bluuebunny/crossref_metadata_2025 huggingface dataset:
Organic-film photovoltaic cell with electroluminescence
Authors: Chu B. , Fan D. , Li W. L. , Hong Z. R. , Li R. G.
Published: July 2002

An organic-film photovoltaic (PV) cell, in which N,N′-bis-(1-naphthyl)- N,N′- diphenyl- 1,1′- biphenyl-4,4′-diamine (NPB) and tris(acetylacetonato)-(monophenothroline) yttrium [Y(ACA)3phen] were used as electron-acceptor and donor, respectively, has been fabricated. Under UV light (4 mW/cm2), the short-circuit current (Isc), open-circuit voltage (Voc), fill factor (FF) and the overall power conversion efficiency of the optimum PV cell were 46 μA/cm2, 2.15 V, 0.30%, and 0.7%, respectively. The photocurrent response region of the cell parallels the adsorption of NPB. The PV effect is attributed to exciplex formation at the interface between the two organic films. The PV cell described displays electroluminescence (EL) emission of blue light upon application of a dc voltage. The maximum luminance was 750 cd/m2 at 15 V driving voltage.
A molecular based continuum approach for the dielectric permittivity of liquids and liquid crystals
Authors: di Matteo Andrea , Ferrarini Alberta
Published: July 2002

A method for the calculation of the dielectric permittivity of isotropic and anisotropic homogeneous fluids is presented which, in the framework of the continuum approximation, adopts a realistic description of the molecular features, so overcoming some of the limits of the Onsager model. The Poisson equation for the molecular charge distribution contained in a cavity in a dielectric continuum in the presence of an external field is solved by a boundary element technique, which allows a detailed description of the cavity shape associated with a given molecular structure. The charge distribution is described in terms of point charges derived from ab initio calculations in vacuum, in addition to a set of interacting atom dipoles induced by all the electric fields experienced by the molecule in the condensed phase. The link between molecular features and bulk properties is established in a general way suitable for isotropic liquids and nematic phases, through the orientational distribution function of the molecule interacting with the applied field and the surrounding fluid. Numerical results are reported for the liquid phase of a set of selected organic compounds of different shape and polarity, and for the isotropic and nematic phases of 4-n-pentyl-4′-cyanobiphenyl (5CB). They show that a realistic description of the molecular features can have dramatic effects in the case of strongly anisometric molecules.
Anomalous magnetotransport properties of epitaxial full Heusler alloys
Authors: Lund M. S. , Dong J. W. , Lu J. , Dong X. Y. , Palmstrøm C. J. , Leighton C.
Published: July 2002

We report the magnetotransport properties of epitaxial films of the full Heusler alloys Ni2MnGa, Ni2MnGe, and Ni2MnAl, grown by molecular beam epitaxy on (001) GaAs. The ferromagnetic alloys (Ni2MnGa,Ni2MnGe) exhibit an anomalous temperature dependence of the resistivity and a negative magnetoresistance peaking near the Curie temperature due to spin disorder scattering. Considering the absolute values of the resistivity, the anomalous high temperature behavior and an upturn in the resistivity below 20 K, we suggest that these Heusler alloys rather than being conventional metals are in fact strongly disordered electronic systems.
Synchrotron x-ray scattering study on the evolution of surface morphology of the InN/Al2O3(0001) system
Authors: Lee Ik Jae , Kim Jin Woo , Hur Tae-Bong , Hwang Yoon-Hwae , Kim Hyung-Kook
Published: July 2002

Dynamic scaling behavior was studied for InN films grown on sapphire(0001) substrates using high-resolution synchrotron x-ray reflectivity and atomic force microscopy measurements. In the early stage of growth, highly strained planar InN films were grown. As the film thickness approaches an effective critical thickness, the growth gradually crosses over to the island growth. Concurrently, the relaxation of the lattice strain begins and the growth front becomes rougher. The roughness increases mostly during the intermediate crossover regime where the strain is relieved. In this regime, the dynamic scaling exponent, β, is estimated as 1.754±0.071. The evolution of the surface roughness in the final-stage growth can be described by the dynamic scaling exponent of 0.236±0.022.
Structure study of Se species in channels of AlPO4-5 crystals
Authors: Li Irene L. , Tang Z. K.
Published: July 2002

The thermal adsorption/desorption process of selenium species in the channels of AlPO4-5 (AFI) single crystals is studied by simultaneous thermogravimetry (TG) and differential scanning calorimetry (DSC). There exist two peaks in the TG and DSC curves, indicating that two different structures of Se species, helical chain and 8-member ring, coexist in the AFI channels. Polarized Raman and optical absorption spectra show that the helical chains are highly aligned along the channel direction, while the Se8 rings are randomly oriented in the channels. The helical chains have a lowest optical absorption band at 2.6 eV, which is strongly polarization dependent, while the 8-member rings have a lowest absorption band at 3.0 eV that varies slowly with the light polarization.
Large deuterium isotope effect in the rotational diffusion of anthraquinone dyes in liquid solution
Authors: Paparo D. , Manzo C. , Marrucci L. , Kreuzer M.
Published: July 2002

The effect of hydrogen–deuterium substitution on the rate of rotational diffusion of two species of amino-substituted anthraquinone dyes dissolved in the isotropic liquid phase of 4′-n-pentyl-4-cyanobiphenyl (5CB) and in a liquid mixture of alkanes (paraffin) has been studied as a function of temperature. The rotational dynamics was probed by time-resolved detection of fluorescence depolarization. In 5CB we observe a reduced rotational mobility of the deuterated species with respect to the protonated ones by up to 43% (at 311 K) and a corresponding increase of 4–5 kJ/mol in the activation energy, as deduced from the temperature dependence. To our knowledge, this is the largest isotopic effect ever reported for the molecular rotational diffusion in liquids. In liquid paraffin the effect vanishes. We attribute our findings to an isotopic effect in the breaking kinetics of the hydrogen-bond between the amino groups of dye molecules and the cyano group of 5CB that cannot be explained with the known isotopic effect of hydrogen-bond stability.
Arsenic surface segregation during the molecular-beam epitaxial growth of GaAs embedded in wurtzite GaN
Authors: Kim Hyonju , Andersson T. G.
Published: July 2002

We report on the growth of a thin GaAs layer embedded in wurtzite GaN prepared on (0001) sapphire substrate by rf-plasma molecular-beam epitaxy. It was found that the As surface segregation during the growth of the GaN cap layer can be controlled by the growth parameters, i.e., the growth interruption and subsequent in situ annealing process implemented in this study. The arsenic profile of the cap layer was accurately evaluated using an empirical segregation model. The surface morphology was also investigated using atomic force microscopy.
Polarization-sensitive photoconductivity in aligned polyfluorene layers
Authors: Zen A. , Neher D. , Bauer C. , Asawapirom U. , Scherf U. , Hagen R. , Kostromine S. , Mahrt R. F.
Published: July 2002

Polarization-sensitive photodiodes have been constructed utilizing an aligned polyfluorene layer oriented on a photoaligned liquid crystalline photoaddressable polymer. The diodes exhibit a pronounced dependence of the photocurrent on the polarization of the incident light, yielding a polarization sensitivity of more than a factor of 10 at the onset of the absorption. The action spectrum is symbatic for light polarized perpendicular to the polymer alignment direction whereas it does not simply correlate with the absorption characteristics for parallel polarized light. The data can be explained utilizing Ghosh’s model taking into account interference effects within the polyfluorene layer.
Changes in Al-related photoluminescence in 4H-SiC caused by hydrogenation
Authors: Koshka Yaroslav , Mazzola Michael S. , Draper William A.
Published: July 2002

Both reduction of the intensity of aluminum-related photoluminescence after hydrogenation and the phenomenon of the optical quenching of the Al bound exciton (Al–BE) previously reported for hydrogenated 6H-SiC was observed now in the 4H-SiC polytype. Hydrogenation caused also a reduction of the nitrogen–aluminum donor-acceptor pair emission. Prolonged excitation of hydrogenated samples with above-band gap light resulted in additional quenching of the residual Al–BE photoluminescence. The quenching kinetics and some differences between the 6H and 4H polytypes are investigated. The observed phenomena are attributed to optically enhanced passivation of Al acceptors with hydrogen.
Miniaturization and integration of photoacoustic detection
Authors: Firebaugh Samara L. , Jensen Klavs F. , Schmidt Martin A.
Published: July 2002

Photoacoustic spectroscopy is an absorption spectroscopy technique that is currently used for low-level gas detection and catalyst characterization. It is a promising technique for chemical analysis in mesoscale analysis systems because the detection limit scales favorably with miniaturization. This work focuses on the scaling properties of photoacoustic spectroscopy, and on the miniaturization of gas-phase photoacoustic detection of propane in a nitrogen ambient. The detection system is modeled with a transmission line analogy, which is verified experimentally. The model includes the effects of acoustic leaks and absorption saturation. These two phenomena degrade the performance of the photoacoustic detector and must be controlled to realize the scaling advantages of photoacoustic systems. The miniature brass cells used to verify the model employ hearing aid microphones and optical excitation from a mechanically chopped, 3.39 μm He–Ne laser, transmitted into the cells with an optical fiber. These cells are able to detect 10 ppm of propane in nitrogen (a signal level of ∼1 Pa/W). We also describe the development of a miniaturized photoacoustic system formed by microfabrication. In this case, the pressure-driven deflection of the detection membrane is measured optically. These systems show that photoacoustic detection may be inappropriate for systems with large variations in gas concentration because of absorption saturation and changing gas acoustic properties. Nevertheless, photoacoustic spectroscopy is a promising technique for the analysis of dilute mixtures in miniature chemical systems.
Decomposition of trisdimethylaminoarsenic and As nucleation on GaAs(001)-2×4 at low temperature
Authors: Kuramochi Hiromi , Cui Jie , Ozeki Masashi , Uchida Hironaga , Akinaga Hiroyuki , Yoshida Hiroaki , Sanada Noriaki , Fukuda Yasuo
Published: July 2002

Adsorption and decomposition of trisdimethylaminoarsenic (TDMAAs) on a GaAs(001)-2×4 surface have been studied in the temperature range from 100 to 500 K by high-resolution electron energy loss spectroscopy (HREELS) and scanning tunneling microscopy (STM). TDMAAs is adsorbed molecularly on the surface at 100 K. Decomposition starts at about 300 K and vibration peaks of HREELS spectra due to adsorbates almost disappear between 400 and 500 K. STM images indicate that TDMAAs is adsorbed on As dimers of the GaAs(001)-2×4 surface. Only As atoms and clusters remain at the edge of the As dimer rows and nucleation of As starts at about 470 K. Decomposition of TDMAAs and As nucleation at relatively low temperature are advantageous as a source molecule in metalorganic chemical vapor deposition for nanofabrication.
Thermo-piezochemistry of InAs on GaAs(001)
Authors: Bottomley D. J.
Published: July 2002

We show that the large biaxial epitaxial stress (−5.3 GPa) of InAs on GaAs(001) strongly affects the chemical reaction between elemental In and As on the GaAs(001) surface at temperatures between 200 and 500 °C. In contrast to the reaction in the bulk at zero stress, solid InAs is not the sole reaction product, but rather a mixture of elemental and compound phases including liquid In and liquid InAs result. This view differs radically from the conventional view that solid InAs is the sole stable reaction product formed at the growing surface on a GaAs(001) substrate.
Supercooling of Peltier cooler using a current pulse
Authors: Snyder G. Jeffrey , Fleurial Jean-Pierre , Caillat Thierry , Yang Ronggui , Chen Gang
Published: July 2002

The operation of a Peltier cooler can be temporarily enhanced by utilizing the transient response of a current pulse. The performance of such a device, using (Bi,Sb)2Te3-based thermoelectric elements, was examined from −70 to 55 °C. We establish both theoretically and experimentally the essential parameters that describe the pulse cooling effect, such as the minimum temperature achieved, maximum temperature overshoot, time to reach minimum temperature, time while cooled, and time between pulses. Using simple theoretical and semiempirical relationships the dependence of these parameters on the current pulse amplitude, temperature, thermoelectric element length, thermoelectric figure of merit and thermal diffusivity is established. At large pulse amplitudes the amount of pulse supercooling is proportional to the maximum steady-state difference in temperature. This proportionality factor is about half that expected theoretically. This suggests that the thermoelectric figure of merit is the key materials parameter for pulse cooling. For this cooler, the practical optimum pulse amplitude was found to be about three times the optimum steady-state current. A pulse cooler was integrated into a small commercial thermoelectric three-stage cooler and it provided several degrees of additional cooling for a period long enough to operate a laser sensor. The improvement due to pulse cooling is about the equivalent of two additional stages in a multistage thermoelectric cooler.
Models of electron trapping and transport in polyethylene: Current–voltage characteristics
Authors: Anta J. A. , Marcelli G. , Meunier M. , Quirke N.
Published: July 2002

We present a unified method to estimate current–voltage characteristics of insulators starting from ab initio electronic calculations of the properties of the dielectric material. The method consists of three stages: (1) computation of trap energy distributions for excess electrons by means of density functional theory, (2) computation of local electron mobilities from a multiple trapping electron transport model which includes trap filling effects and (3) macroscopic integration of the Poisson and current–field equations, using local electron mobility data from stage (2) to predict the current–voltage characteristics for a material of a given width. The only input to this procedure is the chemical composition of the insulating material. We compare our model results with experimental studies of the current–voltage curve of cross-linked polyethylene.
Optical investigations of GaInNAs/GaAs multi-quantum wells with low nitrogen content
Authors: Sun H. D. , Hetterich M. , Dawson M. D. , Egorov A. Yu. , Bernklau D. , Riechert H.
Published: July 2002

The optical properties of GaInNAs/GaAs multi-quantum wells were investigated by photoluminescence excitation (PLE) spectroscopy, as well as by photoluminescence (PL), under various excitation intensities and at various temperatures. The PLE spectra demonstrated pronounced excitonic features and the corresponding transitions were identified. At low temperatures the PL spectra were sensitive to the excitation intensity. Under fixed excitation intensity, both the peak energy and the linewidth of photoluminescence showed anomalous temperature dependence, specifically an S-shaped temperature dependence of the peak energy and a N-shaped temperature dependence of linewidth in the PL spectra. The observed results are explained consistently in terms of the exciton localization effect due to the local fluctuations of nitrogen concentration.
Effect of vacancy and interstitial excess on the deactivation kinetics of As in Si
Authors: Solmi S. , Attari M. , Nobili D.
Published: July 2002

The effect of a point defect excess, vacancies, or, respectively, interstitials, on the deactivation kinetics of As in Si was verified on silicon on insulator (SOI) substrates uniformly doped at concentrations in the range 1.8–7×1020 cm−3. SOI samples can provide more accurate carrier density measurements, moreover the buried oxide layer can prevent recombination of excess vacancies with the interstitials near the projected range. A dose of 5×1015 cm−2 Si+ ions was implanted at 100 keV to obtain interstitials supersaturation (Iex), while the same Si+ dose, at an energy of 2 MeV, was used to generate an excess of vacancies (Vex). These specimens were isothermally heated at different temperatures and their annealing behavior was compared with the one of reference undamaged samples of the same composition, but without point defect excess. Our results indicate that neither the excess of interstitials nor the one of vacancies appreciably affect the deactivation rate. Once the implantation damage has annealed out, the samples with point defect excess are found in the same situation of the reference ones, and the subsequent deactivation kinetics results coincident.
Electron–intramolecular-phonon coupling and possible superconductivity in negatively charged coronene and corannulene
Authors: Kato Takashi , Yamabe Tokio
Published: July 2002

Electron–intramolecular-phonon coupling and its role in the occurrence of possible superconductivity in the monoanions of coronene (C24H12) and corannulene (C20H10) are studied. Electron–intramolecular-phonon coupling constants are calculated from the intramolecular vibronic coupling constants. The C–C stretching E2g mode of 1668 cm−1 plays an essential role in the electron–intramolecular-phonon coupling in the monoanion of coronene while the low frequency E2 modes, which have a radial character, and the C–C stretching mode of 1669 cm−1, which has a tangential character, afford large electron–intramolecular-phonon coupling constants in the monoanion of corannulene. The calculated total electron–intramolecular-phonon coupling constant for the monoanion of corannulene (0.269 eV) is much larger than that for coronene monoanion (0.076 eV). The intermediate characteristics between σ and π orbital interaction properties owing to the bowl-shaped structure of corannulene make orbital interactions between two neighboring carbon atoms strong, and thus electron–intramolecular-phonon coupling is stronger in the monoanion of corannulene than that in the monoanion of coronene. Possible superconducting transition temperature Tc value for the monoanion of corannulene is estimated to be an order of magnitude larger than that for the monoanion of coronene; for example, the calculated values of Tcs for the monoanions of coronene and corannulene are about 0.05–8.86 and 29.17–65.56 K, respectively, in the range of n(0)=4–5 and μ*=0.10–0.20, where n(0) is the density of states at the Fermi level (states per eV, per spin, and per molecule) and μ* is the Coulomb pseudopotential. The relationships between structures and Tcs in alkali-doped A3C60 complexes and graphite intercalation compounds (GICs) are also discussed from a viewpoint that corannulene can be viewed as a fragment of C60 and coronene as a fragment of one-sheet graphite.
Tunable terahertz generation using femtosecond pulse shaping
Authors: Sohn J. Y. , Ahn Y. H. , Park D. J. , Oh E. , Kim D. S.
Published: July 2002

Femtosecond pulse shaping and photomixing methods are combined to make a tunable terahertz source. Grating-pair pulse shaper selects two main frequency components to produce pulses that are modulated with a period which is inversely proportional to the frequency separation. The shaped pulse is then photomixed in semi-insulating GaAs under external bias. Terahertz (THz) emission frequency follows the modulation frequency of the excitation pulses, from 0.5 to 3 THz. Contrary to cw photomixing, this method can easily be combined with electro-optic and photoconductive sampling detection methods.
Quantum wells with atomically smooth interfaces
Authors: Yoshita Masahiro , Akiyama Hidefumi , Pfeiffer Loren N. , West Ken W.
Published: July 2002

By a cleaved-edge overgrowth method with molecular beam epitaxy and a (110) growth-interrupt anneal, we have fabricated a GaAs quantum well exactly 30 monolayers thick bounded by atomically smooth AlGaAs heterointerfaces without atomic roughness. Microphotoluminescence imaging of this quantum well indeed shows spatially uniform and spectrally sharp emission over areas of several tens of microns in extent. By adding a fractional GaAs monolayer to our quantum well we are able to study the details of the atomic step-edge kinetics responsible for flat interface formation.
Local hysteresis and grain size effect in Pb(Mg1/3Nb2/3)O3–PbTiO3 thin films
Authors: Shvartsman V. V. , Emelyanov A. Yu. , Kholkin A. L. , Safari A.
Published: July 2002

The local piezoelectric properties of relaxor ferroelectric films of solid solutions 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 were investigated by scanning force microscopy (SFM) in a piezoelectric contact mode. The piezoelectric hysteresis loops were acquired in the interior of grains of different sizes. A clear correlation between the values of the effective piezoelectric coefficients, deff, and the size of the respective grains is observed. Small grains exhibit slim piezoelectric hysteresis loops with low remanent deff, whereas relatively strong piezoelectric activity is characteristic of larger grains. Part of the grains (∼20–25%) is strongly polarized without application of a dc field. The nature of both phenomena is discussed in terms of the internal bias field and grain size effects on the dynamics of nanopolar clusters.
Precision polarization measurements of the 6s 2S1/2→6p 2Pj Rayleigh scattering spectrum in atomic Cs
Authors: Markhotok A. , Bayram S. B. , Sieradzan A. , Havey M. D.
Published: July 2002

A precision polarimeter has been developed to measure the linear polarization degree of Rayleigh scattered light. Application is made to measurement of the linear depolarization spectrum of atomic Cs in the vicinity of the 6s 2S1/2→6p 2Pj resonance transition. For measurements in an ∼220 cm−1 range around the 6s 2S1/2→6p 2P3/2 resonance transition, the ratio of line strengths for the two resonance transitions has been found to be 2.003(52), in excellent agreement with other methods.
Thermoelectric efficiency in graded indium-doped PbTe crystals
Authors: Dashevsky Z. , Shusterman S. , Dariel M. P. , Drabkin I.
Published: July 2002

High efficiency thermoelectric conversion is achieved by using materials with a maximum figure of merit Z=S2σ/k, where S is the Seebeck coefficient, σ and k, the electrical and thermal conductivities, respectively. High quality homogeneous thermoelectric materials, based on PbTe crystals, usually display an elevated value of Z over a narrow temperature range. A maximal value of figure of merit Z, as a function of electron density, is attained only for one specific location of the Fermi level, EF, with respect to the conduction band edge, EC. In order to maintain this optimal Z value, namely, maintain a constant location of the Fermi level, the electron density, which is determined by the dopant concentration, must increase with increasing temperature. We present a method for the generation of a dopant (indium) concentration profile in n-type PbTe crystals that gives rise to a constant location of the Fermi level, and hence, to an optimal value of Z over a wide temperature range. The resulting functionally graded material, based on PbTe〈In〉, displays a practically constant value of the Seebeck coefficient, over the 50–600 °C temperature range.
Constitutional supercooling and the growth of 200 nm Bi–Sb wire array composites
Authors: Huber T. E. , Onakoya O. , Ervin M. H.
Published: July 2002

Bi 1−x Sb x  (x≈0.1) alloy crystals present a high thermoelectric figure of merit at T≈100 K. The alloys are difficult to grow successfully and require special techniques to avoid constitutional supercooling and the resulting segregation. We present a study of the growth of 200 nm wire arrays of Bi–Sb by high-pressure injection of the melt of the alloy in a porous template. We find that the concentration of Sb in the nanowires is much lower than the composition of the starting alloy and discuss the results in terms of a model of constitutional supercooling in restricted geometries.
Electrical behavior of isolated multiwall carbon nanotubes characterized by scanning surface potential microscopy
Authors: Schujman S. B. , Vajtai R. , Biswas S. , Dewhirst B. , Schowalter L. J. , Ajayan P.
Published: July 2002

We measured the surface electric potential distribution on individual, electrically contacted and biased, multiwall carbon nanotubes using scanning surface potential microscopy. The voltage varies linearly along the nanotube and the voltage drop is directly proportional to the bias applied between electrodes. Its resistance decreases four times when a 4.5 V bias is applied between the nanotube and the substrate. Under these conditions, we were able to resolve the voltage drop along the nanotube and at the contacts, providing a unique way of measuring contact resistance, which is observed in this case to be on the order of 50 kΩ.
N
2
–
H
2
remote plasma nitridation for GaAs surface passivation
Authors: Losurdo Maria , Capezzuto P. , Bruno G. , Perna G. , Capozzi V.
Published: July 2002

A remote N2–H2 (a mixture of 97% N2–3% H2) rf plasma nitridation procedure has been developed to form a very thin (∼5Å) GaN layer successful in the electronic and chemical passivation of GaAs (100) surfaces. The interaction of the plasma with the GaAs surface has been controlled in situ and in real time by spectroscopic ellipsometry. The stability of the chemical and electronic passivation is demonstrated by the nonoxidation and by the nondecaying behavior of the photoluminescence efficiency of the GaAs passivated surface over months of air exposure.
Atomic oxygen surface loss coefficient measurements in a capacitive/inductive radio-frequency plasma
Authors: Gomez S. , Steen P. G. , Graham W. G.
Published: July 2002

Spatially resolved measurements of the atomic oxygen densities close to a sample surface in a dual mode (capacitive/inductive) rf plasma are used to measure the atomic oxygen surface loss coefficient β on stainless steel and aluminum substrates, silicon and silicon dioxide wafers, and on polypropylene samples. β is found to be particularly sensitive to the gas pressure for both operating modes. It is concluded that this is due to the effect of changing atom and ion flux to the surface.
Magnetoresistance decay and switching-field change in SiO2–Ni dusted Co/Cu/Co structures
Authors: Wang H. , Zhao Z. C. , Xia Y. X. , Jin Q. Y.
Published: July 2002

The effect of interfacial planar doping with an insulating granular layer in a sandwiched structure was studied. By inserting a thin SiO2–Ni layer at either the Co/Cu (top) or Cu/Co (bottom) interface in sandwiched Co/Cu/Co structures, the magnetoresistance curve as a function of the magnetic field changed significantly. This change was due to a reduction of interlayer coupling and to a change in the switching mechanism of the magnetizations caused by modification of the interface. In addition, the dusted Co/Cu/Co structures showed flat peaks and small switching fields that would be very useful for practical applications.
Bonding of NO to NiO(100) and NixMg1−xO(100) surfaces: A challenge for theory
Authors: Di Valentin Cristiana , Pacchioni Gianfranco , Bredow Thomas , Dominguez-Ariza David , Illas Francesc
Published: July 2002

The NO/NiO(100) system represents an excellent test case for the theory of surface chemical bond since accurate information about geometry, adsorption strength, and spin properties is available from experiments performed on NiO and Ni-doped MgO powders, single crystals, and thin films. We used cluster models to describe the NO/NiO interaction in combination with density functional theory (DFT) and wave function-based methods. We have identified four major aspects of the interaction: (1) the bonding cannot be described by a single determinant; (2) a spin-polarized DF-B3LYP approach gives reasonable adsorption properties at the price of a physically incorrect spin distribution; (3) a key ingredient of the interaction is the Coulomb repulsion within the Ni 3d shell; since this term is described very differently depending on the exchange-correlation functional it can result in overbound generalized gradient approach or Becke, Lee, Yang, and Parr or in strongly unbound (HFLYP) systems depending on the DFT approach; (4) the proper inclusion of the dynamical correlation is essential to treat the on-site Coulomb repulsion within the Ni 3d shell and to provide an accurate bond strength. In fact, the explicitly correlated complete-active-space second-order perturbation theory method gives results in overall agreement with the experiment. This shows the importance of treating on the same footing spin and electron correlation as well as the multiconfiguration character of the wave function.
Effects of film morphology and gate dielectric surface preparation on the electrical characteristics of organic-vapor-phase-deposited pentacene thin-film transistors
Authors: Shtein Max , Mapel Jonathan , Benziger Jay B. , Forrest Stephen R.
Published: July 2002

Organic vapor phase deposition was used to grow polycrystalline pentacene channel thin-film transistors. Substrate temperature, chamber pressure during film deposition, and growth rate were used to vary the crystalline grain size of pentacene films on O2-plasma treated SiO2 from 0.2 to 5 μm, leading to room-temperature saturation regime field-effect hole mobilities (μeff) from 0.05±0.02 to 0.5±0.1 cm2/V s, respectively. Surface treatment of SiO2 with octadecyltrichlorosilane (OTS) prior to pentacene deposition resulted in μeff⩽1.6 cm2/V s, and drain current on/off ratios of ⩽108 at room temperature, while dramatically reducing the average grain size. X-ray diffraction studies indicate that the OTS treatment decreases the order of the molecular stacks. This suggests an increased density of flat-lying molecules, accompanying the improvement of the hole mobility at the pentacene/OTS interface.
Experimental and theoretical study of the electronic structures of Ni3Al, Ni3Ga, Ni3In, and NiGa
Authors: Hsu Li-Shing , Wang Y.-K. , Guo G. Y.
Published: July 2002

The electronic structures of Ni3Al, Ni3Ga, Ni3In, and NiGa are studied by x-ray absorption near-edge spectra (XANES) at the Ni and Ga K edges. The XANES spectra are compared with those calculated with theory. The experimental XANES features for these compounds reflect the Ni- and Ga-p unoccupied density of states. The calculated magnetic moments for Ni3Al, Ni3Ga, and Ni3In are between 0.7–0.8 μB/cell. The number of 3d holes per Ni atom is calculated for Ni3Al, Ni3Ga, and Ni3In. These numbers show correlation with heats of formation of the bulk compounds.
Thermal conductivity of a GaAs single crystal grown in microgravity
Authors: Ivanov A. I. , Luk’yanov A. N. , Merisov B. A. , Sologubenko A. V. , Khadjai G. Ya.
Published: July 2002

A comparative study of the thermal conductivity in the temperature interval 2–300 K is carried out for single-crystal GaAs samples grown on Earth and grown under analogous conditions in microgravity on the manned space station Mir. It is found that the heat transfer in the samples is due to phonons. A consistent processing of the temperature dependence of the thermal conductivity of the Earth-grown and space-grown samples is carried out in the framework of the Debye model of the phonon spectrum with allowance for boundary and resonant scattering and for scattering on “planar defects” and phonon–phonon U-processes. The difference in the behavior of the thermal conductivity space-grown and Earth-grown samples is due to the presence of excess arsenic in the Earth-grown sample, resulting in both resonant scattering and scattering on planar defects, which may be clusters of arsenic atoms.
Density functional theory for inhomogeneous mixtures of polymeric fluids
Authors: Yu Yang-Xin , Wu Jianzhong
Published: July 2002

A new density functional theory is developed for inhomogeneous mixtures of polymeric fluids by combining Rosenfeld’s fundamental-measure theory for excluded volume effects with Wertheim’s first-order thermodynamic perturbation theory for chain connectivity. With no adjustable parameters, theoretical predictions are in excellent agreement with Monte Carlo simulation data for the density distributions and for the adsorption isotherms of hard-sphere chains near hard walls or in slit-like pores. This theory is applied to calculate the force between two parallel hard walls separated by hard-sphere chains at different densities. Calculated results indicate that the chain-mediated force is attractive and decays monotonically with separation at low chain densities, it oscillates at high chain densities and in between, it is attractive at small separation and repulsive at large separation. This new density functional theory is simpler than similar theories in the literature and is directly applicable to mixtures.
Hot electron tunneling in femtosecond laser-assisted scanning tunneling microscopy
Authors: Merschdorf M. , Pfeiffer W. , Thon A. , Gerber G.
Published: July 2002

The combination of scanning tunneling microscopy with femtosecond laser spectroscopy yields simultaneously ultimate spatial and temporal resolution. One possibility to realize this combination is the direct excitation of the tunnel junction in a pump–probe configuration and the detection of a tunnel current component that depends nonlinearly on the laser intensity. The laser-induced signal is expected to be very small, therefore a suitable sample material and a modulation technique is required. In measurements on a GaP(100) surface evidence for tunneling of hot electrons is obtained giving the possibility for local time-resolved tunneling spectroscopy.
Time-resolved studies of photoluminescence in GaNxP1−x alloys: Evidence for indirect-direct band gap crossover
Authors: Buyanova I. A. , Pozina G. , Bergman J. P. , Chen W. M. , Xin H. P. , Tu C. W.
Published: July 2002

Time resolved photoluminescence spectroscopy is employed to monitor the effect of N incorporation on the band structure of GaNP alloys. Abrupt shortening in radiative lifetime of near-band gap emissions, arising from excitonic radiative recombination within N-related centers, is found to occur at very low N compositions of around 0.5%, i.e., within the same range as the appearance of the direct-band gap-like transitions in the photomodulated transmission spectra of GaNP reported previously. The effect has been attributed to an enhancement in oscillator strength of optical transitions due to band crossover from indirect to direct-band gap of the alloy.
Highly coherent solid-state quantum bit from a pair of quantum dots
Authors: Li Xin-Qi , Yan YiJing
Published: July 2002

In this letter, we propose a scheme to buildup a highly coherent solid-state quantum bit (qubit) from two coupled quantum dots. Quantum information is stored in the state of the electron-hole pair with the electron and hole located in different dots, and universal quantum gates involving any pair of qubits are realized by effective coupling interaction via virtually exchanging cavity photons.
Effect of doping on photoluminescence upconversion in GaAs/AlxGa1−xAs heterostructures
Authors: Cheong Hyeonsik M. , Kim Doseok , Hanna Mark C. , Mascarenhas Angelo
Published: July 2002

We report on the effect of doping on the efficiency of the upconverted photoluminescence (UPL) from GaAs/AlGaAs heterostructures. The UPL intensity is enhanced when the AlGaAs layer is remotely doped with holes. It indicates that the rate-limiting process for the upconversion is the excitation of holes. Our result shows that one has to be careful in interpreting the excitation power dependence of the UPL intensity because dissimilar excitation efficiencies for electrons and holes and a possible background doping of the samples can significantly modify the excitation power dependence.
Growth kinetics and mechanisms of aluminum-oxide films formed by thermal oxidation of aluminum
Authors: Jeurgens L. P. H. , Sloof W. G. , Tichelaar F. D. , Mittemeijer E. J.
Published: July 2002

The growth kinetics and mechanisms of thin aluminum-oxide films formed by the dry, thermal oxidation of a bare Al(431) substrate at a partial oxygen pressure of 1.33×10−4 Pa in the temperature range of 373–773 K were studied using x-ray photoelectron spectroscopy. The initial oxidation of the bare Al substrate proceeds by an island-by-layer growth mechanism, involving the lateral diffusion over the bare Al substrate surface of mobile oxygen species. At low temperatures (T⩽573 K), an amorphous oxide film develops that attains a limiting (uniform) thickness. At high temperatures (T>573 K), growth is not impeded at a limiting thickness. Kinetic analysis established the occurrences of two different oxide-film growth regimes: an initial regime of very fast oxide-film growth and a second, much slower oxidation stage that is observed only at T>573 K. These results could be discussed in terms of electric-field controlled, interstitial, outward transport of Al cations through a close packing of O anions in the amorphous films, and inward diffusion of O along grain boundaries in the crystalline films, respectively. For the electric-field controlled Al cation motion, a value of 2.6 eV was determined for the rate-limiting energy barrier, which is located at the metal/oxide interface. This corresponds with a Mott potential of −1.6 V.
Large remanent polarization of Bi4Ti3O12-based thin films modified by the site engineering technique
Authors: Watanabe Takayuki , Kojima Takashi , Sakai Tomohiro , Funakubo Hiroshi , Osada Minoru , Noguchi Yuji , Miyayama Masaru
Published: July 2002

The site engineering approach based on site-selective substitutions was utilized to improve the ferroelectric properties in Bi4Ti3O12 thin film. Thin films of (Bi4−xNdx)(Ti3−yVy)O12 with both A- and B-sites substitutions were deposited on (111)Pt/Ti/SiO2/Si substrates at 600 °C by metalorganic chemical vapor deposition. Although the films substituted for the A site by Nd, (Bi4−xNdx)Ti3O12, showed no significant improvement in ferroelectricity, additional substitution for the B site by V contributed to a large ferroelectricity. Superior properties compared to (Bi4−xLax)(Ti3−yVy)O12 were confirmed for (Bi4−xNdx)(Ti3−yVy)O12 films. We also showed orientation control using Ru-based substrates. (104)-preferred orientation was obtained on (111)Pt/Ti/SiO2/Si substrates, while (110) and (111) orientations with an advantage for large polarization were stabilized on (001)Ru/SiO2/Si substrates and actually a larger ferroelectricity was obtained; the remanent polarization (2Pr) and coercive field (2Ec) of the (Bi3.35Nd0.65)(Ti2.87V0.13)O12 film were 34 μC/cm2 and 290 kV/cm, respectively. No fatigue phenomenon was also observed for the (Bi3.35Nd0.65)(Ti2.87V0.13)O12 film up to 109 switching cycles.
Intermediate states of superconducting thermometers for x-ray microcalorimeters
Authors: Pressler H. , Koike M. , Ohkubo M. , Fukuda D. , Noguchi Y. , Ohno M. , Takahashi H. , Nakazawa M.
Published: July 2002

Intermediate states near the normal–superconducting transition at ∼100 mK, important for a sensitive thermometer in x-ray calorimetry, have been studied by an imaging technique based on low temperature scanning synchrotron microscopy (LTSSM). In conjunction with iridium superconducting transition-edge sensors (TES) operating in electrothermal feedback mode, the LTSSM visualizations reveal that a normal–superconducting phase separation actually takes place in the intermediate states.
Strain induced stabilization of stepped Si and Ge surfaces near (001)
Authors: Shenoy V. B. , Ciobanu C. V. , Freund L. B.
Published: July 2002

We report on calculations of the formation energies of several [100] and [110] oriented step structures on biaxially stressed Si and Ge (001) surfaces. It is shown that a novel rebonded [100] oriented single-height step is strongly stabilized by compressive strain compared to most well-known step structures. We propose that the side walls of “hut”-shaped quantum dots observed in recent experiments on SiGe/Si films are made up of these steps. Our calculations provide an explanation for the nucleationless growth of shallow mounds, with steps along the [100] and [110] directions in low- and high-misfit films, respectively, and for the stability of the (105) facets under compressive strain.
Impurity photovoltaic effect: Fundamental energy conversion efficiency limits
Authors: Brown Andrew S. , Green Martin A.
Published: July 2002

In the past, minimal improvements have been predicted for efficiency enhancement of solar cells using the impurity photovoltaic (IPV) effect, where optical excitation through midgap defect levels allows the use of long wavelength photons to increase the conversion efficiency of sunlight to electricity. In the present work, the principle of detailed balance is used to calculate the limiting efficiency of solar cells with the inclusion of the impurity photovoltaic effect, in the idealized case when all transitions are assumed to be radiative. Based on these calculations, the limiting efficiency of the IPV device with a large number of different defect species is determined to be 77.2%. The terrestrial performance of the IPV device is also investigated by comparing its spectral sensitivity with that of tandem solar cell designs.
Metamorphosis of photonic bandgaps
Authors: Kim Jideog , Lee Sukhan
Published: July 2002

We found a nontrivial metamorphosis of photonic band gaps during interpolation between two photonic crystals with two different length scales. The interpolation is parametrized by a dielectric constant of the columns inserted at the positions that are appropriately chosen in each unit cell of the starting photonic crystal. For an instance of the metamorphosis, the first band gap that is present at the beginning gradually shortens and disappears during the interpolation, while the second band gap which is initially absent begins to appear and eventually becomes a first band gap. This mechanism of opening and closing of the photonic band gaps can be used when designing tunable photonic crystals.
Effect of electrical boundary conditions on ferroelectric domain structures in thin films
Authors: Li Y. L. , Hu S. Y. , Liu Z. K. , Chen L. Q.
Published: July 2002

The domain structures in a ferroelectric thin film are studied using a phase-field model. A cubic-to-tetragonal ferroelectric phase transition in lead titanate thin film is considered. Both elastic interactions and electrostatic interactions are taken into account. The focus is on the effect of electrical boundary conditions on the domain morphologies and volume fractions. It is shown that different electric boundary conditions may have a significant effect on the domain structures.
Integrating suspended quantum dot circuits for applications in nanomechanics
Authors: Kirschbaum J. , Höhberger E. M. , Blick R. H. , Wegscheider W. , Bichler M.
Published: July 2002

We present an integrated nanoelectromechanical circuit designed for achieving ultrasensitive displacement detection. It consists of a suspended quantum dot defined in the two-dimensional electron system of an AlGaAs/GaAs heterostructure and a mechanical resonator located in close vicinity. Operation of the individual components is demonstrated: Mechanical as well as transport properties of the resonator and the electron system are specified, respectively. Coulomb blockade in a freely suspended quantum dot is revealed. The data are used to estimate the maximum displacement sensitivity of the device to be 0.029 Å/Hz.
Optoelectronic characteristics of polymer light emitting diodes with poly(2-methoxy-5-(2′ethyl-hexoxy)-1,4-phenylene-vinylene) and hydrogenated amorphous silicon alloy heterointerfaces
Authors: Lin Cha-Shin , Yeh Rong-Hwei , Huang Chih-Ping , Hong Jyh-Wong
Published: July 2002

In order to investigate the feasibility of combining polymer and inorganic films for light-emitting diode (LED) fabrication, the inorganic p-amorphous-Si:H/n-amorphous-SiCGe:H layer was employed as hole/electron injection layer (HIL/EIL) in the poly(2-methoxy-5-(2′ ethylhexoxy)-1,4-phenylene-vinylene) polymer LEDs (PLEDs). In contrast to the PLED without any amorphous HIL/EIL, which had an electroluminescence (EL) threshold voltage (Vth) of 10 V and a brightness of 1231 cd/m2 at an injected current density (J)=0.6 A/cm2, the EL Vth could be reduced to 6.9 V for PLED with a 6 nm p-a-Si:H HIL only, also, the brightness could be enhanced to 6450 cd/m2 (at J=0.3 A/cm2 only) for PLED with both p-amorphous-Si:H HIL and n-a-SiCGe:H EIL.
Charge carrier mobility and photorefractive grating buildup in bipolar organic glasses
Authors: Sohn Jiwon , Hwang Jaehoon , Park Soo Young , Noh Yong-Young , Kim Jang-Joo
Published: July 2002

Two-component organic glasses consisting of photoconductive molecule, N′-[9-(2-ethylhexyl)-9H-carbazol-3-ylmethylene]-N,N-diphenyl-hydrazine (EHCzHy), and multifunctional photorefractive molecules, 9-(2-ethyl-hexyl)-3-[2-(4-methanesulfonyl-phenyl)-vinyl]-9H-carbazole (EHCS) [J. Sohn et al., Appl. Phys. Lett. 77, 1422 (2000)] and 9-(2-ethylhexyl)-3-[2-(4-nitro-phenyl)-vinyl]-9H-carbazole (EHCN), were prepared, and their charge carrier mobility and photorefractive grating buildup were investigated. EHCzHy is a unipolar hole-transporting (μh=4.16×10−6 cm2/V s) molecule with no electro-optic property. EHCS and EHCN are nonlinear optical molecules with unipolar (μh=2.42×10−6 cm2/V s) and bipolar (μh=2.30×10−6 cm2/V s,μe=2.86×10−6 cm2/V s) carrier mobilities, respectively. Different behaviors of the photorefractive grating buildup in two-component organic glasses were investigated as a function of EHCN or EHCS content.
Terahertz frequency Hall measurement by magneto-optical Kerr spectroscopy in InAs
Authors: Shimano R. , Ino Y. , Svirko Yu. P. , Kuwata-Gonokami M.
Published: July 2002

We report on the time-domain terahertz (THz) magneto-optical Kerr spectroscopy in the frequency range from 0.5 to 2.5 THz. The developed technique employs reflection geometry, enabling high-frequency noncontact Hall measurements in opaque samples. We also present a method to reveal the off-diagonal component of the complex dielectric tensor from the measured polarization-dependent THz wave forms. At a static magnetic field of 0.48 T, a large Kerr rotation over 10° originating from magnetoplasma resonance is observed in an n-type undoped InAs wafer at room temperature. This indicates the strong potential of this material for the polarization modulator in the THz regime.
Inversion behavior in Sc2O3/GaN gated diodes
Authors: Kim Jihyun , Mehandru R. , Luo B. , Ren F. , Gila B. P. , Onstine A. H. , Abernathy C. R. , Pearton S. J. , Irokawa Y.
Published: July 2002

The capacitance–voltage (C–V) characteristics of Sc2O3/p-GaN gate-controlled diodes show unusual hook shapes due to the charging of surface states. From the drain–voltage dependence of the C–V curves, the total surface state density was estimated to be ∼8.2×1012 cm−2 for diodes undergoing an implant activation anneal at 950 °C. The accumulation capacitance showed a significant dependence on measurement frequency and is suggested to result from the presence of an interfacial dielectric between the Sc2O3 and GaN. The Si-implanted n+ regions in the gated diode structure are effective in providing a source of inversion charge.