bluuebunny/crossref_metadata_2025 huggingface dataset:
Defects and Interfaces in III-Nitride/Sapphire Thin Film Heterostructures
Authors: Oktyabrsky S. , Dovidenko K. , Sharma A. K. , Narayan J.
Published: April 2011

High quality III-nitrides and zinc oxide thin film heterostructures have been fabricated using pulsed laser deposition (PLD) on highly lattice mismatched sapphire substrates. The structural quality of PLD films grown at 700-800°C is comparable with those grown by MOCVD in the temperature range 950-1050°C. The PLD films were found to be single crystalline with columnar structure having the dislocation density of ~1010 cm2 at a film thickness 150-200 nm. The typical structure of defects and interfaces in GaN and AIN films is analyzed and compared with those obtained from specimens grown by MOCVD at much higher temperatures. Low temperature PLD (< 780°C) does not require additional atomic nitrogen source to compensate for the loss of nitrogen due to thermal decomposition of the nitride layer. At slightly higher temperatures the GaN films exhibit rough morphology and formation of the cubic zinc-blende phase which is more stable against nitrogen deficiency than wurtzite phase. A mechanism of formation of zinc-blende phase has been identified. Growth of GaN on lattice-matched ZnO/sapphire buffer layers showed the formation of cubic spinel ZnGa2O4 intermediate layer.
In Situ Initial Growth Studies of SrTiO3 on SrTiO3 by Time Resolved High Pressure RHEED
Authors: Koster Gertjan , Rijnders Guus J.H.M. , Blank Dave H.A. , Rogalla Horst
Published: April 2011

The initial growth of pulsed laser deposited SrTiO3 on SrTiO3 has been studied using high pressure Reflection High Energy Electron Diffraction (RHEED) and Atomic Force Microscopy (AFM). For this, we developed a Pulsed Laser Deposition (PLD)-RHEED system, with the possibility to study the growth and to monitor the growth rates, in situ, at typical PLD pressures (10-50 Pa). Using perfect single crystal SrTiO3 substrate surfaces, we observe true 2D intensity oscillations at different temperatures. Simultaneously, information on the diffusion of the deposited material on the surface could be extracted from the relaxation of the intensity after each laser pulse. The characteristic times depend on pressure and temperature as well as the 2D coverage during growth.
A Theoretical Model for Steam Laser Cleaning
Authors: Lu Y. F. , Zhang Y. , Song W. D. , Low T. S.
Published: April 2011

A theoretical model for removal of tiny particles from solid surface covered with a thin liquid film by laser cleaning is established by taking adhesion force and cleaning force into account. When pulsed laser irradiates on the solid surface coated with a thin liquid film, a sheet of liquid near the liquid/substrate interface can be superheated through thermal diffusion. The rapid growth of vapor bubbles inside the superheated liquid can generate transient stress wave with high pressure, large enough to expel micron and sub-micron particles from the contaminated surface. By calculating the adhesion force and cleaning force, the cleaning threshold of laser fluence can be predicted from this theoretical model. The difference between cleaning force and adhesion force increases quickly along with the laser fluence and leads to higher cleaning efficiency.
Refractive Index Modification During Deposition of Silicon Oxynitride Films Prepared by Reactive Laser Ablation
Authors: Machorro R. , Soto G. , Samano E. C. , Cota-Araiza L.
Published: April 2011

Inhomogeneous low-k thin films of SiOxNy have been deposited by laser ablation of a Si3N4 sintered target in presence of oxygen gas. The high oxidation rate of silicon nitride has been used to control the stoichiometry of the films by modifying the oxygen partial pressure. The refractive index of the deposited material was able to be tailored at any value between 1.47 (SiO2) to 2.03 (Si3N4) by this approach. In situ optical characterization of the growing films was possible using kinetic and spectro ellipsometry. The refractive index was determined by applying the Effective Medium Approximation (EMA) and considering a mixture of SiO2, Si3N4, and voids. The volumetric composition obtained by ellipsometry was compared to the results determined by AES and XPS characterization. The purpose of this application is to show that reactive PLD can be used to produce high quality optical filters.
Determination of The Diffusion Mechanism by A Method with New Possibilities: Nuclear Scattering of Synchrotron Radiation
Authors: Sepiol B.
Published: April 2011

The elementary diffusion jump in crystalline solids can be determined by methods derived from nuclear physics. With these methods not only diffusion rate(s) but also diffusion vector(s), i.e. the complete diffusion mechanism can be deduced. We report on a new method for probing the elementary diffusion jumps in crystalline lattices on an atomistic scale and demonstrate its potential by a study of 57Fe diffusion in different intermetallic alloys. Compared to the results of conventional tracer (macroscopic) technique, the new method provides clear and doubtless statements concerning the direction and distance of elementary jumps. One can also determine (though less precisely than with tracer diffusion), iron diffusion coefficients.
Grain Boundary Diffusion and Solute Segregation in Polycrystals and Oriented Bicrystals
Authors: Herzig Chr. , Surholt T.
Published: April 2011

Measurements of solute GB diffusion in Cu and Ag polycrystals were carried out in Harrisons type-B and type-C kinetic regime using 75Se, 195Au, 63Ni radioisotopes. In the B-regime at high temperatures, the GB diffusion parameter sδDGB was determined (s being the GB segregation factor, δ the GB width and DGB the GB diffusion coefficient), while in the C-regime at low temperatures DGB values were measured directly. The investigations of these differently behaving solute–solvent systems on Cu and Ag basis yield information on solute GB diffusion and segregation in relation to solute–solvent atomic binding and solute–vacancy interaction in the bulk and in GBs. The GB diffusion of 64Cu and 195Au in a series of near ∑ = 5, Θ = 36.9° (310) [001] Cu tilt GBs and of 71Ge in a series of near ∑ = 7, Θ = 38.2° (123) [111] Al tilt GBs in dependence on the tilt angle Θ and on the temperature was studied. A characteristic minimum in the GB diffusion parameter and a maximum in the activation energy was observed at the ideal CSL GBs.
Modeling the Effect of Irradiation and Post-Irradiation Annealing on Grain Boundary Composition in Austenitic Fe-Cr-Ni Alloys
Authors: Allen T.R. , Busby J.T. , Kenik E. A. , Was G.S.
Published: April 2011

Many irradiation effects in Fe-Cr-Ni alloys such as radiation-induced segregation, radiation-enhanced diffusion, and void swelling are known to vary with bulk alloy composition. The development of microstructural and microchemical changes during irradiation and during post-irradiation annealing is determined by the rate of diffusion of point defects and alloying elements. To accurately predict the changes in grain boundary chemistry due to radiation-induced segregation and post-irradiation annealing, the composition dependence of diffusion parameters, such as the migration energy, must be known. A model has been developed which calculates migration energies using pair interaction energies, thereby accounting for the effect of composition on diffusivity. The advantages of this calculational method are that a single set of input parameters can be used for a wide range of bulk alloy compositions, and the effects of local order can easily be incorporated into the calculations. A description of the model is presented, and model calculations are compared to segregation measurements from seven different iron-chromium-nickel alloys, irradiated with protons to doses from 0.1 to 3.0 dpa at temperatures between 200°C and 600°C. Results show that segregation trends can be modeled using a single set of input parameters with the difference between model calculation and measurement being less than 5 at%, but usually less than 2 at%. Additionally, model predictions are compared to grain boundary composition measurements of neutron irradiated 304 stainless steel following annealing. For the limited annealing data available, model calculations correctly predict the magnitude and time scale for recovery of the grain boundary composition.
New Tight-Binding Method for Simulation of Defect Configurations, Creation and Diffusion Mechanisms in Solids: Application to Silicon
Authors: Khakimov Zokirkhon M.
Published: April 2011

This paper presents the self-consistent tight-binding method of new generation which, unlike other tight-binding methods, allows one to calculate structural energies of multiatomic systems (molecules, clusters, defects in solids) and their spectroscopic energies in the framework of the same computational scheme and with comparable accuracy. Reliability of the method is illustrated considering defect state problems in crystalline and amorphous silicon (electron-enhanced-atomic diffusion, metastable defect creation, defects with effective-negative correlation energies, etc.) and comparing obtained results with ab initio calculations and experimental data.
Properties of Vacancies in Silicon Determined by Out-Diffusion of Zinc from Silicon
Authors: Giese A. , Bracht H. , Walton J.T. , Stolwijk N.A.
Published: April 2011

We present out-diffusion of Zn in Si as a new method to study properties of Si vacancies. Out-diffusion experiments were performed on homogeneously Zn-doped Si samples at 1107°C and 1154°C. The resulting concentration-depth profiles were measured by means of spreading-resistance profiling. Based on a diffusion model in which Zn migrates simultaneously via the kick-out and the dissociative mechanism all experimental profiles were modeled by computer simulations. The calculations reveal that out-diffusion of Zn from Si occurs to a considerable extent via the dissociative mechanism. Hence, vacancy properties like the equilibrium concentration and the transport capacity can be extracted from profile fittings. The results are compared with literature data deduced from in-diffusion experiments.
Ge Diffusion in SnTe Crystal
Authors: Kaportseva O.E. , Yashina L.V. , Bobruiko V.B. , Safonov D.V. , Kozlovsky V.F. , Shtanov V.I. , Shatalova T.B.
Published: April 2011

This work is devoted to the study of Ge diffusion in crystalline Sn1-δTe1+8 with δ=0.0065±0.0008 in temperature range T=878-973 K by electron probe microanalysis and layer by layer X-ray analysis. For the latter lattice constant dependence on composition was determined: a(Å)=a(SnTe)-(0.368±0.008)× where 0<×<0. 1. Activation energy was found to be about 1.3 eV, much less than in the case of Ge diffusion in PbTe.
Activation Enthalpy of Sb Diffusion in Biaxially Compressed SiGe Layers
Authors: Kuznetsov A.Yu. , Cardenas J. , Svensson B.G. , Larsen A. Nylandsted , Hansen J. Lundsgaard
Published: April 2011

Enhanced Sb diffusion in biaxially compressed Si1-xGex layers is observed. Assuming the prefactors to be stress independent the Sb diffusion coefficients in biaxially compressed Si0.9Ge0.1 and Si0.8Ge0.2 were extracted as 0.4 × 102 exp[−(3.98(eV)±0.12)/kT] and 1.3 × 102 exp[−(3.85(eV)±0.12)/kT] cm2/s, respectively. The activation volume of Sb diffusion in Si1-xGex (× ≤ 0.2) is estimated to be close to Ω, where Ω is the volume corresponding to a silicon lattice site.
Interplay between Anion Rotation and Cation Transport in the Plastic High-Temperature Phase of Sodium Ortho-Phosphate
Authors: Funke K. , Wilmer D. , Banhatti R. D. , Witschas M. , Lechner R. E. , Fitter J. , Jansen M. , Korus G.
Published: April 2011

The high-temperature phase of sodium ortho-phosphate, α-Na3PO4, belongs to the class of ion conducting plastic crystals, i.e., it is characterized by a dynamic rotational disorder of its poly-atomic anions and, at the same time, by a considerable translational mobility of its cations. During the past decade, the possibility, nature, and importance of a dynamic interplay between the two kinds of motion have been a subject of continued controversy. Proponents of a strong interplay coined the expression “paddle-wheel mechanism”. In our present contribution we report, for the first time, on the results ofdynamicexperiments probing the elementary steps of anionic and cationic motion individually. The techniques utilized in this study are coherent quasielastic neutron scattering and high-frequency conductivity spectroscopy, respectively. The data are complemented by anab-initiomolecular-dynamics simulation. Our results provide a view of the movement of anions and cations and of correlations between them. Strong dynamic coupling is detected between the octahedrally coordinated sodium ions and nearby oxygen ions. For translational sodium-ion transport, a chain mechanism appears to be operative.
A Study of Diffusion of Li+ in the [NZP] Network
Authors: Bhuvaneswari G. , Varadaraju U.V.
Published: April 2011

Li+ has been incorporated in the 3D -framework of PbM3+M4+P3O12 by soft chemistry method using n-butyl lithium at RT. It is shown that the framework of the [NZP] structure remains intact upon Li+ insertion and provides a three dimensional interstitial pathway for Li+ ion diffusion.
Positron Studies of Defects in Nitrogen and Carbon Implanted Titanium
Authors: Soltani-Farshi M. , Baumann H. , Anwand W. , Brauer G. , Coleman P.G. , Richter E. , Kreissig U. , BETHGE K.
Published: April 2011

Modification of materials by ion beams creates defects and defect formation processes. The implantation of nitrogen into titanium influences the hydrogen content in this metal. Hydrogen accumulation enhances hydrogenations in the implanted region. This effect may have important consequences, because excessive hydrogen accumulation generally leads to precipitating hydrids in the matrix lattice and the metal undergoes degradation of its mechanical properties. Many studies have shown that defects in metals trap light gas atoms like H or He which are solved or implanted in the sample. Therefore, the decoration of defects with these atoms is a method to trace defect concentrations and to study the trapping and detrapping mechanism. Mobile defects can be trapped at implanted atoms, at inhomogeneities or at inner surfaces like grain boundaries or interfaces of different phases. Using the slow positron beam technique the traps for hydrogen attributed to vacancy-type defects have been investigated in cp-Ti implanted with nitrogen or carbon.

The concentration of hydrogen has been detected by the 15N profiling technique (1H(15N,αγ)12C). The nitrogen and carbon content were measured by 15N(1H,αγ)12C and 13C(p,γ)14N reaction, respectively.
Investigating Islet Immunoisolation Parameters Using Microfabricated Membranes
Authors: Desai T.A. , Hansford D.J. , Chu W.H. , Huen T. , Ferrari M.
Published: April 2011

The immunoisolation of xenogeneic hormone-secreting cells is a promising therapy for a wide variety of diseases including diabetes, Parkinson's, and hemophilia. By utilizing microfabrication technology, silicon biocapsules can be fabricated with membranes having precisely controlled and uniform pore sizes, allowing one to optimize parameters specifically for the encapsulation of specific hormone-secreting cell types. This study investigates immunoisolation parameters using microfabricated silicon-based membranes, with uniform membrane pore sizes in the tens of nanometer range. The permeability of IgG was studied in microfabricated biocapsules with various pore sized membranes. In addition, immunoisolative characteristics were monitored by assessing viability and functionality of islets within biocapsules.
High Temperature Oxide for NVM Interpoly Dielectric Applications
Authors: Yater J. A. , Esho B. , Paulson W. M.
Published: April 2011

High quality interpoly dielectrics are required for non-volatile memories (NVM) in order to achieve long term data retention and endurance over many program/erase cycles. LPCVD high temperature oxide (HTO) deposited at 800°C-900°C is investigated for use in oxide-nitride-oxide (ONO) interpoly dielectric stacks. HTO allows for reduced thermal budgets, improved conformal coverage at edge features and more flexibility in scaling the ONO stack compared to thermal oxide layers. SIMS and atomic force microscopy results indicate that smooth, high quality films are deposited with a rms roughness of 0.12nm. Etch rates of as-deposited films are lowered 35% following several densification anneals. Field strengths (at 1μA) of 7-8MV/cm and leakage currents in the pA range are measured. Centroid measurements on sressed oxides show traps to be located at the HTO/polysilicon interfaces. Finally, double poly flash memory cells fabricated with ONO stacks containing HTO top oxide show improved field strength and data retention.
The Importance Of Pairing Reactions For The Modeling Of Defect-Dopant Interactions In Silicon
Authors: Bork I. , Schwerin A. v. , AG Siemens
Published: April 2011

In this article it is shown that the reactions between dopants and point defects in silicon are slow enough to play a significant role for low temperature transient enhanced diffusion (TED). As a consequence, diffusion models based on the assumption of local equilibrium between dopants and dopant-defect pairs highly overestimate TED at temperatures below 800°C. Without this assumption, i.e. when full dynamic pairing of dopants is included in simulations, good agreement to experimental results is achieved.
Ultra Shallow Junction Formation by RTA at High Temperature for Short Heating Cycle Time
Authors: Saito S. , Shishiguchi S. , Mineji A. , Matsuda T.
Published: April 2011

In accordance with decrease of device size, ultra shallow junctions are required for realizing superior device performance. Enhanced diffusion caused by implantation is a crucial factor to realize ultra shallow junctions. Not only implant but also RTA conditions are key factors to suppress enhanced diffusion. In this paper, process conditions to minimize enhanced diffusion are discussed. Implant ion species, energy, dose and beam current parameters are investigated for implantation and temperature, time and ramping rate parameters are investigated for RTA. Important result is that optimization of not only implant but also RTA conditions should be carried out in order to fabricate ultra shallow junctions.
Defect Tails in GE Implanted Si Probed by Slow Positrons and Ion Channeling
Authors: Knights A P , Nejim A , Barradas N P , Gwilliam R , Coleman P G , Malik F , Kherandish H , Romani S
Published: April 2011

Positron annihilation spectroscopy has been used to profile the distribution of defects following implantation of 120keV Ge+ into (100) Si in the dose range l x 1010 - lx104 cm−2 . The openvolume defect profiles can be adequately fitted assuming a simple rectangular block distribution extending to 350nm. Using anodic oxidation and etching, a procedure is described which allows details of the defect tails beyond the range of the implanted ion, usually inaccessible to positron -2 annihilation measurements, to be determined. For a time averaged dose-rate (Jr) of 0.02μA cm−2 and incident angle of 7°, open-volume defects are found to exist at concentrations exceeding 1016cm−3 at depths upto 600nm whereas the peak of the depth distribution of the implanted ions (Rp) is 76nm, measured using SIMS. When the time-average dose-rate is increased by a factor of 10, defects persist at concentrations in excess of 1017cm−3 beyond lμm and the Rp increases to 101nm. The open-volume defect profiles are compared to those deduced from Rutherford backscattering-channeling using the fitting routine DICADA.
Si/Ge/Si(001) Magnetron Sputter Heteroepitaxy: the Initial Stages of “Hut”- Cluster Overgrowth
Authors: Vögeli B. , Kummer M. , Känel H. Von
Published: April 2011

We show that the capping of Ge/Si(001) hut clusters with epitaxial Si cannot be easily realised. Even for growth temperatures as low as 300°C, Ge segregation during the Si deposition leads to a Si1-xGex alloy in both the wetting layer and the clusters which reduces the misfit of the system. In turn it is no longer beneficial to release the strain energy by forming small islands. On the contrary, there appears to be a metastable state for exhibiting a flat surface. Thus we find a 3D to 2D transition which at low Si coverages is probably thermally activated. At higher coverages, however, the transition becomes a barrier-less, resulting in the complete dissolution of the hut clusters. Annealing experiments prove that the resulting 2×8 reconstructed Si1-xGex surface is not in thermodynamic equilibrium. During the melting of a hut cluster a number of new facets are formed, mainly at the edges of the clusters and at the top, where the apex becomes substituted by a {001} facet. Our results emphasize that the melting of the Ge/Si(001) hut clusters is a serious obstacle to their application, e. g. in electroluminescence devices. It has to be solved, e. g. by the use of surfactants.
Role Of Surface Instability And Anisotropy On Strain Relaxation Of Sige On Si(110)
Authors: Deng X. , Krishnamurthy M.
Published: April 2011

SiGe alloys (< 1.5 % mismatch) were grown on Si (110) substrates at 700°C by molecular beam epitaxy. The structure and surface morphology were studied as a function of SiGe coverage (1.5mn–9nm) using atomic force microscopy, transmission electron microscopy and RHEED. A very high density of nanoscale SiGe ledges (less than 1 nm high and 1 μm long) were observed to form on the step terraces created during buffer layer growth. An interesting phenomenon is that the step bunches generated by the buffer growth and SiGe ledges are aligned along the elastically hardest <111> direction on the Si (110) surface. With increasing alloy thickness, the nanoscale SiGe ledges become longer and straighter. The effects of the surface instability and elastic anisotropy on the elastic relaxation effects are discussed.
Comparative Growth Kinetics Of Sige In A Commercial Reduced Pressure Chemical Vapour Deposition Epi Reactor And Anomalies During Growth of Thin Si Layers on Sige
Authors: Caymax Matity , Loo Roger , Brijs Bert , Vandervorst Wilfried , Howard David J , Kimura Kenji , Nakajima Kaoru
Published: April 2011

A short discussion about growth kinetics of Si and Si1-xGex, epitaxial layers in a reduced pressure CVD reactor using both dichlorosilane and silane is presented. Through careful observations of the growth of very thin Si layers on SiGe, an anomaly in the Si growth ratewas detected such that the thinner the Si layer, the higher the Si growth rate on SiGe. Due to the difficult nature of very thin film characterization, several analysis techniques were used. A possible explanation based on TEM observations is put forward.
Characterizations Of Zr/Si1-x-yGexCy After Rapid Thermal Annealing
Authors: Aubry-Fortuna V. , Barthula M. , Meyer F. , Eyal A. , Cytermann C. , Eizenberg M. , Chaix-Pluchery O.
Published: April 2011

In this work, we have investigated the reaction between Zr and SiGeC alloys after Rapid Thermal anneals performed at 800°C for 5 min. The interactions of the metal with the alloy have been investigated by X-Ray diffraction. Four crystal X-Ray diffraction was also performed to measure the residual strain in the epilayer. The final compound of the reaction is the C49- Zr(Si1-xGex)2 phase. The C49 film contains the same Ge concentration as in the as-deposited Si1-x-yGexCy layer. This suggests that no Ge-segregation occurs during annealing. Only a small strain relaxation is detected in the unreacted SiGe epilayer during the reaction. The addition of C in the epilayer prevents any strain relaxation. These results are in contrast with those observed in systems with Ti and Co, and show that the system Zr-Si-Ge is much more stable. Schottky barrier heights have been also measured: annealing leads to a slight decrease of the barrier without any degradation of the contact. The resistivity of the C49 film is about 80 μΩcm. These results indicate that Zr may be a good candidate for contacts on IV-IV alloys in term of thermal stability.
Evidence of InterDiffusion Effect In Stacked Polycrystalline Sige/Si Layers For Cmos Gate Application
Authors: Hernandez C. , Campidelli Y. , Sagnes I. , Henrisey E. , Martin F. , Bensahel D.
Published: April 2011

Poly-SiGe stacked gates with Ge content ([Ge])varying between zero and 100% have been fabricated using an industriel single-wafer machine. These poly-SiGe layers were characterised and fully integrated in a 0.18 pm CMOS process. Interdiffusion of Si and Ge upon subsequent annealing of the structure has been observed and studied. This interdiffusion effect was found to be responsible for the discrepancy observed between theoretical and practical values of the Ge workfunction ɸms evaluated from our electrical measurements and from those of different authors. A technique for the limitation of this interdiffusion effect has then been developped and is described.
Monolithic Integration of III-V Microcavity Leds on Silicon Drivers using Conformal Epitaxy
Authors: Gérard B. , Marcadet X. , Etienne P. , Pribat D. , Friedrich D. , Eichholz J. , Bemt H. , Hanssen H. , Carlin J-F. , Ilegems M.
Published: April 2011

Conformal epitaxy is an epitaxial growth technique capable of yielding low dislocation density III-V films on Silicon. In this technique, the growth of the III-V material occurs parallel to the silicon substrate, from the edge of a previously deposited III-V seed, the vertical growth being stopped by an overhanging capping layer. As an example, conformal GaAs layers on Silicon, presenting dislocation densities below 105cm−2, have been obtained using selective vapor phase epitaxy. These layers have then been used as high quality GaAs on Si substrates for subsequent vertical MBE regrowth of active structures. In this paper, we report on the integration of surface-emitting microcavity LEDs with their silicon drivers using this conformal growth technique. The global technology concept and the design of the active structures are first presented. The compatibility of the conformal growth technique with CMOS technology is then checked: the impact of the integration process on the performances of the drivers is for example quantified. Characterisations of the high crystalline quality of the conformal layers and of the LEDs structures grown on it are then shown. The electro-optical characteristics of the LEDs on Si are finally compared to those of reference LEDs on GaAs substrates in order to prove the efficiency of the integration procedure.
FE-Modeling and Physical Testing of IGBTs for Press-Packaging
Authors: Pirondi A. , Nicoletito G. , Cova P. , Pasqualetti M. , Portesine M. , Zani P. E. , Camera A.
Published: April 2011

The reliability of press-packed IGBTs strongly depends on the solutions adopted in terms of heat dissipation and thermo-mechanical stress management so as to guarantee satisfactory electrical and thermal contact conditions throughout the service life. In this paper, two aspects concerning the design process of a single-chip press-pack IGBT are outlined: i) cyclic stresses and strains occurring in the package and in the chip are discussed on the basis of finite element (FE) simulations, ii) a testing rig developed for accelerated testing of single IGBT chips under controlled contact pressure conditions is presented.
Kinetic Modelling Of The Selective Epitaxy Of GaAs On Patterned Substrates By Hvpe. Application to the Conformal Growth Of Low Defect Density GaAs Layers On Silicon
Authors: Gil-Lafon E. , Napierala J. , Castelluci D. , Pimpinelli A. , Gérard B. , Pribat D.
Published: April 2011

The selective growth of GaAs by HVPE was studied on (001), (110), (111)Ga and (111)As, GaAs patterned substrates by varying the I1I/V ratio. A kinetic modelling of the growth was developed, based upon the SEM observations of the growth morphologies as well as on experimental curve synthesis. The growth rate is written as a function of the diffusion fluxes of the adsorbed AsGa and AsGaCI molecules and takes into account the chlorine desorption by H2. 1.5 μm thick GaAs films were then fabricated on Si (001) by a confined epitaxial lateral overgrowth technique. These conformal films exhibit intense and uniform luminescence signals, showing that the dislocation densities of GaAs are lower than 105 cm−2. SEM analyses reveal that conformal growth fronts consist in (110) and (111)As A planes under the III/V ratios (superior to 1) which were tested.
Unique Defect-Induced Donor Structure at the Lattice Mismatched InAs/GaP Heterointerface
Authors: Gopal V. , Chen E.-H. , Kvam E. P. , Woodall J. M.
Published: April 2011

We have investigated the direct growth of narrow-gap InAs on wide-gap GaP by Molecular Beam Epitaxy. InAs and GaP have the largest mismatch among all the III-arsenides and the III-phosphides – 11%. A perfect epitaxial relationship is maintained between the InAs and the GaP despite the large lattice mismatch. Moreover, a reproducible defect structure with unique electronic properties is developed at the heterointerface. A point defect associated with the intersection of 90° misfit dislocations may act as an ordered, structural dopant. This dopant is fully ionized with a constant, high sheet carrier density of 1013 cm−2, independent of InAs layer thickness, and exhibits no freeze out even at 5 K. Device applications for such a system include temperature insensitive Hall sensors. We have also demonstrated high electron mobilities (over 10000 cm2/V-sec) in nominally undoped thick InAs layers grown on GaP. The explanation of this effect is presented to emphasize the exciting possibilities of band gap engineering in this system.
Annihilation Radii for Dislocations Intercepting a Free Surface with Application to Heteroepitaxial Thin Film Growth
Authors: Chang M. , Mathis S.K. , Beltz G.E. , Landis C.M.
Published: April 2011

One critical issue in heteroepitaxial, lattice mismatched growth is the inevitable appearance of threading dislocations which reside in the relaxing film and degrade its semiconducting properties. It has been shown in previous work that threading dislocations interact with each other through a series of annihilation and fusion reactions to decrease their density as the film thickness increases and follow a 1/h decay, where h is the film thickness. A characteristic reaction radius is associated with these interactions. In previous simulations, the reaction radius was taken to be a constant value estimated using a simple approximation based on infinite, parallel dislocation lines. Here, a continuum-based elasticity approach is taken to more accurately quantify the reaction radius by comparing the Peach-Koehler force of one dislocation acting on another at a free surface with the lattice resistance to dislocation motion. The presence of the free surface gives rise to a moderate reduction of the interaction force. Results are compared with preliminary experimental data for GaAs films grown on InP.
On Dislocation Reactions and Hardening Mechanisms in 3D Dislocation Dynamics
Authors: Zbiib H.M. , Rhee M. , Hirth J.P. , Rubia T. D. DE LA
Published: April 2011

A 3D dislocation dynamics (DD) model for plastic deformation, which connects the macroscopic mechanical properties to the basic physical laws that govern dislocation mobility and related interaction mechanisms, has been under development. In this model there is a set of critical reactions that, basically, determine the overall results of the simulations. These reactions are annihilation, formation of jogs, junctions, dipoles, and cross-slip. In this paper we discuss these reactions and the manner in which they influence the predicted stress-strain behavior in bcc metals. In particular, the formation (zipping) and strength of dipoles and junctions, and effect of jogs, using the dislocation dynamics model are examined. The strengths (unzipping) of these reactions for various configurations are determined by direct evaluation of the elastic interaction forces.
Computational Modeling of Multiple Domain Pattern Formation
Authors: Huang Tao , Tsuji Tomohiro , Rey Alejandro D. , Kamal Musa R.
Published: April 2011

We propose a physical model of multiple domain nucleation and growth in order to perform the cross-scale simulation on envelope profiles of the domain and the internal structure. The model utilizes two coupled field: a nonconserved envelope vector field of the local macroscopic growth domains and a vector field of the local mesoscopic lamellae directors. The envelope evolution equation includes an envelope profile function, the domain growth and impingement kinetics, the relaxation of the envelope by a surface tension, a lowest-order nonlinear term, a noise term, and a surface diffusion term. The splay, twist and bend deformation and the interaction between lamellae and noise field are considered in the free energy density equation of the lamellar director field. Results of large-scale computer simulation are reported which are compared with experiments.
Coupled Simulations of Mechanical Deformation and Microstructural Evolution Using Polycrystal Plasticity and Monte Carlo Potts Models
Authors: Battaile C.C. , Buchheit T.E. , Holm E.A. , Wellman G.W. , Neilsen M.K.
Published: April 2011

The microstructural evolution of heavily deformed polycrystalline Cu is simulated by coupling a constitutive model for polycrystal plasticity with the Monte Carlo Potts model for grain growth. The effects of deformation on boundary topology and grain growth kinetics are presented. Heavy deformation leads to dramatic strain-induced boundary migration and subsequent grain fragmentation. Grain growth is accelerated in heavily deformed microstructures. The implications of these results for the thermomechanical fatigue failure of eutectic solder joints are discussed.
First-Principles Study of SI(lll) Homoepitaxy
Authors: Cho Kyeongjae , Kaxiras Efthimios
Published: April 2011

Epitaxial growth on the Si(111) surface is studied using first-principles total-energy pseudopotential calculations. The energetics of added Si atoms essentially determines epitaxial growth modes under different growth conditions (surface temperature, Si flux rate, and surface step density). We have determined the surface adatom diffusion barriers and cluster formation energies; we use these microscopic energy parameters to address the possibilities for macroscopic morphological evolution of the surface under different conditions.
AB Initio Calculations of Point Defects in Silicon
Authors: Xie Jianjun , Chen S.P.
Published: April 2011

We use the ab initio plane wave pseudopotential method and the density functional theory (DFT) to study the arsenic(As)-vacancy interactions in silicon. The detailed lattice distortions surrounding the As-vacancy defect and the energetics of As-vacancy reaction around the six-fold ring are investigated. We find that the As displaces its neighboring silicon atoms outward while the vacancy attracts its neighboring atoms inward. The binding energy and the formation energy of an As-vacancy pair are 1.21 eV and 2.37 eV, respectively. Once the vacancy and As binds together, the highest migration barrier for the whole complex is 1.19 eV, which is in good agreement with the experimental measurement of 1.07 eV. The calculated activation energy for the vacancy mediated diffusion of the neutral As in silicon is 3.56 eV. The nature of the binding between As and vacancy is explained from the lattice distortions introduced by the As-vacancy complex.
Accelerating Atomistic Simulations of Defect Dynamics: Hyperdynamics, Parallel Replica Dynamics, and Temperature-Accelerated Dynamics
Authors: Voter Arthur F. , Sørensen Mads R.
Published: April 2011

Obtaining a good atomistic description of diffusion dynamics in materials remains a daunting task due to the time-scale limitations of the molecular dynamics method. We discuss new methods, derived from transition state theory, for accelerating molecular dynamics simulations of these infrequent-event processes. Two of these methods (hyperdynamics and parallel replica dynamics) have been presented previously, and are briefly reviewed here. The third, temperature-accelerated dynamics (TAD), is presented in detail. In TAD, the system temperature is raised to stimulate more rapid escape out of each potential basin, but attempted transitions are filtered to allow only those that would have occurred at the normal temperature. The characteristics of the methods are compared.
Nonlinear Response of Solids and Molecules in Intense Infrared Radiation
Authors: Pantelides Sokrates T. , Winkler Roland , Ferconil Maurizio , Alvarez J. J. Vicente
Published: April 2011

Infrared radiation, which couples with vibrations in molecules and solids, has long been considered as a promising way to selectively control chemical reactions, materials processing, and biomedical applications. The expectation is that particular frequencies couple to specific atomic motions, in contrast to heat which imparts energy indiscriminately. The promise has yet to be fulfilled, but recent successes and the development of new lasers have rekindled interest. We report calculations of the dynamics of solids and molecules under intense infrared radiation. In solids, using a model calculation, we find that narrowly-defined “windows of opportunity” exist for resonant enhancement of impurity diffusion at moderate intensities. In molecules, using a generalization of Car-Parrinello dynamics, we find that, for very intense fields, energy absorption at normal modes is not very efficient and that selective bond-breaking may occur at nonresonant frequencies by “concerted kicks”. Finally, time-dependent density functional theory is used to show that the effect of very intense infrared radiation on atoms and molecules is to produce very high harmonics as found experimentally.
Imposing Field Boundary Conditions in MD Simulation of Fluids: Optimal Particle Controller and Buffer Zone Feedback
Authors: Li Ju , Liao Dongyi , Yip Sidney
Published: April 2011

We formulate a method for imposing continuous field boundary conditions on an MD simulation with little or no disturbance to its dynamics. Our approach combines the previously developed thermodynamic field estimator, which extracts macroscopic fields from particle data, with a novel procedure, optimal in the sense of least disturbance, for imposing prescribed continuum field boundary conditions on the atomistic system. By means of feedback control and assuming molecular chaos in fluids, that disturbance can further be eliminated entirely, thus providing an exact solution for general steady-state fluid problems, where the desired fields at the real boundary is achieved by adjusting actions in a region separated from the real boundary by a buffer zone.
Electronic Structure Boundary Value Problems Without All of the Atoms
Authors: Johnson H. T. , Phillips R. , Freund L. B.
Published: April 2011

A mixed atomistic/continuum technique, in the spirit of the quasicontinuum method, is formulated and used to solve boundary value problems in strained semiconductor structures in which the mechanical fields and the local electronic structure are fully coupled. The technique is implemented by means of a standard structural mechanics finite element package. Within each element in the mesh, tight binding calculations are made for mechanical properties based on the local electronic structure. The finite element program calculates equilibrium mechanical fields based on this atom-istic constitutive information; the underlying electronic properties can then be extracted at the element level. The technique is demonstrated by examining several simple plane strain boundary value problems for coherently strained silicon.
Pressure Waves in Microscopic Simulations of Laser Ablation Leonid
Authors: Zhigilei V. , Garrison Barbara J.
Published: April 2011

Laser ablation of organic solids is a complex collective phenomenon that includes processes occurring at different length and time scales. A mesoscopic breathing sphere model developed recently for molecular dynamics simulation of laser ablation and damage of organic solids has significantly expanded the length-scale (up to hundreds of nanometers) and the time-scale (up to nanoseconds) of the simulations. The laser induced buildup of a high pressure within the absorbing volume and generation of the pressure waves propagating from the absorption region poses an additional challenge for molecular-level simulation. A new dynamic boundary condition is developed to minimize the effects of the reflection of the wave from the boundary of the computational cell. The boundary condition accounts for the laser induced pressure wave propagation as well as the direct laser energy deposition in the boundary region.
Atomic Scale Simulation of Cross Slip and Screw Dislocation Dipole Annihilation
Authors: Rasmussen Torben
Published: April 2011

Atomistic simulations are used to study cross slip of a single screw dislocation as well as screw dislocation dipole annihilation in Cu. A configuration space path techniquex is applied to determine, without presumptions about the saddle point, the minimum energy path of transition for cross slip. The cross slip process is that proposed by Friedel and Escaig, and the energy of the in-plane constriction initiating cross slip is determined. A minimum stable dipole height much smaller than previously inferred from experimental studies is found. Relaxed screw dislocation dipoles adopt a skew configuration due to the anisotropy of Cu. The path technique is applied to investigate annihilation of stable screw dislocation dipoles, and the energy barrier for annihilation as a function of dipole height is determined for both homogeneous and heterogeneous cross slip leading to the annihilation. The results might be used as quantitative input into meso-/macro-scopical modelling approaches which rely on parameters deduced from either simulation or experiment.
First Attempt to Develop A Material Model Describing Single Crystal Creep Behavior
Authors: Brehmn Holger , Glatzel Uwe
Published: April 2011

A material model is proposed which describes single crystal creep behavior by evolution equations for dislocation densities on individual slip systems. An interaction matrix determines the influence from one glide system to the other. Assuming a face centered cubic crystal, allowing deformation on octahedral glide planes and cube glide planes with a Burgers vector of the type a/2 <110>, nine independent parameters of the interaction matrix can be distinguished. A parameter check of the nine independent parameters has been carried out, showing the influence of parameters on specific orientations of the load axis. If one assumes dislocation interaction of a glide system only with itself a smooth behavior is predicted with a maximum creep rate for [001] orientation, followed by [011] and [111]. If a strong interaction is assumed, the orientation dependent creep behavior is not at all smooth, instead it shows a sharp drop in creep rates mainly in symmetric positions of the standard orientation triangle. The orientations with highest creep rates are in this case those which favor single glide. Highly symmetric orientations, such as [001], [011] and [111] have strongly decreased stationary creep rates.
Coupled Meso-Macro Simulations of Plasticity: Validation Tests
Authors: Lemarchand C. , Devincre B. , Kubin L.P. , Chaboche J.L.
Published: April 2011

A new numerical model has been developed in order to study the plastic deformation of mesoscopic crystalline samples under complex boundary conditions. The model is based on the coupling of two types of simulations, a dislocation dynamics and a finite element code. The former accounts for the dislocation-based plastic properties of the material, thus replacing the usual constitutive form, while the latter treats the boundary value problem and cares of the mechanical equilibrium. In order to test the hybrid simulation and examine its accuracy, the self-stress fields of straight dislocations have been computed in a single crystal of finite size and compared with the predictions of the isotropic elasticity theory. The excellent agreement obtained emphasizes the enormous potential of such hybrid methods for a rigorous treatment of meso-macro problems in plasticity.
Thermal Wave Analysis: A Tool for Non-Invasive Testing in Ion Beam Synthesis of Wide Band Gap Materials
Authors: Teichert G. , Schleicher L. , Knedlik Ch. , Voelskov M. , Skorupa W. , Yankov R.A. , Pezoldt J.
Published: April 2011

Photothermal methods provide a valuable complement to the destructive measurement techniques for the detection of the optimal process conditions in ion beam synthesis of wide band gap semiconductor compounds. In addition to their nondestructive and non contact qualities, they are highly sensitive to changes of thermophysical properties due to structural changes. Analyses have been carried out with (SiC)l-x(AIN)x compounds, formed by ion beam synthesis.
Instability of Nanocavities in Disordered and Amorphous Silicon Under Ion Irradiation
Authors: Zhu X. , Williams J.S. , McCallum J.C.
Published: April 2011

It has recently been shown that a band of nanocavities in crystalline silicon is eliminated during amorphization of the silicon surrounding this band [4]. In this study, we examine the effect of irradiation dose on nanocavity stability. Gettering of Au is used as a detector for open volume defects following annealing of irradiated samples. Rutherford backscattering and channeling and cross-sectional transmission electron microscopy have been used to analyse the samples. Cavities are only completely removed when the region surrounding the cavities is totally amorphized up to the surface. Partial amorphization leaves residual open volume defects.
Effect of Electron Beam Irradiation-Enhanced Diffusion on A W-Aluminum Oxide-Ti/Cu Multilayer
Authors: Ohta S. , Takahashi H.
Published: April 2011

Tungsten, aluminum oxide and Ti films were deposited onto a Cu substrate by means of a rf magnetron sputtering method. TEM thin foils for cross-sectional irradiation were prepared using a focused ion beam (FIB). Electron irradiation was carried out in a Hitachi H- 1300 electron microscope at I MV. The specimen temperatures during irradiation were 300, 473, 623 and 703K. The phases of W, aluminum oxide and Ti films were identified from selected area diffraction patterns as bcc, amorphous and hcp phases, respectively. The phases of W and Ti did not change due to irradiation. However, the amorphous aluminum oxide phase transformed to crystalline γ-Aluminum oxide. Electron irradiation caused no change in the composition of the interface of W/aluminum oxide, but diffusion was enhanced on the interfaces of aluminum oxide/Ti and Ti/Cu.
Radiation-Induced Defects in Electron and Proton Irradiated ZnS
Authors: Brunner S. , Puff W. , Mascher P. , Balogh A.G.
Published: April 2011

In this contribution, we present a study aimed at investigating the microstructural changes of ZnS single crystals and CVD (chemical vapour deposition) grown crystals after electron and proton irradiation. Positron lifetime and Doppler-broadening measurements were performed to investigate the stability of the radiation induced defects and possible clustering mechanisms during isochronal annealing. After electron as well as proton irradiation the significant changes in the annihilation characteristics are indications of radiation induced open-volume-type defects. It is found that electron and proton irradiation causes different changes in the positron annihilation characteristics. After electron irradiation a significant defect component is observed which can be attributed to the annihilation in monovacancies. During isochronal annealing agglomerations to divacancy-type defects take place. Proton irradiation reveals a significantly different defect structure. Isochronal annealing causes agglomerations to larger defect complexes. The observed annealing stages are indications of the annealing of variously sized vacancy complexes.