bluuebunny/crossref_metadata_2025 huggingface dataset:
Fast and sensitive suspended YBaCuO microbolometers using silicon separated by implanted oxygen substrates
Authors: Méchin Laurence , Villégier Jean-Claude , Bloyet Daniel
Published: July 2002

A process to fabricate epitaxial YBaCuO suspended microbolometers using silicon separated by implanted oxygen (SIMOX) substrates is reported. Unlike the other micromachining techniques, no process step is needed after film deposition and the YBaCuO air-bridge retains excellent superconducting qualities. The support membrane is constituted of a 80-nm-thick CeO2/YSZ buffer layer on a 150-nm-thick silicon layer. The critical temperature Tc(R=0) is 88 K and the transition width is 2.5 K. Current-voltage characteristics directly measured on a 10-μm-wide, 100-μm-long suspended bridge show a critical current density of 7.5×105 A/cm2 at 80 K. The thermal conductance G of this bridge is 2×10−5 W/K. An optical noise equivalent power (NEP) of 6.1×10−12 W/Hz at 1 kHz is deduced from optical sensitivity and noise measurements. The time constant is 6 μs, which is the lowest value reported on such suspended structures. Comparison with a similar suspended bridge fabricated without a silicon layer shows that the sensitivity–bandwidth product of the SIMOX bridge is improved by one order of magnitude, thus demonstrating the feasibility of sensitive and fast YBaCuO bolometers.
Simplified evaluation method for light-biased effective lifetime measurements
Authors: Schuurmans F. M. , Schönecker A. , Burgers A. R. , Sinke W. C.
Published: July 2002

In this letter, we present a simplified evaluation method for light-biased photoconductance decay measurements. The measured effective lifetime is shown to be a differential quantity τeff,d, which may differ significantly from the actual effective lifetime τeff. However, the actual effective lifetime can be approximated by integrating τeff,d directly over the incident power density of the bias light. The quality of the approximation depends mainly on the surface recombination velocity and the wavelength of the used bias light. However, the inaccuracy remains well below 10% for most practical cases.
Distributed–feedback, tunable Ce3+-doped colquiriite lasers
Authors: Pinto Joseph F. , Esterowitz Leon
Published: July 2002

Distributed-feedback (dfb) laser oscillation is reported in Ce3+-doped LiSrAlF6 (LiSAF). Tunable operation of the Ce3+:LiSAF dfb laser is achieved in the ultraviolet wavelength region from 289 to 293 nm. At the peak emission wavelength of 290 nm, dfb lasing linewidths are less than the 1 Å resolution limit of the 1 m spectrometer.
Silicon single-electron quantum-dot transistor switch operating at room temperature
Authors: Zhuang Lei , Guo Lingjie , Chou Stephen Y.
Published: July 2002

We fabricated a silicon single-electron quantum-dot transistor, which showed drain current oscillations at room temperature. These oscillations are attributed to electron tunneling through a single silicon quantum dot inside a narrow wire channel. Analysis of its current–voltage characteristic indicates that the energy level separation is about 110 meV and the silicon dot diameter is about 12 nm.
Origin of conductivity and low-frequency noise in reverse-biased GaN p-n junction
Authors: Kuksenkov D. V. , Temkin H. , Osinsky A. , Gaska R. , Khan M. A.
Published: July 2002

We study the origins of conductivity and low-frequency noise in GaN p-n junctions under reverse bias. Carrier hopping through defect states in the space charge region is identified as the main mechanism responsible for low bias conductivity. Threading dislocations appear the most likely source of such defect states. At higher bias hopping is supplemented with Poole–Frenkel emission. A relatively high level of 1/f-like noise is observed in the diode current. The bias and temperature dependencies of the noise current are investigated.
SiGe-based semiconductor-on-insulator substrate created by low-energy separation-by-implanted-oxygen
Authors: Fukatsu S. , Ishikawa Yukari , Saito T. , Shibata N.
Published: July 2002

SiGe-based semiconductor-on-insulator (SOI) substrates have been successfully created by the separation-by-implanted-oxygen technique. Low-energy oxygen ion implantation was performed at 25 kV on a strain-relieved Si0.82Ge0.18 virtual substrate grown on Si(001). A good SOI geometry with a 25-nm top SiGe layer was obtained over a dose window of 2–2.5×1017 cm−2, and a flat SiGe surface and sharp SiGe/SiO2 interfaces were achieved at a low substrate temperature of 550°C. Compositional analysis shows that the top SiGe layer nearly conserves the same composition as the underlying alloy substrate.
ZnSTe-based Schottky barrier ultraviolet detectors with nanosecond response time
Authors: Ma Z. H. , Sou I. K. , Wong K. S. , Yang Z. , Wong G. K. L.
Published: July 2002

ZnSTe-based Schottky barrier photovoltaic detector arrays were fabricated on GaP(100) using a two-step molecular beam epitaxy growth approach. These detectors exhibit visible blind and ultraviolet (UV) sensitive response with a peak UV responsivity of 0.13 A/W and 1.2×106 V/W at 320 nm. The built-in potential of these detectors was determined to be 1.7 V. The temporal photocurrent response of a 400×400 μm2 detector was measured to be 1.2 ns, limited apparently by the resistance-capacitance (rc) constant of the detector structure.
Measurement of the substitutional nitrogen activation energy in diamond films
Authors: Li B. B. , Tosin M. C. , Peterlevitz A. C. , Baranauskas V.
Published: July 2002

We show that the electrical properties of nitrogen-doped nominally undoped polycrystalline chemical vapor deposited diamond films are modified by post-deposition heating in an oxidizing atmosphere. We found that the first heating cycle in air in the temperature range of 300–673 K decreased the graphitization content still present in the diamond surface and that after the second heating cycle the electrical resistance versus temperature curves became stabilized. Using a flow of argon with residues of oxygen over the surface of the sample during the heating cycles, the stabilization of the resistance-temperature dependence also occurred but only after the fourth heating cycle. The results suggest the existence of an oxidation mechanism of the nondiamond carbon atoms present at the diamond surface. After stabilization, the deep donor ionization energy was found to be Ed=1.62±0.02 eV. All results brought together strongly suggest that this level is due to single nitrogen atoms that occupy substitutional lattice sites in diamond.
Laser-based ultrasound detection using photorefractive quantum wells
Authors: Lahiri I. , Pyrak-Nolte L. J. , Nolte D. D. , Melloch M. R. , Kruger R. A. , Bacher G. D. , Klein M. B.
Published: July 2002

We demonstrate a laser-based adaptive ultrasonic homodyne receiver using dynamic holography in AlGaAs/GaAs photorefractive multiple quantum wells. The dynamic hologram acts as an adaptive beamsplitter that compensates wavefront distortions in the presence of speckle and requires no path-length stabilization. The photorefractive quantum wells have the unique ability to achieve maximum linear homodyne detection regardless of the value of the photorefractive phase shift by tuning the excitonic spectral phase. We achieve a root mean square noise-equivalent surface displacement of 6.7×10−7 Å(W/Hz)1/2.
Using printing and molding techniques to produce distributed feedback and Bragg reflector resonators for plastic lasers
Authors: Rogers John A. , Meier Martin , Dodabalapur Ananth
Published: July 2002

We describe the use of low cost printing and molding techniques to fabricate distributed feedback and Bragg reflector resonators for photoexcited plastic lasers that have narrow emission profiles in the visible range. The good performance of the lasers demonstrates the suitability of these techniques for fabricating certain types of functional optoelectronic devices that require submicron features patterned over macroscopic areas.
Bend strain tolerance of critical currents for YBa2Cu3O7 films deposited on rolled-textured (001)Ni
Authors: Park C. , Norton D. P. , Budai J. D. , Christen D. K. , Verebelyi D. , Feenstra R. , Lee D. F. , Goyal A. , Kroeger D. M. , Paranthaman M.
Published: July 2002

Superconducting YBa2Cu3O7/CeO2/YSZ/CeO2 multilayer structures were grown on rolled-textured (001)Ni using pulsed-laser deposition. Critical current densities greater than 1 MA/cm2 were obtained for relatively thick YBa2Cu3O7 films. The compressive and tensile bend strain tolerance of critical currents for the YBa2Cu3O7 films deposited on these rolling-assisted biaxially textured substrates was also determined. These conductors retained up to 80% of their unstrained critical currents for applied compressive bend diameters as small as 1.5 cm and tensile bend diameters of 3.2 cm. The degradation of Jc is caused by the formation and propagation of transverse cracks. The results also suggest a correlation between bend-strain tolerance for these coated conductors and total oxide layer thickness.
Cracking behavior of xerogel silica films on silicon substrates
Authors: Chow L. A. , Xu Y. H. , Dunn B. , Tu K. N. , Chiang C.
Published: July 2002

An analysis of the cracking behavior of sol–gel derived silica, “xerogel,” films on silicon substrates is presented. At the onset of film cracking, xerogel films on 〈100〉 Si substrates show a crosshatched crack pattern, while such films on 〈111〉 Si substrates show a random pattern. This is explained by the fact that for an isotropic film the critical film thickness for cracking decreases for increasing substrate compliance. For a 〈100〉 Si wafer, the directions of highest compliance in the plane of the wafer are in the 〈100〉 directions, which lead to cracks in the film parallel to them. A 〈111〉 Si substrate is isotropic in the plane of the wafer and, hence, there is no preferred direction for film cracking. A random pattern is the result.
Terahertz pulse-induced switching of a quantum-well optical amplifier
Authors: Hughes S. , Citrin D. S.
Published: July 2002

We demonstrate theoretically that an intense half-cycle terahertz pulse can be used conjunctively with a quantum-well optical amplifier to form the basis of an efficient ultrahigh-speed optical switch. Our procedure implies terahertz and subterahertz switching and recovery rates, respectively. Constraints and practicalities are discussed.
Two-band tunneling currents in metal-oxide-semiconductor capacitors at the transition from direct to Fowler–Nordheim tunneling regime
Authors: Okhonin S. , Fazan P. , Guegan G. , Deleonibus S. , Martin F.
Published: July 2002

The conduction and valence band tunneling currents in ultrathin SiO2 films are studied. The slopes of the current–voltage characteristic agree well with the simulations performed. Conduction band current oscillations due to interference of the electrons from the inversion channel at the oxide/gate interface are observed. The shape of the slope of the valence band current in Fowler–Nordheim regime can be explained by the interference of the valence band electron wave at the oxide/gate interface.
Femtosecond response times and high optical nonlinearity in beryllium-doped low-temperature grown GaAs
Authors: Haiml M. , Siegner U. , Morier-Genoud F. , Keller U. , Luysberg M. , Specht P. , Weber E. R.
Published: July 2002

We have investigated the effect of beryllium doping on the optical nonlinearity and on the carrier dynamics in low-temperature (LT) grown GaAs for various growth temperatures and doping levels. Pump–probe experiments with 20 fs pulses and quantitative measurements of the nonlinear absorption show that in undoped LT GaAs, ultrafast response times are only obtained at the expense of low absorption modulation. In contrast, in Be-doped LT GaAs, high absorption modulation is maintained for response times as short as 100 fs. These results are qualitatively explained accounting for the point-defect-related optical transitions in LT-GaAs.
Passivation of boron in diamond by deuterium
Authors: Zeisel R. , Nebel C. E. , Stutzmann M.
Published: July 2002

Capacitance–voltage measurements have been performed on deuterated boron-doped synthetic-type IIb diamond. They demonstrate the electrical passivation of the boron acceptors, which was manifested by a persistent decrease in capacitance after deuteration. The capacitance–voltage dependence is explained by means of a two-layer depletion width model.
Nanocrystalline titanium dioxide-dispersed semiconducting polymer photodetectors
Authors: Narayan K. S. , Singh Th. B.
Published: July 2002

Photodetection properties of nanocrystalline titanium dioxide, TiO2, dispersed in poly-2-methoxy-5-(2′-ethyl-hexoxy)-1,4 paraphenylenevinylene MEHPPV are studied. Responsivity as high as 50 mA/W is observed in the single-layered composite device. The spectral response is sensitive to the magnitude of the bias in the low-voltage range and the crossover from a symbatic to antibatic response is closely followed. At higher reverse bias, the response is relatively uniform throughout the entire spectral range. Differences in the switching response in the forward bias and reverse bias are observed with a pronounced effect of the persistence photocurrent in the forward bias.
A microscope for hard x rays based on parabolic compound refractive lenses
Authors: Lengeler B. , Schroer C. G. , Richwin M. , Tümmler J. , Drakopoulos M. , Snigirev A. , Snigireva I.
Published: July 2002

We describe refractive x-ray lenses with a parabolic profile that are genuine imaging devices, similar to glass lenses for visible light. They open considerable possibilities in x-ray microscopy, tomography, microanalysis, and coherent scattering. Based on these lenses a microscope for hard x rays is described, that can operate in the range from 2 to 50 keV, allowing for magnifications up to 50. At present, it is possible to image an area of about 300 μm in diameter with a resolving power of 0.3 μm that can be increased to 0.1 μm. This microscope is especially suited for opaque samples, up to 1 cm in thickness, which do not tolerate sample preparation, like many biological and soil specimens.
Low-temperature organometallic epitaxy and its application to superlattice structures in thermoelectrics
Authors: Venkatasubramanian Rama , Colpitts Thomas , O’Quinn Brooks , Liu Sandra , El-Masry Nadia , Lamvik Michael
Published: July 2002

We describe a simple, yet phenomenologically very different, low-temperature modification to the conventional metal–organic chemical vapor deposition. It has been applied to the epitaxy of hexagonal-phased Bi2Te3/Sb2Te3 superlattices on zinc-blende GaAs substrates. The modification enables a two-dimensional, layer-by-layer, epitaxy instead of a three-dimensional islanded growth. Therefore, this approach is of generic importance to the epitaxy of many electronic and magnetic materials and their superlattices. High-resolution transmission electron microscopy studies indicate that the interface between the GaAs substrate and Bi2Te3 film is qualitatively defect free and that periodic structures are formed in the Bi2Te3/Sb2Te3 superlattices, with one of the individual layers as small as 10 Å. Such ultra-short-period superlattices offer significantly higher carrier mobilities than their respective solid-solution alloys, apparently due to the elimination of alloy scattering and the minimal effects of random interface scattering on carrier transport. This represents one of the successful observations of enhanced carrier mobilities in monolayer-range superlattices.
Thin-film CuIn1−xGaxSe2 photovoltaic cells from solution-based precursor layers
Authors: Bhattacharya R. N. , Batchelor W. , Hiltner J. F. , Sites J. R.
Published: July 2002

We have fabricated 15.4% and 12.4% efficient thin-film CuIn1−xGaxSe2 (CIGS)-based photovoltaic devices from solution-based electrodeposited (ED) and electroless-deposited (EL) precursors. As-deposited precursors are Cu-rich CIGS. Additional In, Ga, and Se are added to the ED and EL precursor films by physical vapor deposition (PVD) to adjust the final semiconductor film composition to CuIn1−xGaxSe2. The ED and EL device parameters are compared with those of a 17.7% PVD device. The tools used for comparison are current voltage, capacitance voltage, and spectral response characteristics.
High-power, reliable operation of 730 nm AlGaAs laser diodes
Authors: Singh R. , Bull D. , Dabkowski F. P. , Clausen E. , Chin A. K.
Published: July 2002

High-power operation of AlGaAs multi-quantum-well laser diodes emitting near 730 nm is reported. 1000 h, reliable operation at a power of 1.0 W for 100 μm emission aperture and 500 μm cavity length devices has been demonstrated at room temperature. These devices have threshold current densities of 770 A/cm2 with the characteristic temperature coefficients of threshold current, T0, and external differential quantum efficiency, T1, of 152 and 167 K, respectively.
Nanoconstriction microscopy of the giant magnetoresistance in cobalt/copper spin valves
Authors: Theeuwen S. J. C. H. , Caro J. , Wellock K. P. , Radelaar S. , Marrows C. H. , Hickey B. J. , Kozub V. I.
Published: July 2002

We use nanometer-sized point contacts to a Co/Cu spin valve to study the giant magnetoresistance (GMR) of only a few Co domains. The measured data show strong device-to-device differences of the GMR curve, which we attribute to the absence of averaging over many domains. The GMR ratio decreases with increasing bias current. For one particular device, this is accompanied by the development of two distinct GMR plateaus, the plateau level depending on bias polarity and sweep direction of the magnetic field. We attribute the observed behavior to current-induced changes of the magnetization, involving spin transfer due to incoherent emission of magnons and self-field effects.
Recovery of forming gas damaged Pb(Nb, Zr, Ti)O3 capacitors
Authors: Aggarwal S. , Perusse S. R. , Kerr C. J. , Ramesh R. , Romero D. B. , Evans J. T. , Boyer L. , Velasquez G.
Published: July 2002

We report on the recovery of fully integrated Pb(Nb, Zr, Ti)O3 ferroelectric capacitors damaged during forming gas (4% H2, balance N2) annealing. The capacitors were encapsulated using TiOx and SiO2 as interlevel dielectrics to prevent any loss of oxygen or lead. Hydrogen, however, diffused into the ferroelectric film leading to the loss of ferroelectricity. To recover the properties of the capacitor, the fully integrated structure was annealed in N2 ambient to drive the hydrogen out. Raman scattering experiments performed in the high frequency regime to detect the [OH−] stretching vibration mode confirmed the removal of hydrogen after annealing in N2. The ferroelectric properties, including polarization and resistivity of the capacitors, were restored to their initial values prior to damage. This shows that the process of hydrogen damage is reversible with the time to recovery being dependent on the amount of hydrogen in the forming gas.
PbS quantum-dot-doped glasses for ultrashort-pulse generation
Authors: Wundke K. , Pötting S. , Auxier J. , Schülzgen A. , Peyghambarian N. , Borrelli N. F.
Published: July 2002

We investigate the use of PbS quantum-dot-doped glasses as saturable absorbers for ultrashort-pulse lasers by means of absorption bleaching experiments and numerical analysis of the pulse shaping process using the Haus’ master equation. We explain the mode-locking mechanism and the limitations of these absorbers. The generation of transform-limited fs pulses is predicted by soliton mode locking initiated by the absorption saturation of higher excited states of the quantum-dot saturable absorber.
Spontaneous polarization effects in wurtzite GaN/AlGaN quantum wells and comparison with experiment
Authors: Park Seoung-Hwan , Chuang Shun-Lien
Published: July 2002

Electronic and optical properties of wurtzite GaN/AlGaN quantum well (QW) structures with the spontaneous (SP) and piezoelectric (PZ) polarizations are investigated. Although the PZ field in the well is zero where there is no strain if the QW structures are grown on a thick GaN layer, there may still exist a strong field in the well due to the difference between the SP polarizations in the well and barrier regions. It is shown that the transition energies have significant dependence on both the well and the barrier widths and the many-body optical gain is reduced largely due to the SP polarization. In particular, in the case of a QW structure with a large well width, the reduction of the optical gain is dominant due to larger spatial separation between the electron and hole wave functions. These results suggest that a QW structure with a thin well width below 30 Å is desirable for QW lasers. We show that the theoretical transition energies agree very well with the experimental results for several Al compositions and barrier widths. The estimated SP polarization constant for AlN is about −0.040 C/m2, which is smaller than the value (−0.081 C/m2) predicted by previous theory.
Electrical conduction in diamond after vacancy generation by means of carbon-ion implantation
Authors: Prins Johan F.
Published: July 2002

Electrical resistance measurements have been made on high-purity, type-IIa diamonds which had been implanted with carbon ions at liquid-nitrogen temperature followed by rapid thermal annealing to 500 °C. The initial damage density was estimated by appropriate computer simulation to exceed the equivalent damage of 1022 vacancies per cm3. Although above this threshold, such layers did not graphitize during subsequent higher temperature isochronal anneals. Furthermore, their resistance behavior as a function of temperature can be explained in terms of the known properties of vacancies in diamond. This is the expected result if the damage created in the collision cascades during implantation consists primarily of individual vacancies and interstitial atoms.
Polycrystalline nitride semiconductor light-emitting diodes fabricated on quartz substrates
Authors: Bour D. P. , Nickel N. M. , Van de Walle C. G. , Kneissl M. S. , Krusor B. S. , Mei Ping , Johnson N. M.
Published: July 2002

We demonstrate the feasibility of polycrystalline nitride semiconductor light-emitting diodes (LEDs). Here, polycrystalline LEDs were deposited on quartz substrates, incorporating a layer structure identical to that used for epitaxially grown LEDs. The deposition exhibits a tendency to produce c-oriented crystallites. Violet-blue (430 nm) operation of a polycrystalline LED is demonstrated, with spectral width of 38 nm, and emission efficiency approximately two orders of magnitude lower than for single-crystal LEDs. These LEDs could potentially be incorporated in large-area displays, since the deposition of polycrystalline materials avoids single-crystal substrates required for conventional nitride semiconductor light emitters.
Evidence of strong carrier localization below 100 K in a GaInNAs/GaAs single quantum well
Authors: Grenouillet L. , Bru-Chevallier C. , Guillot G. , Gilet P. , Duvaut P. , Vannuffel C. , Million A. , Chenevas-Paule A.
Published: July 2002

We report an anomalous temperature dependence of the photoluminescence (PL) spectrum of a 7 nm Ga0.72In0.28N0.028As0.972/GaAs single quantum well. The PL peak energy exhibits an inverted S-shape dependence with temperature. Below 100 K, the PL integrated intensity shows a temperature dependence similar to that of amorphous semiconductors. The observed anomalous behavior is explained by a strong localization of carriers at low temperatures that could be induced by the presence of nitrogen. Thermal annealing does not significantly change the anomalous temperature dependence.
Defect-assisted tunneling current: A revised interpretation of scanning tunneling spectroscopy measurements
Authors: Grandidier B. , de la Broise X. , Stiévenard D. , Delerue C. , Lannoo M. , Stellmacher M. , Bourgoin J.
Published: July 2002

Scanning tunneling microscopy (STM) is used to study low temperature grown GaAs layers. Excess As gives rise to a high concentration of As antisites (AsGa). On these point defects, tunneling spectroscopy reveals a band of donor states. In fact, the measured tunneling current results from a pure tunneling current between the energy levels of the STM tip and the AsGa energy level Et followed by an exchange of carriers between Et and the bands or between Et and donor states of neighbor point defects. We determine the influence of both contributions on the current. We show that hopping conduction is required to explain the observation of the midgap states in tunneling spectroscopy.
Influence of the valence-band offset on gain and absorption in GaNAs/GaAs quantum well lasers
Authors: Hader J. , Koch S. W. , Moloney J. V. , O’Reilly E. P.
Published: July 2002

The dependence of the gain and absorption in GaNAs/GaAs quantum well lasers on the valence-band offset is investigated. The calculated absorption strength, gain amplitudes, and gain bandwidth are found to depend crucially on the value of this offset. The shift of the peak gain transition energy with carrier density is shown to depend strongly on the magnitude of the offset, providing what should be a useful means to determine the offset experimentally.
Band-tail photoluminescence in nanocrystalline Si
Authors: Kalkan A. Kaan , Fonash S. J. , Cheng Shang-Cong
Published: July 2002

The characteristic subgap photoluminescence (PL) observed in nanocrystalline Si films was found to shift to higher energies with increasing optical gap and decreasing crystallite size. This behavior, along with the temperature dependence of the PL, is consistent with transitions between band-tail states in the energy gap of the crystallites. The PL bandwidth is too broad to be explained by the tail width as deduced from the temperature behavior. Hence, the broadening is ascribed to electron–phonon coupling, while the related Stokes shift was found to increase with decreasing crystallite size, possibly due to decreasing exciton size.
Trapping processes in CaS:Eu2+,Tm3+
Authors: Jia Dongdong , Jia Weiyi , Evans D. R. , Dennis W. M. , Liu Huimin , Zhu Jing , Yen W. M.
Published: July 2002

CaS:Eu 2+ ,Tm 3+ is a persistent red phosphor. Thermoluminescence was measured under different excitation and thermal treatment conditions. The results reveal that the charge defects, created by substituting Tm3+ for Ca2+, serve as hole traps for the afterglow at room temperature. Tm3+ plays the role of deep electron trapping centers, capturing electrons either through the conduction band or directly from the excited Eu2+ ions. These two processes, in which two different sites of Tm3+ are involved, correspond to two traps with different depths.
Photoacoustic effect in stressed elastic solids
Authors: Muratikov K. L. , Glazov A. L. , Rose D. N. , Dumar J. E.
Published: July 2002

A multimode approach based on the simultaneous application of several photothermal and photoacoustic methods is proposed for the study of thermal and thermoelastic effects in solids with residual stress. It includes photoacoustic gas microphone, photodeflection, photoreflectance and photoacoustic piezoelectric microscopy methods. This approach provides complementary information about thermal, elastic and thermoelastic properties of samples with residual stress. Some experimental results obtained within the framework of this approach for Vickers indentation zones in silicon nitride ceramic are presented. The model of the photoacoustic thermoelastic effect in solids with residual stress is proposed. It is based on the modified Murnaghan model of nonlinear elastic bodies which takes into account a possible dependence of the thermoelastic constant on stress. It is demonstrated that the developed theoretical model for the photoacoustic piezoelectric effect agrees qualitatively with the available experimental data.
Influence of a finite level of ion acoustic waves on Raman gain in inhomogeneous plasma
Authors: Barr H. C. , Boyd T. J. M. , Lukyanov A. V.
Published: July 2002

A five-wave Raman scattering process is examined in inhomogeneous plasma in the presence of a finite level of ion acoustic waves. This five-wave interaction is a composite of conventional three-wave stimulated Raman scattering (SRS) followed by the decay of the Raman Langmuir wave into either a second Langmuir wave (LD) or a second scattered light wave (ED) and an ion acoustic wave. The presence of a suprathermal level of ion acoustic waves detunes SRS and inhibits the temporal growth. In inhomogeneous plasma these decays take place locally so that phase matching for both decays is assumed to be coincident. The net Raman convective gain is shown to be sensitive to the source levels of the amplifying waves. For LD the gain may be suppressed or enhanced depending on the relative intensities of the two Langmuir waves. For ED the gain is invariably suppressed. In either case, this occurs for very modest levels of ion acoustic waves.
Salt-induced volume phase transition of poly(N-isopropylacrylamide) gel
Authors: Annaka Masahiko , Motokawa Keiko , Sasaki Shigeo , Nakahira Takayuki , Kawasaki Hideya , Maeda Hiroshi , Amo Yuko , Tominaga Yasunori
Published: July 2002

The salt effect on the phase transition of N-isopropylacrylamide (NIPA) gel was studied. The swelling behavior of the NIPA gel strongly depends on the salt concentration and is well described as a function of the chemical potential difference of water molecules in solution from that at the transition. From the analysis of the OH stretching, Raman spectra in water and in various aqueous solutions in terms of collective proton motions reveals that the presence of salts tends to disrupt or distort the water molecules in hydrophobic hydration shell around the NIPA gel. This leads to inducing the growth of the cluster shell around the salts, which leads to gel collapse. The volume phase transitions due to the different types of perturbation (temperature, salt) are induced by the same mechanism, hydrophobic hydration and dehydration, and therefore can be described in a unified manner in terms of the chemical potential and the collective proton motions of water molecules.
Large magnetic entropy change in TbAl2 and (Tb0.4Gd0.6)Al2
Authors: Wang F. W. , Zhang X. X. , Hu F. X.
Published: July 2002

Rare-earth intermetallic compounds (Tb1−xGdx)Al2, have been successfully fabricated and characterized. Large magnetic entropy change, ΔSM, was observed in these alloys. The maximum −ΔSM(=14.3 J/kg K) is achieved in the TbAl2 alloy at its Curie temperature, TC=105 K, with a 5 T magnetic field change. It is shown that the alloys are very soft magnetic alloys. The magnetic entropy change is thus completely reversible in the whole temperature range of 80–180 K. An interesting feature in (Tb1−xGdx)Al2 compounds is that the Curie temperature can be easily shifted by tuning the composition. These alloys, therefore, appear to be an attractive material candidate for realizing the active magnetic regenerator magnetic refrigerator consisting of eight stages, as suggested by K. A. Gschneidner, Jr. and V. K. Percharsky. [Rare Earth: Science, Technology, and Applications III, edited by R. G. Bautista, C. O. Bounds, T. W. Ellis, and B. T. Kilbourn (The Minerals, Metals and Materials Society, 1997), p. 209.]
Monolithic photorefractive molecule with excellent transparency in the visible region
Authors: Sohn Jiwon , Hwang Jaehoon , Park Soo Young , Lee Jin-Kyung , Lee Jai-Hyung , Chang Joon-Sung , Lee Geon Joon , Zhang Bo , Gong Qihuang
Published: July 2002

9-(2-Ethyl-hexyl)-3-[2-(4-methanesulfonyl-phenyl)vinyl]-9H-carbazole (EHCS) was synthesized as a monolithic photorefractive molecule forming low Tg(∼25 °C) organic glass. It showed distinct photoconductivity due to the carbazole moiety and the optical nonlinearity by the push–pull structure. The EHCS glass showed excellent optical quality and great stability as the ethylhexyl group effectively suppressed the crystallization in the films. Also, the extremely simple structure of EHCS contributed to a very high chromophore concentration. This single-component photorefractive glass of EHCS showed large net gain (56.3 cm−1) and diffraction efficiency (12%). Photorefractivity of EHCS glass was greatly improved by doping with a small amount (<1%) of 2,4,7-trinitrofluorenone (TNF). The two-beam coupling gain of samples with TNF was 113 cm−1 and the diffraction efficiency was 29%.
A comparison of the Hall-effect and secondary ion mass spectroscopy on the shallow oxygen donor in unintentionally doped GaN films
Authors: Meister D. , Böhm M. , Topf M. , Kriegseis W. , Burkhardt W. , Dirnstorfer I. , Rösel S. , Farangis B. , Meyer B. K. , Hoffmann A. , Siegle H. , Thomsen C. , Christen J. , Bertram F.
Published: July 2002

We report on temperature-dependent Hall-effect measurements and secondary ion mass spectroscopy on unintentionally doped, n-type conducting GaN epitaxial films. Over a wide range of free carrier concentrations we find a good correlation between the Hall measurements and the atomic oxygen concentration. We observe an increase of the oxygen concentration close to the interface between the film and the sapphire substrate, which is typical for the growth technique used (synthesis from galliumtrichloride and ammonia). It produces a degenerate n-type layer of ≈1.5 μm thickness and results in a temperature-independent mobility and Hall concentration at low temperatures (<50 K). The gradient in free carrier concentration can also be seen in spatially resolved Raman and cathodoluminescence experiments. Based on the temperature dependence of the Hall-effect, Fourier transform infrared absorption experiments, and photoluminescence we come to the conclusion that oxygen produces a shallow donor level with a binding energy comparable to the shallow Si donor.
Room-temperature gain and differential gain characteristics of self-assembled InGaAs/GaAs quantum dots for 1.1–1.3 μm semiconductor lasers
Authors: Hatori Nobuaki , Sugawara Mitsuru , Mukai Kohki , Nakata Yoshiaki , Ishikawa Hiroshi
Published: July 2002

This letter presents an explanation of the optical gain and differential gain of two types of self-assembled quantum dots in the laser active region, which shows 1.16 and 1.31 μm spontaneous emission from the ground state at room temperature. The gain spectrum was measured using the Hakki–Paoli method up to the lasing threshold. The maximum optical gain of the ground state was found to be 150–400 cm−1 and the differential gain to be 3×10−15–1×10−16 cm2, which agrees quite well with the calculation, taking into account both homogeneous broadening and inhomogeneous broadening. Our results will be a guide to the design of laser structures.
Multidimensional replica-exchange method for free-energy calculations
Authors: Sugita Yuji , Kitao Akio , Okamoto Yuko
Published: July 2002

We have developed a new simulation algorithm for free-energy calculations. The method is a multidimensional extension of the replica-exchange method. While pairs of replicas with different temperatures are exchanged during the simulation in the original replica-exchange method, pairs of replicas with different temperatures and/or different parameters of the potential energy are exchanged in the new algorithm. This greatly enhances the sampling of the conformational space and allows accurate calculations of free energy in a wide temperature range from a single simulation run, using the weighted histogram analysis method.
Growth of Si delta-doped GaN by metalorganic chemical-vapor deposition
Authors: Zhao G. Y. , Adachi M. , Ishikawa H. , Egawa T. , Umeno M. , Jimbo T.
Published: July 2002

Si delta-doped GaN has been grown by metalorganic chemical-vapor deposition. A very high peak density and narrow full width at half maximum (FWHM) of the carrier profile are obtained. It is found that the peak carrier density of Si delta doping increases with the doping time and SiH4 flow rate, while the FWHM of the carrier profile decreases with both increasing doping time and SiH4 flow rate. Some saturation in the carrier density has also been observed for relatively longer doping time. Except for a broadened carrier distribution in GaN induced by Si diffusion due to high growth temperature, the Si delta-doping properties in GaN are found to be similar to those of GaAs.
Tuning of second-harmonic generation in waveguides induced by photorefractive spatial solitons
Authors: Lan Song , Anastassiou Charalambos , Segev Mordechai , Shih Ming-feng , Giordmaine J. A. , Mizell Greg
Published: July 2002

We demonstrate experimentally wavelength tuning of second-harmonic generation in waveguides induced by photorefractive solitons in a KNbO3 crystal. By rotating the crystal, we can attain as large wavelength tunability as in bulk but with considerable improvement in conversion efficiency and elimination of walk-off.
Finite size effects in carbon nanotubes
Authors: Wu Jian , Duan Wenhui , Gu Bing-Lin , Yu Jing-Zhi , Kawazoe Yoshiyuki
Published: July 2002

The low-energy theory for finite long carbon nanotube is derived and numerically examined. It shows that the electronic structure is dominated by the quantum confining, which determines the profile of wave functions as well as the eigen energies; while the details of the wave functions are resolved by the structure of the nanotubes. This behavior is attributed to the peculiar electronic structure of the nanotubes. Because of the slow variation of the profile of electron wave functions, the measured conductance is NOT independent of the position to measure it, which is evident in the multiprobe experiment.
Optical microcavities using highly luminescent films of semiconductor nanocrystals
Authors: Finlayson Chris E. , Ginger David S. , Greenham Neil C.
Published: July 2002

Colloidally grown CdSe nanocrystals with epitaxial ZnS shells show highly efficient, size-tunable luminescence. We report the incorporation of films of these core-shell nanocrystals into wavelength-scale, high-Q, planar microcavities. Under optical excitation, we find that emission from the nanocrystals couples to the discrete optical modes of the microcavity. The broad free-space emission spectrum of the nanocrystals is modified by the presence of the microcavity, giving a series of sharp emission lines with wavelengths determined by the cavity dimension. Our experiments demonstrate that microcavities with semiconductor emitters can be conveniently fabricated using spin-coating techniques. We find that, at room temperature, the microcavity emission spectrum is independent of excitation intensity for excitation densities up to approximately one electron–hole pair per nanocrystal.
Oblique alignment of columns of self-organized Ge/Si(001) islands in multilayer structure
Authors: Huang C. J. , Li D. Z. , Cheng B. W. , Yu J. Z. , Wang Q. M.
Published: July 2002

Ge/Si multilayer structures with a bimodal distribution of the island spacing in the first layer have been investigated by atomic-force microscopy and transmission electron microscopy. Besides the vertical alignment, some oblique alignments of stacked islands are observed. The presence of the elastic interaction between islands is responsible for the oblique alignment of stacked islands.
Scanning tunneling spectroscopy study of silicon and platinum assemblies in an opal matrix
Authors: Dı́az-Guerra C. , Piqueras J. , Golubev V. G. , Kurdyukov D. A. , Pevtsov A. B. , Zamoryanskaya M. V.
Published: July 2002

Scanning tunneling microscopy and scanning tunneling spectroscopy (STS) are used to investigate the local electronic behavior of Pt–Si nanostructures fabricated in an opal matrix formed by silica spheres of 250 nm diameter. Si and Pt are regularly distributed inside the opal pores and form nanoscale metal-semiconductor-metal junctions. Normalized differential conductance curves enable us to study the distribution of Pt and Si and to detect the presence of regions showing a surface band gap in the range 0.5–0.8 eV, possibly associated with the formation of silicides. STS appears as a suitable technique for the electrical characterization of opal-based nanostructures.
Gd
2
O
3
/
GaN
metal-oxide-semiconductor field-effect transistor
Authors: Johnson J. W. , Luo B. , Ren F. , Gila B. P. , Krishnamoorthy W. , Abernathy C. R. , Pearton S. J. , Chyi J. I. , Nee T. E. , Lee C. M. , Chuo C. C.
Published: July 2002

Gd 2 O 3 has been deposited epitaxially on GaN using elemental Gd and an electron cyclotron resonance oxygen plasma in a gas-source molecular beam epitaxy system. Cross-sectional transmission electron microscopy shows a high concentration of dislocations which arise from the large lattice mismatch between the two materials. GaN metal-oxide-semiconductor field-effect transistors (MOSFETs) fabricated using a dielectric stack of single crystal Gd2O3 and amorphous SiO2 show modulation at gate voltages up to 7 V and are operational at source drain voltages up to 80 V. This work represents demonstrations of single crystal growth of Gd2O3 on GaN and of a GaN MOSFET using Gd2O3 in the gate dielectric.
Extended kinetic models with waiting-time distributions: Exact results
Authors: Kolomeisky Anatoly B. , Fisher Michael E.
Published: July 2002

Inspired by the need for effective stochastic models to describe the complex behavior of biological motor proteins that move on linear tracks, exact results are derived for the velocity and dispersion of simple linear sequential models (or one-dimensional random walks) with general waiting-time distributions. The concept of “mechanicity” is introduced to conveniently quantify departures from simple “chemical,” kinetic rate processes, and its significance is briefly indicated. The results are extended to more elaborate models that have finite side branches and include death processes (to represent the detachment of a motor from the track).
Ti-catalyzed Si nanowires by chemical vapor deposition: Microscopy and growth mechanisms
Authors: Kamins T. I. , Stanley Williams R. , Basile D. P. , Hesjedal T. , Harris J. S.
Published: July 2002

Si nanowires grow rapidly by chemical vapor deposition on Ti-containing islands on Si surfaces when an abundant supply of Si-containing gaseous precursor is available. The density of wires is approximately the same as the density of the nucleating islands on the Si surface, although at least two different types of islands appear to correlate with very different wire growth rates. For the deposition conditions used, a minority of long, defect-free wires form, along with more numerous wires containing defects. Energy-dispersive x-ray spectroscopy shows that the Ti-containing nanoparticles remain at the tip of the growing wires. The estimated diffusion coefficient of Si in TiSi2 is consistent with the catalyzing nanoparticle remaining in the solid phase during nanowire growth.
Pressure–volume–temperature paths in the laser-heated diamond anvil cell
Authors: Kavner Abby , Duffy Thomas S.
Published: July 2002

The temperature, pressure, and stress conditions in the diamond anvil cell sample chamber before, during, and after laser heating are mapped by employing standard materials as in situ pressure markers. Unit cell volumes of Pt, MgO, and NaCl were monitored by synchrotron-based x-ray diffraction at temperatures between 300 and 2290 K and pressures ranging from 14 to 53 GPa. To aid in interpreting the resulting pressure–volume–temperature paths, we perform a series of model calculations of the high-temperature, high-pressure x-ray diffraction behavior of platinum subjected to a general stress state. Thermal pressure and thermal expansion effects within the laser-heated volume are observed but are not sufficient to fully explain the measured paths. Large apparent pressure changes can also result from relaxation of deviatoric stresses during heating and partial reintroduction of those stresses during quench. Deviatoric stresses, monitored from both diffraction peak widths and lattice parameter shifts as a function of (hkl), may significantly distort equation of state results if it is assumed that the sample is under hydrostatic pressure. Large-scale, nearly isothermal pressure relaxation events are observed at ∼2000 K. It is proposed that these arise from relaxation of heated components (pressure medium, gasket, cell itself) outside of the directly laser-heated volume.
Influence of counterion valency on the scattering properties of highly charged polyelectrolyte solutions
Authors: Zhang Yubao , Douglas Jack F. , Ermi Brett D. , Amis Eric J.
Published: July 2002

Light and neutron scattering measurements on highly charged polyelectrolyte solutions have recently provided firm evidence for the existence of “domain structures” containing many chains, even at rather low-polymer concentrations. In the present paper, we systematically investigate the influence of counterion charge valency Zc on the scattering properties of sulfonated polystyrene (PSS) solutions in water with monovalent and divalent counterions. This study is part of a larger effort to identify essential factors governing polyelectrolyte domain formation and the geometric properties of these transient structures. Neutron scattering measurements indicate that the interchain correlation length ξd within the domains becomes larger by a factor of 1.5–2 for divalent relative to monovalent counterions. This observation is consistent with the Manning model estimate of the change in effective polymer charge density Γ* with Zc and with previous observations linking ξd [from the peak position in the scattering intensity [I(q)] with the bare polymer charge density, Γ. Light scattering measurements of the radius of gyration Rg,d of the domains indicate that their size becomes smaller for divalent counterions and with a reduction of Γ. We observe that the “fractal” dimension of the domains measured from the low-angle scaling of I(q) depends on the chemical structure of the polyelectrolyte. Zero average contrast (ZAC) neutron scattering measurements show that the radius of gyration Rg,c of individual polyelectrolyte chains is also reduced for divalent counterions, suggesting that chain rigidity is strongly influenced by Γ*. Charge valency effects on relaxation times are investigated by dynamic light scattering. As usual, two diffusive modes are observed in the light intensity autocorrelation function, G(τ). The “fast” mode becomes slower and the “slow” mode becomes faster for the divalent counterion (Mg2+), relative to the monovalent counterion (Na+). Counterion valence has a large influence on the structure and dynamics of highly charged polyelectrolyte solutions through its influence on Γ*.
Imaging of defect-related charges in a Pb(Zr, Ti)O3 film
Authors: Takata K.
Published: July 2002

This letter presents results on imaging of charges causing the freezing of spontaneous polarization in a hydrogen-annealed lead–zirconate–titanate (PZT) film with platinum electrodes. It has been reported that the positive charges, produced by deoxidization of the PZT at the boundary with the platinum upper electrodes due to hydrogen annealing, fixed spontaneous polarization downward and thus greatly degraded remanent polarization. To date, such defect-related charges could not be observed directly. Imaging after removing the upper electrodes shows that the charges are located at the grain boundaries of the PZT and the charge density in the degraded region is much higher than that in the other region.
Parallel heat diffusion and subdiffusion in random magnetic fields
Authors: Albright B. J. , Chandran B. D. G. , Cowley S. C. , Loh M.
Published: July 2002

Stochastic magnetic fields in a weakly collisional plasma enhance the collisionality of electrons in the medium compared to the classical Spitzer values. The reduction in the thermal conductivity may play an important role in the evolution of galaxy clusters. This work extends prior work by examining the parallel propagation of electrons in plasma media where the magnetic field amplitude sampled along a field line is a stochastic function of the distance. New physics is exhibited when many large-amplitude magnetic mirrors are present; the parallel transport becomes subdiffusive rather than diffusive in this limit. The transition between diffusion and subdiffusion can be obtained from properties of the asymptotic solution to the kinetic equation in the vicinity of a solitary magnetic mirror. This transition has been computed as a function of the power law exponent in the distribution of mirror maxima, and the asymptotic scaling of the mean-squared electron displacement with time has been obtained in these systems as a function of this exponent. Monte Carlo comparisons are in agreement with these results.