bluuebunny/crossref_metadata_2025 huggingface dataset:
Excitons bound to structural defects in GaN
Authors: Reshchikov M. A. , Huang D. , Yun F. , Morkoç H. , Molnar R. J. , Litton C. W.
Published: April 2011

We analyzed the photoluminescence (PL) spectra of undoped GaN films grown by molecular beam epitaxy on sapphire substrates. While the PL spectra from high-quality samples contain free and bound exciton peaks only, the spectra from some samples involve sharp unidentified peaks in the energy range of 3.0 – 3.45 eV, specifically at 3.21, 3.32, 3.36, and 3.42 eV. We attribute these peaks to excitons bound to defects because of the linear and sometimes superlinear increase in their intensity with excitation density without saturation up to 100 W/cm2. With increasing temperature these peaks quench in a well-known fashion similar to that for excitons. In order to relate the observed peaks to the structural defects, we etched selected samples in hot H3PO4 acid or, alternatively, with photo-electrochemical (PEC) etching at room temperature in the presence of UV-illumination in a dilute KOH solution. In the former case the dislocations were etched leaving etched pits on the surface, while in the latter case the dislocations remained unetched due to a deficit of photogenerated holes at dislocation sites. We found that the 3.42 eV peak disappeared after both hot wet and PEC etching suggesting that the associated defect is at the GaN surface. Peaks at 3.21 and 3.36 eV could be enhanced greatly by PEC etching, which were correlated to bulk dislocations.
Optical properties of cubic AlGaN
Authors: Fanget Stéphane , Bru-Chevallier Catherine , Guillot Gérard , Martinez-Guerrero Esteban , Jalabert Denis , Daudin Bruno , Mariette Henri , Dang Le Si , Ferro Gabriel , Monteil Yves
Published: April 2011

In this work we report optical characterization on several cubic c-AlGaN layers grown by MBE on SiC on Si pseudo-substrates, with different aluminum concentrations ranging from 0 to 70 %. Excitation power evolution of AlGaN photoluminescence (PL) spectra as well as reflectivity spectra allow to attribute PL peak to band gap recombination. PL energy dependence versus aluminum concentration is given. Reflectivity investigations are performed in the energy range between 1.5 eV and 4 eV on the samples. Theoretical calculations of multilayered structure reflectivity are fitted to experimental results, allowing an accurate determination of refractive index evolution versus Al concentration. From this analysis, qualitative information about interface roughness at AlGaN/SiC is also be derived.
Thermal Desorption of Deuterium from GaN(0001)
Authors: Yang Y. , Lee J. , Thoms B. D. , Koleske D. D. , Henry R. L.
Published: April 2011

The recombinative desorption of deuterium from GaN(0001) has been investigated using temperature programmed desorption (TPD) with variable heating rates. With a heating rate of 1°C/s, molecular deuterium desorption peaks at 410 C in agreement with related previous work. However, the shape of the curve indicates a secondary peak at around 280°C which is merged into the lower temperature shoulder of the dominant peak. By changing linear heating rate from 0.05 C/s to 8°C/s desorption peak temperatures from 380°C to 570°C were observed. Fitting to a pseudo-first-order desorption model results in a hydrogen desorption barrier, Ed, from surfaceof 1.1eV and a pre exponential factor, n, of 2 x 106s-1. Both are below expected values and are assumed to be due to a variation of desorption barrier heights. If a typical pre-exponential factor of 1 x 1013s-1is assumed, reanalysis of the desorption data produce a desorption barrier of 2.0 eV, in agreement with the existence of the surface adsorption barrier at room temperature.
Characteristics of deep traps in freestanding GaN
Authors: Fang Z-Q. , Look D.C. , Visconti P. , Lu C. , Wang D. , Morkoç H. , Park S.S. , Lee K.Y.
Published: April 2011

Deep traps in a 300-m m-thick freestanding GaN sample were characterized by deep level transient spectroscopy (DLTS), using Schottky barrier diodes (SBDs) fabricated on the Ga polarity surface. Most of the SBDs show nearly ideal current-voltage characteristics, with both forward and reverse currents controlled by the thermionic emission mechanism. Five common traps, which include A1 (1.0 eV), A (0.66 eV), B (0.59 eV), C (0.35 eV), and D (0.25 eV), can be consistently observed in all SBDs. Two of them, A1 and C, are related to surface damage. Surprisingly, some new traps can be found in the DLTS spectra of some SBDs if higher reverse biases are used in the measurements. However, they cannot be fitted by DLTS simulations, and are likely associated with parasitic capacitance somewhere in the cryostat.
Determination and Critical Assessment of the Optical Properties of Common Substrate Materials Used in III-V Nitride Heterostructures with Vacuum Ultraviolet Spectroscopic Ellipsometry
Authors: Edwards N.V. , Lindquist O.P.A. , Madsen L.D. , Zollner S. , Järrehdahl K. , Cobet C. , Peters S. , Esser N. , Konkar A. , Aspnes D. E.
Published: April 2011

As a first step toward enabling the in-line metrology of III-V nitride heterostructure and materials, we present the optical constants of the two common substrate materials over an unprecendented spectral range. Vacuum Ultraviolet spectroscopic ellipsometry (VUVSE) was used to obtain the optical constants for Al2O3 and the ordinary and extra-ordinary component of the dielectric function for both 4H- and 6H-SiC. The results are discussed in the context of anisotropy, polytypism, bandstructure, optical transitions, and preparation/characterization of abrupt surfaces, where appropriate.
Intersubband optical absorption and electron relaxation rates in GaN/AlGaN coupled double quantum wells
Authors: Heber J. D. , Gmachl C. , Ng H. M. , Cho A. Y. , Chu S.-N. G.
Published: April 2011

Recent interest in intersubband (IS) transitions in semiconductor heterostructures with large band offset has been fueled by attempts to extend the wavelength range of IS-based optical devices to the fiber-optics wavelength range around ~ 1.55 μm. GaN/AlGaN-based heterostructures are of particular interest due to their large effective electron mass and large longitudinal optical phonon energy. Both are essential to achieve ultrafast electron relaxation at large transition energies. IS absorption in GaN/AlGaN single and coupled double quantum wells (DQWs) has been measured. The samples were grown by molecular beam epitaxy on sapphire substrate and with a large (0.65 or 0.9) AlN-mole fraction in the barriers. Peak absorption wavelengths as short as 1.35 μm and 1.52 μm were measured for a symmetric DQW of 12 Å wide wells coupled by a 10 Å wide barrier, which also showed evidence of excited-state anti-crossing. As expected, asymmetric DQWs displayed no such anti-crossing, and the ground-state anti-crossing energies were found to be much smaller – as a result of the comparatively large effective electron mass – than the energy broadening of individual transitions. The asymmetric DQWs displayed peak absorption wavelengths between 1.5 and 2.9 μm. The electron relaxation time, attributed to longitudinal optical phonon scattering has been measured by pump-probe technique as 240 fs for a coupled DQW sample.
Polarization Effects in the Photoluminescence of AlGaN and AlInGaN Based Quantum Well Structures
Authors: Ryu Mee-Yi , Kuokstis E. , Chen C. Q. , Zhang J. P. , Yang J. W. , Simin G. , Khan M. Asif
Published: April 2011

We have analyzed photoluminescence (PL) dynamics of GaN/AlGaN and AlInGaN/AlInGaN multiple quantum wells (MQWs) with different well and barrier widths. The quantum structures were grown by conventional metalorganic chemical vapor deposition and novel pulsed atomic layer epitaxy. In both types of MQWs a blueshift followed by a redshift of the PL peak position were observed with increasing excitation power, which we attributed to the screening of built-in electric fields and band gap renormalization, respectively. In bulk AlInGaN material or in MQWs with thin well widths the blueshift was not observed. This means that the incorporation of In into AlInGaN material in the amount required to fabricate smooth layers with strong emission at 330-350 nm does not create significant concentration of band-tail states. We have also evaluated the internal field in the MQW structures by comparing the experimental PL data to the simulations based on triangular quantum well model resulting from the polarization fields.
Hybrid active polymer/silica microstructured photonic crystal optical fibers
Authors: Eggleton B. J. , Kerbage C.
Published: April 2011

We review several applications of microstructured photonic crystal optical fibers that incorporate active materials infused into the air-holes. The tunable optical characteristics of the materials combined with the unique structure of the fiber enable a number of functionalities including reconfigurability and tunability for various fiber device applications. We describe a brief characterization of the modes and discuss the experimental results of the fibers.
Simulations of Realizable Photonic Bandgap Structures with High Refractive Contrast
Authors: Gersten Bonnie , Synowczynski Jennifer
Published: April 2011

The transfer matrix method (TMM) software (Translight, A. Reynolds [1]) was used to evaluate the photonic band gap (PBG) properties of the periodic arrangement of high permittivity ferroelectric composite (40 wt% Ba0.45Sr0.55TiO3 /60 wt% MgO composite, εR= 80, tanδ = 0.0041 at 10 GHz) in air (or Styrofoam, εR~ 1) matrix compared to a lower permittivity material (Al2O3, εR= 11.54, tanδ = 0.00003 at 10 GHz) in air. The periodic structures investigated included a one-dimensional (1D) stack and a three-dimensional (3D) face centered cubic (FCC) opal structure. The transmission spectrum was calculated for the normalized frequency for all incident angles for each structure. The results show that the bandgaps frequency increased and the bandgap width increased with increased permittivity. The effects of orientation of defects in the opal crystal were investigated. It was found by introducing defects propagation bands were introduced. It was concluded that a full PBG is possible with the high permittivity material.
Microscopic Phenomenology of Plastic Deformation in Polymer-Metal Laminates.
Authors: Vellinga W.P. , Rastogi R. , Meijer H.E.H.
Published: April 2011

Microscopic studies of the plastic deformation of a simple polymer-metal laminate (PET-steel) are outlined. Representative examples of deformation and failure modes are illustrated with qualitative results from birefringence microscopy. The relation between bulk PET mechanical behaviour and laminate properties is discussed.
Assessment of New Relation for the Elastic Compliance of a Film-Substrate System
Authors: Rar Andrei , Song H. , Pharr G.M.
Published: April 2011

A new closed-form relation for the elastic compliance of a film-substrate system indented by a flat cylindrical punch is presented. The relation is an extension of the work of Gao at al. [1] modified to increase the applicable range of modulus mismatch between the film and substrate. The accuracy of the relation is assessed by comparison to finite element simulations, other approximate numerical solutions, and nanoindentation experiments performed on a fluorinated silicate glass (FSG) film on a silicon substrate. The benefits and limitations of the relation are discussed.
Stressed States and Self-Organized Structuring of W/C Multilayers
Authors: Meyer D.C. , Klingner A. , Leisegang T. , Holz Th. , Dietsch R. , Paufler P.
Published: April 2011

Characterization and quantitative analysis of stressed states of a series of W/C multilayers (10-40 periods prepared by pulsed laser deposition on Si (111) substrates of different thickness) were carried out by means of X-ray reflectometry, wide angle diffractometry and a novel laser mapping device. As the W/C multilayers were dedicated to technical applications as X-ray optics and subjected to optimization of stacking parameters (thickness and number of layers) for a long term (mechanical) stability also further investigations will be discussed. Comparison of wafer distortion as evaluated by laser scanning and strain of the W layer as deduced from X-ray diffraction let us conclude that W layers are under compressive and C layers under tensile stress. The investigation of the thermally stimulated relaxation behavior of the multilayers provided a confirmation of these results. Additional information could be obtained by comparative relaxation experiments under external mechanical constraints. Furthermore, we report on a self-organized process of structuring of the multilayers under investigation, which might be of interest also from a technical point of view. The entire surface area (diameter 2') could be converted from the smooth (as-deposited) to a structured (relaxed) state stable at room temperature. Investigations using optical and atomic force microscopy showed that the topology of the surface consists of a mountain range where the valleys are on the level of the as-deposited non-debonded surface and that long wrinkled ridges of about the same height run along arbitrary directions.
Highly Oriented NiTiCu Shape Memory Thin Films Grown by Molecular Beam Epitaxy
Authors: Hassdorf R. , Feydt J. , Pascal R. , Thienhaus S. , Boese M. , Sterzl T. , Winzek B. , Moske M.
Published: April 2011

We present a study demonstrating the capability for controlled shape memory thin film growth using molecular beam epitaxy. Here, NiTiCu alloy films were grown which are known to exhibit the martensitic transformation well above room temperature. Remarkably, the microstructure of these films was found to be very different compared to conventionally sputtered polycrystalline films: here, the crystallites are highly oriented within 3° along the film plane normal. Moreover, a splitting of the martensite orientation is detected indicating the selection of only two specific martensite variants. Mechanical stress measurements reveal a high ratio of recoverable stress even for films below 500 nm thickness. These results open up the possibility for tailoring microstructure and crystallographic orientation of shape memory thin films and thus suggest promising characteristics, especially in regard to their superelastic behavior.
Depth Profile of Structure, Strain, and Composition in an Annealed Al-Cu-Fe-Cr Quasicrystalline Film
Authors: Daniels M.J. , King D. , Zabinski J.S. , Rek Z.U. , Bilello J.C.
Published: April 2011

Quasicrystalline films were formed by RF sputtering from a powder composite target onto Inconel substrates, which produces a polymorphic nanoquasicrystalline grain structure, ~2.5 - 10 nm. Subsequent annealing at 500°C for 4 hours, at base pressures of below 5*10-5 Torr, and with Ar flow to 5 - 10 mT, fully develops the quasicrystalline structure with decagonal phase predominating, except near the termination surface. Analysis by XPS indicated extensive oxygen incorporation and limited aluminum enrichment at the termination surface. These results are correlated with structure and strain analysis via synchrotron grazing incidence x-ray scattering (GIXS). By varying the incident angle, hence the x-ray penetration depth, the evolution of an amorphous and crystalline crystalline secondary phases at the surface of the film has been detected. Residual strain analysis shows that this second phase induces a compressive residual strain of 0.10% as measured from the displacement of the major quasicrystalline peaks in the surface layers of the film.
Fundamental Criteria for the Propagation of Telephone Cord Buckles Beneath DLC Films on Glass Substrates
Authors: Moon Myoung-Woon , Lee Kyang-Ryel , Chung Jin-Won , Oh And Kyu Hwan
Published: April 2011

The topology of telephone cord buckles that form beneath compressed diamond-like carbon films (DLC) on glass substrates has been characterized with Atomic Force Microscopy (AFM) and with the Focused Ion Beam (FIB). Using AFM with 2nm resolution, the wavelength and amplitude of the buckles and their profiles have been measured. It has been found that, within each wavelength, the profile has symmetric and asymmetric segments. These changes have been related to differences in local mode mixity around the periphery of each repeat unit along the buckle, resulting in a fundamental rationale for the factors governing the wavelength. Sections made through various segments of the buckle by using the FIB imaging system result in local changes in the shape and size of the buckles that provide further insight into the buckle propagation criterion.
Accurate Determination of the Elastic properties of Near Surface Regions and Thin Films Using Nanoindentation and Acoustic Microscopy
Authors: Bamber Matthew , Mann Adrian , Derby Brian
Published: April 2011

Nanoindentation has been successfully used as a mechanical properties microprobe to characterise the elastic properties of materials. However, in an isotropic material it is not possible to measure the two independent elastic constants by nanoindentation. Normally, a value of Young's modulus is determined using an assumed value for Poisson's ratio. It is also possible to use the acoustic microscope in its z-contrast mode to measure the elastic constants of a surface. This too produces a composite measurement of the elastic properties, which can be represented in terms of Young's modulus and Poisson's ratio. By using both techniques on the same sample area, it is possible to make two independent measurements of the elastic properties and thus determine both Young's modulus and Poisson's ratio. This method has been used on well-characterised bulk materials, e.g. silica glass, to demonstrate that it produces consistent results. It has also been uused to characterise thin films of TiN/NbN multilayers. These results show that, although for thin films there is a need to improve the analysis of the mechanics, the combination of nanoindentation and acoustic microscopy shows promise.
Tribological Behavior of Nanocomposite Diamondlike Carbon-Aluminum Films
Authors: Liu E. , Gao J.X. , Zeng A.P. , Tay B.K. , Shi X.
Published: April 2011

Tetrahedral amorphous carbon (ta-C) contains a large percentage of sp3 carbon bonding. However, high internal stresses develop in ta-C films with a high sp3 content, which limits their applications for wear protection. In this study, Al containing ta-C films were produced using Filtered cathodic vacuum arc (FCVA) technique. The structure of films was studied using Micro-Raman spectroscopy in terms of aluminum content. The pure carbon ta-C films generally contained a relatively high residual compressive stress, which was related to its sp3 content, amorphous structure and preparation conditions. For the Al containing ta-C films, the stress reduction is significant with increase of aluminum content in the film. A decrease of the mechanical properties of the ta-C:Al nanocomposite films was noticed with the decrease of the internal stress in the films. The tribological behavior of the films was measured using ball-on-disk tribometer. The wear rate and friction coefficient were determined correspondingly. In the ball-on-disk testing, different loads are applied to the sapphire counterbody. All the tests were performed in ambient air (RH50%) and at room temperature (22°C). It was noted that the friction coefficient of ta-C:Al films increased at the beginning of the testing before reaching a peak value of about 0.25. After the peak, the friction coefficient dropped until reaching a steady state value. The original surface roughness of the counterfaces, surface smoothening due to successive wear, and wear debris produced during the testing are all responsible for the tribological behavior of ta-C:Al films.
Spherical Indentation Testing of Polymers at Various Temperatures
Authors: Jardret Vincent D. , Morel Pierre , Conté Nicolas
Published: April 2011

Contact mechanics for indentation testing with spherical indenter is very attractive. Numerous projects have established equations to define strain and stress distribution in order to obtain stress-strain relationship from a single indentation experiment. Also a large number of studies have focused on metallic materials with the objective of estimating the yield point.

The subject of this work is to analyze the behavior of various polymeric materials during spherical indentation testing at various temperature in order to observe the relationship between the indentation behavior and compression stress-strain behavior of the same materials as a function of temperature. Thermal effects on the indentation data are used to understand the actual effects of the mechanical properties on the indentation behavior. In addition to the load, displacement, and frequency specific stiffness information, topographic analysis of the residual indentation print is used to accurately estimate the contact area, therefore, validate the indentation models for contact depth calculations using spherical indentation. Results presented in this article include spherical indentation data obtained on PMMA and Polycarbonate over a range of temperature from 5°C to l00°C.
An Analytical Model for Intrinsic Residual Stress Effect on Out-of-Plane Deflection in Chemical-Vapor-Deposited Free-Standing Thick Film
Authors: Jeong Jeung-hyun , Baik Young-Joon , Kwon Dongil
Published: April 2011

The effect of residual stress on the out-of-plane deflection in a free-standing thick diamond films was investigated theoretically and experimentally. The deflection is believed to be caused by the variation in residual stress with film thickness. Key idea of this study is that the stress variation may be produced by gradually increasing substrate deformation resulting from the layer-by-layer deposition of the film. The layer-by-layer deposition was modeled by using infinitesimal plate-bending theory, considering the two deformation modes of contraction or expansion and bending. To verify the suggested model, several hundred micron thick diamond films were fabricated on Si, Mo and W substrates of varying thicknesses by microwave plasma assisted chemical vapor deposition. The model's predictions on bowing, based on intrinsic stress value measured by the curvature method, were in good agreement with the experimentally measured curvature of the as-released films. Finally, it is concluded that the bowing of CVD thick films depends on the intrinsic stress variation of the film associated with gradual increase in substrate deformation.
Thin Film Fracture During Nanoindentation of Hard Film – Soft Substrate Systems
Authors: Pang M. , Weaver K.D. , Bahr D.F.
Published: April 2011

Nanoindentation testing of hard film – soft substrate systems can exhibit permanent deformation prior to a yield excursion, indicating that the occurrence of this sudden discontinuity is predominantly controlled by the hard film cracking rather than dislocation nucleation and multiplication. In a previous paper, a model was developed to predict the mechanical response prior to hard film fracture. In the current study a this model, which superimposes large deflection of the hard film and plastic deformation of the substrate the model, is further refined by testing a variety of materials with different film formation conditions. The tested materials include anodic titanium oxide on titanium, thermal aluminum oxide on aluminum and sputtered tungsten films on aluminum. The film fracture strength of titanium oxides on titanium is estimated as 15 GPa and that of aluminum oxides on aluminum is around 10 GPa. In the case of vacuum sputtered tungsten film on aluminum, the tungsten layer is likely plastically deformed. The strain at film fracture is roughly estimated to be 3.4%.
Adsorption-Induced Failure Modes of Thin-Film Resonators
Authors: Kazinczi R. , Mollinger J.R. , Bossche A.
Published: April 2011

The mechanical and resonant properties of thin film resonators are influenced by the surrounding environment. Adsorption- and surface oxidation-induced stiffening effect was observed on silicon nitride and silicon carbide cantilever beams. The resonance frequency increased logarithmically in time upon exposure to ambient air. The variations of surface stress and spring constant of the SiNx cantilever beam were calculated. Further oxynitride formation on the surface increased the stability of the resonators. The shock response of the structures was studied in various environments. The resonance frequency abruptly dropped due to cracking of the absorbed surface layer, than recovered logarithmically. The initial drop and the recovery rate is environment dependant. Humidity increased, while argon and nitrogen rich environments mitigated the degrading effects. The SiCx is more inert to the environmental effects and proved to be a promising candidate as structural material in resonant MEMS devices.
Low Temperature Silicon Dioxide Thin Films Deposited Using Tetramethylsilane for Stress Control and Coverage Applications
Authors: Lin Xin , Fonash Stephen J.
Published: April 2011

Low temperature silicon dioxide thin films have been prepared by plasma-enhanced chemical vapor deposition (PECVD) using tetramethylsilane (TMS) as the silicon precursor at 100- 200°C in the pressure range of 2- 8 Torr. PECVD TMS oxide thin films deposited at these temperatures and pressures exhibit adjustable stress. The type of stress, including tensile stress, zero stress, and compressive stress, as well as the stress level can be tailored as desired by changing the deposition conditions and film thickness. In addition, the conformality of PECVD TMS oxide thin films varies significantly with the deposition conditions. It improves when the deposition pressure is raised and the substrate temperature is reduced. The mechanisms for the variations of stress and conformality with respect to deposition conditions are discussed in this study. The adjustable stress and conformality of the PECVD TMS oxide make it a promising material for many low temperature applications, such as inter-level dielectric, micro-electro-mechanical systems (MEMs), microfabrication, and large area electronics.
Self organized array of quantum nanostructures via a strain induced morphological instability
Authors: Montiel David , Müller Judith , Poiré Eugenia Corvera
Published: April 2011

Motivated by the work of Li et al. [1], we have studied the strain induced morphological instability at the submonolayer coverage stage of heteroepitaxial growth on a vicinal substrate with regularly spaced steps. We have performed a linear stability analysis and determined for which conditions of coverage a flat front is unstable and for which conditions it is stable. For low coverages the instability will cause the front to break in an array of islands. Assuming that the fastest growing mode of the instability determines the properties of the array, we make an estimation of the islands sizes and aspect ratios as well as an estimation of the separation length between islands of the array formed when the dominant mechanism for transport of matter is diffusion of particles along the growing front. These estimations are given as functions of the terrace width and coverage. Since these ones are experimentally controllable parameters, our results could be used to tailor the spontaneous formation of quantum nanostructures.
Lateral Composition Modulation in Mixed Anion Short Period Superlattices
Authors: Dorin C. , Wauchope C. , Millunchick J. Mirecki
Published: April 2011

In this work, we demonstrate for the first time lateral composition modulation in GaAs/GaSb short period superlattices (SPS). Several different structures were investigated in order to study the role of Group V overpressure on the final composition and uniformity of the structure. Cross sectional transmission electron diffraction and xray diffraction reciprocal space maps reveal that the film is indeed phase separated. The composition modulation observed in these films is a result of the SPS growth and not due to spinodal decomposition, as evidenced by the fact that an alloy grown at the same conditions results in a homogeneous layer.
Strain Relaxation in SiGe Thin Films Studied by Low-Energy Electron Microscopy
Authors: Woll A.R. , Moran P. , Rehder E.M. , Yang B. , Kuech T.F. , Lagally M.G.
Published: April 2011

We demonstrate the use of low-energy electron microscopy (LEEM) as a tool for studying dis-location formation in low-Ge-content SiGe films on Si(001) and silicon-on-insulator. Compared to TEM, sample preparation for LEEM consists only of conventional surface cleaning. Yet, because of its sensitivity to local variations in surface strain on Si(001), LEEM can detect dislocations at the earliest stages of strain relaxation. In identically prepared SiGe films, the typical dislocation extends over the entire viewable region of several hundred microns in SiGe/Si, but is less than 100 microns in SiGe/SOI. In addition, dislocation cross-slip and threading segments are common in SiGe/SOI, but virtually non-existent in SiGe/Si. We have also observed dislocation formation in real-time during high temperature annealing. Preliminary results appear to demonstrate dislocation multiplication and blocking at a perpendicular glide plane. The applicability of LEEM to strain relaxation in other Si-based systems will be discussed.
Nucleation and dynamics of dislocations in mismatched heterostructures
Authors: Patriarca Marco , Kuronen Antti , Kaski Kimmo
Published: April 2011

In this paper we have investigated, through computer simulations, dislocation nucleation and dislocation dynamics in a heterostructure system with the lattice-mismatch interface, i.e. a system with internal strain. In particular, we have studied the dependence of the nucleation thresholds on the basic parameters of the crystals, such as the amount of mismatch and the system temperature. These studies have been carried out by using the simulation code with a graphical user interface developed at our laboratory. This on-line simulation system produces a real time interactive visualization of the 3-D Molecular Dynamics model. Furthermore, it detects the presence of dislocations and tracks them by an algorithm based on potential energy mapping.
X-ray Analysis of Si/Ge/Si(001) Heterolayer Structures Grown by Surfactant Mediated Epitaxy.
Authors: Tinkham Brad P. , Goodner Duane M. , Walko Donald A. , Bedzyk Michael J.
Published: April 2011

X-ray diffraction and x-ray standing waves (XSW) have been used to investigate the quality of epitaxial ultra-thin Ge films grown on Si(001) with and without Te as a surfactant. The efficacy of Te as a surfactant in this application has been debated. We measured samples between 1 and 10 ML in thickness and our results clearly indicate that Ge films grown with Te are superior to those grown without Te. The coherent positions and coherent fract ons determined from XSW analysis agree well with those predicted by linear elasticity theory for Ge/Si(001). Furthermore, grazing incidence diffraction measurements (GIXD) suggests that 9 ML Ge grown on Si(001) with Te is strained in-plane while the same film grown without Te is relaxed.
A Multi-Component Model for the Growth and Relaxation of Epitaxial Thin Films
Authors: Gill Simon P.A. , Long Fei , Cocks Alan C.F.
Published: April 2011

A variational approach to the modelling of growth and relaxation in elastically strained epitaxial thin films and islands is presented. The continuum model is derived for a twocomponent material system evolving via the kinetic mechanisms of evaporation/condensation and surface and lattice diffusion under the influence of elastic mismatch strain, surface tension and combinational phase energy. Phenomena such as surface segregation and interdiffusion between the film and the substrate are incorporated into the model. This approach is illustrated by a 2D simulation of the onset of instabilities during the growth of epitaxial thin films.
Quantum-dot Cellular Automata
Authors: Snider Gregory L. , Orlov Alexei O. , Kummamuru Ravi K. , Ramasubramaniam Rajagopal , Amlani Islamshah , Bernstein Gary H. , Lent Craig S.
Published: April 2011

An overview is given of the quantum-dot cellular automata (QCA) architecture, along with a summary of experimental demonstrations of QCA devices. QCA is a transistorless computation paradigm that can provide a solution to such challenging issues as device and power density. The basic building blocks of the QCA architecture, such as AND, OR gates and clocked cells have been demonstrated and will be presented here. The quantum dots used in the experiments to date are metal islands that are coupled by capacitors and tunnel junctions, and devices operate only at very low temperatures. For QCA to be used in practical devices, the operating temperature must be raised, and issues such as background charge must be addressed. An introduction will be given to these issues and possible solutions.
Processing and Characterization of Al2O3 and Al2O3 / Cu Alloy Composites
Authors: McCuiston R.C. , Danforth S.C. , Niesz D.E.
Published: April 2011

Fused deposition of ceramics (FDC) has been used to fabricate macroscopically graded ceramic skeletons. These Al2O3 skeletons were subjected to binder removal and sintering treatments. The sintered skeletons were infiltrated with copper alloys. Both the skeletons and composites were characterized for micro and macrostructure using scanning electron microcopy. Potential applications for the macroscopically graded ceramic/metal composites focus primarily on impact.
Cell-by-Cell Construction of Living Tissue
Authors: Ringeisen Bradley R. , Kim Heungsoo , Young H. Daniel , Spargo Barry J. , Auyeung R.C.Y. , Wu Peter K.
Published: April 2011

This paper outlines investigations into a potentially revolutionary approach to tissue engineering. Tissue is a complex three-dimensional structure that contains many different biomaterials such as cells, proteins, and extracellular matrix molecules that are ordered in a very precise way to serve specific functions. In order to replicate such complex structure, it is necessary to have a tool that could deposit all these materials in an accurate and controlled fashion. Most methods to fabricate living three-dimensional structures involve techniques to engineer biocompatible and biodegradable scaffolding, which is then seeded with living cells to form tissue. This scaffolding gives the tissue needed support, but the resulting tissue inherently has no microscopic cellular structure because cells are injected into the scaffolding where they adhere at random. We have developed a novel technique that actually engineers tissue, not scaffolding, that includes the mesoscopic cellular structure inherent in naturaltissues. This approach uses a laser-based rapid prototyping system known as matrix assisted pulsed laser evaporation direct write (MAPLE DW) to construct living tissue cell-by-cell. This manuscript details our efforts to rapidly and reproducibly fabricate complex 2D and 3D tissue structures with MAPLE-DW by placing different cells and biomaterials accurately and adherently on the mesoscopic scale
Damage Detectability on Aluminum Alloy Panels Under Composite Patching by Various NDT Techniques
Authors: Moropoulou Antonia , Kouloumbi Niki , Marioli-Riga Zaira P. , Avdelidis Nicolas P. , Pantazopoulou Paraskevi
Published: April 2011

Composite patching method is widely applied for repair cases of metallic aircraft structures due to more efficient performance than conventional repairs. However, the detection of structures integrity under patch, during the service life of aircraft, by non-destructive means is considered of great importance. In the present study, different NDT techniques such as active infrared thermography, eddy currents and electrical impedance spectroscopy, were applied for the detection of simulated artificially introduced damages - notches, on the surface of aluminum aircraft skin panels, Al 2024-T3, under composite patching (carbon reinforced laminates). The detection sensitivity of each technique was investigated based on the relation between thickness of composite patch and specific parameters of each method aiming at the development of a reliable, for this purpose, quality inspection technique.
Improving GaAs Chip yield And Enhancing Reliability of GaAs Devices
Authors: Prasad Kanti
Published: April 2011

The purpose of this research is to improve GaAs yield and enhance the reliability of GaAs MMICs (Monolithic Microwave Integrated Circuits) by first understanding the physical mechanisms of GaAs, Ni,Au-Ge eutectic and Au alloying process. Ohmic ooze has been driving force for this research. Variety of innovative experiments has been designed, so that contact resistance may be guaranteed to be within the permissible range. This resulted into the development of analytical techniques to measure contact resistance to GaAs as a result of alloying process employing Ni,Au-Ge and Au.
Depth Dependence of Dopant Induced Features on The Si(100)2x1:H Surface and Its Application for Three Dimensional Dopant Profiling
Authors: Liu Lequn , Yu Jixin , Lyding Joseph W.
Published: April 2011

The lack of surface states within the band gap of the perfect Si(100)2x1:H surface opens the way to scanning tunneling microscopy studies of dopant atom sites in Si(100). In this paper, Boron and Arsenic induced features are studied by ultrahigh vacuum scanning tunneling microscopy. The values of their amplitudes naturally group such that several subsurface layers can be identified. This technique for producing atom-resolved three-dimensional maps of electrically active dopants in silicon may be a useful metric for characterizing dopant profiles in ultra-small electronic device structures.
Magnetic Force Microscopy Signatures of Defects in Current-carrying Lines
Authors: Yongsunthon Ruchirej , Williams Ellen D. , Stanishevsky Andrei , McCoy Jonathan , Pego Robert , Rous Philip J. , Peckerar Martin
Published: April 2011

The spatial variation of current density in lines with model void defects fabricated using Focused-ion Beam (FIB) milling has been investigated using Magnetic Force Microscopy (MFM). The model defects were designed to systematically simulate the natural void shapes that occur in electromigration failure of current-carrying lines. Inhomogenous current density around the defects manifests itself in the form of atypical asymmetry in the MFM signal near the defects. The extent of the asymmetry is greatly dependent upon the defect geometry. At current densities of 3-5×106 A/cm2, an asymmetry in the MFM signal is clearly visible around defects, such as a (1×1) μ,2 notch and a (1×9) μm2 45°-slanted slit, at the edge of a 10μm wide line. We present a survey of MFM images of various defect structures in current-carrying lines that perturb the homogenous current flow of a straight line.
Surface Roughness and Surface-Induced Resistivity of Thin Gold Films On Mica
Authors: Munoz Raúl C. , Kremer German , Moraga Luis , Vidal Guillermo , Arenas Claudio
Published: April 2011

Abstract. We report measurements of the surface topography of a 70 nm gold film deposited on mica preheated to 300 oC in UHV performed with a Scanning Tunneling Microscope (STM). From these measurements we determine the rms roughness amplitude and the lateral correlation length characterizing the average height-height autocorrelation function on a nanometric scale.We also report a method of analyzing thin film resistivity data that departs sharply from the traditional method of parameter fitting. This method allows the determination of the resistivity and mean free path characterizing the bulk from the measured thin film resistivity by means of a new iteration procedure, that uses as input data the roughness parameters experimentally determined with the STM and any of the available quantum transport theories, without adjustable parameters. We examine the resistivity data reported by Sambles et al. [Philos. Trans. R. Soc. London, Ser. A304, 365 (1982)] for gold films deposited on mica under similar substrate temperature and similar speed of evaporation. The remarkable outcome is that any of the quantum transport theories available, describe approximately both the temperature as well as the thickness dependence of the resistivity data without any adjustable parameter. Another surprise is that the parameters characterizing the bulk—assumed to be independent of thickness for many years—turn out to be thickness dependent.
Improvement of Characteristics in Highly Reliable thin Film Diode with Anodic Tantalum Pentoxide by Low Temperature Annealing Conditions
Authors: Lee Chan-Jae , Hong Sung-Jei , Park Sung-Kyu , Kim Yong-Hoon , Kwak Min-Gi , Kim Won-Keun , Han Jeong-In
Published: April 2011

In this study, the quality of thin film diode (TFD) as a switching device for active-matrix liquid-crystal-displays (AM-LCDs) was enhanced by low temperature annealing conditions with high reliability and good electrical properties. Device was composed with Ta as bottom electrode, anodic Ta2O5 as insulator layer and top electrode. Two types of material such as Ti and Cr were evaluated as a top electrode of the TFD device to optimize the symmetry of current-voltage characteristic curve, respectively. The annealing was done at low temperature conditions below 350°C. The low temperature annealing improved the TFD device with nearly perfect symmetry under high electric field.
The Structure and Electrical Properties of Polyaniline
Authors: Ou Runqing , Samuels Robert J. , Gerhardt Rosario A.
Published: April 2011

The anisotropic structure of EB polyaniline films cast from N, N'-Dimethylpropylene urea (DMPU) solution and stretched to different draw ratios was studied by infrared dichroism. The drawn films were doped with 1.0 M HCl and the electrical properties were studied by impedance spectroscopy. A new test fixture has been developed to measure the in-plane impedance of polymeric films. A Debye-like conductivity relaxation was observed for all the HCl-doped PANI samples. The conductivity along the stretch direction increases with molecular orientation. The in-plane conductivity is significantly higher than the through-plane conductivity.
Development of combinatorial PLD-STM system for the quick nano-fabrication and evaluation
Authors: Matsumoto Y. , Ohsawa T. , Takahashi R. , Koinuma H.
Published: April 2011

A combinatorial PLD-STM system has been developed for quick nano-fabrication and evaluation of epitaxial thin films and catalysts. Through a gate valve, a combinatorial nano-materials library fabricated in a PLD chamber can be transferred to a surface analysis chamber equipped with LEED and STM. The LEED pattern and atomically resolved STM image for systematically varied samples in the library are measured by locating one sample to another at the electron beam and STM probe position. In this way, we have quickly elucidated the initial growth process of SrO on SrTiO3(001) as well as surface structures of transition metal deposited TiO2 anatase. The combinatorial PLD-STM system has been verified to be very efficient technique for the exploration and optimization of nano-materials.
Co-doping Approach for β-type ZnO with Combinatorial Laser MBE Method
Authors: Tsukazaki A. , Saito H. , Sumiya M. , Tamura K. , Fuke S. , Koinuma H. , Kawasaki M.
Published: April 2011

We show here three different ways of combinatorial experiments for achieving precise control of dopant concentration in ZnO thin films. Alternating ablation of highly pure single crystal target and ceramics target doped with concentrated Ga dopant yielded in a systematic control of Ga concentration with keeping minimal contamination of undesired impurity. Secondary ion mass spectroscopy for a stack of several ZnO films grown at different temperatures under a constant exposure of N radical gave us a systematic calibration curve for the N concentration. With using a special heating method to give a controlled temperature gradient on a substrate, we demonstrate that continuous spread of N concentration can be built in a film on a substrate with keeping constant Ga concentration. Such systematic experiments taking into account the “combinatorial” concept enable us to make ZnO films with controlled dopant concentrations.
High throughput characterization of magnetic semiconductor thin films with a scanning SQUID microscope
Authors: Fan X. J. , Murakami M. , Takahasi R. , Koida T. , Matsumoto Y. , Hasegawa T. , Fukumura T. , Kawasaki M. , Ahmet P. , Chikyow T. , Koinuma H.
Published: April 2011

Magnetic properties of Co-doped rutile (Ti1-xCoxO2) film in combinatorial composition-spread form have been surveyed by means of a Scanning Superconducting-quantum-interference-device Microscope (SSM). As a consequence, we found magnetic domains in the spatial regions with x>0.05 without external field, giving strong evidence for ferromagnetism with finite spontaneous magnetization. The magnetic moment was monotonously increased with increasing doping level x from 0.05 to ∼ 0.13. On the other hand, it was almost unchanged for x > ∼ 0.13, suggesting that Co does not dissolve into rutile film beyond x ∼ 0.13. The SSM results on the rutile Ti0.95Co0.05O2 thin films with different thickness showed that the magnetic moment is proportional to film thickness, leading to a conclusion that the presently observed ferromagnetism does not result from Co or Co-based oxide particles on the film surface.
Development of Functional Materials by a Composition Spread Approach
Authors: Cremer R. , Neuschütz D.
Published: April 2011

In this paper, the deposition and characterization of laterally graded materials libraries (composition spread) is presented. The films have been deposited by reactive magnetron sputtering, using two or three metallic targets at a low angle to the substrate surface as well as a system of apertures. To illustrate the advantages of combinatorial approaches for the development of advanced materials, the multicomponent metastable hard coatings (Ti,Al)N and (Ti,Al,Si)N were investigated with respect to the relations between structure and composition on the one hand and the oxidation resistance of these coatings on the other.
Growth Temperature and Oxygen Ambient Dependency of SrTiO3/Si(100) InterfaceStructures
Authors: Ahmet Parhat , Koida Takashi , Yoshimoto Mamoru , Koinuma Hideomi , Chikyow Toyohiro
Published: April 2011

A systematical growth temperature and oxygen ambient dependency of SrTiO3/Si interface structures were investigated using a growth temperature gradient pulse laser deposition (PLD) system and cross sectional high resolution transmission electron microscopy (HRTEM). A SiO2 interfacial layer and an amorphized SrTiO3 layer were observed at the interface for the thin films grown on Si (100) at growth temperatures above 600°C. Our results show that at growth temperatures higher than 600°C, the formation of the amorphized SrTiO3 layer is strongly growth temperature and also oxygen partial pressure dependent.
Fabrication and characterization of in-situ grown epitaxial Ba1-xSrxTiO3 composition spreads
Authors: Chang K. S. , Aronova M. , Famodu O. , Hattrick-Simpers J. , Lofland S. E. , Takeuchi I. , Lu C. J. , Bendersky L. A. , Chang H.
Published: April 2011

We have used our combinatorial pulsed laser deposition system to in-situ fabricate epitaxial Ba1-xSrxTiO3 thin film composition spreads on (100) LaAlO3 substrates. Multimode quantitative microwave microscopy was used to perform dielectric characterization of the spreads at multiple microwave frequencies simultaneously. Systematic variation in dielectric properties as a function of composition is studied. The multi-mode measurements allow frequency dispersion studies. We observe strong composition-dependent dielectric dispersion in Ba1-xSrxTiO3.