bluuebunny/crossref_metadata_2025 huggingface dataset:
Metal-organic chemical vapor deposition growth of InGaN/GaN high power green light emitting diode: Effects of InGaN well protection and electron reservoir layer
Authors: Ju Jin-Woo , Kang Eun-Sil , Kim Hwa-Soo , Jang Lee-Woon , Ahn Haeng-Keun , Jeon Ju-Won , Leea In-Hwan , Baek Jong Hyeob
Published: September 2007

We investigated the effects of the well protection layer (WPL) and electron reservoir layer (ERL) on the emission properties of InGaN/GaN green multiple quantum wells (MQWs). In order to increase their emission wavelength by preventing the volatile InGaN well, a thin GaN WPL was coated subsequently on each well layer at the same temperature before ramping-up the temperature to grow the GaN barrier. It was found that the WPL directly influenced the indium content and optical properties of the MQW. The indium content was in fact increased, as was evident from the x-ray diffraction and photoluminescence experiments. Then, to explore the possibility of enhancing the quantum efficiency by increasing the electron capture rate, a superlattice ERL composed of ten pairs of InGaN/GaN was embedded between the MQW and n-GaN. The electroluminescence intensity of the green light emitting diode with the ERL was up to three times higher than that of the diode without the ERL. These results imply that the carrier capture by the MQW is significantly improved by the additional superlattice ERL, which consequently leads to the enhancement of the quantum efficiency.
Avalanche multiplication phenomenon in amorphous semiconductors: Amorphous selenium versus hydrogenated amorphous silicon
Authors: Reznik A. , Baranovskii S. D. , Rubel O. , Juska G. , Kasap S. O. , Ohkawa Y. , Tanioka K. , Rowlands J. A.
Published: September 2007

Although the effect of the impact ionization and the consequent avalanche multiplication in amorphous selenium (a-Se) was established long ago and has led to the development and commercialization of ultrasensitive video tubes, the underlying physics of these phenomena in amorphous semiconductors has not yet been fully understood. In particular, it is puzzling why this effect has been evidenced at practical electric fields only in a-Se among all amorphous materials. For instance, impact ionization seems much more feasible in hydrogenated amorphous silicon (a-Si:H) since the charge carrier mobility in a-Si:H is much higher than that in a-Se and also the amount of energy needed for ionization of secondary carriers in a-Si:H is lower than that in a-Se. Using the description of the avalanche effect based on the lucky-drift model recently developed for amorphous semiconductors we show how this intriguing question can be answered. It is the higher phonon energy in a-Si:H than that in a-Se, which is responsible for the shift of the avalanche threshold in a-Si:H to essentially higher fields as compared to a-Se.
Field emission from silicon nanowires: Conditioning and stability
Authors: Riccitelli Riccardo , Di Carlo Aldo , Fiori Angelamaria , Orlanducci Silvia , Terranova Maria Letizia , Santoni Antonino , Fantoni Roberta , Rufoloni Alessandro , Villacorta Felix Jimenex
Published: September 2007

We report the low-pressure chemical vapor deposition growth and field emission characterization of silicon nanowires (SiNWs). Our field emission results show the importance of the so called conditioning process on the reproducibility of the emission performance itself; this effect has proven to be reversible for the investigated current regime. We explained this behavior by invoking a current-driven desorption of residual adsorbed gases. A highly reproducible turn-on electric field of 27 V∕μm is found for a diode-connected SiNW planar sample. Furthermore, stability analysis is performed showing the technologically promising field emission behavior of the samples.
Micromagnetic study of picosecond dephasing of spin waves in a square magnetic element
Authors: Barman Anjan , Sharma Ramesh C.
Published: September 2007

We present the micromagnetic simulation of picosecond dephasing of spin waves in a square micromagnetic element. Application of a fast magnetic field pulse produces stripes in the time-dependent magnetic images due to the presence of multiple spin waves. Frequencies of the spin waves were extracted from spatial sampling of time-dependent magnetization. We excite the individual spin waves by monochromatic harmonic fields oscillating at frequencies of the observed spin waves in micromagnetic simulations. This allows us to understand the spatial nature of the spin waves and that the observed stripes in time-resolved magnetic images are a consequence of superposition of a number of noninteracting resonant modes in that magnetic element.
Vacancy-fluorine complexes and their impact on the properties of metal-oxide transistors with high-k gate dielectrics studied using monoenergetic positron beams
Authors: Uedono A. , Inumiya S. , Matsuki T. , Aoyama T. , Nara Y. , Ishibashi S. , Ohdaira T. , Suzuki R. , Miyazaki S. , Yamada K.
Published: September 2007

Vacancy-fluorine complexes in metal-oxide semiconductors (MOS) with high-k gate dielectrics were studied using a positron annihilation technique. F+ ions were implanted into Si substrates before the deposition of gate dielectrics (HfSiON). The shift of threshold voltage (Vth) in MOS capacitors and an increase in Fermi level position below the HfSiON/Si interface were observed after F+ implantation. Doppler broadening spectra of the annihilation radiation and positron lifetimes were measured before and after HfSiON fabrication processes. From a comparison between Doppler broadening spectra and those obtained by first-principles calculation, the major defect species in Si substrates after annealing treatment (1050 °C, 5 s) was identified as vacancy-fluorine complexes (V3F2). The origin of the Vth shift in the MOS capacitors was attributed to V3F2 located in channel regions.
Polymer composite electrolytes containing ionically active mesoporous SiO2 particles
Authors: Wang Xiao-Liang , Mei Ao , Li Ming , Lin Yuan-Hua , Nan Ce-Wen
Published: September 2007

In exploring approaches to enhance ionic conductivity of solid polymer electrolytes, adding inert fillers has proved to be effective. In the present work, by doping ionically active SiO2 particles, which were designed by absorbing and confining liquid plasticizers in the nanosized pores of SBA-15 (a kind of mesoporous SiO2) to the polyethylene oxide (PEO)-LiClO4 matrix, about 10-fold further enhancement in the ionic conductivity was achieved as compared with adding inert SBA-15. The fillers dispersed homogenously in the PEO matrix. The ambient ionic conductivity reaches about 1.5×10−4 S cm−1 for (PEO)8-LiClO4∕10 wt % active SBA-15 films. Moreover, other properties such as the ambient lithium transference number, long-term stability of bulk and lithium metal electrode/electrolyte interfacial resistance, and the electrochemical window also showed good results. Application of such electrolytes in lithium batteries was asserted by cell testing with LiCoO2 cathode and lithium metal anode.
Improved high-field reliability for a SiC metal–oxide–semiconductor device by the incorporation of nitrogen into its HfTiO gate dielectric
Authors: Lin L. M. , Lai P. T.
Published: September 2007

Materials with high dielectric constant (k) have been used in SiC-based metal–oxide–semiconductor (MOS) devices to reduce the electric field in the gate dielectric and thus suppress a high-field reliability problem. In this work, high-k gate dielectrics HfxTi1−xO2 and HfxTi1−xON are applied in SiC MOS devices and an ultrathin thermally grown SiO2 is used as an interlayer between SiC and the high-k materials to block electron injection from SiC into the low-barrier high-k materials. Incorporating nitrogen into the Hf-Ti oxide (by adding nitrogen gas during its sputtering) stacked with a SiO2 interlayer (HfxTi1−xO∕SiO2) results in a better gate dielectric for the MOS capacitor, such as smaller frequency dispersion in the capacitance-voltage curve, less oxide charges, and better interface quality. Moreover, the nitrogen incorporation increases the dielectric constant of the oxide, but causes higher dielectric leakage, which can be suppressed by the SiO2 interlayer. High-field stress under constant electric field is performed on the stacked/nonstacked Hf-Ti oxides and oxynitrides, and it turns out that the two oxynitrides show a much smaller flatband shift and a less stress-induced leakage current compared with the two oxides. Based on these results, the HfxTi1−xON∕SiO2 stack could be a promising high-k gate dielectric for SiC MOS devices with enhanced reliability.
X-ray microbeam probing of elastic strains in patterned He+ implanted single-crystal LiNbO3
Authors: Djukic Djordje , Roth Ryan M. , Osgood R. M. , Evans-Lutterodt Kenneth , Bakhru Hassaram , Bakhru Sasha , Welch David
Published: September 2007

X-ray microprobing is used to investigate buried elastic strain resulting from deep He+ implantantion in LiNbO3. The implantation regions are defined lithographically and strain fields mapped with spatial- and energy-resolved x-ray microdiffraction to characterize the resulting structures. The structurally modified regions are found to retain their lateral lithographic definition, with the buried implantation being in strong compression.
Ultrafast optical switching of three-dimensional Si inverse opal photonic band gap crystals
Authors: Euser Tijmen G. , Wei Hong , Kalkman Jeroen , Jun Yoonho , Polman Albert , Norris David J. , Vos Willem L.
Published: September 2007

We present ultrafast optical switching experiments on three-dimensional photonic band gap crystals. Switching the Si inverse opal is achieved by optically exciting free carriers by a two-photon process. We probe reflectivity in the frequency range of second order Bragg diffraction where the photonic band gap is predicted. We find good experimental switching conditions for free-carrier plasma frequencies between 0.3 and 0.7 times the optical frequency ω: we thus observe a large frequency shift of up to Δω∕ω=1.5% of all spectral features including the peak that corresponds to the photonic band gap. We deduce a corresponding large refractive index change of ΔnSi′∕nSi′=2.0%, where nSi′ is the refractive index of the silicon backbone of the crystal. The induced absorption length is longer than the sample thickness. We observe a fast decay time of 21 ps, which implies that switching could potentially be repeated at GHz rates. Such a high switching rate is relevant to future switching and modulation applications.
Optical response and electronic structure of Zn-doped MgAl2O4
Authors: Izumi K. , Mizokawa T. , Hanamura E.
Published: September 2007

We have studied the optical response and the electronic structure of Zn-doped MgAl2O4 using optical transmission, emission, and excitation spectroscopies, x-ray photoemission spectroscopy, and unrestricted Hartree-Fock calculation. Emission lines at 710, 650, and 470nm observed in pure MgAl2O4 are related to the Mg vacancies and Mg–Al antisite defects. Interestingly, the intensities of these emission lines are enhanced by Zn doping. Unrestricted Hartree-Fock calculation for Zn-doped MgAl2O4 shows that in-gap impurity states are formed just above the valence-band maximum of MgAl2O4 when the Zn ion is substituted for the B-site Al ion. On the other hand, no in-gap state is formed when the Zn ion is substituted for the A-site Mg ion. The position of the Zn (3d) impurity level is identified by the photoemission measurement. The broad spectral features of the defect-induced states in pure MgAl2O4 is dramatically reduced by the Zn doping, indicating that holes supplied from the Zn ions at B site are trapped by the defect-induced states.
Triboelectricity evaluation of single toner particle by electron holography
Authors: Okada H. , Shindo D. , Kim J. J. , Murakami Y. , Kawase H.
Published: September 2007

Understanding electrification is particularly important in materials science since the use of charged particles, e.g., the electrophotographic printer with toner particles, is one of the most successful applications of electrification. However, the charge generation mechanism still remains unclear due to the lack of an appropriate method for evaluating individual fine particles. In this study, we describe an approach for determining the charge of a single toner particle that uses electron holography in combination with a shielding technique. Two long-standing problems in holographic studies—namely, perturbation of the reference electron wave and unwanted charging by illumination—have been overcome by introducing two types of shields in a microscope. Using this method, the amount of charge on a single toner particle was determined, and the surface charge distribution was found to be inhomogeneous. Furthermore, an in situ observation of triboelectricity was conducted inside the microscope.
The intermixing and strain effects on electroluminescence of SiGe dots
Authors: Liao M. H. , Lee C.-H. , Hung T. A. , Liu C. W.
Published: September 2007

Secondary-ion mass spectroscopy, energy dispersion spectrometry, and Raman spectroscopy reveal that SiGe dots grown by ultrahigh-vacuum chemical vapor deposition at 600°C exhibit significant intermixing with an average Ge composition of ∼50%. Raman spectroscopy shows the top SiGe quantum dots of the 20-layer sample to be more relaxed than those of the 5-layer samples. As a result, the electroluminescence from the top SiGe quantum dots of the 20-layer sample has the higher peak energy at ∼0.84eV as compared to ∼0.82eV for the 5-layer sample. The external tensile mechanical strain can compensate the built-in compressive strain of SiGe quantum dots and increase electroluminescence energy.
Angle dependence of external and internal quantum efficiencies in bulk-heterojunction organic solar cells
Authors: Dennler Gilles , Forberich Karen , Scharber Markus C. , Brabec Christoph J. , Tomiš Igor , Hingerl Kurt , Fromherz Thomas
Published: September 2007

The realization of highly efficient organic solar cells requires the understanding and the optimization of the light path in the photoactive layer. We present in this article our approach to measure and model the optical properties of our bulk-heterojunction devices, and to control them in order to enhance the photovoltaic performances. We report our recent observations on the dependence of the external quantum efficiency (EQE) on the incidence angle of the light, and our results on the determination of internal quantum efficiency based on EQE measurement and optical modeling cross-checked by reflection measurements. We investigate poly(3-hexylthiophene): 1-(3-methoxy-carbonyl) propyl-1-phenyl[6,6]C61 based solar cells with two different thicknesses of the active layer (170 and 880nm), and show that in the thin ones the absorption is enhanced for oblique incident radiation.
Nitrogen dissociation degree in the diffusion region of a helicon plasma source obtained by atomic lines to molecular band intensities ratio
Authors: Biloiu Costel , Scime Earl E. , Biloiu Ioana A. , Sun Xuan
Published: September 2007

Estimates of the dissociation degree in the diffusion region of a nitrogen helicon plasma source based on optical emission spectroscopy and Langmuir probe measurements are presented. The estimation procedure relies on measurements of the ratios of the intensities of the atomic triplet 3pS04→3sP4 (742.36, 744.23, and 746.83nm) to the intensity of the 4-2 band of the first positive system (AΣu+3→BΠg3) at 750.39nm and the measured relative vibrational distribution of the BΠg3 state. The electron energy distribution function, obtained from the second derivative of the Langmuir probe characteristic, and published excitation cross sections are used to calculate the electron-impact excitation rate coefficients—which are then compared to the atomic line and molecular band intensities to calculate the dissociation degree. For two distinct operating regimes, capacitively and inductively coupled, dissociation fractions of 5% and 13% are obtained in the expansion region of a 10mTorr, 500W, 10.74MHz helicon generated nitrogen plasma. The dissociation degree results suggest that the helicon source can provide excited molecular species for plasma assisted molecular beam epitaxy of III-group nitrides.
Oxygen vacancies induced room temperature ferromagnetism in hydrogenated Mn-doped ZnO
Authors: Sharma V. K. , Varma G. D.
Published: September 2007

We report room temperature ferromagnetism (RTFM) in the hydrogenated bulk samples of Zn0.98Mn0.02O synthesized via standard solid state reaction route. The effect of hydrogenation time (tH) on the structural and magnetic properties of the samples has been explored. Magnetic measurements show that the samples synthesized by sintering in air at ∼800°C exhibit paramagnetic behavior and the same samples on annealing in a gaseous mixture of argon (90%) and hydrogen (10%) at ∼800°C become ferromagnetic at room temperature. Based on the structural and magnetic properties of the samples, it is argued that RTFM in the hydrogenated samples originates from the oxygen vacancies and some defects at Zn sites.
Electron acceleration by a short laser beam in the presence of a long-wavelength electromagnetic wave
Authors: Gupta Devki Nanadan , Kumar Sandeep , Yoon Moohyun , Hur Min Sup , Suk Hyyong
Published: September 2007

A scheme for laser-induced acceleration of an electron injected initially at an angle to the direction of a short-wavelength laser is investigated, where an additional long-wavelength electromagnetic wave is introduced to achieve high energy gain. Due to the beating effect of the electromagnetic waves, the electron can gain additional energy. Some computational results are presented to estimate the electron energy gain by the proposed scheme, where the gain increases by increasing the difference of the wavelengths.
Modeling of cold emission cathode by inclusion of combined field and thermionic emission processes
Authors: He Xin , Scharer John , Booske John , Sengele Sean
Published: September 2007

Emission currents have been measured at elevated temperatures for a periodic copper knife-edge cathode. To model the emission process of the cathode, we have combined thermionic and field emission processes. Electron tunneling is calculated using a transfer matrix method. Using this model, we accurately reproduced the experimental deviation and minimum in the ln(J∕Eg2) vs 1∕Eg plot, where Eg is the gap electric field and J is the emitted current density. This phenomenon has been widely observed but no comprehensive explanation has been put forth. Unambiguous and realistic estimates of the cathode effective field enhancement factor beta averaged (βeff=Esurface∕Egap) and effective work function Φ are obtained from experimental measurements using this model.
Observation of two-dimensional holographic photovoltaic bright solitons in a photorefractive-photovoltaic crystal
Authors: Liu Jinsong , Liu Shixong , Zhang Guangyong , Wang Cheng
Published: September 2007

Two-dimensional holographic photovoltaic bright spatial solitons are observed in a Cu:K0.25Na0.75Sr1.5Ba0.5Nb5O15 crystal in which two coherent laser beams, a signal beam, as well as a strong and uniform pump beam at 532nm are coupled to each other via two-wave mixing. Such solitons originate from the self-trapping of the signal beam due to both photovoltaic effects and holographic focusing effects.
Control of the electrode work function and active layer morphology via surface modification of indium tin oxide for high efficiency organic photovoltaics
Authors: Kim Jong Soo , Park Jong Hwan , Lee Ji Hwang , Jo Jang , Kim Dong-Yu , Cho Kilwon
Published: September 2007

Indium tin oxide (ITO) substrates modified with self-assembled monolayers (SAMs) were used to control the anode work function and active layer morphology of organic solar cells based on poly(3-hexylthiophene)/[6:6]-phenyl-C61 butyric acid methyl ester heterojunctions. By using SAMs with the terminal groups –NH2, –CH3, and –CF3, the authors were able to control the hole injection barrier of the ITO closer to the highest occupied molecular orbital level of active layer and surface energy of the ITO substrate. A solar cell device with CF3 SAM treated ITO was found to exhibit high efficiency performance, about 3.15%.
Electro-optical circuits based on light-sensing ambipolar organic field-effect transistors
Authors: Anthopoulos Thomas D.
Published: September 2007

The author reports on light-sensing ambipolar organic field-effect transistors (LS-OFETs) based on blends of[6,6]-phenyl-C61-butyric acid methyl ester and poly [2-methoxy-5-(3′,7′-dimethyloctyloxy)]-p-phenylene vinylene. By carefully tuning the ambipolar transport character of the LS-OFETs, their photoresponsivity can be controlled and optimized. By combining LS-OFETs with ordinary unipolar organic transistors, fabrication of electro-optical switches and logic circuits, such as NOT and OR gates, is demonstrated. This is the only report of organic electro-optical circuits in which signal processing involves the use of both optical and electrical input signals. Such circuits could one day be explored in optical sensor array applications.
Notes on coherent backscattering from a random potential
Authors: Spohn Herbert
Published: September 2007

We consider the quantum scattering from a random potential of strength λ1∕2 and with a support on the scale of the mean free path, which is of order λ−1. On the basis of maximally crossed diagrams, we provide a concise formula for the backscattering rate in terms of Green’s function for the kinetic Boltzmann equation. We briefly discuss the extension to wave scattering.
Poincaré and sl(2) algebras of order 3
Authors: Goze M. , Rausch de Traubenberg M. , Tanasa A.
Published: September 2007

In this paper, we initiate a general classification for Lie algebras of order 3 and we give all Lie algebras of order 3 based on sl(2,C) and iso(1, 3) the Poincaré algebra in four dimensions. We then set the basis of the theory of the deformations (in the Gerstenhaber sense) and contractions for Lie algebras of order 3.
Convolution theorem for nonlinear optics
Authors: Garcia Hernando , Kalyanaraman Ramki
Published: September 2007

The authors have expressed the nonlinear optical absorption of a semiconductor in terms of its linear spectrum. They determined that the two-photon absorption coefficient in a strong dc electric field of a direct gap semiconductor can be expressed as the product of a differential operator times the convolution integral of the linear absorption without a dc electric field and an Airy function. They have applied this formalism to calculate the two-photon absorption coefficient and nonlinear refraction for GaAs and ZnSe using their linear absorption and have found excellent agreement with available experimental data.
Dynamics of the F2+CH3SCH3 reaction: A molecule-molecule reaction without entrance barrier
Authors: Lu Yu-Ju , Lee Lance , Pan Jun-Wei , Witek Henryk A. , Lin Jim J.
Published: September 2007

The F2+CH3SCH3 reaction was studied with crossed molecular beam techniques and high level ab initio calculations. Significant reactivity was observed even at low collision energies, consistent with the negligible barrier height obtained from the ab initio calculations. All experimental findings are consistent with a weakly bound reaction intermediate of F–F–S(CH3)2 structure, which possesses a special type of three-center four-electron bonding. Analogous intermediates can also explain the reactions of F2 with CH3SH and CH3SSCH3.
A laboratory apparatus for streaming potential and resistivity measurements on soil samples
Authors: Sheffer M. R. , Reppert P. M. , Howie J. A.
Published: September 2007

We describe an apparatus designed to perform streaming potential and resistivity measurements on unconsolidated soil samples. The apparatus enables the use of both unidirectional and oscillatory flow methods to measure the streaming potential coupling coefficient (C); the direct current resistivity method is used to measure the bulk resistivity (ρ) of the soil sample. Measuring both of these parameters on the same sample under the same conditions enables us to properly characterize the streaming current cross-coupling coefficient (L). The apparatus is designed to test reconstituted saturated soil samples up to a maximum grain size of 9.5 mm, and hydraulic gradients from less than 0.1 up to a maximum of 4 m of H2O∕m in flow-through experiments. Excellent agreement between C values measured using the unidirectional and oscillatory flow methods validates the oscillatory flow method for unconsolidated samples.
Calibration of the normal spring constant of microcantilevers in a parallel fluid flow
Authors: Lubarsky Gennady V. , Hähner Georg
Published: September 2007

We demonstrate a novel approach to determine the normal spring constant of microcantilevers. The cantilevers are placed parallel to a fluid flow thus establishing one of the walls of the flow channel. Resonance frequencies are recorded depending on the velocity of the fluid. The pressure gradient resulting from the flow causes the resonance frequency to change. This change can be exploited to deduce the cantilever spring constant with high precision. The method we present can be performed in situ and does not involve any contact of the cantilever with a surface thus having great potential for the calibration of modified probes and for being incorporated in microfluidic systems. In case the spring constant is known, the setup can also be employed to determine the velocity of fluid flows and the flow rate with high precision and up to high speeds.
Surface plasmon increase absorption in polymer photovoltaic cells
Authors: Tvingstedt Kristofer , Persson Nils-Krister , Inganäs Olle , Rahachou Aliaksandr , Zozoulenko Igor V.
Published: September 2007

The authors demonstrate the triggering of surface plasmons at the interface of a metal grating and a photovoltaic bulk heterojunction blend of alternating polyfluorenes and a fullerene derivative. An increased absorption originating from surface plasmon resonances is confirmed by experimental reflection studies and theoretical modeling. Plasmonic resonances are further confirmed to influence the extracted photocurrent from devices. More current is generated at the wavelength position of the plasmon resonance peak. High conductivity polymer electrodes are used to build inverted sandwich structures with top anode and bottom metal grating, facilitating for triggering and characterization of the surface plasmon effects.
Chemical phase transitions of a Si oxide film on SiC by MeV electron beam irradiation
Authors: Jeon C. , Nam J. H. , Song W. , Park C.-Y. , Ahn J. R. , Jung M.-C. , Shin H. J. , Han Y. H. , Lee B. C.
Published: September 2007

An ultrathin Si oxide film grown on a 6H-SiC(0001) wafer was irradiated with 1MeV electron beam to examine its effect on the chemical species of a Si oxide/SiC wafer, where the Si oxide film was composed of SiO2, Si suboxides (Si3+, Si2+, and Si1+), and Si oxycarbides (Si–C–O). Scanning photoelectron microscopy and Si 2p core-level spectroscopy show that e-beam irradiation induces chemical phase transitions from the Si suboxides and Si oxycarbides to SiO2. This suggests that e-beam irradiation is an efficient and simple method of producing a chemically uniform SiO2 film on SiC without thermal and chemical treatments.
Dielectric anisotropy in relaxor ferroelectric Pb0.775La0.15(Zr0.4Ti0.6)O3 ceramics
Authors: Buixaderas E. , Kamba S. , Petzelt J. , Drahokoupil J. , Laufek F. , Kosec M.
Published: September 2007

A huge dielectric anisotropy has been found in Pb0.775La0.15(Zr0.4Ti0.6)O3 (PLZT 15∕40∕60) ceramics by analysis of infrared reflectivity spectra. Anisotropy is present in both the relaxor and the ferroelectric phases. Infrared spectra were evaluated using Bruggeman’s formula within the effective medium approximation. X-ray structural analysis supports a local [110] shift of Pb cations and factor-group analysis taking into account the 4d site symmetry for the effective Pb∕La atom gives more infrared and Raman active phonons than the previous published factor-group analysis, in agreement with our infrared data. Symmetry of all observed polar phonons was assigned.
Study of structural transition from metastable zinc blende to rocksalt crystal in molecular beam epitaxy MgS∕ZnSe∕GaAs multilayer system
Authors: Nasi L. , Bocchi C. , Catellani A. , Germini F. , Morrod J. K. , Prior K. A. , Calestani G.
Published: September 2007

The stable crystal structure of magnesium sulfide (MgS) is rocksalt. However, the metastable zinc-blende structure is obtained when MgS is deposited by molecular beam epitaxy (MBE) on (001) zinc-blende substrates with a relatively small misfit. In the present work, the zinc blende to rocksalt phase transition is analyzed in MgS∕ZnSe∕GaAs multilayer samples grown by MBE with different MgS layer thicknesses. By x-ray diffraction and transmission electron microscopy methods, a partial nucleation of MgS rocksalt is evidenced and correlated to the presence of stacking faults at the bottom interface. The unexpected coexistence of both rocksalt and zinc-blende MgS structural phases is discussed.
Photocontrolled wettability changes in polymer microchannels doped with photochromic molecules
Authors: Caprioli Lucia , Mele Elisa , Angilè Francesco Elio , Girardo Salvatore , Athanassiou Athanassia , Camposeo Andrea , Cingolani Roberto , Pisignano Dario
Published: September 2007

The authors demonstrate the possibility to control the fluid flow inside microfluidic networks by photoresponsive capillaries. The approach relies on the use of photochromic molecules undergoing reversible changes in their polarity when irradiated with light of specific wavelength, thus varying the wettability of cyclic olefin copolymer microchannels. The realized photoresponsive elements exhibit a decrease up to 20° between the water contact angles of the native and the irradiated surfaces, which could be exploited for enhancing the penetration flow rate of fluids inside microfluidic channels up to 25%. The photocontrollable microfluidic circuitry presents on-off valve behavior, allowing or blocking liquid filling processes on the base of optical control, thus allowing one to manipulate liquid flow within microfluidic networks without mechanical actuation parts.
Highly anisotropic Zeeman splittings of wurtzite Cd1−xMnxSe quantum dots
Authors: Zhang Xiu-Wen , Fan Wei-Jun , Chang Kai , Li Shu-Shen , Xia Jian-Bai
Published: September 2007

The electronic structure and Zeeman splittings of wurtzite Cd1−xMnxSe quantum spheres are studied using the k∙p method and mean-field model. It is interesting to find that the Zeeman splittings of some hole states in quantum spheres are highly anisotropic due to the spin-orbit coupling and wurtzite crystal structure. The anisotropy of the Zeeman splittings of hole ground states in large dots is large, while that in small dot is small because the hole ground states vary with radius. An external electrical field can change the Zeeman splitting significantly, and tune the g factor from nearly 0 to about 100.
Effective passivation of highly aluminum-doped p-type silicon surfaces using amorphous silicon
Authors: Bock Robert , Schmidt Jan , Brendel Rolf
Published: September 2007

Highly aluminum-doped p-type emitters prepared by screen printing on crystalline silicon wafers are effectively passivated by plasma-enhanced chemical-vapor deposited amorphous silicon layers. Using the photoconductance decay technique, the authors measure emitter saturation current densities of 800±200fA∕cm2 for nonpassivated emitters and of 490±120fA∕cm2 for Al-p+ emitters passivated with a 20nm thick amorphous silicon layer deposited at 225°C. An additional annealing step at 300°C for 10min reduces the emitter saturation current density down to only 246±60fA∕cm2. The measured saturation current densities are the lowest values achieved so far for Al-doped p+ emitter.
Fabrication of screen-printed field electron emitter using length-controlled and purification-free carbon nanotubes
Authors: Yoshihara Kumiko , Fujii Shunjiro , Kawai Hideyasu , Ishida Kazuhiro , Honda Shin-ichi , Katayama Mitsuhiro , Oura Kenjiro
Published: September 2007

The authors fabricated a screen-printed field electron emitter using purification-free and length-controlled carbon nanotubes (CNTs). They used vertically aligned CNTs grown on Si substrates by thermal chemical vapor deposition as the source material for fabricating CNT paste. The length of CNTs was controlled by adjusting the growth time. The amounts of amorphous carbon and catalyst in the source material were less than 1 and 0.5wt%, respectively, which obviated the need to purify the CNTs. The emitter fabricated using source CNTs with a length of over 80μm showed good reproducibility of current density (J)–electric field (E) characteristics. With a low threshold field Eth of 1.5V∕μm, J=1mA∕cm2 was produced. The emitter exhibited good emission stability for 100h. It was found that the length distribution of the standing CNTs was determined in a precise manner when long CNTs were used as the source material, which led to a highly reproducible fabrication of field emitters.
The role of hydrostatic stress in determining the bandgap of InN epilayers
Authors: Kadir Abdul , Ganguli Tapas , Kumar Ravi , Gokhale M. R. , Shah A. P. , Ghosh Sandip , Arora B. M. , Bhattacharya Arnab
Published: September 2007

This letter establishes a correlation between the internal stress in InN epilayers and their optical properties such as the measured absorption band edge and photoluminescence emission wavelength. By a careful evaluation of the lattice constants of InN epilayers grown on c-plane sapphire substrates under various conditions by metalorganic vapor phase epitaxy, the authors find that the films are under primarily hydrostatic strain. The corresponding stress results in a shift in the band edge to higher energy. The effect is significant and may be responsible for some of the variations in InN bandgap reported in the literature.
In
Ga
As
∕
Al
As
Sb
quantum cascade detectors operating in the near infrared
Authors: Giorgetta F. R. , Baumann E. , Hofstetter D. , Manz C. , Yang Q. , Köhler K. , Graf M.
Published: September 2007

The authors report on short-wavelength In0.53Ga0.47As∕AlAs0.56Sb0.44 quantum cascade detectors (QCDs). At room temperature, one device detects at 505meV (2.46μm) with a responsivity of 2.57mA∕W, while a second QCD is sensitive at 580meV (2.14μm) with a responsivity of 0.32mA∕W.
Study of radial growth rate and size control of silicon nanocrystals in square-wave-modulated silane plasmas
Authors: Nguyen-Tran Th. , Roca i Cabarrocas P. , Patriarche G.
Published: September 2007

The growth of silicon nanocrystals in high pressure and high dilution silane plasmas is investigated by using the temporal evolution of the self-bias on the radio frequency electrode and transmission electron microscopy. A square-wave-modulated plasma was used in order to control the growth of monodispersed nanoparticles with sizes smaller than 12nm. To this end, the plasma on time was kept below 1s. The radial growth rate of nanoparticles was varied in the range from 7.5to75nm∕s by changing silane partial pressure. Nanoparticles grown in silane-helium discharges have been found amorphous while they are crystalline in silane-hydrogen-argon discharges. Surprisingly, the crystallization in the gaseous phase does not depend on how slow or fast the particles grow but on the presence of atomic hydrogen.
Hydrogenated microcrystalline silicon electrodes connected by indium phosphide nanowires
Authors: Kobayashi Nobuhiko P. , VJ Logeeswaran , Saif Islam M. , Li Xuema , Straznicky Joseph , Wang Shih-Yuan , Stanley Williams R. , Chen Yong
Published: September 2007

The authors report the connection of two planar hydrogenated silicon (Si:H) electrodes by intersecting and bridging indium phosphide nanowires (InP NWs). A simple metal-semiconductor-metal photoconductor was used as a test vehicle to measure electrical and optical characteristics of the connected InP NWs. This implementation of III-V compound semiconductor nanowires on Si:H combines the characteristics of a direct bandgap semiconductor with the flexible fabrication processes of non-single-crystal silicon platforms that do not require single-crystal substrates.
Temperature dependence of the interlayer exchange coupling in epitaxial Fe1∕MgO∕Fe2∕Co tunnel junctions
Authors: Popova E. , Keller N. , Gendron F. , Tiusan C. , Schuhl A. , Lesnik N. A.
Published: September 2007

The temperature dependence of the interlayer exchange coupling has been investigated in epitaxial tunnel junctions Fe1∕MgO∕Fe2∕Co∕V with thin MgO layers using X-band ferromagnetic resonance (FMR) in the range 2–300 K. Variations of FMR parameters allow concluding that the coupling strength increases with temperature. This is in agreement with predictions of the theories considering pure tunneling mechanisms and contradicts the model of a resonant assisted tunneling related to defects in the insulator. The temperature dependence of the FMR linewidth shows the line narrowing under the sample heating. This may be due to the additional mechanism associated with the coupling.
Effects of focused ion beam milling on the nanomechanical behavior of a molybdenum-alloy single crystal
Authors: Bei H. , Shim S. , Miller M. K. , Pharr G. M. , George E. P.
Published: September 2007

Nanoindentation was performed on a Mo-alloy single crystal to investigate effects of focused ion beam (FIB) milling on mechanical behavior. On a non-FIB-milled surface, pop-ins were observed on all load-displacement curves corresponding to a transition from elastic to plastic deformation. Similar pop-ins were not detected on surfaces subjected to FIB milling. This difference indicates that FIB milling introduces damage that obviates the need for dislocation nucleation during subsequent deformation. A second effect of FIB milling is that it increased the surface hardness. Together, these effects could be the source of the size effects reported in the literature on micropillar tests.
Deposition of lanthanum zirconium oxide high-κ films by liquid injection atomic layer deposition
Authors: Gaskell J. M. , Jones A. C. , Aspinall H. C. , Taylor S. , Taechakumput P. , Chalker P. R. , Heys P. N. , Odedra R.
Published: September 2007

Thin films of lanthanum zirconium oxide, LaxZr1−xO2−δ (x=0.22,0.35,0.63), have been grown by liquid injection atomic layer deposition using [(PriCp)3La] and [(MeCp)2ZrMe(OMe)] precursors. At lower La atomic fractions (x=0.22) films were stabilized in the cubic phase after annealing at 700°C in air. At higher La atomic fractions (x>0.35), the films remained amorphous after annealing. The films deposited showed good dielectric properties with low hysteresis voltages and negligible flatband voltage shifts. The relative permittivity (κ) ranged from 11 to 14 with leakage current densities at 1MVcm−1 in the range of 2.6×10−6–5.3×10−7Acm−2.
Formation, thermal stability, and surface composition of size-selected AuFe nanoparticles
Authors: Naitabdi A. , Roldan Cuenya B.
Published: September 2007

The surface composition of isolated Au0.5Fe0.5 nanoparticles (NPs) synthesized by micelle encapsulation and supported on TiO2(110) has been investigated. The study reveals that phase-segregated structures are present after annealing at 300°C. A subsequent thermal treatment at 700°C resulted in the formation of a AuFe alloy. At this temperature, a state characteristic of Fe was identified at the NPs’ surface. Annealing at 900°C resulted in the disappearance of the Fe surface state, which is attributed to Au segregation to the surface. The initial hexagonal NP arrangement on the TiO2(110) surface was preserved up to 900°C. At 1000°C, Au desorption was observed.
Observation of exchange bias in the martensitic state of Ni50Mn36Sn14 Heusler alloy
Authors: Li Zhe , Jing Chao , Chen Jiping , Yuan Shujuan , Cao Shixun , Zhang Jincang
Published: September 2007

Exchange bias was observed in the Ni50Mn36Sn14 Heusler alloy after field cooling by means of hysteresis loop measurement. The hysteresis loops shift along the axis of an applied field and its magnitude significantly increased with decreasing temperature below 70K. This effect could be understood as a result of exchange anisotropy created at the interface between an antiferromagnet and a ferromagnet in the phase separated of martensitic state. Above 70K, however, the exchange bias field disappeared and the coercivity significantly reduced owing to the fact that the pinning between an antiferromagnet and a ferromagnet becomes weaker with increasing temperature.
The use of ZnO as optical spacer in polymer solar cells: Theoretical and experimental study
Authors: Gilot Jan , Barbu Ionuţ , Wienk Martijn M. , Janssen René A. J.
Published: September 2007

For organic solar cells, insertion of an optical spacer between the active layer and the reflective electrode results in a redistribution of the optical electric field. In this paper, theoretical calculations using optical modeling are compared with experimental results for devices with ZnO as optical spacer. An excellent agreement was found for blends composed of poly(3-hexylthiophene) and a fullerene derivative [6,6]-phenyl C61 butyric acid methyl ester.
Load induced stresses and plastic deformation in 450mm silicon wafers
Authors: Fischer A. , Kissinger G.
Published: September 2007

The authors present the physical basis for estimation of gravitational constraints in 450mm silicon wafers subjected to high temperature processes. They have identified and quantified the relevant phenomena to predict the mechanical behavior of very large silicon wafers horizontally stacked and ring- or pointlike supported in a vertical-type furnace. It is shown that load induced stress at the supports increases directly proportional with increasing wafer diameter, although the weight of the wafer increases with the square of diameter. The results allow the optimization for a defect-free high temperature treatment of 450mm wafer used for leading edge device fabrication in future.
Coplanar waveguide metamaterials: The role of bandwidth modifying slots
Authors: Ibraheem Ibraheem A. , Koch Martin
Published: September 2007

The authors propose a coplanar waveguide stopband metasurface based on the Babinet principle. The resulting layout is a compact planar metal structure with complementary split ring resonators, which exhibits a high rejection stop band. The complementary rings provide a frequency band with an effective negative dielectric permittivity. Moreover, the rejected bandwidth can be expanded by introducing slots close to the rings. The authors provide a simple physical model which explains the impact of the slots. Simulations confirm the expected behavior and are in excellent agreement with the measurements.
Ultrabroad stimulated emission from quantum-dash laser
Authors: Djie H. S. , Tan C. L. , Ooi B. S. , Hwang J. C. M. , Fang X.-M. , Wu Y. , Fastenau J. M. , Liu W. K. , Dang G. T. , Chang W. H.
Published: September 2007

The authors demonstrate the generation of ultrabroadband stimulated emission in the quasi-zero-dimensional InAs∕InAlGaAs quantum-dash laser grown on InP substrate. The laser exhibits lasing wavelength coverage of up to 76nm at ∼1.64μm from simultaneous multiple confined states lasing at room temperature. Unlike the conventional interband diode laser, the rule changing broadband lasing signature is achieved from the quasicontinuous interband transition formed by the inhomogeneous quantum-dash nanostructure.
White p-i-n organic light-emitting devices with high power efficiency and stable color
Authors: Ho Meng-Huan , Hsu Shih-Feng , Ma Jia-Wei , Hwang Shiao-Wen , Yeh Pu-Cheng , Chen Chin H.
Published: September 2007

Highly efficient p-i-n two-component white organic light-emitting devices have been fabricated with a thin dual emission layer system comprised of one codeposited emitting layer with blue and yellow dyes and one blue emitting layer, which gives rise to a balance white emission. The p-i-n white device achieved an electroluminescence efficiency of 10cd∕A and a power efficiency of 9.3lm∕W at 1000cd∕m2 and a low voltage of 3.4V with a Commission Internationale de l’Eclairage coordinates of (0.32, 0.43). The electroluminescent color of this p-i-n white organic light-emitting diode device has been shown to be immune to drive current density variations.
Output analysis of materials inkjet printer
Authors: Deravi Leila F. , Gerdon Aren E. , Cliffel David E. , Wright David W. , Sumerel Jan L.
Published: September 2007

Piezoelectric inkjet printing was used to deposit single spot microarrays onto Au-coated quartz crystals or SiN3 substrates in air. Composite solutions of nanospheres, carbon nanotubes, and colloidal silver were printed, and their properties were quantitated. The average weight of one spot of each deposited material was determined directly using a quartz crystal microbalance. The calculated weight was used in conjunction with density to estimate the average volume of a single spot of each deposited material. It was shown that inkjet printing reproducibly formed sub-15pl spots, suggesting this deposition method to be highly reproducible for rapid, small-scale fabrication of microarrays.
Coherent magnetization precession in GaMnAs induced by ultrafast optical excitation
Authors: Qi J. , Xu Y. , Tolk N. H. , Liu X. , Furdyna J. K. , Perakis I. E.
Published: September 2007

The authors use femtosecond optical pulses to induce, control, and monitor magnetization precession in ferromagnetic Ga0.965Mn0.035As. At temperatures below ∼40K, they observe coherent oscillations of the local Mn spins, triggered by an ultrafast photoinduced reorientation of the in-plane easy axis. The amplitude saturation of the oscillations above a certain pump intensity indicates that the easy axis remains unchanged above ∼TC∕2. The authors find that the observed magnetization precession damping (Gilbert damping) is strongly dependent on pump laser intensity, but independent of ambient temperature. They provide a physical interpretation of the observed light-induced collective Mn-spin precession and relaxation.
Ultrafast carrier dynamics on conjugated poly(3-hexylthiophene)/[6,6]-phenylC61-butyric acid methyl ester composites
Authors: Lioudakis Emmanouil , Othonos Andreas , Alexandrou Ioannis , Hayashi Yasuhiko
Published: September 2007

The authors have studied ultrafast carrier dynamics on poly(3-hexylthiophene)/[6,6]-phenylC61-butyric acid methyl ester (PCBM) composites up to 50% PCBM concentration. They have resolved the exciton intraband relaxation of composites as well as the subsequent electron/polaron relaxation of dissociated excitons. The observed fast time constant of intraband relaxation is found to be 0.5–1.5ps, whereas the second long-live relaxation (0.5–1ns) is strongly fullerene related. A wavelength dependent ultrafast study is performed giving fundamental information on the nonradiative exciton relaxation, exciton dissociation, and electron relaxation of PCBM-related states.
Prediction of ferroelectricity in BaTiO3∕SrTiO3 superlattices with domains
Authors: Li Y. L. , Hu S. Y. , Tenne D. , Soukiassian A. , Schlom D. G. , Xi X. X. , Choi K. J. , Eom C. B. , Saxena A. , Lookman T. , Jia Q. X. , Chen L. Q.
Published: September 2007

The phase transitions of superlattices into single- and multidomain states were studied using a mesoscale phase-field model incorporating structural inhomogeneity, micromechanics, and electrostatics. While the predictions of transition temperatures of BaTiO3∕SrTiO3 superlattices into multidomains show remarkably good, quantitative agreement with ultraviolet Raman spectroscopic and variable-temperature x-ray diffraction measurements, the single-domain assumption breaks down for superlattices in which the nonferroelectric layer thickness exceeds the characteristic domain size in the ferroelectric layers.