bluuebunny/crossref_metadata_2025 huggingface dataset:
Nature of ‘‘incubation pulses’’ in the ultraviolet laser ablation of polymethyl methacrylate
Authors: Srinivasan R. , Braren Bodil , Casey Kelly G.
Published: July 2002

The term ‘‘incubation pulses’’ is applied to the pulses of ultraviolet laser radiation which are directed initially at a polymer surface and which etch less depth per pulse than subsequent pulses which remove identical depths of material at each pulse. This incubation effect which is particularly strong when the polymer absorbs moderately at the laser wavelength has been studied in polymethyl methacrylate at 193 and 248 nm. The transformation of the surface during incubation pulses has been followed by 1) photographing the blast wave that is produced by the products that emerge from the surface, using a fast (<1 ns) dye laser that is electronically triggered to light the ablation plume after a set delay, and 2) by treating the exposed surface with a solvent to dissolve out any photolyzed material that is left in the exposed area. The results show that ablation, as shown by the ejection of the products at high velocity, occurs even during the incubation period but the surface is not etched at all or only partially. The etch pit is filled with chemically transformed material which is removed by dipping the sample in a solvent. In the photochemical model of Sutcliffe and Srinivasan [J. Appl. Phys. 60, 3315 (1986)], if UV laser ablation is seen as a volume explosion, then incubation can be attributed to insufficient production of the gaseous products that are needed to build up sufficient pressure to eject the large fragments into which the solid is largely broken up.
On optimizing maximum-power heat engines
Authors: Gordon J. M. , Huleihil Mahmoud
Published: July 2002

For a general class of heat engines operating at maximum power, in which the generic sources of irreversibility are finite-rate heat transfer and friction only, we investigate (1) the time-dependent driving functions that maximize power when heat input and heat rejection are constrained to be nonisothermal, as is the case in many conventional heat engines, and (2) the specific impact of friction on the nature of the engine cycle that maximizes power, and on the engine’s power-efficiency characteristic. The extent to which maximum power is affected by the constraints on the driving function is evaluated, as well as the time divisions on the different branches of the optimal cycle. The fundamental differences in engine performance that arise from frictional losses being internally dissipative, as opposed to externally dissipative, are derived, and illustrative examples are presented.
Measurement of deflection-mode damping in an accelerating structure
Authors: Chojnacki E. , Gai W. , Ho C. , Konecny R. , Mtingwa S. , Norem J. , Rosing M. , Schoessow P. , Simpson J.
Published: July 2002

We have directly measured the damping of wake-field deflection modes in a slow-wave accelerating structure consisting of a dielectric-lined waveguide with segmented conducting boundaries wrapped with rf absorbing material. Such damping of deflection modes is desired to prevent beam breakup instabilities. Attenuation e-folding times of 246 ps were recorded for deflection modes at the Advanced Accelerator Test Facility while the quality of the desired accelerating mode remained unaffected.
Magnetic properties of RCo4M (R=Y, Nd and Ho; M=B, Al and Ga)
Authors: Ido H. , Konno K. , Ito T. , Cheng S. F. , Sankar S. G. , Wallace W. E.
Published: July 2002

Magnetic and crystallographic measurements have been made for the compounds RCo4M (R=Y, Nd, and Ho; M=B, Al, and Ga) to intercompare the magnetic properties of RCo4B, RCo4M (M=Al and Ga) and RCo5. The compounds RCo4B crystallize in the CeCo4B type structure, while RCo4M (M=Al and Ga) in the CaCu5 type. The following main conclusions have been obtained: (1) the Curie temperature and the averaged Co-moment of RCo4M (R=Y, Nd, and Ho; M=B, Al, and Ga) are lower and smaller than those of RCo5, respectively, and 6i-site Co-moment in RCo4B is smaller than the 2c-site Co-moment by the influence of the neighboring B-layer; (2) magnetocrystalline anisotropy of R-sublattice of RCo4B is stronger than that of RCo5, while that of RCo4Al is remarkably weaker than that of RCo5; (3) the Co-sublattice anisotropy constants of YCo4M (M=B and Al) are 20% or less of that of YCo5; and (4) JR-Co and JCo-Co, which are the exchange parameters of the atomic pairs in NdCo4M (M=B and Al), have been estimated to be JR-Co/k ≂ 7 K and JCo-Co/k ≂ 200 K, where k is the Boltzman constant.
On the composition of 110 K superconductor in a (Bi,Pb)-Sr-Ca-Cu-O system
Authors: Bai V. Seshu , Ravi S. , Rajasekharan T. , Gopalan R.
Published: July 2002

The 110 K superconductor of the (Bi,Pb)-Sr-Ca-Cu-O system is prepared in single-phase form at the composition Bi1.6Pb0.4Sr2.01Ca2.71Cu3.93Oy. Disks of the material with favorable texture can be obtained in relatively short durations in an O2 poor atmosphere. The literature on the compositions of the 110 K superconductor is reviewed to reveal that the material can be synthesized in single-phase form in a range of compositions, which is poorer simultaneously in Bi and Sr with respect to the nominal composition (Bi,Pb)2Sr2Ca2Cu3Oy, (2:2:2:3). At compositions close to 2:2:2:3, intermediate cold pressing steps between heat treatments are necessary to promote the formation of single-phase material. We find that at compositions which are considerably poorer in Bi and Sr with respect to 2:2:2:3, no such steps are necessary. In this paper, the microstructural reasons for this difference in behavior are also investigated.
Photokinetic etching of polymethyl methacrylate films by continuous wave ultraviolet laser radiation
Authors: Srinivasan R.
Published: July 2002

Continuous wave laser radiation from an argon-ion laser in the wavelength range 300–330 nm or 350–380 nm can be used to etch doped polymethyl methacrylate (PMMA) films with as little thermal damage as from a pulsed, ultraviolet laser (308 or 351 nm) provided the laser spot is moved over the surface at speeds at which the transit time over its own diameter is in the order of 1–200 μs. The laser beam is focused to a spot so that its power density at the polymer surface is between 10 and 100 kW/cm2. There is an optimum speed at a given power density and wavelength at which the cutting action is a maximum. The material that is removed is vaporized in part and the rest collects in and around the cut channel as a low melting polymer. There is no acoustic report similar to that seen in ablative photodecomposition. The process is quite similar to the photokinetic etching that has been described in polyimide films using cw ultraviolet laser radiation. The mechanism of the process can be a thermal depolymerization of the PMMA that is brought about by the photon energy.
Progress towards spin-polarized scanning tunneling microscopy
Authors: Shvets I. V. , Wiesendanger R. , Bürgler D. , Tarrach G. , Güntherodt H.-J. , Coey J. M. D.
Published: July 2002

Solutions to the main problems in operating a spin-polarized scanning tunneling microscope are discussed. Preliminary experimental results obtained in the course of implementing these solutions are reported. Atomic resolution on Si(111) and Si(100) is achieved with a scanning tunneling microscope (STM) using chromium and iron tips. Fabrication of antiferromagnetic tips of Cr, MnNi, and MnPt is described. A technique of preparation of clean (100) surfaces of Fe3O4 (magnetite) is given. Low-energy electron diffraction patterns were obtained on Fe3O4 for the first time. The first STM experimental results obtained on magnetite in air and in ultrahigh vacuum are reported. Atomic resolution is obtained on Fe3O4 (100) with an ultrahigh vacuum scanning tunneling microscope using iron and tungsten tips. This is the first successful observation of atomic resolution on a ferromagnetic sample using a ferromagnetic tip.
Kinetics of platinum silicide formation during rapid thermal processing
Authors: Pant A. K. , Murarka S. P. , Shepard C. , Lanford W.
Published: July 2002

Silicide formation in Pt-Si bilayers was induced by rapid isothermal annealing using incoherent light from tungsten halogen lamps. The growth sequence of the Pt2Si and the PtSi phases that evolved as the result of the solid state reaction in the bilayers was monitored by Rutherford backscattering spectroscopy. The identification of the phases was confirmed by x-ray diffraction. Activation energies for the formation of the Pt2Si and the PtSi phase were determined to be 1.38 and 1.67 eV, respectively. These values agree with the reported results of the formation of these silicides by furnace anneals. Hence, rapid isothermal anneals do not appear to enhance the solid-state reactions in Pt-Si bilayers. It was also found that even rapid thermal anneals led to the formation of a very thin oxide that provided protection to the underlying silicide during selective Pt etch.
Resistivities of conductive composites
Authors: Ruschau G. R. , Yoshikawa S. , Newnham R. E.
Published: July 2002

A steady-state model for the resistivity of composites is presented, based on the idea that the resistance through a composite is the result of a series of a large number of resistors combined in series and parallel. There are three separate contributions to the resistance: constriction resistance at the contacts, tunneling resistance at the contacts, and the intrinsic filler resistance through each particle, with tunneling resistance generally dominating the magnitude of the overall resistance. The model predicts resistivity increases with increasing filler hardness and/or elastic modulus and insulating film thickness, while resistivity decreases with increasing particle size and intrinsic stress. The room-temperature dc resistivity behavior of conductor-filled silicone rubber composites was investigated to verify the model. Comparison of the model to this experimental data showed that good agreement could be obtained for filler materials in which the tarnish layer was a known quantity for a given powder; for other cases, the experimental values were higher than predicted.
Temperature dependencies of the lattice constants and thermal expansion coefficients of Sm3Fe5O12 and Er3Fe5O12 single crystals
Authors: Guillot M. , Rodic D. , Mitric M.
Published: July 2002

The temperature dependencies of the lattice parameters of erbium and samarium iron garnets (ErIG and SmIG) are refined from precise x-ray diffraction data obtained on small spherical single crystals in the 20–300 K temperature range; the variations of the thermal linear expansion coefficients are immediately deduced. For ErIG, the lattice constants and the expansion coefficients present anomalies observed at 20, 59, 75, and 105 K, respectively, whereas for SmIG singularities are situated at 40 and 68 K. These anomalies are interpreted as originating from spin-reorientation transition and from the onset of noncollinear magnetic structure of the rare-earth magnetic moments.
Neutron diffraction and Mössbauer effect study of several Nd2Fe17−xAlx solid solutions
Authors: Yelon W. B. , Xie H. , Long Gary J. , Pringle O. A. , Grandjean F. , Buschow K. H. J.
Published: July 2002

Aluminum-substituted Nd2Fe17−xAlx solid solutions with x=2.30, 4.44, 6.13, 8.00, and 9.40 have been studied by neutron diffraction and Mössbauer spectroscopy. A Rietveld analysis of the neutron scattering indicates that the aluminum atoms have a high initial affinity for the 18h site, show a high affinity for the 6c site at high aluminum concentrations, and are absent from the 9d site at all aluminum concentrations. The Mössbauer spectra show a maximum in both the maximum and weighted average hyperfine field for x≊2. This indicates that the lattice expansion which occurs with aluminum substitution is sufficient, at least for small values of x, where magnetic dilution is small, to enhance the intrasublattice coupling between the iron moments, and to increase the magnetic hyperfine fields.
Conductivity and magnetoresistance in magnetic granular films (invited)
Authors: Zhang Shufeng , Levy Peter M.
Published: July 2002

The transport properties in magnetic granular films are modeled by considering the spin-dependent impurity scattering within the granules and the interface roughness scattering at the boundaries of the granules. The magnetoresistance for these films is derived by using the formalism developed for layered structures with currents perpendicular to the plane of the layers and which is applicable to random systems. With this model, various features of the magnetoresistance observed in recent experiments can be explained and the optimal choice of parameters to maximize the magnetoresistance can be determined.
Optical properties of lithium tungsten bronze thin films
Authors: Green Mino , Hussain Z.
Published: July 2002

The optical absorption spectra of fine grained polycrystalline thin films of lithium tungsten bronze are reported. The results are similar to the previously reported data for hydrogen and sodium tungsten bronzes: pointing to the secondary importance of the nature of the guest atom. The visible blue color band, peaking in the near infrared, is attributed to electron transitions from the conduction band to higher lying states. The absence of free carrier absorption in material, which is very small grained polycrystalline material (tens of nm and less) is tentatively attributed to a modified electron–phonon interaction. The main gap of the bronze, which is indirect, is effectively increased by electron filling of the conduction band, this effect is overwhelmed by bandgap shifts associated with crystallographic changes, which are a function of the guest atom type and concentration.
Nonlinear electrical properties of lead-lanthanum-titanate thin films deposited by multi-ion-beam reactive sputtering
Authors: Fox G. R. , Krupanidhi S. B.
Published: July 2002

The high-field electrical properties of ferroelectric lead-lanthanum-titanate (PLT) thin films deposited by multi-ion-beam reactive sputtering are presented. Space-charge-limited (SCL) conduction theory incorporating the effects of electron traps is used to explain the current-density dependence on applied voltage. Three conduction mechanisms, ohmic, SCL, and trap-filled limited (TFL), were observed with increasing applied potential. Increasing excess PbO content in PLT films increases the absolute current density measured in all three conduction regions and decreases the potential required for the transition between ohmic conduction and SCL conduction. Electron trap density is increased with the addition of La, resulting in increased voltages for the transition between SCL and TFL conduction. Permittivity and polarization hysteresis behavior is interpreted by considering the conduction contributions under linear low-field conditions and nonlinear high-field conditions. Hysteresis loop frequency dependence, asymmetry, and off-center shift from the zero-field and zero-polarization origin arise from conduction contributions and nonlinear internal fields, which are explained by applying SCL theory. Pulse switching results, which eliminate conduction contributions, are compared with normal Sawyer–Tower hysteresis loop measurements of the remanent polarization.
LiNbO3 film with a new epitaxial orientation on R-cut sapphire
Authors: Fujimura Norifumi , Kakinoki Masami , Tsuboi Hiroshi , Ito Taichiro
Published: July 2002

It is argued that epitaxial films of ionically bonded materials are more easily achieved than those of covalently bonded materials. Good epitaxy can be achieved despite a relatively large lattice mismatch with the substrate. However, a strong influence of interfacial energy can result in difficulty in controlling the orientation of epitaxial films. The crystallographic orientation of ionically bonded LiNbO3 films was therefore studied. Growth orientation was controlled by paying attention to the sharing between octahedra in the structure and to the formation of the octahedra containing lithium and niobium ions. The orientation of the film on R-cut (011̄2) sapphire changed from (011̄2) to (101̄0) via (112̄0) by increasing the Li concentration in the film. Lithium concentration could be increased by increasing the rf power, O2 partial pressure and total gas pressure, and decreasing the substrate temperature during sputtering deposition. The (011̄2) and (101̄0) films were epitaxial with respect to the substrate. The (101̄0) epitaxial film, in particular, was of excellent quality. Detailed studies on the crystallinity and the formation mechanism of LiNbO3 film with a new epitaxial orientation (101̄0) on (011̄2) sapphire was performed. Based on the degree of freedom in sharing of octahedra containing Li and Nb, sufficient Li concentration and effects of the interfacial restriction are considered to promote the formation of the (101̄0) epitaxial film. If deposition is performed under the conditions by which (101̄0) epitaxial film is formed, excellent epitaxial film can be formed because the interface between (101̄0) LiNbO3 and (011̄2) sapphire has a larger cohesive energy.
Optical and thermal properties of electron- and hole-trapping sites in the x-ray storage phosphor RbI:X (X=Tl+, In+, Pb2+, Eu2+)
Authors: Thoms M. , Seggern H. von , Winnacker A.
Published: July 2002

Two-dimensional x-ray detectors based on x-ray storage phosphors are utilized in the field of medicine, biology, and physics. Defect centers and mechanisms contributing to the photostimulated luminescence (PSL) process of the x-ray storage phosphor RbI:X (X=Tl+, In+, Pb2+, Eu2+) are reported. By optical and thermoluminescence spectroscopy the electron and hole storage centers involved in the PSL process were identified. F- and Z type and Tl0 centers turned out to be the occupied electron storage centers, VK− and dopant-related VKA centers the hole-trapping sites. The specific choice of the dopant yields storage phosphors with different physical properties, such as emission characteristics, photostimulation characteristics, and thermal stability of the information storage. At T=300 K the information stored in the RbI:Tl+ phosphor is unstable, in RbI:Eu2+ it is nearly stable and in RbI:In+ stable. A physical model for the PSL as well as for the thermoluminescence is derived. In the Tl+-doped material Tl0 centers were found to act as efficient electron storage centers at temperatures below 180 K with a 4.7 times larger storage capability than F centers.
Effect of longitudinal flow velocity on the emission spectrum of CO2 lasers
Authors: Biswas D. J. , Nilaya Padma , Sehgal S. K. , Chatterjee U. K.
Published: July 2002

By operating a low-pressure cw CO2 laser at very high flow velocities in order to smooth out the nonuniform spatial distribution in the inverted population, experimental evidence has been provided of the important role played by the velocity of flow in the longitudinal direction in determining the rotational emission spectrum of CO2 lasers.
Thermoelectric properties of Cu-doped dysprosium sesquisulfide
Authors: Han S. H. , Gschneidner K. A. , Cook B. A.
Published: July 2002

The electrical resistivity, thermoelectric power (Seebeck coefficient), Hall effect, and thermal conductivity have been measured in Cux(Dy2S3)1−x compounds with the η-orthorhombic structure in the composition range 0.006≤x≤0.15 in order to determine their potential as high temperature (300–1000 °C) thermoelectric materials. In this temperature and composition range Cu-doped Dy2S3 behaves as a degenerate semiconductor and shows itinerant conduction. The electrical resistivity and the Seebeck coefficient increased with increasing temperature and reach a maximum value of 4.35–7.13 mΩ cm and −163 to −177 μV/°C, respectively, depending upon the Cu concentration. A maximum power factor of 7.9 μW/cm °C2 for the Cu0.039(Dy2S3)0.961 alloy was observed at 690 °C. The 300–1000 °C integrated average power factor shows that the optimum Cu-doping level in the Dy2S3 matrix is 5–7 at. %. The thermal conductivity of Cu0.039(Dy2S3)0.961 decreased with increasing temperature from 300 to 1000 °C and was governed by both electronic and lattice contributions up to 600 °C, but above 600 °C the electronic contribution is predominant. The 300–1000 °C integrated average thermal conductivity of Cu0.039(Dy2S3)0.961 is 18.38 mW/cm °C, which gives this material a maximum figure of merit Z of 0.440×10−3/°C at 800 °C.
Compositional dependence of AlAsySb1−y ternaries on the ratio of Sb/As fluxes and on the substrate temperature
Authors: Németh S. , Grietens B. , Borghs G.
Published: July 2002

We present a systematic study of the compositional dependence of AlAsySb1−y layers, grown by molecular beam epitaxy, on the ratio of As to Sb fluxes and on the substrate temperature. The initial results clearly demonstrate that variations in the composition can be observed by changing the above-mentioned parameters. The emphasis of this communication is on understanding the qualitative trends of these dependencies.
Comparison of polycrystalline Cu(In,Ga)Se2 device efficiency with junction depth and interfacial structure
Authors: Nelson Art J. , Gabor A. M. , Contreras M. A. , Tuttle J. R. , Noufi R. , Sobol P. E. , Asoka-Kumar P. , Lynn K. G.
Published: July 2002

X-ray photoemission spectroscopy (XPS) and positron annihilation spectroscopy (PAS) have been used to characterize the surface versus bulk composition, electronic, and physical structure of polycrystalline Cu(In,Ga)Se2 thin-film interfaces. Angle-resolved high-resolution photoemission measurements on the valence-band electronic structure and Cu 2p, In 3d, Ga 2p, and Se 3d core lines were used to evaluate the surface and near surface chemistry of CuInSe2 and Cu(In,Ga)Se2 device grade thin films. XPS compositional depth profiles were also acquired from the near surface region. PAS was used as a nondestructive, depth-sensitive probe for open-volume-type defects. Results of these measurements are related to device efficiencies to show the effects of compositional variations and defect concentrations in the near surface region on device performance.
Two-dimensional device modeling and analysis of GaInAs metal–semiconductor–metal photodiode structures
Authors: Averin S. , Sachot R. , Hugi J. , de Fays M. , Ilegems M.
Published: July 2002

A two-dimensional self-consistent time-dependent simulation technique has been developed to investigate electron-hole transport processes in the active region of metal–semiconductor–metal (MSM) interdigitated photodiode structures and to analyze their high-speed response. The distribution of the electric field inside the MSM device is determined by numerically solving the two-dimensional Poisson’s equation by the modified fast elliptic solver method. A set of superparticles photogenerated at a particular wavelength is analyzed with a given initial distribution of the potential and given boundary conditions, and the evolution of the particles is traced in time through the active region of the MSM device. Circuit loading, electric field effects in the MSM structure with various finger separations, background doping, carrier trapping, and recombination are included in the simulation program. Owing to miniaturization of devices, the classical scaling laws lose their validity while various performance degrading effects appear. The simulations show that the main problem in MSM devices with a small contact separation is the low electric field penetration depth. This results in different electron and hole collection rates and in a poor response time. The trade-off between the high-speed response and the internal quantum efficiency is examined and ways to improve the high-speed response are indicated. Modeling results are compared with experimental data on Ga0.47In0.53As based MSM photodiodes.
Stress relaxation in highly strained InAs/GaAs structures as studied by finite element analysis and transmission electron microscopy
Authors: Benabbas T. , François P. , Androussi Y. , Lefebvre A.
Published: July 2002

Finite element (FE) analysis and transmission electron microscopy (TEM) observations have been used to model stress relaxation in InAs quantum dots deposited on (001) GaAs. TEM observations show that these islands are coherently strained and the corresponding strain contrast is simulated using the dynamical electron diffraction contrast theory. The dot strain fields used for the TEM contrast simulations are deduced from FE calculations. These calculations show that elastic stress relaxation mainly occurs at the crest of the island and that the underlying substrate is under tension. That experimental TEM images and simulated images should match shows that the FE method of determination of the dot strain fields is valid (even in the case of microscopic objects), and that the shape of islands can be specified.
Evidence of preferential diffusion of impurities along grain boundaries in very pure niobium used for radio frequency cavities
Authors: Antoine C. , Bonin B. , Safa H. , Berthier B. , Tessier C. , Tocellier P. , Chevarier A. , Chevarier N. , Roux B.
Published: July 2002

In order to overcome dissipation due to impurity segregation at grain boundaries, niobium cavities are submitted to a purification annealing (1300 °C±200 °C under vacuum) during which titanium is evaporated onto the Nb surface. The resulting titanium layer acts as a solid state getter reacting with light impurities (H, C, N, O), thereby removing these impurities from the bulk of the niobium. Preferential titanium diffusion along grain boundaries have been studied using proton-induced x-ray emission.
Extraction of metal-oxide-semiconductor field-effect-transistor interface state and trapped charge spatial distributions using a physics-based algorithm
Authors: Chim W. K. , Leang S. E. , Chan D. S. H.
Published: July 2002

In this article, a new charge-extraction algorithm is proposed for extracting the spatial distributions of hot-carrier-induced interface states and trapped charges in p- and n- metal-oxide-semiconductor field-effect transistors, based on the charge-pumping measurement data. This extraction algorithm is physics based and provides a better understanding of how the presence of hot-carrier-induced trapped charges and interface states affect the charge-pumping curves. The extraction time for this new algorithm is very fast (typically 30 s) and does not require very tedious computer simulation. The verification of this method was performed using TSUPREM-4 and MEDICI simulations. With this new extraction method, one can gain better insight into the degradation mechanisms taking place under different hot-carrier stressing conditions.
Phases, morphology, and diffusion in CuInxGa1−xSe2 thin films
Authors: Marudachalam M. , Birkmire R. W. , Hichri H. , Schultz J. M. , Swartzlander A. , Al-Jassim M. M.
Published: July 2002

CuIn x Ga 1−x Se 2 thin films, with various Ga/(Ga+In) ratios, suitable for solar cells were processed by selenizing stacked Cu, Ga, and In precursor layers in a H2Se reactor in the temperature range of 400–500 °C. Cu/Ga/In and Cu/In/Ga precursors were obtained by sequential sputtering of the elemental layers. The Cu/Ga/In and Cu/In/Ga precursors, and the selenized films were characterized by scanning electron microscopy, x-ray diffraction, energy dispersive spectroscopy, and Auger electron spectroscopy. The precursors contained only binary and elemental phases in the as-deposited condition and after annealing. The selenized films had a nonuniform distribution of Ga and In. The surface of the selenized films were In rich, while the Mo/film interface in these films was Ga rich. The selenized films with Ga/(Ga+In) ratios greater than 0.25 contain graded Ga and In compositions, and the selenized films with Ga/(Ga+In) ratios less than 0.6 contain a phase-separated mixture of CuInSe2 and CuGaSe2 with the CuInSe2 near the surface and the CuGaSe2 near the Mo/film interface. Single phase, homogeneous CuInxGa1−xSe2 films were obtained by annealing the as-selenized films in argon in the temperature range of 500–600 °C for 60 min. Interdiffusion of In and Ga between the CuGaSe2 and the CuInSe2 phases was found to be responsible for the homogenization process. This homogenization process does not occur in the presence of a selenium atmosphere. Diffusion measurements yielded similar interdiffusion coefficients for Ga and In. The annealing temperature and time to effect homogenization depends on the Ga/(Ga+In) ratio of the absorber films. Films with lower Ga/(Ga+In) ratios require a homogenization temperature of 600 °C or more and films with higher Ga/(Ga+In) ratios homogenize at a lower temperature of 400–500 °C, for an annealing time of 60 min.
Investigation of aluminum nitride grown by metal–organic chemical-vapor deposition on silicon carbide
Authors: Zetterling C.-M. , Östling M. , Wongchotigul K. , Spencer M. G. , Tang X. , Harris C. I. , Nordell N. , Wong S. S.
Published: July 2002

Undoped single crystalline aluminum nitride films were grown on 4H and 6H SiC substrates using metal–organic chemical-vapor deposition at 1200 °C. From in situ reflection high-energy electron diffraction, x-ray diffraction rocking curves, and cathodoluminescence spectra, the crystallinity of the films was confirmed. Atomic force microscopy showed that some films were substantially dominated by island growth, rather than step flow growth. Aluminum was evaporated to form metal–insulator–semiconductor (MIS) capacitors for high-frequency capacitance voltage measurements carried out at room temperature. Low leakage made it possible to measure the structures and characterize accumulation, depletion, deep depletion, and, in some cases, inversion. From independent optical thickness measurements, the relative dielectric constant of aluminum nitride was confirmed at 8.4. The flatband voltage of the AlN MIS capacitors on p-type SiC was close to the theoretical value expected. The films were stressed up to 60 V (3 MV/cm) without breakdown, but excessive leakage currents (>0.1 A/cm2), probably dominated by grain-boundary conduction, shifted the flatband voltage of the capacitors. These results indicate the possibility of replacing silicon dioxide with aluminum nitride in SiC field effect transistors using insulated gates.
Concerning superconducting inertial guidance gyroscopes inside superconducting magnetic shields
Authors: Satterthwaite James C. , Gawlinski Edward T.
Published: July 2002

Superconductors can in theory be used to detect rotation by Josephson interference or by detection of the London field, a magnetic induction that fills the interior of any rotating bulk superconductor. One might hope to use these properties of superconductors to build a practical inertial guidance gyroscope. A problem arises from the necessity of surrounding the device with superconducting magnetic shielding: the London field generated by a co-rotating shield eliminates the response of the superconducting device within the shield. The present article demonstrates this point more rigorously than has been done before, discussing solutions of Ampère’s law for rotating and nonrotating superconductors and paying careful attention to boundary conditions. Beginning with a supercurrent density derivable from either the Ginzburg-Landau or the London theory of superconductivity, the article shows: 1) that a superconducting device cannot distinguish between rotation and an applied magnetic field: 2) that a superconducting device surrounded by a co-rotating superconducting shield cannot detect rotation. The term “superconducting gyroscope” in this article refers only to a device whose working principle is the response of the superconductor itself to rotation, not to any device in which superconducting electronic components are used to detect some other effect.
Hole transport and doping states in epitaxial CuIn1−xGaxSe2
Authors: Schroeder David J. , Hernandez Jose Luis , Berry Gene D. , Rockett Angus A.
Published: July 2002

Temperature dependent mobility, resistivity, and carrier concentration measurements were made on epitaxial single crystal thin films of group III-rich CuIn1−xGaxSe2 (CIGS). The films were produced using a hybrid sputtering and evaporation process on GaAs substrates. Samples with average Ga/(In+Ga) values between 0.03 and 1.0 and Cu/(In+Ga) between 0.73 and 1.00 were measured. All films were p type with room-temperature carrier concentrations between 4×1016 and 2×1019 cm−3. Fits to electrical measurements were consistent with the presence of two acceptor levels, with activation energies of 167±20 and 42±8 meV at low Ga contents, and compensating donors in all samples. Increasing Ga content was found to increase acceptor density and decrease acceptor level depth. Hole mobilities near room temperature were found to be between 167 and 311 cm2/V s and peak mobilities were between 439 and 1760 cm2/V s. Mobility behavior did not change significantly as a function of composition or the presence of a Ga gradient.
Correlation of Seebeck coefficient and electric conductivity in polyaniline and polypyrrole
Authors: Mateeva N. , Niculescu H. , Schlenoff J. , Testardi L. R.
Published: July 2002

We have measured the Seebeck coefficient and electric conductivity in the air-stable conducting polymers polyaniline and polypyrrole at different doping levels. We find, at 300 K, the general correlation that the logarithm of the electrical conductivity varies linearly with the Seebeck coefficient on doping, but with a proportionality substantially in excess of a prediction from simple theory for a single type of mobile carrier. The correlation is unexpected in its universality and unfavorable in its consequences for thermoelectric applications. A standard model suggests that conduction by carriers of both signs may occur in these doped polymers, which thus leads to reduced thermoelectric efficiency. We also show that polyacetylene (which is not air stable), does exhibit the correlation with the expected proportionality, and, thus, its properties could be more favorable for thermoelectricity.
Luminescence due to electron-hole condensation in silicon-on-insulator
Authors: Tajima Michio , Ibuka Shigeo
Published: July 2002

We show that photoluminescence from a thin silicon-on-insulator (SOI) layer with a thickness of about 0.1 μm excited by ultraviolet (UV) light originates from the recombination of electrons and holes in their condensed phase. The UV excitation increases the density of electrons and holes in the SOI layer, since the UV light is predominantly absorbed in that layer and the excited carriers cannot diffuse into the base wafer because of the presence of a buried oxide layer which acts as a diffusion barrier. As a result, a characteristic “condensate luminescence” is emitted from the SOI layer. The spectral shape of the condensate luminescence agrees with the calculated shape using convolution of the occupied electron and hole densities of states. A systematic spectral change induced by the variation of excitation intensity and the sample temperature is consistently explained by the phase diagram of the electron-hole system in Si which consists of free exciton (FE), electron-hole plasma (EHP), and electron-hole liquid (EHL). The phase transition from FE to EHP or that from FE to EHL is observed with increasing excitation intensity, according to whether the temperature is higher or lower than the critical temperature of EHL. These results confirm for us that the condensate luminescence in the SOI layer is due to the recombination of electrons and holes in their condensed phase.
Finite element analysis of strain effects on electronic and transport properties in quantum dots and wires
Authors: Johnson H. T. , Freund L. B. , Akyüz C. D. , Zaslavsky A.
Published: July 2002

Lattice mismatch in epitaxial layered heterostructures with small characteristic lengths induces large, spatially nonuniform strains. The components of the strain tensor have been shown experimentally to affect the electronic properties of semiconductor structures. Here, a technique is presented for calculating the influence of strain on electronic properties. First, the linear elastic strain in a quantum dot or wire is determined by a finite element calculation. A strain-induced potential field that shifts and couples the valence subbands in the structure is then determined from deformation potential theory. The time-independent Schrödinger equation, including the nonuniform strain-induced potential and a potential due to the heterostructure layers, is then solved, also by means of the finite element method. The solution consists of the wave functions and energies of states confined to the active region of the structure; these are the features which govern the electronic and transport properties of devices. As examples, two SixGe1−x submicron resonant tunneling devices, a quantum wire with two-dimensional confinement and a quantum dot with three-dimensional confinement, are analyzed. Experimentally measured resonant tunneling current peaks corresponding to the valence subbands in the material are modeled by generating densities of confined states in the structures. Size and composition-dependent strain effects are examined for both devices. In both the quantum dot and the quantum wire, the strain effects on the wave functions and energies of confined states are evident in the calculated densities of confined states in the structures, which are found to be consistent with experimentally measured tunneling current/voltage curves for resonant tunneling diodes.
Transmission and gain of singly and doubly focusing refractive x-ray lenses
Authors: Lengeler B. , Tümmler J. , Snigirev A. , Snigireva I. , Raven C.
Published: July 2002

Multiple refractive lenses with a focal length of 1 to 2 m are a new tool for focusing hard x rays to a spot size in the micrometer range. They may be used for microdiffraction, microfluorescence, and coherent imaging. The lenses may be focusing in one or two dimensions. In this article, we have calculated the transmission and the gain for linear lens arrays, for crossed linear arrays and for doubly focusing lenses with parabolic profile. It is essential to minimize the mass absorption coefficient μ/ρ by choosing low Z materials in order to optimize the transmission. The gain of the lenses can be as high as 5000 and more, i.e., the intensity in the focal spot can be 5000 times higher than that behind a pinhole of size equal to the spot size. In real lenses the gain is smaller and the focal spot is blurred by lens imperfections, by Compton scattering, and by small angle x-ray scattering (SAS). In the present investigation different low Z materials have been tested for SAS. Different linear and crossed linear lenses made of beryllium, boron nitride, pyrographite, plexiglass, polycarbonate, polyoxymethylene, Vespel, and aluminium have been tested for focal spot size, gain, and background. The maximum gain obtained up to now was 13. The focal spot size is slightly larger than the value expected from demagnification of the source size. Possibilities for improving the lens performance are discussed.
Effect of phonon confinement on the thermoelectric figure of merit of quantum wells
Authors: Balandin Alexander , Wang Kang L.
Published: July 2002

Recently, it has been shown that the thermoelectric figure of merit is strongly enhanced in quantum wells and superlattices due to two-dimensional carrier confinement. We predict that the figure of merit can increase even further in quantum well structures with free-surface or rigid boundaries. This additional increase is due to spatial confinement of acoustic phonons and corresponding modification of their group velocities. The latter leads to an increase of the phonon relaxation rates and thus, a significant drop in the lattice thermal conductivity.
Arsenic for antimony exchange on GaSb, its impacts on surface morphology, and interface structure
Authors: Xie Qianghua , Van Nostrand J. E. , Brown J. L. , Stutz C. E.
Published: July 2002

We quantify the rates and total amounts of the arsenic for antimony exchange on both the Sb-terminated and Ga (or In)-terminated GaSb (001) surfaces using in situ real time line-of-sight mass spectrometry (LOS-MS) during molecular beam epitaxy. On the Sb-terminated GaSb (001) surface, an As for Sb exchange is observed to occur at all values of incident As2 flux considered. At high substrate temperature, three-dimensional (3D) nanometer-sized clusters from as a consequence of As/Sb exchange and lattice mismatch strain between GaAs and GaSb. The 3D clusters are found to have lateral dimensions of ∼10–30 nm and heights of 1–3 nm by atomic force microscopy (AFM). By contrast, at lower substrate temperatures a two-dimensional surface morphology is maintained, and AFM reveals an array of atomically flat terraces. On the surface terminated by one monolayer (ML) of Ga or In, there exists a critical As2 flux below which the As/Sb exchange is greatly diminished. The net amounts of Sb leaving the surface during one period of InAs/GaSb type-II superlattice growth are measured in real time by LOS-MS and estimated to be in the range of 0–0.4 ML for the various conditions used. By supplying only an As2 beam to a GaSb surface covered by InAs, the Sb riding over the InAs layer is replaced by arsenic and the total amount of such Sb is measured. The amount of Sb riding on the InAs can be as large as 0.8 ML for the first 1 ML of InAs and it gradually decreases to zero as the number of InAs monolayer increases. X-ray diffraction data show that all the InAs/GaSb superlattices coherently match with the GaSb substrate in the growth plane. The average lattice constant along the growth direction reduces with decreasing Sb mole fraction shown by the increased Sb desorption signal. Using the information on As/Sb exchange and Sb riding on the InAs surface, we predict an average lattice constant along the growth direction to be consistent with the measured one to within 2×10−4.
Formation of uniform nanoscale Si islands on a Si(111)-7×7 substrate
Authors: Shigeta Yukichi , Fujino Hiroyuki , Maki Kunisuke
Published: July 2002

During epitaxial growth on a substrate with a stable surface structure, rearrangement from a superlattice to a normal 1×1 lattice is essential when the substrate is covered with a growing layer. Since rearrangement needs activation energy, lateral growth is prevented at the boundary of the structure unit. In the growth of Si on a stable 7×7 structure composed of a dimer-adatom-stacking- fault layer, lateral growth of islands is prevented at the dimer row. Such a restriction on the lateral growth leads to a discontinuous size distribution of islands, whose size depends on the size of the 7×7 structure unit. In the initial stage of nucleation and growth, we have observed many rounded Si islands with uniform diameters of 3.8 nm when the substrate temperature has been held at 380 °C for 10 min after deposition of Si. From the change in the island size distribution as a function of annealing time after the deposition, we conclude that the rounded island is constructed of atoms created by the dissociation of unstable small islands (<40 atoms).
Incomplete structural phase transition and lattice destruction in Sr1−xLaxTiO3+δ
Authors: Arao M. , Koyama Y. , Inoue Y.
Published: July 2002

In situ observation of the R3̄c-to-Pbnm structural phase transition in Sr0.5La0.5TiO3+δ has been made mainly by transmission electron microscopy to understand features of the transition in samples with relatively large nonstoichiometry of more than δ=0.05. Instead of the appearance of the superlattice reflection spot exhibiting the phase transition, only the diffuse maximum appears at q=1/2〈110〉 and q=1/2〈100〉 in the simple cubic reciprocal lattice below a transition temperature Ts of about 190 K. The subsequent aging below Ts also results in the local lattice destruction. These features found in the samples with large nonstoichiometry should be caused by both the nonstoichiometry and the strain field induced by the change in the crystal system in the transition.
Self-diffusivity of liquid silicon measured by pulsed laser melting
Authors: Sanders P. G. , Aziz M. J.
Published: July 2002

The silicon liquid self-diffusivity was determined by pulsed laser melting of 30Si ion implanted silicon-on-insulator thin films. Secondary ion mass spectrometry was employed to measure the 30Si+ concentration-depth profile before and after melting and solidification. Melt depth versus time and total melt duration were monitored by time-resolved lateral electrical conductance and optical reflectance measurements. One-dimensional diffusion simulations were utilized to match the final 30Si+ experimental concentration spatial profile given the initial concentration profile and the temporal melt-depth profile. The silicon liquid self-diffusivity at the melting point is (4.0±0.5)×10−4 cm2/s. Calculations of buoyancy and Marangoni convection indicate that convective contamination is unlikely.
Photoemission study of aluminum/tris-(8-hydroxyquinoline) aluminum and aluminum/LiF/tris-(8-hydroxyquinoline) aluminum interfaces
Authors: Le Quoc Toan , Yan Li , Gao Yongli , Mason M. G. , Giesen D. J. , Tang C. W.
Published: July 2002

We have investigated the interfaces of aluminum on tris-(8-hydroxyquinoline) aluminum (Alq3) and aluminum on LiF/Alq3, using x-ray and ultraviolet photoemission spectroscopy (UPS). Aluminum appears to react destructively with Alq3 causing significant modification of the oxygen, nitrogen, and aluminum spectra. The well-defined UPS spectrum of Alq3 is quickly destroyed by very low coverages of aluminum. With only a 5 Å layer of LiF on the Alq3, the reaction with aluminum is significantly suppressed. The Alq3 molecular orbital features in the UPS shift to higher binding energy but remain easily recognizable. In addition, a well-defined gap-state forms which is significantly different from that produced without LiF. Both the core-level spectra and the gap-state suggest that the Alq3 anion is formed in the presence of aluminum and LiF.
Investigations of harmonic generations in aperiodic optical superlattices
Authors: Gu Ben-Yuan , Zhang Yan , Dong Bi-Zhen
Published: July 2002

We envisage the enhanced effect of harmonic generations from aperiodic optical superlattices (AOSs) achieved by inverting poled ferroelectric domains in samples. The search of optimal AOSs structures belongs to solving a difficult inverse source problem in nonlinear optics. We employ the simulated annealing method to successfully deal with this problem. We present the analyses of the design idea in the real-space representation and carry out several model designs. The constructed AOSs implement multiple wavelength second-harmonic generation and the coupled third-harmonic generation with an identical effective nonlinear coefficient at the preassigned wavelengths. The simulations show that the harmonic generations in the constructed AOSs can approach the prescribed goal better than those with the Fibonacci optical superlattice. The effective nonlinear coefficients versus the optical wave propagating distance from the impinging surface of incident light in the sample exhibit monotonically increasing behavior. This clearly infers that the contribution from each and every block to the optical parametric processes is in the constructive interference state. We also investigate the influence of the random fluctuation of the thickness of blocks on the performance of the constructed AOSs in detail for simulating practical fabrications and testing the performance stability of the sample.