bluuebunny/crossref_metadata_2025 huggingface dataset:
The Effect of Hydrogen-Based, High Density Plasma Etching on the Electronic Properties of Gallium Nitride
Authors: Eddy C.R. , Molnar B.
Published: April 2011

Development of devices based on the wide gap semiconductor gallium nitride (GaN) requires the realization of reliable, high fidelity, low damage pattern transfer processes. In this work, GaN thin films grown by OMVPE have been subjected to both chlorine- and methane/hydrogen-based etch chemistries in an electron cyclotron resonance microwave plasma reactive ion etching system. Both n-type and semi-insulating thin films have been utilized to examine the effect of these etch processes on the electronic properties of the materials. The methane/hydrogen-based etch system (CH4/H2/Ar) induced considerable changes in the electrical properties of both n-type and semi-insulating films, causing the former to become more insulating and the latter to become conducting. In both cases, the original electrical properties were recoverable after a short, high temperature anneal. In the chlorine-based etching system (Cl2), no changes in the electrical properties were observed and etch rates five times greater than in the methane/hydrogen-based system were achieved. Proposed mechanisms responsible for the observed behavior will be discussed. These results show that pattern transfer processes based in chlorine etch chemistries are more suitable for the generation of high performance GaN devices.
Schottky Barrier Heights of Ni, Pt, Pd, and Au on n-type GaN
Authors: Schmitz A.C. , Ping A.T. , Asif khan M. , Adesida I.
Published: April 2011

Schottky barrier heights of Ni, Pt, Pd, and Au on n-type GaN were measured using current-voltage and capacitance-voltage techniques. Measurements from the I-V technique yielded barrier heights of 0.95, 1.01, 0.94, and 0.87 eV for Ni, Pt, Pd, and Au, respectively. Barrier heights of 1.13,1.16, 1.07, and 0.98 eV, for Ni, Pt, Pd, and Au, respectively, were obtained using C-V measurements.
Recent Progress in AlGaN/GaN Based Optoelectronic Devices
Authors: Khan M. A. , Chen Q. , Sun C. J. , Yang J. W. , Shur M. S.
Published: April 2011

We review our recent results on GaN based optoelectronic devices, which include InGaN-AlGaN Light Emitting Diodes (LEDs), GaN photoconductive, Schottky barrier, and p-n junction ultraviolet detectors, and optoelectronic AlGaN-GaN Heterostructure Field Effect Transistors. GaN-based optoelectronic devices cover a wide spectral range and demonstrate visible blind operation. A high quality of the epitaxial layers, the recent development of high performance GaN-based heterostructure field effect transistors, and transparent substrates make this material system uniquely suited for optoelectronic integrated circuits operating in visible and ultraviolet range.
Laser-Induced Voltages at Room Temperature in YBa2Cu3O7 and PrxY1-xBa2Cu3O7 Thin Films
Authors: Habermeier H.- U. , Jisrawi N. , Jäger-Waldau G. , Sticher U. , Leibold B.
Published: April 2011

Recent reports on high transient transverse voltages at room temperature in YBa2Cu3O7 and PrxYl-xBa2Cu3O7 thin films grown on SrTiO3 single crystal substrates, with a tilt angle between the [ 001 ] cubic axis and the substrate surface plane, have been interpreted by thermoelectric fields transverse to a laser-induced temperature gradient which are caused by the non-zero off diagonal elements of the Seebeck tensor. We have studied this effect in epitaxially grown Pr- doped, as well as undoped YBa2Cu3O7, thin films and observed for a 2 mm long YBa2Cu3O7 strip exposed to a UV photon fluence of 100 mJ/cm2 signals as large as 30 V. The unexpected high values for the signals and their doping dependence are discussed within the frame of a model based on a thermopile arrangement, the growth induced defect structure and the doping induced modifications of the material properties.
Giant Magnetoresistance Phenomenon in Laser Ablated La0.6y0.07ca0.33mnox Thin Films
Authors: Kumar Dhananjay , Kalyanaraman R. , Narayan J. , Christen David K.
Published: April 2011

Microstructural and magnetoresistance properties of La0.6Y0.07Ca0.33MnOx (Y-doped LCMO) thin films grown in-situ by pulsed laser ablation have been studied. Transmission election microscopy and x-ray diffraction measurements have shown that the Y-doped LCMO thin films grow epitaxially on (100) LaAl03 substrates and are cubic with a lattice parameter of 3.849 ?. The as-deposited films exhibited a metal-insulator transition at 130 K and a giant magnetoresistance (GMR) at 125 K with a MR ratio (dR/RH) of 1500% in the presence of a magnetic field of 6 Tesla. Such a colossal value of MR ratio for as-deposited Y-doped LCMO films is quite promising keeping in view the fact that these films were unannealed and not optimized. We ascribe this magnetoresistance to spin-dependent electron scattering coupled with the presence of intervening O2. ions across Mn3+ and Mn4+ with suppressed separation between Mn-O layers caused by smaller sized Y-dopant. The effect of annealing on the positive-shift of metal-insulator transition temperature and the improvement in GMR ratio has also been discussed. We also report a non-ohmic response in the Y-doped LCMO films which is observed only in the region of the resistance peak and lends support to a conduction mechanism in these materials based on spin-dependent scattering of electrons.
Laser Induced Avalanche Ionization and Electron-Lattice Heating of Silicon with Intense Near IR Femtosecond Pulses
Authors: Pronko P.P. , VanRompay P.A. , Singh R.K. , Qian F. , Du D. , Liu X.
Published: April 2011

A two temperature finite difference model has been developed and is used to describe the response of materials under ultrafast femtosecond laser pulses in the energy regime where melting and vaporization can occur. In applying this model to silicon it is observed that, for 800 nm light, laser pulse intensities that are just sufficient to achieve threshold for vaporization are also at the level of optical electric field strength where electron avalanche breakdown at the surface of the material can occur. For sub-picosecond pulses the physical response of the material is associated with a strongly temperature dependent coupling coefficient connecting electron and phonon thermal distributions. The results of these analyses demonstrate that a very thin near solid density plasma, caused by avalanche ionization, is responsible for the surface heating and subsequent thermodynamic response of the material. This interpretation is consistent throughout the pulse duration range from 80 femtoseconds to 0.2 nanoseconds. The proposed mechanism for absorption, at the near infra-red wavelength being used here, is very different from the types of mechanisms usually considered for nanosecond laser heating of semiconductors. Surface damage threshold is determined by atomic force microscopy and the threshold for plasma optical emission by photomultiplier detection . Melt deths are probed with SIMS impurity diffusion profiles and high resolution cross sectional TEM.
Test of Kinetic Models for Interface Velocity, Temperature, and Solute Trapping in Rapid Solidification
Authors: Kittl J.A. , Aziz M.J. , Brunco D.P. , Thompson M.O.
Published: April 2011

During rapid solidification, deviations from local interfacial equilibrium are manifested by solute trapping and interfacial undercooling. Both the solute trapping function and the interface velocity-temperature function have been measured in the Si:As alloy system following pulsed laser melting, permitting us to test models for nonequilibrium interface kinetics. The results are consistent with the Continuous Growth Model “without solute drag” of Aziz and Kaplan and are inconsistent with models that incorporate solute drag effects during solidification.
Kinetic Analysis of Nonisothermal Crystallization
Authors: Kelton K. F.
Published: April 2011

A realistic computer model for polymorphic crystallization under isothermal and nonisothermal conditions, which takes proper account of time-dependent nucleation behavior and cluster-size-dependent growth, is presented. A new correction to the standard Johnson-Mehl-Avrami-Kolmogorov (JMAK) statistical analysis that takes account of finite sample size is incorporated to simulate data taken from fine particles and nano-structured materials. Model predictions compare well with experimental data obtained from calorimetric studies of the polymorphic crystallization of lithium disilicate glass. The computer model is employed to evaluate commonly used methods of analysis for calorimetric data and to suggest new approaches for extracting kinetic parameters.
The Time Cone method for Nucleation and Growth Kinetics on a Finite Domain
Authors: Cahn John W.
Published: April 2011

The Kolmogorov-Johnson-Mehl-Avrami theory is an exact statistical solution for the expected fraction transformed in a nucleation and growth reaction in an infinite specimen, when nucleation is random in the untransformed volume and the radial growth rate after nucleation is constant until impingement. Many of these restrictive assumptions are introduced to facilitate the use of statistics. The introduction of “phantom nuclei” and “extended volumes” are constructs that permit exact estimates of the fraction transformed. An alternative, the time cone method, is presented that does not make use of either of these constructs. The method permits obtaining exact closed form solutions for any specimen that is convex in time and space, and for nucleation rates and growth rates that are both time and position dependent. Certain types of growth anisotropies can be included. The expected fraction transformed is position and time dependent. Expressions for transformation kinetics in simple specimen geometries such as plates and growing films are given, and are shown to reduce to expected formulas in certain limits.
Kinetics of Phase Transitions in Superionic Pbsnf4 Versus Temperature
Authors: Denes Georges , Madamba M. Cecilia , Milova Galina
Published: April 2011

PbSnF4 is the highest performance fluoride ion conductor known to date and is starting to be used for the fabrication of chemical sensors. Although several phase transitions have been reported, with conflicting results from different groups, the exact sequence of phase transitions versus temperature and the kinetics of the phase transformations remain unclear. We have prepared the three phases that can be stabilized at ambient temperature, i.e. o-, α- and β-PbSnF4, and studied the stability of each phase versus temperature. It appears that some of the phases transform very slowly to another under prolonged heating at constant temperature. This is very important since the conductivity is not necessarily the same for each phase, and therefore, a phase transition taking place slowly in the PbSnF4 used for the fabrication of a device, might not be detectable by a short time evaluation but, it could well alter its properties after prolonged use.
High Quality Heteroepitaxial ß-SiC Deposited from Methyltrichlorosilane at 1200°C without any Buffer Layer
Authors: Vepřek S. , Kunstamann Th. , Hofmann J. , Volm D. , Meyer B.
Published: April 2011

Using methyltrichlorosilane diluted with hydrogen and a modified temperature program during the initial stage of the deposition we have improved the deposition process and obtained high quality heteroepitaxial films on Si (100) without any carbon buffer layer. The maximum temperature of 1200°C which is used for the substrate cleaning and film deposition is significantly lower than that used in the conventional deposition system SiH4/C3H8 of about 1400°C for the buffer layer and 1350°C for deposition.
Semiconductor Epitaxial and Nonepitaxial Overgrowth from Solutions
Authors: Köhler R. , Raidt H. , Banhart F. , Jenichen B. , Gutjahr A. , Konuma M. , Silier I. , Bauser E.
Published: April 2011

Having grown semiconductor-on-insulator (SOI) layers by liquid phase epitaxy (LPE) we have investigated their properties by x-ray topography, optical-, atomic force- and high resolution electron microscopy and by etching experiments.

A first series of experiments was carried out on (111)-oriented Si substrate wafers which were partially masked by thermal oxide. Area- selective LPE, which was then applied and followed by epitaxial lateral overgrowth (ELO), yielded SOI layers of high quality. Silicon SOI layers had aspect ratios of up to 65 and were typically dislocation-free. These highly perfect SOI structures facilitated studies of the specific processes for semiconductor lateral growth over amorphous layers. Vertical stresses were detected in the substrate-layer systems. They were found to influence the shapes of the SOI layers. Our studies suggest that here is a model case for solution growth of thin semiconductor lamellae on dissimilar substrates.

Subsequently, a second series of experiments consisted in having SiGe lamellae grown in the same fashion. These experiments confirm that the same morphology is observed for Ge-contents up to 2 percent.
Low-Temperature Growth and Structural Characterization of GaAs Using Ionized Source Beam Epitaxy
Authors: Roh D.-W. , Kim K.
Published: April 2011

Single-crystal GaAs films were grown on SI (100) GaAs at substrate temperatures below 200 °C by using ionized source beam epitaxy. The correlation between the properties of the films and the growth parameters, in particular, the substrate temperature, the amount of As-source beam ionization, and the acceleration voltage of the As beam was investigated to elucidate the possible benefits of source beam ionization and acceleration on low-temperature thin film growth. The use of ionized and accelerated As-source beam greatly improved the quality of the low-temperature grown GaAs film. The surface morphology, crystallinity, and micro structure of the low temperature grown GaAs films were evaluated using in situ reflection high energy electron diffraction, double crystal X-ray diffraction, and cross section transmission electron microscopy.
Uniform Quantum Dots by Self-Organizing Process in Atomic Hydrogen-Assisted MBE
Authors: Chun Y.-J. , Nakajima S. , Okada Y. , Kawabe M.
Published: April 2011

The effects of atomic hydrogen (H) on formation of In(Ga)As quantum dots (QDs) by self-organizing process have been investigated. The low size fluctuation and uniform-shaped QDs are obtained at growth temperature above 450°C. The average size of InGaAs QDs are decreased from 40 nm to 20 nm by atomic H irradiation. The InGaAs QDs are formed uniformly on growth surface in with-H condition while preferentially formed and distributed along the step edges in without-H case. The photoluminescence (PL) peak intensities and full width at half maximum (FWHM) are also improved by atomic H irradiation. The waiting time before GaAs cap layer deposition is a important factor on the optical properties of QDs.
A Model for Ion-Assisted Molecular Beam Epitaxy and Application to Synthesis of Epitaxial SNxGE1-x Alloy Films
Authors: He Gang , Atwater Harry A. , Watson Thomas J.
Published: April 2011

In this work, we develop a model for ion-assisted molecular beam epitaxy with application to synthesis of epitaxial SnxGe1-x/Ge/Si(001) with compositions up to x=0.34. The model describes the effect of energetic beam deposition on surface segregation during thin film growth. As the value of a Péclet number expressing the ratio of surface ion-mixing rate to growth rate exceeds unity, the segregation coefficient during ion-assisted growth exceeds the value during thermal growth. A comparison of the model with experiments for SnxGei.1-x synthesis is reported. The experimental results are consistent with the model.
Dislocation Structure Evolution in Thin Single Crystal Plates Under Cyclic Loading
Authors: Stepanov Yury. N. , Scorupsky Alexander M.
Published: April 2011

The effect of cyclic uniaxial loading applied to surfaces of thin single crystal plates on dislocation structure is analyzed analytically and numerically. This work investigates detailed changes in dislocation structure near surfaces and in size and concentration of dislocation loops inside the crystal. The results may be helpful in choosing the optimum regime for material processing and in predicting material properties.
Scaling of Island Size Distributions in the Growth of Ni on GaAs(110)
Authors: Quesenberry P. E. , First P. N.
Published: April 2011

Island size distributions have been derived from scanning tunneling microscope (STM) images of Ni deposited on cleaved GaAs(110) at room temperature and above. For submonolayer coverages, this system forms 3-dimensional (3-D) reacted islands with the degree of reaction dependent upon the growth temperature. As has been found for other systems, the average island size (sαυ) increases with temperature. The high temperature data (∼ 150° C) shows two distinct island types, each with substantially different average size. The island size distributions have maxima at the smallest island sizes. For different coverages, plots of the area-normalized island size distributions versus the scaled variable s/sαυ show significant differences. However, above a cutoff value for s/sαυ the distributions can be renormalized to fall on a common curve. These characteristics and direct atomic-scale evidence are consistent with nucleation of islands via adatom-substrate exchange, but the temperature dependence of the total island density appears to be inconsistent with this being the only first-order rate process taking place.
Decomposition of Unstable Supersaturated Cu90Co10 Solid Solutions
Authors: Busch Ralf , Gärtner Frank , Borchers Christine , Haasen Peter , Bormann Rüdiger
Published: April 2011

Homogeneous Cu90Co10 alloys were prepared by rapid solidification using the meltspin technique. The decomposition process of this highly supersaturated unstable solid solution was investigated on a nanometer scale using a combination of atom probe field ion microscopy (AP/FIM) analyses and transmission electron microscopy.

Annealing of the Cu90Co10 alloys at 440°C for various times leads to the formation of a compositional modulated microstructure within the grains. The composition profiles determined by AP/FIM analyses clearly exclude a classical nucleation and growth behavior of Co-rich particles. The microstructure is modulated with two different wavelengths. In addition, chemical ordering perpendicular to [111] directions of the fee lattice is observed.

At the grain boundaries of this alloy, heterogeneous nucleation of pure Co particles is observed. This heterogeneous nucleation process competes with the decomposition within the grains.
Which Systems Can Undergo Inverse Melting?
Authors: Sinkler W. , Michaelsen C. , Bormann R.
Published: April 2011

In the process of inverse melting, a crystalline solid solution on cooling becomes less energetically stable than the isocompositional undercooled liquid or amorphous phase. Due to this, the crystal may transform polymorphously to the amorphous or liquid phase. A thermodynamic requirement for this process is the presence of two crossing points of the free energy curves G(T) of the crystalline and liquid or amorphous phases. Fulfillment of the thermodynamic requirement is however not a sufficient criterion for inverse melting to occur. In the present work the question posed in the title is addressed from both a thermodynamic as well as a kinetic standpoint. It is shown that in bcc phases which would otherwise undergo inverse melting, the presence of an energetically favorable B2 phase may prevent inverse melting due to its fast ordering kinetics. This imposes an e/a limit to the occurrence of inverse melting.
Mechanisms of Phase Formation During Milling in the Ternary Immiscible AG-CU-FE System
Authors: Klassen T. , Herr U. , Averback R.S.
Published: April 2011

Powder blends consisting of prealloyed FCC Ag-Cu solid solutions and elemental Fe powders were ball milled in overall compositions of Ag25Cu50Fe25, andAg40Cu20Fe40. The phase evolution with milling time was investigated by x-ray diffraction and differential scanning calorimetry. For the sample with higher Cu concentration, a ternary FCC alloy phase was formed, while milling the other sample resulted in a two phase mixture consisting of a Ag-rich FCC and an Fe-rich BCC solid solution. About the same amount of enthalpy between 12 and 13 kJ/g-atom is stored in the final states for the two different compositions. Two models based on kinetic and energetic considerations will be discussed and compared to the results. The kinetic model is based on the competition between forced atomic motion during shearing, which is driving the system towards a homogeneous alloy, and thermally activated diffusion, which favors phase separation. For the energetic model, the energy balance is calculated and a phase transformation is expected, if the required energy can be stored in phase boundaries.
Enhanced Electrical Conductivity and Nonstoichiometry in Nanocrystalline CeO2-x
Authors: Lavik E.B. , Chiang Y.-M. , Kosacki I. , Tuller H.L.
Published: April 2011

Dense nanocrystalline CeO2-x. of ∼10 nm grain size exhibits enhanced, PO2-dependent electronic conductivity indicative of intrinsic nonstoichiometric behavior under conditions where coarse-grained counterparts are extrinsic. The enthalpy of reduction is lowered by over 2.4 eV per oxygen vacancy. The nanocrystals also exhibit greatly reduced grain boundary resistance, attributed to grain-size-dependent segregation. We propose that interface doping by selected low energy defect sites dominates the defect and transport properties of nanocrystalline ceria, and possibly other nanocrystalline compounds.
Synthesis and Characterization of Nanocrystalline M50 Steel Powders by Cryomilling
Authors: Lau M. L. , Huang B. , Perez R. J. , Nutt S. R. , Lavernia E. J.
Published: April 2011

The present study investigated the synthesis of nanocrystalline high speed steel M50 (4.5% Mo, 4.0% Cr, 1.0% V, 0.8% C, balance Fe) by cryogenic high energy ball milling (cryomilling). Pre-alloyed M50 steel was spray atomized, and subsequently cryomilled in liquid nitrogen for 25 hours. Elemental Al powder was added prior to cryomilling in order to promote the formation of nanoscale Al2O3 and AlN particles, which improved the thermal stability of the nanocrystalline M50 steel. Following annealing at 1373 K, the M50 steel was found to retain its nanocrystalline microstructure with the average grain size of 40–80 nm.
Production and Technical Characteristics of a New Class of Amorphous Metals
Authors: Malkhassian R.T.
Published: April 2011

A new technology for obtainment of amorphous single-component metals is presented.

For the first time the reduction of molybdenum oxide with formation of its amorphous phase is realized in conditions of a given quantum-chemical technology by means of vibrationally excited to the third quantum level hydrogen molecules with 1.5 ± 0.2 eV energy. The evidences of formation of this nonequilibrium amorphous phase are presented along with certain physicochemical properties of the obtained amorphous molybdenum.

A model is proposed for the origin of amorphous phase under the influence of nonequilibrium quantum-chemical technology.
Kinetics and Mechanism of the C49 to C54 Titanium Disilicide Phase Transformation in Nitrogen Ambient
Authors: Nagabushnam R. V. , Sharan S. , Sandhu G. , Rakesh V. R. , Singh R. K. , Tiwari P.
Published: April 2011

The kinetics of the C49 to C54 TiSi2phase transformation in nitrogen ambient have been investigated in a temperature range from 700 °C to 800 °C for a range of titanium film thicknesses (135 Å to 350 Å) using sheet resistance measurement, Auger electron spectroscopy(AES), Rutherford backscattering spectroscopy(RBS) and transmission electron microscopy(TEM). About 80% of the titanium converts to titanium silicide with the rest converting to titanium nitride. The activation energies obtained for the C49 to C54 transformation in nitrogen ambient are lower, at least by 2–2.25 eV, than that obtained for the transformations occuring either in argon ambient or vacuum environment. This has been explained with a model involving stress state of titanium silicide film with titanium nitride overlayer.
Titanium Germanosilicide Phase Formation During The Ti-Si1-xGex Solid Phase Reactions
Authors: Aldrich D. B. , Chen Y. L. , Sayers D. E. , Nemanich R. J.
Published: April 2011

The effect of Si1−xGex alloy composition on the titanium germanosilicide phase formation sequence during the Ti-Si1−xGex solid phase reaction was examined. For the Ti-Si reaction the initial formation of C49 TiSi2 is followed, at higher temperatures, by the formation of C54 TiSi2. For the Ti-Ge reaction the initial formation of Ti6Ge5 is followed, at higher temperatures, by the formation of C54 TiGe2. It was determined that the Ti-Si1−xGex reaction follows three different reaction paths depending on the composition of the initial Si1−xGex alloy. For Si rich Si1−xGex alloys the Ti-Si1−xGex reaction follows a “Ti-Si like” reaction path (Ti+M ↠ C49 TiM2 ↠ C54 TiM2, where M = Si1−xGex). For Ge rich Si1−xGex alloys the reaction follows a “Ti-Ge like” reaction path (Ti+M ↠ Ti6M5 ↠ C54 TiM2). Both Ti6M5 and C49 TiM2 form during the reaction of titanium with Si1−xGex alloys in an intermediate composition range. Properties of the final C54 phase were observed to be strongly dependent on the phase formation sequence. Smooth continuous C54 titanium germanosilicide forms during the “Ti-Si like” reaction and discontinuous islanded C54 titanium germanosilicide forms during the “Ti-Ge like” reaction. An optimum Si1−xGex alloy composition range of 0.00 ≤ x ≤ 0.36 was determined for the formation of continuous- low-resistivity- C54 titanium germanosilicide films from the solid phase reaction of Ti and Si1−xGex alloy.
Effects of Growth Parameters on the Epitaxy of CoSi2/Si(100) Formed by Reactive Deposition Epitaxy
Authors: Vantommela André , Degroote Stefan , Dekoster Johan , Bender Hugo , Langouche Guido
Published: April 2011

Epitaxial CoSi2(100) layers in the thickness range of 20 to 50 nm have been formed by reactive deposition epitaxy (i.e. Co deposition onto a hot Si substrate) without the use of either a template or an intermediate Ti layer. It is explained how growth parameters such as the deposition rate and substrate temperature are crucial in determining the epitaxial nature of the silicide. According to this model, good CoSi2/Si(100) alignment is only achieved when very low deposition rates are used (0.1 Å/s or less), combined with relatively high substrate temperatures during deposition (∼ 600°C or higher). Using these conditions, highly strained, continuous CoSi2 layers with a channeling minimum yield of χmin = 9% could be formed. Using higher rates and/or lower deposition temperatures, an increasing fraction of misoriented CoSi2 grains is presumed from backscattering/channeling and x-ray experiments, the nature of which is under investigation with plan view and cross sectional transmission electron microscopy.
Disorder and Strain Effects in the Optical Response of Thin CoSi Epitaxial Films on Si(111)
Authors: Bocelli S. , Guizzetti G. , Marabelli F. , Schwarz C. , Goncalvesconto S. , Von Känel H.
Published: April 2011

Thin (CsCl)CoSi films of thicknesses from 44 to 105 Å were grown by MBE at room temperature onto ultrathin annealed CoSi2 templates. Also two CoSi2 films of about 100 Å with the CaF2 structure and with the defected CsCl structure were obtained epitaxially.

Far infrared reflectance and transmittance measured on such samples show optical structures between 240 and 370 cm-1. In the range 1.4–5 eV spectroscopic ellipsometry was performed, too. A change in the optical response was revealed among the different samples, which can be ascribed to strain, roughness and/or disorder at the interfaces.
Indentation Fracture Studies of a BaTiO3 Thin Film on a Silicon Substrate
Authors: Ho Wai-Ming , Fu Ran , Wan Kai-Tak , Chou Ji , Zhang Tong-Yi
Published: April 2011

It has been experimentally and theoretically found that the critical applied stress intensity factor for indentation cracking also depends linearly on the reciprocal of the square root of crack length when the indentation fracture technique is used to measure residual stresses in thin films.
Polycrystalline β-SiC Film Growth on Si by ECR-CVD at 178 - 500°C
Authors: Cheng Kuan-Lun , Liu Chih-Chien , Fu Chung-Min , Cheng Huang-Chung , Lee Chiapyng , Yew Tri-Rung
Published: April 2011

Polycrystalline β-SiC, with grain size up to 0.2 μm, was grown on silicon substrate by electron cyclotron resonance chemical vapor deposition (ECR-CVD) from SiH4/CH4/H2 at 178–500 °C. The nucleation process and surface structure of polycrystalline SiC were investigated via observing the film surface by atomic force microscopy (AFM). Reaction species which promote polycrystalline SiC was in-situ monitored by quadruple mass spectrum analysers during deposition process, which is crucial for the control of polycrystalline SiC growth. The microstructure of SiC films were inspected by bright-field imaging, dark-field imaging, and electron diffraction in cross-sectional transmission electron microscopy. This paper will also discuss the key parameters for the nucleation and growth of polycrystalline β-SiC at very low temperature in ECR-CVD system.
Growth and Characterization of Hard Nitrides Films Prepared by Pulsed Laser Deposition
Authors: Inturi R. B. , Kumar Ashok , Ekanayake U. , Shu N. , Barnard J. A.
Published: April 2011

Transition metal of nitrides have many desirable properties for application at elevated temperatures because of extremely high melting point, hardness, high temperature strength, good thermal shock resistance, and high thermal conductivity. We have investigated the thin film coatings of nitrides (titanium nitride, and composite carbon nitride) on various commercial useful substrates by the pulsed laser deposition (PLD) method. The PLD method is unique process for depositing high quality thin films with novel microstructure and properties. The laser parameters: energy density, pulse rate, target/substrate distance and substrate temperature have been optimized to improve the quality of thin films. The mechanical properties of the films have been evaluated at different processed conditions by nanoindentation technique. The films were characterized by X-ray diffraction, scanning electron microscope and FTIR techniques. Optimization of laser deposition parameters to obtain high quality thin films will be discussed in detail.
Development of High-Performance GaAs Solar Cells on Large-Grain Polycrystalline Ge Substrates
Authors: Venkatasubramanian R. , O'Quinn B. , Hills J. S. , Timmons M. L. , Malta D. P.
Published: April 2011

The characterization of MOCVD-grown GaAs-AlGaAs materials and GaAs p+n junctions on poly-Ge substrates is presented. Minority carrier lifetime in GaAs-AIGaAs double-hetero (DH) structures grown on these substrates and the variation of lifetimes across different grainstructures are discussed. Minority-carrier diffusion lengths in polycrystalline GaAs p+-n junctions were evaluated by cross-sectional electron-beam induced current (EBIC) scans. The junctions were also studied by plan-view EBIC imaging. Optimization studies of GaAs solar cell on poly-Ge are discussed briefly. The effect of various polycrystalline substrate-induced defects on performance of GaAs solar cells are presented.
Effects of Cu and Si Dopants on Electromigration Mass Transport in Al Interconnects for VLSI
Authors: Lee C. H. , Fejes P. L. , York B. R. , Elwell S. A. , Carnes R. O. , Lee J. Y. , Grivna G. M. , Bauguess S. W. , Dreyer M. L. , Edwards S. R.
Published: April 2011

We have investigated the effects of Cu and Si dopants on electromigration mass transport in Al interconnects for VLSI technology. Four Al alloys with different Cu and Si dopant concentrations (AI-1.5%Cu, AI-1.5%Cu-I.5%Si, AI-0.5%Cu-I.0%Si, and AI-I.O%Cu-I.0%Si) were sputter deposited onto Si substrates covered with 0.55 pRm of Si02 (tetraethylorthosilicate). All metal films were deposited using an MRC903 sputter deposition system to a thickness of 6800Å. Deposition parameters were held constant for each metallurgy, with a base pressure of 7x 10−2 torr, deposition pressure of 102 torr, forward power of 7 Kwatts, and substrate bias of 125 volts. Test structures designed according to National Institute of Standards design guidelines were fabricated in each metallurgy using conventional photolithography and reactive ion etching methods for line widths of 1.0, 1.8, 3.0, 5.0 and 10.0 μm. Accelerated test conditions of T=200°C and DC current density of 2x106 A/cm−2 were used. The results show that electromigration resistance increases with increasing Cu content and decreases as Si content increases. These results are explained in terms of precipitate, grain size distribution, orientation and stress by Transmission Electron Microscopy (TEM) and X-ray Diffraction (XRD). Our results provide a general guideline relating Cu and Si dopant concentrations, film microstructure and the intrinsic reliability of the metallization system.
Microstructure and Optical Properties of Aluminum Nitride Thin Films
Authors: Watanabe Yoshihisa , Nakamura Yoshikazu , Hirayama Shigekazu , Naota Yuusaku
Published: April 2011

Polycrystalline aluminum nitride (AIN) thin films have been synthesized by ion-beam assisted deposition method and the effect of ion beam energy on the film structure and optical properties has been studied. The kinetic energy of nitrogen ion beam was varied from 0.05 to 1.5 keV under the constant current density. Microstructure of films was examined by thin film X ray diffraction (TFXRD) and optical transmission spectrum from 220 to 2200 nm was measured by UV-visible spectrometer. The TFXRD studies show that the (00*02) plane of hexagonal AIN grows preferentially with the ion beam energy of 0.05 keV and the intensity of the (10*0) and (10*1) planes becomes strong with increasing the ion beam energy. The optical measurements reveal that the wavy structures due to the interference effect are observed in the transmission spectra and the wavy pattern decreases with increasing the ion beam energy, resulting in the decrease of refractive index.
Dielectric and Pyroelectric Characteristics of Plzt (9.5/65/35) Relaxor Thin Films
Authors: Deb K. K.
Published: April 2011

Ferroelectric relaxor thin films of lead lanthanide zirconate titanate (PLZT) (9.5/ 65/35) have been deposited by the laser ablation technique onto metallized silicon and lanthanum aluminate substrates. The dielectric and pyroelectric properties of PLZT films and their temperature dependencies have been investigated in some detail in an evaluation of their potential as prospective pyroelectric infrared (IR) detector materials. The temperature at which the dielectric constant of a PLZT film reaches its peak is different from that for the peak pyroelectric coefficient. A lower dielectric constant at which the pyroelectric peak appears contributes to a high figure of merit for PLZT films, so that it is competitive with its ceramic counterpart, as well as with Pb(Mg1/3Nb2/3)O3 (PMN) relaxor ferroelectric ceramic.
GMR and Structure in Sputtered Co90Fe10/Ag Multilayers
Authors: Jarratt J. D. , Barnard J. A.
Published: April 2011

Giant magnetoresistance (GMR), structure, and magnetic properties of sputtered (Co90Fe10X Å/Ag Y Å) multilayer films have been investigated. Distinct GMR behaviors including granulartype (GGMR) and ‘discontinuous’ (DGMR) are observed which are strongly dependent on the individual CoFe and Ag layer thicknesses; however, standard multilayer GMR and the associated antiferromagnetic (AFM) coupling is absent. The multilayer structure, individual layer thicknesses, and growth texture were investigated using high and low angle x-ray diffraction (HXRD & LXRD).
Magnetic Coupling in Spin-Valves from Magnetic Circular Dichroism
Authors: Bedrossian P. J. , Tobin J. G. , Waddill G. D. , Jankowski A. F. , Anthony T. C. , Brug J. A.
Published: April 2011

We demonstrate the detection of Magnetic X-ray Circular Dichroism (MXCD) from commercial, thin-film spin valves. Measured x-ray absorption spectra on these devices indicate magnetic polarization of the ferromagnetic constituents, Fe, Co, and Ni.
A Parallel Implementation of Tight-Binding Molecular Dynamics Based on Reordering of Atoms and the Lanczos Eigen-Solver
Authors: Colombot Luciano , Sawyer William , Marict Djordje
Published: April 2011

We introduce an efficient and scalable parallel implementation of tight-binding molecular dynamics (TBMD) which employs reordering of the atoms in order to maximize datalocality of the distributed tight-binding (TB) Hamiltonian matrix. Reordering of the atom labels allows our new algorithm to scale well on parallel machines since most of the TB hopping integrals for a given atom are local to the processing element (PE) therefore minimizing communication. The sparse storage format and the distribution of the required eigenvectors reduces memory requirements per PE. The sparse storage format and a stabilized parallel Lanczos eigen-solver allow consideration of large problem sizes relevant to materials science. In addition, the implementation allows the calculation of the full spectrum of individual eigen-values/-vectors of the TB matrix at each time-step. This feature is a key issue when the dielectric and optical response must be computed during a TBMD simulation. We present a benchmark of our code and an analysis of the overall efficiency.
Parallel Simulations of Rapid Fracture
Authors: Abraham Farid F.
Published: April 2011

Implementing molecular dynamics on the IBM SP2 parallel computer, we have studied the fracture of two-dimensional notched solids under tension using million atom systems. Brittle materials are modelled through the choice of interatomic potential function and the speed of the failure process, and our interest is to learn about the dynamics of crack propagation in ideal materials. Recent laboratory findings occur in our simulation experiments, one of the most intriguing is the dynamic instability of the crack tip as it approaches a fraction of the sound speed. A detailed comparison between laboratory and computer experiments is presented, and microscopic processes are identified. In particular, an explanation for the limiting velocity of the crack being significantly less than the theoretical limit is provided.
Gradient-Driven Diffusion Using Dual Control Volume Grand Canonical Molecular Dynamics (DCV-GCMD)
Authors: Swol Frank Van , Heffelfinger Grant S.
Published: April 2011

Recently we developed a new nonequilibrium molecular simulation method [1] that allows the direct study of interdiffusion in multicomponent mixtures. The method combines stochastic insertion and deletion moves characteristic of grand canonical (GC) simulations with molecular dynamics (MD) to control the chemical potential μi of a species i. Restricting the insertions and deletions to two separate control volumes (CV's) one can apply different μ's in distinct locations, and thus create chemical potential gradients. DCV-GCMD can be used to study transient phenomena such as the filling of micropores or used in steady-state mode to determine the diffusion coefficients in multicomponent fluid mixtures. We report on the effects of molecular interactions and demonstrate how in a sufficiently nonideal ternary mixture this can lead to up-hill or reverse diffusion. In addition we introduce a novel extension of DCV-GCMD that is specifically designed for the study of gradient-driven diffusion of molecules that are simply too large to be inserted and deleted.
The SEISM* Project: A Software Engineering Initiative for the Study of Materials
Authors: Mula G. , Angius C. , Casula F. , Maxia G. , Porcu M. , Yang Jinlong
Published: April 2011

Structured programming is no longer enough for dealing with the large software projects allowed by today's computer hardware. An object-oriented computational model has been developed in order to achieve reuse, rapid prototyping and easy maintenance in large scale materials science calculations. The exclusive use of an object-oriented language is not mandatory for implementing the model. On the contrary, embedding Fortran code in an object-oriented language can be a very efficient way of fulfilling these goals without sacrificing the huge installed base of Fortran programs. Reuse can begin from one's old Fortran programs. These claims are substantiated with practical examples from a professional code for the study of the electronic properties of atomic clusters. Out of the about 20,000 lines of the original Fortran program, more than 70% of them could be reused in the C++ objects of the new version. Facilities for dealing with periodic systems and for scaling linearly with the number of atoms have been added without any change in the computational model.
Clustering and Extended Range Order in Binary Network Glasses
Authors: Nekhayev Dmitry , Kieffer John
Published: April 2011

The clustering in alkali silica glasses (M2O)x(SiO2)1-x, where M is either Na or Rb, and x ranges between 0 and 0.4, was studied using molecular dynamics simulations. Computations were performed using a semi-empirical potential, including two- and three-body terms as well as dynamic partial charge transfer. Characterization of the structures was based on pair correlation functions, neutron static structure factor and ring statistics. Results have shown a much stronger tendency to cluster in case of Na than of Rb. The irregular arrangement of Na is evidenced by a decay pattern in the Na-Na pair correlation function, which can be associated with a fractal dimension. The clustering tendency can be attributed to the differences in the way the network structure is able to accommodate the introduction of modifying cations. As opposed to Rb, the smaller Na cation can fit on sites only slightly larger than is required for Si. This would distort the structure but hardly alter its topology. Rb on the other hand, requires larger interstices, which results in a reconstruction of the network and a significantly different intermediate range order. Na can achieve a similar influence on the network structure if they group in pairs.
Theory of Initial Oxidation Stages on Si(100) Surfaces by Spin-Polarized Generalized Gradient Calculation
Authors: Kato K. , Yamasaki T. , uda T. , Terakura K.
Published: April 2011

The chemical reaction of an O2 molecule on the Si(001) surfaces is studied by the ab initio molecular dynamics with the spin-polarized generalized gradient approximation. The dissociative adsorption of an O2 molecule has been found to occur over two adjacent Si(001) dimers with a spin flip-flop transition. The spin-state transition could cause a substantial retardation for dissociative adsorption of an O2 molecule. The C-type defect after chemisorption of a dissociated O2 molecule, however, shows a spin-triplet state, indicating a barrier-less dissociative chemisorption. These results explain well the overall aspects including preferential oxidation of C-type defects found by recent experimental reports.
Investigation of Crystalline Quartz and Molecular Silicon-Oxygen Compounds with a Simplified LCAO-LDA Method
Authors: Kaschner R. , Seifert G. , Frauenheim Th. , Köhler Th.
Published: April 2011

Crystalline and molecular silicon-oxygen compounds are investigated using a simplified LCAO-LDA scheme for the construction of a nonorthogonal tight-binding (TB) Hamiltonian within a two-center approximation. The applicability of this method to the important class of silicon oxides and related molecules is demonstrated. In particular, the properties of the equilibrium structure of α-quartz and several Siloxane molecules are calculated and found to agree well with experiments and self-consistent calculations. To obtain such a good agreement it is necessary to include also the 3d states of Si into the LCAO basis of the wavefunctions.
O(N) Scaling Simulations of Silicon Bulk and Surface Properties Based on a Non-Orthogonal Tight-Binding Hamiltonian
Authors: Bernstein Noam , Kaxiras Efthimios
Published: April 2011

We have implemented a molecular-dynamics algorithm for silicon using a non-orthogonal tight-binding Hamiltonian with the functional form of Menon and Subbaswamy. Parameters for this Hamiltonian were determined by fitting to a database of first-principles total energy calculations of bulk phases and point defect formation energies. These geometries were chosen to reproduce the configurations seen in defective crystalline and amorphous silicon. We have also implemented the non-orthogonal density-matrix method, paying particular attention to data motion locality to facilitate efficient parallelization of the algorithm. The necessary sparse matrix operations (trace, transpose, matrix multiplication) have also been implemented on a single processor workstation with an algorithm which takes O(N) time. Tests of the method's accuracy involved calculations of surface energies and structural reconstructions and activation energies for bulk diffusion through concerted exchange. We present results of a simulation of the melting and rapid quenching of a silicon sample using molecular-dynamics, and examine the resulting structures.
Computer Simulation of Cluster Ion Impacts on a Solid Surface
Authors: Insepov Z. , Yamada I.
Published: April 2011

The sputtering probabilities for normal and oblique cluster ion impacts were calculated by the use of two-dimensional molecular dynamics (MD) calculations. These simulations have revealed the angular dependence of ejecting surface atoms on the cluster incidence angle. The ejecting flux has a symmetrical form with an essential lateral component in the case of normal cluster incidence. For an oblique cluster indidence we found a sharp asymmetry of sputtering orientation. We obtained that the ejecting flux consists of three components: a) fast flying atoms with the velocities higher than the cluster velocity ν ∼ 2.3ν0, b) approximately self-similar component with ν ≈ ν0, and, finally, c) slowly moving tail with ν ≈ 0.2ν0, where ν0 is the cluster velocity. According to our MD results we developed a new model of surface modification phenomena which consists of the Langevin Dynamics based on the Kardar-Parisi-Zhang equation, combined with a Monte-Carlo procedure for crater formation at normal and oblique cluster impacts. We obtained that for a symmetrical crater shape with a size in the order of 20 Å, a significant smoothing occurs after irradiation by ∼ 103 cluster impacts which has been supported by experiment at dose of ∼ 1014 ion/cm2. The rate of the smoothing process can be significantly accelerated if the lateral sputtering phenemenon is taken into account. Simulation of oblique cluster impact on a surface at a grazing angle of 30° by constructing of asymmetric crater shape gives an opposite result: the surface roughness increases. The latter obtaining qualitatively agrees with the experiment.
Dynamic Embrittlement Of Beryllium-Strengthened Copper In Air
Authors: Muthiah Ranjani C. , McMahon C. J. , Guha Amitava
Published: April 2011

A precipitation-hardened Cu-0.26%Be alloy is used as a low-temperature model material for “dynamic embrittlement”, or quasi-static diffusion-controlled intergranular brittle fracture. This alloy is shown to undergo intergranular cracking in air at 150°C and to be almost free of this cracking in 2×10−6 Torr vacuum at 200°C. The time to failure is highly stress dependent. The temperature dependence of cracking was found to be 30 kcal/mole. This is about 50% greater than the activation energy for oxygen diffusion in copper, but the present experiments also include an unknown incubation time.
Experimental Analysis Of Instability Effects On The Surface Of Schallamach's Waves Produced By Sliding Friction Of Rubber-Like Materials
Authors: Koudine A. , Barquins M.
Published: April 2011

It is shown that a small ridge exists on the surface of the detachment fold or Schallamach's wave (1971). Such waves take place in the contact area between a moving transparent hemispherical asperity and the smooth, flat surface of a soft elastomer sample. The space-time evolution diagram, composed by juxtaposition of contact area cross-sections, recorded one after the other, proves that there is a correlation between the appearance of these ridges and microdynamic shifting of forward and backward detachment fold limits. A rolling of a rubber band, compressed between two rigid plates, enables us to reproduce the ridges in the macroscale approach.