bluuebunny/crossref_metadata_2025 huggingface dataset:
Retardation in the Oxidation Rate of Nanocrystalline Silicon Quantum Dots
Authors: Omachi J. , Nakamura R. , Nishiguchi K. , Oda S.
Published: April 2011

Using very-high-frequency (VHF) plasma decomposition of SiH4 and pulsed gas technique, we have successfully prepared nanocrystalline silicon (nc-Si) quantum dots having average diameter of 8 nm and dispersion of 1 nm. The role of natural oxide is very important. It controls the size of nc-Si dots. Of particular interest is that the oxidation of these dots can be self limited, due to the stress induced near Si/oxide interface, which would allow further reduction of size and improvement in dispersion. This paper deals with the systematic study of oxidation process of nc-Si dots. Nc-Si dots formed in an Ar plasma with SiH4 gas pulses are deposited onto a Pt mesh The dots are then oxidized at 750, 800 and 850°C from 20 minutes to 15 hours. The dimensions of the residual nc-Si and the grown oxide are measured directly from the TEM micrographs and analyzed. For comparison, field oxide is investigated using ellipsometry. Retardation in the oxidation rate of nc-Si is observed. The mechanism of the reduction of oxidation rate in nc-Si is discussed taking into account the effect of stress.
Identification of the luminescence center of porous silicon under low temperature thermal oxidation
Authors: Goda Kazuo , Tanaka Ryuji , Honda Yukako , Inoue Kazuhisa , Ohno Hideki
Published: April 2011

The relationship between chemical states and optical properties for porous silicon (PS) under initial oxidation by heating at low temperatures (150, 200, 250 °C) in air was investigated using an infrared (IR) spectroscopy and photoluminescence (PL) measurements. The measurements in the IR region show that the IR absorption peaks for the Si-H stretching band (2050-2150 cm−1) change with appearance of the Si-O-Si-H stretching band (2100-2300 cm−1) when the thermal oxidation time is increased. On the other hand, the peak in the PL spectrum shows a blue shift from 820 nm to 710 nm with the oxidation time. The observed blue shift of the PL spectrum is due to the decrease in the initial PL peak intensity at 820 nm and the increase at 710 nm. Moreover, the peak intensities in the PL spectra at 820 nm and 710 nm have clear relationship to the amounts of the Si-H bonds and Si-O-Si-H bonds, respectively, as a function of the oxidation time.

These results indicate that luminescence center (LC) for as-prepared PS is ascribed to complexes including Si-H bonds. Also the LC for oxidized PS under the oxidation process at low temperatures is ascribed to complexes including Si-O-Si-H bonds covering the inner surface of PS.
Fabrication of Silicon Nano-Crystal Dots on SiO2 by Ultrahigh-Vacuum Chemical Vapor Deposition
Authors: Oishi Masato , Kawashima Takayuki , Mashiro Supika , Sakai Junro , Madhukar Sucharita , Nguyen Bich-Yen , White Bruce E.
Published: April 2011

We report a process for Silicon (Si) nano-crystal dots fabrication using a cold-wall Ultrahigh-Vacuum Chemical Vapor Deposition (UHV-CVD) system. Si2H6 gas was used as the pre-curser and irradiated upon SiO2 film on Si wafer to form Si nano-crystal dots.

In our system, nucleation, growth, and coalescence phases of nano-crystal dots on SiO2 were found to be related with the optical pyro-meter's read-out curve.

At first, the optimum gas irradiation time which gives the highest dot density without coalescence was decided for every gas irradiation condition by using an optical pyro-meter. Then, the dependence of optimum gas irradiation time, dot diameter and dot density on gas flow rate and wafer temperature were investigated. A decrease of wafer temperature or an increase of gas flow rate during the nucleation and growth phase results in a decrease of dot diameter and an increase of dot density. The optimum gas irradiation time was prolonged by decreasing wafer temperature or gas flow rate.

Finally, a reproducible process of forming non-coalesced, small size, and high-density Si nano-crystal dots of about 5.5nm in diameter with density of 1x1012 dots/cm2 were obtained. Typical process time to get such dot formation characteristics was about 10 minutes, which were long enough for ensuring reproducibility.
Stimulated Blue Emission and Second Harmonic Generation From Films of Ultrasmall Si Nanoparticles
Authors: NAYFEH Munir H. , Therrien Joel , Belomoin Gennadiy , Akcakir Osman , Barry Nicolas , Gratton Enrico
Published: April 2011

We describe a procedure for dispersion bulk Si into ultrasmall nanoparticles, much smaller than what is available today, with high throughput and excellent size definition and control. The quantum dots are ∼ 1 nm in diameter and contain 29 Si atoms[1-2]. We show most unexpected and totally surprising phenomenon. Unlike bulk Si, an optically inert indirect gap material, the particles are extremely active optically in the blue, exceeding the activity of fluorescein, such that single particles are readily detected and imaged, using two-photon near- infrared femto second excitation. This comes nearly a decade after Canham first surprised the scientific community by reporting red luminescence from anodized Si[3]. In addition to being ultrabright, the particles have other useful properties that are unlike those of bulk including being a source of stimulated emission and collimated beam emission[4-5], and harmonic generation[6]. Single electron charging and the confinment energy spacing, both are much larger than the thermal energy[7]. They can be formed into colloids, crystals, films, and collimated beams for applications in the electronics, optoelectronics and biomedical industries. The synthesis utilizes straightforward, low-cost, environmentally benign present-day commercial technologies and material: abundant Si, common chemicals and electricity. The procedure is readily amenable to large-volume production, easy recovery, and controlled delivery.

Using density functional with exchange correlation, configuration interaction and Monte Carlo theory, we constructed a structural propotype for 29 atoms (magic number for the Td symmetry and spherical shape), giving a size, an energy gap and an absorption spectrum that agree with experiment[8]. In the prototype, twenty four atoms form a network of reconstructed Si-Si species on the surface, with five atoms constituting a tetrahedral core.

The results suggest that the surface reconstructed network is the source of the new properties. In these particles, the elasticity drops, atoms become amenable to large displacement, and are subjected to unballanced atomic forces, conditions under which novel molecular structure or configuration, distinct from bulk, and which otherwise does not exist may form. This structure may constitute a transition between the bulk and atomistic phases.
Structural Evolution of Ni/Au Contact on GaN(0001)
Authors: Kim Chong Cook , Kim Jong Kyu , Lee Jong-Lam , Yi Min-Su , Kim Jin-Woo , Noh Do Young , Hwu Yeukuang , Ruterana Pierre , Je Jung Ho
Published: April 2011

We investigated the structural behavior of the Ni/Au contact on GaN(000l) during annealing in N2, using in-situ x-ray diffraction, anomalous x-ray scattering, and high resolution electron microscopy. Thermally activated atomic mobility caused the two metal atoms, Au and Ni, to interdiffuse during annealing and form solid solutions. At temperature higher than 500°C, GaN decomposition and reactions occurred mostly along GaN dislocations. By decomposed nitrogen reacted with Ni, interestingly, epitaxial Ni4N phase was formed. The epitaxial relationship of the Ni4N, Au, and Ni was identified as M(111)//GaN(0002) and M[0 1 1]//GaN[0211] (M= Ni4N, Au, and Ni).
Chemical ordering in AlGaN layers grown by MOCVD
Authors: Ruterana P. , Omnes F.
Published: April 2011

One of the applications of wurtzite gallium based nitride compounds will be to provide optoelectronic devices from 6.2eV (AlN) to 1.89eV (InN). This will depend on the possibility to grow wurtzite AlGaN and InGaN ternary alloys. As expected, the most difficult region is InGaN due to the large misfit between GaN and InN (= 10%), in this case, ordering, phase separation and growth instabilities have been reported. In the case of AlN and GaN, the misfit is smaller (∼ 2.5%) and one would expect more stable growth. However, it was in this system that ordering along the c axis between AlN and GaN was reported for the first time.

In this work, we have found that the growth of AlGaN may be more complicated. Not only the wurtzite lattice can be decreased to simple hexagonal by AlN/GaN ordering along the c axis, but the growth can lead to other types of stackings. Even in the low Al composition range, 10 - 15%, we have found that three processes can operate:

1. Ordering into AlN/GaN as two simple hexagonal sublattices.

2. The 3:1 ordering which has been recently reported to occur in InGaN.

3. A new type of ordering where diffraction experiments (XR and electron diffraction) detect superlattice reflection with a period close to 3 nm. The most adequate model which was found to take this into account shows that, in these growth conditions, the system has preferred to form one AlN cell in between 5 GaN cells, leading to a 5:1 ordering.
Highly selective photoenhanced wet etching of GaN for defect investigation and device fabrication
Authors: Visconti P. , Reshchikov M. A. , Jones K. M. , Yun F. , Wang D. F. , Cingolani R. , Morkoç H. , Litton C. W. , Molnar R. J.
Published: April 2011

Photoenhanced electro-chemical (PEC) wet etching has been shown to be suitable for dislocation-density estimation in n-GaN films as well as for GaN-based device fabrication. We report on PEC etching of n-GaN samples grown by MBE and HVPE methods in unstirred aqueous KOH solution under He-Cd laser illumination. Characterization of the etched samples was carried out using atomic force microscopy (AFM) in both cross-sectional and plan-view configurations and scanning electron microscopy (SEM). At moderate illumination densities, the SEM and AFM analyses reveal sub-100 nm scale threading vertical wires on the etched surfaces. The calculated density (∼1×10 9cm−2) is in agreement with dislocation density found by transmission electron microscopy. Using cross-sectional AFM, we find that these vertical wires are ∼1[.proportional]m high and are perpendicular to the sapphire surface. Applying a higher illumination density or an external voltage, we obtain a higher etch rate with a smooth free-feature etched surface. Some highly resistive samples that cannot be etched under normal conditions because the band bending is too small to confine the holes to the surface for them to participate in the PEC process, can be etched with the application of a voltage to the sample. In this case, the etch rate depends on both the polarity and the magnitude of the voltage applied. In an MBE-grown sample with an AlN/GaN superstructure inside, we report on high selectivity between AlN and GaN (AlN is an etch stop); the selectivity is due to the etching mechanism of the PEC process.
Atomic Force Microscopy Study of GaN Grown on Al2O3(0001) by LP-MOVPE
Authors: Xu K. , Lim D. H. , Liu B. L. , Du X. L. , Yu G. H. , Jia A. W. , Yoshikawa A. , Takahashi K.
Published: April 2011

There is currently a high degree of interest in understanding the diverse mechanisms that determine the growth morphology of epitaxial films. To understand and control over these mechanisms, it is essential to know how the growth mechanisms are correlated with morphologies. In GaN MOVPE processes, growth temperature has a remarkable effect on the film morphologies and properties. In present work, the temperature dependency of surface morphology of GaN epilayers grown by low-pressure metal-organic vapor phase epitaxy (LP-MOVPE) was studied using atomic force microscopy. It was demonstrated that dislocations strongly influence the growth mechanisms and the morphology of the films. Three growth modes were identified at different growth temperature ranged from 1030°C to 1100°C, which are, (1) spiral growth dominated by dislocation with screw type, (2) monolayer step flow, and (3) nucleation-assisted step flow growth. The significant effect of dislocation on the surface morphologies and growth mechanisms may be attributed to the high dislocation density and higher activation energy for a step overcoming the dislocation pinning in GaN. Because few dislocations will be introduced under monolayer step flow growth mode, we can infer the useful information about GaN initial growth.
A Comparison of Magnesium and Beryllium Acceptors in GaN Grown by rf-Plasma Assisted Molecular Beam Epitaxy
Authors: Ptak A.J. , Myers T.H. , Wang Lijun , Giles N.C. , Moldovan M. , Da Cunha C.R. , Hornak L.A. , Tian C. , Hockett R. A. , Mitha S. , Van Lierde P.
Published: April 2011

Step-doped structures of both magnesium and beryllium were grown in GaN and analyzed using secondary ion mass spectrometry. Dopant incorporation was studied as a function of substrate temperature and dopant flux for Ga-polarity and N-polarity GaN. Incorporation is different for each polarity, with Mg incorporating by up to a factor of 20 times more (30 times more with atomic hydrogen) on the Ga-face, while Be incorporates more readily on the N-face. The effect of atomic hydrogen on the incorporation kinetics of both Mg and Be is also discussed. Mg and Be both undergo surface segregation during growth. Photoluminescence measurements suggest that Be is a p-type dopant with an optical activation energy of approximately 100 meV.
Growth of quaternary AlInGaN/GaN Heterostructures by Plasma Induced Molecular Beam Epitaxy with high In Concentration
Authors: Lima A. P. , Miskys C. R. , Görgens L. , Ambacher O. , Wenzel A. , Rauschenbach B. , Stutzmann M.
Published: April 2011

Growth of AlInGaN/GaN heterostructures on sapphire substrates was achieved by plasma induced molecular beam epitaxy. Different alloy compositions were obtained by varying the growth temperature with constant Al, In, Ga and N fluxes. The In content in the alloy, measured by Rutherford Backscattering Spectroscopy, increased from 0.4% to 14.5% when the substrate temperature was decreased from 775 to 665°C. X-Ray reciprocal space maps of asymmetric AlInGaN (2.05) reflexes were used to measure the lattice constants and to verify the lattice match between the quaternary alloy and the GaN buffer layers.
Phase Separation in Multiple ZnO /Cubic- MgxZn1−xO Superlattice Heterostructures Observed Via High Resolution Transmission Electron Microscopy
Authors: Kvit A. , Dusher G. , Sharma A. K. , Jin C. , Narayan J. , Muth J. , Teng C.W
Published: April 2011

We have synthesized ZnMgO alloy of wurtzite (Mg content equals to 0.0=0.34) and cubic (Zn content equals to 0.0- 0.18) phases using nonequilibrium pulsed laser deposition method. Epitaxial films of ZnMgO wurtzite structure have been grown on (0001) sapphire substrates. Using JEOL-2010 field-emission transmission electron microscope equipped with STEM and Gatan image filter, we can perform atomic structure, STEM-Z, electron energy loss spectroscopy and imaging simultaneously. Such studies on the ZnO/MgZnO superlattices provide first direct evidence of phase-separation in the range 3 nm.
Microwave AlxGa1−xN/GaN Power HEMT's
Authors: Eastman Lester F.
Published: April 2011

GaN in its hexagonal Wurtzite crystal has 3.4 eV bandgap and is capable of withstanding over 3 MV/cm electric field intensity. It is grown on either sapphire or SiC and the lattice mismatches cause threading dislocations. If it is grown along the c-axis on its Ga face, a thin pseudomorphic AlxGa1−xN barrier layer grown on top causes a substantial electrical polarization, which induces a substantial 2DEG in the GaN [1]. With x =.3 this 2 DEG is about 1 × 1013/cm2 without any doping impurities. The ∼ 109/cm2 threading dislocations accept electrons from the low density ambient donors, yielding very low net electron densities (< 1014/cm3) in the GaN buffer layer used as the HEMT channel [2]. The 2DEG electron mobility can be 1500-1700 cm2/V-s with ≥ 1 × 1013/cm2 electron sheet density. This 2DEG sheet density charge is less than the electrical polarization charge of 1.7 × 1013/cm2 for Al.3Ga.7N/GaN, due to some surface depletion and due to the fact that each dislocation traps ≥ 2 × 103 electrons. The design, processing, and measured performance of such undoped, polarization-induced HEMT's are presented below.
Surface Morphology of SiGe Epitaxial Layers Grown on Uniquely Oriented Si Substrates
Authors: Ware Morgan E. , Nemanich Robert J.
Published: April 2011

The 4% lattice mismatch between Si and Ge creates strain in epitaxial layers of SiGe alloys on Si, and this strain can manifest itself in the morphological structure of the surface of the epitaxial layer. This study explores the relationship of the evolution of the surface morphology of SiGe layers grown on a range of Si surface orientations. We have grown thin, strained and thick, relaxed layers of Si0.7Ge0.3 by solid source molecular beam epitaxy on substrates with surface normals rotated from [001] towards [111] by angles of θ = (0, 2, 4, 10, 22) degrees. The surface morphology was investigated by atomic force microscopy, which showed considerable ordering of surface features on relaxed samples. These features evolve from hut-like structures at 0 degrees to large mesa-like structures separated by pits and crevices at 22 degrees. The organization of these features is also shown to vary with the substrate orientation. Each surface has characteristic directions along which features are aligned, and these directions vary continuously with the angle of rotation of the substrate. Transmission electron microscopy confirmed that misfit dislocations had formed along those same directions. The state of relaxation of each layer is quantified by Raman spectroscopy in order to make a direct correlation between residual strain and surface morphology.
Optical properties of self-organised SSMBE and GSMBE Ge nanostructures grown on SiGe template layers on Si (118)
Authors: Brémond G. , Ferrandis P. , Souifi A. , Ronda A. , Berbezier I.
Published: April 2011

This work reports on the photoluminescence properties of self-organized fully strained Ge dots fabricated using two different growth techniques: gas source molecular beam epitaxy (GS-MBE) and solid source molecular beam epitaxy (SS-MBE). Variable temperature photoluminescence (PL) measurements were carried out on Si/(n)Ge/SiGe/Si structures (n varying from 1 to 7 monolayers) consisting in double layer structures (Ge(n)/Si1-xGex) deposited on (118) oriented Si substrates. The process used consists in realising in a first step a Si1-xGex template layer with a “self-patterned” morphology. Such patterning, based on periodic morphological modulation of the surface is used to confine and organise the Ge dots in a second deposition step. Similar series of experiments with various growth temperatures, Ge coverage levels and Si1-xGex concentrations (x) were done by both GS-MBE and SS- MBE. The PL from the 2D wetting layer in the case of n = 3 ML has been found to be more intense in GS-MBE thanks to the passivating role of the hydrogen atoms. The 2D to 3D growth transition is accompanied by the occurrence of a red-shifted broad PL band (L) attributed to the Ge dots. While broadened luminescence is obtained from dots directly deposited on the Si substrate, a narrower band is obtained from dots deposited on the template layer. Moreover the red-shift of the (L) band observed in the latter case is attributed to higher Ge concentration in the dots. On the other hand, there is no effect of the hydrogen on the formation of the islands which show similar optical and structural properties in both growth techniques. The main difference between GS-MBE and SS-MBE concerns a low energy shift in the PL of the SSMBE Ge dots we interpret as due to size, dispersion and Ge concentration or strain effects
Strain dependence and magnetic anisotropy in chromium dioxide thin films
Authors: Spinu L. , Srikanth H. , Li X. W. , Gupta A. , Xiao Gang
Published: April 2011

Field-dependent transverse susceptibility measurements based on a novel tunnel-diode oscillator (TDO) method were used to probe the magnetic anisotropy and switching in epitaxially grown CrO2 films on TiO2 substrates. The static field was applied in three different orientations (0°, 45° and 90° ) with respect to the magnetic easy axis. Singular peaks in the transverse susceptibility were observed that are associated with the anisotropy and switching fields in CrO2. Theoretical calculations based on a coherent rotation model display remarkable agreement with the experimental data. A new peak emerges at low temperatures and is identified to be a consequence of temperature-dependent magnetoelastic contributions resulting from the in-plane tensile strain in the films due to varying coefficients of thermal expansion in the film and the substrate.
Pulsed laser deposited nanocrystalline ZnO thin films
Authors: Thareja R. K. , Mitra A. , Ganesan V. , Gupta A. , Sahoo P. K. , Kulkarni V.N.
Published: April 2011

We report on random laser emission in pulsed laser deposited nanocrystalline ZnO thin films. The deposition was done on silicon and glass substrates in ambient oxygen pressure ranging from 10 mTorr to 1 Torr at room temperature. The deposited films were characterized using XRD, RBS, PL and AFM. The films grown at pressures less than 300 mTorr are found to be preferentially oriented along (002) plane. Morphology of the films deposited at different background pressures showed that the films deposited at lower pressures are significantly smoother than those at higher back ground pressure. PL intensity depends on the stoichiometry of the films and hence on oxygen pressure. Laser action was observed on optically pumping the films with 355 nm radiation. At excitation intensity above threshold, very narrow peaks are observed in the emission spectrum. The dependence of the laser emission on the size of nanocrystallites at different pressures of the ambient gas is presented.
Mesoscopic Patterning Induced by Co-deposition of C60 and Ni on the MgO(100) Single Crystal
Authors: Vacik Jiri , Naramoto Hiroshi , Narumi Kazumasa , Yamamoto Shunya , Myashita Kiyoshi
Published: April 2011

We report on an unusual phenomenon of pattern formation in the mixture of Ni and C60 codeposited on the single crystal of MgO(100). Under certain deposition kinetics an array of periodic stripes was formed. The stripes consist of sub-half-micrometer Ni droplets (embedded in the complex Ni-C base) and encompassed with a polymerized C60 rind. The stripes are bound to the thin double-layer platform consisting of the epitaxial Ni and amorphous C (a-C) layers. During co-deposition the C60 molecules partly disintegrate and transform towards the a-C structures. Accumulation of a-C (immiscible with Ni) may induce thermodynamic instability in the system and trigger the process of spontaneous partitioning. As a principal mechanism of the stripe formation a sequential drift and co-ordinate release of the incited instability was proposed.
Surface Evolution of NiTi and NiTiHf Thin Films
Authors: Zhang Chen , Zee Ralph H. , Thoma Paul E.
Published: April 2011

The microstructure evolution of Ti-rich NiTi thin films and (TiHf)-rich NiTiHf thin films containing 9at% Hf was investigated. These films were deposited from single NiTi and NiTiHf targets using a DC magnetron sputtering system. Free-standing films were obtained by using single crystal silicon substrates. The thickness of these films was controlled between 10-12 μm. In this investigation, the effects of deposition temperature on the surface and cross-sectional microstructures of these films were studied. Substrate temperature during deposition was varied between 300°C to 700°C at 100°C intervals. The influence of post deposition heat treatment (HT) temperature on the microstructure of these films was also studied. The post deposition HT temperature was varied between 300°C and 800°C at 100°C intervals. Both surface and cross- sectional microstructures were examined using a scanning electron microscope (SEM). The crystallinity and the phases present were determined using x-ray diffractometry. All the as- deposited films were found to be crystalline, even when the substrate temperature was as low as 300°C. Results from the microstructure analysis show that all the films have a relatively fine grain size ranging from 0.2 μm to 2.5 μm, and the grain size increases with increasing substrate deposition temperature. The effect of post deposition HT on grain size was found to be minimal.
The Early Oxynitridation Stages of Hydrogen-Terminated Single-Crystalline Silicon in N2O Ambient
Authors: Cerofolini G. F. , Camalleri M. , Condorelli G. G. , Fragalà I. L. , Galati C. , Lorenti S. , Renna L. , Viscuso O.
Published: April 2011

Oxynitridation of hydrogen-terminated silicon with N2O has been studied by x-ray photoemission spectroscopy. Our analysis has given evidence that the broad N(1s) peak at 398.3 eV usually reported in the literature is preceded by the formation of a very narrow peak at 397.3 eV, attributed to the moiety Si3N in which nitrogen is only marginally oxidized, and two other peaks (previously never reported) at 400.0 eV and 401.5 eV, attributed to the moieties Si2NOSi and SiNO, respectively.
A Study of Nucleation and Growth in MOCVD: The Growth of Thin Films of Alumina
Authors: Singh M.P. , Mukhopadhayay S. , Devi Anjana , Shivashankar S.A.
Published: April 2011

We have studied the nucleation and growth of alumina by metalorganic chemical vapor deposition (MOCVD). The deposition of alumina films was carried out on Si(100) in a horizontal, hot-wall, low pressure chemical vapor deposition (CVD) reactor, using aluminum acetylacetonate{Al(acac)3}as the CVD precursor. We have investigated growth of alumina films as a function of different CVD parameters such as substrate temperature and total reactor pressure during film growth. Films were characterized by optical microscopy, X-ray diffractometry (XRD), scanning electron microscopy (SEM), cross-sectional SEM, and secondary ion mass spectrometry (SIMS) compositional depth profiling. The chemical analysis reveals that the carbon is present throughout the depth of the films.
Modeling MOCVD Growth of YBCO Thin Films
Authors: Lo Tak Shing , Kohn Robert V.
Published: April 2011

We develop a new approach to the modeling of thin film growth. Our model treats the adatom density on the surface of the film as an explict unknown. This approach (1) clarifies the role of the “uphill current” associated with the Schwoebel barrier; (2) facilitates simple, physically natural coupling to the mechanism of deposition; and (3) permits discussion of multispecies effects. Our implementation focuses on spiral mode growth of YBCO thin films, with MOCVD (Metallorganic chemical vapor deposition) as the deposition mechanism.
Heteroepitaxial Growth Mode Transitions in the Presence of a Schwoebel Barrier at an Island Edge
Authors: Trofimov Vladimir I. , Mokerov Vladimir G.
Published: April 2011

A recently developed model for homoepitaxial growth on a singular surface in the presence of a Schwoebel barrier at an island edge is extended to the case of heteroepitaxy (without lattice mismatch) by introducing different adatom mobilities in the first layer (heterodiffusion) and in all the next ones (self-diffusion).The model consists of an infinite set of rate equations for adatom and 2D island areal densities and coverage in successive layers. Growth behaviour depending on the model parameters is studied with emphasis on the transient growth regimes: layer-by-layer (LL) to a smooth multilayer (SML) and SML to a rough 3D growth. It is shown that a slower heterodiffusion relative to the self-diffusion leads to a smoother initial growth and thus retards the LL → SML growth trasition, whereas a faster heterodiffusion leads to a rougher initial growth and assists that transition and especially SML → 3D growth transitition at a higher Schwoebel barrier. It is found also that in a smooth growth regime nucleation kinetics eventually acquires a universal scaling form and the corresponding exponents are determined.
Structure of Charged Polymer Chains in Confined Geometry
Authors: Gilbert Elliot P. , Auvray Loïc , Lal Jyotsana
Published: April 2011

The intra- and interchain structure of sodium poly(styrenesulphonate) when free and when confined in contrast matched porous Vycor has been investigated by SANS. When confined, a peak is observed whose intensity increases with molecular weight and the 1/q scattering region is extended compared to the bulk. We infer that the chains are sufficiently extended, under the influence of confinement, to highlight the large scale disordered structure of Vycor. The asymptotic behavior of the observed interchain structure factor is ≍ 1/q2 and ≍ 1/q for free and confined chains respectively.
Dynamics of Sol-Gel Clusters and Branched Polymers
Authors: Zilman A. G. , Granek R.
Published: April 2011

We study the dynamics of sol-gel clusters. We find that the dynamic structure factor decays in time as a stretched exponential, and the viscoelastic modulus is an algebraic function of time. The difference beween screened and non-screened systems in the context of cluster dynamics is discussed.
Proton Spin-Relaxation Induced by Localized Spin-Dynamical Coupling in Proteins And in Other Imperfectly Packed Solids
Authors: Korb J.-P. , Van-Quynh A. , Bryant R. G.
Published: April 2011

The magnetic field dependence of 1H spin lattice relaxation rates in noncrystalline macromolecular solids including engineering polymers, proteins, and biological tissues is described by a power law, 1/T1 = Aω0-b, where ω0 is the Larmor frequency, A and b are constants. We show that the magnetic field dependence of the proton 1/T1 may be quantitatively related to structural fluctuations along the backbone that modulate proton-proton dipolar couplings. The parameters A and b are related to the dipolar coupling strength, the energy for the highest vibrational frequency in the polymer backbone, and the fractal dimensionality of the proton spatial distribution.
Mechanism for Ferromagnetic Resonance Line Width Broadening in Nickel-Zinc Ferrite
Authors: Hong Hee Bum , Byun Tae Young , Byeon Soon Cheon , Hong Kug Sun
Published: April 2011

The systematic variation in line width of ferromagnetic resonance with the Fe content was observed at X band (9.78GHz) in (Ni0.5Zn0.5)1−xFe2+xO4 (-0.2 ≤ x ≤ 0.2). The line width of the stoichiometric composition (x = 0) showed minimum value, 50 Oe. In contrast, the line width of the non-stoichiometric compositions sharply increased to 210 Oe with increasing nonstoichiometry (x). The mechanism for this line width broadening was investigated using thermoelectric power and electrical resistivity, since the contribution of anisotropy and porosity to the line width was negligible in this compositional region. In Fe excess region, Fe2+ ion concentration increased with increasing Fe content, resulting in line width broadening due to relaxation. But, it was suggested that Ni3+ and Fe2+ ions coexist in Fe deficient region. Therefore the increase of line width in nickel-zinc ferrites originated from the Fe2+ /Fe3+ magnetic relaxation in Fe excess region, and the Fe2+/Fe3+, Ni2+/Ni3+ magnetic relaxation in Fe deficient region.
Rock Wetting Condition Inferred from Dielectric Response
Authors: Araujo Yani Carolina , Araujo Mariela , Guzmán Hernán
Published: April 2011

Wettability is a manifestation of rock-fluid interactions associated with fluid distribution in porous media. Conventional wettability evaluation is performed by a sequence of spontaneous and forced displacements of different fluids into a porous sample, a method which is costly and time consuming. A new attractive approach is to estimate this quantity from dielectric measurements, since they can be done rapidly and economically.

The dielectric frequency response of several rock samples of known wettability condition was studied in the range from 10 Hz to 100 MHz. Samples were saturated with brine and oil. The results confirm the strong influence of wetting condition on dielectric response. Water wet samples have significantly higher values of and (real and imaginary parts of generalized complex permitivity) than oil wet samples. In particular, the high frequency behavior of is most affected. Different regimes are identified as a function of frequency. They correspond to zones where different polarization effects are manifested. We quantify this effect and find a correlation with the modified Amott wettability index. Based on these findings, we propose an experimental protocol for the indirect measurement of wettability at laboratory scale.
Powder Diffraction Refinements of the Structure of Magnetite (Fe3O4) Below the Verwey Transition
Authors: Wright Jon P. , Attfield J. Paul , Radaelli Paolo G.
Published: April 2011

Magnetite is a classic example of a mixed-valent transition metal oxide, in which electronic conductivity and ferromagnetism result from electron hopping between octahedrally coordinated Fe2+ and Fe3+ states. Below the 122 K Verwey transition, the conductivity falls by a factor of ∼100 and a complex monoclinic (or triclinic) superstructure of the high temperature cubic spinel arrangement is adopted. This is assumed to be the result of Fe2+/Fe3+ charge ordering on the octahedral sites, but this has not been confirmed crystallographically, as single crystal refinements have been hampered by the extensive twinning that accompanies the Verwey transition. We have used very highly resolved powder diffraction data to attempt Rietveld refinements of the low temperature structure. The powder sample was prepared by grinding a single crystal of stoichiometric magnetite. Data were collected at 90 K on instruments HRPD at the ISIS neutron source, UK, and BM16 at the European Synchrotron Radiation Facility, France. The very high resolution of these data enables the monoclinic distortion to be observed, and the structure has been refined on the supercell proposed by Iizumi et al (Acta Cryst. B38, 2121 (1982)) with Pmca pseudosymmetry, giving parameters a = 5.94443(1), b = 5.92470(2), c = 16.77518(4) Å, β = 90.236(1)°. The mean octahedral site Fe-O distances differ from each other significantly, but the maximum difference between values is only 20% of that expected for ideal Fe2+/Fe3+ charge ordering.
Stability and Structural Characterization of Epitaxial NdNiO3 Films Grown by Pulsed Laser Deposition
Authors: Doan Trong-Duc , Abramowski Cobey , Salvador Paul A.
Published: April 2011

Thin films of NdNiO3 were grown using pulsed laser deposition on single crystal substrates of [100]-oriented LaAlO3 and SrTiO3. X-ray diffraction and reflectivity, scanning electron microscopy, and atomic force microscopy were used to characterize the chemical, morphological and structural traits of the thin films. Single-phase epitaxial films are grown on LaAlO3 and SrTiO3 at 625°C in an oxygen pressure of 200 mTorr. At higher temperatures, the films partially decompose to Nd2NiO4 and NiO. The films are epitaxial with the (101) planes (orthorhombic Pnma notation) parallel to the substrate surface. Four in-plane orientational variants exist that correspond to the four 90° degenerate orientations of the film's [010] with respect to the in-plane substrate directions. Films are observed to be strained in accordance with the structural mismatch to the underlying substrate, and this leads, in the thinnest films on LaAlO3, to an apparent monoclinic distortion to the unit cell.
Quantum Critical Phenomena in the Ni S2−xSexSystem
Authors: Honig J.M.
Published: April 2011

Electrical conductivity experiments on NiS2−xSex(x = 0.45) subjected to moderate pressure in the 30 – 800 mK range permit the investigation of quantum critical phenomena in this system. Detailed data are presented in the context of standard theories of correlation effects in the vicinity of a critical point. The critical exponents pertaining to these effects have been evaluated; the significance of these findings require further study.
Bonding of Guest Molecules in the Tubes of Nanoporous Cetineite Crystals
Authors: Krasovskii E.E. , Tiedje O. , Brodersen S. , Schattke W. , Starrost F. , Jockel J. , Simon U.
Published: April 2011

A theoretical study of the optical excitation processes in cetineites is presented. This new exciting class of crystals with tubular structures displays photoconductivity, which strongly depends on the presence of guest molecules within the tubes. Based on self-consistent electronic structure calculations we present calculated dielectric function and photoelectron spectra of the (Na;Se) cetineite with the tube fllled with different guest molecules. Calculations are performed with the extended linear augmented plane wave kp method. The configuration and the arrangement of the filling molecules is derived from available crystallographic data and from heuristic arguments on chemical binding.
Theoretical Analyses of Spin Exchange Interactions in Extended Magnetic Solids Containing Several Unpaired Spins per Spin Site
Authors: Dai D. , Koo H.-J. , Whangbo M.-H.
Published: April 2011

For extended magnetic solids containing several unpaired spins per spin site, we reviewed briefly recent progress in quantitative and qualitative methods of describing their spin exchange interactions on the basis of electronic structure calculations for their spin dimers.
Assembly of Metal-Anion Arrays within Dion-Jacobson-Type Perovskite Hosts
Authors: Kodenkandath Thomas A. , Viciu Marilena L. , Zhang Xiao , Sims Jessica A. , Gilbert Erin W. , Augrain François-Xavier , Chotard Jean-Noël , Caruntu Gabriel A. , Spinu Leonard , Zhou Weilie L. , Wiley John B.
Published: April 2011

The construction of metal-anion arrays within Dion-Jacobson-type (DJ) perovskites was studied. A variety of hosts readily react with CuCl2 to produce compounds of the form (CuCl)[A'n-1(M,M')nO3n+1] (A' = alkaline earth, rare earth or Bi; M/M' = Nb, Ta, and/orTi; n = 2, 3). In contrast, CuBr2 only reacts with some of these hosts; this difference relative to the chloride may simply be a size effect. The formation of metal-anion arrays with other transition metals has also been examined. (FeCl)LaNb2O7 can be readily prepared from FeCl2, while reactions with FeBr2and NiCl2 under similar reaction conditions result in compounds of the formM0.5LaNb2O7 (M = Fe, Ni). The reductive intercalation of lithium and sodium into (CuCl)LaNb2O7 was also examined and was found to produce new compounds that have a large layer expansion.
A New Microporous Silicate With 12-Ring Channels
Authors: Plévert Jacques , Kubota Yoshihiro , Honda Takahisa , Okubo Tatsuya , Sugi Yoshihiro
Published: April 2011

A new molecular sieve GUS-1 is synthesized under hydrothermal conditions using a new organic compound acting as structure-directing agent. The structure determination reveals a framework with one-dimensional 12-membered ring channels running parallel. The structure exhibits the same channel net as zeolite ZSM-12 and shows the same projection of the framework along the pores as mordenite.
Preparation, Electrical Properties, and Water Adsorption Behavior of (1-x)Sb2O5·xM2O3·nH2O (M = Al, Bi, and Y; 0 ≤ x ≤ 1)
Authors: Ozawa Kiyoshi , Sakka Yoshio , Amano Muneyuki
Published: April 2011

Bi, Y, and Al-doped antimonic acids with the empirical formula (1-x)Sb2O5·xM2O3·nH2O (M =Al, Bi and Y; 0 ≤ x ≤ 1) have been prepared by the direct reaction of an aqueous H2O2 solution with metal alkoxides. The electrical properties have been investigated by ac and dc conductivity measurements using a polycrystalline compactdisc as a sample, in connection with the water adsorption behavior. The (1-x)Sb2O5·xM2O3·nH2O (M = Bi and Y) materials exhibit a solid solution of cubic antimonic acid (Sb2O5·nH2O) with Bi2O3 and Y2O3 in the range of x = 0 – 0.1, and the ionic transference numbers of the materials are all over 0.98. The proton conductivity of (1-x)Sb2O5·xM2O3·nH2O (M = Bi and Y; x = 0.1) at room temperature increases to over 10-3 Scm-1 with an increase in the relative humidity. The conductivity is closely affected by the water adsorption behavior, which seems to be related to the introduction of oxygen vacancies.
A Composite Ionic-Electronic Conductor in the (Ca,Sr,Ba)-Bi Oxide System
Authors: Meen James K. , Gökçen Oya A. , Lin I-C. , Müller Karoline , Nguyen Binh
Published: April 2011

The rhombohedral alkaline earth-bismuth oxide phase, an oxygen ion conductor, does not coexist stably with electronic conductors in any of the three binary systems, Ca-Bi-O, Sr-Bi-O, Ba-Bi-O. A thermodynamically stable composite of a rhombohedral phase that contains Ba and Sr or Ca or both with the electronic conductor BaBiO3 may be synthesized. The rhombohedral phase appears to have complete mutual miscibility of the alkaline earth elements. The composi- tions of rhombohedral phase that coexist with BaBiO3 in the Sr-Ba-Bi ternary system and the Ca- Sr-Ba-Bi quaternary systems are described. The value of ionic conductivity of the rhombohedral phase (at a constant Bi: [Ca+Sr+Ba]) is not dependent on the relative amounts of Ca, Sr, and Ba. The temperature at which the rhombohedral phase undergoes a polymorphic transformation from a low-temperature (2) form that is a weak ion conductor to a high-temperature (β1) form that is a much better oxygen ion conductor. The temperatures of the polymorphic transformation and of the upper stability limit of the rhombohedral phase both depend strongly on Ca: Sr: Ba. The β1 form develops in the Ba-Bi system at the lowest temperatures and at the highest ones in the Ca- Bi system. On the other hand, the Ca-Bi phase has greater thermal stability than its Ba ana- logues. The temperature range over which a useful composite conductor can operate is, there- fore, strongly dependent upon the bulk composition of the system.
Higher efficiency of n-i-p solar cells by Hot-Wire CVD at moderate temperatures
Authors: van Veen Marieke K. , Schropp Ruud E.I.
Published: April 2011

Hot-Wire deposited amorphous silicon is an excellent material for the incorporation as the absorbing layer in n-i-p solar cells. We decreased the deposition temperature from 430 °C to 250 °C, keeping device quality (opto-)electrical properties of the a-Si:H layers. This enables application of Hot-Wire deposited a-Si:H in p-i-n structures and tandem solar cells. Layers deposited at 250 °C have been applied in efficient n-i-p and n-i-p/n-i-p solar cells. The deposition rate of the intrinsic layer was about 10 Å/s. No optical enhancements, like texturing or back reflectors, were used. Single-junction cells with a fill factor of 0.72 and an open-circuit voltage of 0.89 V have been produced. On a flexible stainless steel substrate, an initial efficiency of 7.2 % was recorded. Tandem cells also show a high fill factor (0.71) and open-circuit voltage (1.70 V), resulting in an initial efficiency of 8.5 %.
Light-induced Creation of Defects and Lifetime Distribution of Photoluminescence in a-Si:H Based Films
Authors: Ogihara Chisato , Takemura Hitoshi , Morigaki Kazuo
Published: April 2011

We have studied the lifetime distributions of photoluminescence (PL) at 10K after the pulsed excitation for a-Si:H based films. Effects of light-induced creation of defects on the lifetime distributions have been studied. The lifetime distributions of PL in a-Si:H based films have a distinct component at about 10 ns together with a longer lifetime component seen in microsecond region. The PL in a-Si:H decreases in intensity after the illumination of visible light. The decreasing of the nanosecond component is slower than that of microsecond component. The decreasing of the PL intensity and increasing of the defect density have also been observed in a-Si:H after illumination of sub-bandgap light, although the absorption coecient is much smaller than that of visible light. The quenching of PL is discussed with distribution of non-radiative lifetime calculated by assuming random distribution of the DBs.
Femtosecond Carrier Dynamics in Nanocrystalline Silicon Films: The Effect of the Degree of Crystallinity
Authors: Myers K.E. , Wang Q. , Dexheimer S.L.
Published: April 2011

We present studies of the ultrafast dynamics of photoexcited carriers in HWCVD nanocrystalline silicon thin films to address the underlying physics of carrier relaxation and recombination processes in this heterogeneous material. The degree of crystallinity is controlled by varying the H-dilution during deposition, yielding materials with increasingly larger grain size and crystalline fraction at higher dilution values. Time-resolved measurements of the carrier dynamics were made using a femtosecond pump-probe method, in which a short pump pulse excites carriers in the sample and a time-delayed probe pulse measures the resulting change in the optical properties as a function of the pump-probe delay time. Photoexcitation of carriers with pulses 35 fs in duration centered at 1.55 eV results in a net induced absorbance signal in the near-infrared that is analyzed in terms of a multi-component response that includes contributions from the silicon crystallites and the amorphous matrix.
Mechanisms of cross-talk in large area a-Si:H continuous image sensors
Authors: Mulato M. , Lu J. P. , Ready S. E. , Schuylenbergh K. Van , Ho J. , Lau R. , Boyce J. B. , Street R. A.
Published: April 2011

We report studies of the image-blur effects caused by lateral cross-talk between neighboring pixels of large-area amorphous silicon (a-Si:H) image sensors. The lateral conduction is attributed to three effects: conduction along the interface between the a-Si:H film and the underlying passivation; field-dependent electron injection at the edge of the sensor; and a field enhancement of the interface conduction due to the bias applied to the address lines. We show that the cross-talk can be controlled by choice of the operating conditions and optimization of the materials.
Tailored Laser scanned photodiodes (LSP) for image recognition
Authors: Vieira M. , Fernandes M. , Louro P. , Vygranenko Y. , Schwarz R. , Schubert M.
Published: April 2011

A tailored ZnO:Al/a-p-i-n SiC:H/Al configuration for the laser scanned photodiode (LSP) imaging detector is proposed. The LSP utilizes light modulated depletion layers as detector and a laser beam for readout. When highly resistive a-SiC:H doped layers are used its higher optical gap when compared with the active layer are responsible by charge accumulation at the illuminated interfaces which blocks the carrier collection under illumination. Those insulator-like layers act as MIS structures that prevent excess signal charge from blooming to the nearby dark regions avoiding the image smearing.

The optical-to-electrical transfer characteristics show reciprocity between light intensity and image signal intensity only limited by the doped layers composition. Data reveal that the sensitivity, the responsivity and the spatial resolution are limited by the cell configuration while the linearity depends on the light source flux used to map the image onto the sensor. By using tailored SiC:H/Si:H/SiC:H p-i-n heterostructures an increase in the image signal optimized to the blue is achieved with a responsivity of 0.2 mW/cm2 and a spatial resolution of 20 µm.