bluuebunny/crossref_metadata_2025 huggingface dataset:
Doping effect of blue light-emitting electron transport molecule in blue organic light-emitting devices
Authors: Kim Youngkyoo , Moon Byoungseok , Ha Chang-Sik
Published: October 2006

Here we report the doping effect of blue light-emitting electron transport molecules on the performance of blue organic light-emitting devices (OLEDs). 4,4′-N,N′-dicarbazole-biphenyl (CBP) was chosen as a blue emission material (layer) for two main reasons: its wide band gap makes it suitable for blue emission; and its shifted energy band structure allows for easy electron injection. However, the main drawback of CBP is the low electron mobility and the large hole injection barrier. In order to overcome these weak points, the CBP nanolayer (50nm thick) in blue OLEDs has been doped with 2-(2-hydroxyphenylbenzoxazole) lithium (LiPBO), which is known to be a blue light-emitting electron transport material. The result showed that the device performance (charge injection voltage, turn-on voltage, and current efficiency) was improved by doping the LiPBO molecule, an improvement that can be attributed to the enhanced electron mobility and the reduced hole injection barrier of the LiPBO-doped CBP nanolayer. However, the blue color purity of OLEDs marginally deteriorated as a result of the LiPBO doping. The worst Commission Internationale de l'Eclairage (CIE) color coordinate of OLEDs was x=0.16 and y=0.18 at 16V, while the standard coordinates are x=0.14 and y=0.08, which is possibly due to the formation of exciplexes in a nanoscale.
Cobalt cluster-assembled thin films deposited by low energy cluster beam deposition: Structural and magnetic investigations of deposited layers
Authors: Dumas-Bouchiat F. , Nagaraja H. S. , Rossignol F. , Champeaux C. , Trolliard G. , Catherinot A. , Givord D.
Published: October 2006

Cobalt cluster-assembled thin films were deposited on amorphous-carbon-coated copper grids and on silicon substrates at room temperature by low energy cluster beam deposition. Characterizations using high-resolution transmission electronic microscopy and atomic force microscopy reveal randomly stacked agglomerates of 9–11nm diameter, which are themselves composed of small 3.6nm diameter fcc cobalt clusters. The films are ferromagnetic up to room temperature and above, which implies that the clusters are exchange coupled. The approach to saturation is analyzed within the random anisotropy model. The values of the exchange coefficient A and the anisotropy constant K then derived are discussed. The temperature dependence of the coercivity below 100K is discussed in terms of thermal activation effects. All results indicate that the fundamental entity governing the magnetic behaviors is constituted by the 9–11nm diameter agglomerates rather than by the clusters themselves.
Tunable high-frequency magnetostatic waves in Thue-Morse antiferromagnetic multilayers
Authors: Zhang X. F. , Peng R. W. , Kang S. S. , Cao L. S. , Zhang R. L. , Wang Mu , Hu A.
Published: October 2006

We theoretically studied the magnetostatic excitation in self-similar antiferromagnetic(AF)/nonmagnetic(NM) multilayers, where the AF and NM layers were arranged in a Thue-Morse sequence. The dispersion relation of magnetostatic spin waves and the precession amplitude of the total magnetization were achieved. It is shown that the distribution of eigenfrequencies possesses two bands of dual structures and each subband presents a hierarchical feature. The states in the finite system can be categorized to three types: critical states in the subband, extended states in the band, and localized surface states in the gaps. The multiformity in frequency spectra leads to the tunable magnetostatic wave, which may have potential applications in designing devices of magnetostatic waves for microwave communications.
Virtual piezoforce microscopy of polycrystalline ferroelectric films
Authors: García R. Edwin , Huey Bryan D. , Blendell John E.
Published: October 2006

An innovative methodology is presented that utilizes the experimental results of electron backscattered diffraction to map the crystallographic orientation of each grain, the finite element method to simulate the local grain-grain interactions, and finally piezoforce microscopy to infer the local properties of polycrystalline ferroelectric materials by comparing the output of the numerical calculation(s) with the experimental results. The proposed combined method resolves the local hysteretic and electromechanical interactions in polycrystalline ferroelectric films, thus quantifying the effects of grain corners and boundaries on the polycrystal’s macroscopic response. For a polycrystalline lead zirconate titanate sample, a finite range of crystallographic orientations and epitaxial strains is found to enhance the out-of-plane electrical response of the film with respect to its single-crystal, stress-free counterpart. Results show that {111} oriented grains parallel to the normal of the surface of the film yield the largest polarization magnitude enhancement, compressive stresses, and built-in electric fields, as well as an asymmetry in the quasistatic coercive field. In the absence of epitaxial strains, {001} oriented grains will be enhanced in their out-of-plane hysteretic response through the in-plane compressive stresses provided by the local neighboring grains. For the studied sample, grain corners and boundaries become favorable sites for pinning or nucleation of ferroelectric domains, depending on the local state of stress and polarization.
Trimodal island distribution of Ge nanodots on (001)Si
Authors: Balle Jacob , Hansen John Lundsgaard , Larsen Arne Nylandsted
Published: October 2006

Molecular beam epitaxy (MBE) grown Ge nanodots are found to come in a clear trimodal island distribution of huts, pyramids, and domes when grown on (001)Si at 550°C. The island types appear in this order as Ge coverage increases and for a certain coverage all three types are found to coexist at this growth temperature. Previously Ge nanodots have mostly been divided into huts and domes at growth temperatures below 600°C, or pyramids and domes above 600°C. The {105} faceted pyramidal and elongated huts and the multifaceted domes are well known, but a distinction has not previously been seen between huts and a separate size distribution of similarly {105}-faceted pyramidal nanodots twice the size of huts, at temperatures below 600°C. The 20–25nm wide huts also appear to be the smallest obtainable self-assembled Ge dots on (001)Si, in accordance with predictions based on Si1−xGex nanodots on (001)Si. They are about a factor of two too large for quantum dot applications.
High temperature dc field poling effects on the structural phase transformations of (1−x)Pb(Mg1∕3Nb2∕3)O3–xPbTiO3 single crystal with morphotropic phase boundary composition
Authors: Shabbir Ghulam , Kojima Seiji , Feng Chude
Published: October 2006

The complex dielectric permittivity of the morphotropic phase boundary [001]-oriented PMN–x%PT single crystal with x=33%, grown by the modified Bridgman technique, has been investigated as a function of both temperature and dc poling field. Structural phase transformation sequence is discussed in the light of polarization rotation process. A remarkable shift in the rhombohedral-tetragonal phase transition temperature is observed with changing of the poling field. It is found that high temperature poled state remained stable even after removal of the field.
Reverse tilt domains in liquid crystal cells with a splayed director configuration
Authors: Lee Seo Hern , Yoon Tae-Hoon , Kim Jae Chang , Lee Gi-Dong
Published: October 2006

When an electric field is applied to a nematic liquid crystal cell with splay alignment layers, which is realized by mechanical parallel rubbing, the system exhibits abnormal reverse tilt domains. Even though the system has pretilt angle due to symmetry breaking by rubbing, existence of singular point in a cell by splay geometry and shift of that by locally asymmetric pretilt angle configurations result in the abnormal reverse tilt domains. On the basis of Oseen-Frank theory, dependence of singular point upon the abnormal reverse tilt domains is theoretically investigated for a splay alignment liquid crystal cell. By using the Dickman’s Q-tensor method [SID Int. Symp. Digest Tech. Papers 24, 638 (1993)], liquid crystal director field is calculated for the splay-aligned liquid crystal cell that an electric field is applied to. We presume a model to explain the abnormal reverse tilt domains, and derive the conditions required for the generation and elimination of these domains.
Stoichiometry and phase purity of Pb(Zr,Ti)O3 thin films deposited by metal organic chemical vapor deposition
Authors: Aggarwal S. , Udayakumar K. R. , Rodriguez J. A.
Published: October 2006

( Pb , Zr ) Ti O 3 (PZT) films have been prepared by metal organic chemical vapor deposition on 200mm wafers. Phase pure perovskite films were deposited in a self-correcting region where the Pb stoichiometry is relatively insensitive to increasing Pb content in the gas phase. Films deposited with Pb flows lower than those used in the self-correcting region showed second phase ZrO2 whereas films deposited at Pb flows higher than those used in the self-correcting region showed second phase PbO. The PZT grains are columnar, extending from the bottom electrode to the top electrode. In the self-correcting region, PZT films of 70nm nominal thickness show good ferroelectric behavior with switched polarization of ∼40μC∕cm2 at 1.5V and saturation voltage of ∼1.2V. The films have an average roughness of ∼4nm with grain size of ∼700Å. The impact of the deposition parameters such as deposition temperature, pressure, precursor flow, and oxygen flow during deposition on the self-correcting region was investigated. Increasing the deposition temperature increases the width of the self-correcting region whereas increasing the oxygen flow narrows it. Pressure and the precursor flow do not impact the width although variation in pressure does shift the location of the self-correcting region.
Scalar dispersion by a large-eddy simulation and a Lagrangian stochastic subgrid model
Authors: Wei Guoxin , Vinkovic Ivana , Shao Liang , Simoëns Serge
Published: October 2006

A hybrid Eulerian-Lagrangian large-eddy simulation (LES) is used to compute scalar dispersion in a turbulent flow. Instead of resolving the passive scalar transport equation, fluid particles are tracked in a Lagrangian way. In order to obtain the subgrid scale velocity component of fluid particles, a Lagrangian stochastic subgrid model is coupled with the Eulerian LES. The Lagrangian stochastic subgrid model is written in terms of subgrid scale statistics. The coupling is applied to the study of turbulent scalar dispersion. The results of our simulation are compared with a direct numerical simulation and with experimental results.
Circulation rate of change: A vortex approach for understanding accelerated inhomogeneous flows through intermediate times
Authors: Lee Dong-Kee , Peng Gaozhu , Zabusky Norman J.
Published: October 2006

We present a new simple diagnostic formula, Eq. (4) in this paper, involving a pressure difference, for determining the rate-of-change of total circulation, Γ̇D (within a domain D) for accelerated inhomogeneous flows, e.g., as may occur in Rayleigh-Taylor (RT) and Richtmyer-Meshkov (RM) environments. We examine the RM, two-dimensional, shock-accelerated (Mach number M=1.3), single-mode, inviscid environment with an initially finite-width interfacial transition layer (ITL), and various postshock Atwood numbers, 0.2≤A*≤0.75. We show that for intermediate times (up to t∕tM<8.0), Γ̇D∕(Γshock∕tM), has several consistent behaviors and scaling laws depending on A*. Here Γshock, is the circulation deposited by the initial shock on the region of finite density gradient and tM is the time at which the single mode interfacial midcontour becomes multivalued.
Self-assembled In0.22Ga0.78As quantum dots grown on metamorphic GaAs∕Ge∕SixGe1−x∕Si substrate
Authors: Hsieh Y. C. , Chang E. Y. , Luo G. L. , Chen S. H. , Biswas Dhrubes , Wang S. Y. , Chang C. Y.
Published: October 2006

Self-assembled In0.22Ga0.78As quantum dots (QDs) grown on Si substrate with Ge∕SiGe as buffer layer grown by metal organic vapor phase epitaxy were investigated. Transmission electron microscopy and atomic force microscopy images were used to observe the size and space distribution of the In0.22Ga0.78As QDs grown on the GaAs∕Ge∕GeSi∕Si layer structure. The influence of the growth temperature on the QDs size and density distribution was investigated. For QDs grown at 450°C, the density of the In0.22Ga0.78As dots was estimated to be 1×1011cm−2 and the In0.22Ga0.78As QDs thickness was 5 ML (monolayer) thick.
Analysis of wafer stresses during millisecond thermal processing
Authors: Smith M. P. , Seffen K. A. , McMahon R. A. , Voelskow M. , Skorupa W.
Published: October 2006

A flash lamp has been used to uniformly anneal large wafers with diameters approaching 100mm. The equipment applies a pulse, with duration of 3–20ms, resulting in large transient thermal gradients in the wafer. In this paper, we present separate models of the thermal reaction of this process and its effect upon the mechanical behavior, in order to predict stresses and shape changes, and to capture practical phenomenon. We further use the model to follow changes in the expected response consequent on altering process conditions, such as preheating and pulse duration, as well as exploring important issues associated with scaling to large wafer sizes. This work presents an initial description of the thermomechanical response of wafers to flash lamp annealing in the millisecond time regime and is therefore fundamental to the use of this technique in the fabrication of semiconductor devices.
Growth of TiN∕GaN metal/semiconductor multilayers by reactive pulsed laser deposition
Authors: Rawat Vijay , Sands Timothy
Published: October 2006

Ti N ∕ Ga N metal/semiconductor multilayers were grown by reactive pulsed laser deposition in an ammonia ambient on sapphire and MgO substrates for potential application in solid-state thermionic direct energy conversion devices. Crystallographic analysis of the multilayers by high-resolution x-ray diffraction and cross-sectional transmission electron microscopy revealed that, despite the difference in the crystal structures of rocksalt TiN and wurtzite GaN, it is possible to grow thick (micron scale) uniaxially textured columnar-grained multilayers with nanoscale periods and without agglomeration. X-ray scattering suggests that epitaxial growth of TiN∕GaN multilayers on (100) MgO substrates stabilizes ultrathin (1–2nm) GaN layers in the high-pressure rocksalt polymorph yielding (100) oriented rocksalt TiN∕GaN superlattices. The challenges in growth and the chemical and morphological stability of lattice- and structure-mismatched multilayers are discussed on the basis of kinetic and thermodynamic factors.
Transverse thermal conductivity of porous materials made from aligned nano- and microcylindrical pores
Authors: Prasher Ravi
Published: October 2006

Nanoporous and microporous materials made from aligned cylindrical pores play important roles in present technologies and will play even bigger roles in future technologies. The insight into the phonon thermal conductivity of these materials is important and relevant in many technologies and applications. Since the mean free path of phonons can be comparable to the pore size and interpore distance, diffusion-approximation based effective medium models cannot be used to predict the thermal conductivity of these materials. Strictly speaking, the Boltzmann transport equation (BTE) must be solved to capture the ballistic nature of thermal transport; however, solving BTE in such a complex network of pores is impractical. As an alternative, we propose an approximate ballistic-diffusive microscopic effective medium model for predicting the thermal conductivity of phonons in two-dimensional nanoporous and microporous materials made from aligned cylindrical pores. The model captures the size effects due to the pore diameter and the interpore distance and reduces to diffusion-approximation based models for macroporous materials. The results are in good agreement with experimental data.
Effect of dislocations on minority carrier diffusion length in practical silicon solar cells
Authors: Kieliba Thomas , Riepe Stephan , Warta Wilhelm
Published: October 2006

In 1998, Donolato presented an analytical model describing the effect of dislocation density on minority carrier effective diffusion length [J. Appl. Phys. 84, 2656 (1998)]. While this analysis was derived for a “semi-infinite” specimen, our objective is the appropriate description of thin devices, such as wafer or thin-film based crystalline Si solar cells with back surface field or passivating layer on the rear side. Therefore, Donolato’s model is extended for specimen of finite thickness and finite recombination velocity at the back surface. Since the associated boundary value problem does not allow a straightforward analytical solution, we derive an approximate expression, which is validated by numerical simulations. In the original work, Donolato uses a special definition of an “effective diffusion length.” This definition is different from the quantity usually referred to as effective diffusion length when analyzing quantum efficiency data. Furthermore, Donolato’s definition does not refer to the typical operation conditions of a solar cell. We therefore modify Donolato’s model for the effect of dislocations consistently using the quantum efficiency effective diffusion length. Finally, our model is applied to the determination of dislocation recombination strength in thin-film solar cells with back surface field from effective diffusion length maps.
Physics considerations in the design of three-dimensional and multilevel magnetic recording
Authors: Khizroev S. , Hijazi Y. , Amos N. , Chomko R. , Litvinov D.
Published: October 2006

The purpose of this paper is to explore three-dimensional magnetic recording as a next generation recording technology. To defer the superparamagnetic limit in magnetic recording substantially beyond the 1Tbit∕in.2 mark, it is proposed to stack magnetic bits in a third (vertical) dimension. The vertical stacking underlies the concept of three-dimensional (3D) magnetic memory and recording—the primary subject of this paper. A clear distinction between absolute 3D memory and its trivial multilevel implementation is drawn. The paper focuses on the study of the media design and write and read processes. To minimize the intersymbol interference and improve stability, it is proposed to pattern the recording media in all three dimensions. Basic Co∕Pd-based 3D recording media necessary for this study are fabricated using cosputter deposition. Focused-ion-beam-based fabrication is used to pattern the recording media into nanoscale bit cells. The physics of 3D magnetic recording is also investigated theoretically with Landau-Lifshits-Gilbert-based micromagnetic modeling. The ultimate goal of this paper is to help understand the physics of 3D and multilevel magnetic recordings and trigger wide interest in the studied concept.
Spintronics for electrical measurement of light polarization
Authors: Dery H. , Cywiński Ł. , Sham L. J.
Published: October 2006

The helicity of a circularly polarized light beam may be determined by the spin direction of photoexcited electrons in a III-V semiconductor. We present a theoretical demonstration on how the direction of the ensuing electron spin polarization may be determined by electrical means of two ferromagnet/semiconductor Schottky barriers. The proposed scheme allows for the time-resolved detection of spin accumulation in small structures and may have a device application.
Hopping conductivity in p-CuGaSe2 films
Authors: Arushanov E. , Siebentritt S. , Schedel-Niedrig T. , Lux-Steiner M. Ch.
Published: October 2006

The results of resistivity measurements on p-type CuGaSe2 films are presented and analyzed within the framework of different hopping conductivity models. Both the Mott [N. Mott and E. A. Davies, Electron Processes in Non-Crystalline Materials (Clarendon, Oxford, 1979); N. F. Mott, Metal-Insulator Transitions (Taylor & Francis, London, 1990)] and the Shklovski-Efros [Electronic Properties of Doped Semiconductors (Springer, Berlin, 1984)] regimes of variable-range hopping are observed. The values of the characteristic and transition temperatures as well as the complete set of parameters describing the properties of the localized holes (the localization radius, the dielectric permeability, the width of the Coulomb gap, and the values of density of states at the Fermi level) are determined.
Growth of p-type and n-type m-plane GaN by molecular beam epitaxy
Authors: McLaurin M. , Mates T. E. , Wu F. , Speck J. S.
Published: October 2006

Plasma-assisted molecular beam epitaxial growth of Mg-doped, p-type and Si-doped, n-type m-plane GaN on 6H m-plane SiC is demonstrated. Phase-pure, m-plane GaN films exhibiting a large anisotropy in film mosaic (∼0.2° full width at half maximum, x-ray rocking curve scan taken parallel to [112¯0] versus ∼2° parallel to [0001]) were grown on m-plane SiC substrates. Maximum hole concentrations of ∼7×1018cm−3 were achieved with p-type conductivities as high as ∼5Ω−1cm−1 without the presence of Mg-rich inclusions or inversion domains as viewed by cross-section transmission electron microscopy. Temperature dependent Hall effect measurements indicate that the Mg-related acceptor state in m-plane GaN is the same as that exhibited in c-plane GaN. Free electron concentrations as high as ∼4×1018cm−3 were measured in the Si-doped m-plane GaN with corresponding mobilities of ∼500cm2∕Vs measured parallel to the [112¯0] direction.
Luminescence characteristics of sol-gel derived Y3Al5O12:Eu3+ phosphors excited with vacuum ultraviolet
Authors: Lu Chung-Hsin , Hsu Wei-Tse , Cheng Bing-Ming
Published: October 2006

The luminescence characteristics of europium-ion activated Y3Al5O12 (YAG:Eu3+) phosphors prepared via a sol-gel process were investigated in the vacuum ultraviolet (VUV) region using synchrotron radiation. An intense red emission due to D05→F17 transition of Eu3+ ions is produced under excitation at 147nm. The intensity of this emission significantly increases with calcination temperature and activator concentration. Deconvolution of the VUV excitation spectra reveals the presence of several peaks that can be accordingly assigned to the charge transfer band, exciton transition bands, and 4f6→4f5-5d transition band. The excitation spectra of YAG:Eu3+ phosphors vary markedly with the phosphor particle size. With a decrease in the particle size, the peak positions of the charge transfer band and exciton transition bands shift towards low energy region, and the f→d transition band of Eu3+ ions shift towards the high energy region. The results imply that the excitation behavior of the sol-gel derived YAG:Eu3+ phosphors can be tailored by controlling the calcination temperature and the particle size.
On characterization of reversible Markov processes by monotonicity of the fluctuation spectral density
Authors: Chen Yong , Qian Min-Ping , Xie Jian-Sheng
Published: October 2006

For the system driven by a stationary Markov process {ξt:t∊R+}, if it is in nonequilibrium steady state (i.e., irreversible), then there exists a function φ such that the fluctuation spectrum (or say: power spectrum density) of {φ(ξt)} is nonmonotonic in [0,+∞) under mild conditions, which means that there exist nonzero spectrum peaks of the fluctuation spectrum. For the system driven by a Markov chain with discrete time {ξt:t∊Z+}, even if it is in equilibrium state (i.e., reversible), one cannot distinguish the equilibrium and nonequilibrium steady state in terms of the monotonicity of the fluctuation spectrum any more.
A general elastic-anisotropy measure
Authors: Ledbetter Hassel , Migliori Albert
Published: October 2006

We propose an elastic-anisotropy measure. Zener’s familiar anisotropy index A=2C44∕(C11−C12) applies only to cubic symmetry [Elasticity and Anelasticity of Metals (University of Chicago Press, Chicago, 1948), p. 16]. Its extension to hexagonal symmetry creates ambiguities. Extension to orthorhombic (or lower) symmetries becomes meaningless because C11−C12 loses physical meaning. We define elastic anisotropy as the squared ratio of the maximum/minimum shear-wave velocity. We compute the extrema velocities from the Christoffel equations [M. Musgrave, Crystal Acoustics (Holden-Day, San Francisco, 1970), p. 84]. The measure is unambiguous, applies to all crystal symmetries (cubic-triclinic), and reduces to Zener’s definition in the cubic-symmetry limit. The measure permits comparisons between and among different crystal symmetries, say, in allotropic transformations or in a homologous series. It gives meaning to previously unanswerable questions such as the following: is zinc (hexagonal) more or less anisotropic than copper (cubic)? is alpha-uranium (orthorhombic) more or less anisotropic than delta-plutonium (cubic)? The most interesting finding is that close-packed-hexagonal elements show an anisotropy near 1.3, about half that of their close-packed-cubic counterparts. A central-force near-neighbor model supports this finding.
Strain effects in ZnO layers deposited on 6H-SiC
Authors: Ashrafi A. B. M. A. , Segawa Y. , Shin K. , Yao T.
Published: October 2006

Correlation in crystallite sizes and defects of epitaxial ZnO layers deposited on 6H-SiC substrates has been addressed. The biaxial strain governs the ZnO crystallites for the layer thickness of ∼400nm. The misfit dislocations were observed in nucleation and theater is the columnar growth mode diffracted in transmission electron microscopy. The columnar growth mode is a symbol of stacking faults that appear due to imbalanced interface chemistry in the II-VI/IV materials system, together with the complex impurity matrix. These defects are the main source of nonradiative recombination centers in ZnO epitaxy resulting in shorter exciton lifetimes examined in time-resolved photoluminescence measurements.
Intermixing at the heterointerface between ZnS∕Zn(S,O) bilayer buffer and CuInS2 thin film solar cell absorber
Authors: Bär M. , Ennaoui A. , Klaer J. , Kropp T. , Sáez-Araoz R. , Lehmann S. , Grimm A. , Lauermann I. , Loreck Ch. , Sokoll St. , Schock H.-W. , Fischer Ch.-H. , Lux-Steiner M. C. , Jung Ch.
Published: October 2006

The application of Zn compounds as buffer layers was recently extended to wide-gap CuInS2 (CIS) based thin-film solar cells. Using an alternative chemical deposition route for the buffer preparation aiming at the deposition of a single-layer, nominal ZnS buffer without the need for any toxic reactants such as hydrazine has helped us to achieve a similar efficiency as respective CdS-buffered reference devices. After identifying the deposited Zn compound, as ZnS∕Zn(S,O) bilayer buffer in former investigations [M. Bär et al., J. Appl. Phys. 99, 123503 (2006)], this time the focus lies on potential diffusion/intermixing processes at the buffer/absorber interface possibly, clarifying the effect of the heat treatment, which drastically enhances the device performance of respective final solar cells. The interface formation was investigated by x-ray photoelectron and x-ray excited Auger electron spectroscopy. In addition, photoelectron spectroscopy (PES) measurements were also conducted using tunable monochromatized synchrotron radiation in order to gain depth-resolved information. The buffer side of the buffer/absorber heterointerface was investigated by means of the characterization of Zn(S,O)∕ZnS∕CIS structures where the ZnS∕Zn(S,O) bilayer buffer was deposited successively by different deposition times. In order to make the (in terms of PES information depth) deeply buried absorber side of the buffer/absorber heterointerface accessible for characterization, in these cases the buffer layer was etched away by dilute HClaq. We found indications that while (out-leached) Cu from the absorber layer forms together with the educts in the chemical bath a [Zn(1−Z),Cu2Z]S-like interlayer between buffer and absorber, Zn is incorporated in the uppermost region of the absorber. Both effects are strongly enhanced by postannealing the Zn(S,O)∕ZnS∕CIS samples. However, it was determined that the major fraction of the Cu and Zn can be found quite close to the heterointerface in the buffer and absorber layer, respectively. Due to this limited (in the range of one monolayer) spatial extent, these “diffusion” mechanisms were rather interpreted as a chemical bath deposition induced and heat-treatment promoted Cu-Zn ion exchange at the buffer/absorber interface. Possible impacts of this intermixing on the performance of the final solar cell devices will also be discussed.
Generation-recombination noise in forward biased 4H-SiC p-n diodes
Authors: Rumyantsev Sergey L. , Dmitriev Alexander P. , Levinshtein Michael E. , Veksler Dmitry , Shur Michael S. , Palmour John W. , Das Mrinal K. , Hull Brett A.
Published: October 2006

Low frequency noise has been studied in forward biased 4H-SiC p+-n diodes at current densities from 10−4to10A∕cm2. At small current densities j⩽10−3A∕cm2, the spectral noise density SI follows the law SI∝1∕f3∕2. At 10−3A∕cm2 <j<10−2A∕cm2, the generation-recombination (GR) noise predominates. The amplitude of this GR noise nonmonotonically depends on current. At j⩾10−2A∕cm2, the 1∕f (flicker noise) dominates. It has been shown that the recombination time in the space charge region of the p+-n junction, τR, is about 70ns. This value is approximately one order of magnitude larger than that reported earlier for SiC p-n structures. A model of GR noise in forward biased p-n junctions has been proposed. The model links the GR noise with fluctuations of the charge state of a trap in the space charge region.
Investigation of stress-induced defects in shallow trench isolation by cathodoluminescence and Raman spectroscopies
Authors: Sugie R. , Matsuda K. , Ajioka T. , Yoshikawa M. , Mizukoshi T. , Shibusawa K. , Yo S.
Published: October 2006

We have applied cathodoluminescence (CL) and Raman spectroscopies to shallow trench isolation (STI) processes in large scale integration to investigate crystalline defects and stresses by extracting the wafer after each process. A sample with high standby leakage current clearly showed dislocation-related luminescence lines (D lines) in the regions surrounding the memory cell. These regions contain various sizes of active and field areas ranging from 0.5to5μm. However, a normal sample showed no D lines in any region through the STI processes. The D lines were observed after liner oxidation, which was performed after trench patterning. Raman microprobe measurements showed that the trench patterning process generated a large inhomogeneous stress distribution up to 200MPa in the regions surrounding the memory cell. The inhomogeneous stress distributions caused by the trench patterning and the subsequent thermal processes are likely the main causes of dislocations. A comparison between the D lines in the CL spectra and the stress measured by a Raman microprobe clarified the mechanism of generation of crystalline defects. We can thus optimize the process conditions in STI processes by these spectroscopic methods.
Complex dynamic behaviors of nonequilibrium atmospheric dielectric-barrier discharges
Authors: Zhang Yuan Tao , Wang De Zhen , Kong M. G.
Published: October 2006

In this paper, a one-dimensional fluid model is used to investigate complex dynamic behaviors of a nonequilibrium dielectric-barrier discharge (DBD) in atmospheric helium. By projecting its evolution trajectory in the three-dimensional phase space of gas voltage, discharge current density, and electrode-surface charge density, the atmospheric DBD is shown to undergo a sequence of complex bifurcation processes when the applied voltage is increased from prebreakdown to many times of the breakdown voltage. Once the gas voltage exceeds the breakdown voltage, the discharge plasma is found to acquire negative differential conductivity and as a result its stability is compromised. For atmospheric DBD, however, the resulting low plasma stability is mitigated by a rapid accumulation of surface charges on the electrodes, thus allowing the atmospheric DBD to retain their character as a glow discharge. At certain values of the applied voltage, a highly complex phenomenon of period multiplication is observed in which the period of the discharge current is three times that of the applied voltage. This suggests that nonequilibrium atmospheric DBD may support evolution patterns that are quasiperiodic or even chaotic. These complex dynamic behaviors are likely to be critical to a full understanding of plasma stability of nonequilibrium atmospheric discharges and to the development of their instability control strategies.
Holographic capture of femtosecond pulse propagation
Authors: Centurion Martin , Pu Ye , Psaltis Demetri
Published: October 2006

We have implemented a holographic system to study the propagation of femtosecond laser pulses with high temporal (150fs) and spatial resolutions (4μm). The phase information in the holograms allows us to reconstruct both positive and negative index changes due to the Kerr nonlinearity (positive) and plasma formation (negative), and to reconstruct three-dimensional structure. Dramatic differences were observed in the interaction of focused femtosecond pulses with air, water, and carbon disulfide. The air becomes ionized in the focal region, while in water long plasma filaments appear before the light reaches a tight focus. In contrast, in carbon disulfide the optical beam breaks up into multiple filaments but no plasma is measured. We explain these different propagation regimes in terms of the different nonlinear material properties.
Influence of ambient atmosphere on metal-insulator transition of strained vanadium dioxide ultrathin films
Authors: Nagashima Kazuki , Yanagida Takeshi , Tanaka Hidekazu , Kawai Tomoji
Published: October 2006

The effect of ambient atmosphere on metal-to-insulator transition (MIT) in strained vanadium dioxide (VO2) ultrathin films (7–8nm) grown epitaxially on TiO2 (001) single crystal substrate by pulsed laser deposition was investigated by varying the ambient oxygen pressure and substrate temperature with the intention being to control arbitrarily the MIT events of strained VO2 ultrathin films, including the MIT temperature and the resistivity change. When reducing the ambient oxygen pressure, the MIT temperature remained almost constant, whereas the change of resistivity during MIT tended to be smaller. Contrary, varying the substrate temperature resulted in the variation of the MIT temperature with keeping the abruptness of transition. The lower the substrate temperature, the lower the MIT temperature, and in addition the MIT temperature ∼290K with keeping the abrupt change of resistivity is found to be the lowest compared with previously reported values. The reduction of the MIT temperature was found to correlate with shortening c-axis length when decreasing the substrate temperature, indicating the significant strain effects. Thus these results highlight that controlling appropriately the ambient atmosphere enables us to modulate arbitrarily the MIT events of strained VO2 thin films near room temperature.
Atomic-level study of melting behavior of GaN nanotubes
Authors: Wang Zhiguo , Zu Xiaotao , Gao Fei , Weber William J.
Published: October 2006

Molecular dynamics simulations with a Stillinger-Weber potential have been used to investigate the melting behavior of wurtzite-type single-crystalline GaN nanotubes. The simulations show that the melting temperature of the GaN nanotubes increases with the thickness of the nanotubes to a saturation value, which is close to the melting temperature of a GaN slab. The results reveal that the nanotubes begin to melt at the surface, and then the melting rapidly extends to the interior of the nanotubes as the temperature increases. The melting temperature of a single-crystalline GaN nanotube with [100]-oriented lateral facets is higher than that with [110]-oriented lateral facets for the same thickness.
Surface plasmon coupling in hexagonal textured metallic microcavity
Authors: Tam H. L. , Li K. F. , Cheah K. W. , Xia J. B. , Huber R. , Wong W. H. , Pun Y. B.
Published: October 2006

The coupling of surface plasmons to the photonic modes in hexagonal textured metallic microcavity was studied. The modified photonic modes enable efficient coupling with the luminescence source in the microcavity. Hexagonal photonic crystal lattice has higher folding symmetry providing more channels for surface plasmon coupling in different in-plane directions, i.e., more isotropic light extraction profile than one—or two-dimensional gratings. Results show that strong coupling between surface plasmon modes and the waveguide mode in the microcavity has led to angle-selective enhanced light extraction and it was as much as 12 times more light extracted compare to planar microcavity.
Magnetic properties and structure of (Co–Ni)3Pt alloy films
Authors: Lauhoff G. , Suzuki T. , Toporov A.
Published: October 2006

A study of the magnetic and structural properties of (Co–Ni)3Pt alloy thin films epitaxially grown onto Al2O3(00∙1) substrates has been carried out. The (CoNi)3Pt and Ni3Pt films deposited in a temperature range of 300–450°C show a partial superlattice ordering. The perpendicular magnetic anisotropy Ku is found to increase with order parameter S. For deposition temperatures of about TS=400°C, the Ku becomes 8×105erg∕cc at S=0.3 for Ni3Pt and 4×106erg∕cc for (CoNi)3Pt at S=0.4 compared to 2×107erg∕cc at S=0.7 for Co3Pt films.
Josephson current in a ferromagnetic semiconductor/semiconductor/ferromagnetic semiconductor junction with superconducting contacts
Authors: Tao Y. C.
Published: October 2006

On the basis of a general expression for dc Josephson current, we study the Josephson current flowing through a ferromagnetic semiconductor (FS)/semiconductor (SM)/FS trilayer connected to two superconducting electrodes. It is demonstrated that the Josephson current in the junction strongly depends not only on the kinds of holes (the heavy or light) in the FS’s and scattering potential strengths at the interfaces but also on the relative orientations of the effective exchange field h of the two FS’s. We find that in the coherent regime, the oscillatory dependences of the critical Josephson current IC on the FS and SM layer thicknesses and of the Josephson current on the macroscopic phase difference ϕ for the heavy and light holes in different orientations of the effective exchange field h of the two FS’s are much different due to the different mismatches in the effective mass and Fermi velocity between the FS and superconductor, which is related to the transition from positive (0) to negative (π) coupling and can be exploited for the design of devices such as π superconducting quantum interference devices with improved accuracy and convenience.
The response of the intermetallic compound Ni3Al to one-dimensional shock loading
Authors: Millett J. C. F. , Meziere Y. J. E. , Gray G. T. , Cerreta E. K. , Bourne N. K.
Published: October 2006

The shock response of the intermetallic compound Ni3Al (doped with boron to enhance ductility) has been investigated to determine equation of state and shock induced mechanical properties, and the response is compared to that of pure nickel. Examination of the Hugoniot data suggests that the Ni3Al has a higher compressibility than pure nickel. Elastic precursor decay has been observed, although due to the limited number of specimens available, we were not able to determine the point where the elastic amplitude became stable. Spall and shear strengths were observed to increase to a peak value (at a shock stress of between 6 and 7GPa) before dropping again. We believe that this may be due to increasing amounts of damage behind the shock front as shocks of increased stress are applied. In a similar manner, spall strength was observed to decrease with pulse duration, again, most likely due to damage accumulating behind the shock front. Finally, it was observed that this material was brittle under all shock loading conditions studied in this investigation. We have proposed, due to the extremely high dislocation velocities imposed by the shock induced strain rates, that boron has effectively been removed from dislocations and hence cannot enhance ductility.
Ferromagnetism in inhomogeneous Zn1−xCoxO thin films
Authors: Tay Maureen , Wu Yihong , Han Gu Chang , Chong Tow Chong , Zheng Yuan Kai , Wang Shi Jie , Chen Yanbin , Pan Xiaoqing
Published: October 2006

We report on a systematic study of structural, optical, electrical, and magnetic properties of Zn1−xCoxO (x=0.05–0.29) thin films codoped with Al (<0.1%). Both codoped (in which Co is cosputtered with other elements) and δ-doped (in which Co is doped digitally in the host matrix) samples have been prepared and studied. Prior to doping of Co, growth conditions were optimized to produce ZnO:Al films with a resistivity of about 1.3mΩcm. Although all the films with x in the range of 0.05–0.29 showed clear hysteresis at room temperature in magnetometry measurement and absorption peaks associated with the d-d transitions of Co2+ ions, only the most heavily doped samples have shown clear anomalous Hall effect. The latter also showed strong, but photon energy dependent, magnetic circular dichroism and negative magnetoresistance at room temperature. These results in combination with detailed structural analysis by transmission electron microscope and x-ray diffraction study revealed that the ferromagnetic properties of Zn1−xCoxO were mostly originated from secondary phases and Co precipitates. The influence of inhomogeneity on the interpretation of various measurement results is also discussed.
Blue-green small-signal gain and saturation in a luminescent polymer gain medium
Authors: Kobayashi Takeyuki , Flämmich Michael , Jordan Grace , D’Arcy Rebekah , Rüther Manuel , Blau Werner J. , Suzuki Yasuhiro , Kaino Toshikuni
Published: October 2006

The authors study the optical gain and saturation behavior in a blue-green-emitting luminescent polymer gain medium. Based on the results of amplified spontaneous emission measurements, the gain coefficients, the gain-length product, and the corresponding small-signal gain are determined. By the use of the variable stripe length method, large net gain coefficients of up to 106±6cm−1 have been measured under nanosecond photopumping. The large gain has favorable implications for the development of short wavelength lasers and amplifiers. Their study shows that a small-signal gain of 19dB is achievable with a very compact optical amplifier with a 400μm length.
Structural and optical properties of In(Ga)As∕GaAs quantum dots treated by partial capping and annealing
Authors: Wang L. , Rastelli A. , Schmidt O. G.
Published: October 2006

We tune the emission energy of self-assembled InAs∕GaAs(001) quantum dots (QDs) by partial GaAs capping and annealing. During the annealing step, the surface above the QDs flattens substantially. The existence of In-rich cores below the thin GaAs cap is directly probed by using in situ selective etching. The blueshift of the QD emission energy, resulting from the reduction of the QD height, is tuned by increasing the annealing time or by reducing the capping layer thickness. For long annealing times, the ensemble photoluminescence (PL) displays a multipeak behavior which is attributed to monolayer fluctuations of the QD height. Single-QD micro-PL shows that the quality of the QDs can be improved by desorbing the In accumulated around the QDs by performing the annealing at high substrate temperatures.
Polarization-dependent optical characterization of poly(phenylquinoxaline) thin films
Authors: Ksianzou V. , Velagapudi R. K. , Grimm B. , Schrader S.
Published: October 2006

Linear optical properties of two types of poly(phenylquinoxaline) (PPQ) are studied by multiwavelength prism coupling technique and optical absorption spectroscopy. Surface roughness measurements are done using atomic force microscopy. PPQs form smooth films of high optical quality having refractive indices above 1.7 in the visible and near infrared spectral ranges. Enhanced birefringence of Δn∼0.04 has been observed in both PPQ films prepared by spin coating. Sellmeier coefficients are derived for the wavelength range starting from 0.532to1.064μm for both TE and TM polarizations. Quantum chemical calculations both on the semiempirical and on the ab initio level are carried out in order to calculate the first-order molecular polarizability tensors of the polymer repeat units. From the obtained tensor elements, theoretical values for both the average refractive indices and the maximum expectable birefringence are calculated. Based on these values a more detailed interpretation of the experimental findings is carried out. The dispersion of refractive index is quantified by the value of Abbe’s constant (νd). In our case the value νd≈11 indicates high dispersion in the visible spectral range. The imaginary part k of the complex refractive index n*=n−ik reaches values of k⩽10−3 in the wavelength range from 0.5to1μm.
Design and fabrication of planar multilayer structures with coherent thermal emission characteristics
Authors: Lee B. J. , Zhang Z. M.
Published: October 2006

A large number of recent publications dealt with enhanced emission properties of micro/nanostructures by the excitation of surface plasmon or phonon polaritons. Some used grating structures to demonstrate coherent thermal emission in a narrow spectral band and towards a well-defined direction. Others suggested that planar layers could also be used to achieve coherent emission. In the present paper, we describe two alternative designs with fewer layers for the application as coherent emission sources in planar multilayer structures. One design is a composite of several unit cells of one-dimensional photonic crystal atop a highly reflective material. Coherent emission can be obtained by the excitation of surface waves between the photonic crystal and the reflector. The other design employs a Fabry-Pérot resonance cavity constructed by coating a dielectric layer onto the reflector and then a thin metallic film on the dielectric layer. When standing waves exist in the cavity, the emissivity plots show sharp spectral peaks and narrow angular lobes. By optimizing the film thicknesses, a close-to-unity emissivity can be achieved from the proposed structures. The theoretical predictions are supported by the measured spectral reflectance from fabricated samples.
Dynamics of an atmospheric pressure plasma plume generated by submicrosecond voltage pulses
Authors: Lu XinPei , Laroussi Mounir
Published: October 2006

Nonequilibrium plasmas driven by submicrosecond high voltage pulses have been proven to produce high-energy electrons, which in turn lead to enhanced ionization and excitations. Here, we describe a device capable of launching a cold plasma plume in the surrounding air. This device, “the plasma pencil,” is driven by few hundred nanosecond wide pulses at repetition rates of a few kilohertz. Correlation between current-voltage characteristics and fast photography shows that the plasma plume is in fact a small bulletlike volume of plasma traveling at unusually high velocities. A model based on photoionization is used to explain the propagation kinetics of the plasma bullet under low electric field conditions.