bluuebunny/crossref_metadata_2025_split huggingface dataset:
The Adsorption of Liquid and Vapor Water on TiO2 (110) Surfaces: The Role of Defects
Authors: Wang Li-Qiong , Baer Don R. , Engelhard Mark. H. 
Published: April 2011

AbstractThe adsorption of liquid and vapor water on defective and nearly defect-free TiO2 (110) surfaces has been examined using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS). The study has focused on examining electronic defects as created in vacuum and after exposure to both liquid and vapor water. The defective surfaces were prepared by electron-beam exposure and Ar+ bombardment. With exposure up to 104 L low vapor pressure (<10−5 Torr) water to defective surfaces, little or no changes on Ti 3d defect intensities were observed, in agreement with the previous studies. However, defect intensities were greatly reduced after exposing defective surfaces to ∼ 108 L higher vapor pressure (0.2 - 0.6 Torr) water for 5 min. More significantly, XPS and UPS spectra showed that electron-beam induced defects were completely removed upon liquid water exposure, while defects created by Ar+ bombardment were only partially removed. It was found that surface defects created by Ar+ bombardment were removed more readily than sub-surface defects. For a nearly defect-free surface, water coverages were ∼ 0.02 ML at 104 L exposure to low vapor pressure water, ∼ 0.07 ML at 108 L exposure to higher vapor pressure water, and ∼ 0.125 ML with liquid water exposure, respectively.
Magnetic Properties of Heisenberg Antiferromagnetic EuTe/PbTe Superlattices
Authors: Chen J.J. , Dresselhaus G. , Dresselhaus M.S. , Springholz G. , Bauer G. 
Published: April 2011

ABSTRACTBulk EuTe is a type II Heisenberg antiferromagnet (AF2) with a single magnetic phase transition temperature at 9.6 K. However, for several isolated EuTe (111) monolayers (MLs) as can be achieved in a superlattice (SL) structure, both ferrimagnetic-like and antiferromagnetic-like phase transitions can take place, depending on the SLs configuration. The temperature-dependent magnetization M(T) of such SLs has been studied near the transition temperature (Tc) by SQUID magnetometry. The functional forms of M(T) at T ≤ Tc can be described by mean-field theory for SLs with 3, 4 and 5 EuTe MLs per SL cell. The magnetic transition temperatures obtained by mean-field analysis, using bulk exchange coupling values, are in close agreement with observed Tc values for SLs with 2, 3, 4 and 5 EuTe MLs. The qualitative behavior of the surface specific heat can be deduced from M(T) data for SLs with three EuTe monolayers.
Effects of Landau Level Coupling on the Magnetic Band Structure for Electrons in a Two-Dimensional Hexagonal Lattice
Authors: Kühn O. , Fessatidis V. , Cui H.L. , Horing N.J.M. 
Published: April 2011

ABSTRACTThe single-particle energy spectrum of two-dimensional electrons in a lateral surface superlattice of hexagonal symmetry, subject to a normal uniform magnetic field is investigated. Special attention is focused on the coupling of different Landau levels due to the periodic potential. It is shown that the inclusion of this coupling causes strong modifications of the spectrum compared with the limit of no coupling investigated previously. It is found that the importance of Landau level coupling is mainly determined by the relation between the potential amplitude and the cyclotron energy, as well as the Landau level index.
Quantum Confinement in Coated Semiconductor Nano-Particles
Authors: Zhou H.S. , Sasahara H. , Honma I. , Komiyama H. , Sasabe H. , Haus J. W. 
Published: April 2011

ABSTRACTWe previously reported the synthesis and some properties of heterostructure nanoparticles, i.e. CdS coated with PbS (CdS/PbS). We report further characterization and investigation of the infrared photoluminescence (PL) of the CdS and coated CdS/PbS nanoparticles. This allows us to provide a more stringent test of the quantum confinement model. We consider the energy shift of the PL peak and the variation of the PL intensity correlated with the predictions of the quantum confinement model. The experimental results can be explained by this model of coated semiconductor nanoparticles.
Controlled Recrystallization of Hematite from Two Highly Different Phases of Ferric Trihydroxide
Authors: Denes Georges , Kabro P. , Madamba M.C. 
Published: April 2011

ABSTRACTHematite Fe2O3, is a semiconductor, and its electrical properties are highly sensitive to the preparation methods and purity. Its precursors can be hydrated ferric oxides; however, these are usually obtained from poorly defined ferric gels, which are obtained by hydrolysis of an aqueous solution of a ferric salt by a base. We have designed a novel synthetic route to ferric hydroxide, by reaction of a peroxo-compound with an aqueous solution of a ferrous salt, which involves simultaneous oxidation of Fe(ll) to Fe(lll), and hydrolysis, in the same reaction process. The two kinds of ferric hydroxide are highly different, however, both give hematite by dehydration/recrystallization, however, the way this occurs for each is different.
Er-Implanted Porous Silicon: a Novel Material for Si-Based Infrared LEDs
Authors: Namavar Fereydoon , Lu F. , Perry C.H. , Cremins A. , Kalkhoran N.M. , Daly J.T. , Soref R.A. 
Published: April 2011

ABSTRACTWe have demonstrated a strong, room-temperature, 1.54 μm emission from erbium-implanted at 190 keV into red-emitting porous silicon. Luminescence data showed that the intensity of infrared (IR) emission from Er implanted porous Si annealed at ≤ 650°C, was a few orders of magnitude stronger than Er implanted quartz produced under identical conditions, and was almost comparable to IR emission from In0.53Ga0.47As material which is used for commercial IR light-emitting diodes (LEDs).The strong IR emission (much higher than Er in quartz) and the weak temperature dependency of Er in porous Si, which is similar to Er3+ in wide-bandgap semiconductors, suggests that Er is not in SiO2 or Si with bulk properties but, may be confined in Si light-emitting nanostructures. Porous Si is a good substrate for rare earth elements because: 1) a high concentration of optically active Er3+ can be obtained by implanting at about 200 keV, 2) porous Si and bulk Si are transparent to 1.54 μm emission therefore, device fabrication is simplified, and 3) although the external quantum efficiency of visible light from porous Si is compromised because of self-absorption, it can be used to pump Er3+.
Light Controlled Photoluminescence Relaxation in Porous Silicon
Authors: Czaputa R. , Fritzl R. , Popitsch A. 
Published: April 2011

ABSTRACTWe report results of photoluminescence (PL), FTIR and ESR investigations on nanoporous silicon (PS) where a reversible PL intensity relaxation effect in the chemically oxidised material is observed. To be activated the effect needs, however, additional preparation steps including light irradiation and ageing in ambient atmosphere. After illumination with visible light, the PL intensity is remarkably diminished. However it recovers in the dark within the time scale of minutes to hours under ambient atmosphere at room temperature. This cycle can be repeated several times. We show that the variation of the PL intensity is anticorrelated to an ESR signal attributed to silicon dangling bonds. From the IR spectrum, however, no significant change of the pore surface chemical structure can be observed during a cycle. Therefore we conclude that the variation of the PL intensity is rather controlled by a metastable change in the number of dangling bond centers than by modification of the surface chemistry in the porous silicon system.
Investigation Of Efficiency Improvement on Silicon Solar Cells Due to Porous Layers
Authors: Sun Gregory , Li Yuxin , Lu Yicheng , Khan Babar , Tompa Gary S. 
Published: April 2011

ABSTRACTObservation of light emission from porous Si has demonstrated that the optical properties of Si can be drastically altered by the quantum size effects. We have investigated the improvement of absorption properties of Si material by forming a porous Si layer. Shallow-junction commercial crystalline as well as polycrystalline Si solar cells without anti-reflective coatings have been processed into porous Si solar cells by a wet chemical etching technique. Our best results have demonstrated more than 15% improvement in short-circuit current with no change in open-circuit voltage. The performance of the porous Si solar cells has been found to be sensitive to the porous layer thickness. The efficiency can be reduced when the porous layer is relatively deep, presumably due to the penetration of pores through the shallow junction. We believe porous Si can be optimized for photovoltaic applications by properly controlling its porosity and thickness.
On the Origin of the Electrically-Induced Spectral Shift of Porous Silicon Photo- and Electro- Luminescence
Authors: Bsiesy A. , Hory M.A. , Gaspard F. , Herino R. , Ligeon M. , Muller F. , Romestain R. , Vial J.C. 
Published: April 2011

ABSTRACTExperimental results showing two electrically-induced phenomena, namely the voltage-tunable electroluminescence (VTEL) and the voltage-induced quenching of porous silicon photoluminescence(QPL) are given. In both cases, a spectral shift as large as 300 nm can be recorded for an external bias variation of only 0.5V. This spectral shift is characterised by a blue-shift of the whole EL line in the case of the VTEL whereas it results from a progressive and selective quenching starting by the low-energy part of the luminescence line in the case of the QPL experiments. The origin of this spectral shift is discussed in relation with an electrically-induced selective carrier injection into the silicon nanocrystallites accompanied with an enhancement of the non-radiative recombination which might take place by an Auger relaxation process. Finally, it is shown that a partial oxidation of the porous silicon layer leads to a complete loss of the selectivity of these two phenomena. This result is qualitatively discussed by considering the voltage drop distribution between the substrate and the silicon nanocrystallites. The voltage drops are modified by the growth of the oxide layer on the nanocrystallite surface leading to a modification of the energy barriers at the crystallite boundaries.
Large Grain Size and High Deposition Rate for Microcrystalline Silicon Prepared by VHF-GD
Authors: Hapke P. , Finger F. , Luysberg M. , Carius R. , Wagner H. 
Published: April 2011

ABSTRACTThe growth mechanism and material properties of -type µc-Si:H prepared with plasma enhanced chemical vapour deposition in the very high frequency range is investigated. By increasing the plasma excitation frequency the grain size, deposition rate and Hall mobility can be simultaneously increased without having to adjust other deposition parameters in particular the temperature. This effect is explained by an enhanced selective etching of amorphous tissue and grain boundary regions together with a sufficient supply of growth species at high frequency plasmas.
Spin Resonance Studies on free Electrons and Defects in Microcrystalline Silicon
Authors: Malten C. , Finger F. , Hapke P. , Kulessa T. , Walker C. , Carius R. , Flückiger R. , Wagner H. 
Published: April 2011

ABSTRACTThe effect of micro-doping, defect creation, and non-steady state occupation through optical transitions on the electron spin resonance signals found in undoped and weakly doped microcrystalline silicon with a high degree of crystallinity is investigated. The experimental results are in agreement with the assignment of the resonance at g=1.9983 to conduction electrons in the crystalline grains and the resonanccs around g=2.0052 to dangling bonds in the remaining amorphous phase and at the grain boundaries. The simultaneous presence of both resonances can result from a large conduction band offset between crystalline grains and grain boundaries or the amorphous phase. The presence of conduction electron spin resonance in compensated and even p-type material points also to potential fluctuations. Free electrons in interconnected crystalline grains are in agreement with the weakly activated transport found in µc-Si:H at low temperatures.
Microcrystalline Cubic Boron Nitride/Amorphous Hydrogenated Boron Nitride Mixed Phase Thin Films
Authors: Lin Shu-Han , Feldman Bernard J. 
Published: April 2011

ABSTRACTTransparent and insulating thin films have been grown by the plasma decomposition of B2H6, NH3, and H2, at a substrate temperature of 250°C. From chemical composition, transmission electron microscopy, infrared absorption, and optical absorption measurements, the thin films are determined to be a mixed phase of crystalline cubic boron nitride and amorphous hydrogenated boron nitride. Also, the films have significantly more boron than nitrogen, a large concentration of hydrogen, a very large bandgap, strong infrared aborption due to both hexagonal boron nitride and boron icosahedra, and good adhesion to various substrates.
Electrical Characteristics of Ultra-Thin Multi-Layers of Poly-Si and Silicon Dioxide
Authors: Chan Kevin K. , Lee Young H. , Stanis Carol L. 
Published: April 2011

ABSTRACTRough-surface polycrystalline silicon films have been used for fabrication of electrically erasable-programmable read-only-memories (EEPROM). Silicon-Rich Oxides (SRO), also known as semi-insulating polycrystalline silicon (SIPOS), have been recommended for use as electron injectors. The advantage of SRO as injectors is that both the bottom and top surfaces of the film can be used to obtain high field enhancement. An ultra-thin multi-layer structure of silicon and silicon dioxide has been fabricated by low pressure chemical vapor deposition (LPCVD). High resolution TEM shows alternating layers of 50 A thick SiO2 and polycrystalline Si (grain size ∼ 80 A) films were deposited and MOS capacitors using this multilayer dielectric were studied to understand their electrical characteristics. Both I-V and C-V measurements show that the Fowler-Nordheim tunneling current is proportional to the number of polycrystalline layers. The “turn-on” voltage of the tunneling current is determined by the thickness of first SiO2 layer, and the effective dielectric constant varies from 3.9 to 9.1, proportional to the number of poly-Si layers within the multi-layer structure (MLS). For a 350 Å stack (7 layers), the interface trap density, Dit, was 3×1010 traps/cm2, and the effective dielectric constant was roughly 9.
Effect of the Hetero-Interface on the Photoresponse of A-Si/C-Si Solar Cells
Authors: Jagannathan B. , Yi J. , Wallace R. , Anderson W. A. 
Published: April 2011

ABSTRACTHeterojunction solar cells were fabricated by glow discharge deposition of amorphous silicon on p-type crystalline silicon resulting in a n/i/p structure. Dark I-V-T data on the devices show that the conduction in the forward bias regime (<0.4 volts) for better devices agrees with a multi-tunnelling-capture-emission process. The photoresponse was evaluated (under 100 mW/cm2) for various a-Si thicknesses and substrate resistivities. Spectral response tests showed an increased low wavelength absorption as the a-Si thickness was decreased. The blue response of the devices have better fill-factors than the red response indicating defects at the interface. Further, I-V-T and C-V measurements also corroborate the presence of defect states which seem to prevent the spread of the depletion region in crystalline silicon. The photoresponse was found to be very sensitive to the interface defects and the fill-factors ranged from 0.42, for the sample in which the depletion region had spread, to 0.1 in those where the depletion region had been reduced in thickness by the interface states.
Scalable Fabrication and Optical Characterization of Nm Si Structures
Authors: Zaidi Saleem H. , Chu An-Shyang , Brueck S. R. J. 
Published: April 2011

ABSTRACTObservations of efficient room temperature photoluminescence (PL) from porous Si have generated a great deal of interest in the optical properties of nm-scale Si structures. The stochastic character of porous-Si fabrication results in a distribution of crystal sizes and shapes. We report on a scalable (to large areas) and manufacturable (to high volumes) fabrication technology for uniform, nm-linewidth Si structures providing an important testbed for controlled studies of these optical properties. Large areas ( ∼ 1 cm2) of extreme sub-μm structures (to ∼ 5 nm) are re-producibly fabricated. Both walls (1-D confinement) and wires (2-D confinement) are reported. The fabrication process includes: interferometric lithography, highly anisotropic KOH etching, and structure dependent oxidation. For the walls, nearly perfect <111> crystal planes form the sidewalls and very high width/depth aspect ratios (> 50) have been achieved. Raman scattering results on the walls demonstrate three regimes: 1) lineshapes and cross sections similar to bulk Si for line widths, W > 200 nm; 2) electromagnetic resonance enhancement of the cross section ( to - 100x) for W from 50-200 nm; and 3) highly asymmetric lineshapes and splittings from W < 30 nm. Photoluminescence is observed for the thinnest samples (W < 10 nm) and is as intense as that observed from porous Si with a spectral linewidth ∼ 50 % smaller than that of porous Si.
Luminescence Properties of Periodic Disordered Thin Layer GaAs/AlAs Superlattices
Authors: Zhang Ruth Y. A. , Strozier J. , Horton C. , Ignatiev A. 
Published: April 2011

ABSTRACTDisordered thin layer GaAs/AlAs superlattices with various disorder periods have been grown using MBE. The disorder was introduced by varying the thickness of GaAs and AlAs layers in the growth direction in various specific but randomly generated finite disorder sequences. The photoluminescence spectra of these disordered superlattice samples showed a sharp peak at the high energy side and a broad peak at the low energy side. The temperature, excitation, and disorder sequence dependence of the photoluminescence spectra indicates that the sharp peak is due to the pseudo-direct exciton emission, and the broad peak is strongly related to the localized states induced by disorder. In addition, the results demonstrate that the luminescence intensity of disordered superlattices can be improved by up to two orders of magnitude over that of ordered short period superlattices. Finally, we propose a kinetic model for the state population and find that the photoluminescence spectra can be well described by this model.
Isolation and Characterization of Two Esr–Active La@C82 Isomers
Authors: Yamamoto Kazunori , Funasaka Hideyuki , Takahashi Takeshi , Akasaka Takeshi , Suzuki Toshiyasu , Maruyama Yusei 
Published: April 2011

ABSTRACTTwo abundant ESR-active lantanofullerene species have been isolated with an efficient HPLC method. Each of isolated species was well identified as a stable metallofullerene molecule. Both mass and ET 9 spectra confirmed that they were La@C82 with 1.159-G and 0.836-G 139La hyperfine splittings. Visible and near-IR absorption spectra, cyclic and differential pulse voltammograms, and 13 C hyperfine splitting structures in ESR spectra for the samples showed features quite different from each other. Their differences were discussed on the basis of their structures.
Properties of Novel Fullerene Tubule Structures: A Computational Study
Authors: Sinnott Susan B. , White Carter T. , Brenner Donald W. 
Published: April 2011

ABSTRACTTheoretical Young's moduli have been estimated for several fullerene carbon tubule fibers along the tubule axis. These results indicate that carbon fibers composed of nested tubules could have a modulus nearly 1.4 times that of a conventional graphite whisker. The results also indicate that the modulus of the tubule fiber can be further increased by decreasing the distance between the close-packed tubules in the fiber. This is true for both nested and single-shell tubules, but is only significant for the latter. Taking advantage of this property of single-shell tubules, we have examined a hypothetical tubule-diamond composite that, if it could be produced, would yield a modulus greater than diamond while at the same time stabilizing the fiber against shear distortions.
Atomic Force Microscopy of Carbon Nanotubes and Nanoparticles
Authors: Li Ping , Satftler Klaus 
Published: April 2011

ABSTRACTVery hot carbon vapor was deposited on highly-oriented pyrolytic graphite (HOPG) and on sapphire substrates. Using atomic force microscopy, the obtained carbon structures are analyzed. It is found that carbon nanotubes and cones had formed on the graphite substrate but not on sapphire. Instead, spherical-type nanoparticles are analyzed on sapphire. A particle with double-conical shape is found on the graphite substrate. The two observed cone angles 19° and 60° are explained by perfect network closure of sectors of curled graphene sheets.
Control of Heteroepitaxy in Sol-Gel Derived LiNbO3 Thin Layers
Authors: Clem P.G. , Payne D.A. 
Published: April 2011

ABSTRACTLithium niobáte (LiNDO3) in single crystal form is useful for the fabrication of acoustooptic and active waveguide devices. In this paper, the feasibility of integrating LiNbO3 thin layers on sapphire is reported. The performance of signal modulators, surface acoustic wave devices, and second harmonic generators relies on control of crystallographic orientation, so a thin-layer deposition method must meet high standards of crystallographic perfection and optical quality. Solution deposition of lithium niobium ethoxide was evaluated on (110) and (006) sapphire substrates for heteroepitaxy. Atomic force microscopy was used to determine the development of microstructure during the transition from the amorphous to crystalline state. Slab waveguides were formed and evaluated for optical quality and loss. Optical losses in the TEo mode of 500nm (110) LiNbO3 thin layers were determined to be 6 dB/cm. Preliminary results are given for the heteroepitaxial growth of α-Ga2O3 buffer layers.
Growth and Properties of PbTiO3/PLT Heterostructures
Authors: Kim Yeongkwan , Han Y.H. , Erbil A. , Boatner L.A. 
Published: April 2011

ABSTRACTFerroelectric superlattice structures composed of three-dimensionally epitaxial PbTiO3 and PLT thin films have been successfully grown on SrTiO3 substrates by metalorganic chemical vapor deposition. The modulation structures were confirmed by θ-2θ XRD, and the excellent in-plane orientational relationship between the superlattice film and the substrate by (100), (110), and (111)-pole figures. The φ-scans through the (110) and (111) reflections were used as additional evidence for three-dimensional epitaxy. The substrate dependence of the epitaxial orientation of PLT and PbTiO3 single-layered thin films was investigated. PbTiO3 thin films with very high crystalline perfection can be successfully grown on KTaO3 substrates.
A Low Temperature Vaporization of Pb(THD)2 with NH3
Authors: Lee June Key , Lee Wan In , Desu Seshu B. 
Published: April 2011

ABSTRACTPb(THD)2 is one of the most popular CVD precursors for lead. It was found in this study that the volatility of Pb(THD)2 can be considerably improved by using ammonia as a carrier gas. An adduct between Pb(THD)2 and NH3 is generated in-situ, when the NH3 carrier gas is passed through the bubbler containing Pb(THD)2. The ammoniated precursor was isolated and its molecular structure was characterized by elemental analysis, infrared and NM.R. PbO thin films could be prepared with this precursor system while maintaining the bubbler temperature at 110° C, which is considerably lower than the temperature used with the Pb(THD)2 precursor in the commonly used carrier gas.