bluuebunny/crossref_metadata_2025 huggingface dataset:
Experimental investigation of surface wave propagation in collisional plasma columns
Authors: Dias F. M. , Tatarova E. , Henriques J. , Ferreira C. M.
Published: July 2002

An experimental study of surface wave propagation in a low-pressure collisional plasma column generated by the wave itself is presented. It is experimentally demonstrated that the maximum wave number value attainable decreases with increasing collisional effects. The separate effects of collisions and radial plasma inhomogeneity on the wave propagation are investigated. Some practical considerations concerning the correct measurement of the phase and attenuation diagrams are pointed out.
Thermal stability of Ir–Mn as exchange biasing material
Authors: van Driel J. , Coehoorn R. , Lenssen K.-M. H. , Kuiper A. E. T. , de Boer F. R.
Published: July 2002

We have investigated the strength and the thermal stability of Ir–Mn (18 at. % Ir) as an exchange biasing material for Ni80Fe20 and Co90Fe10, for configurations with the Ir–Mn layer both below and above the ferromagnetic (F) layer. The highest interfacial exchange energy (Jeb=0.13 mJ/m2 at 300 K) was found for 10 nm Ir–Mn deposited above a Co90Fe10 layer. For all configurations investigated the blocking temperature was approximately 560 K. For 10 or 30 nm Ir–Mn above and for 30 nm Ir–Mn below the F layer a remarkable decrease of the exchange biasing at room temperature was found after heating the film, whereas such an effect was not observed for a 10 nm thick Ir–Mn layer deposited below the F layer. This observation, in combination with the results of a relaxation experiment and the lack of experimental evidence for structural changes upon heating, suggests that heating can induce an irreversible change in the magnetic structure of Ir–Mn layers.
Structures with improved magnetic characteristics for giant magnetoresistance applications
Authors: Lubitz P. , Cheng Shu-Fan , Bussmann K. , Prinz G. A. , Krebs J. J. , Daughton J. M. , Wang D.
Published: July 2002

Giant magnetoresistance (GMR) structures of various thicknesses with layers of different commonly used fcc NiCoFe alloys and Cu layers were made and several approaches aimed at improving their magnetic properties were investigated. Atomic force microscopy was used to observe roughness and grain size; x-ray diffraction indicated crystalline orientation. Vibrating sample magnetometry was the principal method used to observe the magnetic characteristics. Roughness increased for thicker layers, but in itself did not strongly degrade magnetic switching. Grain size also increased with thickness and was found to correlate strongly with poor switching for Co rich alloys. Because these alloys have large magnetocrystalline anisotropy, larger grains apparently act more independently, leading to increased coercivity and slow saturation. Strategies were implemented that achieved small grain size even in thick structures and therefore resulted in abrupt switching in low fields and in higher remanent magnetization. Thin Fe, Cr and especially Ta interlayers were found to be effective in suppressing grain growth and maintaining [111] texture. This significantly improved the switching for structures containing significant amounts of Co. Even for rough GMR structures with >100 nm thickness the coercivity could be reduced by roughly an order of magnitude by use of 0.5 nm Ta interlayers.
Domain wall trapping probed by magnetoresistance and magnetic force microscopy in submicron ferromagnetic wire structures
Authors: Xu Y. B. , Vaz C. A. F. , Hirohata A. , Yao C. C. , Lee W. Y. , Bland J. A. C. , Rousseaux F. , Cambril E. , Launois H.
Published: July 2002

The magnetoresistance (MR) and domain structure of submicron NiFe wires and crosses fabricated using advanced electron beam lithography techniques have been studied in order to investigate the dependence of MR on the detailed domain configurations. While the 0.5 μm wire shows almost no longitudinal MR, the cross sample clearly shows a variation of the resistance upon sweeping the magnetic field, indicating an MR effect associated with the domain structures which form at the junction. By correlating the MR curves with the domain configurations obtained from magnetic force microscopy, we found that a 180° domain wall trapped in the junction of this 0.5 μm cross contributes a negative MR effect.
Buffer layer strain transfer in AlN/GaN near critical thickness
Authors: Kim Chinkyo , Robinson I. K. , Myoung Jaemin , Shim Kyu-Hwan , Kim Kyekyoon
Published: July 2002

X-ray diffraction has been employed to investigate the strain relaxation of both components of a GaN/AlN bilayer on sapphire (0001) as a function of the GaN layer thickness. Below a critical thickness, GaN and AlN both relax with the same in-plane lattice constant, consistent with the energy minimum condition of elasticity theory for a bilayer. Above the critical thickness, however, the strain relaxations in the two layers were different. We can fit this strain relaxation behavior with a free standing bilayer model with an additional term describing the interaction of dislocations.
Synthesis and characterization of magnetic nanocomposite films
Authors: Kodama R. H. , Edelstein A. S.
Published: July 2002

Nanocomposite films were made consisting of Co nanoparticles, with coatings of amorphous Al2O3. Nanoparticles were deposited on room-temperature substrates by sputtering in 0.10–0.20 Torr Ar. Transmission electron microscope images showed average Co particle sizes between 70 and 223 Å, for different sputtering rates and pressures. Selected-area electron diffraction and x-ray diffraction showed that the particles had a fcc structure for sizes ⩽122 Å and a mixture of fcc and hcp phases for larger sizes. Magnetization measurements on a sample with 122 Å mean-particle size showed significant hysteresis at room temperature, with a coercivity of 390 Oe. At 10 K, the coercivity increased to 3145 Oe, and a loop shift of 2020 Oe was measured when the sample was field cooled. This indicates a fraction of the Co was oxidized by reaction with the Al2O3 coating, resulting in a surface layer of antiferromagnetic Co oxide.
Optical distortion and birefringence in a heated trigonal crystal rod
Authors: Greninger Charles E.
Published: July 2002

Analytic expressions for the optical distortion and the fractional polarization purity loss when a perfectly plane, perfectly linearly polarized moderate power laser beam traverses a heated, rectangular trigonal class 3m, 32, or 3̄m crystal rod are given. The expressions are in terms of the crystal physical parameters of the rod, as well as the local stresses and strains that are developed in the rod by heating from the beam. When the crystal temperature variation is symmetric in the rod’s transverse coordinates, these expressions become analytic in the temperature variation and two simple temperature integrals. An example with a symmetric temperature variation in a lithium niobate crystal is given.
Self-organization of surface wave sustained discharges in the pressure range from 10 to 200 Torr
Authors: Djermanova N. , Grozev D. , Kirov K. , Makasheva K. , Shivarova A. , Tsvetkov Ts.
Published: July 2002

Experiments showing the dynamics in the self-organization of surface wave sustained discharges are presented. Microwave (2.4 GHz) discharges maintained in an argon gas in a continuous wave regime at a constant applied power and varying gas pressure are studied. The evolution of the discharge from a stationary plasma column at comparatively low pressure (p⩽10 Torr) to a plasma torch at atmospheric pressure passes through different stages of self-organization of the wave-field↔plasma nonlinear structure showing evidence of the general trends of behavior of nonequilibrium dissipative systems. The measurements are carried out at the stage of the discharge self-organization into a filamentary structure with an azimuthal rotation. Macroscopic characteristics (number, size, velocity of rotation) of the filaments and their dependence on the gas pressure and its time variation are given. The total light emission of the plasma considered as giving information about the plasma density is measured and different methods of signal processing (including correlation-spectrum analysis) are applied. Oscillations of the filament ends are also observed. The different types of interrelation between plasma density and field intensity, registrated in the different pressure ranges, call for variety in the instability mechanisms. Although the scenario of the discharge self-organization is stressed in the discussions, the observations are important with their relation to the discharge applications, which require avoiding conditions of development of instabilities.
Coercivity in amorphous Tb–Fe alloys
Authors: Hellman F. , Messer M. , Abarra E. N.
Published: July 2002

The perpendicular magnetic anisotropy, coercive field, and initial magnetization curves of amorphous TbxFe1−x (x between 0.15 and 0.32) were measured at room temperature for samples prepared under a wide variety of preparation conditions, including both e-beam co-evaporation and dc magnetron cosputtering. The effect of growth temperature, annealing, and thickness were investigated. The perpendicular magnetic anisotropy shows little dependence on substrate type, sample thickness, or details of the deposition such as sputtering or e-beam evaporation, but is strongly dependent on growth temperature, increasing with increasing growth temperature up to nearly 300 °C. Coercivity on the other hand is extremely dependent on microstructure, and hence, on details of preparation, substrate type, and thickness. It is much larger in evaporated films than in similarly prepared dc magnetron sputtered films. Normalized coercivity decreases monotonically with increasing growth temperature. The dominant mechanism appears to be domain wall pinning in the bulk of the film due to columnar microstructure. High growth temperature stabilizes the material against subsequent annealing which tends to eliminate the anisotropy and, more slowly, the coercivity.
Defect generation under substrate-hot-electron injection into ultrathin silicon dioxide layers
Authors: DiMaria D. J.
Published: July 2002

Point-defect generation in ultrathin silicon dioxide layers is studied for various initial hot-electron distributions at the cathode/oxide interface using injection modes dependent on the device structure. Consistent with thicker gate oxides studies, these experiments show unequivocally that defect buildup leading to destructive breakdown depends on electron energy, not oxide electric field (or inverse field). Bulk oxide electron-trap generation is shown to depend on the energy delivered to the anode by the hot electrons transported through the oxide layer after injection from the cathode contact. However, defect generation near the cathode/oxide interface is shown to also depend on the energy of the hot electrons delivered to this interface from the silicon bulk, particularly for nonthermal distributions. By comparing bulk oxide-defect generation due to substrate–hot-electron injection to that due to thermal Fowler–Nordheim injection, direct information about the electron energy distribution at the cathode/oxide interface is obtained for any biasing configuration. The implications of these studies on the reliability of actual device operation where channel–hot-electron effects may occur are discussed.
Characterization of the luminescence center in photo- and electroluminescent amorphous silicon oxynitride films
Authors: Price K. J. , McNeil L. E. , Suvkanov A. , Irene E. A. , MacFarlane P. J. , Zvanut M. E.
Published: July 2002

We have studied the photoluminescence (PL) mechanism of photo- and electroluminescent amorphous silicon oxynitride films grown by plasma enhanced chemical vapor deposition. The composition of the films was determined by Rutherford backscattering spectrometry and monitored by the index of refraction with single-wavelength ellipsometry. Two sets of samples were grown, each with different reactant gas residence times in the deposition chamber. For samples grown with a residence time of about 5 s, the energy of the PL peak for 2.54 eV excitation is 2.3 eV for stoichiometric films and redshifts with increasing silicon content to 1.7 eV for the most silicon-rich films. The energy of the PL peak for 3.8 eV excitation is 2.8 eV for stoichiometric films and 2.1 eV for the most silicon-rich films. For stoichiometric films, the PL intensity is independent of temperature between 80 and 300 K using 2.54 eV excitation, but the PL intensity decreases by a factor of two over the same temperature range for 3.8 eV excitation. The authors interpret these aspects of the PL as consistent with tail-state recombination. Other results imply the PL is due to a specific luminescence center related to Si–Si or Si–H bonding. A 450 °C anneal reduces the paramagnetic defect density in the films, as detected by electron paramagnetic resonance, by an order of magnitude, but does not increase the PL intensity, while a 950 °C anneal increases both the defect density and the PL intensity. In addition, films in a second set of samples, grown with a residence time of 1.8 s, display very different PL behavior than samples in the first set with the same composition. Samples near stoichiometry in the second set have a PL peak at 2.06 eV and are 20 times less intense than stoichiometric samples in the first set. Optical absorption measurements indicate both types of samples contain Si–Si bonds, with the second set containing many more Si–Si bonds than the first. Fourier-transform infrared measurements indicate the presence of a Si–H bond that is stable at temperatures of 950 °C in the first set, but not in the second set. Thus, the study as a whole suggests a complete picture of luminescence in our silicon oxynitride films must incorporate elements of both tail-state and luminescence center models. The relation of the results to other PL studies in silicon alloys and porous silicon is discussed.
Microwave intermodulation of a superconducting disk resonator
Authors: Dahm T. , Scalapino D. J. , Willemsen Balam A.
Published: July 2002

We calculate the third order intermodulation products for the TM010 mode of a thin film disk resonator and discuss the particular case of a 1 GHz resonator in order to obtain a quantitative idea of the performance that can be obtained. The linear and nonlinear microwave responses of a TM010 disk resonator are then compared to that of an equivalent half wavelength microstrip resonator. This analysis allows one to independently quantify the contributions to the nonlinear device performance from the material properties, device size and field configuration.
Photo- and electroluminescence of SrS:Cu and SrS:Ag,Cu,Ga thin films
Authors: Li Wei-Min , Ritala Mikko , Leskelä Markku , Niinistö Lauri , Soininen Erkki , Sun Sey-Shing , Tong Wusheng , Summers Christopher J.
Published: July 2002

Photoluminescence (PL) and electroluminescence (EL) of blue and green SrS:Cu thin films prepared by atomic layer epitaxy and molecular beam epitaxy, and blue SrS:Ag,Cu,Ga thin films prepared by sputtering were studied in a temperature range of 80–320 K. Two bands are present both in the PL and EL spectra of SrS:Cu films. The low energy band (L band) at about 520 nm can be observed throughout the temperature range studied. The high energy band (H band) at about 460 nm is seen only at higher temperatures and it disappears at 80 K. Decay studies at 80 K reveal at least two types of centers having lifetimes of 100 and 17 μs with a 10% variation. Several luminescence processes are proposed for further discussions. The L band may be attributed to the emission from an isolated Cu+ ion replacing the host Sr2+ ion but at an off-center site, and it undergoes a redshift caused by aggregated Cu centers. The H band is tentatively described as due to the Cu+ ion at different site symmetry. Blue luminescence of SrS:Ag,Cu,Ga most probably originates from Ag+ pairs. There is no direct evidence of energy transfer from Cu+ to Ag+ but Cu does contribute to the EL.
Theoretical study of the interactions of SiH2 radicals with silicon surfaces
Authors: Ramalingam Shyam , Mahalingam Pushpa , Aydil Eray S. , Maroudas Dimitrios
Published: July 2002

Silylene (SiH2) radicals created by electron impact dissociation of silane in reactive gas discharges can play an important role in plasma deposition of amorphous and nanocrystalline silicon thin films. In this article, we present a systematic computational analysis of the interactions of SiH2 radicals with a variety of crystalline and amorphous silicon surfaces based on atomistic simulations. The hydrogen coverage of the surface and, hence, the availability of surface dangling bonds is shown to exert the strongest influence on the radical-surface reaction mechanisms and the corresponding reaction probabilities. The SiH2 radical reacts with unit probability on the pristine Si(001)-(2×1) surface which has one dangling bond per Si atom; upon reaction, the Si atom of the radical forms strong Si–Si bonds with either one or two surface Si atoms. On the H-terminated Si(001)-(2×1) surface, the radical is found to react with a probability of approximately 50%. The SiH2 radical attaches itself to the surface either by forming two bonds with Si atoms of adjacent dimers in the same dimer row or through Si–Si bonds with one or both atoms of a surface dimer. In addition, the SiH2 radical can attach itself in the trough between dimer rows, forming two Si–Si bonds with second-layer Si atoms. The energetics and dynamics of these surface reactions are analyzed in detail. A reaction probability of approximately 70% is calculated for SiH2 radicals impinging on surfaces of hydrogenated amorphous silicon (a-Si:H) films with varying concentrations of hydrogen. Recent experimental measurements have reported a 60% loss probability for the SiH2 radical on the reactor walls through laser induced fluorescence. The experimentally obtained reaction probability falls within the range for the sticking coefficients on the H-terminated and amorphous film surfaces as determined by our atomistic calculations. Molecular-dynamics (MD) simulations of a-Si:H film growth by repeated impingement of SiH2 radicals have revealed adsorption reactions at early stages to occur with similar energetics as the corresponding reactions of isolated radicals on crystalline surfaces. The reaction probability of SiH2 on a-Si:H films deposited through MD simulations is approximately 30%. Finally, it is found that the SiH2 radical is much more mobile on surfaces of a-Si:H films than on crystalline surfaces, especially when the hydrogen concentration in the amorphous film and, thus, on the surface is high.
Structural characterization of rapid thermal oxidized Si1−x−yGexCy alloy films grown by rapid thermal chemical vapor deposition
Authors: Choi W. K. , Chen J. H. , Bera L. K. , Feng W. , Pey K. L. , Mi J. , Yang C. Y. , Ramam A. , Chua S. J. , Pan J. S. , Wee A. T. S. , Liu R.
Published: July 2002

The structural properties of as-grown and rapid thermal oxidized Si1−x−yGexCy epitaxial layers have been examined using a combination of infrared, x-ray photoelectron, x-ray diffraction, secondary ion mass spectroscopy, and Raman spectroscopy techniques. Carbon incorporation into the Si1−x−yGexCy system can lead to compressive or tensile strain in the film. The structural properties of the oxidized Si1−x−yGexCy film depend on the type of strain (i.e., carbon concentration) of the as-prepared film. For compressive or fully compensated films, the oxidation process drastically reduces the carbon content so that the oxidized films closely resemble to Si1−xGex films. For tensile films, two broad regions, one with carbon content higher and the other lower than that required for full strain compensation, coexist in the oxidized films.
Partitioning behavior of Al in a nanocrystalline FeZrBAl soft magnetic alloy
Authors: Chen M. W. , Sakai A. , Inoue A. , Wang X. M. , Watanabe Y. , Sakurai T.
Published: July 2002

Atom probe field ion microscopy was employed to investigate the distribution of Al in a nanocrystalline Fe88Zr7B3Al2 soft magnet with optimal magnetic performance. The atom probe concentration depth profiles show that the Al atoms preferentially partition into the residual amorphous phase, and the partitioning factor of Al is approximately five times larger in the amorphous phase than in the bcc Fe phase. Based on the experimental results, the beneficial effect of the addition of Al on the soft magnetic properties is attributed to the change of the inherent magnetostriction constant of the residual amorphous phase induced by Al partitioning.
Mechanism for the generation of interface state precursors
Authors: Zhang J. F. , Sii H. K. , Degraeve R. , Groeseneken G.
Published: July 2002

The generation of interface states plays an important role in the degradation of submicrometer devices. Previous attention was mainly focused on the conversion between interface states and their precursors. The total number of defects, which is the sum of precursors and interface states, is often implicitly assumed to be constant. However, recent work indicates that this number could be increased. The mechanism for the generation of new precursors is still not clear and the objective of this article is to throw light on it. The work is concentrated on investigating the roles played by hydrogen and the holes trapped in the oxide. It is found that, although the H2 or the trapped hole alone does not create precursors, their simultaneous presence causes the damage. The hydrogen species can be either supplied externally or released within the device. The generation is thermally activated, but saturates at a defect-limited level. The generation kinetics is studied and the rate limiting mechanism is discussed. Efforts have been made to unveil the differences between the generated precursors and those originally in the device, in terms of their existing forms, thermal stability, annealing behavior, dependence on the hole fluence, and the hydrogen involvement. It is concluded that they originate from different defects.
Regional approximation approach to space charge limited tunneling injection in polymeric devices
Authors: Koehler M. , Hümmelgen I. A.
Published: July 2002

We developed a theory that explains the charge injection and transport in conjugated polymer films such as those used in organic light emitting diodes. Using the regional approximation the effect of the space charge on the current density of electrons tunneling from metal electrodes to the lowest unoccupied molecular orbital of a polymer film is calculated. The space charge is considered to decrease with increasing distance of the injecting electrode. If the space charge occupies only a limited region between the tunneling distance and the collecting electrode , the current (I) is found to depend on a power law of the applied voltage (V). However, if the space charge occupies all the region between the tunneling distance and the collecting electrode, I is found to vary exponentially on V for lower V values and follow approximately the law of Child at high V. The space charge limited tunneling current (SCTC) theory gives the same results of the space charge limited conduction theory when the energy barrier for charge carrier injection is small or when the polymer layer is thick. The SCTC theory is compared to the experimental data. It is shown that there is a good agreement between theory and experiment, concerning both current magnitude and current versus voltage dependence.
Interfacial roughness effects on interlayer coupling in spin valves grown on different seed layers
Authors: Parks D. C. , Chen P. J. , Egelhoff W. F. , Gomez Romel D.
Published: July 2002

We have studied the behavior of interlayer coupling in giant magnetoresistance spin valves as a function of seed layer composition and spacer layer thickness. Using in situ scanning tunneling microscopy, we have measured directly the roughness of the top surface of the lower ferromagnetic layer. We find that the seed layer composition is correlated to the roughnesses of the interfaces inside the spin valve. Interlayer coupling increases with decreasing Cu spacer layer thickness and with increasing interfacial roughness. Results favorably compare to a topographically derived magnetostatic interaction as described by a modified version of Néel’s “orange peel” model.
Strain and photoluminescence characterization of cubic (In,Ga)N films grown on GaAs(001) substrates
Authors: Sun X. L. , Wang Y. T. , Yang Hui , Zheng L. X. , Xu D. P. , Li J. B. , Wang Z. G.
Published: July 2002

Cubic InxGa1−xN films were successfully grown on GaAs(001) substrates by metalorganic chemical-vapor deposition. The values of x content ranging from 0.10 to 0.24 obtained at different growth conditions were measured by double-crystal x-ray diffraction (XRD). The perpendicular and parallel elastic strain of the In0.2Ga0.8N layer, ε⊥=0.4% and ε∥=−0.4% for GaN and ε⊥=0.37% and ε∥=−0.37% for InGaN, respectively, were derived using the XRD measurements. The inhomogeneous strain and the average grain size of the In0.2Ga0.8N/GaN films were also studied by XRD. Photoluminescence spectra were used to measure the optical characterization of the InxGa1−xN thin films with different In composition, and the near-band-edge emission dependence of cubic InxGa1−xN on the x value is nearly linear with In content x⩽0.24.
Surfactant controlled growth of GaInP by organometallic vapor phase epitaxy
Authors: Lee R. T. , Shurtleff J. K. , Fetzer C. M. , Stringfellow G. B. , Lee S. , Seong T. Y.
Published: July 2002

The effect of the surfactant Sb has been studied for GaInP semiconductor alloys grown by organometallic vapor phase epitaxy. Dramatic changes in the optical and electrical properties of GaInP with CuPt ordering have been observed. A small concentration of triethylantimony (TESb) in the vapor is found to cause Sb to accumulate at the surface. In situ surface photoabsorption analysis indicates that Sb changes the surface bonding by replacing the [1̄10] P dimers that are responsible for the formation of the CuPt structure during growth with [1̄10] Sb dimers. As a result, the degree of order for the GaInP layers is decreased, as shown by transmission electron diffraction studies. The 20 K photoluminescence spectra show a 131 meV peak energy increase for GaInP layers grown on vicinal substrates when a small amount of Sb [Sb/P(v)=4×10−4] is added to the system during growth. The use of surfactants to control specific properties of materials is expected to be a powerful tool for producing complex structures. In this article, the growth of heterostructures by modulating the Sb concentration in the vapor is demonstrated.
Photoluminescence in delta-doped InGaAs/GaAs single quantum wells
Authors: Dao L. V. , Gal M. , Li G. , Jagadish C.
Published: July 2002

We have studied the time integrated (cw) and time resolved photoluminescence (PL) spectra of Si δ-doped In0.2Ga0.8As/GaAs quantum wells (QWs), in which the δ doping layer was either at the center of the quantum well or outside the well, in the barrier region. We found that both the cw and the time resolved PL depended significantly on the position of the doping sheet. When the doping was at the center of the quantum well the luminescence spectrum displayed the characteristic features of the Fermi edge singularity, while in the case of barrier-doped QW, the PL spectra showed well-defined emission lines originating from transitions between subbands in the conduction and valence bands. From low-temperature time resolved PL experiments, we determined the effective hole capture times, the interband relaxation times (for holes), and the radiative decay times for both types of δ doping. We found that the interband relaxation time in the center-doped QWs is nearly two orders of magnitude shorter (τ=3 ps) than in samples doped in the barrier (τ=200 ps).
Precise measurement of the microwave surface impedance of a YBa2Cu3O7−δ film on copper substrate
Authors: Liu Jian-Fei , Asano Kiyomitsu , Ezura Eizi , Inagaki Shigemi , Isagawa Shigeru , Nakanishi Hiroshi , Fukutomi Masao , Komori Kazunori , Saito Masakazu
Published: July 2002

In measurement of the microwave surface resistance, Rs(T), of a high-Tc film with a host-cavity method, in which the cavity material is usually copper, the systematic error in Rs at low temperature can be significantly reduced through calibration with the data obtained by a niobium host cavity. Using a cavity excited in the TE011 mode at 13.6 GHz, the procedure is illustrated for a c-axis oriented YBa2Cu3O7−δ film fabricated on 36 mm diameter copper disk with yttria-stabilized-zirconia and chromium buffer layers. The temperature dependence in Rs(T) was consistent with that of the penetration depth; both quantities behavior could be fit well by a modified two-fluid model, in which the fraction of the pairing normal carriers obeyed (T/Tc)2 rather than (T/Tc)4 with some 20% of the charge carriers remaining normal.
Structural and electrical characterization of heteroepitaxial Pb[Yb1/2Nb1/2]O3–PbTiO3 thin films
Authors: Bornand V. , Trolier-McKinstry S.
Published: July 2002

The pulsed laser deposition process has been used to prepare heteroepitaxial (1-x) Pb[Yb1/2Nb1/2]O3–xPbTiO3 (PYbN–PT, x=0.4,0.5) thin films on single crystalline (001)pcSrRuO3/LaAlO3 and (111)pcSrRuO3/SrTiO3 substrates (the subscript pc refers here to the pseudocubic subcell). High laser frequencies (f−16 Hz) and 300 mTorr of background O3/O2 in the chamber during deposition provide stoichiometric and high crystalline quality heterostructures. Temperatures in the 560–660 °C range lead to improved microstructures as well as good dielectric and ferroelectric properties consistent with those of PYbN–PT ceramics. In particular, films show room temperature dielectric constants greater than 1300 and exhibit well-developed hysteresis loops with remanent polarizations (Pr) as high as 40–50 μC cm−2. Results are discussed in terms of film composition and crystallinity.
Low temperature annealing of 4H–SiC Schottky diode edge terminations formed by 30 keV Ar+ implantation
Authors: Knights A. P. , Lourenço M. A. , Homewood K. P. , Morrison D. J. , Wright N. G. , Ortolland S. , Johnson C. M. , O’Neill A. G. , Coleman P. G. , Hilton K. P. , Uren M. J.
Published: July 2002

Edge termination of Schottky barrier diodes has been achieved using 30 keV Ar+ ions implanted at a dose of 1×1015 cm−2. The reverse-bias leakage current is reduced by 2 orders of magnitude following postimplant annealing at a temperature of 600 °C. The thermal evolution of the implantation induced defects was monitored using positron annihilation spectroscopy and deep-level transient spectroscopy. Two distinct defect regions are observed using the positron technique. The depth of the first is consistent with the range of the implanted Ar+ ions and consists of clustered vacancies. The second extends to ∼250 nm, well beyond the range of the incident ions, and is dominated by point defects, similar in structure to Si–C divacancies. An implant damage related deep level, well defined at Ec−Et=0.9 eV, is observed for both the as-implanted and the 600 °C annealed sample. The effect of annealing is a reduction in the concentration of active carrier trapping centers.
Plasma-chemical vapor deposition of wide band gap a-SiC:H films: An ab initio molecular-orbital study
Authors: Cheng Shih-Hung , Sato Kota , Kumashiro Yukinobu
Published: July 2002

Plasma surface reactions for preparing a-SiC:H films system have been quantum chemically studied by using an ab initio molecular orbital method. The calculated results suggest that the sp2 carbons are preferentially formed in a-SiC:H films which have high carbon contents if silane and methane are used as source gases. It makes the band gap of a-SiC:H films with high carbon contents narrow. In order to produce a-SiC:H with wide band gap, a quantum chemical reaction design has been performed. The effect of chlorine and fluorine substitution in carbon sources on plasma surface reactions have been studied. The activation energy for sp3 network formation reactions of carbons which bonds to chlorine with dangling bond site is much lower than that of hydrocarbons with dangling bond site (20.3 and 59.2 kcal/mol, respectively). The results indicate that chloro-substituted methanes are good carbon source gases for preparing a-SiC:H films with a wide band gap.
Independent contacts to a double-electron gas using mesoscopic surface gates
Authors: Nield S. A. , Nicholls J. T. , Tribe W. R. , Simmons M. Y. , Ritchie D. A.
Published: July 2002

We demonstrate how two two-dimensional electron gases can be independently contacted using only surface gates defined by electron-beam lithography: macroscopic bar gates to contact the lower layer, and mesoscopic split gates to contact the upper layer. When the technique is applied to a sample consisting of two 150 Å GaAs wells separated by 300 Å Al0.33Ga0.67As barrier, there is more than 30 MΩ isolation at 5 K for interlayer voltages up to ±30 mV; the interlayer isolation operates up to 30 K. The independent contacts are used for a preliminary investigation of two parallel one-dimensional quantum wires.
Dyakonov–Shur plasma excitations in the channel of a real high-electron mobility transistor
Authors: Crowne Frank J.
Published: July 2002

The arguments leading to the prediction of instability of plasma excitations in a drifting 2D electron gas given by Dyakonov and Shur are made more rigorous by treating a real high-electron mobility transistor (HEMT) channel biased in its ohmic region (i.e., below pinchoff), including nonuniformity of the channel charge density. Expressions are derived for the changes in resonant frequency and gain of the Dyakonov–Shur modes arising from finite channel mobility and density nonuniformity to first order in the momentum relaxation time. It is found that channel nonuniformity (constriction) weakens the instability, in keeping with the recent results of Cheremisin and Samsonidze [Semiconductors 33, 578 (1999)], possibly explaining why it has not been observed in real HEMTs.
Minority carrier lifetime and iron concentration measurements on p-Si wafers by infrared photothermal radiometry and microwave photoconductance decay
Authors: Rodrı́guez M. E. , Mandelis A. , Pan G. , Garcı́a J. A. , Gorodokin V. , Raskin Y.
Published: July 2002

A comparative study of electronic transport properties of p-Si wafers intentionally contaminated with Fe was performed using infrared photothermal radiometry (PTR) and microwave photoconductance decay (μ-PCD). Strong correlations were found between PTR and μ-PCD lifetimes in a lightly contaminated wafer with no significant PTR transient behavior. The absolute PTR lifetime values were larger than the local averaged μ-PCD values, due to the different excitation wavelengths and probe depths. In a heavily contaminated wafer the μ-PCD and PTR lifetime correlation was poorer. PTR measurements were highly sensitive to iron concentration, most likely due to the dependence of the bulk recombination lifetime on it. Rapid-scanned (nonsteady-state) PTR images of the wafer surface exhibited strong correlations with both μ-PCD lifetime and [Fe] concentration images in both heavily and lightly contaminated wafers. For the lightly and uniformly contaminated wafer, PTR scanning imaging was found to be more sensitive to iron concentration and lifetime variations than μ-PCD images.
An elastic/plastic analysis of the intrinsic stresses in chemical vapor deposited diamond films on silicon substrates
Authors: Yu Jin , Kim J. G. , Chung J. O. , Cho D. H.
Published: July 2002

Diamond films were grown over Si substrate at 1223 K by the hot filament chemical vapor deposition (CVD) method using CH4/H2 gas mixtures (1%, 2%, 3% CH4,) and the intrinsic stresses in the film were deduced using the ex situ curvature method. After subtracting the curvature change during the cooling process, average stresses in the film during the CVD process, 〈σf〉, were calculated using the elastic/plastic analysis which treated the creep deformation of the substrate. The intrinsic stress kept increasing during the CVD process and was generally several times larger than the 〈σf〉 which tended to saturate around the film thickness (tf) of 10 μm. For thicker films, substrate creep became significant and the substrate stress was substantially relaxed by creep. Fraction of the creep strain with respect to the total strain at the film–substrate interface was around ∼1/3 when tf∼10 μm, and increased as large as 2/3 during the film deposition. The intrinsic stress was believed to arise from the grain growth during the CVD process, because the stress deduced from the actual grain size measurements agreed reasonably with calculated values from the above analysis. Later, the diamond film layer was removed by O2 electron cyclotron resonance etching, and the remaining curvatures of the substrate were compared with those deduced from the elastic/plastic analysis. Residual stresses in the substrate after the film removal were tensile near the interface and the substrate bottom but compressive in the middle.
Thermal diffusion factors for the 4He–20Ne system at low temperatures
Authors: Taylor W. L. , Cain David , Meeks F. R. , Pickett P. T.
Published: July 2002

Thermal diffusion factors for the 4He–20Ne system have been measured in a low temperature region previously unexplored (down to 31 K) using a trennschaukel. Both quantum and classical transport collision integrals were evaluated for a recently proposed helium–neon intermolecular potential of the Hartree–Fock dispersion (HFD) type which has been shown to correlate cross section and transport property data quite well. The present data agree extremely well with the quantum mechanically calculated thermal diffusion factors. The classically calculated values fall considerably lower.
Electron spin relaxation rates in nitrosyl ferrous myoglobin
Authors: Muench P. J. , Stapleton H. J.
Published: July 2002

Electron spin relaxation rates of ferrous myoglobin complexed with nitric oxide were measured in a frozen solution in the temperature range between 4.2 and 20 K using pulse saturation and recovery techniques. These measurements were made to determine if the relaxation rate of such a paramagnetic complex could probe the fractal dimension of the protein, as with low spin ferric myoglobin complexes. The relaxation rate varied linearly with temperature as 1/T1=376 T over the entire range, indicating a direct relaxation mechanism, unusable as a structural probe of myoglobin.
The polymer-induced coil–globule transition
Authors: Tanaka Fumihiko
Published: July 2002

In a dilute solution of large polymers, each polymer shrinks when shorter polymers of different species are added, provided that the molecular interaction between the polymers of the two kinds is repulsive. The existence of a sharp transition of the large polymer from an extended random coil to a compact globule is examined in a statistical mechanical way. We show that a first-order-like sharp transition induced by the existence of the smaller polymers takes place under the conditions such that (i) the intermolecular interaction is sufficiently strong compared with the intramolecular interaction between the segments in a large polymer, (ii) the degree of polymerization (DP) of the large polymer is much larger than that of the smaller polymers. The critical concentration of the smaller polymers, above which the large polymer collapses to a closely packed globule, is shown to be inversely proportional to their DP. Detailed numerical analyses on a model system are presented.
The polarizability and second hyperpolarizability of some azabenzenes
Authors: Papadopoulos M. G. , Waite J.
Published: July 2002

The average polarizability and second hyperpolarizability of some azabenzenes are reported and interpreted. A rule allowing the development of basis sets for complex molecules from those of appropriately defined fragments or models of them has been proposed and successfully tested.
The phase space subdivision of the second virial coefficient
Authors: Friend Daniel G.
Published: July 2002

The division of two-body relative phase space into free, bound, and metastable subspaces is presented in terms of fundamental variables. This enables us to examine the differences among the various partitions of the second virial coefficient presented by Rainwater, as boundary effects. Explicit evaluations of these boundary contributions are presented in certain cases and agreement is obtained between this and the earlier approach to the problem. For the bound subspace, in particular, an identity is established which relates the partition function and potential formulations of the second virial coefficient.
An alternative to the magnus expansion in time-dependent perturbation theory
Authors: Salzman W. R.
Published: July 2002

An alternative to the Magnus expansion in time-dependent quantum mechanical perturbation theory is presented. An exponential form of the time-development operator, given as the exponential of a sum of products of integrals well known in time-dependent quantum mechanics, is derived from the standard perturbation expansion. The derivation is simple and the form of the terms in the exponential is so straightforward that the Nth order term can be written by inspection. The standard Magnus terms can be obtained by rearrangement of the terms of the new expansion probably easier than they can be obtained by iteration of the original equations. A proof by induction is presented to establish the form of the Nth term and it is shown how the new expansion terms relate to the Magnus terms. Some apparent differences between Magnus terms in the literature are resolved.
On the manifestation of retardation effects in diagonally polarized light scattering
Authors: Harris Robert A. , McClain Wm. M.
Published: July 2002

It is shown, as an exact consequence of the nonrelativistic quantum theory of light scattering, that the dependence of differential cross section on the incident and scattered wave vectors and polarization vectors is the same as it is in the Kramers–Heisenberg approximate result. Combining this result with previous work, we conclude that in systems of randomly oriented scattering molecules (with no external fields) there is a necessary condition for the diagonally polarized elements of the Perrin matrix 〈Pi3〉 and 〈P3i〉 (with i=1,2,4) to be nonzero. The condition is that the molecule be radiatively and/or nonradiatively damped, and/or that retarded multiple scattering is occurring within the molecule. To show observable retardation effects, molecular size need only be comparable to wavelength. The retarded component of the 〈P3i〉 elements has a frequency prefactor of ω6, in contrast to the total intensity prefactor of ω4. Therefore, the bluest possible laser should be used for their measurement.
Origin of the 454 nm bound→free absorption band of cadmium monoiodide (114CdI)
Authors: Greene D. P. , Eden J. G.
Published: July 2002

The bound→free absorption band recently observed in a 114CdI laser discharge [Greene and Eden, Opt. Comm. (1985)] is shown to arise from absorption by the B 2Σ excited state of 114CdI to a previously unobserved level lying ∼4.3×104 cm−1 above ground. Pumping of the absorptive transition with a pulsed blue dye laser strongly suppresses gain (and, hence, laser output power) on the B→X band in the red (657 nm). Therefore, this constitutes the first known report of an ionic excited state→covalent absorption band [i.e., (M+X−)+ℏω→M*+X; M, X=metal and halogen atoms, respectively] for any of the group IIB metal halides or rare gas–halide laser molecules. Also, ωe for the B 2Σ state of 114CdI is ≳110 cm−1 (considerably larger than an existing value in the literature) and the terminal level for this blue absorption band is tentatively assigned to a dissociative 2Σ state correlated with the Cd(5p 3PJ)+I separated atom limit.
Dynamics of helical worm-like chains. VI. Fluorescence depolarization
Authors: Yoshizaki Takenao , Fujii Motoharu , Yamakawa Hiromi
Published: July 2002

Fluorescence depolarization of both flexible and stiff chain polymers in dilute solution is studied on the basis of the discrete helical worm-like chain such that a fluorescent probe is attached rigidly or with a rotational degree of freedom to one of the subbodies composing the chain. The emission anisotropy r(t) and the average anisotropy r̄ are formulated, considering in general the difference between the directions of absorption and emission dipoles. For flexible chains, proper account is also taken of the difference between the subbodies with and without the probe in size, and for stiff chains, e.g., DNA, effects of wobbling of the probe are considered. If the absorption and emission dipole moment vectors are parallel to each other, r(t) is equal to the magnetic autocorrelation function multiplied by a factor with the internuclear vector being regarded as the emission vector. Naturally, therefore, the results of analysis of experimental data for the present and magnetic cases are then consistent with each other. For flexible chains, the present theory can well explain the dependences of r(t) on the direction of the emission dipole and on the location of the probe, and there is also correlation between the static stiffness parameter λ−1 and the dynamic stiffness as defined as the ratio of a properly defined fluorescence correlation time τF to that of the isolated subbody. For DNA, there is rather good agreement between the estimates of the torsional constant from dynamic properties (magnetic relaxation and fluorescence depolarization) and from equilibrium properties (cyclization and topoisomer distribution).
Discretized propagators in Hartree and Hartree–Fock theory. II. Responses to static electric and magnetic fields
Authors: Harris Robert A. , Pratt Lawrence R.
Published: July 2002

Discrete propagator methods that take account of the Pauli exclusion principle through the level of many-body Hartree theory are used to investigate the linear responses of inhomogeneous electron systems to static scalar and vector potentials. The induced densities and currents are obtained in the form of configurational integrals over Bessel functions, in close analogy with classical statistical mechanical partition functions that are configurational integrals over Boltzmann weight functions. In self-consistent field treatments the arguments of these Bessel functions are themselves functionals of the electron densities in the absence of external potentials. Qualitative differences between the magnetic responses predicted by Hartree and Hartree–Fock equations are pointed out. A simplified Hartree–Fock theory for magnetic responses is also derived. Here too, the responses are shown to be configurational integrals over Bessel functions.
Simulation of a relativistic soliton model of crystalline polymer dynamics
Authors: Wahlstrand Karna J. , Wolynes Peter G.
Published: July 2002

The study of the sine-Gordon soliton model of the dielectric α relaxation in crystalline polymers is continued. We examine the intermediate range of the coupling constant, which is essentially the ratio of intramolecular to intermolecular interaction strengths, via another stochastic molecular dynamics computer simulation. The model is that of Currie et al. of the soliton as a pseudo-relativistic particle moving in an effective periodic potential. Comparison with the previous sine-Gordon chain simulation results in increasingly poor agreement with decreasing coupling constant, probably because of the neglect of multiple soliton effects. However, we find in the process the interesting result that relativity lowers the effective rate constant in such barrier-crossing problems. Finally, we also note that the continuum limit theory which is successful for nonpolar polymers may apply to other soliton-related physical systems as well.