bluuebunny/crossref_metadata_2025 huggingface dataset:
Initial growth dynamics of homo epitaxial (100) GaAs using pulsed laser deposition
Authors: Pun A. F. , Wang X. , Meeks J. B. , Zheng J. P. , Durbin S. M.
Published: November 2004

The homoepitaxial pulsed laser deposition of GaAs on thermally desorbed (100) GaAs substrates is studied utilizing reflection high-energy electron diffraction and contact atomic force microscopy as a function of growth time. Interestingly, results indicate an unexpected initial roughening due to preferential growth on terrace regions around pits, resulting in a rough surface structure. As film growth proceeds, the surface structures become progressively more coherent up to a point, after which further growth results in smoothing by pit filling, and eventually resulting in an ordered undulatory surface topology. An explanation for this change in preferred growth region is set forth in which the initial oxide desorption process leaves regionally varying chemical stochiometries on the surface, which subsequently affect relative growth rates.
Scattering and dissociative adsorption of H2 on the armchair and zigzag edges of graphite
Authors: Arboleda N. B. , Kasai H. , Nakanishi H. , Diño W. A. , Sugimoto T.
Published: November 2004

We performed quantum dynamics calculations on the scattering and dissociative adsorption of hydrogen molecules incident on the armchair and zigzag edges of graphite layers, using relevant potential-energy surfaces (PESs) recently obtained by Diño et al. [e-J. Surf. Sci. Nanotech. 2, 77 (2003), and references therein]. By employing the coupled channel method to determine the reflection and sticking probabilities, we compared the hydrogen scattering and dissociative adsorption dynamics on the two graphite surfaces. Our findings show the different scattering behaviors of H2 for the armchair edge and for the zigzag edge, which enable the identification of an unknown graphite edge from its interaction with H2. The scattering on the zigzag edge is due to the highly curved region of the PES reaction path for H2 interacting with the zigzag edge, whereas the scattering for the armchair edge is caused by a potential barrier. The reflection probability initially decreases with increasing the kinetic energy in both cases but gradually increases for the zigzag edge. Our findings also indicate that the zigzag edge can adsorb hydrogen better than the armchair edge, mainly due to the absence (presence) of an activation barrier in the zigzag (armchair) edge. There is a very weak dependence of the sticking probability on the initial vibrational state of H2 for both graphite edges. The difference in the vibrational effect is due to the relative position of the curved region with respect to the potential barrier (well).
A comparative study of inverted-opal titania photonic crystals made from polymer and silica colloidal crystal templates
Authors: Kuai Su-Lan , Truong Vo-Van , Haché Alain , Hu Xing-Fang
Published: November 2004

Photonic crystals with an inverted-opal structure using polymer and silica colloidal crystal templates were prepared and compared. We show that the behaviors of the template during the removal process and heat treatment are determinant factors on the crystal formation. While both templates result in ordered macroporous structures, the optical quality in each case is quite different. The removal of the polymer template by sintering causes a large shrinkage of the inverted framework and produces a high density of cracks in the sample. With a silica template, sintering actually improves the quality of the inverted structure by enhancing the template’s mechanical stability, helping increase the filling fraction, and consolidating the titania framework. The role of the other important factors such as preheating and multiple infiltrations is also investigated.
Controlled growth of ZnS:Mn nanophosphor in porous silica matrix
Authors: Haranath D. , Bhalla Nitesh , Chander Harish , Rashmi  , Kar Meenakshi , Kishore Ram
Published: November 2004

The development of nanophosphors of desired sizes and properties for various practical applications and its growth in quantitative amounts inside the pores of an inorganic matrix is presented. By doing so, nanophosphors get surface passivated and are stabilized against environmental attacks. Accordingly, in the present study, the growth parameters for ZnS:Mn nanophosphors were systematically studied inside a SiO2 gel matrix, which can act as a capping agent as well. The samples were prepared using the sol-gel technique, followed by annealing at different temperatures to remove the trapped fluid inside the amorphous silica cage. Two categories of samples with lower (3.11×10−4) and higher (1.5×10−1) ZnS∕SiO2 molar ratios were studied. The x-ray diffraction and scanning electron microscopy observations show that upon annealing, the nanocrystals grow in size and undergo a phase transition from cubic to hexagonal at temperatures between 700 and 900°C. This is one of the very few known reports published on nano hexagonal ZnS formation. The observed phase transition is possibly the combined effect of the high-temperature (∼900°C) and annealing-related compressive stress induced on the nano-ZnS by the silica cage. There has been formation of an intermediate metastable phase of the zinc silicate at annealing temperatures around 700°C. The particle size distribution and emission properties were correlated using the optical absorption and photoluminescence (PL) results. The unannealed cubic nano-ZnS:Mn samples gave a broad PL, peaking at ∼585nm, whereas the samples annealed at 900°C for 5h gave a narrow and sharp PL at ∼590nm. This is attributed to the more efficient T14→A16 transitions of Mn in the resultant hexagonal nano-ZnS matrix.
Direct-write patterning of microstructured porous silicon arrays by focused-ion-beam Pt deposition and metal-assisted electroless etching
Authors: Chattopadhyay Soma , Bohn Paul W.
Published: November 2004

Photoluminescent porous silicon (PSi) patterns of micrometer dimension were produced by the Pt-assisted electroless etching of Si in 1:1:2 methanol:HF:H2O2. Pt-containing squares with side lengths ranging from 1.25to20μm were defined by a focused-ion-beam-assisted maskless deposition of Pt from an organometallic precursor, trimethylmethylcyclopentadienyl platinum. The Pt-patterned Si samples were then etched to produce photoluminescent pixel arrays with high fidelity transfer of the Pt deposition pattern into luminescent pixels of varying size. The morphology of the PSi patterns was correlated with the spatial luminescence characteristics at the individual pixel level. Luminescent pixels with feature sizes down to ca. 1μm were largely confined to the areas initially coated with Pt, and the morphologies produced within any one set of equal-sized Pt squares were similar. For 5-μm pads and larger, the morphologies obtained were an admixture of a porous structure coexisting with deeper heavily etched crater regions. Only the porous areas were observed to emit, with the deeper crater areas being dark in a two-photon luminescence. The smaller 1.25- and 2.5-μm pads exhibited a common morphology, in which a brightly luminescent outer ring surrounds a weaker but still distinguishable luminescence in the center of the etched structure. These results are in contrast with the spatial luminescence patterns and morphologies for the millimeter-scale Pt pads [S. Chattopadhyay, X. Li, and P. W. Bohn, J. Appl. Phys. 91, 6134 (2002)], in which electroless etching and, thus, PSi formation is observed in the regions not initially coated with Pt.
Response time analysis of SiGe∕Si modulation-doped multiple-quantum-well structures for optical modulation
Authors: Marris Delphine , Cassan Eric , Vivien Laurent
Published: November 2004

The response time of SiGe∕Si modulation-doped multiple-quantum-well modulators is investigated. A refractive index variation is achieved by the depletion of the free carriers initially present in the wells. Both the tunneling and thermionic emissions are taken into account to study the time needed for the free carriers to escape from and to be captured into the wells. Results are presented for an optimized three-Si0.8Ge0.2-quantum-well (QW) (10-nm-thick) device. Such a QW structure can intrinsically reach an operation frequency around 13GHz.
Fabrication of aluminum single-electron transistors with low resistance-capacitance product
Authors: Brenning Henrik , Kubatkin Sergey , Delsing Per
Published: November 2004

The optimum speed and sensitivity of a single-electron transistor (SET) depend crucially on the resistance-capacitance (RC) product of the tunnel junctions. We present a fabrication method of aluminum single-electron transistors with a high percentage of working devices (80%) and record low RC products: SETs with a typical charging energy of 15K and a resistance of 100kΩ. The oxygen pressure during junction oxidation was very low, 8∗10−4mBar, which resulted in devices with a high cut-off frequency (up to 40GHz). The devices were characterized at 4.2K and at 90mK. The noise was typical for an aluminum single-electron transistor (2.5∗10−4e∕Hz), and the gain (dID∕dQg) was high (54.5nA∕e).
Size dependence of thermal stability of TiO2 nanoparticles
Authors: Li W. , Ni C. , Lin H. , Huang C. P. , Shah S. Ismat
Published: November 2004

Anatase TiO2 nanoparticles with average particle size ranging between 12 and 23 nm were synthesized by metalorganic chemical vapor deposition. The structure and particle size were determined by x-ray diffraction and transmission electron microscopy. The specific surface areas were measured by Brunauer-Emmett-Teller and ranged from 65 to 125m2∕g. The size effects on the stability of TiO2 in the air were studied by x-ray diffraction and transmission electron diffraction for isochronally annealed samples in the temperature range of 700–800 °C. Only anatase to rutile phase transformation occurred. With the decrease of initial particle size the onset transition temperature was decreased. An increased lattice compression of anatase with the raising of temperature was observed by the x-ray peak shifts. Larger distortions existed in samples with smaller particle size. The calculated activation energy for phase transformation decreased from 299 to 180 kJ∕mol with the decrease of initial anatase particle size from 23 to 12 nm. The decreased thermal stability in finer nanoparticles was primarily due to the reduced activation energy as the size related surface enthalpy and stress energy increased.
Field and temperature induced effects in the surface modification process
Authors: Csonka Sz. , Halbritter A. , Mihály G. , Jurdik E. , Shklyarevskii O. I. , Speller S. , Kempen H. van
Published: November 2004

We used the mechanically controllable break junction technique to discriminate between the electric field and temperature effects in the process of surface modification. The electric field strength at the surface of electrodes was accurately determined using the field emission resonance spectra and was gradually raised to the point where surface modification starts. We found that only a limited number of metals with large values of the work function, ≳4.5–5eV, and evaporation fields, ≲1.5–2V∕Å, can be modified in this way. Adsorption of He on the surface drastically increases the local work function of material and enables the field induced nanostructuring of the electrode surface practically for all metals.
Atomic scale characterization of HfO2∕Al2O3 thin films grown on nitrided and oxidized Si substrates
Authors: Nishimura T. , Okazawa T. , Hoshino Y. , Kido Y. , Iwamoto K. , Tominaga K. , Nabatame T. , Yasuda T. , Toriumi A.
Published: November 2004

One and three bilayers of HfO2(9Å)∕Al2O3(3Å) thin films were grown by atomic layer chemical-vapor deposition on Si(001) substrates whose surfaces were nitrided or oxidized. The films as-grown and postannealed in an ultrahigh vacuum were analyzed by atomic force microscopy, photoelectron spectroscopy, and medium energy ion scattering. For the one- and three-bilayer films grown on the nitrided Si substrates, the HfO2 and Al2O3 layers are mixed to form Hf aluminates at temperatures above 600°C. The mixed Hf aluminate layer is partly decomposed into HfO2 and Al2O3 grains and Al2O3 segregates to the surface by postannealing at 900°C. Complete decomposition takes place at 1000°C and the surface is covered with Al2O3. The surfaces are uniform and almost flat up to 900°C but are considerably roughened at 1000°C due to the complete decomposition of the Hf aluminate layer. In contrast, for one- bilayer films stacked on the oxidized Si substrates, Hf silicate layers, including Hf aluminate, are formed by annealing at 600–800°C. At temperatures above 900°C, HfSi2 grows and Al oxide escapes from the surface.
Theory of the backward beam displacement on periodically corrugated surfaces and its relation to leaky Scholte-Stoneley waves
Authors: Declercq Nico F. , Degrieck Joris , Briers Rudy , Leroy Oswald
Published: November 2004

A demonstration of the capability of the inhomogeneous wave theory to simulate backward displacement of ultrasonic-bounded beams [M. A. Breazeale and M. Torbett, Appl. Phys. Lett. 29, 456 (1976)] has been demonstrated recently [N. F. Declercq, J. Degrieck, R. Briers, and O. Leroy, Appl. Phys. Lett. 82, 2533 (2003)]. The current report applies the theory of the diffraction of inhomogeneous waves and shows how this theory is capable of simulating, explaining, and understanding the experiments mentioned above. The theory reveals the existence of leaky Scholte-Stoneley waves, a phenomenon suggested theoretically [N. F. Declercq, J. Degrieck, R. Briers, and O. Leroy, J. Acoust. Soc. Am. 112, 2414 (2002)] and observed experimentally [A. A. Teklu, M. A. Breazeale, J. Acoust. Soc. Am. 113, 2283 (2003)]. Moreover, the present paper shows that the classical Fourier decomposition of bounded beams is unable to simulate the backward beam displacement. This work also elucidates the nature of Wood anomalies in Diffraction spectra.
The triplet state in tris-(8-hydroxyquinoline)aluminum
Authors: Cölle M. , Gärditz C. , Braun M.
Published: November 2004

This paper presents the characterization of the triplet state in tris-(8-hydroxyquinoline)aluminum (Alq3). An emission spectrum of Alq3, namely, the phosphorescence, is presented for crystalline and amorphous Alq3, which enables direct investigation of the T1→S0 transition. The assignment of this spectrum to the phosphorescence is further confirmed by temperature-dependent measurements and comparison with the delayed fluorescence. The triplet energies of the meridional and facial isomer (in α- and δ-Alq3) are determined from the well-resolved vibronic progressions of the phosphorescence as 2.11±0.1 and 2.16±0.1eV, respectively. Furthermore, the lifetime of the triplet state is measured for a temperature range from 6to150K. These temperature-dependent measurements also identify a so far unknown phase transition of Alq3 at about 50K. Optically detected magnetic resonance at a zero field is used to measure the characteristic zero-field splitting parameters (∣E∣=0.0114cm−1 and ∣D∣=0.0630cm−1), and these are discussed in terms of a molecular symmetry and contribution of different ligands and suggest a mini-exciton-like behavior of the triplet state on the three ligands of the Alq3 molecule. All these measurements are performed on different crystalline phases (α- and δ-Alq3) and on evaporated amorphous films. The differences observed for the δ-phase are consistent with the reduced intersystem crossing and thus with the facial isomer in this phase.
Far-infrared study of high-dielectric constant CaCu3Ti4O12 films
Authors: Kafadaryan E. A. , Cho Kyuho , Wu Naijuan
Published: November 2004

The far-infrared reflectivity spectroscopy with a normal and oblique (45°) light incidence has been performed with the spectra of CaCu3Ti4O12∕SrRuO3∕LaAlO3 (CCTO/SRO) and CaCu3Ti4O12∕La0.5Sr0.5CoO3∕LaAlO3 (CCTO/LSCO) heterostructures in the frequency range 200–4000cm−1 with a view to study the electronic and phonon properties of the high-dielectric CCTO thin films grown on the SRO and LSCO electrodes by pulsed laser deposition. The frequency dependence of optical conductivity, dielectric, and energy-loss functions of the CCTO films have been calculated by the Kramers-Kronig transformation. Analysis of the optical functions yields two different trends for two compositions, for CCTO/SRO, the electronic behavior is semiconductorlike (carrier concentration N is in the order of 1020cm−3, optical mass m*∕m=0.6, free-carrier relaxation time τ=3∙10−14s) and can be described by applying the Drude-Lorentz model on a plasma of free carriers, whereas for the CCTO/LSCO composition, Lorentz alone oscillator is enough to simulate the electronic part of the conductivity, revealing the presence of bound optically active electrons. We report also on the observation of additional longitudinal phonons at 756, 732, and 708cm−1 by reflectivity measurements with an oblique beam incidence on CCTO/SRO. The phonons are assigned to Berreman modes. The CCTO/LSCO did not reveal longitudinal phonon modes. Based on the optical spectroscopy results and dc resistivity measurements, we can infer that the CCTO films deposited on a SRO electrode consist of semiconducting grains which are influenced by deposition processing and substrate material.
Generation of focused electron beam by pyroelectric and photogalvanic crystals
Authors: Kukhtarev N. , Kukhtareva J. D. T. , Bayssie M. , Wang J. , Brownridge J. D.
Published: November 2004

We have developed a model to explain the phenomena of electron focusing by pyroelectric crystals. The pyroelectric crystals used to compare the experiments with theory were the Fe-doped and undoped LiNbO3. The crystals were either heated from the +z end or illuminated with a laser. Heating the crystals by passing a current through a resistor attached to the +z end produced the pyroelectric effect, a change in polarization in response to a change in temperature. The crystal illumination with a cw solid-state diode-pumped laser (532 nm and 100 mW) produces the photogalvanic current, which creates charges on polar surface of the LiNbO3:Fe. In both cases, the polar ends of the crystal becomes electrically charged and produced self-focusing electron beams that were imaged on a ZnS screen. In addition, we have demonstrated x-ray imaging using the pyroelectric effect.
Photobleaching Comparison of Poly(methylphenylsilylene) and Poly(phenylsilyne)
Authors: Thomes William J. , Simmons-Potter Kelly , Phifer Carol C. , Potter Barrett G. , Jamison Gregory M. , Jones Jess E. , Casadonte Dominick J.
Published: November 2004

The effect of dimensionality on the photobleaching characteristics of Si-based polymers with phenyl substituents was investigated by studying the change in absorption upon exposure to 248- and 337-nm excitation. Poly(methylphenylsilylene) was chosen as an archetype of the linear-chain polysilanes, and poly(phenylsilyne) was used as the comparable network polysilyne. Differences in bleaching behavior of the two types of polymers are explained in terms of Si–Si bonding and electronic interactions between the phenyl groups and the silicon frameworks. Wavelength-dependent effects include the degree of photodegradation of the phenyl rings and changes in polymer configuration due to photoscission and crosslinking. Absorption changes were accompanied by changes in the refractive index of the exposed regions of the samples. The refractive index modification was used to write embedded strip waveguides in the photosensitive thin films.
Thermally induced sp2 clustering in tetrahedral amorphous carbon (ta-C) films
Authors: Orwa J. O. , Andrienko I. , Peng J. L. , Prawer S. , Zhang Y. B. , Lau S. P.
Published: November 2004

Tetrahedral amorphous carbon films with 70%–88% sp3 content are studied by atomic force microscopy (AFM), transmission electron microscopy (TEM), and Raman spectroscopy as a function of annealing temperature in the range 25–1100°C. Using a high-resolution AFM current imaging, we directly image the formation and growth of conducting graphitic (sp2-bonded) nanoclusters in the ta-C films. Overall results from all the techniques used show that the structural and electronic changes in the films depend sensitively on the initial sp3 content. Cross-sectional TEM confirms that the clusters appear not only at the surface of the films but in the bulk as well. The growth and, perhaps, the partial orientation of the sp2-bonded nanoclusters in the size range of 1–3nm is accompanied by a large reduction in the film stress, which decreases sharply in the temperature range 500–600°C.
Ion species and electron behavior in capacitively coupled Ar and O2 plasma
Authors: Seo Hyungtak , Kim Jung-Hyung , Shin Yong-Hyeon , Chung Kwang-Hwa
Published: November 2004

We investigated the change in electron density using the plasma frequency by the wave cutoff method, and the behavior of ion species with a quadrupole mass spectrometer (QMS) in pure Ar and O2 and mixed O2∕Ar plasmas. The change in electron and ion density in pure Ar and O2 plasmas was evaluated while varying such process conditions as rf power and pressure. We found that electron density in a pure Ar and O2 discharge is closely correlated to loss and generation of ions. The electron densities in both pure Ar and O2 plasmas increase with rf plasma power but show different dependence on pressure due to different loss mechanism for each type of gas. The addition of Ar to an O2 plasma significantly enhances the electron density due to the rapid increase of Ar+ ions regardless of the pressure. Also, Ar addition results in more dissociation of O2, which gives more atomic O. These results indicate that the electron density calculated from the plasma frequency, measured by the wave cutoff method, is well explained by the ion behavior, as characterized by QMS.
Phase sensitive enhancement for biochemical detection using rotating paramagnetic particle chains
Authors: Vuppu Anil K. , Garcia Antonio A. , Hayes Mark A. , Booksh Karl , Phelan Patrick E. , Calhoun Ronald , Saha Sanjoy K.
Published: November 2004

Paramagnetic particle suspensions placed in a rotating unidirectional magnetic field form magnetic chains that rotate with the same frequency as the field. The motion of the fluid and particles surrounding the chain differs in phase and frequency from the chain rotation, a phenomenon that forms the basis of a sensitive detection scheme. Fluorescent particles that bind to the paramagnetic particles through their surface chemistry are used to demonstrate the concept. Epifluorescence video microscopy is used to capture images of the rotating chains. View windows placed over sequential images of rotating chains allows for measurement of the fluorescence brightness in the window, which is composed of periodic signal from the steady rotation of the chain plus the background. A lock-in reference synchronized to the chain rotation is used to enhance the fluorescence signal from chain and improve signal to noise. Two different modes of chain rotation and signal collection are demonstrated. This technique can be used to develop a fast and sensitive, homogenous microdevice based solid-phase immunoassay.
Conversion of infrared radiation into visible emission in YVO4 crystals doped with ytterbium and holmium
Authors: Lisiecki R. , Dominiak-Dzik G. , Ryba-Romanowski W. , Łukasiewicz T.
Published: November 2004

Conversion of the infrared (IR) radiation at 975 nm into the visible emission in the Czochralski grown YVO4 crystals containing several different concentrations of Yb3+ and Ho3+ has been investigated. Unlike other matrices doped with ytterbium and holmium, the YVO4:Yb,Ho system exhibits intense red upconverted emission originating from the F55 level, whereas, green upconverted emission originating from the S25 level of Ho3+ is found to be unusually weak at room temperature but dominating at 4.2K. Power dependencies of the emissions are consistent with two-step excitation process but time dependencies differ and imply that the longlived I75 level of Ho3+ is involved in the second-step excitation of red emission. Intensity distribution of σ-polarized emission band associated with the F55-I85 transition with peak value of effective stimulated emission cross section amounting to 10−20cm2 combined with the possibility of tailoring of optical pumping efficiency by appropriate choice of dopant concentrations offer a potential of IR-pumped visible laser emission.
Thermal stability of the HfO2∕SiO2 interface for sub-0.1μm complementary metal-oxide-semiconductor gate oxide stacks: A valence band and quantitative core-level study by soft x-ray photoelectron spectroscopy
Authors: Barrett N. , Renault O. , Damlencourt J.-F. , Martin F.
Published: November 2004

Synchrotron-radiation photoelectron spectroscopy is used to study the valence-band structure and the core-level photoemission spectra of HfO2 ultrathin films grown onto SiO2∕Si substrates by atomic layer deposition (ALD). We determine the band offsets (valence and conduction) of HfO2 to Si as a function of postdeposition annealing treatments (under an inert N2 atmosphere or in situ in ultrahigh vacuum) and find a significant evolution, the conduction-band offset remaining larger than 1.5eV. The Si2p and the Hf4f core-level spectra give detailed information on the composition and the spatial extent of the interfacial Hf silicate layer formed between the SiO2 bottom oxide and the HfO2 ALD thin film. By a quantitative treatment of the Si2p core-level intensities, we examine the thermal stability of the interface silicate after postdeposition annealing under N2 and in situ annealing in ultrahigh vacuum (UHV), both at 800°C. The as-deposited layer gives rise to a HfO2∕Hf0.35Si0.65O2∕SiO2 stack with corresponding thicknesses of 0.74∕0.51∕0.73nm. After postdeposition annealing at 800°C in a N2 atmosphere, this becomes a HfO2∕Hf0.31Si0.69O2∕SiO2 stack with corresponding thicknesses of 0.71:0.58:0.91nm. In situ annealing in UHV, on the other hand, gives a HfO2∕Hf0.35Si0.65O2∕SiO2 stack with corresponding thicknesses of 0.65:0.70:0.76nm. The former favors an extension of both the silicate and the SiO2 interface layers, whereas the latter develops only the silicate layer.
Resistance degradation due to interstitial hydrogen in photorefractive potassium lithium tantalate niobate single crystals
Authors: Ivker M. , Agranat A. J.
Published: November 2004

Resistance degradation in potassium lithium tantalate niobate (KLTN) doped with iron and titanium was measured in a single sample containing various concentrations of interstitial hydrogen. In this crystal the degradation arose from the migration of interstitial hydrogen and not oxygen vacancies, as reported in previous research. Interstitial hydrogen and oxygen vacancy defects both arise to compensate the valence shortfall of the substitutional iron impurities and the thermodynamic balance between the two compensation mechanisms can be controlled using reaction chemistry techniques. Through appropriate annealing treatments a single crystal of KLTN was prepared in three states: hydrogen-rich oxidized, hydrogen-poor reduced, and hydrogen-rich reduced. The characteristic degradation times for the three cases were 29, 2710, and 26min, respectively. The degradation rate is correlated with hydrogen concentration and not oxidation state of the crystal. Infrared absorption from near the two electrodes of the hydrogen-rich reduced crystal after degradation confirmed polarization of the hydrogen concentration. Electrocoloration was also found to correlate with hydrogen—it was observed in both hydrogen-rich states, but was absent from the hydrogen-poor crystal.
The effects of buoyancy convection on the measured solute diffusion coefficients in dilute metallic liquids
Authors: Huang W. D. , Yang B. J. , Smith R. W.
Published: November 2004

In order to determine the effect of gravity on the results from earlier liquid diffusion experiments in low earth orbit, comparative experiments involving gold, silver, and antimony diffusing in liquid lead have been carried out using the Queen’s University Experiments in liquid diffusion ground-based unit under terrestrial conditions. It was found that buoyancy-driven convection still persists in the liquid even when conditions are arranged for a continuously decreasing density gradient up the long axis of a melted long capillary diffusion couple. It is suggested that radial temperature gradients in the diffusion samples cause such buoyancy-driven convection, and so enhance the solute transport and increase the measured diffusion coefficient (D) values; however, it is noted that if the density of the solute is considerably less than that of the solvent, then the convective flow and the corresponding solute transport are much reduced.
Tunable terahertz-frequency resonances and negative dynamic conductivity of two-dimensional electrons in group-III nitrides
Authors: Kim K. W. , Korotyeyev V. V. , Kochelap V. A. , Klimov A. A. , Woolard D. L.
Published: November 2004

We investigated the terahertz (THz)-frequency resonances of two-dimensional electron conductivity under the streaming transport in a GaN quantum well at the nitrogen temperature. The calculation results found that the negative microwave mobility can occur in the narrow windows near the optical-phonon transit-time resonance frequencies, which can be tuned electrically in the 0.2–2.5THz range with the static electric fields of 1–10kV∕cm. The estimated magnitude of the negative mobility reaches hundreds of cm2∕Vs. These effects suggest that the nitride-based heterostructure may enable the development of an electrically pumped, tunable THz source operating at or above 77K.
Suppression of the metal-insulator transition temperature in thin La0.7Sr0.3MnO3 films
Authors: Angeloni M. , Balestrino G. , Boggio N. G. , Medaglia P. G. , Orgiani P. , Tebano A.
Published: November 2004

In this paper, we illustrate an approach to discriminate between epitaxial strain and other factors responsible for the decrease of the metal–insulator transition temperature (TP) in thin La0.7Sr0.3MnO3 films grown by pulsed laser deposition. Using this approach, we have estimated the effect of the biaxial strain on TP. Ultrathin films, independent of epitaxial strain, do not show any metal–insulator transition over the full temperature range. This finding confirms the existence of an interface dead layer. The strain-independent decrease in TP, relative to its bulk value, observed for a much wider thickness range (up to about 1000Å) can most likely be attributed to oxygen deficiency.
Recombination activity and electrical levels of dislocations in p-type Si∕SiGe structures: Impact of copper contamination and hydrogenation
Authors: Vyvenko O. F. , Kittler M. , Seifert W.
Published: November 2004

The impact of copper contamination and subsequent hydrogenation on recombination activity and hole-trap levels of misfit dislocations were investigated in p-type Si∕Si0.98Ge0.02∕Si structures. In the as-grown (noncontaminated) samples, dislocations were found to exhibit very low recombination activity, detectable with the electron-beam-induced current technique only at low temperatures. Deep-level transient spectroscopy revealed a dislocation-related hole-trap level at Et=Ev+0.2eV. The position of the observed level is close to the theoretically predicted hole-trap state of the intrinsic stacking fault of a dissociated dislocation. Contamination with a low copper concentration [5 (parts per 109) ppb] gave rise to a large increase of the recombination activity of the dislocations and to the appearance of another dislocation-related defect level at Et=Ev+0.32eV. Hydrogenation of the samples by a treatment with an acid solution and subsequent reverse-bias anneal at 380K resulted in the evolution of the levels of substitutional copper and its complexes with hydrogen.
Indentation of a square elastomeric thin film by a flat-ended cylindrical punch in the presence of long-range intersurface forces
Authors: Ju Bing-Feng , Wan Kai-Tak , Liu Kuo-Kang
Published: November 2004

This paper reports an experimental study of the elastic deformation of a 200μm thick microfabricated square membrane with sides ranging from 8.5mmto15mm as indented by a fine cylindrical, flat-ended punch with diameter 500μm in the presence of long-range intersurface forces. An apparatus was constructed to allow simultaneously measurements of the indenter displacement and the applied force. A number of interesting phenomena such as the “jump-into-contact” and the “pull-off” between the membrane and the moving cylinder were observed. A simple theoretical analysis using linear elasticity was adopted to fit the load-displacement curve (compliance) and thus estimated the Young’s modulus of the membrane. The measured parameters are consistent with data reported in literature.
Repair of phase defects in extreme-ultraviolet lithography mask blanks
Authors: Hau-Riege Stefan P. , Barty Anton , Mirkarimi Paul B. , Baker Sherry , Coy Michael A. , Mita Masaaki , Robertson Vernon E. , Liang Ted , Stivers Alan
Published: November 2004

Phase defects that introduce errors in the printed image are one of the major yield limiters for reticles in extreme-ultraviolet lithography. The basis for a reticle is a mask blank, consisting of an ultra-low-expansion substrate and a reflective multilayer stack that is made up of Mo and Si. A potential methodology for repairing phase defects in these mask blanks is to locally irradiate it with a high-resolution electron beam to induce structural deformations that cancel out the distortion of the multilayer. We present experimental and modeling results that show that the interaction of an electron beam with a mask blank can only be understood when the contraction of the multilayer through silicide formation and substrate compaction, as well as the mechanical response of the mask blank, are considered. One of the consequences is that electron beams with energies around 10 keV that are less than 20 nm in diameter induce depressions in multilayers made up of 50 Mo∕Si bilayers that have a surprisingly large diameter of about a micrometer. Our models suggest that these unacceptably large diameters can only be reduced if the compaction of the substrate is prevented.
High electrical activation efficiency obtained from Si-implanted Al0.18Ga0.82N
Authors: Ryu Mee-Yi , Yeo Y. K. , Marciniak M. A. , Hengehold R. L. , Steiner T. D.
Published: November 2004

Si-implanted Al0.18Ga0.82N has been studied by Hall-effect measurements to produce good n-type layers for use in both electronic and optoelectronic devices. Silicon ions were implanted at 200keV with a dose ranging from 5×1014to5×1015cm−2 at room temperature, and the samples were annealed from 1100to1250°C for 5–25min with a 500-Å-thick AlN cap. Nearly 100% electrical activation efficiency for the sample having a dose of 5×1014cm−2 and 94% for a dose of 1×1015cm−2 were achieved after annealing at 1250 and 1200°C for 25min, respectively. Furthermore, this excellent electrical activation was obtained with much lower anneal temperature than the generally perceived 1350°C or higher anneal temperatures. This proves that a longer anneal time at lower anneal temperature (1200°C) is a better alternative than a shorter anneal time at higher anneal temperature (⩾1350°C). We believe that this accomplishment is very important in that the ion implantation technology can now be utilized for device fabrication of group III-nitrides much more readily.
Emission rates for electron tunneling from InAs quantum dots to GaAs substrate
Authors: Fu Y. , Engström O. , Luo Yi
Published: November 2004

Tunneling emission rates of electrons from InAs quantum dots (QD) in a GaAs matrix depletion region have been calculated for application in the analysis of experimental data from deep-level transient spectroscopy (DLTS). The hybridization among metastable states localized in the InAs QD and continuum states in the GaAs substrate in the DLTS measurement is evaluated from the local densities of states. Two physical quantities have been calculated, the broadening of the metastable state localized in the InAs QD, i.e., the charge-transfer rates from the quantum dot to the substrate, and the corresponding hybridization coefficients. The metastable state is broadened significantly only when the bias is large enough, whereas the hybridization coefficients increase almost linearly with the bias. The effects of the direct Coulomb interactions and exchange energies among electrons initially confined in the QD have been included and found to be very significant. Increasing the number of electrons initially confined in the QD from 1 to 4, the charge-transfer rates increase by a factor of 1.8.
Growth mechanism of AlN by metal-organic molecular beam epitaxy
Authors: Gherasoiu I. , Nikishin S. , Kipshidze G. , Borisov B. , Chandolu A. , Ramkumar C. , Holtz M. , Temkin H.
Published: November 2004

The phenomena that accompany the growth of aluminum nitride (AlN) by metal-organic molecular beam epitaxy with trimethylaluminum and ammonia as sources of aluminum and nitrogen, respectively, have been systematically investigated. Optimizing the growth temperature, flux ratios, and the ammonia injector temperature, we obtained an efficient growth with a rate of 500nm∕h and a low consumption of ammonia. Layers of AlN with x-ray diffraction linewidth as low as 141arcsec for the (0002) reflection, and 800arcsec for the (11-24) reflection are demonstrated on Si(111) substrates. High temperatures of ammonia injector result in lower growth rates but facilitate transition to the two-dimensional growth. These phenomena are discussed in terms of surface hydrogen, manifested through surfactant effect, and passivation of nitrogen bonds.
Imaginary branches of surface acoustic wave slowness curves
Authors: Laude Vincent , Masson Mathias , Ballandras Sylvain , Solal Marc
Published: November 2004

The imaginary branches of surface acoustic wave slowness curves are needed in many modal or spectral models that account for waveguides or diffraction based on the angular spectrum of waves approach. Their determination for an arbitrary anisotropic piezoelectric substrate considering either free or shorted surface boundary conditions is discussed. In the case of true, i.e., lossless, surface acoustic wave, the imaginary branches are obtained by a search in the complex transverse slowness plane as a function of the propagation slowness. As a side result, the parabolic approximation is compared with the exact solution and it turns out that its quality depends dramatically on the particular material cut considered. The case of pseudo- or leaky surface acoustic waves is also analyzed and it is found that difficulties arise in connection with the partial-wave selection rule for semi-infinite substrates.
Structural and electric properties of ultrathin SiOxNy layers with posttreatment in N2 plasma
Authors: Guo Ruofeng , Ooi T. , Kurata Y. , Inokuma T. , Hasegawa S.
Published: November 2004

Amorphous SiO2 (a-SiO2) layers were prepared using a thermal oxidation method. The SiO2 layers were nitrided at a temperature Tn between 450 and 650°C utilizing a plasma generation system, in which the layers before the nitridation were simultaneously grown on two different n-type crystalline Si substrates, one having a high resistivity for electron-spin resonance (ESR) and Fourier transform infrared absorption measurements and another having a low resistivity for current–voltage (I–V) measurement. We found the breakdown strength, estimated based on the I–V characteristics, increased with increasing Tn until 550°C, but the annealing at a further high Tn caused also a decrease in the breakdown strength. We found a strong correlation between the changes in the I–V characteristics and those in the bonding properties, and this correlation was also discussed in connection with the change in the ESR properties for the a-SiOxNy layers. Furthermore, incorporation of N atoms into SiO2 layers at Tn=550°C was suggested to improve the structural properties and the electrical properties.
Room-temperature tunable microwave properties of strained SrTiO3 films
Authors: Chang Wontae , Kirchoefer Steven W. , Pond Jeffrey M. , Bellotti Jeffrey A. , Qadri Syed B. , Haeni Jeffrey H. , Schlom Darrell G.
Published: November 2004

Structural distortion of ferroelectric thin films caused by film strain has a strong impact on the microwave dielectric properties. SrTiO3 thin films epitaxially grown on (110)DyScO3 substrates using molecular beam epitaxy are extremely strained (i.e., ∼1% in-plane tensional strain) from 3.905Å of bulk SrTiO3. The room-temperature in-plane dielectric constant and its tuning of the films at 10GHz are observed to be 6000 and 75% with an electric field of 1V∕μm, respectively. The control of strain in SrTiO3 provides a basis for room-temperature tunable microwave applications by elevating its phase-transition peak to room temperature.
Dielectric properties of barium titanate ceramics doped by B2O3 vapor
Authors: Qi Jian Quan , Chen Wan Ping , Wang Yu , Chan Helen Lai Wah , Li Long Tu
Published: November 2004

The effects of B2O3 vapor doping on the dielectric properties of barium titanate ceramics were studied. After the doping, the Curie point of BaTiO3 ceramics was increased from 128 °C to 130 °C and the maximum dielectric constant was considerably increased. X-ray diffraction showed that both the grain lattice parameters and the tetragonality were increased by the boron oxide vapor doping. It was proposed that boron interstitial was introduced into the grain lattice of barium titanate ceramics by B2O3 vapor doping and the dielectric properties were thus obviously changed.
Structure and photostimulated luminescent properties of Eu-doped M2BaX4 (M=Cs, Rb; X=Br, Cl)
Authors: Appleby G. A. , Edgar A. , Williams G. V. M.
Published: November 2004

The crystalline structures of the ternary halides M2BaX4 (M=Cs, Rb; X=Br, Cl), and their luminescence when doped with Eu2+ ions, are reported. These compounds crystallize with the Th3P4 body-centered cubic structure, where the Rb or Cs and Ba cations randomly occupy the Th sites in a 2:1 ratio. The Cs2BaCl4 and Rb2BaCl4 diffraction patterns show a small rhombohedral distortion from this cubic structure, which is accentuated when the compounds are prepared with excess alkali halide. When doped with divalent europium, each compound shows intense photoluminescence with a single emission band centered between 423 and 441 nm. We also find a significant photostimulated luminescence (PSL) effect in all four compounds, with an efficiency of up to 7% relative to the well-known x-ray storage phosphor BaFBr:Eu2+. The PSL effect in Rb2BaBr4 is enhanced by off-stoichiometry preparation or by fluorine doping. Attempts to prepare the related compounds MBa2X5 in single-phase form were unsuccessful. The PSL relative efficiency of a sample containing 34% Rb2BaBr4 and 66% RbBa2Br5 was 1.2% and it increased to 5.3% by doping with 1000 ppm BaF2.
Spin-dependent magnetophotoconductivity in silicon-on-sapphire
Authors: O’Raifeartaigh C. , Alhourani M. , Leonard F. , Barklie R. C.
Published: November 2004

We report a magnetic-field dependence of the photoconductivity of (100) silicon films that have been deposited on sapphire by chemical vapor deposition. The main effect is a fractional decrease of 0.1% in photoconductivity as the magnetic field is increased from zero to 30mT, observed in both as-deposited and epitaxially regrown films. The effect can be explained in the context of the trapped electron-hole pair model of carrier recombination of Kaplan, Solomon, and Mott, and arises from a difference in the precession rate of electrons and holes of recombining pairs. A quantum mechanical version of the model by L’vov, Mima, and Tretyak [Sov. Phys. JETP 56, 897 (1982)] gives a convincing fit to the data and suggests a value of 0.008⩽(ge−gh)⩽0.03 for the difference in the g values of recombining electrons and holes.
Analysis for discharge-radiation dynamics in alternating current plasma display panels
Authors: Suzuki Keizo , Yamamoto Kenichi , Kajiyama Hiroshi , Ho Shirun , Uemura Norihiro , Muraoka Katsunori
Published: November 2004

An analytical method to study the discharge-radiation dynamics (DRD) in alternating current plasma display panels was developed. The input parameters for this DRD analysis were experimentally determined panel voltage and current wave forms. Discharge voltage, current, and power wave forms in the discharge volume of a cell were first obtained from the measured panel voltage and current wave forms using known geometrical configurations and electric circuit calculations. Intrinsic discharge parameters, such as electron temperature and density, were then determined to satisfy these discharge wave forms under the assumption of a hydrodynamic approach. A one-dimensional discharge structure with two regions (cathode fall and positive column) and several other assumptions which are plausible from the discharge physics point of view were also adopted. These assumptions took account of known cross sections and energies of electron-impact excitation and ionization of discharge gas atoms, and a secondary electron emission coefficient of the dielectric surface at the cathode side induced by ion bombardment. Radiation intensities from the discharge were calculated using the determined intrinsic discharge parameters, and the results were compared with those measured for the respective panel conditions used in the calculations, yielding a fair agreement. The luminous efficiency, defined as the radiation intensity divided by the discharge power, was also determined using the intrinsic discharge parameters. Discussion on the luminous efficiency change for different panel operating conditions revealed that the efficiency improvement at a lower voltage was attributable to a lower electron temperature for this condition.
Neutron production by fast protons from ultraintense laser-plasma interactions
Authors: Yang J. M. , McKenna P. , Ledingham K. W. D. , McCanny T. , Robson L. , Shimizu S. , Singhal R. P. , Wei M. S. , Krushelnick K. , Clarke R. J. , Neely D. , Norreys P. A.
Published: November 2004

Tens of MeV proton beams have been generated by interactions of the VULCAN petawatt laser with foil targets and used to induce nuclear reactions in zinc and boron samples. The numbers of C11, Ga66, Ga67, Ga68, Cu61, Zn62, Zn63, and Zn69m nuclei have been measured and used to determine the proton energy spectrum. It is known that (p,n) reactions provide an important method for producing neutron sources and in the present experiment up to ∼109neutronssr−1 have been generated via B11(p,n)C11 reactions. Using experimentally determined proton energy spectra, the production of neutrons via (p,n) reactions in various targets has been simulated, to quantify neutron pulse intensities and energy spectra. It has been shown that as high as 4×109neutronssr−1 per laser pulse can be generated via Li7(p,n)B7 reactions using the present VULCAN petawatt laser-pulse conditions.
Model calculation for the field enhancement factor of carbon nanotube
Authors: Wang X. Q. , Wang M. , He P. M. , Xu Y. B. , Li Z. H.
Published: November 2004

To estimate the apex field enhancement factor associated with carbon nanotube on a planar cathode surface, the simple model of floated sphere between parallel anode and cathode plates was studied. When the anode-cathode distance d is much larger than the height h of carbon nanotube, the field enhancement factor of carbon nanotube was given as the following expression β0=h∕ρ+3.5 with the image method, which is a little than the early work of Miller. Finally, the influence of the anode-cathode distance on the enhancement factor was also discussed and the expression of enhancement factor of carbon nanotube should be modified to be β=h∕ρ+3.5+A(h∕d)3, where A is the constant. This shows a much weaker influence of the relatively smaller anode-cathode distance on the field enhancement factor of carbon nanotube than that estimated earlier. Thus, we can lower the threshold voltage to some extent by the decrease of the anode-cathode distance. The higher aspect ratio of carbon nanotube and the lower anode-cathode distance (still larger than a few times of the height of carbon nanotube) are the main factors leading to the stronger field at the apex.