bluuebunny/crossref_metadata_2025 huggingface dataset:
Cathodoluminescence properties of blue-emitting SrGa2S4:Ce thin-films grown by low-temperature process
Authors: Tanaka Katsu , Okamoto Shinji , Kominami Hiroko , Nakanishi Yoichiro , Du Xiaolong , Yoshikawa Akihiko
Published: July 2002

The cathodoluminescence (CL) properties of Ce-activated strontium thiogallate (SrGa2S4:Ce) thin films have been investigated aiming towards phosphor screen application in an emissive display under low to medium accelerating voltage (<5 kV) excitation. The SrGa2S4:Ce thin films were grown by multisource deposition using a molecular beam epitaxy system. The growth temperature of the CL phosphor thin films was 472.5 °C, which is the lowest value ever achieved. Several related properties were observed, including the CL spectra, luminance, luminous efficacy, and surface morphology of the thin films. A luminance of 1000 cd/m2 and an efficacy of 1 lm/W were obtained at an accelerating voltage of 5 kV with a current density of 60 μA/cm2. The light-outcoupling efficiency (ηout) of the film was 0.0531. In spite of this small ηout of the films compared to that of powder phosphors, the luminance and efficacy were comparable to the conventional ZnS:Ag, Cl powder phosphor. The average grain size of the films was 0.14 μm×0.04 μm. The high luminance and luminous efficacy of the films with their submicron grain size is useful to realize high-resolution emissive displays.
Microstructure evolution and failure mechanism for Cu/Au Schottky contacts to InGaP layer
Authors: Liu Day-Shan , Lee Ching-Ting
Published: July 2002

We present the microstructure evolution and failure mechanism of Cu/Au Schottky contacts to the InGaP layer using x-ray diffractometry (XRD) and transmission electron microscopy (TEM). At annealing temperatures varying from 500 to 600 °C for 1 min, the diffraction peak of the CuP2 alloy can be found from XRD measurement. This indicates that the Cu is unstable, because it diffused into the InGaP layer and formed a CuP2 alloy at annealing temperatures higher than 500 °C. Furthermore, when the rapid thermal annealing temperature reached 500 °C, seen from a typical cross-sectional structure in the TEM measurements, we also found a sublayer in the originally grown InGaP layer. This sublayer was identified as a mixed structure of InGaP and CuP2, the thickness of which increased with a rise in annealing temperature. Moreover, the failure mechanism of Cu/Au Schottky contacts to the InGaP layer could be determined from the evolution of the thickness of the sublayer at an annealing temperature of 500 °C for various times. At the annealing temperature of 500 °C, the activation energy between the Cu and InGaP semiconductor was 0.93 eV and the diffusion constant was 1.37×103 nm2/s.
Fourier imaging study of efficient near-field optical coupling in solid immersion fluorescence microscopy
Authors: Yoshita Masahiro , Koyama Kazuko , Baba Motoyoshi , Akiyama Hidefumi
Published: July 2002

We measured images and Fourier images of fluorescence for 0.11- and 0.22-μm-diameter dye-doped polystyrene microsphere beads on a solid immersion lens, and experimentally verified strongly angle-dependent fluorescence intensities due to efficient near-field optical coupling in solid immersion fluorescence microscopy. The results are interpreted in comparison with calculated emission patterns of an emission dipole placed near a solid surface, which establish a basic model for high-collection efficiency in solid-immersion fluorescence microscopy.
Thermoluminescence study of stoichiometric LiNbO3 crystals
Authors: Chirila M. M. , Garces N. Y. , Halliburton L. E. , Evans D. R. , Basun S. A. , Meltzer R. S. , Yen W. M. , Rutkowski S. A. , Shumov D. , Cahill J. S.
Published: July 2002

Thermoluminescence (TL), optical absorption, and electron paramagnetic resonance (EPR) have been used to characterize point defects in Mg-doped stoichiometric LiNbO3. A broad TL emission, peaking at 440 nm, is observed near 94 K when these crystals are irradiated at 77 K and then rapidly warmed. X rays and below-band-gap lasers (325 and 355 nm) are equally effective in producing the TL peak. During excitation, holes are trapped on oxygen ions adjacent to lithium vacancies and electrons are trapped on niobium ions at regular lattice sites. These defects both have characteristic EPR spectra, and the trapped electron center has an optical absorption band peaking at 1200 nm. Upon warming, the electrons become thermally unstable near 94 K and migrate to the trapped-hole sites where radiative recombination occurs. We suggest that the near-edge charge-transfer absorption band from Fe3+ impurities, nominally present, provides the mechanism by which below-band-gap light initially produces the separately trapped holes and electrons.
Control of flow through porous media using polymer gels
Authors: Zitha Pacelli L. J. , Botermans C. Wouter , Hoek Jeroen v. d. , Vermolen Fred J.
Published: July 2002

We examine the effect of a dynamic stress on the reduction of flow in porous media using polymer gels formed in situ. To develop the theory for the response of the gel, we consider three dominant factors: (a) compressive (elastic) deformation of the gel and porous medium, (b) microscopic flow in this system, and (c) gel displacement. The latter occurs when the stress p is larger than a certain critical value pc, satisfying pcR2=constant (R=effective pore radius), where the constant is an increasing function of elastic modulus of the gel and its cross-linking energy. The expulsion of the gel above pc is reminiscent of growing Saffman-Taylor instabilities. To derive analytic expressions for the macroscopic saturation profiles we use the formalism for fully miscible two-phase flow. The equation of evolution of the pressure, established by mass balance arguments, was solved analytically. For p<pc, the pressure obeys an exponential saturation function while for p<pc, it first increases, reaches a maximum value, and then decreases towards an asymptotic value. These theoretical predictions are supported by our experiments consisting of injecting (salt) water at a constant flow rate in porous samples containing a organically cross-linked polymer gel (co-polymer of acrylamide and t-butyl-acrylate cross-linked using poly-ethylene-imine). The data confirms further that the product pcR2 is constant and prove that both pc and the maximum pressures increase with intrinsic gel strength.
Dynamics of a local algorithm for simulating Coulomb interactions
Authors: Maggs A. C.
Published: July 2002

Charged systems interacting via Coulomb forces can be efficiently simulated by introducing a local, diffusing degree of freedom for the electric field. This paper formulates the continuum electrodynamic equations corresponding to the algorithm and studies the spectrum of fluctuations when these equations are coupled to mobile charges. I compare the calculations with simulations of a charged lattice gas, and study the dynamics of charge and density fluctuations. The algorithm can be understood as a realization of a mechanical model of the ether.
Hyperfine coupling constants and electron-spin g-factors of B2+, Al2+, Ga2+, BAl+, BGa+, and AlGa+: An ab initio study
Authors: Bruna Pablo J. , Grein Friedrich
Published: July 2002

The hyperfine coupling constants (hfcc) and electron-spin g-factors (magnetic moments) calculated for B2+, Al2+, Ga2+, BAl+, BGa+, and AlGa+ are reported. The hfcc’s are obtained with single-reference configuration interaction, second-order Møller–Plesset, density functional (B3LYP, PW91PW91) methods, and 6-311+G(2df ) basis sets. The 2σg/3σ SOMOs of X 2Σg+(1σg21σu22σg)/X 2Σ+(1σ22σ23σ) mainly have a pσ–pσ composition, leading in most cases to similar values of Adip and Aiso. As a result, |A∥| is up two orders of magnitude larger than |A⊥|. The A⊥’s are slightly negative (ca. −10 MHz) for Al2+, Ga2+, and AlGa+. The g-shifts (Δg=g−ge) are evaluated with multireference CI wave functions, perturbation expansions up to second-order, and 6-311+G(2d) basis sets. Both Δg∥ and Δg⊥ are negative, but Δg∥ lies close to zero. The Δg⊥’s of B2+, Al2+, Ga2+ are about −1 300, −12 800, −97 300 ppm, respectively, while for BGa+, BAl+, AlGa+, they are much smaller (−800, −2 800, −47 400 ppm). The reduced Δg⊥’s for XY+ result from the mutual cancellation between a positive contribution from the 1 2Π(3σ→1π) state but a negative one from 2 2Π(3σ→2π). The positive contribution is at variance with the rule-of-thumb stating that SOMO→virtual MO excitations should contribute negatively. The variation of the hfcc’s with bond distance is analyzed for all systems, and that of the Δg⊥ component for B2+ and BAl+. Experimental or previous theoretical electron-spin resonance data are not available for comparison.
Laser ionization and radio frequency sustainment of high-pressure seeded plasmas
Authors: Kelly K. L. , Scharer J. E. , Paller E. S. , Ding G.
Published: July 2002

The feasibility of using a photoionized, low-ionization potential organic seed gas to initiate a high pressure plasma discharge is examined and compared to radio frequency breakdown of high pressure argon alone. The seed gas, tetrakis(dimethylamino)ethylene, which has an ionization potential of 6.1 eV is ionized by an ultraviolet laser through 6.4 eV photon absorption, and forms a plasma column inside a vacuum chamber. The plasma absorbs additional power through inductive coupling of 13.56 MHz helical antenna radio frequency wave fields to the plasma through electron acceleration, ionization, and collisional damping. Laser initiation of 2–6 mTorr of the seed gas in 1–150 Torr of argon is accomplished and produces steady-state line-average plasma densities of ne≈4×1012 cm−3 in a volume of 300 cm3. The two-body recombination coefficient of the organic seed gas and its optimum partial pressure when mixed with argon are experimentally determined and analyzed. Particle loss and power requirements for maintaining the discharge are evaluated by examining ionization, diffusion, and recombination processes.
Factors affecting an efficient sealing of porous low-k dielectrics by physical vapor deposition Ta(N) thin films
Authors: Iacopi F. , Tőkei Zs. , Le Q. T. , Shamiryan D. , Conard T. , Brijs B. , Kreissig U. , Van Hove M. , Maex K.
Published: July 2002

The deposition of homogeneous thin films on porous substrates has been investigated. The thin film deposition of Ta(N) by physical vapor deposition on porous films with different average pore sizes and material compositions has been studied. The continuity of Ta(N) films on top of porous low-k dielectrics is evaluated by means of ellipsometric porosimetry combined with sheet resistance and atomic force microscopy measurements. Interface reactions are analyzed by x-ray photoelectron spectroscopy profiling. It has been observed that the minimal Ta(N) thickness required to obtain a continuous metal layer on top of the porous film depends, on the one hand, on the porosity and pore size and, on the other hand, on the chemical interaction of the thin film with the porous substrate. The sealing of pores is favored by the presence of carbon in the dielectric matrix. This is explained through a mechanism of local enhancement of the degree of crosslinking in the dielectric matrix, catalyzed by Ta.
Vacuum level alignment in organic guest-host systems
Authors: Mäkinen A. J. , Hill I. G. , Kafafi Z. H.
Published: July 2002

Using a combination of ultraviolet and x-ray photoelectron spectroscopies, we have studied the relative energy level alignment of two phosphorescent guest molecules, 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine platinum (PtOEP) and tris(2-phenylpyridine)iridium (Ir(ppy)3), doped in an electron transport host, tris(8-hydroxyquinolinato) aluminum (III) (Alq3), and in a hole transport host, 4,4′-bis(carbazol-9-yl)biphenyl. In each of the guest–host systems, we find that the vacuum levels of the guest and the host molecules align, and that the position of the highest occupied molecular orbital of the phosphorescent guest is independent of the guest molecule concentration (0.8%–56% by mass). The vacuum level alignment implies free movement of the Fermi level within the gaps of the guest and the host molecules manifesting the intrinsic nature of these organic semiconductors. These results give insights into the role that the phosphorescent molecules play in trapping and transporting charges within the emissive layer of an organic light-emitting device.
Gold elliptical nanoantennas as probes for near field optical microscopy
Authors: Sqalli O. , Utke I. , Hoffmann P. , Marquis-Weible F.
Published: July 2002

We investigate the light scattering by individual nanometer-sized gold particles attached at the apex of fiber-based probes for near field optical microscopy. The dependence of the light scattering by the gold nanoparticle on the wavelength, the shape, and the surrounding medium dielectric profile are theoretically described and experimentally investigated, demonstrating that the tuning of the particle’s size and shape plays a crucial role in the light scattering process. In the case of gold spherical nanostructures, the plasmon resonance occurs at 540 nm in air, and 600 nm in water. A higher surrounding medium refraction index leads to a redshift of the plasmon resonance in the gold particle. Moreover, for elliptical structures, the orientation of the polarization of the incident field, as well as the relative ratio of the ellipse dimensions along its main axis, govern the position of the plasmon resonances. The light transmission spectrum for several probes where a single elliptical gold particle has been grafted exhibits at least one resonance in the range 600–650 nm. Surface plasmon resonances are of importance for the field enhancement at the end of the modified tip, thus leading to improvement of the achievable resolution for near field optical microscopy. An optimized design of the metallic nanostructure deposited at the apex of the near field probe is needed to maximize the resonance and would therefore lead to a high-performance probe for scanning near field optical microscopy.
Dynamics of dust particles in plasma sheaths
Authors: Resendes D. P. , Sorasio G. , Shukla P. K.
Published: July 2002

A large number of laboratory observations reveal the formation of ordered linear chains and sheets composed of charged dust grains levitated in the sheath region of radio-frequency or dc plasma discharges at very low pressures. At higher pressures, three-dimensional structures are typically formed. Recently, a number of low gas pressure experiments have explored the generation of large amplitude vertical oscillations by lowering either the background pressure or the plasma power below a threshold value, or by imposing an external force that arises from a low-frequency sinusoidal voltage in the sheath region. A theoretical model is presented that successfully describes the phenomenology in various experiments, including the observed self-excited oscillations, nonlinear resonance and parametric oscillations. A new forcing mechanism, namely the modulational forcing, for exciting nonlinear resonances is proposed.
Infrared study of anomalous volume behavior of water–benzene mixtures in the vicinity of the critical region
Authors: Furutaka Seiya , Ikawa Shun-ichi
Published: July 2002

Volume behavior of water–benzene mixtures at temperatures and pressures in the 473–623 K and 100–350 bar ranges, respectively, has been studied by infrared in situ measurements. The densities of the benzene-rich phase were estimated from the spectroscopically determined concentrations of water and benzene and compared with the average densities before mixing, which were calculated using literature densities of neat water and neat benzene at the same temperature and pressure. Anomalously large volume change for mixing has been found in the vicinity of the critical region of the water–benzene mixtures.
Reptation in the Rubinstein–Duke model: The influence of end-reptons dynamics
Authors: Carlon Enrico , Drzewiński Andrzej , van Leeuwen J. M. J.
Published: July 2002

We investigate the Rubinstein–Duke model for polymer reptation by means of density-matrix renormalization group techniques both in the absence and presence of a driving field. In the former case the renewal time τ and the diffusion coefficient D are calculated for chains up to N=150 reptons and their scaling behavior in N is analyzed. Both quantities scale as powers of N:τ∼Nz and D∼1/Nx with the asymptotic exponents z=3 and x=2, in agreement with the reptation theory. For an intermediate range of lengths, however, the data are well fitted by some effective exponents whose values are quite sensitive to the dynamics of the end reptons. We find 2.7<z<3.3 and 1.8<x<2.1 for the range of parameters considered and we suggest how to influence the end reptons dynamics in order to bring out such a behavior. At finite and not too small driving field, we observe the onset of the so-called band inversion phenomenon according to which long polymers migrate faster than shorter ones as opposed to the small field dynamics. For chains in the range of 20 reptons we present detailed shapes of the reptating chain as function of the driving field and the end-repton dynamics.
Energy dependence of diffractive and rotationally inelastic scattering of D2 from NiAl(110)
Authors: Farías D. , Miranda R. , Rieder K. H.
Published: July 2002

Diffraction and rotational transitions of D2 from NiAl(110) have been measured at incident energies between 88 and 157 meV. The measurements were done along the [11̄0] azimuth and using a set up which allows the recording of diffraction patterns at a fixed angle of incidence. Elastic first-order diffraction and rotationally inelastic diffraction peaks were observed over the whole incident energy range explored. The elastic diffraction data could be fitted using the simple Eikonal approximation only over a very narrow range of incident energies. Similarly, a Debye–Waller-type attenuation of diffraction intensities as a function of surface temperature was only verified at low incident energies (∼100 meV). The absolute 0→2 transition probability was found to increase from 10 to 20% in the energy range investigated, whereas the one corresponding to the 2→0 transition remained constant at 10%. An important conclusion of our work is that the behavior exhibited by these two transitions as a function of incident energy is independent of angle of incidence.
Energy- versus amplitude-corrected coupled-cluster approaches. III. Accurate computation of spectroscopic data exemplified on the HF molecule
Authors: Li Xiangzhu , Paldus Josef
Published: July 2002

The so-called energy-corrected coupled-cluster method with singles and doubles (CCSD), which is closely related to the recently proposed method of moments of Kowalski and Piecuch, accounts for the nondynamic correlation effects, which are missing in the standard single-reference CCSD, by projecting onto the modest-size multireference (MR) configuration-interaction SD (CISD) wave function when evaluating the energy, rather than onto the single-determinantal CCSD reference. The ability of this approach to generate reliable potential energy curves (PECs) or surfaces over a wide range of geometries is examined using a realistic ab initio model of the HF molecule. The PECs obtained with the correlation-consistent polarized valence double (triple, quadruple) zeta cc-pVXZ (T,Q) basis sets are used, together with the PEC extrapolated to the complete basis set limit, to compute the rotational and vibrational levels, which are then compared with the experimentally determined values as well as with the directly measured vibrational frequencies. A comparison is also made with the earlier results [X. Li, J. Mol. Struct.: THEOCHEM 547, 69 (2001)] that were obtained with the amplitude-corrected CCSD method, namely the so-called reduced MR CCSD that exploits the same MR CISD wave function, as well as with recently published renormalized CCSD(T) results [P. Piecuch et al., J. Chem. Phys. 115, 5796 (2001)]. It is shown that both the amplitude- and energy-corrected CCSD PECs may be used to generate highly accurate and reliable spectroscopic data once the basis set limit is achieved.
Ge/Si quantum-dot metal–oxide–semiconductor field-effect transistor
Authors: Yakimov A. I. , Dvurechenskii A. V. , Kirienko V. V. , Nikiforov A. I.
Published: July 2002

We report on the operation of Si metal–oxide–semiconductor field-effect transistor with an array of ∼103 10 nm diameter Ge self-assembled quantum dots embedded into the active channel. The drain current versus gate voltage characteristics show oscillations caused by Coulomb interaction of holes in the fourfold-degenerate excited state of the dots at T⩽200 K. A dot charging energy of ∼43 meV (i.e., >kT=26 meV at T=300 K) and disorder energy of ∼20 meV are determined from the oscillation period and the temperature dependence study of current maxima, respectively.
Do quasicrystals follow Wiedemann–Franz’s law?
Authors: Maciá Enrique
Published: July 2002

In this work we present a theoretical study on the thermal and electrical conductivities of quasicrystals. By considering a realistic model for the spectral conductivity we derive closed analytical expressions for the transport coefficients which allow us to study the temperature dependence of the Lorenz ratio L(T)=κe(T)/Tσ(T) at different temperature regimes. We conclude that quasicrystals closely follow Wiedemann–Franz’s law over a wide temperature range.
Measuring thin film and multilayer elastic constants by coupling in situ tensile testing with x-ray diffraction
Authors: Badawi K. F. , Villain P. , Goudeau Ph. , Renault P.-O.
Published: July 2002

A direct determination of the Young’s modulus and the Poisson’s ratio in a 140 nm polycrystalline tungsten thin film deposited by ion-beam sputtering on a polyimide substrate has been performed by coupling x-ray diffraction measurements with in situ tensile testing. The method described in this article to extract the Young’s modulus of thin films from the evolution of the sin2 ψ curves as a function of applied load only requires to know the substrate Young’s modulus. The determination of the thin film Poisson’s ratio can be realized without knowing any of the substrate elastic constants. In the case of the tungsten thin film, the obtained Young’s modulus was close to the bulk material one whereas the Poisson’s ratio was significantly larger than the bulk one.
An accurate semiclassical method to predict ground-state tunneling splittings
Authors: Tautermann Christofer S. , Voegele Andreas F. , Loerting Thomas , Liedl Klaus R.
Published: July 2002

A new method for calculating the ground-state tunneling splitting is presented. It is based on the semiclassical theory including recently derived corrections and it is the first method, which explicitly takes into account the whole conformational space between the minima and the transition state. The density-functional theory is used to determine the qualitative shape of the potential energy surface (PES) and high level ab initio calculations provide information about the stationary points. With a dual level scheme, the low-level energy surface is mapped onto the high-level points to get a good quantitative description of the high-level PES. Therefore, the new method requires no adjustment of additional parameters like scaling of the energy barrier as is necessary in other methods. Once the high-level PES is calculated, the most probable tunneling paths are determined with a global optimization procedure. Along this representative tunneling path, the tunneling splitting is calculated with additional consideration of zero-point vibrational effects. The method is applied to three molecular systems, namely hydrofluoric acid dimer, malonaldehyde, and tropolone. These systems were chosen because their energy barriers differ strongly (1 kcal/mol–7 kcal/mol). The predicted tunneling splittings agree very well with the experimental ones, therefore, we expect our method to be generally applicable, independent of the magnitude of the energy barrier.
Interplay between partial incoherence, partial inelasticity, resonance, and heterogeneity in long-range electron transfer and transport
Authors: Yang Yu Xiang , Yu Zhang Hou , Han Ping , Li Xin-Qi , Yan YiJing
Published: July 2002

A generalized scattering matrix formalism is constructed to elucidate the interplay of electron resonance, coherence, dephasing, inelastic scattering, and heterogeneity, which play important roles in the physics of long-range electron transfer/transport. The theory consists of an extension of the standard Büttiker phase-breaking model and an analytical expression of the electron transmission coefficient for donor–bridge–acceptor systems with arbitrary length and sequence. The theory incorporates the following features: Dephasing-assisted off-resonance enhancement, inelasticity-induced turnover, resonance enhancement and its dephasing-induced suppression, dephasing-induced smooth superexchange-hopping transition, and heterogeneity effects.
In situ photoluminescence investigation of doped Alq
Authors: Zhong G. Y. , He J. , Zhang S. T. , Xu Z. , Xiong Z. H. , Shi H. Z. , Ding X. M. , Huang W. , Hou X. Y.
Published: July 2002

We report the photoluminescence (PL) properties measured in situ from vacuum-deposited organic films of tris-(8-hydroxyquinoline) aluminum (Alq) doped with 4-(dicyanomethylene)-2-methyl-6(p-dimethylaminostyryl)-4H-pyran (DCM), where the red emission from the guest molecules is due to Förster energy transfer of excited state energy from host to guest. Both bare DCM-doped Alq (Alq:DCM) and bilayer Alq/Alq:DCM films have been studied, with the thickness of the Alq overlayer continuously varied in the latter case. The PL spectra from the bilayer structure contain no Alq contribution when its thickness is below 2.4 nm. Taking the value as the maximum distance for which the Alq exciton can travel in the film and still transfer its energy to a DCM molecule, the minimum DCM concentration in Alq:DCM necessary to produce red emission only can be estimated at 0.31 wt %. The most efficient red emission appears at the DCM concentration of about 1.7 wt %, at which more than 90% Alq-originated excitons are involved in the Förster energy transfer process. There exists a gradual red shift of the PL spectra with increasing DCM concentration, which can be attributed to the polarization and accompanying aggregation tendency of DCM molecules in Alq during the formation of the doped films.
Field electron emission from individual carbon nanotubes of a vertically aligned array
Authors: Semet V. , Binh Vu Thien , Vincent P. , Guillot D. , Teo K. B. K. , Chhowalla M. , Amaratunga G. A. J. , Milne W. I. , Legagneux P. , Pribat D.
Published: July 2002

Field electron emission behavior of individual multiwalled carbon nanotubes (MWNTs), that are elements of a vertically aligned array grown on a Si wafer, were analyzed with a scanning anode field emission microscope. The electron emission of each MWNT followed the conventional Fowler–Nordheim field emission mechanism after their apexes were freed from the erratic adsorption species using a conditioning process at room temperature. The conditioning process led to stable emission currents and reduced their variations ΔI/I to less than 30% between different MWNTs of the array. This opens the possibility for using MWNTs in an array as independent electron sources for massively parallel microguns.
Nucleation and growth of pores and photoluminescence in p-type porous silicon
Authors: Pascual A. , Fernández J. F. , Sánchez C. R.
Published: July 2002

The photoluminescence characteristics (peak intensity, peak wavelength and full width at half maximum of the emission band) of p-type porous silicon have been measured as a function of the etching time. Experimental data are discussed in the light of a pore nucleation and growth mechanism recently proposed by the authors. The steps of the formation of the porous layer are clearly apparent in the photoluminescence characteristic evolution.
Study of self-limiting oxidation of silicon nanoclusters by atomistic simulations
Authors: Dalla Torre J. , Bocquet J.-L. , Limoge Y. , Crocombette J.-P. , Adam E. , Martin G. , Baron T. , Rivallin P. , Mur P.
Published: July 2002

We present molecular dynamics simulations directed at understanding self-limiting oxidation of nanoclusters. Atomic oxygen is inserted in an atom-by-atom way in the silicon bonds to form silicon oxide. First, we focus on planar oxidation to calibrate our model and test its capabilities. Then, we present results on oxidation of 50 Å diam silicon spheres. Kinetic causes of self-limitation are investigated by drawing a map of the local stress in the Si/SiO2 system. We obtain stresses in contrast to in continuum models. For thin oxides, we find in particular tensile pressure in the silicon core and a pressure gradient in the oxide shell. We investigate the effect of pressure gradient on the O2 transport within the framework of Nerst–Eintein’s transport equation. We find that a pressure gradient compatible with experimental estimates yields self-limitation of the oxidation kinetics.
Sensitivity of a platinum-polyyne-based sensor to low relative humidity and chemical vapors
Authors: Caliendo Cinzia , Fratoddi Ilaria , Russo Maria Vittoria
Published: July 2002

The sensitivity to relative humidity (RH) variations in the range 0%–90% of a platinum polyyne, namely poly-[1,4-dihexadecyloxy-2,5-diethynylbenzene-bis(triphenylphosphine) platinum(II)] (Pt-P-HDOB) membrane was investigated by means of a surface acoustic wave (SAW) based sensor. A thin film of polymeric membrane was spin deposited on the free surface of the device and the resulting acoustic velocity and attenuation perturbations allowed the acoustic characterization of the membrane by means of the perturbation theory. The SAW sensor was able to reveal even very low (<10% RH) humidity conditions at room temperature, with high reproducibility, repeatability and stability.
Reliability of laser-activated low-temperature polycrystalline silicon thin-film transistors
Authors: Peng Du.-Zen , Chang Ting-Chang , Zan Hsiao-Wen , Huang Tiao-Yuan , Chang Chun-Yen , Liu Po-Tsun
Published: July 2002

In this letter, the characteristics and reliability of laser-activated polycrystalline silicon thin-film transistors (poly-Si TFTs) have been investigated by stressing the devices under Vds=12 V and Vgs=15 V. In comparison with traditional furnace-activated poly-Si TFTs, the leakage current is relatively large for laser-activated poly-Si TFTs. Further, while the degradation rates of threshold voltage and subthreshold swing are comparable to those of traditional furnace-activated TFTs, the post-stress leakage and on/off current ratio for laser-activated poly-Si TFTs degrade much faster than those of furnace-activated counterparts. The laser activation modifies the grain structure between the drain and the channel region, and causes grain discontinuity extending from the drain to the channel region. As a result, an inferior reliability with extra trap state density and larger leakage current was observed in the laser-activated poly-Si TFTs.
Quantum effects in the solid–liquid phase diagram of Ne13 and (para-H2)13
Authors: López Gustavo E.
Published: July 2002

Applying the Fourier path-integral formalism to the isothermal-isobaric ensemble, the melting transition for Ne13 and (para-H2)13 was characterized at various pressures. All multidimensional integrals were solved using the parallel tempering Monte Carlo algorithm. The volume of the system was defined with respect to the centroids of the quantum particles, and the maximum value of the constant pressure heat capacity at a given temperature was used to identify the melting temperature. Pressure versus temperature phase diagrams were constructed for these systems with and without the inclusion of quantum effects. It was observed that for neon the quantum contribution slightly decreases the melting temperature in the phase diagram, in particular at high pressure. For (para-H2)13, quantum effects considerably reduce the melting temperature at all pressures and pressure effects are more pronounced than in Ne13.
Above-band gap two-photon absorption and its influence on ultrafast carrier dynamics in ZnTe and CdTe
Authors: Schall M. , Jepsen P. Uhd
Published: July 2002

We present measurements of transient carrier dynamics subsequent to intense above-band gap femtosecond excitation of the II–VI compounds ZnTe and CdTe, probed in the far infrared by transient terahertz time-domain spectroscopy. The intensity and temperature dependence of the carrier dynamics illuminates the role of two-photon absorption (TPA) of pump photons. At cold temperatures and high excitation intensities TPA results in a photoexcited carrier distribution which requires several picoseconds to thermalize. As a consequence, the dielectric function of the carrier distribution deviates strongly from the Drude model for as long as 20 ps after excitation.
Leakage current of amorphous silicon p-i-n diodes made by ion shower doping
Authors: Kim Hee Joon , Cho Gyuseong , Choi Joonhoo , Jung Kwan-Wook
Published: July 2002

In this letter, we report the leakage current of amorphous silicon (a-Si:H) p-i-n photodiodes, of which the p layer is formed by ion shower doping. The ion shower doping technique has an advantage over plasma-enhanced chemical vapor deposition (PECVD) in the fabrication of a large-area amorphous silicon flat-panel detector. The leakage current of the ion shower diodes shows a better uniformity within a 30 cm×40 cm substrate than that of the PECVD diodes. However, it shows a higher leakage current of 2–3 pA/mm2 at −5 V. This high current originates from the high injection current at the p-i junction.
Test of response linearity for magnetic force microscopy data
Authors: Yongsunthon R. , Williams E. D. , McCoy J. , Pego R. , Stanishevsky A. , Rous P. J.
Published: July 2002

The utility of vertical propagation by the Green’s function to test response linearity has been explored for magnetic force microscopy (MFM) data from current-carrying wires, by comparing the measured signal at various tip heights to the corresponding propagated MFM signals. Application of a one-dimensional Green’s function was found to be sufficient to predict signal height variation for sample regions of high to moderate field symmetry. For regions of high field asymmetry, the two-dimensional Green’s function was required to obtain good prediction of the height variation. Agreement between the measured and propagated signals was generally within 5%, except at the tails where the signal is not well behaved. The quality of agreement deteriorates gradually with the size of the height propagation. The good agreement spanning a decade of tip and sample separation suggests that the MFM signal is not significantly affected by nonlinearities and can thus be interpreted in terms of classical electromagnetic relations governing current flow.
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An oxyfluoride phase with wide band gap and electrochromic properties
Authors: Mizoguchi Hiroshi , Orita Masahiro , Hirano Masahiro , Fujitsu Satoru , Takeuchi Tomonari , Hosono Hideo
Published: July 2002

It was demonstrated that the oxyfluoride NbO2F with an ReO3-type structure is a promising electrochromic material with wide band gap. Measurement of diffuse reflectance spectra revealed the optical band gap of NbO2F (∼3.1 eV) to be wider than that of WO3 (∼2.6 eV). It was rendered electron conductive by heating at 770 K in the presence of Pt powder in H2. Electrical conductivity for the blue-colored sintered sample of NbO2F obtained was 6×10−3 S cm−1 at 300 K, and a temperature dependence showed the semiconductive behaviors. A reversible electrochromism between pale-blue and deep-blue-color was found in H2SO4 and Na2SO4 aqueous solution.
Strain-electric field hysteresis measurement in a Pb0.65Ba0.35Nb2O6 thin film using optical reflection
Authors: Yang Xi , Wood Lowell T. , Miller John H. , Strikovski Mikhail
Published: July 2002

We present a simple method using direct laser beam reflection from the MgO substrate of a PBN:65 thin film to measure strain–electric field hysteresis produced by electric field-induced bending. We obtained both the strain–electric field and time–electric field relationships from these hysteresis curves. In addition, we determined time constants for strain saturation, obtained a quadratic relationship between strain and electric field, and calculated a value of 0.000 875 μm2/V2 for the electrostrictive constant.
Effect of metal–oxide–semiconductor processing on the surface roughness of strained Si/SiGe material
Authors: Olsen S. H. , O’Neill A. G. , Bull S. J. , Woods N. J. , Zhang J.
Published: July 2002

The impact of metal–oxide–semiconductor processing on strained Si/SiGe device structures has been examined. Material was grown by gas-source molecular beam epitaxy and ultra low pressure chemical vapor deposition, with different as-grown surface roughness. The effects of RCA cleaning, gate oxidation and rapid thermal annealing on this material were studied by atomic force microscopy (AFM) and optical profilometry. Certain processes caused reactions common to both material types, whereas others yielded dissimilar responses. Filtering AFM roughness data of specific wavelengths enabled the effects of processing on large-scale surface roughness dominated by the cross-hatching morphology and smaller scale microroughness to be investigated. The results suggest that as-grown Si/SiGe material quality is not a good indicator of processed device performance, rather morphological changes which occur during processing must be considered.
Microstructural and electrochromic properties of sputter-deposited Ni oxide films grown at different working pressures
Authors: Ahn Kwang-Soon , Nah Yoon-Chae , Sung Yung-Eun
Published: July 2002

The electrochromic properties of Ni oxide films grown at different working pressures (2.7, 5, 15, and 20 mTorr) by rf sputtering were investigated by means of in situ transmittance measurement and the resulting data were related to the crystallographic structure, surface morphology, and film density. At working pressures of over 5 mTorr, the sputter-deposited Ni oxide films crystallized gradually due to the plasma heating effect at a low growth rate. Although the Ni oxide film grown at 2.7 mTorr had the same amorphous crystallographic structure as the film grown at 5 mTorr, the former had a considerably more inhomogeneous surface and a much lower film density due to the rapid growth rate, and resulted in a defect-rich Ni oxide film. The electrochromic properties, such as the transient cycling period, coloration efficiency, and coloring/bleaching response times, were best for the sample grown at 5 mTorr and they are discussed in terms of defect-rich and crystalline Ni oxide films.
Self-assembled two-dimensional superlattice of Au–Ag alloy nanocrystals
Authors: He S. T. , Xie S. S. , Yao J. N. , Gao H. J. , Pang S. J.
Published: July 2002

The self-assembled two-dimensional superlattice of octanthiol-passivated Au–Ag alloy nanoparticles is reported in this letter. These nanoparticles show good composition-tunable properties. In the UV-visible spectra, the absorption peaks shifted linearly from the high-energy to the lower-energy side with increasing molar faction of Au.
Radial aspect of local dynamics in polybutadiene melts as studied by molecular dynamics simulation: To hop or not to hop
Authors: Kim Eung-Gun , Mattice Wayne L.
Published: July 2002

The radial local dynamics in poly(1,4-trans-butadiene) melts is studied using molecular dynamics simulations. In this work, the hopping peak is observed in the Van Hove space-time correlation function Gs(r,t) for the first time in polymeric systems. The hopping motion, observed only for methine hydrogen, is also identified in the mean-square displacement 〈Δr2(t)〉 and the intermediate scattering function Fs(k,t) through the relative dynamics of the hopping methine hydrogen and the nonhopping methylene hydrogen. The hopping motion is found to cause an unusual broadening of the dispersion width in the dynamic structure factor Sinc(k,ω). Active free volume is proposed in terms of Gs(r,t) at a short time, which offers a consistency to the relationship between free volume and local dynamics. Fast counterrotation at a pair of CH–CH2 bonds across a CH=CH bond is found responsible for the hopping peak in this polymer, and a new hopping criterion modified for polymeric liquids is proposed.
Exotic isomers of dicyanoacetylene: A density functional theory and ab initio study
Authors: Kołos Robert
Published: July 2002

Prospects for the existence and detection of yet unknown dicyanoacetylene (NCCCCN) isomers are discussed, based on quantum-chemical calculations for linear, hexagonal and branched C4N2 structural variants. It is concluded that apart from dicyanoacetylene itself and its two already discovered isomers, NCCCNC and CNCCNC, at least two other species are of importance: linear CCCNCN and Y-shaped CC(CN)CN (dicyanovinylidene). Combined CCSD(T) and MP4 calculations predict CC(CN)CN and CCCNCN to be 57 kcal/mol and 66 kcal/mol less stable than dicyanoacetylene, respectively. The height of the energy barrier for dicyanoacetylene←dicyanovinylidene isomerization is about 5 kcal/mol. Density functional theory calculations indicate that CCCNCN should give rise to prominent IR absorption bands, two orders of magnitude stronger than those of dicyanoacetylene.
Growth of Fe doped semi-insulating GaN by metalorganic chemical vapor deposition
Authors: Heikman Sten , Keller Stacia , DenBaars Steven P. , Mishra Umesh K.
Published: July 2002

Iron doped GaN layers were grown by metalorganic chemical vapor deposition (MOCVD) using ferrocene as the Fe precursor. Specular films with concentrations up to 1.7×1019 cm−3, as determined by secondary ion mass spectrometry, were grown. The Fe concentration in the film showed a linear dependence on the precursor partial pressure, and was insensitive to growth temperature, pressure, and ammonia partial pressure. Memory effects were observed, similar to Mg doping of GaN by MOCVD. The deep acceptor nature of Fe was used for growth of semi-insulating GaN films on sapphire substrates. A 2.6-μm-thick GaN film with a resistivity of 7×109 Ω/sq was attained when the first 0.3 μm of the film was Fe doped. X-ray diffraction rocking curves indicated high crystalline quality, very similar to an undoped film, showing that Fe doping did not affect the structural properties of the film. Fe doping allows for growth of semi-insulating GaN on sapphire without the high threading dislocation densities and/or high carbon levels that are normally necessary to achieve insulating films.
Electron spin-polarized alkaline-earth ions produced by multiphoton ionization
Authors: Nakajima Takashi , Yonekura Nobuaki
Published: July 2002

We propose two realistic schemes for producing electron spin-polarized alkaline-earth ions. One of the schemes is based on one-photon resonant two-photon ionization via fine structure manifolds of triplet states, while the other is based on one-photon resonant one-photon ionization. For both schemes we find that the ionization must take place from a triplet state to produce spin-polarized photoions. Photoions produced from a singlet state are always unpolarized regardless of the laser polarization and a magnetic sublevel from which ionization takes place. We carry out specific analysis for both schemes with Sr atoms, and find 85%–90% and 66% polarizations for the former and the latter schemes, respectively, if the wavelengths and polarizations of lasers are judiciously chosen. In particular, if all the produced photoions are in the ground s state, we find an exact one-to-one correspondence between the spin-polarization of photoions and ejected photoelectrons.
Space-charge-limited current in electron diodes under the influence of collisions
Authors: Akimov P. V. , Schamel H.
Published: July 2002

An analytical theory is given for the space-charge-limited (SCL) current of an electron diode in the presence of collisions. It extends the improved SCL current formula, which already takes into account the effect of nonzero electron injection velocities v0, by treating collisions within a simple model, with a friction parameter ν. The general current–voltage characteristic is obtained numerically. Within the Child–Langmuir limit v0→0, however, our model not only recovers analytically both the ballistic Child–Langmuir law (as ν≪1) and the nonballistic drift Mott–Gurney law (as ν≫1), but also provides SCL for any ν. Based on the present analysis, the nonreflective nature of SCL is emphasized. Extensive use is made of the Lagrangian description of the cold-fluid equations and from proper knowledge about the correct definition of the SCL.
Conduction processes in conjugated, highly regio-regular, high molecular mass, poly(3-hexylthiophene) thin-film transistors
Authors: Raja M. , Lloyd G. C. R. , Sedghi N. , Eccleston W. , Di Lucrezia R. , Higgins S. J.
Published: July 2002

Highly regio-regular poly(3-hexylthiophenes) (P3HT) thin films have been produced using the Tzrnadel method. They have head to tail counts approaching very close to 100% and high molecular mass. Thin-film transistors and Schottky diodes have been used to study the effects of counter ion and carrier density on field-effect and bulk mobility, respectively. The density of counter ions was increased using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and had a profound effect on both the field effect and bulk mobility with a ratio between the two of 102, in a similar way to DDQ in poly(β′-dodecyloxy-α,α′,-α′,α″terthienyl) (polyDOT3), but with substantially higher drift mobilities. A method is described, using Schottky barriers, of simply and accurately determining carrier drift mobility. The effect of carrier density on hole mobility is believed to be indirect, filling traps in the regions separating the highly ordered domains until trap free conduction and hence high mobility is reached.
Direct determination of grain sizes, lattice parameters, and mismatch of porous silicon
Authors: Martı́n-Palma R. J. , Pascual L. , Herrero P. , Martı́nez-Duart J. M.
Published: July 2002

High-resolution transmission electron microscopy and digital image processing were used to investigate in detail the structure of porous silicon (PS). It was found that PS is composed of rounded Si nanocrystals with characteristic sizes between 21 and 80 Å, embedded into an amorphous matrix and with no preferential orientation. We have determined that the size distribution of the nanocrystals can be fitted to a Gaussian distribution centered at 45.89 Å. Furthermore, the structure of the individual Si grains was studied, which allowed us to determine that the interplanar distance varies from 3.17 to 3.41 Å, with Gaussian distribution centered at 3.24 Å. Finally, the lattice parameter of the individual Si grains that compose PS was also measured, showing a Gaussian distribution centered at 5.61 Å. The origin of the structure of PS is also discussed.
Electronic and field emission properties of boron nitride/carbon nanotube superlattices
Authors: Meunier Vincent , Roland Christopher , Bernholc J. , Nardelli Marco Buongiorno
Published: July 2002

BN/C nanotube superlattices are quasi one-dimensional heterostructures that show unique physical properties derived from their peculiar geometry. Using state-of-the-art ab initio calculations, we show that BN/C systems can be used for effective band-offset nanodevice engineering, polarization-based devices, and robust field emitters with an efficiency enhanced by up to two orders of magnitude over carbon nanotube systems.
Synthesis and properties of Cu2O quantum particles
Authors: Borgohain Kavita , Murase Norio , Mahamuni Shailaja
Published: July 2002

Cuprous oxide quantum particles as small as 2 nm (comparable to the Bohr exciton radius) were synthesized using an electrochemical route. Quantum confinement effects are evident from a blueshift in the optical absorption. The optical absorption spectra of Cu2O nanoparticles of different sizes are discussed. Structural analysis by x-ray diffraction as well as electron diffraction shows the nanoparticles to be cubic and single phased Cu2O. X-ray photoelectron spectroscopic studies indicate the presence of CuO on the surface of Cu2O core nanoparticles.