bluuebunny/crossref_metadata_2025 huggingface dataset:
Preparation of BaTiO3 thin films by backside pulsed ion-beam evaporation
Authors: Sonegawa T. , Grigoriu C. , Masugata K. , Yatsui K. , Shimotori Y. , Furuuchi S. , Yamamoto H.
Published: July 2002

Barium titanate (BaTiO3) thin films were successfully prepared in situ on Al/SiO2/Si(100) substrates by backside deposition from intense, pulsed, ion-beam evaporation using a 1.3 MeV, 50 ns, 25 J/cm2 ion beam. Good morphology of the films prepared was observed, where no droplets appear compared to normal frontal-side deposition. The deposition rates were typically 100 nm/shot. The films were perovskite polycrystals. The capacitance of the thin films (at 1 kHz) increased from 3 to 10 nF/mm2 with increasing substrate temperature from 25 to 250 °C, respectively.
Optical characteristics of p-type GaN films grown by plasma-assisted molecular beam epitaxy
Authors: Myoung J. M. , Shim K. H. , Kim C. , Gluschenkov O. , Kim K. , Kim S. , Turnbull D. A. , Bishop S. G.
Published: July 2002

Using a molecular beam epitaxy system equipped with an inductively coupled radio frequency nitrogen plasma source, p-type GaN films were grown on sapphire substrates with no postgrowth treatment. Uniformity of the surface morphology and spatial homogeneity of the luminescence of the films were investigated using scanning electron microscopy and cathodoluminescence (CL) imaging, respectively. By examining the dependence of photoluminescence on the excitation laser power density at 6 and 300 K, three different emissions having different origins were identified. A blue emission at ∼3.25 eV is associated with shallow Mg impurities, while two different lower-energy emissions at ∼2.43 and ∼2.87 eV are associated with deep Mg complexes. The spatial distributions of the shallow and deep Mg impurities dominating the optical properties of the p-type GaN films were also examined along the growth direction by low- and room-temperature CL using an electron beam with a range of penetration depths
Optical near-field lithography on hydrogen-passivated silicon surfaces
Authors: Madsen Steen , Müllenborn Matthias , Birkelund Karen , Grey François
Published: July 2002

We report on a novel lithography technique for patterning of hydrogen-passivated amorphous silicon surfaces. A reflection mode scanning near-field optical microscope with uncoated fiber probes has been used to locally oxidize a thin amorphous silicon layer. Lines of 110 nm in width, induced by the optical near field, were observed after etching in potassium hydroxide. The uncoated fibers can also induce oxidation without light exposure, in a manner similar to an atomic force microscope, and linewidths of 50 nm have been achieved this way.
Beam divergence and waist measurements of laser diodes by near-field scanning optical microscopy
Authors: Herzog W. D. , Ünlü M. S. , Goldberg B. B. , Rhodes G. H. , Harder C.
Published: July 2002

We demonstrate the use of near-field scanning optical microscopy (NSOM) for the measurement of the beam properties of single quantum well, graded index separate confinement heterojunction ridge laser diodes. Using NSOM, we measure the field intensity in the transverse plane at near field and as a function of distance from the facet. The divergence of the laser beam and the beam waists in vertical and lateral dimensions are directly measured and the astigmatism of the mode is determined. In the near field, we observe a nearly ideal Gaussian shape in the vertical dimension which is consistent with the beam divergence as measured in the far field. In the lateral dimension, the beam shape deviates from the ideal Gaussian since the mesa structure of the laser diode provides an effective step-index waveguide. The non-Gaussian structure of the mode is also observed in the beam divergence properties.
High-frequency resonance in acoustic superlattice of periodically poledLiTaO3
Authors: Chen Yan-Feng , Zhu Shi-Ning , Zhu Yong-Yuan , Ming Nai-Ben , Jin Biao-Bing , Wu Ri-Xing
Published: July 2002

An electric poling method has been used to prepare microstructured LiTaO3 crystals with periodically inverted-ferroelectric domains. By using these crystals as acoustic superlattices, both an “in-line” scheme and a “cross-field” scheme for acoustic excitation have been realized. The experimental results are in good agreement with the theoretical analysis. It is expected that these results may be applied to a bulk-acoustic device operating at a frequency high above 450 MHz.
Surface states and space charge layer dynamics on Si(111)2×1: A free electron laser-synchrotron radiation study
Authors: Marsi M. , Couprie M. E. , Nahon L. , Garzella D. , Hara T. , Bakker R. , Billardon M. , Delboulbé A. , Indlekofer G. , Taleb-Ibrahimi A.
Published: July 2002

Combining the use of a UV storage ring free electron laser and of synchrotron radiation, a time resolved core level spectroscopy study has been performed on photoexcited Si(111)2×1 surfaces with subnanosecond resolution. This enabled us to measure band bending fluctuations, caused by surface carrier dynamics, during the first nanosecond after photoexcitation; differences in the Si2p core level lineshape, dependent on the pump-probe time delay, were also observed. The presence of defects was found to reduce the fluctuations and make the carrier recombination process faster.
Strain stabilized metal–insulator transition in epitaxial thin films of metallic oxide CaRuO3
Authors: Rao R. A. , Gan Q. , Eom C. B. , Cava R. J. , Suzuki Y. , Krajewski J. J. , Gausepohl S. C. , Lee M.
Published: July 2002

We report the observation of both metallic and semiconducting behavior in epitaxial thin films of the metallic oxide CaRuO3 deposited under identical conditions. X-ray diffraction studies showed that while semiconducting films with enlarged unit cells were obtained on single-crystal (100) SrTiO3 substrates, metallic films with lattice parameters close to the bulk material grew on (100) LaAlO3 substrates and poor crystalline quality SrTiO3 substrates. It is believed that a strain induced substitution of the small Ru4+ cations by the larger Ca2+ cations occurs, breaking the conduction pathway within the three-dimensional network of the RuO6 octahedra and leading to a metal–insulator transition. This unique phenomenon, which is not observed in bulk material, can be significant in technologically important epitaxial perovskite oxide heterostructures.
Crystal growth, spectroscopic characterization, and laser performance of a new efficient laser material Nd:Ba5(PO4)3F
Authors: Loutts G. B. , Bonner C. , Meegoda C. , Ries H. , Noginov M. A. , Noginova N. , Curley M. , Venkateswarlu P. , Rapaport A. , Bass M.
Published: July 2002

High quality single crystals of neodymium doped barium fluorapatite have been grown by the Czochralski technique and evaluated as an optical gain medium. A product of the emission cross section and lifetime of Nd3+ luminescence of more than 1.8×10−22 cm2 s at 1055 nm, and laser performance with slope efficiency up to 65% have been obtained.
Cr
K
edge x-ray absorption study of Cr dopants in Mg2SiO4 and Ca2GeO4
Authors: Miyano K. E. , Woicik J. C. , Sujatha Devi P. , Gafney H. D.
Published: July 2002

X-ray absorption above the Cr K edge has been used to study two Cr-doped olivine-structure materials. For Mg2SiO4, the extended fine structure shows that Cr resides in octahedral and tetrahedral sites with Cr–O distances of 1.98±0.03 and 1.68±0.03 Å, while the near edge structure indicates that Cr is in 3+ and 4+ oxidation states. For Ca2GeO4, site-size constraints limit Cr to the tetrahedral site with a Cr–O distance of 1.745±0.02 Å, and this Cr is only in the 4+ oxidation state. This study illustrates the utility of x-ray absorption for probing and relating the oxidation states and sites of constituent elements. In this specific case of Cr in olivine hosts, such understanding is critical to the development of these materials as tunable infrared solid-state lasers: Cr in the 4+ state provides the desired emission centered near 1.3 μm.
Synthesis of silicon nitride nanorods using carbon nanotube as a template
Authors: Han Weiqiang , Fan Shoushan , Li Qunqing , Gu Binglin , Zhang Xiaobin , Yu Dapeng
Published: July 2002

A method to prepare silicon nitride nanoscale rods using carbon nanotube as a template has been presented in this letter. The products of the reaction of carbon nanotubes with a mixture of Si and SiO2 powder in nitrogen atmosphere are β-Si3N4, α-Si3N4, and Si2N2O nanorods. The sizes of the nanorods are 4–40 nm in diameter and up to several microns in length. The formation mechanism of the nanorods has also been discussed.
Growth of b-axis rare earths on sapphire by molecular beam epitaxy
Authors: Ritley K. A. , Flynn C. P.
Published: July 2002

A process is described whereby hcp rare-earth metals can be grown heteroepitaxially as high-quality single-crystal films with the b axis normal to the growth plane. The growth employs molecular beam epitaxy, starting from available sapphire substrates. The results of characterization by several techniques are described.
Visible-blind GaN Schottky barrier detectors grown on Si(111)
Authors: Osinsky A. , Gangopadhyay S. , Yang J. W. , Gaska R. , Kuksenkov D. , Temkin H. , Shmagin I. K. , Chang Y. C. , Muth J. F. , Kolbas R. M.
Published: July 2002

We report novel GaN detectors grown by molecular beam epitaxy on Si(111) substrates. Wurtzite structure epitaxial GaN exhibits room-temperature photoluminescence with a band-edge-related emission width as narrow as 7 nm and intensities comparable to high quality layers grown on sapphire by metalorganic chemical vapor deposition. Spectral response of lateral geometry Schottky detectors shows a sharp cutoff at 365 nm with peak responsivities of ∼0.05 A/W at 0 V, and ∼0.1 A/W with a −4 V bias. The dark current is ∼60 nA at −2 V bias. The noise equivalent power is estimated to be 3.7×10−9 W over the response bandwidth of 2.2 MHz.
Surface wave dispersion beyond cutoff for a fast layer on a slow substrate
Authors: Lefeuvre O. , Zinin P. , Briggs G. A. D. , Every A.
Published: July 2002

The velocity and attenuation of pseudosurface waves beyond cutoff have been calculated for several layers on a slow substrate and unexpectedly different kinds of behavior can be distinguished. When the elastic properties of the layer and the substrate are not very different, the velocity of the pseudosurface wave beyond cutoff increases up to the Rayleigh wave velocity of the layer. When the elastic properties of the two materials are quite dissimilar, the pseudosurface wave tends towards an attenuated interfacial mode. In addition, another mode appears evolving into the Rayleigh wave of the layer material.
Optical second-harmonic probe for ultra-high frequency on-chip interconnects with benzocyclobutene
Authors: Nagel M. , Meyer C. , Heiliger H.-M. , Dekorsy T. , Kurz H. , Hey R. , Ploog K.
Published: July 2002

We report the application of electric field-induced second-harmonic (EFISH) generation for time-resolved measurement of ultra-fast electrical pulses propagating on thin-film microstrip lines fabricated with a polymer based on benzocyclobutene as a dielectric layer on a silicon substrate. This contactless field detection enables the characterization of electric pulses on the transmission line up to 1.95 THz. In comparison to external electro-optic sampling the EFISH technique provides a better characterization tool concerning time-resolution and noninvasiveness.
Electrical wavelength tunable and multiwavelength actively mode-locked fiber ring laser
Authors: Li Shenping , Chan K. T.
Published: July 2002

Electrical single- and dual-wavelength tuning and three-wavelength operation of an actively mode-locked fiber ring laser generating picosecond pulses were demonstrated by using a cavity with large dispersion. A continuous tuning range up to ∼37 nm of single-wavelength picosecond pulses at ∼1 GHz was achieved by only changing the modulation frequency. Continuous wavelength tuning of dual-wavelength (18.5 nm spacing) 3 GHz picosecond pulses was also demonstrated with a tuning range up to ∼17 nm. The tuning ranges were mainly limited by the gain bandwidth of the Er-doped fiber. Simultaneous generation of three-wavelength picosecond pulses at ∼2 GHz was also demonstrated by further increasing the cavity dispersion.
White light emission from exciplex in a bilayer device with two blue light-emitting polymers
Authors: Chao Ching-Ian , Chen Show-An
Published: July 2002

Bilayer polymer light-emitting diodes with two blue light-emitting materials, poly(N-vinylcarbazole) (PVK) and poly(2-dodecyl-p-phenylene) (C12O-PPP), can emit blue or white light, depending on the solvent used in the fabrication of the second layer, C12O-PPP. If hexane (the nonsolvent for PVK) is used, the device emits blue light as a single layer device with C12O-PPP. However, if toluene (the cosolvent for the two polymers) is used, the device emits white light originating from an exciplex emission at the bilayer interface in addition to the exciton emission from C12O-PPP. At low temperatures, the intensity of the exciplex emission drops and that of the exciton emission becomes dominant.
Deposition of (Zn, Mn)2SiO4 for plasma display panels using charged liquid cluster beam
Authors: Cich M. , Kim K. , Choi H. , Hwang S. T.
Published: July 2002

Films of manganese doped zinc silicate have been deposited using the charged liquid cluster beam technique. The deposition conditions were found to have a large impact on the morphology and photoluminescence intensity of these films. The photoluminescence intensity was maximized at a manganese content near δ=0.04 in (Zn1−δMnδ)2SiO4. No phases other than zinc silicate were detectable with manganese contents up to δ=0.08.
Influence of tunneling voltage on the imaging of carbon nanotube rafts by scanning tunneling microscopy
Authors: Biró L. P. , Thiry P. A. , Lambin Ph. , Journet C. , Bernier P. , Lucas A. A.
Published: July 2002

The influence of bias voltage on the quality of scanning tunneling microscopy images of carbon nanotube “rafts” was investigated in the range from −1 to 1 V in combination with scanning tunneling spectroscopy (STS) measurements. While for positive tip polarity only a slight voltage dependence was found in the image quality, for negative polarity a strong increase of the noise was observed with increasing voltage. STS showed that, for negative tip polarity, the tunneling current may be different in different locations by several orders of magnitude.
Near-field scanning optical nanolithography using amorphous silicon photoresists
Authors: Herndon M. K. , Collins R. T. , Hollingsworth R. E. , Larson P. R. , Johnson M. B.
Published: July 2002

Near-field scanning optical microscopy (NSOM) patterning of hydrogenated amorphous silicon (a-Si:H) has been explored. Our sample preparation technique produces films that are stable over several days. The etching process used is highly selective, allowing the unexposed a-Si:H to be completely removed while patterns with line heights equal to the original film thickness remain in exposed regions. We are able to generate patterns with and without the use of light. We have found that the probe dither amplitude greatly affects the linewidth and height of patterns generated without light. We also find that the exposure required for the NSOM to optically generate patterns agrees with threshold dosages determined by far-field exposure studies. Feature sizes of approximately 100 nm, comparable to the probe diameter, were obtained.
Local vibrational modes in Mg-doped GaN grown by molecular beam epitaxy
Authors: Kaschner A. , Siegle H. , Kaczmarczyk G. , Straßburg M. , Hoffmann A. , Thomsen C. , Birkle U. , Einfeldt S. , Hommel D.
Published: July 2002

Local vibrational modes in the region of the acoustic and optical phonons are reported for Mg-doped GaN grown by molecular beam epitaxy. The modes, studied by Raman spectroscopy, appear in addition to the known modes in the high-energy region around 2200 cm−1. We suggest disorder-activated scattering and scattering from Mg-related lattice vibrations to be the origin of the low-energy modes. Our assignment is supported by calculations based on a modified valence-force model of Kane. Temperature-dependent measurements between 4 and 300 K exclude an electronic Raman-scattering mechanism. We also report a new line at 2129 cm−1 and discuss the origin of all five observed high-energy modes.
Analysis of deep levels in a phenylenevinylene polymer by transient capacitance methods
Authors: Gomes H. L. , Stallinga P. , Rost H. , Holmes A. B. , Harrison M. G. , Friend R. H.
Published: July 2002

Transient capacitance methods were applied to the depletion region of an abrupt asymmetric n+−p junction of silicon and unintentionally doped poly[2-methoxy, 5 ethyl (2′ hexyloxy) paraphenylenevinylene] (MEH-PPV). Studies in the temperature range 100–300 K show the presence of a majority-carrier trap at 1.0 eV and two minority traps at 0.7 and 1.3 eV, respectively. There is an indication for more levels for which the activation energy could not be determined. Furthermore, admittance data reveal a bulk activation energy for conduction of 0.12 eV, suggesting the presence of an additional shallow acceptor state.
Interference effects in bilayer organic light-emitting diodes
Authors: So S. K. , Choi W. K. , Leung L. M. , Neyts K.
Published: July 2002

The electroluminescent spectra and the light intensities of Alq3-based organic light-emitting diodes were measured as a function of the thickness of the light-emitting layer. A maximum in the light intensity occurs when the thickness of this layer is approximately 600 Å, whereas a minimum in the light intensity occurs at about 1400 Å. The electroluminescent spectra were also found to vary strongly with the thickness of the light-emitting layer. Computer simulation, based on wide-angle interference of light-emitting dipoles from a reflective mirror, can account for the experimental observations.
Lateral ordering of quantum dots by periodic subsurface stressors
Authors: Romanov A. E. , Petroff P. M. , Speck J. S.
Published: July 2002

We investigate the possibility of using subsurface dislocation arrays as a tool for controlling the nucleation of self-assembled quantum dots (SAQDs). A quantitative model predicts that periodic nonuniform elastic fields on the surface induced by dislocations may control the lateral ordering SAQDs. The effect of dislocations is shown to be comparable to the interaction between buried and surface dots which leads to vertical dot stacking. The periodic subsurface dislocation arrays necessary for dot ordering can be produced by twist wafer bonding and backside substrate removal.
Ta
2
O
5
thin films with exceptionally high dielectric constant
Authors: Lin Jun , Masaaki Nakabayasi , Tsukune Atsuhiro , Yamada Masao
Published: July 2002

We have achieved tantalum pentoxide (Ta2O5) thin films with extremely highly predominant 〈001〉 orientation. The Ta2O5 thin films have an exceptionally high dielectric constant of 90–110, and capacitors using these Ta2O5 films as a dielectric layer show the high capacitance and low leakage current meeting the requirements for the new generation of memory devices.
Defect-related infrared photoluminescence in Ge+-implanted SiO2 films
Authors: Wu X. L. , Gao T. , Siu G. G. , Tong S. , Bao X. M.
Published: July 2002

SiO 2 films with Ge+ implantation at an energy of 60 keV and a dose of 1×1016 cm−2, followed by annealing at different temperature, exhibit a broad infrared photoluminescence (PL) at room temperature under an excitation of the 514.5 nm line of Ar+ laser. With increasing the annealing temperature, the intensity of the infrared PL band decreases, its full width at half maximum increases, and its energy redshifts. Spectral analysis and some experimental results from Raman scattering, electron spin resonance, and infrared spectroscopy strongly suggest that the infrared PL is mainly related to interfacial oxygen-deficient-type defects between the oxide and Ge nanocrystals.
Thermal stability and relaxation in diamond-like-carbon. A Raman study of films with different sp3 fractions (ta-C to a-C)
Authors: Kalish R. , Lifshitz Y. , Nugent K. , Prawer S.
Published: July 2002

The thermal stability and relaxation processes in hydrogen free diamond-like-carbon films with different sp3 fractions (40%, 60%, and 80%) are comparatively studied for the first time by visible Raman spectroscopy. The 80% sp3 film is very stable under annealing in the entire temperature region investigated (300–1270 K) and shows only a minor change of the optical transmission, most likely due to a mild sp2 clustering, but no graphitization. This is very important for practical applications requiring a high thermal stability. The films with lower sp3 fraction show a thermal stability which decreases with the decreasing sp3. Graphitization starts at 700 K for the 40% sp3 film.
A solid-state process for formation of boron nitride nanotubes
Authors: Chen Ying , Chadderton Lewis T. , Gerald John Fitz , Williams James S.
Published: July 2002

The formation of boron nitride (BN) nanotubes via a solid-state process is demonstrated. The nanotubes are produced by first ball-milling hexagonal BN powder to generate highly disordered or amorphous nanostructures, followed by annealing at temperatures up to 1300 °C. The annealing leads to the nucleation and growth of hexagonal BN nanotubes of both cylindrical and bamboo-like morphology. Unlike previous mechanisms for nanotube formation, the reordering and solid-state growth process of our nanotubes does not involve deposition from the vapor phase nor chemical reactions.
Compositional fluctuations in GaInN/GaN double heterostructures investigated by selectively excited photoluminescence and Raman spectroscopy
Authors: Wieser N. , Ambacher O. , Felsl H.-P. , Görgens L. , Stutzmann M.
Published: July 2002

We report on compositional fluctuations in GaN/GaInN/GaN double heterostructures investigated by photoluminescence excitation and resonant Raman spectroscopy. The energy and line-shape of both luminescence and longitudinal optical (LO) phonon Raman peaks strongly depend on excitation energy due to selective excitation of regions with different In content. For a given excitation energy, luminescence from In-rich regions takes place and in addition, with increasing sample temperature, resonantly excited luminescence from regions of lower In content is superposed. Thus, the luminescence strongly broadens and on average shifts to higher energies with increasing temperature. The spectral variation of the photoluminescence and Raman cross sections is determined and correlated with the GaInN absorbance as measured by photothermal deflection spectroscopy.
Self-organized growth of Fe nanowire array on H2O/Si(100)(2×n)
Authors: Kida A. , Kajiyama H. , Heike S. , Hashizume T. , Koike K.
Published: July 2002

By evaporating Fe on to a water-terminated Si(100)(2×n) surface, we formed an Fe wire array reflecting the 2×n surface reconstruction. The average wire width was 2 nm and the period was 3 nm. The formation was caused by the deposited Fe atoms diffusing over the water-terminated flat area and being trapped at dimer vacancy lines. This array is applicable to magnetic devices.
Growth of a single freestanding multiwall carbon nanotube on each nanonickel dot
Authors: Ren Z. F. , Huang Z. P. , Wang D. Z. , Wen J. G. , Xu J. W. , Wang J. H. , Calvet L. E. , Chen J. , Klemic J. F. , Reed M. A.
Published: July 2002

Patterned growth of freestanding carbon nanotube(s) on submicron nickel dot(s) on silicon has been achieved by plasma-enhanced-hot-filament-chemical-vapor deposition (PE-HF-CVD). A thin film nickel grid was fabricated on a silicon wafer by standard microlithographic techniques, and the PE-HF-CVD was done using acetylene (C2H2) gas as the carbon source and ammonia (NH3) as a catalyst and dilution gas. Well separated, single carbon nanotubes were observed to grow on the grid. The structures had rounded base diameters of approximately 150 nm, heights ranging from 0.1 to 5 μm, and sharp pointed tips. Transmission electron microscopy cross-sectional image clearly showed that the structures are indeed hollow nanotubes. The diameter and height depend on the nickel dot size and growth time, respectively. This nanotube growth process is compatible with silicon integrated circuit processing. Using this method, devices requiring freestanding vertical carbon nanotube(s) such as scanning probe microscopy, field emission flat panel displays, etc. can be fabricated without difficulty.
Correlation between nano-scale friction and wear of boron carbide films deposited by dc-magnetron sputtering
Authors: Prioli R. , Reigada D. C. , Freire F. L.
Published: July 2002

The analysis of friction and wear of boron carbide films scanned with a light load atomic force microscope Si3N4 tip is presented. The results show that nano-scale friction and wear are strongly correlated, and that friction decreases fast at the beginning of the scratching tests. We propose that the energy dissipated at the tip-surface interface during scanning induces the formation of a lubricant material at the surface of the boron carbide film that reduces friction.
Local vibrational modes as a probe of activation process in p-type GaN
Authors: Harima H. , Inoue T. , Nakashima S. , Ishida M. , Taneya M.
Published: July 2002

Raman spectra for a series of Mg-doped GaN films grown by metal organic chemical vapor deposition and annealed in N2 ambiance at different temperatures have been investigated. Some local vibrational modes related to hydrogen were observed, showing drastic changes with the annealing temperature. The spectra show clearly that H impurities incorporated in as-grown films, which passivate Mg acceptors, are released from the Mg–N bonding at above ∼600 °C, and diffuse in the film to form new chemical bondings. We have also observed a local mode related to activated Mg acceptors. This mode is conveniently used as a probe of the activation process of Mg acceptors.
Precise control of 1.55 μm vertical-cavity surface-emitting laser structure with InAlGaAs/InAlAs Bragg reflectors by in situ growth monitoring
Authors: Baek Jong-Hyeob , Choi In Hoon , Lee Bun , Han Won Seok , Cho Hyung Koun
Published: July 2002

The vertical-cavity surface-emitting laser (VCSEL) structure designed at 1.55 μm was grown by a low-pressure metalorganic chemical vapor deposition method. In situ laser reflectometry, using both 0.633 and 1.53 μm wavelengths simultaneously, was employed to control the exact optical thickness over the whole growth time. The distributed Bragg reflectors (DBRs) were grown with alternate In0.53Al0.13Ga0.34As and In0.52Al0.48As λ/4 wavelength layers. The oscillatory reflection signals obtained by the monitoring laser at 1.53 μm gave information for designing the center wavelength of the DBR. The reflectance spectrum of the VCSEL structure showed an excellent square shaped wide flatband (greater than 90 nm) where the reflectivity reached a plateau as expected by the in situ monitoring data.
Memory properties of SrBi2Ta2O9 thin films prepared on SiO2/Si substrates
Authors: Xiong Si-Bei , Sakai Shigeki
Published: July 2002

We prepared SrBi2Ta2O9(SBT) ferroelectric thin films on SiO2/Si and on MgO buffered SiO2/Si substrates using laser ablation, in situ annealing and postannealing, and observed retention time longer than 7 days. On SiO2/Si substrates, only the samples annealed at 800 °C occasionally exhibited good memory properties. On MgO buffered SiO2/Si substrates, successful probability for finding good measuring dots increased largely even while postannealing temperature was as low as 650 °C; their typical memory window was 3.1 V and retention time over 7 days without serious degradation. The MgO buffer layer inserted between SBT and SiO2/Si may play three positive roles on the quality of metal–ferroelectric–insulator–semiconductor structure: preventing bismuth loss, lowering crystallization temperature, and decreasing leakage current.
Experimental demonstration of electrically controllable photonic crystals at centimeter wavelengths
Authors: de Lustrac A. , Gadot F. , Cabaret S. , Lourtioz J.-M. , Brillat T. , Priou A. , Akmansoy E.
Published: July 2002

Electrically controllable photonic crystals have been fabricated by inserting p-i-n diodes in two-dimensional metallic lattices. A first structure uses a square lattice of thin and discontinuous metallic wires. A second structure is fabricated using stacks of printed circuits with metallic strips. The p-i-n diodes are soldered along the different metallic wires or strips. The crystals have been characterized between 1 and 20 GHz. We show that they can be operated as wideband switchable electromagnetic windows with high transmission or reflection contrast between on and off states. A ∼25 dB transmission modulation is reported within the first transmission band of a two-period crystal. We also show that the switching domain and modulation rate can be varied with a separate bias control for each crystal plane. Finally, the distance between crystal planes is used to tune the operating frequency range.
Preferential nucleation of GaN quantum dots at the edge of AlN threading dislocations
Authors: Rouvière J. L. , Simon J. , Pelekanos N. , Daudin B. , Feuillet G.
Published: July 2002

We report on transmission electron microscopy and optical characterization of GaN dots embedded in an AlN matrix. GaN dots were grown by molecular beam epitaxy on top of an AlN layer deposited on (0001) sapphire. We found that the GaN dots (average diameter of 16 nm, average height of 4 nm) are coherently grown on AlN, but nucleate next to threading edge dislocations that were propagating in AlN. The presence of these adjacent dislocations does not inhibit the optical emission of the GaN dots: contrary to the photoluminescence of thick GaN layers or GaN/AlGaN quantum wells, the photoluminescence of these GaN dots is found to be temperature independent.
Fabrication of transparent p–n heterojunction thin film diodes based entirely on oxide semiconductors
Authors: Kudo Atsushi , Yanagi Hiroshi , Ueda Kazushige , Hosono Hideo , Kawazoe Hiroshi , Yano Yoshihiko
Published: July 2002

All oxide-based, transparent polycrystalline p–n heterojunctions on a glass substrate were fabricated. The structure of the diode was n+-ZnO electrode/n-ZnO/p-SrCu2O2/In2−xSnxO3 electrode on the substrate. The contact between the n- and p-type semiconducting oxides was found to be rectifying. The ratio of forward current to the reverse current exceeded 80 within the range of applied voltages of −1.5 to +1.5 V and the estimated diode factor (n value) was 1.62. The diode structure was fabricated on a glass plate with the total thickness of 1.3 μm and possessed an optical transmission of 70%–80% in the visible region.
High coercivity in nanostructured PrCo5-based powders produced by mechanical milling and subsequent annealing
Authors: Chen Zhongmin , Meng-Burany X. , Hadjipanayis G. C.
Published: July 2002

Nanostructured PrCo5-based PrxCo100−x (x=15.4–20.5) powders have been synthesized by mechanical milling and subsequent annealing cast alloys. The best overall properties have been developed in stoichiometric PrCo5 powders milled for 4 h and annealed at 800 °C for 1 min with a high coercivity of 16.3 kOe along with a high Mr/Ms ratio of 0.66 and medium-strength maximum-energy product of 11.6 MGOe. The highest coercivity of 23.7 kOe has been obtained in Pr19Co81 powders. Microstructural studies reveal that a uniform PrCo5 microstructure with an average grain size of about 15 nm is developed in the powders, which have an average particle size of about 5 μm. The observed magnetic hardening is believed to arise from the high anisotropy field of the PrCo5 phase and the uniform nanoscale microstructure developed by the processing used.
Oxygen pressure-tuned epitaxy and optoelectronic properties of laser-deposited ZnO films on sapphire
Authors: Choopun S. , Vispute R. D. , Noch W. , Balsamo A. , Sharma R. P. , Venkatesan T. , Iliadis A. , Look D. C.
Published: July 2002

Influence of oxygen pressure on the epitaxy, surface morphology, and optoelectronic properties has been studied in the case of ZnO thin films grown on sapphire (0001) by pulsed-laser deposition. Results of Rutherford backscattering and ion channeling in conjunction with atomic force microscopy clearly indicate that the growth mode, degree of epitaxy, and the defect density strongly depend on the oxygen background pressure during growth. It is also found that the growth mode and the defects strongly influence the electron mobility, free-electron concentration, and the luminescence properties of the ZnO films. By tuning the oxygen pressure during the initial and the final growth stages, smooth and epitaxial ZnO films with high optical quality, high electron mobility, and low background carrier concentration have been obtained. The implication of these results towards the fabrication of superlattices and controlled n- and p-type doping is discussed.
Optical and acoustic phonon modes in self-organized Ge quantum dot superlattices
Authors: Liu J. L. , Jin G. , Tang Y. S. , Luo Y. H. , Wang K. L. , Yu D. P.
Published: July 2002

Raman scattering measurements were carried out in self-organized Ge quantum dot superlattices. The samples consisted of 25 periods of Ge quantum dots with different dot sizes sandwiched by 20 nm Si spacers, and were grown using solid-source molecular-beam epitaxy. Optical phonon modes were found to be around 300 cm−1, and a dependence of the Raman peak frequency on the size of dots was evidenced in good agreement with a prediction based on phonon confinement and strain effects. Acoustic phonons related to the Ge quantum dots have also been observed.
DyFe
2
(110)
nanostructures: Morphology and magnetic anisotropy
Authors: Mougin A. , Dufour C. , Dumesnil K. , Maloufi N. , Mangin Ph.
Published: July 2002

Single-crystalline DyFe2(110) nanosystems have been obtained by molecular-beam epitaxy. From reflection high-energy electron diffraction observations, the systems have been shown to grow in a Stranski–Krastanov mode. Depending on elaboration conditions (substrate temperature and nominal thickness), dots with anisotropic shape or continuous films with low surface roughness are obtained. Compared to the bulk compounds, the epitaxial systems are strained because of thermal differential contraction and exhibit modifications of easy-magnetization direction compared to bulk. The magnetization reversal process is correlated to the morphology of the layers.
Piezoelectric properties of rhombohedral Pb(Zr, Ti)O3 thin films with (100), (111), and “random” crystallographic orientation
Authors: Taylor David V. , Damjanovic Dragan
Published: July 2002

The longitudinal d33 piezoelectric coefficient was studied in rhombohedral Pb(Zr0.6Ti0.4)O3 thin films with (111), (100), and “random” orientation. The largest d33 was found in (100)-oriented films and the smallest along the polarization direction in (111)-oriented films. These results are in a good qualitative agreement with recent theoretical predictions [Du, Zheng, Belegundu, and Uchino, Appl. Phys. Lett. 72, 2421 (1998)]. The field dependence of d33 was also investigated as a function of crystallographic orientation of the films. It was found that (100)-oriented films with the highest piezoelectric coefficient exhibit the weakest nonlinearity. Observed variation in the piezoelectric nonlinearity with film orientation can be fully explained by taking into account domain-wall contributions, which are dependent on film orientation.
Surface plasmon resonance phase imaging
Authors: Notcovich Ariel G. , Zhuk V. , Lipson S. G.
Published: July 2002

We present a promising optical system and technique for detection, imaging, and visualization of minimal changes, of the order of 10−6 in refractive index, in the near field. The system is based on phase imaging of excited surface plasmons in a conducting layer in contact with a sample. This technique is shown here to be sensitive enough to image the flow of helium and argon gas in a nitrogen atmosphere. Its application to biological and chemical research is suggested.
Detection of luminescent single ultrasmall silicon nanoparticles using fluctuation correlation spectroscopy
Authors: Akcakir O. , Therrien J. , Belomoin G. , Barry N. , Muller J. D. , Gratton E. , Nayfeh M.
Published: July 2002

We dispersed electrochemical etched Si into a colloid of ultrasmall blue luminescent nanoparticles, observable with the naked eye, in room light. We use two-photon near-infrared femtosecond excitation at 780 nm to record the fluctuating time series of the luminescence, and determine the number density, brightness, and size of diffusing fluorescent particles. The luminescence efficiency of particles is high enough such that we are able to detect a single particle, in a focal volume, of 1 pcm3. The measurements yield a particle size of 1 nm, consistent with direct imaging by transmission electron microscopy. They also yield an excitation efficiency under two-photon excitation two to threefold larger than that of fluorescein. Detection of single particles paves the way for their use as labels in biosensing applications.
Distinctly different thermal decomposition pathways of ultrathin oxide layer on Ge and Si surfaces
Authors: Prabhakaran K. , Maeda F. , Watanabe Y. , Ogino T.
Published: July 2002

The thermal decomposition pathway of an ultrathin oxide layer on Ge(100) and Si(100) surfaces is examined by synchrotron radiation photoelectron spectroscopy and ultraviolet photoelectron spectroscopy with helium I radiation. The as-prepared oxide layer consists of a mixture of oxides, namely, suboxides and dioxides, on both the surfaces. Upon annealing, the oxide layers decompose and desorb as monoxides. However, we find that the decomposition pathways are different from each other. On annealing Ge oxides, GeO2 species transform to GeO and remain on the surface and desorb at >420 °C. In contrast, annealing of Si oxides results in the transformation of SiO to SiO2 up to temperatures (∼780 °C) close to the desorption. At higher temperatures, SiO2 decomposes and desorbs, implying a reverse transformation to volatile SiO species.
Formation of silicon islands on a silicon on insulator substrate upon thermal annealing
Authors: Legrand B. , Agache V. , Nys J. P. , Senez V. , Stievenard D.
Published: July 2002

Starting from silicon on insulator substrates, we show that a thermal treatment (in the 600–900 °C range) induces the creation of silicon islands. To characterize the island formation as well as the initial silicon layer thickness, we use in situ Auger electron spectroscopy analysis in an ultrahigh vacuum chamber. The island size and density are studied with an ex situ atomic force microscope. We show that the formation temperature of the islands increases from 575 to 875 °C as the initial silicon layer thickness increases from 1 to 19 nm. For the 1 nm thickness, the minimum island size is reached (semispherical shape with a 16 nm diameter). The phenomena involved in the island formation are discussed and the study of the variations of the calculated stress tensor (IMPACT software) as a function of the thermal treatment explain the behavior of the top silicon layer.
Schottky effect model of electrical activity of metallic precipitates in silicon
Authors: Plekhanov P. S. , Tan T. Y.
Published: July 2002

A quantitative model of the electrical activity of metallic precipitates in Si is formulated with an emphasis on the Schottky junction effects of the precipitate–Si system. Carrier diffusion and carrier drift in the Si space charge region are accounted for. Carrier recombination is attributed to the thermionic emission mechanism of charge transport across the Schottky junction rather than the surface recombination. It is shown that the precipitates can have a very large minority carrier capture cross-section. Under weak carrier generation conditions, the supply of minority carriers is found to be the limiting factor of the recombination process. The plausibility of the model is demonstrated by a comparison of calculated and available experimental results.
Influences of interfacial intrinsic low-dielectric layers on the dielectric properties of sputtered (Ba,Sr)TiO3 thin films
Authors: Lee Byoung Taek , Hwang Cheol Seong
Published: July 2002

The influences of low-dielectric interfacial layers on the dielectric properties of Pt/(Ba,Sr)TiO3/Pt capacitors were investigated before and after postannealing. The interfacial layer is believed to be the intrinsic dead layer (low-dielectric layer) due to the termination of chemical bonds of the (Ba,Sr)TiO3 (BST) material at the interfaces. The dielectric constant of the capacitor decreases with decreasing BST film thickness owing to the low dielectric constant of the dead layer. The dead-layer capacitance varies with processes such as film deposition temperature, and postannealing. Higher deposition temperatures result in a larger dead-layer capacitance and a higher bulk dielectric constant. Although annealing under a N2 atmosphere is less effective in reducing the dead-layer effect than under an O2 atmosphere, it is more effective in increasing the bulk dielectric constant. Therefore, a N2, rather than an O2 atmosphere, results in a larger increase in the overall dielectric constant.
Putting an edge to the Poisson bracket
Authors: Bering K.
Published: July 2002

We consider a general formalism for treating a Hamiltonian (canonical) field theory with a spatial boundary. In this formalism essentially all functionals are differentiable from the very beginning and hence no improvement terms are needed. We introduce a new Poisson bracket which differs from the usual “bulk” Poisson bracket with a boundary term and show that the Jacobi identity is satisfied. The result is geometrized on an abstract world volume manifold. The method is suitable for studying systems with a spatial edge like the ones often considered in Chern–Simons theory and General Relativity. Finally, we discuss how the boundary terms may be related to the time ordering when quantizing.
Log spiral of revolution highly oriented pyrolytic graphite monochromator for fluorescence x-ray absorption edge fine structure
Authors: Pease D. M. , Daniel M. , Budnick J. I. , Rhodes T. , Hammes M. , Potrepka D. M. , Sills K. , Nelson C. , Heald S. M. , Brewe D. I. , Frenkel Anatoly , Grigorieva I. , Antonov A.
Published: July 2002

We have constructed an x-ray monochromator based on a log spiral of revolution covered with highly oriented pyrolytic graphite. Such a monochromator is used for obtaining x-ray absorption edge fine structure by the fluorescence method, and is particularly useful for measuring the fine structure of dilute element A in a concentrated matrix of element B, where B is to the left of A in the Periodic Table. Using the log spiral monochromator, we measure good Cr x-ray fine structure in an alloy of 1% Cr in a V matrix, whereas the corresponding spectrum is severely distorted by the V background if nonmonochromatized fluorescence is used. We also obtain excellent rejection of Mn fluorescence relative to Cr fluorescence in a Cr80Mn20 alloy, and can tune the monochromator such that the entire Mn step height is significantly smaller than the Cr x-ray absorption edge fine structure oscillations for this system.
Statistical representation of a spray as a point process
Authors: Subramaniam S.
Published: July 2002

The statistical representation of a spray as a finite point process is investigated. One objective is to develop a better understanding of how single-point statistical information contained in descriptions such as the droplet distribution function (ddf), relates to the probability density functions (pdfs) associated with the droplets themselves. Single-point statistical information contained in the droplet distribution function (ddf) is shown to be related to a sequence of single surrogate-droplet pdfs, which are in general different from the physical single-droplet pdfs. It is shown that the ddf contains less information than the fundamental single-point statistical representation of the spray, which is also described. The analysis shows which events associated with the ensemble of spray droplets can be characterized by the ddf, and which cannot. The implications of these findings for the ddf approach to spray modeling are discussed. The results of this study also have important consequences for the initialization and evolution of direct numerical simulations (DNS) of multiphase flows, which are usually initialized on the basis of single-point statistics such as the droplet number density in physical space. If multiphase DNS are initialized in this way, this implies that even the initial representation contains certain implicit assumptions concerning the complete ensemble of realizations, which are invalid for general multiphase flows. Also the evolution of a DNS initialized in this manner is shown to be valid only if an as yet unproven commutation hypothesis holds true. Therefore, it is questionable to what extent DNS that are initialized in this manner constitute a direct simulation of the physical droplets. Implications of these findings for large eddy simulations of multiphase flows are also discussed.
Microstructure and dielectric properties of Ba1−xSrxTiO3 films grown on LaAlO3 substrates
Authors: Gim Y. , Hudson T. , Fan Y. , Kwon C. , Findikoglu A. T. , Gibbons B. J. , Park B. H. , Jia Q. X.
Published: July 2002

We report a systematic study of the microstructure and dielectric properties of barium strontium titanate, Ba1−xSrxTiO3, films grown by laser ablation on LaAlO3 substrates, where x=0.1–0.9 at an interval of 0.1. X-ray diffraction analysis shows that when x<0.4, the longest unit-cell axis is parallel to the plane of the substrate but perpendicular as x approaches 1. Dielectric constant versus temperature measurements show that the relative dielectric constant has a maximum value and that the peak temperatures corresponding to the maximum relative dielectric constant are about 70 °C higher when x⩽0.4 but similar when x>0.4, compared with the peak temperatures of the bulk Ba1−xSrxTiO3. At room temperature, the dielectric constant and tunability are relatively high when x⩽0.4 but start to decrease rapidly as x increases.
Stability of a four-vortex aircraft wake model
Authors: Fabre David , Jacquin Laurent
Published: July 2002

The stability of an aircraft wake model composed of an external vortex pair (modeling the wing tip vortices) and an internal vortex pair rotating in the opposite direction (modeling the vortices generated by the fuselage and the horizontal tail) in a stationary configuration is investigated with the vortex filament stability method used by Crow [AIAA J. 8, 2172 (1970)] and Crouch [J. Fluid Mech. 350, 311 (1997)]. It is shown that this configuration is unstable with respect to two-dimensional and three-dimensional disturbances. For long wavelength three-dimensional symmetric perturbations, the rapid growth observed in the numerical simulations of Rennich and Lele [J. Air. 36, 398 (1999)] is found. Moreover, the analysis allows one to show that without an excitation of the long-wave mode, the system will naturally develop short wavelength instabilities localized within the inner vortices which do not affect the outer vortices. Inspection of the initial value problem shows that the long-wave modes can be efficiently initiated by the introduction of perturbations on the internal vortices.