bluuebunny/crossref_metadata_2025 huggingface dataset:
Vibrational spectroscopy of HOD in liquid D2O. VI. Intramolecular and intermolecular vibrational energy flow
Authors: Lawrence C. P. , Skinner J. L.
Published: July 2003

In a previous theoretical study [J. Chem. Phys. 117, 5827 (2002)] we calculated the vibrational lifetimes of the three fundamentals of HOD in liquid D2O. In that calculation the D2O solvent was treated as rigid, not allowing for the possibility of intermolecular vibrational energy transfer as a relaxation mechanism. In this paper we use both flexible and rigid solvent models, enabling us to include the possibility of intermolecular vibrational energy transfer, and also to estimate branching ratios for vibrational and nonvibrational relaxation channels. Our theoretical value for the lifetime of the OH stretch decreases modestly from 2.7 ps (in the original calculation) to 2.3 ps, which should be compared to the experimental value of about 1 ps. The lifetime of the OD stretch decreases dramatically from 18 ps to 390 fs due to resonant energy transfer to the solvent stretch. Our lifetime value for the bend actually increases from 220 to 380 fs, not because of the vibrational energy transfer channel, but rather because we find that Fermi’s Golden Rule (used in the original calculation) breaks down for this very fast process. We have calculated all the state-to-state rate constants for the low-lying vibrational energy levels of HOD, which allows us to construct and solve the vibrational master equation. We find that after excitation of the OH stretch, population flows into the HOD bend states (and to a lesser extent the OD stretch of HOD) on the time scale of 1 ps, in agreement with recent infrared pump/Raman probe measurements of Dlott and co-workers. From our results we estimate that for each quantum of OH stretch excitation, 0.26 quanta of solvent stretch is excited by direct intermolecular energy transfer, and yet we find, surprisingly, that there is almost no direct excitation of solvent bend. On the other hand, we suggest that because of the intramolecular Fermi resonance in D2O, rapid intramolecular vibrational relaxation occurs from solvent stretch to solvent bend. This would account for the experimental observation of solvent stretch and bend excitations on the time scale of 1 ps after the OH stretch is excited.
Pore size distribution, survival probability, and relaxation time in random and ordered arrays of fibers
Authors: Tomadakis Manolis M. , Robertson Teri J.
Published: July 2003

We present a random walk based investigation of the pore size probability distribution and its moments, the survival probability and mean survival time, and the principal relaxation time, for random and ordered arrays of cylindrical fibers of various orientation distributions. The dimensionless mean survival time, principal relaxation time, mean pore size, and mean square pore size are found to increase with porosity, remain practically independent of the directionality of random fiber beds, and attain lower values for ordered arrays. Wide pore size distributions are obtained for random fiber structures and relatively narrow for ordered square arrays, all in very good agreement with theoretically predicted limiting values. Analytical results derived for the pore size probability and its lower moments for square arrays of fibers practically coincide with the corresponding simulation results. Earlier variational bounds on the mean survival time and principal relaxation time are obeyed by our numerical results in all cases, and are found to be quite sharp up to very high porosities. Dimensionless groups representing the deviation of such bounds from our simulation results vary in practically the same range as the corresponding values reported earlier for beds of spherical particles. A universal scaling expression of the literature relating the mean survival time to the mean pore size [S. Torquato and C. L. Y. Yeong, J. Chem. Phys. 106, 8814 (1997)] agrees very well with our results for all types of fiber structures, thus validated for the first time for anisotropic porous media.
The thermal chemistry of saturated layers of acetylene and ethylene on Ni(100) studied by in situ synchrotron x-ray photoelectron spectroscopy
Authors: Neubauer R. , Whelan C. M. , Denecke R. , Steinrück H.-P.
Published: July 2003

Temperature-programmed x-ray photoelectron spectroscopy was used to study the thermal chemistry of acetylene (C2H2) and ethylene (C2H4) on Ni(100) in the temperature range 90–530 K. The use of a third generation synchrotron light source facilitated the measurement of high-resolution C 1s photoelectron spectra within a few seconds, approaching the ideal of real-time analysis. In a quantitative and quasi-continuous manner, the thermal dehydrogenation pathways are followed. For the acetylene decomposition, acetylide (CCH) and methylidyne (CH) are confirmed as intermediates. For the dehydrogenation of ethylene, a vinyl species (C2H3) is observed. Using the fingerprint capabilities of x-ray photoelectron spectroscopy, acetylene can be identified as the subsequent dehydrogenation product. Upon further heating, acetylide and methylidyne are successively formed on the surface, as in the decomposition experiment starting with acetylene adsorbed at 100 K. For both systems carbidic carbon is formed as the final dehydrogenation product, although with different transition temperatures. Species identification is based on observed vibrational fine structure data and correlation of core-level binding energies with previous literature.
Dynamic density functional theory for steady currents: Application to colloidal particles in narrow channels
Authors: Penna F. , Tarazona P.
Published: July 2003

We present the theoretical analysis of the steady state currents and density distributions of particles moving with Langevin dynamics, under the effects of periodic external potentials, with dependence on only one direction, which are displaced at a constant rate. The dynamic density functional (DDF) formalism is used to introduce the effects of the molecular interactions, from the equilibrium Helmholtz free energy density functional. We analyzed the generic form of the DDF for external potential with one-dimensional dependence, and discuss the limits of strong and weak potential barriers. The ideal gas case is solved in a closed form and compared with the numerical results for hard-rods in one dimension, using the exact equilibrium free energy. The results may be of relevance for microfluidic devices, with colloidal particles moving along narrow channels, if external driving forces have to compete with the Brownian fluctuations and the interaction forces of the particles.
Microelastic properties of minimally adhesive surfaces: A comparative study of RTV11™ and Intersleek elastomers™
Authors: Terán Arce Fernando , Avci Recep , Beech Iwona B. , Cooksey Keith E. , Wigglesworth-Cooksey Barbara
Published: July 2003

A comparative study of two commercially available elastomers, RTV11™ and Intersleek™, has been conducted employing atomic force microscopy (AFM) and surface and bulk analysis techniques. The results confirmed the presence of CaCO3 (rhombohedral calcite) filler particles in RTV11 and revealed TiO2 (tetragonal rutile) and Fe3O4 (cubic magnetite) as filler particles in Intersleek formulation. Time-of-flight secondary ion mass spectroscopy (TOF-SIMS) depth profiling revealed average threshold depths of ∼100 nm for the onset of filler particles. Indentation curves obtained from AFM force-volume imaging demonstrated that the presence of these particles significantly alters the local elastic properties of the coating. While the particle-free, high-compliance regions followed Hertzian behavior to a reasonable approximation, the low-compliance domains, where the subsurface filler particles reside, showed a significant deviation from this model. The Hertzian model, applied to the particle-free regions, gave values of 3.0, 1.9, and 1.4 MPa, respectively, for the elastic moduli of thin RTV11, thick RTV11, and Intersleek. For thin and thick RTV11 the presence of subsurface particles caused a local increase in the elastic moduli of the polymers, resulting in values 2.1 and 1.7 times higher than those for the particle-free regions of the corresponding polymers. For Intersleek, this increase was only 1.3 times. TOF-SIMS analysis did not reveal local differences in the chemical composition of the polymers, hence the inhomogeneities in the microelastic properties of these polymers are explained in terms of differences in the physical properties of these regions. Subsurface filler particles undergo substantial elastic displacements under the influence of the penetrating AFM tip.
Practical doping principles
Authors: Zunger Alex
Published: July 2003

Theoretical investigations of doping of several wide-gap materials suggest a number of rather general, practical “doping principles” that may help guide experimental strategies for overcoming doping bottlenecks.
Electrical characterization of redox-active molecular monolayers on SiO2 for memory applications
Authors: Li Qiliang , Surthi Shyam , Mathur Guru , Gowda Srivardhan , Misra Veena , Sorenson Thomas A. , Tenent Robert C. , Kuhr Werner G. , Tamaru Shun-ichi , Lindsey Jonathan S. , Liu Zhiming , Bocian David F.
Published: July 2003

Hybrid silicon capacitors have been successfully fabricated by attaching monolayers of redox-active molecules via self-assembly to ultrathin silicon dioxide layers. Capacitance, conductance, and cyclic voltammetric measurements have been used to characterize these capacitors. The presence of distinct capacitance and conductance peaks associated with oxidation and reduction of the monolayers at low gate voltages indicates discrete electron storage states for these capacitors, suggesting their feasibility in memory devices. The inherent molecular scalability and low-power operation coupled with existing silicon technology support the approach of hybrid molecule-silicon devices as a strong candidate for next generation electronic devices.
Different dynamic and static magnetic anisotropy in thin Permalloy™ films
Authors: Lopusnik R. , Nibarger J. P. , Silva T. J. , Celinski Z.
Published: July 2003

The values of uniaxial anisotropy Hk in thin polycrystalline Permalloy™ films measured by static and dynamic methods differ by as much as a factor of 1.5. The anisotropy obtained with a pulsed inductive microwave magnetometer in 2.5 to 100 nm thick Permalloy films exhibits an additional isotropic component of 120 to 240 A/m not observed in static measurements. The static value of anisotropy was obtained with an inductive magnetic hysteresis loop tracer. The time-resolved precessional response was measured as a function of in-plane applied magnetic bias field and the angle between the easy axis and that of the applied bias field. We interpret the constant-offset field as a transient component of the magnetic susceptibility that affects only dynamical response at time scales below 10 ns.
Origin of a pair of stacking faults in pseudomorphic ZnSe epitaxial layers on GaAs
Authors: Ohno Y. , Adachi N. , Takeda S.
Published: July 2003

We have revealed the origin of typical extended defects in semiconductor heterostructures with heterovalent interfaces, namely pairs of stacking faults in pseudomorphic ZnSe epitaxial layers grown on a GaAs(001) substrate. We have taken structural images of the defects by means of high-resolution transmission electron microscopy. Analyzing the images combined with first-principles energy calculations, we have clarified the atomistic structure of the extended defects: An intrinsic stacking fault on (111) intersects the other one on (111̄) on the interface between the epitaxial layer and the substrate, forming an intersecting line along [11̄0]. Around the intersecting line, we have found that Ga, As, and Se atoms form the specific reconstructed structure with relatively low formation energy, which corresponds well to a reconstructed surface structure on GaAs(001) with excess As and additional Se atoms. We have therefore attributed the stability of extended defects to the reconstructed surface structure on the substrate formed before epitaxial growth.
Investigation of V-Defects and embedded inclusions in InGaN/GaN multiple quantum wells grown by metalorganic chemical vapor deposition on (0001) sapphire
Authors: Florescu D. I. , Ting S. M. , Ramer J. C. , Lee D. S. , Merai V. N , Parkeh A. , Lu D. , Armour E. A. , Chernyak L.
Published: July 2003

We have examined the nature of V-defects and inclusions embedded within these defects by atomic force microscopy (AFM) and high-resolution scanning electron microscopy (SEM)/cathodoluminescence (CL) in InGaN/GaN multiple quantum wells (MQWs). To date, indium distribution nonuniformity in the well or GaN barrier growth temperature have been identified as the main factors responsible for the V-defect occurrence and propagation. Further complicating the matter, inclusions embedded within V-defects originating at the first InGaN-to-GaN interface have been observed under certain growth conditions. Our AFM and high-resolution SEM/CL findings provide evidence that some V-defects occur merely as direct results of barrier temperature growth, and that there are additional V-defects associated with In-rich regions, which act as sinks for further indium segregation during the MQW growth. Both types of V-defects have a tendency of promoting inclusions at low-temperature (800 °C) GaN barrier growth in an H2-free environment. Localized strain-energy variations associated with the apex of V-defects may be responsible for the inclusion occurrence. Adding H2 during the GaN barrier growth reduces V-defect formation and suppresses inclusion propagation entirely, rendering a uniform nanoscale CL signal.
Oxygen-stabilized glass formation in Zr80Pt20 melt-spun ribbons
Authors: Sordelet D. J. , Yang X. Y. , Rozhkova E. A. , Besser M. F. , Kramer M. J.
Published: July 2003

The as-quenched structure of Zr80Pt20 melt-spun ribbons containing measured oxygen contents ranging from 184 to 4737 ppm mass was studied. Ribbons containing less than 500 ppm mass oxygen are fully crystallized and consist predominantly of a metastable ordered β-Zr phase with significant solution of Pt. Increasing oxygen content to 1053 and 1547 ppm mass produces a transition to fully amorphous and to mixed amorphous and quasicrystalline structures, respectively. Samples containing 4737 ppm mass consist of quasicrystalline and crystalline phases in an amorphous matrix. The results from this study suggest a critical level of oxygen is required to promote glass formation in Zr80Pt20 melt-spun ribbons produced at a specific quench rate.
Optimization of broadband quantum cascade lasers for continuous wave operation
Authors: Soibel Alex , Gmachl Claire , Sivco Deborah L. , Peabody Milton L. , Sergent A. Michael , Cho Alfred Y. , Capasso Federico
Published: July 2003

We have optimized the design of the broadband quantum cascade laser for cw operation. The improved design leads to a gain ripple of only about 4 cm−1 over more than a 0.5-μm spectral range. Simultaneous cw emission at several wavelengths spanning the range between 6.7 and 7.4 μm has been achieved in a temperature interval from 20 to 77 K.
Two-dimensional terahertz photonic crystals fabricated by deep reactive ion etching in Si
Authors: Jukam Nathan , Sherwin Mark S.
Published: July 2003

Two-dimensional terahertz photonic crystals were manufactured from Si using deep reactive ion etching. Arrays of square holes with widths of 80 (100) μm and lattice constants of 100 (125) μm were etched through 500-μm-thick wafers with high resistivity. Stop bands with transmittance <1% and widths >200 GHz were observed near 1 THz for light with an electric field vector in the plane of the wafers (TE polarization). The observed stop bands are close to TE photonic band gaps predicted by a two-dimensional calculation.
Effects of electron correlation on the photocurrent in quantum dot infrared photodetectors
Authors: Chang Yia-Chung , Kuo David M.-T.
Published: July 2003

The effect of electron correlation on the photocurrent of self-assembled InAs/InGaAs quantum-dot infrared photodetector is studied. It is found that Coulomb interaction and level mixing in the many-body open system lead to double peaks associated with the intraband transitions involving two lowest levels of the quantum dot. Furthermore, the photocurrent displays a negative differential photoconductance due to Coulomb blockade.
Effect of annealing environment on the memory properties of thin oxides with embedded Si nanocrystals obtained by low-energy ion-beam synthesis
Authors: Normand P. , Kapetanakis E. , Dimitrakis P. , Tsoukalas D. , Beltsios K. , Cherkashin N. , Bonafos C. , Benassayag G. , Coffin H. , Claverie A. , Soncini V. , Agarwal A. , Ameen M.
Published: July 2003

The effect of annealing in diluted oxygen versus inert environment on the structural and electrical characteristics of thin silicon dioxide layers with embedded Si nanocrystals fabricated by very low-energy silicon implantation (1 keV) is reported. Annealing in diluted oxygen increases the thickness of the control oxide, improves the integrity of the oxide and narrows the size distribution of the nanocrystals without affecting significantly their mean size (∼2 nm). Strong charge storage effects at low gate voltages and enhanced charge retention times are observed through electrical measurements of metal-oxide-semiconductor capacitors. These results indicate that a combination of low-energy silicon implants and annealing in diluted oxygen allows for the fabrication of improved low-voltage nonvolatile memory devices.
Variation of magnetization and the Landé g factor with thickness in Ni–Fe films
Authors: Nibarger J. P. , Lopusnik R. , Celinski Z. , Silva T. J.
Published: July 2003

We have measured the Landé g factor, the effective magnetization Meff, the uniaxial anisotropy Hk, and the Gilbert damping parameter α, as a function of Permalloy film thickness from 2.5 to 50 nm. We used a pulsed inductive microwave magnetometer capable of generating dc bias fields of 35.2 kA/m (440 Oe). A significant decrease in g is observed with decreasing thickness below 10 nm. Also, Meff decreases with decreasing thickness consistent with a surface anisotropy constant of 0.196±0.025 mJ/m2. The decrease in g can arise from the orbital motion of the electrons at the interface not being quenched by the crystal field. We also compare our data to a model of an effective g factor suggesting that the decrease in g factor might also stem from the Ni–Fe interface with a Ta underlayer.
Domain wall motion induced by spin polarized currents in ferromagnetic ring structures
Authors: Kläui M. , Vaz C. A. F. , Bland J. A. C. , Wernsdorfer W. , Faini G. , Cambril E. , Heyderman L. J.
Published: July 2003

We present an experimental study of the influence of spin-polarized currents on the displacement of domain walls in submicrometer permalloy ring structures. Using magnetoresistance (MR) measurements with multiple nonmagnetic contacts, we can sense the displacement of a domain wall and, by injecting large dc current densities (1011 A/m2), we can increase or decrease the magnetic field needed to move a single domain wall, depending on the direction of the current with respect to the applied field direction. Using rings with and without notches and by measuring the MR with the magnetic field applied along different directions, we show that we can exclude the possibility that the dominating effect is a classical Oersted field. We conclude that our observations can be explained by a directional spin torque effect.
Wide-band-tunable photomixers using resonant laser-assisted field emission
Authors: Hagmann Mark J.
Published: July 2003

Simulations and experiments show a resonant interaction of tunneling electrons with a radiation field, and photomixing (optical heterodyning) in laser-assisted field emission can cause current oscillations from dc to 100 THz with this effect. Recent simulations by others are shown to be consistent with Fowler–Nordheim theory when the radiation is at low frequencies, and also confirm the existence of the resonance. The relationship of these simulations to photomixing is demonstrated, and estimates are given for the power of signals that could be obtained by this means.
Efficient field emission from ZnO nanoneedle arrays
Authors: Zhu Y. W. , Zhang H. Z. , Sun X. C. , Feng S. Q. , Xu J. , Zhao Q. , Xiang B. , Wang R. M. , Yu D. P.
Published: July 2003

Well-aligned arrays of ZnO nanoneedles were fabricated using a simple vapor phase growth. The diameters of the nanoneedle tips are as small as several nanometers, which is highly in favor of the field emission. Field-emission measurements using the nanoneedle arrays as cathode showed emission current density as high as 2.4 mA/cm2 under the field of 7 V/μm, and a very low turn-on field of 2.4 V/μm. Such a high emission current density is attributed to the high aspect ratio of the nanoneedles. The high emission current density, high stability, and low turn-on field make the ZnO nanoneedle arrays one of the promising candidates for field-emission displays.
Visible luminescence of porous amorphous Si1−xCx:H due to selective dissolution of silicon
Authors: Rerbal K. , Jomard F. , Chazalviel J.-N. , Ozanam F. , Solomon I.
Published: July 2003

Room-temperature photoluminescence of porous hydrogenated amorphous silicon-carbon alloys (a-Si1−xCx:H) has been studied for different carbon concentrations. Porous a-Si1−xCx:H luminesces at energies much higher than the bulk material. Secondary ion mass spectroscopy reveals that the carbon content is higher in the porous layer than in the starting material, which accounts for the blueshifting of the luminescence into the visible range.
High-mobility thin-film transistors based on aligned carbon nanotubes
Authors: Xiao Kai , Liu Yunqi , Hu Ping’an , Yu Gui , Wang Xianbao , Zhu Daoben
Published: July 2003

Thin-film transistors based on aligned carbon nanotubes (CNTs) were fabricated by directly growing highly ordered CNTs on silicon dioxide surface. The transistor shows a pronounced field effect. Electric transport through the aligned carbon nanotubes is dominated by the holes at room temperature. A hole mobility (μp) of the CNT thin-film transistor was estimated to be as high as 61.6 cm2/V s. Such a mobility is comparable to that of heavily doped n-Si and is larger than individual CNT field-effect transistor. Thus, it reveals a potential application of the aligned CNTs in electronics.
Charge injection/ejection and trapping in low-density polyethylene at low and medium fields
Authors: Neagu Eugen R. , Marat-Mendes Jose N.
Published: July 2003

Very long time dc measurements have been carried out to analyze charge injection/ejection and trapping in low-density polyethylene. The basic idea was to compare the total charge involved in the charging step and the total charge stored in the material. The equality of the two charges demonstrates that the charge carriers do not pass through the sample. The injected electrons are trapped very close to the cathode to form a negative layer. It is suggested that near the anode the field ionizes the impurity atoms and/or the neutral molecules associated with structural defects to produce positive ions.
Luminescent and structural properties of ZnO nanorods prepared under different conditions
Authors: Roy V. A. L. , Djurišić A. B. , Chan W. K. , Gao J. , Lui H. F. , Surya C.
Published: July 2003

The morphology and optical properties of ZnO nanostructures prepared by thermal evaporation of Zn under different conditions was investigated. ZnO nanostructures prepared in air, dry and humid argon flow, and dry and humid nitrogen flow were characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, and photoluminescence. Tetrapod nanorods were obtained for fabrication in air, while for fabrication in argon or nitrogen flow nanowires and tetrapod nanorods were obtained. Growth of nanowires from the end of the tetrapod nanorod was observed. Influence of the preparation conditions on the structure and the room-temperature photoluminescence is discussed.
High-performance polymer light-emitting diodes fabricated with a polymer hole injection layer
Authors: Gong X. , Moses D. , Heeger A. J. , Liu S. , Jen A. K.-Y.
Published: July 2003

Enhanced luminance (L) and luminous efficiency (LE) of polymer light-emitting diodes (PLEDs) is demonstrated using poly(bis(tetraphenyldiamino)biphenyl-perfluorocyclobutane), poly(BTPD-Si-PFCB), as the hole injection layer. The monomer is cast directly onto the indium-tin-oxide anode and thermally polymerized in situ. Hole injection from poly(BTPD-Si-PFCB) into the “super yellow” derivative of poly(phenylene vinylene) and into several blue-emitting polymers is comparable to or better than that from poly(ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSSA). With poly(BTPD-Si-PFCB) as the hole injecting layer into “super yellow,” the performance is identical to that obtained with PEDOT:PSSA (LE≈8 cd/A with little fall-off, even at L>104 cd/m2).
Anomalous phase transition in strained SrTiO3 thin films
Authors: He Feizhou , Wells B. O. , Shapiro S. M. , Zimmermann M. v. , Clark A. , Xi X. X.
Published: July 2003

We have studied the cubic-to-tetragonal phase transition in epitaxial SrTiO3 films under various biaxial strain conditions using synchrotron x-ray diffraction. Measuring the superlattice peak associated with TiO6 octahedra rotation in the low-temperature tetragonal phase indicates the presence of a phase transition whose critical temperature is a strong function of strain, with TC as much as 50 K above the corresponding bulk temperature. Surprisingly, the lattice constants evolve smoothly through the transition, with no indication of a phase change. This signals an important change in the nature of the phase transition due to the epitaxial strain and substrate clamping effect. The internal degrees of freedom (TiO6 rotations) evolve independently from the overall lattice shape.
Low-temperature growth of carbon nanotubes by plasma-enhanced chemical vapor deposition
Authors: Hofmann S. , Ducati C. , Robertson J. , Kleinsorge B.
Published: July 2003

Vertically aligned carbon nanotubes were grown at temperatures as low as 120 °C by plasma-enhanced chemical vapor deposition. A systematic study of the temperature dependence of the growth rate and the structure of the as-grown nanotubes is presented using a C2H2/NH3 system and nickel as the catalyst. The activation energy for the growth rate was found to be 0.23 eV, much less than for thermal chemical vapor deposition (1.2–1.5 eV). This suggests growth occurs by surface diffusion of carbon on nickel. The result could allow direct growth of nanotubes onto low-temperature substrates like plastics, and facilitate the integration in sensitive nanoelectronic devices.
pH response of GaN surfaces and its application for pH-sensitive field-effect transistors
Authors: Steinhoff G. , Hermann M. , Schaff W. J. , Eastman L. F. , Stutzmann M. , Eickhoff M.
Published: July 2003

The pH-sensitivity of GaN surfaces in electrolyte solutions has been determined. For this purpose, GaN field-effect transistors and AlGaN/GaN high-electron-mobility transistor (HEMT) structures were used to measure the response of nonmetallized GaN gate regions to changes of the H+-concentration in an ambient electrolyte. We found a linear response to changes in the pH between pH=2 and pH=12 for both as-deposited and thermally oxidized GaN surfaces. Both surfaces showed an almost Nernstian behavior with sensitivities of 57.3 mV/pH for GaN:Si/GaN:Mg and 56.0 mV/pH for GaN/AlGaN/GaN HEMT structures. This suggests that the native metal oxide on the III-nitride surface is responsible for pH-sensitivity. The investigated devices showed stable operation with a resolution better than 0.05 pH over the entire pH range.
Irradiance and temperature dependence of the charge carrier lifetimes in bulk Hg0.6Cd0.4Te
Authors: Guha Shekhar , Zakel Andrew , Blackshire James L. , Krishnamurthy Srinivasan
Published: July 2003

We have measured the temperature and irradiance-dependent charge carrier lifetime in bulk Hg0.6Cd0.4Te using an all-optical pump-probe technique. The carrier lifetime is shown to increase by 50% when the ambient sample temperature is increased from 80 to 300 K. At irradiance levels below 1 MW/cm2, the lifetime decreases sharply with increasing incident irradiance. However, for further increase in irradiance level, the lifetime values reach a saturation. A theoretical model is used to explain both temperature and irradiance dependence of the lifetime.
Controllable optical properties of Au/SiO2 nanocomposite induced by ultrasonic irradiation and thermal annealing
Authors: Fu Ganhua , Cai Weiping , Kan Caixia , Li Cuncheng , Zhang Lide
Published: July 2003

Nanostructured gold/monolithic mesoporous silica assembly was synthesized by a sonochemical method. The optical absorption maximum of the as-prepared nanocomposite varies from the visible to near-infrared region, depending on the reduction rate of AuCl4− ions in solution. There exists a critical reduction rate of AuCl4− ions in solution. When the reduction rate is lower than the critical value, the absorption maximum redshifts from 560 up to 1130 nm with increase of the reduction rate; otherwise, it blueshifts. The subsequent step thermal annealing of the as-prepared nanocomposite with a maximum in the near-infrared results in a blueshift in a large region down to 550 nm. We can thus realize control of the optical absorption in a large region by ultrasonic irradiation and subsequent step thermal annealing. Further experiments reveal that the position of the absorption maximum depends on the aggregate degree of Au nanoparticles supported in the surface layer of silica and that subsequent step annealing induces disaggregation of Au nanoparticles.
Electrical and optical proprieties of photodiodes based on ZnSe material
Authors: Bouhdada A. , Hanzaz M. , Vigué F. , Faurie J. P.
Published: July 2003

Based on the spectral response and I–V measurements, the physical mechanism responsible for electrical conduction and optical response have been suggested. The Schottky diode response remains quite flat for energies above the gap, an advantage in comparison with the p–i–n photodiode. The obtained result showed a high leakage resistance for Schottky photodiode explained by the presence of the defect at the metal/semiconductor interface.
Lead-zirconate-titanate-based piezoelectric micromachined switch
Authors: Gross S. J. , Tadigadapa S. , Jackson T. N. , Trolier-McKinstry S. , Zhang Q. Q.
Published: July 2003

A piezoelectric microelectromechanical switch actuated by lead zirconate titanate (PZT) is reported. A PZT unimorph cantilever actuator, fabricated on a sacrificial polysilicon layer and released using a xenon difluoride dry etch, was used to realize the switch. The PZT thin film was poled and driven with interdigitated electrodes to exploit the d33 coefficient for switching actuation. Preliminary dc and rf switching characteristics are reported. Measurements indicate a fast switching “on” time of <2 μs and “off” time of 2 μs at an actuation voltage of <40 V. An on/off isolation of >25 dB was achieved up to 100 MHz.
Local optical imaging of nanoholes using a single fluorescent rare-earth-doped glass particle as a probe
Authors: Aigouy L. , De Wilde Y. , Mortier M.
Published: July 2003

We have developed a local optical imaging technique that uses a fluorescent rare-earth-doped fluoride glass particle as a probe. This particle is glued at the end of an atomic force microscope tip and scanned over the surface of a nanostructured sample illuminated by a laser beam. The intensity of the laser-induced fluorescence of the particle is then recorded as a function of the position on the sample surface. This method has enabled us to image the light scattered by 250-nm large nanoholes made in a thin chromium film. The advantages of this material over other fluorescent probes is that it has a strong fluorescence when excited at 980 nm, it operates at room temperature, and does not present any evidence of photobleaching.
Controlling the native stacking fault density in II-VI/III-V heterostructures
Authors: Colli A. , Pelucchi E. , Franciosi A.
Published: July 2003

We performed a systematic study of the native stacking fault density in ZnSe/GaAs(001) heterostructures implemented using several of the interface fabrication procedures proposed in the literature. Only two of the procedures examined reproducibly lead to stacking fault densities below 104 cm−2. Despite the apparent differences, the two procedures were found to yield quantitatively similar defect densities, and qualitatively similar interface compositions and band alignments.
Memory and negative photoconductivity effects of Ge nanocrystals embedded in ZrO2/Al2O3 gate dielectrics
Authors: Wan Q. , Zhang N. L. , Liu W. L. , Lin C. L. , Wang T. H.
Published: July 2003

Metal–insulator–semiconductor (MIS) structures containing Ge nanocrystals embedded in high permittivity dielectrics (ZrO2/Al2O3) are fabricated by electron-beam evaporation method. Capacitance–voltage (C–V) and I–V characteristics of the fabricated MIS structures are investigated in the dark and under illumination. Charge storing and negative photoconductivity effects of the Ge nanocrystals are experimentally demonstrated by the hysteresis in the C–V curves and the decrease of the current under illumination at a given voltage, respectively.
Surface plasmon polariton modified emission of erbium in a metallodielectric grating
Authors: Kalkman J. , Strohhöfer C. , Gralak B. , Polman A.
Published: July 2003

The spectral shape and bandwidth of the emission of Er3+ ions in silica glass around 1.5 μm is strongly modified by the presence of a silver grating. The metallodielectric grating was made by a sequence of ion implantation in silica glass, dry etching, and silver sputter deposition. Spectral enhancements are observed that are attributed to near-field coupling of Er3+ ions to surface plasmon polaritons that subsequently reradiate at well-defined resonance conditions. Qualitative agreement is observed between these resonance conditions and calculations based on the surface plasmon polariton dispersion relation.
Generalized equation of phonon radiative transport
Authors: Prasher Ravi
Published: July 2003

We write the equation of phonon radiative transport (EPRT) and the Boltzmann transport equation by including an in-scattering term under a relaxation time approximation. The in-scattering term contains a phase function that is exactly analogous to the phase function in the equation of radiative transport (ERT), thereby making EPRT exactly same as ERT. This formulation of EPRT is called the generalized EPRT (GEPRT). GEPRT is totally consistent with EPRT and transport theory; however, GEPRT shows that the mean free path due to particle scattering depends on the acoustic thickness of the medium if scattering is anisotropic and asymmetrical for heat flow in the thickness direction of the medium, irrespective of the boundary condition. GEPRT can be solved using the techniques used for solving ERT.
Narrow linewidth intervalence-band emission from germanium terahertz lasers
Authors: Chamberlin D. R. , Bründermann E. , Haller E. E.
Published: July 2003

We demonstrate narrow linewidth emission from p-Ge terahertz lasers at low magnetic fields where intervalence-band emission dominates. The volume of these laser crystals is 32 mm3, which is one order of magnitude smaller than the lasers for which the emission spectrum has been measured previously. The frequencies emitted are shown to be selected by the optical cavity and do not shift with the magnetic field, as has been observed for larger laser crystals.
Properties of arsenic-doped p-type ZnO grown by hybrid beam deposition
Authors: Ryu Y. R. , Lee T. S. , White H. W.
Published: July 2003

As-doped ZnO (ZnO:As) films have been characterized. ZnO:As films show p-type characteristics determined by Hall-effect and photoluminescence (PL) measurements. The hole concentration can be increased up to the mid-1017-cm−3 range. The thermal binding energy of the As acceptor (EAth-b) is 120±10 meV, as derived from temperature-dependent Hall-effect measurements. The PL spectra reveal two different acceptor levels (EAopt-b), located at 115 and 164 meV, respectively, above the maximum of the ZnO valence band, and also show the binding energy of the exciton to the As-acceptor (EAXb) is about 12 meV. The values of the ratio EAXb/(EAth-b or EAopt-b) are located in the range from 0.07 to 0.11.
A turnstile electron-spin entangler in semiconductors
Authors: Sifel Claudia , Hohenester Ulrich
Published: July 2003

We propose a single-electron doped quantum dot in a field-effect structure as an optically triggered turnstile for spin-entangled electrons. A short laser pulse excites a charged exciton, whose quantum properties are transferred through tunneling and relaxation to the spin entanglement between electrons in the dot and contact. We identify the pertinent disentanglement mechanisms, and discuss experimental detection and possible application schemes.
Improved synthesis of (In,Ga)N/GaN multiple quantum wells by plasma-assisted molecular-beam epitaxy
Authors: Brandt Oliver , Sun Yue Jun , Schönherr Hans-Peter , Ploog Klaus H. , Waltereit Patrick , Lim Sung-Hwan , Speck James S.
Published: July 2003

We present a simple strategy that minimizes the impact of surface segregation of In during the growth of (In,Ga)N/GaN multiple quantum wells by plasma-assisted molecular-beam epitaxy and simultaneously results in abrupt interfaces. The two ingredients of this strategy are (i) the use of a higher substrate temperature than commonly employed, that is, well above the In desorption point and (ii) the use of a modulated stoichiometry, that is, N-rich during growth of the well and Ga-stable during growth of the barrier.
Dynamics of a metallic particle bouncing between alternating high voltage electrodes
Authors: Park Kijun , Goo Sun-Geun , Yoon Jin-Yul , Kye Won-Ho , Kwon Tae-Yoon , Rim Sunghwan , Kim Chil-Min , Park Young-Jai
Published: July 2003

We study the dynamics of the motion of a metallic particle bouncing between alternating high-voltage electrodes in a compressed SF6 gas coaxial insulator. Through the study of the bifurcation diagram and attractors, we find that the dynamics of a metallic particle develops from a stable periodic motion to chaos as the strength of the electric field increases. Based on these results, we show numerically that the chaotic motion of the particle can be stabilized by using a chaos control method.
Blue and ultraviolet cathodoluminescence from Mn-doped epitaxial ZnO thin films
Authors: Jin Zheng-Wu , Yoo Y.-Z. , Sekiguchi T. , Chikyow T. , Ofuchi H. , Fujioka H. , Oshima M. , Koinuma H.
Published: July 2003

Combinatorial laser molecular-beam epitaxy method was employed to fabricate epitaxial Zn1−xMnxO thin films in a high throughput fashion. Local structures around Mn were investigated for these c-axis-oriented epitaxial films by fluorescence x-ray absorption fine structure measurements. It was shown that Mn substitutionally replaces Zn in Zn1−xMnxO (x<0.22) films. Well-structured blue and ultraviolet cathodoluminescence peaks corresponding to the intra-d-shell transitions of Mn2+ were observed, especially for smaller x. The luminescence is quenched rapidly as x is increased. By comparing the relative absorption strength per mole Mn2+ with the statistical probability of isolated Mn2+, it was concluded that the quick decrement of isolated Mn2+ with increasing x is responsible for the severe suppression of the blue and ultraviolet luminescence.
Surface treatment effects on the thickness dependence of the remanent polarization of PbZr0.52Ti0.48O3 capacitors
Authors: Rodrı́guez Contreras J. , Kohlstedt H. , Poppe U. , Waser R. , Buchal Ch.
Published: July 2003

In this letter, we report on the thickness dependence of the remanent polarization of Pt/PbZr0.52Ti0.48O3/SrRuO3 capacitors. Two different patterning techniques were used to fabricate the capacitors. For lift-off processed capacitors, the remanent polarization decreased with decreasing thickness. Ion-beam-etched capacitors, however, showed a constant remanent polarization for all PbZr0.52Ti0.48O3 film thicknesses down to 23 nm. Remarkably, this constant remanent polarization for ion-beam-etched capacitors corresponds to the spontaneous polarization expected for a stress-free bulk PbZr0.52Ti0.48O3 crystal.
Effect of implantation temperature on dislocation loop formation and origin of 1.55-μm photoluminescence from ion-beam-synthesized FeSi2 precipitates in silicon
Authors: Gao Y. , Wong S. P. , Cheung W. Y. , Shao G. , Homewood K. P.
Published: July 2003

Iron implantation into Si using a metal vapor vacuum arc ion source has been performed at various temperatures to synthesize nanometer scale β-FeSi2 precipitates in Si. Transmission electron microscopy (TEM) results showed that for high-temperature-implanted samples, there were a large number of dislocation loops formed. For intermediate-temperature-implanted samples, only a row of dislocation loops located at the end of implantation range was observed. For low-temperature-implanted samples, however, no dislocation loops were observed at all. From the differences in the photoluminescence spectra, in conjunction with the TEM results, the origins of the photoluminescence peaks in different samples could be distinguished and identified to be from β-FeSi2 precipitates or from crystal defects in the samples.
Large nonlinear magnetoimpedance in amorphous Co80.89Fe4.38Si8.69B1.52Nb4.52 fibers
Authors: Duque J. G. S. , de Araujo A. E. P. , Knobel M. , Yelon A. , Ciureanu P.
Published: July 2003

Fourier analysis of the nonlinear response of the impedance signal of amorphous Co80.89Fe4.38Si8.69B1.52Nb4.52 fibers is presented. The harmonic components of the voltage signal present a strong asymmetry. A strong variation (around 1400%) of the second-harmonic signal (V2 f) as a function of the applied magnetic field is observed. Nonlinear effects and the asymmetric behavior in harmonics can be associated with the anisotropies induced during the fabrication process. These results can lead to improvement of the performance of future magnetoimpedance sensors.
Standing-wave interferometer
Authors: Stiebig H. , Büchner H. , Bunte E. , Mandryka V. , Knipp D. , Jäger G.
Published: July 2003

An interferometric position sensor was developed using the concept of sampling a standing wave. Interference of a standing wave created in front of a plane mirror can be detected by thin, partly transparent sensors based on amorphous silicon. The optical thickness of the absorption layer is thinner than the wavelength λ of the incident light. Detection of minima and maxima of the standing wave can be used to determine the relative displacement of the plane mirror and the detector. For determination of bidirectional fringe counting, two detectors with a certain phase shift were introduced into the standing wave. An integrated solution of two stacked n-i-p diodes and a phase shifter will be presented. The operation principle of the device will be demonstrated by measured Lissajous figures.
Model of transport nonuniversality in thick-film resistors
Authors: Grimaldi C. , Maeder T. , Ryser P. , Strässler S.
Published: July 2003

We propose a model of transport in thick-film resistors which naturally explains the observed nonuniversal values of the conductance exponent t extracted in the vicinity of the percolation transition. Essential ingredients of the model are the segregated microstructure typical of thick-film resistors and tunneling between the conducting grains. Nonuniversality sets in as a consequence of wide distribution of interparticle tunneling distances.
Critical current density and resistivity of MgB2 films
Authors: Rowell J. M. , Xu S. Y. , Zeng X. H. , Pogrebnyakov A. V. , Li Qi , Xi X. X. , Redwing J. M. , Tian W. , Pan Xiaoqing
Published: July 2003

The high resistivity of many bulk and film samples of MgB2 is most readily explained by the suggestion that only a fraction of the cross-sectional area of the samples is effectively carrying current. Hence, the supercurrent (Jc) in such samples will be limited by the same area factor, arising for example from porosity or from insulating oxides present at the grain boundaries. We suggest that a correlation should exist, Jc∝1/Δρ300–50 K, where Δρ300–50 K is the change in the apparent resistivity from 300 to 50 K. We report measurements of ρ(T) and Jc for a number of films made by hybrid physical-chemical vapor deposition which demonstrate this correlation, although the “reduced effective area” argument alone is not sufficient. We suggest that this argument can also apply to many polycrystalline bulk and wire samples of MgB2.
Photo-oxidation-enhanced coupling in densely packed CdSe quantum-dot films
Authors: Wang Xiaoyong , Zhang Jiayu , Nazzal Amjad , Xiao Min
Published: July 2003

Photo-oxidation of densely packed monolayer of CdSe quantum dots (QDs) was studied by time-resolved photoluminescence (PL) spectroscopy. Electrons yielded in QDs by the strong laser-pulse irradiation can assist the oxidation of CdSe QDs. Such rapid photo-oxidation does not introduce more nonradiative defects, instead, it enhances the coupling between QDs through surface modification and the PL intensity can be significantly improved.
Luminescence decay in highly excited GaN grown by hydride vapor-phase epitaxy
Authors: Juršėnas S. , Miasojedovas S. , Kurilčik G. , Žukauskas A. , Hageman P. R.
Published: July 2003

Carrier recombination dynamics in GaN grown by hydride vapor-phase epitaxy has been studied by means of transient photoluminescence under high photoexcitation conditions that are close to stimulated emission regime. The luminescence transient featured an exponential decay with the time constant of 205 ps at room temperature. The transient was shown to be in good agreement with a model of saturated centers of nonradiative recombination with the trap density of ∼1017 cm−3 and carrier recombination coefficients of ∼10−8 cm3/s. In such a regime, the lifetimes of electrons and holes have a common value of 410 ps.
Ultrahigh-speed scanning near-field optical microscopy capable of over 100 frames per second
Authors: Humphris Andrew D. L. , Hobbs Jamie K. , Miles Mervyn J.
Published: July 2003

Scanning near-field optical microscopy is a powerful technique offering subdiffraction-limit optical resolution. However, the range of applications is limited by slow image acquisition rates. In this letter we demonstrate an implementation of a near-field optical microscope capable of scan speeds of 150 mm/s producing images of an area 20 μm2 in less than 10 ms, i.e., over 100 frames/s. To achieve this, a method of measuring the optical near-field intensity with a high bandwidth of greater than 1 MHz has been developed. A second original aspect is that the scan system uses a mechanical resonance of the probe to address the sample. The presented microscope is over 1000 times faster than a conventional scanning near-field optical microscope and ∼10 times faster than any scanning probe microscope to date.
Red emission from Eu-doped GaN luminescent films grown by metalorganic chemical vapor deposition
Authors: Pan M. , Steckl A. J.
Published: July 2003

Bright red emission has been obtained at room temperature from Eu-doped GaN films pumped by 325 nm HeCd laser. The luminescent films were grown by metalorganic chemical vapor deposition on GaN/Al2O3 substrates. Trimethylgallium (TMGa), ammonia (NH3), and europium 2,2,4,4-tetramethyl-3,5-heptanedionate were used as sources for Ga, N, and Eu dopant, respectively. The influence of the V/III ratio during growth on the photoluminescence (PL) intensity has been studied using a fixed TMGa flow rate of 92 μmol/min and varying the NH3 flow rate. The film growth rate (∼2 μm/h) is nearly constant with V/III ratio over the range from ∼30 to ∼1000. The Eu incorporation in GaN films was found to decrease with increasing V/III ratio. The Eu PL intensity (normalized to the Eu concentration) exhibited a maximum at a V/III ratio of ∼100.