bluuebunny/crossref_metadata_2025 huggingface dataset:
Sub-40 nm PtSi Schottky source/drain metal–oxide–semiconductor field-effect transistors
Authors: Wang C. , Snyder John P. , Tucker J. R.
Published: July 2002

PtSi source/drain p-type metal–oxide–semiconductor field-effect transistors (MOSFETs) have been fabricated at sub-40 nm channel lengths with 19 Å gate oxide. These devices employ gate-induced field emission through the PtSi ∼0.2 eV hole barrier to achieve current drives of ∼350 μA/μm at 1.2 V supply. Delay times estimated by the CV/I metric extend scaling trends of conventional p-MOSFETs to ∼2 ps. Thermal emission limits on/off current ratios to ∼20–50 in undoped devices at 300 K, while ratios of ∼107 are measured at 77 K. Off-state leakage can be reduced by implanting a thin layer of fully depleted donors beneath the active region to augment the Schottky barrier height or by use of ultrathin silicon-on-insulator substrates.
Effects of As/P exchange reaction on the formation of InAs/InP quantum dots
Authors: Yoon Sukho , Moon Youngboo , Lee Tae-Wan , Yoon Euijoon , Kim Young Dong
Published: July 2002

InAs self-assembled quantum dots (SAQDs) were grown on InP at various temperatures and V/III ratios by metalorganic chemical vapor deposition. The density, size distribution, and shape of the InAs SAQDs changed significantly with temperature and V/III ratio. Careful analysis of the total volume of the dots grown at various conditions showed that the volume far exceeded the amount of deposition supplied from the gas-phase sources. The amount of excess InAs and the aspect ratio (height/lateral size) of the SAQD increased with temperature and V/III ratio, strongly suggesting that the As/P exchange reaction at the surface played an important role in the kinetics of SAQD formation. Insertion of a lattice-matched InGaAs buffer layer suppressed the excess InAs formation, and lowered the aspect ratio, confirming the effect of the As/P exchange reaction.
Enhanced brightness and efficiency in organic electroluminescent devices using SiO2 buffer layers
Authors: Deng Z. B. , Ding X. M. , Lee S. T. , Gambling W. A.
Published: July 2002

Organic electroluminescent devices using SiO2 as a hole-injecting buffer have been fabricated. With the presence of the buffer, the luminance of the device reaches 1820 cd/m2 at the current density of 20 mA/cm2, which corresponds to an efficiency of 9.1 cd/A. The enhancements in brightness and efficiency are attributed to an improved balance of hole and electron injections due to blocking of the injected holes by the buffer layer and a more homogeneous adhesion of the hole transporting layer to the anode.
Deposition of passivated gold nanoclusters onto prepatterned substrates
Authors: Parker A. J. , Childs P. A. , Palmer R. E. , Brust M.
Published: July 2002

Gold nanoclusters, chemically passivated with decanethiol, have been deposited from solution onto silicon dioxide surfaces prepatterned by photolithography. After lift-off of the photoresist, preferential cluster accumulation is observed along the edges of the resist structures. Elsewhere on the hydrophilic surface, islands of clusters are observed. By contrast, HF treatment, creating a hydrophobic surface, leads to wetting of the unmasked regions of the substrate by the passivated clusters.
Structure and optical properties of ZnO/Mg0.2Zn0.8O superlattices
Authors: Ohtomo A. , Kawasaki M. , Ohkubo I. , Koinuma H. , Yasuda T. , Segawa Y.
Published: July 2002

ZnO/Mg 0.2 Zn 0.8 O superlattices with a band-gap offset of about 0.5 eV were epitaxially grown by laser molecular-beam epitaxy on a sapphire(0001) substrate using a ZnO buffer layer. The superlattice structure with a period ranging from 8 to 18 nm was clearly verified by cross-sectional transmission electron microscopy, Auger depth profile, and x-ray diffraction. As the well layer thickness decreased below 5 nm, the photoluminescence peak and absorption edge in the photoluminescence excitation spectra showed a blueshift, indicating a quantum-size effect.
Blue emission from Tm-doped GaN electroluminescent devices
Authors: Steckl A. J. , Garter M. , Lee D. S. , Heikenfeld J. , Birkhahn R.
Published: July 2002

Blue emission has been obtained at room temperature from Tm-doped GaN electroluminescent devices. The GaN was grown by molecular beam epitaxy on Si(111) substrates using solid sources (for Ga and Tm) and a plasma source for N2. Indium–tin–oxide was deposited on the GaN layer and patterned to provide both the bias (small area) and ground (large area) transparent electrodes. Strong blue light emission under the bias electrode was observable with the naked eye at room temperature. The visible emission spectrum consists of a main contribution in the blue region at 477 nm corresponding to the Tm transition from the G41 to the H63 ground state. A strong near-infrared peak was also observed at 802 nm. The relative blue emission efficiency was found to increase linearly with bias voltage and current beyond certain turn-on levels.
Photonic-crystal spot-size converter
Authors: Kosaka Hideo , Kawashima Takayuki , Tomita Akihisa , Sato Takashi , Kawakami Shojiro
Published: July 2002

We have demonstrated a spot-size converter (SSC) that is made of photonic crystals (PCs) and has a conversion ratio of 10:1 for a 1-μm-wavelength light beam. Its real-spatial distribution was narrowed by intentionally broadening its wave vector distribution and increasing effective refractive index. The advantage of this PC-based SSC over conventional bulk-based SSC are compactness (monolithic integration), positional independence, and extremely deep depth of focus. This PC-SSC is a candidate for an interface between photonic-crystal waveguides and conventional optical waveguides.
Polarity-dependent rejuvenation of ferroelectric properties of integrated SrBi2Ta2O9 capacitors by electrical stressing
Authors: Hong Suk-Kyoung , Hwang Cheol Seong , Kwon Oh Seong , Kang Nam Soo
Published: July 2002

The electric-field-induced rejuvenation behavior of the degraded ferroelectric properties of integrated Pt/SrBi2Ti2O9/Pt capacitors was investigated. Integration processes, especially plasma-enhanced chemical vapor deposition of the passivation layers, generate hydrogen ions and electrons which act as domain pinning centers and a source of a negative internal electric field. Domain pinning was found to reduce the remanent polarization (Pr) and internal field that induces an imprint to the positive bias direction. Alternating current cyclings with peak voltages of +/−6 V rejuvenated the degraded ferroelectric performance of the capacitors. Cycling with a negative bias was more effective in fixing the damage than was a positive bias. Baking at 125 °C again degraded the rejuvenated ferroelectric performance. The degree of re-degradation was also dependent on the polarity of the rejuvenating bias. The polarity-dependent behavior of rejuvenation was explained on the basis of a negative-internal-field model due to preferential capture of electrons from the plasma at the top electrodes.
Narrow photoluminescence peaks from localized states in InGaN quantum dot structures
Authors: Moriwaki Osamu , Someya Takao , Tachibana Koichi , Ishida Satomi , Arakawa Yasuhiko
Published: July 2002

Microscopic photoluminescence spectra were measured for self-assembled InGaN quantum dots (QDs) grown by metalorganic chemical vapor deposition. A thin aluminum layer with 400 nm square apertures was formed on the sample surface to reduce the number of QDs measured. We observed very sharp peaks whose spectral linewidths were typically 170 μeV at 3.5 K, the linewidth being limited by spectral resolution. Such sharp lines were not observed in similar experiments on a reference sample having single InGaN quantum well structure. These experimental results suggest that excitons are strongly confined in our InGaN QD structure.
Ionic contribution to the viscosity of dilute electrolyte solutions: Towards a microscopic theory
Authors: Chandra Amalendu , Bagchi Biman
Published: July 2002

The concentration dependence of viscosity of an electrolyte solution has remained largely an ill-understood problem of solution chemistry. Here we present a microscopic study of the problem aimed at removing this lacuna. A new microscopic expression for the ionic contribution to the viscosity of an electrolyte solution has been derived which expresses it in terms of the static and dynamic structure factors of the charge and the number densities of the electrolyte solution. This ionic contribution becomes the excess viscosity for extremely dilute solutions. The celebrated expression of Falkenhagen follows exactly from the microscopic expression in the limit of very low ion concentration. The present theory is a self-consistent theory which also includes the concentration dependence of the electrolyte friction on the ions. Numerical results reveal that the viscosity of a solution at finite concentration can be very different from that given by the Falkenhagen expression. The present theory predicts a stronger increase of viscosity with increase of ion concentration, especially for ions of higher valence which is in qualitative agreement with experimental results. The theory suggests that, for viscosity, the molecular nature of the ion–solvent interactions could be important even at very low ion concentration.
Langmuir–Taylor surface ionization of alkali (Li, Na, K) and alkaline earth (Ca, Sr, Ba) atoms attached to helium droplets
Authors: Stienkemeier F. , Wewer M. , Meier F. , Lutz H. O.
Published: July 2002

A beam of superfluid helium droplets (HeN, N≈104) doped with alkali and alkaline earth atoms is detected by Langmuir–Taylor surface ionization. We measured detection efficiencies and time constants as a function of the temperature of a hot rhenium surface. Compared to results on free K, Na, and Ba atoms we find practically no differences in the detection properties; apparently the helium host environment does not significantly influence the mechanism of surface ionization. In addition, data for Li, Ca, and Sr on detection probabilities and residence times of the ions on the hot rhenium surface will be presented for the first time. Time constants and desorption energies are extracted; for alkaline earth atoms the latter are found to be significantly higher compared to the desorption of the alkalis.
Conformal contact and pattern stability of stamps used for soft lithography
Authors: Bietsch Alexander , Michel Bruno
Published: July 2002

Patterning in soft lithography techniques such as microcontact printing or light-coupling mask lithography is mediated by surface topographical patterns of elastomeric stamps: intimate contact with the substrate is achieved locally at the protruding areas, whereas a gap remains between the substrate and recessed zones. This principle challenges the properties of the stamp, especially when printing high-resolution or extreme aspect-ratio patterns with high accuracy. On the one hand, the stamp must be soft enough to enable conformal contact with the substrate, which means that it must adapt elastically without leaving voids created by the natural roughness of the substrate. On the other hand, a precise definition of micropatterns requires a rigid material. In this article, we analyze the conditions of elasticity, roughness, and energy of adhesion to establish conformal contact between an elastomer and the target surface. Furthermore, we address questions of replication accuracy and evaluate local elastic deformation induced by normal forces using model calculations for simple pattern geometries. Pressure applied during contact leads to a sagging or collapse of the unsupported areas. We discuss implications on both material and pattern design that allow spontaneous propagation of conformal contact while inhibiting the spreading of collapse.
The dynamics of O2 adsorption on Pt(533): Step mediated molecular chemisorption and dissociation
Authors: Gee Adam T. , Hayden Brian E.
Published: July 2002

The dissociative adsorption of oxygen on the stepped Pt(533) surface has been investigated using supersonic molecular beams by measuring the initial dissociative sticking probability S0 as a function of incident kinetic energy Ei, angle Φ, surface temperature TS, and S as a function of coverage Θ. By comparison with dynamical data available on the Pt(111) surface we have been able to establish that step sites dominate the dissociative adsorption process. S0(Ei) for oxygen on Pt(533) at TS=200 in the energy range 52 meV–1.4 eV shows a similar functional dependence to results on Pt(111), however, the magnitude of S0 on Pt(533) is significantly greater at all energies. The measurement of S0(Φ), scattering in a plane perpendicular to the step direction at Ei=1.18 eV at TS=350 K, reveals a strong and asymmetric angular dependence which contains contributions associated with activated adsorption and dissociation of the chemisorbed precursor on the (111) terraces, and a second contribution associated with activated dissociation through a similar channel at the step sites. The latter exhibits a maximum in S0(Φ) at 35°, near the angle corresponding to the normal of the (100) step plane. S0(TS) at Ei=1.18 eV and Φ=0° reveals a much smaller temperature dependence in the range 150>TS(K)>800 on Pt(533) than on Pt(111). At Ei=1.18 eV and Φ=0° ca. 15%–25% of dissociation takes place through molecules impinging directly at step sites. The remaining fraction dissociate through activated adsorption of the chemisorbed species on the (111) terrace and subsequent partition between desorption, and dissociation at step sites. Dissociation of the chemisorbed precursor on the (111) terrace appears highly activated, a result which is consistent with theory. The rapid decrease in S0(Ei) observed below 0.15 eV on Pt(533), observed also on Pt(111), is consistent with a trapping mechanism where the need to dissipate energy limits the probability of adsorption, and subsequent dissociation, of the physisorbed precursor. Kinetic modelling of this partition on Pt(533), between the conversion of the physisorbed precursor to the chemisorbed species, and desorption yields ΔE=120 meV and vd/vpc=80. We conclude that the effective barrier to conversion of the physisorbed to chemisorbed species on Pt(533) is effectively zero. We conclude that defects will tend to dominate this conversion process on the close packed surface. In addition to this channel, at Ei=0.05 meV ca. 50% of molecules dissociate through the same channel operating at higher energy on Pt(533).
Dielectric and piezoelectric properties of sol–gel derived lead magnesium niobium titanate films with different textures
Authors: Park Jeong Hwan , Xu Fei , Trolier-McKinstry Susan
Published: July 2002

The piezoelectric and dielectric constants for Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT, 70/30) films with different orientations were measured. PMN–PT films were deposited on Pt(111)-passivated silicon substrates using a modified sol–gel process. The room temperature dielectric constants K for the {100}-oriented films were 2500–2600, while K for {111}-oriented films were 1900–2000. In both cases tan δ was less than 0.03. The dependence of the piezoelectric coefficient d31 of the PMN–PT films on the poling fields was investigated. The d31 coefficients of {100}-oriented PMN–PT films were found to range from −28 to −69 pC/N with poling field. The {100}-oriented PMN–PT films showed larger piezoelectric coefficient than {111}-oriented films. The d33 coefficients of the 1.5 μm thick {100} oriented PMN–PT films were ∼170–183 pC/N. The aging rate of −d31 was ∼4%–10%/decade.
Energy level alignment in organic-based three-layer structures studied by photoelectron spectroscopy
Authors: Greczynski G. , Kugler Th. , Salaneck W. R.
Published: July 2002

The alignment of energy band edges at interfaces in three-layer structures, important in polymer-based electroluminescent devices, have been studied using photoelectron spectroscopy. Careful choice of the materials allows investigation of the role of the substrate work function. These results are in agreement with the energy level diagram commonly assumed for the metal/polymer/metal structure, with the potential drop over the polymer layer being equal to the difference between the work functions of the metal contacts. Direct indication of the alignment of the electrochemical potential across the three-layer structure has been observed by photoelectron spectroscopy.
Charge storage and interface states effects in Si-nanocrystal memory obtained using low-energy Si+ implantation and annealing
Authors: Kapetanakis E. , Normand P. , Tsoukalas D. , Beltsios K. , Stoemenos J. , Zhang S. , van den Berg J.
Published: July 2002

Thin SiO2 oxides implanted by very-low-energy (1 keV) Si ions and subsequently annealed are explored with regards to their potential as active elements of memory devices. Charge storage effects as a function of Si fluence are investigated through capacitance and channel current measurements. Capacitance–voltage and source–drain current versus gate voltage characteristics of devices implanted with a dose of 1×1016 cm−2 or lower exhibit clear hysteresis characteristics at low electric field. The observed fluence dependence of the device electrical properties is interpreted in terms of the implanted oxide structure.
Scanning thermal microscopy of carbon nanotubes using batch-fabricated probes
Authors: Shi Li , Plyasunov Sergei , Bachtold Adrian , McEuen Paul L. , Majumdar Arunava
Published: July 2002

We have designed and batch-fabricated thin-film thermocouple cantilever probes for scanning thermal microscopy (SThM). Here, we report the use of these probes for imaging the phonon temperature distribution of electrically heated carbon-nanotube (CN) circuits. The SThM images reveal possible heat dissipation mechanisms in CN circuits. The experiments also demonstrate that heat flow through the tip-sample nanoscale junction under ambient conditions is dominated by conduction through a liquid film bridging the two surfaces. With the spatial resolution limited by tip radius to about 50 nm, SThM now offers the promising prospects of studying electron-phonon interactions and phonon transport in low dimensional nanostructures.
Efficient red electroluminescence from devices having multilayers of a europium complex
Authors: Hu Wenping , Matsumura Michio , Wang Mingzhao , Jin Linpei
Published: July 2002

In order to get red electroluminescence from a europium (Eu) complex with high efficiency, a hole-injection layer was inserted between the Eu-complex layer and an indium–tin–oxide electrode, and a hole-blocking layer was inserted between the Eu-complex and electron-transporting layers. To further improve the efficiency, devices having multiple-stacked Eu-complex (2.5 nm)/hole blocking (2.5 nm) units were fabricated. By stacking six units, the maximal luminance and emission efficiency of the red emission were increased to more than twice that from a device with a single Eu-complex layer.
Holographic encoding of fine-pitched micrograting structures in amorphous SiO2 thin films on silicon by a single femtosecond laser pulse
Authors: Kawamura Ken-ichi , Sarukura Nobuhiko , Hirano Masahiro , Hosono Hideo
Published: July 2002

Fine-pitched micrograting structures were holographically encoded in amorphous (a-) SiO2 thin films on silicon wafers by colliding a pair of focused pulses split from a single, mode-locked Ti: sapphire, femtosecond laser. A method enhancing the third-harmonic generation resulting from the nonlinearity of air adjusted the optical paths of the two pulses. Surface-relief-type gratings were formed on SiO2 glasses due to laser ablation when the laser power exceeded more than 0.3 mJ/pulse, while shallow grating structures were imprinted on a-SiO2 thin films by volume compaction (∼3%) when the irradiation power was reduced to ∼50 μJ/pulse. The postirradiation deepening of the valley of the grating structure was possible with chemical etching. The minimal spacing of 430 nm was encoded using the 800 nm laser.
Measurements of ion energy distributions by Doppler shift spectroscopy in an inertial-electrostatic confinement device
Authors: Khachan J. , Collis S.
Published: July 2002

Doppler shift spectroscopy was carried out on the discharge in a spherically symmetric inertial-electrostatic confinement system. This enabled the ion energy distributions, types, and densities of ionic species to be determined. A weakly ionized hydrogen radio-frequency discharge was used as the ion source for two spherical and concentric electrostatic grids. The inner and outer grids were the cathode and anode, respectively. It was found that the ion energy distribution consisted of a non-Maxwellian directional component, as well as a spatially isotropic Maxwellian distribution. The directional component consisted of three broadened energy peaks belonging to H3+ (20%), H2+ (60%), and H+ (20%). These ions had energies approximately 20% of the cathode potential. The temperature (in electronvolts) of the Maxwellian distribution was approximately 15% of the cathode potential.
Quantum-confined Stark shift in electroreflectance of InAs/InxGa1−xAs self-assembled quantum dots
Authors: Hsu T. M. , Chang W.-H. , Huang C. C. , Yeh N. T. , Chyi J.-I.
Published: July 2002

Electroreflectance was employed to study the electric-field effect on the interband transitions of InAs quantum dots embedded in an In0.16Ga0.84As matrix. The electric field caused an asymmetric quantum-confined Stark shift, which revealed a nonzero built-in dipole moment in the quantum dots. We found the ground-state and excited-state dipole moments to be in the same direction. The electron wave functions are distributed near the base of the quantum dot, with their centers located below the hole wave functions. We also observed a symmetric Stark shift in the wetting-layer transitions. This implies that the wetting-layer potential is symmetric, despite its being capped with quantum dots.
Visible emission from AlN doped with Eu and Tb ions
Authors: Jadwisienczak W. M. , Lozykowski H. J. , Berishev I. , Bensaoula A. , Brown I. G.
Published: July 2002

We report the observation of visible cathodoluminescence (CL) from AlN thin films grown on sapphire (0001) substrate by molecular beam epitaxy and doped by implantation with Eu3+ and Tb3+ ions. The strongest rare earth (RE) CL was observed from samples annealed at 1100 °C for 0.5 h in N2 ambient. The sharp characteristic emission lines corresponding to Eu3+ and Tb3+ intra-4fn shell transitions are resolved in the spectral range from 350 to 900 nm. The CL spectra were recorded over 1–16 keV electron energy in the temperature range of 8–330 K. The depth resolved CL spectral analysis gives the luminescence surface a dead layer thickness of ∼16 nm for implanted AlN samples. We observed several different recombination centers luminescing in the 286–480 nm spectral region due to the presence of structural defects and oxygen impurities. The time resolved spectra and the CL kinetics were studied. The decay times for 5D0→7F2 (Eu3+), 5D3→7F5 (Tb3+), and 5D4→7F6 (Tb3+) transitions at 300 K are ∼0.4, ∼0.9, and ∼0.4 ms, respectively. We also discuss possible excitation mechanisms of RE ions in AlN.
Compression of the dc drain current by electron trapping in AlGaN/GaN modulation doped field-effect transistors
Authors: Nozaki S. , Feick H. , Weber E. R. , Micovic M. , Nguyen C.
Published: July 2002

The frequently observed dc drain current compression of AlGaN/GaN modulation doped field-effect transistors is associated with partial loss of the two-dimensional electron gas caused by electron trapping. The behavior of the temperature-dependent electron concentration and persistent photoconductivity at low temperature in the AlGaN/GaN modulation doped heterostructure are indicative of the presence of DX centers in the AlGaN layer. Deep-level transient spectroscopy of the drain current reveals carrier trapping with activation energy of 0.28 eV. However, this value appears to be too small to explain the compression of the dc drain current or to attribute these traps to DX centers in AlGaN.
Nonlinear coaxial photonic crystal
Authors: Poirier Louis , Haché Alain
Published: July 2002

We demonstrate that large-scale photonic crystals can be used to simulate nonlinear optical effects occurring in real photonic crystals. A crystal made of coaxial segments with periodic impedance is used to create a stop band in transmission near 10 MHz. When a semiconductor rectifying diode is added to the crystal, a nonlinear mode of propagation appears within the forbidden band gap. It originates from a breaking of symmetry and an intensity-dependent attenuation similar to that encountered in saturable absorbers. Experimental results agree well with a theory based on a simple coupled-mass model with nonlinear resistive force.
Silicon nanocrystals and Er3+ ions in an optical microcavity
Authors: Iacona Fabio , Franzò Giorgia , Ceretta Moreira Eduardo , Priolo Francesco
Published: July 2002

The characteristics of Si nanocrystals (nc) and Er-doped Si nc embedded within Si/SiO2 Fabry–Perot microcavities are investigated. It is shown that very narrow (Δλ∼1.5 nm) and intense luminescence peaks can be obtained within the 600–1000 nm wavelength range for Si nc and at around 1.54 μm for Er-doped Si nc by properly varying the cavity resonance. The luminescence intensity of the on-axis emission is over an order of magnitude above that of similar samples without a cavity and the overall luminescence is confined within a 30° cone from the sample normal. The properties of these cavities are presented and the implications of the results discussed.
Protonated nitrous acid (H2ONO+): Molecular structure, vibrational frequencies, and proton affinity
Authors: Francisco Joseph S.
Published: July 2002

The equilibrium structures, harmonic vibrational frequencies of HONO, and protonated HONO have been investigated using ab initio methods. The ab initio methods include second-order Møller-Plesset (MP2) perturbation theory, single-and double-quadratic configuration interaction (QCISD) theory, and the QCIDS(T) method, which incorporates a perturbational estimate of the effects of connected triple excitations. The QCISD(T) equilibrium geometry and vibrational frequencies for nitrous acid are in good agreement with experiment. The lowest energy form of protonated HONO is a complex between water and NO+. The next lowest isomer is 35.1 kcal mol−1 higher in energy. The ab initio proton affinity (PA) of HONO is predicted to be 191.5 kcal mol−1 at the QCISD(T)/6-311++G(3df,3pd) level of theory.
Hardness of irradiated poly(methyl methacrylate) at elevated temperatures
Authors: Lu K.-P. , Lee Sanboh , Cheng Cheu Pyeng
Published: July 2002

The decrease in hardness induced by gamma irradiation in poly(methyl methacrylate) (PMMA) has been investigated. The hardness is assumed to decrease linearly with the concentration of radiation-induced defects. Annealing at high temperatures induces defect annihilation as tracked by an increase in hardness. The annihilation follows first-order kinetics during isothermal annealing. The dependence of hardness on the reciprocal of the time constant satisfies the Arrhenius equation, and the corresponding activation energy of the kinetic process decreases with increasing dose. The hardness of postannealed PMMA decreases linearly with increasing dose.
GaAs dot-wire coupled structures grown by selective area metalorganic vapor phase epitaxy and their application to single electron devices
Authors: Nakajima Fumito , Ogasawara Yuu , Motohisa Junichi , Fukui Takashi
Published: July 2002

We describe a method for fabricating GaAs dot arrays and dot-wire coupled structures having periodic nanofacets which uses selective area metalorganic vapor phase epitaxy. First, a thin GaAs buffer layer and an AlGaAs layer are grown on a masked substrate having wirelike openings with periodic width modulation. The width of AlGaAs wirelike structure is naturally squeezed by the periodic combination of nanofacets, and its top (001) surface is partially isolated by a self-limited region. Next, an AlGaAs/GaAs quantum well structure is fabricated on the substrate to form dots on the narrower top terraces, wires on the wider terraces, and ridge wires in the self-limited region. Cathodoluminescence images clearly showed dot arrays and dot-wire coupled structures were formed using this method. A single electron transistor with the same structure was also fabricated, and clear Coulomb blockade oscillation was observed. We also describe single electron tunneling devices with these dot arrays and dot-wire coupled structures.
Temporal evolution of GaSb/GaAs quantum dot formation
Authors: Müller-Kirsch L. , Heitz R. , Pohl U. W. , Bimberg D. , Häusler I. , Kirmse H. , Neumann W.
Published: July 2002

The formation of GaSb quantum dots in a GaAs matrix in the Stranski–Krastanow growth mode under metalorganic chemical vapor deposition conditions is investigated. Transmission electron microscopical images and photoluminescence measurements show the islands to nucleate during the GaSb deposition and to grow subsequently by mass transfer from the two-dimensional wetting layer. The evolving surface morphology indicates local equilibria between quantum dots and the surrounding wetting layer regions.
Control systems with stochastic feedback
Authors: Allison Andrew , Abbott Derek
Published: July 2002

In this paper we use the analogy of Parrondo’s games to design a second order switched mode circuit which is unstable in either mode but is stable when switched. We do not require any sophisticated control law. The circuit is stable, even if it is switched at random. We use a stochastic form of Lyapunov’s second method to prove that the randomly switched system is stable with probability of one. Simulations show that the solution to the randomly switched system is very similar to the analytic solution for the time-averaged system. This is consistent with the standard techniques for switched state-space systems with periodic switching. We perform state-space simulations of our system, with a randomized discrete-time switching policy. We also examine the case where the control variable, the loop gain, is a continuous Gaussian random variable. This gives rise to a matrix stochastic differential equation (SDE). We know that, for a one-dimensional SDE, the difference between solution for the time averaged system and any given sample path for the SDE will be an appropriately scaled and conditioned version of Brownian motion. The simulations show that this is approximately true for the matrix SDE. We examine some numerical solutions to the matrix SDE in the time and frequency domains, for the case where the noise power is very small. We also perform some simulations, without analysis, for the same system with large amounts of noise. In this case, the solution is significantly shifted away from the solution for the time-averaged system. The Brownian motion terms dominate all other aspects of the solution. This gives rise to very erratic and “bursty” behavior. The stored energy in the system takes the form a logarithmic random walk. The simulations of our curious circuit suggest that it is possible to implement a control algorithm that actively uses noise, although too much noise eventually makes the system unusable.
Defect-induced lateral chemical heterogeneity at Ni/GaN interfaces and its effect on the electronic properties of the interface
Authors: Barinov A. , Gregoratti L. , Kaulich B. , Kiskinova M. , Rizzi A.
Published: July 2002

Scanning photoemission microscopy (SPEM) has been used to investigate the effect of morphological defects in GaN films grown on a 6H–SiC substrate on the composition and electronic properties of Ni/GaN interfaces in the temperature range of 25–600 °C. The SPEM imaging and spectroscopy identified a direct relation between the defects and the development of spatial heterogeneity in the interfacial composition, best pronounced after moderate annealing at 300 °C. The Schottky barrier height measured at these heterogeneous interfaces changes with advancement of the Ni–GaN reaction at elevated temperatures but exhibits negligible spatial variations.
Room-temperature ultraviolet emission from an organic light-emitting diode
Authors: Qiu C. F. , Wang L. D. , Chen H. Y. , Wong M. , Kwok H. S.
Published: July 2002

Ultraviolet emission was obtained from N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine. Gallium nitride was used as a hole-blocking layer to contain the holes. A peak emission wavelength of 400 nm was measured. The external quantum efficiency is about 0.35%.
Pattern formation in the flow of thin films down an incline: Constant flux configuration
Authors: Kondic L. , Diez J.
Published: July 2002

We present fully nonlinear time-dependent simulations of a thin liquid film flowing down an inclined plane. Within the lubrication approximation, and assuming complete wetting, we find that varying the inclination angle considerably modifies the shape of the emerging patterns: Finger-shaped patterns result for the flow down a vertical plane, while saw-tooth patterns develop for the flows down an inclined plane. However, in all of our simulations, the roots always move, indicating that the shape of the patterns is not necessarily related to the surface coverage, a technologically important feature of the flow. Furthermore, we find that triangular steady-state patterns may be produced for the flow down an incline, while the fingers typically grow in length for all explored times. We find quantitative agreement with reported experiments, and suggest new ones.
The pure rotational spectra of SrSH (X̃ 2A′) and SrS (X 1Σ+): Further studies in alkaline-earth bonding
Authors: Halfen D. T. , Apponi A. J. , Thompsen J. M. , Ziurys L. M.
Published: July 2002

The pure rotational spectrum of the SrSH radical in its ground electronic (X̃  2A′) and vibrational states has been measured using millimeter/submillimeter-wave direct absorption techniques. This work is the first observation of SrSH with rotational resolution. The spectrum of its deuterium isotopomer SrSD and SrS (X  1Σ+) has been recorded as well. These species were created by the reaction of strontium vapor and H2S, in the presence of a dc discharge. SrS was also made with CS2. For SrSH and SrSD, eight rotational transitions were recorded, respectively, for which asymmetry components up to Ka=8 were measured; fine structure was also resolved in each component. Thirteen transitions of SrS in each of its v=0, 1, and 2 states have additionally been observed. These data have been analyzed and spectroscopic parameters determined for all three species, including spin-rotation terms for the strontium hydrosulfides. From an r0 structure calculation, the bond angle in SrSH was determined to be 91.48(3)°, very close to that of H2S and CaSH. This geometry indicates that SrSH is a covalently bonded molecule, as opposed to linear (and ionic) SrOH. The Sr–S bond length in SrSH was also found to be greater than that of SrS (rSr—S=2.705 Å versus 2.441 Å), indicating a change in bond order. In addition, the spin-rotation interaction in SrSH and SrSD includes a small contribution from the off-diagonal term, (εab+εba)/2, resulting from the crossing of energy levels with ΔJ=0, ΔKa=±1. Second-order spin-orbit coupling appears to make a significant contribution to the spin-rotation splitting, as well, which must arise from mixing of the Ã 2A′ and B̃ 2A″ excited states.
Low temperature synthesis of MgB2
Authors: Rogado N. , Hayward M. A. , Regan K. A. , Wang Yayu , Ong N. P. , Zandbergen H. W. , Rowell John M. , Cava R. J.
Published: July 2002

We report the synthesis of MgB2 with bulk superconducting properties by conventional solid state methods at temperatures as low as 550 °C. Mg deficiencies of the type Mg1−xB2 were tested. Tc was found to decrease by about 1 K at large x, though the amount of nonstoichiometry, if any, is likely to be very small. For specific processing conditions, indications of the 25–30 K transition often seen in thin films were seen in the bulk materials. The lower temperature transition may be associated with the grain boundaries. These results indicate that it should be possible to fabricate MgB2 with bulk properties in in situ thin films at temperatures of 600 °C or less.
Effect of Fe doping on the magnetotransport properties in Nd0.67Sr0.33MnO3 manganese oxides
Authors: Chang Y. L. , Huang Q. , Ong C. K.
Published: July 2002

We have performed magnetic and transport measurements on a series of Nd0.67Sr0.33Mn1−xFexO3 polycrystalline compounds with x=0.0, 0.05, 0.10, and 0.15. For the Fe-undoped Nd0.67Sr0.33MnO3 (NSMO) materials, a magnetoresistance (MR) as high as ∼33% was observed at the metal–insulator transition temperature, Tp=273 K, in a magnetic field of 10 kOe. Fe substitution in Mn sites leads to a reduction in Tp and an increase in the overall MR. A 10% Fe contribution increases the MR up to about 65% and lowers Tp to 88 K. The calculated magnetic moment at 5 K and an applied field at 9 T for the parent NSMO is 4.21 μB and decreases continuously with an increasing amount of iron added. It was found that with the same amount of Fe doping, the Curie temperature, TC, decreases much faster in the Nd-based system than in the corresponding La-based system. The enhanced colossal magnetoresistance and the suppression of ferromagnetism observed in this compound can be interpreted as due to the weakening of the double exchange mechanism by Fe3+ ions, which causes the localization of the hopping electrons.
Terahertz gain in a SiGe/Si quantum staircase utilizing the heavy-hole inverted effective mass
Authors: Soref Richard A. , Sun Gregory
Published: July 2002

Modeling and design studies show that a strain-balanced Si1−xGex/Si superlattice on Si1−yGey-buffered Si can be engineered to give an inverted effective mass HH2 subband adjacent to HH1, thereby enabling a 77 K edge-emitting electrically pumped p–i–p quantum staircase laser for THz emission at energies below the 37 meV Ge–Ge optical phonon energy. Analysis of hole-phonon scattering, lifetimes, matrix elements, and hole populations indicates that a gain of 450 cm−1 will be feasible at f=7.3 THz during 1.7 kA/cm2 current injection.
Fabrication of GaAs symmetric pyramidal mesas prepared by wet-chemical etching using AlAs interlayer
Authors: Kicin S. , Cambel V. , Kuliffayová M. , Gregušová D. , Kováčová E. , Novák J. , Kostič I. , Förster A.
Published: July 2002

We present a wet-chemical-etching method developed for the preparation of GaAs four-sided pyramid-shaped mesas. The method uses a fast lateral etching of AlAs interlayer that influences the cross-sectional profiles of etched structures. We have tested the method using H3PO4:H2O2:H2O etchant for the (100) GaAs patterning. The sidewalls of the prepared pyramidal structures together with the (100) bottom facet formed the cross-sectional angles 25° and 42° for mask edges parallel, resp. perpendicular to {011} cleavage planes. For mask edges turned in 45° according to the cleavage planes, 42° cross-sectional angles were obtained. Using the method, symmetric and more than 10-μm-high GaAs “Egyptian” pyramids with smooth tilted facets were prepared.
Improved surface imaging with a near-field scanning microwave microscope using a tunable resonator
Authors: Hong Sunghyuk , Kim Jooyoung , Park Wonkyun , Lee Kiejin
Published: July 2002

We report a microwave surface imaging technique using a near-field scanning microwave microscope with a tunable resonance cavity. By tuning the resonance cavity, we could demonstrate improved sensitivity and spatial resolution of the topographic image of YBa2Cu3Oy thin films on MgO substrates. By measuring the shift of resonant frequency and the change of quality factor, we obtained near-field scanning microwave images with a spatial resolution better than 4 μm at an operating frequency of f=1–1.5 GHz. The principal of operation could be explained by the perturbation theory of a coaxial resonant cavity, considering the radius of the probe tip, the sample–tip distance, and the impedance matching.
Photoluminescence properties of MgS/CdSe quantum wells and quantum dots
Authors: Funato M. , Balocchi A. , Bradford C. , Prior K. A. , Cavenett B. C.
Published: July 2002

The optical properties of MgS/CdSe quantum structures grown by molecular beam epitaxy are characterized by photoluminescence (PL) spectroscopy. The increase in the CdSe thickness from 1 to beyond 3 ML results in the formation of, at first, quantum wells (QWs) and then quantum dots (QDs) by Stranski–Krastanov growth. The PL temperature dependence measurements reveal that, in the QWs, excitons localized by potential fluctuations principally govern the PL properties, which is in strong contrast to the QD PL properties.
Hole and electron field-effect mobilities in nanocrystalline silicon deposited at 150 °C
Authors: Cheng I-Chun , Wagner Sigurd
Published: July 2002

Field-effect structures were made from nanocrystalline silicon (nc-Si:H) deposited at a substrate temperature of 150 °C by plasma-enhanced chemical vapor deposition excited at 80 MHz. The nc-Si:H channel layer was grown on top of a separate nc-Si:H buffer and seed layer that serves to develop the crystalline structure. Staggering the contacts and the gate ensures that mobilities are measured precisely in the last-to-grow nc-Si:H layer. The hole mobility in saturation reaches 0.06–0.2 cm2 V−1 s−1 and the electron mobility ∼12 cm2 V−1 s−1. These results suggest that large-area circuits of complementary p- and n-channel devices can be made from nc-Si:H deposited on low-temperature substrates.
Observation and analysis of high-J o1−e1 inter-state transitions in CH2DOH
Authors: Mukhopadhyay Indranath , Perry David S. , Duan Yun-Bo , Pearson John C. , Albert S. , Butler Rebecca A. H. , Herbst Eric , DeLucia Frank C.
Published: July 2002

We report the observation and assignments of several series of transitions in CH2DOH, including a new and intense series of Q-branch c-type transitions between the e1 and o1 torsional substates through high values of the rotational quantum number J. Other transitions assigned are c-type R-branch and P-branch transitions between these two substates and a-type lines within the e1 and o1 substates. The assignments were facilitated by initial analysis of the strong Q-branch series, as recorded by the fast scan submillimeter spectroscopy technique (FASSST). The assigned lines of CH2DOH include the first inter-state transitions in the sub-millimeter-wave region and the first to possess high J-values. The completeness of the data generated by FASSST and the success of a simple power series analysis suggest that many parts of the spectrum of CH2DOH may be far more tractable than previously believed. The data should be useful in the development of a full Hamiltonian and in the assignment of astronomical emission features.
Mechanism of enhanced luminescence in InxAlyGa1−x−yN quaternary alloys
Authors: Chen C. H. , Huang L. Y. , Chen Y. F. , Jiang H. X. , Lin J. Y.
Published: July 2002

We report a firm evidence of enhanced luminescence from InGaN-like clusters in InxAlyGa1−x−yN quaternary alloys. Photoluminescence (PL) and Raman scattering measurements have been employed to study the optical properties of these alloys. The excellent correlation between the phonon replica structures accompanying luminescence line and the observed InGaN-related phonon modes in Raman spectra provide a powerful evidence showing that the existence of InGaN-like clusters is responsible for the enhanced luminescence in InxAlyGa1−x−yN quaternary alloys. In addition, the dependence of the PL emission energy on temperature in the low-temperature regime and on excitation power density can also be explained consistently with recombination mechanisms involving the localized states attributed to InGaN-like cluster size fluctuations.
Raster-scan imaging with normal-incidence, midinfrared InAs/GaAs quantum dot infrared photodetectors
Authors: Stiff-Roberts A. D. , Chakrabarti S. , Pradhan S. , Kochman B. , Bhattacharya P.
Published: July 2002

We demonstrate normal incidence infrared imaging with quantum dot infrared photodetectors using a raster-scan technique. The device heterostructure, containing multiple layers of InAs/GaAs self-organized quantum dots, were grown by molecular-beam epitaxy. Individual devices have been operated at temperatures as high as 150 K and, at 100 K, are characterized by λpeak=3.72 μm, Jdark=6×10−10 A/cm2 for a bias of 0.1 V, and D*=2.94×109 cm Hz1/2/W at a bias of 0.2 V. Raster-scan images of heated objects and infrared light sources were obtained with a small (13×13) interconnected array of detectors (to increase the photocurrent) at 80 K.
Influence of temperature and drive current on degradation mechanisms in organic light-emitting diodes
Authors: Ishii Masahiko , Taga Yasunori
Published: July 2002

We measured luminance decay and half life for green organic light-emitting diodes composed of materials with high glass transition temperature. The devices were driven at dc, constant current density ranging from 3.8 to 130 mA/cm2 at 25, 85, and 120 °C. The shapes of the luminance decay curves were compared by redrawing the curves on a time scale normalized by each half life. Temperature and drive current shortened the half life. However, the normalized decay curve of the device driven at 120 °C had the same shape as that of the device driven at 25 °C. Drive current also left the shape of the decay curves unchanged. Therefore, we conclude that the temperature and drive current do not change the relative contribution of multiple degradation mechanisms.
Luminescence mechanisms in quaternary AlxInyGa1−x−yN materials
Authors: Ryu Mee-Yi , Chen C. Q. , Kuokstis E. , Yang J. W. , Simin G. , Khan M. Asif
Published: July 2002

Low-temperature photoluminescence investigations have been carried out in the quaternary AlInGaN epilayers and AlInGaN/AlInGaN multiple quantum wells (MQWs) grown by pulsed metalorganic chemical-vapor deposition (PMOCVD). With increasing excitation power density, the emission peaks in both AlInGaN epilayers and MQWs show a strong blueshift and their linewidths increase. The luminescence of the samples grown by PMOCVD is attributed to recombination of carriers/excitons localized at band-tail states. We also demonstrate the luminescence properties of AlInGaN and AlGaN materials grown by a pulsed atomic-layer epitaxy and conventional MOCVD, respectively.
Exploring invariant sets and invariant measures
Authors: Dellnitz Michael , Hohmann Andreas , Junge Oliver , Rumpf Martin
Published: July 2002

We propose a method to explore invariant measures of dynamical systems. The method is based on numerical tools which directly compute invariant sets using a subdivision technique, and invariant measures by a discretization of the Frobenius-Perron operator. Appropriate visualization tools help to analyze the numerical results and to understand important aspects of the underlying dynamics. This will be illustrated for examples provided by the Lorenz system.
A novel nonlinear car-following model
Authors: Addison Paul S. , Low David J.
Published: July 2002

The mathematical models used to describe the dynamical behavior of a group of road vehicles traveling in a single lane without overtaking are known as car-following models. These models are widely used in many commercially available microscopic traffic simulation software packages. They attempt to mimic the interactions between individual vehicles that are traveling sufficiently close together for the behavior of each vehicle to be dependent upon the motion of the vehicle immediately in front. In this paper we modify the traditional car-following model by adding a new nonlinear term to take account of the driver attempting to achieve a certain desired intervehicle separation distance as well as the traditional aim of matching the velocity of the vehicle ahead. Numerical solution of the resulting coupled system of nonlinear differential equations is used to analyze the stability of the equilibrium solution to a periodic perturbation. For certain parameter values chaotic oscillations are generated, consisting of a broad spectrum of frequency components. Such chaotic motion produces extremely complicated dynamical behavior that has an inherent lack of predictability associated with it. The results of simulating over a range of parameter values are presented and, where it is present, the degree of chaos is estimated.