bluuebunny/crossref_metadata_2025 huggingface dataset:
High-quality visible-blind AlGaN p-i-n photodiodes
Authors: Monroy E. , Hamilton M. , Walker D. , Kung P. , Sánchez F. J. , Razeghi M.
Published: July 2002

We report the fabrication and characterization of AlxGa1−xN p-i-n photodiodes (0⩽x⩽0.15) grown on sapphire by low-pressure metalorganic chemical vapor deposition. The devices present a visible rejection of six orders of magnitude with a cutoff wavelength that shifts from 365 to 338 nm. Photocurrent decays are exponential for high load resistances, with a time constant that corresponds to the RC product of the system. For low load resistances, the transient response becomes non-exponential, with a decay time longer than the RC constant. This behavior is justified by the strong frequency dependence of the device capacitance. By an admittance analysis, we conclude that speed is not limited by deep levels, but by substitutional Mg capture and emission time.
Raman microscopy of lateral epitaxial overgrowth of GaN on sapphire
Authors: Pophristic M. , Long F. H. , Schurman M. , Ramer J. , Ferguson I. T.
Published: July 2002

We have used confocal Raman microscopy to investigate lateral epitaxially overgrown (LEO) GaN on sapphire substrates. The one-phonon Raman spectra are consistent with pyramidal growth of the GaN before coalescence has occurred. The position and asymmetric line shape of the A1 longitudinal optical (LO) phonon demonstrate that the LEO GaN is doped. The dopant is most likely Si from the SiN mask used to produce the LEO GaN. The carrier concentration is estimated to be 1×1017 cm−3. We have also used Raman microscopy to spatially resolve the yellow emission from different regions of the LEO GaN.
Self-assembled carbon-induced germanium quantum dots studied by grazing-incidence small-angle x-ray scattering
Authors: Stangl J. , Holý V. , Mikulı́k P. , Bauer G. , Kegel I. , Metzger T. H. , Schmidt O. G. , Lange C. , Eberl K.
Published: July 2002

We present a structural investigation of buried C-induced Ge quantum dot multilayers grown on (001) Si by molecular-beam epitaxy. Using grazing-incidence small angle x-ray scattering, we determine the shape, the mean radius, height, and dot distance. The dot distribution is isotropic within the (001) interfaces, and no correlation of the dot positions along growth direction was found.
GaN grown on Si(111) substrate: From two-dimensional growth to quantum well assessment
Authors: Semond F. , Damilano B. , Vézian S. , Grandjean N. , Leroux M. , Massies J.
Published: July 2002

We report on the epitaxial growth of high quality GaN films on Si(111) substrates by molecular beam epitaxy using ammonia. The surface morphology and crystallinity of thick undoped GaN films are characterized by reflection high-energy electron diffraction (RHEED), scanning electron microscopy, and x-ray diffraction. Films having compact morphologies and flat surfaces are obtained and RHEED intensity oscillations are demonstrated for GaN and (Al, Ga)N alloys indicating two-dimensional growth. This has been applied to the growth of AlGaN/GaN quantum well (QW) structures. Low-temperature photoluminescence (PL) spectra of GaN are dominated by a strong and narrow (full width at half maximum=5 meV) band edge luminescence intensity at 3.471 eV assigned to donor bound exciton recombination. PL properties of AlGaN/GaN QW are also very similar to those obtained on equivalent structures grown on sapphire.
Chirp dependence in InGaAs/InAlAs multiple quantum well electro-absorptive modulators near polarization-independent conditions
Authors: Pamplona Pires M. , Yavich B. , Souza P. L.
Published: July 2002

We study the dependence of the chirp parameter on heavy- and light-hole energy splitting by analyzing the change in the absorption curve of different InGaAs/InAlAs p-i-n multiple quantum well structures designed for use in amplitude modulators. We observe, for the transverse electric mode, a high chirp parameter for the sample whose fundamental transition involves the light hole, whereas for samples whose fundamental transition involves the heavy hole, the more polarization sensitive the samples are, the smaller the chirp parameter is. This indicates that it is not possible to have tensile strained InGaAs/InAlAs multiple quantum well structures for electro-absorptive modulators which are simultaneously chirp-free and polarization independent.
Tunable, narrow, and directional luminescence from porous silicon light emitting devices
Authors: Chan Selena , Fauchet Philippe M.
Published: July 2002

Controlled room-temperature photoluminescence (PL) and electroluminescence (EL) from porous silicon (PSi) multilayer structures are achieved. The basic structure is composed of two PSi multilayer mirrors sandwiching a highly luminescent PSi film. This creates an active microcavity resonator, in which a significant PL and EL line narrowing is observed. EL from a microcavity resonator is shown to have a high angular concentration of the output emission (30° around the normal axis). A tunable and narrow EL device, from an all-PSi microcavity resonator is presented, which offers the possibility for high spectral purity, flat-panel displays.
Midinfrared second-harmonic generation in p-type InAs/GaAs self-assembled quantum dots
Authors: Brunhes T. , Boucaud P. , Sauvage S. , Glotin F. , Prazeres R. , Ortega J.-M. , Lemaı̂tre A. , Gérard J.-M.
Published: July 2002

Resonant second-harmonic generation is reported in InAs/GaAs self-assembled quantum dots. Frequency doubling is observed between confined states in the valence band of the quantum dots. The second-order nonlinear susceptibility is maximum at 168 meV (7.4 μm wavelength) and is observed for an in-plane polarized excitation. A value of χzxx(2) as large as 2×10−7 (m/V) is measured for one dot plane. A three-dimensional numerical calculation of the valence band states shows that the second-harmonic generation involves a resonant excitation between the h000 and h101 states and a state close to the continuum wetting layer states.
Influence of the laser fluence on the electrical properties of pulsed-laser-deposited SrBi2Ta2O9 thin films
Authors: Bu S. D. , Park B. H. , Kang B. S. , Kang S. H. , Noh T. W. , Jo W.
Published: July 2002

Polycrystalline SrBi2Ta2O9 ferroelectric thin films were grown on Pt/Ti/SiO2/Si substrates using pulsed-laser deposition. By adjusting the laser fluence, we could successfully control remnant polarization of the films. In a narrow fluence range of 1.0–1.5 J/cm2, films with large remnant polarizations (as high as 18.7 μC/cm2) could be obtained. The choice of an optimal laser fluence was found to be very important to control electrical properties of the films. From electron-probe microanalysis, it was demonstrated that the Bi content is closely related with the remnant polarization.
Built-in field electroabsorption spectroscopy of polymer light-emitting diodes incorporating a doped poly(3,4-ethylene dioxythiophene) hole injection layer
Authors: Brown T. M. , Kim J. S. , Friend R. H. , Cacialli F. , Daik R. , Feast W. J.
Published: July 2002

We report electroabsorption measurements of polymer light-emitting diodes, (LEDs), fabricated with poly(4-4′-diphenylene diphenylvinylene), PDPV, as the emissive layer, Ca–Al cathodes, and indium tin oxide (ITO) anodes, with and without a doped conducting polymer hole injection/transport layer, namely poly(3,4-ethylene dioxythiophene), PEDOT, doped with poly(styrene sulfonate), PSS−. In these structures, the bias at which the electroabsorption signal is null corresponds to the difference between the electrodes’ work functions. We find that such a built-in voltage increases by 0.5 V when a PEDOT:PSS film is incorporated between the ITO electrode and the emissive layer. This leads to a marked reduction of the anode barrier height at the hole-injecting interface, and accounts for a variety of improvements brought about by the PEDOT insertion, namely: (a) the increase of luminescence efficiency, (b) the reduction of the turn-on voltage, and (c) the increase of the device lifetime.
Reduction of band-gap energy in GaNAs and AlGaNAs synthesized by N+ implantation
Authors: Shan W. , Yu K. M. , Walukiewicz W. , Ager J. W. , Haller E. E. , Ridgway M. C.
Published: July 2002

We have studied the optical properties of nitrogen implanted GaAs and AlGaAs samples. The fundamental band-gap energy has been found to decrease with the increasing N+ implantation dose in a manner similar to that commonly observed in GaNAs and GaInNAs alloys grown by molecular beam epitaxy or metal organic chemical vapor deposition. Our results indicate that GaNxAs1−x and AlxGa1−xNyAs1−y alloys can be formed by implantation of nitrogen followed by appropriate postimplantation annealing treatments. As inferred from the magnitude of the band gap shift, the percentage of the implanted N atoms incorporated on the substitutional As sites is estimated to be around 12%.
Phase separation and ordering coexisting in InxGa1−xN grown by metal organic chemical vapor deposition
Authors: Behbehani M. K. , Piner E. L. , Liu S. X. , El-Masry N. A. , Bedair S. M.
Published: July 2002

We have recently reported the occurrence of phase separation in InxGa1−xN samples with x>0.25. Theoretical studies have suggested that InxGa1−xN can phase-separate asymmetrically into a low InN% phase and an ordered high InN% phase. In this letter, we report on the existence of simultaneous phase separation and ordering of InxGa1−xN samples with x>0.25. In these samples, phase separation was detected by both transmission electron microscopy selected area diffraction (TEM-SAD) and x-ray diffraction. Ordering was detected by both imaging and TEM-SAD.
Vacuum ultraviolet absorption spectroscopy employing a microdiacharge hollow-cathode lamp for absolute density measurements of hydrogen atoms in reactive plasmas
Authors: Takashima Seigou , Hori Masaru , Goto Toshio , Kono Akihiro , Ito Masafumi , Yoneda Katsumi
Published: July 2002

We have developed a measurement technique for absolute H-atom densities in process plasmas using vacuum ultraviolet absorption spectroscopy employing a high-pressure microdischarge hollow-cathode lamp (MHCL) as a Lyman α (Lα, 121.6 nm) emission light source. Characterization of the Lα emission-line profile could be simplified by using a high-pressure discharge at about 1 atm. The effect of self-absorption in the MHCL was reduced to an insignificant level by decreasing the H2 partial pressure. The contribution of the collisional broadening to the Lα emission profile was estimated from the saturation characteristics of the absorption intensity when the optical thickness of the plasma was varied. The technique was applied to the measurement of the absolute H-atom density in an inductively coupled H2 plasma.
WS
2
nanotubes as tips in scanning probe microscopy
Authors: Rothschild A. , Cohen S. R. , Tenne R.
Published: July 2002

WS 2 nanotubes a few microns long were attached to microfabricated Si tips and tested afterwards in an atomic force microscope by imaging a “replica” of high aspect ratio, i.e., deep and narrow grooves. These WS2 nanotube tips provide a considerable improvement in image quality for such structures when compared with commercial ultrasharp Si tips. The nanotube tip apex shape was extracted by blind reconstruction from an image of Ti spikes, showing a smooth cylindrical profile up to the end.
The role of Er3+–Er3+ separation on the luminescence of Er–doped Al2O3 films prepared by pulsed laser deposition
Authors: Serna R. , de Castro M. Jiménez , Chaos J. A. , Afonso C. N. , Vickridge I.
Published: July 2002

Erbium-doped Al2O3 films have been deposited in a single step process by pulsed laser deposition using independent ablation of Al2O3 and Er targets. This procedure allows to control the Er3+ ions in-depth distribution. The characteristic Er3+ photoluminescence at 1.54 μm shows lifetime values which increase from 6.0 to 7.1 ms when the Er3+–Er3+ in-depth separation is increased from 3 to 9 nm. These results are discussed in terms of the ion–ion interaction and clustering for separations shorter than 6 nm.
Refractive indices and absorption coefficients of MgxZn1−xO alloys
Authors: Teng C. W. , Muth J. F. , Özgür Ü. , Bergmann M. J. , Everitt H. O. , Sharma A. K. , Jin C. , Narayan J.
Published: July 2002

Indices of refraction for MgxZn1−xO epitaxial films grown by pulsed-laser deposition on sapphire substrates with x up to 0.36 were determined in the range of wavelength 457–968 nm by analysis of optical transmission spectra and prism-coupled waveguide measurements. The dispersion follows the first-order Sellmeier dispersion equation. Absorption coefficients, exciton energy gaps, and binding energies of MgxZn1−xO alloys were determined by transmission spectroscopy. The excitonic absorption features were clearly visible at room temperature despite alloy broadening. These results provide important information for the design and modeling of ZnO/MgZnO heterostructure optoelectronic devices.
Suppression of exchange bias by ion irradiation
Authors: Mewes T. , Lopusnik R. , Fassbender J. , Hillebrands B. , Jung M. , Engel D. , Ehresmann A. , Schmoranzer H.
Published: July 2002

The exchange bias effect in ferromagnetic/antiferromagnetic sandwich structures is generally believed to be sensitive on the interface exchange interaction, the magnetization, and the thickness of the ferromagnetic layer. Also the interface structure plays a crucial role. We show that, by irradiating samples with He ions, we can manipulate the exchange bias field in a controlled manner. Depending on the dose (1014–1017 ions/cm2) and the acceleration voltage (10–35 kV) of the ions, the shift of the hysteresis can be reduced or even fully suppressed. Potential applications of this effect for magnetic patterning on the nanoscale will be discussed.
Photoreflectance spectra of excitonic polaritons in GaN substrate prepared by lateral epitaxial overgrowth
Authors: Chichibu S. F. , Torii K. , Deguchi T. , Sota T. , Setoguchi A. , Nakanishi H. , Azuhata T. , Nakamura S.
Published: July 2002

Photoreflectance (PR) spectra of high-purity, nearly free-standing GaN substrate were compared with emission and reflectance spectra, which were analyzed based on a model exciton-polariton picture in which A, B, and C free excitons couple simultaneously to an electromagnetic wave. The GaN substrate with reduced dislocation density was prepared by lateral epitaxial overgrowth technique and it exhibited predominant excitonic emissions with the decay time nearly 1 ns even at room temperature. The transition energies obtained from the PR spectrum agree with the energies of bottlenecks of the excitonic polariton branches. The result means that perturbation-induced change in the dielectric function is mainly due to polaritons. Temperature dependence of the A-exciton energy was well described using a model which assumes Einstein phonons.
Intersubband transitions in GaAs coupled-quantum-wells for use as a tunable detector at THz frequencies
Authors: Tomlinson A. M. , Chang C. C. , Stone R. J. , Nicholas R. J. , Fox A. M. , Pate M. A. , Foxon C. T.
Published: July 2002

We demonstrate a detection mechanism for 2–5 THz radiation using intersubband transitions between anticrossing electron levels in a GaAs/AlGaAs coupled-quantum-well photodiode. The THz radiation is detected as a modulation in the photocurrent generated by a visible laser. This modulation is caused by carrier heating effects due to absorption of energy by intersubband transitions. Since the frequency of the intersubband transitions varies with the electric-field strength, the device can function as a voltage-tunable THz detector.
Large, nitrogen-induced increase of the electron effective mass in InyGa1−yNxAs1−x
Authors: Skierbiszewski C. , Perlin P. , Wisniewski P. , Knap W. , Suski T. , Walukiewicz W. , Shan W. , Yu K. M. , Ager J. W. , Haller E. E. , Geisz J. F. , Olson J. M.
Published: July 2002

A dramatic increase of the conduction band electron mass in a nitrogen-containing III–V alloy is reported. The mass is found to be strongly dependent on the nitrogen content and the electron concentration with a value as large as 0.4m0 in In0.08Ga0.92As0.967N0.033 with 6×1019 cm−3 free electrons. This mass is more than five times larger than the electron effective mass in GaAs and comparable to typical heavy hole masses in III–V compounds. The results provide a critical test and fully confirm the predictions of the recently proposed band anticrossing model of the electronic structure of the III–N–V alloys.
Flux flow in YBa2Cu3O7−δ grain-boundary Josephson junctions with a four-terminal configuration
Authors: Lombardi Filomena , di Uccio U. Scotti , Ivanov Z. , Claeson T. , Cirillo M.
Published: July 2002

We report on the observation in current–voltage characteristics of YBa2Cu3O7−δ (YBCO) step-edge Josephson junctions of resistive branches generated by a current injected close to the grain-boundary region through two symmetrical Au/YBCO contacts. The current amplitude and voltage tunability of the branches is measured as a function of injected current and sample temperature and the observed effects are explained in terms of flux-flow dynamics in long Josephson junctions. As the voltage of the branches can be tuned up to 2.5 mV, the investigated phenomenon is also appealing for practical applications in the submillimeter-wave range of the electromagnetic spectrum. We estimate the available power and the linewidth of the electromagnetic radiation associated with typical bias points.
Local transport and trapping issues in Al2O3 gate oxide structures
Authors: Ludeke R. , Cuberes M. T. , Cartier E.
Published: July 2002

The bias dependence of interfacial barriers in Al2O3-based metal–oxide–semiconductor structures was studied by ballistic electron emission spectroscopy. Strong image force reductions of the barriers were observed. A conduction band offset between Al2O3 and Si of 2.78 eV was obtained. Electron trapping into levels that overlap the Si band gap and are located near the Si–Al2O3 interface led to charge densities of ∼2.5×1012 cm−2.
Coherent terahertz emission from optically pumped intersubband plasmons in parabolic quantum wells
Authors: Bratschitsch R. , Müller T. , Kersting R. , Strasser G. , Unterrainer K.
Published: July 2002

We report on few-cycle terahertz (THz) emission from modulation-doped parabolic quantum wells. The quantum wells are optically excited by near-infrared femtosecond laser pulses. The observed THz emission corresponds to the intersubband plasmon of the parabolic quantum well. The emission frequency is independent of the number of optically generated carriers. We identify the excitation mechanism of the intersubband plasmon and hence THz emission to be ultrafast field screening. This mechanism allows for an optically driven THz emission from a completely symmetric nanostructure, in contrast to quantum beats which require a broken symmetry for their excitation.
Self-assembled growth of epitaxial erbium disilicide nanowires on silicon (001)
Authors: Chen Yong , Ohlberg Douglas A. A. , Medeiros-Ribeiro Gilberto , Chang Y. Austin , Williams R. Stanley
Published: July 2002

By choosing a material that has an appropriate asymmetric lattice mismatch to the host substrate, in this case ErSi2 on Si(001), it is possible to grow one-dimensional epitaxial crystals. ErSi2 nanowires are less than one nanometer high, a few nanometers wide, close to a micron long, crystallographically aligned to 〈110〉Si directions, straight, and atomically regular.
Electro-optic field mapping system utilizing external gallium arsenide probes
Authors: Yang K. , Katehi L. P. B. , Whitaker J. F.
Published: July 2002

External electro-optic probes fabricated from two different crystal orientations of GaAs have been implemented in an electro-optic sampling system that is capable of mapping three independent orthogonal components of free-space electric fields. The results obtained for the radiated field from a microstrip patch antenna by the GaAs probes are compared with results on the same antenna obtained using bismuth silicate and lithium tantalate probes. An 8 μm spatial resolution has also been demonstrated for the electro-optic field-mapping system, and the capability for the system to measure field patterns at frequencies up to 100 GHz has been shown.
Nature of binding in the alkaline–earth clusters: Be3, Mg3, and Ca3
Authors: Kaplan Ilya G. , Roszak Szczepan , Leszczynski Jerzy
Published: July 2002

The study of the interaction energy and its many-body decomposition in a broad distance interval for the Ben, Mgn, and Can (n=2,3) clusters at the SCF and MP4 levels are performed. A comparative analysis of the obtained results allows one to conclude that the only stabilization factor in the dimers is the dispersion forces. So, the alkaline–earth dimers can be attributed to the van der Waals molecules. The trimers are stabilized by the two-body localized dispersion forces and three-body delocalized exchange forces. The binding in the alkaline–earth trimers has a mixed physical (van der Waals) and chemical (nonadditive exchange) nature. An NBO population analysis reveals a relatively large p-population in all clusters. A surprisingly large p-population at the MP4 level is also obtained for the isolated atoms.
Mechanism for pillar-shaped surface morphology of polysilicon prepared by excimer laser annealing
Authors: Shih An , Meng Chao-Yu , Lee Si-Chen , Chern Ming-Yau
Published: July 2002

Surface morphology of polycrystalline silicon prepared by excimer laser annealing has been investigated. It was found that when a thin amorphous Si film is irradiated by excimer laser, pillars are formed on the surface of the crystallized polysilicon. To find out the mechanism, various preparation parameters such as laser power densities, shot numbers and the thickness of the amorphous Si have been studied. Several intermediate patterns have been found which illustrate the sequential formation of pillars. A surface tension model is proposed to explain these patterns. This mechanism can be applied to prepare the self-assembled silicon quantum dots, whose average size and height are 14.2 and 3.7 nm, respectively.
Yellow luminescence and Fermi level pinning in GaN layers
Authors: Shalish I. , Kronik L. , Segal G. , Shapira Yoram , Eizenberg M. , Salzman J.
Published: July 2002

A correlation between Fermi level pinning and yellow luminescence in Pt/n-GaN junctions has been studied using Schottky barrier measurements by internal photoemission spectroscopy and complementary deep level spectroscopies. The results show that illumination by photons with energies in the yellow luminescence range causes an unpinning of the interface Fermi level, accompanied by a significant increase of the Schottky barrier height from ∼1 to ∼1.9 eV. This strongly suggests the presence of acceptor states related to the yellow luminescence at the Pt/GaN interface. These states are charged in equilibrium and pin the interface Fermi level but can be optically discharged, resulting in a nearly unpinned interface.
Mechanisms of transition-metal gettering in silicon
Authors: Myers S. M. , Seibt M. , Schröter W.
Published: July 2002

The atomic process, kinetics, and equilibrium thermodynamics underlying the gettering of transition-metal impurities in Si are reviewed. Methods for mathematical modeling of gettering are discussed and illustrated. Needs for further research are considered.
Raman spectra of Ge nanocrystals embedded into SiO2
Authors: Kolobov A. V. , Maeda Y. , Tanaka K.
Published: July 2002

We start with an analysis of the Raman spectra of Ge nanocrystals obtained in previous studies and demonstrate that in many cases the observed experimental peak attributed to Ge in fact originates from the Si substrate. We further compare various experimental ways to separate the Ge signal from that of the substrate and suggest optimum conditions for such measurements. Finally, we demonstrate that upon the annealing of an amorphous Ge–Si–O film, Ge nanocrystals are formed. The nanocrystals are randomly oriented and Ge–Si mixing takes place only at the interface with the Si substrate.
Atmospheric pressure plasma chemical vapor deposition system for high-rate deposition of functional materials
Authors: Mori Y. , Yoshii K. , Kakiuchi H. , Yasutake K.
Published: July 2002

The atmospheric pressure plasma chemical vapor deposition (CVD) system has been developed to fabricate functional thin films at very high deposition rate. The atmospheric pressure plasma, in which high-density radicals are created, has been effectively used to deposit thin films. Combination of the newly designed rotary electrode and the 150 MHz very high frequency power supply makes it possible not only to generate the high-density atmospheric pressure plasma but also to avoid ion bombardment against the film. By virtue of these noble characteristics of the system, high quality films can be fabricated at an unprecedented high deposition rate. In order to demonstrate the effectiveness of the atmospheric pressure plasma CVD system, hydrogenated amorphous silicon (a-Si:H) films were prepared in gas mixtures containing He, H2, and SiH4. The results showed that homogeneous a-Si:H films grew when substrates were heated at 200 °C. Extremely high deposition rate, which was more than 100 times faster than that of the conventional low-pressure plasma CVD technique, was realized.
Room-temperature InAs0.89Sb0.11 photodetectors for CO detection at 4.6 μm
Authors: Gao H. H. , Krier A. , Sherstnev V. V.
Published: July 2002

An InAs0.89Sb0.11 photovoltaic detector that operates at room temperature in the 2.5–5 μm mid-infrared wavelength region is reported. The photodiode has an extended spectral response compared with other currently available III–V room-temperature detectors. In order to accommodate the large lattice mismatch between the InAs0.89Sb0.11 active region and the InAs substrate, a buffer layer with an intermediate composition was introduced into the structure. In this way, we obtained room-temperature photodiodes with a cutoff wavelength near 5 μm, a peak responsivity of 0.8 A/W, and a detectivity of 1.26×109 cm Hz1/2/W. These devices could be effectively used as the basis of an optical sensor for the environmental monitoring of carbon monoxide at 4.6 μm, or as a replacement for PbSe photoconductors.
The Yang–Yang relation and the specific heats of propane and carbon dioxide
Authors: Orkoulas G. , Fisher Michael E. , Üstün Cevat
Published: July 2002

The Yang–Yang relation expresses the heat capacity at constant volume, CV(T,ρ), of a fluid linearly in terms of the second temperature derivatives of the pressure and the chemical potential, p″(T,ρ) and μ″(T,ρ). At a gas–liquid critical point CV diverges so, on approaching Tc from below, either pσ″(T), or μσ″(T), or both must diverge, where the subscript σ denotes the evaluation of p and μ on the phase boundary or vapor-pressure curve. However, previous theoretical and experimental studies have suggested that μσ″(T) always remains finite. To test these inferences, we present an analysis of extensive two-phase heat capacity data for propane recently published by Abdulagatov and co-workers. By careful interpolation in temperature and subsequently making linear, isothermal fits vs specific volume and vs density, we establish that the divergence is shared almost equally between the derivatives pσ″(T) and μσ″(T). A re-examination of the analysis of Gaddy and White for carbon dioxide leads to similar conclusions although the singular contribution from μσ″(T) is found to be of opposite sign and probably somewhat smaller than in propane.
Nonlinear processes responsible for nondegenerate four-wave mixing in quantum-dot optical amplifiers
Authors: Akiyama Tomoyuki , Wada Osamu , Kuwatsuka Haruhiko , Simoyama Takashi , Nakata Yoshiaki , Mukai Kohki , Sugawara Mitsuru , Ishikawa Hiroshi
Published: July 2002

Wavelength conversion using nondegenerate four-wave mixing in quantum-dot optical amplifiers is investigated. From the detuning frequency dependence of χ(3), derived from the conversion efficiency, we consider that, within the range of detuning in the experiment, spectral-hole burning and carrier heating are responsible, and that their time constants, i.e., carrier relaxation time to the ground state and the phonon scattering time, are in the range of 60–140 and 200–400 fs, respectively. This indicates that the carrier supply to the ground level via relaxation from the higher levels is very fast and that a broad conversion bandwidth comparable to that of quantum-well devices is ensured.
Infrared and EPR spectroscopic study of open-shell reactive intermediates: F+NH3 in solid argon
Authors: Misochko E. Ya. , Goldschleger I. U. , Akimov A. V. , Wight C. A.
Published: July 2002

Mobile F atoms react with NH3 molecules in an argon matrix at temperatures T=7–35 K. The open-shell NH2–HF complex was observed by EPR and infrared spectroscopies as the main product of this reaction. The hyperfine constants of the NH2–HF complex aN=1.20 mT, aH=2.40 mT, and aF=0.70 mT were determined from the EPR spectra of samples using NH3, NH153 and ND3 isotopomers. Prominent features of the infrared spectrum of NH2–HF are a strongly red-shifted HF stretching mode (Δv≈−720 cm−1 relative to that for isolated HF) and strong absorptions at 791 and 798 cm−1 attributed to HF librational modes in the complex. Quantum chemistry calculations reveal that the hydrogen-bonded NH2–HF complex has a planar C2v structure and a binding energy of 51 kJ/mol. Calculated hyperfine constants and vibrational frequencies of the complex are in good agreement with those observed in the EPR and IR experiments.
Luminescence enhancement of ZnS:Mn nanoclusters in zeolite
Authors: Chen Wei , Sammynaiken Ramaswami , Huang Yining
Published: July 2002

The luminescence intensity of Mn2+ in the ZnS:Mn nanoclusters formed in an ultrastable zeolite-Y is seven orders of magnitude stronger than that of other nanoparticles deposited out of solutions. This remarkable effect is attributed to a strong quantum size confinement, the location of Mn2+ ions at the near-surface sites, and good surface passivation. The lowering of the energy loss in the E4(D4 or/and T24(D4) states during the energy transfer from the ZnS host to the emitting state T14(G4) of Mn2+ may also contribute to the observed fluorescence enhancement.
Effect of polyatomic ion structure on thin-film growth: Experiments and molecular dynamics simulations
Authors: Wijesundara Muthu B. J. , Ji Yuan , Ni Boris , Sinnott Susan B. , Hanley Luke
Published: July 2002

The experiments described here examine 25–100 eV CF3+ and C3F5+ ion modification of a polystyrene (PS) surface, as analyzed by x-ray photoelectron spectroscopy. The molecular dynamics computer simulations probe the structurally and chemically similar reactions of 20–100 eV CH3+ and C3H5+ with PS. CF3+ and C3F5+ each form a distribution of different fluorocarbon (FC) functional groups on PS in amounts dependent upon the incident ion energy, structure, and fluence. Both ions deposit mostly intact upon the surface at 25 eV, although they also undergo some crosslinking upon deposition. Fragmentation of the two ions increases as the ion energies are increased to 50 eV. Both ions show increases in total fluorine and fluorinated carbon content when changing the ion energy from 25 to 50 eV. The simulations predict that CH3+ and C3H5+ behave in a similar fashion to their FC analogs, remaining mostly intact and either embedding or scattering from the surface without reacting at 20 eV. At 50 and 100 eV, the simulations predict fragmentation most or all of the time. The simulations also show that the chemical products of the collisions depend significantly on the structure of the incident isomer. The simulations further illustrate how the maximum penetration depth of ion fragments depends on ionic structure, incident energy, and the identity of the penetrating fragment. These ion–surface results are discussed in terms of their possible role in plasmas.
Electronic excitation spectra of furan and pyrrole: Revisited by the symmetry adapted cluster–configuration interaction method
Authors: Wan Jian , Meller Jaroslaw , Hada Masahiko , Ehara Masahiro , Nakatsuji Hiroshi
Published: July 2002

Electronic excitation spectra of furan and pyrrole are reinvestigated by the symmetry-adapted cluster configuration-interaction method. The 47 and 46 lowest singlet and triplet electronic states are computed for furan and pyrrole, respectively. Two series (1a2 and 2b1) of low-lying Rydberg states and the valence π–π* excited states strongly influence each other in both furan and pyrrole. The present calculations give detailed and satisfactory theoretical assignments of the vacuum ultraviolet spectra and the electron energy-loss spectra of the two molecules. The similarities and differences in the electronic excitations between furan and pyrrole are discussed in detail. The accuracy and assignments of recent theoretical studies, i.e., complete active space second-order perturbation, multireference Møller–Plesset perturbation, second-order algebraic-diagrammatic construction, multireference double configuration interaction, and CC3, are compared.
Continuous-wave operation of phase-coupled vertical-cavity surface-emitting laser arrays
Authors: Monti di Sopra F. , Brunner M. , Gauggel H.-P. , Zappe H. P. , Moser M. , Hövel R. , Kapon E.
Published: July 2002

Coupled arrays of vertical-cavity surface-emitting lasers were realized by patterning the reflectivity of the top-distributed Bragg reflector using a phase-matching layer and a metal grid. For improved current injection and better heat dissipation the devices were selectively oxidized. Continuous-wave room-temperature operation of these arrays has been achieved at 960 nm. Near- and far-field measurements confirm the phase-coupling in a typical out-of-phase mode pattern. Analysis of the differential efficiency reveals a strong decrease when the number of lasing pixels and hence the mesa size is increased.
Probing capped and uncapped mesoporous low-dielectric constant films using positron annihilation lifetime spectroscopy
Authors: Petkov Mihail P. , Weber Marc H. , Lynn Kelvin G. , Rodbell Kenneth P.
Published: July 2002

We have measured uncapped mesoporous low-dielectric-constant films using positron annihilation lifetime spectroscopy (PALS), a nondestructive technique, which yields both pore-size distributions and the threshold for pore interconnectivity. Pairs of fully cured capped and uncapped identical films, initially containing 5%–50% porogen additions, exhibited similar signatures in PALS, establishing a technique in which film capping is unnecessary. We also found that it was possible to distinguish between closed and percolated pores, without film capping, by comparing lifetime spectra of a film taken using different detector configurations. Interconnected pores were observed in cured samples, which had greater than 20% porogen additions.
Scanning calorimeter for nanoliter-scale liquid samples
Authors: Olson E. A. , Efremov M. Yu. , Kwan A. T. , Lai S. , Petrova V. , Schiettekatte F. , Warren J. T. , Zhang M. , Allen L. H.
Published: July 2002

We introduce a scanning calorimeter for use with a single solid or liquid sample with a volume down to a few nanoliters. Its use is demonstrated with the melting of 52 nL of indium, using heating rates from 100 to 1000 K/s. The heat of fusion was measured to within 5% of the bulk value, and the sensitivity of the measurement was ±7 μW. The heat of vaporization of water was measured in the scanning mode to be within ±23% of the bulk value by actively vaporizing water droplets from 2 to 100 nL in volume. Results within 25% were obtained for the heat of vaporization by using the calorimeter in a heat-conductive mode and measuring the passive evaporation of water. Temperature measurements over a period of 10 h had a standard deviation of 3 mK.
Molecular dynamics calculations of the electrochemical properties of electrolyte systems between charged electrodes
Authors: Crozier Paul S. , Rowley Richard L. , Henderson Douglas
Published: July 2002

We investigate the interfacial electrochemical properties of an aqueous electrolyte solution with discrete water molecules in slab geometry between charged atomistic electrodes. Long-range intermolecular Coulombic interactions are calculated using the particle–particle–particle–mesh method with a modification to account for the slab geometry. Density distribution profiles and potential drops across the double layer are given for 0, 0.25, and 1 M aqueous electrolyte solutions each at 0, ±0.1, ±0.2, and ±0.3 C/m2 electrode surface charges. Results are compared qualitatively with experimental x-ray scattering findings, other computer simulation results, and traditional electrochemistry theory. The interfacial fluid structure characteristics are generally in good qualitative agreement with the conclusions obtained in some integral equation theories and in the experimental x-ray study. The potential in the simulations shows an oscillatory behavior near the electrode, which theories that do not include the molecular nature of water cannot reproduce for the given conditions. Surprisingly, the results also show that the water structure near the electrode is dominated by the charge on the electrode and is fairly insensitive to the ion concentrations. Except at large electrode charge, the potential drop across the double layer does not depend significantly upon the concentration of the ions.
Analysis of a transient region during the initial stage of atomic layer deposition
Authors: Lim Jung-Wook , Park Hyung-Sang , Kang Sang-Won
Published: July 2002

In atomic layer deposition (ALD), it is well known that a linear relationship exists between the deposited film thickness and the number of deposition cycles, which is due to its inherent characteristics of self-limited surface reaction between reactants. However, during the initial stage of ALD, the outermost surface is gradually converted from pre-deposited substrates into an actual film as ALD of the film proceeds. Therefore, a transient region should exist, which causes a nonlinear dependence of film thickness on the number of deposition cycles, because the characteristics of the surface adsorption of reactants is dependent on the exposed film surface. To estimate the accurate film thickness, especially for film thickness less than 10 nm, we propose a simple analytical kinetic model in the transient region. The experimental results of TiN–ALD performed on the SiO2 substrate are consistent with the existence of the transient region. Furthermore, it has been found that the probability of adsorption of a reactant to predeposited SiO2 is considerably lower than that to TiN films.
Piezoelectric effects in In0.5Ga0.5As self-assembled quantum dots grown on (311)B GaAs substrates
Authors: Patanè A. , Levin A. , Polimeni A. , Schindler F. , Main P. C. , Eaves L. , Henini M.
Published: July 2002

Photocurrent spectroscopy is used to investigate the quantum-confined Stark shift of In0.5Ga0.5As/GaAs self-assembled quantum dots grown on (100) and (311)B planes. By comparing the Stark shift for dots grown on (100) and (311)B planes, we find that in the (311)B dots, the electron and hole wave functions are displaced by a strain-induced piezoelectric field directed from the apex to the base of the dots.
Monte Carlo study of the microphase separation of cross-linked polymer blends
Authors: Lay Stefan , Sommer Jens-Uwe , Blumen Alexander
Published: July 2002

We present large scale Monte Carlo simulations of polymer blends, cross-linked in the absence of interactions and quenched into the two-phase region. Our results show, in agreement with the experiments, that the characteristic length of the microphases is much larger than the mean radius of gyration of the network strands. We calculate the structure function at different times and for different interaction parameters and compare the results to various theoretical models. We find that the individual strands do not deform much; such deformations cannot explain the scale of microseparation. On the other hand, we find large cooperative rearrangements of the network under quenching; this stresses the role played by the topological disorder in the formation of microphases.
Reverse-rotational domains in in-plane switching-mode liquid crystal displays
Authors: Suzuki Teruaki , Nishida Shin-ichi , Suzuki Masayoshi , Kaneko Setsuo
Published: July 2002

Characteristic dark domains are observed around the tips of electrodes in in-plane switching-mode liquid crystal displays. In these domains, the liquid crystal molecules rotate in the reverse direction because of the radially distorted electric field. We found that these domains extend toward the central area when a higher voltage is applied. This article discusses these reverse-rotational domains.
Electrical conductivity enhancement in nanocrystalline (RE2O3)0.08(ZrO2)0.92 (RE=Sc, Y) thin films
Authors: Zhang Y. W. , Jin S. , Yang Y. , Li G. B. , Tian S. J. , Jia J. T. , Liao C. S. , Yan C. H.
Published: July 2002

Dense, crack-free, uniform, and homogeneous (RE2O3)0.08(ZrO2)0.92 (RE=Sc, Y) nanocrystalline thin films were fabricated by a simple sol-gel method and characterized by impedance studies. At temperatures beyond 600 °C, the electrical conductivity of (Sc2O3)0.08(ZrO2)0.92 and (Y2O3)0.08(ZrO2)0.92 nanocrystals in pure cubic phase was ten times higher than that of the corresponding bulk materials. The decrease of grain boundary resistance related to interfacial effect is predominately responsible for the electrical conductivity enhancement.
Thermal conductivity of skutterudite thin films and superlattices
Authors: Song D. W. , Liu W. L. , Zeng T. , Borca-Tasciuc T. , Chen G. , Caylor J. C. , Sands T. D.
Published: July 2002

Experimental results on the temperature-dependent cross-plane thermal conductivity of skutterudite thin films are presented. The films examined include IrSb3, CoSb3, and Ir0.5Co0.5Sb3 single layers, and IrSb3/CoSb3 superlattices that are grown by pulsed-laser deposition. A differential 3ω method is used to measure the cross-plane thermal conductivity of these films from 80 to 300 K. The experimental results show a significant reduction in their thermal conductivity values compared to those of their corresponding bulk samples reported in literature. Possible mechanisms contributing to the thermal conductivity reduction are discussed.
Charged colloidal heteroaggregation kinetics
Authors: Puertas A. M. , Fernández-Barbero A. , De Las Nieves F. J.
Published: July 2002

A model for describing heteroaggregation kinetics has been developed and tested with experiments in a colloidal system composed of positively and negatively charged particles. At first order, the model reduces to the classical Hogg, Healy, and Fuerstenau approximation. Static light scattering was used to monitor the aggregations. The dependence of the initial aggregation rate on the relative particle fraction and on the surface charges was studied and compared with the theoretical predictions. The experimental results indicated that first-order approximation cannot be used to describe the kinetics. However, good agreement was found when second order was introduced in the theoretical model.
Growth and optical properties of InxAlyGa1−x−yN quaternary alloys
Authors: Li J. , Nam K. B. , Kim K. H. , Lin J. Y. , Jiang H. X.
Published: July 2002

In x Al y Ga 1−x N quaternary alloys with different In and Al compositions were grown by metalorganic chemical vapor deposition. Optical properties of these quaternary alloys were studied by picosecond time-resolved photoluminescence. It was observed that the dominant optical transition at low temperatures in InxAlyGa1−xN quaternary alloys was due to localized exciton recombination, while the localization effects in InxAlyGa1−xN quaternary alloys were combined from those of InGaN and AlGaN ternary alloys with comparable In and Al compositions. Our studies have revealed that InxAlyGa1−xN quaternary alloys with lattice matched with GaN epilayers (y≈4.8x) have the highest optical quality. More importantly, we can achieve not only higher emission energies but also higher emission intensity (or quantum efficiency) in InxAlyGa1−x−yN quaternary alloys than that of GaN. The quantum efficiency of InxAlyGa1−xN quaternary alloys was also enhanced significantly over AlGaN alloys with a comparable Al content. These results strongly suggested that InxAlyGa1−x−yN quaternary alloys open an avenue for the fabrication of many optoelectronic devices such as high efficient light emitters and detectors, particularly in the ultraviolet region.
Temperature dependence of the surface plasmon resonance of Au/SiO2 nanocomposite films
Authors: Dalacu Dan , Martinu Ludvik
Published: July 2002

The optical response of Au/SiO2 nanocomposite films at temperatures spanning the transition to the liquid state has been measured. The change in the optical constants of gold from those corresponding to the crystalline material to those of the liquid occurs gradually and at temperatures below the bulk melting point. The similarity of the temperature-induced changes in the Au interband absorption edge to that observed with decreasing particle size suggests that a gradual transition to the liquid phase also occurs in small particles.
ZnCdTe/ZnTe/ZnMgSeTe quantum-well structures for the application to pure-green light-emitting devices
Authors: Chang J. H. , Song J. S. , Godo K. , Yao T. , Shen M. Y. , Goto T.
Published: July 2002

A ZnCdTe/ZnTe/ZnMgSeTe quantum-well (QW) structure lattice matched to ZnTe is proposed for the light-emitting devices in the pure-green wavelength region. Thin ZnTe layers are inserted in between the ZnCdTe QW layer and ZnMgSeTe cladding layers, which improve the quality of the QW structure as demonstrated by its narrow photoluminescence line width (6.5 meV at 10 K). Optically pumped lasing at 552 nm at room temperature with a threshold optical power of 215 kW cm−2 is achieved. The present results clearly show the feasibility of ZnTe-based QW structures for the application to light-emitting devices in the pure-green wavelength region.
Highly doped thin-channel GaN-metal–semiconductor field-effect transistors
Authors: Gaska R. , Shur M. S. , Hu X. , Yang J. W. , Tarakji A. , Simin G. , Khan A. , Deng J. , Werner T. , Rumyantsev S. , Pala N.
Published: July 2002

We report on the influence of the channel doping on dc, high frequency, and noise performance of GaN metal–semiconductor field-effect transistors (MESFETs) grown on sapphire substrates. The devices with the channel thicknesses from 50 to 70 nm and doping levels up to 1.5×1018 cm−3 were investigated. An increase in the channel doping results in the improved dc characteristics, higher cutoff, and maximum oscillation frequencies, and reduced low frequency and microwave noise. The obtained results demonstrate that the dc and microwave performance characteristics of short-channel GaN MESFETs may be comparable to those for conventional AlGaN/GaN heterostructure FETs.
Atomic-level study of the robustness of the Si(100)-2×1:H surface following exposure to ambient conditions
Authors: Hersam M. C. , Guisinger N. P. , Lyding J. W. , Thompson D. S. , Moore J. S.
Published: July 2002

The in situ hydrogen-passivated Si(100)-2×1 surface is characterized with x-ray photoelectron spectroscopy (XPS) and ultra-high-vacuum scanning tunneling microscopy (STM) following exposure to ambient conditions. The XPS measurements illustrate the chemical inertness of this surface as the onset of oxidation is not observed for the first 40 h of ambient exposure. After 15 min of contact with atmospheric conditions, the STM images reveal that the Si(100)-2×1:H surface remains atomically pristine. This exceptional stability is of relevance to a wide variety of applications that require ultrapure Si(100) substrates (e.g., microelectronics, semiconductor processing, nanofabrication, etc.).