bluuebunny/crossref_metadata_2025_split huggingface dataset:
TEM Analysis of Threading Dislocations in ELO-GaN Grown with Controlled Facet Planes
Authors: Kuwano Noriyuki , Horibuchi Kayo , Miyake Hideto , Hiramatsu Kazumasa 
Published: April 2011

ABSTRACTCross sectional transmission electron microscope (TEM) observation has been performed for specimens of ELO-GaN (ELO: epitaxial-lateral-overgrowth) in order to analyze the behavior of dislocations, with special reference to the effect of facet plane orientation and the size of mask. An ELO-GaN layer was grown overlying on a thick GaN layer with a patterned mask by MOVPE with a carrier gas of hydrogen (H2) under a low-ambient pressure. The growth temperature and the reactor-pressure were controlled in a two-step way during the growth of ELO-GaN layer in order to change the dominant facet-planes and the aspect ratio in growth rate. The experimental results revealed that (a+c)-type threading dislocations (TDs) show a 90-degree-bending in the specimen with slanting facets (2 1 1 2), but not in those with vertical ones (2110). a-type TDs run upward without bending irrespective of the orientation of the facet planes. Dislocations lying on (0001) planes, or horizontal dislocations (HDs), have been generated in the specimens with wide mask-terraces. It is thought that the formation of HDs relieved stresses in the ELO-GaN and then suppressed the bending of a-type TDs. In the specimens with narrow terraces, the both type TDs penetrate upward without bending and few HDs are generated. The behavior of dislocations is attributable to the fact that the small size of terrace generates small stresses and promotes a fast meeting of wings of ELO-GaN.
Integration of InxGa1−xN Laser Diodes with Dissimilar Substrates by Laser Lift-off
Authors: Wong William S. , Kneissl Michael , Mei Ping , Treat David W. , Teepe Mark , Johnson Noble M. 
Published: April 2011

ABSTRACTContinuous-wave (cw) indium-gallium nitride (InGaN) multiple-quantum-well (MQW) laser diodes (LDs) were successfully transferred from sapphire onto copper and diamond substrates using a two-step laser lift-off (LLO) process. Reduced threshold currents and increased differential quantum efficiencies were measured for LDs on Cu due to a 50% reduction of the thermal impedance. Light output for LDs on Cu was three times greater than comparable LDs on sapphire with a maximum output of 30 mW. Increased light output for LDs on diamond were also measure with a maximum output of 80 mW.
The Investigation of InGaN MQW Electroabsorption Modulator using the LED/Modulator/Detector Monolithically Integrated Structure
Authors: Chung S.W. , Zhao Y.S. , Lin C.H. , Lee H.P. 
Published: April 2011

ABSTRACTThe strong piezoelectric effect and quantum confined stark effect (QCSE) in the InGaN/GaN quantum well structures allow one to modify the free exciton absorption by the extrinsic field. The QCSE is investigated using a monolithically integrated three-section device comprising an LED, electroabsorption modulator and a detector section. The experimental results show that the LED output can be modulated as indicated by the detector signal. The strength of modulation decreases monotonically with increasing In composition in the InGaN/GaN MQW. The result can be explained on the basis of the Stokes' shift between the emission and absorption spectra in the InGaN/GaN QW structure, and a blue shift of the absorption spectrum due to the QCSE as a result of the piezoelectric effect.
Investigation of Buffer Layers for GaN Grown by MBE
Authors: Yun Feng , Reshchikov Michael A. , Visconti Paolo , Jones Keith M. , Wang Dongfeng , Redmond Marc , Cui Jie , Litton Cole W. , Morkoç Hadis 
Published: April 2011

ABSTRACTThe structural quality of the buffer layer juxtaposed to the substrate is pivotal in attaining high quality GaN layers. In MBE deposition, low temperature, medium temperature and high temperature AlN buffer layers are at the disposal of the grower. There are quite a few reports, some discussing the benefits of high temperature buffer layers and others doing the same for low temperature buffer layers. The reports emanate from different laboratories; and due to stringent parameter control required, it is difficult to compare one type of buffer with another. To gain some insight, we undertook an investigation wherein these varieties of buffer layers were grown on nitridated sapphire substrate under similar conditions for a comparative analysis. In addition to the single buffer layers of both GaN and AlN varieties, some combinations of stacked buffer layers, including cases where these buffer layers were separated by GaN layers, were employed. Structural analysis by high resolution X-ray diffractometry and topological analysis by AFM were carried out to assess the quality of the epilayers grown on these buffers. Hall measurements at room temperature were carried out to characterize the electrical transport properties.
Surface Segregation and Composition Fluctuations in ammonia MBE and MOVPE of InGaN
Authors: Karpov Sergey Yu. , Talalaev Roman A. , Yakovlev Eugene V. , Makarov Yuri N. 
Published: April 2011

ABSTRACTSurface segregation and phase separation are investigated as processes limiting the indium incorporation in InGaN grown by ammonia Molecular Beam Epitaxy (MBE) and Metal- Organic Vapor Phase Epitaxy (MOVPE). It is shown that a significant concentration of indium on the growing surface may prevent the adsorption of ammonia via site blocking mechanism and result in appearance of In droplets instead of InGaN growth. Another conclusion is that the composition fluctuations in InGaN are related to coexistence of strained and relaxed InGaN islands rather than to the phase separation as commonly assumed.
Strain and Compositional Analysis of InGaN/GaN Layers
Authors: Pereira Sérgio , Correia Maria. R. , Pereira Estela , Trager-Cowan C. , Sweeney F. , Edwards P.R. , O'Donnell K.P. , Alves E. , Sequeira A.D. , Franco N. 
Published: April 2011

ABSTRACTWe investigate strain and composition of epitaxial single layers of wurtzite InxGa1−xN (0<x<0.25) grown by MOCVD on top of GaN/Al203 substrates. It is shown that significant inaccuracies may arise in composition assessments if strain in InxGa1−xN/GaN heterostructures is not properly taken into account. Rutherford backscattering spectrometry (RBS) measures composition, free from the effects of strain and with depth resolution. Using X-ray diffraction (XRD) we measure both a- and c- parameters of the strained wurtzite films. By measuring both lattice parameters and solving Hooke's equation, a good estimation for composition can be obtained from XRD data. The agreement between RBS and XRD data for composition allows reliable values for perpendicular (εzz) and parallel strain components ( (εxx) to be determined. RBS and depth resolved cathodoluminescence (CL) measurements further indicate that the indium content is not uniform over depth in some samples. This effect occurs for the most strained layers, suggesting that strain is the driving force for compositional pulling.
Direct Observation of Bulk and Interface States in GaN on Sapphire grown by Hydride Vapor Phase Epitaxy
Authors: Goss S. H. , Young A. P. , Brillson L. J. , Look D. C. , Molnar R. J. 
Published: April 2011

ABSTRACTWe have used an ultrahigh vacuum scanning electron microscope to carry out cross sectional secondary electron imaging, cathodoluminescence spectroscopy, and cathodoluminescence imaging on GaN grown on sapphire by hydride vapor phase epitaxy. These measurements provide evidence for deep level defects highly localized at the GaN, sapphire interface as well as defects extending into both the semiconductor film and the substrate. The different spatial distributions of these radiative defects provide information on the physical origin of these electrically active features.
Relationship between Microscopic Structure and Optical Property of Polycrystalline GaN on Silica Glass
Authors: Kagatsume Hidetaka , Aono Hiroaki , Araki Tsutomu , Nanishi Yasushi 
Published: April 2011

ABSTRACTWe have investigated relationships between the microscopic structure and optical property of polycrystalline GaN grown by (electron-cyclotron-resonance plasma-excited molecular beam epitaxy) ECR-MBE on silica glass substrates, using scanning electron microscope (SEM) and cathodoluminescence (CL). It was found that CL intensity was stronger for the samples with a large columnar domain size. These individual columnar domains showed clear luminescence. It was found that the origin of strong luminescence from polycrystalline GaN is due to such a columnar domain. That luminescence was closely related to the morphology of the columnar domains. It was revealed that the columnar domain with a homogeneous and hexagonal shape showed clear luminescence.
The Influence of Defects and Piezoelectric Fields on the Luminescence from InGaN/GaN Single Quantum Wells
Authors: Henley S. J. , Cherns D. 
Published: April 2011

ABSTRACTHigh spatial resolution cathodoluminescence (CL) studies have been carried out on GaN/InGaN/(0001)GaN single quantum well (SQW) structures in a field emission scanning electron microscope at 5kV and temperatures down to 8K. Direct comparison of QW CL maps with transmission electron microscope studies of plan-view samples showed that edge type threading dislocations act as non-radiative recombination centers. Spectra taken from extended areas showed a progressive blue shift in the QW emission from around 460nm at low beam intensities to about 445nm as the beam intensity was increased. This effect which correlated with a decrease in the spatial resolution is interpreted as due to an increase in the diffusion length of carriers in the SQW due to a combination of screening of the piezoelectric field and band filling effects.
UV Raman Study of A1(LO) and E2 Phonons in InGaN Alloys Grown by Metal-Organic Chemical Vapor Deposition on (0001) Sapphire Substrates
Authors: Alexson Dimitri , Bergman Leah , Nemanich Robert J. , Dutta Mitra , Stroscio Michael A. , Parker C.A. , Bedair S.M. , El-Masry N.A. , Adar Fran 
Published: April 2011

ABSTRACTWe report on UV Raman spectroscopy of InxGa1−xN thin films grown on (0001) sapphire substrates using a specially designed metal-organic chemical vapor deposition (MOCVD) reactor. Eight films were examined in the compositional range 0<×<0.50. Mid and deep-UV Raman spectroscopy was done with the 325.2 nm line of the HeCd laser and the 244 nm line of a double frequency Ar ion laser. The mode behavior of the A1(LO) and E2 phonons was also investigated. We have found compelling evidence for one-mode behavior for the A1(LO) phonon mode, while our data for the E2 mode deviates from the predictions for one-mode behavior. The results for the A1(LO) mode are consistent with the previously reported phonon mode behavior in AlGaN alloys. Also, evidence regarding the presence of compositional inhomogeneities and spinodal decomposition in InGaN thin films is presented.
Morphological Instabilities during Explosive Crystallization of Germanium Films
Authors: Chojnacka Aleksandra , Thompson Michael O. 
Published: April 2011

AbstractExplosive crystallization, a self-sustaining transformation of an amorphous phase to a crystalline phase mediated by a thin liquid layer, exhibits three distinct kinetic and morphological regimes in germanium. Dynamics of these growth processes and the resulting morphologies have been examined in detail. Steady-state crystallization velocities were measured as a function of heat loss into the substrate. Dark field optical microscopy, tapping mode atomic force microscopy, transmission electron microscopy, and x-ray diffraction were used to examine the crystallized films. Analyses of the experimental results provide evidence for two distinct processes governing explosive crystallization in limits of high substrate temperature (low heat loss) and low substrate temperature (high heat loss). The low temperature growth mode produces a “scalloped” structure with propagation velocities that monotonically increase with temperature. At high substrate temperatures, the velocity is independent of temperature and a “columnar” pattern with preferred texture is formed.
Microstructure and Optical Properties of Co Sputter Deposited Si-Al Nanocomposite Thin Films
Authors: Niu F. , Dobson P.J. , Cantor B. 
Published: April 2011

AbstractNovel Si-Al nanocomposite thin films were made by radio frequency co-sputtering of Si and Al with Al content from 0 at.% to 69 at.%. Microstructure and optical properties of the films were characterised by conventional and high resolution transmission electron microscopyand spectrometry in the wavelength range from 200 to 3000 nm. The film microstructure consisted of Al nanoparticles (2-9 nm) embedded in an amorphous Si-Al matrix. Optical absorption spectra of the films up to 50 at.% Al exhibited a sharp absorption peak below500 nm and relatively low absorption above 500 nm. In addition, the absorption peak shifted towards longer wavelengths and the general absorption above 500 nm increased remarkably as Al content increased. For the Si-69at.%Al films, however, an absorption plateau appeared between 300 nm to 700 nm and a second weak and broad absorption peak appeared at around 900 nm. The results are analysed and compared with the optical absorption predicted by various effective medium theories.
Self-Assembly of Ruthenium Porphyrins into Monolayer and Multilayer Architectures via Heterogeneous Coordination Chemistry
Authors: Sarno David M. , Matienzo Luis J. , Jones Wayne E. 
Published: April 2011

AbstractSelf-assembly chemistry is finding use as a means of controlling the nanoscale structures at materials interfaces. Building on the success of a previous methodology, Ruthenium porphyrins have been coordinated to the pyridine-terminated surface of a self-assembled alkylsilane coupling layer on silica glass. These arrays are built in a stepwise fashion in which the surface of each layer allows further reaction to yield the next layer. Characterization by UV-vis and XPS confirms the deposition of the alkyl chains and pyridine moieties of the coupling layer, as well as coordination of the subsequent Ru-porphyrin layer. AFM analysis indicates very smooth surfaces, with roughness measured at 0.43 nm and 1.43 nm for the pyridine and porphyrin layers, respectively. The occasional appearance of large surface features (15-30 nm diameter) suggests aggregation among Φ-systems, with further characterization underway. Strategies toward the construction of a multilayer porphyrin architecture are also reported.
Investigation of the Layered Structure of Polycrystalline Diamond Thin Films Grown by ECR-Assisted CVD by Spectroscopic Phase Modulated Ellipsometry
Authors: Gupta S. , Weiner B. R. , Morell G. 
Published: April 2011

AbstractPolycrystalline diamond thin films deposited by the electron cyclotron resonance-assisted chemical vapor deposition (ECR-CVD) were investigated using spectroscopic ellipsometry (SE) from the near IR to UV range (830 nm-270 nm). Employing the conventional Bruggeman effective medium approximation (EMA) and linear regression analysis (LRA) to the raw ellipsometry data (ψλi), δ(λi)) provided the details about the film microstructure: (i) the multilayer structure and the ovearall thickness of the films; (ii) the volume fraction of the constituents (sp3 - and sp2- bonded carbon) and of voids in the bulk layer; (iii) the inhomogeneity of the structure along the growth axis and its variation with the seeding density; and (iv) the surface roughness layer. A simplified three-layer structural model consisting of an interfacial layer, an intermediate (or bulk) layer and the top surface roughness layer has been proposed to simulate the ellipsometry data. The results obtained through ellipsometry modeling such as surface roughness layer and overall film thickness were compared with rms surface roughness from atomic force microscopy (AFM) and profilometry respectively, in order to validate the model employed. The results such as fv and fsp2C for the bulk layer and its behavior with respect to process parameters are discussed.
Hydrothermal Growth Kinetics of BaTiO3 on TiO2 Single Crystal Surfaces
Authors: Fuenzalida Víctor M. , Lisoni Judit G. 
Published: April 2011

AbstractTiO2 (rutile) single crystal plates with (001) and (110) orientation were immersed in an aqueous solution of Ba(OH)2 0.5 M at temperatures of 100 and 150 °C for 1 and 4 hours in order to grow BaTiO3on them. SEM micrographs of the samples fabricated on the (001) surface of rutile displayed isolated grains with an average height ranging from 200 nm at 100 °C to 700 nm at 150 °C. On the other hand, samples with the (001) orientation exhibited no growth at 100 °C and only a few grains along lines attributed to the polishing process of the substrate at 150 °C. The image of backscattered electrons indicated that barium is concentrated on the grains in all cases. Only the (001) samples exhibited reflections of cubic BaTiO3, as indicated by x-ray dif- fraction, as well as distinct Ba signals under x-ray photoelectron spectrometry. These results agree with the hypothesis of a dissolution-precipitation growth mechanism, in which dissolution is possible for the (001) face, but not for the (110) one, which is the most stable of the low-index faces of this material. Similar treatments were applied to ZrO2:Y2O3 crystals, leading to no film growth.
Sliding Friction between Polymer-brush-bearing Surfaces: Crossover from Brush-brush Interfacial Shear to Polymer-substrate Slip.
Authors: Klein Jacob 
Published: April 2011

AbstractA model is presented for the shear (or frictional) forces Fs between two surfaces a distance D apart as they slide past each other while bearing mutually compressed polymer brushes, on the assumption that sliding takes place at the brush-brush interface. The predictions of the model for the rapid increase in Fs at increasing compressions are in reasonable agreement with experiments on polystyrene brushes immersed in toluene over two decades in Fs. At higher compressions (smaller D) the experimental shear forces increase only slowly, and diverge from the calculated ones which continue to increase rapidly; at the same time the form of the shear force response at these higher compressions reverts from a viscous-like one to a stick-slip behaviour. These observations strongly indicate that at sufficiently high compressions the plane of slip crosses over from the brush-brush interface to the polymer-solid surface.
Towards a Microscopic Description of Friction
Authors: Porto Markus , Zaloj Veaceslav , Urbakh Michael , Klafter Joseph 
Published: April 2011

AbstractWe investigate the response of an embedded system subject to an external drive using a microscopic model. The shear is shown to excite “shearons”, which are collective modes of the embedded system with well defined spatial and temporal patterns that dominate the frictional properties of the driven system. We demonstrate that the slip relaxation in stick-slip motion and memory effects are well described in terms of the creation and/or annihilation of shearons.
Propagation Dynamics of a Particle Phase in a Single-File Pore.
Authors: Lacasta A.M. , Sancho J.M. , Sagues F. , Oshanin G. 
Published: April 2011

AbstractWe study propagation dynamics of a particle phase in a single-file pore connected to a reservoir of particles (bulk liquid phase). We show that the total mass M(t) of particles entering the pore up to time t grows as (Mt) = 2m(J,ρF) √D0t, where D0 is the “bare” di usion coeffcient and the prefactor m(J,ρF) is a non-trivial function of the reservoir density ρF and the amplitude J of attractive particle-particle interactions. Behavior of the dynamic density pro les is also discussed.
207Pb NMR of PbZrO3 and PbZr1-xTixO3 Solid Solutions
Authors: Rossi Paolo , Dvorak Matthew R. , Harbison Gerard S. 
Published: April 2011

ABSTRACTPbZrO3 undergoes a phase transition to a paraelectric phase at 230°C. During this phase transition the unit cell changes from orthorhombic to cubic. Structural changes of PbZrO3 have been monitored using solid state NMR by measuring the variation in the 207Pb chemical shielding tensor as a function of temperature. The two distinct lead sites show rather different behavior as a function of temperature. The less shielded lead maintains an almost constant asymmetry parameter η = 0.2 from 0°C to 200°C while the more shielded lead resonance becomes more axially symmetric as the temperature is raised going from η = 0.24 at 0°C to [.eta] = 0.08 at 200°C. Powder pattern singularities become less distinct near the phase transitions, but the temperature dependence of the chemical shift tensor principal values remains continuous, and there is no evidence of an intervening higher-symmetry phase. At the phase transition, both resonances collapse into a single narrow line characteristic of a nucleus at a high-symmetry site. Results of a preliminary study of the PbZr1−xTixO3 solid solutions by 207Pb solid state NMR are also presented.
Fine-Tuning the Physical Properties of Perovskite Related La-Ti-Oxides by Merely Altering the Oxygen Stoichiometry
Authors: Becker Olav , Ebbinghaus Stefan , Renner Bernd , Reller Armin 
Published: April 2011

ABSTRACTThe isostructural oxides LaTiO3.5, SrNbO3.5 and CaNbO3.5 adopt layered structures made up of perovskite related slabs intermitted by layers of non-linking BO6-octahedra (B=Ti, Nb). In these phases the transition metals Ti and Nb are in their highest oxidation states +IV and +V, respectively. They are insulators, in the case of the Ti phase a ferroelectric insulator with the highest known Tc (above 1700°C). By changing the oxygen stoichiometry, i.e. by controlled reduction or re-oxidation processes in the range of 2ABO3.5 ø 2ABO3 + ½ O2 (A=La or Sr, B=Ti or Nb) mixed valence phases are obtained. Accordingly, the physical properties of the different phases alter from insulating to semiconducting or to conducting. Detailed studies on the structural changes reveal, however, that the sublattice of the metal cations is basically conserved. In principle the reduction corresponds to a condensation of the perovskite layers leading to intermediate phases such as the semiconducting LaTiO3.4, in which five TiO6-octahedra thick perovskite slabs constitute the structural framework and 20% of the Ti+IV cations are reduced to Ti+III. Conductivity measurements using single crystals of the corresponding Ti and Nb phases reveal that these mixed valence oxides must be considered as one-dimensional conductors. The fully reduced phases La+IIITi+IIIO3 and Sr+IINb+IVO3 adopt distorted perovskite structures. Extensive high resolution electron microscopic and light microscopic investigations have been carried out in order to characterize the structural mechanism of the reversible, highly topotactic reduction and re-oxidation processes. Thermogravimetric measurements in reducing and oxidizing atmospheres have been performed for the identification of the temperature ranges wherein the decisive mass changes take place. The results of the described experiments support that the properties of such metal oxides can be finely and reversibly tuned by merely changing the oxygen stoichiometries.