bluuebunny/crossref_metadata_2025 huggingface dataset:
Field fluctuations and spectral line shape in semiconductor lasers subjected to optical feedback
Authors: Ahmed Moustafa , Yamada Minoru
Published: May 2004

This article reports on extensive investigations of the influence of external optical feedback (OFB) on the spectral line shape and the associated spectra of relative intensity noise (RIN) and frequency noise (FN) in semiconductor lasers. The study is based on a generalized simulation model that treats the OFB as a multiple-roundtrip time delay of the lasing field in the external cavity. We present criteria to apply the famous Lang–Kobayashi model to predict the operation characteristics of lasers under OFB. The accuracy of approximated models of calculating the spectral linewidth from the low-frequency components of the frequency noise is examined. We characterize the spectral line shape and the spectra of the RIN and FN in five distinct operating regions, namely, continuous-wave (cw), weak OFB-induced pulsation, period-doubling route-to-chaos, chaos or coherence collapse, and strong OFB-induced pulsation. InGaAsP lasers emitting in a wavelength of 1.5 μm are included in the calculations. The results show that the line shape is most enhanced with sharp central and satellite peaks when strong OFB induces either high-frequency pulsation or cw operation. The moderate range of OFB induces coherence collapse operation, which is characterized by most suppressed and most broadened central and satellite peaks. The cw operation induced under very strong OFB is characterized by RIN and FN levels lower than those of the case of cw operation induced under low OFB.
Temperature dependence of energy transfer mechanisms in Eu-doped GaN
Authors: Lee Chang-Won , Everitt Henry O. , Lee D. S. , Steckl A. J. , Zavada J. M.
Published: May 2004

The temperature dependent behavior of continuous-wave and time-resolved photoluminescence of Eu-doped GaN in the visible region is measured for both the D05→F27 and D05→F37 transitions. The radiative decay of these transitions, following pulsed laser excitation of the GaN host, is monitored by a grating spectrometer and photomultiplier tube detector system. In addition to these two radiative energy transfer pathways within Eu3+, the data reveal two nonradiative energy transfer paths between Eu3+ and the host GaN. Decay constants for the relaxation processes are extracted from the data using a numerically solved rate equation model. Although the dominant radiative relaxation processes decayed with a temperature insensitive decay constant of 166 μs, a prominent role for nonradiative transfer between Eu3+ and impurities within the GaN host was deduced above 180 K.
Effect of network on tunneling magnetoresistance in a half-metallic granular system
Authors: Zhou Li-Ping , Ju Sheng , Li Zhen-Ya
Published: May 2004

The effect of resistor network on the characteristic features of tunneling magnetoresistance of half-metallic granular materials is studied by using a numerical simulation method. We assume the nanosized magnetic particles are well separated by insulating and nonmagnetic grain boundaries and that they are distributed in an n-dimensional resistor network. The value of the tunneling magnetoresistance is found to depend sensitively on the value of n, which is more obvious for higher spin-polarization materials. This phenomenon was also observed in experiments.
Soft x-ray magnetic circular dichroism study of [Co/Pd] multilayered perpendicular magnetic films
Authors: Agui A. , Mizumaki M. , Matsushita T. , Asahi T. , Kawaji J. , Sayama J. , Osaka T.
Published: May 2004

The electronic and spin states of [Co/Pd] multilayered perpendicular magnetization films with various seedlayers have been investigated by means of soft x-ray absorption and magnetic circular dichroism spectroscopy at the Co L2,3-edges. The expectation values of the orbital angular momentum 〈Lz〉 and the spin angular momentum 〈Sz〉 of Co atom in the [Co/Pd] multilayered film were estimated using the sum rule. It was found that the seedlayer changes macroscopic magnetic properties of the [Co/Pd] multilayered film without affecting the electronic and spin states of the upper layers of Co.
On the origin of photogenerated terahertz radiation from current-biased superconducting YBa2Cu3O7−δ thin films
Authors: Lin Po-Iem , Luo Chih-Wei , Liu Hsin-Shan , Chen Shyh-Feng , Wu Kaung-Hsiung , Juang Jenh-Yih , Uen Tseng-Ming , Gou Yih-Shung , Lin Jiunn-Yuan
Published: May 2004

The origin of photogenerated terahertz radiation pulse emitted from current-biased superconducting YBa2Cu3O7−δ thin films excited by femtosecond optical laser pulses is delineated. By investigating the performance of the transient terahertz radiation generated under different operating parameters, pulse reshaping in the measured terahertz electric field caused by the kinetic inductance of the superconducting charge carriers is identified. After recovering the original wave forms of the emitted terahertz pulses, the transient supercurrent density directly correlated to the optically excited quasiparticle dynamics is obtained. A fast decreasing component of about 1.0 ps and a slower recovery process with a value of 2.5 ps are unambiguously delineated in the optically induced supercurrent modulation. The radiation mechanism of the transient terahertz pulse related to nonequilibrium superconductivity is discussed.
Effect of energetic ion irradiation on CdI2 films
Authors: Rawat R. S. , Arun P. , Vedeshwar A. G. , Lee P. , Lee S.
Published: May 2004

The effect of energetic argon ion irradiation, using a 3.3 kJ pulsed plasma focus device, is studied systematically on a 4H polytype (002)-oriented CdI2 stoichiometric film having compressive residual stress. The CdI2 films were exposed to energetic ions from the plasma focus device at different distances from the top of the central electrode. The irradiation was found to change the orientation of the film to (110) at certain moderate irradiation distances. A linear decrease in grain size and residual stress was observed with decreasing irradiation distance (or increasing ion energy), consistent with both structural and morphological observations. The direct optical energy gap Eg was found to increase linearly at the rate 15 μeV/atm with the compressive stress. The combined data of present compressive stress and from earlier reported tensile stress show a consistent trend of Eg change with stress. The iodine–iodine distance in the unit cell could be responsible for the observed change in Eg with stress.
Profile of optical constants of SiO2 thin films containing Si nanocrystals
Authors: Chen T. P. , Liu Y. , Tse M. S. , Fung S. , Dong Gui
Published: May 2004

For optoelectronic and photonic applications of Si nanocrystals (nc-Si) embedded in a SiO2 matrix, the information of the depth profiles of the optical constants for the thin film system is necessary. In this work, an approach of the depth profiling for the thin film synthesized with ion implantation is developed. In this approach, the nc-Si depth distribution obtained from secondary ion mass spectroscopy measurement is modeled with the approximation of many sublayers, and for a given wavelength the optical constants of each sublayer are formulated with the nc-Si volume fraction in the sublayer and the nc-Si optical constants as variables based on the effective medium approximation. After the above procedures the nc-Si optical constants are obtained from the spectral ellipsometric fittings. Finally the optical constants of each sublayer are calculated, and thus the depth profiles of the optical constants for the SiO2 thin film containing the nc-Si are obtained.
Improved blue-green electroluminescence of metal-oxide-semiconductor diode fabricated on multirecipe Si-implanted and annealed SiO2/Si substrate
Authors: Lin Gong-Ru , Lin Chun-Jung
Published: May 2004

The defect-enhanced blue-green photoluminescence (PL) and electroluminescence (EL) of a metal-oxide-semiconductor (MOS) diode made on 500-nm-thick Si-ion-implanted SiO2 (SiO2:Si+) on Si substrate are demonstrated. A multienergy/multidose implantation and 1100 °C annealing process is employed to enhance the 415–455 nm PL contributed by weak oxygen bond and neutral oxygen vacancy defects. The Ag/SiO2:Si+/n-Si/Ag MOS diode exhibits a negative-differential resistance effect with threshold field strength of 300 kV/cm. The threshold pulsed current of deep-blue EL from Ag/SiO2:Si+/n-Si/Ag diode is 280 mA (or 3 V), which turns to white-light emission at saturation current of 680 mA and further shifts to green as the biased current increases up to 3 A. The 3 dB power decay within 3 h is also observed.
Coulomb scattering with remote continuum states in quantum dot devices
Authors: Wetzler R. , Wacker A. , Schöll E.
Published: May 2004

Electron capture and emission by Coulomb scattering in self-assembled quantum dot (QD) devices is studied theoretically. While the dependence of the Coulomb scattering (Auger) rates on the local wetting layer electron density has been a topic of intense research, we put special interest on the remote scattering between QD electrons and continuum electrons originating from a quantum well, doped bulk layers, or metal contacts. Numerical effort is made to include all microscopic transitions between the Fermi distributed continuum states. The remote Coulomb scattering is investigated as a function of the electron density, the distance from the QDs, and the temperature. Our results are compared with experimental observations, considering lifetime limitations in QD memory structures as well as the electron emission in pn diodes.
Insights into “near-frictionless carbon films”
Authors: Johnson Jacqueline A. , Woodford John B. , Chen Xidong , Andersson Joakim , Erdemir Ali , Fenske George R.
Published: May 2004

A form of hydrogenated diamond-like-carbon, “near-frictionless carbon,” developed at Argonne National Laboratory has been studied by several spectroscopic techniques to determine the hydrogen content and carbon bonding within the film. The techniques used include hydrogen forward scattering, ultraviolet Raman spectroscopy, Fourier transform infrared spectroscopy, near-edge x-ray absorption fine structure, and fluctuation microscopy. These complementary techniques reveal the different types of carbon bonding, such as sp2 and sp3, the medium-range order in the film, and its composition.
Thomas–Fermi approximation for a two-dimensional electron gas at low temperatures
Authors: Tang Ting-Wei , O’Regan Terrance , Wu Bo
Published: May 2004

The main purpose of this article is to cast the quantum mechanical electron density, obtained via the Schrödinger–Poisson solver, into a classical form. The Thomas–Fermi (T–F) equation states that the electron density n is dependent on the electrostatic potential φ. The electrostatic potential is determined by Poisson’s equation. In order to account for quantum effects, such as confinement and tunneling, we set out to dervive an effective potential, φ*, that when used in place of Poisson’s potential in the original T–F equation will mimic the solution of the Schrödinger equation. This total potential is then used directly to calculate the electron density. Thus, φ* effectivley washes out the intricacies of the wave functions and yields the electron density. The validity of the T–F equation, for a two-dimensional electron gas at low temperatures, is demonstrated through the solution of a single-electron distribution in fixed square and triangular potential wells. Self-consistent inversion-layer charge densities for single-gate and double-gate metal–oxide–silicon structures are also presented.
Synthesis and physical properties of arc melted NiMnSb
Authors: Gardelis S. , Androulakis J. , Migiakis P. , Giapintzakis J. , Clowes S. K. , Bugoslavsky Y. , Branford W. R. , Miyoshi Y. , Cohen L. F.
Published: May 2004

Several polycrystalline samples of the half-Heusler alloy NiMnSb were grown by arc melting of stoichiometric and nonstoichiometric amounts of high-purity constituent elements. The structure and the phase-purity of the prepared samples were examined systematically by powder x-ray diffraction. The transport properties of the best sample, with saturation magnetization Ms(5 K)=4 μB/formula unit, were studied by measuring electrical resistivity, thermal conductivity, and thermopower. Features in both magnetic and transport data are consistent with NiMnSb being in a half-metallic state at low temperatures, i.e., the conduction electrons are fully spin polarized. However, point-contact Andreev reflection measurements on the same sample at 4.2 K demonstrate only ∼45% spin polarization.
Importance of lattice matching and surface arrangement for the helicon-wave-excited-plasma sputtering epitaxy of ZnO
Authors: Koyama T. , Chichibu S. F.
Published: May 2004

Importance of lattice matching and proper surface arrangement of the substrate was shown to obtain better epilayer qualities of ZnO grown by an epitaxy method, the helicon-wave-excited-plasma sputtering epitaxy. The a-axis-locked single-domain (0001) ZnO epitaxy was accomplished on the uniaxially nearly lattice-matched (112̄0) Al2O3 substrate. The epilayer had approximately 50-100-nm-diam atomically smooth two-dimensional terraces with 0.26-nm-high monolayer steps, and exhibited a predominant near-band-edge photoluminescence peak, whose full width at half maximum value was 107 meV at room temperature. The a-axis-locked epitaxy was also realized on a lattice-mismatched (0001) Al2O3 substrate, indicating the compatibility of oxide substrates. However, the ZnO epilayer grown on the (0001) AlN epitaxial template prepared on (0001) Al2O3 had an in-plane 12-fold multidomain structure, though the effective lattice-mismatch between [112̄0] ZnO and [112̄0] AlN is smaller than that between [112̄0] ZnO and [101̄0] Al2O3. Proper purification and polarity-control techniques for the AlN surface, and/or insertion of an appropriate buffer layer are required to form single-domain heterovalent ZnO/AlN interfaces.
Formation of plasma induced surface damage in silica glass etching for optical waveguides
Authors: Choi D. Y. , Lee J. H. , Kim D. S. , Jung S. T.
Published: May 2004

Ge, B, P-doped silica glass films are widely used as optical waveguides because of their low losses and inherent compatibility with silica optical fibers. These films were etched by ICP (inductively coupled plasma) with chrome etch masks, which were patterned by reactive ion etching (RIE) using chlorine-based gases. In some cases, the etched surfaces of silica glass were very rough (root-mean square roughness greater than 100 nm) and we call this phenomenon plasma induced surface damage (PISD). Rough surface cannot be used as a platform for hybrid integration because of difficulty in alignment and bonding of active devices. PISD reduces the etch rate of glass and it is very difficult to remove residues on a rough surface. The objective of this study is to elucidate the mechanism of PISD formation. To achieve this goal, PISD formation during different etching conditions of chrome etch mask and silica glass was investigated. In most cases, PISD sources are formed on a glass surface after chrome etching, and metal compounds are identified in theses sources. Water rinse after chrome etching reduces the PISD, due to the water solubility of metal chlorides. PISD is decreased or even disappeared at high power and/or low pressure in glass etching, even if PISD sources were present on the glass surface before etching. In conclusion, PISD sources come from the chrome etching process, and polymer deposition on these sources during the silica etching cause the PISD sources to grow. In the area close to the PISD source there is a higher ion flux, which causes an increase in the etch rate, and results in the formation of a pit.
Imaging from one-bit correlations of wideband diffuse wave fields
Authors: Larose Eric , Derode Arnaud , Campillo Michel , Fink Mathias
Published: May 2004

We present an imaging technique based on correlations of a multiply scattered wave field. Usually the Green’s function hAB between two points (A,B) is determined by direct transmit/receive measurement. When this is impossible, one can exploit an other idea: if A and B are both passive sensors, hAB can be retrieved from the cross correlation of the fields received in A and B, the wave field being generated either by deterministic sources or by random noise. The validity of the technique is supported by a physical argument based on time-reversal invariance. Though the principle is applicable to all kinds of waves, it is illustrated here by experiments performed with ultrasound in the MHz range. A short ultrasonic pulse, sent through a highly scattering slab, generates a randomly scattered field. Behind the slab is the medium to image: it consists of four liquid layers with different sound speeds. The cross correlation of the field received on passive sensors located within the medium is used to estimate the speed of sound. The experimental results show that the sound-speed profile of the layered medium can be precisely imaged. We emphasize the role of wideband multiple scattering and of source averaging in the efficiency of the method, as well as the benefit of performing one-bit correlations. Applications to seismology are discussed.
Behavior of a movable electrode in piezo-response mode of an atomic force microscope
Authors: Xu C. H. , Woo C. H. , Shi S. Q. , Wang Y.
Published: May 2004

The objective of this work was to understand the effect of the movable electrode (the tip of an atomic force microscope) on a piezoelectric-induced (PEI) image. Local polarization is induced on a lead zirconate titanate (PZT) thin film using an atomic force microscope (AFM), by applying dc voltage between the movable electrode and the Pt bottom electrode. The polarized PZT film is then characterized by the AFM using a two-pass method, in which both the topography and PEI image are obtained. The surface morphology is recorded in the first pass under contact mode with a fixed setpoint. A PEI image is obtained in the second pass in piezo-response mode. In this mode, the sample surface is scanned by applying ac voltage between the AFM tip and the Pt bottom electrode at sample displacement. PEI images of various sample displacement, corresponding to different stress exerted by the tip on the sample surface, are obtained and analyzed using force–sample displacement curves. It is found that PEI images can be detected if the tip adheres to the sample. The asymmetry of the A cos θ signal is improved as the force changes from repulsive to attractive.
Electrical and radiation assisted passivation of Ta2O5/Si interface
Authors: Hutson J. M. , Devine R. A. B. , Schrimpf R. D.
Published: May 2004

An unusual interface state passivation effect has been observed in Al/Ta2O5/Si capacitors. The effect is found for negative bias stress fields ∼−0.4 MV cm−1, it is also observed for positive bias fields if the structures are simultaneously x irradiated. The passivation relaxes if the structure is heated to moderate temperatures (⩽70 °C) for relatively short times (∼tens of minutes). Bias stress measurements also evidence the presence of positive and negative trapped charge in the Ta2O5 films in the “as-deposited” state.
Crystal growth, structural, and optical characterization of the ordered defect compound CuGa5Se8
Authors: Marı́n G. , Wasim S. M. , Rincón C. , Pérez G. Sánchez , Bocaranda P. , Molina I. , Guevara R. , Delgado J. M.
Published: May 2004

The crystal structural and optical properties of the ordered defect compound CuGa5Se8 grown by the vertical Bridgman–Stockbarger method have been studied by x-ray diffraction and optical absorption techniques, respectively. The analysis of the x-ray diffraction data shows that the material crystallizes in a tetragonal chalcopyrite-related structure with unit cell parameters a=5.4732(8) and c=10.931(6) Å. Optical data shows that the band gap energy EG varies from 1.917 to 1.811 eV in the temperature range between 10 and 300 K. It is found that the variation of EG with T is mainly due to the contribution of optical phonons with a characteristic phonon energy of about 19 meV. The optical absorption coefficient just below the absorption edge varies exponentially with photon energy confirming the existence of the expected Urbach’s tail. The phonon energy hνp associated with Urbach’s tail, which is found to be 54 meV, is about two times higher than the highest optical phonon mode reported for ternary semiconductors of the Cu–Ga–Se system. The origin of higher energy is attributed to the contribution of localized modes produced by structural disorders of low energy formation mainly due to deviation from ideal stoichiometry.
20 meV-deep donor level in InN film of 0.76 eV band gap grown by plasma-assisted nitrogen source
Authors: Wang S. Z. , Yoon S. F. , Xia Y. X. , Xie S. W.
Published: May 2004

We report the photoluminescence (PL) properties of InN epilayers grown by solid-source molecular beam epitaxy on (0001) sapphire substrates with AlN buffer layers. The reactive nitrogen species were generated by a plasma-assisted radio frequency activated nitrogen plasma source. Hall measurements showed the electron concentration in unintentionally doped InN epilayers at room temperature to be around 5×1018 cm−3. Room temperature PL measurements revealed a strong emission peak at 0.76 eV with no signals detected within an energy range from 1.81 to 2.16 eV, suggesting a band gap of around 0.76 eV. The PL spectrum at 10 K showed a main emission peak at 0.74 eV and shoulder peak at 0.72 eV, with a 20 meV interval. Temperature-dependent PL measurements indicated a 20 meV-deep donor level in the InN epilayers.
Response of ultra-low dislocation density GaN photodetectors in the near- and vacuum-ultraviolet
Authors: Pau J. L. , Rivera C. , Muñoz E. , Calleja E. , Schühle U. , Frayssinet E. , Beaumont B. , Faurie J. P. , Gibart P.
Published: May 2004

The spectral response of metal-semiconductor-metal (MSM) and Schottky barrier photodiodes have been studied in the near- and vacuum ultraviolet (VUV). Devices were fabricated on micro-epitaxial lateral overgrowth GaN layers, which presented dislocation densities as low as 7×107 cm−2. Experimental results indicate that the surface properties become critical for the optical response at short wavelengths. Schottky barrier photodiodes showed a lower VUV sensitivity than MSMs as a result of the radiation absorption in the semitransparent Au top-layer. However, Schottky photodiodes yielded a better time stability operating in photovoltaic mode. For photon energies above 10.5 eV, the quantum efficiency of the MSM photodiodes was enhanced as a consequence of the different nature of light-semiconductor interactions, which provoke a decrease of the absorption coefficient and the generation of multiple electron-hole pairs for each impinging photon. The ionization energy for GaN has been also estimated.
Influence of internal fields on radiative and nonradiative processes in AlN/GaN superlattices
Authors: Martinez C. E. , Stanton N. M. , Kent A. J. , Staddon C. R. , Novikov S. V. , Foxon C. T.
Published: May 2004

We have investigated the effect of the internal electric fields on radiative and nonradiative processes in a series of AlN/GaN superlattices. The results of measurements involving direct detection of phonons emitted as a result of nonradiative recombination and carrier energy relaxation are compared with time resolved photoluminescence studies. Using these complementary techniques we have observed directly the effect of free carrier screening on the radiative and nonradiative recombination processes in the superlattice samples. We find that at high excitation power, photoinjected carriers screen the strong internal fields, resulting in an enhanced radiative recombination. As the carriers recombine, the descreening effect results in an increase in recombination via nonradiative processes, which we observe directly as a delayed phonon signal in the time of flight measurements.
Composition and temperature dependence of the crystal structure of Ni–Mn–Ga alloys
Authors: Lanska N. , Söderberg O. , Sozinov A. , Ge Y. , Ullakko K. , Lindroos V. K.
Published: May 2004

The crystal structure of ferromagnetic near-stoichiometric Ni2MnGa alloys with different compositions has been studied at ambient temperature. The studied alloys, with five-layered (5M) and seven-layered (7M) martensitic phases, exhibit the martensitic transformation temperature (TM) up to 353 K. Alloys with these crystal structures are the best candidates for magnetic-field-induced strain applications. The range of the average number of valence electrons per atom (e/a) was determined for phases 5M, 7M, and nonmodulated martensite. Furthermore, a correlation between the martensitic crystal structure, TM and e/a has been established. The lattice parameters ratio (c/a) as a function of e/a or TM has been obtained at ambient temperature for all martensitic phases. That the paramagnetic-ferromagnetic transition influences the structural phase transformation in the Ni–Mn–Ga system has been confirmed.
Effects of coatings on temporal cathodoluminescence quenching in ZnS:Ag,Cl phosphors
Authors: Bang Jungsik , Abrams Billie , Wagner Brent , Holloway Paul H.
Published: May 2004

Powder phosphors of ZnS:Ag,Cl coated with SiO2 (22 or 130 nm nanoparticles), SnO2 or Al2O3 showed different cathodoluminescent (CL) brightness versus time (temporal CL quenching) behavior as compared to noncoated phosphors. At high current density (e.g., 300–800 μA/cm2), the CL emission intensity of coated ZnS:Ag,Cl decayed over the first ∼15 s of electron beam irradiation, which was postulated to result from a large concentration of nonradiative surface centers generated during surface modification of the phosphor, and from localization of generated electrons at the surface due to primary beam-induced internal electric fields. During the first ∼15 s of excitation, generated electrons are postulated to be redistributed by this induced internal electric fields, resulting in increased nonradiative surface recombination between electrons and holes. The formation of a nonradiative surface layer either from electron-stimulated surface chemical reactions on coated or from heat treatment of noncoated ZnS:Ag,Cl powder phosphors were shown to affect temporal CL quenching.
Three-layer photocarrier radiometry model of ion-implanted silicon wafers
Authors: Li Bincheng , Shaughnessy Derrick , Mandelis Andreas , Batista Jerias , Garcia Jose
Published: May 2004

A three-dimensional three-layer model is presented for the quantitative understanding of the infrared photocarrier radiometry (PCR) response of ion-implanted semiconductors, specifically Si. In addition to the implanted layer and intact substrate normally assumed in all existing two-layer theoretical models to describe the photothermal response of ion-implanted semiconductors, a surface layer is considered in this three-layer model to represent a thin, less severally damaged region close to the surface. The effects on the PCR signal of several structural, transport, and optical properties of ion-implanted silicon wafers affected significantly by the ion implantation process (minority carrier lifetime, diffusion coefficient, optical absorption coefficient, thickness of the implanted layer, and front surface recombination velocity) are discussed. The dependence of the PCR signal on the ion implantation dose is theoretically calculated and compared to experimental results. Good agreement between experimental data and theoretical calculations is obtained. Both theoretical and experimental results show the PCR dependence on dose can be separated into four regions with the transition across each region defined by the implantation-induced electrical and optical degrees of damage, respectively, as the electrical and optical damage occurs at different dose ranges. It is also shown that the PCR amplitude decreases monotonically with increasing implantation dose. This monotonic dependence provides the potential of the PCR technique for industrial applications in semiconductor metrology.
Self-diffusion of C12 and C13 in intrinsic 4H–SiC
Authors: Linnarsson M. K. , Janson M. S. , Zhang J. , Janzén E. , Svensson B. G.
Published: May 2004

Self-diffusion of carbon (12C and C13) in low-doped (intrinsic) 4H–SiC has been studied using secondary ion mass spectrometry. A two layer C13 enriched structure with C13/12C ratios of 0.01 and 0.1, respectively, have been prepared by vapor phase epitaxy. Subsequent anneals have been carried out in Ar atmosphere in a rf heated furnace between 2100 and 2350 °C for 15 min–40 h. The C13 depth profiles reveal a strict t evolution for the diffusion, and the extracted carbon self-diffusion coefficients closely follow an Arrhenius temperature dependence: D*=8.4×102 exp(−8.50 eV/kT) cm2/s. The extracted D* are found to be 5 orders of magnitude lower than previously reported for the same temperatures in C14 radio-tracer experiments.
Magnetization processes in exchange-biased MnPd/Fe bilayers
Authors: Blomqvist P. , Krishnan Kannan M. , Girt Er.
Published: May 2004

The magnetization processes in MnPd/Fe bilayers have been investigated using vibrating sample and torque magnetometry. An analytical model based on coherent magnetic moment rotation has been used to qualitatively explain and describe the magnetization processes. The shift of the hysteresis loop, the increased coercivity, the easy and hard axis behavior as well as the intermediate magnetic state seen in the hysteresis loops are reproduced in the model. However, the magnitude of the bias and the coercivity are not strictly in agreement with the measured values. The discrepancies are attributed to the simplified model which does not take into account the role of magnetic domains and incoherent spin rotation.
Normally black reflective twisted-nematic cell for microdisplay application
Authors: Zhu Xinyu , Wu Shin-Tson
Published: May 2004

A normally black reflective twisted-nematic mode is proposed for microdisplay application. This mode is based on the polarization rotation effect of a twisted-nematic liquid crystal. In the dark state, the cell exhibits a large gap tolerance and weak color dispersion. For twist angle ranging from 45° to 65°, the maximum normalized reflectance can achieve 100% through d/p (cell gap over pitch) ratio optimization. Particularly, when the twist angle equals to 52°, the normalized reflectance can reach 100% without any chiral dopant. The twist angle and polarizer angle also make important contributions to the bright state optical efficiency and dark state spectral bandwidth.
Impact of high B concentrations and high dislocation densities on Au diffusion in Si
Authors: Rodriguez A. , Bracht H. , Yonenaga I.
Published: May 2004

We report diffusion experiments of Au in highly dislocated, undoped and homogeneously doped Si with B concentrations of 3.0×1019 and 1.7×1020 cm−3. After Au diffusion at temperatures between 800 and 1100 °C, Au profiles were measured by means of neutron-activation analysis in conjunction with serial sectioning. The diffusion of Au in dislocated Si is accurately described on the basis of the kick-out mechanism and an additional reaction which takes into account the capture of interstitial Au at dislocations. The profiles yield data for the boundary concentration CAu0 and the Au diffusion coefficient DAu whose product CAu0DAu equals the transport capacity CAuieqDAui of interstitial Aui. With increasing B doping Au diffusion increases showing that interstitial Au is singly positively charged. An enhanced boundary concentration CAu0 and simultaneously retarded Au diffusivity was observed at temperatures below 1000 °C which provides direct evidence of Au segregation at dislocations. A segregation enthalpy of 2.7 eV for the energy difference of Au atoms dissolved on substitutional lattice sites and Au atoms trapped at dislocations was determined.
Characterization of submicrometer ferromagnetic NOT gates
Authors: Allwood D. A. , Xiong Gang , Cooke M. D. , Faulkner C. C. , Atkinson D. , Cowburn R. P.
Published: May 2004

We present operation phase diagrams of all-metallic submicrometer ferromagnetic NOT–gate devices. The phase diagrams summarize four different types of behavior, in which devices can operate correctly with either one or three domain walls propagating through them, nucleate domain walls, or pin a domain wall, leading to its annihilation with a succeeding domain wall. We use these phase diagrams to investigate the influence of junction dimensions on domain wall nucleation and pinning, and determine optimized junction parameters for NOT–gate operation. Furthermore, we demonstrate how changing the NOT–gate orientation to an applied field affects the operating phase diagram and may assist the integration of NOT-gates with other types of junction in the near future to realize a full magnetic logic scheme. By fabricating the NOT–gate junctions within a magnetic feedback loop, the direction of domain wall propagation is shown to be reversible and the input and output wires therefore interchangeable.
Simulations with a dynamic reaction–diffusion model of the polymer grating preparation by patterned ultraviolet illumination
Authors: Leewis Christian M. , de Jong Arthur M. , van IJzendoorn Leo J. , Broer Dirk J.
Published: May 2004

Simulations of volume fraction profiles formed during the lithographic preparation of polymer gratings are made with a reaction/diffusion model, based on the Flory–Huggins theory. Monomer migration is driven by a gradient in the chemical potential rather than a gradient in the concentration. If the chemical potential is used as the driving force, monomer migration is not only driven by a difference in concentration, or volume fraction, but also by other entropic effects: the differences in monomer length and the degree of crosslinking of a polymer network. The monomer volume fractions are simulated as a function of position for different ultraviolet intensities and various grating pitches. Profound edges of the monomer volume fractions caused by the fact that the reaction rate is high compared to the diffusion rate are both measured and simulated. An excellent agreement with nuclear microprobe measurements on the polymer gratings is obtained.
Collinear phase-matched terahertz-wave generation in GaP crystal using a dual-wavelength optical parametric oscillator
Authors: Taniuchi T. , Nakanishi H.
Published: May 2004

Tunable terahertz (THz)-wave generation in GaP crystal was theoretically and experimentally investigated using collinear difference frequency mixing. The wavelength range and length of GaP crystal for efficient difference frequency generation (DFG) were determined by calculating the phase-matching conditions. THz waves from 0.5 to 4.5 THz were generated by collinear phase-matched DFG using a dual-wavelength optical parametric oscillator with two KTiOPO4 crystals in the range of 930–1000 nm. A maximum THz output energy of 5.3 nJ/pulse (peak power of 0.66 W) at 1.9 THz was obtained when the pump energy was 1.2 mJ/pulse.
Structure and luminescent properties of CeO2:rare earth (RE=Eu3+ and Sm3+) thin films
Authors: Fujihara Shinobu , Oikawa Masashi
Published: May 2004

Luminescent thin films were fabricated using CeO2 as a host crystal and Eu3+ and Sm3+ as activators. The films were deposited on glass substrates by a spin-on/pyrolysis technique at temperatures up to 700 °C using ethanolic solutions of rare-earth acetates. A cubic fluorite-type CeO2 phase was formed at the deposition temperatures between 400 and 700 °C. Increases in the temperature promoted the grain growth of CeO2 and improved its crystallinity. Eu3+ was practically doped in the CeO2 lattice as indicated by a dominant magnetic-dipole D05→F17 transition in a site with inversion symmetry. The efficiency of ultraviolet light excitation at 330–340 nm was promoted by the charge transfer from O2− to Ce4+ and the subsequent energy transfer to Eu3+ in photoluminescence. Sm3+ doping was found to cause unusual emissions with a dominant G5/24→H5/26 transition centered at 573 nm.
Luminescence excitation study of the higher energy states of Pr3+ and Mn2+ in SrAlF5, CaAlF5, and NaMgF3
Authors: van der Kolk E. , Dorenbos P. , van Eijk C. W. E. , Vink A. P. , Weil M. , Chaminade J. P.
Published: May 2004

The possibility to use Mn2+ co-doping to modify the emission properties of Pr3+-based quantum cutting phosphors—i.e., phosphors that emit one UV and one visible photon for each absorbed vacuum UV (VUV) photon—into phosphors that emit two visible photons, was studied experimentally. In this respect a luminescence excitation study, using synchrotron radiation, between 500 and 50 nm of Pr3+ and Mn2+ impurities in SrAlF5, CaAlF5, and NaMgF3 was performed. Excitation of Pr3+ into the 4f5d excited states results in emission of UV photons from the S01 state followed by visible photons from the P03 state with an internal quantum efficiency exceeding 100%. It was found that a favorable overlap between the S01→I61 Pr3+ emission and the A16→Eg4, A1g4 Mn2+ absorption exists that should promote energy transfer from Pr3+ to Mn2+. However no such energy transfer could be observed in SrAlF5. At higher excitation energies (>7 eV) intense structured Mn2+ excitation bands are found that are assigned to 3d5→3d44s transitions, Mn2+ related charge transfer transitions, and Mn2+ bound excitons. These bands have an unfavorable overlap with the Pr3+ 4f5d states, preventing efficient selective Pr3+ excitation, prerequisite for the quantum cutting process.
Superresolution size determination in fluorescence microscopy: A comparison between spatially modulated illumination and confocal laser scanning microscopy
Authors: Spöri Udo , Failla Antonio Virgilio , Cremer Christoph
Published: May 2004

Recently developed far field light optical methods are a powerful tool to analyze biological nanostructures and their dynamics, in particular including the interior of three-dimensionally conserved cells. In this article, the recently described method of spatially modulated illumination (SMI) microscopy has been further extended to the online determination of the extension of small, subwavelength sized, fluorescent objects (nanosizing). Using fluorescence excitation with 488 nm, the determination of fluorescent labeled object diameters down to 40 nm corresponding to about 1/12th of the wavelength used for one-photon excitation could be shown. The results of the SMI nanosizing procedure for a detailed, systematic variation of the object diameter are presented together with a fast algorithm for online size evaluation. In addition, we show a direct comparison of the diameter of “colocalization volumes” between SMI nanosizing and conventional confocal laser scanning microscopy.
Submicron-scale characterization of poly-Si thin films crystallized by excimer laser and continuous-wave laser
Authors: Kitahara K. , Ohashi Y. , Katoh Y. , Hara A. , Sasaki N.
Published: May 2004

Stress and defects in polycrystalline silicon (poly-Si) for thin-film transistor (TFT) were investigated on the aspect of difference of techniques between excimer laser annealing (ELA) with extremely short pulse duration and continuous-wave laser lateral crystallization (CLC). Micro-Raman spectroscopy was used for characterization. Mapping measurements with high spatial resolution were carried out for the optical-phonon mode at ∼520 cm−1 to observe distribution of stress and defects. Defects were characterized also by local-vibration modes (LVMs) in ∼2000 cm−1 region, where LVMs were induced by terminating dangling bonds at defects with hydrogen atoms. Significant tensile stress was found for both ELA and CLC, which magnitude is dominated by the thermal stress. The stress in ELA poly-Si tends to relax in the vicinity of grain boundaries. Densities of defects in grains and dangling bonds at grain boundaries in CLC poly-Si films were apparently smaller than those in ELA films. Those differences are attributed to cooling rates of ELA and CLC. The defects in CLC poly-Si distribute extending along the laser scan direction parallel to TFT channel current, which is effective to prevent the deterioration of TFT characteristics by residual defects.
Three-dimensional simulations of self-assembly of hut-shaped Si–Ge quantum dots
Authors: Ramasubramaniam A. , Shenoy V. B.
Published: May 2004

This article presents the results of three-dimensional modeling of heteroepitaxial thin film growth with the objective of understanding recent experiments on the early stages of quantum dot formation in SiGe/Si systems. We use a continuum model, based on the underlying physics of crystallographic surface steps, to study the growth of quantum dots, their spatial ordering and coarsening behavior. Using appropriate parameters, obtained from atomistic calculations, the (100) orientation is found to be unstable under compressive strains. The surface energy now develops a minimum at an orientation that may be interpreted as the (105) facet observed in SiGe/Si systems. This form of the surface energy allows for the growth of quantum dots without any barrier to nucleation—dots are seen to start off via a surface instability as shallow stepped mounds, which steepen continuously to reach their low energy orientations. During the very initial stages of growth, mounds are seen to grow in a dense array with several of them impinging on each other and subsequently coalescing to form larger mounds. This behavior occurs due to the competition between surface energy which seeks to minimize the free-energy by the formation of islands with side-walls at the strain stabilized orientations and repulsive elastic interactions between such closely spaced islands. Using simple analytical calculations, we show the existence of a critical island size for this coalescence behavior. A key result of our analysis is the inverse scaling of this critical size with the misfit strain in the film. While energetic analyses may be used to obtain useful insights, the growth of quantum dots is essentially a nonequilibrium process and requires a fundamental understanding of the kinetics. Numerical studies show that the growth kinetics has a profound effect on surface morphology: arrays of well-separated islands or, alternatively, intersecting ridges are obtained in different kinetic regimes. We also study an alternative model of a stable but nonfacet (100) orientation and point out the inconsistencies of this assumption.
Magnetic properties of metastable phase from undercooled Nd–Fe–B melts
Authors: Ozawa Shumpei , Li Mingjun , Kuribayashi Kazuhiko , Sugiyama Suguru , Jimbo Itaru , Hirosawa Satoshi
Published: May 2004

Nd 14 Fe 79 B 7 alloys were solidified from the undercooled melts by containerless processing. The resultant alloys contained a large amount of a metastable phase with a chemical composition near that of the bulk alloy and a crystal structure similar to that of the Nd2Fe17 phase. The metastable phase had low magnetic properties, a saturation magnetization of 86.7 emu/g, a remanence of 4.5 emu/g and a coercivity of 0.14 kOe. The magnetic properties of the sample annealed at 1020 K were markedly improved due to the increase in the amount of the Nd2Fe14B phase.
Filtered pulsed carbon cathodic arc: Plasma and amorphous carbon properties
Authors: Liu Dongping , Benstetter Günther , Lodermeier Edgar , Zhang Jialiang , Liu Yanhong , Vancea Johann
Published: May 2004

The carbon plasma ion energies produced by the filtered pulsed cathodic arc discharge method were measured as a function of filter inductance. The energy determination is based on the electro-optical time-of-flight method. The average ion energies of the pulsed ion beams were found to depend upon the rise time and duration of pulsed arc currents, which suggests that a gain of ion kinetic energy mainly arises from the electric plasma field from the ambipolar expansion of both electrons and ions, and an electron drag force because of the high expansion velocity of the electrons. The tetrahedral amorphous carbon (ta-C) films with a sp3 fraction of ∼70% were deposited on silicon substrates at the average ion energies of >6 eV in the highly ionized plasmas. The ta-C films were found to be covered with a few graphitelike atomic layers. The surface properties of ultrathin carbon films, such as nanoscale friction coefficients, surface layer thickness, and silicon contents were strongly dependent on the ion energies. The growth of amorphous carbon films was explained in terms of the thermal spike migration of surface carbon atoms. In terms of this model, the thermal spike provides the energy required to release surface atoms from their metastable positions and leads to the formation of the sp3 bonded carbon on a sp3 bonded matrix. The experimental results indicate that the low-energy (<3 eV) carbon ions have insufficient energies to cause the rearrangement reaction within the film and they form graphitelike structures at film surface.
Field effect on planar devices made of epitaxial manganite perovskites
Authors: Pallecchi I. , Pellegrino L. , Bellingeri E. , Siri A. S. , Marré D.
Published: May 2004

Planar side gate devices are fabricated with epitaxial perovskite oxides by means of the voltage biased tip of an atomic force microscope. The high performance of the insulating gate barriers as well as the high dielectric permittivity of the chosen substrate are exploited to obtain a remarkable value of accumulated/depleted charge. We fabricate field effect devices having a La0.67Ba0.33MnO3 channel on a SrTiO3 substrate. By applying a gate voltage of +/−60 V we observe a modulation of the manganite channel resistance up to 150% at low temperatures. Furthermore, a field effect induced reversible shift by 3.2 K of the metal-insulator transition temperature is observed. This technology for prototype devices based on epitaxial oxides opens perspectives in oxide electronics applications.
Evaluation of film biaxial modulus and coefficient of thermal expansion from thermoelastic film stress measurements
Authors: Thurn Jeremy , Hughey Michael P.
Published: May 2004

The so-called “double-substrate technique,” a popular method for determining thermoelastic properties of films, involves deposition of a film on two (or more) substrates of differing coefficient of thermal expansion (CTE) and measuring the elastic stress response to thermal cycling to determine the film biaxial modulus and CTE. However, the substrate CTE variation with temperature is often neglected. As an unintended consequence, the calculated film properties will depend on the temperature range used during thermal cycling and data analysis. An analysis technique that allows the film CTE to vary with temperature is proposed, ensuring that the calculated film properties are independent of the temperature range used during thermal cycling. The technique is demonstrated for a NiFe (permalloy) film electroplated on Al2O3–TiC, Si, and glass substrates.
Carbon doping of GaN with CBr4 in radio-frequency plasma-assisted molecular beam epitaxy
Authors: Green D. S. , Mishra U. K. , Speck J. S.
Published: May 2004

Carbon tetrabromide (CBr4) was studied as an intentional dopant during rf plasma molecular beam epitaxy of GaN. Secondary ion mass spectroscopy was used to quantify incorporation behavior. Carbon was found to readily incorporate under Ga-rich and N-rich growth conditions with no detectable bromine incorporation. The carbon incorporation [C] was found to be linearly related to the incident CBr4 flux. Reflection high-energy electron diffraction, atomic force microscopy and x-ray diffraction were used to characterize the structural quality of the film’s postgrowth. No deterioration of structural quality was observed for [C] from mid 1017 to ∼1019 cm−3. The growth rate was also unaffected by carbon doping with CBr4. The electrical and optical behavior of carbon doping was studied by co-doping carbon with silicon. Carbon was found to compensate the silicon although an exact compensation factor was difficult to extract from the data. Photoluminescence was performed to examine the optical performance of the films. Carbon doping was seen to monotonically decrease the band edge emission. Properties of carbon-doped GaN are interpreted to be consistent with recent theoretical work describing incorporation of carbon as function of Fermi level conditions during growth.