bluuebunny/crossref_metadata_2025_split huggingface dataset:
Nature of the Green Luminescent Center in Zinc Oxide
Authors: Vanheusden K. , Warren W. L. , Seager C. H. , Tallant D. R. , Caruso J. , Hampden-Smith M. J. , Kodas T. T. 
Published: April 2011

ABSTRACTWe apply a number of complementary characterization techniques including electron paramagnetic resonance, optical absorption, and photoluminescence spectroscopies to characterize a wide range of different ZnO phosphor powders. We generally observe a good correlation between the 510-nm green emission intensity and the density of paramagnetic isolated oxygen vacancies. In addition, both quantities are found to peak at a free-carrier concentration ne, of about 1.4 × 1018 cm-3. We also find that the green emission intensity can be strongly influenced by free-carrier depletion at the particle surface, especially for small particles and/or low doping. Our data suggest that the green PL in ZnO phosphors is due to the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes in the valence band.
Photoluminescence Properties Of δ-Doped ZnS:Mn Grown By Metal-Organic Molecular Beam Epitaxy
Authors: Park W. , Tran T. K. , Tong W. , Kyi M. M. , Schön S. , Wagner B. K. , Summers C. J. 
Published: April 2011

AbstractA detailed study is reported of the optical properties of homogeneously and δ-doped ZnS:Mn thin films grown by metal-organic molecular beam epitaxy. Fine structure in the Mn luminescence was observed at low Mn concentrations. The energy of the zero-phonon 4T1→6A1, transition of the Mn ion was determined to be 2.215eV, and the phonon energies associated with the transition to be 10meV and 37meV for the TA and TO phonons, respectively. It was found that the PL intensity depended on the Mn concentration and the δ-doped ZnS:Mn showed much brighter luminescence than the homogeneously doped samples with comparable Mn concentrations. For homogeneously doped ZnS:Mn with low Mn concentrations, the temperature dependence of the linewidth and the peak position was well described by the configuration coordinate model. For δ-doped ZnS:Mn, the antiferromagnetic interaction between Mn ions was invoked to explain the low temperature behavior. The luminescence lifetime measurements suggested that the δ-doping technique resulted in better incorporation of Mn ions in the ZnS host and less defect formation.
An Amorphous Silicon Thin Film Transistor Fabricated at 125°C by dc Reactive Magnetron Sputtering
Authors: McCormick C. S. , Webe C. E. , Abelson J. R. 
Published: April 2011

AbstractWe deposit hydrogenated amorphous silicon-based thin film transistors using dc reactive magnetron sputtering at a substrate temperature of 125°C, which is low enough to allow the use of plastic substrates. We characterize the structural properties of the a-Si:H channel and a-SiNx:H dielectric layers using infra-red absorption, thermal hydrogen evolution, and refractive index measurements, and evaluate the electrical quality using capacitance-voltage and leakage current measurements. Inverted staggered thin film transistors made with these layers exhibit a field effect mobility of 0.3 cm2/V-s, a Ion/Ioff ratio of 5 × 105, a sub-threshold slope of 0.8 V/decade, and a threshold voltage of 3 V.
Improved a-Si:H TFT Performance Using a-SixN1-x / a-SixC1-x Stack Dielectrics
Authors: Lavareda G. , Fortunato E. , Carvalho C. Nunes , Martins R. 
Published: April 2011

AbstractIn this paper we present a study on the electrical characteristics (conductivity, σ and relative dielectric constant, εr,.) of amorphous silicon nitride (a-SixN1-x) and carbide (a-SixC1-x) films deposited by PECVD, used as dielectric materials in TFT devices, aiming to select the most adequate alloy that lead to improve device performances. Besides that, double stack a-SixN1-x/a-SixC1-x structures were developed and applied as dielectric layers on TFTs, whose performances show to be superior to those ones using single silicon nitride or silicon carbide as dielectric.
High Deposition Rate a-Si:H for the Flat Panel Display Industry
Authors: Hautala J. , Saleh Z. , Westendorp J. F. M. , Meiling H. , Sherman S. , Wagner S. 
Published: April 2011

AbstractHigh deposition rates and good quality electrical properties and thickness uniformities over large areas are required for all three films (SiNx, a-Si:H and n+ a-Si:H) composing the thin film transistors (TFTs) for the AMLCD industry, while maintaining high tool up-time and low particle formation. Generally these conditions have been achieved for most single-panel multichamber PECVD systems; however, it has become increasingly apparent that a compromise is drawn between the TFT mobility and the deposition rate of the a-Si:H layer. Thus it becomes essential to clearly assess the industry requirements for both deposition rates as well as TFT performance for the different device structures used for AMLCDs, and to discover and control these underlying material properties.The TEL VHF (40/60 MHz) PECVD system produces high quality, low defect density a- Si:H at deposition rates exceeding 1500 Å/min when analyzed by FTIR, CPM, photo and dark conductivity. Even though the low deposition rate a-Si:H exhibits very similar bulk properties, higher mobility TFTs are produced with a-Si:H deposited at lower RF power. Having both a high ion flux and low ion energy in the SiH4 discharge are likely the most critical conditions for controlling the a-Si:H quality and thus the TFT mobility. Increasing the RF frequency enhances both of these effects, as well as provides a higher deposition rate for a given power density and a higher power threshold for particle/powder formation. For these reasons it is likely a 40/60 MHz plasma will produce better performing TFTs for a given deposition rate when compared with a conventional 13.56 MHz system. Other process conditions such as diluting the SiH4 in H2 or Ar also seem to play an important role in the optoelectronic properties of the a-Si:H film and ultimately the TFT performance.
Energy Levels of Defects in a-Si:H From Optical and Electrical Characteristics
Authors: Globus T. 
Published: April 2011

AbstractTwo novel characterization techniques for hydrogenated silicon thin films have been recently proposed which show promise in providing critical feedback for evaluating materials and monitoring the device fabrication process. The first technique is the optical interference spectroscopy for a quick non-destructive measurement of absorption coefficient and refractive index spectra of amorphous- and poly-Si thin films in a wide range of the incident photon energies (0.5–3.5 eV) [1]. By using this technique, the absorption related to defects in the subgap energy region has been determined for device quality thin films. The second technique is the novel version of the field effect conductivity (FEC) method for the direct density-of-states (DOS) determination from analysis of thin film transistor (TFT) quasi-static transfer characteristics [2]. This sensitive, fast, and easy to use, method makes it possible to resolve fine-scale features in the midgap DOS of a-Si:H. In this work, data from two methods of spectroscopy are analyzed together. Very close correlation of results is demonstrated which provides a unique opportunity to identify midgap defect states and to understand the fundamental physics of hydrogenated silicon films. The energy map of defect states in the upper half of a-Si:H bandgap is presented. These results permits to use TFT transfer characteristics and optical interference technique measurements as effective tools to control the quality of TFF manufacturing process.
Synthesis of Ferroelectric Liquid Crystalline Polymers by Ring Opening Polymerization
Authors: Hsu C. S. , Lee C. J. 
Published: April 2011

AbstractThe synthesis of two series of side-chain liquid crystalline polyoxetanes and polyoxiranes containing 4-alkanyloxybiphenyl-4′ -yl (2S,3S)-2-chloro-3-methylvalerate side groups is presented. Differential scanning calorimetry, polarizing optical microscopy, and X-ray diffractometry reveal smectic mesomorphism for all obtained polymers. Most of the prepared polyoxetanes present smectic A and chiral smectic C phases. The polyoxetane containing tweleve methylene units in the spacer is the only one showing two enantiotropic smectic A and B phases. All of the obtained polyoxiranes display two enantiotropic smectic A and B phases. Although the polyoxetane and polyoxirane backbones are more flexible than the polymethacrylate backbone, side-chain crystallization do not occur in any of the synthesized polymers.
Novel Vitrified Liquid Crystals and Potential Applications
Authors: Chen S. H. , Shi H. , Conger B. M. , Katsis D. , Mastrangelo J. C. 
Published: April 2011

AbstractAn overview is presented for a generic approach to low molar mass liquid crystalline materials capable of vitrification into morphologically stable, optically anisotropic thin films. Potential applications as polarization control devices, optical notch filters, and luminescent elements will be explored. Preliminary data for defect formation and annihilation will also be presented to illustrate an advantage of low molar mass materials over polymer analogs.
A Microstructural Model for Liquid Crystalline Polymers
Authors: Hobdell John R. , Windle Alan H. 
Published: April 2011

AbstractWe have developed a numerical technique to investigate the effect of different Frank elastic constants on the microstructure of liquid crystalline materials. In particular the technique is applied to predict microstructure in polymeric materials where the splay elastic constant is high and the twist elastic constant is low. It is found that such a combination of elastic constants leads naturally to the twist escaped features seen experimentally.
High Modulus Liquid Crystalline Thermosetting Resins through Novel Processing Techniques
Authors: Smith M. E. , Douglas E. P. , Benicewicz B. C. , Earls J. D. , Priester R. D. 
Published: April 2011

AbstractWe have demonstrated the effects of magnetic field alignment for the epoxy resin system, the diglycidyl ether of dihydroxy-α-methylstilbene cured with sulfanilamide. With increasing magnetic field, the tensile modulus of the thermoset can be enhanced by a factor of three, without compromise to the transverse value. Measurements of the coefficients of thermal expansion and the order parameter as determined by x-ray diffraction elucidate the high degree of alignment obtained at field strengths of 15–18 Tesla.The interrelationship between the reaction kinetics of curing with the molecular alignment process is central to successful processing of this class of thermosets. Preliminary relationships between the field strength, time in the magnetic field, reaction rate, and extent of reaction have been developed. Process control and sequencing allow a wide adjustment in the properties of the produced samples. Our studies demonstrate the increased value of multi-mode processing in the development of tailored property, lightweight materials.
Liquid Crystal Photonics: Optical Switching and Image Storage by Means of Azobenzene Liquid-Crystal Films
Authors: Shishido A. , Tsutsumi O. , Ikeda T. 
Published: April 2011

AbstractOptical switching and image storage were explored by means of azobenzene liquid crystals (LCs). Azobenzene derivatives with monomeric and polymeric forms were prepared, which show nematic LC phase in the trans form while no LC phase in the cis form. On pulse irradiation at 355 nm, which causes trans-cis isomerization of the azobenzene moiety, these azobenzene LC films underwent nematic (N) to isotropic (1) phase transition in 100 ˜ 200 μs as probed by change in transmittance of a He-Ne laser through crossed polarizers (transmission-mode analysis) or change in reflectivity at the interface between the sample and glass substrate (reflection-mode analysis). In the reflection-mode analysis, the decay was faster than that observed in the transmission-mode analysis. The N phase recovered through the cis-trans thermal backisomerization process in the transmission-mode analysis, while it was restored through diffusion of the cis form, followed by reorientation of the trans form which replaced the cis form in the interface region, in the reflection-mode analysis. The photochemical N-I phase transition was also induced in 200 μs below Tg of the polymer azobenzene LC. Image could be stored into the polymer azobenzene LC film on photoirradiation to bring about the N-I phase transition and the stored image remained unchanged even after one year when it was kept below Tg of the polymer.
Theory of Circularly Polarized Light Emission from Chiral Nematic Liquid Crystalline Films
Authors: Shi H. , Chen S. H. 
Published: April 2011

AbstractChiral nematic liquid crystalline films with fluorescent chromophores are capable of circularly polarized emission. A theory is presented in which the effects of anisotropic absorption and film thickness are accounted for. The predicted dissymmetry factor is compared with experimental data reported by Stegemeyer et al. and with the theory for the two limiting cases treated by Pollmann et al.
Electro-Optics of Polymer-Dispersed Liquid Crystal Materials
Authors: Drzaic Paul S. 
Published: April 2011

AbstractThe electro-optical properties of polymer dispersed liquid crystal (PDLC) materials depend on both the chemical nature of the liquid crystal and polymer, as well as the morphology of the liquid crystal/polymer composite. The relationship between several aspects of film structure and PDLC film properties are described, emphasizing scattering effects and reorientation fields. In particular, the idea of relative orientation of neighboring droplets as controlling light scattering, and the role of anchoring in determining film voltages, are highlighted.
Surface Anchoring and Electro-Optics in Polymer-Dispersed Liquid-Crystal Films
Authors: Amundson Karl , Srinivasarao Mohan 
Published: April 2011

AbstractPolymer-dispersed liquid crystal (PDLC) films are composed of micrometer-scale droplets of liquid crystal (typically nematic) in a polymeric matrix. They can be switched with an electric field from a scattering to a transparent state, and are of interest for use in flat-panel displays. The electro-optics of PDLC films, and the director field pattern within the nematic droplets, are strongly influenced by the surface anchoring condition at the liquid crystal - polymer matrix interface. To understand the role of surface anchoring, we studied the electro-optics of photopolymerized PDLC films composed of a liquid crystal and acrylates as a function of temperature. Properties depend strongly upon the choice of the acrylate. With several acrylate matrix materials, the nernatic director field undergoes a reversible, temperature-driven configurational transition, accompanied by dramatic changes in electro-optical properties. We find that the surface anchoring behavior is very sensitive to the polymer structure as well as several parameters of PDLC film formation. Several surface anchoring transitions are used in a study that shows strong evidence for surface memory in PDLC films.
Synthesis of New Chiral Disubstituted Binaphthol Derivatives and their use as the Chiral Dopants
Authors: Deussen H.-J. , Shibaev P. V. , Vinokur R. A. , Schaumburg K. , Bjornholm T. , Bechgaard K. , Shibaev V. P. 
Published: April 2011

AbstractA number of new chiral binaphthol (BN) derivatives with different substituents in the 6,6'-positions in open and bridged forms have been synthesized. Their possible liquid crystalline properties and their helical twisting power (B) of three different nematic liquid crystals (LCs) were investigated. Derivatives with spatially extended substituents in the 6,6'-positions (e.g. styryl or vinil) show unusual high helical twisting power (up to 120 μm–1). A direct correlation between the magnitude of β and the length of the substituents was found.From the different temperature dependence of β of the open and bridged BNs, a molecular model was developed relating the molecular conformation and twisting power
Silicon Films Deposited by LPE on Low Cost Multicrystalline Silicon Substrates
Authors: Stemmer M. , Wagner G. , Martinuzzi S. 
Published: April 2011

AbstractP type silicon layers for photovoltaic applications have been deposited on low cost multicrystalline silicon substrates prepared by directional solidification or by electromagnetic casting with solar grade charges. The thickness is between 4 to 20 μm. The layers were characterized by Light Beam Induced Current (LBIC) mappings at different wavelengthes. Collecting structures are p-n junctions or Al-Si semitransparent diodes. The layer and the substrate can be investigated separately by changing the wavelength of the LBIC maps, or/and by realizing a collecting structure on the free surface of the substrate.It is found that the same extended crystallographic defects are detected by LBIC in the layer and in the substrate. However their recombination strength in the layer is less marked probably due to a higher purity of the layer. The computed values of minority carrier diffusion length are close or larger than the film thickness and the investigated layers could be used to make solar cells.
ZnO/c-Si Heterojunction Interface Tuning by Interlayers
Authors: Fenske F. , Kliefoth K. , Elstner L. , Selle B. 
Published: April 2011

AbstractThe junction properties of isotype and anisotype n+-ZnO/c-Si heterostructures have been studied by electrical and photoelectrical methods. We present evidence that the junction properties are strongly affected by a 10–30 nm thick ZnO layer closest to the heterointerface with distinctively different properties than those of the ZnO film bulk. This layer supports a dominant current flow via multistep tunnelling-recombination. When a 10 nm thin ZnS or ZnSe interlayer is inserted charge transport is controlled by thermionic emission. The interlayer acts as spacer and increases the band bending in the silicon absorber. However, there is still a too high trap density at the interlayer/c-Si interface, so that Voc does not exceed 0.25–0.32 V.
XPS Analysis of Chemically Treated I-III-VI Semiconductor Surfaces and the Relation to II-VI/I-III-VI Heterojunction Formation
Authors: Nelson A. J. , Schwerdtfeger C. R. , Herdt G. C. , King D. , Contreras M. , Ramanathan K. , O'Brien W. L. 
Published: April 2011

AbstractDevice-grade thin-film CuInSe2 was subjected to various chemical treatments commonly used in photovoltaic device fabrication to determine the resulting microscopic surface composition/morphology and the effect on II-VI/CuInSe2 heterojunction formation. HCI (38%), Br-MeOH (<1% Br), (NH4)2S, and NH4OH/thiourea solutions were used separately to modify the surface chemistry of the CuInSe2 polycrystalline films. Scanning electron microscopy was used to evaluate the resultant surface morphology. Angle-resolved high-resolution photoemission measurements on the valence band electronic structure and Cu 2p, In 3d, Ga 2p and Se 3d core lines were used to evaluate the chemistry of the chemically treated surfaces. CdS overlayers were then deposited in steps on these chemically treated surfaces. Photoemission measurements were acquired after each growth to determine the resultant heterojunction valence-band discontinuity between the CdS and the chemically modified CuInSe2 surface.
Effects of Oxygen During Close-Spaced Sublimation of CdTe Solar Cells
Authors: Rose D. H. , Albin D. S. , Matson R. J. , Swartzlander A. B. , Li X. S. , Dhere R. G. , Asher S. , Hasoon F. S. , Sheldon P. 
Published: April 2011

AbstractThe presence of oxygen during close-spaced sublimation (CSS) deposition of CdTe has been regarded as necessary for high-efficiency CdS/CdTe solar cells. To date, research has focused on the effect of oxygen on the acceptor density in the CdTe absorber. We find that although oxygen may influence the doping density, a perhaps more significant effect is its influence on nucleation and growth of the film. Oxygen partial pressures of 0 to 24 torr and total pressures of 5 to 45 torr (balance He) have been explored for CSS depositions. We find that increasing the O2 partial pressure increases the density of CdTe nucleation sites, thereby suppressing pinhole formation and likely reducing defects at the junction. However, increasing O2 also tends to decrease grain size and faceting in the films. In addition to influencing the film properties, O2 adversely affects the source material, producing non-uniformly oxidized surfaces that reduce deposition rates and impair device uniformity and run-to-run reproducibility.We have also determined that the graphite susceptors typically used with CSS deposition convert a portion of the oxygen ambient to CO and CO2 during deposition. The amount converted is highly dependent on the type and age of the graphite susceptor used. The COx byproducts are not efficiently incorporated in the CdTe film and do not influence the deposition as oxygen does. We describe the effect of the changing oxygen partial pressure during the course of deposition. Finally, we report high-efficiency solar cells (12.8% AM 1.5) made without oxygen.
Float Line Deposited Transparent Conductors -Implications for the Pv Industry
Authors: Gerhardinger Peter F. , Mccurdy Richard J. 
Published: April 2011

AbstractThe flat glass industry successfully commercialized pyrolytically deposited low emissivity glass based on color-suppressed fluorine-doped tin oxide in 1990. The current production capability of this product approaches 500,000 m2 of coated glass per year. Spin-off products for the photovoltaic (PV) industry were introduced by LOF in 1991 and enjoy considerable success in this emerging technology. Many PV researchers, however, still prefer to produce optimized tin oxide off-line in CVD furnaces as they can be produced on higher performing but higher cost substrates.
In this paper, we discuss the characteristics of on-line production of PV transparent conductors (TCO), including the tradeoffs required when a coating process is incorporated onto a float glass line. We conclude by suggesting that eventual large scale commercialization of thin film photovoltaics dictates the use of on-line TCO based on economics, the excellent uniformity and repeatability, and ready availability. We believe that the PV researchers are best served by understanding the capabilities of on-line tin oxide production, and tailoring their programs towards the characteristics of these new cost effective TCOs.
Electrical Characteristics Of The Transparent Conductive Oxide / p-a-Si:H Interface
Authors: Guse Stephan , Peros Dimitrios , Wagner Michael , Böhm Markus 
Published: April 2011

AbstractThe current-voltage (I-V) characteristics of the interfaces between boron-doped hydrogenated amorphous silicon (p-a-Si:H) and transparent conductive oxides (TCOs) such as tin oxide (SnO2) and indium tin oxide (ITO) are experimentally determined. The measurements are performed on especially developed Kelvin cross bridge structures which allow the direct sensing of the contact region. All contacts exhibit for high dopant concentrations an almost linear I-V characteristic. The values of the contact resistance depend on the TCO type and range between 0.5 and 4 Ω· cm2 at 25°C. Gradual reduction of the boron doping level leads to a remarkable increase of the contact resistance and finally to a rectifying behavior. The results indicate that tunnelling across the interface contributes significantly to the current transport. This conclusion is also supported by the behavior of the I-V characteristics at low temperatures. Regarding degradation of the contacts a systematic reduction of the contact resistance is observed after ageing them for 1000 hours at 80°C.