bluuebunny/crossref_metadata_2025 huggingface dataset:
Indirect excitation of 1.5 μm emission from Er3+ in silicon-rich silica
Authors: Kenyon A. J. , Chryssou C. E. , Pitt C. W.
Published: July 2002

We report the observation of near-IR emission from erbium in silicon-rich silica, excited using a filtered white-light source. The characteristic I13/24–4I15/2 intra-4f transition at 1535 nm is observed even when excitation wavelengths corresponding to the principal erbium optical absorption bands are removed using selective filtering. We ascribe this effect to an efficient transfer mechanism between silicon nanoclusters present in the silicon-rich silica films and the rare-earth ions. This is in good agreement with our previous work in this area and suggests the possibility of obtaining flashlamp-pumped erbium optoelectronic devices.
Microwave response near zero magnetic field in transition-metal-doped silicate glasses
Authors: Rakhimov R. R. , Ries H. R. , Jones D. E. , Glebov L. B. , Glebova L. N.
Published: July 2002

A sharp microwave response centered at zero magnetic field was observed in manganese- and iron-doped Na2O–CaO–MgO–SiO2 glasses with phase opposite to the normal Mn+2 and Fe+3 paramagnetic signals. The response can be described as magneto-induced microwave conductivity in the dielectric glass that derives from spin-dependent charge migration within the first coordination sphere of the paramagnetic ion. In contrast to the spin-polarized tunneling in ferromagnets between different valence states of metals, the observed effect is due to spin-dependent tunneling that occurs in the vicinity of manganese or iron in diluted paramagnetic systems.
Determination of the concentration of hot-carrier-induced bulk defects in laser-recrystallized polysilicon thin film transistors
Authors: Brown T. M. , Migliorato P.
Published: July 2002

We present a method to investigate hot carrier induced defects in laser recrystallized polysilicon thin film transistors, based on the use of a structure with front and back gate and the analysis of the off current. The maximum process temperature for these devices was 425 °C. We find that both positive and negative VGS produce similar degradation of the characteristics and the main effect is bulk, rather than interface, state creation. The changes in the I–V characteristics are accounted for by an increase of the dangling bond concentration, which is obtained as a fitting parameter, giving a good agreement between experiment and simulation.
Conductance quantization in nanoscale vertical structure silicon field-effect transistors with a wrap gate
Authors: Nishiguchi K. , Oda S.
Published: July 2002

Experimental results of quantum ballistic transport in single quantum contact by using vertical structure silicon field effect transistors with a wrap gate are presented. Based on dc measurement, the conductance–voltage characteristics show quantized plateaus at multiples of 2e2/h. The devices were prepared by electron beam lithography and by combinations of various types of etching. The channel is fabricated by the chemical vapor deposition of amorphous silicon and solid-phase crystallization. The vertical structure allows a channel length as short as 30 nm, which is defined by the film thickness. The effective channel is reduced by the depletion potential, resulting in a much narrower channel width compared to the geometrical width of 60 nm. Thus, the effective size of the silicon transistor is smaller than the elastic mean free path of 40 nm, resulting in the conduction quantization at 3–5 K.
Electrochemical passivation of gallium arsenide surface with organic self-assembled monolayers in aqueous electrolytes
Authors: Adlkofer Klaus , Tanaka Motomu , Hillebrandt Heiko , Wiegand Gerald , Sackmann Erich , Bolom Tibor , Deutschmann Rainer , Abstreiter Gerhard
Published: July 2002

Self-assembled monolayers of octadecylthiol (ODT) were reconstituted on freshly etched gallium arsenide (n-GaAs) for the electrochemical stabilization against decomposition of the surfaces (passivation) in aqueous buffers. The surface composition was evaluated by x-ray photoelectron spectroscopy to optimize the surface treatment before ODT deposition. Electrochemical properties of the monolayers were monitored by cyclic voltammetry and impedance spectroscopy. The impedance spectrum of the photoetched n-GaAs after the deposition of the ODT monolayer was stable in an aqueous electrolyte at pH=7.5 for more than 24 h within the sensitivity of our experimental technique. The effective passivation of GaAs surfaces is an essential step towards biosensor applications.
Maskless fabrication of field-emitter array by focused ion and electron beam
Authors: Yavas O. , Ochiai C. , Takai M. , Hosono A. , Okuda S.
Published: July 2002

Niobium-gated field-emitter arrays with Pt tips were fabricated using focused ion and electron beams. A promising approach, based on a two-step etch process in a Nb/SiO2/Si structure, has been implemented for the suppression of beam-induced damage and contamination in the processed area during the production of the gate openings. Only the top Nb layer was removed for gate openings by physical sputtering using the focused ion beam. The underlying SiO2 was subsequently removed by wet etching. Deposition of Pt pillars into these gate openings using electron-beam-induced chemical reaction resulted in field emission at an applied gate bias of 50 V even without any thermal annealing process.
Nanocrystalline p-type transparent Cu–Al–O semiconductor prepared by chemical-vapor deposition with Cu(acac)2 and Al(acac)3 precursors
Authors: Gong Hao , Wang Yue , Luo Yi
Published: July 2002

P-type transparent copper–aluminum–oxide semiconductor films are prepared by the use of the chemical-vapor deposition (CVD) technique with Cu(acac)2 and Al(acac)3 precursors. Transmission electron microscopy and electron diffraction suggest that the films contain nanocrystalline phases of CuAlO2 and Cu2O, in which CuAlO2 is dominant. Both Hall technique and Seebeck measurement reveal that the film is p type, and a very high room-temperature conductivity of 2 S cm−1 is achieved. This success of high-conductive p-type transparent semiconductor using CVD further paves the way for transparent semiconductor p–n junctions as well as industrial mass production of the relevant devices. The Hall measurement of the film shows a sheet mobility of 0.16 cm2 V−1 S−1 and a carrier concentration of 1.8×1019 cm−3. A wide optical gap of 3.75 eV due to quantum confinement is found, and the activation energy for the positive holes is 0.12 eV.
Magnetic tunnel junction performance versus barrier thickness: NiFe/AlOx/NiFe junctions fabricated from a wedged Al layer
Authors: Covington M. , Nowak J. , Song D.
Published: July 2002

The resistance–area product (R*A) and the magnetoresistance (MR) of NiFe/AlOx/NiFe spin-dependent tunnel junctions exhibit a strong dependence on the thickness of Al before oxidation. We obtain these data from wafers where we uniformly oxidize an Al layer with a wedged thickness profile, enabling us to reliably characterize the effect of Al thickness variations with subangstrom precision. The R*A drops from 104 to 102 Ω μm2 as the Al thickness decreases from 9 to 4 Å, respectively. The MR is highest (21%) for an Al thickness of 7 Å, where the Al layer is fully oxidized and the oxidation of the bottom NiFe electrode is minimal.
Improvement of the interface quality during thermal oxidation of Al0.98Ga0.02As layers due to the presence of low-temperature-grown GaAs
Authors: Ferrer J. C. , Liliental-Weber Z. , Reese H. , Chiu Y. J. , Hu E.
Published: July 2002

The role of a low-temperature-grown GaAs (LT GaAs) layer on the lateral oxidation of an Al0.98Ga0.02As/GaAs layer structure has been studied by transmission electron microscopy. Results show that structures incorporating LT GaAs develop better quality oxide/GaAs interfaces compared to reference samples without LT GaAs. While the latter have As accumulation in the vicinity of these interfaces, the structures with LT layers display sharper oxide–GaAs interfaces with a reduced concentration of As. These results are explained in terms of the high Ga vacancy concentration in the LT GaAs and the possible influence of those vacancies in enhancing As diffusion away from the oxide–semiconductor interface.
Visible emission from electroluminescent devices using an amorphous AlN:Er3+ thin-film phosphor
Authors: Dimitrova V. I. , Van Patten P. G. , Richardson H. H. , Kordesch M. E.
Published: July 2002

Electroluminescence (EL) studies of AlN:Er alternating-current thin-film electroluminescent (ACTFEL) devices were performed at 300 K. Thin films of Er-doped AlN, ∼200 nm thick, were grown on indium–tin–oxide/aluminum–titanium–oxide/glass substrates using rf magnetron sputtering in a nitrogen atmosphere. The turn-on voltage was found to be around 70–80 and 100 V for ACTFEL devices without and with a top insulator layer. Sharp emission lines in the visible region were observed which correspond to known transitions of the Er3+ ion. Temperature-dependent cathodoluminescence studies corroborate the EL results, and show that optimum device performance is attained near 300 K.
Effect of surface steps on the microstructure of lateral composition modulation
Authors: Follstaedt D. M. , Reno J. L. , Jones E. D. , Lee S. R. , Norman A. G. , Moutinho H. R. , Mascarenhas A. , Twesten R. D.
Published: July 2002

Growth of InAs/AlAs short-period superlattices on appropriately miscut (001) InP substrates is shown to alter the microstructure of composition modulation from a two-dimensional organization of short compositionally enriched wires to a single dominant modulation direction with wire lengths up to ∼1 μm. The effects of miscut are interpreted in terms of surface step orientation and character. The material is strongly modulated and exhibits intense optical emission. The one-dimensional modulations appear potentially useful for new devices that take advantage of the preferred direction formed in the growth plane.
Magneto-optical effects from nanophase α-Fe and Fe3O4 precipitates formed in yttrium-stabilized ZrO2 by ion implantation and annealing
Authors: Honda S. , Modine F. A. , Meldrum A. , Budai J. D. , Haynes T. E. , Boatner L. A.
Published: July 2002

Magneto-optically active nanocomposite structures have been created by using ion implantation and thermal processing to form precipitated layers of ferromagnetic α-Fe or ferrimagnetic Fe3O4 that are embedded in the near-surface region of (100)-oriented yttrium stabilized ZrO2 (YSZ). When Fe-implanted YSZ is annealed at 1100 °C in Ar+4%H2, the redox conditions are sufficiently reducing so that metallic Fe is the stable phase. At lower temperatures the annealing conditions become less reducing and Fe3O4 becomes the stable phase. Transmission electron microscopy and x-ray diffraction studies established that each α-Fe or Fe3O4 particle is a single crystal that is crystallographically aligned with respect to the YSZ host. Magneto-optical effects due to both the α-Fe and Fe3O4 nanophase precipitates were found and characterized using magnetic circular dichroism. These magneto-optical effects have potential applications in integrated-optical devices.
The dissociative recombination of hydrocarbon ions. III. Methyl-substituted benzene ring compounds
Authors: Rebrion-Rowe C. , Mostefaoui T. , Laubé S. , Mitchell J. B. A.
Published: July 2002

The recombination of electrons with cyclic ions produced via ion–molecule reactions between atomic precursor ions and methyl-substituted benzene ring compounds (toluene, ortho-, and para-xylene and mesitylene) has been studied at 300 K using a flowing afterglow Langmuir probe-mass spectrometer apparatus. Differing amounts of energy can be deposited into the daughter ions depending upon which atomic precursor is used. It has been found that same-mass daughter ions formed from different precursors displayed different recombination rate coefficients indicating that different isomeric forms were reacting. In particular, the benzene ring of the toluene cation expands to a seven-membered ring following isomerization to the cycloheptatriene form. H atom abstraction allows two different isomeric daughter ions to be formed that do not interconvert and that display different recombination rates. A similar behavior was observed for the xylenes and for mesitylene. All recombination rates lie in the range from 10−7 to 10−6 cm3 s−1 and display no apparent relation with size nor with the aromaticity of the ions.
A structural, magnetic, and Mössbauer spectral study of Dy2Fe17 and its hydrides
Authors: Isnard O. , Hautot Dimitri , Long Gary J. , Grandjean F.
Published: July 2002

The structural and magnetic properties of the Dy2Fe17Hx compounds, where x is 0, 1, 2, 3, and 3.8, have been investigated by means of powder x-ray diffraction, thermomagnetic and ac magnetic susceptibility measurements, and iron-57 Mössbauer spectroscopy. The Dy2Fe17Hx compounds crystallize in a hexagonal Th2Ni17 -like structure which has both an iron-rich stoichiometry and disorder of the Dy and Fe–Fe dumbbell sites. The increase in the lattice parameters, the magnetic ordering temperature, the saturation magnetization, and the dependence of the Mössbauer hyperfine parameters upon hydrogen content support a two-step filling by hydrogen of the interstitial sites with hydrogen first filling the octahedral 6h sites for x<3 and then partially filling the tetrahedral 12i sites for x=3 and 3.8. Neither the Mössbauer spectra nor the ac magnetic susceptibility measurements reveal any spin reorientation in any of the compounds. The extent of the excess iron and the disorder observed in all the compounds is confirmed by the Mössbauer spectra and the hyperfine parameters of the iron 4e sites are reported herein for the first time. Finally, the Mössbauer spectra indicate that the interstitial hydrogen atoms partially occupying the tetrahedral 12i sites are jumping between these sites on the Mössbauer time scale.
Optical absorption of transition-metal island films: Correlation interaction between conduction electrons of transition metals
Authors: Anno E. , Tanimoto M.
Published: July 2002

Optical plasma-resonance absorption of Pt island films consisting of Pt particles larger than about 25 Å in diameter has been measured in the photon energy range of 0.5–6.5 eV. As in Rh and Pd island films reported previously, the broadening of the optical plasma-resonance absorption reflects a correlation interaction between conduction electrons. Comparison of the broadening for the Pt island films with that for the Rh island films shows that the correlation interaction is strong when the conduction-electron density n is low. In an electron-gas model, the correlation interaction between electrons becomes stronger with lowering electron density, because the magnitude ratio of the Coulomb to kinetic energy increases as the electron density lowers. Thus, the strong correlation-interaction at low n proves that the correlation interaction in transition metals becomes stronger with magnitude ratio. The magnitude ratio in transition metals is pointed out to increase with lowering n and/or with strengthening d character of conduction electrons. Based on the correlation interaction, reflected by the broadening for the Pt, Rh, and Pd island films, and on the strong correlation interaction, found previously for Ir, the order of magnitude ratio is Ir>Pt>Pd>Rh.
Suppression of runaway electron avalanches by radial diffusion
Authors: Helander P. , Eriksson L.-G. , Andersson F.
Published: July 2002

The kinetic theory of runaway electron avalanches caused by close Coulomb collisions is extended to account for radial diffusion. This is found to slow down the growth of avalanches. An approximate analytical formula for the growth rate is derived and is verified by a three-dimensional Monte Carlo code constructed for this purpose. As the poloidal magnetic flux that is available to induce an electric field in a tokamak is limited, avalanches can be prevented altogether by sufficiently strong radial diffusion. The requisite magnetic fluctuation level is sensitive to the mode structure and the speed of the disruption. It is estimated to be δB/B∼10−3 for parameters typical of large tokamaks.
How a quantum-dot laser turns on
Authors: Grundmann Marius
Published: July 2002

The turn-on dynamics of quantum-dot lasers is modeled theoretically. The impact of the—so far technologically inevitable—inhomogeneous broadening σ, the homogeneous broadening Γ, and the interlevel relaxation time on the relaxation oscillations (ROs) is investigated. When the inhomogeneous broadening is dominant (Γ≪σ), the individual nanostructures in the ensemble exhibit independent ROs. In the case of significant homogeneous (Γ⩾σ) broadening, the quantum-dot ensemble exhibits collective, synchronized ROs, leading to a stronger intensity modulation and a larger turn-on delay.
Topography-induced contributions to friction forces measured using an atomic force/friction force microscope
Authors: Sundararajan Sriram , Bhushan Bharat
Published: July 2002

Most friction studies using an atomic force/friction force microscope, while concentrating on material-induced effects, often present users with conflicting and confusing interpretations of the topography-induced friction forces. It has been generally reported that topography-induced contributions are independent of scanning direction and can be removed by subtracting friction data from forward and backward scans. In this article, we present friction studies on samples with well-defined topography variations and find that the above-given statement is not generally true. At surface locations involving significant changes in topography, the topography-induced contributions to friction forces are found to be different between forward and backward scanning directions. This is explained by the ratchet mechanism of friction and due to the additional torsion generated by “collision” of the tip when traversing up an increase in topography, which is absent in the downward travel. Topography-induced contributions to the friction force always correspond to transitions in the surface slope. Moreover, these contributions will be of the same sign in both Trace and Retrace friction profiles of the friction loop whereas changes due to material effects are in opposite directions. These characteristics of topography-induced friction forces will aid in differentiating them from other effects.
Determination of the direction of the “easy” axis at a twisted nematic liquid crystal-wall using half-leaky guided modes
Authors: Yang F. Z. , Cheng H. F. , Gao H. J. , Sambles J. R.
Published: July 2002

Theoretical analysis and numerical modeling indicate that, under high fields, the director of a thin region of liquid crystal close to the wall of a twisted nematic cell returns to the “easy” axis on the interface. Polarization conversion reflectivity signals are recorded for half-leaky guided modes from a twisted nematic cell subject to high ac fields. By fitting model theory to reflectivities recorded for a particular region of incidence angles, the director twist at the cell walls is deduced. In the high-field limit, this then gives the original director alignment axis, the easy axis, at the wall of a twisted nematic cell.
Boron-interstitial silicon clusters and their effects on transient enhanced diffusion of boron in silicon
Authors: Solmi S. , Bersani M. , Sbetti M. , Hansen J. Lundsgaard , Larsen A. Nylandsted
Published: July 2002

The nature of ion-implantation induced clusters of boron and silicon-self interstitials (BICs), and their effects on transient enhanced diffusion (TED) of B in Si have been investigated in samples predoped with B at different concentrations. Excess Si interstitials have been introduced by Si+ implantation at 60 keV with doses of 1 and 5×1014 cm−2. The B diffusivity and the amount of B trapped in the clusters have been evaluated from the best fits of simulation-prediction profiles to experimental B profiles, after annealing at 740 and 800 °C for different times. Our results show that the BICs in the beginning act as a sink for interstitials, strongly reducing the TED in the early phases of the annealing. However, being more stable than the Si-interstitial clusters and the {113} defects, they dissolve slowly and can, therefore, sustain a moderate Si-interstitial supersaturation for longer annealing times, even when the Si-interstitial defects are completely dissolved. The data show that the amount of B in the BICs is higher than that of the interstitials; we estimate an average ratio between the B and interstitial concentrations to be about 1.5.
Size and molecular configuration of dye aggregates in mixed Langmuir–Blodgett films based on merocyanine dye
Authors: Hirano Yoshiaki , Okada Takuya M. , Miura Yasuhiro F. , Sugi Michio , Ishii Toshio
Published: July 2002

We have studied the blue-shifted and the red-shifted bands formed in mixed Langmuir–Blodgett films of the merocyanine dye (MS)-arachidic acid (C20)-n-octadecane (AL18) ternary system with the molar mixing ratio [MS]:[C20]:[AL18]=1:2:x(0.5≦x≦5.0). The formation of the blue-shifted and the red-shifted bands depends on the AL18 content, and shows that the aggregation state can be modulated by changing the AL18 content. The observed overlapping spectra of the blue-shifted and the red-shifted bands are deconvoluted into two original bands. The extended dipole model has been applied to examine the aggregation state of MS referring to the deconvoluted spectra. Thus the estimated minimum aggregation number Nmin and the slip angle α between the long axis of the aggregate and the transition dipole moment are Nmin=40 and α=30° and Nmin=40 and α=50° for fully-developed J- and H-aggregates, respectively, seen for x≦1.5, and Nmin tends to decrease with increasing x.
Formation of Co filled carbon nanocapsules by metal-template graphitization of diamond nanoparticles
Authors: Tomita Satoshi , Hikita Masahiro , Fujii Minoru , Hayashi Shinji , Akamatsu Kensuke , Deki Shigehito , Yasuda Hidehiro
Published: July 2002

Co filled carbon nanocapsules, which are formed by a heat treatment of the mixture of Co and diamond nanoparticles, have been studied by in situ transmission electron microscopy (TEM), x-ray diffraction, and Raman spectroscopy. Raman studies show that the heat treatment reduces the surface native oxide (Co3O4) of Co nanoparticles. The reduction is accompanied by graphitization of diamond nanoparticles, indicating that diamond nanoparticles being in contact with the metallic Co are transformed into graphitic coating. The in situ TEM studies show that the graphitic coating is formed in the heating process, not in the cooling process. Furthermore, once the coating is completed, the number of the graphitic layers is almost constant on further heating and cooling. These results allow us to conclude that metallic Co particles simply act as templates for graphitic coating.
Matrix infrared spectra and quantum chemical calculations of the MCO− (M=Si, Ge, Sn) anions
Authors: Zhang Luning , Dong Jian , Zhou Mingfei
Published: July 2002

Laser-ablated Si, Ge, and Sn atoms and electrons have been codeposited with CO in excess argon at 11 K. In addition to SiCO, GeCO, SnCO, and bent Si(CO)2, Ge(CO)2, and Sn(CO)2 molecules, carbonyl anions as SiCO−, GeCO−, and SnCO− were produced via electron capture by the neutral carbonyls during cocondensation process. The anion species were identified by isotopic substitutions, CCl4 electron trapping, and density functional calculations. All these carbonyl anions are calculated to have Π2 ground state with linear geometry.
Comparison of nitrogen incorporation in molecular-beam epitaxy of GaAsN, GaInAsN, and GaAsSbN
Authors: Harmand J. C. , Ungaro G. , Largeau L. , Le Roux G.
Published: July 2002

The incorporation of nitrogen in the low percentage range is investigated in a different III–V compound matrix. The materials are grown by molecular-beam epitaxy with a nitrogen radio-frequency plasma source. For equivalent growth conditions, the same rate of N incorporation is found for GaAsN and GaInAsN. However, this N incorporation rate is significantly enhanced in the GaAsSbN alloy. These observations support a discussion on the reactive nitrogen species.
Theory of magnetostatic coupling in thin-film rectangular magnetic elements
Authors: Tsymbal E. Y.
Published: July 2002

A theory of magnetostatic coupling in thin-film uniformly magnetized arrays of rectangular magnetic elements is presented. Analytic expressions for the magnetostatic energy and the dipolar fields are derived. The influence of the aspect ratio and the spacing between the elements on the magnetostatic coupling is investigated. It is found that increasing the aspect ratio reduces the critical distance between the elements, above which the magnetostatic inter-element coupling can be neglected.
Microscopic model for enhanced dielectric constants in low concentration SiO2-rich noncrystalline Zr and Hf silicate alloys
Authors: Lucovsky G. , Rayner G. B.
Published: July 2002

Dielectric constants, k, of Zr(Hf) silicate alloy gate dielectrics obtained from analysis of capacitance–voltage curves of metal–oxide–semiconductor capacitors with 3–6 at. % Zr(Hf) are significantly larger than estimates of k based on linear extrapolations between SiO2 and compound silicates, Zr(Hf)SiO4. Analysis of infrared spectra of Zr silicate alloys with 3–16 at. % Zr indicates increases in the coordination of Zr to O atoms from 4 to approximately 8 with increasing Zr content. The major contributions to enhancements in k in these low Zr(Hf) content alloys are explained by a transverse infrared effective charge that scales inversely with increasing Zr–O bond coordination.
N-dimensional dynamical systems exploiting instabilities in full
Authors: Rius J. , Figueras M. , Herrero R. , Farjas J. , Pi F. , Orriols G.
Published: July 2002

We report experimental and numerical results showing how certain N-dimensional dynamical systems are able to exhibit complex time evolutions based on the nonlinear combination of N-1 oscillation modes. The experiments have been done with a family of thermo-optical systems of effective dynamical dimension varying from 1 to 6. The corresponding mathematical model is an N-dimensional vector field based on a scalar-valued nonlinear function of a single variable that is a linear combination of all the dynamic variables. We show how the complex evolutions appear associated with the occurrence of successive Hopf bifurcations in a saddle-node pair of fixed points up to exhaust their instability capabilities in N dimensions. For this reason the observed phenomenon is denoted as the full instability behavior of the dynamical system. The process through which the attractor responsible for the observed time evolution is formed may be rather complex and difficult to characterize. Nevertheless, the well-organized structure of the time signals suggests some generic mechanism of nonlinear mode mixing that we associate with the cluster of invariant sets emerging from the pair of fixed points and with the influence of the neighboring saddle sets on the flow nearby the attractor. The generation of invariant tori is likely during the full instability development and the global process may be considered as a generalized Landau scenario for the emergence of irregular and complex behavior through the nonlinear superposition of oscillatory motions.
Energy of an array of dislocations in a strained epitaxial layer deposited on a finite substrate
Authors: Wang Shing-Dar
Published: July 2002

By adopting the superposition principle and Fourier integral methodology, this work studies the energy of a dislocation array in a strained epitaxial layer deposited on a finite substrate with the same elastic constants. The total energy comprises the self-energy of the dislocations, the strain energy arising from the lattice mismatch, and the interaction energy between the dislocations and the mismatch. The sum of the self-energy and the interaction energy constitute the dislocation formation energy. Zero formation energy is used as the criterion to determine the epilayer critical thickness hc. No dislocation can appear when the epilayer thickness is below hc. When the epilayer thickness equals the critical thickness and the dislocation density is extremely low, the total energy is independent of the dislocation spacing p. If the critical thickness is less than the substrate thickness and the epilayer thickness is greater than the critical thickness, the total energy has a local minimum at dislocation spacing p=pmin; in addition, the corresponding dislocation density is the critical dislocation density. When p>pmin, the total energy decreases by decreasing the dislocation spacing, i.e., increasing the dislocation density. The total energy curve near p=pmin changes to a steep valley when the epilayer’s thickness approaches that of the substrate thickness. This corresponds to the experimental observation that a fast relaxation of misfit strain occurs when the epilayer thickness grows to a sufficient thickness. If p<pmin, the total energy markedly increases by decreasing the dislocation spacing. This phenomenon implies that work hardening is inevitable due to the dislocation–dislocation core interaction.
Aluminum and GaN contacts on Si(111) and sapphire
Authors: Zhao Z. M. , Jiang R. L. , Chen P. , Li W. P. , Xi D. J. , Xie S. Y. , Shen B. , Zhang R. , Zheng Y. D.
Published: July 2002

Al and GaN contacts on both Si(111) and sapphire substrates were studied in this letter. After annealing at 450, 520, 600, and 650 °C for 35 min under a N2 flowing ambient, the current–voltage characteristics of Al/GaN contacts on Si(111) substrates were different from those on sapphire ones. The interfacial properties were discussed using the Schottky emission model in which the current is proportional to the square of the applied voltage. The metallurgical reactions were analyzed using photoluminescence (PL) spectra and x-ray diffraction (XRD). After annealing, blueshifts in the PL spectra and new peaks in the XRD data indicated that the wide-band gap interfacial phase AlGaN was formed at the Al/GaN interface.
Photoluminescence spectroscopy on annealed molecular beam epitaxy grown GaN
Authors: Bell A. , Harrison I. , Korakakis D. , Larkins E. C. , Hayes J. M. , Kuball M. , Grandjean N. , Massies J.
Published: July 2002

Photoluminescence (PL) spectroscopy has been used to investigate the effect that annealing temperature and ambient has on annealed molecular beam epitaxy grown GaN. Significant differences induced by the different annealing conditions occur in the PL spectra in the 3.424 eV region as well as the deep level band (2.0–3.0 eV). Power resolved measurements indicate that the 3.424 eV emission is a donor–acceptor pair transition. In the deep level region peaks are observed in all spectra at 2.3 and 2.6 eV. This suggests that the 2.3 and 2.6 eV peaks are related and a model is proposed to explain this luminescence.
On the gradual character of the first-order spin reorientation transition in DyFe11Ti
Authors: Kuz’min M. D.
Published: July 2002

Proceeding from recently published crystal field parameters, I demonstrate that the first-order spin reorientation transition point in DyFe12−xTix depends strongly on the value of titanium content x. This leads, due to inevitable small spatial variation of composition, to significant broadening of the first-order transition, giving it an appearance of a continuous process. Yet it can be unmistakably identified as a first-order phase transition, thanks to the characteristic shape of the temperature dependence of the spin orientation angle θ. What makes first- and second-order spin reorientation transitions clearly distinct is the shape, rather than width, of the “step” in the θ(T) dependence. Small differences of composition are also found to account for the outstanding discrepancies between the experimental studies carried out on different single crystals of DyFe11Ti, thus bringing the decade-long controversy to an end.
High magnetic field studies of two-dimensional electron gas in a GaN/GaAlN heterostructure: Mechanisms of parallel conduction
Authors: Contreras S. , Knap W. , Frayssinet E. , Sadowski M. L. , Goiran M. , Shur M.
Published: July 2002

We report on quantum Hall effect (QHE) and Shubnikov–de-Haas (SdH) measurements for a GaN/GaAlN heterostructure in high magnetic fields (up to 35 T). The observed well defined SdH oscillations, correlated with QHE plateaus, confirm the existence of a two-dimensional electron gas at the GaN/GaAlN interface. The Shubnikov–de-Haas oscillations are superimposed on a strong positive magnetoresistance. We show that this positive magnetoresistance can be linked to a parallel conduction channel. The analysis of the high magnetic field data yields information about the carrier concentration and mobility in this channel. The concentration and mobility of two-dimensional gas and the parallel conduction layer are analyzed in a wide temperature range (1.9–77 K). A physical model (based on the band diagram and mobility calculations) explaining the origin of parallel conduction is presented.
Sequence rotation in N-dimensional space and the folding of hydrophobic protein models: Surpassing the diagonal unfolded state approximation
Authors: Pereira de Araújo Antônio F.
Published: July 2002

The hydrophobic protein model is defined by an energy function intended to resemble the hydrophobic effect, in the sense that the energetic contribution of each monomer is the product of its degree of exposure and its “hydrophobicity.” This simple function was recently shown to produce proteinlike folding behavior for adequate native conformations in Monte Carlo simulations of lattice models. Sequences were originally designed based on the assumption of a diagonal average unfolded state, i.e., under the assumption that the average number of contacts made by each monomer in the unfolded state is the same for all monomers. The present study explores the unfolded state of this model by explicitly considering unfolded conformations sampled during very long equilibrium trajectories. It is found that the average unfolded state is not diagonal around the folding temperature and that the average number of contacts made by each monomer depends primarily on its own hydrophobicity. It is shown that sequences can be “rotated” away from the nondiagonal unfolded state by varying the hydrophobicity of polar monomers. Folding cooperativity is maximal when polar residues become so hydrophilic that their average number of contacts in the unfolded state approaches zero. Most derivations in the model can be understood as operations in a N-dimensional vector space, where both sequences and structures are represented. Many properties of the model can be visualized in a two-dimensional particularly important sub-space in the form of a sequence-structure diagram. Potential implications for the thermodynamics and sequence design of real proteins are discussed.
Phase behavior of hyperbranched polymer solutions with specific interactions
Authors: Jang Jeong Gyu , Bae Young Chan
Published: July 2002

We propose a molecular thermodynamic framework to describe liquid–liquid equilibria of hyperbranched polymer solutions. The model is based on the lattice cluster theory and the hydrogen-bonding model. We examine phase behaviors of hyperbranched polymer solutions in the effect of a branched structure and hydrogen bonding formations among endgroups of hyperbranched polymer and solvent molecules. The solvent–solvent hydrogen bonding dominates phase behaviors of hyperbranched polymer/water systems. The endgroups of hyperbranched polymers also play a great role in determining phase separation of highly branched polymer structure.
Translational and orientational order in lead zinc niobate: An optical and Raman study
Authors: Lebon A. , El Marssi M. , Farhi R. , Dammak H. , Calvarin G.
Published: July 2002

Raman and optical experiments have been performed on a lead zinc niobate [Pb(Zn1/3Nb2/3)O3 or PZN] single crystal oriented along (111) under zero-field cooling, and zero-field heating after field cooling conditions. At high temperature, the Raman spectra originate from a phonon–polarization coupling whose selection rules depend upon the wave vector of the quasistatic polarization, and that activates modes in the entire Brillouin zone. At room temperature, ferroelectric macrodomains were induced and stabilized under field cooling conditions. Their optically uniaxial behavior along [111] reflects a rhombohedral orientational order. However, Raman analysis shows that a zone center line assignment on the basis of the C3v point group is not appropriate, because of translational symmetry breaking due to chemical and structural disorder. Raman spectroscopy was also performed on a single crystalline sample of PZN oriented along (001). Raman measurements evidence the similarity of the spectra for both crystals in the paraelectric phase. This confirms the idea of a second order-like process. At room temperature, this crystal configuration, with a dc electric field parallel to [001], is found according to Raman experiments to give way to an equivalent distribution of the four rhombohedral domains selected by the applied electric field.
Theoretical analysis of interband optical absorption in an external electric field
Authors: Chakraborty P. K. , Ghatak K. P.
Published: July 2002

We study theoretically the interband optical absorption coefficient of semiconductors described within the two-band Kane model in the presence of an external electric field for photon energy below and above the semiconductor band gap. The optical matrix element depends on the momentum vector, and this important aspect has been incorporated in the present analysis. For photon energies below the band gap, the absorption coefficient exhibits an exponential falloff with the electric field and the photon energy. For the opposite inequality, the same coefficient oscillates with the photon energy without consideration of the Wannier–Stark levels, which generally exists in a band due to the presence of an external electric field. Additional oscillations are expected in the above two cases because of the formation of the discrete Stark ladders.
Tunneling through ultrathin SiO2 gate oxides from microscopic models
Authors: Städele M. , Tuttle B. R. , Hess K.
Published: July 2002

We investigate theoretically coherent electron tunneling through three-dimensional microscopic Si[100]/SiO2/Si[100] model junctions with oxide thicknesses between 0.4 and 4.6 nm. The transmission probabilities of these structures were calculated using a semiempirical tight-binding scattering method. Our calculations provide a basis for the microscopic understanding of the observed independence of tunneling transmission on the orientation of the bulk silicon and on the nature of inelastic defect-assisted tunneling. We document significant differences between transmission coefficients obtained with the present scheme and with the popular effective-mass-based approaches. The energy dependence of the effective tunneling mass in bulk silicon dioxide is predicted.
Elastic properties of textured diamond and silicon
Authors: Anastassakis E. , Siakavellas M.
Published: July 2002

We have used invariance properties of fourth-rank tensors within the Voigt–Reuss–Hill averaging procedure, to calculate the elastic stiffness and compliance of cubic polycrystalline materials exhibiting texture. The computation is extended to the bulk and shear modulus as well as to Young’s modulus, volume compressibility and Poisson’s ratios. Numerical values and polar plots are given for all texture directions in the plane (11̄0) of diamond and silicon.
Dependence of substitutional C incorporation on Ge content for Si1−x−yGexCy crystals grown by ultrahigh vacuum chemical vapor deposition
Authors: Kanzawa Y. , Nozawa K. , Saitoh T. , Kubo M.
Published: July 2002

Si 1−x−y Ge x C y crystals were grown by ultrahigh vacuum chemical vapor deposition (UHV-CVD) using Si2H6, GeH4, and SiH3CH3 as source gases. Although the total C content in the grown crystals increased with increasing the partial pressure of SiH3CH3 gas, the substitutional C content saturated at a certain value. The maximum substitutional C content was found to change depending on the Ge content. As the Ge content was increased from 13 to 35 at. %, the maximum substitutional C content linearly decreased from 2.0 to 0.8 at. %. These results clearly demonstrate that the existence of Ge atoms prevents the substitutional incorporation of C atoms in Si1−x−yGexCy growth by CVD.
Epitaxy of HgBa2CaCu2O6 superconducting films on biaxially textured Ni substrates
Authors: Xie Y. Y. , Aytug T. , Wu J. Z. , Verebelyi D. T. , Paranthaman M. , Goyal A. , Christen D. K.
Published: July 2002

Superconducting HgBa2CaCu2O6 (Hg-1212) films have been coated on biaxially textured Ni substrates buffered with CeO2/YSZ/CeO2 trilayers. A two-step cation-exchange process was employed for Hg-1212 coatings, whereby epitaxial Tl2Ba2CaCu2O8 precursor films grown in a fast temperature ramping annealing process were converted to Hg-1212 via diffusive Hg–Tl cation exchange. This minimizes chemical diffusion/reaction of Hg with the substrates and allows Hg-1212 films to inherit the epitaxy and smooth surface morphology of the Tl2Ba2CaCu2O8 precursor films. The superconducting transition temperature (Tc) of these Hg-1212 coated conductors is up to 124 K. Self-field critical current densities (Jc) of 2.2×106 A/cm2 at 77 K have been obtained, with Jc values of 0.7×106 A/cm2 achieved at 100 K. These results show Hg-1212 coated conductors may hold promise for electrical power applications above 77 K.
Influence of dual incorporation of In and N on the luminescence of GaInNAs/GaAs single quantum wells
Authors: Sun B. Q. , Jiang D. S. , Pan Z. , Li L. H. , Wu R. H.
Published: July 2002

The optical properties of above- and below-band-edge transitions have been investigated by incorporating In atoms into GaNAs/GaAs single quantum wells. The experimental results show that with increasing In concentration the interband luminescence is improved and the luminescence intensity below the band edge in GaInNAs/GaAs decreases significantly. An interpretation is given that N atoms are preferable to form a covalent bond with In than with Ga atoms in a GaInNAs alloy, due to the compensation of the atomic-size difference between In and N atoms on the GaAs substrate. The photoreflectance spectra of the GaInNAs/GaAs single quantum well support the assignment of an intrinsic mechanism to the high-energy luminescence peak.
Minority-carrier effects in poly-phenylenevinylene as studied by electrical characterization
Authors: Stallinga P. , Gomes H. L. , Rost H. , Holmes A. B. , Harrison M. G. , Friend R. H.
Published: July 2002

Electrical measurements have been performed on poly[2-methoxy, 5 ethyl (2′ hexyloxy) paraphenylenevinylene] in a pn junction with silicon. These included current–voltage measurements, capacitance–voltage measurements, capacitance–transient spectroscopy, and admittance spectroscopy. The measurements show evidence for large minority-carrier injection into the polymer possibly enabled by interface states for which evidence is also found. The shallow acceptor level depth (0.12 eV) and four deep trap level activation energies (0.30 and 1.0 eV majority-carrier type; 0.48 and 1.3 eV minority-carrier type) are found. Another trap that is visible at room temperature has point-defect nature.
Estimation of trap levels in SrTiO3 epitaxial films from measurement of (LaSr)MnO3/SrTiO3/(LaSr)TiO3 p-i-n diode characteristics
Authors: Sugiura Masanori , Uragou Kazuyuki , Tachiki Minoru , Kobayashi Takeshi
Published: July 2002

We have explored electrical trap levels inside SrTiO3 epitaxial films grown by the pulsed laser deposition method from electrical measurements of semiconducting oxide p-i-n diodes consisting of La0.85Sr0.15MnO3/SrTiO3/La0.05Sr0.95TiO3 trilayers. The prepared p-i-n diodes exhibited a marked temperature dependence in their current–voltage (I–V) characteristics. By attributing the temperature dependence of I–V curves to the space-charge limitation due to the trapped charges inside the i-SrTiO3 layer, we estimated crudely the level of the dominant trap in the SrTiO3 layer as ∼0.09 eV. The trap density was also estimated from the curve-fitting technique. At this moment, the trap densities were 1016 and 1018/cm3 when SrTiO3 layers were grown at 650 and 630 °C, respectively.
Influence of ribbon thickness on the formation and magnetic properties of melt-spun Fe–Co–Nd–Dy–B metallic glasses
Authors: Zhang Wei , Inoue Akihisa
Published: July 2002

The glass-forming ability, crystallized structure, and magnetic properties have been investigated for a Fe67Co9.5Nd3Dy0.5B20 glassy alloy with a large supercooled liquid region prepared by the melt-spinning technique. The glassy phase is formed in the wide ribbon thickness range below about 250 μm, and the volume fraction of the glassy phase decreases gradually with increasing ribbon thickness to above 250 μm. The crystallized nanocomposite structure consists of Nd2Fe14B, (Fe,Co)3B, and α-(Fe,Co), and the remanence Br, coercivity Hci, and maximum energy product (BH)max are 1.24 T, 247 kA/m, and 105 kJ/m3, respectively, for the ribbon of 250 μm in thickness annealed at 903 K for 600 s. The magnetic properties remain almost unchanged in the thickness range below 250 μm, and decrease gradually in the ribbons with thicknesses above 250 μm.
Investigation of deep levels in semi-insulating GaAs by means of the temperature change piezoelectric photo-thermal measurements
Authors: Fukuyama Atsuhiko , Memon Aftab , Sakai Kentaro , Akashi Yoshito , Ikari Tetsuo
Published: July 2002

The temperature variation of the piezoelectric photo-thermal (PPT) signal intensity of semi-insulating (SI) GaAs from 20 to 150 K was measured. Four peaks at 50, 70, 110, and 125 K were observed in the PPT signal. From the theoretical analysis based on the rate equations of electrons in the conduction band and deep levels, we concluded that the observed four peaks were due to the nonradiative electron transitions through EL6, EL7, EL15, and an unspecified deep level, respectively. Deep levels with extremely low concentration (1012–1015 cm−3) were clearly identified in SI GaAs by using the PPT method.
A portable ultrahigh vacuum organic molecular beam deposition system for in situ x-ray diffraction measurements
Authors: Ritley K. A. , Krause B. , Schreiber F. , Dosch H.
Published: July 2002

A portable UHV molecular beam deposition system has been developed for synthesis, in situ, and real-time x-ray diffraction measurements of organic thin films, multilayers, and superlattices. The system has been optimized for small size, while still incorporating full features necessary to achieve thin film growth under molecular beam epitaxy (MBE) conditions. It can be used independently for thin film growth, or it can be transported and mounted on standard diffractometers. Additionally, it can be docked to a stationary multipurpose MBE growth system for sample transfer, thus permitting more extensive growth and characterization. The design and performance of this system are reported, with emphasis on modifications required to deposit organic materials. To demonstrate the capabilities for real-time x-ray scattering experiments, some preliminary results of a study of epitaxial growth of 3,4,9,10-perylene-tetracarboxylic dianhydride on Ag(111) substrates are given.