuming topside illumination. A comprehensive description of the recombination current components is incorporated including the effects of optical absorption on recombination. The model is then used to calculate the dc and small signal current gain and the device’s optical gain, and to examine the effects of dc biasing and the optical power level. The simulation results are compared with the available experimental results and reasonable agreement is found.
Field dependence of coupling efficiency between electromagnetic field and ultrasonic bulk waves
Authors: Ogi Hirotsugu
Published: July 2002

The coupling mechanism of the electromagnetic acoustic transducer (EMAT) for bulk waves has been studied by examining the magnetic-field dependence of the wave amplitudes. A spiral elongated coil placed on a thin plate of low carbon steel excites and receives the longitudinal and the shear waves propagating in the thickness direction in the presence of the bias magnetic field. The field dependences of the bulk-wave amplitudes are measured using the electromagnetic acoustic resonance both for the normal and tangential bias fields, which showed different features, depending on the bias-field direction and the wave mode. A two-dimensional model is presented for the explanation of the observed results. The present analysis emphasizes the inclined total field in the derivation of the magnetostriction constants, which is revealed to play an essential role for the wave generation. Both the measurement and the model analysis conclude that the magnetostrictive effect dominates the EMAT phenomena for the bulk waves in ferromagnetic metal, regardless of the bias field direction.
Simulations of sputtering induced roughening of amorphous diamond films deposited with mass separated kiloelectronvolt ion beams
Authors: Koponen I. , Lappalainen R. , Hakovirta M. , Sievänen O.-P. , Hautala M.
Published: July 2002

Atomic scale simulations are performed for the ion bombardment induced roughening of the amorphous diamond films, when the films are deposited by using kiloelectronvolt mass separated ion beams operated in the upper limit of practical deposition energies up to 20 keV. The results show that the roughness due to sputtering erosion is by a factor of 3 to 4 larger than roughness observed in experiments. The experimentally observed roughness is obtained only when moderate surface relaxation within the distance of next nearest neighbors is included in the simulations. The results suggest that atomic scale relaxation of sputtering induced topography occurs during the deposition.
Electrical properties of n-GaSe single crystals doped with chlorine
Authors: Micocci G. , Serra A. , Tepore A.
Published: July 2002

Hall-effect and space-charge-limited-current (SCLC) measurements were performed on Cl-doped GaSe single crystals grown by the Bridgmann–Stockbarger method. The temperature dependence of the free electron density shows the characteristics of a partially compensated n-type semiconductor. The electrical properties are dominated by a deep donor level at about 0.57 eV below the conduction band. An electron trapping level between 0.56 and 0.62 eV below the conduction band has been observed by SCLC measurements. The trapping level concentration depends on the amount of dopant. Finally, the conduction band density-of-states effective mass was estimated to be 1.1 m0.