The Experts below are selected from a list of 11349 Experts worldwide ranked by ideXlab platform
B. Beaumont - One of the best experts on this subject based on the ideXlab platform.
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Transmission electron microscopy study of the nitridation of the (0001) Sapphire Surface
Applied Physics Letters, 1999Co-Authors: P. Vennéguès, B. BeaumontAbstract:The purpose of this letter is to study, using transmission electron microscopy, the nitridation of the (0001) Sapphire Surface which is a key step for the fabrication of high-quality GaN materials. A nitridation conducted at 1080 °C during 7 min at atmospheric pressure with a 2 slm NH3 flow, results in the formation of a fully crystalline 10-atomic-planes-thick AlN film by the chemical transformation of the Al2O3 Surface. From measurements of interplanar distances in high-resolution images, we show that this chemical transformation is incomplete, i.e., that a few Al vacancies and/or O atoms remain in the AlN structure.
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the effect of the si n treatment of a nitridated Sapphire Surface on the growth mode of gan in low pressure metalorganic vapor phase epitaxy
Applied Physics Letters, 1998Co-Authors: S Haffouz, P. Vennéguès, B. Beaumont, H Lahreche, P De Mierry, F Omnes, P GibartAbstract:In this letter, we studied the effect of the high-temperature Si/N treatment of the nitridated Sapphire Surface followed by the deposition of a low-temperature GaN nucleation layer on the growth mode of GaN in low-pressure metalorganic vapor phase epitaxy. It was shown that the nucleation layer, initially flat and continuous, converts to wide isolated truncated hexagonal islands having {1–101} facet planes and a top (0001) plane, after heating up to 1150 °C. The coalescence of these GaN islands yields a reduction of the total number of extended defects from the 1010–1011 cm−2 range usually obtained down to the low 109 cm−2 range for the best samples.
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influence of in situ Sapphire Surface preparation and carrier gas on the growth mode of gan in movpe
Journal of Crystal Growth, 1998Co-Authors: P. Vennéguès, B. Beaumont, S Haffouz, M Vaille, P GibartAbstract:Abstract A comparative study of GaN layers grown on Sapphire (0 0 0 1) by metalorganic vapor phase epitaxy with different Sapphire Surface preparations and carrier gas combinations is reported. Observation of the growth modes at several stages of the growth process is carried out by in situ laser reflectometry measurements and ex situ transmission electron microscopy. It is shown that the combination involving both an in situ Sapphire Surface preparation consisting in the deposition of a thin Si x N 1− x film and a H 2 –N 2 mixture as carrier gas, induces a three dimensional mode in the first stage of the growth of GaN epilayer. The growth occurs then at high temperature by coalescence of GaN islands limited by (0 0 0 1) top Surface and {1 0 1 1} lateral facets. This island formation is initiated by the modification of the morphology of the GaN buffer layer, due to the dual action of a “morphactant” effect of H 2 and a modification of the Surface energy of Sapphire by the Si x N 1− x coating. This three-dimensional growth mode yields a reduction of the total number of extended defects from above 10 10 cm −2 usually obtained down to 7×10 8 cm −2 for the best samples.
P. Vennéguès - One of the best experts on this subject based on the ideXlab platform.
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Transmission electron microscopy study of the nitridation of the (0001) Sapphire Surface
Applied Physics Letters, 1999Co-Authors: P. Vennéguès, B. BeaumontAbstract:The purpose of this letter is to study, using transmission electron microscopy, the nitridation of the (0001) Sapphire Surface which is a key step for the fabrication of high-quality GaN materials. A nitridation conducted at 1080 °C during 7 min at atmospheric pressure with a 2 slm NH3 flow, results in the formation of a fully crystalline 10-atomic-planes-thick AlN film by the chemical transformation of the Al2O3 Surface. From measurements of interplanar distances in high-resolution images, we show that this chemical transformation is incomplete, i.e., that a few Al vacancies and/or O atoms remain in the AlN structure.
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the effect of the si n treatment of a nitridated Sapphire Surface on the growth mode of gan in low pressure metalorganic vapor phase epitaxy
Applied Physics Letters, 1998Co-Authors: S Haffouz, P. Vennéguès, B. Beaumont, H Lahreche, P De Mierry, F Omnes, P GibartAbstract:In this letter, we studied the effect of the high-temperature Si/N treatment of the nitridated Sapphire Surface followed by the deposition of a low-temperature GaN nucleation layer on the growth mode of GaN in low-pressure metalorganic vapor phase epitaxy. It was shown that the nucleation layer, initially flat and continuous, converts to wide isolated truncated hexagonal islands having {1–101} facet planes and a top (0001) plane, after heating up to 1150 °C. The coalescence of these GaN islands yields a reduction of the total number of extended defects from the 1010–1011 cm−2 range usually obtained down to the low 109 cm−2 range for the best samples.
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influence of in situ Sapphire Surface preparation and carrier gas on the growth mode of gan in movpe
Journal of Crystal Growth, 1998Co-Authors: P. Vennéguès, B. Beaumont, S Haffouz, M Vaille, P GibartAbstract:Abstract A comparative study of GaN layers grown on Sapphire (0 0 0 1) by metalorganic vapor phase epitaxy with different Sapphire Surface preparations and carrier gas combinations is reported. Observation of the growth modes at several stages of the growth process is carried out by in situ laser reflectometry measurements and ex situ transmission electron microscopy. It is shown that the combination involving both an in situ Sapphire Surface preparation consisting in the deposition of a thin Si x N 1− x film and a H 2 –N 2 mixture as carrier gas, induces a three dimensional mode in the first stage of the growth of GaN epilayer. The growth occurs then at high temperature by coalescence of GaN islands limited by (0 0 0 1) top Surface and {1 0 1 1} lateral facets. This island formation is initiated by the modification of the morphology of the GaN buffer layer, due to the dual action of a “morphactant” effect of H 2 and a modification of the Surface energy of Sapphire by the Si x N 1− x coating. This three-dimensional growth mode yields a reduction of the total number of extended defects from above 10 10 cm −2 usually obtained down to 7×10 8 cm −2 for the best samples.
Martial Ducloy - One of the best experts on this subject based on the ideXlab platform.
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Resonant van der Waals Repulsion between Excited Cs Atoms and Sapphire Surface
Physical Review Letters, 1999Co-Authors: H. Failache, Solomon M. Saltiel, Michèle Fichet, Daniel Bloch, Martial DucloyAbstract:We explore a situation where the van der Waals long-range atom-Surface interaction is repulsive. This repulsion originates in a resonant coupling between a virtual emission at 12.15 mm of a Cs 6D3 2 atom and a virtual excitation of a Surface polariton in Sapphire. The experimental evidence is based upon the analysis of the spectroscopic response of Cs* in the near-infrared range with a technique that probes a distance range 100 nm away from the Sapphire Surface. We also demonstrate the critical dependence of atom-Surface forces on the Sapphire crystal orientation.
Toshio Ogino - One of the best experts on this subject based on the ideXlab platform.
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Graphene etching controlled by atomic structures on the substrate Surface
Carbon, 2012Co-Authors: Takahiro Tsukamoto, Toshio OginoAbstract:Abstract We etched graphene on a Sapphire (1 −1 0 2) Surface using the reaction between graphene and hydrogen catalyzed by metal nanoparticles. To investigate effects of the atomic structure of the Sapphire substrate on graphene etching, we used Sapphire substrate with as-polished, air-annealed, and step-ordered Surfaces. We investigated the relationship between the atomic arrangement of Sapphire and graphene etching and found that graphene is selectively etched in the [1 −1 0 −1] direction of Sapphire. This indicates that atomic structure of the Sapphire Surface can be used as a template to control graphene etching. By combining the transfer method for graphene sheets grown on metal substrates with the present etching technique, graphene nanoribbons can be fabricated at a wafer level.
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Selective adsorption of protein molecules on phase-separated Sapphire Surfaces.
Journal of colloid and interface science, 2011Co-Authors: Kenji Yamazaki, Takayuki Ikeda, Toshinari Isono, Toshio OginoAbstract:Site-selective adsorption of protein molecules was found on Sapphire Surfaces that exhibit a phase separation into two domains: weakly charged hydrophobic domain and negatively charged hydrophilic one. Ferritin and bovine serum albumin molecules, which are negatively charged in a buffer solution, are adsorbed to the hydrophobic domains. Avidin molecules, which are positively charged, are adsorbed to the other domain. Fibrinogen molecules, which consist of both negative and positive modules, are adsorbed to the whole Sapphire Surface. Hemoglobin molecules, whose net charge is almost zero, are also adsorbed to the whole Surfaces. These results indicate that electrostatic double layer interaction is the primary origin of the observed selectivity. Dependence of protein adsorption or desorption behaviors on the pH value can also be interpreted by the proposed model.
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‘Graphene-on-insulator’ fabricated on atomically controlled solid Surfaces
Journal of Physics D: Applied Physics, 2010Co-Authors: Takahiro Tsukamoto, Toshio OginoAbstract:Graphene or few layer graphene (FLG) was attached on a single-crystalline Sapphire Surface on which a regularly ordered step/terrace structure was formed. The height of graphene from the substrate Surface was observed to be about 0.35 nm, which is almost equal to the layer spacing of graphite. On the attached graphene or FLG Surface, the step/terrace structure originating from the Sapphire Surface was clearly observed because the graphene flake tightly adhered to the Sapphire Surface. FLGs were etched by a reaction between carbon of the FLG and H2 gas at 900 °C using Fe nanoparticles as catalysts. When a FLG flake is thick, the etching direction is subject to the crystallographic directions of graphene. As the FLG is thinner than 6 nm, strain induced on the FLG Surface by the step/terrace structure of the substrate Surface influences the etching direction, and etching along a buried step occurs when the Fe nanoparticle size is small. The etching directions of FLG can be controlled by the ordered atomic step arrangement on Sapphire Surfaces owing to the Surface flatness and tight adhesion of graphene to the Surface.
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Evolution of Supported Planar Lipid Bilayers on Step-Controlled Sapphire Surfaces
Langmuir : the ACS journal of surfaces and colloids, 2010Co-Authors: Toshinari Isono, Takayuki Ikeda, Toshio OginoAbstract:Self-organized step/terrace structures on a Sapphire Surface were used to investigate interface properties between a solid Surface and a supported planar lipid bilayer (SPB). We prepared random-stepped, single-stepped and multistepped Sapphire Surfaces. Some multistepped Surfaces covered with crossing steps exhibit phase-separation into hydrophilic and hydrophobic domains. We studied evolution of self-spreading lipid bilayers that are subject to the atomic structures and chemical states on the Surfaces. The growth direction of SPBs in the self-spreading method is regulated by the atomic steps. While the SPBs were apparently uniform after a 1 h self-spreading, a density gradient of the lipid molecules was observed even after 24 h spreading. We found that various patterns of the SPBs that depend on the density of the lipid molecules are self-assembled on the phase-separated Surfaces. Although the SPB is supported on the Sapphire Surface via an about 1 nm water layer, the self-spreading direction and the mor...
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Morphology of Graphene on Step-Controlled Sapphire Surfaces
Applied Physics Express, 2009Co-Authors: Takahiro Tsukamoto, Toshio OginoAbstract:Graphene attached on a Sapphire Surface with regularly ordered step-terrace structure was observed using atomic force microscopy (AFM). We found that graphene tightly adheres to a Sapphire Surface and the buried step structure on the Sapphire Surface was clearly observed on the graphene Surface. Height of a single-layer graphene was estimated to be approximately 0.36 nm on Sapphire Surface, which is in good agreement with the theoretical height. These results indicate that Sapphire is suitable for the substrate that supports graphene because we can obtain undistorted graphene that is tightly fixed on a substrate Surface.
Thomas F. Kuech - One of the best experts on this subject based on the ideXlab platform.
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The Effects of Sapphire Surface Treatments and Nitridation on GaN Nucleation Grown using Hydride Vapor Phase Epitaxy
MRS Proceedings, 2003Co-Authors: F. Dwikusuma, Thomas F. KuechAbstract:AbstractWe have studied the effects of Sapphire Surface treatments and nitridation on GaN nucleation grown using the hydride vapor phase epitaxy technique. The Surface treatments used were airannealing at 1400°C, etching in pure H2SO4 at 250°C, and etching in a 3:1 mixture of H2SO4:H3PO4 solution at 250°C. A nitridation step was carried out using 20% NH3 in N2 gas mixture at 1100°C. GaN nucleation and the early stage s of growth was investigated by short time growth and quench experiments. Atomic force microscopy and double crystal x-ray diffraction were used to examine the Sapphire Surface morphology, GaN island density, and GaN island structure. A lower density of GaN islands was grown on the air-annealed Sapphire compared to the H2SO4-etched and 3:1 H2SO4:H3PO4-etched Sapphire. GaN islands grown on the 3:1 H2SO4:H3PO4-etched Sapphire had a broad mosaic spread due to preferential growth on Surface pits. Sapphire nitridation resulted in a higher GaN island density with a smaller mosaic spread. A high density and uniform nucleation of GaN islands is critical in producing high quality thick GaN films. The H2SO4-etched Sapphire and nitridation resulted in a high density of uniform GaN islands.
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Study on Sapphire Surface Preparation for III-Nitride Heteroepitaxial Growth by Chemical Treatments
Journal of The Electrochemical Society, 2002Co-Authors: F. Dwikusuma, D. Saulys, Thomas F. KuechAbstract:The etching of Sapphire substrates using H2SO4,H3PO4 , and a 3:1 H2SO4 :H3PO4 mixture, as a function of temperature and etching time, was systematically studied using atomic force microscopy. The Sapphire preparation by liquid-based etchings was compared with H2 etching at 1100°C and air-annealing at 1400°C. In liquid-based treatments, the smoothest, pit-free Surface was obtained by etching in pure H2SO4 at 300°C for 30 min. Sulfuric acid etching at higher temperatures or for longer periods generated an insoluble mixture of Al2(SO4)3 and Al2(SO4)3•17H2O crystalline deposits on the Surface. Phosphoric acid and the 3:1 H2SO4 :H3PO4 mixture, which is the routinely employed chemical treatment for Sapphire preparation, etched the Sapphire preferentially at defect sites and resulted in pit formation on the Surface. Sapphire treatment using H2 at 1100°C did not remove the Surface damage. Air annealing the Sapphire at 1400°C for 1 h produced an atomically smooth Surface consisting of a terrace-and-step structure. The results of this study were described in terms of the chemistry of the Sapphire etching process.
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The Nature and Impact of ZnO Buffer Layers on the Initial Stages of the Hydride Vapor Phase Epitaxy of GAN
MRS Internet Journal of Nitride Semiconductor Research, 2000Co-Authors: Rong Zhang, Jingxi Sun, L. Zhang, Thomas F. KuechAbstract:The nature and impact of ZnO buffer layers on the initial stages of the hydride vapor phase epitaxy (HVPE) of GaN have been studied by x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), x-ray diffraction (XRD) and photoluminescence (PL). During pre-growth heating, the Surface ZnO layer was found to both desorb from ZnO-coated Sapphire and react with the underlying Sapphire Surface forming a thin ZnAl{sub 2}O{sub 4} alloy layer between ZnO and Sapphire Surface. This ZnO-derived Surface promotes the initial nucleation of the GaN and markedly improves material Surface morphology, quality and growth reproducibility.
