Search Results - (Author, Cooperation:L. Romano)

2013-10-19

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

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Alleles ; Base Sequence/genetics ; Binding Sites/genetics ; Chromatin/chemistry/*metabolism ; DNA/chemistry/*metabolism ; *Gene Expression Regulation ; *Genetic Variation ; Histones/chemistry/metabolism ; Humans ; Polymorphism, Single Nucleotide ; Promoter Regions, Genetic ; Transcription Factors/*metabolism ; *Transcription, Genetic

2014-08-01

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

Alleles ; Cell Transformation, Neoplastic/genetics/pathology ; Colorectal Neoplasms/*genetics/pathology ; DNA Methylation ; Gene Expression Profiling ; Gene Expression Regulation, Neoplastic/*genetics ; Genes, Neoplasm ; Genome-Wide Association Study ; Genotype ; Germ-Line Mutation/genetics ; Humans ; Intestinal Mucosa/cytology/metabolism/pathology ; Quantitative Trait Loci/genetics ; Regulatory Sequences, Nucleic Acid/*genetics ; Sequence Analysis, RNA ; Transcription Factors/metabolism ; Transcriptome/genetics

1089-7550

AIP Digital Archive

Physics

Interface dislocations present in a Si0.85Ge0.15/Si sample have been imaged using the channeling scanning transmission ion microscopy (CSTIM) method with a 2 MeV proton beam 200 nm across. Groups of parallel dislocations gave dark bands of contrast down to ∼1.5 μm across, the contrast arising from dechanneling of the beam by the bent lattice planes. Tilting of the sample caused the band contrast to change and gave quantitative data concerning the local bending of the lattice planes. A low-angle boundary model was developed to describe the effect of the groups of dislocations on the channeling contrast. Channeling and topography contrast were obtained from mesa structures present on the sample. Improvements in the sensitivity of the CSTIM method are discussed. The dislocations in the sample were initially characterized by transmission electron microscopy.

Electronic Resource

1089-7550

AIP Digital Archive

Physics

The effect of Si doping on the strain and microstructure in GaN films grown on sapphire by metalorganic chemical vapor deposition was investigated. Strain was measured quantitatively by x-ray diffraction, Raman spectroscopy, and wafer curvature techniques. It was found that for a Si concentration of 2×1019 cm−3, the threshold for crack formation during film growth was 2.0 μm. Transmission electron microscopy and micro-Raman observations showed that cracking proceeds without plastic deformation (i.e., dislocation motion), and occurs catastrophically along the low energy {11(underbar)00} cleavage plane of GaN. First-principles calculations were used to show that the substitution of Si for Ga in the lattice causes only negligible changes in the lattice constant. The cracking is attributed to tensile stress in the film present at the growth temperature. The increase in tensile stress caused by Si doping is discussed in terms of a crystallite coalescence model. © 2000 American Institute of Physics.

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1089-7550

AIP Digital Archive

Physics

We have investigated threading dislocation (TD) removal from GaAs films on Si by introduction of additional InGaAs graded strain layers in combination with growth on patterned substrates. The substrate patterns consisted of mesas with 10–34 μm widths. The mesa sidewalls were either overhanging (concave), leading to free sidewalls for the film on the mesas, or outward sloping (convex) sidewalls with {111} orientation. The dislocation structure was studied using transmission electron microscopy. It was found that the graded strained layers led to a reduction of dislocation density by a factor of ∼5 in films grown both on mesas with concave sidewalls and on unpatterned substrates. This reduction was due to dislocation reactions leading to annihilation of TDs. For films with graded strained layers on mesas with convex sidewalls, an additional factor of ∼3 reduction in TD density was observed in the part of the film that was grown on top of the mesas. In this case all mobile TDs (TDs associated with 60° misfit dislocations, i.e., TDs that could glide to relieve misfit stress) were removed from the film on top of the mesas to the regions above the sidewalls and only TDs associated with 90° misfit dislocations remained. We suggest that this is due to pinning of the TDs associated with 60° misfit dislocations at the mesa edges and we have presented an explanation for this pinning in terms of the stress conditions at the {111} oriented mesa edges. In addition, this leads us to suggest that in order to obtain minimum TD density it is imperative to prevent formation of 90° misfit dislocation during lattice mismatched heteroepitaxial growth.

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1077-3118

AIP Digital Archive

Physics

Thick (225 nm) InxGa1−xN layers, grown on 5 μm thick GaN, were found by x-ray diffraction (XRD) measurements to be pseudomorphic up to x=0.114. Transmission electron microscopy showed that no misfit or additional threading dislocations were created at the InxGa1−xN/GaN interface. Composition of the overlayers was determined by Rutherford backscattering spectrometry and correlated to both the a and c lattice constants from XRD. It was found that Vegard's law is applicable at these compositions, if the biaxial strain is included. Biaxial strain must also be considered to accurately determine the bowing parameter as shown by optical transmission measurements. © 1998 American Institute of Physics.

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1077-3118

AIP Digital Archive

Physics

Evidence is presented for phase separation in In0.27Ga0.73N/GaN multiple quantum wells. After annealing for 40 h at a temperature of 950 °C, the absorption threshold at 2.95 eV is replaced by a broad peak at 2.65 eV. This peak is attributed to the formation of In-rich InGaN phases in the active region. X-ray diffraction measurements show a shift in the diffraction peaks toward GaN, consistent with the formation of an In-poor phase. A diffraction peak corresponding to an In-rich phase is also present in the annealed material. Nanoscale In-rich InGaN precipitates are observed by transmission electron microscopy and energy dispersive x-ray chemical analysis. © 1998 American Institute of Physics.

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1077-3118

AIP Digital Archive

Physics

Growth of GaN by rf-plasma molecular beam leads to different surface morphologies for nitrogen-rich growth versus gallium-rich growth. Nitrogen-rich growth produces a significant density of pyramidal hillocks while gallium-rich growth results in flat surfaces. Differences in surface morphology were directly linked to the presence of inversion domains which originated in the nucleation layer. Nitrogen-rich growth and growth under atomic hydrogen enhanced the growth rate of inversion domains with respect to the surrounding matrix, while growth under Ga-rich conditions resulted in a more nearly equal growth rate. © 1997 American Institute of Physics.

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1077-3118

AIP Digital Archive

Physics

The structure of GaN films grown by hydride vapor phase epitaxy on sapphire substrates has been studied by x-ray diffraction, transmission electron microscopy (TEM), and atomic force microscopy. Films, 15–80 μm thick, were grown on c-plane sapphire that were either pretreated with GaCl or contained a ZnO sputter deposited layer. The defect density, for both types of films, was found by plan view TEM to range between mid-107 to mid-108 dislocations/cm2 despite very different structural defects at the film/substrate interface. Nanovoids were found; however, no cracks were observed in the films that were investigated by TEM. © 1997 American Institute of Physics.

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1077-3118

AIP Digital Archive

Physics

High resolution electron microscopy (HREM) has been used to study misfit dislocations of ZnSe films grown on vicinal Si(001) substrates tilted 4° towards the [11¯0] axis. In images taken with the electron beam parallel to the [11¯0] direction, 60° dislocations were found to predominate whereas mostly Lomer dislocations or closely spaced 60° dislocations (separated by 〈2 nm) were observed in images taken in the orthogonal direction. A model is presented here to explain the formation of the asymmetric dislocation structure on the basis of mechanisms for propagation and formation of misfit dislocations.

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1077-3118

AIP Digital Archive

Physics

Planar defects observed in GaN films grown on (0001) sapphire have been identified as inversion domain boundaries (IDBs) by a combination of high resolution transmission electron microscopy, multiple dark field imaging, and convergent beam electron diffraction techniques. Films grown by molecular beam epitaxy (MBE), metalorganic vapor deposition (MOCVD), and hydride vapor phase epitaxy (HVPE) were investigated and all were found to contain IDBs. The IDBs in the MBE and HVPE films extended from the interface to the film surface and formed columnar domains that ranged in width from 3 to 20 nm in the MBE films and up to 100 nm in the HVPE films. For the films investigated, the MBE films had the highest density, and the MOCVD films had the lowest density of IDBs. The nucleation of inversion domains (IDs) may result from step-related inhomogeneities of the GaN/sapphire interface. © 1996 American Institute of Physics.

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1077-3118

AIP Digital Archive

Physics

The threshold current density of narrow-stripe gain-guided nitride laser diodes increases very rapidly as the stripe width is made narrow. To examine this behavior, waveguide simulations, incorporating the complex refractive indices associated with optical gain, have been used to analyze the lateral optical modes of gain-guided laser diodes. Threshold current was then determined from the gain–current relationship of our laser material, which was obtained experimentally. These evaluations reveal that gain guiding, coupled with a carrier-induced index depression, offer a reasonable explanation for the rapid increase in threshold when the stripe width becomes less than 5 μm. © 1999 American Institute of Physics.

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1077-3118

AIP Digital Archive

Physics

Current-injection InGaAlN heterostructure laser diodes grown by metalorganic chemical vapor deposition on sapphire substrates are demonstrated with mirrors fabricated by chemically assisted ion beam etching. Due to the independent control of physical and chemical etching, smooth vertical sidewalls with a root-mean-squared roughness of 4–6 nm have been achieved. The diodes lased under pulsed current-injection conditions at wavelengths in the range from 419 to 423 nm. The lowest threshold current density was 25 kA/cm2. Lasing was observed in both gain-guided and ridge-waveguide test diodes, with cavity lengths from 300 to 1000 μm; and output powers of 10–20 mW were achieved. Laser performance is illustrated with light output-current and current–voltage characteristics and with a high-resolution optical spectrum.© 1998 American Institute of Physics.

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1077-3118

AIP Digital Archive

Physics

The electronic and structural properties of GaN were investigated for heteroepitaxial layers grown by hydride vapor phase epitaxy. Uniform film nucleation on the sapphire substrates was facilitated by a GaCl pretreatment. The films were all unintentionally doped n type. Variable temperature Hall effect measurements reveal electron concentrations as low as 2×1017 cm−3 and electron mobilities as high as 460 cm2/V s at 300 K. The films exhibit bound exciton photoluminescence lines with a full width at half-maximum (FWHM) of 2.42 meV at 2 K. Transmission electron microscopy studies of the GaN/sapphire interface reveal a ∼200 nm thick, highly defective GaN layer consisting predominantly of stacking faults. The excellent quality of these GaN films is attributed to this "auto-buffer'' layer which enables growth of GaN cells with a dislocation density of ∼3×108 cm−2 after ∼12 μm of film growth. © 1996 American Institute of Physics.

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1077-3118

AIP Digital Archive

Physics

We present first-principle calculations of the structure and energetics of the GaN(101(underbar)1) surface, and present models for the reconstructions. A strong preference for In surface segregation and occupation of specific surface sites is demonstrated. We argue that inverted pyramid defect formation is enhanced by segregation of In on (101(underbar)1) facets. We propose that the chemical ordering recently observed in InGaN alloys is driven by the preference for In incorporation at the sites of reduced N coordination present at step edges during growth on the (0001) and (0001(underbar)) surfaces. © 1999 American Institute of Physics.

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1077-3118

AIP Digital Archive

Physics

Interdiffusion of In and Ga is observed in InGaN/GaN multiple quantum wells for annealing temperatures of 1300–1400 °C. Hydrostatic pressures of up to 15 kbar were applied to prevent surface decomposition. In as-grown material, x-ray diffraction spectra show InGaN diffraction peaks up to the fourth order. After annealing at 1400 °C for 15 min, only the zero-order peak is observed, as a result of compositional disordering of the quantum well superlattice. Transmission electron microscopy confirms that the superlattice is completely disordered after annealing at 1400 °C for 15 min. © 1998 American Institute of Physics.

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1077-3118

AIP Digital Archive

Physics

Band gap measurements have been performed on strained InxGa1−xN epilayers with x≤0.12. The experimental data indicate that the bowing of the band gap is much larger than commonly assumed. We have performed first-principles calculations for the band gap as a function of alloy composition and find that the bowing is strongly composition dependent. At x=0.125 the calculated bowing parameter is b=3.5 eV, in good agreement with the experimental values. © 1998 American Institute of Physics.

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1077-3118

AIP Digital Archive

Physics

Phase separation was found to occur in In0.33Ga0.67N/GaN multiple-quantum-well structures after annealing at 975 °C in a hydrostatic pressure of 5 kbar N2 for 4 h. X-ray diffraction (XRD) spectra of the as-grown samples showed superlattice peaks that were replaced by a broad, single-phase peak after annealing. Transmission electron microscopy (TEM) images of the annealed samples show In-rich precipitates and voids that are found only within the quantum-well region. Both TEM and XRD measurements indicated that the formation of voids and second phases were suppressed after annealing in a hydrostatic pressure of 15 kbar. In addition, optical absorption measurements on these samples showed no indication of a peak at 2.65 eV that was observed in previous annealing studies. © 1999 American Institute of Physics.

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1077-3118

AIP Digital Archive

Physics

Room-temperature Hall effect measurements of (0001) Mg-doped GaN films grown on sapphire substrates by metalorganic chemical vapor deposition show a reduction in hole concentration for Mg concentrations greater than 1020 cm−3. A combination of secondary ion mass spectrometry and transmission electron microscopy indicates a steadily increasing Mg incorporation during growth and the formation of inversion domains at these high concentrations. We discuss mechanisms that could give rise to a reduction of the hole concentration at high Mg doping levels. © 2001 American Institute of Physics.

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1077-3118

AIP Digital Archive

Physics

Homoepitaxial GaN films, doped with Mg, were grown by rf-plasma molecular-beam epitaxy on Ga-polarity (0001) templates. Convergent-beam electron diffraction analysis establishes that the film polarity changes from [0001] to [0001(underbar)] when the Mg flux during growth is approximately 1 ML/s. Secondary ion mass spectrometry indicates a doping concentration of ∼1020 cm−3 in the film where the inversion occurs, and a reduced Mg incorporation in the [0001(underbar)] material. Transmission electron microscopy shows that the inversion domain boundary is faceted predominantly along the {0001} and {h,h,−2h,l} planes, with l/h approximately equal to 3. Using first-principles total energy calculations, we show that the {h,h,−2h,l} segments of the boundary are stabilized by the incorporation of Mg in threefold coordinated lattice sites. © 2000 American Institute of Physics.

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