bluuebunny/crossref_metadata_2025 huggingface dataset:
Chemical and electrical characterization of Gd2O3∕GaAs interface improved by sulfur passivation
Authors: Yang Jun-Kyu , Kang Min-Gu , Park Hyung-Ho
Published: October 2004

We report the formation of a gate dielectric film on the sulfur passivated GaAs(001). The Gd2O3 films were deposited on the n-GaAs substrates using the electron-beam evaporation. The sulfur passivated GaAs metal-oxide-semiconductor diode exhibited the improved electrical properties compared to a similar GaAs diode that was cleaned with HCl including an enhanced conduction-band barrier and a reduced-leakage current. We investigated the relation between the interfacial bonds and the electrical properties by a systematic characterization using the photoemission spectroscopy. Sulfidation was shown to preserve a stable Gd2O3∕GaAs interface and to prevent an interfacial GaAs oxide formation. The stable interface and the passivated surface were caused by the bonding transition from As–S to Ga–S. The As-oxide was dissociated to form the interfacial Ga2O3 and to volatilize the elemental As in the Gd2O3 deposition, conducted without sulfidation. The interfacial oxides and excess As degraded the electrical properties and resulted in the decrease of the conduction-band offset. It was found that the Gd2O3 film was composed of two oxidation states: the amorphous Gd–O and the crystalline Gd2O3. The bonding transition with a substrate heating implies that the Gd2O3 film on the GaAs surface completely crystallizes above 400°C. Electrical properties were understood in relation to the band alignment and were estimated from the valence band and the core-level spectra analysis. The sulfidation introduced a shift of the Fermi level toward the conduction-band minimum on the GaAs surface and increased the conduction-band offset at the Gd2O3∕GaAs interface, followed by a saturated accumulation capacitance with a low-leakage current.
Wet etching fabrication of photonic quantum ring laser
Authors: Kim Moojin , Kim Dongkwon , Lee Seongeun , Kwon O’Dae
Published: October 2004

We present a wet etching process to fabricate good vertical mesa structures that result in high quality (Q) factors up to 2×104, important for smooth sidewall cavities such as the photonic quantum ring (PQR) laser. Q factor analyses also indicate that it can be improved much more once the internal scattering from the wafer materials is minimized. We use an etching solution of H3PO4:CH3OH:H2O2 with a volume ratio of 3:1:1, and a single-layer photoresist etch mask for etching GaAs∕AlGaAs structures of the PQR laser several micrometer deep. As the etching temperature is varied from 20to40°C, the etched surface roughness decreases from 4.690to0.703nm according to scanning electron microscope and atomic force microscopy studies. From the activation energy analysis for the above etching process and the temperature dependence, the etching process is shown to be reaction limited. The PQR lasers with an active diameter of 10μm, fabricated by the wet etching process, show the spectral linewidth of 0.04nm. Three-dimensional Rayleigh-Fabry-Perot mode spectra for the PQR laser are also reported for the angle-resolved emission modes.
Regular and random magnetic resonance force microscopy signal with a cantilever oscillating parallel to a sample surface
Authors: Berman G. P. , Gorshkov V. N. , Tsifrinovich V. I.
Published: October 2004

We study theoretically the magnetic resonance force microscopy (MRFM) technique of oscillating cantilever-driven adiabatic reversals, for the case when the cantilever tip oscillates parallel to the surface of a sample. The main contribution to the MRFM signal is associated with a part of the resonance slice near the surface of the sample. The regular (approximately exponential) decay of the MRFM signal is followed by the nondecaying random signal. The spectrum of the random signal is characterized by the high level of noise at low frequencies and a roll off at the characteristic frequency related to the inverse relaxation time.
Femtosecond laser ablation properties of borosilicate glass
Authors: Ben-Yakar Adela , Byer Robert L.
Published: October 2004

We study the femtosecond laser ablation properties of borosilicate glass using atomic force microscopy and laser pulses of 200 fs duration, centered at 780 nm wavelength. We show that both single-shot and multishot ablation threshold fluences can be determined by studying the diameter and the depth of single-shot ablated craters. The linear relationship between the square of the crater diameter and the logarithm of the laser fluence in the form of D2=2w02ln(F0∕FthN=1) provides the single-shot ablation threshold, FthN=1, whereas the linear relationship between the ablation depth and the logarithm of laser fluence in the form of ha=αeff−1ln(F0∕FthN>1) provides the multishot ablation threshold, FthN>1. The results depict a multishot ablation threshold of ≈1.7J∕cm2 independent of the atmospheric conditions. The slopes of the linear fits also provide a precise estimate of the beam radius at the surface, w0≈5.9μm, and the “effective optical penetration depth,” αeff−1≈238nm in air. The method is systematic, provides results that are consistent with the literature, and eliminates uncertainties because of instrument sensitivities. We also show that threshold measurement based on the extrapolation of volume to zero, a method used often in previous studies, is somewhat questionable. Finally, the measured dimensions of ablated craters reveal that the ablation volume per unit input energy is about 1.3–1.5μm3∕μJ at an intermediate fluence regime of 10<F0av<40J∕cm2. This value represents an order of magnitude larger ablation efficiency when compared to the ablation of glass with nanosecond ultraviolet laser pulses.
Temperature stability of the piezoelectric and elastic response of dc biased [001] and [110] oriented Pb(Zn1∕3Nb2∕3)O3-PbTiO3 single crystals
Authors: Amin Ahmed , Viehland Dwight
Published: October 2004

Temperature and field dependent investigations of the longitudinal electromechanical properties of [001]- and [110]-oriented 0.955Pb(Zn1∕3Nb2∕3)O3-0.045PbTiO3 single crystals have been performed. Electromechanical equivalence was found between [001] and [110] orientations in the ferroelectric rhombohedral FEr phase region, but not in the ferroelectric tetragonal FEt one. A dc bias of 0.4 MV∕m did not affect this equivalence. Softening of the dielectric constant (K), piezoelectric coefficient (d33), and Young’s modulus (Y) were observed along both the [001] and [110] orientations in the FEr phase region on heating towards the FEt one. Above the FEr-FEt phase transition, a significant anisotropy was observed in the electromechanical and elastic coefficients. These results demonstrate that the enhanced length extensional (33-mode) electromechanical coupling k33 of rhombohedral crystals far from the morphotropic phase boundary is not constrained to the ⟨001⟩ orientation, but rather to the (110) plane.
A comparison of the optical constants of aligned and unaligned thin polyfluorene films
Authors: Lyons Benjamin P. , Monkman Andrew P.
Published: October 2004

The optical constants of aligned and unaligned thin polyfluorene films have been investigated using variable angle spectroscopic ellipsometry over the spectral range 250–1000nm. Films were prepared by spin coating, with alignment achieved by deposition on a rubbed polyimide substrate followed by annealing. The films could be well modeled by layers with uniaxial anisotropy. The unaligned film had its optical axis oriented normal to the film surface and was optically negative. There was large absorption and dispersion of the refractive index perpendicular to the optical axis and small absorption and dispersion of the refractive index parallel to it. The aligned film had its optical axis oriented in plane, normal to the rubbing direction, and was optically positive. There was large absorption and dispersion of the refractive index parallel to the optical axis and small absorption and dispersion of the refractive index perpendicular to it. These results will be useful in the design of polarized light-emitting devices using polyfluorene as an active layer.
Measuring microwave properties of laminated dielectric substrates
Authors: Egorov V. N. , Masalov V. L. , Nefyodov Yu. A. , Shevchun A. F. , Trunin M. R.
Published: October 2004

The theoretical approach is developed and the test rig is designed for measuring the microwave parameters of dielectric substrates at frequencies between 30 and 40 GHz and over a temperature range from −50 to +70°C. The test rig is based on sapphire resonators and allows the measurements of the following parameters of commonly used PCB materials: (i) dielectric constant (ε) of the dielectric substrate in the range from 2 to 10, (ii) loss tangent (tan δ) of the substrate in the range from 10−4 to 10−2, and (iii) ranging from 0.03 to 0.3 Ωsurface resistance of the laminating metal layer adjoining the dielectric material.
Production of high-quality ZnO films by the two-step annealing method
Authors: Ye J. D. , Gu S. L. , Zhu S. M. , Qin F. , Liu S. M. , Liu W. , Zhou X. , Hu L. Q. , Zhang R. , Shi Y. , Zheng Y. D.
Published: October 2004

In this study, a two-step annealing method is advanced to produce high-quality ZnO films with excellent structural, electrical, and optical properties. The effects of oxygen and nitrogen annealing on the properties of undoped ZnO films are reversible to each other and are attributed to the creation and annihilation of extrinsic trap states of antisite oxygen OZn and oxygen vacancies VO, which result from the chemisorption and desorption of oxygen, respectively. Moreover, annealing in nitrogen causes slight nitrogen incorporation, subsequently increasing the resistivity and inducing compressive stress in the film. The key to this two-step method is to keep the chemisorption and desorption of oxygen in equilibrium. Due to the similarity of annealing ambient with the growth condition, this process can be transplanted and employed in the in situ preparation of high-quality ZnO epilayers.
Absorption and photoluminescence properties of Er-doped and Er∕Yb codoped soda-silicate laser glasses
Authors: Li S. F. , Zhang Q. Y. , Lee Y. P.
Published: October 2004

Er -doped and Er∕Yb codoped soda-silicate laser glasses with various concentrations of Er and Yb were fabricated. The absorption and the photoluminescence (PL) spectra were measured and analyzed. For the Er- doped soda-slilicate glasses, the optimum Er concentration for the PL intensity at 1536nm turns out to be 0.5at.%, and the full width at half maximum (FWHM) of PL spectrum increases from 18 to 26nm, with the increase of the concentration from 0.1 to 0.8at.%. The PL intensity of Er∕Yb codoped soda-silicate glasses with an Er concentration of 0.5at.% is enhanced approximately by four times, and the optimum Yb concentration for the PL intensity at 1536nm is analyzed to be 3.0at.%. The PL spectrum becomes broader with increasing the Yb concentration, up to a FWHM of 80nm at 6.0at.%. Yb. The relation between the absorption and PL spectra, together with the mechanism of PL broadening, has also been addressed.
Low-energy nitrogen ion implantation of InSb
Authors: Mahboob I. , Veal T. D. , McConville C. F.
Published: October 2004

The modification of the electronic properties of InSb by implantation of low-energy N2+ ions and annealing have been investigated. A non-uniform electron density depth profile is observed in the near-surface region. Detailed measurements of the conduction-band electron-plasma frequency as a function of temperature combined with carrier statistics reveal that the electron concentration profile in the near-surface region cannot be explained solely by donor-type defects induced by the nitrogen implantation. However, these experimental observations can readily be explained in terms of InNxSb1−x band structure, the different distributions of damage-induced donor defects, and the acceptor-type nitrogen.
Are microbubbles necessary for the breakdown of liquid water subjected to a submicrosecond pulse?
Authors: Joshi R. P. , Qian J. , Zhao G. , Kolb J. , Schoenbach K. H. , Schamiloglu E. , Gaudet J.
Published: October 2004

Electrical breakdown in homogeneous liquid water for an ∼100ns voltage pulse is analyzed. It is shown that electron-impact ionization is not likely to be important and could only be operative for low-density situations or possibly under optical excitation. Simulation results also indicate that field ionization of liquid water can lead to a liquid breakdown provided the ionization energies were very low in the order of 2.3eV. Under such conditions, an electric-field collapse at the anode and plasma propagation toward the cathode, with minimal physical charge transport, is predicted. However, the low, unphysical ionization energies necessary for matching the observed current and experimental breakdown delays of ∼70ns precludes this mechanism. Also, an ionization within the liquid cannot explain the polarity dependence nor the stochastic-dendritic optical emission structures seen experimentally. It is argued here that electron-impact ionization within randomly located microbubbles is most likely to be responsible for the collective liquid breakdown behaviors.
Effect of residual catalyst on the vibrational modes of single-walled carbon nanotubes
Authors: McNeil L. E. , Park H. , Lu J. P. , Peters M. J.
Published: October 2004

Raman scattering measurements of single-walled carbon nanotubes prepared by laser ablation with Ni∕Co catalyst show that samples that have not been purified have a graphitic mode frequency that is 8cm−1 lower than that of samples from which most of the catalyst has been removed. The shift is attributed to charge transfer from the catalyst particles to the nanotubes. The charge transfer from the residual catalyst also affects the temperature dependence of the radial breathing mode.
Versatile UHV compatible Knudsen type effusion cell
Authors: Shukla A. K. , Banik S. , Dhaka R. S. , Biswas C. , Barman S. R. , Haak H.
Published: October 2004

A versatile Knudsen type effusion cell has been fabricated for growing nanostructures and epitaxial layers of metals and semiconductors. The cell provides excellent vacuum compatibility (10−10 mbar range during operation), efficient water cooling, uniform heating, and moderate input power consumption (100 W at 1000 °C). The thermal properties of the cell have been determined. The performance of the cell has been assessed by x-ray photoemission spectroscopy (XPS) for Mn adlayer growth on Al(111). We find that this Knudsen cell has a stable deposition rate of 0.17 monolayer per minute at 550 °C. From the XPS spectra, we show that the Mn adlayers are completely clean, i.e., devoid of any surface contamination.
Magnetism and nuclear magnetic resonance of hectorite and montmorillonite layered silicates
Authors: Levin E. M. , Hou S.-S. , Bud’ko S. L. , Schmidt-Rohr K.
Published: October 2004

The temperature and magnetic-field (H) dependencies of the bulk dc magnetization (M) and the M∕H ratio of montmorillonite (MMT), hectorite (HCT), and synthetic mica-montmorillonite (SMMT) clays have been measured and compared with the signal intensity of H1 and Si29 nuclear magnetic resonance (NMR) spectra. MMT exhibits Langevin paramagnetism with an effective magnetic moment of 5.5±0.1μB per Fe ion whereas SMMT has diamagnetic properties. At 300K, M∕H of HCT measured in a magnetic field of H⩽1kOe is larger than that of MMT, whereas in a field of 50kOe, the inverse situation is observed. The difference arises because the magnetization of HCT is dominated by a contribution from ferromagneticlike impurities. The H1 and Si29 NMR signals of MMT are broadened beyond detectability due to the paramagnetic effect. Although HCT contains ferromagneticlike components that result in a large M∕H in low field, it yields H1 and Si29 NMR spectra with signal intensities similar to those of diamagnetic SMMT. Our data highlight that the quality of the NMR spectra is not related to the low-field magnetic susceptibility but to the bulk magnetization in the high magnetic field used for NMR.
Calorimeter for adsorption energies of larger molecules on single crystal surfaces
Authors: Ajo Henry M. , Ihm Hyeran , Moilanen David E. , Campbell Charles T.
Published: October 2004

A calorimeter for measuring heats of adsorption of large molecules on single crystal surfaces is described. It extends previous instrumentation for single crystal adsorption calorimetry by adding the capability for measuring larger (lower vapor pressure) molecules. This is achieved using a chopped and collimated (∼4 mm diameter) molecular beam capable of stable 100 ms pulses of low vapor pressure substances, and a line-of-sight modification of the King and Wells method for measuring their sticking probabilities at the single crystal’s surface. The heat input to the single crystal due to adsorption is detected using a pyroelectric polymer ribbon pressed against the back of the single crystal, following our previous calorimeter design. Measurements of benzene adsorption on Pt(111) prove the capability to produce a highly stable beam of flux ∼2×1014 molecules/(cm2 s) and measure adsorption energies with an absolute accuracy of ∼5% and a pulse-to-pulse standard deviation of 2 kJ/mol.
Noncontact inspection laser system for characterization of piezoelectric samples
Authors: Jiménez F. J. , De Frutos J.
Published: October 2004

In this work measurements on a piezoelectric sample in dynamic behavior were taken, in particular, around the frequencies of resonance for the sample where the nonlineal effects are accentuated. Dimension changes in the sample need to be studied as that will allow a more reliable characterization of the piezoelectric samples. The goal of this research is to develop an inspection system able to obtain measurements, using a noncontact laser displacement transducer, also able to visualize, in three-dimensional graphic environment, the displacement that takes place in a piezoelectric sample surface. In resonant mode, the vibration mode of the sample is visualized.
Brillouin scattering and x-ray photoelectron studies of germanium nanoclusters synthesized in SiO2 by ion implantation
Authors: Żuk J. , Krzyżanowska H. , Clouter M. J. , Bromberek M. , Bubert H. , Rebohle L. , Skorupa W.
Published: October 2004

Brillouin scattering and x-ray photoelectron spectroscopy (XPS) have been utilized to characterize Ge+-implanted thermal SiO2 layers on a Si substrate with subsequent annealing at 500°C and 1100°C. Sputtering depth profiling in conjunction with XPS studies have been applied to identify the chemical state of elemental Ge and GeO2 precipitations in the SiO2 matrices. The presence of a subsurface GeOx zone as predicted in kinetic 3-dimensional lattice simulations has been confirmed. It is concluded that the intermediate step of Ge oxide formation seems necessary for the creation of Ge nanoclusters. The Ge atomic concentrations obtained from XPS were used to compute the bulk and shear moduli, and consequently the surface acoustic wave (SAW) velocities, for the Ge∕GeO2∕SiO2 systems. These calculations confirm the character of SAW velocity softening as determined from the Brillouin scattering investigations.
Vibrational properties of hydrogen atom adsorbed on Cu(111) and on Ir(111) surfaces
Authors: Nobuhara Kunihiro , Kasai Hideaki , Nakanishi Hiroshi , Diño Wilson Agerico
Published: October 2004

We investigate the quantum mechanical behavior, in particular, the vibrational properties, of H atoms adsorbed on metal surfaces. We carry out density functional theory–based calculations of the relevant potential energy curves (PECs) for the hydrogen on Cu(111) and Ir(111) systems and construct the adiabatic three-dimensional potential energy surfaces (PESs) based on the obtained PECs. The wave functions and the corresponding energies for the hydrogen motion on the PESs are calculated within the framework of the variation method. The results show that the H atom is adsorbed at the threehold hollow site of Cu(111) and it is strongly localized. On the other hand, on the Ir(111), the H atom is adsorbed at the top site and it exhibits delocalized features. Furthermore, our calculated energies for vibrationally excited hydrogen and deuterium adsorbed on Cu(111) and Ir(111) agree well with the corresponding recently observed high-resolution electron energy-loss spectroscopy loss peaks.
YAP scintillators for resonant detection of epithermal neutrons at pulsed neutron sources
Authors: Tardocchi M. , Gorini G. , Pietropaolo A. , Andreani C. , Senesi R. , Rhodes N. , Schooneveld E. M.
Published: October 2004

Recent studies indicate the resonance detector (RD) technique as an interesting approach for neutron spectroscopy in the electron volt energy region. This work summarizes the results of a series of experiments where RD consisting of YAlO3 (YAP) scintillators were used to detect scattered neutrons with energy in the range 1–200 eV. The response of YAP scintillators to radiative capture γ emission from a U238 analyzer foil was characterized in a series of experiments performed on the VESUVIO spectrometer at the ISIS pulsed neutron source. In these experiments a biparametric data acquisition allowed the simultaneous measurements of both neutron time-of-flight and γ pulse height (energy) spectra. The analysis of the γ pulse height and neutron time of flight spectra permitted to identify and distinguish the signal and background components. These measurements showed that a significant improvement in the signal-to-background ratio can be achieved by setting a lower level discrimination on the pulse height at about 600 keV equivalent photon energy. Present results strongly indicate YAP scintillators as the ideal candidate for neutron scattering studies with epithermal neutrons at both very low (<5°) and intermediate scattering angles.
Inverted hysteresis in exchange biased Cr2O3 coated CrO2 particles
Authors: Zheng R. K. , Liu Hui , Wang Y. , Zhang X. X.
Published: October 2004

Exchange bias and inverted hysteresis loops were observed in the Cr2O3 coated CrO2 particles with a wide distribution of the blocking temperature. The ferromagnetic coupling between the CrO2 cores and Cr2O3 shell is responsible for exchange bias observed below 292K. In the temperature range of 261K–∼340K, the particles show inverted hysteresis loops, i.e., the negative remanence and coercivity. The most interesting observation is that the particles show an exchange-biased (or shifted) inverted hysteresis loops in between 261K and 292K; and that the maximum negative remanence appears at 292K where exchange bias disappears. An antiferromagnetic type of interaction between the blocked particles and the superparamagnetic particles is believed to be responsible to the inverted hysteresis loops.
Evidence for capture of holes into resonant states in boron-doped silicon
Authors: Yen S. T. , Tulupenko V. N. , Cheng E. S. , Chung P. K. , Lee C. P. , Dalakyan A. T. , Chao K. A.
Published: October 2004

The variation of hole population in the resonant states of B-doped Si excited by sequences of short electric-field pulses has been investigated by the technique of time-resolved step-scan far-infrared spectroscopy. From the variation of the p3∕2 absorptions, we find that the hole population in the ground state decreases continuously with the sequential electric pulses, as a result of the breakdown delay and hole accumulation in long-lived excited states. The measured time-varying spectra of the p1∕2 series have been analyzed and attributed to a significant variation of the hole population in the resonant states. We have also observed a new absorption line at 676cm−1 which is probably caused by the electric-field induced mixing of the resonant states.
Correlation between magnetization performance and magnetic microstructure of patterned permalloy films fabricated by microcontact printing
Authors: Sannomiya Takumi , Shi Ji , Nakamura Yoshio , Nittono Osamu
Published: October 2004

Permalloy films were patterned by two simple methods, direct etching and lift-off, which are based on the microcontact printing method. Several kinds of patterns, including chessboard, stripe, grid, and rectangular dot patterns, have been produced. These patterned permalloy films reveal varied magnetization behavior that originate from the differences in the shape of the pattern-consisting elements. The magnetic structure of the chessboard pattern film was investigated by Fresnel and Foucault modes of Lorentz transmission electron microscopy. It is found that the square elements of the patterned film exhibit vortexlike magnetization closures, although some connections between the elements remained due to insufficient etching. The macroscopic magnetization process of the chessboard-patterned film was explained in reference to the in situ magnetization-distribution observation of the specimen along a minor magnetization loop. We also tried to control the rotational sense of the “vortices” in the square elements by using a gradient field.
Burstein-Moss shift in impurity-compensated bulk Ga1−xInxSb substrates
Authors: Pino R. , Ko Y. , Dutta P. S. , Guha Shekhar , Gonzalez Leonel P.
Published: October 2004

The optical and electrical properties of tellurium- (Te) compensated Ga1−xInxSb bulk crystals with alloy compositions ranging from x=0.37 to x=0.98 have been investigated. It has been observed that the Burstein-Moss shift plays an important role in the optical properties of Ga1−xInxSb crystals for the alloy compositions greater than x=0.5 and net donor concentrations in the 2.9×1017 to 2.6×1018cm−3 range at 300K. A good agreement has been obtained between the theory and experimental observations. Furthermore, electrical characterizations at 300 and 77K show that Te compensates the native defects in Ga1−xInxSb, irrespective of the alloy composition.
Large nonlinear optical properties of semiconductor quantum dot arrays embedded in an organic medium
Authors: Gao Y. , Huong N. Q. , Birman J. L. , Potasek M. J.
Published: October 2004

We show that an array of semiconductor quantum dots in an organic host leads to a large two-photon absorption. The optical nonlinearity depends on the semiconductor, the dot size, and the dot-to-dot spacing. Using numerical simulations, we demonstrate that a large optical limiting is possible using thin films of this hybrid material.
Electrical properties and microstructure of ternary Ge∕Ti∕Al ohmic contacts to p-type 4H–SiC
Authors: Tsukimoto S. , Sakai T. , Murakami Masanori
Published: October 2004

The high-power SiC devices require ohmic contact materials, which are prepared by annealing at temperatures lower than 800°C. Recently, we demonstrated in our previous paper [J. Appl. Phys. 95, 2187 (2004)] that an addition of a small amount of Ge to the conventional binary Ti∕Al contacts reduced the ohmic contact formation temperature by about 500°C, and this ternary contacts yielded a specific contact resistance of approximately 1×10−4Ωcm2 after annealing at a temperature as low as 600°C. In this paper, the electrical properties and the microstructures of the Ge∕Ti∕Al contacts (where a slash “/” indicates the deposition sequence) were investigated by current-voltage measurements and transmission electron microscopy observations, respectively, in order to understand the ohmic contact formation mechanism. Ti3SiC2 compound layers (which were previously observed at the metal/SiC interface in the Ti∕Al ohmic contacts after annealing at temperatures higher than 1000°C) were observed to grow epitaxially on the SiC surface after annealing at temperatures as low as 600°C. The Ti3SiC2 layers were believed to act as a p-type intermediate semiconductor layer, which played a key role to reduce the Schottky barrier height at the contacting metal/SiC interface. Further reduction of the contact resistances of the Ge∕Ti∕Al contacts would be achieved by increasing the coverage of the Ti3SiC2 layers on the SiC surface.
Soft-imprint technique for multilevel microstructures using poly(dimethylsiloxane) mold combined with a screen mask
Authors: Choi Won Mook , Park O Ok
Published: October 2004

This letter presents a microfabrication technology of multilevel microstructures via the soft-imprint technique using poly(dimethylsiloxane) (PDMS) mold attached with a screen mask, TEM grid. A prepolymer and monomer mixture, after short UV exposure, rises only up to the open spot of TEM grid, sequentially into the groove of the PDMS mold, then multilevel microstructures are formed in one-step process. The proposed technique is an inexpensive, simple, and reliable method to fabricate multilevel microstructures without expensive and complex lithographic tools. Thus, using this multilevel microfabrication method, various multilevel microstructures of the combination of TEM grid pattern and PDMS mold groove are easily generated with good pattern fidelity.
Description of unconventional electronic transport in mesoscopic structures
Authors: Craco L. , Cuniberti G.
Published: October 2004

Inspired by a fair amount of experimental and theoretical works describing nonlinear transport in hybrid mesoscopic structures, we study a confined state coupled to different types of noninteracting leads. It is shown that specific properties of the electron bath, here simulated by the presence of van Hove singularities in the spectral density of the reservoirs, determine sharp resonances in the differential conductance at finite applied voltages. For both magnetic and nonmagnetic single impurities, the two-channel network exhibits a resonant negative differential conductance behavior in a region of parameters which is not expected for ideal electrodes. This result may have important ramifications in probing resonant tunneling experiments.
Vertical electrochemical transistor based on poly(3-hexylthiophene) and cyanoethylpullulan
Authors: Taniguchi Masateru , Kawai Tomoji
Published: October 2004

We have fabricated an all-solid state and vertical electrochemical transistor, which employs poly(3-hexylthiophene) and cyanoethylpullulan as the semiconducting layer and solid electrolyte, respectively. The device has the same structure as a vertical field effect transistor, but can function using a different mechanism of operation in which the doping and dedoping to polythiophene are controlled by the gate voltage and the source–drain current is modulated by the doping–dedoping process. The transistor behaves like p-channel accumulation-mode devices and has a switching speed of 50Hz in vacuum.
Damage profiles of ultrashallow B implants in Si and the Kinchin-Pease relationship
Authors: Berg J. A. van den , Carter G. , Armour D. G. , Werner M. , Goldberg R. D. , Collart E. J. H. , Bailey P. , Noakes T. C. Q.
Published: October 2004

Damage distributions resulting from 0.1–2keV B+ implantation at room temperature into Si(100) to doses ranging from 1×1014 to 2×1016cm−2 have been determined using high-depth-resolution medium-energy-ion scattering in the double alignment mode. For all B+ doses and energies investigated a 3–4nm deep, near-surface damage peak was observed while for energies at and above 1keV, a second damage peak developed beyond the mean projected B+ ion range of 5.3nm. This dual damage peak structure is due to dynamic annealing processes. For the near-surface peak it is observed that, at the lowest implant energies and doses used, for which recombination processes are suppressed due to the proximity of the surface capturing interstitials, the value of the damage production yield for low-mass B+ ions is equal or greater than the modified Kinchin-Pease model predictions [G. H. Kinchin and R. S. Pease, Rep. Prog. Phys. 18, 1 (1955); G. H. Kinchin and R. S. Pease, J. Nucl. Energy 1, 200 (1955); P. Sigmund, Appl. Phys. Lett. 14, 114 (1969)].
Effect of thermal annealing on switching dynamics of fatigued bulk lead zirconate titanate
Authors: Verdier Cyril , Lupascu Doru C. , von Seggern Heinz , Rödel Jürgen
Published: October 2004

The effect of thermal annealing on switching kinetics of electrically fatigued bulk lead-zirconate-titanate ceramics is demonstrated. Fatigue induces stretched exponential switching behavior reaching time constants in the range of hours. In the final stages of fatigue, all switching is highly retarded. Thermal annealing does not yield recovery of the fast switching components. The slow components recover in different temperature intervals. The degree of randomness of the fatigued state does not change upon annealing, while the characteristic time constants re-decrease considerably.
Recombination processes in CdTe quantum-dot-doped glasses
Authors: Padilha L. A. , Neves A. A. R. , Cesar C. L. , Barbosa L. C. , Cruz C. H. Brito
Published: October 2004

Electron-hole recombination in CdTe quantum dots was studied by photoluminescence and resonant femtosecond pump–probe measurements. A dependence of recombination times with pump pulse intensity was observed and we attribute this to the Auger recombination process. The overall kinetic processes that we observed are a fast decay from the initial excited state to surface trap states, the Auger recombination, the recombination of electrons from the surface states, and a longer time recombination which we attribute to electrons in the deep traps states.
Pt
∕
ZnO
nanowire Schottky diodes
Authors: Heo Y. W. , Tien L. C. , Norton D. P. , Pearton S. J. , Kang B. S. , Ren F. , LaRoche J. R.
Published: October 2004

Pt Schottky diodes were formed on single ZnO nanowires grown by site-selective molecular-beam epitaxy and then transferred to SiO2-coated Si substrates. The diodes exhibit excellent ideality factors of 1.1 at 25 °C and very low (1.5×10−10A, equivalent to 2.35Acm−2, at −10V) reverse currents. The nanowire diodes show a strong photoresponse, with the current–voltage characteristics becoming ohmic under ultraviolet illumination (366 nm light). The on-off current ratio of the diodes at 0.15∕−5V was ∼6. These results show the ability to manipulate the electron transport in nanoscale ZnO devices.
Field ionization of argon using β-phase W nanorods
Authors: Singh J. P. , Karabacak T. , Lu T.-M. , Wang G.-C. , Koratkar N.
Published: October 2004

We report testing of β-phase W nanorods as a gas ionizer for neutral argon atoms. These W nanorods having square-base pyramidal apexes were grown on oxidized Si(100) substrate using glancing angle sputter deposition technique with substrate rotation. Only a few volts of positive anode voltage (3–4V) applied to the W nanorods generates a high electric field, which ionizes gas-phase argon atoms and generates ion currents up to several tenths of microamperes. The low anode voltage and high ion current observed in this study indicates that β-phase W nanorods can be used as battery-operated gas ionizers.
Microphotoluminescence spectra of hillocks in Al0.11Ga0.89N films
Authors: Ke W. C. , Ku C. S. , Huang H. Y. , Chen W. C. , Lee L. , Chen W. K. , Chou W. C. , Chen W. H. , Lee M. C. , Lin W. J. , Cheng Y. C. , Cherng Y. T.
Published: October 2004

The spatial variation of the optical properties of hillocks in Al0.11Ga0.89N films has been studied by using microphotoluminescence (μ-PL) microscopy. The μ-PL spectrum revealed a strong emission (IH) at 351nm from the hillock, besides the near-band-edge emission (Inbe) at 341nm. Moreover, the IH intensity increases significantly and its full width at half maximum decreases from ∼76to∼53meV by probing across the hillock center. These indicated that the hillock structure is a strong emission center. The temperature-dependent μ-PL measurements showed that the IH also has the S-shape behavior with a transition temperature of ∼120K which is lower than that of Inbe. The redshift of IH is also smaller than Inbe. Both indicated that the Al composition in hillocks is lower than the surrounding area.
Electrically pumped single-photon sources in lateral p-i-n junctions
Authors: Xu Xiulai , Williams D. A. , Cleaver J. R. A.
Published: October 2004

Electrically pumped single-photon sources using semiconductor quantum dots are of interest as they can be integrated with other semiconductor devices, using standard processing techniques. In this letter, we report electroluminescence from single quantum dots in a lateral p-i-n junction. Exciton and biexciton emission from a single quantum dot can be achieved under different electrical bias conditions. Antibunching effects from exciton and biexciton emission are observed using cw and pulsed electrical injection, indicating single-photon emission; this can be used for quantum information processing.
Patterning conductive copper by nanotransfer printing
Authors: Felmet Kimberly , Loo Yueh-Lin , Sun Yangming
Published: October 2004

We describe a method for patterning conductive copper over large areas by nanotransfer printing (nTP). This technique is purely additive and yields feature sizes in the 1–500 μm range. Unlike gold patterns printed in a similar manner, oligomers from poly(dimethylsiloxane) (PDMS) stamps used in nTP permeate through the entire thickness of printed copper resulting in nonconductive patterns. Hot leaching the PDMS stamps in toluene prior to printing removes residual oligomers; printing with pretreated stamps reproducibly yields conductive copper patterns with an average resistivity of 31μΩ-cm.
The improvement of discharge characteristics and lifetime of alternate current plasma display panel by MgO deposition on the phosphor
Authors: Ha Chang Hoon , Kim Jae Sung , Jeong Dong Chul , Whang Ki Woong
Published: October 2004

An alternate application of MgO film for the improvement of discharge characteristics and lifetime in an ac plasma display panel (PDP) is suggested. In the normal ramp reset, the accumulated wall charges on the phosphor surface just after the reset period are different as to their color (red, green, blue) of each phosphor, which reduces the common voltage margin of ac PDP. In general, MgO is used for the protective layer of dielectrics from ion sputtering and the improvement of secondary electron emission which affects the driving voltage in an ac PDP. In this research, we deposited MgO on the phosphor to get the same addressing characteristics irrespective to the color of each phosphor, high speed addressing, and prolonged lifetime of phosphor despite the vacuum ultraviolet irradiation and ion bombardment. An optimized MgO thickness was used to avoid the excessive luminance and luminous efficacy degradation by the MgO deposition on the phosphor. Results showed that PDP with MgO coated phosphor has more uniform formative delays in address discharges and improved degradation characteristics.
Injection of holes from the silicon substrate in Ta2O5 films grown on silicon
Authors: Novkovski N. , Atanassova E.
Published: October 2004

In this paper a compact model for leakage currents of metal∕Ta2O5∕SiO2∕Si structures is presented, considering tunneling and hopping conduction in a SiO2 layer and Poole-Frenkel emission in a Ta2O5 layer. Theoretical calculations fit very well the experimental results obtained on thermally grown Ta2O5 films over an unintentionally grown ultrathin SiO2 layer for both bias polarities. The following parameters were fitted: the SiO2 layer thickness, the hopping conductivity of a SiO2 layer, and the Poole-Frenkel defect-related constant. Experimentally observed asymmetry between different polarities was explained by the injection of holes from the silicon substrate at negative bias.
Loss characterization in microcavities using the thermal bistability effect
Authors: Rokhsari H. , Spillane S. M. , Vahala K. J.
Published: October 2004

We demonstrate a powerful method based on the thermal bistability effect to characterize distinct loss mechanisms limiting the quality factor of microresonators. The relative importance of absorption and scattering losses are investigated in toroidal microcavities using this technique. Empirical results on thermal nonlinearity of these structures have been used to study the interaction of microtoroids with their ambient environment.
Enhanced temperature uniformity by tetrahedral laser heating
Authors: Schroers Jan , Bossuyt Sven , Rhim Won-Kyu , Li Jianzhong , Zhou Zhenhua , Johnson William L.
Published: October 2004

Temperature profile on a spherical sample that is heated by laser beams in various geometries while processed in vacuum is analyzed. Sample heating by one or four laser beams was considered. An analytical expression was derived for directional sample heating cases. It suggests an enhanced temperature uniformity over the samples when heated with four diffuse laser beams arranged in a tetrahedral geometry. This was experimentally verified by heating a spherical stainless steel sample by laser beams. Both the calculated and experimentally determined temperature variations over the sample suggest that use of diffuse four beams arranged in tetrahedral geometry would be effective in reducing temperature variation to within 1 K. The enhancement in the temperature uniformity for four diffuse beams arranged in a tetrahedral geometry by a factor of 50 over a single focused beam is promising to accurately measure of thermophysical properties. This drastic improvement in temperature uniformity might even enable atomic diffusion measurements in the undercooled liquid states of the bulk glass forming alloys since Marangoni and gravity driven convection will be substantially reduced.
Time-resolved scanning near-field optical microscopy with supercontinuum light pulses generated in microstructure fiber
Authors: Nagahara Tetsuhiko , Imura Kohei , Okamoto Hiromi
Published: October 2004

A novel apparatus for time-resolved near-field optical microscopy is described. The apparatus consists of a mode-locked Ti:sapphire laser, a microstructure fiber, and a scanning near-field optical microscope equipped with an apertured optical fiber probe. The probe pulses in visible to near-infrared regions are generated by focusing laser pulses in the microstructure fiber. The broadband continuum can be used as a wavelength-tunable light source for fluorescence excitation as well as for probing absorption of excited states at arbitrary wavelengths by applying pump-probe scheme, in high spatial-resolution (∼100 nm) predominantly determined by the aperture size of the tip. Time resolution obtained with 740–830 nm probe pulses was in 1–2 ps range, while that with 570 nm pulses was 5 ps without precompensation of the group delay dispersion for the probe pulses. Results on the excited-state dynamics of molecular aggregates are presented.
Interaction of two circular cylinders in inviscid fluid
Authors: Wang Qian Xi
Published: October 2004

The unsteady problem of two parallel circular cylinders, moving in an inviscid fluid, is analyzed analytically by exploring a conformal mapping. Exact solutions are obtained for the flow induced and the hydrodynamic forces acting on them, as the two circular cylinders, with any radii and at any locations, expand (contract) and translate arbitrarily at time dependent speeds. As the two bodies are far apart, the solutions of the flow and forces reduce to those for a single circular cylinder moving in an unbounded inviscid flow. The force components along the line of centers are inversely proportional to the distance between them, whereas the force components perpendicular to the line of centers are inversely proportional to the square of the distance between them. Numerical analyses are performed for the two circular cylinders deforming and translating at constant speeds and in various ways, as well as a circular cylinder falling to a wall. It has been noticed that they are attracted to each other, as one of them expands and the other contracts, or as they translate perpendicular to the line of centers; whereas they are repelled from each other, as both of them expand, contract, or as they translate along the line of centers. The force on one of them increases with the size and speed of the other, as well as their proximity.
Direct numerical simulations of isotropic compressible turbulence: Influence of compressibility on dynamics and structures
Authors: Pirozzoli S. , Grasso F.
Published: October 2004

In the present paper the statistical properties of compressible isotropic turbulence are analyzed by means of direct numerical simulations. The scope of the work is to evaluate the influence of compressibility on the time evolution of mean turbulence properties and to quantify the statistical properties of turbulent structures, their dynamics and similarities with the incompressible case. Simulations have been carried out at various turbulent Mach numbers and compressibility ratios by using a conservative hybrid scheme that relies on an optimized weighted essentially nonoscillatory approach for the convective terms and compact differencing for the viscous contributions. In order to identify similarities with incompressible turbulence we have also carried out an analysis in the plane of the second (Q*) and third (R*) invariants of the anisotropic part of the deformation rate tensor. The simulations show that the joint probability density function (Q*,R*) has a universal structure, as found in incompressible turbulence. The study confirms that the enstrophy obeys a two-stage evolution, due to the competing mechanisms of vortex stretching and viscous dissipation; however, at high turbulent Mach numbers, compressibility effects, associated to the occurrence of shocklets, become important. Furthermore, the analysis of the controlling mechanisms of vorticity generation has shown that even for compressible turbulence the growth of enstrophy is associated to a preferential alignment of the vorticity with the intermediate eigenvector of the anisotropic part of the strain-rate tensor.
Field emission properties of self-assembled silicon nanostructures on n- and p-type silicon
Authors: Johnson S. , Markwitz A. , Rudolphi M. , Baumann H. , Oei S. P. , Teo K. B. K. , Milne W. I.
Published: October 2004

This letter considers field emission from self-assembled silicon nanostructure arrays fabricated on n- and p-type silicon (100) substrates using electron beam rapid thermal annealing. Arrays of nanostructures with an average height of 8 nm were formed by substrate annealing at 1100 °C for 15 s. Following conditioning, the Si nanostructure field emission characteristics become stable and reproducible with Fowler–Nordheim tunneling occurring for fields as low as 2Vμm−1. At higher fields, current saturation effects are observed for both n-type and p-type samples. These studies suggest that the mechanism influencing current saturation at high fields acts independently of substrate conduction type.
A grating-assisted resonant-cavity-enhanced optical displacement detection method for micromachined sensors
Authors: Lee Wook , Hall Neal A. , Degertekin F. Levent
Published: October 2004

We present an integrated optical displacement sensing method for microscale sensors which is based on an asymmetric Fabry–Perot etalon structure with an embedded phase-sensitive diffraction grating. Analytical modeling of the structure shows that the etalon significantly improves the detection sensitivity as compared to a regular optical interferometer and the embedded diffraction grating enables integration of optoelectronics in a small volume. The efficacy of the method is experimentally validated on a surface micromachined diffraction-based opto-acoustic sensor fabricated on a quartz wafer. A 15 nm silver layer is used to form the bottom mirror of the etalon structure with a sensor membrane and embedded diffraction grating made of aluminum. Comparison of the results with and without the etalon shows an 8 dB increase in detection sensitivity with the etalon structure, which should be further enhanced with the use of low-loss dielectric mirrors.