bluuebunny/crossref_metadata_2025 huggingface dataset:
Multiple-phonon relaxation in GaAs-AlGaAs quantum well dots
Authors: Wang P. D. , Sotomayor Torres C. M.
Published: July 2002

We report resonant Raman scattering (RRS) and ‘‘hot’’ exciton luminescence (HEL) from dry etched GaAs-AlGaAs quantum dots in which quantum confinement effects have been found with decreasing dot sizes. Up to fourth-order multiphonon processes have also been observed for the first time via photoluminescence and photoluminescence excitation in quantum dots. These results are direct evidence of the bottleneck effect. Hot carriers are maintained due to the slowed-down electron-phonon scattering in one- and zero-dimensional semiconductors. The competition between resonant relaxation via LO phonons and sidewall nonradiative recombination results in stronger multiphonon processes with increasing quantum confinement. Furthermore, lateral patterning of quantum wells strongly localizes excitons in three-dimensional quantum confinement regime. The observation of RRS and HEL up to high temperature demonstrates the high stability of localized excitons.
Improved bounds on the conductivity of composites by translation in a variational principle
Authors: Helsing Johan
Published: July 2002

The problem of combining different methods of deriving bounds on the conductivity of composite materials is addressed. We combine the translation method and the Hashin-Shtrikman variational principle and apply the combined method to a composite where the two methods, separately, give bounds that partly improve on each other; an equiaxed polycrystal. As a result we get a lower bound which improves on all previously known bounds for this composite.
Structure and magnetic properties of sputtered thin films of Fe0.79Ge0.21
Authors: Hamdeh H. H. , Oliver S. A. , Fultz B. , Gao Z. Q.
Published: July 2002

Films of Fe0.79Ge0.21 with thicknesses of 300 nm were synthesized by ion beam sputtering, and were annealed at temperatures from 200 to 550 °C. The materials were characterized by x-ray diffractometry, Mössbauer spectrometry, vibrating sample magnetometry, ferromagnetic resonance spectrometry, and electrical resistivity measurements. The as-prepared materials comprised chemically disordered bcc crystallites of sizes less than 20 nm, and were found to have a distribution of internal strains. Upon annealing at temperatures of 250 °C and below, there occurred strain relaxation, some evolution of short range chemical order, and an improvement in soft magnetic properties. The coercive field was a minimum for the sample annealed at 250 °C. Crystallite growth occurred at higher annealing temperatures, accompanied by a transition in several measured parameters from those of ultrafine grained materials to those typical of polycrystalline materials. This trend can be explained with the random anisotropy model. Mössbauer and magnetization measurements indicated that the Ge atoms behave as magnetic holes. The 57Fe hyperfine magnetic field distribution, and its change during chemical ordering, can be calculated approximately with a model of magnetic response. The large local isomer shifts at 57Fe atoms near Ge atoms suggest that a local depletion of 4s conduction electron density should be incorporated into the model.
Fine silicon oxide particles in rf hollow magnetron discharges
Authors: Chu J. H. , I Lin
Published: July 2002

The formation and the properties of fine silicon oxide particles in a hollow post-type rf magnetron discharge in SiH4/O2/Ar gas mixtures were studied. For P≳30 mTorr, primary fine particles (PFPs) with a diameter of about 20 nm are formed through homogeneous reactions. Their diameter increases with the system pressure. PFPs with sufficient negative charge are suspended in the plasma. They can further aggregate with other PFPs to form aggregated fine particles (AFP) with nearly spherical shape and larger diameter. The size of the AFP depends on the duration of the rf power. In the cw mode, AFPs gradually drift axially to both ends of the discharge system. The accumulation of AFPs at the end trap causes low-frequency oscillation of the discharge. The films consisting of fine particles are loose and brittle. Infrared-absorption spectrum shows that oxide formed in the homogeneous reaction has similar Si—O bond strain relaxation to that of the thermal oxides and the annealed oxide from chemical-vapor deposition.
Phase transitions and epitaxial Ni3Si formation on Ni(100) after high dose silicon ion implantation
Authors: Rao Z. , Williams J. S. , Sood D. K.
Published: July 2002

Structural changes and phase transitions during high dose Si implantation into single crystal Ni(100) and subsequent annealing at temperatures up to 800 °C have been characterized using Rutherford backscattering spectrometry and channeling. A complete amorphous layer is produced after implantation to doses above about 4.5×1017 Si/cm2. The recrystallization of the amorphous layer during isochronal annealing leads to the formation of various Ni-rich silicides at different temperatures. Ion channeling suggests that an epitaxial Ni3Si layer on the Ni(100) substrate can form at a temperature of 400 °C. Further annealing at higher temperatures up to 600 °C improves the epitaxial quality, as indicated by a channeling minimum yield χmin of 8% in the Ni3Si layer. Higher temperature annealing at 700 °C and above results in the dissolution of the Ni3Si layer into the Ni substrate.
Low-angle x-ray diffraction with in situ annealing: Application to W/Cu multilayers
Authors: Chiang Wen-C. , Baxter David V.
Published: July 2002

A high-temperature x-ray-diffractometer stage specifically designed for low-angle work has been built. With careful alignment, the stage provides resolution sufficient to probe temperature-induced structural changes in multilayers. A series of studies using the stage was performed on W/Cu multilayers with modulation wavelengths between 65 and 110 Å over the temperature range 25–180 °C. Reversible, temperature-dependent changes in the modulation wavelength, and an irreversible increase in the intensity of the scattered radiation along both specular and nonspecular directions in reciprocal space, have been observed. The observed thermal-expansion coefficients in the growth direction are marginally greater than would be expected from bulk behavior even when interactions with the Si substrate are taken into account. Analysis of the intensity changes indicates that, upon annealing, the interfaces within these multilayers become more rough, but that this roughness comes from a thermally induced buckling of the film. This results in a mosaic texture and increased reflectivity with a concomitant increase in diffuse scattering.
Influence of bismuth as a surfactant on the growth of germanium on silicon
Authors: Kawano Akira , Konomi Ichiro , Azuma Hirozumi , Hioki Tatsumi , Noda Shoji
Published: July 2002

The influence of a Bi surfactant layer on the growth of Ge on Si(100) substrates was investigated by using a medium-energy ion-backscattering spectrometer and a transmission electron microscope. A monolayer of Bi predeposited on the Si substrates suppressed islanding in the subsequent molecular-beam-epitaxial growth of Ge. The Bi atoms moved on top of the Ge film during the growth process.
Ultrafine copper particles grown in a glass ceramic
Authors: Roy B. , Chakravorty D.
Published: July 2002

Copper particles having diameters in the range 4.2–7.2 nm have been grown within a glass ceramic by subjecting the latter to an alkali/copper ion exchange reaction followed by a reduction treatment in hydrogen. A wide range of surface resistivity has been induced in the glass-ceramic samples by a change of the heat treatment schedule. The dc electrical resistance has been measured over the temperature range 80–300 K. These data when analyzed by Ziman’s equation show that the effective Debye temperature decreases as the metal particle size is reduced.
Influence of annealing on magnetic properties of a Ge-doped garnet film
Authors: Stashkevich A. A. , Gouzerh J.
Published: July 2002

An investigation of the influence of annealing (furnace annealing in an O2 atmosphere and local laser annealing) on the major magnetic properties of an Y1.5Sm0.3Ca0.9Lu0.3Fe4.1Ge0.9O12 epitaxial single-crystal bubble garnet film has been carried out. After furnace annealing a Ge-substituted film has shown a considerable drop of the saturation magnetization 4πMs and of the constant of the uniaxial anisotropy Ku. The kinetics of redistribution of Ge cations between the octa- and tetrahedral sites turned out to be comparatively slow, the activation energy Ea amounting to 6.2 eV. Laser annealing caused a small increase of the collapse field Hc, which can be explained by slight randomization of the distribution of Y-Sm ions in the dodecahedral sublattice, while the Ge cations governing 4πMs remain on their sites.
Effect of plasma treatment on the triboelectric properties of polymer powders
Authors: Kodama J. , Foerch R. , McIntyre N. S. , Castle G. S. P.
Published: July 2002

Surfaces of fine polystyrene (PS) and polymethyl methacrylate (PMMA) powders were modified by exposure to the downstream products of a nitrogen or oxygen microwave plasma. The effects of nitrogen and oxygen incorporation in the powder surface were studied with emphasis on variations in the triboelectric properties of the powder. X-ray photoelectron spectroscopy was utilized to determine the changes in surface elemental composition. After nitrogen plasma treatment, the C 1s peak shapes suggested the formation of amines in the case of PS, and the formation of imines and amides in the case of PMMA. Oxygen plasma treatment appears to result in the formation of hydroxyl and carbonyl groups on the surfaces of both PS and PMMA. After treatment with a nitrogen or oxygen plasma, the charge-to-mass ratio (Q/M) of PS and PMMA powders in contact with carrier particles was measured using the cage blowoff method. The surface charge density (Q/A) was calculated from Q/M. The Q/A of nitrogen plasma-treated PS powder was seen to shift towards positive charge with small increases in the nitrogen concentration. The Q/A of oxygen plasma-treated PS powder initially shifted toward negative charge, but changed towards positive charge with higher oxygen concentrations. Plasma-treated PMMA powder showed a different behavior and the variation of Q/A on PMMA was much less than that of PS. Results suggest that triboelectrification of the polymer powder may be related to changes in the electrical surface states, and that nitrogen may act as a group V modifier within the PS surface.
Disordering induced by impurity diffusion in ZnSe-based superlattices and optical waveguides fabricated by disordering
Authors: Yokogawa T. , Floyd P. D. , Merz J. L. , Luo H. , Furdyna J. K.
Published: July 2002

The phenomenon of layer disordering in CdZnSe/ZnSe and ZnSe/ZnS strained layer superlattices (SLSs) by Ge diffusion and the fabrication of ZnSe-based optical waveguides using the Ge-induced layer disordering in the superlattices has been studied. Both the as-grown sample and the sample annealed without a Ge layer showed several orders of well-resolved double crystal x-ray satellite peaks due to SLS periodic structure. However, the satellite peaks completely disappeared in the Ge-diffused sample, indicating that the SLS structure was disordered by the Ge diffusion and not caused by the annealing process. Photoluminescence (PL) measurements at 1.4 K of both the as-grown and the annealed samples without Ge diffusion show identical, sharp excitonic emission around 483 and 420 nm in CdZnSe/ZnSe SLS and ZnSe/ZnS SLS, respectively. After Ge diffusion, the PL peaks shift to higher energy, confirming the layer disordering of the SLS. The optically guided mode in the SLS guiding layer confined by the disordered alloy was confirmed, and a propagation loss α as low as 0.96 cm−1 was obtained.
Microstructural dependence of annealing temperature in magnetron-sputtered Al–Si–Cu films
Authors: Liang Ming-Kaan , Ling Yong-Chien
Published: July 2002

The effect of sputtering temperature, sputtering bias, and annealing temperature upon the sheet resistance, WO3 formation at the Al–Si–Cu/Ti–W interface, and diffraction intensity of the Al2Cu precipitates of magnetron-sputtered Al–Si–Cu films were investigated. Statistical methods and microcharacterization techniques were applied to study these effects. Statistical analysis verifies the effect of annealing temperature on the measured sheet resistance. Annealing temperature alone is the dominant factor upon the WO3 formation at the Al–Si–Cu/Ti–W interface and the Al2Cu (211) plane diffraction intensity. Annealed samples are of higher sheet resistance. Increase in sheet resistance is ascribed to the formation of interfacial WO3. Reduced electromigration is related to the formation of Al2Cu precipitates. Secondary ion mass spectrometry (SIMS) analysis of the as-deposited sample depicts the presence of an excess amount of oxygen atoms at the surface and the Al–Si–Cu/Ti–W and Ti–W/Ti interfaces. Rutherford backscattering spectrometry and SIMS analyses reveal the outdiffusion of W from the Ti–W layer toward the Al–Si–Cu layer, the presence of Si nodules at the Al–Si–Cu/Ti–W interface, and the formation of Ti silicides at the Ti/Si interface. These phenomena are confirmed by transmission electron microscopy, energy dispersive x-ray analysis, and scanning electron microscopy analyses. It is concluded that interfacial oxygen, which reacts with W to form WO3 upon annealing, warrants further reduction to yield films of better sheet resistance.
The evolution of growth stresses in chemical vapor deposited tungsten films studied by in situ wafer curvature measurements
Authors: Leusink G. J. , Oosterlaken T. G. M. , Janssen G. C. A. M. , Radelaar S.
Published: July 2002

An in situ study of the evolution of the biaxial state of intrinsic stress during nucleation and growth of polycrystalline tungsten chemical vapor deposition films deposited by the hydrogen reduction of tungsten hexafluoride is presented. The evolution of biaxial stress was determined from in situ wafer curvature measurements. It is shown that the intrinsic stress is a growth stress, i.e., a stress developing in close vicinity to the advancing surface of the film due to metastable film growth processes. The stress developing depends strongly on the thickness of the film. High tensile stress (≊4 GPa) is observed during the initial stage of growth, compressive stress (up to −1 GPa) is observed in an intermediate thickness regime after film closure and tensile stress (0.1–1 GPa) is observed in the thick film regime. The associated stress gradients in the film are preserved during and after growth. The development of growth stress is determined by deposition temperature and growth rate. The tensile stress in the thick film regime is larger at a higher growth rate or a lower deposition temperature, while the compressive stress in the intermediate thickness regime showed the opposite dependency. Film properties as the evolution of grain size, impurity content, and resistivity are found not to vary significantly with the growth conditions. Therefore, the development of growth stress is ascribed to kinetical processes. The development of tensile stress in the thick film regime is described with a (kinetic) grain boundary formation and relaxation model. The compressive stress in the intermediate thickness regime is tentatively ascribed to compressive coherency strains induced by interfacial tensions of the grains in the stage of island growth.
Temporal synchronization of independently frequency tunable dual CO2 laser pulses
Authors: Oman D. M. , Sakthivel P. , Mukherjee P.
Published: July 2002

We describe a new method for the simultaneous emission of two CO2 laser pulses from a single transversely excited atmospheric discharge. The transient gains of the pulsed low-pressure sections of two spatially separate hybrid cavities aligned on the discharge were temporally manipulated to allow low-jitter pulse synchronization. Independently frequency tunable dual laser pulses over arbitrarily selected rotational transitions spanning the available P and R branches of the 9 and 10 μm CO2 laser transitions were obtained, overcoming limitations on possible dual frequency combinations that restricted previous techniques. The result of numerical calculations for the hybrid laser, using a four-level rate equation model, supports the observed delay characteristics and provides a physical basis for understanding the dynamics of the synchronization.
Comment on ‘‘Evaluation of the minority carrier diffusion length and edge surface-recombination velocity in GaAs p/n solar cells’’ [J. Appl. Phys. 72, 2919 (1992)]
Authors: Luke Keung L.
Published: July 2002

The recent findings of Hakimzadeh, Möller, and Bailey [J. Appl. Phys. 72, 2919 (1992)] regarding the determination of minority carrier diffusion length from electron-beam-induced current data are compared to the results of an earlier study by Luke, von Roos, and Cheng [J. Appl. Phys. 57, 1978 (1985)]. The differences are briefly discussed.
High-purity intense lithium-ion-beam sources using glow-discharge cleaning techniques
Authors: Struckman C. K. , Kusse B. R.
Published: July 2002

High purity, 1 MV, 50 ns, 300 A/cm2 lithium beams have been produced using an extraction diode driven by Cornell’s Light Ion Accelerator. Both passive and active anodes were studied with the highest-purity beams being generated by active anodes. These anodes were approximately 200 cm2 in area and consisted of 0.1 μm films of Al and LiF evaporated onto a glass substrate. The anode plasma was formed by using a plasma opening switch to divert 50 kA from the main power pulse through the Al foil for 20 ns. Without glow-discharge cleaning, the ion beam produced by the LiF Al anode was over 55% hydrogen and only 20% Li+. In order to remove impurities, especially the hydrogen, the anodes were cleaned with a 110 mA, 33 W, 60 Hz glow discharge. The stoichiometry of the ion beam after discharge cleaning was 65% Li+, 20% Al+2, and 15% H+. The results are presented and a model of the cleaning mechanism is discussed.
Radial optical emission profiles of radio frequency glow discharges
Authors: Pender J. , Buie M. , Vincent T. , Holloway J. , Elta M. , Brake M. L.
Published: July 2002

Radial optical emission profiles are determined from Abel inverted emission spectroscopy of a parallel plate radio frequency system known as a GEC Reference Cell. These profiles in general show a nonuniform plasma, annular in shape. Etching results of silicon wafers also follow this annular pattern. This effect is explained by numerically computed large radial and axial electric fields near the edge of the electrodes, produced by the presence of the grounded dark shields.
Theoretical study on optimum barrier height of GaAs/AlxGa1−xAs multiple quantum well modulator: Inhomogeneous broadening effects
Authors: Lim Kyu Nam , Hong Songcheol , Singh Jasprit
Published: July 2002

The optimum barrier height of GaAs/AlxGa1−xAs multiple quantum well for the static property of the optical modulator is studied including exciton line broadening effects. The exciton binding energy, oscillator strength, and radius are calculated with respect to Al composition x in the barrier. Inhomogeneous line broadening of exciton lines due to interface roughness and alloy disorder are also calculated. Interface roughness broadening increases substantially as x increases while oscillator strength is not changed so much. It is shown that the optimum barrier height is strongly dependent on the dominant broadening mechanism. The optimum barrier height occurs at the lower barrier as the inhomogeneous broadening becomes more dominant.
Reactive ion etching profile and depth characterization using statistical and neural network analysis of light scattering data
Authors: Krukar Richard , Kornblit Avi , Clark Linda A. , Kruskal Joseph , Lambert Diane , Reitman Edward A. , Gottscho Richard A.
Published: July 2002

As device design rules continue to shrink for the manufacturing of integrated circuits, unprecedented challenges for process inspection appear. No longer is optical microscopy adequate for determining if process results meet specifications. On the other hand, the alternative—scanning electron microscopy—is time consuming, destructive, and costly. Another approach is to measure scattered light intensity as a function of scattering angle, as opposed to imaging, to obtain distinct signatures for submicron structures. In this work, a set of Si wafers with photolithographically defined lines and spaces are reactively ion etched. By varying process conditions, a range of depths and sidewall profiles is generated and then inspected by detecting visible scattered laser light over 180°. The resultant scattergrams are then analyzed both by using discriminant analysis and by training a neural network to catalog the microstructures according to depth and profile. We find that this approach is a viable alternative to destructive sampling and off-line inspection by scanning electron microscopy: depth and profile are correctly classified with better than 95% accuracy.
Comparison of interface trap densities measured by the Jenq and charge pumping techniques
Authors: Saks N. S.
Published: July 2002

The validity of the Jenq technique for measuring the average density of interface traps Dit in metal-oxide-semiconductor devices at cryogenic temperatures (78 K) is assessed. Dit has been determined using both the Jenq technique, which is based on measurement of hysteresis in high-frequency capacitance-voltage data, and the more standard charge pumping technique. Excellent agreement (±10%) is found between the two techniques for a wide range of Dit values from 5×1010 to 1×1012 traps/cm2 eV.
Electronic-structure effects on the intermetallic compounds Al3Ti with the addition of the alloying elements Cr or Mn
Authors: Liu Senying , Hu Rongze , Wang Chongyu
Published: July 2002

By use of the self-consistent field Xα scattered-wave method, the electronic structure was calculated for two models of the Ti6Al12X (X=Al, Cr, or Mn) cluster for the tetragonal DO22 crystal strcuture as well as the Ti4Al14X (X=Al, Cr, or Mn) cluster for the cubic L12 crystal structure in Al3Ti-base intermetallic compounds for contrast. The results are presented for density of states and one-electron properties, such as the relative binding tendency between the atom and the model cluster and the hybrid bonding tendency between the atoms introduced by us. Comparisons are made among the three models of the Ti6Al12X cluster, among those of the Ti4Al14X cluster, and between Ti6Al12X and Ti4Al14X clusters. The effect of the Cr or Mn atom on the physical properties of Al3Ti-base intermetallic compounds is analyzed. The calculations indicate that the addition of the Cr or Mn atom weakens the relative binding tendency between the Ti2 atom and the Ti6Al12X cluster and intensifies that between the Ti1 atom and the Ti4Al14X cluster. Thus the addition of the Cr or Mn atom may promote crystal structure transformation from DO22 to L12 in Al3Ti-base intermetallic compounds. Comparison of the density of states between the Ti6Al12X and Ti4Al14X clusters shows that the stable crystal structure for a given compound is the one in which the Fermi level EF lies in or comes near a minimum in the density of states. The calculation also shows that the Cr or Mn atom strengthens the hybrid bonding tendency between the central atom and the host atoms for the Ti6Al12X and Ti4Al14X clusters and thereby may lead to the constriction of the lattice constant of Al3Ti-base intermetallic compounds. The results also show that the addition of the Cr or Mn atom provides more d electrons to strengthen the p-d and d-d directional bondings for the Ti6Al12X and the Ti4Al14X clusters. Thus, Al3Ti-base intermetallic compounds with the addition of the Cr or Mn atom still remain brittle.
Correlation of the structural properties with the device characteristics of hydrogenated amorphous silicon deposited at different substrate temperatures
Authors: Bhattacharyya T. K. , Chaudhuri P. , Banerjee Ratnabali
Published: July 2002

The structural properties of hydrogenated amorphous silicon (a-Si:H) have a profound effect on the characteristics of the devices based on this material. The structural properties of the films were varied by depositing the samples at different substrate temperatures and characterized by secondary neutral mass spectroscopy, hydrogen evolution technique, and photothermal deflection spectroscopy. The results were correlated with the primary photocurrent data of a-Si:H Schottky barrier devices. It was observed that there is an optimum in both the rigidity of network and hydrogen content for the realization of the best film and device properties.
YBa2Cu3O7−δ thin films and microstrip resonators on MgLaAl11O19 substrates
Authors: Xiong G. C. , Lian G. J. , Zhu X. , Li J. , Gan Z. Z. , Jing D. , Shao K. , Guo H. Z.
Published: July 2002

Superconducting YBa2Cu3O7−δ thin films with Tc=89.5 K were deposited on MgLaAl11O19 (11̄0) substrates by pulsed laser deposition. X-ray diffraction patterns indicate that the YBa2Cu3O7−δ films were epitaxial films, with the c axis perpendicular to the substrate surfaces. Microstrip resonators of YBa2Cu3O7−δ thin films deposited on MgLaAl11O19 substrates were fabricated. The loaded quality factor of the resonator was 1007 at 77 K and 4.28 GHz. As a new substrate for high Tc oxide superconducting films, MgLaAl11O19 substrates are especially suitable for superconducting-microwave applications.
Ion-beam-assisted deposition of ferroelectric PbTiO3 films
Authors: Qu B. D. , Zhong W. L. , Wang K. M. , Zhang P. L. , Wang Z. L. , Li W. Z.
Published: July 2002

Ferroelectric PbTiO3 films were deposited by ion-beam-assisted deposition (O2+Ar 75–150 eV). The effects of ion bombardment on the Pb/Ti ratio and the structures of the film are discussed. For a given target-substrate distance and substrate temperature, the Pb/Ti ratio decreased with increasing bombarding beam energy. Compared with the films deposited without ion bombardment, the deposition rate was increased under ion bombardment, which is attributed to an increase in the surface reaction rate. The crystal grains are larger for films deposited under ion bombardment, which implies that ion bombardment enhances the surface mobility of adatoms and hence the growth kinetics of the growing films. Dielectric and ferroelectric properties of the as-deposited films are also reported.
Epitaxial CeO2 buffer layers for YBa2Cu3O7−δ films on sapphire
Authors: Maul M. , Schulte B. , Häussler P. , Frank G. , Steinborn T. , Fuess H. , Adrian H.
Published: July 2002

Epitaxial CeO2 buffer layers and YBa2Cu3O7−δ thin films have been grown in situ on (11̄02) sapphire by electron beam evaporation. Buffer layers of only 20 nm thickness inhibit interdiffusion between YBa2Cu3O7−δ and Al2O3 as determined by depth profiling using x-ray photoelectron spectroscopy. The layers show smooth surfaces and narrow interfaces. High lattice perfection of the CeO2 layer has been shown by x-ray diffraction. Laue oscillations up to ninth order have been observed in thin CeO2 buffer layers on sapphire. We found only one epitaxial orientation with YBa2Cu3O7−δ (001) ∥ CeO2 (001) ∥ Al2O3 (11̄02) and YBa2Cu3O7−δ [110] ∥ CeO2 [100] ∥ Al2O3 [112̄0]. YBa2Cu3O7−δ films grown on these buffer layers reveal Tc=88±0.5 K, ρ(300 K)=380 μΩ cm, and jc(77 K, 0 T)=1.3×106 A/cm2.
A rate equation formalism for electron and phonon dynamics in undoped quantum wells
Authors: Educato J. L. , Leburton J.-P.
Published: July 2002

A theoretical investigation of the electron and phonon dynamics during intersubband transitions in undoped multiple GaAs-AlxGa1−xAs quantum well structures is presented within a rate equation formulation where particle and energy flow equations are derived from Boltzmann’s equation using Fermi statistics. This work focuses upon the role played by quantized polar optical phonons known as slab modes and interface modes. Photoexcited carrier behavior is analyzed in relation with recent Raman measurements and shows time constants for electron relaxation in agreement with experimental data.
Non-steady-state photoelectromotive force in semiconductor crystals with high light absorption
Authors: Korneev N. A. , Stepanov S. I.
Published: July 2002

The influence of a high light absorption on the photoelectromotive force excited by moving interference fringes in photoconductive crystals is considered. It is shown that due to a deep penetration of the electric field pattern into the sample volume, the absolute magnitude of the output signal and its dependence on the spatial frequency of the grating may practically be the same as for low absorption. This conclusion is confirmed experimentally for semi-insulating GaAs:Cr for λ=633, 840–900, and 1064 nm. This means, in particular, that films or thin specimens of photoconductive materials can be used for fabrication of adaptive detectors of phase-modulated optical signals with the same efficiency as bulk specimens.
Shock-induced mechanisms of phase transformation from rhombohedral BN to cubic BN
Authors: Sekine Toshimori , Sato Tadao
Published: July 2002

Rhombohedral forms of layered boron nitride (r-BN) as mixtures with copper powders were shock compressed and quenched from pressures in the range 8–50 GPa. Recovered specimens were investigated by x-ray diffraction and electron microscopy. The graphite-like BN (h-BN) and turbostratic BN (t-BN) phases are observed in the pressure range 8–22 GPa and wurtzite-type BN (w-BN) in the range 17–39 GPa. The zincblende-type BN (c-BN) phase is identified from specimens subjected to high shock temperatures and consists of very fine grains mostly less than 10 nm. The mechanism of phase transformation of r-BN to c-BN is proposed to occur through two paths: (i) direct conversion by relatively strong shocks and (ii) indirect, kinetically controlled conversion via intermediate phases by shock loadings generating relatively weak pressures (<50 GPa). The possible intermediate phases are h-BN, t-BN, and w-BN.
A nonequilibrium thermodynamic theory of viscoplastic materials
Authors: Bernstein Barry , Fong Jeffrey T.
Published: July 2002

A nonequilibrium thermodynamic theory for predicting the mechanical behavior of materials beyond the elastic range is formulated. The theory incorporates the idea of a ‘‘concealed’’ parameter α, originally due to Bridgman [Rev. Mod. Phys. 22, 56 (1950)], where the constitutive equations are governed by (a) a thermodynamic potential such as a generalized Gibbs function, G, or Helmholtz free-energy function, F, each with an explicit dependence on α, and (b) a prescription for α̇, the time rate of change of α, such that α̇ is directly proportional to the negative of Gα or Fα, the partial derivative of G or F with respect to α, respectively. The theory is found to be consistent with (1) the second law of thermodynamics regarding entropy production; (2) the concept of Lyapunov stability at equilibrium; (3) the rule of invariance with respect to a transformation of parameters; and (4) the powerful law of invariance with respect to the Legendre transformation. Significance of the new formulation is discussed by solving a class of one-dimensional creep and fatigue modeling problems and by comparing the new theory with several similar approaches in the literature.
Violet photoluminescence in Zn-doped CuAlS2
Authors: Aksenov Igor , Yasuda Takashi , Segawa Yusaburo , Sato Katsuaki
Published: July 2002

The observation of a violet emission peaked at 390 nm from Zn-doped CuAlS2 crystals is reported. The emission has been attributed to a donor-acceptor pair recombination involving native acceptor (VCu) with activation energy 0.18 eV and extrinsic donor (ZnCu) with activation energy 0.32 eV.
Backward volume wave solitons in a yttrium iron garnet film (invited) (abstract)
Authors: Chen Ming , Tsankov Mincho A. , Nash Jon M. , Patton Carl E.
Published: July 2002

Solitons are nonlinear propagating wave pulses that preserve their shape without spreading. Recently, microwave envelope solitons have been observed in thin yttrium iron garnet (YIG) films for magnetostatic surface wave and forward volume wave configurations. In this work, we report the first observation of microwave envelope of magnetostatic backward volume waves (MSBVW) for an in-plane magnetized YIG film with waves propagating parallel to the magnetization direction. Both soliton profiles and the nonlinear peak power response were observed. The experiments were conducted using a microstrip magnetostatic wave delay line structure. A single crystal YIG film of 7.2 μm thickness, 2 mm×15 mm in size, and with unpinned surface spins was magnetized in-plane by a static external field of 1343 Oe along the long 15 mm edge. The 10 GHz ferromagnetic resonance linewidth of the film was 0.6 Oe. Square microwave pulses with pulse widths from 2 to 200 ns and a carrier frequency of 5.78 GHz were launched parallel to the field through a microstrip antenna. The output signal was received by a second microstrip antenna placed 4 mm downstream and analyzed in the time domain with a microwave transition analyzer. Envelope soliton behavior evident from the time resolved waveforms was observed for various input-power/pulse-width combinations.

At relatively low power levels, one sees a broad output signal with a peak power increasing linearly with the input power. As the input power is increased above some threshold, a sharp soliton pulse emerges and the peak power increases more rapidly with input power than in the low power regime. The threshold varies with pulse width, as expected for solitons. A further increase of the input power produces multiple soliton profiles and a corresponding drop in the peak power. These results clearly demonstrate the existence of MSBVW solitons in YIG films. Dr. J. D. Adam of Westinghouse is acknowledged for providing the YIG films. This work was supported in part by the National Science Foundation, Grant No. DMR-8921761 and by the U. S. Army Research Office, Grant No. DAAL03-91-G-0327.
Speed properties of a semiconductor-discharge gap IR image converter studied with a streak camera system
Authors: Astrov Yu. A. , Portsel L. M. , Teperick S. P. , Willebrand H. , Purwins H.-G.
Published: July 2002

Under certain experimental conditions a semiconductor-discharge gap structure can be used as detector for spatiotemporal resolved measurements on IR radiation. With a streak camera system and a semiconductor laser diode (λ=1.3 μm), we investigate experimentally the speed properties of this kind of converter. The experimental results are compared with the predictions of a simple theoretical model.
Contributions to the dielectric constant of the system BaLn2Ti4O12 from packing fraction and nephelauxetic ratio
Authors: Raju K. C. James , Sivasubramanian V. , Pragasam R. , Viswanathan B. , Murthy V. R. K.
Published: July 2002

An attempt has been made to identify the optimum parameters for good dielectric resonator materials. The dependence of dielectric constant of the system BaLn2Ti4O12 (Ln=La, Pr, Nd, Sm) on packing fraction and nephelauxetic ratio is discussed. The reported dielectric data for this system were treated using the multiregression method.
Origin of saturated light-induced defect density in hydrogenated amorphous silicon
Authors: Yoon Jong-Hwan
Published: July 2002

The saturation of light-induced defect density in hydrogenated amorphous silicon (a-Si:H) has been studied as a function of various material parameters, such as initial defect density Ninit, Urbach energy Eu, hydrogen content cH, hydrogenated void fraction R, and deposition-induced annealable defect density ΔNann. These parameters were varied by depositing samples at various substrate temperatures or by postgrowth anneals of samples grown at low substrate temperatures. It is found that total defect density of saturated light-soaked state Nsat is well correlated with the Ninit, Eu, cH, R, and ΔNann in the as-grown states but no correlations between Nsat and parameters other than the Ninit, Eu, and ΔNann in the annealed states are found. It is also observed that annealing before light soaking reduces the Nsat substantially. In particular, it is found that the value of Nsat from the fully annealed state is much lower than that from the as-grown state with similar Ninit and Eu. A possible parameter that plays a primary role in the saturation of light-induced defects is discussed.
Influence of impurity and phonon scattering effects in resonant tunneling structures
Authors: Fu Y. , Chen Q. , Willander M. , Brugger H. , Meiners U.
Published: July 2002

Tunneling properties and their temperature variations of molecular bean epitaxy grown symmetric AlAs/GaAs/AlAs resonant tunneling diodes with thin barriers are studied theoretically and experimentally. The measured peak and valley current densities show strong dependences on temperature. A Monte Carlo simulation including impurity and optical-phonon scatterings is developed for the calculation of the current-voltage behavior of the double barrier structures. This approach reveals pronounced temperature dependent tunneling features which agree well with measured results.
Parameter fitting in grazing incidence x-ray reflectometry
Authors: Spirkl W.
Published: July 2002

A method for parameter identification in grazing incidence x-ray reflectometry is presented. It is based on a nonlinear least-squares approach with on-line low-pass filtering and fits the measured reflectivity curve as a function of the incidence angle to a simulation model. The filter is applied in the (angular) frequency domain; it introduces, even for strong filtering, no bias and renders the result independent of the angular stepsize of the data. Automatic restarting helps to determine the global solution. The commonly used roughness model is generalized to non-normal distributions in order to assess data with large scattering vectors. The method is demonstrated for data from single and double film samples. Thickness and roughness can be determined with an accuracy of about 0.2 nm. However, only in special cases it is possible to determine accurately materials constants such as compositional fractions or densities. Absorption cannot be neglected, but the corresponding parameters are difficult to determine separately.
New controllable optical switching by the frequency modulation method in nematic liquid crystals
Authors: Sugimura Akihiko , Ou-Yang Zhong-can
Published: July 2002

A new type of sensitive electro-optic effect in a nematic liquid-crystal (4-cyano-4′-5-alkyl-bipheneyl) cell with polymer thin films is reported. Optical switching in a transmitted intensity of light passing through a cell is experimentally observed by dual low-frequency modulation method using a triangular wave application. Dielectric relaxation effects in the thin polymer layer as part of the cell serve to reduce the effective voltage acting on the liquid-crystal layer for lower frequencies but not for higher frequencies. The mean effective applied voltage to a liquid-crystal layer is efficiently controlled by changing only the frequency.
Analysis of aluminum tungsten oxide ceramics synthesized by a high-power cw CO2 laser
Authors: Li Xingjiao , Zheng Fang , Zheng Qiguang , Li Jiarong , Li Zaiguang , Li Shaopin
Published: July 2002

Ceramics in the Al2O3-WO3 system have been produced by laser irradiation. Examination of the products by scanning electron microscopy showed that a cell structure had formed in the specimens. A comparison of the properties of the samples synthesized by laser and by conventional sintering methods showed that the laser technique can produce new materials with some unique properties such as the linear decrease in resistivity of Al2O3-50 mol % WO3 specimens with increasing temperature over the range of 10–150 °C. X-ray photographic powder analysis indicated that the conducting phase in the specimen is a nonequilibrium product: AlxWO3 tungsten bronze.
Growth of epitaxial 3C-SiC films on (111) silicon substrates at 850 °C by reactive magnetron sputtering
Authors: Wahab Q. , Glass R. C. , Ivanov I. P. , Birch J. , Sundgren J.-E. , Willander M.
Published: July 2002

Reactive magnetron sputtering in a mixed Ar/CH4 discharge has been used to deposit 3C-SiC films on (111)-oriented Si substrates. The carbon content as well as the crystalline structure was found to depend on both the CH4 and Ar pressures. At a total pressure of 3 mTorr and a CH4 partial pressure of 0.6 mTorr, epitaxial stoichiometric films were obtained at growth temperatures as low as 850 °C. The epitaxial nature of the films was established by x-ray diffraction using a combination of reciprocal space maps, texture scans, and 360° φ scans. Based on these analyses it could also be concluded that double-positioning domains rotated 60° to one another as well as other defects, giving rise to a mosaic broadening in the reciprocal space maps, were present in the films. Furthermore, based on plasma probe measurements and determination of the electron energy distribution functions in the near-substrate vicinity, the low growth temperature of 850 °C is suggested to be a consequence of an effective decomposition of CH4 molecules in the plasma.
Ion-beam adhesion effects in the Cu/sapphire interfacial system
Authors: Skinner W. M.
Published: July 2002

The effects of 2 MeV H+, B+, O++, and Au++ ion irradiation on the adhesion energy of thin Cu films deposited on sapphire has been investigated. Adhesion energy was measured by the observation of the resultant laterally segregated Cu particle shape after vacuum annealing. Film/substrate adhesion is shown to increase with increasing ion dose and exhibits saturation behavior at high fluences. The adhesion energy is shown to be a linear function of the electronic stopping over the entire ion mass range—suggesting a purely electronic mechanism for the adhesion enhancement in this ion energy regime.
Influence of HF-H2O2 treatment on Si(100) and Si(111) surfaces
Authors: Gräf D. , Bauer-Mayer S. , Schnegg A.
Published: July 2002

The time depending influence of HF-H2O2 mixtures on the Si(100) and Si(111) surface was studied by means of x-ray photoelectron spectroscopy and high resolution electron energy loss spectroscopy (HREELS). The H2O2 concentration was varied in the range between 0% and 30%, for the HF concentration 0.5% was used. The oxygen coverage of the silicon surface increases with H2O2 concentration which shows up in HREELS spectra as Si–O–Si bridge bonded oxygen and O–H groups. Oxidation of Si–Si backbonds proceeds as can be seen by changes of the Si–H stretching mode. The oxygen uptake of Si(100) with a substantial higher amount of Si–O–Si is faster as compared with Si(111) which reveals Si–OH. Further on we find changes in the morphology of Si(100) surfaces. The dihydride termination decreases with increasing H2O2 concentration and immersion time which indicates the formation of Si(111)-like facets.
Effect of structural incoherence on the low-angle diffraction pattern of synthetic multilayer materials
Authors: Xu Zengli , Tang Zizhou , Kevan S. D. , Novet Thomas , Johnson David C.
Published: July 2002

The sequential, layering techniques used to prepare multilayer materials result in significant structural incoherence due to deviations from the intended thicknesses within an elemental layer and local deviations from the average due to islanding of the depositing elements during deposition. We demonstrate that if the domain size of the structural incoherence is large compared with the wavelength of the scattering radiation, the structural incoherence manifests itself in the low-angle diffraction pattern by attenuating the intensity of the subsidiary maxima relative to the Bragg maxima. We also show that the subsidiary maxima in the low-angle diffraction pattern of a multilayer result from incomplete destructive interference from all of the interfaces, not just from the top and bottom surface of the film. A technique for incorporating structural incoherence when modeling the diffraction pattern of a multilayer structure is presented. The ability of this model to simulate the experimental diffraction pattern of an iron-silicon multilayer is demonstrated.
Comment on ‘‘A theory on the x-ray sensitivity of a silicon surface-barrier detector including a thermal charge-diffusion effect’’ [J. Appl. Phys. 72, 3363 (1992)]
Authors: Donolato C.
Published: July 2002

It is shown that a recent analysis by Cho et al. [J. Appl. Phys. 72, 3363 (1992)] of surface-barrier detectors does not represent a new approach and is affected by gross errors, which lead to a wrong factor of 1/2 in the charge-collection probability and to the incorrect conclusion that one- and three-dimensional charge-diffusion models disagree.
Inductive noise thermometer: Theoretical aspects
Authors: Seppä Heikki , Varpula Timo
Published: July 2002

We describe a new noncontacting method for measuring the temperature of electrically conducting objects by sensing the magnetic field noise. When a high-Q antenna is placed close to a conductive material, e.g., a hot metal plate, the effective noise temperature of the antenna becomes proportional to the temperature of the material. Contrary to the conventional noise thermometer in which a resistor is embedded in the material, the method requires neither galvanic nor thermal contact. If the antenna impedance at different frequencies is known, the temperature of the object can be calculated from the total voltage noise of the antenna. We show, however, that for high-Q antennae the knowledge of the impedance at resonance is adequate to determine the unknown temperature. Consequently, the temperature of moving objects can be measured via synchronous monitoring of the total noise of the antenna and its impedance at resonance. Since only electrically conductive objects create magnetic field noise, the method is immune to nonconductive contamination of the surface.
Transport model of charged particle behavior in the afterglow region of a microwave generated oxygen plasma
Authors: Wei Ta-Chin , Phillips Jonathan
Published: July 2002

In this paper a model of charged particle behavior in a low-pressure oxygen plasma is developed, and compared with experimental results. Agreement is excellent. It is demonstrated that the extremely high temperature (≳1 eV) of electrons in these plasmas results in diffusion totally dominating the transport of charged species. It is also shown that charged particle recombination on the walls of a quartz reactor is insignificant. Finally, the influence of the electron temperature profile must be fully considered for accurate results. This work complements an earlier model of radical behavior in these plasmas. Both are needed to fully understand materials modification in these plasmas, which has been shown to involve a synergism between radicals and charged species.
Lateral diffusion of sources during selective growth of Si-doped GaAs layers by metalorganic vapor phase epitaxy
Authors: Hara Naoki , Shiina Kazushige , Ohori Tatsuya , Kasai Kazumi , Komeno Junji
Published: July 2002

Lateral diffusion of sources during selective growth of Si-doped GaAs layers by metalorganic vapor phase epitaxy was analyzed. The diffusion lengths of gallium and silicon species were estimated from carrier concentration profiles measured by Raman spectroscopy and thickness profiles. Using the diffusion lengths obtained, it is speculated that the diffusion materials are monomethylgallium and silylarsine. From their identical diffusion lengths, it was determined that there is no difference in diffusion materials between arsine and tertiarybutylarsine.