bluuebunny/crossref_metadata_2025 huggingface dataset:
AlxGa1-xas Growth by OMVPE Using Trimethylamine Alane
Authors: Hobson W. S. , Abernathy C. R. , Pearton S. J. , Harris T. D.
Published: March 2011

We have investigated the growth of AlxGal-xAs (0.1 ≤ x ≤ 1) by organometallic vapor phase epitaxy using trimethylamine alane (TMAA1) as the aluminum precursor. A low pressure (30 Torr) reactor was used with hydrogen as the carrier gas. At the high gas velocities (> 1 m · s-1) employed there was no visible deposition upstream of the substrate. AIGaAs epilayers with featureless surface morphology could be obtained over the entire range of composition. The layers exhibited very strong room-temperature photoluminescence and excellent compositional uniformity (x = 0.235 ± 0.002 over a 40 mm diameter). A comparison was made between the electrical and optical characteristics of AlGaAs grown with either trimethylgallium (TMGa) or triethylgallium (TEGa). The hole concentration of the layers grown using TMGa was significantly higher than that with TEGa (e.g., 6–7 × 1017cm-3vs. 1 × 1016cm-3) for the same TMAA1 and AsH3mole fractions. High-purity AlGaAs was achieved with TMAAl and TEGa at higher AsH3flow rates.
Deposition of GaN by Remote Plasma-Enhanced Chemical-Vapor Deposition (Remote PECVD)
Authors: Choi S.W. , Bachmann K.J. , Lucovsky G.
Published: March 2011

Films of GaN have been grown on silicon by remote plasma-enhanced chemical-vapor deposition using trimethyl gallium and ammonia. The ammonia is rf plasma excited along with He, and the trimethyl gallium is introduced downstream from the plasma generation zone. The activation energy for the growth of GaN is 0.95±0.05 eV. This is tentatively assigned to the activation of NH groups, extracted from the plasma as primary precursors for the surface reaction with trimethyl gallium (TMG), or adsorbed fragments of this molecule. At high He flow rates, an abrupt increase in the growth rate is observed corresponding to a change in the reaction mechanism which is attributed to the formation of atomic nitrogen.
Calculation of Gallium-Arsenic-Metal Phase Diagrams
Authors: Scofield James D. , Davison J. E. , Smith S. R.
Published: March 2011

Contact metallizations for space-based GaAs solar cells must survive at high temperatures for several minutes. Which metallizations will survive can be predicted by properly calculated phase diagrams instead of performing hundreds of reactions. A method for calculating the ternary temperature constitution phase diagrams Is briefly explained and the phase diagrams are calculated for two Ga-As-X alloy systems. The free energy functions of the liquid and solid phases are approximated by the regular solution theory. The phase diagrams of the three binary alloy systems which form the boundaries of the ternary alloy system are utilized to calculate the binary regular solution parameters. The free energy functions for the ternary liquid and solid phases are expressed In terms of the binary regular solution parameters. The temperature and composition of the liquidus and solidus boundaries for the ternary alloy systems are calculated from these free energy functions. Calculated results are presented for the Ga-As-Ag and the Ga-As-Ge systems.
Deposition of Heavily-Doped μc-Silicon Thin Films by Remote Plasma-Enhanced Chemical-Vapor Deposition Process (remote PECVD)
Authors: Wang C. , Bjorkman C.H. , Lee D.R. , Williams M.J. , Lucovsky G.
Published: March 2011

We have succeeded in depositing both activated n- and p-type μc-Si, by a low temperature, 250°C, remote PECVD process in which dopant gases (PH3 or B2H6)/Silane (SiH4) mixtures are injected downstream from the exit port of a He/H2 plasma. The room temperature conductivities and activation energies for the n- and p-type μc-Si are respectively, 40 S/cm with Eaa=0.018 eV, and 5 S/cm with Ea =0.040 eV. Doped μc-Si is obtained for PH3/SiH4 ratios up to 1%, and for B2H6/SiH4 ratios to 0.1%. For B2H6/SiH4 ratios < 0.1%, the deposited p-type material is doped a-Si rather than doped μc-Si. We have shown that these heavily doped μc-Si film are a viable candidate for the gate electrode in MOS devices. The application of these doped μc-Si films in p-i-n diode devices has also been studied.
Effect of Plasma Modulation in the Glow Discharge Deposition of Amorphous Silicon Films
Authors: Bruno G. , Capezzuto P. , Cicala G. , Manodoro P.
Published: March 2011

The plasma deposition of hydrogenated and fluorinated amorphous silicon (a-Si:H,F) and silicon-germanium alloys (a-Si,Ge:H,F) from SiF4-H2 and SiF 4-GeH 4-H2 mixtures, respectively, has been studied in continuous (CW) and modulated wave (MW) r.f. discharges. It has been found that the period and duty cycle of the modulated wave strongly affect the plasma composition, the surface homogeneity and the material properties. The plasma-phase characterization, performed by time resolved optical emission spectroscopy (TR-OES), supplies arguments on the origin of emitting species and on their formation kinetics. It has been found that H* and SiFx are formed by a direct electron impact process involving the same species in the ground state. In addition, the surface homogeneity and some material properties are strongly improved by plasma modulation.
Properties of in Situ Doped Amorphous and Polycrystalline Silicon Deposited by Chemical Vapor Deposition from Tertiarybutylarsine and Tertiarybutylphosph INE
Authors: Kotccki David E. , Blouse Jeffrey L. , Parks Christopher C. , Sarkozy Robcrt F.
Published: March 2011

Tcrtiarybutylarsine (TBA) and tertiarybutylphosphine (TBP) were evaluated as n-type dopant precursors for low pressure chemical vapor deposition (LPCVD) of in situ doped amorphous and polycrystalline silicon from silanc. The deposition kinetics, film composition, and electrical properties obtained with TBA and TBP dopant precursors are compared to that obtained when using arsine and phosphine as dopant precursors. Compared to the hydride dopant sources, TBA and TBP provide increased safety due to their sub-atmospheric vapor pressure (≃ 200 Torr at 26°C) and lower toxicity. Using mixtures of TBA and TBP with silane and hydrogen, high quality films of arsenic and phosphorus in situ doped amorphous and polycrystalline silicon were obtained. Dopant incorporation levels ranged from 5×1019 to l.5×1021 cm−3 as measured by SIMS. The deposition rate of polycrystallinc films deposited from TBA and TBP was enhanced by a factor of-≃-3X compared to the hydride dopants for the same mole fraction of dopant precursor. The minimum resistivitics of 5.0 and 1.4 mΩ-cm were obtained, following a 1000°C, 30 second rapid thermal anneal, for ≃500Å thick films on SiO2 deposited with TBA and TBP respectively. A drawback of TBA and TBP is the incorporation of ≃1.0 atomic percent carbon which is > 15X higher than obtained when using the hydride dopant sources. Carbon was observed under all deposition conditions examined.
Adsorption and Desorption Kinetics for Chlorosilanes on Si(111) 7×7
Authors: Gupta P. , Coon P.A. , Koehler B.G. , Wise M.L. , George S.M.
Published: March 2011

The adsorption and desorption kinetics for SiCl4 and SiCl2H2 on Si(111) 7×7 were studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. Both LITD and TPD experiments monitored SiCl2 as the main desorption product at 950 K at all coverages of SiCl4 and SiCl2H2 on Si(111) 7×7.HC1 desorption at 850 K and H2 desorption at 810 K were also observed following SiCl2H2 adsorption. Isothermal LITD measurements of SiCl4 and SiCl2H2) adsorption on Si(111) 7×7 revealed that the initial reactive sticking coefficient decreased with increasing surface temperature for both molecules. The temperature-dependent sticking coefficients were consistent with precursor-mediated adsorption kinetics. Isothermal LITD studies of SiC12 desorption revealed second-order SiCl2 desorption kinetics. The desorption kinetics were characterizedby a desorption activation energy of Ed = 67 kcal/mol and a preexponential of vd = 3.2 cm2/s. TPD studies observed that the HCI desorption yield decreased relative to H2 and SiCl2 desorption as a function of surface coverage following SiCl2H2 exposure. These results indicate that when more hydrogen desorbs as H2 at higher coverages, The remaining chlorine is forced to desorb as SiCl 2.
An Investigation of in Situ Chlorination of Tungsten
Authors: Thomas N. , Blanquet E. , Vahlas C. , Bernard C. , Madar R.
Published: March 2011

A thermodynamic and experimental study of in-situ chlorination of tungsten is presented. When gaseous tungsten chlorides are used as metal precursors for tungsten and tungsten silicide CVD, their direct synthesis provide a reproductible volatile chloride source.

The conditions leading to the best production of volatile tungsten chlorides have been optimized by means of thermodynamic calculations. The influence of the different production parameters (temperature, nature of reactant gas, …) have been determined.

During some chlorination experiments, volatile chlorides have been detected in-situ by a quadrupole mass spectrometer.

Good agreement between theoretical and experimental results is obtained.
Photoreactions in Polyalkylsilynes Induced by ArF-Laser Irradiation
Authors: Kunz R. R. , Bianconi P. A. , Horn M. W. , Smith D. A. , Freed C. A.
Published: March 2011

Photoreactions in polyalkylsilyne thin films induced by ArFlaser (193 nm) irradiation have been examined. Photoexcitation of the σ-conjugated Si-network at 193 nm (6.42 eV) results in Si-Si bond scission and alkyl-group desorption when irradiated in a vacuum. In addition to these processes, efficient (up to 7% quantum efficiency) insertion of oxygen into the Si backbone occurs when the irradiation is performed in air, resulting in the formation of a siloxane. Both infrared and X-ray photoelectron spectroscopies indicate a higher oxygen coordination about the Si atoms in the oxidized product than observed for linear polysilanes. This higher oxygen coordination indicates a siloxane network. The polysilynes have been demonstrated as deep UV photoresists and may have additional applications as precursors for thin film or binary optical components.
Increased Volatility of Barium Metal Organics by the Use of Nitrogen Lewis Bases
Authors: Buriak Jillian M. , Cheatham Linda K. , Graham John J. , Gordon Roy G. , Barron Andrew R.
Published: March 2011

Substituted β-diketonate complexes of barium have limited volatility even at reduced pressures. The addition of nitrogen Lewis bases to the CVD carrier gas allows barium β-diketonates, even those with no reported volatility, to be transported in the vapor phase at temperatures as low as 70°C (atmospheric pressure) with no decomposition. No increase in volatility is observed, however, for barium carboxylate complexes.
Chemical Phase Separation in Binary Iron-Chromium Alloys
Authors: Hawick K.A. , Epperson J.E. , Windsor C.G. , Rainey V.S.
Published: March 2011

A study of chromium-enriched domain growth occurring in binary Fe-Cr alloys quenched from above to various temperatures within the miscibility gap has been made. We present kinetic sequences of in-situ small angle neutron scattering (SANS) data for ageing times up to 75 hours on alloys containing 20, 30 and 40 atomic percent chromium. The SANS measurements are compared with partial structure functions obtained from computer simulations performed on a distributed array processor (DAP). We use a pair-potential lattice model, but simulate large systems containing up to 16 million lattice sites. We find good agreement between the scaled structure factors for our SANS data and computer simulated system.
Time-Resolved X-ray Scattering Investigation of Ordering Kinetics in Fe3Ai
Authors: Park B. , Stephenson G. B. , Ludwig K. F. , Allen S. M.
Published: March 2011

We have investigated the kinetics of the B2/DO3 transition in Fe3AI (28 at. %) using in situ time-resolved x-ray scattering. In these experiments, the evolution of the diffuse and/or Bragg intensity near the (½ ½ ½) DO3 superlattice peak is observed after the temperature of the sample is abruptly changed. H-ere we present results for the kinetics of re-equilibration of short-range order within the disordered (B2) phase, and of short- and long-range order within the ordered (DO3) phase. The short-range order is characterized by the diffuse peak intensity IDIFFUSE and correlation length ξ;the long-range order is characterized by the Bragg intensity IBRAGG. For quenches within the disordered phase, IDIFFUSE and ξ both relax exponentially at the same rate. The temperature dependence of the relaxation time shows evidence of a divergence at the critical temperature. For shallow quenches within the ordered phase, IBRAGG, IDIFFUSE and ξ all relax exponentially, but with different rates. However, for deep quenches within the ordered phase, IDIFFUSE and ξ do not show simple exponential relaxation. Instead, coarsening of short-range order into long-range order is seen, as in quenches from the disordered phase into the ordered phase. Investigation of up-quenches and down-quenches to the same temperature within the ordered phase indicates that disordering is faster than ordering.
Kinetics of Sro and Lro in FeAl Alloys with A2 and DO3 Structures
Authors: Cadeville M.C. , Pierron-Bohnes V. , Vennegues P. , Afyouni M. , Kim T.H.
Published: March 2011

The kinetics of short and long-range ordering in FeAlx alloys have been investigated using resistometry. The results were obtained in the A2 solid solution (x = 0.10 and 0.17), in the DO3 ordered phase (x = 0.23, 0.27, 0.30 and 0.33) and in the A2 + DO3 two-phase region (x = 0.19). The ordering kinetics curves are well described in a classical model with a lognormal distribution of relaxation times whose center varies with the temperature according to an Arrhenius law. One thus gets the thermodynamic quantities which drive the atomic mobility. The data display a strong decrease of the activation energy with Al content and with the formation of the D03 structure, as well as very short relaxation times. Such an abnormally high atomic mobility can be attributed to a decrease of the migration energy through a lattice dynamics effect, similar to that recently proposed by several authors to explain the enhanced diffusion in some bcc transition metals and alloys.
Structural Defects in Amorphous Silicon Probed by Sub-Picosecond Photocarrier Dynamics
Authors: Stolk P.A. , Roorda S. , Calcagnile L. , Sinke W.C. , Van Linden Van Den Heuvell H.B. , Saris F.W.
Published: March 2011

The dynamics of a photogenerated electron-hole plasma in pure amorphous silicon (a-Si) in different stages of structural relaxation have been studied with sub-picosecond resolution using pump-probe reflectivity measurements. For high plasma densities (> 1020/cm3) the plasma evolution is dominated by Auger recombination. At lower plasma densities (≈ 1018/cm3) the plasma decays exponentially with a time constant τ, suggesting that carrier trapping dominates in this regime. The decay time τ increases with the temperature at which the a-Si has been annealed, ranging from τ = 1 ps for as-implanted a-Si to τ=14 ps for a-Si annealed at 500 °C. This observation is consistent with a reduction in the number of defects in a-Si upon thermal annealing.
Interface Structure, Grain Morphology, and Kinetics of Growth of the Superconducting Intermetallic Compound Nb3Sn Doped with ZrO2and Copper
Authors: Hall Ernest L. , Benz Mark G. , Rumaner Lee E. , Jones Karen D.
Published: March 2011

One method for the fabrication of the superconducting compound Nb3Sn involves interdiffusion of a surface coating of Sn alloyed with Cu on Nb containing Zr and O. In this study, the kinetics and microstructure associated with this reaction have been studied in detail. The results show that small Nb3Sn grains nucleate at the Nb3Sn/Nb interface, and that the Nb3Sn grains experience grain growth immediately after they are formed. ZrO2precipitates are observed in the Nb3Sn at the Nb3Sn/Nb interface and throughout the Nb3Sn. The ZrO2precipitates occur in the form of small partially-coherent spheres in the Nb3Sn. No ZrO2precipitates are observed by TEM in the unreacted Nb. The grain boundaries in the Nb3Sn region are coated with a Sn-Nb-Cu alloy which would have been liquid at the diffusion/reaction temperature. The thickness of the Nb3Sn reaction layer formed during the isothermal diffusion anneal is proportional to time to the first power, indicating “reaction”-controlled rather than “diffusion”-controlled kinetics. The absence of diffusion-controlled kinetics can be explained by the presence of the liquid coating on the Nb3Sn grains. Diffusion of Sn in this liquid layer is apparently fast enough to not be the limiting kinetic step.
Formation of Palladium Silicide and Germanide in the Pd/a-Si/Si., Pd/a-Ge/Si, and Pd/a-GeSi/Si Systems During Thermal Treatment
Authors: Edelman F. , Cytermann C. , Brener R. , Eizenberg M. , Weil R. , Beyer W.
Published: March 2011

X-ray diffraction and transmission electron microscopy have been used to study the kinetics of phase transformations and the structure of Pd/a-Si, Pd/a-Ge and Pd/a-GeSi thin films deposited on Si substrates. Different kinds of amorphouis structures were used: a-Si:H:D, a-Si.:F, a-Ge:H:D, and a-GeSi:H:D. The first stage of phase transformation during heat treatment was palladium silicide (Pd2Si) and palladium germanide (Pd2Ge) formation at temperatures above 200°C. Annealing studies demonstrated that the presence of F in a-Si promotes the Pd2Si formation. The study of the Pd2Si crystallization process showed that: a) when the Pd layer and the a-Si layer are thin, then c-PdSi grows in a fractal-]ike form; b) when the Pd and a-Si both are thick, then c-Pd2Si grows in a globular structure; c) in both above mentioned cases a well-oriented [0011 texture forms. The growth of the silicide and germanide layers in the temperature range of 200-300°C was found to be controlled by a diffusion limited process. It was found that c-Pd2Ge transforms to c-PdGe above 200°C. The a-Ge,.,Si,. 5 alloy behaved similarly to a-Si forming only [001] textured c-Pd2(Ge,Si).
Measurements of to Temperatures of Supersaturated Si-As Alloys
Authors: Lee Kwang-Ryeol , West Jeffrey A. , Smith Patrick M. , Aziz M. J. , Knapp J. A.
Published: March 2011

The congruent melting point, or To curve, of crystalline Si-As alloys has been measured in the range of 1.6 to 18.1 at. % arsenic by line source electron beam annealing. Alloys were created by ion implantation of As into 0.1mm Si-on-sapphire and crystallized by pulsed laser melting. To temperatures decrease from 1673±10K at 2.0 at.% As to 1516±30K at 18.1 at.% As. The results of these measurements are significantly higher than the previous results of studies using pulsed laser melting techniques. Advantages of the e-beam technique over previous techniques are discussed. Chemical free energy functions of the solid and liquid phases were calculated from existing thermodynamic data. The calculated To curve agrees with the measured values only in low concentration region (less than 8 at.%).
Phase Transformation Kinetics During Precipitation of an Ordered Intermetallic from a Disordered Matrix -a Computer Simulation
Authors: Chen Long-Qing , Khachaturyan A.G.
Published: March 2011

The precipitation kinetics of an ordered intermetallic from a disordered matrix, which involves simultaneous ordering and decomposition, is studied by a computer simulation technique based on the microscopic diffusion theory. It is found that the precipitation starts from a congruent ordering transition, which may be continuous or nucleation and growth. This congruent ordering transition transforms the initially disordered state into a single phase nonstoichiometric ordered state with antiphase domains. The next stage is the decomposition which starts from the antiphase domain boundaries and then propagates into the ordered domains. And the final process is the coarsening of the order/disorder two-phase mixture. The predicted kinetics of precipitation is in excellent agreement with recent experimental observations in important alloy systems.
A Survey of Segregation at Interphase Boundaries in Ternary Ni-Ag-X Alloys
Authors: Rao G. , Wynblatt P.
Published: March 2011

This paper presents an alternative derivation of a discrete lattice regular solution model for computing the composition profile across an interphase boundary in a three component system. The model is applied to the calculation of the composition profiles across interfaces separating Ni-rich and Ag-rich phases in several Ni-Ag-X ternary systems (where X stands for Cu, Pd or Au). The results show that segregation to a (001) Ni-Ag interface is expected to be strong in the case of Cu, moderate in the case of Pd, and weak in the case of Au. It is also found that the Gibbsian interfacial excess of these segregants decreases with increasing temperature.
Molecular Dynamics Studies of Tweed and ω -Phase Instabilities in B2 NI62.5AL37.5 Alloys
Authors: Becquart C. , Clapp P.C. , Rifkin J.A.
Published: March 2011

Diffuse TEM and x-ray data has been reported by several research groups that has been interpreted in terms of both tweed and (ω -phase instabilities. Computer simulations of 1,024 atom arrays were run at various temperatures using the Voter-Chen Embedded Atom Method (EAM) interactions. If the extra Ni atoms were placed at random on the Al sub-lattice of the B2 structure, at low temperatures both <110>* and <112>* diffuse streaking were observed in reciprocal lattice plots, indicative of tweed and ω - phase instabilities, respectively. The atomic displacements causing the <110> diffuse streaking could generally be classified as {110}<1-10> local shears, in agreement with accepted models of tweed. However, the displacements producing the <112>* streaking were best described as coordinated atom row motions in <111> directions, which does not agree with traditional ω -phase models based on (111) interplanar collapse. Both types of diffuse streaking decreased with increasing temperature, but the <110> type was stronger at all temperatures and persisted to higher temperatures. Furthermore, the atomic displacements producing the streaking were static at low temperatures ( persistence times greater than 1000 vibrational periods) but became dynamic at higher temperatures ( persistence times of the order of a few vibrational periods). Another interesting result was that by merely rearranging the extra Ni atoms into an ordered superlattice pattern, virtually no diffuse streaking of any type was observed at any temperature, strongly suggesting that compositional disorder ( or other kinds of point defects) are essential to induce tweed instabilities.

*The asterisk represents a quantity in reciprocal space.
Nonstationary Nucleation in the Supersaturated Vapor.
Authors: Bulatov Alexi
Published: March 2011

The problem of relaxation to the quasistationary mode of homogenious nucleation in the supersaturated vapor is being studied analytically. A simple analytical function of the nucleation rate behaviour is obtained, which is in a good agreement with the results of the numerical analysis.
Dissolution Dynamics of Sub-Critical Clusters in Liquid Silicon
Authors: Uttormark Michael J. , Cook Stephen J. , Thompson Michael O. , Clancy Paulette
Published: March 2011

Previous attempts to simulate by Molecular Dynamics the spontaneous nucleation and growth of a crystalline Stillinger-Weber ‘silicon’ from the liquid have been essentially impossible because of constraints on system size and time scales. We have overcome these limitations by studying the related problem of the disintegration of crystalline ‘embryos’ into the liquid phase at temperatures slightly above the melting point. Molecular Dynamics simulations using the Stillinger-Weber potential were performed by embedding crystallites of 400 atoms in a liquid consisting of approximately 3600 atoms. During each simulation, the time-evolution of the size and shape of the embryo was followed until it became indistinguishable from the liquid. These simulations provide intriguing new information on the atomic processes involved in dissolution and on the macroscopic kinetics of small clusters. Comparisons of results at different temperatures, system sizes and initial configurations are shown and the implications of these cluster dynamics for crystal growth in supercooled liquids, homogeneous nucleation, and transient nucleation are discussed.
Evidence for the Direct Ejection of Clusters from Non-Metallic Solids During Laser Vaporization
Authors: Bloomfield L. A. , Yang Y. A. , Xia P. , Junkin A. L.
Published: March 2011

We have studied the formation of molecular scale particles or “clusters” of alkali halides and semiconductors during laser vaporization of solids. By measuring the abundances of cluster ions produced in several different source configurations, we have determined that clusters are ejected directly from the source sample and do not need to grow from atomic or molecular vapor. Using samples of mixed alkali halide powders, we have found that unalloyed clusters are easily produced in a source that prevents growth from occurring after the clusters leave the sample surface. However, melting the sample or encouraging growth after vaporization lead to the production of alloyed cluster species.

The sizes of the ejected clusters are initially random, but the population spectrum quickly becomes structured as hot, unstable-sized clusters decay into smaller particles. In carbon, large clusters with odd numbers of atoms decay almost immediately. The hot even clusters also decay, but much more slowly. The longest lived clusters are the magic C50 and C60 fullerenes. The mass spectrum of large carbon clusters evolves in time from structureless, to only the even clusters, to primarily C50 and C60. If cluster growth is encouraged, the odd clusters reappear and the population spectrum again becomes relatively structureless.

The laser vaporization process in non-metallic systems closely resembles multiphoton laser photodissociation of clusters themselves. As a result, the best cluster source samples are powders or powder-like materials. We have found that carbon powder or exfoliated graphite sheet (Union Carbide Grafoil) are the best samples for the production of large carbon clusters. Many previous studies of carbon clusters formed in vacuum may have relied on the vaporization of carbon dust produced by repeated vaporizations in a small region of the sample.
Synthesis and Optical Properties of Cadmium- and Lead-Chalcogenide Semiconductor Clusters (Diameter < 5 Nm) In Liquid Solutions and Silicate Glasses
Authors: Micic O. I. , Rajh T. , Comor M. I. , Zec S. , Nedeljkovic J. , Patel R. C.
Published: March 2011

Small-particle clusters of CdS, CdSe, CdTe, PbS, and PbSe have been synthesized in aqueous solution. Absorption spectra with very sharp maxima were obtained for extremely small clusters (diameters < 30 Å). However, when clusters grow the excitonic absorption bands gradually decrease and the sharp maxima are replaced by broad shoulders. Cadmium chalcogenide clusters capped with 3-mercapto-l, 2-propanediol can be collected as stable solids with the same properties as original clusters. A method for the incorporation of semiconductor clusters in transparent silicate glasses has been developed. This technique relies on the low temperature sol-gel process, and silane alkoxide as the starting material. The absorption spectra of the clusters in the glasses are identical to those of the starting aqueous solutions. Excess charge carriers on PbS and CdS clusters lead to a blue shift in the optical absorption edge of semiconductors. The appearance of this shift depends critically on the method of colloid preparation and surface chemistry.
Structure and Dynamics of Aluminum Clusters
Authors: Bernholc J. , Yi Jae-Yel , Oh Dirk J. , Sullivan D. J.
Published: March 2011

The energetics and structures of metallic clusters containing up to 55 atoms are studied by the Car-Parrinello (quantum-molecular dynamics) method with Al chosen as a paradigm. It is found that the structural energy differences between small cluster (icosahedral) and bulk (fee) structures are surprisingly small, indicating that the icosahedron-fee structural transition can occur very early in Al clusters. Simulated annealing studies for 13- and 55-atom clusters, where both perfectly symmetric icosahedral and cuboctahedral (fee) structures exist, show that the distortions from the ideal structures are substantial. For the 55-atom cluster several inequivalent but energetically nearly degenerate structures are found, which is likely to lead to floppiness at finite temperatures as well as a low melting point. Although the structure factors for the annealed structures are nearly identical, they are very different from those of either of the ideal structures. The computed IPs and EAs differ by less than 0.1 eV for weakly annealed cuboctahedral and icosahedral clusters, making the structural identification on the basis of IP and EA measurements very difficult. The quantum-mechanical results are also used to develop a classical potential for long-time molecular dynamics simulations of large clusters.
Interplay Between Local Environment Effect and Electronic Structure Properties in Close Packed Structures
Authors: Turchi P. E. A.
Published: March 2011

Within a tight binding framework, the interplay between local atomic arrangement, orbital directionality and phase stability properties is discussed for a series of tetrahedrally close packed (tcp) structures. The common features which characterize the local densities of states of tcp phases can be explained by a moment analysis up to fourth order terms. On the contrary, it is shown that structural energy differences between simple and tcp crystalline structures requires the knowledge of at least the fifth order moments, a fact which has been underestimated in the past. Finally, the relative stability of various representative geometries, including the ones of simple crystalline structures is examined as a function of the number of valence electrons.
Observation of Melting in 30 ÅDiameter CdS Nanocrystals
Authors: Goldstein A. N. , Colvin V. L. , Alivisatos A. P.
Published: March 2011

Temperature dependent electron diffraction studies on 30 Å diameter CdS nanocrystals are described. The linear thermal expansion coefficient of the nanocrystals is 2.75 * 10−5 Å/K,and the melting point is 575 K. These data are in contrast to bulk CdS which has a melting point of 1750 K and a linear expansion coefficient of 5.5 * 10−6Å/K. The observed depression in the melting point of these semiconductor clusters is similar to effects observed in metals and molecular crystals, indicating that the phenomenon of reduced melting point in small systems is a general one regardless of the type of material. The observation of melting point depression in these clusters also has far reaching implications for the preparation of highly crystalline clusters of CdS, as well as for the use of these nanocrystals as precursors to thin films.
Development of an Inert Gas Condensation Based Ion Source for the Generation of Strong Beams of Large Cluster Ions
Authors: Sarkas H. W. , Kidder L. H. , Eaton J. G. , McHugh K. M. , Bowen K. H.
Published: March 2011

We report on the further development of an ion source for producing intense, continuous beams of large positive and negative cluster ions comprised of high temperature materials. This device, the Smoke-Ion Source, is the result of combining inert gas condensation methods with techniques for injecting electrons directly into expanding jets. We demonstrate the capability of this ion source to generate strong beams of cluster ions comprised of materials including metals, semiconductors, and metal oxide ceramics.
Simulation of Gas Phase Clustering of Nanocrystals in Sputter Discharges
Authors: Choi Seung J. , Kushner Mark J.
Published: March 2011

The preparation of nanocrystalline particles or clusters (sizes 1–50 nm) is of interest to the study of small systems and for use in sintering or compacting of high purity bulk materials. Recently, a method whereby these crystals can be fabricated using a sputter discharge has been reported. We have developed a computer model to simulate the formation of homogeneous (e.g., Si, Cu, Ti) gas phase clusters in these devices as precursors to larger nanocrystals. The model combines Monte Carlo and drift-diffusion algorithms to simulate the sputtering of atoms from the target, their thermalization in the buffer gas, and gas phase nucleation reactions. Densities of clusters having one to many hundreds of atoms are obtained as a function of position in the discharge. We find that the experimentally observed particle sizes cannot be explained by clustering involving solely neutral reactants due to their short residence times in the plasma. Negatively charged clusters which are trapped in the plasma have correspondingly longer residence times and most likely are responsible for the growth of large particles. Scaling laws for the growth of homogeneous clusters will be presented based on the results of the model.
A New Type of Cluster-Ion Source for Thin Film Deposition
Authors: Haberland Hellmut , Karrais Martin , Mall Martin
Published: March 2011

Atoms are gas discharge sputtered from a solid target. They are condensed to form clusters using the gas aggregation technique. An intense beam of clusters of all solid materials can be obtained. Up to 80 % of the clusters can be ionised without using additional electron impact ionisation. Total deposition rates vary between 1 and 1000 Å per second depending on cluster diameter, which can be varied between 3 and 500 nm. Thin films of Al, Cu, and Mo have been produced so far. For non accelerated beams a weakly adhering mostly coulored deposit is obtained. Accelerating the cluster ions this changes to a strongly adhering film, having a shiny metallic appearance, and a very sharp and plane surface as seen in an electron microscope. The advantages compared to Kyoto ICB-method are: easy control of the cluster size, no electron impact ionisation, high degree of ionisation, and sputtering is used instead of thermal evaporation, which allows the use of high melting point materials.
Controlled Formation of Quantum Size Metal and Semiconductor Particles from Aqueous Solutions
Authors: Patel Ramesh C. , Nedeljkovic Jovan , Micic Olga
Published: March 2011

Photochemical and chemical reduction methods are described for the controlled formation of metallic silver in aqueous solutions. The former approach is capable of generally depositing a number of metals. When spherical silver bromide particles are the substrates, the resulting silver coated composite particles exhibit optical absorption spectra which vary with the coat thickness as theoretically predicted. In the case of spherical silica particles of uniform size, it was possible to produce both quantum size silver particles supported on silica, as well as a silver coat of variable thickness, depending on the rate of the deposition process. In addition to silica, substrates such as latex and chromium hydroxide could be used.

CdS particles with two different particle diameters (50–200 Å and < 30 Å) were subjected to 308 nm excimer laser irradiation at 77 K, and the subsequent charge carrier processes studied by ESR. Dramatic differences in the ESR signals as a function of decreasing particle size could be observed, consistent with the localization of charge carriers on numerous surface sites.
The Physical Properties of Ge Clusters and Ultrathin Films
Authors: Yu R. Q. , Fortner J. , Chase S. J. , Lannin J. S.
Published: March 2011

Raman scattering and ultraviolet photoelectron spectroscopy on Ge clusters and ultrathin films before and after H chemisorption reveal the significant changes in the vibrational and electronic densities of states with size. These effects are attributed to the large fraction of surface dangling bonds and the important role of short range order changes on physical properties.
Solid C60 – How we Found It
Authors: Huffman Donald R. , Krätschmer Wolfgang
Published: March 2011

In May of 1990, crystals of a new form of carbon were observed crystallizing from a solution of the newly-produced macroscopic quantities of fullerene molecules. This discovery followed many years of work we had done in producing and measuring the optical properties of small particles of graphitic carbon, a work which was aimed, in part, at a better understanding of the various spectral features of interstellar material. Among three distinctly different types of carbon particulates, distinguished by their differing ultraviolet and Raman spectra, was one that later proved to contain macroscopic quantities of C60 and C70. Evidence for fullerenes came from mass spectra, infrared spectra, and X-ray and electron diffraction. More, recent experiments with our collaborators have included scanning tunneling microscopy and gas phase emission spectroscopy in the infrared. At the moment, there is no obvious connection with interstellar spectral features.
Photoelectron Spectroscopy of Si2
Authors: Kitsopoulos Theofanis N. , Neumark Daniel M.
Published: March 2011

In this report we discuss the photoelectron spectrum of Si2− obtained at 355 nm. The spectrum shows a series of transitions to the low-lying triplet states of Si2 as well as transitions to the higher-lying singlet states. Several possible assignments of the anion and neutral triplet states are presented.
Modelling of The Intramolecular Structure Of Monolayer C60 Molecules Observed with Scanning Tunneling Microscopy
Authors: Chen T. , Howells S. , Gallagher M. , Yi L. , Sarid D. , Lichtenberger D. L. , Nebesny K. W. , Ray C. D.
Published: March 2011

Images of monolayer coverages of C60 on gold substrates, using scanning tunneling microscopy, reveal intermolecular arrangement and intramolecular features. In this paper we concentrate on the interpretation of these internal features, present a model showing that they arise from local charge density variations of high-lying π orbitals on approximately every other carbon atom, and discuss also other possible mechanisms for this effect.
Vibrational Properties of Si Microclusters and Comparison to the Amorphous Material
Authors: Feldman Joseph L. , Kaxiras Efthimios
Published: March 2011

The lattice vibrational properties of two clusters of Si atoms are studied using the Stillinger-Weber phenomenological interatomic potential to calculate the dynamical matrix. The clusters contain 33 and 45 atoms, and their structure was postulated from considerations of surface reconstructions in analogy to crystalline Si surfaces. A remarkable similarity to the vibrational density of states of amorphous Si is demonstrated especially for the 45 atom cluster.
Optical (Hyper)Polarizabilities of Small Silicon Clusters
Authors: Rantala Tapio T. , Stockman Mark I. , Jelski Daniel A. , George Thomas F.
Published: March 2011

Electronic contributions to the optical (hyper)polarizabilities of small silicon clusters are theoretically determined. Geometries and the electronic structures of the clusters are established using the tight-binding model. The nonlinear polarizabilities are found to depend primarily on the symmetry of the cluster and prove to be high for the low-symmetry clusters. Possible experiments and applications are discussed.
Proposal of Metastable Spin-Polarized He as a Probe of Antiferromagnetic Transition Metal Oxide Surfaces
Authors: Swan Anna , Franzen W. , El-Batanouny M. , Martini K. M.
Published: March 2011

We suggest a new application for elastic scattering of a metastable spin-polarized atomic helium beam at thermal energy. We demonstrate how angle-resolved measurements of the Bragg peaks of scattered surviving metastable atoms can give information about the spin-ordering of an antiferromagnetic (AF) transition metal oxide surface. In this paper, we discuss the feasibility of such measurements for NiO(100) and MnO(100), based on available information about their electronic structure and the properties of spin-polarized metastable helium. On impact with a surface, the survival probability of metastables is generally very low (<10-2). There are two possible decay mechanisms for metastables, a resonance ionization followed by auger neutralization, or an auger de-excitation process. For AF surfaces which fulfill certain requirements on their electronic structure, spin-selection rules will inhibit the decay of the metastable atoms from a favourably aligned magnetic sublattice. The survival probability will then be dramatically enhanced from the chosen sublattice, and the coherently scattered surviving metastables will reflect the periodicity of that magnetic sublattice. In contrast to other methods currently applied to magnetic systems, this method does not rely on difference spectra. Consequently, reversal of spinorientation is not necessary for the observation of magnetic ordering.
Evidence of Quantum Motion of Hydrogen on Pd(111) in He-Diffraction data
Authors: Hsu C-H. , Larson B. E. , El-Batanouny M. , Willis C. R. , Martini K. M.
Published: March 2011

We present elastic He-beam scattering data of the Pd(111)/H system. Diffraction intensities were measured as a function of surface temperature in the range 140°K–320°K. Two remarkable features are observed : the first is the presence of C3v symmetry at (1 × 1) saturation coverage (140°K) and its transformation to C6v symmetry at lower coverages (270°K). The second feature is the anomalous attenuation of the specular He beam accompanying this transformation. Taken together these features provide strong evidence of a fundamental change in the surface charge density corrugation. A classical interpretation of the motion of hydrogen either fails to reproduce the measured attenuation or leads to contradictory and unphysical conclusions regarding the H-metal bond length or surface equilibrium. An alternative quantum mechanical interpretation is developed and is shown to provide consistent and satisfactory explanation of the measurements.
Use of Atom Beama Scattering to Study Structures of and Phase Transitions in Ultra-Thin Films
Authors: Vidali G. , LI W. , Lin J.-S. , Moses C.
Published: March 2011

We have used neutral helium beam scattering to probe structural and thermodynamic properties of in-situ deposited ultra-thin films of metals on well characterized substrates. We show the capabilities of this technique by presenting some recent results of our studies of Hg adsorption of Cu(001) and on monolayer phases of Pb on Cu(001).
The Electronic Structure of Grain Boundaries in NB
Authors: Sowa Erik C. , Gonis A. , Zhang X. -G.
Published: March 2011

We present first-principles calculations of the electronic structure of Nb grain boundaries. These are the first such calculations for a bcc metal using the real-space multiple-scattering theory (RSMST). Local densities of states near a Σ5 twist grain boundary are compared to those for bulk Nb.
Defect Generation and Motion in Polyethylene-Like Crystals, Analyzed by Simulation With Supercomputers
Authors: Wunderlich Bernhard , Xenopoulos A. , Noid D. W. , Sumpter B. G.
Published: March 2011

Defects in polymers were observed by high resolution electron microscopy and inferred from their mechanical and dielectrical behavior. The details of their generation was not known, however, in the past. During the last few years we have been able to extend the molecular dynamics simulation of polyethylene to crystals containing up to 6100 atoms and to times as long as 100 ps. The major observation was that single bond rotations of more than 90° become possible already more than 100 K below the melting temperature. These defects have lifetimes of only a few ps. By coupling to kinks (2g1) they can extend their lifetime considerably. Addition of a thermal, mechanical or dielectric free energy gradient to the thermally created defects seems to be able to account for the microscopic motion needed to explain the macroscopically observed annealing, deformations and relaxation effects. Key to the mechanical and dielectric properties is thus the existence of conformational disorder (condis crystal).
The Surface Green's Function in Semiconductors by the Tight-Binding Linear Muffin-Tin Orbital Method
Authors: Šob M. , Kudrnovský J.
Published: March 2011

A computationally efficient method for the determination of the Green's function (GF) of an ideal semi-infinite semiconductor crystal within the first-principles TB-LMTO approach is developed. Our sample is described by a semi-infinite stack of principal layers with only nearestneighbor interaction between them. The projection of the GF of the ideal semi-infinite crystal onto the top principal layer is the surface GF (SGF), which is evaluated from the condition of removal invariance: by adding (removing) a principal layer of bulk atoms to (from) the semi-infinite crystal we recover the same semi-infinite crystal. This approach avoids the knowledge of the bulk GF and the surface-normal k-integration common in other treatments. Our method is illustrated on the evaluation of k11-resolved densities of states, both on the surface and deep in the sample, for the (110) and (001) faces of typical elemental and AIIIBV semiconductors.
Contributions of Conformational and Structural Defects to the Hyperpolarizabilities of Polymers.
Authors: Risser Steven M. , Ferris Kim F.
Published: March 2011

The extent of the delocalized π electron network is of prime importance in determining hyperpolarizabilities of conjugated molecules. Thus, for conjugated polymers, disruptions to this continuous order such as structural and/or conformational defects may have a large influence on the hyperpolarizabilities. We have performed semiempirical electronic structure calculations for a series of model compounds such as polyenes and polyphosphazenes to determine the effects of structural and conformational defects on the hyperpolarizabilities. Briefly, we find that structural and conjugational defects enhance the hyperpolarizabilities of polyenes, but have little influence on polyphosphazenes due to their limited π delocalization.
Simulation of the Crystal-To-Amorphous Transformation in Irradiated Quartz
Authors: Jain Uma , Powell Adam C. , Hobbs Linn W.
Published: March 2011

Quartz and other crystalline polymorphs of silica transform from the crystalline to an aperiodic state under irradiation. There is a need to understand the structural changes involved during this amorphization process. We have built an engineering modelwhich simulates the growth of amorphous regions within a crystalline matrix during the crystal-to-amorphous transition in irradiated quartz. The resulting crystal structure is displayed on the computer screen or plotted on a printer with the orthogonal coordinates of all the atoms in the cluster and the interatomic distances stored in a file. We find the bond lengths increase by about 3%, which is a reasonable value to expect since quartz expands 14% by volume during the amorphization. The results also show the crystal structure surrounding the strained region to be somewhat disturbed, consistent with what is observed experimentally.
Vibrational Frequencies for Model Silicates: Extensions Beyond Molecular Properties
Authors: Ferris Kim F. , Risser Steven M.
Published: March 2011

The investigation of surface properties for ceramic materials often focuses on molecular properties, both in terms of model systems and methods. Typically, we use molecular species (i.e. SiO4H4 , H3SiOH) to represent silica surfaces, often resulting in poor prediction of absorption phenomena. Dielectric effects, even when approximated by electrostatic and dipolar interactions, can have significant effects on the charge distribution and surface absorption characteristics of model molecular complexes. In this paper, we report on the effect of the surrounding matrix on the harmonic vibrational frequencies by employing reaction field techniques in electronic structure calculations. Comparisons of molecular-based to reaction field affected properties will be made to illustrate the extensions of the molecular to the extended network domain.