bluuebunny/crossref_metadata_2025 huggingface dataset:
Gas density measurements with heavy ion beams
Authors: Neff S. , Tauschwitz A. , Niemann C. , Penache D. , Hoffmann D. H. H. , Yu S. S. , Sharp W. M.
Published: March 2003

High current discharge channels are a promising option for the final stage of beam transport in a heavy ion fusion reactor. The creation and the properties of the channel are studied at the Gesellschaft fuer Schwerionenforschung accelerator facility. This article discusses a diagnostic technique to determine the neutral gas density channel created by a laser in a neutral gas. The concept is to measure the spreading of an ion beamlet due to collisional scattering with the gas, and thereby deduce the average line-integrated gas density. The results show a density reduction near the axis, which compares favorably with one-dimensional code calculations.
Photoinduced transformations at semiconductor/metal interfaces: X-ray absorption studies of titania/gold films
Authors: Lahiri Debdutta , Subramanian Vaidyanathan , Shibata Tomohiro , Wolf Eduardo E. , Bunker B. A. , Kamat Prashant V.
Published: March 2003

We have measured x-ray absorption fine structure of pre- and post-ultraviolet(UV)-irradiated gold (Au) deposited- titania (TiO2) nanocomposites in order to study the effect of UV irradiation on the charge state and local structures around Au in TiO2. Our results indicate a positive oxidation state of Au in TiO2 following UV irradiation and, in addition, a remarkable change is observed in the local environment between these samples as an effect of UV irradiation. The environment around Au, which is comprised mostly of Ti and O atoms prior to UV illumination, is seen to form Au clusters following UV illumination. The photoinduced chemical transformation established in this study demonstrates the nature of semiconductor/metal interface under UV irradiation, and its role in dictating long-term photoelectrochemical performance of nanocomposite photocatalysts.
Magnetic force microscopy of skew angle dependencies in perpendicular magnetic recording
Authors: Svedberg E. B. , Litvinov D. , Gustafson R. , Chang C. H. , Khizroev S.
Published: March 2003

Magnetic force microscopy measurements of perpendicular media with a CoCr-based hard layer and a CoFe soft underlayer has been made to investigate skew angle effects. The recorded tracks were produced using a focused ion beam made single pole head with a track width of approximately 400 nm and a gap thickness of approximately 1 μm at a 25 nm flying height. Magnetic force microscopy images of tracks recorded at write currents above and below the saturation value of approximately 100 mA turn were studied at different values of the linear density, 20–100 kfci, at zero skew angle as well as −15° and +15°. Calculated perpendicular recording fields versus the distance down the track, and across the track, near the trailing pole edge was performed using three-dimensional boundary element modeling and compared to the results from the microscopy data. Drawbacks and possible solutions to the skew angle problem for perpendicular recording are discussed.
Internal structure and expansion dynamics of laser ablation plumes into ambient gases
Authors: Harilal S. S. , Bindhu C. V. , Tillack M. S. , Najmabadi F. , Gaeris A. C.
Published: March 2003

The effect of ambient gas on the expansion dynamics of the plasma generated by laser ablation of an aluminum target has been investigated using frequency doubled radiation from a Q-switched Nd:YAG laser. The diagnostic tools include fast photography of overall visible plume emission using a 2 ns gated intensified charged coupled device and space and time resolved emission spectroscopy using a 50 cm monochromator/spectrograph and photomultiplier tube. The expansion behavior of the plasma was studied with ambient air pressure ranging from 10−6 to 100 Torr. Free expansion, plume splitting and sharpening, hydrodynamic instability, and stagnation of the plume were observed at different pressure levels. Space and time resolved emission spectroscopic studies showed a twin peak distribution for Al and Al+ species at farther distances illustrating plume splitting at pressures higher than 100 mTorr. Combining imaging together with time resolved emission diagnostics, a triple structure of the plume was observed. The expansion of the plume front was compared with various expansion models and found to be generally in good agreement.
Re-entrant spin glass and magnetoresistance in Co0.2Zn0.8Fe1.6Ti0.4O4 spinel oxide
Authors: Bhowmik R. N. , Ranganathan R.
Published: March 2003

We have investigated the static and dynamical response of magnetic clusters in Co0.2Zn0.8Fe1.6Ti0.4O4 spinel oxide, where a sequence of magnetic states, i.e., paramagnetic to ferromagnetic state at TC⩽270 K and ferromagnetic to canted spin glass (CSG) state at Tf⩽125 K are observed. The time dependence of remanent magnetization shows nonequilibrium spin dynamics in the CSG state, whereas a weak time dependent relaxation is observed in the ferromagnetic state. There is no relaxation effect in the paramagnetic state. The field dependence of the magnetization confirms the absence of long range ferromagnetic order in the system. We considered the magnetic clusters as ferromagnetic domains, in which the spins are not necessarily infinite ordered due to spin canting effects. The variation of the ferromagnetic and antiferromagnetic components and magnetic disorder inside the clusters shows some interesting magnetic and electrical properties in the system, viz. field induced transition in M vs H data, re-entrant magnetic transition in ac susceptibility vs T data, and re-entrant semiconducting behavior in resistivity vs T data.
Soft x-ray absorption spectroscopy studies of (110) YBa2Cu3O6.9 thin film
Authors: Liu S. J. , Juang J. Y. , Wu K. H. , Uen T. M. , Gou Y. S. , Chen J. M. , Lin J.-Y.
Published: March 2003

Unoccupied states of the (110) YBa2Cu3O6.9 (YBCO) thin films were investigated by the E∥ab and E∥c polarized O 1s x-ray absorption near edge structure (XANES) spectroscopy. With correction of the self-absorption effect, the E∥ab O 1s XANES spectra of the (110) YBCO films obtained from the normal incidence geometry and grazing incidence geometry are very similar. After the self-absorption correction, the E∥c spectrum of the (110) YBCO film obtained from the normal incidence geometry is almost identical to that of the (001) YBCO film obtained from the grazing incidence geometry. Hole distribution over the different oxygen sites has been derived. The hole number in the apical sites in some thin films obtained in the present investigation is larger than that from the previous x-ray absorption studies of single crystals and from the theoretical calculations. The comparison of the XANES spectra between the (001) and (110) YBCO thin films is also presented.
Temperature dependence of the dielectric response of anodized Al–Al2O3–metal capacitors
Authors: Hickmott T. W.
Published: March 2003

The temperature dependence of capacitance, CM, and conductance, GM, of Al–Al2O3–metal capacitors with Cu, Ag, and Au electrodes has been measured between 100 and 340 K at seven frequencies between 10 kHz and 1 MHz. Al2O3 films between 15 and 64 nm thick were formed by anodizing evaporated Al films in borate-glycol or borate-H2O electrolyte. The interface capacitance at the Al2O3–metal interface, CI, which is in series with the capacitance CD due to the Al2O3 dielectric, is determined from plots of 1/CM versus insulator thickness. CI is not fixed for a given metal–insulator interface but depends on the vacuum system used to deposit the metal electrode. CI is nearly temperature independent. When CI is taken into account the dielectric constant of Al2O3 determined from capacitance measurements is ∼8.3 at 295 K. The dielectric constant does not depend on anodizing electrolyte, insulator thickness, metal electrode, deposition conditions for the metal electrode or measurement frequency. By contrast, GM of Al–Al2O3–metal capacitors depends on both the deposition conditions of the metal and on the metal. For Al–Al2O3–Cu capacitors, GM is larger for capacitors with large values of 1/CI that result when Cu is evaporated in an oil-pumped vacuum system. For Al–Al2O3–Ag capacitors, GM does not depend on the Ag deposition conditions.
Optical properties of Mn-doped GaAs layers grown on (100) GaAs substrate
Authors: Yoon I. T. , Leem Jae Hyeon , Kang T. W.
Published: March 2003

Mn-doped Ga1−xMnxAs epilayers grown on semi-insulating (100) GaAs substrates using the liquid phase epitaxy technique were investigated using photoluminescence and Hall effect measurements from 20 to 300 K. Transitions involving shallow Mn acceptors were identified through photoluminescence measurements and the ionization energy of Mn acceptor was determined to be 104.6 meV which is in good agreement with values calculated from a hydrogenic hole model including sp–d exchange contribution. Also, it was concluded that Ga1−xMnxAs alloys were obtained from samples with low Mn concentration (below x≈1%) and these layers have a simple impurity band merging to the valence band.
Trapping with anisotropic diffusion in an applied field: H+ in Mg-doped GaN
Authors: Myers S. M. , Wright A. F.
Published: March 2003

The anisotropy of H+ diffusion in wurtzite GaN is treated using the density-functional theory, and the effects of this anisotropy on trapping by charged and neutral sinks are examined through a numerical solution of the time-dependent diffusion problem. The results clarify recent findings from modeling of experimentally observed H+ drift in reverse-biased GaN p–n junctions, where the fitted effective capture radius for trapping of H+ by Mg− was much larger than expected for isotropic diffusion. More generally, diffusion anisotropy is found to introduce qualitative changes in the solution of the classic Onsager problem of trapping by electrostatic attraction in the presence of an externally applied electric field.
Room temperature gravitational wave bar detector with optomechanical readout
Authors: Conti L. , De Rosa M. , Marin F. , Taffarello L. , Cerdonio M.
Published: March 2003

We present the full implementation of a room-temperature gravitational wave bar detector equipped with an optomechanical readout. The bar mechanical vibrations are read by a Fabry–Pérot interferometer whose length changes are compared with a stable reference optical cavity by means of a resonant laser. The detector performance is completely characterized in terms of spectral sensitivity and statistical properties of the fluctuations in the system output signal. This kind of readout technique allows for wide-band detection sensitivity and we can accurately test the model of the coupled oscillators for thermal noise. Our results are very promising for cryogenic operation and represent an important step towards significant improvements in the performance of massive gravitational wave detectors.
Magnetic and Mössbauer spectral study of ErFe11Ti and ErFe11TiH
Authors: Piquer Cristina , Hermann Raphaël P. , Grandjean Fernande , Long Gary J. , Isnard Olivier
Published: March 2003

X-ray diffraction, isothermal magnetization at 5 and 300 K, ac magnetic susceptibility measurements between 5 and 200 K, and iron-57 Mössbauer spectral measurements between 4.2 and 295 K have been carried out on ErFe11Ti and ErFe11TiH. Hydrogen uptake has been measured by gravimetric analysis and the insertion of hydrogen into ErFe11Ti increases its magnetization, magnetic hyperfine fields, and isomer shifts as a result of the associated lattice expansion. Peaks and steplike changes in both the real and imaginary components of the ac magnetic susceptibility are observed at ∼50 and 40 K for ErFe11Ti and ErFe11TiH, respectively, and are assigned to spin-reorientation transitions resulting from the temperature dependence of the sixth-order Stevens crystal-field term of erbium. The Mössbauer spectra have been analyzed with a model which considers both these spin reorientations and the distribution of titanium atoms in the near-neighbor environment of the three crystallographically distinct iron sites. The assignment and the temperature dependencies of the hyperfine fields and isomer shifts are in complete agreement with the Wigner–Seitz cell analysis of the three iron sites in ErFe11Ti and ErFe11TiH. The changes in the hyperfine field and isomer shift with the number of titanium near neighbors of the three iron sites are in agreement with the values observed for related titanium–iron intermetallic compounds.
Cation order and magnetic properties of double perovskite Sr2FeMoO6
Authors: Sakuma Hiroshi , Taniyama Tomoyasu , Kitamoto Yoshitaka , Yamazaki Yohtaro
Published: March 2003

A double perovskite type compound Sr2FeMoO6 was synthesized by a solid-state reaction. The order parameter (x) of Fe and Pt ions on the B sites increased from 68% to 82% and the spontaneous magnetization at 5 K increased from 2.1 to 3.0 μB per formula unit by lowering the calcination temperature from 1100 to 500 °C. The dependence of the spontaneous magnetization on x was discussed using two models considering the local magnetic interactions between ions on the B sites. Fe57 Mössbauer analysis suggested the existence of tiny areas of SrFeO3−δ in Sr2FeMoO6.
Anomalous tunneling magnetoresistance in junctions with an Au dusting layer at the Co/Al2O3 interface
Authors: Shim Heejae , Cho B. K. , Kim Jin-Tae
Published: March 2003

We report an anomalous magnetotransport in a magnetic tunnel junction (MTJ) containing an Au dusting layer with Au thickness 0.6 nm at the Co/Al2O3 interface, where the Co layer is pinned by the underlying FeMn layer. Despite its structure being exchange biased, the junction with the 0.6 nm Au layer exhibits symmetric magnetoresistance (MR) for both the applied magnetic field (H) directions, which is a typical MR feature without a pinning effect, and a low resistance state near H≈0 Oe. It is also found that the same MR features, i.e., symmetric MR and a low resistance state near H≈0 Oe, can be achieved in the MTJ without an Au insertion layer when the pinned Co moments are at an angle to the applied H direction. Based on measurements of the angular dependence of MR, we surmise that the anomalous MR is due to the effect of spin disorder in the Co layer, induced by the deposited Au layer. It is not likely that the anomalous effect is due to the quantum well state in the Au layer.
Noise analysis and optimization of a charge transformer, a noise-matching device for single electron transistors
Authors: Kurdak Ç. , Lewis K. M.
Published: March 2003

Operation and noise analysis of a recently proposed noise-matching device, called a charge transformer, are presented. The charge transformer consists of N identical capacitors and 3N+1 switches that enable the capacitors to be connected either in series or in parallel. The device is operated by switching back and forth between these two configurations at speeds faster than the signals that will be measured. We show that an ideal charge transformer can achieve perfect noise matching between any single-electron transistor and a high capacitance device that is under test. We also discuss how a realistic charge transformer made using switches with finite capacitance and resistance should be operated to achieve optimum noise performance.
Comparing organic to inorganic photovoltaic cells: Theory, experiment, and simulation
Authors: Gregg Brian A. , Hanna Mark C.
Published: March 2003

Charge carriers are photogenerated with very different spatial distributions in conventional inorganic photovoltaic (IPV) cells and in organic photovoltaic (OPV or excitonic) cells. This leads to a fundamental, and often overlooked, mechanistic difference between them. Carriers are generated primarily at the exciton-dissociating heterointerface in OPV cells, resulting in the production of electrons in one phase and holes in the other—the two carrier types are thus already separated across the interface upon photogeneration in OPV cells, giving rise to a powerful chemical potential energy gradient ∇μhv that promotes the photovoltaic effect. This occurs also in high-surface-area OPV cells, although their description is more complex. In contrast, both carrier types are photogenerated together throughout the bulk in IPV cells: ∇μhv then drives both electrons and holes in the same direction through the same phase; efficient carrier separation therefore requires a built-in equilibrium electrical potential energy difference ∅bi across the cell. The open-circuit photovoltage Voc is thus limited to ∅bi in IPV cells, but it is often greater than ∅bi in OPVs. The basic theory necessary to compare IPVs to OPVs is reviewed. Relevant experiments are described, and numerical simulations that compare semiconductor devices differing only in the spatial distribution of photogenerated carriers are presented to demonstrate this fundamental distinction between the photoconversion mechanisms of IPV and OPV devices.
Influence of boundary roughness on the magnetization reversal in submicron sized magnetic tunnel junctions
Authors: Meyners D. , Brückl H. , Reiss G.
Published: March 2003

The reproducible magnetic switching of submicron magnetic tunnel junctions (MTJ’s) is an important requirement for their application in highly integrated magnetic memory devices. We have investigated the switching of small MTJ’s by atomic and magnetic force microscopy (AFM/MFM) combined with micromagnetic numerical simulations. The latter are carried out with the real (AFM) shape as input mask, including the boundary roughness of the MTJ’s. MFM reveals S-, C-, and K- shaped magnetization patterns for rectangular submicron sized junctions in saturation. In general, the magnetization loops and switching fields are different for individual junctions. The simulations show that the detailed boundary shape, which is specific for each junction, has a significant influence on the nucleation and location of domain walls and vortices, and hence, on the magnetic switching.
2.5–3.5 μm optically pumped GaInSb/AlGaInSb multiple quantum well lasers grown on AlInSb metamorphic buffer layers
Authors: Pease E. A. , Dawson L. R. , Vaughn L. G. , Rotella P. , Lester L. F.
Published: March 2003

Room-temperature emission is observed as long as 3.26 μm in optically pumped type-I quantum well lasers on relaxed epitaxial layers grown by molecular-beam epitaxy. A superlattice is used to filter dislocations in the metamorphic buffer to reduce Shockley–Read–Hall losses. The longest wavelength emission of 3.45 μm from these structures is observed at 170 K, and the brightest room-temperature laser emits 0.5 W/facet peak power at 2.81 μm. It has a low threshold power density of 169 W/cm2 and a differential quantum efficiency of 28%. The characteristic temperatures, T0 and T1, are 119 K and 171 K, respectively. Stimulated emission is observed in this sample at a maximum operating temperature of 370 K.
High-pressure luminescence studies in CaS doped with Eu2+
Authors: Chen C. , Teo K. L. , Chong T. C. , Wu Y. H. , Low T. S. , Carey R. , Newman D. M. , Viney I. , Wu J. P. , Bickerton J.
Published: March 2003

The luminescence of CaS doped with Eu2+ was studied under hydrostatic pressure at room temperature. Besides the main Eu2+ emission peak, a low-energy peak emerges with increasing pressure. The results are explained in terms of a single configuration coordination model.
Termination mechanism of inversion domains by stacking faults in GaN
Authors: Iwamoto C. , Shen X. Q. , Okumura H. , Matsuhata H. , Ikuhara Y.
Published: March 2003

GaN film was grown by molecular-beam epitaxy (MBE) with intermittent indium exposure during film growth, and the microstructures were investigated by atomic-resolution high-voltage electron microscopy (ARHVEM). Epitaxial wurzite GaN (0001) film grew on a sapphire (0001) substrate. Although many inversion domains (ID’s) were generated at the film/substrate interface, stacking faults on the basal plane of GaN, which was formed at the position of indium exposure, were found to play an important role in terminating the growth of ID’s. ARHVEM observation revealed that stacking faults I2 with two stacking violations were introduced in the ID while stacking faults I1 with one stacking violation were introduced in the matrix. The difference in the number of stacking violations was related to the change in the ID boundary (IDB) structure and to the reduction of the IDB area. The termination mechanism is considered to be important to the realization of the high-quality GaN film by MBE.
Sp
2
bonding distributions in nanocrystalline diamond particles by electron energy loss spectroscopy
Authors: Okada Katsuyuki , Kimoto Koji , Komatsu Shojiro , Matsumoto Seiichiro
Published: March 2003

Nanocrystalline diamond particles with 200–500 nm in diameter have been prepared in a 13.56 MHz low pressure inductively coupled CH4/CO/H2 plasma. The chemical bonding map was investigated by electron energy loss spectroscopy (EELS). The EEL spectrum shows a peak at 290 eV due to σ* states and the energy loss near-edge structure is similar to that of diamond. A slight peak appears at ∼285 eV corresponding to π* states. The mapping of sp2 states by π* peak reveals that sp2-bonded carbons are identified around the 20–50 nm subgrains of nanocrystalline diamond particles at approximately 1 nm resolution.
Dielectric study of water/methanol mixtures for use in pulsed-power water capacitors
Authors: Bolund Björn F. , Berglund Mats , Bernhoff Hans
Published: March 2003

Large pulsed-power systems normally use a solid capacitive primary storage and a water capacitor for secondary storage. It is shown that the electrical breakdown strength is increased and the maximum storage time is enhanced from the microsecond level to the millisecond level when the water is replaced by a water/methanol mixture. The increased time scale in the second storage enables the use of an inductive primary storage. The mixing of methanol in the water decreases the relative permittivity. However, due to the increased electrical breakdown strength, a higher energy density is nevertheless possible. Previous work with water/ethylene glycol has manifested the problem of charge injection that occurs when energy is stored for millisecond time scales at high electrical stress. Water/methanol mixtures show less tendency for charge injection and a better ability to store energy at millisecond time scales.
Azimuth-dependent reflection anisotropy spectroscopy
Authors: Macdonald B. F. , Law J. S. , Cole R. J.
Published: March 2003

A detailed explanation of “azimuth dependent reflection anisotropy spectroscopy” (ADRAS) [B. F. Macdonald and R. J. Cole, Appl. Phys. Lett. 80, 3527 (2002)] is presented. Experimental data and simulations are used to illustrate the greater information content of ADRAS compared to the standard reflection anisotropy technique. The application of ADRAS to the study of chiral surface modifiers, relevant to biomolecular adsorption and enantioselective catalysis, is discussed.
Compensation walls in gallium and aluminum substituted gadolinium–bismuth–iron garnet films created by laser annealing: Measurements and simulations
Authors: Wilkens L. , Träger D. , Dötsch H. , Alexeev A. M. , Popkov A. F. , Korneev V. I.
Published: March 2003

Magnetic garnet films of composition (Gd2 Bi1) (Fe4.3 Ga0.2 Al0.5) O12 are grown by liquid phase epitaxy on (111) oriented substrates of gadolininium–gallium–garnet close to magnetic compensation. Local compensation walls (CW) are fabricated by laser annealing. Images of parallel and antiparallel compensation walls and of Bloch walls in annealed regions are obtained by magnetic force microscopy (MFM). The MFM signals associated with the CWs turned out to be smaller in amplitude but broader in space than the signals created by Bloch walls. To simulate the experimental results a theoretical model is developed taking into account the broadening of the CW by the preparation technique. Very good agreement between measurements and numerical simulations is obtained.
Spectra and energy levels of Er3+(4f11) in Gd3Ga5O12
Authors: Gruber John B. , Sardar Dhiraj K. , Zandi Bahram , Hutchinson J. Andrew , Trussell C. Ward
Published: March 2003

A detailed crystal-field splitting analysis is presented for the energy levels of Er3+(4f11) incorporated into Gd3Ga5O12 single crystals. Spectra are reported between 1600 and 350 nm and were observed at temperatures between 8 K and room temperature. It is most likely that Er3+ ions substitute for Gd3+ ions in cationic sites of D2 symmetry in the cubic garnet host. A parameterized model Hamiltonian including atomic and crystal-field terms is diagonalized over the entire electronic configuration of Er3+(4f11). The diagonalization of the total energy matrix accounts for the intrinsic J–J mixing between states described in terms of the basis set of SLJMJ wave functions. The resolved absorption spectra, representing some 120 transitions from the ground state, I415/2, to excited multiplet manifolds, L2S+1.J, are compared with calculated Stark levels. A final standard deviation of 5.3 cm−1 is obtained between 60 calculated and experimental Stark levels.
Transmission of low-energy electrons in boron-doped nanocrystalline diamond films
Authors: Yater J. E. , Shih A. , Butler J. E. , Pehrsson P. E.
Published: March 2003

Transmission electron spectroscopy is used to examine the low-energy electron transport and emission properties of nanocrystalline chemical-vapor-deposited diamond films. In particular, the intensity and energy distribution of transmitted electrons are measured as a function of film thickness and incident-beam parameters. Low-energy transmission is detected in measurements from two films of thickness 0.15 and 2.5 μm with similar boron concentrations. The transmitted energy distributions are very similar for the two samples and are nearly identical to those obtained in reflection measurements. The full width at half maximum of the transmitted distribution is slightly broader for the thinner film (∼0.8−0.9 eV) than for the thicker film (∼0.6−0.7 eV), and the maximum transmission yields are similar (∼3−5 emitted electrons per incident electron). However, different beam energies are required to produce the low-energy transmission. The energy-dependent data is interpreted using Monte Carlo simulations along with a qualitative model of the diamond nanostructure. From this analysis, the low-energy-electron escape depth is confirmed to be as long as ∼1 μm in the 2.5-μm-thick sample.
Frequency and temperature dependence of the relative permittivity in ferroelectrics: Monte-Carlo simulation study
Authors: Bolten D. , Böttger U. , Waser R.
Published: March 2003

In this article, the domain wall densities calculated from simulated domain configurations are used to estimate the domain wall contributions to the dielectric permittivity of a ferroelectric material. The configurations were calculated using a Monte-Carlo model proposed by B. G. Potter et al. [J. Appl. Phys. 87, 4415 (2000)]. The temperature and frequency dependence of the contributions are analyzed and compared to experimental results obtained on PbZrxTi1−xO3 thin films.
Wave-mechanical calculations of leakage current through stacked dielectrics for nanotransistor metal-oxide-semiconductor design
Authors: Le Roy M. , Lheurette E. , Vanbésien O. , Lippens D.
Published: March 2003

Quantum calculations of leakage current through ultrathin dielectric heterostructures are used to propose design criteria for the next generation of metal-oxide-semiconductor devices in a deep submicron technology. By using as input parameters both the dielectric constant and the barrier height of different dielectrics, including at first stage SiO2, Si3N4, TiO2, and Ta2O5, but also such emerging materials as HfO2 and ZrO2, we show that, depending on the voltage range investigated, the hierarchy between those dielectrics is not simply given by the increase of their permittivity. Deeper considerations based on resonant tunneling mechanisms, of prime importance when dielectric heterostructures are used for fabrication purposes, must be taken into account especially for the future low-consumption nanotransistors with operating voltages below 1 V.
Melt-threshold method to determine the thermal conductivity of thin films in phase-change optical recording stacks
Authors: Meinders Erwin R. , Peng Chubing
Published: March 2003

We report on the dynamic melt-threshold method for the in situ determination of the effective thermal conductivity of thin layers in phase-change optical recording stacks. The method is based on the systematic variation of some of the thermal parameters of the thin films in the recording stack such that the temperature of the phase-change layer, calculated with a multilayer numerical model, equaled the measured melt temperature of the phase-change material. The laser power that causes the onset of melting, the melt-threshold power, was determined from the detectable reflection difference between the crystalline and molten state of the phase-change film and served as input for the model calculations. Melt-threshold experiments were performed for different recording velocities, stack structures, layer thicknesses and optical spot sizes. The melt-threshold method provided a consistent set of thermal parameters that accurately predicts the temperature distribution in a recording stack during erasing and recording of data. Hence, the method is very suitable to calibrate a multilayer thermal model that is used for the optimization of, for example, write strategies or thermal stack designs. The accuracy of the determined effective thermal conductivity of the dielectric layers is estimated to be within 10%–20%, that of the phase-change layer is determined with less accuracy. The melt-threshold method is easily extended to recording stacks with metal heat sink layers.
Stability analysis for liquid-bridging induced contact
Authors: Fan H. , Wang G. F.
Published: March 2003

Microscale contact between two solid surfaces induced by liquid-bridging force was studied in this article. Attention was placed on configurational stability of the liquid between the two contacting bodies. By examining the solutions for describing the shape of the liquid under equilibrium, it was found that these solutions become physically infeasible as the volume of the liquid vanishes. A linear stability analysis was carried out by perturbing the relevant equilibrium configurations and examining the change of the free energy of the system under consideration. The analysis leads to the conclusion that the equilibrium shape of the liquid, that bridges two solids in contact, may become unstable under certain conditions. As its volume diminishes, the liquid may break into smaller droplets.
Low-temperature resistivities of nanotubular polyaniline doped with H3PO4 and β-naphthalene sulfonic acid
Authors: Long Yunze , Chen Zhaojia , Zheng Ping , Wang Nanlin , Zhang Zhiming , Wan Meixiang
Published: March 2003

The temperature dependence of the resistivity of nanotubular polyaniline (PANI) doped with H3PO4 and β-naphthalene sulfonic acid (NSA) synthesized by a “template-free method” has been investigated. The molar ratios of H3PO4 (NSA) to aniline monomer (represented by H3PO4/An and NSA/An) of the samples are H3PO4/An=0.01, 0.05, 0.3, 0.5 and NSA/An=1, 2, respectively. It is found that ln ρ(T) is linear in T−1/2 and the resistivity is sensitive to the doping concentration. For NSA/An=1 sample, a crossover at 57 K in the plot of ln ρ(T)∼T−1/2 has been observed. On the basis of detailed analysis of the structure characteristics of the PANI nanotubes, we suggest a hopping–tunneling model to explain the experimental results.
The stationary Monte Carlo method for device simulation. I. Theory
Authors: Kosina H. , Nedjalkov M. , Selberherr S.
Published: March 2003

A theoretical analysis of the Monte Carlo method for steady-state semiconductor device simulation, also known as the single-particle Monte Carlo method, is presented. At the outset of the formal treatment is the stationary Boltzmann equation supplemented by boundary conditions, which is transformed into an integral equation. The conjugate equation has been formulated in order to develop forward Monte Carlo algorithms. The elements of the conjugate Neumann series are evaluated by means of Monte Carlo integration. Using this mathematically-based approach, the single-particle Monte Carlo method is derived in a formal way. In particular, the following are recovered: the probability densities for trajectory construction, both the time averaging and the synchronous ensemble methods for mean value calculation, and the rule that the initial points of the trajectories have to be generated from the velocity weighted boundary distribution. Furthermore, the independent, identically distributed random variables of the simulated process are identified.
The stationary Monte Carlo method for device simulation. II. Event biasing and variance estimation
Authors: Nedjalkov M. , Kosina H. , Selberherr S.
Published: March 2003

A theoretical analysis of the Monte Carlo method for the solution of the stationary boundary value problem defined by the Boltzmann equation has been presented in Part I. Based on this analysis, the independent, identically distributed random variables of the simulated process are identified. Estimates of the stochastic error of the single-particle Monte Carlo method are derived. An event-biasing technique for carrier transport across an energy barrier is developed and its suitability for variance reduction is demonstrated.
Mg-acceptor activation mechanism and transport characteristics in p-type InGaN grown by metalorganic vapor phase epitaxy
Authors: Kumakura Kazuhide , Makimoto Toshiki , Kobayashi Naoki
Published: March 2003

The Mg-acceptor activation mechanism and transport characteristics in a Mg-doped InGaN layer grown by metalorganic vapor phase epitaxy are systematically investigated through their structural, optical, and electrical properties. The In mole fraction was from 0 to 0.13, and the Mg concentration varied from 1×1019 to 1×1020 cm−3. X-ray rocking curves for Mg-doped InGaN layers indicate that the structural quality is comparable to that of undoped and Si-doped InGaN layers. Their photoluminescence spectra show emissions related to deep donors emerged at lower energy when Mg doping concentrations are above 2−3×1019 cm−3. The electrical properties also support the existence of these deep donors in the same Mg concentration range because the hole concentration starts to decrease at around the Mg concentration of 2−3×1019 cm−3. These results indicate that self-compensation occurs in Mg-doped InGaN at higher-doping levels. The temperature dependence of the hole concentration in Mg-doped InGaN indicates that the acceptor activation energy decreases with increasing In mole fraction. This is the reason the hole concentration in Mg-doped InGaN is higher than that in Mg-doped GaN at room temperature. In addition, the compensation ratio increases with doping concentration, which is consistent with the deep donor observed in PL spectra. For Mg-doped InGaN, impurity band conduction is dominant in carrier transport up to a relatively higher temperature than that for Mg-doped GaN, since the acceptor concentration for Mg-doped InGaN is higher than that of Mg-doped GaN.
Thermal expansion of the isostructural PtSi and NiSi: Negative expansion coefficient in NiSi and stress effects in thin films
Authors: Detavernier C. , Lavoie C. , d’Heurle F. M.
Published: March 2003

NiSi displays the rare occurrence of contracting during heating along the axis with the smallest unit cell dimension. Because of stress due to the thermal mismatch between the film and the Si substrate, the unit cell dimensions reported in the Joint Committee on Powder Diffraction Standards record for NiSi are faulty. They seem to be reproducible in thin films prepared by reactive diffusion, but do not correspond to relaxed equilibrium conditions. For PtSi (isostructural with NiSi), there is neither expansion nor contraction along the axis with the small dimension during heating. In the present PtSi film, x-ray diffraction does not reveal any presence of residual thermal stresses at low temperature.
Temperature dependent exciton photoluminescence of bulk ZnO
Authors: Hamby D. W. , Lucca D. A. , Klopfstein M. J. , Cantwell G.
Published: March 2003

Temperature dependent (4.2–300 K) photoluminescence (PL) of bulk (0001)-oriented ZnO in the range of free- and bound-exciton emission is presented. Emission from several bound excitons and the free A exciton were observed from the low temperature (20 K) PL spectrum. The temperature dependence of the free-exciton peak position was fit using the Manoogian-Woolley equation and the coefficients obtained show reasonable agreement both with first-principle theoretical calculations and empirical values of the coefficients for other II–VI semiconductors. The strongest bound-exciton line with a width (full width at half maximum) of about 1 meV exhibited a thermal activation energy of approximately 14 meV, consistent with the exciton-defect binding energy. It was not observed at temperatures above 150 K. Additional analysis of this particular bound-exciton peak suggests it dissociates into a free exciton and a neutral-donor-like defect-pair complex with increasing temperature.
Effect of NiSb on the thermoelectric properties of skutterudite CoSb3
Authors: Katsuyama S. , Watanabe M. , Kuroki M. , Maehata T. , Ito M.
Published: March 2003

The phase equilibrium and thermoelectric properties of Co1−xNixSb3 ternary system were examined up to high nickel content x=0.40. NiSb compound with nickel arsenide structure was observed as the impurity phase in the samples with x>0.10 by x-ray diffraction. The sign of the Seebeck coefficient of Co1−xNixSb3 is negative, and the absolute value of the Seebeck coefficient and electrical resistivity generally decreases with increasing x. The thermal conductivity is reduced in the samples with x⩽0.06, but above x=0.06 it increases with increasing x. These behaviors of the thermoelectric properties in the samples with low nickel content are ascribed to the substituted Ni, while those in the samples with high nickel content are ascribed to the precipitated NiSb compound. In order to more reduce the thermal conductivity of the material, we have prepared a sintered CoSb3–NiSb composite where the NiSb particles are dispersed in the CoSb3 matrix by mechanical grinding and hot pressing. The thermal conductivity of the composite is lower than that of the Co1−xNixSb3 system especially at low temperature. The decrease of the thermal conductivity is ascribed to the enhancement of the phonon scattering caused by the dispersion of NiSb particles in the CoSb3 matrix.
Infrared absorption by conical silicon microstructures made in a variety of background gases using femtosecond-laser pulses
Authors: Younkin R. , Carey J. E. , Mazur E. , Levinson J. A. , Friend C. M.
Published: March 2003

We show that the near-unity infrared absorptance of conical microstructures fabricated by irradiating a Si(111) surface with 100 fs laser pulses depends on the ambient gas in which the structures are formed. SF6 produces an absorptance of 0.9 for radiation in the 1.2–2.5 μm wavelength range, higher than any of the other gases. Use of Cl2, N2, or air produces surfaces with absorptances intermediate between that for microstructures formed in SF6 and that for flat crystalline silicon, for which the absorptance is roughly 0.05–0.2 for a 260 μm thick sample. Secondary ion mass spectrometry shows that elements from the ambient gas are incorporated into the silicon surface in high concentration.
Low-temperature dielectric behavior of Nb2O5–SiO2 solid solutions
Authors: Choosuwan H. , Guo R. , Bhalla A. S. , Balachandran U.
Published: March 2003

Dielectric properties of Nb2O5(0.92):SiO2(0.08) ceramic were measured in the temperature range of 10–300 K by the cryostat system. Frequency-dependent dielectric loss suggests the relaxation behavior of this material. The relaxation mechanism was analyzed by the Arrhenius relationship and the Cole–Cole plot. Calculated distribution of relaxation time reveals deviation from the pure Debye relaxation.
Calculation of dielectric constant and loss of two-phase composites
Authors: Ang Chen , Yu Zhi , Guo Ruyan , Bhalla A. S.
Published: March 2003

The field distribution, dielectric constant, and loss in a two-phase composite, in which phase A is distributed inside a square matrix of phase B, have been calculated using the finite-element method (FEM). The calculation was carried out by taking into account different shapes for phase A, such as circles, triangles, and rings with different sizes. The modeling by FEM in the dielectric composite indicates that the shape for phase A has an influence on the electric-field distribution. In the case of the triangular shape of phase A, the electric flux condensed on the sharp angles of phase A. In a particular case of a ring for phase A, the electric-field distribution in the composite was quite different from that of circles/triangles due to the shielding effect of flux, and hence the dielectric constant and loss are greatly changed. The calculated values are analyzed and compared with that from the empirical Licktenecher relation. The results indicate that the FEM method can reflect the change in the shape and size of the particles of a composite and provide more reasonable results than that from the empirical Licktenecher relation.
Temperature dependent characteristics of poly(3 hexylthiophene)-fullerene based heterojunction organic solar cells
Authors: Chirvase D. , Chiguvare Z. , Knipper M. , Parisi J. , Dyakonov V. , Hummelen J. C.
Published: March 2003

Electrical and optical properties of poly(3-hexylthiophene-2.5diyl) (P3HT) used as the main component in a polymer/fullerene solar cell were studied. From the study of space-charge limited current behavior of indium-tin-oxide (ITO)/P3HT/Au hole-only devices, the hole mobility and density were estimated to range from 1.4×10−6 cm2/V s and 5.3×1014 cm−3 at 150 K to 8.5×10−5 cm2/V s and 1.1×1015 cm−3 at 250 K, respectively. The highest occupied to lowest occupied molecular orbital energetic difference was estimated from absorption spectrometry to be about 2.14 eV. Strong quenching of photoluminescence when the polymer was mixed with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), provided evidence of photoinduced charge transfer from P3HT to PCBM. Characterization of ITO/PEDOT:PSS/P3HT:PCBM/Al solar cells was done by analyzing the dependence of current density–voltage characteristics on temperature and illumination intensity. The main solar cell characteristics recorded at 300 K under 100 mW/cm2 white-light intensity were: Open-circuit voltage 0.48 V, current density 1.28 mA/cm2, with an efficiency of 0.2%, and fill factor of 30.6%. Open-circuit voltage decreased almost linearly with increasing temperature, while short circuit current density increased with temperature, saturating at around 320 K, and decreased thereafter. Power conversion efficiency and fill factor were maximum around 3 mW/cm2 due to the poor bulk transport properties of the active layer.
Effects of mechanical activation on the formation of PbTiO3 from amorphous Pb–Ti–O precursor
Authors: Yu T. , Shen Z. X. , Xue J. M. , Wang J.
Published: March 2003

We investigate the effects of mechanical activation in triggering PbTiO3 (PT) formation from an amorphous Pb–Ti–O precursor synthesized by coprecipitation. In this work, the amorphous precursor and the samples derived from it by mechanical activation were investigated using Raman spectroscopy, x-ray diffraction, and high-resolution transmission electron microscopy. Our results show that the crystallization of tetragonal PT phase can be considered as a nucleation and growth process described by the Avrami model. In the initial stage of mechanical activation, the main effect is size reduction of the constituent starting materials. For a longer period of milling, perovskite PT crystallites are formed by mechanical activation alone. These crystallites act as seeds, reducing the activation energy from 249±6 kJ/mol for the precursor to 97±7 kJ/mol for the 30-h-milled sample and enhances the crystallization kinetics, during postcalcination. Consequently, the PT phase formation temperature is dramatically lowered. In addition, our results demonstrate that the particle size affects the structure of the PT phase, where the PT shows the pseudocubic to tetragonal transition with increasing particle size.
Full band Monte Carlo study of high field transport in cubic phase silicon carbide
Authors: Nilsson H.-E. , Englund U. , Hjelm M. , Bellotti E. , Brennan K.
Published: March 2003

A full band Monte Carlo study of the electron transport in 3C–SiC is presented based on an ab initio band structure calculation using the local density approximation to the density functional theory. The scattering rates and impact ionization transition rates have been calculated numerically from the ab initio band structure using both energy dispersion and numerical wave functions. This approach reduces the number of empirical parameters needed to a minimum. The two empirical coupling constants used have been deduced by fitting the simulated mobility as a function of lattice temperature to experimental data. The peak velocity was found to be approximately 2.2×107 cm/s with a clear negative differential mobility above 600 kV/cm. The electron initiated impact ionization coefficients were found to be 2–10 times stronger than the reported values for the hole initiated impact ionization.
Pressure dependence of enhanced x-ray line emission from laser-produced plasmas expanding in an ambient gas
Authors: Chowdhury Avijit , Arora V. , Naik P. A , Gupta P. D.
Published: March 2003

An experimental study of parametric dependence of space-resolved x-ray line emission from laser-produced magnesium plasma expanding in a helium background gas has been performed for different pressures up to 125 mbar. A strong enhancement in MgXI He-α resonance (1s2 S10P11 at 9.17 Å) and intercombination (1s2 1S0−1s2p3P1 at 9.23 Å) line emission is observed in the expansion region extending up to ∼ 6 mm from the target. At the optimum background gas pressure of about 40 mbar, there is an order of magnitude increase in the x-ray line intensity compared to that observed for a gas pressure of few mbar. The results of measurements of the distance of the peak emission region from the target and width of the emission zone for different pressures are presented, and the role of the background gas for the observed behavior is discussed.