bluuebunny/crossref_metadata_2025 huggingface dataset:
Multispeckle diffusing-wave spectroscopy: A tool to study slow relaxation and time-dependent dynamics
Authors: Viasnoff Virgile , Lequeux François , Pine D. J.
Published: July 2002

A multispeckle technique for efficiently measuring correctly ensemble-averaged intensity autocorrelation functions of scattered light from nonergodic and/or nonstationary systems is described. The method employs a charge coupled device (CCD) camera as a multispeckle light detector and a computer-based correlator, and permits the simultaneous calculation of up to 500 correlation functions, where each correlation function is started at a different time. The correlation functions are calculated in real time and are referenced to a unique starting time. The multispeckle nature of the CCD camera detector means that a true ensemble average is calculated; no time averaging is necessary. The technique thus provides a “snapshot” of the dynamics, making it particularly useful for nonstationary systems where the dynamics are changing with time. Delay times spanning the range from 1 ms to 1000 s are readily achieved with this method. The technique is demonstrated in the multiple scattering limit where diffusing-wave spectroscopy theory applies. The technique can be combined with a two-cell technique that can measure faster decay times. The combined technique can measure delay times from 10 ns to 1000 s. The method is peculiarly well suited for studying aging processes in soft glassy materials, which exhibit both short and long relaxation times, nonergodic dynamics, and slowly evolving transient behavior.
Blue photoluminescence from in situ Cu-doped porous silicon
Authors: Suh Kahp Y. , Kim Y. S. , Lee Hong H.
Published: July 2002

We have observed weak blue photoluminescence from in situ Cu-doped porous silicon formed by electrochemical etching with a low etching current density that is aided by cuprous chloride [Cu(I)Cl]. Fourier transform infrared spectroscopy, x-ray photoelectron spectroscopy, and Auger electron spectroscopy reveal that the blue emission is associated with formation of the carbonyl group that is catalyzed by Cu on the surface of the porous silicon.
Propagation characteristics of a shear horizontal wave in a liquid-crystal cell having temperature-dependent phase states
Authors: Inoue M. , Yoshino K. , Moritake H. , Toda K.
Published: July 2002

Propagation characteristics of the zeroth-order symmetric (S0) mode shear-horizontal (SH) wave in a cell structure containing a thermotropic liquid crystal layer with chiral smectic-C (SmC*), smectic-A (SmA), chiral nematic (N*) and isotropic (Iso) phase states, are examined. The SH wave propagation is numerically analyzed with consideration of the directional dependence of the viscosity and elastic constant of the SmA liquid crystal. Temperature dependencies of acoustic phase delay and propagation loss of the SH wave are observed. The critical behavior of the acoustic phase delay and the propagation loss of the S0 mode SH wave in the vicinity of the N*-SmA phase transition temperature are explained by the existence of the elastic constant related to layer compression. The layer compression elasticity is predominant in the SmA phase, and is determined from the comparison of numerical analysis with experimental results.
Two different behavior patterns of photoluminescence depending on Mg doping rate in p-type GaN epilayers
Authors: Jeong S. M. , Shim H. W. , Yoon H. S. , Cheong M. G. , Choi R. J. , Suh E.-K. , Lee H. J.
Published: July 2002

Mg-doped p-type GaN epilayers grown by metalorganic chemical vapor deposition exhibit two different types of photoluminescence (PL) characteristics depending on Mg doping rate; the existence of critical Mg concentration is found where both electrical and optical characteristics show an abrupt change in their behavior. In samples with relatively low Mg concentration, a band edge emission, a peak associated with shallow donor-acceptor pair recombination, and a 2.8 eV blue band emission appear in the PL spectrum. Intensities of all three PL peaks decrease with increasing annealing temperature; the blue band quenches most rapidly. In contrast, in samples grown with high ratio of Mg/Ga flow rate, only a strong blue emission band is observed and the intensity of the blue band increases with the annealing temperature suggesting a different origin from the blue band of lightly doped samples. From the annealing temperature dependence of the blue band intensity, we speculate that the emission mechanism of the blue band in relatively lightly Mg-doped GaN epilayers involves Mg-related deep level complexes such as substitutional/interstitial Mg-related complexes. On the other hand, a model involving a formation of pyramidal defects and the presence of Mg-rich inclusions which modifies the dominant optical transition can be a possible explanation for the origin of the blue band in heavily doped ones.
Characterization of band bendings on Ga-face and N-face GaN films grown by metalorganic chemical-vapor deposition
Authors: Jang Ho Won , Lee Jung-Hee , Lee Jong-Lam
Published: July 2002

The change of band banding with the crystal polarity of GaN films was investigated using high-resolution photoemission spectroscopy. Compared with a N-face sample, the Ga-face sample exhibited higher Schottky barrier height and lower contact resistivity of a Ti/Al-based Ohmic contact. It was found that Ga-face GaN has a larger surface band bending than N-face GaN by 1.4 eV due to spontaneous polarization, resulting in higher Schottky barrier height. The lower Ohmic contact resistivity on Ga-face GaN originated from the formation of polarization-induced two-dimensional electron gas at the interface of AlN with GaN.
Universality in the quasiperiodic route to chaos
Authors: Dixon T. W. , Gherghetta T. , Kenny B. G.
Published: July 2002

Numerous physical systems with two competing frequencies exhibit frequency locking and chaos associated with quasiperiodicity. In this paper we review certain universal aspects of the quasiperiodic route to chaos by making use of the standard circle map. Particular attention is paid to the golden mean and silver mean with a view to comparison with experimental work.
Magnetic properties of R2Fe14B single crystals
Authors: Koon N. C. , Das B. N. , Rubinstein M. , Tyson J.
Published: July 2002

A whole new class of high-performance permanent magnet materials is based on the ternary tetragonal structure R2Fe14B, where R is one of the rare-earth elements. We have successfully grown single crystals of this structure with R=Y, Nd, and Tb. Y is a nonmagnetic rare-earth substitute, while Nd and Tb couple ferro- and ferrimagnetically, respectively, relative to the iron moment. All three of the compounds have [001] easy axes at room temperature, although the Nd compound exhibits a spin reorientation away from the [001] below about 150 K. Nd2Fe14B has a saturation induction at room temperature of 16.2 kG, which places an upper limit of approximately 65.6 MGOe on the energy product obtainable by magnets based on that material. While Tb2Fe14B exhibits a smaller magnetization because of ferrimagnetic coupling of the rare earths and the iron, it also has an extremely large magnetic anisotropy which is nearly temperature independent between 4.2 and 300 K.
Nonlinear transient response of extrinsic Ge far-infrared photoconductors
Authors: Westervelt R. M. , Teitsworth S. W.
Published: July 2002

Physical mechanisms responsible for nonlinear phenomena and anomalous transient response of cooled extrinsic far-infrared photoconductors are discussed. A simple model describing carrier generation, trapping, and impact ionization is presented, which describes the transient response on fast time scales 10−3 to 10−4 sec, neglecting changes in space charge. Carrier heating by a dc electric field produces relatively fast, damped oscillatory response to external excitation. A small-signal analysis of these equations is a test of stability. An analysis of the role of ideal electrical contacts and space charge is also presented. The very slow (∼1 sec) overshoot and transient response commonly observed in cooled extrinsic photoconductors is explained by the dynamics of trapped space charge near the injecting electrical contact. A small-signal analysis determines the characteristic time constants for these processes, which are typically ∼1 sec. Calculated examples of the recombination and ionization coefficients, dc I-V curves, differential equation flow diagrams, and transient response are presented for parameters typical of  p-type Ge photoconductors doped with shallow acceptor levels, and suggestions for the design of more stable photoconductors are presented.
Dangling bonds and the Urbach tail in silicon
Authors: Seager C. H. , Lenahan P. M. , Brower K. L. , Mikawa R. E.
Published: July 2002

Electron spin resonance (ESR) and highly sensitive optical-absorption measurements have been performed on plastically deformed silicon, polycrystalline silicon, and high interface state density, oxidized silicon samples. In the first two cases, the response of the Urbach-like subgap optical absorption and the ‘‘dangling bond’’ ESR signal following thermal and atomic hydrogen anneals is found to be distinctly different from that reported previously for polycrystalline silicon. These data suggest that transitions of the dangling bond are not responsible for this Urbach-like behavior. In addition, we find that removal of the interfacial silicon dangling bond by oxide stripping results in no measurable sample absorptance decreases, implying a considerably lower optical cross section than has been previously estimated for dangling bonds in the bulk.
Diffusion phenomena and defect generation in rapidly annealed GaAs
Authors: Pearton S. J. , Cummings K. D.
Published: July 2002

A detailed study of transient thermal processing of S-doped epitaxial GaAs wafers, both with and without ion-implantation damage being present, has been performed. The average diffusivity of S is given for the temperature range 950–1050 °C, and is shown to be dependent on the position of any lattice damage present, the surface condition (capped or capless), and the annealing regime employed (thermal or rapid thermal). The mobility of the S-doped region is degraded by rapid annealing, and the extent of this degradation is a function of the experimental conditions employed. Implantation of Si (which occupies a Ga site) into S-doped n+ regions (S occupies an As site) failed to increase the electrical activity of the region above the often observed limit of 2×1018 cm−3.
Detection of thermal waves through modulated optical transmittance and modulated optical scattering
Authors: Rosencwaig Allan , Opsal Jon , Smith W. L. , Willenborg D. L.
Published: July 2002

We show that variations in local optical constants induced by the presence of thermal waves can be used for thermal wave detection and analysis through measurements of thermal wave-induced modulated transmittance and scattering.
Interface structure evolution and impurity effects during solid-phase-epitaxial growth in GaAs
Authors: Licoppe C. , Nissim Y.I. , Meriadec C. , Krauz P.
Published: July 2002

The time-resolved reflectivity technique is shown to be able to characterize interface structure during solid-phase epitaxy in GaAs. A detailed study of interface structure during regrowth and recrystallization kinetics is made for different implanted impurities and implantation parameters in GaAs. It is shown that the interface roughens on a macroscopic scale during the regrowth process and that this evolution has an intrinsic character in the implanted material. Activation energy is shown to be independent of implantation conditions. Substitutional impurity implantation does not produce variations in regrowth kinetics whereas argon implantation drastically decreases the growth rate. Results are interpreted in terms of interface roughening due to nonrelaxing atomic configurations in the disordered phase. The evolution of the interface has been related to an increase of disorder in the regrown layers.
Spectroscopic studies of plasma during cw laser materials interaction
Authors: Rockstroh Todd J. , Mazumder Jyotirmoy
Published: July 2002

The role of plasma during cw laser materials interaction was studied using emission spectroscopy. The experiments were carried out in two stages: first, with a pure argon plasma; and second, with an aluminum target in an argon atmosphere. The electron-temperature distribution and electron density were determined. The experimental results were used to estimate laser attenuation and refraction as the beam propagated through the plasma column. Thus, transmitted energy and energy distribution on the target surface were determined. This was expected to be a more realistic input for transport phenomena models based on absorbed energy.
Excitation effects in excess Ca-doped epitaxial CaGe:YIG garnets
Authors: Yuan S. H. , Pardavi-Horvath M. , Wigen P. E. , DeGasperis P.
Published: July 2002

The effects of light and electric field on the electrical transport properties of p-type CaGe:YIG epitaxial films, having 0.1 at. f.u. excess Ca2+, were investigated from 30 to 340 K by measurements of dc conductivity, space-charge-limited current, and thermally stimulated current decay. The room-temperature conductivity of the films is of the order of 10−6 mho/cm, the mobility of the holes is 0.5 cm2/V s, and the density of charge carriers is 1012/cm3. The low-temperature conductivity of the films can be increased by six orders of magnitude under the effect of light (∼25 mW/cm2) and electric field (≤104 V/cm) at temperatures below 140 K. This photo- and electric-field-excited high conductivity state can be destroyed by thermal excitation at higher temperatures. It is assumed that below 200 K the holes are localized at Ca2+-Fe4+ centers which constitute an impurity band. Electrons excited from the valence band are trapped under the influence of the field at centers, formed probably by oxygen vacancies. The holes left in the valence band contribute to the high conductivity at low temperature.
A simple technique for measuring the interface-state density of the Schottky barrier diodes using the current-voltage characteristics
Authors: Tseng Hsun-Hua , Wu Ching-Yuan
Published: July 2002

A simple technique for measuring the interface-state density of a Schottky barrier diode fabricated on the moderately doped semiconductor is presented. It requires the I-V characteristics and substrate doping concentration as inputs which can be easily measured by existing techniques. Checked by the Schottky capacitance spectroscopy method, the proposed technique is proved to be highly accurate for various metal-Si and metal silicide-Si Schottky barrier diodes.
Formation of biaxial anisotropy in nickel by plastic deformation
Authors: Tejedor M. , Carrizo J. , Hernando B. , Fernández A.
Published: July 2002

An explanation is given of the behavior with applied magnetic field of the weak biaxial magnetic anisotropy observed in pure polycrystalline nickel after plastic deformation. This explanation is based on the analysis of the superposition of torque curves corresponding to regions of orthogonal uniaxial magnetic anisotropies of magnetoelastic origin, produced by residual microstresses in tensile and compressive regions. These generate a biaxial torque curve in unsaturated specimens. This analysis explains some features of the observed biaxial anisotropy that cannot be understood by supposing that its origin is due only to a preferred orientation (texture) of the subgrain structure.
The magnetic properties of high i
H
c Sm-Co, Nd-Fe-B, and Sm-Ti-Fe films crystallized from amorphous deposits
Authors: Cadieu F. J. , Cheung T. D. , Wickramasekara L. , Kamprath N. , Hegde H. , Liu N. C.
Published: July 2002

Permanent magnet films of the compounds SmCo5, Nd2Fe14B, and a high intrinsic coercive force, iHc, Sm-Ti-Fe phase have been prepared by a two-stage process in which the appropriate compositions were first deposited with a magnetic field applied in the substrate plane, and then subsequently crystallized in situ in this same magnetic field. The resulting samples exhibited high iHc values and specular reflecting surfaces. For the case of SmCo5 and for the high iHc Sm-Ti-Fe phase, samples were synthesized that exhibited extreme in-the-film-plane anisotropy. For these SmCo5 and high iHc Sm-Ti-Fe samples magnetic measurements, made at room temperature, indicated an in-the-film-plane anisotropy constant of at least 107 erg/cm3 for magnetizing the films in the film plane parallel versus perpendicular to the field that had been applied during the sputter deposition and crystallization. A model is presented based upon the crystallite grain sizes and magnetic alignment energy that correlates the differences in the degree of in-the-film-plane anisotropy induced in these systems. The model is applied to these systems as prepared by crystallizing amorphous deposits, and as directly crystallized onto heated substrates.
Temperature dependence of the figures of merit for blocked impurity band detectors
Authors: Szmulowicz Frank , Madarasz Frank L. , Diller Jeffrey
Published: July 2002

In a recent paper [F. Szmulowicz and F. L. Madarasz, J. Appl. Phys. 62, 2533 (1987)] we introduced details of an analytical model for a top side illuminated blocked impurity band infrared detector operating under background limited conditions (BLIP). In the present paper we extend that model to a bottom side illuminated detector operating in the presence of thermally generated carriers (non-BLIP case) as well as the optically generated background carriers. We display results of a parametric study including gain, quantum efficiency, the excess noise factor, and hence the detector figures of merit responsivity and detectivity, as functions of the detector temperature. Our study of the thermal noise includes the Poole–Frenkel effect. The results of the present calculation determine optimal operating temperatures as well as bias voltages. We show that Si:As blocked impurity band detectors, doped to 5×1017 cm−3, will be background limited below 13.5 K at low background, 1010 photons cm−2 s−1 operation, with responsivities and detectivities on the order of 5 A/W and 1014 cm Hz1/2/W, respectively.
A contribution to the ferromagnetic resonance linewidth of M-phase barium ferrite
Authors: Tsantes E. , Silber L. M.
Published: July 2002

Mössbauer and neutron diffraction studies of barium ferrite show that the iron ion in the 2b (trigonal bipyramid) site oscillates or hops between two equilibrium positions on either side of the mirror plane of the bipyramid, for temperatures above 80 K. Since this motion is uncorrelated from one (2b) site to another, it can lead to two-magnon scattering of spin waves due to the time-dependent fluctuation in the anisotropy energy. The contribution of this mechanism to the ferromagnetic resonance linewidth of barium ferrite is calculated, using time-dependent perturbation theory. For reasonable values of the parameters, this linewidth is of the order of 10 Oe at 300 K.
Magnetic and structural studies of erbium-iron-manganese-boron alloy (invited)
Authors: Fuerst C. D. , Meisner G. P. , Pinkerton F. E. , Yelon W. B.
Published: July 2002

We report the results of neutron diffraction and magnetization measurements on powder samples of Er2(Fe,Mn)14B. We find that the unit cell expands with increased concentration of manganese as larger manganese atoms replace the smaller iron atoms within the transition metal sublattice. The preferential occupancy of transition metal sites is also affected since manganese prefers positions with larger site volumes, such as the j2, while iron prefers smaller site volumes, such as the k2. At low temperatures (4.2 K) the simple collinear moment model of the magnetic structure, valid at higher temperatures, is not sufficient to explain the existence of reflections forbidden in the space group P42/mnm. A five-sublattice model is used in which each of the two rare-earth sites splits into two distinct sublattices whose moments are oppositely canted from the direction antiparallel to the transition metal moment. The linear field dependence of the magnetization provides support for this moment configuration. Neutron diffraction analysis of the transition metal moments is compared to the compositional dependence of the magnetization.
Electron-spin-resonance study of radiation-induced paramagnetic defects in oxides grown on (100) silicon substrates
Authors: Kim Yong Yun , Lenahan P. M.
Published: July 2002

We have used electron-spin resonance to investigate radiation-induced point defects in Si/SiO2 structures with (100) silicon substrates. We find that the radiation-induced point defects are quite similar to defects generated in Si/SiO2 structures grown on (111) silicon substrates. In both cases, an oxygen-deficient silicon center, the E′ defect, appears to be responsible for trapped positive charge. In both cases trivalent silicon (Pb centers) defects are primarily responsible for radiation-induced interface states. In earlier electron-spin-resonance studies of unirradiated (100) substrate capacitors two types of Pb centers were observed; in oxides prepared in three different ways only one of these centers, the Pb0 defect, is generated in large numbers by ionizing radiation.
Electrical characteristics of metal-insulator-semiconductor diodes with ZrO2/SiO2 dielectric films
Authors: Fukumoto Hirofumi , Morita Mizuho , Osaka Yukio
Published: July 2002

Thin films of ZrO2 vacuum-deposited on Si(100) substrates were oxidized in dry O2 at 800 °C and then analyzed by infrared absorption and Auger electron spectroscopy. It was found that the SiO2 layer was formed at the ZrO2/Si interface. Mo/ZrO2/SiO2/p-Si(100) metal-insulator-semiconductor diodes were electrically characterized. The static dielectric constant of the ZrO2 layer decreases from 21.1 ε0 to about 15.5 ε0 due to the crystallization of amorphous ZrO2 during the oxidation. The SiO2 layer formed at the ZrO2/Si interface was found to lower the leakage current through the dielectric film.
Out-diffusion of Ga and As atoms into dielectric films in SiOx /GaAs and SiNy/GaAs systems
Authors: Haga T. , Tachino N. , Abe Y. , Kasahara J. , Okubora A. , Hasegawa H.
Published: July 2002

Residual Ga and As atoms in SiOx and SiNy dielectric films deposited on GaAs were investigated by Rutherford backscattering spectroscopy and particle-induced x-ray emission techniques. Both Ga and As atoms were detected in the films after high temperature heat treatment, and even in the films as-deposited. The magnitude of the residual atoms presumably out-diffused from GaAs substrates was of the order of 1×1019 /cm3. The concentration of Ga atoms prevails over that of As atoms in SiOx/GaAs systems as is generally known, and vice versa in SiNy/GaAs systems after heat treatment. Dynamic behavior of Ga and As atoms in the films as functions of annealing temperature and annealing time cannot be explained by a simple diffusion mechanism. A model is proposed that the damaged layer around the interface of the systems is responsible for the anomalous out-diffusion phenomena.
Corona poling and real-time second-harmonic generation study of a novel covalently functionalized amorphous nonlinear optical polymer
Authors: Eich M. , Sen A. , Looser H. , Bjorklund G. C. , Swalen J. D. , Twieg R. , Yoon D. Y.
Published: July 2002

Thin films for optical second-harmonic generation (SHG) were prepared from a newly designed and synthesized amorphous polymer that incorporated a high density of active nonlinear optical groups (p-nitroaniline as attached side groups). For alignment of the nonlinear groups a very high electric field was applied by a corona discharge to the polymer films above Tg (125 °C). The subsequent freezing process resulted in a polymer film initially exhibiting a very high second-order nonlinear coefficient, d33=31 pm/V, measured by the Maker-fringe technique, plus excellent thermal, mechanical, and optical properties. The dynamics of polar alignment and decay, studied by in situ poling (or depoling) and SHG measurements, indicated a multiple exponential behavior with the average relaxation time somewhat longer than expected from extrapolation of the dielectric relaxations data according to the Williams–Landel–Ferry equation. The frozen-in SHG behavior at room temperature (∼100 °C below Tg) relaxed after 5 days to a stabilized value of 19 pm/V for d33.
Enhancement of magneto-optical Kerr effects
Authors: Chen Liang-Yao , McGahan William A. , Shan Z. S. , Sellmyer D. J. , Woollam John A.
Published: July 2002

A general expression for the magneto-optical polar Kerr effect is given for a bilayered configuration in which the optical constants of the two media differ. The equation shows that a giant enhancement of the Kerr effect in magneto-optical/metallic bilayers can be expected if the dielectric constants of the two materials are low or are matched well to the other. An explicit equation is also given to calculate the spectral enhancement due to a dielectric overcoating on magnetic substrates. An example is given for SiO-coated Dy/Co compositionally modulated alloys, yielding a good comparison to the experimental results in both the transparent and absorbing spectral regions of SiO.
Growth of Fe-Cr-Fe (100) sandwiches and their magnetization behavior (abstract)
Authors: Prinz G. A. , Krebs J. J. , Chaiken A.
Published: July 2002

Single-crystal Fe-Cr-Fe sandwiches have been epitaxially grown in the (100) bcc structure on ZnSe epilayers on GaAs substrates. These sandwiches form a simple well-defined unit for studying the coupling between the Fe layers through the Cr. These simple sandwiches show not only the antiferromagnetic coupling reported earlier in (100) superlattices,1 but also competing effects of the magnetocrystalline anisotropies which are of comparable strength. Detailed studies were made of the magnetization in the (100) plane along the [100] and [110] axes for a large set of samples for which the Fe films were held at ≂40 Å, but the Cr thickness (t) was varied from 5 to 85 Å. We observe a narrow window of values 10<t<25 Å for which antiferromagnetic alignment is obtained.
Collective effects of cavitation: Experimental study of bubble-bubble and bubble-shock wave interactions
Authors: Testud-Giovanneschi P. , Alloncle A. P. , Dufresne D.
Published: July 2002

This paper is concerned with a basic study of optical cavitation or cavitation induced by a high intensity laser beam. The work deals with the collective behavior of several bubbles induced independently and simultaneously. The experimental setup to produce and to visualize bubbles and shock waves in the liquid (water) is described. Bubbles dynamics in an infinite medium is displayed in sequential photographs (framing and streak). The visualizations show such interesting results as strong asymmetries, deformations, attraction or repulsion, and premature implosion due to bubble-interface/shock-wave-radiated-upon-collapse impact.
Ferrimagnetic resonance linewidth in single-crystal Mn-doped Ba2Zn2Fe12O22
Authors: Dorsey P. , Sun K. , Vittoria C. , Wittenauer M. A. , Friedlander F. J. , Schindler A.
Published: July 2002

The ferrimagnetic resonance linewidths in single crystals of Mn-doped Ba2Zn2Fe12O22 were measured as a function of frequency (0.01–40 GHz) and temperature (4–300 K). In addition, the saturation magnetization, g factor, and planar anistropy fields were measured. The field linewidth ΔH appears to scale with frequency; however, the field linewidth as a function of temperature for the hexaferrite exhibits maxima. The temperature-dependent resonant field values were used to calculate the anisotropy field as a function of temperature. These measurements were performed in a frequency-swept slot-coplanar device at a fixed value of applied magnetic field and in a traditional ferrimagnetic resonance system.
Measurement of intersubband absorption in multiquantum well structures with monolithically integrated photodetectors
Authors: Hasnain G. , Levine B. F. , Bethea C. G. , Abbott R. R. , Hsieh S. J.
Published: July 2002

Intersubband absorption in GaAs/AlGaAs multiple quantum wells (MQW) is measured using a novel integrated detector-waveguide structure. The responsivity spectra of MQW infrared photodetectors is demonstrated to follow exactly the intersubband absorption spectra at various temperatures and bias. The blue shift of the bound-to-continuum state intersubband absorption peak with lowering of temperature is observed to track the change of the conduction-band offset. The absorption peak showed only a slight Stark shift since the field in the wells is small in these narrow-well/thick-barrier structures and partly screened by the high doping.
High resolution 4.2 K near band-gap photoluminescence spectrum of mercuric iodide
Authors: Bao X. J. , Schlesinger T. E. , James R. B. , Ortale C. , van den Berg L.
Published: July 2002

We have investigated in detail (resolution up to 0.35 Å) the near-band-gap 4.2 K photoluminescence spectrum of undoped Hgl2 in its red tetragonal form. At least 26 emission lines are resolved in the wavelength region between 5290 and 5400 Å. Many of these are reported for the first time. We have also tabulated the steplike emission lines between 5220 and 5290 Å.
Replacement of group-III atoms on the growing surface during migration-enhanced epitaxy
Authors: Yamaguchi Hiroshi , Horikoshi Yoshiji
Published: July 2002

An InAs monolayer is grown between GaAs layers using migration-enhanced epitaxy. The surface chemical characteristics during growth are investigated by reflection high-energy electron diffraction. With low As4 pressure and with a substrate temperature of 500 °C, the oscillation amplitude of the reflected electron beam after growth of one monolayer of InAs vanishes during the growth of GaAs over more than 20 atomic layers. High-resolution secondary-ion-mass spectroscopic analysis and photoluminescence characteristics of fabricated structures indicate an anomalous distribution of In atoms with a gradual slope toward the growth direction only when the substrate temperature is 500 °C. Numerical calculation shows that the experimental results are explained by the In atoms in the InAs monolayer being replaced by subsequently deposited Ga atoms. The comparison between experimental results and calculated ones indicates that replacement occurs with a probability of 98% at 500 °C. An InGaAs layer was also grown on an InP substrate using migration-enhanced epitaxy with the deposition of two monolayer metallic atoms per one cycle. X-ray diffraction analysis shows that the composition is modulated with a period of two monolayers in the grown InGaAs layer. This phenomenon is also explained in terms of metallic atom replacement.
Tunable lasers at 1080 nm for helium optical pumping
Authors: Schearer L. D. , Tin Padetha
Published: July 2002

We describe the current status of tunable, cw, solid-state lasers in the infrared region of the spectrum which may be tuned to the helium resonance transition near 1083 nm. The principal use for these laser devices is the optical pumping of 3He and 4He atoms via the 23S1 metastable state. The tunable lasers we have examined include Nd3+-doped YAlO3, LaxNd1−xMgAl11O19 (x ∼0.15), Nd:LiNbO3, and Nd:silica glass fibers. Power outputs from these lasers range from a milliwatt to more than 1 W at the helium transition. Laser emission bandwidths range from less than 40 MHz (single-mode operation) to several GHz. Applications include magnetometry, dense nuclear targets, and polarized electron beams.
Electro-optic imaging of the internal fields in a GaAs photoconductive switch
Authors: Donaldson W. R. , Kingsley L. , Weiner M. , Kim A. , Zeto R.
Published: July 2002

A technique has been developed that allows single-shot measurement of the electric field over an extended area. A LiTaO3 crystal, covering the entire active area of the GaAs photoconductive switch, coupled the surface field with the polarization of an optical probe pulse. When imaged onto a two-dimensional detector array, this system produced snapshots of the surface field profile with 200-ps time resolution and 3-μm spatial resolution. The technique was used to monitor the collapse of the electric field in the GaAs as it was switched with a λ=1.06 μm optical pulse. The switching speed and generation of field enhancements were found to be dependent on the illumination configuration and the electric field. In particular, the switching efficiency was found to decrease with increasing field.
Fluorocarbon impurities in KrF lasers
Authors: Hwang Helen H. , James Kristopher , Hui Roger , Kushner Mark J.
Published: July 2002

Fluorocarbon impurities are known to have deleterious effects on the operation of excimer lasers; however, the sensitivity limits are poorly known. Absorption at 248.9 nm in an e-beam-pumped KrF laser has been attributed to CF2, produced by plasma fragmentation of precursor molecules such as CF4. In this paper, the effects of CF4 impurities on the gain of an electron-beam-excited KrF laser are investigated theoretically. It is found that the density of KrF(B) significantly decreases and absorption increases when the CF4 concentration exceeds 0.03%. The decrease in the density of KrF(B) is dominantly the result of the interception of precursors to forming the upper laser level, as opposed to direct quenching.
Anisotropy compensation and magnetostriction in TbxDy1−x(Fe1−yTy)1.9 (T=Co,Mn)
Authors: Clark A. E. , Teter J. P. , Wun-Fogle M.
Published: July 2002

From magnetization (M) and magnetostriction (λ) measurements as a function of magnetic field and stress, the temperatures of anisotropy compensation, Tm, for technologically important TbxDy1−x(Fe1−yTy)1.9 [T=Co,Mn (0.3≤x≤0.5) (0≤y≤0.3)] were determined. Measurements of M and λ encompassing Tm were made under compressive stresses from 8.8 to 36 MPa and for temperatures from −196 to +130 °C. In agreement with earlier measurements, Tm decreases with increasing Tb. Substitution of Mn for Fe for fixed x also decreases Tm. In contrast with these observations is the increase of the anisotropy compensation temperature with the replacement of Fe by small amounts of Co. In the cases of both (1) increasing Tb content and (2) increasing Co content, the Curie temperature TC increases, yielding, in general, a higher magnetic moment and saturation magnetostriction of these alloys. Thus, compensation at a given temperature may be obtained in an improved class of Laves phase compounds, R(1)xR(2)1−x(Fe1−yCoy)2, where rare earths R(1) and R(2) are, for example, Tb and Dy.
Magnetic interactions in particulate and thin-film recording media
Authors: Speliotis D. E. , Lynch W.
Published: July 2002

Magnetic interactions between particles or crystallites in recording media can have an important effect on their switching behavior and on their recording performance. If the types of interactions that tend to stabilize the saturated state are defined as positive and those that tend to destabilize it as negative, it can be argued that positive interactions would reduce demagnetization losses and lead to improved recording performance. Furthermore, positive interactions would be expected to give rise to cooperative switching of groups of particles or crystallites that, on the one hand, would tend to reduce the switching field distribution and, on the other hand, would tend to increase noise. Remanence curves, being strongly influenced by interparticle interactions, provide a sensitive way to investigate the magnitude and sign of such interactions. In this paper, the interactions of different particulate and thin-film media have been studied by comparing their demagnetization remanence curves with their isothermal remanence curves. Recording media employing oriented acicular particulate assemblies exhibit negative interactions. Nonoriented particulate media with either acicular or platelet-type particles exhibit very small and slightly negative interactions. Thin-film media and oriented Ba-ferrite media with platelet-type particles usually exhibit strong positive interactions. These large differences in interaction fields are discussed on the basis of the type of coupling (magnetostatic and/or exchange), the particle morphologies, and the anisotropy fields in these media. The polarity, magnitude, and shape of the interaction curves seem to be qualitatively correlated with the recording media noise.
Determination of nanometer structures and surface roughness of polished Si wafers by scanning tunneling microscopy
Authors: Hartmann E. , Hahn P. O. , Behm R. J.
Published: July 2002

Roughness and general surface topography of polished Si wafers were systematically studied on an angstrom to nanometer scale by scanning tunneling microscopy (STM). Evaluation of a large number of STM images by using a simple classification scheme assures statistically relevant results without the disadvantages and loss of information connected with averaging the rms roughness or similar methods. For the flat parts of the surface, a rms roughness of 1.2–1.8 Å is found, comparable to that derived from light scattering and diffraction measurements on similar surfaces. Significant amounts of the surfaces were found to exhibit more pronounced structures, contrasting results of other techniques. The reliability and applicability of STM measurements on technical surfaces, on an angstrom to nanometer scale, is discussed in the light of these results.
Ga out-diffusion in rapid-thermal-processed GaAs with SiO2 encapsulants
Authors: Katayama Masayuki , Tokuda Yutaka , Inoue Yajiro , Usami Akira , Wada Takao
Published: July 2002

Effects of rapid thermal processing (RTP) using halogen lamps on GaAs with 50-, 200-, and 1250-nm-thick SiO2 encapsulants have been studied by capacitance-voltage, secondary-ion-mass spectroscopy, and x-ray photoelectron spectroscopy. RTP has been performed at 760 and 910 °C for 9 s. A decrease of the carrier concentration is observed near the SiO2/GaAs interface in all RTP samples. The decreased carrier concentration profile is fitted to a complementary error function diffusion profile. This indicates that the decrease of the carrier concentration by RTP is related to the Ga out-diffusion through SiO2. Furthermore, the amount of the decreased carrier concentration is found to be proportional to the doped Si concentration. These results show that the origin of the carrier concentration decrease is the formation of VGa-SiGa complex defects called the self-activated center. The amount of the Ga out-diffusion is larger in the RTP samples at 760 °C with thicker SiO2 and at 910 °C with a thinner one. This inverse relation can be explained by taking account of two different kinds of driving force, that is, interfacial thermal stress and the interfacial reaction between GaAs and SiO2. Interfacial thermal stress enhances the Ga out-diffusion in RTP at 760 °C, while interfacial reaction enhances in RTP at 910 °C through As loss.
Time dependence of radiation-induced interface trap formation in metal-oxide-semiconductor devices as a function of oxide thickness and applied field
Authors: Brown D. B. , Saks N. S.
Published: July 2002

This work is a study of the formation mechanisms of interface traps (Nit) in metal-oxide-semiconductor devices. The time-dependence of the Nit formation has been measured as a function of oxide thickness following a short radiation pulse. The Nit formation time is found to increase as t2.6ox when the gate bias is negative during irradiation and positive afterward. This result is in excellent agreement with predictions of a hydrogen transport model where drift of hydrogen ions (H+) is the rate-limiting step. When the gate bias during irradiation is positive, interpretation of the correlation between data and model suggests that the hydrogen ions are preferentially created near the Si-SiO2 interface. Finally, the Nit formation time is found to decrease with increasing oxide field as E−1.73ox. This result is compatible with the hydrogen transport model if the average displacement per hop is assumed to be proportional to Em.
Irreversibility line in the ferromagnet Co70.4Fe4.6Si15B10 alloy
Authors: Montarroyos E. , Machado F. L. A. , Rezende S. M. , de Almeida J. R. L. , Altoé M. V. P. , Missell F. P.
Published: July 2002

We report magnetization measurements in the zero magnetostriction Co70.4Fe4.6Si15B10 amorphous alloy. The measurements were carried out from room temperature up to 430 °C and for applied magnetic fields up to 45 Oe. An irreversible behavior in the zero-field and field-cooled magnetization was observed. The irreversibility line was fitted to a de Almeida–Thouless line with a crossover critical exponent φ=4.66. The experimental results are analyzed in the framework of an extended random anisotropy model for amorphous ferromagnets.
Equilibrium density of defects in hydrogenated amorphous silicon carbon alloys
Authors: Alvarez F. , Sebastiani M.
Published: July 2002

In this work, we compare experimental measured density of defects in amorphous silicon-carbon alloys with a theoretical expression obtained by applying a thermodynamic equilibrium defects model to the intrinsic defect concentration in the semiconductor. The samples were deposited under different conditions of hydrogen dilutions and two substrate temperatures. The density of defects was determined by photothermal deflection and photoconductivity spectroscopy. The expression of the equilibrium density of defects is obtained starting from the canonical partition function assuming two independent systems of Si-Si and Si-C dangling bonds. A weak-bond defect formation reaction is assumed in the analysis. Linear approximations of the equilibrium temperature and the valence band exponential decay (Urbach’s tail) are used for the numerical calculations. The trend of the experimental data are well represented by the theoretical expression. The model, however, overestimates the density of defects in samples deposited onto substrata maintained at the lower of two temperatures.
Modeling of the negative glow and Faraday dark space of a low-pressure Hg–noble-gas discharge
Authors: Coe S. E. , Lister G. G.
Published: July 2002

A model of the negative glow of a low-pressure Hg-Ar discharge is described. The model contains a number of important physical processes (in particular noble-gas excitation and multiple-step ionization) that have not been included in earlier models. The model has also been coupled to a simple description of the Faraday dark space to enable predictions to be made of the axial density and potential profiles across the whole negative-glow Faraday dark-space positive-column region. Results of a number of numerical computations are presented and in general these have been found to be in agreement with experimental observations. The effect of varying the various input parameters to the model on its predictions has also been studied and the results of this will also be presented.
Effects of the structure and composition of lead glasses on the thermal lensing of pulsed laser radiation
Authors: Taheri B. , Munoz F. A. , St. John W. D. , Wicksted J. P. , Powell Richard C. , Blackburn D. H. , Cranmer D. C.
Published: July 2002

The thermal lensing characteristics of several silicate, germanate, phosphate, and borate glasses were studied using a laser with a 7 ns pulse at 457 nm in a tight focus geometry. A geometric model was developed to describe the quadratic radial profile of the refractive index resulting from the laser-induced temperature profile. This model was utilized to interpret the effects of some of the relevant experimental parameters on the fluence transmission experiments. The influence of material properties such as different types of network former and modifier ions on the nonlinear optical properties of these materials were also studied. It was found that: (i) the greatest influence of the network modifier ions was due to their effect on the absorption coefficient of the glasses; (ii) in lead glasses, the thermo-optic coefficients dn/dT of the germanates and silicates with random network structures were greater than those of the borate and phosphate glasses with ring and chain structures; and (iii) the main contribution to the thermo-optic coefficient comes from the thermally induced changes in the electronic polarizability of the glass components. In these glasses, the oxygen polarizability provides the dominant contribution and is affected by the variations in the polarizing power (charge to radius-squared ratio) of the network former ions.
Effective conductivity of suspensions of overlapping spheres
Authors: Kim In Chan , Torquato S.
Published: July 2002

An accurate first-passage simulation technique formulated by the authors [J. Appl. Phys. 68, 3892 (1990)] is employed to compute the effective conductivity σe of distributions of penetrable (or overlapping) spheres of conductivity σ2 in a matrix of conductivity σ1. Clustering of particles in this model results in a generally intricate topology for virtually the entire range of sphere volume fractions φ2 (i.e., 0≤φ2≤1). Results for the effective conductivity σe are presented for several values of the conductivity ratio α=σ2/σ1, including superconducting spheres (α=∞) and perfectly insulating spheres (α=0), and for a wide range of volume fractions. The data are shown to lie between rigorous three-point bounds on σe for the same model. Consistent with the general observations of Torquato [J. Appl. Phys. 58, 3790 (1985)] regarding the utility of rigorous bounds, one of the bounds provides a good estimate of the effective conductivity, even in the extreme contrast cases (α≫1 or α≂0), depending upon whether the system is below or above the percolation threshold.
Fermi level shift in Bi12SiO20 via photon-induced trap level occupation
Authors: Attard Alfred E.
Published: July 2002

Photon-induced local departures from charge neutrality are the origin of the photorefractive effect. When a periodic photorefractive grating is stored in the material, one can associate with it quasi-Fermi levels having corresponding periodic variations. The hopping band structure is modified across the local variation in charge distribution. A model is presented for the shift of the Fermi level with photon-induced trap occupation in Bi12SiO20. The shift of the Fermi level is dependent on the density of trap sites in the band gap, the radiation-induced occupation density, and the energy levels of the traps. Shifts in the Fermi levels with trap occupation have been calculated, using a simple model. Estimations of the change of density of sites in the hopping band with Fermi level shift for typical photoexcited charge concentration are shown to be consistent with experimental data.
Origins of charged particles in vapor generated by electron-beam evaporation
Authors: Nishio R. , Tuchida K. , Tooma M. , Suzuki K.
Published: July 2002

The origins of electrons and ions in uranium vapor generated by electron-beam evaporation have been determined. Measurements were made for the electron emission current due to high-energy electron-beam irradiation on a uranium surface (backscattered electrons, etc.), thermionic emission current from the melt surface, and electron current due to vapor ionization. Comparison of these currents confirmed that vapor ionization was the main electron generation process at evaporation surface temperatures above 2200 K. The ionized vapor formed a weakly ionized plasma of very low electron temperature: The degree of ionization ≤1%, electron temperature ≤0.3 eV. The electron-impact ionization process contributed mainly to plasma formation. Beam electrons, their backscattered electrons, and secondary electrons from the beam-irradiated uranium surface were the source electrons for this process. Thermal ionization was the next major process. In addition to the plasma formation model, plasma behavior in vapor was described by a one-dimensional symmetric expansion model. The calculated degree of ionization was in good agreement with the measured value over a wide range of evaporation temperatures and electron-beam currents.