bluuebunny/crossref_metadata_2025 huggingface dataset:
Dynamic behavior of photocarriers in a GaAs metal-semiconductor-metal photodetector with sub-half-micron electrode pattern
Authors: Koscielniak W. C. , Pelouard J. L. , Littlejohn M. A.
Published: July 2002

Simulation results of the temporal evolution of photocurrent in an interdigitated GaAs metal-semiconductor-metal photodetector are presented. The dependence of response time on the distance between fingers (0.1 and 0.25 μm) is investigated. The solutions of the time-dependent Schrödinger equation and ensemble Monte Carlo calculations are employed. For a device with 0.1 μm finger spacing, the response time of an intrinsic detector is less than 2 ps, with hole current decay being the major limiting factor. The role of parasitics is shown to significantly increase the simulated response time.
Emission of vacuum ultraviolet radiation from neon excimers excited by a heavy ion beam
Authors: Krötz W. , Ulrich A. , Busch B. , Ribitzki G. , Wieser J.
Published: July 2002

The vacuum ultraviolet emission of neon excited with a pulsed 100 MeV 32S9+ ion beam from the Munich Tandem van de Graaff accelerator was studied at pressures between 1.8 and 96.1 kPa. In the wavelength range between 70 and 110 nm the first, second, and third excimer continua were observed. From time- and pressure-dependent studies of the third continuum emission at a wavelength of 99 nm, rate coefficients k2=(3.6±0.3)×10−13 cm3/s for the bimolecular reaction Ne2++Ne→2Ne+ and k3=(2.84±0.09)×10−31 cm6/s for the termolecular reaction Ne2++2Ne→(Ne2+Ne)2++Ne were determined.
Low-voltage electron beam lithography with a scanning tunneling microscope
Authors: Marrian C. R. K. , Colton R. J.
Published: July 2002

Studies of a polydiacetylene negative electron beam resist have been made in a scanning tunneling microscope operated in vacuum at pressures in the 10−8 Torr range. The resist can be imaged if it is applied as a thin film to a conductive flat substrate and the tip bias voltage is chosen appropriately. An exposure threshold energy close to 8 eV has been observed for the formation of raised features in the resist. A minimum feature size of about 20 nm has been measured when written at an energy just above the exposure threshold. Details of the necessary substrate preparation are described together with the operation of the scanning tunneling microscope during imaging and exposure. The exposure dose is applied by raising the bias voltage for a specific time while keeping the tip-sample current constant which has permitted insights into the mechanisms of the resist exposure.
Ion-induced annealing and amorphization of isolated damage clusters in Si
Authors: Battaglia Anna , Priolo Francesco , Rimini Emanuele , Ferla Giuseppe
Published: July 2002

The interaction between high-energy ion irradiation and pre-existing damage clusters dispersed in single-crystal Si is discussed. Silicon substrates were predamaged by low-dose 150 keV Au ions. Post-irradiation by 600 keV Kr2+ ions resulted in either damage annealing or damage accumulation, depending on the substrate temperature. The transition temperature between these two different regimes is 420 K. These data are discussed and compared with the ion beam induced epitaxy and amorphization of continuous surface amorphous layers.
NiSi formation through a semipermeable membrane of amorphous Cr(Ni)
Authors: Rozgonyi G. A. , Lee Ju-Hyeon , Knoesen D. , Adams D. , Patnaik B. , Parikh N. , Salih A. S. M. , Balducci P.
Published: July 2002

Following heat treatments of Pt encapsulated Ni80Cr20 thin films on silicon substrates at temperatures ranging from 300 to 500 °C, it has been discovered that Cr atoms segregate at the original NiCr/Si interface to form an amorphous layer, while Ni atoms diffuse into the Si to form Ni silicide. The Cr-rich amorphous layer acts as a ‘‘semipermeable membrane’’ which selectively passes Ni to form a very uniform NiSi layer.
Microstructure and critical current density of zone melt textured YBa2Cu3O6+x/Y2BaCuO5 with BaSnO3 additions
Authors: McGinn P. , Chen W. , Zhu N. , Tan L. , Varanasi C. , Sengupta S.
Published: July 2002

Additions of BaSnO3 have been made to YBa2Cu3O6+x/Y2BaCuO5 composite wires to examine its potential for improving solidification processing and enhancing pinning. Zone melting has been used to produce texture in extruded YBa2Cu3O6+x (Y-123) wires with Y2BaCuO5(211) and BaSnO3 additions. BaSnO3 additions markedly improve the magnetization properties over similarly textured Y-123 or Y-123+211 wires in low fields, but do not show a noticeable improvement at higher fields (≳2 T). Microstructural effects of BaSnO3 include the growth of much larger Y-123 grains, and a substantial reduction in size of 211 precipitates. Magnetic critical current densities (Jc) in excess of 1.2×105 A/cm2 in zero field, and 3.0×104 A/cm2 at 1 T are observed.
Epitaxial Pt(001), Pt(110), and Pt(111) films on MgO(001), MgO(110), MgO(111), and Al2O3(0001)
Authors: Lairson B. M. , Visokay M. R. , Sinclair R. , Hagstrom S. , Clemens B. M.
Published: July 2002

We have grown epitaxial Pt films, both in oxidizing and nonoxidizing environments, using planar magnetron sputtering onto heated substrates. The out-of-plane orientation relationships we report are Pt(001)∥MgO(001), Pt(110)∥MgO(110), Pt(111)∥MgO(111), and Pt(111)∥Al2O3(0001). We also report a seeded epitaxy technique using Fe for lower temperature epitaxial growth of Pt(001)∥MgO(001).
Measurements of optical-heterodyne conversion in low-temperature-grown GaAs
Authors: Brown E. R. , McIntosh K. A. , Smith F. W. , Manfra M. J. , Dennis C. L.
Published: July 2002

A low-temperature-grown GaAs interdigitated-electrode photomixer is used to generate coherent power at microwave frequencies. An output power of 200 μW (−7 dBm) is generated by pumping the photomixer with two 70-mW modes of a Ti:Al2O3 laser, separated in frequency by 200 MHz. This represents an optical-to-microwave conversion efficiency of 0.14%, which is within 50% of a prediction based on optical-heterodyne theory. When two lasers are used and the frequency of one is tuned with respect to the other, the output frequency of the photomixer increases smoothly and the output power is nearly constant up to 20 GHz. At higher frequencies the power decays because of parasitic capacitance.
p-type CdS thin films formed by in situ Cu doping in the chemical bath
Authors: Sebastian P. J.
Published: July 2002

The formation of p-type CdS thin films, doped with Cu, in a chemical bath, is reported for the first time. The Cu doped films showed amorphous nature and exhibited high electrical conductivity as compared to undoped films, which were near stoichiometric with n-type conductivity and crystalline in nature. Cu doping influenced the photocurrent response, structural, electrical, and optical properties of the films. The optical band gap of the CdS film varied from 2.35 to 2 eV after Cu doping. The potential application of these films as an absorber layer in thin film solar cells is being investigated.
Gas source molecular beam epitaxy of FeSi2/Si(111) heterostructures
Authors: Schäfer H. Ch. , Rösen B. , Moritz H. , Rizzi A. , Lengeler B. , Lüth H. , Gerthsen D.
Published: July 2002

Epitaxial iron disilicide thin layers have been grown on silicon by gas source molecular beam epitaxy (GSMBE) in the temperature range 450–550 °C. Fe(CO)5 and SiH4 are used as sources for the silicide growth on a heated Si(111) surface. The growth phases are characterized in situ by means of high-resolution electron energy loss spectroscopy, ultraviolet and x-ray photoelectron spectroscopies. The formation of an epitaxial metallic γ-FeSi2 layer at the interface with the silicon substrate is revealed and no complete relaxation of this strained metastable interface layer is observed, as the growth proceeds with the semiconducting equilibrium β-FeSi2 phase. The coexistence in the GSMBE grown heterostructures of the metallic (CaF2) and semiconducting (orthorhombic) FeSi2 structures is confirmed by cross-section transmission electron microscopy.
Mechanical alloying process of the zirconia–8 mol % yttria ceramic powder
Authors: Chen Y. L. , Qi M. , Wu J. S. , Wang D. H. , Yang D. Z.
Published: July 2002

The phase transformation process of zirconia–8 mol % yttria powder mixtures during a high energy ball milling process has been studied by means of x-ray diffraction analysis. It has been found that the m-ZrO2 (monoclinic zirconia) transforms first to m-ZrO2 solid solution and then to t-ZrO2 (tetragonal zirconia) solid solution. Finally, a single cubic zirconia solid solution phase forms after prolonged milling. The structural transformation is discussed and explained in terms of the phase relations in zirconia-yttria ceramics and the nonequilibrium nature of the mechanical alloying.
Measuring photocarrier diffusivity near a Si(111) surface by reflective two-color transient grating scattering
Authors: Li C. M. , Ying Z. C. , Sjodin T. , Dai H. L.
Published: July 2002

Photocarrier diffusion within several hundred angstroms of a Si(111) surface was studied by the transient grating technique in the reflection geometry with independently tunable pump and probe picosecond laser pulses. A monotonous decay of the diffraction signal on 10−10 s scale immediately following the carrier excitation pulse was observed and attributed to band edge carrier diffusion. The ambipolar diffusivity at photocarrier density of 2×1019 cm−3 is determined to be 4.7 cm2/s.
Method for the determination of the value of the optical Kerr constant in weakly anisotropic liquids
Authors: Blaszczak Z. , Gauden P.
Published: July 2002

A simple method is demonstrated to significantly improve the typical apparatus currently used in optical Kerr effect measurements and, in particular, to facilitate the investigation of weakly anisotropic liquids. This modified version of the apparatus is highly sensitive and at the same time easy to handle. Additionally, it presents the advantage of reducing errors from absorption, divergence, and inaccurate coaxiality of both laser beams in the measuring cell.
Surface analysis of layered thin films using a synchrotron x-ray microbeam combined with a grazing-exit condition
Authors: Noma Takashi , Iida Atsuo
Published: July 2002

A new surface characterization technique with a lateral spatial resolution has been developed for nondestructive surface analysis using a synchrotron x-ray microbeam. The grazing-exit condition for fluorescent x-ray detection is utilized to attain the surface sensitivity. A focused x-ray microbeam of high photon flux has realized a lateral spatial resolution of a few μm. A layered thin-film electrode is analyzed in both the scanning and point modes. In the point mode, the analyzing position is fixed while the exit angle is changed. The characterization of thin films over a small region is made. In the scanning mode, the exit angle is fixed while the sample is being scanned. The two-dimensional intensity distribution reflects depth information in addition to the lateral elemental distribution. The intensity modulation near to the surface step of the pattern edge is discussed.
Development of a sealed-off cw CO2 laser using a supported gold catalyst
Authors: Tripathi A. K. , Gupta N. M. , Chatterjee U. K. , Bhawalkar D. D.
Published: July 2002

A compact low-power long-life sealed-off cw CO2 laser has been developed by incorporating a catalyst-coated outer jacket that helps to regenerate CO2 from the dissociation products formed during discharge. Maintaining the catalyst at an optimized temperature prevented CO2 adsorption in its bulk and resulted in the required level of CO oxidation activity. The laser system has been operated during the day for the past five months at a constant output level of about 4 W. The gas analysis performed at different catalyst temperatures suggests that although the presence of CO in large amounts is detrimental, an optimum concentration of carbon monoxide supports the CO2 laser operation.
A hydrocarbon detector for the remote sensing of vehicle exhaust emissions
Authors: Guenther Paul L. , Stedman Donald H. , Bishop Gary A. , Beaton Stuart P. , Bean James H. , Quine Richard W.
Published: July 2002

A new remote sensor for measuring on-road carbon monoxide, carbon dioxide, and hydrocarbon exhaust emissions in under 1 s from vehicles passing the sensor is described. The new design adds the capability for measuring exhaust hydrocarbons and eliminates the need for liquid–nitrogen-cooled detectors while improving upon the overall signal to noise. Under typical field operating conditions, sensitivity to 0.05% propane with a precision of 0.014% propane is observed. Two types of water interferences important to the measurement of exhaust hydrocarbons are reported. The water vapor present in all auto exhaust causes a small positive bias dependent on the analytical wavelength chosen. A much larger interference is caused by liquid water (often called ‘‘steam’’) plumes seen behind cold vehicles at low temperatures.
A large volume microwave plasma source
Authors: Bowles Jeffrey H. , Duncan Dwight , Walker David N. , Amatucci William E. , Antoniades John A.
Published: July 2002

We report on the design, construction, and use of a large cavity microwave plasma source. The source is designed to provide a range of space-plasma-like conditions in the Naval Research Laboratory Space Physics Simulation Chamber. A new feature of the source design incorporates hard anodized aluminum as the internal cavity area surface and does not use the conventional quartz cavity liner which is prone to overheating and cracking with extended use. By placing a number of small plasma outlet holes around the surface of a 36.8 cm output plate, we are able to provide a fairly radially uniform plasma; by further surrounding the production region and exit with an axial pinch magnetic field we are able to extend this region of plasma uniformity further toward the chamber walls and cover a significant portion of the experimental area. The source provides plasmas with selectable densities between 104 and 108 cm−3 and electron temperatures vary from about 0.5 to 2.0 eV.
Time-of-flight analysis of light pulses with a temporal resolution of 100 ps
Authors: Nielsen T. , Bormann F. , Wolbeck S. , Spiecker H. , Burrows M. D. , Andresen P.
Published: July 2002

This article describes a new, direct way to measure the time evolution of single light pulses in the subnanosecond region using time-of-flight analysis. The light pulse is coupled into a set of 40 optical fibers of increasing length arranged in an array. The output of the fiber array is imaged onto a fast gated optical imager (gatewidth 100 ps) coupled to a CCD camera (12 bit). The intensified part of the pulse of each fiber corresponds to the intensity of the light pulse at a different time due to the different transmission time of each fiber. Additional, two-dimensional (2D) spatial resolution is possible if image guides are used instead of fibers. The temporal resolution is limited by the gatewidth of the detector. The difference in length between the longest and the shortest fiber determines the total period of time displayed by the system (1 ns). The pulse characteristics of a recently developed short-pulse Raman-excimer laser are analyzed as an example of an application. Laser parameters like pulse length, energy stability, and jitter are measured with the fiber array.
Chemical reactivity of size-selected supported clusters: An experimental setup
Authors: Heiz U. , Vanolli F. , Trento L. , Schneider W.-D.
Published: July 2002

We describe an experimental setup for the investigation of the chemical reactivity of size-selected, supported clusters. The clusters are generated by a high frequency laser vaporization source. After mass selection and energy control the clusters are deposited under ultrahigh-vacuum conditions and with low kinetic energy onto thin oxide films grown on metal single crystals. Such films are ideal in their function as supports for size-selected deposited metal clusters. Measured currents of mass-selected niobium cluster ions were of the order of 1 nA and are high enough for fast deposition rates. Adsorbed molecules used on the one hand as probes to characterize the supported clusters and on the other hand for studying chemical reactions on these clusters are investigated in situ by Fourier transform infrared spectroscopy, temperature desorption spectroscopy, and Auger electron spectroscopy. The performance of these analytical tools is illustrated with CO adsorbed on Ni atoms supported on thin MgO(100) films grown on a Mo(100) single crystal.
Construction of a dual mode scanning near-field optical microscope based on a tapping mode atomic force microscope
Authors: Lin H.-N. , Chen S. H. , Lee L. J. , Tsai Din Ping
Published: July 2002

We present the modification of a commercial tapping mode atomic force microscope into a reflection and transmission dual mode scanning near-field optical microscope. In the configuration, the normal force detection unit is replaced by a shear force detection module and an interfacing circuit. The tip-sample distance control is therefore similar to tapping mode operation. Detection of the near-field signals is based on photodiodes and the lock-in technique, and the resolutions obtained for the topography and the near-field signal are around 80 and 150 nm, respectively.
Fiberoptic displacement sensor with a short hollow metal optical waveguide
Authors: Branzalov P. P. , Ivanov L. M. , Pavlov L. I.
Published: July 2002

A fiberoptic displacement sensor is described that is based on a combination of a pair of emitter and receiver fibers and a short hollow metal waveguide. The role of the hollow metal waveguide and its influence on the sensor characteristics are studied experimentally. It is shown that adding such a waveguide changes the shape of the sensor’s output signal and increases its amplitude by more than a factor of two. The optimum length of the metal optical waveguide that ensures improved sensor’s parameters is also estimated. The proposing sensor scheme can modify and improve the parameters of a large variety of fiber optic sensors that employ the same emitter-receiver fiber arrangement.
Compact laser molecular beam epitaxy system using laser heating of substrate for oxide film growth
Authors: Ohashi S. , Lippmaa M. , Nakagawa N. , Nagasawa H. , Koinuma H. , Kawasaki M.
Published: July 2002

A high-temperature, oxygen compatible, and compact laser molecular beam epitaxy (laser MBE) system has been developed. The 1.06 μm infrared light from a continuous wave neodymium-doped yttrium aluminum garnet (Nd:YAG) laser was used to achieve a wide range and rapid control of substrate temperature in ultrahigh vacuum and at up to 1 atm oxygen pressure. The maximum usable temperature was limited to 1453 °C by the melting point of the nickel sample holder. To our knowledge, this is the highest temperature reported for pulsed laser deposition of oxide films. The efficient laser heating combined with temperature monitoring by a pyrometer and feedback control of the Nd:YAG laser power by a personal computer made it possible to regulate the substrate temperature accurately and to achieve high sample heating and cooling rates. The oxygen pressure and ablation laser triggering were also controlled by the computer. The accurate growth parameter control was combined with real-time in situ surface structure monitoring by reflection high energy electron diffraction to investigate oxide thin film growth in detail over a wide range of temperatures, oxygen partial pressures, and deposition rates. We have demonstrated the performance of this system by the fabrication of homoepitaxial SrTiO3 films as well as heteroepitaxial Sr2RuO4, and SrRuO3 films on SrTiO3 substrates at temperatures of up to 1300 °C. This temperature was high enough to change the film growth mode from layer by layer to step flow.
A study on background subtraction in Auger and photoelectron time coincidence spectroscopy using third generation synchrotron radiation source
Authors: Calicchia P. , Lagomarsino S. , Scarinci F. , Martinelli C. , Formoso V.
Published: July 2002

Auger and photoelectron time coincidence spectroscopy measurements on Si(100) have been carried out employing third generation synchrotron radiation source. This work has been focused on the particular treatment of the collected data required for this kind of experiment. The time structure characteristics of the European Synchrotron Radiation Facility light source have been taken into account in time coincidence data analysis, and compared with other sources. This study has shown the particular influence of the source in the distribution of accidental coincident events, revealing the need to introduce a suitable method for background subtraction in coincidence spectra.
On the origin of the 1.5 μm luminescence in ion beam synthesized β-FeSi2
Authors: Leong D. N. , Harry M. A. , Reeson K. J. , Homewood K. P.
Published: July 2002

In this letter we present photoluminescence results on β-FeSi2/Si using excitation energies above and below the silicon band gap. These results show that the luminescence emission observed at 1.5 μm can be firmly attributed to band edge related emission from the β-FeSi2. This result confirms the potential of β-FeSi2 as a strong contender for a silicon compatible optoelectronics technology that matches the conventional optical fiber transmission wavelength at 1.5 μm.
Low resistance Zn3P2/InP heterostructure Ohmic contact to p-InP
Authors: Park Moon-Ho , Wang L. C. , Cheng J. Y. , Deng Fei , Lau S. S. , Palmstro/m C. J.
Published: July 2002

A low resistance Zn/Pd Ohmic contact scheme to p-InP based on solid phase reactions has been investigated. Contact resistivity in the mid-10−5 Ω cm−2 could be obtained for contacts with an atomic ratio of Zn to Pd of ∼1.5. In addition to the solid phase regrowth process, a Zn3P2/InP heterojunction is formed at the contact/semiconductor interface. The Ohmic contact formation mechanism is rationalized with the formation of Zn3P2/InP heterojunction.
Chemistry, diffusion, and electronic properties of a metal/organic semiconductor contact: In/perylenetetracarboxylic dianhydride
Authors: Hirose Y. , Kahn A. , Aristov V. , Soukiassian P.
Published: July 2002

We present a photoemission investigation of the interface between In and 3, 4, 9, 10 perylenetetracarboxylic dianhydride (PTCDA). The interfacial region is very wide due to an anomalously fast diffusion of In into the organic layer. In also reacts with the anhydride end groups of the molecules. The absence of metal clustering, which permits diffusion, is believed to be due to the ionization of In and ion–ion repulsion in the PTCDA matrix. Finally, the ohmicity of the In/PTCDA contact is attributed to reaction-induced electronic gap states created throughout the wide interfacial region upon formation of the interface. This study provides the first direct correlation between chemistry and electronic properties of a metal contact on an organic molecular semiconductor.
Low pressure synthesis of bulk, polycrystalline gallium nitride
Authors: Argoitia Alberto , Hayman Cliff C. , Angus John C. , Wang Long , Dyck Jeffrey S. , Kash Kathleen
Published: July 2002

Thick films of polycrystalline GaN were grown at low pressures by direct reaction of atomic nitrogen with liquid Ga without the presence of a substrate. The crystals were confirmed to be wurtzitic GaN by x-ray diffraction, transmission electron microscopy, Raman spectroscopy, and elemental analysis. Photoluminescence spectra showed near band edge peaks and broad yellow band emission at both 298 and 10 K. The results show that atomic nitrogen is an attractive alternative to high pressure N2 for the saturation of liquid gallium with nitrogen for the synthesis of bulk GaN.
Synthesis and characterization of PbSe quantum dots in phosphate glass
Authors: Lipovskii A. , Kolobkova E. , Petrikov V. , Kang I. , Olkhovets A. , Krauss T. , Thomas M. , Silcox J. , Wise F. , Shen Q. , Kycia S.
Published: July 2002

The controlled synthesis of PbSe nanocrystal quantum dots with narrow size distributions was achieved through phase decomposition of the PbSe solid solution in a phosphate glass host. Structural characterization by electron microscopy and x-ray diffraction shows that the dots have mean diameters between 2 and 15 nm. The exciton Bohr radius aB=46 nm in PbSe, so these quantum dots provide unusual and perhaps unique access to the regime of strong quantum confinement. The optical absorption spectra are compared to the predictions of a theoretical treatment of the electronic structure. The theory agrees well with experiment for dots larger than ∼7 nm, but for smaller dots there is some deviation from the theoretical predictions.
The Meyer–Neldel conductivity prefactor for chalcogenide glasses
Authors: Yelon Arthur , Movaghar Bijan
Published: July 2002

K. Shimakawa and F. Abdel-Wahab [Appl. Phys. Lett. 70, 652 (1997)] have reported that electrical conductivity in chalcogenide glasses obeys the Meyer–Neidel (MN) rule: σ=σ00eΔE/EMNe−ΔE/kT. They also found that σ00 increases extremely rapidly as a function of EMN, the MN energy. Using Emin’s model for optical phonon induced electron hopping, we show that such a result is in accordance with the multiexcitation model for the origin of the MN rule.
n
-type electrical conduction in transparent thin films of delafossite-type AgInO2
Authors: Otabe Takuya , Ueda Kazushige , Kudoh Atsushi , Hosono Hideo , Kawazoe Hiroshi
Published: July 2002

Thin films of AgInO2 were prepared to find a transparent and n-type conducting oxide with a delafossite structure. This is a candidate material for fabricating a pn junction with the recently found p type conducting and transparent CuAlO2 delafossite. Nondoped and 5% Sn-doped thin films were deposited on a silica glass substrate by radio-frequency sputtering. The crystalline phase in the films was identified to be the delafossite structure by x-ray diffraction and chemical composition was confirmed to be an Ag/In ratio=1.00/0.97 by inductively coupled plasma emission spectroscopy. The optical band gap was estimated from absorption spectra to be ∼4.4 eV, and the thin films were transparent up to near ultraviolet region. Electrical conductivities of the nondoped and 5% Sn-doped AgInO2 films at room temperature were 1×10−5 and 6×100 S cm−1, respectively. Measurements of Hall voltage and Seebeck coefficient (−50 μV K−1) suggested the conduction in the Sn-doped film to be n type. Carrier concentration and Hall mobility in the doped film were 2.7×1019 cm−3 and 0.47 cm2 V−1 s−1, respectively.
Growing carbon nanotubes by microwave plasma-enhanced chemical vapor deposition
Authors: Qin L. C. , Zhou D. , Krauss A. R. , Gruen D. M.
Published: July 2002

A processing route has been developed to grow bundles of carbon nanotubes on substrates from methane and hydrogen mixtures by microwave plasma-enhanced chemical vapor deposition, catalyzed by iron particles reduced from ferric nitrate. Growth takes place at about 900 °C leading to nanotubes with lengths of more than 20 μm and diameters on the nanometer scale.
Experimental evidence of the size effect in thin ferroelectric films
Authors: Vendik Orest G. , Zubko Svetlana P. , Ter-Martirosayn Leon T.
Published: July 2002

The phenomenological model based on the Ginsburg–Devonshire equation has been applied to investigate the size effect in thin SrTiO3 and BaxSr1−xTiO3 films. The size effect is caused by two phenomena: spatial correlation of the polarization and boundary conditions for the ferroelectric polarization on electrodes. The experimental data obtained by different groups were used to investigate the boundary conditions. The most remarkable result was found for the SrRuO3/BaxSr1−xTiO3/SrRuO3 structure which is characterized by free boundary conditions for the ferroelectric polarization. Such boundary conditions provide the highest value of the effective dielectric constant of a dielectric layer in the sandwich structure. The strain dependence of the dielectric permittivity of SrTiO3 at finite temperatures was taken into account as well.
Photoluminescence and upconversion optical properties of the CaS:Sm3+ nanocrystallites
Authors: Gong X. , Chen W. J. , Wu P. F. , Chan W. K.
Published: July 2002

The CaS:Sm3+  nanocrystallites are synthesized by the modified sol-gel technique. The grain size of the nanocrystallites is approximately 60 nm, which is determined by transmission electron microscopy, x-ray diffraction and UV-vis absorption spectrum. Compared to the CaS:Sm3+ macrocrystallites, the absorption and emission of the nanocrystallites have a blueshift in their spectra, and the emission spectrum of the host CaS appears in the region of 400–500 nm. It was also found that the efficiencies of both photoluminescence and upconversion in the nanocrystallites are higher than those in the macrocrystallites.
Scanning tunneling microscopy and spectroscopy of arsenic antisites in low temperature grown InGaAs
Authors: Grandidier B. , Chen Huajie , Feenstra R. M. , McInturff D. T. , Juodawlkis P. W. , Ralph S. E.
Published: July 2002

Scanning tunneling microscopy is used to study low temperature grown (LTG) InGaAs with and without Be doping. The Be-doped material is observed to contain significantly fewer AsGa antisite defects than the undoped material, with no evidence found for Be–As complexes. Annealing of the LTG-InGaAs forms precipitates preferentially in the undoped material. The previously observed dependence of the optical response time on Be doping and annealing is attributed to changes in the As antisite concentration and the compensation effect of the Be.
A cryogenic amplifier for direct measurement of high-frequency signals from a single-electron transistor
Authors: Pohlen S. L. , Fitzgerald R. J. , Hergenrother J. M. , Tinkham M.
Published: July 2002

We have developed a cryogenic voltage amplifier using GaAs metal semiconductor field effect transistors and coupled it to a superconducting single-electron transistor (SET) inside a dilution refrigerator. With this amplifier, we could extend the maximum output frequency of the SET, normally less than a kilohertz, up to 1 MHz. By placing this amplifier off-chip, we could maintain the low SET temperature required for proper SET operation.
(Ba,Sr)TiO
3
thin films with conducting perovskite electrodes for dynamic random access memory applications
Authors: Nagaraj B. , Sawhney T. , Perusse S. , Aggarwal S. , Ramesh R. , Kaushik V. S. , Zafar S. , Jones R. E. , Lee J.-H. , Balu V. , Lee J.
Published: July 2002

Interfaces and hence electrodes determine the performance of (Ba,Sr)TiO3 (BST) capacitors for ultralarge scale integration dynamic random access memories. Electrode materials forming a rectifying contact on BST drastically reduce the dielectric constant and hence the capacitance and charge storage density of the capacitor, when the dielectric thickness is reduced. This can limit the role of Pt as an electrode material for gigabit dynamic random access memories (DRAM). The conducting oxide, La0.5Sr0.5CoO3 (LSCO) with its perovskite structure, has structural and chemical compatibility with BST. Our results in LSCO/BST/LSCO capacitor show that the mechanism of conduction is not interface limited but predominantly bulk limited. A 75 nm BST film with LSCO electrodes shows a leakage current density of 1×10−7 A/cm2 at 1 V, 85 °C. The dielectric constant at 1 V, 105 Hz is 350, making LSCO a potential contact electrode for DRAM memories.
High-efficiency oligothiopene-based light-emitting diodes
Authors: Gigli G. , Barbarella G. , Favaretto L. , Cacialli F. , Cingolani R.
Published: July 2002

We report investigations of the photoluminescence (PL) and electroluminescence (EL) of a thiophene oligomer for which we have devised a variety of substitutions aimed at enhancing the solid-state efficiency. We find that the absolute PL quantum efficiency in the solid state is up 37% for both powders or spin-coated thin films of the compound. The material thus becomes competitive for applications in organic light-emitting diodes (LEDs). EL efficiencies up to 1.2 cd/A are demonstrated in LEDs prepared with indium–tin–oxide and Ca–Al electrodes.
Optical resonance modes in GaN pyramid microcavities
Authors: Jiang H. X. , Lin J. Y. , Zeng K. C. , Yang W.
Published: July 2002

An array of GaN hexagonal pyramids with a side length of 8.0 μm was fabricated by selective epitaxial overgrowth. These microsized pyramids are highly efficient microcavities. Three types of optical resonance modes with mode spacings of 10, 5.0, and 6.0 Å were observed when a single pyramid was pumped optically by an intense ultraviolet laser beam. An optical ray tracing method has been developed for calculating the optical resonance modes inside the pyramid microcavities. It was shown that a single pyramidal cavity can support several different types of optical resonance modes. The calculated mode spacing agrees very well with the observations. The uniqueness and advantages of this class of hexagonal pyramidal microcavities over the other microcavities are discussed. The implications of our finding on the future GaN microcavity light emitters including micro-light-emitting diodes, microcavity lasers, and vertical-cavity-surface emitting lasers are also discussed.
Electrical effects of plasma damage in p-GaN
Authors: Cao X. A. , Pearton S. J. , Zhang A. P. , Dang G. T. , Ren F. , Shul R. J. , Zhang L. , Hickman R. , Van Hove J. M.
Published: July 2002

The reverse breakdown voltage of p-GaN Schottky diodes was used to measure the electrical effects of high density Ar or H2 plasma exposure. The near surface of the p-GaN became more compensated through introduction of shallow donor states whose concentration depended on ion flux, ion energy, and ion mass. At high fluxes or energies, the donor concentration exceeded 1019 cm−3 and produced p-to-n surface conversion. The damage depth was established as ∼400 Å based on electrical and wet etch rate measurements. Rapid thermal annealing at 900 °C under a N2 ambient restored the initial electrical properties of the p-GaN.
Stable zirconium silicate gate dielectrics deposited directly on silicon
Authors: Wilk G. D. , Wallace R. M.
Published: July 2002

Zirconium silicate (ZrSixOy) gate dielectric films with ∼3–5 at. % Zr exhibit excellent electrical properties and high thermal stability in direct contact with Si. We demonstrate an equivalent oxide thickness of about 21 Å for a 50 Å ZrSixOy film sputter-deposited directly on a Si substrate, as measured by capacitance–voltage techniques, with a hysteresis shift less than 10 mV. Leakage currents for these films are very low, approximately 1×10−6 A/cm2 at 1.0 V bias in accumulation. Films ramped to hard breakdown exhibit breakdown fields Ebd ∼10 MV/cm. Excellent electrical properties are obtained with Au electrodes, in particular.
Effect of hydrogen etching on 6H SiC surface morphology studied by reflection high-energy positron diffraction and atomic force microscopy
Authors: Kawasuso A. , Kojima K. , Yoshikawa M. , Itoh H. , Narumi K.
Published: July 2002

Hydrogen-etched 6H SiC (0001) surfaces have been studied by reflection high-energy positron diffraction and atomic force microscopy. It was found that residual damage on the surfaces were effectively removed by the hydrogen etching as compared to the HF etching after the oxidation. The hydrogen-etched surfaces were atomically flat. After the oxidation following the hydrogen etching, the surface roughness was found to increase and an anomalous dip structure appeared in the rocking curve of the reflection high-energy positron diffraction.
Molecular orientation in laser-induced periodic microstructure on polyimide surface
Authors: Lu Qing-hua , Wang Zong-guang , Yin Jie , Zhu Zi-kang , Hiraoka H.
Published: July 2002

The dichroism of the laser-induced periodic microstructure on a polyimide surface was studied with polarized reflectance infrared spectroscopy. The experimental results show that the polar C=O groups in the polyimide exhibit a greater tendency of orienting in the direction parallel to the microlines while the nonpolar C–C–C linkages between the two benzene rings in the diamine moiety tend to orient in a direction perpendicular to the nanolines. This result indicates that the polyimide molecule chains tend to orient in a direction perpendicular to the nanolines.
Individual and collective effects of oxygen and ethanol on the conductance of SnO2 thin films
Authors: Kissine Vladimir V. , Sysoev Victor V. , Voroshilov Sergey A.
Published: July 2002

We study the changes in the conductance of the fully depleted SnO2 films exposed both to oxygen or ethanol vapors in vacuum and to their mixture in the air. The dependence of free carrier concentration on the acceptor-like and/or donor-like gas pressure is discussed assuming the flat-band condition. We show that the gas sensitivity of the depleted semiconductor layer can be explained taking into account the compensation of the sample conductivity by the surface adsorbed species. The results of calculations are in good agreement with our experimental data.
Magnetocaloric effect in Gd2PdSi3
Authors: Sampathkumaran E. V. , Das I. , Rawat R. , Majumdar Subham
Published: July 2002

The influence of the application of a magnetic field (H) on the temperature (T) dependence of heat capacity and isothermal magnetization has been investigated in Gd2PdSi3, a compound ordering antiferromagnetically below (TN=)21 K in zero H. Among other findings, the one to be emphasized is the observation of significant magnetocaloric effect (MCE) over about a 30 K range with a peak in the vicinity of TN. This finding suggests that this material could be useful for magnetic refrigeration below 50 K. The results also establish that either of the two experimental methods—heat capacity or magnetization—can be applied to identify materials with large MCE.
Algebraic solutions for all dihedral groups
Authors: Chen Jin-Quan , Fan Peng-Dong , McAven Luke , Butler Philip
Published: July 2002

The significant problem with using point groups is the dependence upon extensively tabulated results. We present simple algebraic expressions for the primitive characters, matrix irreps, symmetry adapted functions (SAFs), and Clebsch–Gordan coefficients for all dihedral groups, Dn, Cnv, Dnd, and Dnh. Those results, for arbitrary n and for single- and double-valued representations, have been derived in a simple manner without using group tables. Previously incomplete tabulated results are now redundant. In particular the parity dependence of the SAFs of the improper dihedral groups is shown analytically. Simple relations are derived between the SAFs of the proper and improper dihedral groups, so that the SAFs of the latter can be easily obtained from the SAFs of Dn.
The dynamics of supercritical water: A quasielastic incoherent neutron scattering study
Authors: Tassaing T. , Bellissent-Funel M.-C.
Published: July 2002

Results of quasielastic and inelastic incoherent neutron scattering experiments of light water at several thermodynamic states (200<T<400 °C and 184<P<400 bar) corresponding to densities ranging from 0.2 to 0.9 g/cm3 are presented. The results have been analyzed using a jump diffusion model and the two parameters of this model, namely, τ0, the residence time and D, the translational diffusion coefficient have been determined as a function of the density of supercritical water. The values of the diffusion coefficient D reported here are in very good agreement with those measured by NMR and increases strongly as the density of the medium decreases. The residence time τ0 slightly increases as the density of supercritical water decreases, with a value between 0.14 and 0.2 ps (ten times shorter than that measured in ambient liquid water). Moreover, we observe an almost linear increase in the mean jump distance L=6Dτ0 from 1.5 Å at 0.9 g/cm3 to 4 Å at 0.2 g/cm3. Finally, the value of the mean square displacement 〈u2〉1/2 increases from 0.6 Å at 0.9 g/cm3 to 1 Å at 0.2 g/cm3.
Experimental investigation of high-quality synchronization of coupled oscillators
Authors: Blakely Jonathan N. , Gauthier Daniel J. , Johnson Gregg , Carroll Thomas L. , Pecora Louis M.
Published: July 2002

We describe two experiments in which we investigate the synchronization of coupled periodic oscillators. Each experimental system consists of two identical coupled electronic periodic oscillators that display bursts of desynchronization events similar to those observed previously in coupled chaotic systems. We measure the degree of synchronization as a function of coupling strength. In the first experiment, high-quality synchronization is achieved for all coupling strengths above a critical value. In the second experiment, no high-quality synchronization is observed. We compare our results to the predictions of the several proposed criteria for synchronization. We find that none of the criteria accurately predict the range of coupling strengths over which high-quality synchronization is observed.
The structure and properties of H3+Arn (n=1–9) cations
Authors: Kaczorowska Malgorzata , Roszak Szczepan , Leszczynski Jerzy
Published: July 2002

Ab initio post Hartree–Fock studies were performed on the title species. Their structures consist of a tightly bound H3+ core ion which generates progressively weaker bonded distinct shells of Ar atoms. The predicted structures of H3+Arn (n=1–9) clusters explain the experimental pattern of changes in the stepwise enthalpies and entropies of dissociation. The subtle changes in thermodynamic properties reproduced by calculations indicate an accurate location of the global minimum geometries of the clusters and the proper determination of the shells for coordinating argon atoms. The nature of chemical bonding is studied based on the interaction energy decomposition. The importance of the covalent component in the interaction forces is revealed.
Geometry optimization of excited valence states of formaldehyde using analytical multireference configuration interaction singles and doubles and multireference averaged quadratic coupled-cluster gradients, and the conical intersection formed by the 1 1B1(σ-π*) and 2 1A1(π-π*) states
Authors: Dallos Michal , Müller Thomas , Lischka Hans , Shepard Ron
Published: July 2002

Extended MR-CISD (multireference configuration interaction singles and doubles), MR-CISD+Q (multireference configuration interaction singles, doubles, and quadrupole), and MR-AQCC (multireference averaged quadratic coupled clusters) calculations have been performed on the following valence states of formaldehyde: 1 1A1 (planar ground state), 1 1A2 and 1 1A″ (planar and nonplanar n-π* state), 1 1B1 and 2 1A1 (planar σ-π* and π-π* states) and their nonplanar counterparts 2 1A′ and 3 1A′. Full geometry optimizations have been performed using analytic gradient techniques developed for the MR-CISD and MR-AQCC methods as implemented into the COLUMBUS program system. Basis set extrapolation techniques have been used for the determination of high-accuracy geometries and adiabatic excitation energies. Harmonic vibrational frequencies have been computed also. Agreement between calculated and available experimental data is very good. Especially for the σ-π* and π-π* states experimental information is extremely scarce and our results provide reliable predictions. The major new result of our work is the finding that for the 2 1A1(π-π*) state the structure optimized under planarity constraints is only a saddle point and not a minimum. This fact is the result of a conical intersection between the σ-π* and π-π* states (1 1B1 and 2 1A1). The final result is that neither on the 2 1A′ nor on the 3 1A′ surface a stationary minimum can be assigned to the π-π* state.
Structural modification of single-axis-oriented yttria-stabilized-zirconia films under zirconium ion bombardment
Authors: Voevodin A. A. , Jones J. G. , Zabinski J. S.
Published: July 2002

Crystal size and orientation control of yttria-stabilized-zirconia (YSZ) films produced by pulsed-laser deposition under zirconium-ion bombardment are reported. A structure development mechanism is proposed based on the correlation between ablated plasma chemistry, velocity distributions of atomic and molecular species in the plasma, bombardment energy, time, incidence angle, film crystal size, and orientation. A near-room-temperature growth of single-axis-oriented YSZ films with (002) texture on randomly oriented polycrystalline substrates is demonstrated.
Microsolvation effect on chemical reactivity: The case of the Cl−+CH3Br SN2 reaction
Authors: Raugei Simone , Cardini Gianni , Schettino Vincenzo
Published: July 2002

The microsolvatation of the SN2 reaction Cl−+CH3Br with one and two molecules of water has been investigated by ab initio molecular dynamics. The energy profile and the stationary points have been discussed. For the case of one molecule of water the thermal effects have been evaluated and some impact trajectories have been simulated. The results, in agreement with recent experimental results, suggest the noneffectiveness of the SN2 mechanism in the presence of solvatation. The analysis in terms of localized electron density has shown the importance of the H-bond between the chloride ion and the water molecule during the reaction path.
Functional integral formulations for classical fluids
Authors: Woo Hyung-June , Song Xueyu
Published: July 2002

Various functional integral formalisms of classical monatomic fluids are considered with their applicabilities and limitations compared. For length scales large compared to the particle size, the density field theory, in which the action of the functional integral is simply given by the mean field free energy functional expression, is shown to be a well-defined and rigorous formulation. For short range properties of dense fluids, a different version of the functional integral method is developed by explicitly separating out the hard core part of the interaction. The resulting functional integral is seen to require correlation functions of the hard sphere fluid as the input. The generalized van der Waals equation and the random phase approximation of the cluster diagrammatic methods are recovered simply as the stationary-phase approximation and its Gaussian correction to the functional integral, respectively.
A nondestructive technique for determining the spring constant of atomic force microscope cantilevers
Authors: Gibson Christopher T. , Weeks Brandon L. , Lee Jonathan R. I. , Abell Chris , Rayment Trevor
Published: July 2002

We present a simple, accurate, and nondestructive method to determine cantilever spring constants by measuring the resonant frequency before and after the addition of a thin gold layer. The method for resonating the cantilevers uses electrostatic force modulation, which has been described for conductive cantilevers, but we demonstrate it can also be applied to silicon nitride cantilevers. The variations in spring constant for cantilevers of the same type across the same wafer are also explored.
Approximate integral equation theory for classical fluids
Authors: Sarkisov Gari
Published: July 2002

A new approximate integral equation with no empirical or adjustable parameters which was recently proposed [J. Chem. Phys. 110, 3961 (1999)] is used for the systematic study of Lennard-Jones fluid. The Ornstein–Zernike (OZ) equation designed originally to describe the thermodynamically stable states is substantiated to be valid also for predicting and studying the metastable states. Both stable and metastable solutions of the OZ equation are analyzed. The problem of determining the chemical potential is considered within the framework of the exact series of the bridge functionals. The structure and thermodynamic properties calculated from proposed approaches show excellent agreement with simulations for both stable and metastable states. Displayed and discussed is the gas–liquid phase diagram predicted by the new equation. Also, to build the liquid–crystal phase diagram some one-phase empirical structural criteria are utilized.