bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

AbstractIn 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

AbstractThe 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

AbstractDeep 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

AbstractAs 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

AbstractRecent 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

AbstractWe 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

AbstractWe 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

AbstractThe 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

ABSTRACTMicroscopic 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

ABSTRACTA 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

ABSTRACTCharacterization 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

ABSTRACTWe 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

ABSTRACTQuasicrystalline 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

ABSTRACTThe 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

ABSTRACTNanoindentation 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

ABSTRACTTetrahedral 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

ABSTRACTContact 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

ABSTRACTThe 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

ABSTRACTNanoindentation 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

ABSTRACTThe 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.