bluuebunny/crossref_metadata_2025 huggingface dataset:
Bound-to-extended state absorption GaAs superlattice transport infrared detectors
Authors: Levine B. F. , Bethea C. G. , Choi K. K. , Walker J. , Malik R. J.
Published: July 2002

We demonstrate an efficient 10-μm detector based on localized bound-state–to–extended-continuum-state absorption in doped GaAs/AlxGa1−xAs quantum wells designed to contain only one bound state. The voltage dependence of the responsivity is strikingly different from previous intersubband tunneling detectors containing two bound states. This results from the photoexcited electrons being transported over the top of the superlattice barriers, rather than tunneling through them.
Lattice relaxation mechanism of ZnSe layer grown on a (100) GaAs substrate tilted toward 〈011〉
Authors: Ohki Akira , Shibata Noriyoshi , Zembutsu Sakae
Published: July 2002

Using metalorganic vapor-phase epitaxy, zinc selenide films are grown on GaAs surfaces of three different orientations: (100), (100) tilted 2° off toward 〈011〉, and (100) tilted 5° off toward 〈011〉. The properties of these epilayers are determined by x-ray diffraction and photoluminescence. There is remarkable difference in lattice relaxation mechanism between exact (100) case and 5° off, a part of lattice mismatch stress in the ZnSe layer on 5° off substrate is relaxed by lattice inclination. This inclination prevents the generation of defects and dislocations. Photoluminescence properties are also improved by using the tilted substrates. Strong near-band-edge emission and weak deep-level emissions such as Y0, S, and SA lines compared with those grown on exact (100) substrates are observed.
The effect of Ar+ sputtering on the x-ray photoelectron spectra of plasma-deposited silicon nitride films
Authors: Chiang J. N. , Hess D. W.
Published: July 2002

The effects of argon-ion sputtering and atmospheric exposure on the composition and structure of plasma-deposited (PD) silicon nitride films were studied using x-ray photoelectron spectroscopy. PD silicon nitride samples were transferred directly from the plasma deposition chamber into the analysis chamber without ambient exposure. Samples exposed to ambient oxidized, with an oxidation rate that depended upon deposition temperature. Ion energies of 500–4000 eV were used to sputter sample surfaces. Preferential sputtering was observed at all energies, with the lower energies creating less compositional and structural modification. Films deposited at lower temperatures exhibited pronounced preferential sputtering of nitrogen, particularly NHx species. Ion milling was found to be unacceptable as a means of surface cleaning PD silicon nitride since even qualitative N:Si composition trends were modified by sputtering.
Oxidation studies of SiGe
Authors: LeGoues F. K. , Rosenberg R. , Nguyen T. , Himpsel F. , Meyerson B. S.
Published: July 2002

We have studied the kinetics and mechanism of oxidation of SiGe alloys deposited epitaxially onto Si substrates by low-temperature chemical vapor deposition. Ge is shown to enhance oxidation rates by a factor of about 3 in the linear regime, and to be completely rejected from the oxide so that it piles up at the SiO2/SiGe interface. We demonstrate that Ge plays a purely catalytic role, i.e., it enhances the reaction rate while remaining unchanged itself. Electrical properties of the oxides formed under these conditions are presented, as well as microstructures of the oxide/substrate, Ge-enriched/SiGe substrate, and SiGe/Si substrate interfaces, and x-ray photoemission studies of the early stages of oxidation. Possible mechanisms are discussed and compared with oxidation of pure silicon.
Photoinjected hot-electron damage in silicon point-contact solar cells
Authors: Gruenbaum P. E. , King R. R. , Swanson R. M.
Published: July 2002

Initial designs of single-crystal silicon point-contact solar cells have shown a degradation in their efficiency after being exposed to concentrated sunlight. The main mechanism appears to be an increase in recombination centers at the Si/SiO2 interface due to ultraviolet light photoinjecting electrons from the silicon conduction band into the silicon dioxide that passivates the cell’s front surface. The instability of the interface is significantly worse if the oxide is grown in the presence of trichloroethane. Texturization of the surface also leads to more instability. A reasonably good resistance to ultraviolet can be created by putting a phosphorus diffusion at the surface, and can be improved further by stripping off the deposited oxide after the diffusion and regrowing a dry thermal oxide.
Formation of device-grade epitaxial silicon films at extremely low temperatures by low-energy bias sputtering
Authors: Ohmi T. , Ichikawa T. , Iwabuchi H. , Shibata T.
Published: July 2002

Device-grade epitaxial single silicon layers have been formed at extremely low temperatures of about 300 °C by low-energy bias sputtering in conjunction with in situ substrate surface cleaning and an ultraclean processing environment. Dopant impurities in the target material are fully incorporated into the sputter-deposited silicon film and these impurities are 100% electrically activated without any additional heat cycles. An epitaxial silicon film having a resistivity as low as 0.0014 Ω cm has been obtained using a heavily arsenic-doped silicon block as a target material. A p-n junction diode formed by directly depositing a phosphorus-doped n-type epilayer on a p-type substrate indicates a reverse current level as low as 1.88×10−9 A/cm2 at a reverse bias voltage of 5 V, thus verifying that the in situ substrate surface cleaning by low-energy Ar ion bombardment is very effective and damage free. The electrical characteristics of a grown film have shown a good correlation to the crystal structure of the film, which is primarily determined by the energy of ions concurrently bombarding the growing film surface.
Effect of silver and silver oxide additions on the mechanical and superconducting properties of YBa2Cu3O7−δ superconductors
Authors: Singh J. P. , Leu H. J. , Poeppel R. B. , Van Voorhees E. , Goudey G. T. , Winsley K. , Shi Donglu
Published: July 2002

The effects of Ag and Ag2 O additions on the mechanical and superconducting properties of YBa2 Cu3 O7−δ superconductors were studied. The addition of 20-vol % Ag resulted in an increase in strength from ≊40 to 75 MPa and a twofold increase in the critical current density (Jc ) of YBa2 Cu3 O7−δ bar specimens. These improvements are believed to be due to the increased density. Furthermore, Ag particles may relax the residual stresses resulting from the expansion anisotropy of individual grains, and may provide increased resistance to crack propagation by pinning the propagating cracks; these effects would also increase the strength. The addition of 30-vol % Ag resulted in an increase in strength to 87 MPa, but it was associated with a decrease in Jc . This decrease in Jc at high Ag content is believed to be due to the presence of oxygen deficient nonsuperconducting tetragonal phase as well as continuous Ag phase. Similar effects were also observed after Ag2 O additions.
Analysis of a CF4/O2 plasma using emission, laser-induced fluorescence, mass, and Langmuir spectroscopy
Authors: Buchmann L.-M. , Heinrich F. , Hoffmann P. , Janes J.
Published: July 2002

The behavior of CF2 radicals in CF4/O2 plasmas has been studied as a function of the oxygen partial pressure in an rf reactor with 13.56 MHz, 30 W, and 40 mTorr total pressure. The CF2 ground and excited states were detected by the CF2 (A-X) band spectra applying laser-induced fluorescence and optical emission spectroscopy, respectively. Adding oxygen to the CF4 feed gas, the intensity of the CF2 signals in both spectra showed similar decrease. No evidence is found for a simple correlation between the neutral CF2 densities and the CF+2 ions measured by quadrupole mass spectrometry. Electron densities and temperatures were evaluated to be slightly above 8×109 cm−3 and ∼5 eV, respectively. A simplified model, which takes into account different excitation paths, suggests that direct electron impact of the CF2 ground state species was the dominant mechanism for the population of the electronically excited state. Absolute CF2 and O concentrations depending on the oxygen feed could be estimated. We obtained CF2 densities between 2×1013 and 2×1012 cm−3, when the oxygen concentration increases from 2% to 23%. The corresponding O densities varied from 1012 to 6×1013 cm−3.
Particulates in aluminum sputtering discharges
Authors: Jellum G. M. , Graves D. B.
Published: July 2002

Plasma-generated particulates are receiving increasing attention as a possible source of device yield reduction in plasma-assisted etching and deposition processes. We have investigated aluminum sputtering in argon discharges between parallel-plate aluminum electrodes under both direct current and radio frequency excitation. Pulsed laser-induced fluorescence experiments indicate that particulates containing aluminum form in these discharges under conditions in which aluminum sputtering takes place. Under sufficiently powerful laser excitation, the laser acts to dissociate the particles, resulting in an enhanced aluminum-atom laser-induced fluorescence signal. Spatial regions of atomic aluminum fluorescence enhancement coincide exactly with nonresonant light scattering profiles from a low-power helium-neon laser. The pulsed laser was observed to locally deplete particulates on a time scale of seconds. Particle profiles appear to evolve in the discharge on a time scale of tens of minutes to hours. The spatial profile of the particles depends in a sensitive manner on the applied voltage, as well as other discharge variables. At a sufficiently negative dc voltage applied to the cathode, particles were observed to be excluded from the interelectrode region.
Two-dimensional model for material damage due to melting and vaporization during laser irradiation
Authors: Kar A. , Mazumder J.
Published: July 2002

A two-dimensional model is developed for material damage caused by melting and vaporization during pulsed laser irradiation. The problem is formulated by using the energy conservation equation (the Stefan condition) at various points on the solid-liquid and liquid-vapor interfaces. The effect of curvature of the solid-liquid and liquid-vapor interfaces are taken into account and the problem is solved numerically by using the Runge–Kutta method. For determining the maximum damage that can occur during laser irradiation, the laser energy is considered to be utilized only to melt and vaporize the material. The effect of various laser parameters, such as the laser power, laser beam diameter, pulse-on time, and the number of pulses per second on the depth and the radius of the crater is presented. Also, the tapering angle of the crater and the formation of recast layer in the crater during laser irradiation are examined in this study. Finally, a linear relationship between the maximum crater depth and the ‘‘gross’’ laser intensity is derived phenomenologically and verified by using the numerical results of this study.
Modeling and diagnostics of silicon nitride deposition from 254-nm Hg photosensitization of SiH4-NH3 mixtures: Luminescence of HgNH3 excimer and laser-induced fluorescence of NH2(Ã 2A1)
Authors: Fuyuki Takashi , Allain Bruno , Perrin Jérôme
Published: July 2002

Silicon nitride thin films are obtained from the Hg(63P1) -photosensitized decomposition of SiH4 and NH3 using the 254-nm radiation of a low-pressure Hg discharge lamp. Several diagnostics are associated to characterize the physicochemical mechanisms and measure the densities of reactive species or reaction products. The photon flux is obtained by radiometry, the Hg density is determined by absorption of the 254-nm radiation, and the SiH4 gas depletion is measured by mass spectrometry. The luminescence at 340 nm of HgNH3 excimers is used as a probe of the metastable Hg(63P0) density and the NH2 radical is detected by laser-induced fluorescence (LIF) using the 597.8-nm line to excite NH2(Ã2A1) Σ(0,9,0) vibronic states.

The interpretation of LIF results in terms of NH2 density takes into account collisional transfer and quenching of NH2(Ã2A1) revealed by nonexponential time decay of the fluorescence. In addition, the silicon nitride film deposition rate is monitored and the N/Si stoichiometry is determined by x-ray photoelectron spectroscopy. All these data are used to validate a one-dimensional spatially resolved modeling of the photochemistry. The model involves radiative transfer of the 254-nm radiation from the lamp, excitation and quenching of Hg(63P1) and Hg(63P0) by photoabsorption and reactive collisions, dissociation of NH3 and SiH4 into H, NH2, and SiH3 radicals, gas-phase diffusion and secondary reactions of these radicals, and eventually surface reactions of SiH3 and NH2 on the growing silicon nitride film surface, in terms of sticking and recombination probabilities.

It is shown that SiH4 diluted in NH3 is essentially dissociated by reactive collisions with Hg(63P0) metastables produced by collisional deexcitation of Hg(63P1) with NH3. Moreover, within the assumptions and the precision of the model, we derive from LIF data that the rate constant for the reaction of NH2 with SiH4 is lower or equal to 10−13 cm3 s−1, in agreement with a recent determination. It is also shown that the radical-radical reactions between SiH3, NH2, and H are the main cause for the limitation of radical gas-phase density and deposition rate.
Tin selenide films grown by hot wall epitaxy
Authors: Singh J. P. , Bedi R. K.
Published: July 2002

Tin selenide films have been grown by the hot wall epitaxy technique onto glass substrates. Experimental deposition conditions are optimized to obtain better crystallinity and deposition rate. Grain size as large as 4 μm has been obtained. It has been observed that the electrical conductivity and the carrier mobility of SnSe films are comparatively higher than those obtained by conventional evaporation techniques. Analysis of optical absorption measurements indicate that the absorption edge of SnSe films is due to an allowed direct transition of energy about 1.22 eV.
Study of the structure in rf glow discharges in SiH4/H2 by spatiotemporal optical emission spectroscopy: Influence of negative ions
Authors: Tochikubo Fumiyoshi , Suzuki Akira , Kakuta Shigeru , Terazono Yuko , Makabe Toshiaki
Published: July 2002

Radio frequency glow discharges in SiH4(50%)/H2 at 13.56 MHz and 100 kHz have been investigated by using time- and space-resolved optical emission spectroscopy. The relative net-excitation rate is obtained by deconvolving the spatiotemporal emission profile considering the influence of the radiative lifetime. From the results, we infer the behavior of electrons with energy greater than the threshold energy for the dissociative excitation and we discuss the discharge structure. Lines of SiI(4s 11P0→3p 21D), SiH(A2Δ;v′=0→X2Π;v″=0), Hα, and H2(d 3Πu→a 3Σ+g) are selected for observation. It is concluded that there exists a considerable population of negative ions compared with positive ions, which has not been previously proposed in high-frequency discharges in SiH4 because of the small degree of electron attachment. At 13.56 MHz, the optical emission oscillates at 2ω in the bulk plasma region, in addition to the emissions at the plasma-sheath boundaries. It implies that the field in the bulk is strengthened by the external sinusoidal field to make up for the loss of electrons by electron attachment. The bulk field is estimated at 69 sin(ωt) V cm−1 from V-I characteristics. The existence of a double layer by the modulation of electrons is strongly suggested even in a high frequency at 13.56 MHz. At low frequency (100 kHz), a double layer is observed, which is formed by the large modulation of negative and positive ions. Consequently, excitation by electrons accelerated in the field at the double layer is observed in addition to excitation by secondary electrons from ion bombardment. Consideration of the role of negative ions in high-frequency SiH4 discharges at 13.56 MHz is necessary to understand the discharge structure of the silane plasma and the deposition of the hydrogenated amorphous silicon.
Acoustic fields and velocities for surface-acoustic-wave propagation in multilayered structures: An extension of the Laguerre polynomial approach
Authors: Kim Yoonkee , Hunt William D.
Published: July 2002

The propagation of surface acoustic waves (SAW) in multilayered structures is studied by using Laguerre polynomials. Field distributions and velocities are obtained easily by this simple and noniterative method. Formulations are given for open- and short-circuit boundary conditions. The SAW velocity of a ZnO film on a Si substrate is calculated and the result is compared against other calculations from the literature. Numerical results are also given for GaAs/AlGaAs multilayered structures.
Effective mass for strained p-type Si1−xGex
Authors: Manku T. , Nathan A.
Published: July 2002

The density of states and carrier concentration effective masses have been calculated for strained p-type nondegenerate Si1−xGex using the band structure recently proposed by Manku and Nathan (Phys. Rev. B). The results show a significant decrease in both masses compared to pure silicon. It turns out that the total carrier concentration effective mass for a Ge composition, x=0.3 is approximately three times smaller than the corresponding value for silicon. At 300 K, the total carrier concentration effective mass is 0.35 (normalized with the free electron mass) and at 77 K, the corresponding value is 0.22.
Monte Carlo calculation of electron impact ionization in bulk InAs and HgCdTe
Authors: Brennan Kevin F. , Mansour Nabil S.
Published: July 2002

We present calculations based on an ensemble Monte Carlo simulation of the electron impact ionization rate in bulk InAs and Hg0.70Cd0.30Te at 77 K. The Monte Carlo calculation includes an analytic nonparabolic model of the conduction band, all of the dominant scattering mechanisms, and a nonparametrized model of the impact ionization rate. Calculations are first made of the ionization rate in bulk InAs and are compared to both experiment and previous Monte Carlo investigations. Updated material parameters are used as well as calculated values of the threshold energy and the ionization probability in the InAs simulation. Good agreement with previous Monte Carlo studies is achieved. Using a similar model, the electron ionization rate in bulk Hg0.70Cd0.30Te is studied. To our knowledge, this is the first study either theoretically or experimentally of electron impact ionization in long wavelength (5 μm) band-gap HgCdTe.
Plasma points and radiative collapse in vacuum sparks
Authors: Koshelev K. N. , Pereira N. R.
Published: July 2002

This review discusses the intense x-ray emitting regions, called plasma points, that appear in low-inductance vacuum sparks and other high-current discharges. Accurate x-ray spectroscopy indicates the existence of two types of plasma points with different plasma parameters. One type is extremely small (∼microns), dense (∼1023/cm3), and hot (≳1 keV), while the second type is an order of magnitude less extreme. A dynamic model (Vikhrev 1982a) based on radiation cooling with axial outflow of plasma predicts a radiative collapse that is consistent with many features of the plasma points.
Oxide glasses for fast photonic switching: A comparative study
Authors: Lines M. E.
Published: July 2002

New developments involving nonlinear optical effects are conditioned by the improvement of pertinent figures of merit involving the characteristics of nonlinear optical materials within the ‘‘transparency’’ regime of their optic windows. This paper discusses, from a global standpoint, the maximum values of nonlinear refractive index n2, and of its ratio n2/α relative to intrinsic attenuation α, attainable for a multicomponent oxide glass. It also locates and analyses those classes of multicomponent oxide glass that, from the standpoint of these two measures, appear to be the most promising candidates for use as fast photonic switching elements in the 1–2.5-μm wavelength regime.
Antiferromagnetic ordering in MnSe/ZnSe multilayers
Authors: Kl/osowski P. , Giebul/towicz T. M. , Rhyne J. J. , Samarth N. , Luo H. , Furdyna J. K.
Published: July 2002

We report neutron diffraction studies of zinc-blende strained MnSe/ZnSe superlattices. Such systems are examples of a fcc Heisenberg antiferromagnet of the third kind. The results show clearly long-range order in the MnSe layer plane, and no magnetic coupling across the passive layers. The strain affects the relative domain population. Measurements of the diffracted intensity versus T show that the AF phase transition in the MnSe layers is of second order with TM = 107 ± 5 K.
The classical Preisach model of hysteresis and reversibility
Authors: Mayergoyz I. D.
Published: July 2002

A new form for the classical Preisach model of hysteresis which gives the decomposition of this model into reversible and irreversible components is derived. This form reveals that the classical Preisach model does describe (to a certain extent) reversible properties of hysteresis nonlinearities. It is also shown that the Preisach model accounts for nonzero initial slopes of reversal curves.
Photovoltaic properties of iodine-doped magnesium tetraphenylporphyrin sandwich cells. II. Properties of illuminated cells
Authors: Nevin W. A. , Chamberlain G. A.
Published: July 2002

The photovoltaic characteristics of optimized iodine-doped thin-film sandwich cells of the structure Al/Al2O3/magnesium tetraphenylporphyrin/Au, fabricated and tested under high vacuum, are reported. The action spectra of the short-circuit photocurrent show that only light absorbed near the Al-Al2O3 electrode, to a depth of ∼70 nm in the porphyrin layer, is effective in producing charge carriers, consistent with the presence of a depletion region at this contact. Comparison of observed and modeled action spectra indicate an underlying enhancement of carrier generation which increases with increasing excitation energy above ∼2 eV. The photovoltaic efficiency increases with decreasing porphyrin thickness between 200 and 100 nm. Maximum values of ηwh=1×10−3 %, η′ε=7×10−2 % and φε=4.3 % are obtained for a 100-nm thick film. For the thinner cells, the photocurrent varies linearly with light intensity up to 106 mW/cm2, indicating that little recombination of charge carriers occurs during transport to the electrodes. The porphyrin films degrade rapidly in the presence of oxygen/water vapor; however, the stability of the films and devices is good under vacuum. Possible mechanisms of charge-carrier generation are discussed.
Polycrystalline silicon carbide films deposited by low-power radio-frequency plasma decomposition of SiF4-CF4-H2 gas mixtures
Authors: Ganguly Gautam , De Subal C. , Ray Swati , Barua A. K.
Published: July 2002

Polycrystalline silicon carbide thin films have been deposited on amorphous substrates by radio-frequency plasma-assisted decomposition of tetrafluoro silane, tetrafluoro methane, and hydrogen gas mixtures using low-power density and deposition temperatures. The material is shown to possess the α-SiC structure using transmission electron microscopy. It has highly visible transmittance and exhibits bands due to silicon carbide as well as fluorine bonded to carbon and silicon in the infrared transmission spectra. It is easily doped, both types showing high conductivity (∼10 S/cm) and Hall mobility [∼10 cm2/(V s)] for either carrier type. The conductivity is seen to be independent of thickness down to ∼10 nm when deposited on glass. This behavior and the dependence of both structural and electronic properties on deposition parameters is discussed in terms of the chemical reactions in gas phase and on the growth surface.
Diffusion of boron in silicon during post-implantation annealing
Authors: Solmi S. , Baruffaldi F. , Canteri R.
Published: July 2002

The diffusion of B implanted in Si has been investigated at different concentrations in a wide range of experimental conditions (temperature from 800 to 1000 °C and time from 10 s to 8 h) by using furnace and rapid thermal treatments. In particular, the transient enhanced diffusion induced by the implantation damage in the early phase of the annealing and the precipitation occurring in concomitance with the diffusion for dopant concentration exceeding the solid solubility have been extensively analyzed. A simulation program taking these phenomena into account has been developed by modifying the supreme iii code. A satisfactory agreement with experimental data has been obtained for all the investigated conditions. The model represents a significative improvement of the diffusion simulation of B implanted in crystalline Si. In fact, the more commonly used codes of process simulation do not evaluate adequately the effects of the above considered phenomena.
Optical characterizations of (111) oriented InGaAs/InAlAs strained quantum wells grown on InP substrates
Authors: Nishi Kenichi , Anan Takayoshi
Published: July 2002

Strained InGaAs/InAlAs quantum wells with compressively or tensilely strained InGaAs layers were successfully grown on slightly misoriented (111)B InP substrates. Optical properties were systematically studied from comparisons of theoretical and experimental data. The transmission spectra measured on both the (111) and the (100) oriented strained quantum wells showed excitonic peaks even at room temperature, which revealed the high quality of these quantum wells. The peak positions for the (111) oriented quantum wells corresponded well to the calculated excitonic transition energies only when growth orientation dependence of material parameters was taken into account. In the photoluminescence measurements with various excitation intensity, it was found that only the peak from compressively strained (111) InGaAs quantum wells blue-shifted as the excitation intensity increased. This implies the screening of the internal electric field, that is a piezo-electric field, by photogenerated carriers and a consequent decrease in the quantum confined Stark shift. These results provide evidence for the existence of a built-in electric field in the strained InGaAs/InAlAs quantum wells. These strained quantum wells grown along the (111) direction are found to be promising for the realization of optical bistable devices and optical switches in the long wavelength region by the use of the internal electric field.
HgCdTe dual-band infrared photodiodes grown by molecular beam epitaxy
Authors: Arias J. M. , Zandian M. , Williams G. M. , Blazejewski E. R. , DeWames R. E. , Pasko J. G.
Published: July 2002

We report the demonstration of an all molecular beam epitaxy HgCdTe bias-selectable dual-band infrared photodetector. The mesa device, an n-p-n three layer HgCdTe heterostructure, was in situ doped with arsenic and indium for p- and n-type doping. The device design is similar to a heterostructure floating base transistor. The feasibility of the two-color bias-switchable detector was demonstrated by obtaining backside illuminated spectrally pure dual-band detection at 77 K. Wavelength cutoff (λco) selection to 5.2 μm with 60% quantum efficiency (QE) was obtained by applying a negative bias of −250 meV, and to λco=7.9 μm with 36% QE by applying a positive bias of 250 meV. The current-voltage characteristics of this device can be described in terms of a simple back-to-back diode model.
The ammonia sensitivity of Pt/GaAs Schottky barrier diodes
Authors: Lechuga L. M. , Calle A. , Golmayo D. , Briones F.
Published: July 2002

In this work a new ammonia gas sensor is presented. Schottky barrier devices, Pt/n-GaAs, with discontinuous platinum films are sensitive detectors of ammonia gas in a wide temperature range (from room temperature to 150 °C). The effect of parameters such as operation temperature, thickness of the catalytic metal film, background atmosphere, and applied voltage bias on the device sensitivity are investigated. In addition, significant characteristics have been examined including sensitivity limits, linearity of response, and adsorption and desorption kinetics.
Microstructural and magnetic studies on Pr-Fe-B-Cu alloys
Authors: Kwon H. W. , Bowen P. , Harris I. R.
Published: July 2002

A detailed study on the microstructure and magnetic properties of alloys Pr20.5Fe74.0B3.5Cu2.0(A) and Pr17.0Fe76.5B5.0Cu1.5(B) has been carried out as a precursor to a study of the effects of high-temperature mechanical deformation on these materials. In the cast condition, the microstructure of alloy A consisted of three phases in the cast condition: Pr2Fe14B matrix phase, Pr-rich grain-boundary phase, and free Fe inside the matrix phase. The microstructure of alloy B in the cast state consisted of four phases: the same three phases as those observed in alloy A together with the PrFe4B4 phase. On annealing alloy A, an extra phase with needlelike or irregular form was observed in the grain-boundary regions. EDX (energy dispersive x-ray spectrometer)/WDX (wavelength dispersive x-ray spectrometer) analysis indicated that this phase is the Pr(Fe0.94Cu0.06)2 phase. In alloy B, the microstructures were not altered radically by annealing at high temperature except for the disappearance of the free iron. Alloy A exhibited much better permanent magnetic properties than those of alloy B, and this was attributed to grain size and improved magnetic isolation of the individual grains. The effects of low-temperature annealing on the magnetic properties at 500 °C have also been investigated.
Magnetic and crystallographic properties of novel Fe-rich rare-earth nitrides of the type RTiFe11N1−δ (invited)
Authors: Yang Ying-chang , Zhang Xiao-dong , Ge Sen-lin , Pan Qi , Kong Lin-shu , Li Hailin , Yang Ji-lian , Zhang Bai-sheng , Ding Yong-fan , Ye Chun-tang
Published: July 2002

We succeed in inserting a number of nitrogen atoms into the RTiFe11 intermetallics. The nitrides retain the ThMn12-type structure, but with an increase in the unit cell volume. The crystallographic sites located by nitrogen atoms are determined by using neutron diffraction techniques. The nitrogen atoms are found to have an effect of increasing Curie temperature and saturation magnetization. Moreover, an essential change in magnetocrystalline anisotropy is observed upon nitrogenation. By all of these effects, the NdTiFe11N1−δ compounds have excellent intrinsic magnetic properties favorable for permanent magnet applications.
Elimination of misfit dislocations in Si1−xGex/Si heterostructures by limited-area molecular-beam epitaxial growth
Authors: Nishida A. , Nakagawa K. , Murakami E. , Miyao M.
Published: July 2002

Limited-area molecular-beam epitaxial growth of Si1−xGex films, on a Si substrate with patterned SiO2, has been studied in order to eliminate misfit dislocations in Si1−xGex/Si heterostructures. This method is found to dramatically reduce misfit dislocations in Si1−xGex films. Hence, a thicker Si1−xGex film can be grown, without introducing misfit dislocations, on a Si substrate with patterned SiO2 than on a Si substrate without patterned SiO2. This phenomenon is attributed to the blocking of misfit dislocation extension and to a partial relaxation of the residual strain in the boundary between the crystal and polycrystalline Si1−xGex film.
Photothermal deflection measurements for monitoring heat transfer during modulated laser heating of solids
Authors: Shannon Mark A. , Rostami Ali A. , Russo Richard E.
Published: July 2002

Photothermal deflection (PTD) techniques have been used to monitor various laser-heating processes, including melt, vaporization, and ablation of solids. To interpret the complex signal response resulting from transient phase changes at a surface, the temporal profile of the PTD signal response must be considered. In doing so, the case of the linear heating of a target without phase change is first studied here. Numerical and experimental work is presented to show the effect on the shape, magnitude, and phase of a PTD signal due to changes in (1) the thermophysical properties of the target material and deflecting medium, (2) the dimensions and boundary conditions of the target, (3) the distance of the probe beam from the surface of the target, and (4) the modulation frequency of the heating source. Copper and lead target materials heated in air are used in the experimental work. The PTD signals show qualitative agreement with the temperature gradient normal to the surface calculated using a numerical finite-difference two-dimensional thermal-diffusion model. The results also show that an unusual phenomenon occurs when heating with a laser or other finite-sized heating source. When the thermal diffusivity of the target and deflecting medium are different and the probe beam is close to the surface, a local maximum is observed in the time-response profile of the PTD signal during the heating cycle. The maximum occurs as a result of asymmetric changes in the temperature field over time. The shape of the PTD signal, therefore, can provide information about the laser-heating process at a surface in real time.
Deposition of CaF2 and GaF3 on sulfur passivated GaAs(111)A, 100, and (111)B surfaces
Authors: Scimeca T. , Muramatsu Y. , Oshima M. , Oigawa H. , Nannichi Y. , Ohno T.
Published: July 2002

The interfacial chemical bonding has been investigated for GaF3 and CaF2 deposited on S passivated GaAs(111)A, 100, and (111)B surfaces. Synchrotron radiation excited photoelectron spectroscopy S 2p measurements indicate greater S surface segregation for the Ga terminated (111) A and (100) surfaces relative to the As terminated (111)B surface. This S surface segregation correlates well with the Ga—S bond strength as well as the S—Ga coordination number for the three different GaAs surfaces. X-ray photoelectron spectroscopy measurements indicate S may act as a catalyst in the reduction of Ca2+ to metallic Ca at the CaF2–S/GaAs(111)A interface. In contrast to island growth observed for the CaF2/S/GaAs system, more uniform growth is observed for the GaF3/S/GaAs system. In addition, in contrast to the CaF2/S/GaAs system, band bending is considerably reduced for all surfaces upon deposition and annealing for the GaF3/S/GaAs system.
Study of the structure of radio frequency glow discharges in CH4 and H2 by spatiotemporal optical emission spectroscopy
Authors: Tochikubo Fumiyoshi , Makabe Toshiaki , Kakuta Shigeru , Suzuki Akira
Published: July 2002

Radio frequency discharges at 13.56 MHz in CH4(100%), CH4(10%)/H2, and H2(100%) are investigated by time- and space-resolved optical emission spectroscopy. The spatiotemporal net-excitation profile of H(n=3) is presented and discussed. The absolute value of the net-excitation rate, the emission intensity, and the density of the optically allowed electronic excited species are also measured. The self-quenching rate of H(n=3) and CH(A) by H2 and CH4 are estimated and compared with the previous works. The importance of the quenching of CH(A) by atomic hydrogen is also discussed. Excitation by very high-energy electrons is shown from the difference of the emission profile between Hα and H2(d3Πu→a3Σg+) in H2 at 100 kHz.
Annealing studies of low-temperature-grown GaAs:Be
Authors: Bliss D. E. , Walukiewicz W. , Ager J. W. , Haller E. E. , Chan K. T. , Tanigawa S.
Published: July 2002

The isochronal and isothermal annealing characteristics of acceptor-doped GaAs:Be grown at low substrate temperatures (300 °C) by molecular-beam epitaxy (LTMBE) have been studied. The Be was introduced in a range of concentrations from 1016 to 1019 cm−3. Electrical measurements of as-grown material up to the highest Be concentration of 1019 cm−3 show that no free holes are contributed to the valence band even though Raman spectroscopy of the Be local vibrational mode indicates that the majority of the Be impurities occupy substitutional sites. It is proposed that Be acceptors are rendered inactive by the high concentration of AsGa-related native donor defects present in LTMBE material. The concentration of AsGa-related defects in the neutral charge state was estimated from infrared absorption measurements to be as high as 3×1019 cm−3. A distinct annealing stage at 500 °C, similar to that found in irradiation-damaged and plastically deformed GaAs, marks a rapid decrease in the concentration of AsGa-related defects. A second annealing stage near 800 °C corresponds to the activation of Be acceptors. The presence of gallium vacancies VGa was investigated by slow positron annihilation. Results indicate an excess concentration of VGa in LTMBE layers over bulk-grown crystals. Analysis of isothermal annealing kinetics for the removal of AsGa-related defects gives an activation energy of 1.7±0.3 eV. The defect removal mechanism is modeled with VGa-assisted diffusion of AsGa to As precipitates.
CdS/CdSe intrinsic Stark superlattices
Authors: Halsall M. P. , Nicholls J. E. , Davies J. J. , Cockayne B. , Wright P. J.
Published: July 2002

Strained layer superlattices of wurtzite CdS/CdSe have been grown on (111)A GaAs substrates by metalorganic chemical vapor deposition and their optical properties studied by photoluminescence spectroscopy. It is shown that the superlattice layers contain giant strain-induced piezoelectric fields exceeding 2×108 V m−1. These fields are similar to those reported for (111) orientated III–V superlattices, but an order of magnitude greater. The recombination energies from a series of samples provide evidence for a type II conduction band offset of 0.23±0.10 eV (the electron wells being in the CdS), with the band structure heavily modified by the internal electric fields. In addition, the photoluminescence peak emission energy shows a strong dependence on the excitation power. This is interpreted as further evidence for the effect of internal fields. We conclude that this system shows new effects not previously observed in II–VI compound superlattices. The large band-gap tunability and the space-charge effects offer possibilities for all-optical switching devices in the 700–1300-nm region of the spectrum.
Low-temperature transport characteristics of AlGaAs-GaAs in-plane-gated wires
Authors: Hirayama Y. , Wieck A. D. , Ploog K.
Published: July 2002

The transport characteristics of in-plane-gated wires, in which the conducting two-dimensional electron gas (2DEG) channel and the 2DEG gates are separated by focused Ga-ion-beam scanned lines, are studied at low temperature (1.5 K). They are understood as a combination of the normal field-effect-transistor (FET) characteristics and a peculiar resistance jump at the channel pinch-off threshold. In the normal FET region, the depletion region spreading is gradually controlled by a gate voltage applied to the in-plane gates. The channel conductance variation by the gate voltage is explained by the change of the effective channel width rather than by the change of the carrier density. The variable range of the effective wire width is equal to or less than 0.6 μm in the experiments. In spite of this small controllable value, the channel can be pinched off up to W=10 μm with a gate leakage current of less than 1 nA. The pinch off of the wide wires always occurs together with a resistance jump at the threshold. These characteristics are explained by the drastic extension of the depletion region for the case that a small gate leakage occurs through the AlGaAs (Si) layer but not through the two-dimensional electron gas at the heterointerface. This resistance jump produces an interesting negative drain conductance (drain current/drain voltage) in the drain-voltage–drain-current characteristics.
Kinetic analysis of C49-TiSi2 and C54-TiSi2 formation at rapid thermal annealing rates
Authors: Clevenger L. A. , Harper J. M. E. , Cabral C. , Nobili C. , Ottaviani G. , Mann R.
Published: July 2002

We have used in situ resistance versus temperature measurements to demonstrate that a 60 nm titanium thin film on polycrystalline silicon heated at rates up to 3000 °C/min always forms high-resistivity base-centered orthorhombic C49-TiSi2 before the low-resistivity face-centered orthorhombic C54-TiSi2 phase. Kinetic analysis of the shift in transformation temperatures with heating rate indicates that the activation energies for the formation of C49-TiSi2 and C54-TiSi2 are 2.1±0.2 and 3.8±0.5 eV, respectively, when formed during the same annealing cycle. The higher activation energy of formation of C54-TiSi2 as compared to C49-TiSi2 suggests that under very high heating rates and annealing temperatures, the formation of C49-TiSi2 before C54-TiSi2 might be completely or partially bypassed.
Multiplication of misfit dislocations in epitaxial layers
Authors: Beanland R.
Published: July 2002

The conditions for operation of two multiplication mechanisms in strained layers are analyzed. These are spiral and Frank–Read type sources. It is shown that a minimum thickness is required for their operation, of the order of four times the critical thickness for the bending over of threading dislocations. The implications for strain relief of mismatched layers are discussed.
The extraordinary Hall effect in coherent epitaxial τ (Mn,Ni)Al thin films on GaAs
Authors: Sands T. , De Boeck J. , Harbison J. P. , Scherer A. , Gilchrist H. L. , Cheeks T. L. , Miceli P. F , Ramesh R. , Keramidas V. G.
Published: July 2002

Ultrathin coherent epitaxial films of ferromagnetic τ(Mn,Ni)0.60Al0.40 have been grown by molecular beam epitaxy on GaAs substrates. X-ray scattering and cross-sectional transmission electron microscopy measurements confirm that the c axis of the tetragonal τ unit cell is aligned normal to the (001) GaAs substrate. Measurements of the extraordinary Hall effect (EHE) show that the films are perpendicularly magnetized, exhibiting EHE resistivities saturating in the range of 3.3–7.1 μΩ-cm at room temperature. These values of EHE resistivity correspond to signals as large as +7 and −7 mV for the two magnetic states of the film with a measurement current of 1 mA. Switching between the two magnetic states is found to occur at distinct field values that depend on the previously applied maximum field. These observations suggest that the films are magnetically uniform. As such, τ(Mn,Ni)Al films may be an excellent medium for high-density storage of binary information.
Magnetic dead layer in Fe/Si multilayer: Profile refinement of polarized neutron reflectivity data
Authors: Ankner J. F. , Majkrzak C. F. , Homma H.
Published: July 2002

We have used polarized neutron reflectometry to study the magnetic structure of an Fe/Si multilayer film. By simultaneous refinement of both plus and minus reflectivities we have extracted separate nuclear and magnetic scattering density profiles that include a 6-Å-thick magnetically dead layer in Fe at the interface. This result supports the contention that the antiferromagnetic coupling reported in this system is mediated by the presence of Fe in the Si interlayers.
Wavelength dependence of the Faraday effect and magnetobirefringence in ferrofluid thin films
Authors: Pan Y. T. , Du C. W. , Liu X. D. , Li Z. G. , Birngruber R.
Published: July 2002

An enhancement of negative Faraday rotation in a dilute ferrofluid was reported near the wavelength of 0.5 μm by Yusuf. The observed phenomenon was explained in terms of a ‘‘small particle resonance.’’ The premise of the above explanation is debatable. In order to give a more detailed explanation, measurements of Faraday rotation and magnetobirefringence spectra in dense ferrofluid thin films are reported from 0.5 to 2.7 μm, and a mathematical model is derived taking into account a skew-symmetric dielectric tensor for ferrite particles. The experiments confirm that the negative Faraday rotation is predominantly caused by the wavelength dependence of the off-diagonal elements εfxy(λ) of the dielectric tensor of the magnetic particles themselves.
Pulsed plasma-pulsed injection sources for remote plasma activated chemical vapor deposition
Authors: Kushner Mark J.
Published: July 2002

Remote plasma activated chemical vapor deposition (RPACVD) is an attractive fabrication technique owing to the increased selectivity of radical generation which can be obtained compared to deposition techniques in which the substrate is immersed in the plasma. This selectivity can be compromised if the deposition gases, which are typically injected downstream of the plasma zone, back-diffuse into the plasma where indiscriminate electron impact dissociation occurs. In this communication, a new RPACVD technique is described in which the plasma and injected gases are sequentially pulsed to temporally isolate the injected gases from the plasma. This method reduces, or eliminates, indiscriminate dissociation of the injected gases and improves the selectivity of radical fluxes to the substrate.
High-temperature photorefractive effects in LiNbO3:Fe
Authors: Carrascosa M. , Arizmendi L.
Published: July 2002

Photorefractive effect has been measured in LiNbO3:Fe at high temperature in the range 180–300 °C. At temperatures higher than 230 °C only transient effects can be observed. The kinetics of the diffraction efficiency is explained in terms of two kinds of mobile charges; electrons and protons. From the analysis of the final decay of the curves, an activation energy of 1.33±0.02 eV is obtained for the diffusion of protons in lithium niobate.
Characteristics of the complete moving hysteresis model
Authors: Vajda Ferenc , Della Torre Edward
Published: July 2002

Moving-type Preisach-derived models computing both the irreversible and the locally reversible components of the magnetization accurately characterize magnetic recording materials exhibiting hysteresis. Experimental results show that the derivative of the major remanent loop is not a symmetrical function of the applied field. It is shown that the complete moving hysteresis model can compute such an asymmetrical variation with a symmetrical Preisach function. It is also shown that this asymmetry is due to the reversible magnetization. Additional properties of moving-type models are also discussed.
Giant magnetoresistance in heterogeneous Cu–Co and Ag–Co alloy films (invited)
Authors: Berkowitz A. E. , Mitchell J. R. , Carey M. J. , Young A. P. , Rao D. , Starr A. , Zhang S. , Spada F. E. , Parker F. T. , Hutten A. , Thomas G.
Published: July 2002

Giant magnetoresistance in sputtered single films of Cu–Co and Ag–Co heterogeneous alloys is discussed. The films consist of Co-rich precipitates in a nonferromagnetic matrix. The Ag–Co films have higher Δρ/ρ and Δρ values than the Cu–Co films, possibly due to less Co dissolved in the Ag matrix. Δρ scales inversely with precipitate particle size, implying that Co-rich clusters ≤20 Å diameter may be most effective for spin dependent scattering. This trend of the data and a phenomenological model suggest that interfacial spin dependent scattering is significantly stronger than the scattering within the Co-rich particles.
The spectral distribution of the intrinsic radiative recombination in silicon
Authors: Ruff M. , Fick M. , Lindner R. , Rössler U. , Helbig R.
Published: July 2002

Microscopic calculations of the intrinsic radiative recombination probability B(ω,T) of Si have been performed to obtain the absolute values of the spectral distribution and of its temperature dependence. In these calculations we use the concepts of k⋅p theory, consider excitonic effects, and take into account the electron-phonon interaction. The calculated spectra are compared with measured absolute values of the intrinsic radiative recombination spectra obtained from forward biased Si p-i-n diodes and also with spectra obtained from the detailed balance theory of van Roosbroeck and Shockley [W. van Roosbroeck and W. Shockley, Phys. Rev. 94, 1558 (1954)]. Quantitative agreement is obtained for higher temperatures (about 300 K) and deviations for lower temperatures are critically discussed.