bluuebunny/crossref_metadata_2025 huggingface dataset:
Characterization of SixGe1−x/Si heterostructures for device applications using spectroscopic ellipsometry
Authors: Sieg R. M. , Alterovitz S. A. , Croke E. T. , Harrell M. J. , Tanner M. , Wang K. L. , Mena R. A. , Young P. G.
Published: July 2002

Spectroscopic ellipsometry (SE) characterization of several complex SixGe1−x/Si heterostructures prepared for device fabrication, including structures for heterojunction bipolar transistors (HBT), p- and n-type heterostructure modulation-doped field-effect transistors, has been performed. It is shown that SE can simultaneously determine all active layer thicknesses, SixGe1−x compositions, and the oxide overlayer thickness, with only a general knowledge of the structure topology needed a priori. The characterization of HBT material includes the SE analysis of a SixGe1−x layer deeply buried (600 nm) under the silicon emitter and cap layers. In the SE analysis of n-type heterostructures, a silicon layer under tensile strain was examined. It was found that an excellent fit can be obtained using optical constants of unstrained silicon to represent the strained silicon conduction layer. SE was also used to measure lateral sample homogeneity, providing quantitative identification of the inhomogeneous layer. Surface overlayers resulting from prior sample processing were also detected and measured quantitatively. These results should allow SE to be used extensively as a nondestructive means of characterizing SixGe1−x/Si heterostructures prior to device fabrication and testing.
Dielectric constants of silver particles finely dispersed in a gelatin film
Authors: Ashrit P. V. , Bader Georges , Badilescu Simona , Girouard Fernand E. , Nguyen Le-Quang , Truong Vo-Van
Published: July 2002

Optical properties of gelatin with a very low concentration of silver particles were studied in the region from 300 to 800 nm. Such samples clearly showed isotropic properties and offered the advantage of having well-separated and noninteracting silver particles. Detailed optical constants for the gelatin matrix have been determined and their use, via the Mie theory, has enabled the determination of the dielectric constants and the loss function for the metallic particles themselves. An apparent blue shift in the plasma resonance absorption has been observed with the loss function when the particle size varied from 22 to 10 nm. Experimental data of this nature are needed for meaningful comparison between theory and experiment.
Magnetic phase diagram of Nd(Fe,Mo)12 alloys
Authors: Wang Y. Z. , Hu B. P. , Rao X. L. , Liu G. C. , Song L. , Yin L. , Lai W. Y.
Published: July 2002

The magnetic phase diagram of the alloy series NdFe12−xMox (x=1.0∼2.5) is obtained. Below the Curie temperature, the ferromagnetic phase is divided into three regions: Uniaxial, canted 1, and canted 2. At a temperature of Tsr1 (below 200 K) a spin reorientation transition (SRT) was observed for all the samples, and at Tsr2 (above 200 K) another SRT was found for samples with Mo concentration 1.5<x<1.75. At room temperature the easy direction of magnetization (EDM) of NdFe12−xMox changes from the c axis to a canted structure with increasing Mo concentration, with a critical composition of x=1.56. At low temperatures (below 100 K) all NdFe12−xMox compounds exhibit a canted moment structure.
Epitaxial CoSi2 films on Si(100) by solid-phase reaction
Authors: Vantomme André , Nicolet Marc-A. , Theodore N. David
Published: July 2002

The inversion of a bilayer of Co on top of Ti and a Si(100) substrate upon steady-state annealing, and the resultant formation of an epitaxial CoSi2 layer have been studied using both reactive (N2, N2+5% H2, He+14% H2) and nonreactive (vacuum) annealing ambients. In nitrogen, a high-quality, single-crystalline CoSi2 layer forms above 600 °C for 30 min, with an abrupt interface to the substrate. As the fraction of hydrogen in the ambient increases, the abruptness of the interface deteriorates slightly. On top of this silicide, the Ti is chemically bound with oxygen present as a contaminant. In the case of a nonreactive annealing ambient, the Co/Ti inversion still takes place, although it is only partial. Moreover, the interface is very rough. The structure is unstable above 800 °C for 30 min annealing and transforms into a mixed layer of Co0.25Ti0.75Si2 and epitaxial CoSi2 grains. Using isothermal vacuum annealings with varying durations, a square-root time dependence is observed for the growth of the epitaxial CoSi2 in a temperature range around 700 °C. An activation energy of Ea=3.5 eV is found for that process, which is higher than the 2.3 eV reported for the polycrystalline growth of CoSi2 and is attributed to this structural dissimilarity.
Photon scanning tunneling microscope in combination with a force microscope
Authors: Moers M. H. P. , Tack R. G. , van Hulst N. F. , Bölger B.
Published: July 2002

The simultaneous operation of a photon scanning tunneling microscope with an atomic force microscope is presented. The use of standard atomic force silicon nitride cantilevers as near-field optical probes offers the possibility to combine the two methods. Vertical forces and torsion are detected simultaneously with the optical near field, which allows a comparison between topography and the optical signal. Images of an optical thin film (indium tin oxide) and a Langmuir–Blodgett layer (pentacosa diynoic acid) show absorption contrast with a lateral resolution of about 30 nm (based on edge steepness), which is well below the diffraction limit.
Magnetic and structural properties of Co/CoO bilayers
Authors: Lin X. , Hadjipanayis G. C. , Shah S. I.
Published: July 2002

The magnetic and structural properties of Co/CoO bilayers prepared by sputtering have been studied in films with Co thickness in the range of 65 to 250 Å. A two-phase structure consisting of hcp Co and fcc CoO was observed in the films. The coercivity was found to change inversely proportional to the Co thickness and to the sputtering rate. The highest Hc (9 kOe) was obtained in films exposed to ambient conditions for long periods of time. A shift in the hysteresis loop was found in field-cooled samples, indicating a strong exchange coupling between the Co and CoO phases.
Characterization of an AlGaAs/GaAs asymmetric triangular quantum well grown by a digital alloy approximation
Authors: Mathine D. L. , Maracas G. N. , Gerber D. S. , Droopad R. , Graham R. J. , McCartney M. R.
Published: July 2002

An asymmetric triangular quantum well was grown by molecular-beam epitaxy using a digital alloy composition grading method. A high-resolution electron micrograph (HREM), a computational model, and room-temperature photoluminescence were used to extract the spatial compositional dependence of the quantum well. The HREM micrograph intensity profile was used to determine the shape of the quantum well. A Fourier series method for solving the BenDaniel–Duke Hamiltonian [D. J. BenDaniel and C. B. Duke, Phys. Rev. 152, 683 (1966)] was then used to calculate the bound energy states within the envelope function scheme for the measured well shape. These calculations were compared to the E11h, E11l, and E22l transitions in the room-temperature photoluminescence and provided a self-consistent compositional profile for the quantum well. A comparison of energy levels with a linearly graded well is also presented.
Structure of S on a passivated GaP(100) surface
Authors: Lu Z. H. , Graham M. J.
Published: July 2002

X-ray photoelectron spectroscopy (XPS), chemical state-specific photoelectron diffraction (XPD), and photoelectric polarization-dependent sulfur K-edge x-ray absorption near-edge structure (XANES) have been used to determine the structure of S on GaP(100) surfaces passivated in a (NH4)2S solution. XPS shows that S forms chemical bonds only with Ga. XPD and XANES measurements show that the Ga—S bond is a well-ordered bridge bond in the [011] azimuth.
Photoluminescence studies of strained InxGa1−xAs-Al0.28Ga0.72As heterostructures grown by molecular-beam epitaxy
Authors: Radhakrishnan K. , Yoon S. F. , Li H. M. , Han Z. Y. , Zhang D. H.
Published: July 2002

Low-temperature photoluminescence measurements were carried out on pseudomorphically strained InxGa1−xAs-Al0.28Ga0.72As ternary-on-ternary heterostructures grown by molecular-beam epitaxy to investigate the change in the transition energy and linewidth as a function of InGaAs well thickness at two different indium compositions x=0.10 and x=0.15, respectively. Sharp exciton peaks as narrow as 4–6 meV were observed from the InGaAs wells grown at 530 °C with 1 min of growth interruption at the top and bottom heterointerfaces. The linewidth decreases as the well thickness is increased up to 300 Å. In addition, there are signs of linewidth broadening at higher well thicknesses which may indicate the onset of plastic relaxation. Relatively small variations in the transition energy were observed at well thicknesses which are above the theoretical critical thickness as calculated by the Matthews–Blakeslee model [J. Cryst. Growth 27, 118 (1974)], suggesting the existence of a second critical thickness higher than the theoretical value. Good agreement between experimental and calculated transition energy versus well thickness data was obtained, from which the conduction-band offset ΔEc/ΔEg was estimated to be 0.65±0.05 for x=0.10–0.15, consistent with the results derived from other techniques.
Analysis of eddy-current interaction with a surface-breaking crack
Authors: Harfield N. , Bowler J. R.
Published: July 2002

The change in electromagnetic impedance of a conductor due to the presence of a long, perpendicular surface-breaking crack in a normally incident, uniform electric field is calculated in closed form in the high-frequency limit. At high frequencies, where the skin depth is much smaller than the depth of the crack, the fields near the edge and corners of the crack are effectively decoupled. This means that the solution may be formulated as the sum of contributions from the corners, faces, and edge of the crack. Simple analytical expressions for the electric field are found and used to calculate the impedance due to the crack in the high-frequency limit without resorting to numerical methods.
Pulsed laser etching of silicon: Dopant profile modification and dopant desorption induced by surface melting
Authors: Desmur A. , Bourguignon B. , Boulmer J. , Ozenne J.-B. , Budin J.-P. , Débarre D. , Aliouchouche A.
Published: July 2002

Laser-induced modification of dopant profile and desorption of the dopant are measured for silicon and several dopants (B, As, Sb). The experimental conditions are laser melting and chemical etching by chlorine, and laser annealing. The laser fluence is above the melting threshold, and the Si dosing by chlorine is pulsed under a very low static Cl2 pressure. Successive dopant profiles evolve very differently for these three dopants. The desorption efficiency is shown to depend strongly on the dopant atom, increasing from boron to arsenic to antimony. The implications of etching, doping, and annealing experiments are discussed.
Modeling of a low-intensity electro-optical semiconductor switching device due to intrinsic photoconductivity
Authors: Ivanov A. L. , Haug H.
Published: July 2002

An electro-optical device consisting of a stack of undoped alternating layers of narrow- and wide-gap materials (e.g., GaAs/AlGaAs) together with a series resistor under constant voltage bias is analyzed theoretically. The GaAs heterolayers with a width in the order of a carrier mean free-path are a photo-active region of this vertical device. The device operates only in the presence of a low-intensity light beam due to the intrinsic photoconductivity of the active region. The Franz–Keldysh effect, the strong accumulation of the photocarriers in the photo-active layers due to the charge separation in the presence of the static electric field, and the ballistic component of the total current are responsible for the unusually large electro-optical nonlinearity of the device. Kirchhoff’s law for the electrical circuit of the device provides a sensitive feedback between the voltage drop over the layer and the photocurrent. In a Fabry–Perot cavity a room temperature electro-optical bistabitity is obtained at light intensities less than 10 mW/cm2 with a switching time of about 100 ns.
Investigation of the Paschen curve of nitrogen via the application of nanosecond pulsed electromagnetic radiation
Authors: Scholfield D. W. , Gahl J. M. , Mullins B. W.
Published: July 2002

The Paschen curve of nitrogen from pressures of 0.336 to 685 Torr via the application of nanosecond pulsed electromagnetic radiation has been investigated. Breakdown of the gas was achieved via pumping by short electromagnetic pulses and verified through the use of an optical multichannel analyzer. Adjustments for the short pulse nature of this investigation have been derived and are applied to the Paschen curve. The experiments were conducted utilizing a pulser with machine parameters of 1 J delivered during 1.375 ns FWHM into a 50 Ω coaxial line, at a peak power of 1 GW. Investigation of this region has yielded data in the region of the relativistic Paschen curve as hypothesized by Graham and Roussel-Dupré. Experimental results yield no observable relativistic effects under the conditions of this experiment. Additionally, the collisional frequency limit has been experimentally identified.
Stark broadening diagnostics of the electron density in the laser ablation plume of YBa2Cu3O7−x and PbZrxTi1−xO3
Authors: Fuso Francesco , Vyacheslavov Leonid N. , Masciarelli Guglielmo , Arimondo Ennio
Published: July 2002

Diagnostics of pulsed laser ablation and deposition of superconductive YBCO (YBa2Cu3O7−x) and ferroelectric PZT (PbZrxTi1−xO3) thin films has been accomplished by measuring the Stark broadening of the laser produced plume. Fluorescence emission has been collected from regions along the axis of the plume at different distances from the target surface. The line shape of some selected transitions has been analyzed and the electron density ne determined from the observed linewidth. The measured values of ne are in the range 0.4–30×1017 cm−3 for YBCO and 0.1–6×1017 cm−3 for PZT. A scaling law of ne as a function of the distance from the target has been found, which indicates a one-dimensional behavior in the early stage of the electron cloud expansion. The dependence of ne on the presence of the ambient oxygen gas and on the laser fluence has been also investigated. The results of the diagnostics are compared with theoretical predictions and correlated with the deposition parameters.
Strain effects in epitaxial GaN grown on AlN-buffered Si(111)
Authors: Meng W. J. , Perry T. A.
Published: July 2002

Growth of GaN thin films on AlN-buffered Si(111) by ultrahigh-vacuum rf glow discharge reactive magnetron sputtering is reported. Epitaxy of GaN is established by x-ray and electron diffraction. Raman scattering from the epitaxial films consistent with that of wurtzitic GaN is observed. The ion energies involved in the growth process are quantified by measuring the plasma potentials of the Ar/N2 glow discharge by an emissive Langmuir probe technique. As a function of increasing input power, a systematic increase in ion energies and a systematic straining of the GaN lattice are observed. Measured GaN phonon energy scales with lattice strain.
Magnetic properties of nanostructured thin films of transition metal obtained by low energy cluster beam deposition
Authors: Dupuis V. , Perez J. P. , Tuaillon J. , Paillard V. , Mélinon P. , Perez A. , Barbara B. , Thomas L. , Fayeulle S. , Gay J. M.
Published: July 2002

Clusters of iron, cobalt, and nickel are produced in a laser vaporization source. The size distributions of the incident clusters are checked by time-of-flight mass spectrometry before deposition at low energy. Studying the near threshold photoionization, Con and Nin clusters exhibit an icosahedral structure while for iron, no clear structure emerges. Neutral clusters were deposited on different substrates at room temperature with thicknesses up to 100 nm in view to determine their structure and magnetic properties. A limited coalescence of the clusters is observed from high-resolution transmission electron microscopy. No icosahedron has been observed but cuboctahedron and interface twins between adjacent particles have been clearly identified in Ni films. Grazing incidence x-ray diffraction experiments reveal a classical phase with grain size around 6 and 4 nm for Fe and Ni films, respectively but an anomalous fcc phase for Co films and a very low grain size of 2 nm. The density of films determined by x-ray reflectivity was estimated to represent only 60%–65% of the bulk density. Magnetic behaviors studied by ferromagnetic resonance and SQUID magnetization measurements have been interpreted using the correlated spin glass model. Mössbauer spectra performed on Fe films at zero field revealed the presence of 20% of iron in the form of thin nonmagnetic oxide skin surrounding Fe grains which allow to find 2.2 μB per magnetic iron atom in agreement with macroscopic magnetic measurements. Nevertheless we found an anomalous reduced atomic moment for Ni film.
Ferromagnetic-ferromagnetic tunneling and the spin filter effect
Authors: LeClair P. , Moodera J. S. , Meservey R.
Published: July 2002

Tunneling characteristics of a ferromagnetic-antiferromagnetic-ferromagnetic (FM-AFM-FM) thin film tunnel junction were studied in high magnetic fields with a view to investigate magnetic coupling by the tunneling process. Gd2O3, a stable oxide which undergoes antiferromagnetic ordering below about 3.9 K, was chosen as the tunnel barrier between the ferromagnetic electrodes Gd and permalloy. Tunnel characteristics showed as much as 32% decrease in junction resistance in an applied field of 20 T, below 4.2 K. The resistance behavior as a function of H can be explained by two different effects: firstly, the change in tunnel conductance due to change in the relative magnetization of the two FM electrodes in low H; secondly, the spin filter effect in high fields, due to the exchange splitting of the Gd2O3 conduction band.
Anisotropic dc conductivity in stretch-oriented iodine-doped poly[3-(4-octylphenyl)-2,2′-bithiophene]
Authors: Dyreklev P. , Inganäs O.
Published: July 2002

A square four-point probe is constructed to measure anisotropic conductivity of small samples. The method is applied to measurements of the anisotropic conductivity of a conjugated polymer. The polymer poly[3-(4-octylphenyl)-2,2′-bithiophene] is stretch oriented to various elongations and doped with I2 vapor. The conductivity anisotropy is dependent on the degree of orientation and the absolute conductivity. An anisotropy value of 2.4 is obtained for an elongation of four times. For a given elongation the conductivity anisotropy decreases during the spontaneous undoping. The experimental results are discussed in terms of varying charge hopping distances.
The dependence of the electro-optic effect on electric field distribution and light beam propagation direction
Authors: de Kort Kees , Heringa Anco , Vrehen Joris J.
Published: July 2002

A calculation of the electro-optic effect in cubic materials is presented on the basis of Nelson’s perturbation approach and the calculated field distribution in an electro-optic tip used for measurements in integrated circuits. In a strict sense, longitudinal electro-optic sampling is not possible for this application, and it is found that the magnitude of the electro-optic effect depends on both the field distribution and propagation direction of the light beam. This has important consequences for the interpretation of the signal amplitude obtained with longitudinal electro-optic sampling. We show that the adverse effects can be circumvented in a configuration suitable for a null measurement.
Compositional dependence of the ordering probability in GayIn(1−y)P/GaAs grown by metalorganic chemical vapor deposition
Authors: Choi Won-Jin , Kim Jong-Seok , Ko Hyun-Chul , Chung Ki-Woong , Yoo Tae-Kyung
Published: July 2002

Compositional dependence of the ordering probability in GayIn(1−y)P epitaxial layers grown on GaAs substrates is investigated. Experimental results reveal that band-gap energy reduction due to the ordering is varied with the Ga composition of GayIn(1−y)P and shows a dependence on growth temperature in a wide range of Ga compositions where no serious lattice mismatch occurs. In addition, the change of the ordering of GayIn(1−y)P is confirmed by transmission electron diffraction patterns. For the calculation of the relative ordering probability of the GayIn(1−y)P alloy, a model representing an ordering state with alternative stacking of two tetrahedral micro-unit cells (three Ga-P-one In)/(one Ga-P-three In), is proposed. The results of the calculation on the compositional dependence of ordering probability, which are based on internal strain energy states of the unit cells of the GayIn(1−y)P alloy, show good agreement with experimental results.
Interaction in the Cr/amorphous-silicon system
Authors: Masaki Yuichi , Nonomura Shuichi , Kurokawa Takashi , Sakimoto Takahiro , Kawai Hidenori , Gibson Roderick A. G. , Jones David I.
Published: July 2002

The kinetics of a low-temperature silicide formation have been investigated in experiments on the solid-phase interaction between Cr and amorphous Si (a-Si). For the Cr/plasma-enhanced chemical-vapor-deposited a-Si:H system, it was found that amorphous interlayer was formed at the interface at RT. The compositional ratio of Cr to Si in the amorphous interlayer formed at RT was found to be about 5%. On the other hand, for the Cr/sputtered a-Si system at RT, an interfacial amorphous layer with negligible thickness seemed to exist. From the experimental results and from the results previously reported, the kinetics of the low-temperature interaction were discussed and a new model of an internal-field-assisted interaction was proposed.
YBa2Cu3O7−δ infrared bolometers: Temperature-dependent responsivity and deviations from the dR/dT curve
Authors: Fardmanesh M. , Rothwarf A. , Scoles K. J.
Published: July 2002

The phase and amplitude of the response to infrared radiation of superconducting bolometers was investigated. The detectors were fabricated with 120–550 nm Y1Ba2Cu3O7−δ films on MgO, SrTiO3, and LaAlO3 substrates. A model for the response is developed and compared to the experimental results. The response versus frequency of the samples shows bolometric behavior, in agreement with the measured time dependence of the signals at low frequencies. Different techniques were developed to measure the key parameter in the response of the bolometers, G, the thermal conductance. Several anomalies were observed in the study that provide insight into heat conduction in these devices. The dc or low modulation frequency thermal conductance, G(0), of the samples is found to be limited by the substrate/cold-head thermal boundary resistance. It was also found to decrease with increasing substrate thickness, but still limited by the substrate/cold-head interface. This thickness dependence of G(0) is attributed to scattering of phonons within the substrate, that changes their transmission rate through the substrate/cold-head interface. The results from simultaneous measurements of the IR response and dR/dT (using R versus T) at low modulation frequencies (20 Hz) show that the magnitude of the response differs by up to 30% from the dR/dT curve. The discrepancy is found to be frequency dependent, increasing with decreasing modulation frequency.

This can be treated by use of temperature-dependent thermal constants (G and the heat capacity, C) in the model for the bolometric response. The discrepancy is also observed to be dependent on the superconducting transition, which suggests a possible correlation between the heat conduction through the substrate (and its interfaces) and the absorption mechanism in the superconductor. The phase of the response versus temperature shows an abrupt change at the transition. This is evidence for a change in thermal constants in the bolometer as it goes through the superconducting transition, affecting both the phase and magnitude of the response. Joule heating at even high bias currents has little effect on the response (and its deviation from the dR/dT curve) in our samples, and the effect of noise on the response is significant only at very low bias currents.
Deposition of C60 films by partially ionized fullerene beams
Authors: Ren Zhong-Min , Xiong Xia-Xing , Du Yuan-Cheng , Ying Zhi-Feng , Li Fu-Ming , Chen Liang-Yao
Published: July 2002

C60 films have been deposited by partially ionized cluster beam deposition in which a C60 beam is partially ionized by electron impact and then accelerated by an acceleration field Va towards the substrate where the films are deposited. The experimental results show that the ionized C60 molecules in the evaporated beam are fragmented upon collision with the substrate under the elevated accelerating fields Va. Particularly, as Va exceeds about 400 V, almost all the C60 molecules including ionized and unionized ones are broken into fragments in the deposition films and the resulting films turn out to be amorphous carbon layers, as indicated by the measurements of Raman spectra, x-ray diffraction, and ellipsometry.
Modeling of YBa2Cu3O7 weak links produced by oxygen-ion modification
Authors: Tinchev S. S.
Published: July 2002

High-Tc YBa2Cu3O7 rf superconducting quantum interference devices made by oxygen-ion modification have been produced and used successfully since 1990, but the mechanism of their operation remains still unclear. Reported is a computer modeling of the ion modification using the TRIM program. It appears that the depression of the transition temperature is responsible for the operation of these bridges as weak links. At 77 K and modeling using our experimental parameters, only the base of the YBa2Cu3O7 film about 50 nm thick near the substrate is superconducting, which acts as a weak link probably because of the temperature-dependent coherence length near the reduced transition temperature T′c of the modified material.
The effects of Si doping in In0.52Al0.48As layers grown lattice matched on InP substrates
Authors: Yoon S. F. , Miao Y. B. , Radhakrishnan K. , Duan H. L.
Published: July 2002

The characteristics of Si-doped In0.52Al0.48As layers as a function of silicon doping ranging from 1×1017 to 4×1018 cm−3 are analyzed by low-temperature photoluminescence (PL), Raman spectroscopy, and Hall effect measurements. When the sample temperature is increased from 4 K, the PL peak energy exhibits an inverted S-shaped dependence which is characteristic of carrier localization. This effect was more prominent at lower doping levels, but weakened at high doping levels due to a possible reduction in the donor binding energy. The peak energy variation at temperatures higher than ≊100 K follows the usual band-edge variation with temperature, suggesting that the PL arises from band-to-band transitions. While the PL linewidth of the undoped and moderately doped samples decreases and then increases with temperature, a near-monotonic increase in the linewidth due to thermal broadening was observed in highly doped samples. Supported by observations of a reduction in both the AlAs-like and InAs-like longitudinal-optic (LO) phonon frequencies and a broadening of the LO phonon line shape as the doping level is increased, the PL intensity also showed in increasing degrees at higher doping levels, a temperature dependence which is characteristic of disordered and amorphous materials.
Band-edge-related luminescence due to the energy backtransfer in Yb-doped InP
Authors: Taguchi Akihito , Takahei Kenichiro
Published: July 2002

Band-edge-related photoluminescence in Yb-doped InP was investigated by solving rate equations. For InP:Yb, two characteristic properties have been observed in the band-edge-related luminescence. One is that the decay curve of the band-edge-related luminescence has a slowly decaying component above about 100 K, which has not been observed in undoped InP. The other is that the luminesce intensity shows an increase at around 120 K, although that for undoped InP monotonically decreases as temperature increases. These two properties were investigated based on a proposed model of energy transfer between the Yb 4f shell and the InP–host electrical state. In the model, it is assumed that a nonradiative multiphonon transition process assists the energy transfer. We have shown, in a previous article, that the temperature dependence of the decay time and of the intensity of the Yb 4f shell luminescence can be explained by using this assumption. In this article, we calculated the band-edge-related luminescence properties under the same assumption. Good agreement was obtained between the calculated and experimental results. Thus, the experimentally observed characteristic properties in the band-edge-related luminescence are explained by the energy backtransfer from the excited Yb 4f shell to the host electronic state.
Magnetic viscosity investigations of nanograin iron powder
Authors: Atzmony U. , Livne Z. , McMichael R. D. , Bennett L. H.
Published: July 2002

Magnetic properties of three kinds of nanograin Fe powder were studied. The first two, designated as NM(Ar) and NM(N), were obtained by ball milling the iron powder under an argon or nitrogen atmosphere. The third, designated NN, was obtained by evaporation and condensation. They have grain sizes of 20, 7, and 60 nm, respectively. The particle sizes of the first two were in the micrometer range, whereas those of the NN were in the nanometer range. The NN powder exhibited a coercivity of 79 mT; its magnetic moment at saturation was lower than that of bulk iron. The coercivities of the NM(N) and NM(Ar) were much smaller and their magnetic moment values at high fields were closer to that of bulk iron (210 A⋅m2/kg). The NM(N) and NN samples exhibited marked time dependences of the magnetic moments following a relatively abrupt change of the applied field. The moment changes by more than 0.5% of the saturation moment within 50 min and continues to change for many hours. Much smaller effects were detected in the NM(Ar) sample. The viscosities were investigated as a function of temperature and jump field. The results showed differences in the behavior of the three samples. The powders were consolidated by cold isostatic pressing followed by sintering at around 600 °C for several hours. The magnetic behavior of the sintered samples approach that of bulk iron.
Roughness effect on heterojunction photovoltaics
Authors: Palasantzas G. , Koumanakos E.
Published: July 2002

In this work, we present an investigation of the junction interface roughness effect on the open circuit voltage, Voc for thin film heterojunction photovoltaics. The roughness effect is studied for self-affine rough interfaces, which are described in Fourier space by the correlation model ∼σ2ξ2(1+aq2ξ2)−1−H. σ, ξ, and H denote, respectively, the rms roughness, the in-plane roughness correlation length, and the interface irregularity exponent (0<H<1). The roughness effect becomes significant for small H (<0.5), and for large long-wavelength roughness of typical values σ/ξ∼0.1. The junction interface roughness may yield a contribution to Voc even up to 10%. Comparison of the results is performed with predictions in real heterojunctions, e.g., CuxS/(Zn)CdS.
Electromagnetic and microstructural characterization of YBa2Cu3O7 step edge junctions on (001) LaAlO3 substrates
Authors: Yi H. R. , Gustafsson M. , Winkler D. , Olsson E. , Claeson T.
Published: July 2002

Sharp and straight step edges on (001) LaAlO3 (LAO) substrates were ion milled by using an electron beam defined amorphous carbon thin film mask. YBa2Cu3O7 (YBCO) thin films patterned to narrow strips across the step edges gave high quality Josephson junctions. Their current–voltage (I–V) curves could be well described by the resistively shunted junction model with or without excess current. By varying the YBCO film thickness over a fixed step height, the critical current density (jc) of the junction could be changed by several orders of magnitude. For junctions with high jc, typical IcRn (product of critical current and normal resistance) values of around 100 μV at 77 K and more than 1 mV at 4.2 K were obtained. Some excess current was observed. For junctions with low jc, the dependence of Ic on an applied magnetic field was strong even at low temperatures. The Ic showed a main peak in the center and well-defined periods as a function of applied magnetic field. The minimum Ic value suppressed by the magnetic field was about 20% of its maximum value at 4.2 K. Junctions with low jc usually showed hysteretic I–V curves at low temperatures. The McCumber constant βc fell in the range of 0.8–2. Fiske and flux-flow resonances were observed for some junctions. The shunting capacitances of the junctions were estimated from the McCumber constant βc, Fiske resonances, and flux-flow resonances. A shunting capacitance value per unit area of 12–35 fF/μm2 was obtained. High resolution cross-sectional transmission electron microscopy was used to study YBCO films grown across straight and wavy step edges. Two 90° tilt boundaries were formed at the edge of a step. The top and bottom YBCO films had their c axis oriented normal to the (001) plane of LAO. In the edge region, the c axis of the YBCO film was tilted by 90°, the a axis was normal to the (001) plane of LAO, and the b axis was lying along the step edge. For wavy step edges, second phase particles usually appeared in the YBCO film along the step edge region. Defects were found on the surface of the LAO substrate along the wavy step edge region. These defects might act as nucleation centers for the second phase particles in the YBCO film.
Radiation patterns of acoustic phonons emitted by hot electrons in a quantum well
Authors: Mitin V. V. , Paulavičius G. , Bannov N. A. , Stroscio M. A.
Published: July 2002

The acoustic phonon radiation patterns and acoustic phonon spectra due to electron–acoustic- phonon interaction in a double barrier quantum well have been investigated by solving both the kinetic equations for electrons and phonons. The acoustic phonon radiation patterns have strongly pronounced maximum in the directions close to the perpendicular to the quantum well direction. The radiation pattern anisotropy is explained in terms of possible electron transitions, electron distribution function, and the Hamiltonian of electron–phonon interaction. It was shown that, the simple assumption that emitted phonons always have a perpendicular wave-vector component of the order of 2π/a, where a is the width of the quantum well, cannot explain the strong anisotropy of the radiation patterns. More detailed analysis is required and has been carried out. The emitted acoustic phonon spectra have maxima at energies 2πℏu/a, where u is the sound velocity.
Crystal structure and magnetism of LaCo13−x−yFexSiy compounds
Authors: Rao G. H. , Liang J. K. , Zhang Y. L. , Tang W. H. , Cheng X. R.
Published: July 2002

The crystal structure and magnetic properties of LaCo13−x−yFexSiy compounds were investigated by means of x-ray powder diffraction and magnetization measurements. The substitution of Si for Co induces an order–disorder transition from the cubic NaZn13-type to its tetragonal derivative structure, while the substitution of Fe for Co does not induce such a phase transition. After annealing treatment, the homogeneous range of the cubic phase is narrowed and that of the tetragonal phase is extended. From crystallographic and thermodynamic points of view, the sta- bility of the cubic and the tetragonal phases is discussed. The measured magnetic moment of LaCo13−x−yFexSiy coincides well with the theoretical prediction based on the magnetic valence model. Within the framework of this model, LaCo13−x−yFexSiy compounds can be considered as strong ferromagnets and their magnetic moment can be theoretically predicted. The composition dependence of Curie temperature is discussed within the mean field approximation. At higher iron concentration the decrease of Curie temperature with Fe concentration might be attributed to the increase of the number of antiferromagnetically coupled Fe–Fe pairs and the decrease of the ferromagnetic coupling between transition metal atoms.
Infrared study of polarization reversal in a polyurethane and its low molecular weight model compound
Authors: Jayasuriya Ambalangodage Champa , Tasaka Shigeru , Shouko Toshikatu , Inagaki Norihiro
Published: July 2002

Ferroelectric polarization reversal in polyurethane [poly(trimethylene heptamethylene dicarbamate)] and low molecular weight urethane (pentamethylene stearyl dicarbamate) has been studied by a spectroscopic technique; Fourier transform infrared measurement of the samples while applying an external electric field. These samples showed typical electrical displacement versus electric-field hysteresis loops and pyroelectric properties. It is considered that the above properties result from urethane dipole rotation in the crystal under an external electric field.
Reply to ‘‘Comment on: ‘Multiband coupling effects on electron quantum well intersubband transitions’ ’’ [J. Appl. Phys. 80, 600 (1996)]
Authors: Peng L. H. , Fonstad C. G.
Published: July 2002

It is shown that the 14 band k⋅p analysis of TE (x,y)-polarized quantum well intersubband transitions can be categorized as (i) second order (bulk limit), and (ii) first order (quantum limit) perturbation depending on the structures. In the quantum limit, TE-active intersubband transition prevails. This explains the recent reports of normal incident quantum well photodetectors made of InGaAs/GaAs, InGaAs/AlGaAs, and GaAs/AlGaAs.
Copper germanide Schottky barrier contacts to silicon
Authors: Doyle J. P. , Svensson B. G. , Aboelfotoh M. O.
Published: July 2002

Copper germanide Schottky barrier contacts have been fabricated on both n- and p-type silicon via sequential evaporation of thin layers of germanium and copper followed by an annealing schedule in the temperature range up to 673 K. Silicon is found to outdiffuse into the contact as observed by secondary-ion-mass spectrometry as the annealing temperature is increased. The barrier heights of the contacts were examined via capacitance–voltage and current–voltage techniques in the temperature range of 80–300 K while electrically active defects were probed by deep level transient spectroscopy. The Schottky barrier height for copper germanide is found to be independent of the germanium concentration and similar to that reported for copper silicide.
X-ray absorption spectroscopic studies of sputter-deposited LaNiO3 thin films on Si substrate
Authors: Lee Hsin-Yi , Wu Tai-Bor , Lee Jyh-Fu
Published: July 2002

X-ray absorption spectroscopy (XAS) was applied to investigate the growth behavior of LaNiO3 thin films on Si substrate deposited via radio frequency magnetron sputtering at high temperature. The thickness of deposited film was always proportional to the sputtering time. However, the Ni-to-La ratio in the film was found to decrease with increasing substrate temperature. It is due to a loss of Ni on high-temperature deposition which lowers the film growth rate and leads to a gradual structural change. Nevertheless, the oxidation states of both Ni atom and La atom in the thin films were not influenced by the substrate temperature or sputtering time. All the XAS evidence was consistent with the results from x-ray reflectivity, x-ray diffraction, and chemical analysis.
Hall-effect study of an ion-bombarded polymer
Authors: Long V. C. , Washburn S. , Chen X. L. , Jenekhe S. A.
Published: July 2002

We report an extended Hall-effect study of ion-bombarded poly(benzimididazobenzophenanthroline ladder), which resembles a disordered metal. We have investigated the dependence of carrier concentration and mobility on ion energy and sample temperature during bombardment and on measurement temperature. The dramatic increases in mobility and conductivity with bombardment energy and temperature indicate increased cross-link density. Concomitant slight decreases in carrier concentration may result from fewer defects in the cross-linked network. The mobility decreases slightly with measurement temperature while the conductivity is dominated by the gradually increasing carrier density.
Luminescence studies of GaN grown on GaN and GaN/AlN buffer layers by metalorganic chemical vapor deposition
Authors: Turnbull D. A. , Li X. , Gu S. Q. , Reuter E. E. , Coleman J. J. , Bishop S. G.
Published: July 2002

A detailed study of the luminescence properties of GaN layers grown by metalorganic chemical vapor deposition on sapphire substrates with GaN/AlN double buffer layers or GaN single buffer layers was carried out with photoluminescence and cathodoluminescence (CL) spectroscopy. It was discovered that the use of the double buffer layer resulted in an improved surface morphology, but also increased the strain in the samples relative to samples grown on single GaN buffer layers. Free exciton (A exciton), neutral donor-bound exciton, and acceptor-bound exciton photoluminescence peaks were observed for GaN films grown on GaN, AlN, and GaN/AlN buffer layers. Acceptor free-to-bound luminescence was also observed and the thermal activation energy of the acceptors was measured. From these data we are able to determine the acceptor binding energy, EA, to be 231.5 meV and the donor binding energy ED to be 29.5 meV. An exciton peak, the acceptor free-to-bound luminescence, and an unidentified lower energy peak were observed with CL. CL imaging also allowed us to correlate luminescence with surface features of the films. e-beam annealing of both n-type and p-type films was investigated.
Theoretical and experimental analysis of Mach–Zehnder quantum-well modulators
Authors: Bardyszewski Witold , Yevick David , Liu Yong , Rolland Claude , Bradshaw Scott
Published: July 2002

A theoretical model of the electro-optic properties of III–V multiple quantum well systems is compared to photocurrent spectrum measurements. Our technique, which is essentially free of adjustable parameters provides accurate estimates of both the absorption and refractive index far below the band edge. We have found that the proper inclusion of both inter-subband mixing effects and the angular dependence of the optical matrix elements is required to obtain satisfactory agreement with experiment.
A rectifying tungsten-molybdenum foil for positron remoderation
Authors: Jo/rgensen L. V. , van Veen A. , Schut H. , Chevallier J.
Published: July 2002

We report the use of a layered foil as a positron rectifying device for transmission geometry positron moderation and remoderation, thereby increasing the efficiency of such devices. Positron transmission through foils consisting of a 10 nm W layer on top of a 100 nm Mo foil has been measured. The measurements are compared with similar results for a 100 nm W foil. A strong rectifying effect with a forward to reverse current ratio of more than 10 is observed making this type of well matched layered foils an obvious candidate for positron remoderation. Retarding energy spectra of the transmitted positrons were obtained and measurements of Doppler broadening and surface branching ratios were performed to further characterize the foils. The obtained transmission results again reveal the discrepancy in positron implantation profiles for thin foils between experiments and existing theory.
Strain-conserving doping of a pseudomorphic metastable Ge0.06Si0.94 layer on Si(100) by low-dose BF2+ implantation
Authors: Im S. , Eisen F. , Nicolet M.-A. , Tanner M. O. , Wang K. L. , Theodore N. D.
Published: July 2002

A thick (260 nm) pseudomorphic metastable n-type Ge0.06Si0.94 layer grown by molecular beam epitaxy on an n-type Si(100) substrate was implanted at room temperature with 70 keV BF2+ ions to a dose of 3×1013 cm−2, so that a p−n junction was formed in the GeSi layers. The samples were subsequently annealed for 10–40 s in a lamp furnace with a nitrogen ambient, or for 30 min in a vacuum-tube furnace. The samples were characterized by 2 MeV He4 backscattering/channeling spectrometry, double-crystal x-ray diffractometry, transmission electron microscopy, and by Hall effect measurements using the van der Pauw sample geometry. Samples annealed for either 40 s or 30 min at 800 °C exhibit full electrical activation of the boron in the GeSi epilayer without losing their strain. The Hall mobility of the holes is lower than that of p-type Si doped under the same experimental conditions. These results can be attributed to the Hall factor of heavily doped p-type GeSi films which is less than unity while the Hall factor of a heavily doped p-type Si or n-type GeSi film is close to unity. When annealed at 900 °C, the strain in both implanted and unimplanted layers is partly relaxed after 30 min, whereas it is not visibly relaxed after 40 s.
Ballistic electron emission spectroscopy of magnetic multilayers (abstract)
Authors: First P. N. , Bonetti J. A. , Guthrie D. K. , Harrell L. E. , Parkin S. S. P.
Published: July 2002

The giant magnetoresistance observed in magnetic multilayers arises from spin-dependent scattering and transmission of electrons at the Fermi energy. We will describe a method for the measurement of these quantities in a “CPP” geometry at electron energies both above and below the Fermi energy. Initial results will also be presented. The measurements employ ballistic electron emission spectroscopy (BEES) to detect the ballistic electron current transmitted through a multilayer as a function of magnetic field and electron energy. The experiments are similar in concept to the “spin–valve transistor,”1 except that the injector is the tip of a scanning tunneling microscope. This allows the injection energy to be varied over a wide range, and spectra can be correlated with the local surface morphology on a nanometer scale. Spectral broadening due to sample inhomogeneities is also eliminated. We anticipate that BEES measurements and complementary scanning tunneling spectroscopy will provide information that is easily compared with calculations of the multilayer band structure and the electron transmittance versus energy.
Development of unusual magnetic anisotropy in Co-based trilayers
Authors: Kim Hyeong-Jun , Lee Byung-Il , Joo Seung-Ki
Published: July 2002

NiFe/Cu/Co trilayers have been prepared on 4° tilt-cut Si (111) substrate by RF magnetron sputtering method. When a Cu(50 Å) underlayer was deposited prior to the formation of the trilayer, NiFe and Co layers consisting of the trilayer developed in-plane magnetic anisotropy where the easy axes were perpendicular to each other. This unusual fact could be confirmed from the M-H curves and the corresponding R-H curves of the trilayers. When a small amount of Co was added to NiFe layer to form a NiFeCo/Cu/Co trilayer, the easy axes of NiFeCo and Co layers turned out to be aligned along the same direction so that perfectly square R-H curves could be obtained. In [NiFeCo(60 Å)/Cu(25 Å)/Co(30 Å)]/Cu(50 Å)/Si(111, 4° tilt-cut) samples, 5.5% of MR ratio was obtained at room temperature, and the maximum and minimum MR values could be obtained at zero external magnetic field in the minor R-H curves. According to the R-H curves, it has been believed that NiFe/Cu/Co trilayers on 4° tilt-cut Si (111) substrate are suitable for switching devices and NiFeCo/Cu/Co trilayers for a memory application.