bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of Magnesium Oxide Films by Spectroscopic Ellipsometry
Authors: Kimura Hiroshi
Published: April 2011

Spectroscopic ellipsometry was used to evaluate the density and surface roughness of MgO films. The density of a film affects the dispersion of the film's refractive index, and the refractive index can be evaluated by spectroscopic ellipsometry. Among the various fitting models applied to all of the deposited films, the two-layered model gave the best results. For the two-layered model, we found that the thickness of the upper layer was strongly correlated with surface roughness. Thus, spectroscopic ellipsometry can be used to evaluate density and roughness.
Solid Phase Crystallization (SPC) Behavior of Amorphous Si Bilayer Films with Different Concentration of Oxygen: Surface vs. Interface-nucleation
Authors: Ryu Myung-Kwan , Kwon Jang-Yeon , Kim Ki-Bum
Published: April 2011

Solid-phase crystallization (SPC) behavior of a-Si film [a-Si(II)] in which oxygen concentration (CO) is higher at the a-Si/SiO2 interface (CO=5×1021/cm3) than at the film surface (CO=3×1020/cm3) has been investigated. The results were also compared with that of a-Si single layer [a- Si(I), 600 Å] with CO=3×1020/cm3. It has been found that the interface-nucleation was suppressed in the a-Si(II) and the surface-nucleation occurred to make a poly-Si/a-Si (300 Å/300Å) bilayer structure. Many equiaxial grains with sizes of 1∼2 [.proportional]m were formed in the surface- nucleated poly-Si layer. Compared with the results of conventional SPC poly-Si (600 Åthick) in which elliptical grains with sizes of 0.5∼1 [.proportional]m were formed by the interface (a-Si/SiO2)- nucleation, we concluded that the poly-Si/a-Si bilayer scheme is a method to improve the microstructure of SPC poly-Si film.
Low Temperature Polycrystalline Si TFTs Fabricated with Directionally Crystallized Si Film
Authors: Jung Y.H. , Yoon J.M. , Yang M.S. , Park W.K. , Soh H.S. , Cho H.S. , Limanov A.B. , Im J.S.
Published: April 2011

The comparison of TFTs fabricated on films processed by conventional excimer laser an- nealing (ELA) and sequential lateral solidification (SLS) demonstrates the dependence of the device characteristics on the microstructure of the device channel region. We report the perform- ance characteristics of non-self-aligned coplanar n- and p-channel low temperature TFTs fabricated on 1000-Å-thick films on Corning 1737 glass substrates that were directionally solidified using SLS. The devices were aligned so that the grain boundaries were parallel to the direction of the source-drain current flow. These results were compared with those obtained from devices fabricated on conventional ELA-processed polycrystalline Si films (with average grain size of ∼3000 Å) with identical methods. The values for channel mobility obtained from the SLS TFTs are ∼370 cm2/Vsec for n-channel and ∼140 cm2/Vsec for p-channel devices, compared to ∼100 and ∼60 respectively for ELA TFTs. Other device characteristics of SLS TFTs were Ion/Ioff > 107 at Vd=0.1V, and subthreshold slopes less than 0.5V/dec. We further discuss the physical implications of the results and present additional details of the devices.
High-Performance Poly-Si TFTs with Multiple Selectively Doped Regions in the Active Layer
Authors: Lee Min-Cheol , Yoo Juhn-Suk , Park Kee-Chan , Jung Sang-Hoon , Han Min-Koo , Kim Hyun-Jae
Published: April 2011

We have proposed and fabricated a new poly-Si TFT that employs selectively doped regions between the source and drain in order to reduce leakage current without the sacrifice of the on current. In the proposed poly-Si TFTs, the selectively doped regions where doping concentration is identical to that of source/drain, reduce the effective channel length during the on state. Under the off state, the selectively doped regions may reduce the lateral electric field induced in the depletion region near drain so that the leakage current reduces considerably. The experimental data of the proposed TFT shows that it has the high on-current, low leakage current and low threshold voltage when compared with conventional TFT. The fabrication steps for the proposed TFT are reduced because ion-implantation for source/drain and selectively doped regions is performed simultaneously prior to an excimer laser irradiation. It should be noted that, in the proposed TFT, only one excimer laser annealing is required while two excimer laser annealing steps are required in conventional TFT.
The Dependence of Poly-Si TFT Characteristics on the Relative Misorientation Between Grain Boundaries and the Active Channel
Authors: Jung Y.H. , Yoon J.M. , Yang M.S. , Park W.K. , Soh H.S. , Cho H.S. , Limanov A.B. , Im J.S.
Published: April 2011

We present device-related experimental results that quantitatively reveal the effect of varying the active channel/grain-boundary misorientation on the resulting TFT characteristics. Specifically, using low-temperature SLS processes, we have fabricated and analyzed n-channel and p-channel devices (40 μm width × 8 μm length) with three different orientations of the channel with respect to the grain boundaries: parallel, 45° inclined, and perpendicular on Corning 1737 glass substrates.

The results reveal that the TFTs with the best (worst) characteristics were obtained for the devices with parallel (perpendicular) alignment. In general, for both n- and p-channel devices, the most prominent orientation-dependent effects were observed in the values of the field effect mobilities, which were 340, 227, and 141 cm2/Vsec for n-channel devices and 145, 105, and 80 cm2/Vsec for p-channel devices, in the order of increasing orientation mismatch. In contrast, no notable effect was manifested in the leakage currents, while small effects were seen for the sub-threshold slopes and threshold voltages. The degradation of device performance under hot-carrier stress was found to decrease with increasing orientation mismatch.
Tight Binding Modeling of Properties Related to Field Emission from Nanodiamond Clusters
Authors: Areshkin Denis A. , Shenderova Olga A. , Zhirnov Victor V. , Pal Alexander F. , Hren John J. , Brenner Donald W.
Published: April 2011

The electronic structure of nanodiamond clusters containing between 34 and 913 carbon atoms was calculated using a tight-binding Hamiltonian. All clusters had shapes represented by an octahedron with (111) facets with the top and the bottom vertices truncated to introduce (100) surfaces. The tight-binding Hamiltonian consisted of environment-dependent matrix elements, and C-H parameters fit to reproduce energy states of the cyclic C6 and methane. The calculations predict a density of states similar to bulk diamond for clusters with radii greater than ∼2.5nm, and insignificant differences in the potential distribution between the clusters and bulk diamond for radii greater than ∼1nm. Hydrogen passivated nanodiamond clusters are estimated to have an electron affinity of approximately -1.8 eV.
Anomalous Field Enhancement in Planar Semiconducting Cold Cathodes from Spontaneous Ordering in the Accumulation Region
Authors: Bilbro Griff L. , Nemanich Robert J.
Published: April 2011

Wide band gap semiconductors exhibit a low electron affinity and may prove suitable for cold cathode applications. We introduce a simple closed-form analytic approximation for the stability of electrons in the electron accumulation layer of planar Low Electron Affinity (LEA) semiconducting cathodes. This analysis extends our previous results, which used Runge-Kutta numerical integration of the linearized equations of motion for the electric potential and quasi Fermi level. The model shows conditions in which the electrons in the accumulation layer form a two dimensional array of regions of higher and lower electron density. This instability could lead to field enhancement without surface roughness and could account for observed electron emission at low applied fields.
Dispersions and fibers of carbon nanotubes
Authors: Vigolo Brigitte , Pénicaud Alain , Coulon Claude , Sauder Cédric , Pailler René , Journet Catherine , Bernier Patrick , Poulin Philippe
Published: April 2011

We study the phase behavior of single walled carbon nanotubes in aqueous solutions of surfactant molecules or amphiphilic polymers. Homogeneous dispersions can be obtained by using sodium dodecyl sulfate (SDS) in a well-defined concentration range. In contrast, polyvinyl alcohol (PVA) is not efficient at stabilizing the tubes. Carbon nanotubes stick with each other when PVA is added to homogeneous dispersions initially stabilized by SDS. This behavior is the basis of a simple method that we developed to assemble single walled carbon nanotubes into indefinitely long ribbons and fibers. The processing consists of dispersing the nanotubes in SDS solutions, re-condensing the nanotubes in the flow of a PVA solution to form a nanotube mesh, and then collating this mesh to a nanotube fiber. Flow induced alignment may lead to a preferential orientation of the nanotubes in the mesh that has the form of a ribbon. Unlike classical carbon fibers, the nanotube fibers can be strongly bent without breaking. Their obtained elastic modulus is 10 times higher than the modulus of high-quality bucky paper.
Vibrational modes of graphitic fragments and the nucleation of carbon nanotubes
Authors: Volpe Manuela , Cleri Fabrizio , D'Agostino Gregorio , Rosato Vittorio
Published: April 2011

We studied the nucleation mechanism of carbon nanotubes based on the hypothesis that the starting nanotube seed can be nucleated by rolling a small fragment of a graphite sheet (graphene) under thermal fluctuations. The energy barriers for rolling a graphene along different crystallographic directions are calculated from a tight-binding model,. We then estimate the relative weight of the large-amplitude fluctuations corresponding to low-frequency vibrational modes of graphene sheets of increasing size. Direct molecular dynamics simulation of the high- temperature fluctuation of a pair of parallel graphenes demonstrates that a nanotube closed at one end can spontaneously form. We discuss the combined effects due to: (a) the decrease of the energy barriers against rolling with increasing nanotube radius, and (b) the increase of random fluctuations with increasing size of the graphene sheet. The superposition of such effects may lead to a preferential range of nanotube diameters which could nucleate more abundantly than others.
Laser Synthesis of Single-wall Carbon Nanotubes Utilizing High Temperature Induction Heating.
Authors: Gennett T. , Dillon A.C. , Alleman J.L. , Jones K.M. , Parilla P. A. , Heben M.J.
Published: April 2011

An inductive heating system was used to control the temperature of the target during laser synthesis of SWNTs. The position of the target relative to the heating coil, the type of catalyst metals and the synthesis temperatures were all varied in attempts to gain more control over the types of carbon single wall nanotubes grown. Raman spectroscopy results suggest that narrower diameter distributions are produced when the gas-phase species are rapidly condensed. TEM analysis of the raw soot shows that many tube ends are present and that the tubes are short due to incomplete growth. This investigation shows that it is possible to gain further control over SWNT diameter distributions through fine control of the reaction environment made possible with inductive heating.
Structure of Self-Assembled Fe and FePt Nanoparticle Arrays
Authors: Yamamuro S. , Farrell D. , Humfeld K. D. , Majetich S. A.
Published: April 2011

Arrays were self-assembled by evaporating suspensions of 4 nm FePt or 8 nm Fe nanoparticles. The monolayers had a hexagonal close packed (hcp) structure, but the multilayer structure varied. To identify the multilayer structures, transmission electron microscopy (TEM) images were compared with phase contrast image simulations. The results showed that Fe could be grown as both hcp and face-centered cubic (fcc), or fcc-like, structures. The results of image analysis of the FePt arrays were consistent with fcc structures.
Development of Induced Crystallization as a Pattern Transfer Mechanism for Nanofabrication
Authors: Chraska T. , Cabral M.J. , Mesarovic S. , Longo D.M. , Stach E.A. , Bean J.C. , Hull R.
Published: April 2011

A novel pattern transfer technique, based upon direct contact between nanofabricated printheads and amorphous films/substrates is being developed. Transfer of features from printhead to substrate is via an induced crystallization mechanism, with a goal of controlling crystal nucleation at dimensions comparable to the printhead features. To realize such a printing mechanism, appropriate mediums/materials that can be crystallized through contact with the printhead elements must be identified. From a range of candidate materials we focus on Ge and Indium Tin Oxide (ITO) films grown by molecular beam epitaxy or sputtering. The crystallization may be induced directly by heating (thermally), pressure (mechanically), or electrical conduction through the contact points. Crystallization of many amorphous materials is known to be self sustaining (so called “explosive crystallization”), raising the need to identify mechanisms for abruptly stopping crystallization if feature dimensions are to be constrained. A set of amorphous or nanocrystalline samples have been studied during crystallization by a heated printhead fabricated with a Focused Ion Beam (FIB). A simplified continuum model suggests the feasibility of achieving lateral confinement of the induced crystallization. In order to characterize fully the transitional and final substrate states, real-time observation of nanoscale crystallization in the transmission electron microscope (TEM) is in progress. A special TEM goniometer is being developed which incorporates a piezoelectrically driven tip and a heating capability to enable observation of tip-substrate contact at elevated temperatures that should give invaluable insight into crystallization mechanisms.
Electrodeposition of Three-Dimensionally Periodic Metal Meshes and Spheres
Authors: Xu Lianbin , Zhou Weilie L. , Baughman Ray H. , Zakhidov Anvar A. , Wiley John B.
Published: April 2011

Electrochemical methods have been used to produce three-dimensionally periodic metal meshes and spheres. Nickel is initially deposited into porous opal sheets. The opals themselves consist of close-packed silica spheres, which serve as a template for the growth of the nickel arrays within the void space between SiO2 spheres. Dissolution of the SiO2 spheres results in open, three-dimensionally periodic nickel mesh structures. The metal meshes can then be oxidized in air to produce nonconducting NiO meshes. This results in an inverse template that can be used for the growth of three-dimensionally periodic metal sphere arrays. Details on the preparation and characterization of these materials are presented.
Novel Colloidal Assembly Methods for the Preparation of Core-Shell Composite Materials
Authors: Fleming Michael S. , Mandal Tarun K. , Walt David R.
Published: April 2011

Colloidal assembly is a process by which particles ranging in size from nanometers to micrometers are organized into structures by mixing two or more particle types. Assembly is controlled by either specific or non-specific interactions between particles. Examples include chemical bonding, biological interactions, electrostatic interactions, capillary action and physical adsorption. The assembly process is performed such that smaller particles assemble around larger ones. In this paper, we report on colloidal assembly of polymer nanoparticles (50-200 nm diameter) onto silica particles (3-5 μm diameter) using specific chemical interactions (i.e. aminealdehyde). Annealing the assembled composites at temperatures above the glass transition (Tg) of the polymer nanospheres allows polymer to flow and uniformly coat the microsphere surfaces. Polystyrene and poly(methyl methacrylate) nanospheres were used to produce such materials. Shell composites were created by mixing both nanosphere types prior to assembly/annealing. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FTIR) were used to characterize the materials presented herein.
Non-lithographic Nanocolumn Fabrication with Application to Field Emitters
Authors: Colgan M. J. , Vick D. , Brett M. J.
Published: April 2011

A non-lithographic method of fabricating high-density arrays of nanometer-scale vertical columns was investigated. The use of oblique deposition techniques allows the fabrication of isolated vertical columns in a single-step evaporation process without the need for either pre- or post-deposition lithographic processing. Extreme oblique incidence deposition with computer controlled substrate motion was utilized to fabricate columns with diameters near 100 nm and densities exceeding 109 columns/cm2. The desired column geometry may be engineered through choice of deposition angle and substrate spin rate. In one potential application of these microstructures, arrays of vertical columns were fabricated from silicon and carbon and tested for field emission characteristics. Further studies were made on the use of ion milling to modify the tips of the nanocolumns in order to improve the field emission properties.
A FIB Micro-Sampling Technique and a Site-Specific TEM Specimen Preparation Method for Precision Materials Characterization
Authors: Yaguchi Toshie , Urao Ryoichi , Kamino Takeo , Ohnishi Tsuyoshi , Hashimoto Takahito , Umemura Kaoru , Tomimatsu Satoshi
Published: April 2011

A technique to cut out small pieces of samples directly from chips or wafer samples in a focused ion beam (FIB) system has been developed. A deep trench is FIB milled to cut out a small, wedge-shaped portion of the sample from the area of interest A micromanipulator with tungsten (W) probe is employed for lifting the micro-sample. The lifted micro-sample is then mounted on a carrier to prepare electron transparent thin foil specimens for transmission electron microscope (TEM) observation. We have also developed a method for site-specific TEM specimen preparation. In this method, FIB system and TEM/scanning transmission electron microscope (STEM) equipped with secondary electron (SE) detector are employed. An FIB–TEM/STEM compatible specimen holder has also been developed so that a specimen can be milled in the FIB system and observed in a TEM/STEM without remounting the specimen. STEM and scanning electron microscopy (SEM) images are used for locating a specific site on a specimen. SEM image observation at an accelerating voltage of 200kV enabled us to observe not only surface structures but also inner structures near the surface of a cross section with depth of field of around 1 micrometer. The STEM image allows the observation of inner structures of 3-5 micrometer thick specimens. Milling of a specimen by FIB and observation of the milled sample by SEM and STEM are alternately carried out until an electron transparent thin foil specimen is obtained. The position accuracy of the method in TEM specimen preparation is approximately 100nm.
Investigations of Silicon Nano-crystal Floating Gate Memories
Authors: Kumar Arvind , Welser Jeffrey J. , Tiwari Sandip , Rana Farhan , Chan Kevin K.
Published: April 2011

As memory continues to be scaled to ever smaller dimensions, the floating-gate memory transistor, which offers a single-element storage cell, becomes more attractive. Typically, this structure has been reserved for nonvolatile applications, where the comparatively high voltages, slow write speeds, and limited cyclability could be tolerated. However, if the floating-gate, which is usually a continuous film of polysilicon, is replaced with a discontinuous film of small floating islands (nano-crystals), a new set of tradeoffs in these performance factors becomes possible, opening the door to broader applications. If these islands are further reduced to the point where Coulomb charging or energy quantization effects become relevant, it is possible to control the charge on the islands at a single (or few) electron level, which offers very low power operation and may enable new functionality. This paper will discuss design and fabrication of these memories, experimental results on fabricated devices, and modeling of what could ultimately be achieved, as well as what limitations will ultimately be reached.
Comparative SEM and Cathodoluminescence Microanalysis of Porous GaP Structures
Authors: Stevens-Kalceff M.A. , Langa S. , Tiginyanu I.M. , Carstensen J. , Christophersen M. , Föll H.
Published: April 2011

Electron microscopy and cathodoluminescence (CL) microanalysis were used for a comparative study of porous layers fabricated by electrochemical etching of n-GaP substrates in a sulfuric acid solution. Both the CL and morphology of porous layers were found to depend upon the anodic current density. At high current density (100 mA/cm2) anodization leads to the formation of so-called current-line oriented pores and an increase in the CL intensity. We observed self-induced voltage oscillations giving rise to a synchronous modulation of the diameter of pores and CL intensity. When the current density decreased to values as low as 1 mA/cm2 the pores began to grow along <111> crystallographic directions and the CL intensity was observed to be lower than that of bulk GaP.
Raman Spectroscopy of Si Nanocrystals in Nanocrystalline Si Superlattices: Size, Shape and Crystallographic Orientation
Authors: Grom G. F. , Fauchet P. M. , Tsybeskov L. , McCaffrey J. P. , Labbé H. J. , Lockwood D. J. , White B. E.
Published: April 2011

We use Raman spectroscopy to study the size, shape and crystallographic orientation of silicon nanocrystals formed by solid phase crystallization of amorphous Si/SiO2 superlattices (SLs) grown by radio-frequency sputtering. The first and second Raman peaks broadening, their relative positions and intensities indicate the presence of nanoscale Si objects with a degree of disorder (grain boundaries) and strain (Si/SiO2 interfaces). Shapes of Si nanocrystals sandwiched between SiO2 layers strongly influence the Si/SiO2 interface roughness, which is inferred from the intensities of folded acoustic phonon scattering. The averaged crystallographic orientation of Si nanocrystals is determined by polarized Raman analysis. The laterally elongated nanocrystals exhibit <111> preferred crystallographic orientation along the SL axis due to orientation-dependent crystallization rates. These results demonstrate that control over Si nanocrystals structural parameters has been achieved and that solid phase crystallization of nanometer-thick amorphous Si films remains one of the most promising techniques for Si-based nanoelectronic device fabrication.
Growth of Self-Seeded Aluminum Nitride by Sublimation-Recondensation and Substrate Preparation
Authors: Rojo J. Carlos , Slack Glen A. , Morgan Kenneth , Schowalter Leo J. , Dudley Michael
Published: April 2011

Bulk aluminum nitride boules have been grown at driving rates of 0.9mm/h by the self-seeded sublimation-recondensation technique. Up to 15mm diameter substrates cut from those boules present large single crystal grains that have been analyzed using different techniques. X-ray double crystal diffraction shows a full-width-at-half-maximum of around 100 arcsec and X-ray topography reveals extensive areas with a density of dislocations less than 104 cm−2. These substrates have been prepared by chemical mechanical polishing techniques to obtain a surface roughness of 1.4-1.6nm.
Fabrication and Characterization of InxAlyGa1−x−yN Ultraviolet Detectors
Authors: Oder Tom N. , Li Jing , Lin Jingyu , Jiang Hongxing
Published: April 2011

Ultraviolet (UV) photoconductive detectors based on InxAlyGa1−x−yN quaternary alloy lattice-matched to GaN have been fabricated and characterized. The detectors consisted of about 0.1 μm thick InxAlyGa1−x−yN quaternary alloy of different In (x) and Al (y) compositions grown by metalorganic chemical vapor deposition (MOCVD). The films were characterized by different techniques including x-ray diffraction (XRD), Rutherford backscattering spectroscopy (RBS), atomic force microscopy (AFM), Hall-effect and time-resolved photoluminescence measurements. The characteristics of these UV detectors including the cut-off wavelength, photoresponsivity and device speed have been measured. The cut-off wavelength of the InxAlyGa1−x−yN detectors was found to vary to the deep UV range with varying In and Al compositions. The most important and intriguing result is that the responsivity of the InxAlyGa1−xyN quaternary alloy exceeded that of AlGaN alloy of comparable cut-off wavelength by a factor of five. This makes the nitride quaternary alloy very important material for solar blind UV detector applications in the deep UV range where Al rich AlGaN alloys have problems with low quantum efficiency and cracks due in part to lattice mismatch with GaN.
A Comparative Study of the Influence of the Local Density Approximation and the Generalized Gradient Approximation on the Calculated Properties of the III-Nitride (110) Surfaces
Authors: Alves H. W. Leite , Alves J. L. A. , Nogueira R. A. , Leite J. R.
Published: April 2011

We present a systematic theoretical study of several III-nitride (110) surfaces based on accurate parameter-free, self-consistent total energy and force calculations using the density functional theory, the local density approximation (LDA), as parametrized by Perdew and Zunger, and the generalized gradient approximation (GGA), as proposed by Perdew, Burke, and Ernzerhof, for the exchange-correlation term; we use the Full Potential Linear Augmented Plane Wave (FPLAPW) approach (WIEN-97 code) associated with the slab supercell model to simulate the (110) surface. We studied BN, AlN, GaN, InN and compared the theoretical results as related to the use of the LDA and the GGA. We conclude that although the results for both approximations are similar, differences in structural parameters may be as large as 10%.
Optical Characterization of AlGaN/GaN MQW's
Authors: Rocha Ricardo A. , Monteiro Teresa , Pereira Estela , Alves Eduardo
Published: April 2011

AlGaN/GaN multi-quantum wells (MQW's) were optically studied in this work. Photoluminescence spectra revealed a quantum Stark effect in the samples. Calculated builtin electric fields were found to be significantly less than the values expected from theoretical models. An approach concerning the existence of free charges responsible for screening of the electric field is made, where we find carrier densities very similar to values described in other works.
Shallow-impurity-related photoluminescence in homoepitaxial GaN
Authors: Kirilyuk V. , Zielinski M. , Christianen P.C.M. , Zauner A.R.A. , Weyhera J.L. , Hageman P.R. , Larsen P.K.
Published: April 2011

Photoluminescence (PL) spectra of GaN epitaxial layers grown on vicinal (0001) GaN substrates (i.e. N-polar) demonstrate high optical quality showing free exciton transitions in addition to narrow bound-exciton peaks (line width 1-2 meV). In contrast, N-polar films deposited on exactly oriented (0001) substrates exhibit a broad PL emission resulting from high free carrier concentrations. As derived from Secondary Ion Mass Spectroscopy (SIMS), exact (0001) epilayers reveal very high concentrations of oxygen (1.5×1019 atom/cm3), which are about one order of magnitude higher then in the layers grown on a 4°-off substrate. It is therefore concluded that oxygen is responsible for the high free carrier concentration and could be a dominant shallow donor in the homoepitaxial N-polar films. It is also shown that incorporation of oxygen strongly depends on the polarity of the films (Ga or N). Concentrations of other impurities, such as Si, C and H are found to be similar for homoepitaxial films of both polarities being consistent with the results of exciton-related PL.
Development of an AlN Deep UV Detector for Space Application
Authors: Zhong Feng , Huang Changhe , Danylyuk Yuri V. , Auner Gregory W.
Published: April 2011

AlN is a very promising deep UV sensing material suitable for space application such as exoatomospheric solar blind detectors due to its large band gap (6.2 eV), excellent radiation and thermal stability. In this paper, a deep UV detector based on AlN photoconductor was successfully grown and characterized. High quality AlN thin films on Sapphire have been confirmed by in-stu RHEED and X-ray diffraction measurements. The film has an optical Eg of 5.96 eV. The detector has extremely low dark current. Different electrodes were investigated as contact materials. Al and Pt are more suitable electrode contacts for AlN photoconductor compared to Ti. The DC responsivity of the detector starts to rise for photon energies of about 255 nm (4.9eV) and reaches its maximum 7×10-5 A/W at 195 nm (6.5eV).
High Density Plasma Etching Damage Effects on Contacts to n-GaN
Authors: Singh Rajwinder , Eddy C. R. , Moustakas T.D. , Ng H.M.
Published: April 2011

The effects of inductively-coupled plasma etching on the quality of ohmic contacts to etched n- GaN are reported. A high density chlorine plasma and a range of rf bias power levels (incident ion energies) are employed to etch n-GaN films with varying doping levels. Resulting plasma damage degrades contact ohmicity in all cases examined. At moderate levels of rf bias power the extent of this degradation for nitride layers with lower doping levels (mid-1017 cm−3) is similar to the degradation for nitride layers with higher doping levels. With increased rf bias, the degradation becomes more severe in films with higher carrier concentration. Annealing at 700 °C rapidly improves the contact quality. Studies of cumulative annealing time, up to 160 seconds, show that the improvement in contact quality takes place within the first twenty seconds of annealing.
Propagation and Scattering of Exciton-Polaritons in Nitride-Based Multiple Quantum Wells
Authors: Malpuech Guillaume , Zamfirescu Marian , Kavokin Alexey , Carlo Aldo Di
Published: April 2011

We show theoretically that disorder-induced scattering of light between « bright » and « dark » exciton-polariton modes governs the coherent optical response of GaN/AlGaN quantum wells (QWs) on a femtosecond scale. The incident light essentially excites the “bright” modes, they emit the most part of light radiatively during first few hundreds of femtoseconds. The rest of the coherent signal, i.e. the temporal behaviour on a scale from 1 to few picoseconds, comes from the “dark” modes, populated due to the elastic scattering processes in the system, and mixed so that they form two bulk-like polariton branches. The pico-second scale optical response of GaN/AlGaN QWs is essentially incoherent because of a very efficient scattering of exciton-polaritons by a structural disorder.
Epitaxial Iridium Growth on Strontium Titanate
Authors: Dai Z. , Li A.P. , Bednarski C. , McCann L. I. , Golding B.
Published: April 2011

Epitaxial (100) iridium films have been grown on (100) SrTiO3 (STO) substrates by electron beam evaporation. The epitaxial relationship between the iridium film and STO substrate was determined to be Ir(001)[100]//STO(001)[100]. A systematic study of the role of STO substrate surface preparation, Ir thickness, and substrate temperature on Ir film crystallinity and morpholo- gy has been performed. The best Ir films typically have small Ir(200) XRD linewidths < 0.3 °, surface roughness of 0.2 nm, and low ion channeling yields, χmin≤ 4%, when deposited at 800 °C. Films generally become smoother with better crystallinity when the film thickness approaches 300 nm. A growth mode with initial island formation and subsequent layer-by-layer appears to take place at higher substrate temperatures, whereas at lower temperatures the film grows in a 3D mode.
Enzymes and Cells Confined in Silica Nanopores
Authors: Livage Jacques , Roux Cécile , Coradin Thibaud , Fennouh Souad , Guyon Stéphanie , Bergogné Laurie , Coiffier Anne , Bouvet Odile
Published: April 2011

The sol-gel process opens new possibilities in the field of biotechnologies. Sol-gel glasses are formed at room temperature via the polymerization of molecular precursors. Enzymes can be added to the solution of precursors and trapped within the growing silica network. Small substrate molecules can diffuse through the pores allowing reactions to be performed in-situ, within the silica gels. Enzyme are encased by the hydrated silica in a cage tailored to their size, they retain their biocatalytic activity and may even be stabilized within the sol-gel matrix.

Whole cell bacteria have also been immobilized within sol-gel glasses. They behave as a "bag of enzymes" and their membrane protects enzymes against denaturation and leaching. The cellular organization of bacteria cells is preserved upon encapsulation. Experiments performed with Escherichia coli induced to β-galactosidase show that they still exhibit noticeable enzymatic activity. Some degradation of the cell walls may even occur increasing the “measured” activity. However silica gels made from aqueous precursors seem to prevent bacteria from natural degradation upon ageing.

Antibody-antigen recognition has been shown to be feasible within sol-gel matrices. Trapped antibodies bind specifically the corresponding haptens and can be used for the detection of traces of chemicals. Even whole cell protozoa have been encapsulated without any alteration of their cellular organization. For medical applications, trapped parasitic protozoa have been used as antigens for blood tests with human sera. Antigen-antibody interactions were followed by the so-called Enzyme Linked ImmunoSorbent Assays (ELISA).
Spectrophotometric Observations of Gel-Free Reaction Front Kinetics in Confined Geometry
Authors: Park Sung Hyun , Parus Stephen , Kopelman Raou , Taitelbaum Haim
Published: April 2011

We present a new experimental system to study the kinetics of the reaction front in A + B→C reaction-diffusion systems with initially-separated reactants. The set-up is composed of a CCD camera monitoring the kinetics of the front formed in the reaction-diffusion system Cu2++ tetra→1:1 complex (in aqueous, gel-free solution) inside a 150 micron gap between two flat microscope slides. This is basically a two-dimensional system. The results are consistent with the theoretical predictions for the anomalous time dependence of the front's width, height,and location.
Anomalous Surface Conformation for Polymeric Gas-Hydrate-Crystal Inhibitors
Authors: King H.E. , Hutter Jeffrey L. , Lin Min Y. , Sun Thomas
Published: April 2011

We have used both conventional small-angle neutron scattering and contrast variation techniques to characterize the polymer conformations of two non-ionic water soluble polymers: poly(ethylene oxide) and poly(N-vinyl-2-pyrollidone). The second of these is able to kinetically suppress hydrate crystallization, and an objective of these studies is to obtain a understanding of this inhibition mechanism. The dilute-solution polymer conformation in a hydrate-forming tetrahydrofuran/water fluid shows only a small difference between the polymers. The single- chain characteristics are unperturbed, but the hydrate inhibitor polymer seems to show an enhanced tendency to form aggregates in solution. This is evidenced by excess low-q scattering following a q-2.5 power law. Much more evident is the strong perturbation in the conformation of the inhibitor polymer upon crystallization of the hydrate. We utilize contrast variation methods to resolve the scattering of the polymer on the hydrate surface. Unlike expectations from polymer scaling laws, the resulting layer is considerably thicker (550) than the single- chain radius of gyration, 80. Also surprising, only 2 percent of the available crystal surface is covered. Within the covered areas, the polymer concentration is significantly enhanced over that in the surrounding solution, about 2.5c* (where c* is the overlap concentration). This suggests a coverage of clumps of polymer widely separated from one another. Consideration of the concentration and implied spacing of such clumps on the hydrate surface suggests that they can effectively inhibit crystal growth.
Relaxation Method Simulation of Confined Polymer Dispersed Liquid Crystals in an External Field
Authors: Castro J. J. , Valladares R. M. , Calles A.
Published: April 2011

Polymer dispersed liquid crystals (PDLC) are materials formed by nematic liquid crystals droplets with radii of a few hundred Å embedded in a polymer matrix. We discuss the use of relaxation methods for the study of the response of the director of a PDLC under the switching of an external electric field. We simulate the confining system by considering different boundary conditions at the droplet surface.
“Open structure” semiconductors: Clathrate and channel compounds for low thermal conductivity thermoelectric materials
Authors: Nolas George S.
Published: April 2011

In a good semiconductor the electrons (or holes) propagate through the lattice structure of well ordered atoms without being scattered by the coherent vibrations of the crystal. Thus semiconductors are good conductors of electrons (or holes) and as such have given rise to modern microprocessors that are revolutionizing the way we live. In the same semiconductors the vibrations of the lattice atoms mainly carry the heat. Due to the covalent nature of the bonding in these materials the thermal conductivity is very large. These are therefore poor materials for thermoelectric applications. If the atomic vibrations, or phonons, can be localized so that the heat transfer is essentially an atom-to- atom propagation, then the thermal conduction can be drastically reduced. A semiconductor can, in principle, have the thermal conductivity of a glass. Amorphous semiconductors are of course very poor conductors of electricity therefore one does not want the electrons to propagate through a glass-like material. Instead one wants the electrons to travel as though they only “see” the well-ordered, periodic structure of a crystal while the phonons are scattered by localized disorder within the covalently bonded lattice. “Open structure” semiconductors do, in fact, exist and recent research has given rise to new thermoelectric materials.
Micromagnetic and Magnetoresistance Studies of Ferromagnetic La0.83Sr0.13MnO2.98 Crystals
Authors: Popov Guerman , Kalinin Sergei V. , Alvarez Rodolfo A. , Greenblatt Martha , Bonnell Dawn A.
Published: April 2011

Magnetic force microscopy (MFM) and atomic force microscopy (AFM) were used to investigate the surface topography and micromagnetic structure of La0.83Sr0.13MnO2.98 single crystals with colossal magnetoresistance (CMR). The crystals were grown by fused salt electrolysis and characterized by chemical analysis, X-ray diffraction, magnetic and transport measurements. The crystals are rhombohedral (R 3 c). Magnetic and transport measurements indicate that the ferromagnetic ordering at 310 K is associated with an insulator-metal transition at the same temperature. A maximum negative magnetoresistance (-62 %) is observed at 290 K in an applied magnetic field of 5 T. The magnetoresistance increases in magnitude sharply (1.8 %), comparing to the rest of the change, with increasing magnetic field up to 20 G, and then it increases slowly with increasing field.

MFM and AFM were used to study the (110) surface as well as a number of unspecified surfaces. Surface topography of an as-grown crystal exhibits well-developed surface corrugations due to extensive twinning. The corrugation angle at twin boundaries can be related to the unit cell parameters, surface and twinning planes. Magnetic force microscopy images show that magnetic domain boundaries are pinned to the crystallographic twins; a small number of unpinned boundaries are observed. The statistical analysis of domain boundary angle distribution is consistent with cubic magnetocrystalline anisotropy with easy axis along [100] directions for this material. Unusual magnetization behavior in the vicinity of topological defects on the surface is also reported. MFM contrast was found to disappear above the ferromagnetic Curie temperature; after cooling a new magnetic structure comprised of Bloch walls of opposite chiralities developed.
Frameworks of Transition Metals and Linkers with Two or More Functional Groups
Authors: Sevov Slavi C.
Published: April 2011

Hybrid inorganic/organic materials with open-framework or layered structures are known for many transition metals linked by functionalized organic molecules such as organic diphosphonates, polycarboxylates, polynitriles, etc., species with more than one equivalent functional groups. We have studied the effect of pH on such a system of cobaltmethylenediphosphonate and report three new compounds, Na3Co[(O3PCH2PO3)(OH)],Na2Co(O3PCH2PO3)•H2O, and Co2[(O3PCH2PO3)(H2O)], that form at very basic, moderately basic, and acidic conditions, respectively. More interestingstructural chemistry should be expected from linkers with two or more different functionalities. Both the carboxylic and phosphonic groups in carboxyethylphosphonic acid are used to coordinate to cobalt or calcium atoms in the new compounds Co3(O3PCH2CH2COO)2•6H2O and Ca(O3PCH2CH2COOH)•H2O. Taking one more step further in complexity we have also studied linkers with three different functional groups, phosphonated amino acids. The structures of two new compounds, Zn(O3PCH2CH(NH3)COO) and Zn(O3PCH2CH2CH(NH3)COO), are threedimensional frameworks made of zinccoordinated by both the carboxylic and phosphonic ends of the organic molecules. The amino groups are protonated and terminal in the voids of the frameworks.
Manganese Vanadium Oxide Compounds as Cathodes for Lithium Batteries
Authors: Ngala J. Katana , Zavalij Peter Y. , Whittingham M. Stanley
Published: April 2011

A pure form of the compound [(CH3)4N]0.2Mn0.06V2O5+δ•0.2H2O has been synthesized by the hydrothermal technique. A manganese richer compound, [(CH3)4N]0.18Mn0.1V2O5+δ•0.35H2O, has also been synthesized. By comparison to [(CH3)4N]0.17Fe0.1V2O5+δ•0.17H2O, the structure of these compounds consists of double sheets of vanadium oxide with tetramethyl ammonium ions in the interlayer spaces. Both compounds display reversible lithium intercalation with sharp end points. The lithiation capacity is lowered by the increase in manganese content.
Structural Changes in Amorphous Silicon Annealed at Low Temperatures
Authors: Pivac Branko , Dubček Pavo , Milat Ognjen , Zulim Ivan
Published: April 2011

The hydrogen dilution in the course of production of amorphous silicon (a-Si) influences its structural properties, which affect significantly light-induced degradation. We used FTIR, X-ray reflectivity and GISAXS analysis to monitor the structural changes occurring during the low temperature annealing of undoped a-Si:H films. FTIR results show that upon annealing at very low temperatures, hydrogen is moved from its positions (voids) where it was accumulated unbonded to silicon and is subsequently trapped at dangling bonds, enhancing disorder. X-ray reflectivity and GISAXS measurements confirmed the enhancement of the void size.
Excimer Laser Recrystallization of Selectively Floating a-Si Active Layer for Large-Grained Poly-Si Film
Authors: Kim Cheon-Hong , Yoo Juhn-Suk , Song In-Hyuk , Han Min-Koo
Published: April 2011

We report a new excimer laser annealing method by employing selectively floating a-Si active layer structure in order to increase the grain size of poly-Si film. The floating a-Si region blocks the heat conduction into the underlying substrate due to high thermal-insulating property of an air so that the lateral temperature gradient is successfully induced by the proposed simple air-gap structure. Our experimental results show that large grains were grown in the lateral direction from the edge to the center of the floating active region. The large grains exceeding 4 m were successfully obtained with only one laser irradiation.
Growth, Structural Characterization, and Optical Band Gap Anomaly in Cu-III3-VI5 and Cu-III5-VI8 Ternary Compounds
Authors: Wasim S. M. , Rincón C. , Marín G. , Márquez R. , Pérez G. Sánchez , Guevara R. , Delgado J. M. , Nieves L.
Published: April 2011

A comparative study of the chemical composition, crystal structure, carrier concentration, mobility, and the energy gap at room temperature of the selenides and tellurides of normal Cu- III-VI2 and ordered defect Cu-III3-VI5and Cu-III5-VI8 compounds, grown by Bridgman techniques, is made. It is established that these compounds have a chalcopyrite-related structure with the exception of CuIn5Se8 and CuGa5Te8 that have hexagonal and cubic structure, respectively. Smaller volume of the unit cell and lower concentration of the charge carrier of the 1:3:5 and 1:5:8 phases, as compared to the 1:1:2, are attributed to the presence of the ordered vacancies. A new mechanism of the scattering of the charge carriers with the donor-acceptor defect pair (DADP) is suggested to explain the low value of mobility in these compounds. The anomaly observed in the band gap is explained in terms of the relative shift of the conduction band minimum and valence band maximum due to the donor-acceptor defect pairs and cation vacancies, respectively.
Cu(In,Ga)S2 Phase Formation from Metallic Cu-In-Ga Precursor Stacks in Rapid Thermal Processes
Authors: Neisser Axel , Álvarez-García Jacobo , Calvo-Barrio Lorenzo , Klenk Reiner , Matthes Thomas W. , Luck Ilka , Lux-Steiner Martha Ch. , Pérez-Rodríguez Alejandro , Morante J.R.
Published: April 2011

This contribution compares the growth of Cu(Ga,In)S2 based thin film solar cell absorbers in rapid thermal systems using sulfur vapor Sx or H2S/Ar as reactive atmosphere, focusing on Ga-related influences on film growth and phase formation. Cu-In alloying in the precursor is kinetically hindered by the presence of Cu-Ga phases. In sulfur vapor Ga-containing samples sulfurize via an intermediate CuIn2S8 phase, thereby delaying the full sulfurization and recrystallization of the layer. In contrast, in H2S/Ar fast Ga-In interdiffusion and no intermediate chalcogenide phases are observed. The inhomogeneous Ga depth distribution usually reported for sequentially prepared Cu(In,Ga)S2 films can be assigned to the segregation of CuGaS2 prior to CuInS2.
Effect Of Grain Size, Morphology and Deposition Temperature on Cu(InGa)Se2 Solar Cells
Authors: Shafarman William N. , Zhu Jie
Published: April 2011

Cu(InGa)Se2 films have been deposited by multisource elemental evaporation at substrate temperatures from 550 to 400°C to determine the effect of temperature on grain size and morphology and on solar cell performance. Films were deposited with different flux profiles to compare cases where a copper selenide phase is formed during different stages of growth or not at all. The grain size and surface area of the films are determined by analysis of atomic force microscope images. With the Cu-rich growth, the mean lateral grain area decreases from 1.8 to 0.3 μm2 as substrate temperature decreases. Device efficiency decreases with the lower temperature but cannot be simply attributed to changes in grain size, surface area, or the availability of sodium which diffused from the glass substrate. Instead the decrease in open circuit voltage and fill factor are attributed to a greater density of intra-grain trap states in the Cu(InGa)Se2.
The Influence of Stressing at Different Biases on the Electrical and Optical Properties of CdS/CdTe Solar Cells
Authors: Townsend S. W. , Ohno T. R. , Kaydanov V. , Gilmore A. S. , Beach J. D. , Collins R. T.
Published: April 2011

Cadmium Sulfide/Cadmium Telluride (CdS/CdTe) devices are subject to stress under various biases. Striking differences are observed with the Current-Voltage, and Capacitance- Voltage measurements for cells degraded at 100°C in dark under forward (FB), open circuit (OC), and reverse (RB) biases. RB stress provides the greatest degradation, and the apparent doping density profile shows anomalous behavior at the zero bias depletion width. Thin films of CdS, both doped and undoped, with Cu are characterized with photoluminescence (PL). The PL spectra from the CdS films are correlated with the CdS spectra from stressed devices, revealing that Cu signatures in the CdS layer of stressed devices are a function of stress biasing. Device modeling using AMPS-1D produces IV curves similar to that in RB degraded devices, by only varying the trap level concentration in the CdS layer.