bluuebunny/crossref_metadata_2025 huggingface dataset:
Photoconductivity of intrinsic and nitrogen-doped hydrogenated amorphous germanium thin films
Authors: Marcano G. , Zanatta A. R. , Chambouleyron I.
Published: July 2002

In this work the effects of nitrogen doping on the photoconductivity of hydrogenated amorphous germanium (a-Ge:H) thin films are presented and discussed. The quantum efficiency-mobility-lifetime (ημτ) product of majority carriers has been measured as a function of light intensity in samples containing different dopant concentrations. It has been found that, for low dopant concentrations, the incorporation of nitrogen atoms in hydrogenated amorphous germanium films enhance the photoconductivity, as expected from n-type doping in tetrahedrally coordinated amorphous semiconductors. The results have been explained in terms of changes in charge distribution in the mobility gap on doping. The sensitization depends on the position of the Fermi energy, the maximum being at around Ec−EF≤0.2 eV, which corresponds to a nitrogen content of ∼0.03 at %. The recombination kinetics for N-doped samples is always dominated by a monomolecular process, a consequence of the rather large density of electron states in the pseudogap of a-Ge:H.
The microwave surface impedance of granular high T
C superconductors in dc magnetic fields: Its relationship to frequency dependence
Authors: Remillard S. K. , Reeves M. E. , Rachford F. J. , Wolf S. A.
Published: July 2002

The surface impedance of bulk and thick film granular high temperature superconductors has been studied. The static magnetic field, temperature, and frequency dependence of the surface impedance have been analyzed in terms of the weakly coupled grain model. Accurate quantitative predictions of the frequency dependence of the surface resistance are made using this model. The model also quantitatively accounts for the negative magnetic field coefficient of the effective rf penetration depth. The rf penetration depth is a maximum when the ratio of penetration depth to skin depth is 0.930.
Chemical topography of anisotropic etching of polycrystalline Si masked with photoresist
Authors: Guinn K. V. , Donnelly V. M.
Published: July 2002

The chemical constituents present on the surface of small poly-Si features that were etched in a chlorine plasma were determined. Poly-Si films on SiO2-covered Si(100) substrates were masked with photoresist stripes and then etched downstream from a helical resonator, high-density plasma source. rf biasing of the sample provided low-energy ion bombardment to enhance anisotropic etching. These conditions match those in high-density plasma sources that are currently being developed elsewhere for commercial use. After etching, samples were transferred under ultrahigh vacuum to an analysis chamber equipped with an x-ray photoelectron spectrometer. Shadowing of photoelectrons by adjacent etched stripes was used to determine the chemical constituents on the tops, sides, and bottoms of etched features. Monolayer coverages of Cl and SiO2 were found on the sides of the photoresist mask, while the top of the mask was covered with a monolayer of Cl, and no Si was detected. The Si on the side of the mask was deposited as a result of the slight etching of the glass tubing in the plasma source region. The etched poly-Si trench bottoms are also covered with Cl, at levels comparable to those found in unmasked regions that were subjected to equal amounts of ion bombardment. However, a relatively small amount of Cl covers the sides of the etched poly-Si features. A low coverage of O was also found on all exposed poly-Si surfaces, and is also due to etching of the glass tubing. The O coverage was small compared to Cl on horizontal poly-Si surfaces, but comparable to Cl on the side of the poly-Si features. Very little C was detected on etched poly-Si sidewall surfaces, indicating that erosion of the photoresist plays no role in passivating etched sidewalls.
Experimental gain characteristics and barrier lasing in strained-layer InGaAs-GaAs-AlGaAs quantum well heterostructure lasers
Authors: Coleman J. J. , Beernink K. J.
Published: July 2002

Gain characteristics are reported for a series of five separate confinement heterostructure InxGa1−xAs-GaAs-Al0.20Ga0.80As (0.08<x<0.33) strained-layer quantum well lasers with a 70 Å well thickness. The differential current gain β increases with indium composition from 14.2 for x=0.16 to 27.4 cm/A for x=0.33, as calculated from a semilogarithmic gain-current density relation. Data are also presented on emission wavelengths and threshold current as a function of composition and cavity length. Devices with x=0.08 are unable to reach threshold on the quantum well transition, and laser operation in the barriers is observed for all cavity lengths. Devices with higher indium fraction switch from the n=1 transition in the quantum well for long cavity lengths to the n=2 transition and to the GaAs barriers for short cavities due to saturation of the available quantum well gain.
Effects of order and disorder on field-effect mobilities measured in conjugated polymer thin-film transistors
Authors: Holland E. R. , Bloor D. , Monkman A. P. , Brown A. , De Leeuw D. , Bouman M. M. , Meijer E. W.
Published: July 2002

The morphology of conjugated polymer thin films has been controlled by exploiting the processes giving rise to solvatochromism in the initial polymer solutions from which the polymer films were deposited by spin coating. The material used was the substituted polythiophene, 3[2(S2-methylbutoxy)ethyl]-polythiophene. Starting with polymer dissolved in ‘‘good’’ solvent, various quantities of ‘‘bad’’ solvent when added lead to the reorganization of the solubilizing side chains attached to the conjugated polymer backbone. Ordering of these side chains increases with addition of bad solvent decreasing the flexibility of the backbone, yielding more rigid rodlike polymer chains. This in turn causes aggregation and finally precipitation. It is found that this molecular order can be transferred from solution to solid film during the spinning process, ascertained from optical spectroscopy. With these films acting as the active layer in a field-effect transistor structure, the mobility of the carriers injected into the films can be measured. It is found that as the molecular order and aggregation increases, carrier mobility decreases from ∼10−5 cm2 V−1 s−1 to ≤10−8 cm2 V−1 s−1 in films displaying the highest degree of molecular order. This is ascribed to the increased interchain separation in the ordered systems along with effects due to macroscopic aggregate grain boundaries.
A magnetocalorimetric study of spin fluctuations in amorphous FexZr100−x
Authors: Dawson A. LeR. , Ryan D. H.
Published: July 2002

Low temperature calorimetry data on amorphous FexZr100−x samples near the ferromagnetic/paramagnetic phase boundary at x=37.5 are reported, as well as some magnetocalorimetry data in magnetic fields of up to 5 T. The data are analyzed in terms of two models, one describing the effect of spin fluctuations (SF) and the other the effect of superparamagnetic clusters. The data are shown to favor the SF model and taken with earlier resistivity work, provides convincing evidence for the existence of SF in a-FexZr100−x.
Growth and structure of rapid thermal silicon oxides and nitroxides studied by spectroellipsometry and Auger electron spectroscopy
Authors: Gonon N. , Gagnaire A. , Barbier D. , Glachant A.
Published: July 2002

Rapid thermal oxidation of Czochralski-grown silicon in either O2 or N2O atmospheres have been studied using spectroellipsometry and Auger electron spectroscopy. Multiwavelength ellipsometric data were processed in order to separately derive the thicknesses and refractive indexes of rapid thermal dielectrics. Results revealed a significant increase of the mean refractive index as the film thickness falls below 20 nm for both O2 or N2O oxidant species. A multilayer structure including an about 0.3-nm-thick interfacial region of either SiOx or nitroxide in the case of O2 and N2O growth, respectively, followed by a densified SiO2 layer, was found to accurately fit the experimental data. The interfacial region together with the densified state of SiO2 close to the interface suggest a dielectric structure in agreement with the continuous random network model proposed for classical thermal oxides. Auger electron spectroscopy analysis confirmed the presence of noncrystalline Si—Si bonds in the interfacial region, mostly in the case of thin oxides grown in O2. It was speculated that the initial fast growth regime was due to a transient oxygen supersaturation in the interfacial region. Besides, the self-limiting growth in N2O was confirmed and explained in agreement with several recently published data, by the early formation of a very thin nitride or oxynitride membrane in the highly densified oxide beneath the interface. The beneficial effect of direct nitrogen incorporation by rapid thermal oxidation in N2O instead of O2 for the electrical behavior of metal-oxide-semiconductor capacitors is likely a better SiO2/Si lattice accommodation through the reduction of stresses and Si—Si bonds in the interfacial region of the dielectric.
Simulation of the orientation-dependent growth of InGaAs/InP by metalorganic vapor-phase epitaxy
Authors: Bakin A. , Zwinge G. , Klockenbrink R. , Wehmann H.-H. , Schlachetzki A.
Published: July 2002

Growth of InGaAs/InP heterostructures on patterned (100) InP substrates by metalorganic vapor-phase epitaxy (MOVPE) was investigated. Anisotropic growth rates as obtained by measurements were exploited to precisely predict defined growth shapes. We present a model with a complete set of parameters to simulate the growth of In0.53Ga0.47As and InP by MOVPE. The model relies on a graphical construction procedure to determine the cross-sectional structure of heterostructures on patterned substrates.
Characterization of polyester films used in capacitors. I. Transient and steady-state conductivity
Authors: Thielen A. , Niezette J. , Feyder G. , Vanderschueren J.
Published: July 2002

Charging and discharging currents flowing through polyethylene terephthalate (PET) ultrathin films (1.5–12 μm) were measured by the use of a two-electrode configuration involving opposite lateral contacts. A study of the influence of electrification time, applied electric field, film thickness, nature of electrodes, and water content was carried out on both transient and steady-state conduction. The transient behavior can be interpreted in terms of dipolar orientation and relaxation processes while steady-state conductivity can be mainly accounted for in terms of Schottky emission. A comparison between PET and polyethylene naphthalate films is also reported.
Computer simulation of photoconductivity decay in AgBr microcrystals: Relaxation model
Authors: Hailstone R. K. , Erdtmann D. E.
Published: July 2002

Previous workers have explained photoconductivity decay in AgBr microcrystals by proposing a model involving lattice relaxation following shallow surface trapping of electrons. This model is investigated using Monte Carlo simulations in which the electron and hole undergo a random walk in the volume of the microcrystals and are shallowly trapped at the surface. The decay rate is controlled by the thermal barrier that must be overcome to achieve a lower-energy lattice configuration, as well as by the density and cross section of the shallow surface traps. The simulation results lend support to the lattice relaxation model, but also suggest a significant role of free-electron/trapped-hole recombination in the photoconductivity decay. The experimentally observed increase in decay time with increasing microcrystal size is also seen in the simulations. This size dependence is due to the effect of the surface-to-volume ratio on the distribution between free and shallowly trapped states for the electron.
(Double) relaxation oscillation SQUIDs with high flux-to-voltage transfer: Simulations and experiments
Authors: Adelerhof D. J. , Nijstad H. , Flokstra J. , Rogalla H.
Published: July 2002

Relaxation oscillation superconducting quantum interference devices (SQUIDs) (ROSs) and double relaxation oscillation SQUIDs (DROSs) have been fabricated and characterized. The SQUIDs are based on hysteretic Nb/Al,AlOx/Al/Nb Josephson tunnel junctions. The relaxation oscillations were simulated by computer. By these simulations the minimum and maximum values of the shunt inductance at which the relaxation oscillations persist have been determined for different values of the shunt resistance and of the critical current and capacitance of the junctions. The transfer from flux to dc voltage in ROSs increases linearly with the shunt resistance. The operation range of the shunt inductance, however, becomes smaller with increasing shunt resistance. In ROSs with a SQUID inductance of 20 pH, a maximum voltage modulation width of 400 μV and a flux-to-voltage transfer δV/δφ of 4 mV/φ0 have been obtained. Two ROSs have been operated in a simple flux-locked loop with direct voltage readout. The experimental white flux noise spectral density of about 3μφ0/√Hz is in good agreement with the theoretical model. The sensitivity of these SQUIDs is limited by so-called switching noise, which is noise due to the interaction between the relaxation oscillations and the Josephson frequency. In DROSs typical flux-to-voltage transfer values of 10–30 mV/φ0 and maximum values up to 80 mV/φ0 have been obtained. This transfer is determined by thermal noise on the critical currents of the SQUIDs. The effective flux noise density of DROSs in flux-locked-loop operation with direct voltage readout is in good agreement with the theoretical noise level which is mainly determined by thermal noise on the critical currents. In one of the DROSs with a SQUID inductance of 20 pH, a white flux noise of 1μφ0/√Hz has been obtained at an estimated relaxation frequency of about 80 MHz. This corresponds to an energy sensitivity of about 160 h.
Regularization techniques applied to depth profiling with photoacoustic spectroscopy
Authors: Hodgson R. J. W.
Published: July 2002

A variety of regularization procedures have been applied to the inverse problem associated with depth profiling with photoacoustic spectroscopy. This problem is expressed in terms of an inverse Laplace transform along a straight line in the complex plane. In an attempt to determine depth profiles which vary continuously, four regularization methods are investigated in conjunction with three different procedures for the determination of the regularization parameters. Using simulated data, it is observed that as long as there are no discontinuities in the depth profile function, most regularization methods work quite well if the quasioptimality function is employed to determine the associated parameter. Other methods of determining the regularization parameter are found to be less satisfactory.
Dissipation in macroscopic quantum tunneling and coherence in magnetic particles (invited)
Authors: Garg Anupam
Published: July 2002

For quantum phenomena involving a macrovariable such as the total moment of a ferromagnetic particle, or the Néel vector of an antiferromagnetic one, environmental couplings play a crucial role. The effect of such couplings, or dissipation, is studied, with emphasis on the nuclear spin environment. It is shown that magnetic macroscopic quantum tunneling (MQT) and coherence (MQC) both become harder to see. Quantitative calculations are presented for the tunneling rate in MQT, and the tunnel splitting and ac susceptibility for MQC. A recent claim to have seen MQC in a resonance experiment on ferritin is critically discussed and the absorption is found to be at least 1500 times larger than predicted by MQC.
Laser ablation of polymeric materials at 157 nm
Authors: Costela A. , García-Moreno I. , Florido F. , Figuera J. M. , Sastre R. , Hooker S. M. , Cashmore J. S. , Webb C. E.
Published: July 2002

Results are presented on the ablation by 157 nm laser radiation of polytetrafluoroethylene (PTFE), polyimide, polyhydroxybutyrate (PHB), poly(methyl methacrylate) (PMMA), and poly(2-hydroxyethyl methacrylate) with 1% of ethylene glycol dimethacrylate as a crosslinking monomer. Direct photoetching of PHB and undoped PTFE is demonstrated for laser fluences ranging from 0.05 to 0.8 J/cm2. The dependence of the ablation process on the polymer structure is analyzed, and insight into the ablation mechanism is gained from an analysis of the data using Beer–Lambert’s law and the kinetic model of the moving interface. Consideration of the absorbed energy density required to initiate significant ablation suggests that the photoetching mechanism is similar for all the polymers studied.
Structure and solid state reactions of laser-deposited and sputtered Fe/Nb and Fe/Ag multilayers
Authors: Luo Yuansu , Krebs Hans-Ulrich
Published: July 2002

The structure of laser-deposited and sputtered Fe/Nb and Fe/Ag multilayers was investigated by x-ray diffraction. Significant differences in the textures, sharpness of the interfaces, and critical bilayer thicknesses for alloys are obtained for these two deposition processes. For small bilayer thicknesses, the measured x-ray-diffraction patterns were analyzed by computer simulations using a one-dimensional model, which includes the occurrence of amorphous or disordered interlayers between the crystalline layers and takes layer thickness fluctuations into account. These simulations indicate that already in as-prepared Fe/Nb multilayers thin amorphous interlayers exist and describe the thickness increase during an interdiffusion reaction already in the early stages of the transformation. In the case of Fe/Ag multilayers, the interfaces are disordered after deposition and first sharpen during annealing at about 250 °C, before grain coarsening occurs.
Ge(001) gas-source molecular beam epitaxy on Ge(001)2×1 and Si(001)2×1 from Ge2H6: Growth kinetics and surface roughening
Authors: Bramblett T. R. , Lu Q. , Lee N.-E. , Taylor N. , Hasan M.-A. , Greene J. E.
Published: July 2002

The growth rates RGe of epitaxial Ge films deposited on Ge(001)2×1 and Si(001)2×1 substrates from Ge2H6 by gas-source molecular beam epitaxy were determined over a wide range of temperatures Ts (300–800 °C) and impingement fluxes JGe2H6(0.1–1×1016 cm−2 s−1). Steady-state RGe(Ts, JGe2H6) curves were well described at both low and high growth temperatures (Ts≤325 °C and Ts≳500 °C) using a model based upon dissociative Ge2H6 chemisorption followed by a series of surface decomposition reactions with the rate-limiting step being first-order hydrogen desorption from Ge monohydride for which the activation energy was found to be 1.56 eV. At intermediate temperatures, however, experimental RGe results exhibited a large positive deviation from model predictions due, as demonstrated by temperature programmed desorption measurements and transmission electron microscopy (TEM) observations, to kinetic surface roughening. Extensive (113) faceting resulted in both an increase in the number of active surface sites and higher reactive sticking probabilities. With increased growth temperatures, the facets became more rounded and film surfaces appeared sinusoidal in cross section. The zero-coverage Ge2H6 reactive sticking probability on Ge(001) in the high-temperature flux-limited regime was found to be 0.052, more than two orders of magnitude higher than that for GeH4. In situ reflection high-energy electron diffraction and post-deposition TEM examinations showed that Ge films deposited on Ge(001) at Ts≤325 °C grew in a layer-by-layer mode exhibiting a smooth flat surface.
Electrical properties and degradation kinetics of compensated hydrogenated microcrystalline silicon deposited by very high-frequency-glow discharge
Authors: Flückiger R. , Meier J. , Goetz M. , Shah A.
Published: July 2002

Microcrystalline silicon (μc-Si:H) layers deposited by the very high-frequency-glow discharge technique at a radio-frequency excitation of 70 MHz are observed to be basically slightly 〈n〉 type. By doping (so-called ‘‘microdoping’’) with boron in the gas phase volume part per million (vppm) range, compensated material could be obtained. The influence of this doping on the electronic transport properties is documented. A pronounced onset of the boron incorporation into the films measured by secondary-ion-mass spectrometry is observed around 3 vppm (B2H6/SiH4), together with marked changes in the electrical properties. The compensated film obtained for a microdoping of about 1 vppm shows the lowest dark conductivity [3×10−8 (Ω cm)−1], the highest activation energy (517 meV), and, finally, the highest photoconductive gain of 6×103 (photo/dark current ratio). Depending on the value of the activation energy (the critical value is ≊0.2 eV), two different transport models are identified, corresponding to ‘‘Meyer–Neldel’’ or ‘‘anti-Meyer–Neldel’’ behavior. As for light-induced degradation, the compensated film exhibits better stability than undoped films. Finally, the use of slightly boron doped μc-Si:H as photovoltaically active material will be discussed.
Analysis of deposition stress during thin-film growth on a relaxing substrate
Authors: Rosenberg Sara E. , Wong Peter Y. , Miaoulis Ioannis N.
Published: July 2002

Diamond films are becoming more prevalent for application in microelectronic devices, tool bits, and optical coatings because of diamond’s hardness and high thermal conductivity. Growth of these thin films on existing thin film/substrate structures occurs at temperatures high enough to induce structural relaxation in substrates with low melting points, such as germanium. Such relaxation affects the stress of the depositing film. A numerical model of film growth of one film on a multilayered structure that relaxes has been developed. Results show that significant tensile stresses in the depositing film arise during the deposition process, even though the end resulting stress is compressive. With faster deposition rates, the magnitude of this tensile stress is reduced. Further analysis suggests that allowing the substrate to relax initially before the deposition process begins is beneficial to avoid harmful tensile stresses.
Polarization bistability in strained ridge-waveguide InGaAsP/InP lasers: Experiment and theory
Authors: Berger G. , Müller R. , Klehr A. , Voss M.
Published: July 2002

New experimental and theoretical results on TE/TM bistability in 1.3 μm ridge-waveguide InGaAsP/InP bulk lasers at room temperature are presented. Measured polarization resolved light power–current (P–I) characteristics as well as lateral near- and far-field patterns are compared with results from a theoretical model based on the paraxial wave equations for TE- and TM-polarized modes and the diffusion equation for the carrier distribution. The model was numerically evaluated by use of the beam propagation method. The observed TE/TM bistability is explained by the interplay of three different effects: (i) Tensile stress of about 109 dyn/cm2 promotes the TM gain strongly enough to compete with the TE mode. (ii) Improved TM waveguiding due to an enhancement of the effective refractive index near the beam axis caused by carrier depletion with increasing current leads to the onset of TM lasing and TE/TM switching. (iii) The TE/TM transition is accompanied by an abrupt increase of spatial hole burning in the lateral carrier distribution. Because of this nonlinear effect, a lower current is needed to switch the laser back to TE, giving rise to a hysteresis loop in the P–I characteristics and to TE/TM polarization bistability.
Cathodoluminescence studies of exciton localization in GaAs-AlGaAs single quantum wells
Authors: Jahn U. , Fujiwara K. , Menniger J. , Grahn H. T.
Published: July 2002

The temperature dependence of the spectrally resolved cathodoluminescence intensity from a 3.5- nm-thick GaAs-AlGaAs single quantum well prepared by growth interrupted molecular beam epitaxy has been investigated between 5 and 120 K. As the temperature increases, we observe thermally activated carrier transfer from wider quantum well regions to narrower ones. This observation indicates that there is exciton localization within the wider quantum well regions, one or two monolayers larger in width, at low temperatures. The contrast in spectrally resolved cathodoluminescence images of the quantum well is significantly influenced by this exciton localization and therefore does not necessarily reflect the lateral island distribution.
Numerical study of fluxon dynamics in a system of two-stacked Josephson junctions
Authors: Petraglia A. , Ustinov A. V. , Pedersen N. F. , Sakai S.
Published: July 2002

The dynamics of magnetic fluxons in a system of two vertically stacked long Josephson junctions is investigated numerically. The model is based on the approach by S. Sakai, P. Bodin, and N. F. Pedersen [J. Appl. Phys. 73, 2411 (1993)] and is described by two strongly coupled sine-Gordon equations. In agreement with recent experimental data, we confirm numerically the effect of splitting of the fluxon travelling mode into two separated modes with different characteristic velocities. The simulated current-voltage characteristics indicate stable phase-locked flux-flow resonances of two junctions. These results support a possibility of application of the stacked long Josephson junctions as a system of coherent oscillators for millimeter and sub-millimeter wave bands.
Optical and electrical properties of Ag-As-S glasses
Authors: Ohto M. , Itoh M. , Tanaka K.
Published: July 2002

Physical properties of Ag2S-As2S3 glassy alloys have been studied as a function of the Ag atomic concentration x. With an increase in x, the optical-absorption edge exhibits a red shift, while the steepness of the Urbach tail changes little. In the glasses with x≥15, the electrical conductivity is governed by Ag+ ionic conduction. The ionic conductivity exponentially increases with x, and in contrast the hole conductivity hardly changes with x; however, the activation energies for the ionic and the hole conductivity are nearly the same at about 0.4 eV for compositions with 15 ≤x≤35. These observations are discussed on the basis of atomic and electronic structures in Ag2S-As2S3 glasses.
Formation and electronic properties of the CdS/CuInSe2 (011) heterointerface studied by synchrotron-induced photoemission
Authors: Löher T. , Jaegermann W. , Pettenkofer C.
Published: July 2002

The heterointerface p-CuInSe2/CdS was investigated by soft x-ray photoelectron spectroscopy. CdS was deposited sequentially in steps onto CuInSe2 (011) cleavage planes at room temperature (RT) and at elevated temperatures (≳120 °C). At RT a nonreactive interface to cubic CdS is formed. The valence band and conduction-band discontinuities are determined to be 0.8 and 0.7 eV, respectively. A band bending of 0.9 eV is deduced for the p-type substrate. Annealing to temperatures above 120 °C leads to the formation of a CuxS reactive layer at the interface. As a consequence the valence-band offset and band bending is found to be considerably reduced. The experimentally determined band energy diagram is in agreement with heterojunctions of zincblende-type semiconductors, and its consequences for solar cells are discussed.
Electronic states of vapor deposited electron and hole transport agents and luminescent materials for light-emitting diodes
Authors: Schmidt A. , Anderson M. L. , Armstrong N. R.
Published: July 2002

The electronic states of vapor-deposited materials used in electroluminescent devices were measured by ultraviolet and x-ray photoelectron spectroscopy, UV-visible absorbance, and photoluminescence spectroscopy. The combination of these measurements on ultrathin films of these materials allows (1) the determination of the energy (with respect to vacuum) of the highest occupied molecular orbital (HO) and the ionization potential (IP), and (2) the estimation of the lowest unoccupied molecular orbital (LU) energy and an approximation of the electron affinity, (EA). The knowledge of the binding energies of these states is important for the understanding of light-emitting diode properties and the potential optimization of such devices. The luminescent material tris(8-hydroxy-quinoline) aluminum has an IP of 5.9 eV and an apparent EA smaller than 3.5 eV. The IP of both hole transport agents, tri-p-tolylamine and 1,1-bis(4-di-p-tolylaminophenyl)cyclohexane, is 5.4 eV and their EA is estimated to be smaller than 1.8 eV. The electron transport agents 2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, 2-t-butyl-9,10-n,n’-dicyano-anthraquinonediimine and dicyano-diphenylsulfone differ in IP from 7.1 to 7.6 eV and the EA for these materials are estimated to be smaller than 3.5, 4.9 and 5.5 eV, respectively.
The capacitance of microstructures
Authors: Natori Kenji
Published: July 2002

The down sizing of devices has been so rapidly promoted that the device and the circuit feature size will break into the nanometer range less than 1000 Å at the beginning of the next century. It is of critical importance to investigate the characteristics of microstructure capacitances for analysis of future devices and circuits. We analyze the general features of microstructure capacitance based on the self-consistent field approximation, and clarify that it is decomposed into three components. One is the extension of the capacitance of classical perfect conductors discussed in electrostatics. The quantum-mechanical effect plays an important role in the other two components; one of them is proportional to the electronic density of states at the Fermi potential of the conductor in low temperatures. At room temperature this portion is proportional to the electronic charge in the conductor. The other is due to the delocalization of electronic charge off the surface into the bulk of conductor. These two components have only self-capacitance contributions. Various aspects of the microstructure capacitance, e.g., the order of magnitude, the diagrammatic expression, the charging energy, and the quantum dot charging are discussed.
Electron transmission in interacting quantum resonators
Authors: Takagaki Y. , Ploog K.
Published: July 2002

The quantum transport of electrons in a T-shaped waveguide structure is investigated using the tight-binding model. We take into account electron-electron interaction in the resonator region within the mean-field approximation. The scaling rule breaks down in the interacting system and the effect of the interaction is significant when the sample dimensions become large compared to the effective Bohr radius of electrons. The nearly perfect reflection due to the transmission resonances is shifted to higher energy as the repulsive interaction is increased. The interaction effect in the presence of short-range disorder is also examined.
Elastic strain relaxation in GaN–AlN–GaN semiconductor–insulator–semiconductor structures
Authors: Bykhovski A. D. , Gelmont B. L. , Shur M. S.
Published: July 2002

We calculated the elastic strain relaxation in wurtzite GaN–AlN–GaN semiconductor–insulator–semiconductor (SIS) structures. Elastic strain tensor components, elastic energy, the density of the misfit dislocations, and the other parameters of the system were obtained as functions of the AlN layer thickness. Theoretical values of the elastic strain relaxation are in satisfactory agreement with experimental data extracted from the capacitance-voltage (C-V) characteristics of GaN–AlN–GaN SIS structures. Our results confirm that the gradual relaxation process starts from 30 Å AlN film thickness. The uniform contributions to the elastic strain tensor components decrease by approximately an order of magnitude when the film thickness increases from 30 to 100 Å. Commensurate with this decrease is an increase in a nonuniform contribution of the misfit dislocations. The dislocation interactions lead to redistribution of dislocations within the 30–60 Å range of AlN film thicknesses.
High-resolution determination of the stress in individual interconnect lines and the variation due to electromigration
Authors: Ma Qing , Chiras S. , Clarke D. R. , Suo Z.
Published: July 2002

Large tensile stresses usually exist in metallic interconnect lines on silicon substrates as a result of thermal mismatch. When a current is subsequently passed any divergence of atomic flux can create superimposed stress variations along the line. Together, these stresses can significantly influence the growth of voids and therefore affect interconnect reliability. In this work, a high-resolution (∼2 μm) optical spectroscopy method has been used to measure the localized stresses around passivated aluminum lines on a silicon wafer, both as-fabricated and after electromigration testing. The method is based on the piezospectroscopic properties of silicon, specifically the frequency shift of the Raman line at 520 R cm−1. By focusing a laser beam at points adjacent to the aluminum lines, the Raman signal was excited and collected. The stresses in the aluminum lines can then be derived from the stresses in the silicon using finite element methods. Large variations of stress along an electromigration-tested line were observed and compared to a theoretical model based on differences in effective diffusivities from grain to grain in a polycrystalline interconnect line.
Phosphorus cracking efficiency and flux transients from a valved effusion cell
Authors: Johnson F. G. , Wood C. E. C.
Published: July 2002

The transient behavior of the flux from a valved effusion cell containing red phosphorus is shown to result from a low elemental sublimation coefficient (α=1.3×10−7) and not from the presence of white phosphorus in the sublimator region. The criteria for generating white phosphorus in the sublimator are given. The conversion from tetramers to dimers is modeled as a function of temperature and pressure. At higher pressures, higher temperatures are needed to convert P4 to P2. Experimental results are shown to be in qualitative agreement with results from thermodynamic equilibrium arguments.
Design of gate-confined quantum-dot structures in the few-electron regime
Authors: Chen Minhan , Porod Wolfgang
Published: July 2002

Numerical simulations for the design of gated delta-doped AlGaAs/GaAs quantum-dot structures in the few-electron regime are presented. The confining potential is obtained from the Poisson equation with a Thomas–Fermi charge model. The electronic states in the quantum dot are then obtained from solutions of the axisymmetric Schrödinger equation. Our model takes into account the effect of surface states by viewing the exposed surface as the interface between the semiconductor and air (or vacuum). Various gate configurations and biasing modes are explored. The simulations show that the number of electrons can be effectively controlled in the few-electron regime with combined enhancement and depletion gates.
Asymmetric polarization switching in triglycine sulfate crystals with free surface
Authors: Kugel V. D. , Rosenman G. , Shur D.
Published: July 2002

Basic switching parameters (switching transient current, switching time, etc.) observed during electrical polarization reversal of undoped ferroelectric triglycine sulfate crystals with free surface are asymmetric. A comparison of the experimental data with the theoretical models of Fatuzzo [Phys. Rev. 127, 1999 (1962)], Pulvari, and Kuebler [J. Appl. Phys. 29, 1742 (1958)] shows that the classic models cannot to the fullest describe this type of polarization reversal. It is suggested that the asymmetric switching is caused by the difference in the field electron emission from a metal switching electrode and a ferroelectric crystal.
Time-resolved photoluminescence measurement of the energy relaxation process in GaAs/AlGaAs quantum-well structures
Authors: Furuta T. , Tomizawa M. , Yoshii A.
Published: July 2002

The energy relaxation process of electrons in GaAs/AlGaAs multiple-quantum-well structures is investigated by time-resolved photoluminescence measurement using an up-conversion technique. The relaxation time becomes shorter as the well width becomes narrower. This means that the two-dimensional nature of electrons plays an important role in energy relaxation. In addition, the intersubband relaxation process is studied under several excitation conditions. The relaxation rate from the n=2 to the n=1 subband is found to be 1.2 ps, which is in good agreement with theoretical calculations.
Point defect-based modeling of diffusion and electrical activation of ion implanted boron in crystalline silicon
Authors: Jäger H. U.
Published: July 2002

The time evolution of the transient enhanced diffusion and of the electrical activation of boron in crystalline silicon during thermal annealing subsequent to boron ion implantation is modeled by a system of diffusion-reaction equations for the dopant species and the silicon point defects. The concept of point defect impurity pair diffusion under equilibrium conditions is used to describe the diffusion process. Outdiffusion of implantation-induced silicon self-interstitials and the kick-out reaction Bi■Bs+I are assumed to be the leading mechanisms for boron activation. In the case of low-dose boron ion implantation, we start from a defect distribution of Gaussian shape with one interstitial per implanted boron atom. For higher boron doses, the area density of this interstitial distribution is constant, but the depth position of its peak depends on boron dose. Local equilibrium for the reactions between the point defects and the boron species is postulated to be realized before the onset of diffusion. The computed boron depth profiles are compared to data from the literature. Implantation doses from 2×1014 cm−2 up to 5×1015 cm−2 are analyzed, annealing temperatures and times are considered over the ranges 800–1000 °C and 10 s–8 h, respectively. Although this approach is characterized by a number of simple assumptions, essential deficiencies are only found in certain cases of annealing subsequent to high-dose boron implantation. Trapping of free interstitials by extended defects seems to become important at low temperatures and for long annealing times. If the depth region with maximum boron concentration is in its as-implanted state close to amorphization, a boron overactivation which is beyond the present model can be found. For all other cases it is possible to achieve a reasonable modeling of transient enhanced diffusion and of electrical activation.
Simulation of the effect of bulk vacancy diffusion on the shape of sputtered films deposited onto trenches and vias
Authors: Winterton S. S. , Smy T. , Dew S. K. , Brett M. J.
Published: July 2002

We present a model for the shape evolution of a thin metal film due to the bulk diffusion of vacancies. High-temperature Al sputter deposition and postdeposition annealing are currently becoming significant (very large scale integrated circuit metallization processes. The attractiveness of these processes lies in the ability to fill and planarize high-aspect-ratio vias and contacts with very few process steps. Previous attempts to model these processes have not fully emphasized the role played by bulk diffusion and three-dimensional curvature. We present the differential equation and boundary conditions that describe the flow of vacancies in metal films. An approximate method of solution for this equation system is presented and incorporated into the thin-film deposition simulator SIMBAD (simulation of ballistic deposition). This simulator is then used to present results pertaining to both postdeposition annealing and high-temperature sputter deposition. Particular emphasis is placed upon determining the effects of via and contact geometries, wetting angles and deposition rate during sputtering.
Impact of structure on photogating
Authors: Bae Sanghoon , Fonash Stephen J.
Published: July 2002

In previous reports, we predicted with computer simulation that photocarriers from blue bias light can contribute under certain conditions to the quantum efficiency in the red wavelength part of the spectrum for hydrogenated amorphous silicon (a-Si:H) p-i-n and Schottky barrier structures. We termed this phenomenon photogating and subsequently experimentally verified its existence by demonstrating red wavelength quantum efficiency≳1 under blue bias light. In this report, we show how device structure affects this photogating and affects the wavelength ranges where it can appear. We show with computer simulations that certain structures can display a complementary phenomenon of blue quantum efficiencies≳1 under red bias light and discuss supporting experimental data.
The application of the photoacoustic transmittance oscillations for determining elastic constants in gallium and indium selenides
Authors: Ferrer Ch. , Segura A. , Andrés M. V. , Muñoz V. , Pellicer J.
Published: July 2002

Transmittance periodic oscillations are observed in GaSe and InSe on excitation with optical pulses. Such oscillations are explained in terms of photoacoustic generation of dilatational waves, which become resonant within the crystal. Spectral analysis of those oscillations in samples of different thickness has led to an accurate determination of the longitudinal acoustic-wave velocity along the crystallographic axis c.
Optimizing the giant magnetoresistance of symmetric and bottom spin valves (invited)
Authors: Egelhoff W. F. , Chen P. J. , Powell C. J. , Stiles M. D. , McMichael R. D. , Lin C.-L. , Sivertsen J. M. , Judy J. H. , Takano K. , Berkowitz A. E. , Anthony T. C. , Brug J. A.
Published: July 2002

We have attempted to optimize the values of the giant magnetoresistance in symmetric spin valves of the type NiO/Co/Cu/Co/Cu/Co/NiO (achieving 23.4%) and in bottom spin valves of the type Co/Cu/Co/NiO (achieving 17.0%), the largest values ever reported for such structures. The key elements in this achievement are improved vacuum conditions and careful attention to the film thicknesses.
The growth of magnetic Fe overlayers on sulphur passivated GaAs(100)
Authors: Anderson G. W. , Hanf M. C. , Norton P. R. , Kowalewski M. , Myrtle K. , Heinrich B.
Published: July 2002

This paper describes a new approach for producing epitaxial Fe overlayers on GaAs(100), which prevents the intermixing of the semiconductor elements in the overlayer. This is accomplished by sulphur passivation of the substrate surfaces in an aqueous ammonium sulphide solution prior to Fe deposition. bcc Fe(100) is observed to grow epitaxially on S/GaAs(100) substrates, with most of the S floating out as an ordered overlayer. No evidence of semiconductor interdiffusion into the Fe overlayer is observed. The Fe overlayers are observed to be ferromagnetic, with easy axes along the [010] directions. A uniaxial in-plane anisotropy is observed, in addition to the expected fourfold anisotropy, which is attributed to the bonding geometry at the interface. The saturation magnetization of these overlayers is found to be close to that of bulk Fe.
New method of evaluating the explicit magnetic-state-dependent energy of iron in semiempirical calculations
Authors: Krasko Genrich L.
Published: July 2002

The existing semiempirical methods for use in atomic computer simulations in iron (such as various modifications of the embedded atom method) have not explicitly taken account of the ferromagnetism per se. Rather, the adjustable parameters have simply been fitted to mechanical and thermal properties of bcc iron as a ferromagnetic phase. However, when the magnetic properties are essential in affecting localized atomic behavior (e.g., in the vicinity of crystal defects), these methods may be inadequate. To remedy the situation, a procedure is suggested for explicitly calculating the magnetic contribution to the energy of individual atoms, as a function of their atomic environment. The method uses the Stoner approach, as previously developed, and a data base obtained by the ab initio calculations on bcc Fe under hydrostatic and tetragonal deformations, and the (111) Fe free surface. The procedure may be incorporated in any semiempirical method: the ferromagnetic contributions to the bulk moduli, C11, C12, and C44 are easily calculated and may be used subsequently in adjusting the semiempirical method parameters. Preliminary testing shows a good agreement with the magnetic properties obtained by our ab initio calculations.
Magnetic tunneling effect in Fe/Al2O3/Ni1−xFex junctions
Authors: Tezuka N. , Miyazaki T.
Published: July 2002

The dependence of the magnetoresistance ratio, ΔR/Rs, on the Ni content has been studied in Fe/Al2O3/Ni1−xFex (0⩽x⩽1) tunneling junctions. The value of ΔR/Rs at 4.2 K increased with increasing x and exhibited a maximum of ∼35% at x=0.8. The result is discussed by taking into account the spin-polarization of ferromagnetic electrodes.
Wannier–Stark localization in asymmetric double-well superlattices
Authors: Agulló-Rueda F. , Grahn H. T. , Ploog K.
Published: July 2002

The Wannier–Stark localization of miniband states has been investigated in an asymmetric double-well superlattice. The period consists of 3.4 and 2.0 nm GaAs quantum wells separated by 1.4 nm Al0.15Ga0.85As barriers. Photocurrent spectra at 6 K reveal that the lowest excitonic peak near flat band splits up into eight peaks at moderate electric fields. At very high fields only one peak remains. The observed transitions are attributed to the Stark ladder splitting of the wide-well miniband only. This type of superlattice introduces a new degree of freedom for the tailoring of electro-optic devices.
A radiometric investigation of low-pressure rf sulfur discharges
Authors: Gibson N. D. , Kortshagen U. , Lawler J. E.
Published: July 2002

This investigation focuses on low-pressure S2 molecular rf discharges for use as diffuse radiators. Cylindrical fused silica S2–Ar and S2–Ne lamps have been tested at 13.56 MHz and the absolute radiative efficiencies have been measured. The radiation is emitted primarily between 2800 and 3500 Å. The UV radiative efficiencies were studied as a function of input power, buffer gas pressure, discharge radius, and applied external cooling. The maximum observed efficiency for a low power density, diffuse rf discharge is 7%. The measured UV efficiencies of the 13.56 MHz discharges are somewhat lower than those of low-pressure S2–Ar dc discharges. A reentrant lamp design has been tested as well and no improvements in efficiency were observed.
Mass spectrometric measurements of neutral reactant and product densities during Si etching in a high-density helical resonator Cl2 plasma
Authors: Donnelly V. M.
Published: July 2002

Line-of-sight mass spectrometry was used to sample both stable and reactive neutral species near the walls in the downstream region of a high-density helical resonator Cl2 plasma during etching of Si. In this region, where the positive ion density is 1–2×1011 cm−3, the Cl2 number density at a pressure of 8 mTorr decreases by about 20% when the plasma is ignited. At constant pumping speed, this percentage increases with decreasing pressure, reaching 30% at 1 mTorr. A decrease of about 10% is due to expansion of the gas, heated by the plasma to a measured temperature of 400±50 K, integrated over a distance of one mean-free path from the wall. This, therefore, accounts for about one-half to one-third of the drop in Cl2 number density. The remaining half to two-thirds of the decrease in Cl2 number density upon ignition of the plasma can be ascribed to the formation of Cl atoms and SiClx etch products. Cl atoms are detected throughout this pressure range; their percentage increases at the higher pressures at constant pumping speed. SiCl4 is the main etching product in the chamber, though not necessarily a primary product. Smaller amounts of SiCl2 and possibly SiCl are also present in the plasma. Within experimental error, chlorine mass balance is found at all pressures, indicating an overall consistency in the mass spectrometric calibration methods. The percent dissociations measured in this study are much higher than earlier values derived from Cl-atom measurements, and are more in line with recent measurements and model predictions for high-density plasmas. Relationships between the mass spectrometer geometry and detection efficiency were also investigated. The detection configuration with the quadrupole axis perpendicular to the line of sight was found to be superior to one in which the quadrupole axis was parallel to the line of sight. In the latter configuration, signals from Cl and Cl2 are anomolously large due to charge exchange, producing a collimated beam of fast neutrals at the sampling orifice.