bluuebunny/crossref_metadata_2025 huggingface dataset:
Hydrogen desorption from chemical vapor deposited diamond films
Authors: Schulberg Michelle T. , Fox Ciaran A. , Kubiak Glenn D. , Stulen Richard H.
Published: July 2002

Temperature programmed desorption was used to measure the desorption kinetics of hydrogen and its isotopes from chemical vapor deposited diamond surfaces. The desorption spectra are surprisingly simple considering the polycrystalline nature of the sample, exhibiting a single peak at ∼1300 K for a heating rate of 6 K/s. There is no isotope effect to the desorption, and neither the position of the peak maximum nor the peak width change with increasing hydrogen coverage. The maximum surface coverage achieved is approximately one monolayer. The spectra can be represented by a single peak first order desorption model, yielding kinetic parameters of Ea=51 kcal/mol and ν=5×107 s−1. An alternate model of multiple desorption sites with a Gaussian-distributed population gives kinetic parameters of Ea,mean=82 kcal/mol, ν=9×1012 s−1, and σ (the width of the Gaussian distribution)=3 kcal/mol. A comparison to desorption from low-index natural diamond surfaces is presented.
Stark effect and Stark-ladder effect in Al0.4Ga0.6As/GaAs asymmetric coupled multiple quantum wells
Authors: Kim S. J. , Oh Y. T. , Kim S. K. , Kang T. W. , Kim T. W.
Published: July 2002

Photoluminescence (PL) measurements at 10 K were performed in order to investigate the influence of electric fields on the exciton states in Al0.4Ga0.6As/GaAs asymmetric coupled multiple quantum wells (ACMQW) grown by molecular-beam epitaxy. The coupling of the electronic energy levels in the wells led to an enhancement of the quantum-confined Stark effect. The PL intensity decreased as the electric field increased. Calculated values of the intersubband transition energies were in good agreement with the experimental values for the ACMQW, and these values showed a similar behavior as those for the step quantum well. When the external applied field was very strong, Stark-ladder transitions were observed, and the measured dependence of the field-induced energy shifts of the Stark-ladder transitions for the ACMQW agreed with theory. These results indicate that the Stark effect and the Stark-ladder transitions in a unique ACMQW based on the AlxGa1−xAs/GaAs structure were observed simultaneously.
Zinc oxide/n-Si junction solar cells produced by spray-pyrolysis method
Authors: Kobayashi H. , Mori H. , Ishida T. , Nakato Y.
Published: July 2002

Zinc oxide (ZnO)/n-Si junction solar cells were fabricated by a spray-pyrolysis method and high short-circuit photocurrent densities and relatively high photovoltages were obtained by adjusting the conditions of the deposition and the post-deposition heat treatment. Consequently, relatively high conversion efficiencies ranging between 6.9% and 8.5% were obtained. The efficiency of the solar cells with ZnO/n-Si structure decreases slowly with time when they are kept in air in the dark because of the increase in the thickness of the silicon oxide layer between Si and the ZnO film. This degradation can be avoided by forming an indium-tin-oxide (ITO) overlayer on the ZnO film, indicating that the silicon oxide layer grows through the reaction of Si with oxygen diffusing from the atmosphere, not with ZnO. The efficiency of the ZnO/n-Si junction solar cells decreases rapidly with the illumination time. Capacitance-voltage measurements show that this degradation is caused by a decrease in the work function of the ZnO film. The decrease in the work function is caused by desorption of O−2 from the grain boundaries of the ZnO films. When incident light contains no ultraviolet (UV) component, this degradation does not occur, indicating that the desorption is caused by the acceptance of holes generated by UV light.
Simulation of diffusion and trapping in digitized heterogeneous media
Authors: Coker David A. , Torquato Salvatore
Published: July 2002

We present an efficient and fast simulation technique to determine the mean survival time τ of a Brownian particle diffusing among a digitized lattice-based domain of traps. Following the first-passage time ideas of Torquato and Kim [Appl. Phys. Lett. 55, 1847 (1989)], the algorithm is based on the known solution for the mean first passage time of a Brownian particle in a cube. The mean survival time, the inverse of the trapping rate, is obtained for a variety of configurations involving digitized spheres. Since the survival time is highly sensitive to the surface area and associated resolution, the results provide a means of determining the relation between the survival time of a real material and its digitized representation. In general, lower resolution images give rise to a diminished mean survival time and, thus, a lower bound on the true mean survival time τ. Digitization can affect other transport properties in which the interface plays a major role, e.g., the fluid permeability associated with flow in porous media. We demonstrate both analytically and computationally that the mean survival time for the digitized medium converges to the continuum value in the high-resolution limit.
Epitaxial growth of Sb/GaSb structures: An example of V/III-V heteroepitaxy
Authors: Dura J. A. , Vigliante A. , Golding T. D. , Moss S. C.
Published: July 2002

The requirements for heteroepitaxial growth of Sb on both GaSb and GaAs, and the subsequent growth of GaSb on Sb, using molecular-beam epitaxy are described. These systems serve as examples of the heteroepitaxy of group-V elements with III-V compounds, i.e., between materials utilizing different bonding and possessing different electronic properties. The quality of the films was determined using high-resolution four-circle x-ray diffraction, and comparisons were made between different structures. GaSb was found to grow (111) oriented on Sb (111) with an inverted stacking sequence. A simple epitaxial model is proposed to explain this.
Detection of residual defects in silicon doped by neutron transmutation
Authors: Hartung Joachim , Weber Jörg
Published: July 2002

Proton implantation into neutron transmutation doped (NTD) silicon generates three hydrogen-related effective mass-like donors with binding energies between 35 and 44 meV. The donors are formed by the complexing of hydrogen with damage induced by the irradiation. Our results indicate residual defects even in well annealed NTD silicon.
On the mechanism of emission from the ferroelectric ceramic cathode
Authors: Puchkarev Victor F. , Mesyats Gennady A.
Published: July 2002

This article suggests an alternative mechanism for electron emission from the ferroelectric ceramic cathode. This mechanism involves the concept of vacuum breakdown initiation from metal-dielectric cathodes that comprises two stages: the field emission through metal- dielectric-vacuum conjunction followed by the processes resulting in an uncompleted surface discharge. The electron emission thus occurs from the low-density surface plasma.
Preparation and optical properties of transparent glass-ceramics containing β-PbF2:Tm3+
Authors: Hirao Kazuyuki , Tanaka Katsuhisa , Makita Masaya , Soga Naohiro
Published: July 2002

Transparent glass ceramics containing β-PbF2:Tm3+ crystallites have been prepared by heat treatment of glasses in GeO2-PbO-PbF2-TmF3 system. Average crystallite size of β-PbF2 evaluated from the full width at half maximum of x-ray diffraction lines varies from about 16 to 20 nm as the heat treatment temperature increases from 340 to 400 °C when 50GeO2⋅40PbO⋅10PbF2⋅1TmF3 glass is heat treated. The glass ceramics containing β-PbF2:Tm3+ of 16 nm is transparent in the visible region. The incorporation of Tm3+ into β-PbF2 crystalline phase has been demonstrated from the linewidth and peak position of optical absorption spectrum. Frequency upconversion fluorescence has been observed for the transparent glass ceramics at room temperature. The upconversion fluorescence intensities due to the 1D2→3H6 and 1D2→3F4 transitions are higher for the transparent glass ceramics than for the glass. This phenomenon is explainable by assuming that the multiphonon relaxation rate of the 3H4 state is smaller in the glass ceramics than in the glass.
Characterization of deep levels and carrier compensation created by proton irradiation in undoped GaAs
Authors: Tan H. H. , Williams J. S. , Jagadish C.
Published: July 2002

Deep levels and carrier compensation created in undoped metal-organic chemical-vapor deposition grown GaAs by low fluence proton irradiation (1×108−1×1010 cm−2) are investigated by the deep level transient spectroscopy technique and capacitance-voltage profiling. At least five main electron traps are observed after room-temperature irradiation in addition to the EL2 present in the as-grown material. Irradiation generates additional EL2, which annihilate at much lower temperatures than one would expect for isolated EL2. However, with further increase in irradiation fluence, the magnitude for this additional increment begin to decrease. The apparent decrease in the EL2 peak is accompanied by an increase of a broad peak in the deep level transient spectroscopic spectrum. This broad peak has a highly nonexponential capacitance transient and it is suggested to result from the interaction of the additional EL2 with EL6. One of the observed traps, with energy level, (Ec−0.40) eV, has not previously been reported in proton irradiated GaAs. The signature of this trap resembles that of EL5 and is quite stable at moderate annealing temperatures; annihilating completely only at a temperature of ∼600 °C. This level shows a saturation effect with increasing irradiation dose and we believe it is related to complex defect-impurity formation. The temperature dependence of the carrier profiles reveals some complex behavior of carrier compensation, including acceptor- and donor-like properties of the various traps.
Effect of negative dc bias voltage on mechanical property of a-C:H films deposited in electron cyclotron resonance plasma
Authors: Kamata Kiichiro , Inoue Tohru , Sugai Ken-ichi , Saitoh Hidetoshi , Maruyama Kazunori
Published: July 2002

Amorphous C:H films were deposited with CH4-Ar-H2 using a chemical vapor deposition apparatus assisted by electron cyclotron resonance plasma. The effects of applying negative dc bias voltage on the mechanical property and the microstructure of the films were examined. The microhardness of the films increased rapidly at a threshold value of the bias voltage at each gas ratio, [Ar]/([Ar]+[H2]). The results obtained from Raman spectroscopy suggest that the films contain diamondlike structure added to polymer structure over the threshold value on the negative dc bias voltage. Although the dc bias voltage required to form diamondlike carbon (DLC) becomes stronger with the increase in the H2 flow rate, the approximate volume fraction of DLC in the films rapidly increases, implying that hydrogen is more effective to synthesize DLC films.
Optical characterization of GaN films by photoreflectance and photocurrent measurement
Authors: Qin L. H. , Zheng Y. D. , Feng D. , Huang Z. C. , Chen J. C.
Published: July 2002

We report on the room temperature optical characterization of single crystal hexagonal GaN films on (0001) sapphire grown by metalorganic chemical vapor deposition. The energy gap of GaN was determined to be 3.400 eV by photoreflectance and the possible origin of the photoreflectance signal is discussed. Photocurrent measurement exhibited a peak at 3.351 eV and a continued photoresponse through the ultraviolet region. We found that the intensity of photocurrent was dependent upon the chopper frequency in the measurement. Absorption coefficient and film thickness were obtained from the optical transmission spectra.
Electro-optic measurements of thin-film materials by means of reflection differential ellipsometry
Authors: Wang Feiling , Furman Eugene , Haertling Gene H.
Published: July 2002

Electro-optic properties of thin-film materials grown on opaque substrates are determined utilizing the principles of reflection differential ellipsometry. The scheme of the measurement involves the detection of the field-induced phase shift in a probing light beam reflected from thin-film samples. To quantitatively determine the field-induced indices change or birefringence and the field-induced strain it is essential to model the differential ellipsometric process in the stratified structure. The modeling reveals that the field-induced changes of the ordinary and the extraordinary indices contribute to the measured phase shift with different incident-angle dependences. The field-induced strain gives yet another unique incident-angle distribution. By means of an incident-angle-varying technique, therefore, the field-induced changes of ordinary index and extraordinary index in a thin-film material may be determined separately. Detailed descriptions of the measuring technique and the modeling work are presented.
Evolution of microstructure in nanocrystalline Mo-Cu thin films during thermal annealing
Authors: Ramanath G. , Xiao H. Z. , Yang L. C. , Rockett A. , Allen L. H.
Published: July 2002

The evolution of microstructure in Mo-Cu thin films during annealing has been investigated by in situ sheet resistance measurements, ex situ x-ray diffraction, and in situ hot-stage as well as conventional transmission electron microscopy. Mo-Cu thin films, deposited on various glass substrates by magnetron sputtering at ∼30 °C, were supersaturated solid solutions of Cu in Mo with a nanocrystalline microstructure. The as-deposited films had large compressive residual stresses owing to the low homologous deposition temperature and low Ar pressure during deposition. Annealing results showed two distinct sets of microstructural changes occurring in the temperature ranges between ∼300 and 500 °C, and ∼525 and 810 °C. In the lower-temperature range, anisotropic growth of nanocrystallites was accompanied by stress relaxation without any observable phase separation. At temperatures greater than ∼525 °C, the metastable solid solution collapsed and Cu precipitated at the grain boundaries. Increasing temperature resulted in the coarsening of Cu precipitates and simultaneous growth of Mo grains. At temperatures greater than ∼700 °C, phase separation and grain growth approached completion.
Effect of B-site cation stoichiometry on electrical fatigue of RuO2//Pb(ZrxTi1−x)O3//RuO2 capacitors
Authors: Al-Shareef H. N. , Tuttle B. A. , Warren W. L. , Headley T. J. , Dimos D. , Voigt J. A. , Nasby R. D.
Published: July 2002

There have been numerous reports that Pb(ZrxTi1−x)O3 (PZT) thin-film capacitors with RuO2 electrodes and compositions near the morphotropic phase boundary exhibit minimal decrease in switched polarization with electric-field cycling. We show that the fatigue performance of RuO2//PZT//RuO2 capacitors strongly depends on PZT film composition. Specifically, we demonstrate that the rate of polarization fatigue increases with increasing Ti content for PZT thin films of tetragonal crystal symmetry deposited on RuO2 electrodes. As the Ti content of the PZT films increased, the film gain morphology changed from columnar to granular and the volume percent of a fluorite-type second phase decreased. These microstructural trends and the possibility that the electrode material acts as a sink for oxygen vacancies are discussed to explain the fatigue dependence on B-site cation ratio for PZT films with RuO2 electrodes.
Positron beam study of annealed silicon nitride films
Authors: Landheer D. , Aers G. C. , Sproule G. I. , Lawther D. W. , Simpson P. J. , Massoumi G. R. , Tong S. Y.
Published: July 2002

Positron annihilation spectroscopy has been used to study silicon nitride films grown by plasma-enhanced chemical vapor deposition and annealed at different temperatures. For both silicon-rich and nitrogen-rich films, the positron line shape (S) parameter increases after annealing for 15 min at temperatures up to 700–800 °C. This is understood in terms of the fact that removal of the hydrogen by annealing leads to the presence of unpassivated silicon dangling bond sites and vacancy complexes. Annealing at higher temperatures leads to a reduction in the S parameter, consistent with further hydrogen removal producing unpassivated N− sites.
Induced noncolinear frequency doubling: A new characterization technique for electro-optic crystals
Authors: Reichert A. , Betzler K.
Published: July 2002

Induced noncolinear frequency doubling (INCFD) is presented as a new characterization technique for electro-optic crystals. Using the interaction of two noncolinear laser beams for the generation of second harmonic light, the method yields a three-dimensional spatial resolution. First experimental results of INCFD measurements on composition gradients in lithium niobate and on the spatial orientations of 90o-domain walls in potassium niobate are shown.
Group V incorporation in InGaAsP grown on InP by gas source molecular beam epitaxy
Authors: LaPierre R. R. , Robinson B. J. , Thompson D. A.
Published: July 2002

The incorporation of the group V components in In1−xGaxAsyP1−y, grown lattice matched to InP by gas source molecular beam epitaxy, has been studied over the entire alloy range, 0≤y≤1, as a function of the group V source composition, the V/III beam flux ratio, and the substrate surface orientation. Several aspects of the group V incorporation are most easily understood in terms of a simple model involving a constant incorporation coefficient and an As ‘‘underpressure’’ condition. An improved description of the results at lower values of the V/III flux ratio is provided by a thermodynamic model based on equilibrium reactions for the formation of the binary constituents, and using the bulk properties of the solid solution. However, the thermodynamic model is quantitatively incorrect for large values of the V/III flux ratio. Furthermore, the results for different surface orientations reveal additional weaknesses in the thermodynamic model and suggest the need to account for the surface bonding configurations in describing the group V incorporation in epitaxial growth.
Magnetic and crystallographic properties of CrAs1−xSx(0≤x<1)
Authors: Suzuki Takanobu , Ido Hideaki
Published: July 2002

S substitution for As of CrAs compound has been successful up to 90 at. % for the first time in this work. The x-ray powder patterns and magnetic susceptibility curves have been observed for the compounds CrAs1−xSx(0≤x≤0.6) in the temperature region from 80 K to 1100 K. It was found that the crystallographic transition from the orthorhombic MnP to the hexagonal NiAs type structure occurs with increase of temperature in the composition region x≤0.5. From the temperature dependence of lattice parameters, large spontaneous magnetostriction is found to occur mainly along the B axis of the compounds with small x. The magnetostriction decreases with x and disappears at x=0.35. The Neel temperature have also been determined. The magnetic moments per Cr atom in the compounds with x=0.3, 0.4, and 0.5 have been found to be within 2.95±0.10 μB, in the temperature region of the NiAs type structure. These data are compared with the previous ones of similar compounds.
Effects of Cr2O3 or Y2O3 doping in barium ferrite thin-film medium for high-density recording
Authors: Chen Y. J. , Kryder M. H.
Published: July 2002

The effects of Cr2O3 or Y2O3 doping on the surface morphology and magnetic properties in barium ferrite thin films have been studied. Thin films were deposited by co-sputtering a barium ferrite target with a Cr2O3 or Y2O3 target. The deposited amorphous films were homogenized at a temperature of 600 °C for over 10 h followed by annealing at 800 °C in a tube furnace. It was observed that, whereas stoichiometric barium ferrite shows 5000 Å×500 Å acicular grainlike features, films doped with Cr2O3 or Y2O3 had equiaxed topographic features with an average size of 500 Å. Magnetic studies showed that the coercivity of the doped films was over 3500 Oe and was larger than that of stoichiometric films. A weak magnetostatic interaction was found in all the films. Studies of the magnetic after effect indicated excellent thermal stability in all the films.
Magnetic coupling in Co/face-centered-cubic Fe/Co sandwiches
Authors: Kawakami R. K. , Escorcia-Aparicio Ernesto J. , Qiu Z. Q.
Published: July 2002

Ferromagnetic (FC) and antiferromagnetic coupling (AFC) of Co layers across a metastable fcc Fe spacer layer has been observed. Room-temperature-grown Fe on Co/Cu(100) was chosen as a spacer layer because it exhibits three distinct structural and magnetic phases depending on the thickness range: fct and ferromagnetic (region I), fcc and nonferromagnetic (region II), bcc and ferromagnetic (region III) (listed in order of increasing thickness). Co/Fe/Co sandwiches were grown on Cu(100) by molecular beam epitaxy with a base pressure of ∼2×10−10 Torr, and characterized by low-energy electron diffraction and reflection high-energy electron diffraction. The magnetic properties were studied in situ using surface magneto-optic Kerr effect. Using a wedged Fe spacer layer, we investigated the magnetic coupling between Co films across many thicknesses of Fe. We found FC in region I, strong AFC at the boundary between regions I and II, and weak AFC in region II. We also studied the effect of just the Co overlayer on the metastable fcc Fe. We find that Co/Fe/Cu(100) differs qualitatively from Fe/Co/Cu(100). Finally, we find an oscillation in the AFC with a periodicity of ∼12 Å by artificially increasing the thickness range of region II.
Texture in magnetic annealed Terfenol-D films
Authors: Loveless M. , Guruswamy S.
Published: July 2002

The paper will present an approach to obtaining (111) textured Terfenol-D films suitable for use in magnetostrictive devices. Amorphous thin films of Terfenol-D were rf magnetron sputter deposited on (100) silicon single crystal substrates. A composite target was used to obtain the Terfenol-D films. Samples were subsequently annealed in the range of 300 to 600 °C in the presence of a 2000 Oe magnetic field for times of 15 min to 1 h. X-ray diffraction studies were performed using a Siemens D5000 diffractometer to examine the changes in the preferred crystalline orientation in films with changes in annealing time, annealing temperature, and field orientation. The magnetic properties were examined using a Micromag 2900 alternating gradient magnetometer. Magnetic annealing at 450 °C showed the best combination of magnetic properties. The films exhibited in-plane Hc values in the range of near zero to 350 Oe and Ms values in the range of 200 to 550 emu/cc.
Giant magnetic effects in the L-edge extended x-ray absorption fine structure of 3d transition metals
Authors: Chakarian V. , Idzerda Y. U. , Kemner K. M. , Park J.-H. , Meigs G. , Chen C. T.
Published: July 2002

A systematic extended x-ray absorption fine structure (EXAFS) study at the L edges of ferromagnetic Fe, Co, and Ni thin films has been conducted using circular polarized soft x rays. A very large helicity dependence has been observed in the L-edge EXAFS spectra. Surprisingly, in contradiction with prior works, these magnetic EXAFS results indicate a nonlinear dependence of this effect on the magnetic polarization of the scattering atom. The data further indicate that the dominant scattering paths responsible for the effect are different from those responsible for EXAFS. A discussion of the results in relation to the present understanding of the underlying mechanism and its ramifications to the existing theoretical frameworks is also presented.
Giant magnetoresistance in melt-spun Cu80Ni10Fe10 ribbons
Authors: Chen L. H. , Jin S. , Tiefel T. H. , Liao S. Y. , Yao Y. D.
Published: July 2002

Magnetoresistance (MR) behavior in melt-spun ribbons of a Cu80Ni10Fe10 alloy has been studied. The rapid solidified ribbon, when properly heat treated for phase decomposition, exhibits giant magnetoresistance behavior with the MR values of 8.5% at room temperature and 29% at 4.2 K. The observed magnetoresistance in the alloy is attributed to spin-dependent scattering at the two-phase interface and in the ferromagnetic phase. The substantial increase in MR, as the temperature decreases from room temperature to 4.2 K, is most likely caused by the reduction of spin-flip scattering of conducting electrons in the paramagnetic regions as a result of magnetic transformation.
Si ion implantation-induced damage in fused silica probed by variable-energy positrons
Authors: Knights A. P. , Simpson P. J. , Allard L. B. , Brebner J. L. , Albert J.
Published: July 2002

Samples of synthetic fused silica have been implanted at room temperature with silicon ions of energy 1.5 MeV. Fluences ranged from 1011 to 1013 cm−2. Samples were probed using variable-energy positron annihilation spectroscopy. The Doppler-broadening S parameter corresponding to the implanted region decreased with increasing fluence and saturated at a fluence of 1013 cm−2. It is shown that the decrease in the S parameter is due to the suppression of positronium (Ps) which is formed in the preimplanted material, due to the competing process of implantation-induced trapping of positrons. In order to satisfactorily model the positron data it was necessary to account for positron trapping due to defects created by both electronic and nuclear stopping of the implanted ions. Annealing of the 1013 cm−2 sample resulted in measurable recovery of the preimplanted S parameter spectrum at 350 °C and complete recovery to the preimplanted condition at 600 °C. Volume compaction was also observed after implantation. Upon annealing, the compaction was seen to decrease by 75%.
Ion energy and plasma characterization in a silicon filtered cathodic vacuum arc
Authors: Bilek M. M. M. , Chhowalla M. , Weiler M. , Milne W. I.
Published: July 2002

The plasma generated by a silicon filtered cathodic vacuum arc has been investigated using a Faraday cup and Langmuir probes. Ion energy distributions for arc currents ranging from 30 to 80 A were measured. Mean ion energies were found to range from 8 to 18 eV. The ion saturation current density varied from 0.1 to 1 mA/cm2 depending on both the arc and filter coil currents. The energy distributions were fitted by a sum of Gaussians spaced according to the gas dynamic model for ion acceleration at the cathode spot.
Determination of a set of electron impact cross sections in tetrafluoromethane consistent with experimental determination of swarm parameters
Authors: Bordage M. C. , Ségur P. , Chouki A.
Published: July 2002

A set of electron-molecule cross sections in CF4 has been derived using the solution of the Boltzmann equation under the hydrodynamic regime. The set is evaluated by fitting the calculated swarm parameters to the experimental data. The obtained set is consistent with measured swarm parameters (drift velocity, transversal diffusion coefficient, ionization, and attachment coefficients), not only in pure gas but also in argon–CF4 mixtures as well with the data available. Information about cross sections (form and magnitude) coming from direct determination has also been respected, even though it is scarce. In mixtures, the electron swarm parameters drastically change even when a small amount of CF4 is added, showing a negative differential conductivity in the drift velocity. This phenomenon is studied in detail. Furthermore, the introduction of the superelastic collisions in the calculations is also justified. In the gas under study, due to the importance of the vibrational cross sections at low energy, the anisotropy of the electron energy distribution function is so high that it must be taken into account in the method used for solution of the Boltzmann equation. The validity of the set of cross sections (including elastic momentum transfer, vibrations, dissociation, ionization, and attachment) has been tested by comparing calculated and measured swarm data not used in the unfolding procedure.
Simulation of optically pumped mid-infrared intersubband semiconductor laser structures
Authors: Wang Jin , Leburton J.-P. , Moussa Z. , Julien F. H. , Sa’ar A.
Published: July 2002

A theoretical self-consistent investigation of optically pumped mid-infrared intersubband semiconductor laser with hot electron effects is presented. Electron dynamics under optical pumping are investigated within a rate equation formulation where particle and energy flow equations are derived from Boltzmann’s equation using Fermi statistics. Electron-polar optical phonon interactions with suitable screening are calculated by using a macroscopic model with slab and interface phonon modes. Our calculations show that despite hot electron effects, population inversion between the first and second excited states can occur at low temperatures under intersubband optical excitation. It is anticipated that lasing in the mid-infrared can be achieved with asymmetric quantum well structures optimized for electron concentrations exceeding 1011/cm2.
A positron lifetime investigation of InP electron irradiated at 100 K
Authors: Dannefaer S. , Bretagnon T. , Kerr D.
Published: July 2002

Positron lifetime investigations have been made on variously doped InP samples irradiated at 100 K with 2.5 MeV electrons. Three irradiation-produced positron lifetimes were found: 240±10, 265±10, and 330±20 ps which are, respectively, ascribed to VP, VIn, and VP⋅VIn vacancies in close association with interstitials. Total introduction rates of these defects were in the range of 0.6–1.2 cm−1. Observation of the defects depends on the position of the Fermi level. In n-type materials no evidence could be found for VP⋅PI, while in p-type material VIn⋅InI was not observed. Annealing studies up to 300 K show that VP⋅PI anneals slightly below 300 K, while VIn⋅InI anneals in part around 125 K, but a sizable fraction remains at 300 K. Divacancies begin annealing at 125 K, but some can persist to 300 K.
Thermodynamic equation of state and application to Hugoniot predictions for porous materials
Authors: Wu Qiang , Jing Fuqian
Published: July 2002

A thermodynamic equation of state (EOS) is derived which is appropriate for investigating the thermodynamic variations along isobaric paths. By using this EOS, a Hugoniot EOS model with a unified theoretical basis is proposed for predicting the shock compression behavior of porous materials. The model is tested on 2024 aluminum, copper, and tungsten which are the typical materials with low, intermediate, and high shock impedance, respectively, and commonly used as standards. The calculated Hugoniots for these three materials with different initial densities are in good agreement with the corresponding experimental data published previously. It shows that this Hugoniot EOS model can satisfactorily predict the Hugoniot of porous materials over a wide pressure range.
Electrical characterization of radio frequency sputtered hydrogenated amorphous silicon carbide films
Authors: Choi W. K. , Han L. J. , Loo F. L.
Published: July 2002

The electrical results of rf sputtered hydrogenated amorphous silicon carbide (a-Si1−xCx:H) films prepared under different sputtering conditions are presented. It was found that hopping and Poole–Frenkel effects are the conduction mechanisms for low and high applied fields, respectively. From the capacitance versus voltage measurements, the fixed charge density (Qf) and the interface trapped charge density (Dit) were found to be in the range of 5.5–6.81×1010 cm−2 and 5–13×1011 eV−1 cm−2, respectively. Dit decreases with either an increase in the total sputtering pressure, the partial hydrogen pressure, or the substrate temperature, but increases with an increase in the rf sputtering power. The decrease in Dit was found to be closely related to the increase in the number of silicon–hydrogen bonds.
Nanometer surface gratings on Si(100) characterized by x-ray scattering under grazing incidence and atomic force microscopy
Authors: Metzger T. H. , Haj-Yahya K. , Peisl J. , Wendel M. , Lorenz H. , Kotthaus J. P. , Cargill G. S.
Published: July 2002

During rapid melting and resolidification of As-implanted Si(100) by pulsed laser irradiation a periodic lateral grating has been created on the Si surface. Structure and perfection of the grating is investigated by specular and diffuse x-ray scattering under grazing incidence and exit angles. Using synchrotron radiation we find sharp, off-specular diffraction rods perpendicular to the sample surface. Their lateral separation is given by the periodicity of the grating (522±1 nm), which is nearly the same as the light wavelength (530 nm) used in laser annealing the samples. Intensity measurements along the diffraction rods are used to determine the detailed structure of the surface grating by fitting the experimental results with model calculations. A sinusoidal shape is found with an average amplitude of 6±1 nm. This structure is confirmed by atomic force microscopy studies. The x-ray method presented will be a unique tool also applicable in the case of buried lateral nanostructures which are not accessible by surface-sensitive techniques, e.g., scanning probe methods.
Relationship between carrier diffusion lengths and defect density in hydrogenated amorphous silicon
Authors: Sakata I. , Yamanaka M. , Sekigawa T.
Published: July 2002

Experimental studies and numerical analysis have been carried out to clarify the relationship between carrier diffusion lengths and defect density in undoped a-Si:H. It has been confirmed that in device quality plasma-deposited a-Si:H, the diffusion lengths of both electrons and holes under steady-state illumination of the intensity equivalent to normal solar cell operating conditions are determined by the density of Si dangling-bond defects ranging between 3×1015 and 8×1016 cm−3. This rather trivial result, however, has not been obtained in previous studies in which the carrier transport data obtained by the steady-state photocarrier grating method were treated incorrectly. The ratio of the drift mobility of electrons to that of holes and the ratio of electron lifetime to the hole lifetime in a-Si:H under illumination have been determined and their implications discussed.
Structural and defect characterization of GaAs and AlxGa1−xAs grown at low temperature by molecular beam epitaxy
Authors: Fleischer S. , Beling C. D. , Fung S. , Nieveen W. R. , Squire J. E. , Zheng J. Q. , Missous M.
Published: July 2002

We have investigated the structural and defect characteristics of GaAs and AlxGa1−xAs grown at low substrate temperature (250 °C) by molecular beam epitaxy. Using x-ray diffraction we have observed an increase in lattice parameter for all as-grown layers, with the AlxGa1−xAs layers showing a smaller expansion than the GaAs layer. However, infrared absorbtion measurements revealed that the concentration of neutral arsenic antisite defect, [AsGa]0, was not significantly affected by aluminum content (x), with only a small reduction for x=0.36. Positron beam studies showed that the low temperature layers had a higher concentration of vacancy-related defects (∼1017 cm−3) than the semi-insulating substrate, with the AlxGa1−xAs layers having the highest values. After annealing (600 °C, 15 min) the lattice constants relaxed to those of conventionally grown material and [AsGa]0 was reduced in all cases, with the smallest reduction occurring for the x=0.36 layer, indicating that the Al atoms strengthen the lattice against excess arsenic incorporation and hold the arsenic antisite atoms more strongly in position. X-ray photoelectron spectroscopy showed that arsenic diffused out of the surface region and was replaced by oxygen, possibly due to an insufficient overpressure of forming gas during the anneal. This oxygen penetration was greater for the GaAs layer than for the AlxGa1−xAs layers. Extra Raman peaks at 200 and 257 cm−1 confirmed that the surface was very disordered. There was, nevertheless, a large increase (4%) in the positron S parameter in the bulk of the annealed layers, suggesting the formation of vacancy clusters, whereas in the surface region we find evidence that AsGa diffusion proceeded at a faster rate in the x=0.36 than the x=0.2, in agreement with the vacancy-enhanced AsGa diffusion model.
Effect of secondary electron emission on sheath potential in an electron cyclotron resonance plasma
Authors: Yao X. Z. , Jiang D. Y.
Published: July 2002

The sheath potential φM is calculated from the difference between the plasma potential and the floating potential. The plasma potential has been measured by using a retarding field analyzer with an entrance slit located in front of it. The electron temperature Te and ion temperature Ti are also measured with the same analyzer in an electron cyclotron resonance plasma. Comparing the normalized sheath potential, −eφM/kTe, with the different theoretical values, it is found that the secondary electron emission could not be neglected in our experiment and is noncritical. The critical emission corresponds to zero electric field at the emitter. The various calculations have shown that the thermionic electron emission need not be required to be taken into consideration. The total energy transmission factors δT(=W/FTe, where W is the energy flux from the plasma, F is the ion flux, and Te is the electron temperature at the sheath edge) for hydrogen, nitrogen, and argon plasmas are calculated from the estimated secondary electron emission coefficient and the sheath potential, respectively.
On the accuracy of depth-dependent thermal conductivity retrieval in photoacoustic experiments
Authors: Fivez J. , Thoen J.
Published: July 2002

We investigate the intrinsic error sensitivity of photoacoustic experiments in obtaining reliable and accurate information on the inhomogeneous thermal conductivity below the surface. The analysis is based on thermal wave properties and does not depend on any approximate inversion technique. As an example, the theory is applied to the important class of monotonic profiles. The theory can help us in choosing the lower frequency limit in an actual experiment.
Space-charge-limited-current spectroscopy: Possibilities and limitations
Authors: Schauer F. , Novotny R. , Nešpůrek S.
Published: July 2002

Criteria for space-charge-limited currents to be applied as a spectroscopical method for localized electron states elucidation are examined, namely, density of states, their energy position, and temperature. The “visibility’’ of the states is determined by the statistics of the occupation of the injected space charge. The resolving power of the method is increased by decreasing temperature during the experiment.
Magnetoresistance and magnetic properties of Co/Ir multilayers on MgO(110) substrates
Authors: Yanagihara H. , Pettit K. , Salamon M. B. , Kita Eiji , Parkin S. S. P.
Published: July 2002

A Co(12 Å)/Ir(16 Å) multilayer deposited on MgO(110) is found to have a strong uniaxial anisotropy, comparable to bulk hcp-Co. Experimental results suggests the crystallographic relation of (100)[001]Co/(110)[001]Ir/(110)[001]MgO. A remarkable difference is seen between easy and hard axis magnetization processes, due to competition between the in-plane anisotropy and the exchange coupling. Considering the characteristics of the MH loops and magnetoresistance curves, we simulate the magnetization and magnetoresistance processes numerically. The results of our simulation indicate the existence of biquadratic coupling in Co/Ir(110).
Domain and domain wall contributions to optical second harmonic generation in thin magnetic films
Authors: Lyubchanskii I. L. , Petukhov A. V. , Rasing Th.
Published: July 2002

A symmetry analysis of nonlinear-optical imaging of domains and domain walls in thin magnetic films is presented. Different contributions to the second harmonic (SH) response depending on the local magnitude of the film magnetization M as well as on its spatial derivatives are calculated. The polarization of the SH light is shown to be a sensitive function of the relative weight of different magnetization-induced contributions, the film symmetry, and the type of the domain wall.
Magnetic properties of Fe/Cu(001) superlattices
Authors: Zhou Yumei , Zhong Lieping , Zhang Wenqing , Wang Ding-sheng
Published: July 2002

The electronic and magnetic structure of Fe/Cu(001) superlattices are investigated with the first-principles all-electron linearized augmented plane wave method in the local spin-density functional approximation. The results show that the magnetic moment of the interface Fe layer is stabilized at the high spin state of the fcc Fe crystals. When the thickness of Fe layers is larger than two monolayers, the interior layers always exhibit an antiparallel spin configuration in its lowest energy phases.
Determination of the upper limit of the magnetic moment in the magneto-resistance by using its irreversible temperature and square magnetization dependences in (Cu-rich)-TM melt-spun systems
Authors: Vergara J. , Ortega R. J. , Madurga V. , Rao K. V.
Published: July 2002

We have produced Cu80(Fe0.5Ni0.5)20 melt-spun granular ribbon samples whose magnetization is a superposition of superparamagnetic and hysteretical behavior at temperatures between 1.5 and 300 K. In contrast, its magneto-resistance (MR) data show hysteresis only at temperatures below 30 K. Above this temperature, a completely reversible behavior is observed. By considering the temperature dependence of the coercive field of nanoparticles, we determine the particles participating in MR to have a maximum magnetic moment of ≈27000 μB for the 500 °C annealed sample. This value is in agreement with the limits obtained by fitting the data of MR as proportional to the square of the magnetization, assuming a distribution density of magnetic moments proportional to the inverse of the square of the magnetic moment of each cluster.
Quantum computation and spin physics (invited)
Authors: DiVincenzo David P.
Published: July 2002

A brief review is given of the physical implementation of quantum computation within spin systems or other two-state quantum systems. The importance of the controlled-NOT or quantum XOR gate as the fundamental primitive operation of quantum logic is emphasized. Recent developments in the use of quantum entanglement to build error-robust quantum states, and the simplest protocol for quantum error correction, are discussed.
Optical and transport studies on thin microcrystalline silicon films prepared by very high frequency glow discharge for solar cell applications
Authors: Tzolov M. , Finger F. , Carius R. , Hapke P.
Published: July 2002

The initial growth stage of phosphorus doped microcrystalline silicon films prepared by plasma enhanced chemical vapor deposition with different plasma excitation frequencies in the range 13.56–116 MHz was studied by Raman and infrared spectroscopy, optical transmission and reflection, and conductivity measurements. The sensitivity of Raman spectroscopy and optical reflection on Si crystallites in the initial growth regime is compared and optical reflection at 4.5 eV is proposed as an easy and reliable tool for this investigation. While the crystallite formation on amorphous silicon substrates at 13.56 MHz is delayed in comparison with glass, SiO2 and chromium substrates, nucleation of the crystalline phase on amorphous silicon is found to be greatly enhanced at higher plasma excitation frequencies. On the other hand, for deposition on glass, SiO2, and chromium at frequencies equal to or higher than 70 MHz, increased porosity is found in the initial growth region. The results are interpreted within a model that suggests a conelike initial formation of the silicon crystallites and a higher etching rate of disordered material at high plasma excitation frequencies. In addition, the extension of the process of crystallite formation from the film-plasma interface into a growth zone more than 10 nm deep is proposed. The application of the microcrystalline silicon layers prepared at high plasma excitation frequency is demonstrated in amorphous silicon based tandem solar cells.
Si/Si
1−x
Ge
x
heterostructures: Electron transport and field-effect transistor operation using Monte Carlo simulation
Authors: Dollfus Philippe
Published: July 2002

Using an ensemble Monte Carlo simulation, we study the electron transport properties in tensile strained Si grown on a Si1−xGex substrate and in an n-channel Si/Si1−xGex modulation-doped field-effect transistor (MODFET). Owing to the strain-induced modification of the conduction-band structure, the in-plane drift mobility in undoped material reaches 3250 cm2/V s at 300 K (for x⩾0.2) and 31000 cm2/V s at 77 K (for x⩾0.05). The two-dimensional Monte Carlo modeling of 0.18 μm gate Si/SiGe MODFETs reveals a high velocity overshoot of 2.5×107 cm/s related to a significant ballistic transport through the gated part of the strained Si channel. For a 0.18 μm gate and a Ge mole fraction x=0.3, intrinsic transconductance and transition frequency as high as 460 mS/mm and 85 GHz are obtained.
Linear and nonlinear optical properties of CuInSe2 and CuGaSe2 epitaxial thin films on GaAs(001)
Authors: Bottomley D. J. , Mito A. , Niki S. , Yamada A.
Published: July 2002

The linear and nonlinear optical properties of tetragonal CuInSe2 and CuGaSe2 thin films grown epitaxially on GaAs(001) have been investigated as a function of film stoichiometry. The refractive indicies of the films have been measured between 250 and 800 nm using ellipsometry. For a fundamental wavelength of 790 nm, the second order optical nonlinearity χ(2) and optical third harmonic field anisotropy σ are typically reduced strongly in nonstoichiometric thin films and correlate well with each other. The magnitudes of the chalcopyrite film χ(3) values are relatively small, being typically only ∼1% of those of Si. Third harmonic data from an approximately stoichiometric CuInSe2 thin film is consistent with a significant second order surface optical nonlinearity cascading contribution. Spectral features in the room temperature refractive index data from approximately stoichiometric CuInSe2 and CuGaSe2 thin films correlate well with previous experimental work.
Effects of optical penetration and laser pulse duration on laser generated longitudinal acoustic waves
Authors: Enguehard Franck , Bertrand Lionel
Published: July 2002

In this article, we concentrate on the optical penetration and laser pulse duration effects on the features of the ultrasonic waves generated in a solid by a laser impact. We consider the simple case of a uniform irradiation of the sample, which we describe with the help of a simple analytical one-dimensional model. In spite of its simplicity, this model clearly highlights the competition that occurs between two temporal convolution sources, related to the optical penetration and the laser pulse duration, respectively, to produce the longitudinal arrivals on the rear side of the sample. The model is also used for an accurate description of the features of the longitudinal precursor.
Ion-ion neutralization of iodine in radio-frequency inductive discharges of Xe and I2 mixtures
Authors: Barnes Paul N. , Kushner Mark J.
Published: July 2002

Xe/I 2 low-pressure electric discharges are being developed as efficient, long-lived ultraviolet lighting sources. In this work the kinetics of low pressure, 0.5–5 Torr, radio-frequency inductively excited discharges sustained in Xe and I2 were investigated to determine the source of radiating states. The diagnostics applied in this study include optical absorption and emission spectroscopy, microwave interferometry, and microwave absorption. We found that in time modulated discharges, the emissions from excited states of atomic iodine decays with time constants of hundreds of microseconds. These observations are consistent with those states being populated by ion-ion neutralization. Electron-ion recombination leading to excited states appears not to be an important source of emission.