bluuebunny/crossref_metadata_2025 huggingface dataset:
Optical Properties Of Tantalum Oxide Films Deposited On BK7 Substrates By Excimer Laser Ablation
Authors: Boughaba S. , Islam M. U.
Published: April 2011

Thin amorphous films of tantalum oxide were grown on borosilicate crown glass substrates by KrF excimer pulsed laser ablation of a Ta2O5 target, in an oxygen environment. The deposition was performed at a temperature of 250 or 400 °C, while the oxygen pressure was set in the range 5 to 30 mTorr. The optical properties of the tantalum oxide coatings, as evaluated by reflectance/transmittance spectrophotometry, were found to be dependent on the oxygen gas pressure. At a pressure of 5 mTorr, absorbing films were obtained, with extinction coefficients above 10−2 (at λ=633 nm), along with an optical energy band-gap as low as 0.7 eV. At a pressure of 10 mTorr and above, the coatings had refractive indices up to 2.25 (at λ=633 nm), extinction coefficients below 10−4 (for λ>390 nm), and an optical energy band-gap in the range 3.9 to 4.0 eV.
Process Modeling Of The Laser Induced Surface Modification Of Ceramic Substrates For Thermal And Electrical Lines In Microsystems
Authors: Gruhn Herbert , Heidinger Roland , Rohde Magnus , Rüdiger Sabine , Schneider Johannes , Gahr Karl-Heinz Zum
Published: April 2011

Laser induced surface modification has been used to fabricate conducting paths in ceramic substrates. For the purpose of process simulation and prediction of process parameters a finite element model has been developed to simulate the thermal behaviour of the substrate during laser surface interaction. The results of the model calculation have been verified experimentally for alumina and Cordierite substrates. Using this model the width and the depth of the fabricated lines could be predicted as a function of the laser power and velocity. The stresses due to thermal mismatch are estimated and identified as the likely reason for crack formation which reduces the functionality of the conducting paths. Further developments will consider different ceramic substrates such as PZT.
Molecular multilayers: structure and templating effects
Authors: Heutz Sandrine , Bayliss Sallie M. , Cloots Rudi , Middleton Ruth L. , Rumbles Garry , Jones Tim S.
Published: April 2011

Powder X-Ray diffraction (XRD) has been used to study multilayered structures grown by, organic molecular beam deposition based on the molecular materials PTCDA and metal-free phthalocyanine (H2Pc). Double layers of different polymorphic forms (α, β1 and β2) of H2Pc indicate that the structure of the second layer is determined by the properties of the first layer. It is also shown that the first layer completely disrupts the crystallinity of the second layer in heterostructures containing PTCDA and H2Pc. The implication is that a strong templating effect occurs during the growth of multilayer molecular thin film structures.
Biogenic Inspiration for the Controlled Nucleation and Growth of Inorganic Materials
Authors: Heywood Brigid R , Hill Susan , Pitt Kate , Tibble Paul , Williams Stuart
Published: April 2011

The development of effective protocols for the control of crystal structure, size and morphology attracts considerable interest given the requirement for particles of modal size and shape in many areas of particle processing and the importance of crystallochemical selectivity in determining the exploitable properties of crystalline solids. In biological systems there are many examples of advanced “crystal engineering” in which materials are deposited in a highly controlled manner to produce crystal phases that are unique with respect to their structure, habit, uniformity of size and texture. A review of biomineralisation will show that while a complex array of strategies have evolved for regulating crystal growth, one feature is common to the biological paradigm. Interactions between supramolecular organic structures and the nascent inorganic solids play a fundamental role in controlling the deposition of the biominerals and ordering the assembly of these units into hierarchical structures. In order to gain a better understanding of the molecular recognition events, which take place at the organic-inorganic interface, a bio-inspired crystal chemical approach has been adopted. For this work organised organic assemblies (e.g. surfactant aggregates, peptide mimics, dendrimers) of precise molecular design (head group identity, packing conformation, primary sequence etc.) are being assayed for their effectiveness in controlling the nucleation and growth of crystals. It is evident from these studies that the chemical organisation of the polymeric microenvironment operates at the molecular level to control certain aspects of the nucleation, growth and stabilisation of inorganic particles. By systematically changing the molecular motif of the organic template we have established that the size, crystallographic orientation, growth and assembly of the mineral phase can be tailored to function. These results have relevance not only to our understanding of biomineralisation but also suggest a multiplicity of exploitable opportunities for the engineering of crystals.
Surface Modification of Si Field Emitter Arrays for Vacuum Sealing
Authors: Nagao M. , Tanabe H. , Kobayashi T. , Matsukawa T. , Kanemaru S. , Itoh J.
Published: April 2011

Vacuum packaging is a very important issue for vacuum microelectronics devices, especially for field emission displays. Emission current from the field emitter array (FEA), however, is known to decrease significantly after the vacuum packaging process. The current decrease is caused by heating treatment in the vacuum sealing process. In the present paper, the effect of the heating treatment on Si FEA was investigated and CHF3 plasma treatment was proposed for avoiding the problem. The Si FEA was exposed to plasma for 15sec and emission characteristics were measured before and after the vacuum sealing process using frit. It was confirmed that CHF3 plasma treatment was very effective for avoiding the emission degradation of the Si FEA. Details of the heating damage and CHF3 plasma treatment are described.
Experiment and Modeling of Conversion of Substrate-Waveguided Modes to Surface-Emitted Light by Substrate Patterning
Authors: Lu Min-Hao M. , Madigan Conor F. , Sturm J. C.
Published: April 2011

To predict the optical power that could be harvested from light emission that is waveguided in the substrate of organic light emitting devices (OLEDs), a quantitative quantum mechanical model of the light emitted into the waveguided modes has been developed. The model was used to compute the exact distribution of energy in external, substrate and ITO/organic modes as a function of the distance of the emission zone from the cathode. The results are compared to the classical ray optics model and to experiments in two-layer OLED devices. Classical ray optics is found to substantially over-predict the light in waveguided modes.
High Voltage Effects in Top Gate Amorphous Silicon Thin Film Transistors
Authors: Tosic N. , Kuper F. G. , Mouthaan T.
Published: April 2011

In this paper, an analysis of the high voltage induced degradation in top gate amorphous silicon Thin Film Transistors (TFT) will be shown, including the aspect of self-heating. It will be shown through experimental results that the degradation level under high voltages on drain and gate is different for TFT's with different channel lengths. In addition, the temperature distribution over the TFT area for devices with different channel length is simulated. Simulation shows that the peak of temperature distribution is located at the drain/channel edge and that level of thermal heating depends on the channel length.
Nd:YVO4 Laser Crystallization for Thin Film Transistors with a High Mobility
Authors: Dassow Ralf , Köhler Jürgen R. , Nerding Melanie , Strunk Horst P. , Helen Youri , Mourgues Karine , Bonnaud Olivier , Mohammed-Brahim Tayeb , Werner Jürgen H.
Published: April 2011

We crystallize amorphous silicon films with a frequency doubled Nd:YVO4 laser operating at a repetition frequency of up to 50 kHz. A sequential lateral solidification process yields polycrystalline silicon with grains longer than 100 μm and a width between 0.27 and 1.7 μm depending on film thickness and laser repetition frequency. The average grain size is constant over the whole crystallized area of 25 cm2. Thin film transistors with n- type and p-type channels fabricated from the polycrystalline films have average field effect mobilities of μn = 467 cm2/Vs and μp = 217 cm2/Vs respectively. As a result of the homogeneous grain size distribution, the standard deviation of the mobility is only 5%.
13C NMR study of functionalized nanotubes
Authors: Goze-Bac C. , Holzinger M. , Jourdain V. , Latil S. , Aznar R. , Bernier P. , Hirsch A.
Published: April 2011

13C nuclear magnetic resonance on functionalized single wall carbon nanoutubes is reported. We present a possible chemical modification of the nanotube in order to get soluble materials. NMR is used in order to study in details the chemical changes and their reversiblities. In particular, acid groups, inequivalent sp2 and sp3 carbons attributed to functionalized nanotubes are identified and characterized.
1D P-Block Halide Crystals Confined into Single Walled Carbon Nanotubes
Authors: Flahaut E. , Sloan J. , Coleman K.S. , Williams V.C. , Friedrichs S. , Hanson N. , Green M.L.H.
Published: April 2011

The synthesis and characterisation of one-dimensional (1D) crystals that have a well-specified chemistry, size and crystal structure have presented a formidable challenge for materials chemistry and analysis. We report here the filling of single (SWNTs) and double walled carbon nanotubes (DWNTs) by two different p-block halides, TlCl and PbI2. The nanotubes were produced either by the arc synthesis [1] or by a CVD method [2], based on the reduction of a Mg0.9Co0.1O solid solution by a hydrogen-methane mixture. In the case of TlCl, the structure of the crystals observed inside the tubes were all found to be derived from the rocksalt form and bilayer crystals were observed which exhibited reduced coordination relative to the fcc structure, as determined from high resolution transmission electron microscopy (HRTEM). In contrast, the crystal structure of bulk TlCl is a CsCl type structure. These results are consistent with the recently reported reduced coordination KI crystals formed within SWNTs [3]. In the case of PbI2 (i.e. with the CdCl2 structure), the use of HRTEM images combined with image simulations was used to confirm the partially reduced coordination of Pb atoms within the SWNT and DWNT confined 1D crystals.
Bromine Doped Single-walled Carbon Nanotubes
Authors: Liu Bingbing , Carlsten Jan , Sundqvist Bertil
Published: April 2011

Raman spectra for three SWNT samples doped with Br2 were studied using different excitation energies, 2.41 eV and 1.58 eV. One sample was also studied under high pressure up to 2 GPa. The vibrations in the low frequency range behave similarly for all samples. With an excitation energy of 1.58 eV, additional peaks at ca 240 cm−1 and 209 cm−1 are observed. With increasing pressure, the peak at 209 cm−1 gradually softens by 2∼3 cm−1 and becomes very weak at 1.7 GPa, while a new broader peak at ca 250 cm−1 remains constant up to 2 GPa. A number of high intensity overtone modes are observed under high pressure. These results indicate that the vibrations are attributed to bromine, not to the breathing modes of SWNTs, and that bromine resides in the bundles, possibly also inside tubes as a form of polymer.
Synthesis of Carbon, Silicon, and Boron-nitride Nanostructures via Microwave Plasma Enhanced Chemical Vapor Deposition
Authors: Cui Hongtao , Liu Wei , Stoner Brian R.
Published: April 2011

Carbon, silicon nitride and boron nitride nanostructures were synthesized using microwave plasma enhanced chemical vapor deposition. Both scanning and transmission electron microscopies and x-ray diffraction were used to observe the morphology and structures while energy dispersive x-ray was used to determine the composition of the nanostructured materials. Bamboo-like multiwalled carbon nanotube structures were observed while silicon nitride showed novel needle-like solid structures. All materials were deposited using a thin film iron catalyst seeded on either silicon with reactive plasma gases containing either a mixture of methane and ammonia or ammonia. A general discussion of the nucleation and growth mechanisms for the various materials will also be included.
Electron Emission from Pentagons on a Carbon Nanotube Tip Revealed by Field Emission Microscopy
Authors: Hata Koichi , Takakura Akihiro , Saito Yahachi
Published: April 2011

Field emission of electrons from multiwall carbon nanotubes (MWCNTs) has been investigated by field emission microscopy (FEM) in an ultra-high vacuum chamber. An MWCNT whose tip is capped by curved graphite layers gives a FEM pattern consisting of 6 bright pentagons when the surface of the nanotube tip is clean. Even in the ultra-high vacuum with a base pressure of about 10-10 Torr, residual gas molecules, attracted by polarization forces, adsorb on the nanotube tips. The adsorbed molecules reside preferentially on the pentagonal sites, giving bright spots in the FEM pattern. A flash heating the emitter at about 1300 K allows the molecules to desorb, and the nanotube emitter recovers the original clean surfaces. The adsorption and desorption of gas molecules are responsible for stepwise increases and decreases in the emission current, respectively.
Use of Multi-Walled Carbon Nanotubes for Conductive Probe Scanning Force Microscopy (Cp-SFM)
Authors: Stavens Kevin B. , Andres Ronald P.
Published: April 2011

Multi-Walled Carbon Nanotubes (MWNTs) mounted on commercial Scanning Force Microscopy (SFM) cantilevers have proven to be excellent probes for high resolution Tapping Mode-Scanning Force Microscopy (TM-SFM). Because of their robust nature and high electrical conductance, MWNTs are also attractive for use in Conductive Probe-Scanning Force Microscopy (CP-SFM). To be used in this application, however, the MWNT must be mounted via a high conductance contact to a conductive cantilever. A technique has been developed that produces such a contact. First, a MWNT is attached using acrylic adhesive to a commercial SFM cantilever that has been vacuum coated with gold. Then, a cap of gold is deposited over the junction between the MWNT and the SFM cantilever via spatially selective electro-deposition.
Electrochemical Testing of Potential Molecular Devices
Authors: Price David W. , Dirk Shawn M. , Rawlett Adam M. , Chen Jia , Wang W. , Reed Mark A. , Zacarias Angelica G. , Seminario Jorge M. , Tour James M.
Published: April 2011

Cyclic voltammetry (CV) was used to study the reduction potentials of 2,5-di(ethynylphenyl)-4-nitroaniline and 2,5-di(ethynylphenyl)nitrobenzene. Although no absolute reduction potentials can be used in the correlation between solution (CV) and solid state (nanopore) embodiments, each CV plot showed two reductions. The first and second reduction might correspond to switching events of recently reported molecular electronic devices in a nanopore. Cyclic voltammetry results are also reported for other potential molecular scale electronic devices.
Submicron Patterned Anodic Oxidation of Aluminum Thin Films
Authors: Huang Qiyu , Lye Whye-Kei , Longo David M. , Reed Michael L.
Published: April 2011

Alumina formed by the electrochemical anodization of bulk aluminum has a regular porous structure [1]. Sub-100 nm pores with aspect ratios as high as 1000:1 can easily be formed [2] without elaborate processing. Anodization of aluminum thus provides the basis for the inexpensive, high throughput microfabrication of structures with near vertical sidewalls [2]. In this work we explore the patterned anodic oxidation of deposited aluminum thin films, facilitating the integration of this technique with established microfabrication tools. An anodization barrier of polymethylmethacrylate (PMMA) is deposited onto 300 nm thick aluminum films. The barrier film is subsequently patterned and the exposed aluminum anodized in a 10% sulfuric acid solution. Barrier patterning techniques utilized in this study include optical exposure, ion-beam milling and nano-imprint lithography. Sharp edge definition on micron scale patterns has been achieved using optical methods. Extension of this technique to smaller dimensions by ion-beam milling and nano-imprint lithography is presented. We further report on the observation of contrast reversal of anodization with very thin PMMA barriers, which provides a novel means of pattern transfer. Potential applications and challenges will be discussed.
Nanoscale Silicon Microcavity Optical Sensors for Biological Applications
Authors: Chan Selena , Horner Scott R. , Miller Benjamin L. , Fauchet Philippe M.
Published: April 2011

The large surface area of porous silicon provides numerous sites for many potential species to attach, which makes it an ideal host for sensing applications. The average pore size can be easily adjusted to accommodate either small or large molecular species. When porous silicon is fabricated into a structure consisting of two high reflectivity multilayer mirrors separated by an active layer, a microcavity is formed. Multiple narrow and visible luminescence peaks are observed with a full width at half maximum value of 3 nm. The position of these peaks is extremely sensitive to small changes in refractive index, such as that obtained when a biological object is attached to the large internal surface of porous silicon. We demonstrate the usefulness of this microcavity resonator structure as a DNA optical biosensor which displays appropriate sensitivity, selectivity, and response speed. A probing strand of DNA is initially immobilized in the porous silicon matrix, and then subsequently exposed to its sensing complementary DNA strand. Red-shifts in the luminescence spectra are observed and detected for various DNA concentrations. The spectral shifts confirm successful recognition and binding of DNA molecules within the porous structure. Detailed device fabrication procedures and the results of extensive testing will be presented. The detection scheme has also been extended to include the detection of viral DNA, proteins, and potentially bacteria. This work will lead to the development of a “smart bandage”, where the detection of bacteria or viruses can be diagnosed and an antibiotic treatment can be recommended.
Thermal Functionalization of Hydrogen-Terminated Porous Silicon Surfaces with Terminal Alkenes and Aldehydes
Authors: Boukherroub Rabah , Lockwood David J. , Wayner Danial D. M. , Canham Leigh T.
Published: April 2011

H-terminated porous silicon (PSi) surfaces were chemically modified with terminal alkenes and aldehydes at high temperature to yield organic monolayers covalently attached to the surface through Si-C and Si-O-C bonds, respectively. Diffuse reflectance infrared Fouriertransform and X-ray photoelectron spectroscopies have been used to characterize the surfaces. Derivatized surfaces retain the PSi photoluminescence. Chemography was used to monitor the chemical changes of the PSi surface when exposed to 100% humidity in air. Organic monolayers linked through Si-C bonds are found to be highly resistant and have shown a better protection of the surface against corrosion compared to surfaces that are linked through Si-O-C bonds. The surface functionalized with ethyl undecylenate exhibits an even higher passivation of the surface through the presence of small amounts of oxide, which are induced by traces of water present in this chemical reagent, along with organic molecules attached to the surface.
Factors affecting Ge nanocrystal size in co-sputtered Ge+SiO2films
Authors: Choi WK , Ng V , Ho YW , Chen TB , Ho V
Published: April 2011

The high resolution transmission electron microscopy and Raman spectroscopy results of germanium nanocrystals embedded in SiO2 synthesized by rapid thermal processing (RTA) have been presented. From the results of samples with different Ge concentrations, it was concluded that there is a narrow window in the Ge concentration that can produce nanocrystals. We also showed that it is possible to vary RTA duration or temperature to produce Ge nanocrystals with varying sizes. Our results therefore suggest that it is possible to utilize (i) annealing duration and; (ii) temperature to tune crystal sizes for optoelectronic applications.
Novel Solid Nano Diamond/Pyrocarbon Semiconductor Materials
Authors: Gordeev Sergey K. , Belobrov Peter I. , Kiselev Nikolay I. , Petrakovskaya Eleonora A. , Ekstrom Thommy C.
Published: April 2011

Porous solids made of nano diamonds cemented by pyrocarbon show a semiconductor behavior where the properties are controlled by the factor γ = mass ratio of sp2/sp3 phases. The volume electrical conductivity μ (T) was measured in the temperature range 77 - 290 K as a function of γ. By controlling the ratio γ from 0 to 50 %, the electrical conductivity will change by 12 orders of magnitude. The semiconductor activation energy depends on γ-ratio and has values between 0.03 - 0.3 eV. The variable-range hopping conduction was checked by measuring μ against T and to look for a T1/4 dependence. A positive thermoelectric power showed that NDC was p-semiconductor with a narrow band gap. The paramagnetic nanodiamonds, g = 2.0027(1) and δH = 0.86(2) mT, will still have this property in the NDC material and both the g-value and line width are not dependent on γ-ratio and temperature. The concentration of unpaired electrons is 41019 spin/g that equals to one dangling bond per nanodiamond particle. Absences of magnetoresistance and Hall potential were observed for all NDC materials. This was interpreted as resulting from a Tamm - Lifshitz - Pekar electronic conduction mechanism within onedimensional band of surface states.
Effects of the Substrate Temperature, Annealing Temperature, and Hydrogen Presence During Sputtering on the Luminescence of Eu-Doped Nanocrystalline Si/SiO2
Authors: Nery G. A. , Fonseca L. F. , Liu H. , Resto O. , Weisz S. Z.
Published: April 2011

We synthesized RF co-sputtered Eu2O3, Si and SiO2 on quartz substrates at different temperatures, and with and without the presence of H2 gas during sputtering. The photoluminescence of the samples was measured using both the 514.5nm green line and 457.9nm blue line of an Ar laser. The samples were then annealed, their luminescence remeasured, and the process repeated for a range of temperatures from 470°C to 1084°C. The effects of substrate temperature, presence or absence of H2, annealing, and the use of the green or blue line on the observed luminescence are presented.
Effect of Hydrogen Treatment on High Efficiency Electroluminescence Device Using Silicon Nanocrystals
Authors: Sato K. , Izumi T. , Iwase M. , Show Y. , Nozaki S. , Morisaki H.
Published: April 2011

We have fabricated the electroluminescence (EL) device using silicon (Si) nanocrystals, which were formed on Si substrate by co-sputtering of Si and silicon dioxide (SiO2). By treating with the Si nanocrystals in the hydrofluoric (HF) acid solution, the SiO2 region of the luminous layer reduced, and then, the electrons were efficiently injected in the Si nanocrystals. At the same time, Pb-center (non-radiative recombination center) was decreased by hydrogen termination to the Si-dangling bond in the interface between the Si nanocrystals and the SiO2 layer From these effects of the HF treatment, the high efficiency red light emission with the external quantum efficiency (EQE) of 0.35 % was obtained from the HF treated EL device under the operating voltage of +4.5 V.
Optical and structural analysis of Ge quantum dots embedded in strained Si quantum wells grown on patterned substrates
Authors: Beyer A. , Müller E. , Sigg H. , Stutz S. , David C. , Grützmacher D. , Ensslin K.
Published: April 2011

Germanium quantum dots embedded in silicon have been used in the past to improve the opto-electronic properties of Si based materials. The idea is to overcome the limitation of the indirect band gap of Si by a strong localization of the carriers in quantum dots. However, the Ge quantum dots provide a strong carrier confinement only for the holes, the electrons are only weakly confined in the Si. In this study we embedded the Ge quantum dots in strained Si quantum wells grown on relaxed SiGe buffer layers. The strained Si quantum wells provide a confinement of the electrons in the vicinity of the Ge dots.

The structures were deposited on planar as well as on patterned substrates by molecular beam epitaxy. The structural and optical properties of the samples were analyzed using high resolution cross sectional transmission electron microscopy (TEM) as well as low temperature photoluminescence. The size of the mesa structures have been used as experimental parameter. Relaxed buffer layers grown on line shaped mesa structures show a strongly reduced dislocation density. Consequently the deep luminescence attributed to dislocations in the buffer layers is strongly reduced and pronounced photoluminescence of the quantum structures grown on top of the buffer layers can be observed.
Photoluminescence from Nanoscale Si in a-SiOxMatrix
Authors: Umezu I. , Yoshida K. , Inada M. , Sugimura A.
Published: April 2011

We prepared a-SiOx (x < 2.0) films by co-sputtering Si and SiO2 targets and found that these films included nano-scaled a-Si regions in a-SiOx matrix. The structure of the films was evaluated by X-ray photoemission spectroscopy (XPS) and infrared absorption. The XPS spectra in the a-SiOx films showed two Si 2p peaks around 99.6 and 104 eV. This indicates that the a-SiOx films were composed of Si and a-SiO2 regions. The optical gap energy of this system rapidly increased when x exceeded 0.7. The rapid increase in the optical gap of this system indicates the formation of nanometer sized Si islands. The photoluminescence (PL) peak energy of this material decreased with increasing temperature above 60 K when x was less than 0.7. On the other hand, when x exceeds 0.7, the PL peak energy increases with temperature above 60 K. The quenching of PL intensity also takes place at 60 K. These results indicate a film structure change at x = 0.7, and recombination path and carrier distribution changes at 60 K. A correlation between film structure and PL properties are discussed. PL properties in this system are explained by superposition of the PL peak from nanoscale Si and a-SiO2 matrix.
Correlation Between Bulk Morphology and Luminescence in Porous Silicon Investigated by Evaporation Induced Pore Collapse
Authors: Sirbuly Donald J. , Mason Michael D , Buratto Steven K.
Published: April 2011

We use a combination of scanning electron microscopy, laser scanning confocal microscopy and luminescence spectroscopy to correlate the emission properties of anodized porous silicon (PS) with film morphology in samples that have undergone evaporation induced collapse of the underlying porous structure. Several PS samples were investigated as a function of the current density (J) and total etch time, while the total charge (Q) injected per unit area (and the total Si removed) was kept constant during etching. From this data two classes of PS samples emerge. Porous silicon samples produced at high current density have a 3-dimensional pore network with a narrow distribution of blue-green emitting chromophores. In contrast, low current density samples form a 2-dimensional pore network normal to the Si substrate with larger chromophores and exhibit broad red luminescence.
Silicon Nanoclusters in Si-SiO2 System
Authors: Espinoza-Beltrán F.J. , Díaz-Flores L.L. , Yáñez-Limón J.M. , Morales-Hernández J. , Rodríguez-Melgarejo F. , Vorobiev Yuri , González-Hernández J.
Published: April 2011

Low concentration of Si nanoparticles (n-Si) produced by a ball milling method were introduced into a SiO2 matrix by the sol-gel method. These SiO2 sol-gel suspensions were prepared with high water-TEOS ratios. The samples obtained have high silanol concentration as was proved by infrared spectroscopy. The silanol structures of this samples are resistant for temperature as high as 800°C. Raman scattering measurements showed evidence of a photo-oxidation effect of the nSi particles embedded into the SiO2 matrix with high silanol concentration. The nanometric size of the Si particles was computed from Raman scattering measurements. The photo-oxidation effect was observed as a gradual shifting and broadening of the Raman signal due to the reduction of the particle size from 14 to 7 nm.
Fabrication and Characterization of Cold Electron Emitter Based on Nanocrystalline Silicon Dots
Authors: Nishiguchi K. , Zhao X. , Oda S.
Published: April 2011

A cold electron emitter has been made from nanocrystalline silicon (nc-Si) dots. Nc-Si dots are formed in the gas phase by very-high-frequency (VHF) plasma enhanced chemical vapor deposition (CVD). Electrons, accelerated by electric field, are ballistically transported through nc-Si and SiO2, then extracted into vacuum. Electron emission efficiency is optimized through varying nc-Si film thickness, surface roughness, and by short thermal oxidation.
A Significant Change in Refractive Index of Nanocrystalline Porous Silicon Induced by Carrier Injection
Authors: Toriumi Y. , Takahashi M. , Koshida N.
Published: April 2011

To verify the availability of porous silicon (PS) device for application to optical switching, the photonic response of PS for carrier injection has been studied for a microcavity configuration. The observed reflectance spectral shift in the PS microcavity with increasing diode current is analyzedunder the assumption that the refractive index of nanocrystalline silicon is increased by carrier injectionand the subsequent accumulation possibly in localized states. This is consistent with the experimental results obtained from dynamic response measurements: there are fast and slow components in the refractive indexchange. It is also demonstrated that under the pulsed operation, the PS microcavity operates as a current-induced nonlinear optical switching device.
Gas source molecular beam epitaxy of high quality AlGaN on Si and sapphire
Authors: Nikishin S. , Kipshidze G. , Kuryatkov V. , Zubrilov A. , Choi K. , Gherasoiu Íu. , Peralta L. Grave de , Prokofyeva T. , Holtz M. , Asomoza R. , Kudryavtsev Yu. , Temkin H.
Published: April 2011

We report the results of epitaxial growth experiments on AlxGa1−xN (0≤ x ≤ 1) on Si(111) and sapphire substrates aimed at understanding the origin and elimination of cracking. We describe growth procedures resulting in thick layers of AlxGa1−xN, grown by gas source molecular beam epitaxy with ammonia, that are free of cracks. In GaN layers with the thickness of ∼2.5 µm, we find the background electron concentration of (1-2)×1016 cm−3 and mobility of (800±100) cm2/Vs. In AlxGa1−xN (0.2 < x < 0.6) with the film thickness of 0.5-0.7 µm the electron concentration of (2-3)×1016 cm−3 is obtained. Low background concentrations in GaN allow for formation of p-n junctions by doping with Mg. Light emitting diodes with the peak emission at 380 nm have been demonstrated.
Polarity of Hexagonal GaN Grown on GaAs (111)A and (111)B Surfaces by HVPE and Movpe
Authors: Takahashi O. , Namerikawa M. , Tanaka H. , Souda R. , Suemasu T. , Hasegawa F.
Published: April 2011

Dependence of hexagonal GaN polarity on the polarity of GaAs (111) substrates was investigated by CAICISS for HVPE and MOVPE. Although polarity of MOVPE GaN followed the polarity of GaAs substrate; Ga polar for (111)A and N polar for (111)B, both layers of HVPE GaN grown on GaAs (111)A & B surfaces were Ga polarity. Crystal quality was better for Ga polarity layers than for N polarity layers for MOVPE. However, Ga polar GaN grown on GaAs (111)B by HVPE had better crystal quality than that on the (111)A. These results can be understood by difference of the buffer layers and difference of growth kinetics between HVPE and MOVPE.
Surface Modification of Cubic Gan Buffer Layer Grown by Metalorganic Vapor Phase Epitaxy
Authors: Nagayama Akira , Katayama Ryuji , Wu Jun , Onabe Kentaro , Sawada Hidetaka , Takuma Eliko , Ichinose Hideki , Shiraki Yasuhiro
Published: April 2011

Anisotropic X-Ray diffraction (XRD) and transport properties of cubic GaN grown on GaAs substrates correspond to the features of low-temperature grown GaN (LT-GaN) buffer layer. When the LT-GaN layer is grown on the surface tilted from (001) to [1-10] with annealing in arsenic ambient, the macroscopic step edges along [1-10] direction are modified by either the ambient of thermal annealing, or substrate misorientation. A parallel conduction in GaN, GaAs, and GaN/GaAs hetero-interface was observed by photoconductivity measurements. Transmission electron microscope (TEM) observation shows that self-annihilations for (-111) B stacking faults are preferentially occurred near GaAs interface when GaN film grown on the surface tilted from (001) toward [1-10] (As step edge) is annealed in arsenic ambient. TEM observation also shows that stacking faults and dislocations are preferentially generated near GaN/GaAs interface. It is suggested that anisotropic transport properties correspond to the well-like potential generated by band bending at GaN/GaAs interface. The nearly isotropic mobility of 3,000 cm2/Vsec at 77K is obtained by improving interface property.
Growth Mode and Defects in Aluminum Nitride Sublimated on (0001) 6H-SiC Substrates
Authors: Liu Lianghong , Liu Bei , Shi Ying , Edgar J. H.
Published: April 2011

The effect of substrate preparation on the sublimation growth of AlN on 6H-SiC was investigated at about 1800°C and 400 torr. Short and long-time sublimation growths of AlN indicated that the nucleation, growth mode, and defects formed depended on the substrate surface preparation. Growth on an off-axis 6H-SiC substrate with 6H-SiC epilayer was in the step flow growth mode in contrast to the island growth mode on as-received substrates, while the 2-D growth was achieved on substrates first coated with an AlN epitaxial layer. Cracks due to the lattice and mainly large thermal expansion coefficient mismatch were always observed in the deposited AlN crystal, as characterized by SEM and optical microscopy.
Raman spectroscopy studies in InGaN/GaN wurtzite epitaxial films
Authors: Correia Maria R. , Pereira Sérgio , Monteiro Teresa , Pereira Estela , Alves Eduardo
Published: April 2011

In this work we studied a set of nominally undoped epitaxial InxGa1−xN wurtzite films grown on (0001) sapphire substrates. In order to separate the contribution of the strain and indium content in the phonon mode frequency, indium mole fraction was determined using a strain insensitive method, Rutherford backscattering spectrometry (RBS). Strain was evaluated by comparing the lattice constants measured by X-ray diffraction (XRD) with the relaxed lattice parameters given by Vegard's law. Samples with comparable indium content, but under different states of strain were used as reference. This allowed the behaviour of different Raman shift modes for both, strain and composition to be independently established. We also assess the potentiality of Raman spectroscopy for the evaluation of crystalline quality by comparing the results obtained with the ones provided by other well-established methods such as XRD and RBS.
Preparation of 30×30 mm2 Free-Standing GaN Wafer by Mechanical Liftoff and Optical Properties in The Backside of The Free GaN by Cathodoluminescence
Authors: Kim Hwa-Mok , Oh Jae-Eung , Kang Tae-Won
Published: April 2011

In this study, free-standing GaN was produced from 350- to 400-μm-thick GaN films grown on sapphire by using hydride vapor phase epitaxy. The thick films were separated from the substrate by using the mechanical polishing method with a diamond slurry. After liftoff, the bow is slight or absent in the resulting free-standing GaN. Double-crystal X-ray diffraction, hall measurements, and cathodoluminescence were used for characterizing the free-standing GaN wafer. To investigate spatially the backside of the free-standing GaN substrate, we controlled the electron beam energy from 5 keV to 30 keV. As the beam energy increased, dark regions, i.e., nonradiative regions, become smaller than bright regions. We think this means that nonradiative centers, i.e., threading dislocations, merge during film growth.
Persistent Photoconductivity in a High Mobility two Dimensional Electron Gas in an AlGaN/GaN Heterostructure
Authors: Elhamri S. , Saxler A. , Cull D. , Mitchel W. C. , Elsass C.R. , Smorchkova I.P. , Heying B. , Poblenz C. , Fini P. , Keller S. , Petroff P.M. , DenBaars S.P. , Mishra U.K. , Speck J.S.
Published: April 2011

Temperature-variable Hall and Shubnikov- de Haas effects have been used to study persistent photoconductivity in an AlGaN/GaN heterojunction. At liquid helium temperatures, the mobility in this structure was close to 55000 cm2/Vs. A blue GaN-based light emitting diode was used to illuminate the sample. This illumination resulted in a persistent photocurrent, which allowed us to vary the carrier density and study the dependence of the mobility on the carrier concentration. Exposing the sample to this light resulted in an increase in the carrier density. For small increases in the density, the mobility also increased. However, unlike in previous reports by other authors, extended illumination resulted in an increase in the density and a decrease in the mobility. The initial increase in the mobility is attributed to increased screening due to the increase in the carrier density, while the decrease in the mobility may be attributed to alloy scattering.
Time-Resolved Photoluminescence Measurements of In0.15Ga0.85N/In0.015Ga0.985N Quantum Wells with Si-doped Barriers
Authors: Ryu Mee-Yi , Yu Young Jun , Yu Phil Won , Shin Eun-joo , Oh Eunsoon , Sone Chul Soo , Nam Ok Hyun , Park Yong Jo
Published: April 2011

A systematic study of photoluminescence (PL) and time-resolved PL spectra from In0.15Ga0.85N/In0.015Ga0.985N quantum wells (QWs) with different Si doping concentration in the barriers has been carried out. As the Si doping concentration increases, the PL emission intensity was increased and the PL peak energy was blueshifted. The energy separation between the spontaneous emission (SPE) and stimulated emission (SE) peaks decrease with increasing Si doping concentration. We also observed that the slow decay time τ2 in the QWs decreases with increasing Si doping concentration, from ∼ 130 ns for [Si] = 2 × 1018 cm−3 to ∼ 30 ns for [Si] = 1 × 1019 cm−3. The PL emission peak shifts to lower energies with delay time after a pulsed excitation and this shift decreases with increasing Si doping concentration. The increased recombination rate, the decrease of peak shift with delay time, and the reduced separation between the SPE and SE peaks with increasing Si doping concentration are attributed to the screening of piezoelectric field by carriers originated from Si doped barriers.
Island decay on the anisotropic Ag(110) surface
Authors: Morgenstern Karina , Lægsgaard Erik , Besenbacher Flemming
Published: April 2011

We have investigated the decay of two-dimensional islands on the anisotropic Ag(110) surface using variable-temperature scanning tunneling microscopy. Contrary to predictions from traditional Ostwald ripening theory, a quasi-one-dimensional decay mode is observed at low temperatures (175-220 K). A surprisingly sharp transition to the quasi-two-dimensional decay mode is observed around 220 K. This transition is accompanied by a fast equilibration of the island shape. These findings have tentatively been rationalized within a simple model to identify the underlying rate limiting atomistic processes.
Ordering in AlGaN: Thermodynamical and X-Ray Analysis
Authors: Ruffenach-Clur Sandra , Briot Olivier , Moret Matthieu , Aulombard Roger-Louis
Published: April 2011

Up to date, different groups have reported ordering phenomena in nitride alloys like InGaN or AlGaN. These evidences of ordering during and after growth lead us to study the atomic arrangement of Al and Ga on site III in the AlGaN alloy using a thermodynamical approch. We have defined the enthalpy and the entropy of our system using the Keating model and the cluster variation method limited to the first coordination shell. This study gives us a good way to understand ordering, and moreover to see why at three special compositions (25%, 50% and 75%), the alloys have good chances to present a better crystalline quality than in other cases.

We have grown AlGaN samples increasing the Al composition by 10% steps, in the full range of composition from GaN to AlN. All the growth parameters were kept constant except for the alloy composition. X-Ray diffraction measurements performed on these samples indicate that FWHM increases linearly with Al composition, except for three compositions, where according to our thermodynamical study, we observe an improved crystalline quality.
Surfactant-Mediated Growth of MnSi1.7 on Si(001)
Authors: Teichert S. , Hortenbach H. , Sarkar D. K. , Beddies G. , Hinneberg H.-J.
Published: April 2011

The formation of the silicide MnSi1.7 by reactive deposition of Mn onto Si(001) has been studied using Sb as a surfactant. The growth was performed under UHV conditions by simultaneous or consecutive exposure of the Si substrates, held at high temperatures (550°C, 600 °C), to a flux of Sb and Mn atoms. The presence of Sb during the growth strongly increases the island density and changes the crystalline orientation of the MnSi1.7 grains. The morphology and the orientation of the resulting silicide are the same both for the deposition of Mn on a Sb terminated Si(001) surface and for the co-deposition of Mn and Sb on Si(001). A residual Sb coverage close to one monolayer (ML) at the sample surface has been determined for both of the preparation modes at Tsub = 550 °C. The transition from the growth mode without Sb to the surfactant-controlled growth has been studied for Tsub = 600 °C. It has been found that the silicide morphology and orientation strongly depend on the thickness of Sb pre-coverage, which was increased from 0 to about 0.7 ML (1 ML = 6.78·1014 atoms cm-2).
Dynamics of InAs Quantum Dots Formation on AlAs and GaAs
Authors: Yakimov M. , Tokranov V. , Oktyabrsky S.
Published: April 2011

We have studied the formation of InAs quantum dots (QDs) grown by molecular beam epitaxy on top of GaAs and 2 ML-thick AlAs layers in the temperature range from 350 to 500°C. In-situ reflection high energy electron diffraction (RHEED) patterns were recorded in real time during the growth and analyzed to characterize the 2D-to-3D transition on the surface, including QD formation, and ripening process. The kinetics of QD formation was studied using the InAs growth rates ranging from 0.01 to 1 ML/s and different ratios of As2/In fluxes. RHEED patterns and ex-situ atomic force microscopy images were analyzed to reveal the development of sizes and shapes of the single-layer and stacked QD ensembles. The critical InAs coverage for QD formation was shown to be consistently higher for dots grown on the AlAs overlayer than for those grown on GaAs surface. Self-assembly of multilayer QD stacks revealed the reduction of the critical thickness for dots formed in the upper layers.
In Situ Investigation of TixAly Film Growth As A Function of Substrate Temperature and Film Stoichiometry
Authors: Lackner St. , Abermann R.
Published: April 2011

The growth of TixAly-alloy films was investigated by measuring the film stress during as well as after the film deposition under UHV-conditions with a cantilever beam technique. The alloy films were deposited on alumina substrates from two separate evaporation sources. The substrate temperature was varied from room temperature to 500°C and the alloy stoichiometry from Ti25Al75 to Ti75Al25.

At a substrate temperature of 200°C the dependence of the growth stress of the alloy film on stoichiometry is pronounced and varies from tensile stress in the case of titanium rich alloy-films to compressive stress in aluminium rich films. The stress changes after deposition at 200°C are negligible for titanium rich alloys but in the case of aluminium rich alloys small tensile stress changes are measured.

At a substrate temperature of 500°C the film stress is compressive and the dependence on stoichiometry is rather small. The stress vs. thickness curves of Ti25Al75-films, however, show unexpected oscillations during growth while at lower aluminum content stable stress curves are measured. The stress changes after deposition are large in each case, interpreted to indicate film recrystallisation and/or continued alloy formation.

In a second series of experiments we have measured the internal stress changes in Ti25Al75-alloy films during annealing. It can be shown that the changes in the film stress can be correlated with changes in the film microstructure seen in the electron microscope (TEM).
Oblique Stacking of Three-Dimensional Dome Islands in Ge/Si Multilayers
Authors: Sutter P. , Sutter E. , Vescan L.
Published: April 2011

The organization of Ge ‘dome’ islands in Ge/Si multilayers has been investigated by cross-sectional transmission electron microscopy. Ge ‘domes’ are found to spontaneously arrange in oblique stacks, replicating at a well-defined angle from one bilayer to the next. The formation of oblique island stacks is governed by a complex interplay of surface strain - generated by the already buried islands - and surface curvature - caused by the inherent tendency of large ‘domes’ to carve out material from the surrounding planar substrate.
Nanoassembled Model Catalysts: Changing Efficiency and Selectivity Atom-by-Atom
Authors: Heiz U. , Abbet S. , Häkkinen H. , Landman U.
Published: April 2011

Cluster-assembled materials open fascinating new routes for tuning physical and chemical properties by changing cluster size and often these materials behave completely differently than their bulk analogues. By depositing gas phase cluster cations on various surfaces, cluster model catalysts are fabricated, which exhibit remarkable catalytic activity. While inert as bulk material, gold cluster catalysts oxidize carbon monoxide with Au8 to be the smallest active size. In contrast to the high selectivity of Pd(111) surfaces for the cyclotrimerization of acetylene, small supported Pdn clusters reveal a strongly size-dependent selectivity for the polymerization of acetylene and catalyze the cyclotrimerization as well as the hydrogenation of the formed intermediate C4H4.
Anomalous Diffusion in Active Intracellular Transport
Authors: Caspi Avi , Granek Rony , Elbaum Michael
Published: April 2011

The dynamic movements of tracer particles have been used to characterize their local environment in dilute networks of microtubules, and within living cells. In the former case, 300 nm diameter beads are fixed to individual microtubules, so that the movements of the bead reveal undulatory modes of the polymer. The mean square displacement shows a scaling of t3/4 in keeping with mode analysis arguments. Inside a cell, beads show a more complicated behavior that reflects internal dynamics of the cytoskeleton and associated motors.

When placed near the cell edge, 3 micron diameter beads coated by proteins that mediate membrane adhesion are engulfed underneath the membrane and drawn toward the center by a contracting flow of actin. On reaching the region surrounding the nucleus, the beads continue to move but lose directionality, so that they wander within a restricted space. Measurement of the mean square displacement now shows a scaling of t1 up to times of ~1 sec. At longer times the scaling varies between t and t1/2 in the various runs. The data do not fit a crossover between ballistic (t2)and diffusive (t1) behavior. The movement is clearly driven by non-thermal interactions, as it cannot be stopped by an optical trap. Treatment of the cell to depolymerize microtubules restores ordinary diffusion, while actin depolymerization has no effect, indicating that microtubule-based motor proteins are responsible for the motion. Immunofluorescence microscopy shows that the mesh size of the microtubules is smaller than the bead diameter.

We propose that the observations are related, and that the non-trivial scaling in the polymer system leads to time-dependent friction in a network, which in turn leads to a generalized Einstein relation operative in the intracellular environment. This results, in the driven system, in sub-ballistic motion at short times and sub-diffusive motion at longer times.