bluuebunny/crossref_metadata_2025 huggingface dataset:
Modeling Block Copolymer Interactions with Biomimetic Membranes
Authors: Adiga Shashishekar P. , Zapol Peter , Firestone Millicent A.
Published: April 2011

Association of amphiphilic triblock copolymers with lipid membranes results in versatile novel materials with enormous potential in many areas of bionanotechnology. The molecular architecture and concentration of block copolymers along with environmental variables such as temperature and pH provide means to tune these structures for desired applications and also allow for designing signal-responsive materials. Understanding interaction between block copolymers and lipid bilayers is crucial for applications in nanomedicine. Monte Carlo simulations are used to explore the effect of molecular architecture on the mode of insertion of triblock copolymers into lipid bilayers. The results are compared with small angle X-ray scattering data.
Silicon Photonic Devices and Polarisation Independence
Authors: Reed Graham T. , Mashanovich Goran Z. , Timotijevic Branislav D , Gardes Frederic Y. , Headley William R. , Wright Nicholas
Published: April 2011

Silicon Photonics is experiencing a dramatic increase in interest due to emerging application areas and several high profile successes in device and technology development. For both performance and cost reasons, there is a worldwide trend towards miniaturising silicon photonic waveguides. The shrinking of the device dimensions provides advantages in terms of cost and packing density, modulation bandwidth, improved performance in resonant structures, and an increase in optical power density within the devices. However, the size reduction comes at some costs in increased difficulty in maintaining single mode operation of the waveguides whilst controlling the polarisation properties of the device.

For some applications, it is essential that optical modulators and filters, as well as other components, operate in the single mode regime. It is also desirable that these components are polarisation insensitive. Design of the waveguide, which is the basic element of these devices, is therefore crucial. Due to the compatibility with single mode fibre dimensions as well as the possibility of control of polarisation, rib waveguides represent promising candidates for integration in SOI. The main limitation for rib waveguides is that the bend radius cannot always be sufficiently small to minimise the device footprint. Strip waveguides, on the other hand, are often considered as a good choice in this respect, as they allow very small bend radii resulting in a compact footprint. However, polarisation dependence of these waveguides can be significant. Therefore, both waveguide configurations are investigated in this paper, together with the implications for optical modulators.

Following from this discussion, we also consider optical modulators that operate in the depletion mode with the intrinsic bandwidth of several tens of gigahertz. We have previously reported a modulator based on a small rib waveguide with the height of < 500nm for high speed operation. However, in this paper we consider slightly larger designs to accommodate polarisation independence. Finally we discuss the characteristics of ring and racetrack resonators based on both rib and strip waveguides and methods of improving free spectral range whilst considering polarization effects. Both theoretical and experimental results are presented. The maximum free spectral range that we have demonstrated experimentally is ∼43nm.
Photoluminescent Silicon Nanocrystals with Mixed Surface Functionalization for Biophotonics
Authors: Erogbogbo Folarin , Swihart Mark T.
Published: April 2011

Orange-emitting photoluminescent silicon nanocrystals have been grafted with pairs of compounds to produce a mixed monolayer on their surface. Vinyl acetic acid (VA) is used to enhance dispersibility in water by providing carboxyl termination, while alkyl groups are used to improve the stability of the photoluminescence (PL). The grafting of these molecules to the surface is enabled by an etching procedure that leaves a hydrogen terminated surface on the silicon nanocrystals. Multiple molecules can be grafted in series by taking advantage of the residual hydrogen present after initial grafting with a first compound. Multiple molecules can also be grafted in parallel by allowing them to compete for surface reactive sites and varying their concentration in solution to control the composition of the mixed surface layer. Both series and parallel grafting result in particles that have a mixture of two distinct molecules on their surface, and thereby allow control of the particle dispersibility in different solvents and the density of reactive groups for subsequent functionalization steps. Optimizing the surface composition may allow production of particles that can be reacted with proteins via carbodiimide linking chemistries while maintaining their PL and their dispersibility in water and buffer solutions. Here we focus on the parallel grafting approach.
Understanding of the Synthesis and Structure of Si Nanocrystals in an Oxide Matrix from First Principles Based Atomistic Modeling
Authors: Lee Sangheon , Yu Decai , Hwang Gyeong S.
Published: April 2011

Kinetic Monte Carlo simulations were performed to examine mechanisms underlying the formation of Si nanoparticles in Si-rich SiO2. We have determined two important features of the embedded Si nanoparticle growth: “coalescence-like” and “pseudo Ostwald ripening”. The former is mainly responsible for fast Si particle growth at the early stage of annealing where the particles are close to each other, while the latter becomes important when the density of particles is low such that they are separated by large distances. The pseudo ripening process takes place several orders of magnitude slower than the “coalescence-like” growth. The predominance of “coalescence-like” behavior in the growth of Si nanoparticles results in a big variation in the particle size in terms of the Si:O ratio. Overall the predicted growth behavior based on our Monte Carlo simulations agrees well with experiments.
Spectroscopic Characterization of Alkyl-Passivated Si Nanoparticles Synthesized by a Solution Route
Authors: Tanaka Akinori , Kamikake Tadafumi , Imamura Masaki , Murase Yoshiaki , Yasuda Hidehiro
Published: April 2011

We have synthesized the alkyl-passivated Si nanoparticles by a solution route, and have carried out the various spectroscopic studies in order to investigate their intrinsic optical properties, electronic structures, and surface chemistry. Photoluminescence (PL) spectra of the n-butyl-passivated Si nanoparticles with mean diameter less than 2 nm exhibit the strong ultraviolet-blue emission. Moreover, it is found that their valence-band maximum energies directly estimated from the synchrotron-radiation valence-band photoemission spectra correspond to the resonance features in the PL excitation spectra. Therefore, it is concluded that the PL from the present n-butyl-passivated Si nanoparticles originates from the electron-hole pair recombination between the modified valence-band and conduction-band due to the quantum size effect. From the synchrotron-radiation Si 2p core-level photoemission spectra, their interfacial electronic structures of these Si nanoparticles have also been characterized. From these results, we discuss the detailed electronic and chemical properties of alkyl-passivated Si nanoparticles.
Morphology and Optical Properties of ZnO Nanorods Grown by Catalyst-assisted Vapor Transport on Various Substrates
Authors: Avrutin Vitaliy , Ozgur Umit , Izyumskaya Natalia , Chevtchenko Serguei , Leach Jacob , Moore James C. , Baski Alison A. , Everitt Henry O. , Tsen Kong-Thon , Ruterana Pierre , Morkoc Hadis
Published: April 2011

ZnO nanorods were grown by catalyst-assisted vapor phase transport on Si(001), GaN(0001)/c-Al2O3, and bulk ZnO(0001) substrates. Morphology studies showed that ZnO nanorods grew mostly perpendicularly to the GaN substrate surface, whereas a more random directional distribution was found for nanorods on Si. Optical properties of fabricated nanorods were studied by steady-state photoluminescence and time-resolved photoluminescence. Stimulated emission was observed from ZnO nanorods on GaN substrates. Raman spectroscopy revealed biaxial strain in the nanorod samples grown on Si. Conductive atomic force microscopy was applied to study I-V spectra of individual nanorods.
Fabrication and Modeling of Gated Field-Emission Devices Using Carbon Nanotubes on Si Substrates
Authors: Javad - Koohsorkhi  , Shams - Mohajerzadeh  , Yaser - Abdi  , Pouya - Hashemi
Published: April 2011

We report fabrication and modeling of self-defined gated filed-emission devices based on encapsulated vertically aligned carbon nanotubes grown on Si substrates with a PECVD method. The electrical characteristics of such devices have been theoretically modeled using an expanded Fowler-Nordheim tunneling effect. Devices fabricated here, resemble vacuum tubes where CNTs act as cathode. They show a saturation behavior for large distances between anode and cathode electrodes, making them suitable for transistor applications. The physical properties of the CNTs were investigated using SEM, evidencing the evolution of vertical CNTs with a tip-growth mechanism. The triode structure proposed in this paper uses CNT as the field-emission electrode (cathode), the surrounding Cr as the gate and a second Si substrate as the anode electrode. The emission current from the nanotube tips is significantly controlled by applying a negative voltage between CNT and the gate. The effect of the anode-cathode and gate-cathode voltages on the emission current has been experimentally observed and theoretically modeled using an expanded F-N effect.
p- and n-Type Charge Transport in Field-Effect Transistors of Pristine Poly(p-Phenylenevinylene)
Authors: Kayashima Hiroshi , Yasuda Takeshi , Fujita Katsuhiko , Tsutsui Tetsuo
Published: April 2011

Poly(p-phenylenevinylene) (PPV) thin films were prepared by using drop casting under high gravity condition and p- and n-type PPV based field effect transistors (FET) have been fabricated. PPV FETs with gold source-drain electrodes showed the p-channel FET conduction. The FET characteristics of PPV were improved by annealing and the field-effect hole mobility was 8.8×10−4cm2V−1s−1. On the other hand, PPV FET with calcium source-drain electrodes showed the n-channel FET conduction and the field-effect electron mobility was 1.0×10−6cm2V−1s−1.
Enhanced Efficiency and Stability in P3HT:PCBM Bulk Heterojunction Solar Cell by using TiO2 Hole Blocking Layer
Authors: Yoshikawa Osamu , Hayakawa Akinobu , Fujieda Takuya , Uehara Kaku , Yoshikawa Susumu
Published: April 2011

Insertion of TiO2 layer between Al negative electrode and active layer of bulk-heterojunction gave a high solar conversion efficiency and high stability. The TiO2 layer was prepared by spin-coating titanium(IV)isopropoxide (Ti(OC3H7)4) on active layer of blended P3HT:PCBM. The parallel resistance (Rp) is increased by inserting TiO2 layer leading to an increase in Voc. The TiO2 layer works as a hole blocking layer and prevents the physical and chemical damages by the direct contact between P3HT:PCBM active layer and Al electrode resulting in improvement of the parallel resistance and rectification. The efficiency of P3HT:PCBM bulk heterojunction cell with TiO2 hole blocking layer attained 4.05% with a Isc 9.72 mA/cm2, Voc 0.60 V, and FF 0.70 by using optimized TiO2 film thickness and ratio of P3HT:PCBM condition.
Using Atomic Steps to Control Pentacene Crystal Orientation Texture
Authors: Ignatescu Valerian , Hsu Jing-Chih M. , Mayer Alex C. , Blakely Jack M. , Malliaras George G.
Published: April 2011

We have studied the effect of substrate atomic steps on the azimuthal alignment of vapor-deposited pentacene crystals. Si(111) substrates with a low miscut angle were annealed at high temperature in ultra-high vacuum before the pentacene deposition; this produced surfaces with atomically flat terraces and arrays of parallel atomic steps. AFM analysis shows that pentacene deposited on these heated samples, at a low deposition rate, results in significant alignment of the pentacene crystals along the atomic steps.
Enhanced [111] Preferred Orientation of Ag Thin Film on Amorphous SiO2 by Cu Addition
Authors: Zoo Yeongseok , Han Hauk , Alford Terry L.
Published: April 2011

Small amounts of copper were added to enhance the adhesion and minimize agglomeration of Ag thin films on SiO2 substrates. Comparison of texture profiles between Ag and Ag(Cu) thin films showed that Cu additions enhanced (111) texture in Ag thin films. For the case of Ag film, (111) texture was enhanced mainly by recrystallization of amorphous regions during annealing. In addition to the recrystallization, (111) texture enhancement in Ag(Cu) film attributes to the dissociation of grains with other crystallographic orientations.
Evaluation of Copper Oxide to Copper Selectivity of Chemical Systems for BEOL Cleaning Through Electrochemical Investigations
Authors: Venkataraman Nandini , Muthukumaran Ashok Kumar , Raghavan Srini
Published: April 2011

Back End of Line (BEOL) cleaning of copper based structures requires chemical formulations that can remove copper oxide selectively without corroding copper and etching the dielectric. Many commercially available semi-aqueous and all aqueous formulations claim to meet these criteria. These include semi-aqueous fluoride strippers (SAF) and all- aqueous ammonium phosphate based chemical systems.

This paper will report the results from a fundamental study undertaken to evaluate the performance of a semi-aqueous fluoride formulation in removing copper oxide films of controlled thickness grown on copper. The thickness and composition of the oxide films were determined electrochemically using cathodic reduction technique. Electrochemical impedance spectra of samples immersed in the formulation have been measured as a function of time to follow copper oxide dissolution and the data have been analyzed to detect the transition of copper oxide to copper.
Characterization of Atomic Layer Deposited Ultrathin HfO2 Film as a Diffusion Barrier in Cu Metallization
Authors: Majumder Prodyut , Katamreddy Rajesh , Takoudis Christos G
Published: April 2011

Thermally stable, amorphous HfO2 thin films deposited using atomic layer deposition have been studied as a diffusion barrier between Cu and the Si substrate. 4 nm thick as-deposited HfO2 films deposited on Si are characterized with X-ray photoelectron spectroscopy. Cu/HfO2/ samples are annealed at different temperatures, starting from 500 °C, in the presence of N2 atmosphere for 5 min and characterized using sheet resistance, X-ray diffraction and scanning electron microscopy. Ultrathin HfO2 films are found to be effective diffusion barrier between Cu and Si with a high failure temperature of about 750 °C.
Interface Stability of Metal Barrier and low K Dielectrics
Authors: Lu Toh-Ming , Ou Y. , Wang P.-I.
Published: April 2011

It is known that the interface between a refractory metal barrier and a dielectric material is stable against thermal treatment at a conventional IC interconnect processing temperature. However, the interface may not be stable against thermal and electrical stress called the bias temperature stress (BTS) at moderate conditions of 150 °C and 0.5 MV/cm. Massive refractory metal ions are seen to drift into low K dielectric materials that contain a mixture of organic and inorganic elements. It is argued that the oxidation of the metal at the interface creates unstable metal ions that are ready to drift into the dielectric film under an electric filed during the BTS test. Dielectric or dielectric capping materials that do not contain oxygen can prevent metal oxidation and are desirable to create a stable metal and dielectric interface.
Study of Fatigue Behavior of 300 nm Damascene Interconnect Using High Amplitude AC Tests*
Authors: Read David T. , Geiss Roy , Alers Glenn
Published: April 2011

The AC fatigue test technique, which uses cyclic joule heating to apply thermal cycles to thin-film structures, was applied to copper lines and vias in damascene dielectric structures on silicon substrates. Specimen chips with two different types of dielectric, oxide and low-k, were tested. The lines were 300 nm wide; various via widths were tested. At 100 Hz, cyclic temperature ranges from 400 to 900 °C produced line lifetimes between 10 and 1 million seconds. Similar lifetimes were reached in the vias for temperature ranges between 100 and 500 °C. The data were plotted as number of load reversals to failure against cyclic temperature range; the data trends for the two different types of dielectric were indistinguishable. When the line data were fit to the Basquin equation for mechanical fatigue, the temperature ranges for both dielectrics at the one1-reversal intercept were above 1000 °C. The via data were more scattered, but trended toward a lower intercept temperature.
Memory Effect in Organic Diodes Containing Self-assembled Gold Nanoparticles
Authors: Wang Hai Ping , Pigeon Sébastien , Izquierdo Ricardo , Martel Richard
Published: April 2011

Electrical bistability is reported in metal-organic-metal diodes. The device consists of two Al electrodes separated by an organic layer that contains embedded Au nanoparticles (NPs) supported by parylene nanopillars. This paper presents results with two different organic materials: 2-amino-4, 5-imidazoledicarbonitrile (AIDCN) and aluminum tris(8-hydroxyquinoline) (Alq3). Electrical characterization of the diodes shows bistability with two well-defined states with high (OFF) and low (ON) resistances. The ON/OFF ratio is 104 and current-voltage (I-V) curves show a negative differential resistance (NDR). The diodes can be programmed in either the ON or the OFF state and maintained it for at least 8 months in air without any evidence of degradation. This conspicuous memory effect is rationalized in terms of charge storage mediated by the oxidation/reduction of the NPs. The fabrication method is general and provides a good control on both the size-uniformity and the position of the Au NPs embedded in the organic materials. The diode characteristics with different NP density are also addressed.
Electrical Properties of CuTCNQ Based Organic Memories Targeting Integration in the CMOS Back End-of-Line
Authors: Mueller Robert , Billen Joris , Naulaerts Rik , Rouault Olivier , Goux Ludovic , Wouters Dirk J , Genoe Jan , Heremans Paul
Published: April 2011

CuTCNQ (TCNQ=7,7,8,8-tetracyanoquinodimethane) is a resistive switching charge-transfer complex which can be used for organic nonvolatile memories. In this contribution we report on a thorough investigation of the electrical switching of CuTCNQ memories. Our memories currently achieve an endurance of up to 10000 write/erase cycles with a clear distinction between ON and OFF reading currents. ON and OFF threshold voltages follow a Gaussian distribution. Temperature dependent measurements of CuTCNQ based organic memories show a semiconductor like behavior for the ON state. The retention time of the ON state exceeded 60 hours at room temperature. Electrical switching of CuTCNQ memories in air was virtually not affected by temperatures up to 80°C, but becomes erratic at 120°C. The CuTCNQ material itself already starts to decompose around 200°C in presence of oxygen as shown by thermogravimetric analysis.
Oxide-nitride-oxide Dielectric Stacks with Embedded Si-nanoparticles Fabricated by Low-energy Ion-beam-synthesis
Authors: Ioannou-Sougleridis Vassilis , Dimitrakis Panagiotis , Vamvakas Vassilios Em. , Normand Pascal , Bonafos Caroline , Schamm Sylvie , Ben Assayag Gerard
Published: April 2011

This work reports on the formation of Si-nanocrystals within silicon nitride layers by low-energy Si ion implantation. Electrical characterization of oxide/Si-nanocrystal-nitride/oxide dielectric stacks demonstrates regions of negative differential resistance at low-fields. In addition, the memory characteristics in terms of charge trapping, write/erase response and retention properties of the dielectric stacks were recorded. The results indicate the large potential of the low-energy ion beam synthesis method in nitride memory technology.
New Nonvolatile Memory Effect Showing Reproducible Large Resistance Ratio Employing Nano-gap Gold Junction
Authors: Naitoh Yasuhisa , Horikawa Masayo , Shimizu Tetsuo
Published: April 2011

A large negative resistance is observed in the I-V characteristics of gold nanogap junction when high-bias voltages are applied. This phenomenon is characteristic behaviour on the nanometre scale; it only occurs for gap widths slightly under 13 nm. Furthermore, this junction exhibits a non-volatile resistance hysteresis when the bias voltage is reduced very rapidly from a high level to around 0 V, and when the bias voltage is reduced slowly. This non-volatile resistance change occurs as a result of changes in the gap width between the metal electrodes, brought about by the applied bias voltage.
Femtosecond Laser Structuring of As2S3 Glass for Erasable and Permanent Optical Memory
Authors: Juodkazis Saulius , Rode Andrei V. , Kondo Toshiaki , Misawa Hiroaki , Samoc Marek , Luther-Davies Barry
Published: April 2011

The nonlinear absorption coefficient of As2S3 glass has been measured to be 2.0 cm/GW for femtosecond pulses at 800 nm. Femtosecond laser structuring via two photon absorption in bulk As2S3 glass by erasable and permanent photo-darkening is demonstrated using both holographic and direct multi-beam laser writing.
Fabrication and Characterization of Cd1-xMgxTe Thin Films and Their Application in Solar Cells
Authors: Dhere Ramesh , Ramanathan Kannan , Scharf John , Young David , To Bobby , Duda Anna , Moutinho Helio , Noufi Rommel
Published: April 2011

We present our work on development of Cd1-xMgxTe (CMT) polycrystalline thin film for solar cells for tandem cell applications. CMT thin films were fabricated by co-evaporation of CdTe and Mg at a substrate temperature of 400˚C. The spatial separation of the two sources resulted in a compositional gradient, which allowed the deposition of a wide range of compositions from fewer runs. Stable films for compositions up to x=0.73 were fabricated. The structural analysis showed that the as-deposited films have a sphalerite structure and are preferentially oriented in the <111> direction, and the lattice constant varies linearly with composition. Optical analysis of the samples shows that the optical bandgap varies linearly with composition. The optical absorption coefficient of the alloy films for the entire composition range (up to x=0.73) is over 6×104/cm-1 for energies above the bandgap, indicating a direct-gap semiconductor. We have fabricated solar cells using these films with the following structure: glass / SnO2 / CdS / CMT / back contact. Using the same processing conditions used for CdTe solar cells, the device efficiency was less than 2%. With modified contact processing, we were able to improve efficiencies to over 5%. Post-deposition chloride heat-treatment in oxygen ambient resulted in partial dissociation of the CMT alloy and loss of Mg. With modified chloride processing, we were able to reduce alloy decomposition and further improve the efficiencies. We have obtained devices with open-circuit voltages of up to 845 mV and efficiencies of 8% for CMT devices with a bandgap in the range of 1.6-1.62 eV which are highest reported values for CMT alloys.
In-situ Characterization of As-grown Surface of CIGS Films
Authors: Kashiwabara Hirotake , Teshima Shimpei , Kikunaga Kazuya , Takeshita Kazunori , Okuda Tetsuji , Obara Kozo , Sakurai Keiichiro , Ishizuka Shogo , Yamada Akimasa , Matsubara Koji , Niki Shigeru , Terada Norio
Published: April 2011

In-situ characterization of composition, electronic structure and their depth profiles of surface of Cu(In1-xGax)Se2 (CIGS) film grown by three stage co-evaporation has been carried out by means of photoemission and inverse photoemission spectroscopy (PES/IPES), for the purpose of investigating the formation mechanism of the CIGS-side wide band-gap region adjacent to CBD-CdS/CIGS interface in cell structure. Sample-transportation in vacuum below 1 x 10-8 Torr yielded almost contamination-free feature of the CIGS surface. The as-transferred surface of Cu0.93(In0.65Ga0.35)Se2 grown at the identical condition for the high performance solar cell exhibited seriously Cu and Ga deficient composition. Chemical formula of this region was inbetween Cu : (In+Ga): Se = 1 : 3 : 5 and 1 : 5 : 8. In-situ UPS/IPES measurements CIGS showed that the as-grown surface region of the CIGS already had expanded band gap energy up to 1.4 eV and n-type character. A gradual decrease of band energy and a rise of valence band maximum as a function of depth from the original surface were observed. These results have revealed that the surface of CIGS by the three stage method already has the wide band gap, which might originate in so-called Cu-vacancy ordered phase.
Study of Band Alignment at the Interface between CBD-CdS and CIGS grown by H2O-introduced co-evaporation
Authors: Terada Norio , Kashiwabara Hirotake , Kikunaga Kazuya , Teshima Shimpei , Okuda Tetsuji , Niki Shigeru , Sakurai Keiichiro , Yamada Akimasa , Matsubara Koji , Ishizuka Shogo
Published: April 2011

For understanding the origin of the improvements of properties in the CIGS-based cells, of which the CIGS absorber has been fabricated by H2O-introduced co-evaporation [CIGS-H2O], band alignment at the interfaces between chemical bath deposited CdS and CIGS-H2O with Ga substitution ratio ~ 40 % has been studied by photoemission and inverse photoemission spectroscopy. The CdS layer over the CIGS-H2O showed an identical electronic structure with that of CdS on the conventionally grown CIGS; band gap energy of 2.3 ~ 2.4 and a location of conduction band minimum (CBM) and valence band maximum (VBM) relative to Fermi level were + 0.75 eV and -1.6 ~ -1.7 eV, respectively. In the interface region, decreases of CBM and a rise of VBM were observed. Total amount of the decrease of CBM over the interface was 0.2 ~ 0.3 eV. XPS measurements of the core-level signals over the interface showed a small upward bend bending of 0.1 ~ 0.2 eV. Consequently, the conduction band offset (CBO) and valence bad offset (VBO) at the CBD-interface over the CIGS-H2O (Ga~40%) are about +0.1, and 0.9 ~ 1.1 eV, respectively. This positive CBO is contrast with a slightly negative CBO at the interface between CBD-CdS/conventionally grown CIGS with Ga ~ 40 % measured previously. These results indicate that the H2O introduction is effective to extend the upper limit of the Ga substitution ratio where the Type-I conduction band alignment is maintained. The observed band alignments are consistent with the rise of Voc and efficiency in the CIGS-H2O based cells.
On Coincidence Site Lattice Modelling of Twins in the Sphalerite and Chalcopyrite Structures
Authors: Durose Ken
Published: April 2011

Coincidence site lattice (CSL) modelling has been applied to determine the orientations and atomic structures of first order twin boundaries in the chalcopyrite structure. A rotation of 250°32' about [110] generated a CSL similar to that for the Ó = 3 sphalerite boundary, but with one in two cation columns being metal-metal anti-sites. The models predict one coherent boundary type i.e. (112)M-(112)T, and four lateral types, (110)M-(118)T, (1110)M-(112)T, (114)M-(114)T, (001)M-(114)T. Un-relaxed atomic models for the boundaries have been constructed. The coherent boundary is shown to differ from that in sphalerite by the inclusion of one in two Se columns having alternate atoms with either three Cu and one In nearest neighbours or vice versa. All of the [110] tilt boundaries in chalcopyrite are expected to be electrically active. A tentative list of the boundaries ranked by energy, electrical and chemical activity is given, along with comments on the influence of preferred orientation in polycrystalline films in the context of solar cells.
All-Chemically Deposited Solar Cells with Antimony Sulfide-Selenide/Lead Sulfide Thin Film Absorbers
Authors: Messina Sarah , Nair M.T.S. , Nair P. K.
Published: April 2011

Solar cell structures with Sb2SxSe3-x and PbS as absorber layers were fabricated by chemical deposition on commercial transparent conductive oxide coated glass. The solid solution here was prepared by heating at 250°C a Sb2S3 thin film in contact with a chemically deposited Se-thin film. It has a graded band gap of 1-1.8 eV. A PbS thin film deposited on this layer basically fulfils the role of a p+ layer; its role as an absorber is yet to be studied. Open circuit voltage of 560 mV and short circuit current density ¡Ö 1mA/cm2under 1-3 kW/m2 tungsten halogen radiation are characteristics of these cells. Optimization of the film thicknesses and heating may offer prospects for these materials toward alternate thin film solar cell technology.
Hg1-xCdxTe as the Bottom Cell Material in Tandem II-VI Solar Cells
Authors: Parikh Viral , Chen Jie , Marsillac Sylvain , Collins Robert W , Compaan Alvin D
Published: April 2011

We have measured the electrical, optical and morphological properties of as-grown Hg1-xCdxTe films prepared by r.f. sputtering. The Hg1-xCdxTe films were grown at substrate temperatures ranging from 25°C to 150°C. Films grown at temperatures lower than or equal to 70°C were highly resistive (≥105 Φ-cm) and were not measurable by our Hall apparatus. Optical transmission data show that the band gap of these films ranged from 0.8 eV-1.5 eV, satisfying the optimum band gap criteria. Plan view and cross sectional SEM studies indicate that the films grown at 85°C and 100°C have larger grains with compact grain boundaries and these films typically yield the best cell performance. Spectroscopic ellipsometry studies are being used to estimate the band-gap as a function of substrate temperature.

We have also studied the effect of various back contacts and have fabricated complete solar cells. We found that Cu-Au serves as the best ohmic back contact to CdS/HgCdTe solar cell. Our preliminary results of J-V analyses on the complete solar cell show that the efficiency is mainly limited by the short-circuit current. Electrical-bias-dependent QE measurements indicate voltage-dependent current collection mainly in the long wavelength region. Further optimization of growth parameters and CdCl2 treatment needs to be carried out to improve the cell performance.
Confinement Effects of Hard Disk Lubricant Investigation by Atomic Force Microscopy
Authors: Caia George L , Buenviaje-Coggins Cynthia , Munoz Nicole , Faddis David , Florescu Doru I , Park Sung , Lee SungChang
Published: April 2011

As the critical design requirements decrease, we need to understand how confinement affects the performance of the materials we use. One example of this occurs with the lubricant used on magnetic recording disks to reduce wear between the disk and head reader during unexpected intermittent contact. Lubricants for use in hard drives need to have sufficient reflow and redistribution properties which are stable in a wide range of temperatures experienced in today's hard drive. Since the material properties of the confined system often differ from the bulk properties, the tools needed to properly investigate these systems need to be developed and understood. In this study, we utilized atomic force microscopy (AFM) techniques, to study the confinement effects on lubricant stability with temperature. Our studies have demonstrated distinct changes in adhesion force with changes in the thickness of the confining substrate layer. As adhesion is a component of friction, these changes in adhesion are directly related to lubricant performance and its ability to reduce wear between disk and reader.
Influence of Molecular Ordering on Surface Free Energy of Polymer Nanofibres using Scanning Probe Microscopy
Authors: Li Shuangwu , Wang Wei , Barber Asa H
Published: April 2011

Fibrous materials are used in a variety of applications due to their relatively high surface area to volume as well as anisotropic behavior. Electrospinning is a popular fabrication method which produces polymer nanofibres with a potentially high molecular alignment. In this work we examine the surface free energy of electrospun polyvinyl-alcohol nanofibres and its relation to molecular ordering using scanning probe microscopy adhesion measurements. Comparisons are made with bulk polymer material to show that a high degree of molecular orientation is present at least at the surface of the polymer nanofibre. As a result, the surface free energy of electrospun polymer nanofibres is greater than that of a bulk polymer. This effect indicates that the electrospinning process is effective at polymer alignment over a variety of experimental parameters.
Development of Homoepitaxially Grown GaN Thin Film Layers on Freestanding Bulk m-plane Substrates by Metalorganic Chemical Vapor Deposition (MOCVD)
Authors: Jindal Vibhu , Grandusky James , Tripathi Neeraj , Tungare Mihir , Shahedipour-Sandvik Fatemeh , Sandvik Peter , Tilak Vinayak
Published: April 2011

High quality homoepitaxial growth of m-plane GaN films on freestanding m-plane HVPE GaN substrates has been performed using metalorganic chemical vapor deposition. For this a large growth space was studied. Large areas of no-nucleation along with presence of high density of defects were observed when layers were grown under growth conditions for c-plane GaN. It is believed that these structural defects were in large part due to the low lateral growth rates as well as unequal lateral growth rates in a- and c- crystallographic directions. To achieve high quality, fully coalesced epitaxial layers, growth conditions were optimized with respect to growth temperature, V/III ratios and reactor pressure. Higher growth temperatures led to smoother surfaces due to increased surface diffusion of adatoms. Overall, growth at higher temperature and lower V/III ratio decreased the surface roughness and resulted in better optical properties as observed by photoluminescence. Although optimization resulted in highly smooth layers, some macroscopic defects were still observed on the epi-surface as a result of contamination and subsurface damage remaining on bulk substrates possibly due to polishing. Addition of a step involving annealing of the bulk substrate under H2: N2 environment, prior to growth, drastically reduced such macroscopic defects.
Density functional calculations of the binding energies and adatom diffusion on strained AlN (0001) and GaN (0001) surfaces
Authors: Jindal Vibhu , Grandusky James , Tripathi Neeraj , Tungare Mihir , Shahedipour-Sandvik Fatemeh
Published: April 2011

Density functional theory calculations were carried out to study the binding energies and diffusion barriers for various adatoms on AlN and GaN (0001) surfaces. The binding energies and potential energy surfaces were investigated for Al, Ga, and N adatoms on both Al (Ga) terminated and N terminated (0001) surfaces of AlN (GaN). Calculations for the diffusion paths and diffusion energy barriers for Al, Ga, and N adatoms on AlN and GaN were performed. It was found that the N adatom on N terminated AlN and GaN surfaces faces a high diffusion barrier due to strong N-N bond. The Al and Ga adatom on Al (Ga) terminated AlN (GaN) surfaces showed lower diffusion barriers due to the weak metallic bonds. However, the diffusion barrier for an Al adatom was always larger than that of a Ga adatom on any surface. The surfaces were also subjected to a hydrostatic compressive and tensile strain in the range of 0 to 5% to investigate the effect of strain on diffusion barriers. The diffusion energy barrier for N adatom on N terminated AlN and GaN surfaces decreased when the strain state was changed from tensile to compressive. In contrast, Al and Ga adatoms show continuous increase in diffusion barriers from tensile to compressively strained Al (Ga) terminated AlN (GaN) surfaces.
Non-planar Corrugated Layered Heptazine-based Carbon Nitride: The Lowest Energy Modifications of C3N4
Authors: Kroll Peter , Gracia Jose
Published: April 2011

We investigate structure and energy of carbon nitride, C3N4, with a special focus on two-dimensional graphitic-type structures. Our density functional calculations indicate that structures comprising heptazine (C6N7) heterocycles are energetically more favorable than structures comprising triazine (C3N3) heterocycles. Lowest energy modifications exhibit corrugated sheets with a substantial gain in energy in comparison to planar structures. Our analysis indicates that nitrogen-nitrogen non-bonded interaction is the key to understand the corrugation.
Prehistoric Ceramics of Northern Afghanistan: Neolithic through the Iron Age
Authors: Kolb Charles C.
Published: April 2011

For nearly four millennia, Afghanistan has been at the crossroads of Eurasian commerce and remains ethnically and linguistically diverse, a mosaic of cultures and languages, especially in the north, where the Turkestan Plain is a conduit for the so-called Silk Route, a series of “roads” that connected far-flung towns and urban centers and facilitated the transfer of goods and services. The research reported herein involves the comparative analysis of archaeological ceramics from a series of archaeological sites excavated in northern Afghanistan in the mid-1960s by the late Louis Dupree and me. I served as the field director (1965-1966) and analyzed the ceramics excavated from all six archaeological sites. These were Aq Kupruk I, II, III, and IV located in Balkh Province (north-central Afghanistan) and Darra-i-Kur and Hazar Gusfand situated on the border between Badakshan and Tarkar Provinces (extreme northeastern Afghanistan). Ten of the 72 ceramic types from the Aq Kupruk area have been published [1, 2, 3] but none of the 53 wares from northeastern Afghanistan have been described. The majority of the Aq Kupruk materials are undecorated (plain ware) ceramics but there is a unique series of red-painted decorated ceramics (Red/Buff, numbered types 45 through 52) with early first millennium BCE designs but the pottery dates to the BCE-CE period. The results of ceramic typological, macroscopic, binocular and petrographic microscopy (thin-section analysis and point counting) are reported.
Study of Painting Materials and Techniques in the 18th Century St.Athanasius Church in Moschopolis, Albania
Authors: Pavlidou Eleni , Civici N. , Caushi E. , Anastasiou L. , Zorba T. , Hatzikraniotis E. , Paraskevopoulos K. M.
Published: April 2011

In this paper are presented the studies of the paint materials and the technique used in 18th century wall paintings, originated from the orthodox church of St Athanasius, in the city of Maschopolis, a flourishing economical and cultural center, in Albania. The church was painted in 1745 by Konstantinos and Athanasios Zografi, and during the last years, restoration activities are being performed at the church. Samples that included plasters and pigments of different colors were collected from important points of the wall paintings. Additionally, as some parts of the wall-paintings were over-painted, the analysis was extended to the compositional characterization of these areas. The identification of the used materials was done by using complementary analytical methods such as Optical Microscopy, Fourier Transform Infrared spectroscopy (FTIR), Scanning Electron Microscopy (SEM-EDS) and X-ray fluorescence (TXRF).

The presence of calcite in almost all the pigments is indicative for the use of the fresco technique at the studied areas, while the detection of gypsum and calcium oxalate, indicates an environmental degradation along with a biodegradation. Common pigments used in this area at 15-16th centuries, such as cinnabar, green earth, manganese oxide, carbon black and calcite were identified.
Focused-Ion Beam and Electron Microscopy Analysis of Corrosion of Lead-Tin Alloys: Applications to Conservation of Organ Pipes
Authors: Oertel Catherine , Baker Shefford P. , Niklasson Annika , Johansson Lars-Gunnar , Svensson Jan-Erik
Published: April 2011

Across Europe, lead-tin alloy organ pipes are suffering from atmospheric corrosion. This deterioration can eventually lead to cracks and holes, preventing the pipes from producing sound. Organ pipes are found in compositions ranging from >99% Pb to >99% Sn. For very lead-rich (>99% Pb) pipes, organic acids emitted from the wood of organ cases have previously been identified as significant corrosive agents. In order to study the role of alloy composition in the susceptibility of pipes to organic acid attack, lead-tin alloys containing 1.2-15 at.% Sn were exposed to acetic acid vapors in laboratory exposure studies. Corrosion rates were monitored gravimetrically, and corrosion product phases were identified using grazing incidence angle X-ray diffraction. In a new method, focused-ion beam (FIB) cross-sections were cut through corrosion sites, and SEM and WDX were used to obtain detailed information about the morphology and chemical composition of the corrosion layers. The combination of FIB and SEM has made it possible to obtain depth information about these micron-scale layers, providing insight into the influence of acetic acid on alloys in the 1.2-15 at.% Sn range.
Synthesis of Porous Silica Nanotubes using Rosette Nanotubes as Templates
Authors: Danumah Christophe , Hemraz Usha D. , Fenniri Hicham
Published: April 2011

Rosette nanotubes (RNTs) were used as structure-directing agents for the synthesis of microporous and/or mesoporous silica nanotubes. The silica nanomaterials were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and CO2 and nitrogen physisorptions. The results show that the mode of the obtained porous systems as well as the surface areas and volume of micro- and/or mesopores depend on the nature of the pendant attached to the rosette nanotubes surface.
Effects of Stress-relieving AlN Interlayers in GaN-on-Si Grown by Plasma-assisted Molecular Beam Epitaxy
Authors: Adikimenakis Adam , Sahonta Suman-Lata , Dimitrakopulos George , Domagala Jaroslav , Komninou Philomela , Georgakilas Alexander
Published: April 2011

The insertion of an AlN interlayer for tensile strain relief in GaN thin films grown on Si (111) on-axis and vicinal substrates by nitrogen rf plasma source molecular beam epitaxy has been investigated. The 15 nm AlN interlayer was inserted between the bottom 0.5 micron GaN layer and the top 1.0 micron GaN layer. The interlayer was very effective to reduce the tensile stress in the overall 1.5 micron GaN/Si film to the level required for complete avoidance of microcracks, which were present in high densities in GaN/Si heterostructures grown without an AlN interlayer. The strain of the AlN interlayer, as well as the strain in all the layers of the entire GaN/Si heterostructure was analyzed by x-ray diffraction (XRD) and transmission electron microscopy (TEM) measurements. Reciprocal space map in XRD indicated that the 15 nm AlN interlayer was coherently strained with the GaN films. However TEM observations revealed that the AlN interlayer was partially relaxed in local regions. The AlN interlayer was also observed to interfere with the GaN growth process. In particular, above morphological features such as V-defects, GaN was overgrown with a large density of threading dislocations and inversion domain boundaries.
Relaxation Behavior and Breakdown Mechanisms of Nanocrystals Embedded Zr-doped HfO2 High-k Thin Films for Nonvolatile Memories
Authors: Yang Chia-Han , Kuo Yue , Lin Chen-Han , Wan Rui , Kuo Way
Published: April 2011

Semiconducting or metallic nanocrystals embedded high-k films have been investigated. They showed promising nonvolatile memory characteristics, such as low leakage currents, large charge storage capacities, and long retention times. Reliability of four different kinds of nanocrystals, i.e., nc- Ru, -ITO, -Si and -ZnO, embedded Zr-doped HfO2 high-k dielectrics have been studied. All of them have higher relaxation currents than the non-embedded high-k film has. The decay rate of the relaxation current is in the order of nc-ZnO > nc-ITO > nc-Si > nc-Ru. When the relaxation currents of the nanocrystals embedded samples were fitted to the Curie-von Schweidler law, the n values were between 0.54 and 0.77, which are much lower than that of the non embedded high-k sample. The nanocrystals retain charges in two different states, i.e., deeply and loosely trapped. The ratio of these two types of charges was estimated. The charge storage capacity and holding strength are strongly influenced by the type of material of the embedded nanocrystals. The nc-ZnO embedded film holds trapped charges longer than other embedded films do. The ramp-relax result indicates that the breakdown of the embedded film came from the breakdown of the bulk high-k film. The type of nanocrystal material influences the breakdown strength.
Synthesis of Nickel Disilicide Nanocrystal Monolayers for Nonvolatile Memory Applications
Authors: Yoon Jong-Hwan , Elliman Robert G.
Published: April 2011

Nickel silicide nanocrystals (NCs) were formed by thermally annealing SiOxNy films either implanted with Ni or coated with an evaporated Ni film. It is observed that the NCs grow into well-defined single crystalline structures embedded in a SiOxNy matrix, and that their size can be directly controlled by adjusting the concentrations of either silicon or nickel in the SiOxNy layer. The formation of well-defined NC monolayers was also demonstrated by depositing an ultra-thin Ni layer between two SiOxNy layers. These structures are shown to exhibit characteristic capacitance-voltage hysteresis suitable for nonvolatile memory applications.
Dependence on Organic Thickness of Electrical Characteristics Behavior in Low Molecular Organic Novolatile memory
Authors: Kim Yool Guk , Seo Sung Ho , Lee Gun Sub , Park Jea Gun , Kim Jin Kyu
Published: April 2011

Recently, organic nonvolatile memory of nonvolatile memories have attracted attention because application as next generation memory devices. In effort for realizing low-molecular organic nonvolatile memory, the dependence of organic thickness on electrical characteristic behavior in low-molecular organic nonvolatile memory was investigated. We developed an low-molecular organic nonvolatile memory fabricated with the device structure of Al/Alq3 (Aluminum tris(8-hydroxyquinoline))/Ni nanocrystals/Alq3/Al. Four different organic thicknesses, i.e., 30, 40, 50, and 100 nm, with fixing middle layer thickness were deposited by using a high vacuum thermal deposition method. The reason we chose Ni for middle metal layer is that Ni has smaller grain boundary which is benefit for scaling down and larger work function (∼5.15 eV) that can make a deep quantum well in energy band diagram, compared with those of Al. We confirmed that, as an organic thickness increases, a current level linearly decreases by an order of magnitude in a log-scale except for the 100 nm sample case. That is, Ion of 30-nm-thick sample was about 1 mA and 50-nm-thick sample was about 10 ¥ìA. The reason why the decrease in the current with increasing an organic thickness can be attributed to that electron transfer occurs less frequently because of the decrease in the hopping frequency. In addition, a 100-nm-thick sample was not shown the electrical characteristic of low-molecular organic nonvolatile memory. Meanwhile, the switching characteristics of our device showed that Vth of 2V, Vp(program) of 4V, Ve(erase) of 7V, and Ion (after programming)/Ioff (after erasing) of ~6x103. In addition, the interesting behavior of those characteristics is that the switching voltages (e.g; Vth, Vp, Ve Ion/Ioff) were not much changed with a varying an organic thickness. Therefore, we can conclude that an organic thickness does not affect significantly to the switching characteristics but current level. In addition, it was confirmed that a 30-nm-thick organic thickness was the best process condition in realizing low-molecular organic nonvolatile memory fabrication process.