bluuebunny/crossref_metadata_2025 huggingface dataset:
Single-crystal Si films for thin-film transistor devices
Authors: Im James S. , Sposili Robert S. , Crowder M. A.
Published: July 2002

The fact that single-crystal Si would make an ideal material for thin-film transistor devices has long been recognized. Despite this awareness, a viable method by which such a material could be directly produced on a glass substrate has never been formulated. In this letter, it is shown experimentally that location-controlled single-crystal Si regions on a SiO2 surface can be obtained in a glass-substrate compatible manner, via excimer-laser-based sequential lateral solidification of thin Si films using a beamlet shape that self-selects and extends a single grain over an arbitrarily large area. This is accomplished by controlling the locations, shape, and extent of melting induced by the incident excimer-laser pulses, in such a manner as to induce interface-contour-affected sequential super-lateral growth of crystals, during which the tendency of grain boundaries to align approximately orthogonal to the solidifying interface is systematically exploited.
Large excited state Stokes shift in crescent-shaped AlGaAs/GaAs quantum wires
Authors: Wang Xue-Lun , Ogura Mutsuo , Matsuhata Hirofumi , Hamoudi Ali
Published: July 2002

The Stokes shifts of the ground and the excited states in a crescent-shaped AlGaAs/GaAs quantum wire (QWR) are investigated using photoluminescence (PL) and PL excitation spectroscopy. The first excited electron to heavy-hole transition showed a Stokes shift (∼17 meV) considerably larger than that of ground state-related transitions (∼4 meV). This is a quite different phenomenon than that observed in two dimensional quantum well structures, and can be explained by the spatial separation of wave functions with different confinement energies in crescent-shaped QWRs.
Superconducting single flux quantum 20 Gb/s clock recovery circuit
Authors: Kaplunenko V. , Borzenets V. , Dubash N. , Van Duzer T.
Published: July 2002

A clock recovery circuit has been successfully tested at frequencies up to 20 GHz. This cell is designed for a rapid-single-flux-quantum (RSFQ) telecommunication data switch. It serves to set the receiver clock in phase with the incoming digital signal. The circuit consists of a dc-to-SFQ converter, ring oscillator [(RO) is a closed-loop RSFQ Josephson transmission line], confluence buffer, and an 8-bit binary counter. The input signal transforms to SFQ pulses, and each pulse resets the phase of the ring oscillator, giving a locking time of 1 bit. Thus, the pull-in (capture) range and hold-in (tracking) range are the same, and strictly depend on the encoding of the input signal. This range is estimated to be about 1 GHz at frequency 20 GHz, if the sequence of consecutive ONEs or ZEROs does not exceed 20 bits. The quality factor QRO of ring oscillator is about 2000, which gives a jitter of 50 fs for a 35-junction RO. A sampling technique was used to demonstrate phase recovery (phase locking) with only one incoming pulse per 512 clock periods.
Carrier capture in pseudomorphically strained wurtzite GaN quantum-well lasers
Authors: Wang Jin , Kim K. W. , Littlejohn M. A.
Published: July 2002

The generalized photon-carrier rate-equation formalism is used to evaluate the carrier capture process in pseudomorphically strained wurtzite GaN quantum-well lasers. Our results show that both the carrier capture time and the carrier escape time vary significantly with the injected carrier density and strain-induced piezoelectric field. Thus, it is demonstrated that in place of the flatband conditions adopted in most treatments, a self-consistent adjustment of the carrier capture process is essential for achieving an accurate description of carrier dynamics in wurtzite GaN quantum-well lasers.
Possibility of a single electron tunneling diode and a controllable saturated tunneling current
Authors: Richardson W. H.
Published: July 2002

A quantum theory of a degenerate p-n junction coupled to an external circuit shows that the combination of Coulomb blockade, and the intrinsic (constant–voltage bias) nonlinear I–V of the junction results in a new device with unique features. In the limit when the coulomb charging energy is larger than the energy width of the degeneracy on the P side, it is demonstrated that conduction is permitted in only one direction of voltage. It is also shown, that when the energy width of the degeneracy on the N side is much larger than that on the P side, then a controllable tunneling saturation current is possible.
Photoluminescence energy and interface chemistry of GaInP/GaAs quantum wells
Authors: Mesrine M. , Massies J. , Vanelle E. , Grandjean N. , Deparis C.
Published: July 2002

It is shown that the unusual scattering of the currently reported photoluminescence energy of GaInP/GaAs quantum wells (QWs) can be well accounted for by considering the combined effect of In surface segregation and As/P exchange at the interfaces. The resulting chemical modification of the interfaces being strongly growth procedure dependent, can explain the observed dispersion of the experimental results. This is demonstrated by experiments in which different growth switching procedures are used at the GaInP/GaAs QW interfaces.
Characteristics of GaAs and AlGaAs homojunction avalanche photodiodes with thin multiplication regions
Authors: Anselm K. A. , Yuan P. , Hu C. , Lenox C. , Nie H. , Kinsey G. , Campbell J. C. , Streetman B. G.
Published: July 2002

The gain and noise of thin GaAs and Al0.2Ga0.8As homojunction avalanche photodiodes were measured. The gain and the excess noise factor were found to be significantly lower than would be expected using ionization coefficients reported in the literature. The discrepancy is believed to be due to physical effects that become significant in thin multiplication layers. It is shown that the gain and excess noise under electron injection can be accurately fit using conventional models with width-dependent ionization coefficients.
Fuzzy controlled feedback applied to a combined scanning tunneling and force microscope
Authors: Battiston F. M. , Bammerlin M. , Loppacher C. , Lüthi R. , Meyer E. , Güntherodt H.-J. , Eggimann F.
Published: July 2002

A feedback mechanism based on fuzzy logic has been applied to operate a combined atomic force microscope (AFM)/scanning tunneling microscope (STM), which is able to measure the resonance frequency shift Δf of the cantilever-type spring and the mean tunneling current Īt simultaneously. Using a decision making logic, the microscope can be scanned over a heterogeneous surface without tip crash. On the conductive parts of the sample, the STM mode is preferred, whereas the noncontact (nc)-AFM mode is used on the poorly conductive parts of the surface. The transition from the STM mode to nc-AFM mode is performed smoothly with the fuzzy logic feedback.
Interband transition studies on CdxZn1−xTe/ZnTe step quantum wells under applied electric fields
Authors: Kim T. W. , Lee K. H. , Park H. L.
Published: July 2002

Photoluminescence (PL) measurements were carried out to investigate the interband transitions in CdxZn1−xTe/ZnTe asymmetric step quantum wells with and without an applied electric field. Transmission electron microscopy showed that the CdxZn1−xTe/ZnTe step quantum wells consisted of a shallow Cd0.1Zn0.9Te well and a deep Cd0.18Zn0.82Te well bound by two thick ZnTe barriers. The results for the PL data at 300 K for several applied electric fields showed that the excitonic transition from the first electronic state to the first heavy-hole state shifted to the larger energy side as the applied electric field increased. The electronic subband energies and the wave functions in the step quantum wells were calculated by an envelope-function approximation method, taking into account the strain effects, and the calculated values of the interband transitions were in qualitative agreement with those obtained from the PL measurements. The Stark shift of the step quantum well was much more sensitive to the applied electric field than that of the single quantum well. These results indicate that CdxZn1−xTe/ZnTe step quantum wells hold promise for potential applications such as new types of modulators and quantum-well infrared photodetectors in the blue–green region of the spectrum.
Efficient frequency doubling of 1.5 μm femtosecond laser pulses in quasi-phase-matched optical fibers
Authors: Pruneri V. , Bonfrate G. , Kazansky P. G. , Simonneau C. , Vidakovic P. , Levenson J. A.
Published: July 2002

Second-order nonlinear gratings in optical fibers have been produced for efficient quasi-phase-matched frequency conversion around 1.5 μm. Periodic poling was achieved by defining a patterned electrode on a D-shape fiber via standard lithography and applying high voltage (4–5 kV) at elevated temperature (270–280 °C). This fabrication technique has allowed us to produce gratings uniform over 7.5 cm, as indicated by the shape and the bandwidth of the phase-matching curve. By frequency doubling ∼100 fs pulses in a grating ∼4 cm long and with a period of 57.15 μm, ∼1.05 mW average power at 768 nm has been generated, with an average conversion efficiency ∼1.2%.
Large optical nonlinearities near the band gap of GaN thin films
Authors: Schmidt T. J. , Song J. J. , Chang Y. C. , Horning R. , Goldenberg B.
Published: July 2002

The interband optical transitions in single-crystal GaN films grown by metal organic chemical vapor deposition have been studied at 10 K and room temperature using nondegenerate nanosecond optical pump-probe techniques. At low temperatures, strong, well-resolved features are seen in the absorption and reflection spectra corresponding to the 1s A and B exciton transitions. These features broaden and decrease in intensity due to the presence of a high density of photoexcited free carriers and are completely absent in the absorption and reflection spectra as the excitation density, Iexc, approaches 3 MW/cm2, resulting in induced transparency in transmission measurements. The absorption spectra also show induced absorption below the band gap as Iexc is increased. Both the observed induced transparency and induced absorption were found to be extremely large, exceeding 4×104 cm−1 as the pump density approaches 3 MW/cm2 at 10 K.
p
-type conduction in as-grown Mg-doped GaN grown by metalorganic chemical vapor deposition
Authors: Sugiura Lisa , Suzuki Mariko , Nishio Johji
Published: July 2002

We have clarified the effect of H2 and NH3 on the passivation of Mg acceptor in p-type GaN films grown by metalorganic chemical vapor deposition. It has been found that the small amount of H2 carrier gas strongly influences the electrical property of the Mg-doped GaN films. Low-resistivity p-type GaN has been obtained by H2-free growth without any post-treatments. Its acceptor concentration is as high as that obtained by conventional H2-rich growth with subsequent thermal annealing. It has also been clarified that hydrogen produced by NH3 dissociation does not prevent Mg from electrically activating in H2-free growth.
Direct transformation of graphite to cubic diamond observed in a laser-heated diamond anvil cell
Authors: Yusa H. , Takemura K. , Matsui Y. , Morishima H. , Watanabe K. , Yamawaki H. , Aoki K.
Published: July 2002

Graphite samples are heated in a diamond anvil cell with a CO2 laser above 11 GPa. A light transparent phase has appeared after heating, which is quenchable at ambient conditions. The Raman spectrum shows that the phase is a fine-grained cubic diamond. Images by a high resolution transmission electron microscope indicate that the diamond has twins and stacking faults of nanometer size. The electron diffraction pattern shows that the stacking fault cannot be explained by a diamond polytype such as a hexagonal diamond. The cathodoluminescence pattern exhibits the “band-A” emission, which is derived from the defects of the cubic diamond.
Low field magnetic resonance images of polarized noble gases obtained with a dc superconducting quantum interference device
Authors: Augustine Matthew P. , Wong-Foy Annjoe , Yarger Jeffery L. , Tomaselli Marco , Pines Alexander , TonThat Dinh M. , Clarke John
Published: July 2002

Using a low transition temperature superconducting quantum interference device as a detector, we have obtained magnetic resonance images of laser-polarized He3 gas and solid Xe129 at 4.2 K in magnetic fields as low as 0.54 mT (3He) and 1 mT (129Xe), corresponding to Larmor frequencies of 17.6 and 11.8 kHz, respectively. The experimental resolution of the images is ∼500 μm for He3 in the gas phase and ∼950 μm for Xe129 in the solid state.
Two-dimensional arsenic precipitation in superlattice structures of alternately undoped and heavily Be-doped GaAs grown by low-temperature molecular beam epitaxy
Authors: Su Z. A. , Huang J. H. , Hsieh L. Z. , Lee W.-I.
Published: July 2002

The precipitation of arsenic in superlattice structures of alternately undoped and [Be]=2.4×1019 cm−3 doped GaAs grown at low temperatures has been studied using transmission electron microscopy. Novel precipitate microstructures were observed in annealed samples, including preferential accumulation of precipitates toward each interface of Be-doped GaAs and the following grown undoped GaAs. Specifically, after 800 °C annealing, the precipitates are totally confined in Be-doped regions, forming two-dimensional dot arrays near the aforementioned interfaces. Data are also presented to show that the heavily Be-doped GaAs has a smaller lattice constant than the undoped GaAs. A strain-induced mechanism was proposed to account for the segregation of As clusters.
Polarization-dependent efficiency of photoconducting THz transmitters and receivers
Authors: Huggard P. G. , Shaw C. J. , Cluff J. A. , Andrews S. R.
Published: July 2002

The emission and detection efficiencies of photoconducting THz transmitters and receivers are found to be sensitive to the polarization of the optical gating pulse. Signal amplitudes from GaAs coplanar stripline transmitters and silicon-on-sapphire dipole receivers vary by factors of up to 4 and 3, respectively, with rotation of the exciting pulse polarization. In both cases, maximum signal is obtained when the polarization of the normally incident light is perpendicular to the edge of the metal electrodes. This polarization sensitivity, which appears to arise from differences in the spatial distribution of photoexcited carriers in the semiconductor, needs to be considered when optimizing the signal-to-noise ratio in coherent THz spectroscopy.
Local optical spectroscopy of self-assembled quantum dots using a near-field optical fiber probe to induce a localized strain field
Authors: Robinson H. D. , Müller M. G. , Goldberg B. B. , Merz J. L.
Published: July 2002

We introduce and demonstrate a novel operating mode in near-field optical microscopy. The tip is used to simultaneously optically probe the sample and induce a highly localized strain in the area under study by pushing the tip into the sample. From knowledge of total tip-sample compression and tip geometry, we estimate the magnitude of stress, and show that localized uniaxial-like stresses in excess of 10 kbar can be achieved. We apply this method to a sample of InAlAs self-assembled quantum dots. A blueshift of quantum dot emission lines consistent with estimates of the strain is observed, as well as a quenching of the photoluminescence with strain.
High-power inter-miniband lasing in intrinsic superlattices
Authors: Tredicucci Alessandro , Capasso Federico , Gmachl Claire , Sivco Deborah L. , Hutchinson Albert L. , Cho Alfred Y. , Faist Jérôme , Scamarcio Gaetano
Published: July 2002

We report the realization of a mid-infrared (λ≃7 μm) quantum-cascade laser, in which the emission process takes place between the two lowest minibands of an intrinsic superlattice. Contrary to previous lasers based on doped superlattices, here the dopants are located only inside suitably designed injector regions, where positive ionized donors and negative electrons are arranged to compensate the applied external field across the superlattices. This reduces impurity scattering and translates into low threshold currents (4.2 kA/cm2 at 10 K) and into room temperature operation, without compromising the large current-carrying capabilities of the minibands. Peak powers of ∼1.3 W per facet have been obtained from broad-area devices at 10 K, with still more than 1 W at 120 K and 400 mW at 200 K. Effects related to the finite size of the superlattices become visible in the spectral properties, owing to the reduced broadening, and have to be taken into account to accurately describe the laser’s behavior.
In situ ultrasonic measurement of photoresist glass transition temperature
Authors: Morton Susan L. , Degertekin F. Levent , Khuri-Yakub B. T.
Published: July 2002

We have developed a method to measure the glass transition temperature of photoresist in situ. The purpose of photoresist prebake is to evaporate excess solvent from the photoresist and to allow the polymer chains to relax into an ordered matrix. The phase of a high frequency ultrasound signal was monitored as it was reflected from the silicon/photoresist interface during the photoresist prebake. There was an initial decrease in phase followed by a rapid increase once a given temperature was reached. It is believed that this repeatable temperature is the glass transition temperature of the resist/solvent mixture and that the rapid increase in phase following it is a result of solvent evaporation. The glass transition temperature (Tg) for a freshly spun 2.2 μm Shipley 1813 resist was measured to be 50 °C. The Tg measured increased from 50 to 66 °C as the time delay between coating and prebaking was increased from 0 to 20 min. It is believed that this change in Tg is a result of solvent evaporation.
Laser cleaning of field emitter arrays for enhanced electron emission
Authors: Yavas O. , Suzuki N. , Takai M. , Hosono A. , Kawabuchi S.
Published: July 2002

The effect of laser irradiation on the electron emission efficiency of field emitter arrays has been investigated as a function of wavelength using different harmonics of a diode-pumped Nd:YLF laser. While irradiation in the infrared or visible range did not induce any observable change in the emission behavior, ultraviolet laser irradiation, even at a lower fluence compared to infrared or visible, resulted in a significant increase of the emission efficiency, revealing that the photodecomposition of organic contaminants is the main mechanism for cleaning. During laser irradiation, the emission current increases gradually, approaching a saturation level as the surface is cleaned.
Enhanced levitation forces with field cooled YBa2Cu3O7−δ
Authors: Hennig W. , Parks D. , Weinstein R. , Sawh R.-P.
Published: July 2002

We have examined levitation forces of field cooled (2.3 T) and zero field cooled YBa2Cu3O7−δ superconductors opposed by a SmCo magnet. We have achieved unusually large levitation forces (140 N) and pressures (44.6 N/cm2) in field cooled measurements. 5–10 subsequent cycles, of approach and retreat of the magnet, result in a 10–20% decrease in force, which then stabilizes.
Topography measurements of the critical thickness of ZnSe grown on GaAs
Authors: Horsburgh G. , Prior K. A. , Meredith W. , Galbraith I. , Cavenett B. C. , Whitehouse C. R. , Lacey G. , Cullis A. G. , Parbrook P. J. , Möck P. , Mizuno K.
Published: July 2002

Synchrotron-based x-ray topography (XRT) measurements have been used to study the initial stages of relaxation in ZnSe layers grown by molecular beam epitaxy on vertical gradient freeze Bridgman GaAs substrates. The formation of the very first strain-relieving misfit dislocations in the grown ZnSe layers has been detected in a layer of thickness 100 nm. No such dislocations have been observed in a corresponding layer of 95 nm thickness. The critical thickness for this material system is therefore estimated to be 97.5±2.5 nm, which is markedly lower than the widely accepted value of 150 nm. In contrast to the InGaAs/GaAs system, combined XRT and transmission electron microscopy studies indicate that the initial misfit dislocations observed for ZnSe/GaAs are not, in general, formed by the bending over of pre-existing threading dislocations into the interface, but by other mechanisms such as stacking fault decomposition. The critical thickness data obtained have been used to infer the maximum critical thickness of CdZnSe quantum wells possible in II–VI laser diodes.
Effects of piezoelectric field on defect formation, charge transfer, and electron transport at GaN/AlxGa1−xN interfaces
Authors: Hsu L. , Walukiewicz W.
Published: July 2002

We have calculated the donor defect profiles in pseudomorphic AlxGa1−xN layers grown on GaN, taking into account the effects of the strain polarization field on the defect formation energy. Under certain conditions, the defect concentration may be enhanced by more than one order of magnitude. These large concentrations combined with the band bending effects of the piezoelectric field makes the charge transfer from the AlxGa1−xN barrier to the GaN well extremely efficient, resulting in a two-dimensional electron gas of very high density and low mobility. These results explain recent experimental observations of large electron densities found in nominally undoped GaN/AlGaN heterostructures.
Near-field magneto-optical spectroscopy of single self-assembled InAs quantum dots
Authors: Toda Y. , Shinomori S. , Suzuki K. , Arakawa Y.
Published: July 2002

We report observations of Zeeman spin splitting in photoluminescence from single InAs/GaAs self-assembled quantum dots (SAQDs) fabricated by Stranski–Krastanow growth. A low-temperature near-field optical microscope incorporating a superconducting magnet enabled us to resolve the luminescence from individual SAQDs. We have measured Zeeman split components from single SAQDs in magnetic fields up to 10 T. Circularly-polarized photoluminescence resolved carriers of different spin polarity. Dependence of the splitting on the QD structures was also investigated.
Luminescence efficiency measurements of silicon nanoclusters
Authors: Kenyon A. J. , Trwoga P. F. , Pitt C. W. , Rehm G.
Published: July 2002

We present the results of what we believe to be the first study of the power efficiency of room temperature photoluminescence from thin films of silica containing silicon nanoclusters. Films were prepared by plasma enhanced chemical vapor deposition from silane and nitrous oxide precursors. Luminescence was excited using the 476 nm line of an argon-ion laser. We have measured power efficiencies for samples that exhibit luminescence solely due to radiative recombination of quantum confined excitons. Efficiencies around 0.04% are reported.
Submillimeter-band high-power generation using multilayered Josephson junctions
Authors: Ustinov A. V. , Sakai S.
Published: July 2002

We report on numerical simulations of 10- and 19-layer stacked Josephson junctions. Simulations are performed using the experimentally feasible parameters for Nb/Al–AlOx/Nb junctions. With an appropriate choice of parameters such as magnetic field and coupling, the current–voltage curves of N-layer stacks systematically indicate in-phase locking among the inner N−2 junctions. The simulation results suggest that multilayer Josephson junctions are very promising as submillimeter-band high-power oscillators.
Infrared analysis of deuterated carbon–nitrogen films obtained by dual-ion-beam-assisted-deposition
Authors: Alvarez F. , Victoria N. M. , Hammer P. , Freire F. L. , dos Santos M. C.
Published: July 2002

The isotopic effect on the infrared spectra is used to determine the existence of nitrogen–hydrogen bonds in amorphous carbon–nitrogen alloys (a-CNx) prepared by dual-ion-beam-assisted deposition. The deuteration experiments and the evolution of the infrared spectra upon atmospheric exposure show that hydroxyls are incorporated from atmospheric moisture.
Relationship between optical and structural properties in substituted quaterthiophene crystals
Authors: Gigli G. , Lomascolo M. , Cingolani R. , Barbarella G. , Zambianchi M. , Antolini L. , Della Sala F. , Di Carlo A. , Lugli P.
Published: July 2002

The optical properties of polysubstituted α-conjugated quaterthiophene crystals display marked differences depending on substitution pattern and molecular conformation. The combination of optical spectroscopy and x-ray diffraction elucidates the correlation among molecular functionalization, crystalline structure, and electronic states. The data are quantitatively interpreted by a semiempirical technique that provides the excited state energies starting from the measured molecular geometry.
Observation of step bunches in units of 4 ML on vicinal Si(113) surfaces
Authors: Zhu Jian-hong , Brunner K. , Abstreiter G.
Published: July 2002

Step bunching with a bunching unit of a four-atomic step has been observed on vicinal Si(113) surfaces which are misoriented by 0.37° towards a direction about 36° off [1̄10]. A strong short-range attractive interaction and a long-range repulsive interaction between the four-atomic steps have been revealed. The influence of the repulsive interaction is observed even at about 700 °C. The bunching can be enhanced by optimizing the growth conditions followed by annealing. The experiments confirm the low surface energy of the Si(113) plane.
Correlated electron transport in coupled metal double dots
Authors: Orlov Alexei O. , Amlani Islamshah , Toth Geza , Lent Craig S. , Bernstein Gary H. , Snider Gregory L.
Published: July 2002

The electrostatic interaction between two capacitively coupled, series-connected metal double dots is studied at low temperatures. Experiment shows that when the Coulomb blockade is lifted, by applying appropriate gate biases, in both double dots simultaneously, the conductance through each double dot becomes significantly lower than when only one double dot is conducting a current. The conductance lowering seen in interacting double dots is compared to that caused by an external ac modulation applied to the double-dot gates. The results suggest that the conductance lowering in each double dot is caused by a single-electron tunneling in the other double dot. Here, each double dot responds to the instantaneous, rather than average, potentials on the other double dot. This leads to correlated electron motion within the system, where the position of single electron in one double dot controls the tunneling rate through the other double dot.
Selective carrier injection into V-groove quantum wires
Authors: Weman H. , Martinet E. , Rudra A. , Kapon E.
Published: July 2002

We have observed selective carrier injection into GaAs/AlGaAs V-groove quantum wires (QWRs) via self-ordered vertical quantum wells (VQWs). Room-temperature I–V characteristics of QWR diodes show a turn-on voltage lower by 0.2 V as compared with planar QW diodes, consistent with the band-gap reduction of 0.2 eV at the vertical QW. This selective injection results in narrow linewidth electroluminescence (∼5 nm at 300 K) emanating exclusively from the QWR from 10 K up to 300 K.
Metal–insulator transition in SrRuO3 induced by ion irradiation
Authors: Sefrioui Z. , Arias D. , Navacerrada M. A. , Varela M. , Loos G. , Lucı́a M. , Santamarı́a J. , Sánchez-Quesada F. , López de la Torre M. A.
Published: July 2002

We have studied the effect of He+ irradiation on the electrical resistivity and Curie temperature of ferromagnetic SrRuO3 thin films. An evolution from metallic to insulating behavior is observed when He+ ion fluence is increased, suggesting a metal–insulator transition. Damage by ion irradiation produces a strong decrease of the Curie temperature. On the other hand, no significant change in Tc (∼160 K) takes place in fresh samples grown at different substrate temperatures. We discuss the possible correlation between structural changes induced by irradiation, which reflect in an increase of the pseudocubic lattice parameter, and the observed depression of Tc.
The dynamics of tunneling into self-assembled InAs dots
Authors: Luyken R. J. , Lorke A. , Govorov A. O. , Kotthaus J. P. , Medeiros-Ribeiro G. , Petroff P. M.
Published: July 2002

Using frequency-dependent capacitance spectroscopy, the dynamics of tunneling into arrays of self-assembled InAs quantum dots is investigated with respect to sample geometry, Coulomb interaction, and magnetic field. An equivalent resistance-capacitance circuit is derived which allows us to determine the tunneling times for each state of the dots. The different tunneling times for different many-particle states are explained by a reduced tunneling barrier and Coulomb interaction. A magnetic field applied perpendicular to the tunneling direction results in a strong suppression of the charging signal, which is attributed to enhanced localization caused by the magnetic field. Calculations for three-dimensional to zero-dimensional magnetotunneling can account for the experimental data.
Characterization of the inverted Ga0.52In0.48P/GaAs (001) junctions using current–voltage and capacitance–voltage measurements
Authors: Cai C. , Nathan M. I. , Lim T. H.
Published: July 2002

Al /u :GaAs /n :Ga 0.52 In 0.48 P inverted-structure (GaAs on top) Schottky diodes on n+ GaAs (001) substrates have been grown by gas source molecular beam epitaxy with several GaAs thicknesses from 10 to 100 nm. The barrier height determined by the capacitance versus voltage method is substantially higher than the barrier height determined by the current versus voltage method. These results suggest that there is a negative interface charge 6–8×1011/cm−2 at the GaAs/Ga0.52In0.48P interface, which is opposite in sign to the interface charge at the normal structure of the Ga0.52In0.48P/GaAs heterojunction reported previously.
Optically induced bistability in the mobility of a two-dimensional electron gas coupled to a layer of quantum dots
Authors: Shields A. J. , O’Sullivan M. P. , Farrer I. , Ritchie D. A. , Cooper K. , Foden C. L. , Pepper M.
Published: July 2002

We report a bistability in the resistance of a GaAs/AlGaAs modulation doped field effect transistor in which a layer of InAs self-organized quantum dots has been grown near the electron channel. Brief optical illumination causes a large, persistent drop in the two-dimensional electron gas (2DEG) resistance which can be recovered by allowing a current to flow through the Schottky gate. We demonstrate that illumination reduces the number of electrons trapped in the quantum dots, lowering their potential and thereby enhancing the 2DEG mobility. This bistability could be the basis of an optical memory or sensitive phototransistor.
Simulation of hyperthermal deposition of Si and C on SiC surfaces
Authors: Alfonso Dominic R. , Ulloa Sergio E.
Published: July 2002

We describe the adsorption dynamics of Si and C atoms at supersonic velocities on Si- and C-terminated 6H–SiC(0001) substrates using molecular dynamics simulations. The sticking probabilities of adatoms are found to be very high and not to change substantially with increasing incident kinetic energy. We identify two mechanisms responsible for the high sticking probabilities of the adatoms: (a) efficient transfer of adatom energy to the substrate and (b) strong attractive forces experienced by the impinging adatom over the entire surface. The calculated potential energy surfaces reveal possible binding sites of the adatoms on the substrates.
Photoinduced birefringence and reversible optical storage in liquid-crystalline azobenzene side-chain polymers
Authors: Wang Changshun , Fei Haosheng , Qiu Yong , Yang Yanqiang , Wei Zhenqian , Tian Yanqing , Chen Yongmei , Zhao Yingying
Published: July 2002

A liquid crystal with an azobenzene moiety as a photoactive mesogenic unit is investigated in photoinduced birefringence. Measurements reveal that it possesses large magnitude of photoinduced birefringence (Δn∼10−2) and the characteristic of long-term optical storage. The resulting polarization information can be erased either by heating the sample to a temperature much higher than the glass transition temperature, or by optical method with a circularly polarized light. The mechanism of long-term optical storage is discussed in this letter.
Active reflection via a phase-insensitive quadratic nonlinear interaction within a microcavity
Authors: Cojocaru Crina , Martorell Jordi , Vilaseca R. , Trull J. , Fazio Eugenio
Published: July 2002

The reflectivity of a microcavity filled with a quadratic nonlinear material is shown to be actively changed by the interaction of two waves. Within this microcavity, the reflection coefficient of a weak wave at the fundamental frequency is changed from almost 0% to a value in the vicinity of 100% by the simultaneous incidence of an intense wave at the second-harmonic frequency. This change in reflectivity is shown to be in a large degree insensitive to the input phase difference between the two waves.
Temperature dependence of InGaAs/GaAs quantum well microcavity light-emitting diodes
Authors: Takamori T. , Pratt A. R. , Kamijoh T.
Published: July 2002

We report a systematic investigation of the emission properties of semiconductor microcavity light-emitting diodes (MC-LEDs) with different cavity detuning. Evidence that varying the cavity detuning leads to temperature insensitive output characteristics is provided by changes in the temperature dependence of the slope efficiency extracted from the light output versus current characteristics. For resonantly tuned devices the slope efficiency decreases monotonically with increasing temperature. However when the cavity peak is detuned to long wavelength with respect to the room temperature quantum well (QW) emission, temperature insensitive characteristics are achieved. Compared to a noncavity type LED, enhanced efficiency and narrow spectral linewidth have been observed for the MC-LEDs with the highest output efficiency achieved when the QW emission and cavity peak are exact resonant.
ZnSe/GaAs interface state probed by time-resolved reflectance difference spectroscopy
Authors: Wong K. S. , Wang H. , Yang Z. , Sou I. K. , Wong G. K. L.
Published: July 2002

Time-resolved reflectance difference spectroscopy (TRDS) has been applied to study the dynamics and relaxation processes of the 2.7 eV ZnSe/GaAs interface state associated with Zn–As bonds. The instantaneous screening due to the photoexcited carriers and ∼18 ps recovery time of the 2.7 eV interface state is observed in the TRDS spectra. The rapid cooling of the hot carrier in the spectral region above the ZnSe band edge is also observed.
Acoustic field mapping on GaAs using microscopic reflectance and reflectance anisotropy
Authors: Santos Paulo V.
Published: July 2002

The mapping of surface acoustic wave (SAW) fields by means of microscopic reflectance and reflectance anisotropy in GaAs-based structures is reported. The two techniques are complementary with the second being sensitive to the strain and the first both to the strain and to the surface modulation induced by the SAW. Their combination provides information about both the longitudinal and transverse components of the SAW particle displacement vector.
Nonlinear inductive response of high temperature superconducting films measured by the mutual inductance technique
Authors: Claassen J. H. , Booth James C. , Beall J. A. , Rudman D. A. , Vale L. R. , Ono R. H.
Published: July 2002

The dependence of the penetration depth λ on current density J in an unpatterned superconducting film can be measured with a pair of small coaxial coils positioned on opposite sides of the film. Mutual inductance measurements in this configuration with a direct-current (dc) current component in one of the coils provide a means to determine λ(J). It is possible to separate out the effects of heating by initially trapping a persistent current in the film. The nonlinear (current-dependent) coefficients of λ(J) measured by this “dc” technique agree well with those measured by third harmonic generation in coplanar waveguide transmission lines at 5 GHz. This nondestructure technique could be used to screen films before incorporating them into circuits sensitive to nonlinear effects.
Femtosecond pump-probe investigation on relaxation of photoexcitations and spectral narrowing of photoluminescence for poly(para-phenylenevinylene)
Authors: Jeoung Sae Chae , Kim Yong Hee , Kim Dongho , Han Ja-Young , Jang Min Sik , Lee Jung-Ik , Shim Hong-Ku , Kim Cheon Min , Yoon Choon Sup
Published: July 2002

Photoinduced absorption spectra and their temporal profiles of poly(para-phenylenevinylene) thin film exhibit a strong correlation with an appearance of spectrally narrowed emission bands. The dependence of transient absorption signals on excitation and probe power denotes much lower saturation intensity in photoinduced absorption than that in stimulated emission arising from the identical photoexcitations, singlet excitons. We interpreted the peculiar emission bands in terms of superradiance or superfluorescence with a time constant of around 1 ps rather than amplification of spontaneous emission from uncorrelated emitters.
High temperature proton implantation induced photosensitivity of Ge-doped SiO2 planar waveguides
Authors: Hughes P. J. , Knights A. P. , Weiss B. L. , Kuna S. , Coleman P. G. , Ojha S.
Published: July 2002

The possibility of using keV proton implantation at 800 °C to enhance the photosensitivity of Ge-doped silica has been investigated. Room temperature implantation induced defects indicated by absorption at ultraviolet (UV) (<200 nm) and visible wavelengths (>550 nm) were annealed during implantation at 800 °C to leave stable photosensitive neutral oxygen vacancy (NOV) centers with an absorption peak at ∼240 nm. The stable NOV defects were photochemically bleached after UV exposure, a process which is accompanied by a change in UV absorption. Positron annihilation spectroscopy demonstrated the effectiveness of implantation at 800 °C in annealing the implantation induced damage.
Urbach energy dependence of the stability in amorphous silicon thin-film transistors
Authors: Wehrspohn R. B. , Deane S. C. , French I. D. , Gale I. G. , Powell M. J. , Brüggemann R.
Published: July 2002

We investigate the relationship between the stability of amorphous silicon thin-film transistors (a-Si:H TFTs) and the bulk properties of a-Si:H films. Threshold voltage shifts in a-Si:H TFTs are characterized by the thermalization energy Eth for different times and temperatures and fitted by {1+exp[(Eth−Ea)/kT0]}−2. We find that kT0 exhibits a clear correlation to the Urbach energy, but the more significant parameter Ea seems to depend only on the deposition-induced microstructure and not on the Urbach energy, the hydrogen content, or the hydrogen diffusion coefficient.
Band offsets at the ZnSe/CuGaSe2(001) heterointerface
Authors: Bauknecht A. , Blieske U. , Kampschulte T. , Albert J. , Sehnert H. , Lux-Steiner M. Ch. , Klein A. , Jaegermann W.
Published: July 2002

The formation of the ZnSe/CuGaSe2 heterointerface was studied by x-ray photoelectron spectroscopy (XPS). ZnSe was sequentially grown on CuGaSe2(001) epilayers. In situ photoemission spectra of the Ga 3d and Zn 3d core levels as well as XPS valence bands were acquired after each deposition step. The valence-band offset is determined to be ΔEV=0.6±0.1 eV. As a consequence, a nearly symmetric “type-I” band alignment for the ZnSe/CuGaSe2 heterojunction with a conduction-band offset of ΔEC=0.4±0.1 eV is found. Concerning the band alignment ZnSe can, therefore, be expected to be a suitable buffer material for CuGaSe2-based thin-film solar cells.
Interface and bulk defects in SiC/GaN heterostructures characterized using thermal admittance spectroscopy
Authors: Witte H. , Krtschil A. , Lisker M. , Christen J. , Topf M. , Meister D. , Meyer B. K.
Published: July 2002

SiC/GaN p-n and n-n heterostructures grown by low pressure chemical vapor deposition were investigated using thermal admittance spectroscopy. Different kinds of defects were isolated and located. Evidence of a distribution of defects at the p-SiC/n-GaN interface is given as having thermal activation energies of (87±3) meV at 5 V and (72±4) meV at 8 V bias. Additionally, three bulk defects with activation energies between 155 and 175 meV were found. By comparison with admittance spectra of the p-type SiC substrate, one level was identified as Al acceptor in SiC, whereas the other defects are electron traps in the GaN layer.
Large photosensitivity in lead–silicate glasses
Authors: Long X.-C. , Brueck S. R. J.
Published: July 2002

Strong, permanent surface-relief and refractive index gratings are written in lead–silicate glasses by irradiation with the output of a pulsed KrF excimer laser (248 nm) through a phase mask. Diffraction efficiencies as high as 10% are obtained. The diffraction efficiency of the refractive index grating after removal of the surface-relief grating shows that a very large photoinduced refractive index change (Δn0=0.09±0.02) is obtained.
Broad-range, latchable reconfiguration of Bragg wavelength in optical gratings
Authors: Jin S. , Mavoori H. , Espindola R. P. , Strasser T. A.
Published: July 2002

Tunability of optical signal filters such as fiber Bragg gratings is important for high volume data communications. A magnetically tunable and latchable optical fiber Bragg grating structure has been devised, and a fine wavelength control as well as a broad-range wavelength tunability has been demonstrated. A spinodally decomposed, anisotropically elongated microstructure was induced in Fe–Cr–Co alloys to fabricate square-loop, programmable magnets. The adjustable attractive force between the magnetic poles was used to controllably strain the grating and induce a very large shift in the Bragg reflection wavelength by as much as 15 nm (which is equivalent to more than 36-channel span in a high density optical communication system). This novel approach can be useful not only for a variety of optical communication networking applications, but also for imparting latchable reconfiguration of structural periodicity and functional properties in a wide class of materials and devices.
Optical amplification with high gain in hybrid-polymer–liquid-crystal structures
Authors: Kajzar F. , Bartkiewicz S. , Miniewicz A.
Published: July 2002

Efficient coherent light amplification with a very high net exponential gain Γ≈2600 cm−1 has been observed in sandwich-type hybrid-polymer–liquid-crystal structures. The amplification is a result of an energy exchange between two beams in a two-beam coupling experiment. The reported gain coefficient has been measured in a tilted sample configuration within a Raman–Nath diffraction regime at electric fields of the order of 1 V/μm.
Fabrication of electro-optic Pb(Zr, Ti)O3 heterostructure waveguides on Nb-doped SrTiO3 by solid-phase epitaxy
Authors: Nashimoto K. , Nakamura S. , Morikawa T. , Moriyama H. , Watanabe M. , Osakabe E.
Published: July 2002

Pb(Zr, Ti)O 3 (PZT) heterostructure optical waveguides were grown on low resistivity Nb-doped SrTiO3(100) substrates by solid-phase epitaxy. The propagation loss was reduced to 1.7 dB/cm at the wavelength of 1.3 μm by introducing an epitaxial buffer layer between the PZT waveguide and the Nb-doped SrTiO3 substrate. An electro-optic beam deflector with an indium–tin–oxide prism electrode on the surface of the PZT waveguide showed efficient laser beam deflection as great as 3.3° (58 mrad) by applying 20 V between the prism electrode and the substrate. An index change higher than 0.001 at 5 V and an average apparent electro-optic coefficient larger than 46 pm/V were estimated from the deflection characteristic.
Collisional broadening and shift of spectral lines in quantum dot lasers
Authors: Uskov A. V. , Nishi K. , Lang R.
Published: July 2002

We estimate homogeneous broadening and shift of optical transition lines in self-assembled quantum dots (SAQD) caused by elastic Coulomb collisions of carriers in wetting layer with carriers in the SAQD. In particular, we demonstrate that the dephasing time for lasing transitions can be ∼0.1–1 ps at carrier densities in wetting layer ∼1015 m−2.
Influence of a piezoelectric field on the electron distribution in a double GaN/Al0.14Ga0.86N heterojunction
Authors: Ramvall Peter , Aoyagi Yoshinobu , Kuramata Akito , Hacke Peter , Horino Kazuhiko
Published: July 2002

C–V profiling of Al0.14Ga0.86N/GaN heterojunctions was performed. It was found that a heterojunction with the Al0.14Ga0.86N layer on top increases the electron concentration at the Al0.14Ga0.86N/GaN interface, while the reversed structure with the GaN layer on top decreases it. In accordance with this result, an Al0.14Ga0.86N/GaN double heterojunction was found to experience a strongly asymmetric electron distribution with an enhancement of the electron concentration at the interface closest to the sample surface. This effect is attributed to the presence of a piezoelectric field redistributing the electrons in the heterostructure.
Planar contact geometry for far-infrared germanium lasers
Authors: Chamberlin D. R. , Bründermann E. , Haller E. E.
Published: July 2002

We demonstrate operation of p-Ge far-infrared lasers with a planar contact geometry. The smallest laser crystal with this geometry has a maximum pulse length of 22 μs, which is 10% longer than the pulse length of comparably doped lasers with the traditional contact geometry. Calculations of the electric field distribution show that the fraction of active crystal volume for this new geometry is ∼four times larger than that of traditional geometries. Experiments and calculations reveal that in the planar design, the minimum applied voltage necessary for lasing decreases when the crystal geometry approaches a flat planar structure. This is due to a significant increase of the total electric field caused by the Hall effect.
Mechanism for photon emission from Au nano-hemispheres induced by scanning tunneling microscopy
Authors: Liau Yish-Hann , Scherer Norbert F.
Published: July 2002

The photon emission yield observed in scanning tunneling microscopy (STM) measurements of Au hemispheroid-decorated thin films is used to elucidate the interaction of tunneling electrons with local surface plasmon modes. The photon emission probability is found to depend on the surface feature size. The agreement of a model calculation with the experimental results demonstrates that inelastic electron tunneling is the dominant mechanism of STM-induced plasmon excitation for 10–60 nm size metallic features.
Surface acoustic wave impedance element filters for 5 GHz
Authors: Lehtonen S. , Koskela J. , Salomaa M. M. , Plessky V. P. , Honkanen M. , Turunen J.
Published: July 2002

Surface acoustic wave (SAW) impedance element filter prototypes operating in the 5 GHz range are designed, fabricated, and characterized. The patterning is carried out using direct-writing electron-beam lithography and the lift-off technique. The periodicity p of the resonators in the filters for the 5 GHz regime is on the order of 400 nm. Despite the nonideal finger profile, a low minimum insertion loss of 6.5 dB and a flat passband are measured. Our results suggest that SAW technology itself presents no fundamental physical limitations for its extension into the 5 GHz range.
Isotopic study of metalorganic chemical vapor deposition of (Ba, Sr)TiO3 films on Pt
Authors: Gao Y. , Tran T. , Alluri P.
Published: July 2002

Isotopic labeling experiments (18O2) have been carried out to understand the film-formation reactions in the metalorganic chemical vapor deposition of (Ba, Sr)TiO3 (BST) films. Time-of-flight secondary ion mass spectrometry reveals both M 18O and M 16O (M=Ba, Sr, Ti) in the BST films, indicating that the oxygen in the BST films originates from both the gas phase oxidants (18O), and the precursor ligands (16O). Use of a 50% 18O2-50% N2 16O mixture results in a reduction of 18O incorporation in the BST film, indicating direct involvement of N2O in the film-formation reactions. Addition of N2O in O2 also appears to improve film surface morphology and step coverage.