bluuebunny/crossref_metadata_2025 huggingface dataset:
Perturbed angular correlation studies of dopant atom interactions in silicon
Authors: Wichert Th. , Swanson M. L.
Published: July 2002

The perturbed γγ angular correlation (PAC) technique was used to study the interaction of implanted 111 In probe atoms with the donor atoms P, As, and Sb in Si. Nearest-neighbor pairs of In-P, In-As, and In-Sb atoms, characterized by νQ1 =179(1), 229(1), and 271(1) MHz, respectively, and having trigonal symmetry about a 〈111〉 axis (η1 =0), were observed after annealing the samples between 540 and 1170 K. These results indicate a strong interaction between acceptor and donor atoms in Si, thus explaining the present and earlier Rutherford backscattering-channeling results that the In atom solubility in Si was enhanced by the addition of As. The In-donor atom binding energy was about 0.5 eV. For increasing As concentrations, PAC data showed the appearance of small In-As atom clusters characterized by νQ2 (As)=238(1) MHz, η2 = 0.65(1); they were probably In-As2 complexes produced when mobile In atoms were trapped by As2 pairs.
Brittleness of high T
c Bi0.7Pb0.3SrCaCu1.8Ox crystal and glass
Authors: Nishi Yoshitake , Ninomiya Nobuyuki , Kanai Fumiyuki , Uchida Satoshi , Moriya Shuji
Published: July 2002

We suggest a convenient and standard evaluation method of brittleness for high Tc Bi0.7Pb0.3SrCaCu1.8Ox. The crystal oxides are ductile, whereas the liquid-quenched glassy oxide is brittle. The high Tcoff (111.7 K) sample, annealed for 12 h at 1120 K, is ductile.
Local mode spectroscopy and photo-induced effects of oxygen-related centers in semi-insulating gallium arsenide
Authors: Song C. , Pajot B. , Gendron F.
Published: July 2002

Vibrational absorption of semi-insulating O-containing GaAs samples has been investigated under high-resolution spectroscopy at low temperature and many sharp lines due to localized modes have been observed. Some of them, between 714 and 730 cm−1 had already been ascribed to some (Ga,O)-related center having features common with the vacancy-oxygen defect in silicon, but a new state related to this center, giving a local mode near 713 cm−1 is reported here for the first time. Another new low-frequency mode at 604 cm−1 is also reported, involving probably nitrogen or more likely oxygen bonded to two next-nearest-neighbor Ga atoms. The attribution of new modes in the 950–1400 cm−1 spectral region to complexes involving oxygen, arsenic, and eventually impurities with 100% isotopic abundance is discussed. The effects of near-infrared illumination on the observation of the (Ga,O)-related modes between 713 and 730 cm−1 and on a new mode at 983 cm−1 have been studied. Tentative explanations based either on metastability or on photo-induced changes of the charge states of the centers are proposed. These results seem to show that in GaAs, oxygen decorates native centers and that this kind of study could provide a clue to their understanding.
Monte Carlo simulations of CsMnBr3
Authors: Mason T. E. , Collins M. F. , Gaulin B. D.
Published: July 2002

Monte Carlo simulations of the magnetic Hamiltonian of CsMnBr3, an XY antiferromagnet on a simple-hexagonal lattice, have been carried in order to examine the phase diagram in an applied magnetic field and the recently discovered tetracritical point. Results on a 12×12×12 lattice reproduce the observed phase diagram. Application of a magnetic field in the ab plane splits the zero-field transition into two transitions. The structures of the two phases are also in agreement with the measurements. Simulations of lattices with a larger number of spins along the c axis reveal a substantial renormalization of the Néel temperature due to the quasi-one-dimensional nature of the Hamiltonian.
The effect of nonlinear ion transport on the rate of laser-induced electrochemical etching of semiconductors
Authors: Grebel H. , Iskandar B. , Sheppard K. G.
Published: July 2002

Based on a nonlinear coupling between the etchant species and the photo-induced carriers during photoelectrochemical etching of semiconductor surfaces, we propose that an optimum exists for the reaction rate as a function of electrolyte concentration.
Arsenic redistribution during cobalt silicide formation
Authors: Pai C. S. , Baiocchi F. A. , Williams D. S.
Published: July 2002

The redistribution of arsenic during cobalt silicide formation was studied. For the case of cobalt monosilicide (CoSi) formation, arsenic was implanted initially either into the Co or into the Si substrate and for the case of cobalt disilicide (CoSi2) formation, arsenic was implanted initially into either CoSi or into Si substrate. The distribution profiles of arsenic were monitored before and after heat treatment using Rutherford backscattering spectrometry analysis. The redistribution profiles of arsenic were found to be determined by the kinetics of cobalt silicide formation and by annealing at high temperature. It was found that during the silicide formation the motion of arsenic dopants is in the opposite direction to the dominant diffusing species. The details of the dominant diffusing species in cobalt silicide formation and their relationship to the arsenic dopant redistributions are discussed.
Photoeffects in La3+-doped BaTiO3 ceramic electrodes
Authors: Yoon Ki Hyun , Kwon Chul Han , Kim Tae Heui
Published: July 2002

Photoelectrochemical conversion characteristics of semiconducting, polycrystalline BaTiO3 electrodes have been investigated as a function of sintering atmosphere (air/H2) and the amount of dopant (lanthanum). The photocurrents for specimens sintered in air were greater than those of specimens sintered in hydrogen. Also, as the amount of dopant in BaTiO3 sintered in air increased, the photocurrent increased up to 0.2 at. % addition and then decreased. However, for BaTiO3 sintered in hydrogen, the photocurrents decreased consistently with the addition of the dopant. These results could be explained in terms of ceramic microstructure and electrical resistivity. Regardless of the amount of lanthanum, the photoresponse, as the wavelength of the induced light that varied, began to appear around 420–430 and 780–800 nm, which corresponds to the energy band of La-doped BaTiO3 and the oxygen vacancy level within the gap, respectively.
Energy band-gap calculations of short-period (ZnTe)m(ZnSe)n and (ZnS)m(ZnSe)n strained-layer superlattices
Authors: Wu Yi-hong , Fujita Shizuo , Fujita Shigeo
Published: July 2002

We report on the calculations of energy band gaps based on the semiempirical tight-binding model for short-period (ZnTe)m(ZnSe)n and (ZnS)m(ZnSe)n strained-layer superlattices (SLSs). During the calculation, much attention has been paid to the modeling of strain effect. It is found that (ZnTe)m(ZnSe)n superlattices grown on InAs, InP, and GaAs substrates show very different electronic properties from each other, which is consistent with experimental results now available. Assuming that the emission observed for (ZnTe)m(ZnSe)n SLS originates from intrinsic luminescence, we obtain an unstrained valence-band offset of 1.136±0.1 eV for this superlattice. On the other hand, the band gap of (ZnS)m(ZnSe)n superlattice grown coherently on GaP is found to exhibit a much stronger structure dependence than that grown coherently on GaAs. The difference of energy gap between superlattice with equal monolayers (m=n) and the corresponding alloy with equal chalcogenide composition is also discussed.
Characterization of the interface between Ge+-implanted crystalline silicon and its thermally grown oxide by spectroscopic ellipsometry
Authors: Nguyen N. V. , Vedam K. , Narayan J.
Published: July 2002

Using spectroscopic ellipsometry we have studied a series of virgin and germanium-implanted crystalline silicon samples oxidized at 900 and 1000 °C. The thickness of the oxide in the virgin and implanted sample was accurately determined and found to be in agreement with the results previously obtained by single-wavelength ellipsometry. The interface structure between the silicon substrate and the oxide layer of all the samples was investigated in detail. We find that the interposing layer between the oxide and the silicon substrate is composed of a thin layer of crystalline germanium-silicon alloy. A brief discussion on the strain and quantum confinement of the interface that may affect the calculated composition is also presented. The interface thickness was found to be slightly larger to that obtained by high-resolution transmission electron microscopy measurement for samples implanted at higher doses. A plausible explanation for the discrepancy is provided.
Domain structure in the reentrant FexZr100−x amorphous ribbons
Authors: Hadjoudj S. , Senoussi S. , Ryan D. H.
Published: July 2002

The domain structure in amorphous ribbons of FexZr100−x is investigated by Lorentz transmission electron microscopy in the temperature range 18–290 K. The domains appear approximately at the ferromagnetic transition temperature Tc determined by conventional magnetic measurements. Their dimensions (10–50 μm) and shapes stay unchanged whatever the temperature (T<Tc), particularly upon cooling through the reentrant transition. The physical factors limiting the observation of the magnetic structure are also discussed.
Theoretical specific resistance of ohmic contacts to n-GaAs
Authors: Yoo Jae S. , Lee Hong H.
Published: July 2002

An improved version of a prior model is given for specific resistance of ohmic contacts to n-GaAs. Instead of the two-band model used in the prior work for the transmission coefficient, a more rigorous three-band model due to E. O. Kane [J. Phys. Chem. Solids 1, 249 (1957)] is used in the improved version. In the doping range of interest, the theoretical contact resistance can differ by more than an order of magnitude, particularly at high intrinsic barrier heights. The contact resistance is given as a function of doping and the intrinsic barrier height. An ultimate limit to the contact resistance is also given.
In0.53Ga0.47As/AlAs resonant tunneling diodes with peak current densities in excess of 450 kA/cm2
Authors: Broekaert Tom P. E. , Fonstad Clifton G.
Published: July 2002

Extremely high peak current density resonant tunneling diodes, with high peak-to-valley current ratios, have been obtained in the InP-based InGaAlAs material system by using very thin AlAs barriers and In0.53Ga0.47As wells, within In0.53Ga0.47As bounding regions. A peak current density of 460 kA/cm2 has been obtained for diodes with 4-ML AlAs barriers and a 16-ML In0.53Ga0.47As well. The room-temperature peak-to-valley current ratio of these devices is 4. From a measurement of the dependence of the peak current density on barrier thickness, a value for the strength, m*V, of the pseudomorphic AlAs barrier on In0.53Ga0.47As is found to be 0.07±0.01 eV.
Plasma properties of a hydrocarbon arcjet used in the plasma deposition of diamond thin films
Authors: Stalder K. R. , Sharpless R. L.
Published: July 2002

Plasma-deposited thin films of polycrystalline diamond were achieved with an arcjet plasma. The jet operated with hydrogen gas with small admixtures of CH4. The diamond films were highly nonuniform, probably due to the very high gradients of the chemical species and temperatures within the arcjet-substrate system. The arcjet apparatus, and optical emission spectra and Langmuir probe diagnostics of the plasma are described. Also presented are results for the surface chemical analysis of the diamond film using surface analysis by laser ionization.
Interfacial reaction of Ta and Si rich tantalum silicides with Si substrate
Authors: Hara Tohru , Murota Masayuki
Published: July 2002

Properties of tantalum silicide (TaSix) films deposited from a TaSix composite target are studied. Ta-rich (Si/Ta atom ratio, x=1.83) and Si-rich (x=2.60) films are deposited by dc magnetron sputtering on Si substrates. In Ta-rich films, a silicidation reaction occurs with annealing above 700 °C. As a result, Si composition, x, and silicide film thickness increases with consuming Si substrate. Stress changes abruptly from tensile to compressive with this reaction. This stress change leads to peeling in the Ta-rich film during the silicidation reaction. In Si-rich films, excess Si precipitates at the interface at 750 °C. However, the stress changes more gently in the Si-rich film and a more uniform TaSi2/Si interface is formed. Therefore, peeling can be avoided in Si-rich films.
Ex situ Ohmic contacts on thin film YBa2Cu3O7−x
Authors: Pendrick V. , Brown R. , Matey J. R. , Findikoglu A. , Xi X. X. , Venkatesan T. , Inam A.
Published: July 2002

We report on ex situ fabrication of gold contacts onto patterned YBa2Cu3O7−x films. Gold seed layers were blanket sputtered onto photoresist defined over the patterned films. In a second photo process, openings were aligned to the original pads and 8-μm-thick contacts were electroplated. The two layers of photoresist and sputtered gold were removed leaving defined contacts. We obtained specific contact resistance of 2.6×10−7 Ω cm2 on post-annealed samples at 79 K. Microwave filters with contacts showed 4.2 dB loss at 9 GHz and 77 K, while comparable filters in copper showed 18 dB loss.
Crystal structure of Si1−xCx films by plasma-enhanced chemical vapor deposition at 700 °C
Authors: Takeshita T. , Ichige K. , Kurata Y. , Hasegawa S.
Published: July 2002

Using x-ray diffraction and electron-spin-resonance measurements, the crystal structure and bonding configurations of Si1−xCx films by plasma-enhanced chemical vapor deposition at 700 °C are shown to change abruptly at around x=0.5. At around this composition, a polycrystalline film with a cubic SiC 〈111〉 preferred orientation and an average grain size of 95 nm is grown on a fused silica substrate under high hydrogen dilution. The structural change can be attributed to an increase in the probability of occurrence of a Si- C4 tetrahedron in a chemically ordered network.
Rate constants for the reaction of Cl atoms with intrinsic and n+-doped polycrystalline silicon
Authors: Walker Zane H. , Ogryzlo Elmer A.
Published: July 2002

The reaction of Cl atoms with intrinsic and n+-doped polycrystalline silicon has been studied at a Cl partial pressure of 0.17 Torr and in the temperature ranges from 150 to 290 °C and 25 to 90 °C for the two materials, respectively. The reaction with n+-doped silicon was observed to proceed 90 times faster than with intrinsic silicon at any given temperature, i.e. within experimental error the difference in the rate constants for the two materials was found to be entirely attributable to a change in the pre-exponential factor, with the activation energy remaining unchanged. The rate constant for the reaction is given by (9±2)×105 nm min−1 Torr−1 exp−28.2±1.2 kJ/mol)/RT for the intrinsic material and (7±3)×107 nm min−1 Torr−1 exp−(27.8±1.5 kJ/mol)/RT for the phosphorus doped material with a dopant density of 5×1018 cm−3.
Influence of laser annealing on the index of refraction of a Bi, Ga-doped garnet film
Authors: Gouzerh J. , Stashkevich A. A.
Published: July 2002

The purpose of this work is to understand the influence of laser annealing on the index of refraction of a ferrimagnetic film in the near-infrared domain. To carry out the measurements of the variation of the index of refraction a series of diffraction gratings was fabricated by means of laser annealing in a ferrimagnetic film of the following composition: Y2.15Lu0.35Bi0.5Fe3.8Ga1.2O12. Measuring the intensity of the light scattered by a grating the possibility of detecting variations of the index of refraction as small as 10−5 was obtained. An all-lines argon cw laser was used to produce the gratings and a He-Ne laser (λ=1.152 μm) was used to observe the diffraction of the optical waveguide modes. The best results were obtained with the grating with the period of 80 μm. An increase of the diffraction efficiency for the higher optical waveguide modes with respect to the lower ones was observed. The experimental results are in good agreement with the following theoretical interpretation. There exist two phase diffraction gratings for the optical waveguide modes in the film: the grating induced by the variation of the index of refraction of the film (Δn ≂ 2 × 10−4) and that induced by the variations of the thickness of the film (Δh≂5 Å). Both effects are connected with the migration of Ga ions between tetrahedral and octahedral sites under thermal treatment.
Exchange effects in molecular-beam-epitaxy grown iron films
Authors: Chen YouJun , Lottis Daniel K. , Dahlberg E. Dan , Kuznia J. N. , Wowchak A. M. , Cohen P. I.
Published: July 2002

The magnetic properties of molecular-beam-epitaxy iron films grown on (001) GaAs substrates were studied using a SQUID magnetometer. In uncapped films, where there is an oxidized iron layer on top of the film, the M-H loops shift from being symmetric about the origin when the film is cooled in applied fields to temperatures below 100 K. The observed behavior is attributed to a unidirectional exchange anisotropy generated by the antiferromagnetic ordering of the oxidized iron surface. By comparing the magnitude of the exchange coupling observed in these films with that observed in the Co-CoO system, the oxide that forms on the iron surface is most likely FeO. In addition, the magnetic training effect or the effect of cycling through the hysteresis loops at fixed temperatures has been studied. It is noted that the temperature dependence of the measured exchange coupling is different from that which is usually reported. The reason for this difference is unknown.
Influence of DX centers and surface states on δ-doped high-electron-mobility transistor performance
Authors: Tian H. , Kim K. W. , Littlejohn M. A.
Published: July 2002

The roles of DX centers and surface states associated with the n-AlGaAs layer of δ-doped AlGaAs/GaAs high-electron-mobility transistors have been investigated by employing a two-dimensional, self-consistent ensemble Monte Carlo simulation. It is found that both DX centers and surface states degrade device performance, particularly as gate-to-source voltage increases. This degradation is manifested largely through reduced channel electron concentration, increased intervalley scattering, and enhanced real-space transfer. Of the two defect states, DX centers have more detrimental influence on drain current and transconductance.
Thermally stimulated current studies of bismuth germanium oxide crystal
Authors: Takamori Takeshi , Just Dieter
Published: July 2002

Thermally stimulated current (TSC) from nominally undoped Bi12GeO20 (BGO) crystals was measured as a function of the uv excitation temperature and time. The results were compared with the previous work on Bi12SiO20 (BSO). The correlation in trapping kinetics among the three major traps observed previously in BSO appears to hold also for BGO; i.e., the shallowest trap acts as a precursor state for electrons to be trapped into the deeper traps. However, an additional finding on the temperature dependence of trap density in the TSC curves of BGO indicates that the simple model proposed earlier is not strictly valid and further study is required.
Elastic scattering of low-energy positrons by bound silicon and germanium
Authors: Antolak A. J. , Williamson W.
Published: July 2002

Elastic scattering differential and total cross sections for low-energy positron collisions with bound silicon and germanium atoms have been computed using partial wave analysis with static and polarization potentials. The calculations are performed at incident positron energies ranging from 0.2 to 500 eV. For each impact energy, an appropriate number of phase shifts are obtained by numerically integrating the radial wave equation to ensure convergence of the scattering amplitude. A simple parametrization of the total cross section in terms of the screened Rutherford cross section is also presented for use in Monte Carlo codes. Due, in part, to the lack of reliable experimental data, relatively little theoretical work has been done to calculate the inelastic and elastic differential cross sections (DCS) and total cross sections (TCS) for positron scattering. Most of the measurements were made on noble gases where it is observed that low-energy positrons exhibit a Ramsauer–Townsend (RT) minimum for helium, neon, and possibly argon (for electrons, the RT minimum occurs for heavier noble gases).1,2 Calculations of the positron TCS for these atoms agree with the observations. The RT effect has also been identified in a number of other free atoms and molecules for electrons.3,4 Recently, Meredith et al. computed the electron elastic scattering cross sections in solid silicon and germanium.5 In this work we calculate the DCS and TCS for positrons elastically scattered by silicon and germanium atoms which are bound in an amorphous solid using partial waves and the optical model. The results are presented for incident positron energies ranging from 0.2 to 500 eV. Simple fits to each TCS are given for use in Monte Carlo scattering simulations. For the static potential VS, Salvat and Parellada used Dirac–Hartree–Fock–Slater (DHFS) calculations to accurately fit analytic screening functions for atoms.6 They also imposed Wigner–Seitz boundary conditions (WSBC) on their wave functions to simulate atoms bound in a solid. The functional form of the static potential is (in atomic units)
Composition and chemical reactions of titanium oxide films deposited by laser evaporation
Authors: Dai C. M. , Su C. S. , Chuu D. S.
Published: July 2002

Thin films were deposited on Pyrex glass substrates from Ti and TiO2 targets at room temperature by laser evaporation technique. These films were characterized by scanning electron microscopy, Auger electron spectroscopy, and depth profile analysis. The films deposited from Ti target are TiCx and TiOy matrix, while films from TiO2 target are almost stoichiometric in oxygen. The films have a smooth surface morphology under a laser power density of 5 × 106 W/cm2, which is close to the critical intensity for evaporation. The interface reaction of these films is strong, and the Ti atoms diffuse into the substrate.
In situ ellipsometric study of amorphous silicon/amorphous silicon-carbon interfaces
Authors: Chu V. , Fang M. , Drevillon B.
Published: July 2002

An ellipsometric study of the growth of hydrogenated amorphous silicon/hydrogenated amorphous silicon-carbon interfaces and multilayer structures is presented. Kinetic ellipsometry is used at a fixed wavelength to study the growth of the materials at the interface between the a-Si:H and a-Si1−xCx:H layers as a function of the carbon content. It is observed that for samples with carbon content below 36%, the a-SiC:H grows uniformly on top of the a-Si:H. However, as the carbon content is increased beyond this point, the growth of the alloy material becomes nonuniform. Various growth models were used to fit the experimental data and the best fits were obtained using a multilayer model of varying void fraction to describe the inhomogeneous growth. Multilayers were grown using a-Si:H and a-SiC:H. It was observed that for low carbon concentrations (x<0.36), the successive layers of the multilayer structure are reproducible and the growth remains homogeneous for all the layers.
Monte Carlo study of phase transitions in an Ising model superlattice
Authors: Li Jialin , Landau D. P.
Published: July 2002

An importance sampling Monte Carlo method has been used to simulate L×L×nD Ising superlattices consisting of n alternating layers of ferromagnetic and antiferromagnetic material each of which is D atomic planes thick. Spins within each plane are coupled ferromagnetically and the interface interations are assumed to be ferromagnetic. From data on layer properties, the phase diagram is determined as a function of temperature and magnetic field. A series of phase transitions and tricritical points are found.
Fourier transform infrared attenuated total reflection spectra of ion-implanted silica glasses
Authors: Hosono Hideo
Published: July 2002

Fourier transform infrared attenuated total reflection spectra of silica glasses implanted with Si or P were measured using a Ge as an internal reflection element. Calculated penetration depth of the infrared radiation is 0.2–1.0 μm depending on the wavelength. These values are comparable to the depths of ions implanted at conventional acceleration voltages. Upon implantation the frequencies of ω4(LO) and ω4(TO) shifted to lower energy, whereas ω3 moved to higher energy. This opposite direction of the frequency shift was explained by a decrease in the Si-O-Si bridging angle, which is caused by ion-bombarded compaction. The magnitude of frequency shifts observed were much larger than that in thermally compacted silica glasses reported so far, showing that an extremely high fictive temperature state is realized in the implanted silica glass layers.
Analytical approximations for the Fermi energy of an ideal Fermi gas obeying a nonparabolic dispersion relation
Authors: Beresford R.
Published: July 2002

Analytical approximations are derived for treating isotropic nonparabolic bands in the framework of the Joyce–Dixon approximation [W. B. Joyce and R. W. Dixon, Appl. Phys. Lett. 31, 354 (1977)]. The approximations are worked out for three-, two-, and one-dimensional idealized systems. Example calculations illustrate the utility of the methods for the narrow-gap III-V compounds, such as InAs and InSb, which are now being developed for device applications.
Effect of thermally induced stresses on the rapid-thermal oxidation of silicon
Authors: Deaton R. , Massoud H. Z.
Published: July 2002

During rapid-thermal processing (RTP), radiative losses from the edge of silicon wafers which are heated by uniform irradiation create a radial temperature gradient. This temperature gradient induces a stress distribution which is compressive at the center and tensile towards the edge of the wafer. This thermally induced stress affects oxidation kinetics during the rapid-thermal oxidation (RTO) of silicon. The tensile stress enhances the oxidation towards the edge of the wafer, which is opposite to the effect of the radial temperature gradient. For a radially symmetric temperature distribution, the resolved stress and its effect on oxidation are largest along specific crystal directions, namely, the slip directions on slip planes. The stress effect on oxidation is largest at low temperatures and for short times. The temperature dependence relates to the magnitude of the temperature gradient and yield strength of silicon at different temperatures. As the oxidation proceeds, the influence of the thermally induced stress on RTO kinetics diminishes as the effects of the intrinsic stress and the viscous properties of the oxide increase. These observations have implications for RTP temperature measurement and RTO process uniformity.
Photoeffects in SnO2 film electrodes deposited by spray pyrolysis
Authors: Kim Tae Heui , Yoon Ki Hyun
Published: July 2002

The photoelectrochemical conversion behavior of SnO2 film electrodes deposited by spray pyrolysis has been investigated as a function of deposition parameters and the amount of additive Sb. The photocurrent increased with increasing substrate temperature up to 400 °C, and then decreased. As the concentration of the spray solution increased, the photocurrent reached a maximum value at the concentration of 0.05 M. In Sb-doped SnO2 film electrodes, the photocurrent decreased due to the reduction of mobility and depletion layer width. These results could be explained in terms of microstructure and electrical property.
Thermally activated capture of charge carriers into irradiation induced Si/SiO2 interface states
Authors: Ricksand Anders , Engström Olof
Published: July 2002

Deep-level transient spectroscopy (DLTS) measurements of capture cross sections of Co60 induced silicon-silicon dioxide (Si/SiO2) interface states have been performed. For the accurate analysis of measured data a new technique based on numerical solutions of the emission rate equations has been used. Computer simulations of DLTS measurements have been compared with measured data in order to extract reliable results in the case of nonexponential transients. We show that the capture cross sections are thermally activated. Together with earlier consistent results on the same kind of centers, this leads us to the conclusion that vibronic properties play important roles for the electron capture into these traps.
Magnetic properties of Fe–Cr ultrafine particles
Authors: Gong Wei , Li Hua , Zhao Zhongren , Hadjipanayis G. C. , Papaefthymiou V. P. , Kostikas A. , Simopouos A.
Published: July 2002

Fe100−xCrx ultrafine particles (UFP) with x=5–55 and a size in the range of 100–500 Å have been prepared by the gas evaporation method using Ar gas pressures P=2, 5, and 8 Torr. Transmission electron microscopy with energy dispersive x-ray analysis showed that the composition and structure of the UFPs was the same as in the original bulk alloy. The coercivity Hc at room temperature for the Fe95Cr5, Fe90Cr10, and Fe80Cr20 UFPs reached values up to 1250, 780, and 625 Oe, respectively. The temperature coefficients of these UFPs α(Hc) = −0.09%/°C were much smaller than those of Fe in the temperature range of 20–450 °C when x≤20. A strong antiferromagnetic coupling between Fe and Cr appeared in the small particles. Both the coercivity and saturation magnetization were found to decrease with Cr addition. For samples with x=50%–60%, Hc = 200–300 Oe, and Ms = 30–60 emu/g.
Electrical properties of β-FeSi2/Si heterojunctions
Authors: Dimitriadis C. A.
Published: July 2002

The electrical properties of heterojunctions of polycrystalline films of β-FeSi2 grown on n-type single-crystal silicon are investigated. The dark current-voltage and capacitance-voltage characteristics are measured over a wide temperature range. Analysis of the data reveal that multistep tunneling is the current conduction mechanism due to a high density of interface defects.
High-temperature-superconducting magnetic susceptibility bolometer
Authors: Brasunas J. , Lakew B. , Lee C.
Published: July 2002

We present a new type of infrared detector, the magnetic susceptibility bolometer, based on the temperature dependence of the diamagnetic screening of a high transition-temperature (Tc) superconductor film near Tc. Results are reported for the response of a prototype model to modulated blackbody radiation. We discuss possible improvements and the potential sensitivity of an improved device.
A deconvolution procedure for photothermal and photoconductive spectra of amorphous silicon films
Authors: Amato G. , Giorgis F. , Spagnolo R.
Published: July 2002

A simple deconvolution procedure has been applied to photothermal and photoconductive spectra of a-Si:H films. The method requires no assumption about the midgap density of states, thus obtaining results in an unambiguous way. The analysis of the distortion occurring in photothermal spectra due to surface states is shown to be an effective way to separate between surface and bulk absorption, thus enabling a direct comparison between photothermal and photoconductive spectroscopies.
One-dimensional particle-in-cell simulations of sheath erosion in a plasma-filled diode
Authors: Kares Robert J.
Published: July 2002

The effect of electron emission on cathode sheath evolution in a plasma-filled diode (PFD) is studied using a one-dimensional, electrostatic, particle-in-cell simulation of a PFD coupled to an external LC driver circuit. For the case in which space-charge-limited electron emission is allowed from the cathode, the simulation model reveals a variety of new phenomena that are not included in the standard bipolar erosion model for the operation of the PFD as first proposed by Miller, Poukey, and Wright [Phys. Rev. Lett. 35, 940 (1975)]. As a result the simulation model makes a variety of scaling predictions for the operation of the PFD that are different from those of the bipolar model and that may be used to distinguish experimentally between the two. Some of these predictions are discussed.
Cross-property relations for momentum and diffusional transport in porous media
Authors: Torquato S. , Kim In Chan
Published: July 2002

Cross-property relations linking the fluid permeability k associated with viscous flow through a porous medium to effective diffusion properties of the medium have recently been derived. Torquato [Phys. Rev. Lett. 64, 2644 (1990)] found that k≤Dφ1τ, where τ is the ‘‘mean survival time’’ associated with steady-state diffusion of ‘‘reactants’’ in the fluid region of diffusion coefficient D and porosity φ1 of a porous medium containing absorbing walls (i.e., trap boundaries). Subsequently, Avellaneda and Torquato [Phys. Fluids A 3, 2529 (1991)] related k to the electrical formation factor F (inverse of the dimensionless effective electrical conductivity) and the principal (largest) diffusion relaxation time T1 associated with the time-dependent trapping problem, namely, k≤DT1/F. In this study, we compute the aforementioned bounds, using an efficient first-passage-time algorithm, for grain-consolidation models of porous media and compare them to exact results for these models. We also conjecture a new relation connecting k to τ and F for a wide class of porous media, namely, k≤Dτ/F, and show that it gives the sharpest permeability estimate among the existing bounds. The importance of this relation lies not only in its usefulness as an estimator of the permeability but that it involves the diffusional parameters τ and F which can be measured in situ.
Current transport in Pd/n-InP diodes formed at room and low temperature
Authors: Shi Z. Q. , Anderson W. A.
Published: July 2002

Schottky contacts to n-InP have been made by deposition on substrates cooled to low temperature (LT=77 K) with a vacuum close to 10−7 Torr. The Schottky barrier height, φB, was found to be as high as 0.96 eV with Pd metal. This indicated an 0.4 to 0.5 eV increase in φB compared with the room temperature (RT=300 K) deposition. For the diode fabricated at room temperature, the reverse saturation current density, J0, decreased sharply with the decrease of measuring temperature. The φB was increased from 0.48 to 0.71 eV, with the testing temperature decreased from 300 to 100 K, with a temperature coefficient of −6.1×10−4 eV/K. The conduction mechanism was controlled by thermionic emission. For the LT diode, the value of J0 was about six orders smaller than for the RT diode at the same temperature. As testing temperature decreased, the barrier height was increased from 0.96 to 1.15 eV, with a temperature coefficient of −3.2×10−4 eV/K. The forward current had a saturation trend at a larger forward bias. In this region, the current was almost temperature independent and the transport mechanism was controlled by thermionic field emission.
Ferromagnetic resonance and spin anisotropy in iron oxide thin films and iron oxide/nickel oxide superlattices
Authors: Krebs J. J. , Lind D. M. , Berry S. D.
Published: July 2002

Oriented single-crystalline thin films of Fe3O4 and superlattices of Fe3O4/NiO have been grown using molecular-beam-epitaxy techniques on polished substrates of MgO(001). The 1–2-μm-thick superlattices have their Fe3O4 layer thicknesses held constant at 68 Å and the NiO thickness varied from 8.5 to 102 Å. We report the initial ferromagnetic resonance measurements made on these samples to investigate the effects that interfacial lattice distortion and interlayer coupling have on their magnetic ordering and anisotropy. These results are compared with structural and magnetic studies made on the same films using x-ray diffraction and magnetometry techniques. We find clear evidence in the trends of both the ferromagnetic resonance fields and the linewidths as a function of NiO thickness that coupling between the layers can change the in-plane anisotropy and the overall magnetic response. The single-layer Fe3O4-only sample behaves essentially like bulk magnetite.
Magneto-optical properties of Cd1−xMnxSe microcrystallites in SiO2 glass prepared by rf sputtering
Authors: Yanata Kohei , Suzuki Kyoichi , Oka Yasuo
Published: July 2002

Cd1−xMnxSe microcrystallites have been successfully prepared by rf sputtering in SiO2 matrix glass. Optical absorption edge shows high energy shifts of 20–550 meV to the high energy as compared to its position in bulk crystals; these shifts are interpreted as evidence for exciton confinement in the microcrystallites. Annealing of the Cd1−xMnxSe microcrystallites at 450–650 °C for 0.5–5 h increases the size of the microcrystallites from 20 to 250 Å. In an external magnetic field of 7 T the absorption edge shifts by 40 meV towards the low energy side due to the exchange coupling of the excitons with the d electrons of Mn2+ ions in the Cd1−xMnxSe microcrystallites. The spectral red shifts of the exciton luminescence in the Cd1−xMnxSe microcrystallites are also observed in the presence of magnetic fields. The corresponding effective g value varied from 138 at 1 T to 99 at 7 T.
Charge exchange of low energy ions in thin carbon foils. II. Results for ions of B, C, F, Ne, Na, Si, S, Cl, Ar, K, and Fe
Authors: Bürgi Alfred , Gonin Marc , Oetliker Michael , Bochsler Peter , Geiss Johannes , Lamy Thierry , Brenac Ariel , Andrä Horst Jürgen , Roncin Philippe , Laurent Henri , Coplan Michael A.
Published: July 2002

In order to calibrate a novel type of time-of-flight mass spectrometer to be flown in the solar wind, we have continued our investigation of the charge exchange of low energy ions passing thin carbon foils. We analyzed elements with widely different chemical properties: Ions of B, C, F, Ne, Na, Si, S, Cl, Ar, K, and Fe in the energy range 0.5–3 keV/u were passed through carbon foils with thicknesses between 1.1 and 10 μg/cm2, and their charge state distributions and residual energies were determined. It was found that (1) the charge state distribution behind the foil is independent of the charge of the incident projectile, (2) isotopes show the same charge exchange properties at equal velocities as we have found previously, (3) at the lowest energies the charge state distribution is no longer a function of the residual energy alone but depends on both residual energy and foil thickness, (4) probable differences in chemical properties between the front and back surfaces of the foil have no detectable influence on the charge exchange properties, and (5) strong electron shell effects manifest themselves when results for different elements are compared.
Light scattering from a dispersion of aligned nematic droplets
Authors: Whitehead J. B. , Z̆umer S. , Doane J. W.
Published: July 2002

Light-scattering and transmission properties of aligned nematic droplets dispersed in a solid polymer are studied. Theoretical evaluations of the differential and total cross sections are performed in the anomalous diffraction approximation. The corresponding attenuation coefficients are calculated in a single scattering approximation. The effect of interdroplet interference is discussed as well. Theoretical calculations are in agreement with experimental data obtained from thin films of polymer dispersed nematic droplets of micron size, aligned by an applied electric field.
Weak ferromagnetism induced by the external ield above T
N in GdzCuO4
Authors: Steren L. B. , Fainstein A. , Tovar M. , Rouco A. , Pérez F. , Obradors X. , Mira J. , Rivas J. , Oseroff S. B. , Fisk Z.
Published: July 2002

A detailed study of the temperature and field dependence of the dc magnetization near the antiferromagnetic (AF) transition in Gd2CuO4 shows the presence of field-induced weak ferromagnetism (WF) well above the Néel temperature TN. ac susceptibility measurements indicate TN=266(3) and 282(3) K, for single crystals and ceramic samples, respectively. The shift of the electron-spin-resonance line of the Gd3+ ions as a function of temperature T, δHr(T), is compared with the static internal field Hi(T), which describes the coupling between the paramagnetic Gd lattice and the WF component of the Cu moments. Both δHr(T) and Hi(T) show that the WF persists up to ≊50 K above TN. The initial susceptibility (for H≤100 G) presents a contribution in excess of the paramagnetism of Gd ions. For T≥TN, it decays as K(T−TN)−1, with K=0.20(3) emu K mol. The relationship between this result and the strong two-dimensional AF correlations in these cuprates is discussed.
Magnetic multilayers: Quasiclassical transport via the Kubo formula
Authors: Camblong Horacio E. , Levy Peter M.
Published: July 2002

A real-space quantum approach, based on the Kubo formula, is used to describe the quasiclassical transport behavior of metallic multilayers. We emphasize the differences between the cases of current in the plane of the layers, for which size effects play a dominant role and current perpendicular to the planes, for which we provide several proofs that the so-called series resistor model holds for all length scales.
Ensemble Monte Carlo simulation for electron transport in quantum wire structures
Authors: Ando Yuji , Cappy Alain
Published: July 2002

Ensemble Monte Carlo simulations are performed for the GaAs (100) oriented square well wire. The model considers the infinite potential approximation as well as the extreme confinement limit, i.e., one-dimensional subbands are formed in the L and X valleys as well as in the Γ valley. We discuss the electron dynamics in quantum wire structures from steady-state, transient, and diffusion aspects. Even at very low electric fields (10–100 V/cm), the non-Maxwellian distribution, originating from the one-dimensional density of states, leads to electron heating and a deviation from the Einstein relation, as well as mobility enhancement. At high fields, the anisotropic mass effects of the satellite valleys gives rise to a saturation velocity enhancement as well as a diffusivity increase. The low-temperature simulations show temporal oscillations of velocity due to the periodical phonon emissions. This effect possibly suppresses the diffusion noise, since no additional carrier diffusion is introduced with a periodic velocity oscillation. The velocity overshoot phenomena are studied based on the relaxation time approximation and are compared to the transient Monte Carlo results. The satisfactory agreement obtained between these two results indicates the validity of the phenomenological approach to quantum wire structures.
Measurement of intrinsic stresses during growth of aluminum nitride thin films by reactive sputter deposition
Authors: Meng W. J. , Sell J. A. , Eesley G. L. , Perry T. A.
Published: July 2002

Real-time measurements of intrinsic stresses during growth of polycrystalline and epitaxial aluminum nitride (AlN) thin films on Si(111) are reported. Our room-temperature measurements on polycrystalline films corroborate previous post-growth measurements. Our high-temperature measurements provide evidence of large intrinsic stresses during epitaxial growth of AlN on Si(111) and insignificant stress relaxation during growth.
Depth-resolved measurement of lattice relaxation in Ga1−xInxAs/GaAs strained layer superlattices by means of grazing-incidence x-ray diffraction
Authors: Pietsch U. , Metzger H. , Rugel S. , Jenichen B. , Robinson I. K.
Published: July 2002

The state of relaxation in two different superlattices (SLs) of a system with large lattice mismatch, Ga0.8In0.2As/GaAs grown on GaAs [001] by molecular beam epitaxy, has been investigated by surface-sensitive grazing-incidence diffraction (GID). The SL is squeezed between the substrate and a thick GaAs top layer. The thickness of individual GaInAs layers ta (active layer) is the same in both samples, while the GaAs barrier thickness tb is different. We have studied the influence of the thickness ratio tb/ta on the state of relaxation for different distances from the sample surface. We find that for thick barriers the whole SL remains coherently strained and for the thinner barrier thickness the SL is partially relaxed against the the GaAs top layer. The GID technique was applied for the first time to obtain depth resolution of the lateral lattice parameter in a SL. It is demonstrated to be especially well suited for SL systems with a small difference of the average electron density between the sublayers. The scattering contrast is improved by measuring the intensity as a function of the exit angle (‘‘rod scans’’) from the ‘‘weak’’ (200) Bragg reflection. Comparing computer simulations with the measured variation of the scattering contrast between GaAs and GaInAs layers obtained from different ‘‘information depths’’ and at different angular positions of the in-plane rocking curves, the state of relaxation can quantitatively be evaluated. On the basis of these results we propose two models for the partial relaxation of the SL into the state of strain-reduced domains. We believe that the partial relaxation is due to the elastic field interaction between the GaInAs layers accross the GaAs barriers, if tb is small.
Field-effect conductance in amorphous silicon thin-film transistors with a defect pool density of states
Authors: Deane S. C. , Powell M. J.
Published: July 2002

A new computer program to analyze field-effect conductance measurements has been developed. In this program a defect pool model, where the equilibrium density of state is determined by the Fermi level, has been incorporated. Transistors with finite band bending, due to fixed charge in the insulator, will therefore have a density of states that is spatially inhomogeneous. The inhomogeneous density of states means that the subthreshold slope of a device is not always controlled by the density of states near the interface, but can become dominated by the bulk density of states, contrary to simpler models. Both electron and hole branches are modeled simultaneously and self-consistently with no assumptions made about the flatband voltage. Indeed, it is demonstrated that there is no flatband voltage in a transistor with an inhomogeneous density of state; however, a true flatband voltage can be achieved by a process of thermal bias annealing. Finite thickness effects and defect correlation energies are taken into account. The program is used to model the characteristics of thin-film transistors both before and after thermal bias annealing. The model fits the experimental results well, even though there are few free parameters. A key result is that the defect pool parameter Δ is determined, which is the energy separation of the D0/− transition observed in n-type material and the D+/0 transition in p-type material. It is found that Δ=0.44 eV for all devices fitted, which suggests this a fundamental parameter of a-Si:H. This value of Δ implies that there are about four times as many charged as neutral devices in bulk intrinsic amorphous silicon.
Determination of the crystallographic orientation of cubic media to high resolution using optical harmonic generation
Authors: Bottomley D. J. , Lüpke G. , Mihaychuk J. G. , van Driel H. M.
Published: July 2002

Using the tensorial characteristic of optical harmonic generation it is shown that the crystallographic orientation of centrosymmetric and noncentrosymmetric cubic media can be determined to within ±0.1°. As an illustration of the technique the orientation of vicinal GaAs(001) and Si(111) single-crystal wafers is obtained using second- and third-harmonic generation, respectively, with a continuously mode-locked Ti:sapphire laser operating at 775 nm. Intensity measurements were taken as a function of the wafer azimuthal angle and are fit to a truncated Fourier series derived from a phenomenological theory.