bluuebunny/crossref_metadata_2025 huggingface dataset:
Field-Emitter-Array Cold Cathode Arc-Protection Methods - A Theoretical Study
Authors: Parameswaran L. , Murphy R.A.
Published: April 2011

Field-emitter arrays (FEAs) are desirable for use as electron emitters in microwave-tube amplifiers because they can provide such advantages as higher efficiency and faster turn-on compared to their thermionic counterparts. Calculations have shown that Spindt-type metal and semiconductor emitters operate well below intrinsic current limits due to thermal effects, even for high-current applications such as klystrodes, twystrodes, and traveling-wave amplifiers. Nevertheless, the primary barrier to FEA utilization in such applications is premature failure due to arcing. These failures appear to be produced by ionization of gas molecules and/or desorbed contaminants, which are exacerbated by a poor vacuum environment. Lifetime and stability issues have been largely resolved for less stringent applications, such as flat-panel displays, through the use of integrated passive resistors that provide current limiting. However, such an approach is not directly compatible with operation at high frequency and current density. Other more complex approaches, such as the incorporation of active control in the form of integrated transistors, have also been demonstrated, but again, only for FEAs used in displays. This paper will review some of these schemes in the context of their efficacy in improving lifetime and stability of FEA cold cathodes in high-frequency applications. A theoretical analysis will be given of the effect on highfrequency performance of incorporating arc protection structures into Spindt-type metal FEAs. Specifically, two approaches will be considered: passive protection schemes such as the use of a modified thin film resistive layer, and active schemes such as FETs and saturated current limiters.
Electron Emission of Vertically Aligned Carbon Nanotubes Grown on a Large Area Silicon Substrate by Thermal Chemical Vapor Deposition
Authors: Lee Cheol J. , Park Jung H. , Son Kwon H. , Kim Dae W. , Lee Tae J. , Lyu Seung C. , Kang Seung Y. , Lee Jin H. , Park Hyun K. , Lee Chan J. , You Jong H.
Published: April 2011

We have grown vertically aligned carbon nanotubes on a large area of Co-Ni codeposited Si substrates by thermal chemical vapor deposition using C2H2 gas. The carbon nanotubes grown by the thermal chemical vapor deposition are multi-wall structure, and the wall surface of nanotubes is covered with defective graphite sheets or carbonaceous particles. The carbon nanotubes range from 50 to 120 nm in diameter and about 130 μm in length at 950 °C. Steric hindrance between nanotubes at an initial stage of the growth forces nanotubes to align vertically. The turn-on voltage was about 0.8 V/μm with a current density of 0.1 μA/cm2 and emission current reveals the Fowler-Nordheim mode.
Large Field Emission from Vertically Well-aligned Carbon Nanotubes
Authors: Sohn Jung Inn , Lee Seonghoon , Song Yoon-Ho , Choi Sung-Yool , Cho Kyoung-Ik , Nam Kee-Soo , Kang Young-Il
Published: April 2011

We have grown well-aligned carbon nanotube arrays by thermal chemical vapor deposition at 800°C on Fe nanoparticles deposited by a pulsed laser on a porous Si substrate. Porous Si substrates were prepared by the electrochemical etching of p-Si(100) wafers with resistivities of 3 to 6 ωcm. These well-aligned carbon nanotube field emitter arrays are suitable for electron emission applications such as cold-cathode flat panel displays and vacuum microelectronic devices like microwave power amplifier tubes. Field emission characterization has been performed on the CNT-cathode diode device at room temperature and in a vacuum chamber below 10−6 Torr. The anode is maintained at a distance of 60[.proportional]m away from the carbon nanotube cathode arrays through an insulating spacer of polyvinyl film. The measured field emitting area is 4.0×10−5cm2. Our carbon nanotube field emitter arrays emit 1mA/cm2at the electric field, 2V/[.proportional]m. And they emit a large current density as high as 80mA/cm2 at 3V/[.proportional]m. The open tip structure of our carbon nanotubes and their good adhesion through Fe nanoparticles to the Si substrate are part of the reason why we can attain a large field emission current density within a low field. The field emitter arrays in our diode device are vertically well-aligned carbon nanotubes on the Si-wafer substrate.
Thermal Properties of Single-Walled Carbon Nanotubes
Authors: Hone J. , Batlogg B. , Benes Z. , Llaguno M.C. , Nemes N.M. , Johnson A.T. , Fischer J.E.
Published: April 2011

The thermal properties of carbon nanotubes are strongly dependent on their unique structure and size, and show promise as an ideal material for thermal management on the micro- and macro-scale. The specific heat of nanotubes is similar to that of two-dimensional graphene at high temperatures, but is sensitive to the effects of rolling the the graphene sheet into a small cylinder at low temperatures. Specifically, the acoustic phonon modes are stiffened due to the cylindrical geometry, and the phonon spectrum is quantized due to the small diameter of the tube. In bundles of single-walled nanotubes, the specific heat is a sensitive probe of inter-tube mechanical coupling. Measurements of the specific heat show that inter-tube coupling is relatively weak, and show direct evidence for quantum effects. The thermal conductivity of nanotubes should reflect the on-tube phonon structure. Aligned bundles of SWNTs show a high thermal conductivity (>200 W/m-K at room temperature), and possible quantization effects at low temperature.
Three-Dimensional Electronic Surfaces
Authors: Sturm J.C. , Hsu P.I. , Miller S.M. , Gleskova H. , Darhuber A. , Huang M. , Wagner S. , Troian S. , Suo Z.
Published: April 2011

There is an increasing interest in electronics functionality on surfaces which are not planar. This paper examines the critical technologies for fabricating electronic surfaces which have a three-dimensional shape. Two different approaches for achieving such a goal are examined. One can fabricate electronics using conventional technologies on a flat surface, and then after fabrication deform that surface into the desired shape (e.g. a spherical cap). In an alternative approach, one can directly fabricate onto substrates with an arbitrary shape. In this case one must address the issue of pattern formation and transfer on the curved surfaces. The scaling of letterpress printing to micron-scale features on flat and spherically curved surfaces is demonstrated.
Nanofabrication Using Self-Assembled Alumina Templates
Authors: Rabin Oded , Herz Paul R. , Cronin Stephen B. , Lin Yu-Ming , Akinwande Akintunde I. , Dresselhaus Mildred S.
Published: April 2011

A new approach for the use of porous alumina films as a template for nanofabrication is presented. In this process the porous films are prepared on silicon substrates, simplifying both the template fabrication and subsequent processing, and improving the quality of the films and their surfaces. Structural analysis of the film was carried out. Bismuth and bismuth telluride nanowires were prepared by pressure injection and electrochemical deposition, respectively, in alumina films 5-10 μ thick with parallel ordered pores 40 nm in diameter. The films were also patterned by lithography, offering new opportunities for area-selective anodization of non-planar structures. The new approach offers a straightforward method for the fabrication of arrays of nanostructures and their incorporation into electronic and optical devices.
Silicon Single Electron Transistors with Single and Multi Dot Characteristics
Authors: Savin Alexander , Manninen Antti , Kauranen Jari , Pekola Jukka , Prunnila Mika , Ahopelto Jouni , Kamp Martin , Emmerling Monika , Forchel Alfred
Published: April 2011

Silicon single electron transistors (SET) with side gate have been fabricated on a heavily doped silicon-on-insulator (SOI) substrate. Samples demonstrate two types of characteristics: some of them demonstrate multiple dot behavior and one demonstrates single dot behavior in a wide temperature range. SETs demonstrate oscillations of drain-source current and changes in the width of the Coulomb blockade region with change of gate voltage at least up to 100 K. At temperature below 20 K long-term oscillations (relaxation) of source-drain current after switching the gate voltage has been observed in both multiple dot and single dot samples. Illumination affects both the characteristics of the SETs and the relaxation process. Telegraph noise has been observed in a definite range of source-drain and gate voltages.
Pseudo-MOS Operation of Ultra-Narrow Polycrystalline Silicon Wires: Electrical Characterization and Memory Effects
Authors: Anghel C. , Hefyene N. , Ionescu A.M. , Declercq M. , Tringe J.W. , Plummer J.D.
Published: April 2011

This paper presents a new method for the electrical characterization of polycrystalline silicon wires based on the pseudo-MOS effect. The investigated devices are ultra-narrow (0.1- 0.2μm), four-contact polycrystalline silicon wires composed of several individual grains in series. The pseudo-MOS static operation allow the examination of interface and bulk characteristics via MOSFET modeling in terms of electrical parameters (carrier mobility, flat band and threshold voltages, mobility reduction coefficient). Additionally, the measurement of 1/f noise is shown to be a useful complementary characterization tool for material investigation. Particular attention is paid to the exploration of transient phenomena in the dynamic pseudo-MOS operation. A significant I-V hysteresis is experimentally revealed when high voltages are applied on the substrate acting as a gate. Long current relaxation (>100s) in accumulation and depletion, in darkness and under illumination, demonstrate the dominance of carrier trapping at grain boundaries over oxide charging and/or SRH generation-recombination intra-grain phenomena. The dynamic regime analysis shows remarkable data retention duration and suggest further investigations of polycrystalline silicon wires for memory applications may be useful.
Si/Ge Intermixing and Island-Island Interaction in Ge/Si(100) Self-Assembled Islands
Authors: Capellini G. , Seta M. De , Evangelisti F.
Published: April 2011

In this paper we present a study of the influence of the growth rate and deposition temperature on the growth dynamic of self-assembled Ge/Si(100) islands. By combining AFM and XPS measurements we show that the main effect of rising the deposition temperature is an enhancement of the Ge-Si intermixing. The actual composition of the Ge islands as a function of deposition temperature has been measured in the 450 °C- 850 °C range: the Ge content x in the alloyed epilayer was found to decrease from x=1 to x=0.28. By changing the growth rate at fixed deposition temperature we were also able to modify the island density. The influence of the island density on the island size has been investigated. We found that an increase of the island density from 109 to 2×1010 cm−2 at T=600 °C brings about a change of the mean island size from 85 to 55 nm. This reduction is explained in terms of island-island interaction effects on the growth dynamic.
Atomically Sharp Artificially Reordered Structures in Si(100)
Authors: Sen P. , Akhtar J.
Published: April 2011

The ability to organize materials in two or three dimensional structures forms the basis for approach worldwide to construct nanometer sized arrangements. Here we show the interaction of 200 MeV silver ions with a Si( 100) single crystal lattice which has been studied to look for defects with atomic resolution. Employing scanning tunnelling microscopy (STM), we demonstrate that the deposited energy is not stored as random defected arrangements at the irradiation site but as spatially extended structures at predetermined locations. These artificially reordered structures consist of random Si atoms, placed atomically sharp next to the single crystalline lattice.
Shock Synthesis of Nanocrystalline High -Pressure Phases in Semiconductors by High-Velocity Thermal Spray
Authors: Goswami R. , Parise J. , Herman H. , Sampath S. , Gambino R. , Zhu Y. , Welch D.
Published: April 2011

Shock synthesis of nanocrystalline Si, Ge and CdTe was accomplished using high- velocity thermal spray. Si or Ge powders were injected into a high energy flame, created by a thermal spray gun, where the particles melt and accelerate to impact on a substrate. The shock wave generated by the sudden impact of the droplets propagated through the underlying deposits, which induces a phase transition to a high pressure form. The decompression of the high-pressure phase results in the formation of several metastable phases, as evidenced by transmission electron microscopy and x-ray diffraction studies. The peak pressure is estimated to be ≈23GPa with a pulse duration of 1-5 ns. Transmission electron microscopy revealed that the metastable phases of Si with a size range of 2 to 5 nm were dispersed within Si-I. In Ge, a metastable phase, ST-12, was observed. This is a decompression product of Ge-II which possesses the β-Sn type of structure. In the case of CdTe, a fine dispersion of hexagonal CdTe particles, embedded in cubic-CdTe with an average size of 2 nm was obtained.
Nanostructured TiO2 and W:TiO2Thin Films by a Novel Sol-Gel Processing for Alcohol Sensing Devices
Authors: Garzella Carlotta , Comini Elisabetta , Bontempi Elza , Depero Laura E. , Frigeri Cesare , Sberveglieri Giorgio
Published: April 2011

TiO2and W:TiO2 thin films have been prepared by a chemically modified sol-gel technique, that implies hydrolysis and condensation of tetraethylortotitanate in the presence of a polymer dissolved in ethanol. Oxide-polymer films were deposited by dip-coating. Annealing at 500°C results in nanosized structurally stable oxides films. For doping tungsten(V)ethoxide was used in concentration that led to a final W/Ti atomic ratio of 1/33, 5/33, 10/33. The morphological and structural characteristics of thin films were studied through microraman, GIXRD, SEM and TEM. A microstructural comparison between pure and doped TiO2 layers is reported. Ethanol and methanol sensing properties are tested.
Photoluminescence from n-(p-) type impurity doped Si nanocrystals
Authors: Fujii Minoru , Mimura Atsushi , Hayashi Shinji , Kovalev Dmitri , Koch Frederick
Published: April 2011

Effects of impurity (P and B) doping on the photoluminescence (PL) properties of Si nanocrystals (nc-Si) in SiO2 thin films are studied. It is shown that with increasing P concentration, PL intensity first increases and then decreases. In the P concentration range where PL intensity increases, quenching of the defect-related PL is observed, suggesting that dangling-bond defects are passivated by P doping. On the other hand, in the range where PL intensity decreases, optical absorptiondue to the intravalley transitions of free electrons generated by P doping appears. The generation of free electrons andthe resultant three-body Auger recombination of electron-hole pairs is considered to be responsible for theobserved PL quenching. In the case of B doping, the behavior is much different. With increasing B concentration, PL intensity decreases monotonously. By combining the results obtained for P and B doped samples, theeffects of donor and acceptor impurities on the PL properties of nc-Si are discussed.
Low-noise, Low-dark-current GaN Diodes for UV Detectors
Authors: Deelman Peter W. , Bicknell-Tassius Robert N. , Nikishin Sergey , Temkin Henryk
Published: April 2011

We report mesa-isolated Schottky barrier photodetectors fabricated on n-GaN. Single-element detectors were constructed from nitride epilayers grown by gas source molecular beam epitaxy (GSMBE) on Si(111). Chlorine-based reactive ion etching was used to form two-level mesas. The detectors were front-illuminated through 100 Å Pd semitransparent Schottky contacts on the upper mesas; ohmic contact on the lower mesas was made using standard Ti/Al/Ti/Au metallurgy. Silicon dioxide grown by plasma-enhanced chemical vapor deposition provided both surface passivation and electrical isolation. The dark current of an 86 × 86 µm2 single-element detector is 2.10 × 10−8 A/cm2 at –2 V bias, and the zero-bias noise power density at 1Hz is as low as 9 × 10−29 A2/Hz. In addition, we present preliminary results for p-n diodes fabricated from epilayers grown on sapphire. The dark current of a 50 × 50 µm2 single-element detector is 1.6 × 10−6 A/cm2 at –2 V bias.
Analysis of strain in sub-grains with variable misorientation in GaN epilayers by digital processing of HRTEM images
Authors: Kret S. , Ruterana P. , Chen J. , Nouet G.
Published: April 2011

In epitaxial GaN layers on (0001) sapphire, the distance between edge dislocations in sub-grain boundaries is variable. The strain state in such sub-grains is quantitatively measured by processing of HRTEM images. The dislocation core distribution maps and in plane Burgers vectors components are derived from experimental strain tensor by applying the continuum dislocation theorem. For image simulationsthe atomic models of edge dislocations are calculated using a modified Stillinger-Weber potential. It is shown that the strain field extracted from simulated images matches that from experimental ones. The corresponding dislocation density peaks are larger and more splitted than those from simulated images.
Nitride-Rich Hexagonal GaNP Growth Using Metalorganic Chemical Vapor Deposition
Authors: Yoshida Seikoh , Itoh Yoshiteru , Kikawa Junjiroh
Published: April 2011

The growth of GaNP using laser-assisted metalorganic chemical vapor deposition (LA-MOCVD) was carried out for the fabrication of a light-emitting diode (LED). We used an Ar-F laser in order to decompose the source gases at lower temperatures. Trimethylgallium (TMG), ammonia (NH3) and tertialybuthylphosphine (TBP) were used for the growth. GaNP growth was carried out at different temperatures. After that, annealing was carried out at 1273-1373 K to improve the crystal quality.

As a result, N-rich GaNP could be grown at 1123-1223 K. The surface morphologies of GaNP were improved when the growth temperature was increased to above 1173 K. We investigated the photoluminescence (PL) of GaNP. The band-edge emission of GaNP was observed at 77 K upon applying thermal annealing at 1323 K. This peak shifted to about 0.2 eV compared with the GaN band-edge emission. Furthermore, a GaNP LED was fabricated and the electoluminescence spectra were investigated. The band-edge emission at 420 nm was observed.
Optical Properties of a Quantum Well of A1−x Bx Alloy Semiconductor in the Coherent Potential Approximation
Authors: Shinozuka Yuzo , Kida Hirotsugu , Watarikawa Masanori
Published: April 2011

We have theoretically studied optical properties of a quantum well (QW) in which the well region is constructed from a binary alloy semiconductor A1−xBx in the coherent potential approximation (CPA). A tight binding model is used for a single particle (electron, hole, Frenkel exciton) in the well composed by a rectangular array of NxxNyxNz sites. The effect of the diagonal randomness is reduced to the coherent potential σ(E), which is assumed to be the same for all sites, and is selfconsistently determined with the average Green's function. For a slab (∞, ∞, Nz) and wire (∞, Ny, Nz), the density of states (E) is composed of Nz (or NyxNz) subbands, each shows the two (one)-dimensional van-Hove singularity. When x (or 1−x) is small, a B (A) impurity-band always appears at the lower (higher) energy side of the lowest (highest) host-band. The change of (E) and the absorption spectrum by changing the well-width and the dimensionality is discussed in detail.
Luminescence And Structural Properties Of InGaN Epilayer, Quantum Well And Quantum Dot Samples Using Synchrotron Excitation
Authors: O'Donnell K.P. , Martin R.W. , White M.E. , Tobin M.J. , Mosselmans J.F.W. , Watson I.M. , Damilano B. , Grandjean N.
Published: April 2011

The Daresbury synchrotron radiation source (SRS) provides bright, tunable x-rays for scattering and absorption probes of local structure. Scanning confocal microscopy and luminescence decay measurements employ the SRS in alternative ways, as a tunable luminescence excitation engine and as a source of weak, 160 ps pulses with a large pulse-topulse separation, respectively. This report first describes local atomic structure studies of InGaN epilayers by extended x-ray absorption fine structure (EXAFS). In addition, we report photoluminescence (PL) imaging, PL microspectroscopy and photoluminescence decay studies of various nitride samples, including tailored InGaN quantum wells and discs.
Improvement of Thin Palladium and Platinum Silicide Films On (100) Si Substrates By Incorporating Phosphorus Dopant
Authors: Juang M. H. , Ou-Yang C. I. , Cheng H. C. , Lin C. T.
Published: April 2011

Effects of phosphorus dopant on thin Pd and Pt silicide films on (100) Si substrates have been studied. As for the samples formed by implanting BF2 dopant into thin Pd films deposited on Si substrates and then annealing, the thermal stability of thin Pd2Si films can be improved relative to the control samples without dopant incorporation. The degree of improvement is affected by the fluorine concentration around the silicide/Si interface and thus the BF2 implant energy. On the other hand, as for the samples formed by implanting phosphorus dopant into thin Pd films and then annealing, both the thermal stability and the silicide conductivity of thin Pd2Si films can be significantly enhanced. The large improvements in the thermal stability and the silicide conductivity are nearly independent of the phosphorus implant energy, and which are primarily due to the formation of textured Pd2Si structures. In addition, the Pt silicides formed by implanting phosphorus dopant into thin Pt films and then annealing also show considerably improved thermal stability.
Effects of Surface Hydrogenation on Initial Reaction Processes of Transition-Metal Adatoms on Silicon Surfaces
Authors: Higai Shin'ichi , Ohno Takahisa
Published: April 2011

We studied the effects of the surface hydrogenation on the adsorption, penetration, and silicidation, i.e., the initial reaction processes of the transition-metal Ni and Ti adatoms on the Si surfaces by the first-principles theoretical calculations. We found for both Ni and Ti that the surface hydrogenation changes the most stable surface site and reduces the adsorption energy. In addition, it blocks the penetration, and thus prevents the silicidation. Furthermore, we newly propose its interesting effects from our results, i.e., impurity metal atoms existing in the Si subsurface are extracted onto the surface by the surface hydrogenation. Thereby, highly pure and atomically flat Si surfaces are expected to be obtained.
Microstructure and Coarsening in Mazed Bicrystal Films of Au Grown on Ge Substrates
Authors: Radetic Tamara , Dahmen Ulrich
Published: April 2011

Thin films of gold (Au) were grown on single crystal germanium (Ge) or silicon (Si) substrates using physical vapor deposition (PVD). The resulting microstructure was that of a mazed bicrystal in which two equivalent grain orientations, related to each other by a 90° rotation, are arranged in a morphology of irregularly shaped, convoluted grains. Quantitative morphological analysis showed a strong dependence of grain shape on size, with larger grains being more convoluted and smaller grains more compact. The evolution of grain size, anisotropy and shape during heating in the temperature range from 300-340 °C was studied by in-situ transmission electron microscopy (TEM).
The nature of islanding in the InGaAs / GaAs epitaxial system
Authors: Walther T. , Cullis A.G. , Norris D. J. , Hopkinson M.
Published: April 2011

The interest in the phenomenon of islanding in a range of semiconductor systems is in part due to the fundamental importance of the Stranski-Krastanow transition but also driven by potential device applications of self-organized quantum dot arrays. However, the mechanism underlying the island formation is still to a significant degree unclear. In the present work, we focus on the epitaxial InGaAs / GaAs(001) system, with layer deposition by molecular beam epitaxy. Atomic force microscopy is used to measure the surface topography of nominally 4nm thick InxGa1-xAs films. It is shown that the growth mode switches abruptly from flat layer to island growth if a critical Indium composition of x(In)≍0.25 is reached. The structure of such layers during early stages of growth is examined using energy-filtered transmission electron microscopy. Indium gradients in the islanded layers are measured and the driving force for the islanding transition itself is considered.
Kinetic properties of adatoms in Sine-Gordon model
Authors: Baryakhtar I.V.
Published: April 2011

The model for describing kinetic properties of adatoms on an anisotropic substrate is proposed. The kinetic equation for 1D adatom localized structures is constructed in the frame of the Sine-Gordon model. The self-diffusion coefficient for adatoms is calculated and the process of their homogenization is discussed.
Velocity Distribution of Electrons Escaping from a Potential Well
Authors: Lavine James P.
Published: April 2011

Electron escape over a one-dimensional potential barrier is treated with a Monte Carlo method that incorporates simple models for the electron-phonon interaction. The consequences of these models are considered here through the calculation of the escaping electron velocity distribution and the electron energy distribution before escape. Effective temperatures are derived from both distributions. The numerical results are compared with those from the classical model of thermionic emission.
Determination of TN for Ba6Mn4MO15 (M = Cu, Zn) by Neutron Diffraction
Authors: Battle P. D. , Cussen E. J.
Published: April 2011

The Néel temperatures of Ba6Mn4MO15 (M = Cu, Zn) have been determined by neutron powder diffraction to be ∼ 6 K. In both cases, short-range magnetic coupling within the individual chains of this pseudo-1D crystal structure produces a susceptibility maximum at a temperature somewhat higher than that at which the frustrated interchain interactions establish long-range magnetic order.
Synthesis of Mesophase Cerium(IV) Oxides via Surfactant Templating
Authors: Inagaki Masahiko , Hozumi Atsushi , Yokogawa Yoshiyuki , Kameyama Tetsuya
Published: April 2011

Mesostructured cerium(IV) oxides were directly synthesized via the hexamethylenetetramine (HMT) homogeneous precipitation method. Cerium(III) nitrate, a templating agent, HMT and water were mixed in a molar ratio of 1: 0.1-2: 1-15: 100-1000 and kept at 65 to 95°C for 10 to 40 hours. 1-Hexadecanesulfonic acid sodium salt was used as the templating agent. Low-angle XRD patterns showed that the as-prepared powders had a lamellar structure with d100=4.1 nm. Wide-range (2θ=10-70°) XRD patterns also exhibited reflection peaks of CeO2 with a fluorite structure. However, the low-angle reflection peaks disappeared in calcined specimens.
A Novel Route For The Synthesis of LiAlxCo1−xO2 Battery Materials And Their Structural Properties
Authors: Tomar M. S. , Hidalgo A. , Dobal P. S. , Katiyar R. S. , Dixit A. , Melgarejo R. E. , Kuenhold K. A.
Published: April 2011

LiCoO2 is an important cathode material for rechargeable batteries. Al substitution on Co sites was recently found to enhance its intercalation voltage. We have developed a reliable and less expensive method for the synthesis of such LiAlxCo1−xO2 compositions using simple salts of the constituent elements and organic solvents. X-ray diffraction and Raman spectroscopy were used for structural characterization of these materials at various annealing temperatures. The phase evolutions in LiAlxCo1−xO2 compositions were studied using micro-Raman spectroscopy and a phase diagram is proposed based on the observations. The phase diagram suggests that thin films of these materials can be deposited at substrate temperature at about 650°C
Reducibility study of the LaMxFe1−xO3 (M = Ni, Co) perovskites
Authors: Estournès C. , Provendier H. , Bedel L. , Petit C. , Roger A.C. , Kiennemann A.
Published: April 2011

Solid solutions of LaMxFe1−xO3 (with M = Ni and Co) have been used in the Fischer- Tropsch reaction (CO + H2 → Hydrocarbons + CO2) and in the partial oxidation of methane (CH4 + 1/2 O2 → CO + 2 H2). In both catalytic reactions, the active catalyst is reported to be reduced metal particles; their size and their interactions with the support induce large differences in the product distribution.

In the nickel system, after total reduction by TPR all catalysts exhibit ferromagnetic behavior at room temperature. In situ magnetization in 1 Tesla on cooling the sample under reducing atmosphere shows one magnetic transition for each sample indicating one Curie temperature. These Curie temperatures are in between those known for bulk nickel and iron and decrease with the initial nickel content of the perovskite. This indicates that nickel is reduced first and induces the reduction of iron, leading to the formation of an alloy.

In the cobalt system, in situ magnetization on heating the sample shows a sharp increase of the magnetization only for x = 0.25, 0.40 and 1, corresponding to the formation of metallic cobalt nanoparticles. All other materials present only one increase of the magnetization for temperatures similar to those observed for the second reduction in TPR corresponding to the formation of CoFe alloys.
Conversion of an Aurivillius Phase Bi2SrNaNb3O12 into Its Protonated Form via Treatment with Various Mineral Acids
Authors: Shirata Masashi , Tsunoda Yu , Sugimoto Wataru , Sugahara Yoshiyuki
Published: April 2011

A protonated form of a layered perovskite was prepared from an Aurivillius phase Bi2SrNaNb3O12via acid treatments, and the effect of the type of mineral acids was investigated. The treatment with HX (X = Cl, Br, I) resulted in the formation of a protonated form H1.8[Sr0.8Bi0.2NaNb3O10], while no reactions were observed for HNO3 and H2SO4 under the present experimental conditions. All the products obtained by HX-treatments exhibited layered structures and the structures of the perovskite-like slabs were preserved.
Two Different Interactions between Oxygen Vacancies and Dopant Cations for Ionic Conductivity in CaO-Doped CeO2 Electrolyte Materials
Authors: Zhou Yuanzhong , Chen Xi
Published: April 2011

The electrical measurement has demonstrated that conductivity of CaO-doped CeO2 has higher activation energy for low temperature and lower activation energy for high temperature. A model with two different kinds of defect interactions between oxygen vacancy and doped cations has been used to interpret the phenomenon. Diffusion based on hopping of oxygen ions was assumed as the mechanism of electrical conduction. The analysis indicated that at high temperature free oxygen vacancies are dominant and the activation energy is only for oxygen ion hopping. At low temperature, however, oxygen vacancies associated with dopant calcium ions are dominant for high CaO content and the activation energy is the energy for hopping of an oxygen ion plus half of the association energy between one oxygen vacancy and one calcium ion. For low level doping, both free and associated oxygen vacancies are important.
Piezoresistive Sensors on Plastic Substrates Using Doped Microcrystalline Silicon
Authors: Alpuim P. , Chu V. , Conde J. P.
Published: April 2011

The piezoresistive behavior of optimized n-type and p-type microcrystalline silicon films deposited on polyethylene terephthalate plastic substrate by hot-wire and radio-frequency plasma-enhanced chemical vapor deposition, at a substrate temperature of 100 °C, is studied. A 4-point bending jig allowed the application of positive and negative strains in the films. Repeated measurements of the relative changes in the resistance of the samples during the strained condition showed reversible behavior, with p-type microcrystalline films having positive gauge factor in the range from 25 to 30 and n-type [.proportional]c-Si:H films having negative values of gauge factor from -40 to -10. The induced strain in the films was in the range between 0 and ±0.3%. A sensor utilizing the piezoresistive property of doped [.proportional]c-Si:H was used to map a contour with the shape of an Archimedes' spiral.
Photovoltaics on Wire
Authors: Rojahn M. , Rakhlin M. , Schubert M. B.
Published: April 2011

This contribution presents an approch to use wire-like substrates for thin-film devices. Based on plasma deposition processes for metals, hydrogenated amorphous silicon (a-Si:H) and transparent conductive oxides, solar cells are fabricated onto metal wires coated by a dielectric. Photolithography using a N2-laser and etching steps serve to pattern the thin film layers. Scanning electron microscope pictures and current-voltage characteristics of the device are demonstrated. Simple geometric simulations show the spectrally resolved intensity of the light that is scattered into the a-Si:H layers as a function of the diameter ratio of the substrate material and the deposited thin films. Finally, we discuss light concentrating and light trapping designs of sensors on transparent fibre substrates.
Vibrational Stretching Modes of the Si-H and Si-D Bonds in Amorphous Silicon Nitride
Authors: Lin Shu-Ya
Published: April 2011

The vibrational density of states for the SiH(D) and SiH2(D2) groups in amorphous silicon nitride are calculated. The Si-H(D) bond-stretching modes of the SiH2(D2) groups are found to split, with the Si-D bond-stretching mode having larger splitting. The difference between the Si-D bond-stretching frequency of the SiD group and the Si-D asymmetrical stretching frequency of the SiD2 group has about the same value as that of the Si-H bonds. The differences in the effective masses of the Si-H and Si-D bonds and the degrees of the bond-stretching splitting of the SiH2 and SiD2 groups have compensating effects.
Orientation-Dependence of Low Temperature Epitaxial Silicon Growth
Authors: Wagner Thomas A. , Oberbeck Lars , Nerding Melanie , Strunk Horst P. , Bergmann Ralf B.
Published: April 2011

Electronic properties of thin epitaxial silicon films deposited at temperatures below 650°C by means of ion-assisted deposition strongly depend on substrate orientation as well as on deposition temperature: In (100)-oriented epitaxial films we find a low density of structural defects, and the minority carrier diffusion length is only limited by the presence of point defects or point defect complexes. These investigations also show an improvement of the electronic quality with increasing deposition temperature. Epitaxy on non-(100)-oriented substrates results in a significantly higher density of structural defects. The electronic properties of films deposited on stable flat surfaces, such as (111)- and (113)-oriented substrates are inferior as compared to (100)-oriented films, but are still superior to those of films deposited on faceted surfaces, as shown by light beam induced current and electron back-scattering diffraction measurements of polycrystalline thin films.
Effect of Experimental Noise on Recovery of the Electronic Density of States from Transient Photocurrent Data
Authors: Reynolds Steve , Main Charlie , Gueorguieva Mariana J.
Published: April 2011

The effects of random noise on density of states determination from transient photocurrent data are examined by superimposing noise levels similar to those found experimentally (1% to 20%) on computer-simulated current-time data. Mathematically approximate methods based on Fourier and Laplace transformations are found to operate effectively at noise levels of up to 20%. Mathematically exact methods offer higher resolution, but this is compromised by greater susceptibility to noise. A Tikhonov regularisation method yields both high resolution and good noise tolerance.
Properties of silicon films deposited under argon dilution
Authors: Longeaud C. , Roy D. , Chaudhuri P. , Gupta N. Dutta , Ray P. Pratim , Vignoli S. , Meaudre R. , Meaudre M. , Saviot L.
Published: April 2011

Four series of samples, prepared at 250° C by decomposition of a mixture of silane and argon in a radio frequency powered deposition systems (rf-PECVD), have been studied. The dilution rates were 1 %, 1.5 %, 5 % and 10 % of silane in argon and the total pressure was 0.5 Torr for the first series and 0.2 Torr for the others. Structural and transport properties of the materials have been studied as function of power density. Structural studies show the transition from purely amorphous material towards microcrystalline material with increasing rf power density. The transport parameters were measured in the as-deposited, light-soaked and annealed states and compared to those obtained on state of the art material. The best material obtained is clearly device grade material. This study shows that argon dilution allows to tailor the material for a given application.
Structural and Optical Features of Nano-Crystalline Silicon Films Prepared by Pecvd and Rf Magnetron Sputter Techniques
Authors: Park M.-B. , Cho N.-H.
Published: April 2011

Nano-crystalline Si films were prepared by plasma enhanced chemical vapor deposition (PECVD) and rf-magnetron sputter techniques at various deposition conditions, and the relations of the photoluminescence (PL) phenomena of the films with process variables are discussed. The phase of the films prepared at R.T. by PECVD techniques is somewhere between amorphous and crystalline states, consisting of nano-crystallites of size ranging from 3.2 to 5.3 nm. These films exhibit significant PL intensities near blue light region; the PL peaks shift from 510 to 460 nm with decreasing the reaction gas (SiH4) fraction from 4.7 to 2.0%. The films prepared at 500 °C by PECVD are composed of about 5 and 150 nm crystallites, exhibiting little PL phenomena. The PL intensity of the films prepared by sputter techniques was observed to increase with raising the sputter power and the post-deposition heat-treatment temperature.
Increasing The Dark Conductivity Activation Energy in Undoped Microcrystalline Silicon by Post-Growth Anneals
Authors: Yoon Jong-Hwan
Published: April 2011

Undoped µc-Si:H film of the strong n-type character with the dark conductivity activation energy of about 0.28 eV was annealed. Annealing was carried out by slowly increasing the temperature from 25 °C to 450 °C at a constant rate of 12 °C/min (one annealing cycle). Annealing effects were monitored by measuring the changes in dark conductivity, oxygen and hydrogen concentrations, and photoluminescence (PL). Dark conductivity activation energy gradually increases with increasing the number of annealing cycles to a saturation value of about 0.6 eV. There is little or no change in the oxygen concentration, but the hydrogen concentration decreases with increasing the number of annealing cycles. The PL band near 1.2 eV disappears with annealing, while the low energy PL band near 0.85 eV dominates rather as the number of annealing cycles increases. A possible explanation will be discussed.
Optical Simulations of the Effects of Transparent Conducting Oxide Interface Layers on Amorphous Silicon Solar Cell Performance
Authors: Ferreira Gelio M. , Ferlauto Andre S. , Rovira Pablo I. , Chen Chi , Nguyen Hien V. , Wronski Christopher R. , Collins Robert W.
Published: April 2011

Spectroscopic ellipsometry (SE) analysis of so-called “specular” (macroscopically smooth) and “textured” (macroscopically rough) thin film amorphous silicon (a-Si:H) based solar cell structures demonstrates the need to incorporate interface layers into the multilayer stack in order to simulate the observed Stokes vector of the specularly-reflected beam. In most cases, these layers can be attributed to microscopic roughness (e.g., at the SnO2/p-layer/i-layer interface in a-Si:H p-i-n solar cells), as verified by atomic force microscopy (AFM). In limited cases, the layers may include regions wherein chemical intermixing also occurs (e.g., at the ZnO/Ag interface in back-reflectors), particularly for overlying films prepared by sputtering. In spite of the clear evidence for the existence of interface layers, they have been neglected in previous simulations of the optical quantum efficiency (QE) of the solar cells. In this study, we incorporate the experimentally- observed characteristics of interface layers as input into optical models for the p-i-n solar cell structure. In this way, we demonstrate the beneficial effects of SnO2/p/i interface microroughness as an anti-reflector and the detrimental effects of the ZnO/Ag interlayer as a parasitic absorber.
Proton NMR and Magnetic Susceptibility in a-Si:H
Authors: Baugh Jonathan , Han Daxing , Kleinhammes Alfred , Wu Yue
Published: April 2011

Proton nuclear magnetic resonance (NMR) is an important tool for characterizing the structure of a-Si:H over a wide range of length scales including short- and medium-range order as well as nanostructures. Some of the structural information obtained by NMR is based directly on proton NMR characteristics such as the lineshape and dipolar interactions that probe distances between hydrogen atoms and hydrogen environment. Based on such information it is shown that hydrogenated multi-vacancy model for Si-H clusters is fully consistent with both experimental and theoretical observations. It is demonstrated thatchanges of short- and medium-range order can be detected by measuring the magnetic susceptibility χ. Here ï is not directly related to NMR characteristics and NMR is used merely as a sensitive and accurate magnetometer. The result indicates that a-Si:H prepared by hot-wire CVD (HWCVD) and plasma-enhanced CVD (PECVD) with high H-dilution have higher structural order compared to conventional a-Si:H. By carrying out NMR measurements on single a-Si:H thin film new NMR features were observed such as the orientation dependence of the proton NMR spectrum with respect to the magnetic field. Based on such orientation dependence, strong evidence of aligned nano-channels was obtained in some device quality a-Si:H films.
The Properties of Low Hydrogen Silicon Thin Films Deposited by Mesh-type PECVD
Authors: Ryu Se-Won , Kwon Do-Hyun , Park Sung-Gye , Kim Hyoung-June
Published: April 2011

We propose a mesh-type PECVD to minimize the hydrogen concentration in this study. Since in this system deposition rate is very slow, so for increasing a deposition rate, we suggest an applied DC bias enhanced sputtering process. We investigated several conditions to compare with conventional PECVD. Excimer-laser melting and regrowth of thin a-Si films is for fabricating polycrystalline-Si (poly-Si). Furthermore, we fabricate poly-Si thin-film transistor(TFTs) and measure threshold voltage (V), field-effect mobility (cm2 /Vs) and on/off current ratio
Thermal Annealing Study of Variable Band-Gap a-SiN:H Alloy Films
Authors: Banerji N. , Faro E. , Serra J.
Published: April 2011

A study of the effects of thermal annealing on the bond structure, optical properties, morphology and stoichiometry of variable band gap a-SixNyHz alloy films deposited by ArF LCVD has been made. These films were generated through two different photochemical pathways resulting from the use of either SiH4/NH3 or Si2H6/NH3 as precursor gases. Thermal annealing study of these films which span a wide compositional (0.22 < x/y < 1.44) and band-gap (Eg) range (1.7–5.2 eV) has led to the analysis of the stability of different bond configurations. We report here hydrogen bond stability and rearrangement of the near-neighbor environment of SiN bonds analyzed through FTIR, refractive index variations measured by ellipsometry and surface morphological changes observed through AFM during to the process of annealing. Results indicate that in the SiH4/NH3 deposited nitrogen-rich SiN:H films, SiHn bonds persist even upto 900°C and SiH bonds are less thermally stable than SiH2. Also, while the total bonded hydrogen decreases, the extent of SiN bond increases through a reaction mechanism involving: SiH + NH → SiN + H2. In the Si2H6/NH3 deposited silicon-rich film, whose initial spectra shows Si-H stretching and Si-H wagging bands, a rapid loss of Si-H bonds occurs at T ≤ 650°C. Based on the bond configurational interpretations of the Random bonding model proposed by Bustarret, we calculate the variations in film stoichiometry and density with the annealing temperature.
Physical Vapor Deposition of Cu(In,Ga)Se2Films for Industrial Application
Authors: Wada T. , Nishiwaki S. , Hashimoto Y. , Negami T.
Published: April 2011

Cu(In,Ga)Se2 thin films were prepared by physical vapor deposition. The CIGS films were deposited by three kinds of method. The 1st was “2-stage process” in which (In,Ga)2Se3 precursor layer was deposited on Mo coated soda-lime glass at the 1st stage, and then exposed to Cu and Se fluxes to form CIGS films at the 2nd stage. The 2nd method was an ordinary “3-stage process”. The 3rd method was “2-stage deposition and post-annealing process” in which CIGS films were deposited at low substrate temperatures and then the obtained CIGS precursor films were annealed in Se flux at high temperatures. A solar cell using a CIGS film prepared at 400 °C by the “2-stage process” showed an efficiency of 11.8 % and that using a CIGS film deposited at 350 °C by the “3-stage process” showed an efficiency of 12.4 %. The CIGS films deposited by the “2-stage deposition and post-annealing process” have similar microstructures to the device quality CIGS films deposited by the “3-stage process” at high temperatures. The solar cell with an MgF2/ITO/ZnO/CdS/CIGS/Mo/glass structure showed an efficiency of 17.5 % (Voc=0.634 V, Jsc=36.4 mA/cm2, FF=0.756). The thin CIGS films with a smooth and flat surface can be fabricated by the “2-stage deposition and post-annealing process”. The solar cell using a 0.7μm CIGS absorber layer showed an efficiency of over 12 % and a large open circuit voltage of 0.677 V.
Next-Generation Transparent Conducting Oxides for Photovoltaic Cells: an Overview
Authors: Ginley David , Coutts Tim , Perkins John , Young David , Li Xiaonan , Parilla Phil
Published: April 2011

Transparent conducting oxides (TCOs) are becoming a more critical element in thin-film photovoltaic devices. In the continued drive to increase efficiency and stability while reducing cost and optimizing performance, the optical, electrical, and materials properties of TCOs gain increasing importance. TCOs can perform a variety of important functions, including contacts, antireflection coatings, and chemical barriers. In this paper, we will review some of the current advances in the field of transparent conductors and, where possible, will relate these advances to thin-film photovoltaic devices. Highlights will be on the rapidly growing collection of new n- and p-type materials; the implications of these materials on PV have not been fully assessed.
Structural and Optical Properties of Highly Efficient 2- μm Thick CdS/CdTe Thin Film Solar Cells
Authors: Nakamura K. , Gotoh M. , Fujihara T. , Toyama T. , Okamoto H.
Published: April 2011

The structural and optical properties of CdS/CdTe(S) interface region of 2-μm thick CdS/CdTe solar cells have been studied in conjunction with photovoltaic performances of the solar cells. The properties are found to be crucially influenced by the annealing temperature and oxygen concentration of the CdCl2 treatment. An increase in VOC and F.F. found in the solar cells with the CdCl2 treatment at < 360°C is interpreted as due to suppression of interdiffusion of sulfur and tellurium at the CdS/CdTe(S) interface. On the other hand, the electromodulated photoluminescence with UV light excitation (UVE-EMPL) study reveals that the increasing VOC due to increasing oxygen concentration to 5% is likely to be caused by an increase in the built-in electric field in n+-CdTe1−xSx just adjacent to the CdS/CdTe(S) interface. As a result of the modification of the process parameters, we have achieved the conversion efficiency of 13.6% (VOC: 0.817V, JSC: 23.0 mA/cm2, F.F.: 0.725) using 2.3-μm thick PV active layer without anti-reflection coating.