bluuebunny/crossref_metadata_2025 huggingface dataset:
Polysiloxane-based photorefractive polymers for digital holographic data storage
Authors: Poga C. , Lundquist P. M. , Lee V. , Shelby R. M. , Twieg R. J. , Burland D. M.
Published: July 2002

A photorefractive polymer system based on a substituted polysiloxane backbone is described. In addition to high holographic diffraction efficiency and reasonable writing speed, the material reported exhibits excellent optical clarity and low optical scattering characteristics. These latter optical properties are necessary in high density holographic digital data storage applications. The utility of the polysiloxane based photorefractive polymers for storage applications is demonstrated by recording digital data at a density of 0.52 Mbit/cm2 and reading it back without error.
Depression of melting point of multidomained bismuth in aluminum based metallic glass nanocomposites
Authors: Goswami R. , Chattopadhyay K.
Published: July 2002

We report the synthesis of nanocomposites of Bi in an aluminum based metallic glass matrix by rapid solidification. It is shown that constrained melting and solidification of nanometer sized embedded Bi particles lead to the formation of symmetry related multidomained particles. The Bi particles exhibit a significantly large depression of bulk melting point (over 100 K) requiring a free energy gain of greater than 0.7×108 J m−3. This cannot be explained by the size dependence of melting points or other pressure effects and represents an intrinsic characteristic of the multidomained particles.
Epitaxial growth of NiSi2 on (111)Si inside 0.1–0.6 μm oxide openings prepared by electron beam lithography
Authors: Yew J. Y. , Chen L. J. , Nakamura K.
Published: July 2002

Epitaxial growth of NiSi2 on (111)Si inside 0.1–0.6 μm oxide openings prepared by electron beam lithography has been studied by field emission scanning electron microscopy, transmission electron microscopy, and thin-film stress measurement. Striking effects of size and shape of deep submicron oxide openings on the growth of NiSi2 epitaxy were observed. Epitaxial growth of NiSi2 of single orientation on (111)Si was found to occur at a temperature as low as 400 °C inside contact holes of 0.2 μm or smaller in size. Contact holes were found to be more effective in inducing the epitaxial growth of NiSi2 of single orientation than that of linear openings of the same size. The effects of size and shape of lateral confinement on the epitaxial growth of NiSi2 on (111)Si are correlated with the stress level inside oxide openings.
Time-resolved picosecond optical nonlinearity and all-optical Kerr gate in poly (3-hexadecylthiophene)
Authors: Gomes A. S. L. , Demenicis Luciene , Petrov D. V. , de Araújo Cid B. , de Melo Celso P. , Souto-Maior Rosa
Published: July 2002

A time-resolved anisotropic configuration exploiting the Z-scan technique in a one color pump-and-probe arrangement was implemented to measure the time response and the value of the nondiagonal component of the third-order nonlinear susceptibility of a chloroform solution of poly (3-hexadecylthiophene). In addition, an all-optical Kerr gate with picosecond response using the polymeric solution was implemented.
Synthesis of crystalline carbon nitride thin films by laser processing at a liquid–solid interface
Authors: Sharma A. K. , Ayyub P. , Multani M. S. , Adhi K. P. , Ogale S. B. , Sunderaraman M. , Upadhyay D. D. , Banerjee S.
Published: July 2002

Pulsed laser induced reactive quenching at a liquid–solid interface was used for the synthesis of tetrahedrally coordinated crystalline carbon nitride on a tungsten substrate. The crystalline phase was identified by transmission electron diffraction. X-ray photoelectron spectroscopy indicated that the carbon atoms are coordinated only tetrahedrally with nitrogen—as expected for C3N4. The atomic percentage of N (considering only those atoms coordinated with C) is about 35%.
All-optical switching in a dye-doped polymer fiber Fabry–Perot waveguide
Authors: Welker D. J. , Kuzyk M. G.
Published: July 2002

We report on the demonstration of all-optical switching in a 110 μm diam, 1 cm long Fabry–Perot cavity defined in a dye-doped polymer optical fiber waveguide. Such a device is a basic building block of optical logic.
Highly insulative barium zirconate-titanate thin films prepared by rf magnetron sputtering for dynamic random access memory applications
Authors: Wu T. B. , Wu C. M. , Chen M. L.
Published: July 2002

Polycrystalline thin films of Ba(ZrxTi1−x)O3 with x=0–0.4 and a thickness of ∼180 nm were deposited on platinum-coated silicon substrates by rf magnetron sputtering at 500 °C. Ba-rich targets were used to prepare films of stoichiometric composition. The film having x=0.12 exhibited a satisfactory dielectric property of dielectric constant, k≊300, and dissipation factor, tan δ<0.02, at frequencies from 103 to 106 Hz under ambient temperatures ranging from 20 to 180 °C. More importantly, the film showed a very stable and highly insulative characteristic against applied voltage. The leakage current density J increases only smoothly to a value less than 10−7 A/cm2 followed by an Ohmic relation of J=σE with σ=1.4×10−14 (Ω cm)−1 up to an extremely high electric field E of 5.6 MV/cm without any sign of abrupt increase of leakage current or electrical breakdown. Also, no time-dependent electric degradation was observed for the film subjected to an electric field as high as 5 MV/cm at room temperature up to 3600 s of measurement.
Whiskerlike structure growth on silicon exposed to ArF excimer laser irradiation
Authors: Sánchez F. , Morenza J. L. , Aguiar R. , Delgado J. C. , Varela M.
Published: July 2002

The effects of ArF excimer laser irradiation on silicon single crystals in air have been studied. The etch rate versus fluence curve shows three well defined zones, with very different etch rates and dependences. In the intermediate zone (from 1.5 to 2.5 J/cm2), narrow (1–2 μm diameter) and tall columns (3–30 μm) start to grow after irradiation with some hundreds of laser pulses. These whiskerlike columns, with height between one and two orders of magnitude higher than the depth of the crater, have not been formed by preferential etching of the surrounding material, but through hydrodynamical processes.
Low-temperature dopant activation and its application to polycrystalline silicon thin film transistors
Authors: Lee Seok-Woon , Ihn Tae-Hyung , Joo Seung-Ki
Published: July 2002

Low-temperature activation of dopants in amorphous silicon films was achieved by a new method and high-performance polycrystalline silicon thin film transistors were fabricated through its application. It was found that the dopants implanted into amorphous silicon were activated simultaneously with the crystallization of amorphous silicon. With the help of a thin nickel layer, the thermal budget for dopant activation and crystallization was considerably reduced, from 600 °C (30 h) to 500 °C (5 h). Even without plasma hydrogenation, the n-channel polycrystalline silicon thin film transistors fabricated at temperatures below 500 °C showed a mobility of 120 cm2/V s, which is much higher than that of conventional devices fabricated at 600 °C.
Effect of hydrogen on the indium incorporation in InGaN epitaxial films
Authors: Piner E. L. , Behbehani M. K. , El-Masry N. A. , McIntosh F. G. , Roberts J. C. , Boutros K. S. , Bedair S. M.
Published: July 2002

The InN percent in metalorganic chemical vapor deposition (MOCVD) and atomic layer epitaxy (ALE) grown InGaN was found to be significantly influenced by the amount of hydrogen flowing into the reactor. The temperature ranges for this study are 710–780 °C for MOCVD, and 650–700 °C for ALE. For a given set of growth conditions, an increase of up to 25% InN in InGaN, as determined by x-ray diffraction, can be achieved by reducing the hydrogen flow from 100 to 0 sccm. Additionally, the hydrogen produced from the decomposition of ammonia does not seem to change the InN percent in the films, indicating that the ammonia decomposition rate is less than 0.1%. The phenomenon of having hydrogen control the indium incorporation was not reported in the growth of any other III–V compound previously studied.
Measurement of two-photon absorption cross sections of dye molecules doped in thin films of polymethylmethacrylate
Authors: Mukherjee N. , Mukherjee A. , Reinhardt B. A.
Published: July 2002

Two-photon absorption cross sections of two dye molecules, doped in thin films of polymethylmethacrylate, are measured by comparing fluorescence intensities induced by single- and two-photon excitations. Ratio of the single- and two-photon absorption cross sections is related to the ratio of the single- and two-photon pumped fluorescence intensities. Using this sensitive and background-free detection technique, dispersion of the nonlinearity is measured over a wavelength range of 700–800 nm.
Growth of diamond films on boron nitride thin films by bias-assisted hot filament chemical vapor deposition
Authors: Polo M. C. , Sańchez G. , Wang W. L. , Esteve J. , Andújar J. L.
Published: July 2002

We report the growth of continuous diamond thin films by bias-assisted hot filament chemical vapor deposition onto hexagonal boron nitride films prepared by plasma chemical vapor deposition on silicon substrates. Negative substrate biasing during the early stages of diamond growth greatly increased the nucleation density. Values of 1010 cm−2 were achieved at −250 V for bias times as short as 25 min. After the nucleation stage, high quality polycrystalline continuous diamond films, as revealed by scanning electron microscopy and Raman analysis, were grown under standard hot filament deposition conditions.
Single-chip fused hybrids for acousto-electric and acousto-optic applications
Authors: Rotter M. , Rocke C. , Böhm S. , Lorke A. , Wixforth A. , Ruile W. , Korte L.
Published: July 2002

The combination of the electronic and optical properties of a semiconductor hetero-junction and the acoustic properties of a piezoelectric substrate material yields a new class of very promising hybrids for potential acousto-electric and acousto-optic applications. LiNbO3/GaAs hybrids have been fabricated using the epitaxial lift-off technique resulting in unusually large acousto-electric and acousto-optic interaction between the quasi two-dimensional electron system in the semiconductor and surface acoustic waves on the piezoelectric substrate. Field effect tunability of the interaction at room temperature is demonstrated and possible device applications are discussed. Photoluminescence measurements show the influence of the acousto-electric fields on the optical properties of quantum well structures.
Photon tunneling from an optically manipulated microsphere to a surface by lasing spectral analysis
Authors: Sasaki Keiji , Fujiwara Hideki , Masuhara Hiroshi
Published: July 2002

Photon tunneling of lasing emission from a dye-doped microspherical particle to an object was investigated by use of a microspectroscopy system combined with a laser manipulation technique. An emission spectrum drastically changed with approaching the lasing microsphere to a glass plate. The intensity ratio between resonant peaks exhibited exponential dependence on the sphere-object distance, whose decay constant agreed with the penetration depth of an evanescent field just outside of the microsphere. The variation in the spectral profile can be explained with the Mie scattering theory. Applicability of the lasing microsphere as a probe of a near-field scanning optical microscope is discussed.
Cathodoluminescence and photoluminescence of highly luminescent CdSe/ZnS quantum dot composites
Authors: Rodriguez-Viejo J. , Jensen K. F. , Mattoussi H. , Michel J. , Dabbousi B. O. , Bawendi M. G.
Published: July 2002

We report room-temperature cathodoluminescence and photoluminescence spectra originating from ZnS overcoated CdSe nanocrystals, 33 and 42 Å in diameter, embedded in a ZnS matrix. The thin-film quantum dot composites were synthesized by electrospray organometallic chemical vapor deposition. Cathodoluminescence and photoluminescence are dominated by the sharp band-edge emission characteristic of the initial nanocrystals. The emission wavelength can be tuned in a broad window (470–650 nm) by varying the size of the dots. The cathodoluminescence intensity depends on the crystallinity of the ZnS matrix and the voltage and current density applied.
Grain boundary evolution of YBa2Cu3O7−δ in the vicinity of steps on patterned (001) LaAlO3 substrates
Authors: Gustafsson M. , Olsson E. , Yi H. R. , Winkler D. , Claeson T.
Published: July 2002

The microstructure of YBa2Cu3O7−δ(YBCO) grain boundaries obtained at step edges on (001) LaAlO3 substrates have been studied by scanning and transmission electron microscopy. A typical morphology of the YBCO grain boundaries is distinguished by two parts where the parts are expected to have different junction properties. One part of the boundary is caused by the difference in nucleation probability between the (001) surface and the inclined step facet. The second morphology is developing as a result of minimization of the YBCO surface energy.
Atomic nitrogen doping in p-ZnSe molecular beam epitaxial growth with almost 100% activation ratio
Authors: Kimura K. , Miwa S. , Jin C. G. , Kuo L. H. , Yasuda T. , Ohtake A. , Tanaka K. , Yao T. , Kobayashi H.
Published: July 2002

An almost 100% activation ratio {(NA-ND)/[N]} for a nitrogen-doped ZnSe molecular beam epitaxy (MBE) layer with the highest net acceptor concentration (NA-ND) of 1.2×1018 cm−3 was obtained using a high-power rf plasma source. Even at this high NA-ND value, a 4.2 K photoluminescence spectrum shows bound exciton emission and deep donor–acceptor pair emission with well-resolved phonon replicas. The high activation in nitrogen doping could be ascribed to the generation of the predominant atomic nitrogen and to the suppressed extraction of nitrogen ions and excited neutral nitrogen molecules due to the structure of the orifice placed between the MBE growth chamber and the plasma discharge tube of the high-power plasma source.
Growth of high-quality GaAs/AlAs Bragg mirrors on patterned InP-based quantum well mesa structures
Authors: Gebretsadik H. , Kamath K. , Linder K. K. , Zhang X. , Bhattacharya P. , Caneau C. , Bhat R.
Published: July 2002

We have investigated the regrowth of GaAs/AlAs quarter-wave Bragg reflectors on patterned mesa InP-based quantum well heterostructures that can be fabricated into 1.55 μm vertical cavity surface emitting lasers. It is seen from transmission electron and scanning electron microscopy that the multiple layer GaAs-based mirrors can be grown on InP-based heterostructure mesas of diameters 10–40 μm without noticeable propagation of defects into the reflector layers or the quantum well region below. At the same time the photoluminescence from the quantum wells after regrowth indicates that lasers can be fabricated.
Silicon incorporation into chemical vapor deposition diamond: A role of oxygen
Authors: Sakaguchi Isao , Nishitani-Gamo Mikka , Loh Kian Ping , Haneda Hajime , Hishita Shunichi , Ando Toshihiro
Published: July 2002

The suppression effect of oxygen on Si incorporation in homoepitaxial diamond films grown by microwave assisted chemical vapor deposition is investigated by secondary ion mass spectrometry. The Si depth profile in the multilayered diamond films continuously synthesized with different oxygen addition shows that Si incorporation decreases with increasing oxygen addition into the plasma. A change in the interfacial composition at the quartz glass due to oxygen-promoted surface chemistry may be the origin of this suppression effect.
Growth of amorphous-layer-free microcrystalline silicon on insulating glass substrates by plasma-enhanced chemical vapor deposition
Authors: Zhou Jiang-Huai , Ikuta Kazuyuki , Yasuda Tetsuji , Umeda Takahide , Yamasaki Satoshi , Tanaka Kazunobu
Published: July 2002

Using a triode plasma-enhanced chemical vapor deposition (PECVD) system and high H2 dilution of SiH4 (down to a SiH4/H2 gas flow ratio of 0.33/99), amorphous-layer-free μc-Si:H has been successfully grown on insulating glass substrates in the continuous PECVD growth mode. It is demonstrated that an ultrathin layer of such μc-Si:H can serve as a seed layer to facilitate an epitaxial-like growth of μc-Si:H (seeded growth).
Highly ordered nanochannel-array architecture in anodic alumina
Authors: Masuda Hideki , Yamada Haruki , Satoh Masahiro , Asoh Hidetaka , Nakao Masashi , Tamamura Toshiaki
Published: July 2002

The development of the ordered channel array in the anodic porous alumina was initiated by the textured pattern of the surface made by the molding process, and growth of an almost defect-free channel array can be achieved throughout the textured area. The long-range-ordered channel array with dimensions on the order of millimeters with a channel density of 1010 cm−2 was obtained, and the aspect ratio was over 150. The master for molding could be used many times, which makes it possible to overcome problems in the conventional nanolithographic technique, such as low through-put and high cost.
Theoretical analysis of horizontal shear mode piezoelectric surface acoustic waves in potassium niobate
Authors: Nakamura Kiyoshi , Oshiki Mitsuhiro
Published: July 2002

The propagation characteristics of the pure SH (horizontal shear) mode piezoelectric surface acoustic wave (SAW) on rotated Y-cuts about the Z-axis of potassium niobate (KNbO3) are theoretically analyzed. The results show that there exists a SH mode SAW which has a velocity lower than the piezoelectrically inactive Rayleigh mode SAW for most of the rotated cuts. Differing from the so-called Bleustein-Gulyaev wave in 6 mm crystals, the particle displacement of the SH mode SAW on the metalized surface of Y-cut KNbO3 decays with slow oscillation as a function of depth from the substrate surface, while that on the free surface oscillates with a slight decay. The electromechanical coupling factor K2 ranges from a very large value of 0.53 to a relatively small value of 0.026, depending on substrate orientation.
Artificial dielectrics: Nonlinear optical properties of silicon nanoclusters at λ=532 nm
Authors: Vijayalakshmi S. , Shen F. , Grebel H.
Published: July 2002

The nonlinear optical responses of laser-ablated Si nanoclusters were measured at λ=532 nm. Re{χ(3)} values as high as −(1.33±0.33)×10−3 esu were measured for films that were only 200 nm thick. The response time for this nonlinearity was as short as 3.5±0.5 ns, limited by our laser pulse duration.
A two-stack indirect-barrier/triple-coupled quantum well infrared detector for mid-wavelength and long-wavelength infrared dual band detection
Authors: Chiang J. C. , Li Sheng S. , Singh A.
Published: July 2002

We report a two-stack indirect-barrier (IB) GaAs/AlGaAs quantum well infrared photodetector (QWIP) for mid-wavelength infrared (MWIR) and a voltage-tunable InGaAs/GaAs/AlGaAs triple-coupled (TC) QWIP for long-wavelength infrared (LWIR) detection. The peak responsivity of the stacked QWIP at zero bias (PV mode) was found to be 30 mA/W at λp=4.3μm and T=40 K. The maximum peak responsivity for the stacked QWIP was found to be 0.21 A/W at λp=4.3 μm, Vb=4 V, and T=40 K. For the LWIR TC-QWIP, the peak detection wavelength due to (E1→E3) transition shifts from 10 to 9.4 μm as bias voltage increases from 7 to 12 V. The maximum responsivity was found to be 0.085 A/W at λp=9.4 μm, Vb=12 V, and T=40 K. The results show that simultaneously detection of both the MWIR and LWIR bands can be achieved at Vb⩾7 V or Vb⩽−5 V. It is shown that this two-stack QWIP can be used as a voltage-tunable two-color or multicolor QWIP for the MWIR and LWIR dual band detection.
Blueshifting of InGaAsP/InP laser diodes by low-energy ion implantation
Authors: Paquette M. , Beauvais J. , Beerens J. , Poole P. J. , Charbonneau S. , Miner C. J. , Blaauw C.
Published: July 2002

A new method based on low-energy implantation is presented for the fabrication of laser diodes with shifted emission wavelength. The laser diodes are based on InGaAsP/InGaAs/InP material, with compressively strained active layers. Low-energy implantation (18 keV As+) is used to generate vacancies near the surface of an incomplete laser structure, for which the epitaxial growth was interrupted 45 nm above the active layers of the device. The vacancies are subsequently diffused through the quantum wells by rapid thermal annealing. This diffusion causes a local intermixing of atoms at the interfaces of the active layers, which induces an increase of the band gap energy. The implantation/anneal process can be repeated several times to increase the amount of intermixing, thereby further shifting the emission wavelength. Once this process is completed, the upper optical confinement layer of the structure is overgrown using chemical beam epitaxy. Operational lasers with blueshifts as large as 35 nm were obtained.
Submicron resolution measurement of stress in silicon by near-field Raman spectroscopy
Authors: Webster S. , Batchelder D. N. , Smith D. A.
Published: July 2002

A scanning near-field optical microscope (SNOM) has been constructed that is capable of recording Raman spectra with a spatial resolution of ∼150 nm. The SNOM has been used to produce a combined topological and Raman map of a plastically deformed area of a silicon wafer. The variation of the frequency of the 520 cm−1 Raman band with position has been used to estimate the residual stresses associated with the deformation. The measurements demonstrate the feasibility of nondestructive, submicron stress measurement in semiconductors by near-field Raman spectroscopy.
Transport and structural properties of the top and bottom grain boundaries in YBa2Cu3O7−δ step-edge Josephson junctions
Authors: Lombardi Filomena , Ivanov Z. G. , Fischer G. M. , Olsson E. , Claeson T.
Published: July 2002

We present a method to study separately the electrical transport properties of the grain boundaries (GBs) formed at the top and at the bottom edges of YBa2Cu3O7−δ(YBCO) step-edge Josephson junctions. The step-edge junctions were fabricated on (100) LaAlO3 steps using tilted Ar ion milling to define the electrodes and the microbridges. Due to the shadowing effect of the step, a continuous YBCO stripe remains along and at the bottom of the step on both sides of a microbridge. We found that the top GB is responsible for the weak link behavior of our step-edge junctions. The transport properties were correlated with the different microstructural properties of the two GBs formed at the edges of the step.
Doping of AlxGa1−xN
Authors: Stampfl C. , Van de Walle Chris G.
Published: July 2002

N- type AlxGa1−xN exhibits a dramatic decrease in the free-carrier concentration for x⩾0.40. Based on first-principles calculations, we propose that two effects are responsible for this behavior: (i) in the case of doping with oxygen (the most common unintentional donor), a DX transition occurs, which converts the shallow donor into a deep level; and (ii) compensation by the cation vacancy (VGa or VAl), a triple acceptor, increases with alloy composition x. For p-type doping, the calculations indicate that the doping efficiency decreases due to compensation by the nitrogen vacancy. In addition, an increase in the acceptor ionization energy is found with increasing x.
Near-field scanning optical spectroscopy of an InGaN quantum well
Authors: Crowell P. A. , Young D. K. , Keller S. , Hu E. L. , Awschalom D. D.
Published: July 2002

Near-field scanning optical microscopy is used to image photoluminescence (PL) in an InGaN/GaN quantum well (QW) with spatial resolution of approximately 100 nm for temperatures between 50 and 295 K. Strong (∼50%) fluctuations in the quantum well photoluminescence as well as a tenfold enhancement of deep level-related emission at lower energies occur at large (∼500 nm diam) pits in the heterostructure. Regions of smaller (∼15%) fluctuations in the QW PL are not correlated with the presence of pits. The spectrum of the QW PL shows no significant variations on the length scales probed in this experiment. We thus find no spectroscopic signature of the recombination of strongly localized carriers at temperatures above 50 K.
Nature of the 2.8 eV photoluminescence band in Mg doped GaN
Authors: Kaufmann U. , Kunzer M. , Maier M. , Obloh H. , Ramakrishnan A. , Santic B. , Schlotter P.
Published: July 2002

The blue Mg induced 2.8 eV photoluminescence (PL) band in metalorganic chemical vapor deposition grown GaN has been studied in a large number of samples with varying Mg content. It emerges near a Mg concentration of 1×1019 cm−3 and at higher concentrations dominates the room temperature PL spectrum. The excitation power dependence of the 2.8 eV band provides convincing evidence for its donor–acceptor (D–A) pair recombination character. It is suggested that the acceptor A is isolated MgGa while the spatially separated, deep donor (430 meV) D is attributed to a nearest-neighbor associate of a MgGa acceptor with a nitrogen vacancy, formed by self-compensation.
Atom-resolved electronic spectra for Alq3 from theory and experiment
Authors: Curioni A. , Andreoni W. , Treusch R. , Himpsel F. J. , Haskal E. , Seidler P. , Heske C. , Kakar S. , van Buuren T. , Terminello L. J.
Published: July 2002

The electronic structure of Alq3 is investigated using density functional theory-based calculations, photoemission and near-edge x-ray absorption fine structure. The distinct features of the observed spectra are understood in terms of contributions from the different atoms and molecular orbitals. Fingerprints of the molecular bonding and of the individual atoms are identified. These results are meant to be a reference for the monitoring of chemical processes that Alq3 may undergo during fabrication or degradation of light-emitting devices, and for the understanding of the effects of ligand or metal substitution.
Shot noise beyond the Tucker theory in niobium tunnel junction mixers
Authors: Dieleman P. , Klapwijk T. M.
Published: July 2002

The electrical and shot-noise characteristics of high-current-density niobium superconductor–insulator–superconductor (SIS) tunnel junctions suitable for heterodyne detection of THz radiation are studied. It is found that a significant part of the current at voltages V<2Δ/e is carried via barrier defects (pinholes). Due to the transport mechanism in these pinholes, the shot noise is considerably increased. The data presented clarify an often-observed discrepancy between measured and expected heterodyne mixer noise temperatures and predict a 25% increase in the receiver noise temperature of Nb SIS heterodyne detectors at 1 THz.
Imprinting magnetic structures
Authors: Felcher G. P. , Lohstroh W. , Fritzsche H. , Münzenberg M. , Maletta H. , Felsch W.
Published: July 2002

We created helical magnetic structures by depositing in sequence layers of lanthanum and iron (each 30 Å thick) on a substrate rotating in a fixed field of a few Oe. The magnetic field was sufficient to define the magnetization of each layer during growth without disturbing the magnetization of the layers already formed. As a result, the imprinted helical magnetic structure had a chirality given by the direction of rotation of the substrate; the pitch was given by the speed of rotation and the rate of deposition. This finding indicates that it is possible to engineer mesoscopic magnetic structures.
Electrical characteristics of magnesium-doped gallium nitride junction diodes
Authors: Fedison J. B. , Chow T. P. , Lu H. , Bhat I. B.
Published: July 2002

Electrical characteristics of lateral p+n diodes made from gallium nitride epitaxial layers on sapphire substrates are reported. The current–voltage characteristics are observed to have several distinct regions in which a tunneling current has been identified at low forward bias in addition to the conventional temperature-dependent diffusion current observed at moderate forward bias. A tunneling behavior indicates the presence of deep-level traps at the junction, which alter the electrical behavior of these junctions compared to the conventional behavior. In addition, space-charge-limited currents are found to influence these junctions at large forward and reverse bias.
Thin film electroluminescence in highly anisotropic oxide materials
Authors: Xiao T. , Kitai A. H. , Liu G. , Nakua A. , Barbier J.
Published: July 2002

A criterion for high performance electroluminescent (EL) phosphor based on the crystal structure of the phosphor host has been developed. The performance of some best-performing EL oxide phosphors is correlated to the fundamental atomic arrangements of the phosphor host and to the microstructural characteristics of their thin films. It is shown that, contrary to popular beliefs, oxide phosphors with certain crystallographic features do have the capability of transporting significant current densities of hot electrons. Examples of newly developed high brightness and efficiency oxide EL phosphors are presented.
Preparation of atomically flat Co(110) films on Cu(110)
Authors: Tölkes Christian , Struck Rainer , David Rudolf , Zeppenfeld Peter , Comsa George
Published: July 2002

It is shown that the growth mode of cobalt on the Cu(110) surface at 350 K can be changed from three-dimensional to extended layer-by-layer growth by pre- and co-adsorption of oxygen. The shape and the intensity of the growth oscillations observed in thermal energy helium atom scattering experiments depends sensitively on the total oxygen coverage. Helium diffraction reveals that different oxygen induced surface reconstructions of the top-most Co layer are responsible for the presence or absence of layer-by-layer growth in an extended coverage range.
First-principles exploration of possible trap terminators in SiO2
Authors: Yokozawa Ayumi , Miyamoto Yoshiyuki
Published: July 2002

Oxygen vacancies (O vacancies) are considered to be charge trap centers in SiO2. This letter explores possible terminators of Si-dangling bonds associated with the O vacancies by performing first-principles total energy calculations. The present exploration is focused on the terminators which retain their chemical bonds after hole trapping. We have found that Cl atoms, NH and OH molecules firmly terminate the Si-dangling bond while H atoms are dissociated from a Si–H bond after hole trapping. The H dissociation is accompanied by the generation of gap states which can contribute to the leakage currents in the SiO2 films.
Optical switches of gadolinium–magnesium multilayers
Authors: van der Sluis P.
Published: July 2002

It is shown that Pd-coated multilayers built from alternating Gd and Mg layers go through an optical transition by variation of the hydrogen concentration. The multilayer structure is preserved during switching. The optical properties of these stacks are comparable to those of the Pd-coated GdMg alloys. By selecting appropriate multilayer periods, faster switches can be built when compared to alloys with a comparable Mg content.
Significance of tunneling in p+ amorphous silicon carbide n crystalline silicon heterojunction solar cells
Authors: van Cleef M. W. M. , Schropp R. E. I. , Rubinelli F. A.
Published: July 2002

We used the internal photoemission (IPE) technique to accurately determine the valence and conduction band offsets at the a-SiC:H/c-Si interface and investigated with numerical simulations their effects on the photocarrier collection in p+ a-SiC:H/n c-Si heterojunction solar cells. The valence and conduction band discontinuities were found to be 0.60 and 0.55 eV, respectively. However, despite the large barrier at the valence band edge, 30 nm p+ a-SiC:H/n c-Si heterojunction solar cells show no collection problems due to blocking of holes (FF=0.73). Combined IPE measurements and simulation results indicate that tunneling of holes through this barrier at the valence band edge can explain the unhindered collection.
High-transconductance self-aligned AlGaN/GaN modulation-doped field-effect transistors with regrown ohmic contacts
Authors: Chen Ching-Hui , Keller Stacia , Parish Gia , Vetury Ramakrishna , Kozodoy Peter , Hu Evelyn L. , Denbaars Steven P. , Mishra Umesh K. , Wu Yifeng
Published: July 2002

Self-aligned AlGaN/GaN modulation-doped field-effect transistors (MODFETs) with high transconductances have been demonstrated on a sapphire substrate. Source and drain were selectively regrown with ∼1700 Å of n-GaN adjacent to the gate electrode. Source resistance was reduced to 0.95 Ω mm from 1.4 to 1.8 Ω mm with conventional GaN-based MODFETs. These self-aligned devices show a record high value of extrinsic transconductance ∼400 mS/mm for AlGaN/GaN MODFETs with a gate length of 1.2 μm.
Diffusion coefficient of Al in metastable, amorphous Al–Pt phase
Authors: Radi Z. , Lábár J. L. , Barna P. B.
Published: July 2002

Diffusion data for amorphous binary alloys are scarce in the literature. There are no data available for the diffusion of Al in Al–Pt amorphous phase known to the authors. High-temperature successive deposition of components was used in the present experiments to reveal the elementary processes of reactive diffusion forming the metastable amorphous phase and to determine the diffusion coefficient of Al in this phase: D(α)=(1.2±0.4)×10−10×exp[−(6.9±0.1)×104J mol−1/RT]m2 s−1. This value, taken from thin film experiments, can be considered as a good approximation to the bulk diffusion coefficient.
The induced phase transformation and oxygen vacancy relaxation in La-modified bismuth titanate ceramics
Authors: Jiang A. Q. , Hu Z. X. , Zhang L. D.
Published: July 2002

Nanopowders of Bi2Ti2O7 with various La modifications were fabricated by chemical coprecipitation. As sintered at high temperature, the phase structure in powders translates into (La0.22Bi0.78)4Ti3O12 and (La0.26Bi0.74)2Ti4O11 with increasing La concentration, respectively. Dielectric and nonlinear resistivity investigations in bulk ceramics show that oxygen vacancy relaxation or defect dipole polarization contributes considerably to the dielectric permittivity, while the mechanism is shut off under a threshold field dependent on phase transition as well as temperature. Furthermore, the quantitative analysis of x-ray photoelectron spectroscopy indicates the deviation of oxygen element composition at surfaces of (La0.22Bi0.78)4Ti3O12 ceramic grains from the standard ratio.
Fabrication and stimulated emission of Er-doped nanocrystalline Si waveguides formed on Si substrates by laser ablation
Authors: Zhao Xinwei , Komuro Shuji , Isshiki Hideo , Aoyagi Yoshinobu , Sugano Takuo
Published: July 2002

Er-doped nanocrystalline Si (nc-Si) waveguides were fabricated on Si substrates and investigated by optical pumping. A stimulated emission at 1540 nm was demonstrated at room temperature. The sizes of the fabricated Er-doped nc-Si waveguides were 5000 nm×200 nm×L, where L is the cavity length and is changed from 1 to 10 mm. Superlinear optical outputs at 1540 nm were observed for the waveguides longer than 3 mm. The threshold of the optical output where the stimulated emission occurs is in the order of 10 MW/cm2, and is demonstrated to depend on the cavity length of the waveguides. A large reduction of decay lifetimes of the light output from a cleavage facet of the Er-doped nc-Si waveguides was observed when the pumping power density exceeded the thresholds indicating an increase of transition probabilities in intra-4f electrons in Er3+ ions caused by the stimulated emission. Better 1540 nm laser performance and lower pumping power density should be obtained by optimizing the device structure and increasing the Er concentration.
Analysis of the potential distribution in the channel region of a platinum silicided source/drain metal oxide semiconductor field effect transistor
Authors: Snyder John P. , Helms C. R. , Nishi Yoshio
Published: July 2002

An analysis of the effect of the source and drain Schottky diodes on the drain characteristics (Idrain vs Vdrain) of a platinum silicided metal oxide semiconductor field effect transistor is presented. A back to back Schottky diode model is used to explained the “diode like” feature observed in the small Vdrain regime. For two-dimensional device simulations indicate an abrupt voltage drop across the source Schottky diode which can be a significant fraction of the applied drain bias. The resulting virtual source potential (Vsource′, different from the applied source potential Vsource) has been quantified as a function of applied drain bias, temperature, and channel length, based on simulations that have been calibrated with experimentally measured data. The physical basis for Vsource′ is described as the reverse breakdown mechanism of the source Schottky diode and as such is a strong function of source electrode tip sharpness, poly edge to silicide edge gap distance, and temperature. The implications of this effect on device performance is briefly discussed.
Interpretation of x-ray photoelectron spectra of elastic amorphous carbon nitride thin films
Authors: Holloway B. C. , Kraft O. , Shuh D. K. , Kelly M. A. , Nix W. D. , Pianetta P. , Hagström S.
Published: July 2002

We report the synthesis and characterization of amorphous carbon nitride (CNx) thin films using a direct current magnetron reactive sputter system. Nanoindentation of the CNx films and amorphous carbon films deposited under similar conditions shows the CNx films are extremely elastic, that the addition of nitrogen fundamentally changes the mechanical properties of the films, and that traditional methods of calculating the hardness and Young’s modulus may not be valid. X-ray photoelectron spectroscopy (XPS) of the N(1s) and C(1s) core levels show multiple bonding arrangements. In a new interpretation of the XPS data, the two predominant N(1s) spectral features have been identified, based on comparison to reference data in the literature, as those belonging to nitrogen in a four-bond arrangement and nitrogen in a three-bond arrangement, independent of hybridization. The formation of a fourth bond allows nitrogen to substitute for C atoms in a carbon-based graphitic system without the formation of dangling bonds or unfilled states. The relationship between nitrogen incorporation in a carbon-based ring structure and measured film properties is rationalized based on previously published models.
Time-resolved imaging of radiative recombination in 4H–SiC p-i-n diode
Authors: Galeckas A. , Linnros J. , Breitholtz B.
Published: July 2002

The time-resolved imaging of electron-hole recombination radiation from a forward-biased 4H–SiC p-i-n diode is reported. A novel approach of combining gated charge couple device technique with cross-sectional emission microscopy is demonstrated as a fast and informative method for characterization of both 4H–SiC material properties and the overall power device performance. We present the capability of the technique to visualize structural defects, to characterize spatial distribution and dynamics of injected carriers, and to provide effective carrier diffusion and lifetime parameters. From the results of lateral and in-depth imaging of the light emission we conclude that at low currents the injection of holes from the p+ emission is dominant. Furthermore, an effective carrier lifetime of 350 ns in the active n− region and a diffusion length of 15 μm in the substrate are readily obtained.
Spin injection into semiconductors
Authors: Oestreich M. , Hübner J. , Hägele D. , Klar P. J. , Heimbrodt W. , Rühle W. W. , Ashenford D. E. , Lunn B.
Published: July 2002

The injection of spin-polarized electrons is presently one of the major challenges in semiconductor spin electronics. We propose and demonstrate a most efficient spin injection using diluted magnetic semiconductors as spin aligners. Time-resolved photoluminescence with a Cd0.98Mn0.02Te/CdTe structure proves the feasibility of the spin-alignment mechanism.
Modified Stranski–Krastanov growth in stacked layers of self-assembled islands
Authors: Schmidt O. G. , Kienzle O. , Hao Y. , Eberl K. , Ernst F.
Published: July 2002

In a stack of vertically aligned Stranski–Krastanov grown islands, the critical thickness for planar growth for all but the initial dot layer is reduced, if the thickness of the spacer layer ts is smaller than a certain value t0. We present structural and photoluminescence results on the basis of the extensively studied lattice-mismatched material system Si/Ge.
Molecular dynamics simulation study of the fluence dependence of particle yield and plume composition in laser desorption and ablation of organic solids
Authors: Zhigilei Leonid V. , Garrison Barbara J.
Published: July 2002

Two distinct regimes of molecular ejection separated by a well-defined threshold fluence are observed in molecular dynamics simulation of pulsed laser irradiation of an organic solid. At fluences above the threshold a collective multilayer ejection or ablation occurs where large liquid droplets are ejected and the total yield of the ablated material follows a critical volume density of the deposited energy. Below threshold thermal desorption from the surface is observed and the desorption yield has an Arrhenius-type dependence on the laser fluence. The yield of monomers does not have a step increase at the threshold and thus deceptively does not give a straightforward interpretation of the ejection mechanisms.
In situ annealing studies of molecular-beam epitaxial growth of SrS:Cu
Authors: Tong W. , Xin Y. B. , Park W. , Summers C. J.
Published: July 2002

Annealing studies are reported on molecular-beam epitaxial grown SrS:Cu which has potential as a blue phosphor for full color electroluminescent (EL) displays. It was found that annealing under a sulfur flux at 650 °C greatly improved film quality and luminescent brightness. This was attributed to the reduction of sulfur vacancies, and a large enhancement in the grain size of these thin-film phosphors. Using this procedure, EL devices with a luminance of 26 cd/m2 at 40 V above the turn-on threshold voltage with chromaticity coordinates of x=0.17, y=0.29 were obtained.
Photoreflectance study of H2S plasma-passivated GaAs surface
Authors: Shen H. , Zhou W. , Pamulapati J. , Ren F.
Published: July 2002

Photoreflectance is used to study the effect of H2S plasma passivation on the GaAs surface. GaAs samples are treated with a H2S plasma in an electron cyclotron resonance chemical vapor deposition system and in-situ encapsulated with a SiNx film. The surface Fermi level moves towards the conduction band after H2S plasma passivation and a surface state density of 6×1010 cm−2 is achieved under optimal passivation conditions. The surface state density is highly dependent on the sample temperature during passivation. The movement of the surface Fermi level is due to the reduction of the surface state density and not due to a shift of midgap surface states, suggesting that S–Ga bonds play the major role in H2S plasma passivated GaAs surfaces. This work demonstrates the need to measure both the surface Fermi level and the density of surface states.
The photoluminescence from hydrogen-related species in composites of SiO2 nanoparticles
Authors: Glinka Yuri D. , Lin Sheng-Hsien , Chen Yit-Tsong
Published: July 2002

Measurements of photoluminescence (PL) from composites of silica nanoparticles (the primary particle size 7 and 15 nm) as a function of heat treatment temperature show that the PL results from hydrogen-related species and thermally produced structural defects. The PL was induced by an ArF or Nd:YAG (yttrium–aluminum–garnet) laser (λexc=193 or 266 nm). The green PL exhibits a progression with spacings of about Δν=630 cm−1 assigned to the bending vibration of ≡Si–H on the surface of particles. The spacings increase up to Δν=1200 cm−1 when ≡Si–H and nonbridging oxygen (≡Si–O•) form interfacial water species.
Kinetics of radiative recombination in strongly excited ZnSe/BeTe superlattices with a type-II band alignment
Authors: Maksimov A. A. , Zaitsev S. V. , Tartakovskii I. I. , Kulakovskii V. D. , Yakovlev D. R. , Ossau W. , Keim M. , Reuscher G. , Waag A. , Landwehr G.
Published: July 2002

We report results of a detailed investigation of type-II superlattices under high density photoexcitation. A strong spectral shift (≈0.5 eV) of the recombination band corresponding to the indirect transition from the ZnSe conduction band to the BeTe valence band in ZnSe/BeTe superlattices with increasing carrier density has been found at T=300 K. The dynamical characteristics of this transition are studied by time-resolved spectroscopy. A model which accounts for the dependence of band bending and lifetimes of spatially separated electrons and holes on the concentration of the photoexcited carriers is developed. Numerical simulations of the photoluminescence kinetics are in very good agreement with experimental results. It turns out that despite the huge band offsets involved, the radiative recombination under high excitation conditions can be nearly as fast as in spatially direct quantum wells.
Self-assembled ErAs islands in GaAs: Growth and subpicosecond carrier dynamics
Authors: Kadow C. , Fleischer S. B. , Ibbetson J. P. , Bowers J. E. , Gossard A. C. , Dong J. W. , Palmstrøm C. J.
Published: July 2002

We report the growth of self-assembled ErAs islands embedded in GaAs by molecular beam epitaxy. The nucleation of ErAs on GaAs occurs in an island growth mode leading to spontaneous formation of nanometer-sized islands. Several layers of ErAs islands separated by GaAs can be stacked on top of each other to form a superlattice. X-ray diffraction shows superlattice fringes from such samples. Pump–probe measurements indicate carrier capture times as short as 120 fs. These capture times are strongly correlated with the period of the superlattice.
All solid-state diode-pumped Raman laser with self-frequency conversion
Authors: Grabtchikov A. S. , Kuzmin A. N. , Lisinetskii V. A. , Orlovich V. A. , Ryabtsev G. I. , Demidovich A. A.
Published: July 2002

Operation of an all solid-state compact pulsed Raman laser pumped by a continuous-wave laser diode is demonstrated. The Stokes and anti-Stokes radiations of stimulated Raman scattering at the 1.181 and 0.973 μm wavelengths, respectively, are generated as a result of self-frequency conversion of the 1.067 μm laser radiation in a Nd3+:KGd(WO4)2 crystal. With Q-switched operation, the Raman laser threshold corresponds to 230 mW of a laser diode light power. The output power of 4.8 mW is achieved at the Stokes wavelength with a kilohertz repetition rate.
Layer-by-layer growth of ZnO epilayer on Al2O3(0001) by using a MgO buffer layer
Authors: Chen Yefan , Ko Hang-Ju , Hong Soon-Ku , Yao Takafumi
Published: July 2002

By introducing a thin MgO buffer, layer-by-layer growth of ZnO epilayers on Al2O3(0001) substrates is achieved by plasma-assisted molecular beam epitaxy. The MgO buffer is very effective on the improvement of surface morphology during the initial growth stage, which eventually leads to an atomically flat surface. As a result, (3×3) surface reconstruction of ZnO is observed and reflection high-energy electron diffraction intensity oscillations are recorded. Structural analysis indicates that the twin defect with a 30° in-plane crystal orientation misaligned is completely eliminated, while the total dislocation density is reduced. Free exciton emissions at 3.3774 eV (XA) and 3.383 eV (XB) are observed in photoluminescence at 4.2 K further indicating the high quality of the resulting ZnO epilayers.
Effects of the postannealing atmosphere on the dielectric properties of (Ba, Sr)TiO3 capacitors: Evidence of an interfacial space charge layer
Authors: Pontes F. M. , Leite E. R. , Longo E. , Varela J. A. , Araujo E. B. , Eiras J. A.
Published: July 2002

The dielectric properties of (Ba, Sr)TiO3 films were found to be remarkably sensitive to the postannealing treatment atmosphere. This study demonstrates that postannealing in an oxygen atmosphere increases the dielectric relaxation phenomenon and that postannealing in a nitrogen atmosphere produces a slight dielectric relaxation. Such dependence of the dielectric relaxation was related both to oxygen vacancies and to the presence of negatively charged oxygen, trapped at the grain boundary and/or at the electrode/dielectric film interface.