bluuebunny/crossref_metadata_2025 huggingface dataset:
Stability of magnesium implanted YBa2Cu3O7 thin films
Authors: Wong André , Liang Ruixing , Badaye M. , Carolan J. F. , Hardy W. N. , Hong S. H. , Ma Q. Y.
Published: July 2002

Magnesium ions were implanted into highly crystalline YBa2Cu3O7 (YBCO) thin films for the purpose of patterning. Films were implanted at doses corresponding to Mg contents of x=0.008, 0.02, and 0.04 in the formula YBa2(Cu1−xMgx)3O7. High temperature annealing (900 °C) of films implanted below the solubility limit was successful in obtaining single phase, Mg doped YBCO films with finite resistivities at 77 K and x-ray (005) rocking curve widths <0.15°. An electron probe microanalysis on a film patterned using Mg implantation revealed that lateral diffusion of Mg ions resulting from annealing was limited to only a few microns. The superior quality of a top layer film indicated that Mg ion implantation is suitable for multilayer patterning.
Effect of negative capacitances on high-temperature dielectric measurements at relatively low frequency
Authors: M’Peko Jean-Claude
Published: July 2002

Barium ferrite (BaFe12O19) was elaborated and investigated in this work in terms of its capacitance-temperature characteristics. The curve taken at the fixed frequency of 1 kHz shows anomalous negative values of capacitance over the high-temperature regime. A complete analysis of this effect is carried out by performing ac measurements at various temperatures. The results indicate that a resonance phenomenon, arising from the external electrical circuit and shifting from the higher frequencies to the lower ones with increasing temperature, is responsible for this unusual effect.
Photoluminescence properties of GaN grown on compliant silicon-on-insulator substrates
Authors: Cao J. , Pavlidis D. , Eisenbach A. , Philippe A. , Bru-Chevallier C. , Guillot G.
Published: July 2002

A compliant substrate approach has been employed to release lattice-mismatch caused strain in GaN epilayers through stress absorption in the substrate. GaN layers have been grown on silicon-on-insulator (SOI) substrates by low-pressure metalorganic chemical vapor deposition. Photoluminescence measurements at 4 K show the spectrum of grown GaN being dominated by UV emission around 3.47 eV related to neutral-donor bound excitons. The much weaker yellow luminescence shows a broad spectrum around 2.16 eV. Peak position of the UV emission changes both with measurement temperature and strain. At room temperature, the UV peak is red shifted by 64 meV corresponding well to the band-gap temperature dependence. Strain-induced blue shift of the peak, compared to unstrained GaN, is much less than for growth on sapphire, indicating strain relief in the GaN by growth on SOI. Further reduction of the blue shift is consistent with increased electron mobility.
Elastic behavior of nanoparticle chain aggregates
Authors: Friedlander Sheldon K. , Jang Hee Dong , Ryu Kevin H.
Published: July 2002

Nanoparticle chain aggregates (NCA) possess elastomeric properties including stretch under tension, and contraction when the tension is relaxed. Individual chain aggregates of 7 nm titania particles a few hundred nm long were observed in the electron microscope. The NCA were stretched across expanding holes in the carbon film on an electron micrograph grid. After stretching up to 90%, the NCA broke loose at one end and contracted to a tightly folded chain. This pattern was observed in repeated tests. Mechanisms for this behavior and reasons for its generality are proposed. Implications are discussed for the ductility of nanoparticle ceramics, and the increased tensile strength and elastic modulus of rubber due to nanoparticle additives.
180° ferroelectric domains in polycrystalline BaTiO3 thin films
Authors: Kim In-Tae , Jang Jin Wook , Youn Hyuk-Joon , Kim Chang Hoon , Hong Kug Sun
Published: July 2002

The presence of 180° ferroelectric domains in polycrystalline BaTiO3 thin films has been observed with the aid of transmission electron microscopy. The 180° domain boundary exhibited a curved appearance, and its orientational relationship was demonstrated. It was found that equiaxial grains did not have 180° domain boundaries due to their higher boundary energy. The grain size effect on dielectric constant of polycrystalline BaTiO3 thin films was also discussed in conjunction with 180° ferroelectric domains.
730-nm-emitting Al-free active-region diode lasers with compressively strained InGaAsP quantum wells
Authors: Al-Muhanna A. , Wade J. K. , Mawst L. J. , Fu R. J.
Published: July 2002

0.73-μm-emitting, Al-free active-region, strained (Δa/a≈1.4%) InGaAsP single-quantum-well diode lasers have been grown by low-pressure metal–organic chemical-vapor deposition. A broad waveguide laser design with In0.5(Ga0.5Al0.5)0.5P cladding layers is utilized to achieve a large effective transverse spot size (d/Γ=0.433 μm) and to minimize carrier leakage from the active region. Threshold current densities of 514 A/cm2 (100-μm-wide stripe, L=1 mm), external differential quantum efficiencies of 60%, and characteristic temperature coefficients for the threshold current, T0, and external differential quantum efficiency characteristic temperature, T1, have values of 72 and 153 K, respectively. Continuous wave output powers of 1.4 W are obtained from facet-coated (90%/10%) devices operating at 735 nm.
Stimulated emission from localized states in partially ordered (AlxGa1−x)0.52In0.48P
Authors: Dörr U. , Kalt H. , Mowbray D. J. , Button C. C.
Published: July 2002

We have investigated the optical properties of metalorganic vapor phase epitaxy grown (Al0.5Ga0.5)0.52In0.48P samples with different degrees of CuPtB-type ordering. With increasing excitation intensity, the photoluminescence (PL) exhibits a blueshift (moving emission) which is typical for ordered material. At high excitation intensities, this blueshift is shown to result from filling of localized tail states. Further evidence for the existence of an exponential tail of localized states is provided by the form of the PL line shape for high, quasistationary excitation and by the carrier hopping relaxation demonstrated by time-resolved PL. Applying the variable stripe-length method, we have determined the spectra of the optical gain. Stimulated emission is shown to occur from localized states.
An improved analysis for band edge optical absorption spectra in hydrogenated amorphous silicon from optical and photoconductivity measurements
Authors: Jiao L. , Chen I. , Collins R. W. , Wronski C. R. , Hata N.
Published: July 2002

The uncertainties inherent in the normalization of subgap photoconductivity spectra to the optical absorption spectra α(hv) in a-Si:H based films have been addressed. An analysis is presented which is based on optical transitions of constant dipole matrix element between parabolic distributions of extended states and exponential distributions of localized tail states. This analysis has been used to normalize the two sets of results accurately, as verified by photothermal deflection spectroscopy measurements, and is shown to be useful in the commonly encountered cases, in which the two spectra do not overlap over an extended region. Improved quantitative fits of α(hv), for photon energy from ∼1.5 to 2.4 eV, obtained on different a-Si:H based films indicate that the localized exponential band tail regions extend ∼60–70 meV above the optical gap.
Infrared photodetection at room temperature using photocapacitance in amorphous silicon structures
Authors: Caputo D. , de Cesare G. , Nascetti A. , Palma F. , Petri M.
Published: July 2002

In this letter we present a device, based on amorphous silicon material, able to detect infrared light through capacitance measurement at room temperature. The device is a p-c-n stacked structure, where c indicates a compensated amorphous silicon absorber layer. Operation is based on transitions between extended states in the valence band and defects in the forbidden gap excited by the infrared radiation. We found that the measured capacitance is sensitive to radiation from 800 nm to 5 μm.
Si/SiGe resonant-cavity photodiodes for optical storage applications
Authors: Seto M. , de Boer W. B. , Sinnis V. S. , Morrison A. P. , Hoekstra W. , de Jager S.
Published: July 2002

We report on a resonant cavity photodiode with a Si/SiGe Bragg mirror grown by low temperature chemical vapor deposition suitable for short wavelength detection around 600–700 nm. The presence of Fabry-Pérot oscillations in the spectral response of the photodiode are indicative of its wavelength selectivity.
Current–voltage characteristics of metal-insulator-semiconductor structures via quantum mechanical tunneling
Authors: Mohaidat Jihad M. , Ahmad-Bitar Riyad N.
Published: July 2002

The current–voltage (I-V) characteristics for a metal-insulator-heavily-doped semiconductor structure are computed numerically by solving the time-dependent Schrödinger equation. The Fowler–Nordheim tunneling expression was found to be inappropriate to estimate the barrier potential nor found to fit the experimental results at both high and low applied fields. It is shown also that the computed I–V characteristic curves agree well with the recently published experimental data for Ta-Sn-O and Ta2O5 films at the high as well as low fields.
Boron-doped carbon fullerenes and nanotubules formed through electron irradiation-induced solid-state phase transformation
Authors: Golberg D. , Bando Y. , Kurashima K. , Sasaki T.
Published: July 2002

Here we report on the in situ creation of single/multi-shelled fullerenes and multi-walled nanotubules by electron irradiation of chemical vapor deposit graphitic BxC1−x (x⩽0.2) in a high-resolution 300 kV electron microscope. Nanostructure formation occurred through irradiation-induced solid-state phase transformation, in contrast to conventional fullerene/nanotube synthesis from the gas phase. The fullerene/nanotubule chemical composition and bonding character were analyzed by electron energy loss spectroscopy applying a ∅0.5 nm electron probe. The B/C ratio of the observed curled, closed nanostructures has never exceeded ∼0.1. Unlike pure carbon “onions,” most of the B-doped fullerenes exhibited a polyhedral faceted circumference suggestive of B-induced buckling.
Electron transfer efficiency of Si δ-modulation-doped pseudomorphic GaAs/In0.2Ga0.8As/AlxGa1−xAs quantum wells
Authors: Li G. , Babinski A. , Chua S. J. , Jagadish C.
Published: July 2002

In Si δ-modulation-doped GaAs/In0.2Ga0.8As/AlxGa1−xAs quantum well structures (QWs), the electrons from the ionized Si donors are initially confined in the V-shaped potential well (V-PW) formed at the position of a Si δ-doped layer. The efficiency of electrons transferring from the V-PW to the QW was investigated as a function of Si δ-doping concentration in the symmetric GaAs/In0.2Ga0.8As/GaAs QW at 1.7 K. The electron density in the QW increases linearly with an increase of Si δ-doping concentration, while the electron transfer efficiency remains unchanged either in the dark or under the illumination. The asymmetric GaAs/In0.2Ga0.8As/Al0.2Ga0.8As QW has a relatively higher electron transfer efficiency. The effect of grading the Al mole fraction over the AlxGa1−xAs spacer layer on the electron transfer efficiency was also reported.
Optical characterization of disordered InxGa1−xP alloys
Authors: González Luisa , González Yolanda , Dotor Maria Luisa , Martinez-Pastor Juan
Published: July 2002

We present results on the optical characterization of InxGa1−xP layers grown by atomic layer molecular beam epitaxy on GaAs (001) substrates at a growth temperature of 420 °C. Our results show that the optical characteristics of these layers, which do not show ordering effects, are strongly dependent on surface stoichiometry during growth. In this way, we can obtain either highly homogeneous alloys with a predictable band-gap energy or layers with optical properties indicative of spatial localization effects, like an anomalous behavior of photoluminescence peak energy with temperature and a large shift between the emission energy and absorption edge.
Pyroelectric properties of (Pb1−xLax)TiO3 thin films deposited using SrRuO3 as a buffer layer
Authors: Tseng Yung-Kuan , Liu Kuo-Shung , Jiang Jian-Der , Lin I-Nan
Published: July 2002

In this study, we observed that the ferroelectric properties of (Pb1−xLax)TiO3, (PLT) thin films deposited on Pt/Ti/Si substrates using SrRuO3 as a buffer layer change markedly with the substrate temperature and the composition. All the films are perovskite with no secondary phases when deposited at 500–600 °C. However, only the films deposited below 520 °C possess a satisfactory small leakage current density, for example, JL⩽10−7 A/cm2, under a 50 kV/cm applied field. Both PLT10 (x=0.10) and PLT5 (x=0.05) thin films thus obtained possess large pyroelectric coefficient (p=0.009–0.018 °C−1). However, the PLT10 thin films show pyroelectric properties markedly superior to the PLT5 thin films, although the PLT5 thin films own much better ferroelectric properties. This phenomenon is explained by the lower Curie temperature (Tc) of the PLT10 materials.
Calculated and measured transmittance of metallodielectric photonic crystals incorporating flat metal elements
Authors: Kao A. , McIntosh K. A. , McMahon O. B. , Atkins R. , Verghese S.
Published: July 2002

A three-dimensional metallodielectric photonic crystal (MDPC) that utilizes planar metal scattering elements in a dielectric medium has been studied in the microwave regime, both experimentally and theoretically. The metal elements are circular copper patches defined on thin dielectric sheets, which are alternately stacked with thicker polyethylene sheets to form a (111)-oriented face-centered-cubic lattice. A photonic stop band has been measured from this “flat-atom” MDPC at 8.4 GHz with a rejection level of 18 dB per lattice period and a width of 50% of the center frequency. Calculations show excellent agreement with measured stop band characteristics, including a similar angular dependence and insensitivity to interplane registration.
Carrier relaxation in an InP/InGaAs nonlinear Bragg reflector
Authors: Ogawa Kensuke , Matsui Yasuhiro , Itatani Taro , Ouchi Kiyoshi
Published: July 2002

Carrier dynamics in absorption saturation in an InP/InGaAs nonlinear Bragg reflector is studied by transient reflectance measurements. An ultrafast process of the relaxation time shorter than 100 fs is observed and identified as carrier-carrier scattering. Such an ultrafast process is not observed in the transient reflectance of bulk InGaAs measured with the same photoexcitation level. The difference between these transient reflectance characteristics indicates an enhancement of carrier-carrier scattering in the nonlinear Bragg reflector.
Effect of the silicon top layer of silicon implanted with oxygen on the uptake and release of deuterium by the buried oxide
Authors: Zimmermann L. , de Nijs J. M. M. , Alkemade P. F. A. , Westerduin K. , van Veen A.
Published: July 2002

The effect of the silicon top layer on the uptake and release of deuterium by silicon implanted with oxygen (SIMOX) was studied using thermal desorption measurements. The deuterium is incorporated in the buried oxide by disruption of the Si–O bridging bonds. The data reveal that the top layer reduces the uptake at 1073 K. Furthermore, it retards release; a moderate (≈1125 K) and a high-temperature (≈1400 K) retention were observed. It is proposed that release is accompanied by the reconstruction of the Si–O bonds and that the bare oxide surface constitutes an abundant source for defects thus enhancing the generation and elimination of Si–O bridging bond defects.
Microwave electric-field imaging using a high-Tc scanning superconducting quantum interference device
Authors: Chatraphorn S. , Fleet E. F. , Black R. C. , Wellstood F. C.
Published: July 2002

We have used a 77 K thin-film YBa2Cu3O7 superconducting quantum interference device (SQUID) in a scanning SQUID microscope to image room-temperature sources of high-frequency electric field. We find that time-varying electric fields capacitively induce currents in the SQUID, which in turn are rectified by the nonlinearity of the SQUID current–voltage characteristics, leading to changes in the quasistatic voltage across the SQUID. By observing changes in the voltage modulation depth ΔV of the SQUID as a sample is scanned past the SQUID, we obtain electric-field images in the 1–15 GHz frequency range with a SQUID-to-sample separation of about 80 μm.
Monte Carlo simulation of the induced signal in semi-insulating GaAs detectors
Authors: Cola Adriano , Reggiani Lino , Vasanelli Lorenzo
Published: July 2002

We develop a Monte Carlo simulator of the charge signal induced by an external radiation on semi-insulating GaAs detectors. Trapping and detrapping processes and the dynamics of generated carriers are consistently accounted for. The relative contributions to the charge signal of fast- and slow-time components are found to depend significantly upon the applied voltage. Calculations provide a physical interpretation of experimental results.
Surface morphological structures in ultra-high-dose self-implanted silicon
Authors: Zhu Xianfang , Williams J. S. , McCallum J. C.
Published: July 2002

In this letter, we report the observation of intriguing, implantation-induced surface morphological structures in Si. Experimental evidence indicates that an extremely rough surface can occur when ultra-high-dose (∼1018 cm−2) self-implantation is performed on a partially preamorphized Si wafer within a temperature range from 175 to 250 °C. However, the same implantation into crystalline or completely amorphized Si does not produce such surface structures. Characteristics of the structures and their temperature dependence suggest a possible mechanism for their evolution.
Ultrafast optical pulse noise suppression using a nonlinear spectral filter: 23 dB reduction of fiber laser 1/f noise
Authors: Friberg Stephen R. , Machida Susumu
Published: July 2002

Nonlinear spectral filtering provides a simple method for reducing noise in optical pulses to below the shot noise level. We show its effectiveness for excess noise reduction in an ultrafast optical pulse train by demonstrating 23 dB reduction of low frequency 1/f noise in pulses from a passively mode-locked erbium-doped fiber laser. The noise reduction is achieved by spectrally filtering pulses propagated as solitons through a 1.5 km length of optical fiber.
Atomic transport across the interfaces during the formation of ultrathin silicon oxide/nitride/oxide films
Authors: Baumvol I. J. R. , Salgado T. D. M. , Radtke C. , Krug C. , de Andrade J.
Published: July 2002

The redistribution of O and N during the final, thermal oxidation in dry O2 step in the formation of ultrathin silicon oxide/nitride/oxide dielectric films (ONO) was investigated using isotopic tracing and depth profiling with nanometer resolution. The results show that the final oxidation step induces atomic transport of O and N species in the system, such that the formed ONO structures are not stacked layer structures, but rather a silicon oxynitride ultrathin film, having moderate concentrations of N in the near-surface and near-interface regions, and a high N concentration in the bulk.
Mesoscopic structure of DNA–membrane self-assemblies: Microdiffraction and manipulation on lithographic substrates
Authors: Wong Gerard C. L. , Li Youli , Koltover Ilya , Safinya Cyrus R. , Cai Zhonghou , Yun Wenbing
Published: July 2002

Using microdiffraction techniques at the Advanced Photon Source, we have found evidence for local molecular alignment within the mesoscopic fibers of DNA–cationic membrane complexes, a system originally conceived as gene delivery vectors. Furthermore, these mesoscopic structures can be manipulated by using lithographically patterned microchannel arrays, so that the optical axes of individual fibers are aligned parallel to the channels. These observations suggest that DNA–membrane complexes may have additional technological applications, such as active electrophoretic media and templates for the fabrication of semiconductor superlattices and nanoporous materials with tunable pore sizes.
Temperature dependence of noise in a GaAs metal-semiconductor field effect transistor at microwave frequencies
Authors: Fendrich J. A. , Feng M.
Published: July 2002

Measurements of the noise parameters of a GaAs metal-semiconductor field effect transistor (MESFET) show the minimum noise figure decreases exponentially into the noise floor with decreasing temperature for most frequencies from 2 to 18 GHz and for most currents. The MESFET noise has a thermal activation energy of ∼55 meV which is almost half the value seen in a pseudomorphic high electron mobility field effect transistor (pHEMT). Unlike in the pHEMT, the high-temperature coefficient to the activated behavior for the MESFET is independent of current and frequency.
Precipitation of As in thermally oxidized ion-implanted Si crystals
Authors: Terrasi A. , Rimini E. , Raineri V. , Iacona F. , La Via F. , Colonna S. , Mobilio S.
Published: July 2002

The As precipitation occurring during thermal oxidation of As implanted Si crystals has been studied by extended x-ray absorption fine structure measurements. (100) Si wafers, implanted with 3×1015/cm2 and 3×1016/cm2 As+ ions at an energy of 70 keV, were oxidized either in H2O ambient (wet) at 920 °C or O2 (dry) at 1100 °C. Precipitation of monoclinic SiAs occurs at the SiO2/Si interface for low temperature oxidation processes. In the case of 3×1016 As/cm2, about 90% of the As forms SiAs precipitates, while for the lower dose a mixing of precipitates and As in substitutional-like sites is observed. On the other hand, when the high temperature oxidation is performed, most of the As (up to 90% for the 3×1015 As/cm2 sample) is found in a substitutional-like configuration.
An equation for thermionic currents in vacuum energy conversion diodes
Authors: Marshall Albert C.
Published: July 2002

Apparent thermionic emission constants A* are commonly reported with values deviating from the theoretical value of 120 A/cm2 K2. For thermionic energy conversion diodes, using A* in the conventional equation for predicting net currents is found to violate basic physics laws and may result in poor predictive accuracy. A general equation for vacuum energy conversion diodes is proposed to prevent misuse of A*.
High-Q whispering gallery modes from a composite system of GaAs quantum well and fused silica microsphere
Authors: Fan Xudong , Doran Andrew , Wang Hailin
Published: July 2002

We demonstrate a composite system consisting of a GaAs quantum well structure placed in the evanescent field of a fused silica microsphere and show evanescent coupling between excitons in the quantum well and high-Q whispering gallery modes of the composite system. The composite system allows separate engineering of photonic and electronic confinement and can take advantage of high-Q factor and small mode volume of fused silica microspheres.
Two-dimensional rough surface couplers for broadband quantum-well infrared photodetectors
Authors: Jandhyala Vikram , Sengupta Deepak , Michielssen Eric , Shanker Balasubramaniam , Stillman Greg
Published: July 2002

Rough surfaces are proposed as a class of couplers for achieving high absorption in broadband quantum-well infrared photodetectors (QWIPs). The performance of these structures is predicted numerically using the steepest-descent fast-multipole method, a recently developed fast electromagnetic analysis technique. This method permits the efficient and accurate solution of the electric-field integral equation governing scattering from a quasiplanar structure. The scattered optical electric-field component along the growth direction at the device layer can be used to predict the QWIP absorption. It is demonstrated that rough-surface couplers exhibit very high absorption sustained over large spectral ranges, in contrast to quasirandom or periodic gratings that are characterized by tradeoffs between peak absorption and spectral range. It is expected that the excellent spectral absorption characteristics of rough-surface couplers will make them particularly suitable for broadband QWIP applications.
Strain relaxation in InAs/GaSb heterostructures
Authors: Bennett Brian R.
Published: July 2002

Lattice strain relaxation in InAs/GaSb heterostructures was investigated by x-ray diffraction. Two types of structures, grown by molecular beam epitaxy, are compared. In the first, GaSb buffer layers were grown on GaAs substrates, followed by 0.05–1.0 μm thick InAs layers. In the second, InAs layers were grown directly on GaSb substrates. For a given thickness, the InAs layers retain significantly more strain when grown on GaSb substrates, reflecting the lower threading dislocation density in the GaSb substrates relative to the GaSb buffer layers grown on GaAs.
Nitrogen radical adsorption on InAs (001) surface studied by scanning tunneling microscopy and x-ray photoelectronic spectroscopy
Authors: Kasu M. , Kobayashi N. , Tanaka H. , Mikami O.
Published: July 2002

The adsorption of nitrogen (N) radicals (nitridation) on InAs (001) surfaces has been studied by ultrahigh-vacuum scanning tunneling microscopy and x-ray photoelectronic spectroscopy. The nitridation proceeds as Langmuir adsorption. The N adsorption at 350 °C is faster than that at 100 °C, but N adsorption rates at 100 °C on InAs and GaAs are almost the same. These results are explained as follows: at 350 °C, N radicals bond mainly with the topmost In atoms on an In-stabilized surface, and at 100 °C, N radicals replace As atoms in the topmost layer on an As-stabilized surface, and subsequently bond with In atoms in the second layer. The amorphous 100 °C nitrided surface layer is found to have an insulating characteristic without surface states.
Magneto-optics of InAs/Ga1−xInxSb infrared superlattice diodes
Authors: Fuchs F. , Ahlswede E. , Weimar U. , Pletschen W. , Schmitz J. , Hartung M. , Jager B. , Szmulowicz F.
Published: July 2002

Spectrally resolved measurements of the responsivity of infrared photodiodes based on InAs/(GaIn)Sb superlattices (SL) were performed in applied magnetic fields. For the field oriented parallel to the growth axis, interband Landau transitions related to both the center and the edge of the SL Brillouin zone in the growth direction were observed, in accordance with the parity selection rules that are expected for the type II system under consideration. For the field oriented perpendicular to the growth axis, the interband Landau resonances are broadened and the widening of the band gap is smaller.
Neutral higher silane molecules in silane plasmas
Authors: Suzuki Atsushi
Published: July 2002

Neutral higher silane molecules in silane radio frequency glow discharge plasmas are investigated using photoionization mass spectroscopy at various radio frequency (rf) powers and total pressures. Densities of higher silane molecules increase with rf power up to 10 W, and then decrease at 10–100 W. The reduction of the neutral higher silane molecules at high rf power suggests that these molecules contribute to the particle formation through secondary reactions in the plasma.
Growth of nonlinear optical thin films of KTa1−xNbxO3 on GaAs by pulsed laser deposition for integrated optics
Authors: Knauss L. A. , Harshavardhan K. S. , Christen H.-M. , Zhang H. Y. , He X. H. , Shih Y. H. , Grabowski K. S. , Knies D. L.
Published: July 2002

We report successful deposition of epitaxial nonlinear KTa0.52Nb0.48O3 (KTN) films on (100) GaAs substrates. A buffer layer scheme consisting of epitaxial MgO and SrTiO3 buffer layers and a Si3Ni4 encapsulation of the substrate was developed to alleviate chemical and structural incompatibilities between the GaAs substrate and KTN film at the growth temperature (∼750 °C). The structure, composition, and preliminary optical properties of the KTN films were evaluated by four-circle x-ray diffraction, Rutherford backscattering spectrometry, and prism coupled optical waveguide mode measurements, respectively. We observed sharp and distinguishable transverse electric and transverse magnetic propagating modes in the KTN films, and measured the refractive index (n0) of the film at 488 nm to be 2.275 which is close to the bulk value of 2.35, all of which indicates a high structural and optical film quality.
Direct current voltage increment due to ac coupling in a high Tc superconducting coil
Authors: Shaked N. , Al-Omari I. A. , Friedman A. , Wolfus Y. , Sinvani M. , Shaulov A. , Yeshurun Y.
Published: July 2002

The voltage–current characteristics of a superconducting coil, made of multifilamentary silver-sheathed Bi2Sr2Ca2Cu3O10+δ tape, are investigated. We find that a small ac current Iac superimposed on a relatively large dc current Idc causes a significant increase in the coil dc voltage, approximately proportional to Idc, Iac2, and the ac frequency. We attribute this effect to the nonlinear magnetoresistance of the coil, and discuss its significance in power applications of high Tc superconducting coils, such as fault current limiters and superconducting magnetic energy storage devices.
Flux flow steps in a triangular cell of long Josephson junctions
Authors: Carapella G. , Costabile G. , Sabatino P.
Published: July 2002

Flux flow steps are experimentally found in a triangular cell of long Josephson junctions that we successfully fabricated. Their behavior at the relevant values of the magnetic flux linked to the cell (Φ=0 and Φ=Φ0/2) is compared with the one predicted by numerical simulations of the coupled sine-Gordon equations that model the system. Qualitative agreement between numerical and experimental results is found.
Control of diffusion of implanted boron in preamorphized Si: Elimination of interstitial defects at the amorphous-crystal interface
Authors: Roth E. G. , Holland O. W. , Thomas D. K.
Published: July 2002

Transient-enhanced diffusion (TED) during thermal annealing of ion-implanted B in Si is well established and attributed to the ion-induced, excess interstitials. On the other hand, the mechanism to account for TED of B in preamorphized (PA) Si remains unclear. Enhanced diffusion of the B persists in regrown layers even though the ion-induced interstitial defects responsible for TED in B+-only implanted Si are eliminated following regrowth. To test the hypothesis that TED in PA Si results from the “excess” interstitial-type defects below the amorphous-crystalline (a-c) interface, a buried PA layer has been recrystallized from the surface inward to the SiO2 interface of silicon-on-insulator material to eliminate all possible sources of excess interstitials. The effect on B diffusion and the role of the residual interstitial-type defects will be discussed.
Application of neutron interferometry to the measurement of thin film density
Authors: Wallace W. E. , Jacobson D. L. , Arif M. , Ioffe A.
Published: July 2002

The application of neutron interferometry to the measurement of the atom density of polymer thin films (<1 μm thick) supported on silicon substrates is described. Polymer films were chosen primarily for their fixed, well-defined stoichiometry; however, the technique is applicable to films of any elemental composition. The wavelength-independent phase shift of a beam of thermal neutrons passing through the sample gives a measure of the product of the film atom density, the film thickness, the lattice spacing of the silicon interferometer, and the scattering lengths of the constituent elements of the film. The film thickness was found by x-ray reflectivity while the other two parameters are well-defined quantities. The technique does not rely on complex mathematical modeling of physical processes nor on thin film standards for data interpretation. With some refinements, neutron interferometry is envisioned as an important tool in the creation of thin films having well-defined densities which will be useful in the calibration of many analysis techniques.
Dielectric properties of fullerene films
Authors: Su J. S. , Chen Y. F. , Chiu K. C.
Published: July 2002

We report the study on the dielectric properties of C60 films by means of capacitance and dissipation factor measurements at temperatures between 5 and 325 K. In addition to the structural phase transition at 260 K, we have observed a clear anomaly at T=90 K, which did not show up in previous dielectric studies. This result confirms the fact that a glass transition exists due to the freezing in of orientational disorder in C60. A Debye-like relaxation in the dielectric response has also been observed, and the relaxation rate is thermally activated with an energy of about 277 meV, which is in good agreement with that obtained from other measurements.
Sixfold-coordinated carbon as a postdiamond phase
Authors: Sekine T.
Published: July 2002

Carbon behavior at extremely high pressures, which at present cannot be investigated by direct experiments, was evaluated based on high-pressure experimental results on various SiC and NaCl-type carbides. Sixfold-coordinated carbon C(VI) was determined to be the most probable candidate for the postdiamond phase. Taking into account structural variations in the corresponding metals, the volume of C(VI) is estimated to be 3.08 cm3/mol at ambient conditions and 2.8 cm3/mol at pressures of 100–250 GPa. The postdiamond phase of C(VI) may display a greater zero-pressure bulk modulus than diamond.
Ultraviolet emission of silicon quantum tips
Authors: Zheng W. H. , Xia Jian-bai , Lam S. D. , Cheah K. W. , Rakhshandehroo M. R. , Pang S. W.
Published: July 2002

Silicon tips used as field emitters have dimensions that are within the quantum confinement regime. Therefore they can be considered as freestanding silicon tips. In this letter, a photoluminescence spectrum of a 100×100 array of silicon tips was taken at 10 K. Narrow ultraviolet luminescence peaks were observed. Using the empirical pseudopotential homojunction model, it is demonstrated that these luminescence peaks come from energy levels arising from quantum confinement. By fitting the theoretical result to the experimental result, we conclude that the luminescence peaks come from Si quantum tips of about 20 Å in width and that they are covered by silicon dioxide.
Heterodyning far-infrared radiation using coherently controlled directional photocurrent in semiconductors
Authors: Khurgin Jacob B.
Published: July 2002

We propose a method of heterodyne detection of far-infrared signals using generation of directional current in semiconductors via a coherent interaction of one- and two-photon absorption, and evaluate the sensitivity and detectivity of this method.
Metastability and switching in the vortex state of 2H-NbSe2
Authors: Banerjee S. S. , Patil N. G. , Ramakrishnan S. , Grover A. K. , Bhattacharya S. , Ravikumar G. , Mishra P. K. , Chandrasekhar Rao T. V. , Sahni V. C. , Higgins M. J.
Published: July 2002

History-dependent metastable states with different bulk properties are formed in the vortex state of the type-II superconductor 2H-NbSe2. Magnetic measurements demonstrate the difference between the shielding responses of a field- and a zero-field-cooled state, and provide a procedure for switching the system from one state to the other.
Electrically active defect centers induced by Ga+ focused ion beam irradiation of GaAs(100)
Authors: Brown S. J. , Rose P. D. , Jones G. A. C. , Linfield E. H. , Ritchie D. A.
Published: July 2002

The electronic nature of defect centers induced by 30 keV Ga+ focused ion beam irradiation of GaAs(100) has been studied in situ by scanning tunneling spectroscopy (STS). The defect centers were identified as electron traps lying below the surface state conduction band, each with an active area of approximately 20 nm2. An areal ion beam dose of 1×1013 cm−2 was sufficiently low that no significant surface sputtering was observed by topographic imaging which suggests that the features observed by STS are not related to gross physical damage. Spatial STS measurements also allow a lateral profile of a focused ion beam patterned line to be determined accurately, thereby setting a resolution limit on the direct write technique for nanoscale lithography.
Issues on the molecular-beam epitaxial growth of p-SiGe inverted-modulation-doped structures
Authors: Sadeghzadeh M. A. , Parry C. P. , Phillips P. J. , Parker E. H. C. , Whall T. E.
Published: July 2002

The influence of boron segregation and silicon cap-layer thickness on two-dimensional hole gases (2-DHGs) has been investigated in Si/Si0.8Ge0.2/Si inverted-modulation-doped heterostructures grown by solid-source molecular-beam epitaxy. Boron segregation, which is significant in structures with small spacer layers, can be suppressed by growth interruption after the boron doping. How growth interruption affected the electrical properties of the 2-DHG and the boron doping profile as measured by secondary ion mass spectroscopy are reported. We report also on the role played by the unpassivated silicon cap, and compare carrier transport at the normal and inverted interfaces.
Metastable surface ordering in strain relaxed Si0.5Ge0.5 epitaxial layers grown at high temperature
Authors: Reichert H. , Moss S. C. , Imperatori P. , Evans-Lutterodt K.
Published: July 2002

We have studied compositional ordering in the near surface region of 3500 Å thick unstrained Si0.5Ge0.5(001) samples grown by chemical vapor deposition. Measuring asymptotic Bragg scattering along integer and half-integer truncation rods, we found a type of metastable ordering at this surface which is characterized by integer/half-integer reflections along the integer order truncation rods. We show unambiguously that those scattering features originate from a thin layer at the surface. Annealing at 750 °C extinguished these reflections irreversibly, while the reflections of the RS3 bulk structure were not affected. Anomalous scattering at the Ge K edge also confirmed the existence of a new structure in the near surface region.
Effects of a Bi4Ti3O12 buffer layer on SrBi2Ta2O9 thin films prepared by the metalorganic decomposition
Authors: Hu G. D. , Xu J. B. , Wilson I. H. , Cheung W. Y. , Ke N. , Wong S. P.
Published: July 2002

Ferroelectric SrBi2Ta2O9 thin films have been deposited on the Bi4Ti3O12 buffered Pt/Ti/SiO2/Si substrates using the metalorganic decomposition technique at annealing temperatures ranging from 600 to 750 °C. No pyrochlore phase was found in the SrBi2Ta2O9 thin films although the Bi2Ti2O7 phase appeared in the Bi4Ti3O12 buffer layers. A SrBi2Ta2O9 film with (200) predominant orientation was formed at 650 °C. The effects of the Bi4Ti3O12 buffer layer and post-annealing temperature on the structure, surface morphology, and electrical properties of SrBi2Ta2O9 thin films were analyzed.
Propagation of light beams along line defects formed in a-Si/SiO2 three-dimensional photonic crystals: Fabrication and observation
Authors: Hanaizumi Osamu , Ohtera Yasuo , Sato Takashi , Kawakami Shojiro
Published: July 2002

We have fabricated optical waveguides in three-dimensional (3D) photonic crystals and observed propagation of light beams. Light beams with wavelengths of 1.15 μm propagate along the line defects formed in the 3D photonic crystals. The 3D photonic crystals consist of a-Si/SiO2 multilayers laminated alternately by rf bias sputtering on a periodically hollowed silica substrate with a triangular lattice. The pit diameter is 0.2 μm and the pitch of the lattice is 0.5 μm. The thickness of each laminated layer is 0.2 μm. Line defects are formed normal to the surface by laminating a-Si/SiO2 multilayers with ten periods on the substrate in which the corrugation patterns have been omitted in a certain area corresponding to the core. The measurements of transmittance normal to the surface show that the wavelength of 1.15 μm used in observation of propagation is in the passband for the one-dimensional periodic region corresponding to the core and in the stop band for the 3D periodic region corresponding to the cladding, respectively. Measurements show good agreement with finite-difference time-domain calculations.
Ballistic phonon transmission across wafer-bonded crystals
Authors: Msall M. E. , Klimashov A. , Kronmüller S. , Kostial H. , Dietsche W. , Friedland K.
Published: July 2002

We have studied phonon transmission through wafer-bonded GaAs–GaAs interfaces using the techniques of phonon imaging. The short wavelength phonons used for imaging (λ≈10 nm) are an extremely sensitive probe of the bond. We report unprecedented transmission of phonons through carefully prepared bonds. This transmission is remarkable since strong phonon scattering is usually observed at virtually any free surface. The dramatic differences between phonon transmission through well bonded interfaces and phonon transmission through poorly bonded interfaces are the basis of an easily determined quality factor of the bond. In contrast to electron microscopy, the phonon measurements of bond quality are not destructive.
Effect of oxygen plasma treatment on field emission characteristics of boron–nitride films
Authors: Sugino Takashi , Tagawa Shigeru
Published: July 2002

Field emission properties of boron–nitride (BN) films synthesized by plasma-assisted chemical vapor deposition are investigated. There appears a hysteresis of the field emission characteristic for as-grown BN films. It is found that the hysteresis is suppressed for the BN films treated with O2 plasma. Moreover, no significant degradation of the field emission characteristics occurs although a slight generation of B2O3 is detected at the BN surface by O2 plasma treatment.
Charging damage during residual metal overetching
Authors: Hwang Gyeong S. , Giapis Konstantinos P.
Published: July 2002

The influence of electron and ion temperatures on charging damage during residual metal (latent antenna) overetching in high-density plasmas is investigated by Monte Carlo simulations. The tunneling current through a thin gate oxide, electrically connected to the antenna, increases significantly with electron temperature, mainly as a result of changes in plasma current and ion energy distribution. However, the current decreases with ion temperature as ion shading: (a) directly decreases the ion flux to the antenna and (b) neutralizes the negative charge at the upper mask sidewalls, thus allowing more electrons to enter the pattern. The role of exposed antenna areas (trench bottoms and perimeter) is examined from the perspective of current imbalance.
Selective area oxide desorption by electron irradiation in a H2 ambient on GaAs (100)
Authors: Brown S. J. , Burke T. M. , Smith M. P. , Ritchie D. A. , Pepper M. , Tang K. B. T. , Palmer R. E.
Published: July 2002

The selective area removal of oxides from the surface of exposed GaAs (100) has been achieved by irradiating the sample with a broad, low energy electron beam in a H2 ambient. It is proposed that electrons dissociate the molecular hydrogen to create ionized species which react with the surface. The surfaces of samples decontaminated at 365 °C, up to electron energies of 200 eV, were undamaged as revealed by atomic force microscopy. Moreover, quantum well structures epitaxially grown on these surfaces exhibited luminescence. A possible reaction mechanism responsible for the oxide removal is described.
Morphological control and structural characteristics of crystalline Ge–C systems: Carbide nanorods, quantum dots, and epitaxial heterostructures
Authors: Nesting David C. , Kouvetakis J. , Smith David J.
Published: July 2002

Chemical precursors are used to grow crystalline Ge–C materials with unusual morphologies that depend on the molecular design of the precursor and the C concentration. Ge–C nanorods with overall C content of about 13–15 at. % and lattice constants close to that of pure Si grew very rapidly from the surface of a 40 nm Ge–C epitaxial film. Coherent carbide islands are formed after epitaxial growth of 20 nm Ge1−xCx (x=9 at. %) on (100)Si. Lower reaction temperatures resulted in extremely low growth rate of epitaxial Ge1−xCx (x=3–5 at. %) heterostructures with very flat surfaces implying two-dimensional layer-by-layer growth. The use of precursor chemistry as reported here to control morphology and composition in the Ge–C system may provide a simple and reliable synthetic route to a new family of Si-based heterostructures.
Interactions between DX centers and hot electrons and holes in Al0.25Ga0.75As/GaAs heterostructure field-effect transistors
Authors: Dieci D. , Canali C. , Menozzi R. , Pavesi M. , Cetronio A.
Published: July 2002

Using power Al0.25Ga0.75As/GaAs heterostructure field-effect transistors (HFETs) and both electrical and electroluminescence measurements, we show here that while at room temperature the electron and hole capture processes at the DX centers present in the AlGaAs are in equilibrium, and therefore no charge buildup leading to drifts in the HFET characteristics may take place, under cryogenic conditions (T<100 K), where the electron and hole capture cross sections of the DX centers are very different, two competing processes of hole and hot electron capture lead to bias-dependent, negative, recoverable HFET threshold voltage shifts.
Preparation of amorphous hydrogenated silicon-germanium material and solar cells using the thermocatalytic chemical vapor deposition
Authors: Lill M. , Schröder B.
Published: July 2002

Hydrogenated amorphous silicon-germanium (a-SiGe:H) films were produced by the “thermocatalytic chemical vapor deposition” [(TC-CVD) also called “hot-wire” CVD] method. With respect to proposed superstrate solar cell application of the material, the substrate temperature was kept around 200 °C. The band gap Eg of the films could be varied between 1.1 and 1.77 eV by changing the ratio of silane to germane flow. The photoelectronical properties of the alloy films have been found sufficient for solar cell application when Eg⩾1.5 eV and deteriorate for further reduction of the band gap. Using the concept of band gap grading, p-i-n solar cells containing a TC-CVD-a-SiGe:H i layer with initial conversion efficiencies up to η=6.42% (Eg=1.5 eV) have been fabricated. The influence of moderate hydrogen dilution of the process gas on the conversion efficiency and stability of the solar cells has also been investigated.
Superconducting transport properties of 2.2-GeV Au-ion irradiated c-axis twist Bi2Sr2CaCu2O8+δ bicrystals
Authors: Li Qiang , Tsay Y. N. , Suenaga M. , Wirth G. , Gu G. D. , Koshizuka N.
Published: July 2002

2.2-GeV Au-ions irradiation is used to study the effect of columnar defects on the transport properties of Bi2Sr2CaCu2O8+δ (Bi2212) bicrystals with high quality c-axis twist grain boundary. The studies show a similar increase in the irreversible temperature determined within the single crystals and across the grain boundaries after irradiation. However, the irradiation enhancement on the grain boundary critical current (Ic) at low temperatures is small, compared to the more than an order of magnitude increase of single crystal Ic.
Low-temperature scanning tunneling microscope-induced luminescence of an InGaN/GaN multiquantum well
Authors: Evoy S. , Harnett C. K. , Craighead H. G. , Keller S. , Mishra U. K. , DenBaars S. P.
Published: July 2002

We report the scanning tunneling microscope-induced luminescence of an InGaN/GaN multiquantum well. Spectral analysis confirms the dominance of quantum well luminescence. This dominance is discussed in terms of the extent of band bending near the surface. The onset of light emission occurs at a bias larger than the emitted photon energy. This observation agrees with a tunneling in the GaN cap prior to a transport to the quantum well. Luminescence images exhibit features 30–100 nm in size and are discussed in relation to previous studies.