bluuebunny/crossref_metadata_2025 huggingface dataset:
Comparison of trimethylgallium and triethylgallium for the growth of GaN
Authors: Saxler A. , Walker D. , Kung P. , Zhang X. , Razeghi M. , Solomon J. , Mitchel W. C. , Vydyanath H. R.
Published: July 2002

GaN films grown by low-pressure metalorganic chemical vapor deposition using trimethylgallium and triethylgallium as gallium precursors are compared. The films were characterized by x-ray diffraction, Hall effect, photoluminescence, secondary ion mass spectroscopy, and etch pit density measurements. GaN layers grown using triethylgallium exhibited superior electrical and optical properties and a lower carbon impurity concentration.
Short-wavelength photoluminescence and electroluminescence in Ga(Al)P/GaP staggered type II quantum wells
Authors: Gerhold M. , Kamath K. , Bhattacharya P.
Published: July 2002

Photoluminescence spectra of tailored Ga(A1)P/GaP quantum well heterostructures exhibit strong short-wavelength peaks at 363, 560, and 600−700 nm. The peak at 560 nm seems to originate from a no-phonon transition. All the transitions are observed up to 200 K. Light emitting diodes made with the same heterostructure predominently emit 560 nm light (green) with a background of 700 nm (red) at room temperature under cw operation.
Four-wave mixing mediated by the capture of electrons and holes in semiconductor quantum-well laser amplifiers
Authors: Paiella Roberto , Hunziker Guido , Vahala Kerry J. , Koren Uzi
Published: July 2002

An experimental technique based on frequency-resolved four-wave mixing is proposed for the investigation of phonon-assisted capture of electrons and holes in electrically pumped semiconductor quantum wells. We show how this technique can be used to directly measure the intrinsic capture lifetime, with no need for involved numerical fits. We also present experimental results from an application of the technique to a multiquantum-well semiconductor optical amplifier. The possible impact of phase matching on the results is discussed.
PbTiO
3
thin films on Si(100) as potential varistor material
Authors: Palkar V. R. , Purandare S. C. , Apte P. R. , Pinto R. , Multani M. S.
Published: July 2002

We have shown that the aqueous sol derived thin films of PbTiO3 on Si(100) substrate exhibit strong varistor type of behavior for certain annealing conditions. Current–voltage (I–V) characteristics depend upon the processing conditions. The behavior is explained by grain boundary limited conduction model. Since the resistance of the grain boundaries varies with the processing conditions, grain boundaries in all samples do not act as potential barriers to the charge carriers. The film postannealed at 700 °C, possessing high nonlinear coefficient (α) with low breakdown voltage per barrier (Egbl), could be used as a protection device in current-sensitive electronic equipment.
Improved flux pinning through Ce–Mg additions in melt textured YBa2Cu3O7−δ
Authors: McGinn P. J. , Yeung S. , Banerjee A. , Fultz J.
Published: July 2002

The effects of Ce-based additions (CeO2 and BaCeO3) in combination with MgO additions on the magnetic properties of melt textured YBa2Cu3O7−δ have been investigated. The additions lead to improvements in the magnetic properties of YBa2Cu3O7−δ compared to samples with either addition alone or with no additions. The Ce–Mg addition combination produces a “peak effect” in the magnetic hysteresis loop without significant Tc degradation. This is postulated to be due to the formation of a new type of pinning center. Both Ce and Mg ions are thought to substitute in the YBa2Cu3O7−δ lattice, creating defects that produce a peak effect in the magnetic hysteresis loop. Mg additions alone lead to a reduced Tc, while Ce additions restore the Tc and enhance the magnitude of the peak.
Reflection of surface acoustic waves by localized wetting liquids
Authors: Newton M. I. , McHale G. , Banerjee M. K.
Published: July 2002

Rayleigh wave propagation on unloaded solid surfaces becomes severely damped when the surface is covered by a liquid. Our experimental results show that, when the liquid is localized on the surface, reflections of the wave may occur. Moreover, if the liquid shape dynamically evolves, these reflections display a distinct series of maxima and minima.
Positron irradiation: A technique for modifying the photoluminescent structures of porous silicon
Authors: Huang Y. M.
Published: July 2002

It has been shown that a two-peak green photoluminescence (PL) of porous silicon (PS) can be obtained from a continuous blueshift of a red PL by positron irradiation. At room temperature, PS samples were irradiated in air by energetic positrons coming from the conventionally used isotope Na22 (∼20 μCi). With increasing positron irradiation time, an originally red PL shifted continuously to green, then a two-peak PL appeared with a weak high-energy emission band (529 nm) and a low-energy dominant band (562 nm). The intensity of this high-energy band was enhanced by prolonged positron irradiation. The electron-spin-resonance signal combined with infrared absorption showed that positron irradiation created dangling bonds and stimulated oxide growth in PS. An interpretation is given on the basis of quantum confinement and atomiclike nature for very small nano-Si crystallites.
Origin of peak effect in the magnetization hysteresis loop of melt-processed REBa2Cu3Oy superconductors
Authors: Viswanath N. V. N. , Rajasekharan T. , Menon Latika , Kumar N. Harish , Malik S. K. , Dey G. K. , Upadhyaya D. D.
Published: July 2002

Melt-processed Sm-123 samples with an excess Sm2BaCuOy (Sm-211) phase is studied by transmission electron microscope (TEM) and superconducting quantum interference device magnetometer, to explain the origin of the peak effect exhibited in the magnetization hysteresis loops of light RE-123 samples in intermediate fields at 77 K. TEM images of the sample showing peak effect reveal dilute concentrations of areas with ortho-II structure distributed on a nanometer scale within the twin regions of the sample. The ortho-II structure is an oxygen deficient 123 with a lower Tc than the bulk of the material and will be a source of flux pinning by turning normal in high fields, thus contributing to the peak effect. With an increase in Sm-211 concentration, the peak effect disappears from the hysteresis loop, as does the ortho-II structure in the TEM images.
Temperature dependence of mobility in n-type short-period Si–Ge superlattices
Authors: Pearsall T. P. , DiVergilio A. , Gassot Pierre , Maude Duncan , Presting Hartmut , Kasper Erich , Jäger W. , Stenkamp Dirk
Published: July 2002

We have studied the mobility and carrier concentration as a function of temperature between 4 and 300 K in n-type Si–Ge short-period superlattices. In the parallel transport configuration, we have measured a mobility ratio μ77/μ300>50 in an n-type strained-layer superlattice with a carrier concentration in the mid 1016 cm−3 range. The peak mobility measured was 17 000 cm2/V s−1 at 70 K. Carrier freeze-out effects frustrated transport measurements below 60 K. Above 80 K the mobility is limited by optical phonon scattering with a characteristic phonon energy of 60 meV. These characteristics indicate electron transport via extended states.
High stability amorphous-silicon–nitride thin-film diode ring switch
Authors: Johnson M. T. , Oversluizen G. , van der Put A. A. , Lodders W. H. M.
Published: July 2002

Hydrogenated amorphous-silicon–nitride thin-film diode (TFD) switches have been shown to degrade electrically at both the cathode (electron injection contact) and anode (noninjection contact) due, respectively, to electron–hole recombination and hot-electron-induced-state creation mechanisms. An antiparallel configuration of two asymmetric TFDs provides an elegant method to minimize the cathodic degradation and avoid the consequences of anodic defect state creation. In this way, extremely stable TFDs may be prepared.
Excitonic transitions in (GaAs)1−x(Ge2)x/GaAs multilayers grown by magnetron sputtering
Authors: Salazar-Hernández B. , Vidal M. A. , Navarro-Contreras H. , Asomoza R. , Merkulov A.
Published: July 2002

We report on excitonic transitions observed at room temperature in multilayers of (GaAs)1−x(Ge2)x/GaAs grown by magnetron sputtering. We attribute the observation of the exciton absorption at room temperature to confinement effects on the bound electron–hole pairs inside the Ge-rich layers of the (GaAs)1−x(Ge2)x alloys. From secondary ions mass spectroscopy these layers are measured to be 50, 33, and 30 nm in full width at half maximum (FWHM), for three different samples. These layers alternate with (GaAs)1−x(Ge2)x alloy layers of very low Ge concentration that can be considered of plain GaAs, ∼360 nm in thickness for two samples and 240 nm for a third one. The observed transition energies of the exciton are very close to ≈1 eV both at 300 and 4 K. A calculation of exciton energy-level spacing and transition probabilities provides acceptable values for the exciton transition energies observed, considering a triangular quantum well as an approximation to the Ge profile in the confinement layers. The observed excitonic FWHM remain nearly constant at all temperatures examined (300–4 K).
Wurtzite GaN epitaxial growth on a Si(001) substrate using γ-Al2O3 as an intermediate layer
Authors: Wang Lianshan , Liu Xianglin , Zan Yude , Wang Jun , Wang Du , Lu Da-cheng , Wang Zhanguo
Published: July 2002

Wurtzite GaN films have been grown on (001) Si substrates using γ-Al2O3 as an intermediate layer by low pressure (∼76 Torr) metalorganic chemical vapor deposition. Reflection high energy electron diffraction and double crystal x-ray diffraction measurements revealed that the thin γ-Al2O3 layer of “compliant” character was an effective intermediate layer for the GaN film grown epitaxially on Si. The narrowest linewidth of the x-ray rocking curve for (0002) diffraction of the 1.3 μm GaN sample was 54 arcmin. The orientation relationship of GaN/γ-Al2O3/Si was (0001) GaN‖(001) γ-Al2O3‖(001) Si, [11–20] GaN‖[110] γ-Al2O3‖[110] Si. The photoluminescence measurement for GaN at room temperature exhibited a near band-edge peak of 365 nm (3.4 eV).
Nonlinear contradirectional coupler
Authors: Campi D. , Coriasso C. , Stano A. , Faustini L. , Cacciatore C. , Rigo C. , Meneghini G.
Published: July 2002

Results of room-temperature experiments with a multiple-quantum-well, nonlinear, contradirectional coupler are reported. A power-dependent, contradirectional coupling condition is demonstrated, inducing an optically controlled switching of optical signals, with a switching energy of 1 pJ.
Loss measurements on semiconductor lasers by Fourier analysis of the emission spectra
Authors: Hofstetter Daniel , Thornton Robert L.
Published: July 2002

We present a study on a novel method for the determination of the cavity losses in semiconductor lasers. The method we use involves Fourier analysis of the Fabry–Pérot mode spectrum when operating the device below lasing threshold. The observation of the decay rate of higher order harmonics in the Fourier analysis of the spectra allows us to determine the amount of cavity propagation loss/gain. A comparison between experimental and calculated data for an AlGaInP laser at 670 nm showed good agreement up to an injection current of 0.93×Ith. This method therefore provides a generalization of the Fabry–Pérot contrast measurement method for extracting cavity losses from spectral information.
Optical and photoelectric properties of CdxHg1−xTe solid solutions (0.35<x<0.40)
Authors: Belogorokhov Alexander I. , Belov Alexander G. , Lakeenkov Vladimir M. , Belogorokhova Lubov I.
Published: July 2002

This analysis is made of the spectral dependences of the position of the fundamental absorption edge, the photoconductivity, and the optical spectra of CdxHg1−xTe (0.35<x<0.40) alloys at temperatures 5–300 K in the wave number region (30–5000) cm−1. The nonmonotonic dependence on temperature of the parameters considered is investigated. The reflection spectra are analyzed mathematically by means of dispersion analysis and the Kramers–Kroning relations. The results obtained are explained by the influence of polaron effects on the processes of charge transport in the crystal. It is conjectured that the polaron configuration changes at the characteristic temperatures 60, 160, and 225 K.
High-Tc direct current SQUIDs on silicon bicrystal substrates operating at 77 K
Authors: Schmidl F. , Linzen S. , Wunderlich S. , Seidel P.
Published: July 2002

We investigated Josephson junctions and dc superconducting quantum interference devices (SQUIDs) on silicon bicrystal substrates with epitaxially grown 24° Y1Ba2Cu3O7−x, (YBCO) grain boundaries. Buffer layers and passivation/contact layers were prepared for the YBCO thin films by laser deposition techniques. The observed current–voltage characteristics of ion-beam etched, as well as direct laser patterned junctions with products of the critical current and the normal resistance up to 150 μV at 77 K, can be described within the resistively shunted junction model. The inductance of dc SQUIDs was varied by additional laser patterning. In this way, dc SQUIDs with transfer functions up to 30 μV/Φ0 and a white noise level of 30 μΦ0/Hz at 77 K were reached.
Grey scale memory in an optically addressed spatial light modulator with a Lu(Pc)2 doped layer
Authors: Guéna M. , Wu Z. Y. , L’Her M. , Pondaven A. , Cadiou C.
Published: July 2002

An optically addressed spatial light modulator with electrolyte bisphtalocyanine of lutetium incorporated in the alignment layer is shown to be capable of memorizing a grey scale image. The relationship between the memorized grey scale and the illumination is shown. The temperature, the concentration of Lu(Pc)2 and the spontaneous polarization of liquid crystals are found to have the effect on the memorization rate.
Measuring the ion current in electrical discharges using radio-frequency current and voltage measurements
Authors: Sobolewski M. A.
Published: July 2002

This letter describes a technique for measuring the ion current at a semiconductor wafer that is undergoing plasma processing. The technique relies on external measurements of the radio-frequency (rf) current and voltage at the wafer electrode. The rf signals are generated by the rf bias power which is normally applied to wafers during processing. There is no need for any probe inserted into the plasma or for any additional power supplies which might perturb the plasma. To test the technique, comparisons were made with dc measurements of ion current at a bare aluminum electrode, for argon discharges at 1.33 Pa, ion current densities of 1.3–13 mA/cm2, rf bias frequencies of 0.1–10 MHz, and rf bias voltages from 1 to 200 V. Additional tests showed that ion current measurements could be obtained by the rf technique even when electrically insulating wafers were placed on the electrode and when an insulating layer was deposited on the electrode.
Measurement of the static error rate of a storage cell for single magnetic flux quanta, fabricated from high-Tc multilayer bicrystal Josephson junctions
Authors: Chong Y. , Ruck B. , Dittmann R. , Horstmann C. , Engelhardt A. , Wahl G. , Oelze B. , Sodtke E.
Published: July 2002

We measured the static error rate of a high-Tc superconductor dc superconducting quantum interference device (SQUID), which, in the form as a storage loop for single flux quanta, is a basic element of rapid single flux quantum circuits. Using high-Tc multilayer bicrystal technology, we fabricated a stacked dc SQUID pair, one SQUID serving as the storage loop, the other one as the readout device. The escape rate of a stored flux quantum was measured as a function of the bias current at a temperature of 28 K. The measured error rates were in good agreement with a model calculation based on thermally activated barrier crossing.
A fiber laser for effective generation of tunable single- and dual-wavelength mode-locked optical pulses
Authors: Zhao Y. , Shu C.
Published: July 2002

A scheme to produce wavelength-tunable pulses with gigahertz bit rate is demonstrated by active mode locking of a fiber laser. Fiber gratings are used in the setup to play the simultaneous roles of wavelength selective mirrors, tunable optical filters, and output fiber couplers. Single- and dual-wavelength picosecond pulses are generated with a flexibility of switching between the outputs by polarization adjustment. A strain tuning process is successfully developed and a total operating range of 13.62 nm towards both shorter and longer wavelengths is demonstrated. Our experimental observations are in good agreement with the calculated results.
Formation of epitaxial CoSi2 spike in Co/Si3N4/Si(100) system and its crystallographic structure
Authors: Park Ji-Soo , Sohn Dong Kyun , Kim Yeongcheol , Bae Jong-Uk , Lee Byung Hak , Byun Jeong Soo , Kim Jae Jeong
Published: July 2002

The formation of CoSi2 spike in the Co/Si3N4/Si(100) system and its crystallographic structure have been investigated. An annealing at 1050 °C caused not only agglomeration of Co film but penetration of Co agglomerates through the Si3N4 layer. The CoSi2 spike of B type epitaxial and twinned orientation of CoSi2[110]∥Si[110], Si(111̄)∥CoSi2(1̄11̄) and Si(1̄1̄1)∥CoSi2(11̄1) was formed in the Si substrate by the penetrated Co source. The formation of the epitaxial CoSi2 spike can be explained by the fast diffusion of Co atoms along defects in Si such as dislocations resulting from stress between the Si3N4 layer and the Si substrate.
Ultraviolet cathodoluminescence from diamond layers after doping by means of boron-ion implantation
Authors: Prins Johan F.
Published: July 2002

Boron-doped diamond layers have been generated in insulating (type IIa) diamonds by means of ion implantation. Cathodoluminescence measurements were used to monitor the layers during this process. It was found that the activation of the boron acceptors, followed by suitable annealing, gave rise to the appearance of two ultraviolet luminescence bands around 3.5 and 4.6 eV. These same bands had been observed previously in natural, as well as synthetically grown, boron-doped, type IIb diamonds. The results reported in this study, support the conclusion that these bands are related to the presence of boron acceptors and show, in addition, that the acceptor density NA need not be more than the density of the compensating donors ND, as had been postulated.
Void-like defects in annealed Czochralski silicon
Authors: Gao M. , Duan X. F. , Peng L.-M. , Li J.
Published: July 2002

Void-like defects of octahedron structure having {111} facets were observed in annealed Czochralski silicon. The amorphous coverage of SiOx and SiCx on the inner surface of the defects was identified using transmission electron microscopy and electron energy-loss spectroscopy. It is suggested that these defects are a kind of amorphous precipitate origin. A mechanism for the generation of these defects and the previously reported solid amorphous precipitates is proposed.
Microstructural evidence on direct contact of Au/Ge/Ni/Au ohmic metals to InGaAs channel in pseudomorphic high electron mobility transistor with undoped cap layer
Authors: Lee Jong-Lam , Kim Yi-Tae , Lee Jeong Yong
Published: July 2002

Microstructural evidence on direct contact of Au/Ge/Ni/Au ohmic metals to a InGaAs channel in AlGaAs/InGaAs pseudomorphic high electron mobility transistor with an undoped GaAs/AlGaAs cap layer was found using transmission electron microscopy, and the results were used to interpret the electrical properties of the contact. The lowest contact resistivity of 3.8×10−6 Ω cm2, obtained at 420 °C annealing, is due to the penetration of the interfacial compounds, Au2Ga and Au2Al, into the buried InGaAs channel. The direct contact of the compounds to the channel causes the reduction of series resistances between the ohmic compounds and the channel, resulting in the low contact resistivity.
Influence of tellurium doping on step bunching of GaAs(001) vicinal surfaces grown by organometallic vapor phase epitaxy
Authors: Lee S. H. , Stringfellow G. B.
Published: July 2002

Atomic force microscopy has been used to investigate the influence of controlled tellurium (Te) incorporation on the step structure of GaAs grown by organometallic vapor phase epitaxy on vicinal (001) surfaces. Te doping, using the precursor diethyltelluride, is found to markedly decrease the surface roughness. Step bunching, observed for undoped layers, is totally eliminated. Only monolayer steps are formed for Te concentrations of >4×1017 cm−3. A model is proposed to account for these effects and the results reported previously for GaInP.
Parametric amplification of electromagnetic waves in low-dimensional electron systems
Authors: Mikhailov S. A.
Published: July 2002

We propose a parametric amplifier (oscillator) of electromagnetic radiation, based on the time-dependent modulation of the confining potential in low-dimensional electron systems like arrays of quantum dots and wires (or classical disks and strips), and calculate their operating characteristics. The operating frequency of the devices can be tuned by external (for instance, magnetic) fields.
Optically induced polarization rotations in CdTe/CdMnTe multiple quantum wells
Authors: Haddad M. , Leisching P. , Frey R. , Flytzanis C. , Cibert J.
Published: July 2002

We report large polarization state rotations of a weak pulsed beam in CdTe/CdMnTe quantum wells photoinduced by intense circularly polarized pulsed pump beam. Time-resolved analysis of the effect shows that, at moderate pump fluences, the polarization state rotation is due to the combined effect of many-body interactions and spectral hole burning at early times and to photoinduced magnetization at longer ones. At higher pump fluences, space filling and degenerate four-wave mixing become important. At low pump fluences (few μJ/cm2) already the polarization rotation photoinduced at early times can be as large as a few degrees making these multiple quantum wells potential candidates for applications.
Growth of CaF2 on Si(111): Imaging of the CaF interface by friction force microscopy
Authors: Klust Andreas , Pietsch Holger , Wollschläger Joachim
Published: July 2002

The initial growth state of CaF2/Si(111) has been investigated in the high-temperature regime (700 °C). At these growth temperatures the interface between CaF2 and Si consists of CaF with the Ca atoms bound to the Si. Using friction force microscopy it is possible to distinguish between the interfacial CaF layer and the overgrowing CaF2 with high lateral resolution: the CaF layer has a higher friction coefficient than the bulklike CaF2. This material contrast has been used to investigate the CaF2 nucleation on the interfacial CaF layer.
A laser pumped ultranarrow bandwidth optical filter
Authors: Hu Zhilin , Zeng Xizhi
Published: July 2002

An atomic optical filter with ultranarrow bandwidth has been developed. This filter has two operating channels generated from two hyperfine energy levels in the Cs ground state with a 9.2 GHz frequency separation. By employing a cw Ti:sapphire laser to alternatively pump the polarized Cs atoms to a selected hyperfine energy level, each of the two channels can be, respectively, used as emitting and receiving terminals. Compared to a normal Faraday anomalous dispersion optical filter, there is an increase in transmittance and a decrease in total equivalent bandwidth in this filter.
Plasma heating in highly excited GaN/AlGaN multiple quantum wells
Authors: Zeng K. C. , Mair R. , Lin J. Y. , Jiang H. X. , Chow W. W. , Botchkarev A. , Morkoç H.
Published: July 2002

Time-resolved photoluminescence (PL) spectroscopy was used to investigate carrier distributions in a GaN/AlGaN multiple quantum well (MQW) sample under high excitation intensities necessary to achieve lasing threshold. Room temperature PL spectra showed optical transitions involving both confined and unconfined states in the quantum well structure. Analysis of the experimental results using a microscopic theory, indicates that at high excitation the carrier distributions are characterized by plasma temperatures which are significantly higher than the lattice temperature. The implications of our findings on GaN MQW laser design are also discussed.
Observation of a leaky wave guide resonance mode in polycrystalline silicon structures using infrared spectroscopic ellipsometry
Authors: Ferrieu F.
Published: July 2002

Optical constants of polycrystalline silicon, used as the gate electrode material in semiconductor technology, have been analyzed by infrared spectroscopic ellipsometry. In the optical refractive indices n, k of the polycrystalline silicon material, we observed an unexpected absorption resonance at 1280 cm−1. The same effect is seen with different layer thicknesses and different polycrystalline texture. The absorption peak is not seen with an additional capping silicon oxide layer on top of the stack. This effect does not arise from an intrinsic absorption of the polysilicon but rather because of a longitudinal mode excitation present in the underlying silicon oxide layer. In the oxide infrared reststrahlen band, the incident electromagnetic radiation is totally reflected at the polysilicon/oxide interface. Further evidence for the mechanism is obtained from observation of a polysilicon/nitride/silicon sample.
A displacement amplifier using mechanical demodulation
Authors: Carotenuto Riccardo , Iula Antonio , Pappalardo Massimo , Lamberti Nicola
Published: July 2002

Piezoelectric ceramics can provide electromechanical transduction with high stresses but low displacements. To obtain larger displacements, several mechanical amplifying structures have been used. High alternating displacements can be obtained using resonant structures. We propose a displacement amplifier based on a mechanical rectifier, able to take advantage of the high displacements of resonant structures, and capable of obtaining a continuous displacement. A prototype composed of a Langevin resonator coupled to a mass–spring–damper system, acting as the rectifier, was constructed. Interferometric measurements on the proposed mechanical demodulator were carried out, validating the principle of operation. The possible applications are in the field of fast valves and micrometric positioning.
Stress and relief of misfit strain of Ge/Si(111)
Authors: Walz J. , Greuer A. , Wedler G. , Hesjedal T. , Chilla E. , Koch R.
Published: July 2002

The intrinsic stress and morphology of the Stranski–Krastanow system Ge/Si(111) have been investigated at deposition temperatures of 700–950 K. In a broad range of intermediate temperatures, only one distinct decline of stress is observed at the onset of three-dimensional islanding. Supported by a recent transmission electron microscopy study, the results demonstrate that the strain of Ge/Si(111), where the substrate surface in contrast to Ge/Si(001) is the glide plane for dislocations, is relieved by incorporation and continuous rearrangement of dislocations during the island stage.
Effect of a local electric field on photogeneration efficiency in a photorefractive polymer
Authors: Wang Liming , Wang Qing , Yu Luping
Published: July 2002

The effect of a local electric field upon photogeneration efficiency was investigated through analyses of the dependence of photocurrent, photorefractive gain, diffraction efficiency, and birefringence on the applied field of a novel photorefractive polymer containing an ionic tri(bispyridyl) ruthenium complex. It was found that since this polymer system possesses a low glass transition temperature, the dipole moments formed between the counter ions are readily aligned and generate ionic dipole field which screen photogeneration sites from the applied field. This local field lowers the photogeneration efficiency and results in the saturation of photocurrent and photorefractive gain at high applied field. This local field’s effect on an ionic polymer is further confirmed experimentally by comparing the photoconduction of a similar nonionic polymer.
Stability of above threshold ionization spectrum during intense-field ionization of H2+ near Rc
Authors: Hu S. X. , Qu W. X. , Xu Z. Z.
Published: July 2002

With the collinear model for H2+, we have numerically investigated the photoelectron spectrum of hydrogen molecular ions exposed to an ultrashort intense laser pulse. Above-threshold ionization (ATI) peaks and their temporal evolution are carefully analyzed. We find that once the photon-energy-spaced photoelectron peaks appear, their positions will not shift during the interaction. The stability of ATI spectrum is attributed to the fact that the Stark-shifted ionization potential of H2+ exhibits a minimum plateau during the internuclear distance Rc=3.6–6 bohr in which the stretching H2+ is significantly ionized.
Suppression of surface cracks on (111) homoepitaxial diamond through impurity limitation by oxygen addition
Authors: Sakaguchi Isao , Nishitani-Gamo Mikka , Ping Loh Kian , Hishita Shunichi , Haneda Hajime , Ando Toshihiro
Published: July 2002

The use of oxygen in improving diamond quality has been investigated by comparing two (111) homoepitaxial diamond films deposited with H2–CH4 and H2–CH4–O2 mixtures by microwave assisted chemical vapor deposition. The (111) diamond deposited using a H2–CH4 mixture showed surface cracks due to the presence of nondiamond phases as well as a significant amount of hydrogen and silicon impurities. The (111) diamond deposited using a H2–CH4–O2 mixture showed an absence of hydrogen and silicon impurities and nondiamond phases, and exhibited a flat surface. The addition of oxygen is one of the suitable methods to produce high-quality (111) homoepitaxial diamond.
Optical characterization of porous silicon embedded with CdSe nanoparticles
Authors: Belogorokhov A. I. , Belogorokhova L. I. , Pérez-Rodrı́guez A. , Morante J. R. , Gavrilov S.
Published: July 2002

Arrays of a few nanometer-size clusters have been realized using a porous silicon (PS) matrix by filling its pores with CdSe. The photoluminescence (PL) peak from the embedded area of the PS samples with different luminescence stabilizes at 1.79 eV. This has been interpreted as due to emission from the CdSe clusters with an average size of about 3–5 nm. Likewise, the PL and Raman scattering spectra of the pure PS area of the samples have been compared with those obtained from the embedded areas. PL spectra were examined as a function of laser irradiation. Finally, to analyze the possibility of the formation of metal/porous semiconductor contacts, cross-section structures have been observed by scanning electron microscopy in the electron-beam-induced current mode.
Mapping the influence of stress on the surface elasticity with an atomic force microscope
Authors: Finot Eric , Lesniewska Eric , Goudonnet Jean-Pierre , Mutin Jean-Claude
Published: July 2002

This letter reports an original experimental observation of the lateral stress effects on pure surface. The surface elasticity has been separated from topographical informations at the microscopic and atomic levels. The stress applied to the sample leads to modify the stiffness of the surface and affect the force modulation images. Measurements also revealed that the spring constant and the quality factor of cantilevers play an important role in the contrast mechanism of the elasticity images.
Imaging of layer by layer growth processes during molecular beam epitaxy of GaAs on (111)A substrates by scanning electron microscopy
Authors: Yamaguchi Hiroshi , Homma Yoshikazu
Published: July 2002

When we use a scanning electron microscope-molecular beam epitaxy system with a GaAs (111)A substrate that dramatically improves the flatness of the growing surface, it makes possible detailed observations of the near-equilibrium growth processes of island nucleation, coalescence, and step motion. These observations allow the quantitative analysis of the growth processes based on the standard model of crystal growth. As an example, the Ga adatom surface diffusion length is directly determined from the dependence of measured step velocity on the Ga arrival rate.
Measurement of the critical thickness of ZnCdSe quantum wells in ZnSe barrier layers by the piezoelectric effect
Authors: Milnes J. S. , Morhain C. , Telfer S. A. , Urbaszek B. , Galbraith I. , Prior K. A. , Cavenett B. C.
Published: July 2002

The occurrence of the piezoelectric effect in a strained zincblende semiconductor layer grown on an (h11) plane results in an internal electric field which can be observed through the optical properties. We report a study of Zn0.85Cd0.15Se/ZnSe quantum wells grown on (211)B GaAs substrates where the quantum confined Stark effect due to the internal field shifts the luminescence to longer wavelengths provided that the quantum well layer is strained. When the well width is greater than the critical thickness the layer begins to relax and the internal field decreases. We have used these measurements to determine that the critical thickness for the onset of strain relaxation of the Zn0.85Cd0.15Se quantum wells grown on (211) oriented substrates is 20 nm. The method will be applicable to materials such as the nitride semiconductors with wurtzite symmetry.
Wavelength tuning of 1/2-rational harmonically mode-locked pulses in a cavity-dispersive fiber laser
Authors: Zhao Y. , Shu C. , Chen J. H. , Choa F. S.
Published: July 2002

We investigate electrical wavelength tuning of a cavity length dispersive fiber ring laser operated in the 1/2-rational harmonic mode-locking condition. By changing the modulator frequency around the optimum mode-locking frequency, a tuning range of 10.45 nm with a tuning rate of 0.067 nm/kHz is achieved. The repetition rate of the mode-locked pulses is twice the modulation frequency (∼1 GHz) and the extinction ratio of the laser spectrum during the full tuning range is above 30 dB. Compared with the conventionally harmonic mode locking, the tuning range is about half reduced but the tuning rate has a same value. The variation of the modulation frequency and the length of the dispersion compensation fiber has a same effect on the wavelength tuning as in the case of conventionally harmonic mode-locking.
Low-threshold amplified spontaneous emission in blends of conjugated polymers
Authors: Gupta R. , Stevenson M. , Dogariu A. , McGehee M. D. , Park J. Y. , Srdanov V. , Heeger A. J. , Wang H.
Published: July 2002

Low thresholds (∼500 W/cm2) for amplified spontaneous emission (ASE) are reported in films of soluble poly(paraphenylene vinylene)-based conjugated polymer blends. Efficient Förster energy transfer from the absorbing host polymer to the emitting guest polymer is observed. Emission in the blends originates predominantly from the guest polymer. The large spectral shift between the absorption and emission wavelengths lowers the self-absorption losses and results in low ASE thresholds. Initial results show an enhancement in photoluminescence quantum efficiency of the blends.
Anisotropic low-field magnetoresistance of polycrystalline manganite sensors
Authors: Xu Yunhui , Dworak V. , Drechsler A. , Hartmann U.
Published: July 2002

Magnetic-field sensors of bulk La0.67Sr0.33MnO3 and La0.67Ba0.33MnO3 were fabricated. The investigations show that a large low-field magnetoresistance (MR) is exhibited by the polycrystalline samples. MR ratios of the sensors as large as 20% at 77 K and 1.5% at 298 K were observed in fields of 700 Oe. Corresponding field sensitivities as high as 170%/T at 3 mT and 298 K, and 700%–960%/T at 3–8 mT and 77 K were obtained. The low-field MR is associated with intergranular transport of spin-polarized electrons. It is found to be highly anisotropic. The phenomenon is discussed in terms of spin-polarized transport through two kinds of grain boundaries. These represent two extremes of grain-boundary environments.
Laser-induced selective area band-gap tuning in Si/Si1−xGex microstructures
Authors: Dubowski J. J. , Rowell N. , Aers G. C. , Lafontaine H. , Houghton D. C.
Published: July 2002

A one-step process is reported for selective area band-gap tuning of as-grown quantum well (QW) material consisting of Si/Si1−xGex microstructures. The process takes advantage of the ability of increasing the local temperature of the wafer, in excess of 900 °C, by applying the beam of a high-power cw Nd:YAG laser, which leads to controlled intermixing between the quantum well and barrier material. A microstructure with the band-gap blueshifted by 142 meV has been fabricated from as-grown 980 meV band-gap material. The results indicate that this approach has the potential for “writing” of Si/Si1−xGex QW microstructures with the selectively tuned band gap required in the fabrication of optoelectronic integrated circuits.
High voltage sheath behavior in a drifting plasma
Authors: Brown I. G. , Monteiro O. R. , Bilek M. M. M.
Published: July 2002

We describe a simple experiment in which a high voltage substrate is immersed in a streaming, vacuum-arc-produced, titanium plasma. We show that high substrate voltages that are orders of magnitude greater than the electron temperature can be sustained, effectively on a direct current basis, by a well-rounded substrate (no sharp points or edges) immersed in the plasma stream, for the case of a plasma with density not too low (greater than about 109 cm−3) and with drift velocity that is equal to or greater than the sheath propagation speed, typically of order the ion acoustic speed. This result is significant not only from a basic plasma physics perspective, but also for the technology of carrying out plasma immersion surface processing.
Waveguide luminescence and Raman spectroscopy: Characterization of an inhomogeneous film at different depths
Authors: Ferrari M. , Montagna M. , Ronchin S. , Rossi F. , Righini G. C.
Published: July 2002

In graded-index planar optical waveguides, different guided modes propagate in layers with different thickness. By comparison of spectra (Raman, luminescence) taken by waveguide excitation in different modes, it is possible to characterize the guide at different depths. The method has been applied to waveguides obtained by ion exchange of silver in a soda-lime substrate. Luminescence from silver ions and Raman scattering from the optical vibrations of the glass and from the acoustic vibrations of silver nanoclusters depend on the silver concentration, providing different spectra for excitation in different modes of the guide.
Third-order nonlinear optical properties of didodecyldimethylammonium–Au(dmit)2
Authors: Wang Shufeng , Huang Wentao , Zhang Tieqiao , Yang Hong , Gong Qihuang , Okuma Yoshihito , Horikiri Masashi , Miura Yasuhiro F.
Published: July 2002

The third-order optical nonlinearity of metal(dmit)2 was investigated with femtosecond optical Kerr effect at 830 nm wavelength. The nonresonant second-order hyperpolarizability for the didodecyldimethylammonium–Au(dmit)2 (2C12–Au) molecule was estimated to be as large as 2.2×10−32 esu.
Generation of first-order terahertz optical sidebands in asymmetric coupled quantum wells
Authors: Phillips C. , Su M. Y. , Sherwin M. S. , Ko J. , Coldren L.
Published: July 2002

We have generated first-order tetrahertz (THz) optical sidebands on a near-infrared (NIR) probe beam by driving an excitonic intersubband resonance with THz electric fields. We use THz radiation polarized along the noncentrosymmetric axis of a quantum well system to generate a comb of sidebands ωsideband=ωNIR+nωTHz. The n=1 process offers an efficient means of modulating a NIR carrier beam at THz frequencies and yields new spectroscopic information on excitonic intersubband transitions.
Piezoelectric doping in AlInGaN/GaN heterostructures
Authors: Khan M. Asif , Yang J. W. , Simin G. , Gaska R. , Shur M. S. , Bykhovski A. D.
Published: July 2002

We report on the piezoelectric doping and two-dimensional (2D) electron mobility in AlInGaN/GaN heterostructures grown on 6H–SiC substrates. The contribution of piezoelectric doping to the sheet electron density was determined using an In-controlled built-in strain-modulation technique. Our results demonstrate that in strained AlGaN/GaN heterostructures, the piezoelectric field generates at least 50% of the 2D electrons. The strain modulation changes the potential distribution at the heterointerface, which, in turn, strongly affects the 2D electron mobility, especially at cryogenic temperatures. The obtained results demonstrate the potential of strain engineering and piezoelectric doping for GaN-based electronics.
Attenuation of optical transmission within the band gap of thin two-dimensional macroporous silicon photonic crystals
Authors: Leonard S. W. , van Driel H. M. , Busch K. , John S. , Birner A. , Li A.-P. , Müller F. , Gösele U. , Lehmann V.
Published: July 2002

The transmissivity within the photonic band gap of two-dimensional photonic crystals of macroporous silicon is reported as a function of crystal thickness. Measurements were carried out for crystals of nominally 1, 2, 3, and 4 crystal layers using a commercial parametric source, with a wavelength tunable from 3 to 5 μm. For wavelengths well within the 3–5 μm photonic band gap, attenuation of approximately 10 dB/crystal layer is obtained, in agreement with calculations based on plane wave expansion methods. For these materials, one should be able to achieve photonic crystal functionality in many applications with very small crystal volumes.
A reduced complexity process for organic thin film transistors
Authors: Klauk Hagen , Gundlach David J. , Bonse Mathias , Kuo Chung-Chen , Jackson Thomas N.
Published: July 2002

A simplified device structure for depletion-mode organic thin film transistors is described in which the gate electrode and the source/drain contacts are prepared in the same process step, thus reducing the number of material depositions and photolithography steps. Based on the simplified device structure and using the small-molecule aromatic hydrocarbon pentacene as the active material, organic thin film transistors were fabricated on glass substrates with carrier mobility of 0.6 cm2/V s, on/off current ratio of 105, and subthreshold slope of 0.5 V/decade.
On the crystallographic characteristics of ion-beam-synthesized Ru2Si3 precipitates
Authors: Shao G. , Ledain S. , Chen Y. L. , Sharpe J. S. , Gwilliam R. M. , Homewood K. P. , Kirkby K. Reeson , Goringe M. J.
Published: July 2002

Semiconducting Ru2Si3 precipitates have been fabricated by ion beam synthesis and a crystallographic study has been carried out using transmission electron microscopy. The results show that the orientation relationship between Ru2Si3 precipitates and Si is: 〈110〉Si∥〈111〉Ru2Si3 and {11̄1}Si∥{1̄10}Ru2Si3, which consists of 16 independent orientation variants.
Nanometer-scale electrochemical lithography on the spinel oxide LiMn2O4
Authors: Kostecki Robert , McLarnon Frank
Published: July 2002

A scheme for nanometer-scale patterning of the surface of a conducting lithium manganese oxide (LiMn2O4) by scanning probe microscopy is described. A significant feature of this work is the demonstration that a localized surface chemical change can be confined to a depth which depends on the oxide-tip voltage bias and ambient humidity. Processing strategies exploiting this localized pattern generation vary from shallow surface property modification to localized etching of the oxide and the formation of nanometer-size pits, wells, or trenches. Unlike the scanning probe based surface oxidation of metals and semiconductors, the electroactive material is altered via electrochemically generated species.
Refractive indices of InGaAsP lattice-matched to GaAs at wavelengths relevant to device design
Authors: Wallace S. G. , Robinson B. J. , Mascher P. , Haugen H. K. , Thompson D. A. , Dalacu D. , Martinu L.
Published: July 2002

The refractive indices of In1−xGaxAsyP1−y grown lattice-matched to GaAs by gas-source molecular-beam epitaxy, have been measured by variable angle spectroscopic ellipsometry. Indices in the transparent regime of these quaternaries, at 980 and 808 nm (relevant to the design of pump sources for erbium-doped fiber amplifiers and Nd:YAG lasers, respectively) and at 850 nm, are presented.
Magnetic-field effects on undriven chaos in a weakly coupled GaAs/AlAs superlattice
Authors: Luo K. J. , Friedland K.-J. , Grahn H. T. , Ploog K. H.
Published: July 2002

We have investigated the effect of a perpendicular magnetic field on the spontaneous current oscillations in a weakly coupled GaAs/AlAs superlattice. At zero magnetic field, voltage regions of periodic as well as chaotic oscillations are observed. With increasing magnetic field B, the voltage regions for chaotic oscillations become larger and those for periodic oscillations smaller. At B=9 T, the whole voltage range consists of chaotic oscillations. At the same time, the time-averaged current–voltage characteristic hardly changes. The experimental observations demonstrate that the quantization by the perpendicular magnetic field results in an equalization of the sequential resonant tunneling process over the whole superlattice, suppressing scattering effects due to disorder.
Vertical motional narrowing of exciton polaritons in GaN based multiple quantum wells
Authors: Malpuech Guillaume , Kavokin Alexey
Published: July 2002

Numerical simulation of time-resolved light reflection from GaN/AlGaN single and multiquantum wells (MQWs) revealed a pronounced vertical motional narrowing effect. This effect consists in reduction of the inhomogeneous broadening of the exciton peaks and increase of the decay time of the time-resolved reflection due to the averaging of the vertical disorder in the structure by extended exciton-polariton modes. The theory predicts pronounced oscillations in the time-resolved reflection of MQWs resulting from the interference of exciton-polariton modes in the structure.
Low-frequency noise and mobility fluctuations in AlGaN/GaN heterostructure field-effect transistors
Authors: Garrido J. A. , Foutz B. E. , Smart J. A. , Shealy J. R. , Murphy M. J. , Schaff W. J. , Eastman L. F. , Muñoz E.
Published: July 2002

The 1/f low-frequency noise characteristics of AlGaN/GaN heterostructure field-effect transistors, grown on sapphire and SiC substrates by molecular beam epitaxy and organometallic vapor phase epitaxy, are reported. The Hooge parameter is deduced taking into account the effect of the contact noise and the noise originating in the ungated regions. A strong dependence between the Hooge parameter and the sheet carrier density is obtained, and it is explained using a model in which mobility fluctuations are produced by dislocations. A Hooge parameter as low as αCH≈8×10−5 is determined for devices grown on SiC substrates.
Surface photovoltage analysis of phase transformation of copper in p-type silicon
Authors: Ramappa Deepak A.
Published: July 2002

Surface photovoltage minority carrier lifetime/diffusion length analysis of copper contaminated p-type silicon was performed. It was observed that an optical or low-temperature thermal activation procedure on Cu-doped silicon significantly degrades the diffusion length. Unlike iron doped p-type silicon no recovery of diffusion length was observed following such an activation procedure. It is proposed that the activation procedure dissociates interstitial copper agglomerations and forms extended substitutional defects in silicon, which have much greater recombination efficiency. The change in phase of copper and the formation of associated defects is a function of activation light intensity, annealing time, and temperature. An activation energy of 0.419 eV is obtained for the process, which is in good agreement with copper diffusivity value in silicon. It is thus concluded that the change in phase of copper and the formation of extended defects with activation is a diffusion limited process.