bluuebunny/crossref_metadata_2025_split huggingface dataset:
Reciprocal Space Analysis of the Microstructure of Luminescent and Nonluminescent Porous Silicon Films
Authors: Lee S.R. , Barbour J.C. , Medernach J.W. , Stevenson J.O. , Custer J.S. 
Published: April 2011

ABSTRACTThe microstructure of anodically prepared porous silicon films was determined using a novel x-ray diffraction technique. This technique uses double-crystal diffractometry combined with position-sensitive x-ray detection to efficiently and quantitatively image the reciprocal space structure of crystalline materials. Reciprocal space analysis of newly prepared, as well as aged, p+porous silicon films showed that these films exhibit a very broad range of crystallinity. This material appears to range in structure from a strained, single-crystal, sponge-like material exhibiting long-range coherency to isolated, dilated nanocrystals embedded in an amorphous matrix. Reciprocal space analysis of n+and p+porous silicon showed these materials are strained single-crystals with a spatially-correlated array of vertical pores. The vertical pores in these crystals may be surrounded by nanoporous or nanocrystalline domains as small as a few nm in size which produce diffuse diffraction indicating their presence. The photoluminescence of these films was examined using 488 nm Ar laser excitation in order to search for possible correlations between photoluminescent intensity and crystalline microstructure.
Dimensions of Luminescent Porous Silicon by Thermal Effusion of Hydrogen
Authors: Nikolov A. , Petrova-koch V. , Polisski G. , Koch F. 
Published: April 2011

ABSTRACTUsing electrochemically prepared porous Si with hydride covered internal surfaces, we determine the H-content by thermal effusion and measurement of the total gas evolution. The amount of Si is obtained by weighing. In this way, we derive a value of the ratio of H- to Si-atoms and from the latter a value of the linear dimension of the luminescing particles
Photoluminescence Quenching of Porous Silicon
Authors: Fisher Diana L. , Gamboa Aurilee , Harper Jessica , Lauerhaas Jeffrey M. , Sailor Michael J. 
Published: April 2011

ABSTRACTPhotoluminescence from porous silicon can be quenched reversibly by a variety of molecular species. Quenching pathways for chemically incorporated surface species, physisorbed species (that undergo no net chemical transformation), and for electron donating reagents have all been identified. For systems involving charge transfer quenching, the concentration dependence typically follows a Stern-Volmer type of relationship, with the more easily oxidized molecules producing the largest Stern-Volmer slopes (most efficient quenching). The slope of the Stern-Volmer plot is dependent upon the particular wavelength that the Stern-Volmer data are obtained from. Data interpretation is complicated by the fact that porous Si shows a wavelength dependent emission lifetime, although when this is taken into account the data can be qualitatively described within the context of a driving-force dependent quenching model. A simple model for understanding the various photoluminescence quenching phenomena observed with porous Si is presented, involving an ensemble of emissive states with energy dependent lifetimes and Stern-Volmer quenching behavior. The model adequately accounts for both red and blue spectral shifts that have been observed upon photoluminescence quenching.
Carrier Dynamics in Porous Silicon: from the Femtosecond to the Second
Authors: Fauchet Philippe M. 
Published: April 2011

ABSTRACTThe luminescence in red-emitting porous silicon exhibits a distribution of lifetimes in the μsec time domain at room temperature and in the msec time domain at cryogenic temperatures. However, the luminescence and carrier dynamics in porous silicon display transients that vary from much less than 1 psec to ∼ 1 sec, depending on the measurement conditions and sample preparation. We have investigated the carrier dynamics in porous silicon by two time-resolved techniques. The blue photoluminescence of oxidized porous silicon has been measured with 100 ps time resolution as a function of the oxidation method, emission wavelength, excitation intensity and measurement temperature. The blue luminescence has a distinct origin from the well-studied red luminescence and we attribute it to defects in the oxide. Femtosecond photoinduced absorption measurements have been performed on thin red-emitting porous silicon films. The wavelength and intensity dependence of the recovery are interpreted in terms of trapping and of Auger recombination at high excitation intensity. Our results also show conclusively that red-emitting porous silicon is not a direct gap semiconductor.
Micron-Size and Submicron-Size Light-Emitting Porous Silicon Structures
Authors: Duttagupta S. P. , Fauchet P. M. , Peng C. , Kurinec S.K. , Hirschman K. , Blanton T. N. 
Published: April 2011

ABSTRACTWe have developed three classes of techniques to produce micron-size and submicron-size light emitting porous Si (LEPSi) patterns and to protect the rest of the wafer. In the 1st class, LEPSi lines down to 2 µm width have been made using a photoresist/silicon nitride trilayer mask, followed by anodization. PL mapping of the structures indicates that the protected regions have not been etched. Using electron beam lithography sub-0.5 micron porous Si lines have been generated. In the 2nd class, formation of porous Si is inhibited by amorphizing Si using ion implantation followed by anodization and annealing. The crystallinity and electrical properties of the implanted region have been fully characterized after annealing. Using focussed ion-beam implantation, LEPSi patterns of the order of 100 nm have been obtained. The 3rd class consists of enhancing the formation of porous Si by a low energy/low dose bombardment (ion-milling) with argon ions prior to anodization. Under appropriate conditions, we have observed a strong enhancement of the formation rate of LEPSi where bombardment took place, possibly due to the generation of a large number of defects on the wafer surface.
Light Emission Versus Excitation from Porous Structures in Ion-Implanted Silicon
Authors: Vázsonyi É. , Barsony I. , Lohner T. , Fried M. , Erostyák J. , Rácz M. , Pászti F. 
Published: April 2011

ABSTRACTThe quantum efficiency of electroluminescence (EL) in porous silicon structures (PSL) is practically limited by the mechanical instability of higher porosity layers. This can be overcome by adding a thin implanted p+ surface region before the formation of PSL. The resulting structure consisted of different morphologies depending on the local doping level. When the implantation doping level and diffusion depth was increased, reduced photoluminescence (PL) from the otherwise strongly emitting buried PSL of 80% porosity was obtained. Spectroscopic ellipsometry provided detailed analysis of optical transmission of PSL prepared on a boron implanted substrate. Since the spectral transmission of PSL is morphology dependent, in this novel structure the excitation wavelength can be absorbed in the upper mesoporous p+ region, which is much more transparent for the visible emission spectrum. The width and peak position as well as integral intensity of EL spectra during anodic oxidation of the novel, workable structure were not affected by the doping level in the thin surface region.
A Monte Carlo Simulation of the Stillinger-Weber Model for Si-Ge Alloys
Authors: Laradji Mohamed , Landau D. P. , Dünweg B. 
Published: April 2011

ABSTRACTThe bulk phase behavior of silicon-germanium alloys is investigated by means of a constant pressure Monte Carlo simulation of the Stillinger-Weber potential in the semi-grand-canonical ensemble. At low temperatures, Si and Ge phase separate into a Si-rich phase and a Ge-rich phase. The two-phase region is terminated by a critical point whose nature is investigated thoroughly by the multihistogram method combined with finite size scaling analysis. These results showed that the critical behavior of the alloy belongs to the mean field universality class, presumably due to the elastic degrees of freedom. We have also studied the structural properties of the mixture and found that the linear thermal expansions of both Si and Ge agree well with experiments. We also verified Végard's law above the critical point and calculated bond length distributions.
Carrier Transport in Porous Silicon Light-Emitting Diodes
Authors: Peng C. , Fauchet P.M. , Hirschman K.D. , Kurinec S.K. 
Published: April 2011

ABSTRACTThis work presents a systematic investigation of carrier injection and transport mechanisms in light emitting porous silicon (LEPSI). In addition to optical characterization of the LEPSI film, the electrical characteristics of LEPSI light emitting diodes (LEDs) and the device parameter effects on electroluminescence (EL) efficiency are explored including junction structure, LEPSI layer thickness, and chemical treatment. Au/LEPSI device structures exhibit a power-law current-voltage relationship (I∼Vm) and LEPSI pn junction device structures exhibit an exponential current-voltage relationship (I∼eeV/nkT). The carrier transport mechanisms are discussed, and theoretical simulations match well with the experimental data. An increase in EL efficiency has been achieved by adjusting the LEPSI layer thickness, implementing chemical treatment, and improving the quality of the contacts. The EL response to the frequency modulation of LEPSI LEDs was studied, resulting in a cutoff frequency in excess of 100 kHz. LEPSI LEDs have demonstrated stable EL at applied voltages as low as 5 volts with EL efficiencies of 0.01%.
Electroluminescence of Yb-Doped InP
Authors: Alshawa A. K. , Lozykowski H. J. , Brown I. 
Published: April 2011

ABSTRACTDetailed experimental and theoretical studies of the electroluminescence excitation mechanism of Yb3+ in InP are presented. The electroluminescence spectra and the kinetics of Yb implanted InP were investigated under pulsed and dc excitations at temperatures over the range 9 - 70 K. The electroluminescence spectrum of the 4f transitions 2F5/2 - 2F7/2,, at 9 K, consists of a peak at 1001 nm and broader peaks in the spectral range between 1003 nm and 1010 nm. The intensity and current versus voltage characteristics have been recorded. The emission intensity increased linearly with current and started to saturate at high currents. The plot of the natural logarithm of intensity versus V−1/2 shows a straight line characteristic over three orders of emission intensity, showing that the direct impact excitation mechanism is a dominant process. At low temperature, the rise and decay times of the 1001 nm emission line were 9.63 µs and 10.50 µs, respectively. The thermal quenching energy, EA, was found to be 100 meV for both the electroluminescence intensity and the decay time.
Preparation and Optical Properties of Ultrathin Silicon Films
Authors: Collins R. W. , Nguyen Hien V. , An Ilsin , Lu Yiwei , Wakagi M. 
Published: April 2011

ABSTRACTUltrathin crystalline silicon (c-Si) and amorphous silicon (a-Si:H) films have been prepared using plasma-enhanced chemical vapor deposition (PECVD) and atomic hydrogen etching methods. The complex dielectric functions (2 < hv < 4.5 eV) of films consisting of isolated clusters have been measured in situ and in real time using spectroscopic ellipsometry. For 12 Å c-Si cluster films, a sharp absorption onset is observed near 3 eV that blue-shifts with decreasing thickness, in consistency with the quantum confinement of electrons. A much broader absorption onset, observed for ∼13 Å a-Si:H cluster films near 2 eV, is attributed to an electron mean free path that is less than the cluster size, which limits the appearance of confinement effects.
Observation of Quantum Size Dependent Blue Shift in the Luminescence of Recrystallized Si/SiNx Superlattices
Authors: Grützmacher D.A. , Steigmeier E.F. , Auderset H. , Morf R. , Delley B. , Wessicken R. 
Published: April 2011

ABSTRACTLight emitting Si nanostructures were fabricated by recrystallizing Si/SiNx superlattices grown by plasma enhanced chemical vapor deposition. After recrystallisation by a laser annealing or by rapid thermal annealing the Raman spectra show clearly a confinement effect. A substantial photoluminescence around 470 to 550 nm and at 620 nm is observed after a hydrogen passivation step. The photoluminescence at 620 nm is assigned to carrier recombination via efficient surface states, whereas the blue emission at 470 to 550 nm shows a behaviour expected for quantum confinement effects.
Carrier Transport in Polycrystalline and Amorphous Silicon Thin Film Transistors
Authors: Sameshima T. , Sekiya M. , Hara M. , Sano N. , Kohno A. 
Published: April 2011

ABSTRACTThe technologies of laser crystallization and methods of SiO2 formation in remote plasma chemical vapor deposition or SiO evaporation with an oxygen ambient realize the fabrication of n-channel polycrystalline and amorphous silicon thin film transistors (poly-Si and a-Si TFTs) at a temperature lower than 300 °C. The defect density was achieved to be 2∼3×1011 cm−2eV−1 and threshold voltage was about IV for both TFTs. The maximum field effect mobility was 600 cm2/Vs for poly-Si TFTs and 2.6 cm2/Vs for a-Si TFTs. The mobility of poly-Si TFT decreased as the gate voltage increases. This is interpreted as that the electrons are confined in the narrow inversion layer and electron scattering with phonon is enhanced for higher normal electric field.
Formation of Highly-Uniform and Densely-Packed Arrays of GaAs Dots by Selective Epitaxy
Authors: Tsai Charles S. , Lee Robert B. , Vahala Kerry J. 
Published: April 2011

ABSTRACTFormation of highly-uniform and densely-packed arrays of GaAs dots by selective epitaxy using diethylgallium-chloride and arsine is reported. The arrays of GaAs dots are imaged using atomic force microscopy (AFM). Accounting for the AFM tip radius of curvature, the smallest GaAs dots formed are 15-20 nm in base diameter and 8-10 nm in height with slow-growth crystal planes limiting individual dot growth. Completely selective GaAs growth within dielectric-mask openings at these small size-scales is also demonstrated. The uniformity of the dots within each array ranged from 6% for the larger dots to 16% for the smallest dots (normalized standard deviations of the areas of individual dots within each array).
Determination of the Strain Status of GaAs/AlAs Quantum Wires and Quantum Dots
Authors: Darhuber A.A. , Bauer G. , Wang P.D. , Song Y.P. , Torres C.M. Sotomayor , Holland M.C. 
Published: April 2011

ABSTRACTWe have investigated periodic arrays of 150 and 175 nm wide GaAs-AlAs quantum wires and quantum dots, fabricated by electron beam lithography and SiCI4/O2 reactive ion etching, by means of reciprocal space mapping using triple axis x-ray diffractometry (TAD). The reciprocal space maps reveal that after the fabrication process the lattice constant along the growth direction slightly increases for the wires and even more so for the dots.
Photoluminescence of a Single-Crystal Silicon Quantum Well
Authors: Saeta Peter N. , Gallagher Alan C. 
Published: April 2011

ABSTRACTSingle crystal-silicon quantum well layers with SiO2 barriers were grown from silicon-on-insulator substrates. Photoluminescence in the red and near-infrared was observed for average layer thickness < 8 nm, with peak signal for 2-nm thickness. The luminescence spectrum was essentially independent of well width for SiO2 barriers, but the photoluminescence intensity decreased sharply after annealing in Ar. These results suggest the importance of radiation from surface states. In contrast to oxide-passivated silicon nanocrystals and to porous silicon, the room-temperature photoluminescence quantum efficiency is low (10-4-10-5), probably due to variations in layer thickness and to diffusion of photoexcited carriers to fast nonradiative recombination centers.
Molecular Form-Factor and Analysis of Diffraction Pattern of Fullerene Crystals
Authors: Shulakov E. V. , Dilanyan R. A. , Rybchenko O. G. , Shekhtman V. Sh. 
Published: April 2011

ABSTRACTC60 molecules, in diffraction terms, is a quite novel object- covered by scattering points large sphere (D≅0.5acub). The peculiarities of the diffraction distribution and its variation near the order-disorder phase transition are defined from the buckybal's molecular form-factor, fmol. The comparison of calculated diffraction distributions and experimental data proved advantage of the C60 “jump rotation” model at middle and high temperatures.
A Molecular Dynamics Investigation of C60 –Rare Gas Mixtures
Authors: Abramo Maria C. , Caccamo C. 
Published: April 2011

ABSTRACTMixtures of krypton atoms and C60, modelled in terms of rigid spherical molecules, are investigated at T ≃ 1800K through classical Molecular Dynamics. The two species do not tend to mix for low to moderate concentrations of krypton, and solid clusters of C60 tend to be stable. Possible implications of these results are shortly discussed.
Molecular Dynamics Studies of Nanotube Growth in a Carbon ARC
Authors: Brabec C. J. , Maiti A. , Roland C. , Bernholc J. 
Published: April 2011

ABSTRACTIt has been shown experimentally that the growth of carbon nanotubes in an arc discharge is open-ended. This is surprising, because dangling bonds at the end of open tubes make the closed tube geometry more favorable energetically. Recently, it has been proposed that the large electric fields present at the tip of tube is the critical factor that keeps the tube open. We have studied the effects of the electric field on the growth of the nanotubes via ab initio molecular dynamics simulations. Surprisingly, it is found that the electric field cannot play a significant role in keeping the tubes open, implying that some other mechanism must be important. Extensive studies of the energetics and simulations of the growth of tubes were performed using a threebody Tersoff-Brenner potential. Our results show that there exists a critical diameter of ∼ 3 nm above which a defect-free growth of a straight tubule is possible. Narrower tubes stabilize configurations with adjacent pentagons that lead to tube-closure and termination of the growth. This explains the absence of tube narrower than 2.2 nm in arc discharge experiments.
Superconductivity at 40K in Cesium Doped C60
Authors: Palstra T.T.M. , Zhou O. , Iwasa Y. , Sulewski P.E. , Fleming R.M. , Zegarski B.R. 
Published: April 2011

ABSTRACTWe report superconductivity in Cs3 C60 at 40K using ac susceptibility measurements under hydrostatic conditions up to 15 kbar. Cs3 C60 was prepared by reaction of C60 with Cs in liquid ammonia, followed by heating at 150°C. This route circumvents formation of the energetically more stable CSl C60 and Cs4 C60 phases. We have studied the synthesis and phase formation by measuring the symmetric Ag pentagonal pinch mode of C60 using Raman spectroscopy. Whereas Raman spectroscopy indicates homogeneous charge transfer of three electrons, x-ray diffraction indicates two phases other than the commonly observed fcc structure. This superconducting transition temperature is considerably higher than for known doped C60 compounds and intermetallic compounds.
Formation of Nanoparticles in A Carbon Arc
Authors: Majetich S. A. , Scott J. H. , Brunsman E. M. , Mchenry M. E. 
Published: April 2011

ABSTRACTDetails of the carbon-coated nanoparticle growth mechanism are revealed by a comparison of product morphology and reactor parameters. Carbon-coated metal or metal carbide clusters nucleate in the gas phase, grow to characteristic sizes, and deposit on surfaces within the reactor. The surface temperature determines the crystallinity of the nanoparticles and the surrounding carbon. Tungsten carbide nanoparticles show that the carbon coating arises due to phase segregation when the nanoparticle has a lower melting point than that of graphite.
Rb1C60: Linear Polymer Chains and Dimers
Authors: Martin Michael C. , Koller Daniel , Rosenberg A. , Kendziora C. , Mihaly L. 
Published: April 2011

ABSTRACTThe infrared- and Raman-active vibrational modes of C60 were measured in the various structural states of Rb1C60. According to earlier studies, Rb1 C60 has an f cc structure at temperatures above ∼ 100°C, a linear chain polymer orthorhombic structure when slowly cooled, and an as yet undetermined structure when very rapidly cooled (“quenched”). We show that the spectra obtained in the polymer state are consistent with each C60 molecule having bonds to two diametrically opposite neighbors. In the quenched state, we find evidence for further symmetry breaking, implying a lower symmetry structure than the polymer state. The spectroscopic data of the quenched phase are shown to be consistent with Rb2(C60)2, a dimerization of C60.
Geometric and Electronic Structure of Fullerene Film Growth as a Function of Coverage
Authors: Reihl B. 
Published: April 2011

ABSTRACTWe have employed scanning tunneling microscopy at room and low temperature, i.e. 300, 50, and 5 K, to study the epitaxy and growth of fullerene films on the noble-metal surfaces Ag(110) and Au(110). Initial island growth occurs on terrace sites away from substrate step edges. Particularly at low temperatures where the rotational and vibrational movements of the fullerene molecules are frozen in, different intra-molecular topographic patterns become visible in ordered films, which are characteristic of particular adsorption sites. Complementary tunneling spectroscopy and direct and inverse photoemission measurements reveal distinct differences between the first adsorbed monolayer and additional fullerene layers indicating differences in bonding and charge transfer. Our results are compared to theoretical calculations.
Structural Characterization of Fullerene thin Films Fabricated by Organic Molecular Beam Deposition
Authors: Yase Kiyoshi , Saraya Takuya , Kudo Kazuhiro 
Published: April 2011

ABSTRACTFullerene C60 thin solid films have been fabricated on the (001) surfaces of alkali halides (NaCl, KCI and KBr) and muscovite by organic molecular beam deposition (OMBD) technique. Pure C60 powder was sublimated from the precisely temperature-controlled K-cell at low pressure (10−7 Torr) and deposited onto the substrates kept in the range 125 –300 °C. Increasing both the temperature of K-cell and the substrate temperature, fine crystals grow with different size, shape and distance between neighbors. The dependence of the size and density of island crystals on the deposition conditions revealed the mechanism of crystal growth and the effect of interaction between molecules and substrate surface.
The Effects of Mechanical Alloying of C60 with Metals (II): Al and Mg
Authors: Zhang Xin-Yu , Hidalgo Rafael Q. , Murphy Richard J. , Markiewicz Robert S. , Giessen Bill C. 
Published: April 2011

ABSTRACTC60 has been subjected to multiple micro-alloying passes by co-rolling together with Al and Mg. Complete amorphization has been found in samples of C60 with Al. However, the structural changes in the C60/Mg combination were more complex. Here, a chemical reaction occurred during which the C60 molecules were apparently broken up into amorphous carbon-like materials, while a Mg-containing oxidation reaction product formed.
Structure and Conductivity of Iodine-Doped C60 Thin Films
Authors: Chen Jun , Zhang Haiyan , Chen Baoqiong , Peng Shaoqi , Ke Ning , Wong S.P. 
Published: April 2011

ABSTRACTWe report here the results of our study on the properties of iodine-doped C60 thin films by IR and optical absorption, X-ray diffraction, and electrical conductivity measurements. The results show that there is no apparent structural change in the iodine-doped samples at room temperature in comparison with that of the undoped films. However, in the electrical conductivity measurements, an increase of more that one order of magnitude in the room temperature conductivity has been observed in the iodine-doped samples. In addition, while the conductivity of the undoped films shows thermally activated temperature dependence, the conductivity of the iodine-doped films was found to be constant over a fairly wide temperature range (from 20°C to 70°C) exhibiting a metallic feature.