bluuebunny/crossref_metadata_2025 huggingface dataset:
Electron Beam Processing of ZnGeP2: A Nonlinear Optical Material for the Infrared
Authors: Schunemann P.G. , Drevinsky P.J. , Ohmer M.C. , Mitchel W.C. , Fernelius N.C.
Published: April 2011

Zinc germanium phosphide, an important frequency-conversion material for producing mid-infrared lasers, is plaqued by a detect-related absorption band extending from the fundamental edge (0.62 microns) to ∼3 microns. The level of absorption varies with melt composition, and can be reduced by post-growth annealing treatments. In these experiments, further reduction of the near-band-edge absorption was achieved by irradiating with 1-1.5 MeV electrons at cumulative fluence levels up to 2.75xlOI8cm2. Ge-rich, ZnP2-rich, and both as-grown and annealed stoichiometric crystals were studied. The near-edge absorption of the higher-loss, nonstoichiometric samples decreased monotonically with each subsequent irradiation, whereas the absorption in the lower-loss, stoichiometric samples was minimized after cumulative electron fluences of 2 x 10I8cnv2 and 1 x 10I8cm2 for as-grown and annealed material respectively. The minimum absorption coefficient achieved at 1/im was ∼4.4cnr’ in both stoichiometric samples, representing a factor two decrease for the as-grown crystal. Further exposure after reaching saturation served only to increase the losses at longer wavelengths.
Niobium Nitride Thin Films Deposition using Radical Beam Assisted Deposition
Authors: Murzin Ivan H. , Hayashl Nobuyuki , Sakamoto Isao , Ohkubo Matsataka
Published: April 2011

We have employed a radical beam assisted deposition technique to prepare single-crystalline niobium nitride thin films on MgO (100) substrates. The radical beam containing excited species of nitrogen was produced by an ECR plasma source and used to irradiate the growing Nb film, which was simultaneously deposited by an electron-gun vapor source. The nitride film was found to grow epitaxially on the substrates heated to 600 – 650°C. It has resulted in the formation of NbN having predominantly Bl structure, resistivity of 44 (μΩcm at 20 K, and almost equiatomic composition.
The Diffusion Bonding Interface Structure of Yttria Stabilized Zirconia
Authors: Kim Young J. , Yoo Young Sung , Kim Hwan
Published: April 2011

The specimens of Yttria Stabilized Zirconia(3Y-TZP) were bonded by a diffusion process. The microstructural evolution at the bonded interfaces was examined for the bonding conditions. We confirmed that above general sintering temperature(1500 °C), grain coarsening, which enhances the size fluctuation at the bonding interface, not only eliminates free surfaces but also is the main mechanism of bonding. In order to enhance the diffusion rate at the bonding interface, Ceria was added as a dopant, and grain boundary undulation was examined. We found that the bonding characteristics were improved by the Ceria doping. Specimens with an undulating bonded interface exhibited transgranular fracture, while specimens with a straight interface showed intergranular fracture. The undulation at the bonded interface is suggested to improve ceramic to ceramic bonding.
Kink and Crack Interfaces in Deformed 6H-SiC Single Crystals
Authors: Ning X. J. , Pirouz P.
Published: April 2011

When a 6H-SiC single crystal is deformed under indentation or uniaxial compression in orientations not favorable for the activation of the 1/3[1120](0001) easy glide system, the secondary slip system is activated. Additionally, for low- temperature deformations, “kinks” and/or micro-cracks form in the crystal. In this paper, experimental results on relatively lowtemperature compression and indentation tests of single crystal 6H-SiC, and the microstructure of the deformed crystals, are presented. Based on the results, the secondary slip system in 6HSiC has been determined to be 1/3[1120](1100), which may actually be a combination of alternate glide of 1/3[1120] dislocations on the (1102) and (1102) planes. Further, dislocation mechanisms for the nucleation of prism-plane and basal-plane cracks, and for the process of kinking, in deformed 6H-SiC are proposed.
Reactive Wetting Taxonomy
Authors: Cannon R. M. , Saiz E. , Tomsia A. P. , Carter W.C.
Published: April 2011

Contact angles and spreading rates of liquids on solids are oft characterized owing to the widespread importance of wetting. Particularly with dissimilar materials, such as liquid metals on ceramics, where wetting can be poor, additives that can induce various reactions often promote wetting, even if it preceeds the reaction. However, excessive or unwanted reaction can equally well be detrimental in other metallic or ceramic systems. Despite obvious needs, mechanisms of reactive wetting are poorly known.

An effort is described to categorize sessile or spreading drop situations having a potential for some degree of reaction and ways wherein the driving force for reaction can actually couple with or parallel that for fluid flow. Several categories where reactivity enhances or retards spreading are described for which the very existence typically depends upon kinetic or nucleation limitations; this includes defining situations in which metastable interfaces have a meaningful context and unique properties. Examples use pairs of inorganic materials, for which the solid may only rarely be truly inert.
Influence of Crystallographic Orientation, Chemical Inhomogeneities, Material Transport Anisotropy and Elastic Strain Energy on the Migration of Grain Boundaries in Chromium-Doped Alumina During Internal Reduction
Authors: Backhaus-Ricoult Monika , Peyrot-Chabrol A. , Chiron R. , Hagege S.
Published: April 2011

Diffusion-induced grain boundary migration is observed during internal reduction of chromium-doped alumina. It occurs because grain boundary diffusion is fast compared to lattice diffusion of oxygen. The oxygen chemical potential relaxes between grain boundaries and adjacent grains. Migration to either side of the boundary is controlled by multiple factors: chemical composition differences between adjacent grains, elastic strain energy differences on the two sides of the boundary plane or by more rapid oxygen relaxation when the c-axis of a grain is perpendicular to the boundary plane.
Formation and Orientation of Low Energy MgO/Pd, Al2O3/Pd and TiO2/Pd Interfaces
Authors: Cosandey Frederic , Lu Ping
Published: April 2011

The results of this study indicate that oriented α-Al2O3 and TiO2 particles with planar facets suitable for HREM studies can be produced in Pd by internal oxidation. A single high symmetry cube-on-cube orientation relationship has been observed for cubic MgO oxide particles in Pd. Six orientation relationships have been observed for trigonal α-Al2O3 oxide particles in Pd. Parallelism of the low misfit [1100]M// [110]O common direction appears to be the dominant factor for the preferred orientations with facets corresponding to parallelism of low index planes of Pd and α-Al2O3. For the tetragonal TiO2 phase in Pd, eight orientation relationships have been observed with orientations controlled primarily by oxide nucleation on {111} plane of Pd. For this system, the particle shape is controlled by anisotropy in growth kinetics.
Mechanisms of Enhanced Ionic Conduction at Interfaces in Ceramics
Authors: Lubben D. , Modine F. A.
Published: April 2011

A large enhancement in the ionic conductivity of certain compounds occurs when the compound is produced as a composite material containing a finely-dispersed non-conductor such as SiO2 or Al2O3 This effect has been reported on for more than 20 years, and it is well established that the enhancement is associated with the presence of interfaces. The popular explanation has been based on a model which contends that the enhancement is due to a space-charge layer which forms to compensate a net charge layer at an interface. A different model proposes that extended defects such as dislocations and grain boundaries, either resulting from or stabilized by the interface, are responsible for the enhancement. This paper describes recent experiments which strongly support the latter model. The ionic conductivities of LiI and CaF2 thin films grown on sapphire(0001) substrates were monitored in-situ during deposition as a function of film thickness and deposition conditions. LiI films grown at 27°C exhibited a region of enhanced conduction within 100 nm of the substrate and a lesser enhancement as the film thickness was increased further. This conduction enhancement was not stable but annealed out with a characteristic log(time) dependence. The observed annealing behavior was fit with a model based on dislocation motion which implies that the increase in conduction near the interface is due to extended defects generated during the growth process. LiI films grown at higher temperatures (100°C) in order to reduce the grown-in defects showed no interfacial conduction enhancement. X-ray diffraction measurements suggest that these high-temperature LiI films nucleate as faceted epitaxial islands with a stable misfit dislocation density defined by the epitaxial relationship between the substrate and film. CaF2 films grown at 200°C showed a behavior similar to the 27°C LiI films, with a region of thermally unstable enhanced conduction that occurs within 10 nm of the substrate. Amorphous Al2O3 films deposited over the CaF2 layers created no additional enhancement but did increase the stability of the conduction, consistent with an extended defect model. Simultaneous deposition of CaF2 and Al2O3 produced films consisting of very-fine-grained CaF2 and particles of amorphous Al2O23 (5-10 nm grain and particle size) and a high defect density which was stable even well above the growth temperature. Measured conduction in the composite at 200°C was approximately 360 times that of bulk CaF2.
Criteria for Misfit Localization at Interfaces
Authors: Lojkowski Witold
Published: April 2011

It is well known that in many cases, the structure of interfaces is well described in terms of “good matching” islands, separated by misfit dislocations. The “0” lattice theory can predict the dislocations' spacing but not their core width. In the present paper, a new physical parameter: p, called further “localization parameter”, is introduced. It permits the assessment of the width of misfit dislocations. p is expressed as p=EAB/E where EAB is the bonding energy of the two crystals and E is their average cohesion energy. The localization parameter was calculated for about 30 interfaces: metal/metal, metal/ceramics, ceramics/ceramics. For p<0.25, misfit dislocations are delocalized.
The Effect of The Space Charge On The Properties of An Inhomogeneous Pyroelectric Ceramic
Authors: Pintilie L.
Published: April 2011

The properties of a series ceramic bimorph structure were studied in this paper. The two components of the bimorph were PLZT ceramics with different transition temperatures. The properties of the series bimorph were strongly influenced by the space charge that appears due to the inhomogenities in resistance and dielectric constant.. Some theoretical considerations were made regarding the electric fields that appear inside the bimorph, when heated. It is shown that the value and the direction of these fields could suffer considerable changes in a bimorph structure, compared with the situation when each of the two components is heated separately. The possibility to obtain structures with improved pyroelectric coefficient was put into evidence.
Defects in Superconductor thin Films
Authors: Nandedkar A. S. , Ganapathi L.
Published: April 2011

Computer aided simulations were used to investigate various grain boundary configurations in superconducting (YBa2Cu3O7) thin films. Four angles for tilt grain boundaries were simulated (12.45°, 53.5°, 66.4°, and 74.6°). Energies of unrelaxed configurations were calculated.
Atomic Scale Simulations of Tensile Failure in Metal Oxides
Authors: Streitz F. H. , Mintmire J. W.
Published: April 2011

We describe atomic-scale simulations of the failure under tensile load of an aluminum-alumina heterostructure, comparing the results with similar simulations of failure in metallic aluminum and the ceramic α-alumina. The simulations were performed using a novel computational method which explicitly includes variable charge transfer between cations and anions in an empirical potential. From our simulations we estimate the theoretical limit of yield stress for the interface to be approximately 2 GPa, at a strain of only a few percent. The theoretical limit for yield stress in α-alumina, for comparison, is about 45 GPa.
Electronic States in Cd1−xZnxTe/CdTe Strained Layer Coupled Double Quantum Wells and their Photoluminescence
Authors: Li Tiesheng , Lozykowski H. J.
Published: April 2011

Experimental and theoretical investigations of electronic states in a strained-layer CdTe/CdZnTe coupled double quantum well structure are presented. The optical properties of this lattice mismatched heterostructure were characterized by photoluminescence (PL), PL excitation and polarization spectroscopies. The influence of electrical field on exciton states in the strained layer CdTe/CdZnTe coupled double quantum well structure is experimentally studied. The confined electronic states were calculated in the framework of the envelope function approach, taking into account the strain effect induced by the lattice-mismatch. Experimental results are compared with the calculated transition energies.
Luminescence from SiOx Nanoclusters
Authors: wickboldt Paul , Cheong Hyeonsik M. , Pang Dawen , Chen Joseph H. , paul William
Published: April 2011

SiOx nanoclusters (7 nm to 17 nm) are produced by evaporation of SiO (or Si) in Ar (+O2) atmospheres. Room temperature photoluminescence (PL) measurements in vacuum reveal a broad band centered at 1.65 eV. Upon exposure to gas this PL band is extinguished in a matter of seconds, and another band centered at 2.12 eV appears. This effect occurs regardless of the gas used (He, Ar, N2, O2, H2O vapor or air) and is entirely reversible upon evacuation.

Transmission electron microscopy (TEM), Raman, infrared transmission, and X-ray photoluminescence spectroscopy (XPS) measurements are used to characterize the clusters. They are noncrystalline, and the oxidation state is a suboxide rather than SiO2 The PL spectra are independent of cluster size. The PL does not occur without sufficient oxidation and does not require the presence of bonded hydrogen. We are led to speculate that the radiative recombination occurs in electron states derived from a suboxide.
Blue Light Emission from Germanium Ultrafine Particles by the Gas Evaporation Technique
Authors: Nozaki Shinji , Sato S. , Denda A. , Ono H. , Morisaki H.
Published: April 2011

The ultrafine particles with diameters in the order of 10 nm were deposited onto Si and SiO2 substrates by evaporation of Ge in a pure hydrogen atmosphere. Although the as-deposited Ge ultrafine particles do not show any detectable luminescence, they emit blue light after being exposed to the UV light for a long time. The blue light is strong enough to be seen with the naked eye even under a room light. The photooxidation, unique to the Ge ultrafine particles, has been identified as a major factor contributing to the blue light emission.
Visible Photoluminescence from Silicon Nanoconstrictions formed by Heavy Hydrogen Implantation and Annealing Treatments
Authors: Pavesi L. , Bisero D. , Corni F. , Frabboni S. , Tonini R. , Ottaviani G.
Published: April 2011

Ion implanted crystalline Si samples show luminescence (PL) in the visible at cryogenic temperatures after annealing treatments. This phenomenon is studied by varying the ion fluences and the sample doping density, and it is discussed considering the damage caused by the H implantation and its evolution with thermal treatments. A working hypothesis about the PL has been formulated which considers the exciton localisation in nanoconstrictions formed in the regions among the different microvoids produced by the implantation.
Control of and Mechanisms for Room Temperature Visible light Emission from Silicon Nanostructures in SiO2 formed by Si+ Ion Implantation
Authors: Komoda T. , Kelly J.P. , Nejm A. , Homewood K.P. , Hemment P.L.F , Sealy B.J.
Published: April 2011

Implantation of Si+ ions into thermal oxides grown on silicon has been used to synthesise a two phase structure consisting of Si nanocrystals in a SiO2 matrix. Various processing conditions have been used in order to modify the size and population distributions of the Si inclusions. Photoluminescence spectra have been recorded from samples annealed in nitrogen, forming gas and oxygen. Both red and blue shifts of the luminescence peaks have been observed. It is concluded that the photoluminescence is a consequence of the effects of quantum confinement but is also dependent on the presence of irradiation-induced defects or Si/SiO2 interface states.
Correlation Between Quantum Nanocrystal Particle Size and Photoluminescence Using Raman Scattering
Authors: Forsvthe E. W. , Whittaker E. A. , Pollak F.H. , Sywe B. S. , Tompa G. S. , Khan B. A. , Khurgin J. , Lee H.W.H. , Adar F. , Schaffer H.
Published: April 2011

The PL (photoluminescence) and EL (electroluminescence) demonstrated for porous silicon and silicon rich SiO2 films have provided exciting opportunities for the integration of optoelectronics and Si digital electronics. Recent work has suggested that the porous silicon luminescence results from either quantum size or surface state effects. In this report, we review PL demonstrated in the visible spectral range from quantum nanocrystals (QNC) formed from Si embedded in an SiO2 matrix. We used Raman scattering to estimate the QNC particle size and correlated the shift in the luminescence spectra to the observed change in the Raman spectra. The PL spectrum peak shifted from 7000 to 8000 Å as the average particle size increased from ∼50 to 70 Å, measured from Raman scattering. Further, High Resolution Transmission Electron Microscopy, HRTEM, and X-ray Diffraction, XRD, measurements confirmed the particle size range. PL lifetime measurements and excitation intensity studies are also presented. The stable nature of the QNC embedded in an insulating or semiconducting matrix offers further advances towards the integration of optoelectronics with Si devices.
Influence of the Dispersion of the Size of the Si Nanocrystals on their Emission Spectra
Authors: Khurgin J. B. , Forsythe E. W. , Kim S. I. , Sywe B. S. , Khan B. A. , Tompa G. S.
Published: April 2011

A systematic study of the PL spectra of Si quantum nanocrystals in the SiO2 matrix has been performed. The results have been fitted to a quantum-confinement model that includes the nanocrystal size dispersion rather than a specific size of the nanocrystal. This serves as a strong confirmation of the confinement-induced nature of the PL. It has been shown that if the dispersion is taken into account, the position of the emission peak as well as the PL width can always be correlated with the average size of the nanocrystal.
Transparent Sol-Gel Matrices Doped with Quantum Sized PbS Particles
Authors: Gacodin T. , Boilot J.P. , Gandais M. , Ricolleau C. , Chamarro M.
Published: April 2011

Sol-gel matrices doped with lead sulfide nanoparticles showing a structured absorption spectrum were synthesized by using the procedure previously reported for CdS doped matrices. The morphology of the PbS aggregates was investigated by high resolution electron microscopy, and the observations are correlated with the absorption spectra. Finally, preliminary results concerning some linear optical properties of the materials are presented.
Synthesis of PbS Semiconductor Microcrystallites In Situ in Reverse Micelles
Authors: De Sanctis O. , Kadono K. , Tanaka H. , Sakaguchi T.
Published: April 2011

Colloid semiconductor particles of PbS have been prepared in reversed micelles of AOT-heptane. The effects of the AOT, water and reactants concentrations have been studied. The initial crystallite size and its growth with the time aging are strongly dependent on the water/AOT ratio and the ionic pool concentration. The absorption spectroscopic characteristics of the PbS particles have been examined. As the particle size decreases the band gap shifts in color from orange-brown to yellow and approaches the transition energy of the first allowed excited state of a PbS molecule. Moreover, the optical nonlinear property of PbS particles was investigated by Degenerate Four-Wave Mixing (DFWM) experiment using a frequency-doubled Nd:YAG laser.
Optical Features of Nanosize Iron and Molybdenum Sulfide Clusters
Authors: Wilcoxon J. P. , Samara G. , Newcomer P.
Published: April 2011

In the bulk state FeS2 and MoS2 are optically opaque, narrow bandgap semiconductors with no optical applications. We demonstrate that nanosize FeS2 and MoS2 have bandgaps that can be adjusted to the visible and even UV region of the spectrum by control of the cluster size. This opens up a host of applications of these materials as inexpensive solar photocatalysts. We demonstrate that the band-gap of both materials shifts to the blue with decreasing size but ceases shifting when a size of ∼ 3 nm (in the case of MoS2) is attained. We interpret this observation as a change from bulk quantum confinement of the hole-electron pair of a tiny semiconductor to a set of discrete molecular-like transitions more characteristic of a large molecule. Room temperature photoemission studies of these clusters demonstrate that, while photoemission shifts to the blue with increasing bandgap for large clusters, small clusters have photoemission exclusively from trapped sub-bandgap surface states. Chemical modification of the surface to introduce hole or electron traps can result in either an enhancement or a decrease in the photoluminescence. In addition, we report our results concerning chemical purification and preliminary surface characterization of MoS2 clusters by chromatography.
Peculiarity of Porous Silicon Formed in the Transition Regime
Authors: Lazarouk S. , Chumash V. , Fazio E. , La Monica S. , Maiello G. , Proverbio E.
Published: April 2011

Electrochemical anodization in the transition regime, between porous silicon formation region and electropolishing region, of monocrystalline silicon was investigated. Using this process bright and stable photoluminescence could be obtained on a very large range of substrate resistivities: p=12-0.005 Ωcm for p-type silicon and p = 20-0.001 Ωcm for n-type substrates.

Photoluminescence spectra, Fourier Transform IR (FTIR) absorbance and X-Ray Diffraction (XRD) measurements are reported. Investigations showed that anodic silicon suboxide was formed on the surface. The porous structure obtained in the transition regime is suggested to consist of silicon crystallites built inside an anodic oxide.
Effect of Geometrical Irregularities on the Band Gap of Porous Silicon
Authors: Sapoval B. , Russ S.
Published: April 2011

We discuss how the geometrical irregularities of small crystallites, quantum dots or wires may play an essential role in the density of electronic states near the band gap in semiconductors. Assuming a possible irregular structure for porous silicon we show that the irregularity has a strong effect on the value of the band gap and of the density of states in the near band gap region. The effect of the irregularity is essentially to enhance the quantum confinement effect and to screen the fundamental wave-function from the surface.
Investigation of Chemical Adsorbate Effects on Blue and Red Emitting Porous Silicon Samples
Authors: Rehm Julie M. , McLendon George L. , Tsybeskov Leonid , Fauchet Philippe M.
Published: April 2011

We have investigated the sensitivity of blue (PLmax = 480 nm) and red (PLmax = 660 nm) emitting porous silicon samples to various chemical adsorbates. Steady-state and time-resolved photoluminescence measurements and FTIR spectroscopy were employed to characterize the photophysical and optical effects induced by chemical exposure. The red samples, which are hydrogen terminated, exhibit quenching and recovery of photoluminescence intensity and broadening of the Si-Hx stretch bands upon exposure to liquid methanol. This behavior is attributed to the ability of the Si-Hx specie on the surface of the PSI to interact with the solvent molecules which temporarily traps the electrons and causes PL loss and Si-Hx broadening. The blue samples, which are oxygen terminated, display similar sensitivity to methanol. This sensitivity is attributed to the solvent's ability to change the surface passivation and thereby introduce competitive radiative and nonradiative recombination channels. The origin of the blue PL is discussed.
Effective Dielectric Function of Porous Silicon: the Transverse Component
Authors: Lugo J.E. , Del Río J.A. , Tagüeña-Martínez J. , Ochoa-Tapia J.A.
Published: April 2011

In previous works we have obtained an expression for the effective electrical conductivity of a columnar model simulating porous silicon. We used the averaging volume method that has proven to be successful in treating fluid transport in porous media. With this method we can calculate the bulk and the surface contribution to an effective transport property. The axial component can be solved analytically, but in the xY plane the calculation can only be performed numerically. However there is a certain approximation called Chang's cell (valid for high porosities) where the transverse component is also analytical. The extension of our original approach to find the axial component of the effective dielectric function is relatively simple, but the transverse component calculation presents interesting features.
Probing Optical Transitions in Porous Silicon by Reflectance Spectroscopy in the Near Infrared, Visible and UV
Authors: Theiβ W. , Arens-Fischer R. , Arntzen M. , Berger M.G. , Frohnhoff S. , Hilbrich S. , Wernke M.
Published: April 2011

Reflectance spectroscopy has been used to obtain the dielectric function of the solid phase of porous silicon. The method is based on a fit of a parameterized dielectric function model to measured spectra. A crucial step in the procedure is the 'dielectric averaging' of the microscopic dielectric function of the pore wall material to the macroscopic effective dielectric function which governs the optical properties.

Results are given for heavily and moderately p-doped samples of various porosities. For the latter large differences to bulk silicon have been found. The obtained dielectric functions are compared to the results of band structure calculations taken from literature.
Comparison of the Band Gap of Porous Silicon as Measured by Photoelectron Spectroscopy and Photoluminescence
Authors: Van Buuren T. , Eisebitt S. , Patitsas S. , Ritchie S. , Tiedje T. , Young J. F. , Gao Yuan
Published: April 2011

The peak energy of the room temperature photoluminescence of porous silicon is compared with the bandgap determined from photoelectron spectroscopy measurements for a series of porous silicon samples prepared under different conditions. The photoluminescence bandgap is found to be smaller than the photoelectron spectroscopy bandgap, but exhibits the same trend with preparation conditions. The width of both the photoluminescence spectrum and the L-absorption edge increases when the current density during the preparation is increased or the sample is allowed to soak in HF after preparation.
The Spectroscopy of Porous Silicon
Authors: Calcott P. D. J. , Nash K. J. , Canham L. T. , Kane M. J.
Published: April 2011

The spectroscopic evidence that the main visible photoluminescence (PL) band of porous silicon (the 'S' band) originates from quantum confined crystalline silicon is presented, and arguments that claim to invalidate this evidence are analysed in detail. We find that a careful study of all the spectroscopic data provides strong support for the quantum confinement model. Additionally we consider the interesting issue of the luminescence spectrum of a single silicon quantum dot.
Localized Nature of Photoluminescence from Anodically Oxidized Porous Silicon
Authors: Ito T. , Furuta K. , Yoneda T. , Arakaki O. , Hatta A. , Hiraki A.
Published: April 2011

Photoluminescence (PL) properties from anodically oxidized porous silicon (PS) have been investigated. Large temperature coefficients (-0.5 meV/K) of PL peak energy are observed for relatively strongly oxidized PS specimens which show saturation of PL peak blue shift upon further anodic oxidation. Time-resolved PL data show that the PL decay is characteristic of thermal activation process with an energy of 9-27 meV at low temperatures between 80 and 180 K. Those results can be explained using a luminescence model which assumes several PL centers in the energy gap and considers thermally activated and tunneling processes.
The Visible and the Infrared Luminescence Bands as a Tool for Characterization of the Porous Silicon Bandstructure
Authors: Petrova-Koch V. , Muschik T. , Polisski G. , Kovalev D.
Published: April 2011

The visible and the infrared photoluminescence bands in porous Si have been studied at low temperature for two series of samples: one in which the size of the crystallites has been varied and another in which the degree of surface degradation has been changed. It is shown that the relation of the two bands can be explored for characterization of the porous Si bandstructure. The size- and the surface dependence of the valence band and of the conduction band related states is discussed. A model is proposed for explanation.
Investigation of Electronic Properties of Porous Silicon by the Pulsed Surface Photovoltage Technique
Authors: Dittrich Th. , Flietner H.
Published: April 2011

Electronic states in por-Si are investigated by the pulsed surface photovoltage technique using the por-Si/c-Si interface as probe. The values of the potentials at the por-Si/n-Si and at the por-Si/p-Si interface are -0.05 V and +0.47 V, respectively. Non-monotonous photovoltage transients related to trapping in por-Si are observed. The traps can be annealed at 150°C and generated in H20 atmosphere.
Porous Silicon as an Ultraviolet Light Source
Authors: Kozlowski F. , Huber B. , Steiner P. , Sandmaier H. , Lang W.
Published: April 2011

In addition to photoluminescence and electroluminescence porous silicon is capable of emitting an ultraviolet line spectrum. This emission can be observed already at ambient conditions. We could identify this line emission as the spectrum of nitrogen. At a pressure of about 10 mbar the UV-intensity exceeds the intensity at ambient pressure about two orders of magnitude. Angular dependent spectroscopy and the light emission behaviour at lowered pressure led us to the conclusion that the silicon structures in the samples behave as sub-micrometer-sized electron guns. Dye covered glass substrates can be excited by the intense UV-light at about 5-20 mbar so that the red and green light of the dyes can easily be recognized under usual laboratory illumination. A luminous density of 240 Cd/m2 and 40 Cd/m2 could be achieved for the green luminescing ZnS:Cu,Al and the red luminescing YVO4:Eu respectively. Continuous UV-light emission could be observed for more than 1.5 hours at 2 mbar.
Visible Electroluminescence from Al-Porous Silicon Reverse Bias Diodes Formed on the Base of Degenerate N-Type Silicon
Authors: Lazarouk S. , Bondarenko V. , Pershukevich P. , La Monica S. , Maiello G. , Ferrari A.
Published: April 2011

We demonstrate current induced visible light emission from Schottky junctions between aluminium electrodes and porous silicon formed by electrochemical etching of degenerate n+ -type silicon. HF concentration and anodizing current were chosen to yield preparation conditions in the transition region between electropolishing and porous silicon formation regimes. The light emitting diodes were formed by magnetron sputtering of aluminum on the porous silicon surface. Visible electroluminescence (EL) was recorded when dc or ac voltages larger than 4 V were applied between the aluminium electrodes. The visible EL appears in the dark, at the edge of the electrodes at a reverse bias of 5-6 V. The intensity of emitted light increases with applied voltage; at applied bias higher than 7 V the light emitted was observable by the naked eye at normal daylight. Compared to forward bias solid state contact porous silicon devices, the structure has an increased stability (after 100 hours of continuous operation under a 7 V reverse bias, no appreciable modification was observed in emission intensity). The main features of this electroluminescence are very similar to the ones observed under avalanche breakdown of silicon p-n junctions.
Optoelectronic Effects in Porous Silicon Related to the Visible Luminescence Mechanism
Authors: Koshida N. , Koyama H. , Ozaki T. , Araki M. , Oguro T. , Mizuno H.
Published: April 2011

Some optoelectronic effects in porous Si (PS) have been investigated in relation to the visible luminescence mechanism. As regards photoluminescence (PL), particular emphasis is placed on the relationship between photoconduction (PC) and PL excitation (PLE) spectra, the interaction of external electric field and PL emission, and polarization properties of PL Main subjects of electroluminescence (EL) studies reported here are the dynamic behavior of EL operation and the formation of a large-area contact by a conducting polymer (polypyrrole: PP). The observed experimental results (almost complete coincidence of PC spectra with PLE ones, linear polarization memory of PL definite correlation between the polarization degree and the PL efficiency, and comparable response time of electrical PL quenching and EL to the PL decay time) are consistent with our hypothesis that the major process of PL takes place within Si nanocrystallites. The electrical characterization of light-emitting PS diodes with PP contacts ensures the usefulness of the contact formation by electropolymerization as a technique for uniform and efficient carrier injection into PS.
Luminescence Properties of Silicon Clusters: Chain, Ladder and Cubic Structures
Authors: Kanemitsu Yoshihiko
Published: April 2011

We have studied the luminescence properties of chemically synthesized Si8 clusters and Si-based polymers with different backbone structures. The optical properties of Si8 clusters depend strongly on the shape of clusters. The luminescence properties of the cubic Si8 cluster are different from those of the chain and ladder Si8 clusters. Moreover, the ladder and network Si polymers whose backbones are constructed by the organosilicon units with three Si-Si bonds exhibit broad photoluminescence spectra in the visible region. These luminescence properties are entirely different from those of the chain structure constructed by the organosilicon units having two Si-Si bonds. The unique optical properties of silicon clusters and polymers are discussed.
Luminescence Properties of Silicon Oxynitride Films
Authors: Fischer T. , Muschik T. , Schwarz R. , Kovalev D. , Koch F.
Published: April 2011

Silicon oxynitride films, deposited by chemical vapour deposition using a high He dilution are investigated with respect to their photoluminescence properties. At low temperatures the PL spectra, which show broad maxima around 2 eV, consist of two contributions with different decay times: a ns-fast and a ms-slow component, the slow one disappearing above 80 K. To account for these findings and for absorption measurements, a model is proposed to describe the structure of the samples consisting of inclusions of Si rich regions in an a-SiOxNy:H matrix with a common molecular aggregate as luminescence center. The different PL lifetimes supposedly result from different feeding mechanisms. Such a behaviour may be present in oxidized porous Si as well.
Si1−xGexY Alloy Nanocluster Materials Chemical Vapor Deposition of Si2H6Ge2H6 Mixtures in Zeolite Y
Authors: Dag Ömer , Kuperman Alex , Ozin Geoffrey A.
Published: April 2011

Si1−xGexY alloy nanocluster materials are synthesized by the chemical vapour deposition of Si2H6/Ge2H6 mixtures within the diamond lattice of 13Å supercages in acid zeolite Y. A multianalytical approach to the study of these Si1−xGexY nanomaterials over the full compositional range 0<x<1 provides information concerning the mode of formation, geometric and electronic structures, trends in the optical properties of the encapsulated alloy nanoclusters.
Control of Grain Size and Texture of Poly-Si with Atomic Hydrogen Under In Situ Ellipsometric Observation
Authors: Nakamura K. , Akasaka T. , He D. , Shimizu I.
Published: April 2011

A systematic study was performed to fabricate poly-Si thin films on glass substrates. Amorphous tissues were efficiently removed by forming a textured structure with the aid of atomic hydrogen. According to the real-time observation by spectroscopic ellipsometry(SE), the grain growth was enhanced by permeation of atomic hydrogen into the sub-surface. Hall measurements were made over a rather wide temperature range: 100 K--350 K. A rather high Hall mobility of 20 cm2/Vs or more was obtained at room temperature due to the reduction of amorphous tissues remaining at grain boundaries.
Grain Boundary Location-Controlled Polt-Si Films for Tft Devices Obtained Via Novel Excimer Laser Process
Authors: Kim H. J. , Im James S.
Published: April 2011

Based on a previously acquired physical understanding of the excimer-laser-induced crystallization process, we have developed a new crystallization technique that produces controlled microstructures and possesses a wide processing window. A patterned oxide capping layer was used as an antireflective coating to induce complete melting of an Si film under an SiO2 pattern, and partial melting of the Si film in the areas not under the capping layer—allowing controlled super lateral growth to proceed from the incompletely melted portion of the film to the completely melted portion. For the simple stripes used in this investigation, when the width of the completely molten region is less than a critical distance (above which nucleation of solids occurs in the middle of the completely melted regions), the resulting microstructure has large and elongated grains with one precisely located grain boundary running parallel to the stripe In the middle of the oxide capped region.

Arrangement of TFT devices on the resulting Grain boundary Location-Controlled (GLC) Si films with one (or zero) grain boundaries located perpendicular to the flow of electrons within the active channel portion of the TFT devices is illustrated. Such devices are expected to possess performance and uniformity characteristics that are superior to currently available poly-Si TFT devices.
Fabrication of Photoluminescent Amorphous Pillar Silicon Structures
Authors: Lazarouk S. , Katsuba S. , Kazuchits N. , De Cesare G. , Monica S.La , Maiello G. , Proverbio E. , Ferrari A.
Published: April 2011

Fabrication of amorphous pillar silicon structures showing visible photoluminescence (PL) by naked eye is reported. Some attempts involving silicon etching processes were previously performed by other authors, but PL from the resulting pillar structures was never observed.

The pillar silicon structures were grown by Plasma Enhanced Chemical Vapour Deposition (PECVD) of hydrogenated amorphous silicon (a-Si:H) over the surface of porous aluminum oxide films, previously formed by anodization process. Alumina pore dimensions varied between 4-200 nm, depending on the parameters of the anodization process. The size of the grown a-Si:H pillars was quite homogeneous and could be tightly controlled by the anodization conditions. Immediately after PECVD process, PL from the pillar structure was very weak and not visible by naked eye, but after a short electrochemical treatment in 1% wt HF at 2 mA/cm2 it became quite bright and visible at normal daylight. A red shift, very similar to the one shown by porous silicon formed on monocrystalline substrates, was observed during samples drying after the electrochemical treatment.
Carrier Confinement Effects in Epitaxial Silicon Quantum Wells Prepared by MOCVD
Authors: Maruska H. Paul , Sudharsanan R. , Bretschneider Eric , Davydov Albert , Yu J.E. , Pathangey Balu , Jones K.S. , Anderson Timothy J.
Published: April 2011

Silicon multiquantum wells ranging in width from 3 to 15 nm were deposited on closely lattice-matched ZnS barriers. MOCVD was used to deposit the ZnS films using diethyl zinc and hydrogen sulfide as the precursors; disilane was used to deposit silicon layers at low temperatures. Single and multiple silicon nano-layers were observed by transmission electron microscopy and secondary ion mass spectrometry. Photoluminesence studies revealed emissions peaks which were blue-shifted with respect to the edge emission from bulk silicon substrates. The observation of emission from silicon nanostructures shifted to wavelengths as short as the 800-850 nm range is consistent with the effects of quantum confinement in silicon nanostructures.
Endohedral Metallofullerenes: Isolation and Characterization
Authors: Dorn H. C. , Stevenson S. , Burbank P. , Sun Z. , Glass T. , Harich K. , Loosdrecht P. H. M. Van , Johnson R. D. , Beyers R. , Salem J. R. , Vries M. S. De , Yannoni C. S. , Kiang C. H. , Bethune D. S.
Published: April 2011

Since the initial discovery of fullerenes nearly a decade ago [1], material scientists have focused attention on the possibility of encapsulating one or more metal atoms inside these spheroidal carbon frames. The experimental realization of macroscopic quantities of endohedral metallofullerenes (Am@C2n, n=30-55) in the early 1990's has heightened interest in developing this new class of tunable materials with possible electronic and/or optical applications [2,3]. They have been characterized by a number of spectroscopic techniques, for example, scanning tunneling microscope [4,5], EXAFS [6,7] and x-ray diffraction and electron microscopy [8]. However, low production yields and purification difficulties have hampered the development of this new class of materials. The soluble product distribution usually consists of high levels of the empty-caged fullerenes C60, C70, C84 and decreasing levels of the higher fullerenes, while the endohedral metallofullerene fraction usually constitutes less than 1% of the total soluble yield. Furthermore, the endohedral metallofullerene fraction consists of molecules with different numbers of metal atoms encapsulated (m=1-3), cage sizes (C2n) and isomers of the same mass (e.g., Er2@C82). The purification process is further complicated by the chemical reactivity of several endohedral metallofullerenes [9] in aerobic environments. For several years, we have been involved in a collaborative effort to develop methodology for detection, isolation, and characterization of endohedral metallofullerenes. The focus of the present study is on fullerenes encapsulating metals from Group II1b, (Sc@C2n, Y@C2n, and La@C2n) and the lanthanide series metal (Er@C2n).
The Crystallographic Modeling of C60 Orientations in a Cubic Lattice
Authors: Shekhtman Veniamin Sh. , Dilanyan Ruben A. , Rybchenko Oksana G.
Published: April 2011

The symmetry conditions which rule the matching of a fulleren molecule's main axis in a translational lattice are considered. The full set of orientational states for the structure of fullerene are obtained. The different levels of correlations between symmetry axes of icosahedral molecule and cubic lattice are considered. This result used for analysis of orientational ordering models, including modulation structure and discrete orientational glass.
Molecular Dynamics Simulations of Hypervelocity Buckminsterfullerene Collisions
Authors: Robertson D.H. , Brenner D.W. , White C.T.
Published: April 2011

Molecular dynamics simulations of high-energy collisions between various combinations of C60 and C70 fullerenes were performed to calculate the threshold for molecular fusion of these clusters as a function of the center-of-mass collision energy. For collision energies below 90 eV, only non-reacting collisions occurred with no observation of any fusion. However, at higher collision energies molecular fusion of the colliding clusters was observed with the fusion probability approaching 1 by 160 eV collision energy. The non-fusing, rebounding collisions showed deeply inelastic behavior with the loss of translational energy to internal energy varying from 50 to 70 percent.
Search for the Ground State of C60B10
Authors: Esfarjani Keivan , Ohno Kaoru , Kawazoe Yoshiyuki
Published: April 2011

Recent experimental discovery of C60B10 has motivated the present search for the possible atomic structures of this system. We have already studied the electronic properties of C58BN within the density functional theory, and are interested in the effects of other substitutions of carbon atoms by boron and nitrogen in fullerenes. In our all-electron calculation, we adopt the mixed-basis approach in which 1s and 2p orbitals in addition to about 3000 plane waves are included. The total energy and the band gap of many possible configurations are calculated in order to predict the most stable structure of this system. The density of states is presented as well for the ground state configuration.
Fullerene Superconductors: Effects of Molecular Orientation and Valence
Authors: Yildirim T. , Barbedette L. , KniaŹ K. , Fischer J. E. , Lin C. L. , Bykovetz N. , Stephens P. W. , Sulewski P. E. , Erwin S. C.
Published: April 2011

We report results of high-resolution x-ray diffraction, dc magnetometry and Raman scattering on the quaternary alkali and ternary mixed alkali-alkaline earth fullerene compounds Na2RbxCs1−x C60 (0 < x < 1) and Rb3−δ BaδC60 (δ ∼ 0.4 and 2.0). These have the same cubic space lattices as K3C60 and Rb3C60 however the former exhibit long-range orientational correlations with Pa3 symmetry and the latter have N > 3 delocalized electrons per molecule. We show that the dependence of Tc on lattice constant is much steeper for the ordered as compared to the merohedrally disordered phases with N = 3, and that within the constraint of cubic symmetry Tc decreases significantly with increasing N > 3, in disagreement with some theoretical predictions.
Medical Applications of Water-Soluble Polyhydroxylated Fullerene Derivatives
Authors: Chiang Long Y. , Lu Fung-Jou , Lin Jaw-Town
Published: April 2011

The promising positive results in reducing superoxide free radicals (O2-.), generated by invitro xanthine and xanthine oxidase, have been demonstrated in the evaluation of potential uses of novel water-soluble [60] fullerenols as free radical scavengers. Observation reveals the inherent application of fullerenols in a wide range of medical and clinical areas as preventive alternatives for a long-term medical treatment.
Metal Film Nucleation and Growth on C60 Interfacial Layers
Authors: Hebard A. F. , Eom C. -B. , Haddon R. C. , Phillips Julia M. , Marshall J. H.
Published: April 2011

The effect that the presence or absence of interfacial C60 layers has on the nucleation and subsequent growth of overlying thin metal films has been studied using in situ resistivity measurements. Comparisons are made for Al and Cu films grown on quartz and yttria stabilized zirconia (YSZ) substrates. Our results indicate that electron donation across M/C60 (M=Al, Cu) interfaces reduces the percolation threshold for conductivity while simultaneously giving rise to an increased resistance in the metal due to electron depletion. Additional physical processes that are taken into account include the effect of different interfacial energies at the M/C60 and M/substrate boundaries and the effect of substrate tunneling between percolating metallic islands during the initial stages of film growth.