bluuebunny/crossref_metadata_2025 huggingface dataset:
Enhanced infrared absorption with dielectric nanoparticles
Authors: Anderson Mark S.
Published: October 2003

Enhanced infrared absorption is demonstrated for anthracene coating polar dielectric nanoparticles of silicon carbide and aluminum oxide. An enhancement factor greater than 100 was measured near the surface of silicon carbide particles. This is the result of the enhanced optical fields at the surface of the particles when illuminated at the surface phonon resonance frequencies. This phonon resonance effect is analogous to plasmon resonance that is the basis of surface enhanced infrared absorption and surface enhanced Raman scattering. The results have implications for near-field microscopy, the characterization of nano-optical devices, and chemical sensing. In addition, the methodology used for surface phonon analysis of particles is useful for simulating comet and interstellar dust spectra.
Low jitter capillary discharge channels
Authors: Greenberg B. , Levin M. , Pukhov A. , Zigler A.
Published: October 2003

Generation of low jitter capillary discharge channels triggered by a relatively low-energy laser pulse is reported. For ablative discharges in polyethylene and boron nitride capillaries, the dependences of the breakdown delay and jitter on the incident laser intensity and discharge voltage are presented. Low jitter (5 ns and less) and the absence of electric noise associated with the discharge ignition make this method very useful for applications such as channel-guided laser-wakefield accelerators and x-ray lasers, where precise synchronization of the discharge with a laser and/or electron beam entering the channel is required.
Direct evidence for structural origin of stress-induced magnetic anisotropy in Fe–Si–B–Nb–Cu nanocrystalline alloys
Authors: Ohnuma M. , Hono K. , Yanai T. , Fukunaga H. , Yoshizawa Y.
Published: October 2003

The structural origin of magnetic anisotropy in Fe–Si–B–Nb–Cu alloys annealed under a tensile stress of 200 MPa is studied by transmission x-ray diffraction. The diffraction peak of the (310) plane, whose normal vector is parallel to the tensile direction (ribbon direction), appears at a lower angle than the one perpendicular to it by about 0.1°. This indicates that the spacing of the (310) plane normal to the tensile direction is about 0.2% larger than the one parallel to it. This is direct evidence for the structural origin of the stress-induced magnetic anisotropy of nanocrystalline soft magnetic alloys.
Explanation of annealing-induced blueshift of the optical transitions in GaInAsN/GaAs quantum wells
Authors: Kudrawiec R. , Se̢k G. , Misiewicz J. , Gollub D. , Forchel A.
Published: October 2003

This letter aims to describe the effect of rapid thermal annealing on a Ga0.64In0.36As0.99N0.01/GaAs single quantum well grown by molecular-beam epitaxy. This effect was investigated using both photoluminescence and photoreflectance. A blueshift of optical transitions and a change of character of the ground-state transition were observed after annealing. We show that this behavior can be explained by a combination of two annealing-induced effects: A change in the nearest-neighbor configuration of nitrogen atoms and a simultaneous change in the quantum well profile due to atom diffusion across the quantum well interfaces.
Planar gallium nitride ultraviolet optical modulator
Authors: Oberhofer A. E. , Muth J. F. , Johnson M. A. L. , Chen Z. Y. , Fleet E. F. , Cooper G. D.
Published: October 2003

A planar optical modulator operating near 360 nm suitable for ultraviolet spatial light modulation has been constructed. The modulator operates in transmission mode with a 18% change in transmission at 305 V. The modulator is based on using the electric field to shift and broaden the room-temperature resonance of the gallium nitride exciton with electric fields.
Superhard self-lubricating AlMgB14 films for microelectromechanical devices
Authors: Tian Y. , Bastawros A. F. , Lo C. C. H. , Constant A. P. , Russell A. M. , Cook B. A.
Published: October 2003

Performance and reliability of microelectromechanical system (MEMS) components can be enhanced dramatically through the incorporation of protective thin-film coatings. Current-generation MEMS devices prepared by the lithographie-galvanoformung-abformung (LIGA) technique employ transition metals such as Ni, Cu, Fe, or alloys thereof, and hence lack stability in oxidizing, corrosive, and/or high-temperature environments. Fabrication of a superhard self-lubricating coating based on a ternary boride compound AlMgB14 described in this letter has great potential in protective coating technology for LIGA microdevices. Nanoindentation tests show that the hardness of AlMgB14 films prepared by pulsed laser deposition ranges from 45 GPa to 51 GPa, when deposited at room temperature and 573 K, respectively. Extremely low friction coefficients of 0.04–0.05, which are thought to result from a self-lubricating effect, have also been confirmed by nanoscratch tests on the AlMgB14 films. Transmission electron microscopy studies show that the as-deposited films are amorphous, regardless of substrate temperature; however, analysis of Fourier transform infrared spectra suggests that the higher substrate temperature facilitates the formation of the B12 icosahedral framework, therefore leading to the higher hardness.
Magnetic out-of-plane component in MnAs/GaAs(001)
Authors: Ney A. , Hesjedal T. , Pampuch C. , Mohanty J. , Das A. K. , Däweritz L. , Koch R. , Ploog K. H.
Published: October 2003

From highly sensitive superconducting quantum interference device magnetometry and magnetic force microscopy, we deduce a small out-of-plane magnetization component of MnAs/GaAs(001) films. Its temperature dependence is substantially different from the dominating in-plane magnetization, particularly as it is still detectable above the phase transition temperature of MnAs films. Our measurements indicate that the out-of-plane component is due to small isolated magnetic “grains” within the film.
Temperature dependent low-field electron multiplication in In0.53Ga0.47As
Authors: Ng W. K. , Tan C. H. , David J. P. R. , Houston P. A. , Yee M. , Ng J. S.
Published: October 2003

Electron multiplication for a series of In0.53Ga0.47As p-i-n diodes and hole multiplication for a n-i-p diode were measured at electric fields of 100–260 kV/cm over a temperature range of 20–300 K. The electron multiplication characteristics consistently showed positive temperature dependence at low electric fields (below ∼200 kV/cm) but exhibited negative temperature dependence at high fields. These observations explain the apparent discrepancies of the temperature dependence of the electron ionization coefficient in In0.53Ga0.47As in literature. In contrast, hole multiplication showed little change with temperature.
Using point-defect engineering to increase stability of highly doped ultrashallow junctions formed by molecular-beam-epitaxy growth
Authors: Shao Lin , Thompson Phillip E. , Bennett Joe , Dharmaiahgari Bhanu P. , Trombetta Len , Wang Xuemei , Chen Hui , Seo Hye-Won , Chen Quark Y. , Liu Jiarui , Chu Wei-Kan
Published: October 2003

Stability of p+/n junctions remains a critical issue for device performance. We report that the technique of point-defect engineering (PDE) can substantially increase the stability of ultrashallow junctions formed by molecular-beam epitaxy. It is shown that an as-grown 15 nm, 2×1020/cm3 B-doped Si layer becomes unstable during 10 min thermal anneal above 650 °C. The thermal stability can be increased by performing a 5×1015/cm2 1 MeV Si ion implantation. The B profile with the MeV Si implant does not show significant diffusion during annealing up to 750 °C, and the final junction depth after an 800 °C/10 min anneal is about half that of an annealed unimplanted sample. Although with Mev implantation the as-implanted B profile becomes slightly deeper due to recoil implantation, and some of the B has been electrically deactivated by the MeV implantation, PDE is advantageous for postgrowth thermal processes above 700 °C. The mechanism causing the instability is discussed.
Effective diffusivity in periodic porous materials
Authors: Berezhkovskii Alexander M. , Zitserman Vladimir Yu. , Shvartsman Stanislav Y.
Published: October 2003

Diffusion of a solute in a periodic porous solid is analyzed. An expression for the effective diffusion coefficient is derived for a solute diffusing in a porous medium formed by a simple cubic lattice of spherical cavities connected by narrow tubes. This expression shows how the effective diffusion coefficient depends on microgeometry of the porous material. Generalizations to nonspherical cavities, other lattices, and nonequal diffusion coefficients in the cavities and in the tubes are discussed.
Coupling between magnetic/nonmagnetic semiconductor quantum dots in double-layer geometry
Authors: Lee S. , Shin D. Y. , Titova L. , Kutrowski M. , Dobrowolska M. , Furdyna J. K.
Published: October 2003

Magneto-photoluminescence (PL) is used to study carrier transfer between self-assembled quantum dots (QDs) fabricated in the form of two adjacent QD layers separated by a thin barrier, one layer consisting of CdSe QDs and one of CdMnSe QDs in a ZnSe matrix. CdMnSe is a diluted magnetic semiconductor (DMS). In contrast to typical behavior of many low-dimensional DMS systems in which the application of a magnetic field B dramatically increases the PL intensity, in double-layer structures described above we observed a striking decrease of the PL intensity as a function of B. This effect indicates carrier tunneling from the CdSe to the CdMnSe dots as the ground state in the latter is lowered via the large Zeeman shift of the CdMnSe band edges.
Extended wavelength region of self-assembled Ge/Si(001) islands capped with Si at different temperatures
Authors: Stoffel M. , Denker U. , Kar G. S. , Sigg H. , Schmidt O. G.
Published: October 2003

We investigate the emission wavelength region of self-assembled Ge/Si(001) islands. The islands were grown between 360 and 840 °C and subsequently capped with Si at low temperatures (300 °C). Under these conditions, the island morphology is preserved as revealed by atomic force microscopy. By decreasing the capping temperature, photoluminescence measurements evidence a systematic redshift enabling us to discuss the relative contribution of Si intermixing during growth and during capping. We also find that the emission wavelength can be extended up to 2.06 μm for hut clusters grown at 400 °C. By further decreasing the Ge growth temperature to 360 °C, the emission energy evidences a blueshift. This result is explained by enhanced charge carrier confinement in extremely small Ge quantum dots.
Technique for writing of nonlinear optical single-crystal lines in glass
Authors: Honma Tsuyoshi , Benino Yasuhiko , Fujiwara Takumi , Komatsu Takayuki , Sato Ryuji
Published: October 2003

The technique for the writing of crystal lines in glass by Nd:YAG laser irradiation, selective atom heat processing, is applied to rare-earth (Sm3+, Dy3+) barium borate glasses, and nonlinear optical β-BaB2O4 (β-BBO) crystal lines with a width of ∼5 μm are fabricated. The crystal lines show uniform colors in polarized optical microscope and second-harmonic generation (SHG) measurements. The azimuthal dependence of SHG signals in the crystal lines is the same as that in commercially available β-BBO bulk single crystals. It is demonstrated that the crystal lines written by YAG laser irradiations are β-BBO single crystals with the c-axis orientation along the laser scanning direction.
Layer-by-layer growth of high-optical-quality ZnO film on atomically smooth and lattice relaxed ZnO buffer layer
Authors: Tsukazaki A. , Ohtomo A. , Yoshida S. , Kawasaki M. , Chia C. H. , Makino T. , Segawa Y. , Koida T. , Chichibu S. F. , Koinuma H.
Published: October 2003

The growth mode of ZnO thin films can be well regulated in a molecular layer-by-layer growth by employing a ZnO buffer layer deposited on a lattice-matched ScAlMgO4 substrate and annealed at high temperature. The annealed buffer layer has atomically flat surface and relaxed (strain-free) crystal structure. The intensity oscillation of reflection high-energy electron diffraction persisted for more than a 100-nm film deposition under optimized conditions on such a buffer layer. Thus prepared thin films show free exciton emissions in a 5 K photoluminescence spectrum and excited-state exciton resonance structures in a reflection spectrum, both indicating very high optical quality.
High-contrast all-optical bistable switching in photonic crystal microcavities
Authors: Yanik Mehmet Fatih , Fan Shanhui , Soljačić Marin
Published: October 2003

We present a bistable photonic crystal configuration consisting of a waveguide sided coupled to a single-mode cavity with instantaneous Kerr nonlinearity. We show that such a configuration can generate extremely high contrast between the bistable states in its transmission with low input power. We also provide an analytic theory that can completely account for the entire transient switching dynamics, as revealed by finite difference time domain simulations.
Effect of strain on the local phase transition temperature of MnAs/GaAs(001)
Authors: Mohanty J. , Hesjedal T. , Ney A. , Takagaki Y. , Koch R. , Däweritz L. , Ploog K. H.
Published: October 2003

We present measurements of the influence of local strain on the phase transition behavior of epitaxial MnAs films on GaAs(001). As shown previously, stripes of ferromagnetic α-MnAs and paramagnetic β-MnAs coexist around room temperature. Temperature-dependent atomic force and magnetic force microscopy reveals that the characteristic temperature T*, at which the as-grown films transform to the paramagnetic β-phase, is locally shifted up towards the value of unstrained bulk MnAs. The film areas exhibiting a higher T* were identified as regions in which the strain in the MnAs film was allowed to relax.
The Nd-nanocluster coupling strength and its effect in excitation/de-excitation of Nd3+ luminescence in Nd-doped silicon-rich silicon oxide
Authors: Seo Se-Young , Kim Mun-Jun , Shin Jung H.
Published: October 2003

The Nd-nanocluster Si (nc-Si) coupling strength and its effect in excitation/de-excitation of Nd3+ luminescence in Nd-doped silicon-rich silicon oxide (SRSO) is investigated. Nd-doped SRSO thin films, which consist of nc-Si embedded inside a SiO2 matrix, were prepared by electron-cyclotron-resonance plasma-enhanced chemical vapor deposition of SiH4 and O2 with cosputtering of Nd and subsequent anneal at 950 °C. Efficient Nd3+ luminescence with moderate temperature quenching is observed. Based on an analysis of the temperature dependence of Nd3+ luminescence lifetime, we find a coupling strength between nc-Si and Nd that is strong enough to result in efficient excitation of Nd3+ via quantum-confined excitons, while weak enough to result in a small back-transfer rate is identified as the key to Nd3+ luminescence.
Three-color (λp1∼3.8 μm, λp2∼8.5 μm, and λp3∼23.2 μm) InAs/InGaAs quantum-dots-in-a-well detector
Authors: Krishna S. , Raghavan S. , von Winckel G. , Stintz A. , Ariyawansa G. , Matsik S. G. , Perera A. G. U.
Published: October 2003

We report a three-color InAs/InGaAs quantum-dots-in-a-well detector with center wavelengths at ∼3.8, ∼8.5, and ∼23.2 μm. We believe that the shorter wavelength responses (3.8 and 8.5 μm) are due to bound-to-continuum and bound-to-bound transitions between the states in the dot and states in the well, whereas the longer wavelength response (23.2 μm) is due to intersubband transition between dot levels. A bias-dependent activation energy ∼100 meV was extracted from the Arrhenius plots of the dark currents, which is a factor of 3 larger than that observed in quantum-well infrared photodetectors operating at comparable wavelengths.
The piezoelectric shear strain coefficient of 〈111〉-oriented Pb (Zn1/3Nb2/3)O3-PbTiO3 piezocrystals
Authors: Liu S-F. , Ren W. , Mukherjee B. K. , Zhang S. J. , Shrout T. R. , Rehrig P. W. , Hackenberger W. S.
Published: October 2003

The piezoelectric d15 coefficients of 〈111〉-oriented (1−x)Pb(Zn1/3Nb2/3)O3–xPbTiO3 single crystals for x=0.045 and x=0.08 (PZN-4.5%PT and PZN-8%PT) have been measured as functions of dc bias field and frequency using a Zygo laser interferometer system. High d15 values (∼5500 pC/N for PZN-4.5%PT and ∼7500 pC/N for PZN-8%PT) were obtained in the absence of dc bias. d15 decreased as the dc bias was increased. d15 was also found to decrease with increasing frequency, and this frequency dependence decreased at sufficiently large bias fields. Our results suggest that these 〈111〉-oriented crystals are promising materials for high performance shear transducers.
Actively controlled oxidation of Cu{100} with hyperthermal O2 molecular beam
Authors: Okada Michio , Moritani Kousuke , Goto Seishiro , Kasai Toshio , Yoshigoe Akitaka , Teraoka Yuden
Published: October 2003

The oxidation of Cu{100} with a hyperthermal O2 molecular beam (HOMB) was investigated using x-ray photoemission spectroscopy in conjunction with a synchrotron light source. The efficiency of oxidation with HOMB is higher than that with ambient thermal O2. Further oxidation under oxygen coverage (Θ)⩾0.5 ML occurs rather inefficiently even for the 2.3-eV-HOMB irradiation. We found that such slow oxidation of Cu corresponding to the initial stage of the Cu2O formation can be interpreted in terms of a collision-induced-absorption mechanism. The kinetics of the dissociative adsorption under Θ⩽0.5 ML is well described using the first-order kinetics in a simple Langmuir-type adsorption model.
Inversion domains in AlN grown on (0001) sapphire
Authors: Jasinski J. , Liliental-Weber Z. , Paduano Q. S. , Weyburne D. W.
Published: October 2003

Al-polarity inversion domains formed during AlN layer growth on (0001) sapphire were identified using transmission electron microscopy (TEM). They resemble columnar inversion domains reported for GaN films grown on (0001) sapphire. However, for AlN, these columns have a V-like shape with boundaries that deviate by 2°±0.5° from the c axis. TEM identification of these defects agrees with the post-growth surface morphology as well as with the microstructure revealed by etching in hot aqueous KOH.
Mg-based bulk metallic glass composites with plasticity and high strength
Authors: Ma H. , Xu J. , Ma E.
Published: October 2003

Composite alloys of (Mg0.65Cu0.075Ni0.075Zn0.05Ag0.05Y0.1)100−xFex(x=9 and 13) have been produced through copper mold casting, based on a good bulk metallic glass former. Upon cooling the melt, an α-Fe solid solution precipitates uniformly with sizes in the 1–10 μm range while the remaining melt undergoes a glass transition. The in situ composite has a compressive strength as high as ∼1 GPa, a factor of 1.6 higher than the single-phase metallic glass. In contrast to all the previous Mg-based monolithic glasses that always fail in the elastic regime, a plastic strain to failure of the order of 1% was obtained for the Fe-toughened composite.
(In)GaAsN-based type-II “W” quantum-well lasers for emission at λ=1.55 μm
Authors: Vurgaftman I. , Meyer J. R. , Tansu N. , Mawst L. J.
Published: October 2003

Whereas laser emission at 1.55 μm is difficult to realize using type-I InGaAsN quantum wells grown on GaAs, we show that it can be achieved with far fewer restrictions on the growth by employing type-II (In)GaAsN/GaAsSb/(In)GaAsN/GaAs structures having a “W” band alignment. We use a 10-band k⋅p formalism that accounts for the N band anticrossing to calculate the gain and spontaneous-emission characteristics of “W” structures optimized for maximum overlap of the electron and hole wave functions. We estimate that one to three wells would be necessary for edge emitters with moderate cavity losses and nonradiative recombination rates, and a somewhat larger number of wells may be required for vertical-cavity surface emitters.
Improvement in ferroelectric properties of Pb(Zr0.35Ti0.65)O3 thin films using a Pb2Ru2O7−x conductive interfacial layer for ferroelectric random access memory application
Authors: Ryoo Sung-Nam , Yoon Soon-Gil , Kim Seung-Hyun
Published: October 2003

Pb 2 Ru 2 O 7−x (PRO), having a similar lattice constant to Pb(Zr0.35Ti0.65)O3 thin films was introduced into the Pt/PZT interface to increase the remanent polarization (Pr) and retention reliability. The grain sizes of PZT thin films deposited onto PRO interlayers increase with increasing PRO film thickness and grain size (∼370 nm) of PZT films on 50 nm thick PRO layers showed a threefold increase, relative to that of PZT films (∼130 nm) without PRO layers. The Pt/PZT/Pt and Pt/PRO/PZT/PRO/Pt capacitors have a remanent polarization (2Pr) of 52 and 75 μC/cm2, polarization degradation for ten years at 100 °C of 82% and 10%, respectively. The introduction of PRO into the Pt/PZT interface can greatly improve the remanent polarization and retention reliability of PZT thin films for 64 Mb FeRAM applications.
Size difference in dielectric-breakdown-induced epitaxy in narrow n- and p-metal oxide semiconductor field effect transistors
Authors: Pey K. L. , Tung C. H. , Tang L. J. , Lin W. H. , Radhakrishnan M. K.
Published: October 2003

The physical dimension of the hillocks formed during gate-dielectric-breakdown-induced epitaxy (DBIE) is found to be dependent on transistor type. When narrow transistors of area between 3.0×10−10 and 8.0×10−10 cm2 with a gate oxide ranging from 16 to 33 Å electrically stressed in inversion mode under the same accelerated stress condition, the DBIEs formed in the n-metal oxide semiconductor field effect transistor (MOSFET) are found to be always about 2 times or more larger than that in the p-MOSFET. The difference in the DBIE dimensions is primarily attributed to a larger percolation leakage current in the n-MOSFET during the gate oxide breakdown transient.
Preparation of parallel capacitor of epitaxial SrTiO3 film with a single-crystal-like behavior
Authors: Takashima Hiroshi , Wang Ruiping , Kasai Naoko , Shoji Akira , Itoh Mitsuru
Published: October 2003

Epitaxially grown SrTiO3 thin films with single-crystal-like dielectric behavior were obtained by the chemical mechanical planarization method. A parallel capacitor of a SrTiO3 film with excellent insulation was fabricated between YBa2Cu3O7−δ electrodes by pulsed laser deposition. Dielectric constants of the parallel capacitor with a thickness of 300–600 nm are 330 and >20 000 at room temperature and 4.2 K, respectively, and obey the Curie–Weiss law above 150 K, being identical to the behavior of SrTiO3 single crystals. The microshort-free insulating film may aid in the development of oxide superconductor multilayer devices, such as superconducting quantum interference devices, three-terminal transistors, and field effect transistors.
Photoluminescence studies of Si-doped AlN epilayers
Authors: Nam K. B. , Nakarmi M. L. , Li J. , Lin J. Y. , Jiang H. X.
Published: October 2003

Si-doped AlN epilayers were grown by metalorganic chemical vapor deposition on sapphire substrates. Deep ultraviolet picosecond time-resolved photoluminescence (PL) spectroscopy has been employed to study the optical transitions in the grown epilayers. The donor bound exciton (or I2) transition was found to be the dominant recombination line in Si-doped AlN epilayers at 10 K and its emission intensity decreases with increasing Si dopant concentration. Doping-induced PL emission linewidth broadening and band-gap renormalization effects have also been observed. Time-resolved PL studies revealed a linear decrease of PL decay lifetime with increasing Si dopant concentration, which was believed to be a direct consequence of the doping-enhanced nonradiative recombination rates.
Responses of glassy structure and properties to pressure and devitrification
Authors: Wang R. J. , Li F. Y. , Wang J. F. , Wang W. H.
Published: October 2003

The pressure (up to 2 GPa) and devitrification-induced properties and structural changes in glasses studied by an ultrasonic method are summarized. The pressure has a larger effect on the longitudinal mode for a crystallized state and on the shear mode for a glassy state in oxide glasses. The crystallization significantly affects the longitudinal mode for oxide glasses and the shear mode for metallic glasses. The different effects are contributed to their different microstructures in short-range order for different glasses.
Temperature measurement using a gallium-filled carbon nanotube nanothermometer
Authors: Gao Yihua , Bando Yoshio , Liu Zongwen , Golberg Dmitri , Nakanishi Haruyuki
Published: October 2003

We report here temperature measurement by means of a Ga-filled C nanotube thermometer with diameter <150 nm and length ∼12 μm. The method relies on the initial identification and calibration of a nanothermometer in a transmission electron microscope (TEM), followed by placing it into an air-filled furnace whose temperature is to be measured, and final TEM reading of a postmeasurement gradation mark visible inside the tubular channel. The mark originates from the fact that, at high temperature, the Ga column tip exposed to the air through the open C nanotube end oxidizes, and a thin Ga oxide layer sticks to the nanotube walls upon cooling. The temperature according to this gradation mark coincides closely with nominal furnace temperature controlled by standard means. The method paves the way for practical temperature measurements using a C nanothermometer in air and within spatially localized regions (e.g., dimensions of tens of micrometers).
Free-space coupling of a light beam into a symmetrical metal-cladding optical waveguide
Authors: Li Honggen , Cao Zhuangqi , Lu Haifeng , Shen Qishen
Published: October 2003

We report on a coupling method, which transfers energy of a light beam from free space into a symmetrical metal-cladding optical waveguide from the top surface without employing the prism, grating, tapered film, and other coupling components. In the experiment, a glass flat with a thickness about 100 μm, sandwiched between two gold films is used as a guiding layer. A top gold film of 46 nm thick serves as a coupling layer as well as the cladding and a base gold film of 200 nm serves as the substrate of the waveguide. Up to 50% of the incident light energy fed into the waveguide has been demonstrated.
Two-photon photoconductive terahertz generation in ZnSe
Authors: Holzman J. F. , Elezzabi A. Y.
Published: October 2003

We report on the generation of free-space terahertz (THz) radiation using polycrystalline ZnSe as the photoconductive (PC) substrate. It is found that the ZnSe emitter can be operated at peak fields up to 125 kV/cm without dielectric breakdown, and can be photoexcited (through two-photon absorption) at pump energy fluences up to 28 mJ/cm2 without saturating. The relationship between the THz field strength and the excitation conditions of the PC gap are interpreted through a photocarrier transport model.
First-principles study of optical properties of barium titanate
Authors: Cai Meng-Qiu , Yin Zhen , Zhang Ming-Sheng
Published: October 2003

The optical properties of perovskite barium titanate in the core-level spectra are investigated by the first principles under scissor approximation. There are nine peaks at the curve of the imaginary part of dielectric function. The optical spectra are assigned to interband contribution from O 2p valence bands to Ti 3d conduction bands in the low-energy region and outer core electron excitation (core level excitation) from near valence band semicore levels Ba 5p and O 2s to conduction band in the high-energy region. In contrast to the calculated results by the tight-binding linear muffin-tin orbitals method, our results are in better agreement with the experimental results.
Far-field emission and feedback origin of random lasing in oligothiophene dioxide neat films
Authors: Anni M. , Lattante S. , Cingolani R. , Gigli G. , Barbarella G. , Favaretto L.
Published: October 2003

We report on random lasing in substituted quinquethienyl S,S-dioxide neat films. Despite the absence of highly efficient scatterers in the film, a fine structure with laser-like peaks as narrow as 5 Å is observed in the emission spectra. The far-field emission pattern is studied through angle-resolved emission measurements, demonstrating that random lasing emission is directional, with a 8° divergence but different individual emission patterns. The origin of the scattering centers providing the feedback for lasing has been analyzed through atomic force microscopy measurements of the film surface. We demonstrate that the random lasing is induced by sequential scattering from 50 nm diameter holes in the film with an average distance of 500 nm, while thickness fluctuations are not relevant.
Formation of luminescent Si nanocrystals by high-temperature rapid thermal chemical vapor deposition
Authors: Cheong Hea Jeong , Kang Jung Hyun , Kim Jae Kwon , Kim Yong , Yi Jae-Yel , Chung Tae Hun , Bark Hong Jun
Published: October 2003

We observe a completely different growth regime of silicon-rich oxide (SRO) layers by rapid thermal chemical vapor deposition for the formation of luminescent nanocrystals. The growth regime is characterized by low [N2O]/[SiH4] ratios (<1) and high growth temperatures (>700 °C). High-resolution cross-sectional transmission electron microscopy (XTEM) shows the bimodal distribution of large polycrystals and nanocrystals after post-deposition annealing. The luminescence is attributed to the nanocrystals. Fourier transform infrared spectroscopy in conjunction with XTEM and energy-dispersive x-ray studies show the phase separation and bonding reconfiguration in as-deposited SRO layers. The effectively increased oxygen content in the oxide matrix by phase separation and bonding reconfiguration reduces the diffusion coefficient of Si in the matrix, resulting in the formation of nanocrystals during post-deposition annealing.
Suppression of current collapse in insulated gate AlGaN/GaN heterostructure field-effect transistors using ultrathin Al2O3 dielectric
Authors: Hashizume Tamotsu , Ootomo Shinya , Hasegawa Hideki
Published: October 2003

We investigated effects of electronic states at free surfaces of AlGaN/GaN heterostructure field-effect transistors (HFETs) on the inner current transport at the heterointerfaces. The analysis on transient currents for the air-exposed and H2-plasma-treated devices showed that N-vacancy-related near-surface traps play an important role in current collapse in AlGaN/GaN HFETs. An Al2O3-based surface passivation scheme including an N2-plasma surface treatment was proposed and applied to an insulated-gate HFET. A large conduction-band offset of 2.1 eV was achieved at the Al2O3/Al0.3Ga0.7N interface. No current collapse was observed in the fabricated Al2O3 insulated-gate HFETs under both drain stress and gate stress.
Generation of powerful electron beams in a dense gas with a dielectric-barrier-discharge-based cathode
Authors: Mitko S. V. , Udalov Y. B. , Peters P. J. M. , Ochkin V. N. , Boller K.-J.
Published: October 2003

An electron beam source based on a dielectric barrier discharge and a perforated anode working in a dense gas is described. Electron beams with current densities up to 60 A/cm2 and pulse durations of 150 ns were generated. Stable operation of the device at a repetition frequency of 200 Hz has been demonstrated.
In situ spectral control of Zn species during helicon-wave-excited-plasma sputtering epitaxy of ZnO
Authors: Koyama T. , Onuma T. , Chichibu SF.
Published: October 2003

Atomic species in the sputtered plumes from undoped ZnO target were identified by means of emission spectroscopy during helicon-wave-excited-plasma sputtering epitaxy. Luminescent Zn species were found to be excited neutral zinc (Zn*) and zinc cations (Zn+*), and Zn* density was independently controlled by the target bias (Vt) that accelerates the velocity of Ar cations while keeping the plasma density constant. The ZnO film formation seemed to have certain threshold Vt (around −200 V at 600 °C), and the growth rate increased with the magnitude of Vt but decreased with the substrate temperature, indicating that the growth rate is limited by the sticking coefficient of Zn. As a result of the supply mode of Zn that is similar to the case for laser molecular-beam epitaxy, the epitaxial (0001) ZnO on (112̄0) sapphire substrate had ultra-smooth surfaces having atomically flat terraces, and exhibited excitonic reflectance anomalies and luminescence peaks at low temperature.
Probing depth of threshold photoemission electron microscopy
Authors: Marx G. K. L. , Jubert P.-O. , Bischof A. , Allenspach R.
Published: October 2003

The probing depth of photoelectron emission microscopy has been determined for photon excitation energies of 5 eV. For Ag overlayers deposited on Fe, it is found that the probing depth is extremely large; that is, 16.2 nm. This result is compared with probing depths of related techniques, and the physical mechanisms responsible for this bulk sensitivity are discussed.
Structural and electrical characterization of erbium oxide films grown on Si(100) by low-pressure metalorganic chemical vapor deposition
Authors: Singh M. P. , Thakur C. S. , Shalini K. , Bhat N. , Shivashankar S. A.
Published: October 2003

We report the structural and electrical properties of Er2O3 films grown on Si(100) in the temperature range 450–600 °C by low-pressure metalorganic chemical vapor deposition using Er(acac)3⋅phen, the phenanthroline adduct of erbium acetylacetonate, as the precursor. The film properties are correlated with the growth and processing conditions. The structural characterization reveals that films grown at lower temperatures are smooth, but poorly crystalline, whereas films grown at higher temperatures are polycrystalline. A dielectric constant in the range 8–20, a minimum total fixed oxide charge density (Nf) of −1×1010 cm−2, and a minimum hysteresis of 10 mV in the bidirectional capacitance–voltage characteristics are demonstrated.
Electrical and optical emission characteristics of radio-frequency-driven hollow slot microplasmas operating in open air
Authors: Yalin A. P. , Yu Z. Q. , Stan O. , Hoshimiya K. , Rahman A. , Surla V. K. , Collins G. J.
Published: October 2003

We employ hollow slot electrodes, with pd values of ∼10 Torr cm and average E/N values of ∼70 Td, to create plasmas in open air. We measure the 13.56 MHz Irf–Vrf electrical characteristics of the plasma. Stable discharges, with sinusoidal currents, are obtained up to power densities of 14 kW/cm3, and root-mean-square radio-frequency (rf) currents of 1.5 A/cm of slot length, before nonsinusoidal currents and rf glow-to-arc transitions occur. We report the absolute optical emission in the vacuum ultraviolet region located between 110 and 155 nm, with a focus on the 149 nm atomic nitrogen line. For this atomic N line alone, we find an emitter efficiency of 0.0024.
Unusually high coercivity and critical single-domain size of nearly monodispersed CoFe2O4 nanoparticles
Authors: Chinnasamy C. N. , Jeyadevan B. , Shinoda K. , Tohji K. , Djayaprawira D. J. , Takahashi M. , Joseyphus R. Justin , Narayanasamy A.
Published: October 2003

Nearly monodispersed CoFe2O4 nanoparticles with average sizes between 8 and 100 nm were synthesized by using seed-mediated growth dominant coprecipitation and modified oxidation methods. X-ray diffraction and Mössbauer spectroscopy analyses confirmed the spinel phase and a stoichiometric composition of (Co0.25Fe0.75)[Co0.75Fe1.25]O4 for powders with different particle diameters. Rotational hysteresis loss (Wr) analysis showed an average switching field (Hp) of 17 kOe and a magnetic anisotropy field (Hk) of 38 kOe for the 40 nm CoFe2O4 particles. The corresponding magnetocrystalline anisotropy energy constant (K) was about 5.1×106 erg/cc. The Hc and Hp results suggest that the critical single-domain size of CoFe2O4 is about 40 nm. The room temperature coercivity (Hc) of the 40 nm CoFe2O4 particles is found to be as high as 4.65 kOe.
Ferromagnetism in Fe-doped Ba6Ge25 chiral clathrate
Authors: Li Yang , Ross Joseph H.
Published: October 2003

We have synthesized a Ba6Ge25 clathrate, substituting 3 Ge per formula unit by Fe. This chiral clathrate has Ge sites forming a framework of closed cages and helical tunnel networks. Fe atoms randomly occupy these sites, and exhibit high-spin magnetic moments. A ferromagnetic transition is observed with Tc=170 K, the highest observed Tc for a magnetic clathrate. However, the magnetic phase is significantly disordered, and exhibits a transformation to a reentrant spin glass phase. This system has a number of features in common with other dilute magnetic semiconductors.
Threading dislocations in epitaxial InN thin films grown on (0001) sapphire with a GaN buffer layer
Authors: Lu C. J. , Bendersky L. A. , Lu Hai , Schaff William J.
Published: October 2003

The density and types of threading dislocations (TDs) in InN thin films grown on (0001) sapphire with a GaN buffer layer were characterized by transmission electron microscopy. Perfect edge TDs with 13〈112̄0〉 Burgers vectors are predominant defects which penetrate the GaN and InN layers. Pure screw and mixed TDs were also observed. Overall the TD density decreases during film growth due to annihilation and fusion. The TD density in GaN is as high as ≈1.5×1011 cm−2, and it drops rapidly to ≈2.2×1010 cm−2 in InN films. Most half-loops in GaN are connected with misfit dislocation segments at the InN/GaN interface and formed loops, while some TD segments threaded the interface. Half-loops were also generated during the initial stages of InN growth.
Metastable rocksalt phase in epitaxial GaN on sapphire
Authors: Lada M. , Cullis A. G. , Parbrook P. J. , Hopkinson M.
Published: October 2003

In a series of GaN epilayers grown by metalorganic chemical vapor deposition on sapphire, the GaN rocksalt structure has been detected by x-ray diffraction (XRD) and directly observed by high resolution transmission electron microscopy. The rocksalt GaN phase was found to coexist with zinc-blende GaN and to have a lattice parameter of a=4.074±0.004 Å. High stresses, measured by XRD, and microtwins in the samples point to an epitaxial stabilization process. The behavior of the intensity variation of the rocksalt phase XRD reflections with epilayer thickness indicates that the phase is confined near the GaN/sapphire interface, and that phase transformation takes place during high-temperature deposition and not earlier, as might otherwise be expected.
Ultraviolet-enhanced photodiode employing n-ZnO/p-Si structure
Authors: Jeong I.-S. , Kim Jae Hoon , Im Seongil
Published: October 2003

We report on the photoelectric properties of n-ZnO/p-Si photodiodes which detect UV photons in the depleted n-ZnO and simultaneously detect visible photons in the depleted p-Si. As characterized by I–V measurements in the photon range of 310 to 650 nm our photodiodes exposed to UV photons show a linear increase in photocurrent with reverse bias. In the visible range, the photocurrent rises rapidly with bias but saturates beyond a critical voltage. Our diodes exhibit strong responsivities of 0.5 and 0.3 A/W for UV (310-nm) and red (650-nm) photons, respectively, under a 30-V bias with a weak minimum near 380 nm, the wavelength corresponding to the band gap of ZnO. It is concluded that our n-ZnO/p-Si diode can be a UV-enhanced photodiode that simultaneously detects UV and visible photons by employing two related photoelectric mechanisms in parallel.
Large asymmetric Stark shift in GaxIn1−xAs/InP/InAsyP1−y composite quantum wells
Authors: Aneeshkumar B. , Silov A. Yu. , Leys M. R. , Wolter J. H.
Published: October 2003

Strong asymmetric Stark shift in excess of 115 meV of the lowest energy transition has been experimentally observed in composite GaxIn1−xAs/InP/InAsyP1−y quantum-well system. In this structure, we can independently control the confinement of electrons and holes by controlling the strain. The photoexcited electrons and holes are confined in spatially separated regions without the application of an electric field. Due to the large asymmetry in the structure, we observed large blueshifts and redshifts of the absorption edge with an applied electric field. All our measurements agree with the calculations within the framework of the Bir–Pikus strain Hamiltonian.
Room-temperature electroluminescence at 1.3 and 1.5 μm from Ge/Si self-assembled quantum dots
Authors: Chang W.-H. , Chou A. T. , Chen W. Y. , Chang H. S. , Hsu T. M. , Pei Z. , Chen P. S. , Lee S. W. , Lai L. S. , Lu S. C. , Tsai M.-J.
Published: October 2003

Room-temperature electroluminescence at 1.3 and 1.5 μm from Ge/Si quantum-dot light-emitting diodes is reported. The devices were fabricated in a mesa-type structure, with a silicon oxide layer on the top for surface/sidewall passivation. Different passivation processes were employed. We found that the integrated electroluminescence intensities were relatively less sensitive to temperature, persisting at nearly the same intensity up to RT. The fabricated device shows an internal quantum efficiency of about 0.015% at RT. The improved emission property is attributed to the reduced nonradiative recombination centers due to the surface passivation and thermal treatment.
Dark count probability and quantum efficiency of avalanche photodiodes for single-photon detection
Authors: Kang Y. , Lu H. X. , Lo Y.-H. , Bethune D. S. , Risk W. P.
Published: October 2003

We propose a physical model that quantitatively describes the behavior of the dark count probability and single-photon quantum efficiency of avalanche diodes under conditions that allow these devices to be used for single-photon detection. The model shows analytically how various physical parameters such as dark current, dc gain, Geiger mode gain, carrier detrap time, pulse repetition rate, etc., can affect the dark count probability and single-photon quantum efficiency of a Geiger mode avalanche photodiode. The theory agrees well with the experimental results.
Origin of asymmetrical magnetoimpedance in a Co-based amorphous microwire due to dc bias current
Authors: Phan Manh-Huong , Yu Seong-Cho , Kim Cheol Gi , Vázquez Manuel
Published: October 2003

Permeability spectra and asymmetrical giant magnetoimpedance (AGMI) have been studied in a Co-based amorphous microwire upon the application of a dc biasing current together with a shift of axial hysteresis loops. The permeability from wall motion of core domains is nearly constant, but the permeability from rotational magnetization of shell domains increases with the bias current. AGMI and its field sensitivity are realized for the bias current due to the influence of a unidirectional bias field on wall motion in the circumferential direction. In addition, helical internal anisotropy in this microwire also plays a significant role in responding differently to the axial magnetization process and thus to AGMI. The helical residual stress is estimated to be smaller in Co-based amorphous wire than that in Fe-based amorphous wire.
Effect of target heating on ion-induced reactions in high-intensity laser–plasma interactions
Authors: McKenna P. , Ledingham K. W. D. , McCanny T. , Singhal R. P. , Spencer I. , Clark E. L. , Beg F. N. , Krushelnick K. , Wei M. S. , Galy J. , Magill J. , Clarke R. J. , Lancaster K. L. , Norreys P. A. , Spohr K. , Chapman R.
Published: October 2003

Measurements of ion-induced nuclear reactions have been used to diagnose ion acceleration from the interaction of high-intensity (>1019 W cm−2) laser light with solid targets. Nuclear activation of catcher materials surrounding the interaction region has been studied using a high-resolution germanium detector. It was found that, when a 100 μm thick Al target foil was preheated, the proton flux produced from the laser–foil interaction was considerably reduced. Observed heavy-ion-induced reactions are used with calculated reaction cross sections to quantify ion acceleration.
Enhancement of current-perpendicular-to-plane giant magnetoresistance by synthetic antiferromagnet free layers in single spin-valve films
Authors: Jiang Y. , Abe S. , Nozaki T. , Tezuka N. , Inomata K.
Published: October 2003

Current-perpendicular-to-plane (CPP) geometrical giant magnetoresistance (GMR) single spin-valve (SPV) structures with synthetic antiferromagnets (SyAFs) as free layers were fabricated. The sample sizes range from micron to submicron. We demonstrate that SyAFs as free layers dramatically enhances the CPP-GMR ratio from 0.83% to 3.56% at room temperature and also double the resistance-area product in single SPV films. A study of the magnetic switching behavior by both experiment and single-domain thermal activation modeling shows that the CPP-GMR SPV with SyAF structure approaches that of a single-domain magnet even with low aspect ratio of 1 as long as its size diminishes to ∼0.18 μm2. The single-domain structure brings size-independent magnetic switching field when the aspect ratio is 1. The large GMR ratio at room temperature, single-domain structure and size-independent magnetic switching field demonstrate the great potential of CPP-GMR SPVs with SyAF free layers for use in future ultrahigh density magnetic storage devices.
Homogeneous carbon nanotube/polymer composites for electrical applications
Authors: Ramasubramaniam Rajagopal , Chen Jian , Liu Haiying
Published: October 2003

Homogeneous carbon nanotube/polymer composites were fabricated using noncovalently functionalized, soluble single-walled carbon nanotubes (SWNTs). These composites showed dramatic improvements in the electrical conductivity with very low percolation threshold (0.05–0.1 wt % of SWNT loading). By significantly improving the dispersion of SWNTs in commercial polymers, we show that only very low SWNT loading is needed to achieve the conductivity levels required for various electrical applications without compromising the host polymer’s other preferred physical properties and processability. In contrast to previous techniques, our method is applicable to various host polymers and does not require lengthy sonication.
Band bending of LiF/Alq3 interface in organic light-emitting diodes
Authors: Ihm Kyuwook , Kang Tai-Hee , Kim Ki-Jeong , Hwang Chan-Cuk , Park Yong-Jun , Lee Ki-Bong , Kim Bongsoo , Jeon Cheol-Ho , Park Chong-Yun , Kim Kibeom , Tak Yoon-Heung
Published: October 2003

The insertion of LiF for an interlayer material between the Al cathode and tris-(8-hydroxyquinoline) aluminum (Alq3) in the organic light-emitting diodes (OLEDs) provides an improved device performance. The highly occupied molecular orbital (HOMO) level lowering in the Alq3 layer induced by a low-coverage LiF deposition results in the reduction of electron injection barrier height. We investigated the electronic structure of the interface between the ultrathin LiF and the Alq3 layer, using synchrotron x-ray photoelectron emission spectroscopy. The results revealed that the major origin of the HOMO level lowering is not the chemical bonding of dissociated fluorine in the Alq3 layer but the band bending caused by charge redistribution driven by work function difference between LiF and Alq3 layer.
Temperature effect on the formation of uniform self-assembled Ge dots
Authors: Jin G. , Liu J. L. , Wang K. L.
Published: October 2003

The effect of the growth temperature on the formation of uniform self-assembled Ge dots on Si (001) substrates was studied. The ratio of pyramid dots to dome dots varies with the growth temperature from 500 to 700 °C. Temperature of 600 °C was optimum to form uniform self-assembled Ge dots, and is attributed to the enhanced diffusion kinetics. Highly uniform Ge dots with height deviation of ±3% were obtained at this growth temperature. Discontinuity in characteristic length was found in an Arrhenius plot between 600 and 625 °C, and it is due to intermixing of Si with Ge which occurred at high temperature.