Search

Your search keyword '"Gass, M. H."' showing total 37 results
Start Over Author "Gass, M. H."
37 results on '"Gass, M. H."'

1. Structural and optical properties of high quality zinc-blende/wurtzite GaAs hetero-nanowires

Author

Spirkoska, D., Arbiol, J., Gustafsson, A., Conesa-Boj, S., Glas, F., Zardo, I., Heigoldt, M., Gass, M. H., Bleloch, A. L., Estrade, S., Kaniber, M., Rossler, J., Peiro, F., Morante, J. R., Samuelson, L., Abstreiter, G., and Morral, A. Fontcuberta i

Subjects
Condensed Matter - Mesoscale and Nanoscale Physics, Condensed Matter - Materials Science
Abstract

The structural and optical properties of 3 different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented. A variety of shorter and longer segments of zinc-blende or wurtzite crystal phases are observed by transmission electron microscopy in the nanowires. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is increased. The downward shift of the emission peaks can be understood by carrier confinement at the interfaces, in quantum wells and in random short period superlattices existent in these nanowires, assuming a staggered band-offset between wurtzite and zinc-blende GaAs. The latter is confirmed also by time resolved measurements. The extremely local nature of these optical transitions is evidenced also by cathodoluminescence measurements. Raman spectroscopy on single wires shows different strain conditions, depending on the wurtzite content which affects also the band alignments. Finally, the occurrence of the two crystallographic phases is discussed in thermodynamic terms., Comment: 24 pages

Published
2009
Full Text
View/download PDF

7. C/CrC nanocomposite coating deposited by magnetron sputtering at high ion irradiation conditions.

Author

Zhou, Z., Rainforth, W. M., Gass, M. H., Bleloch, A., Ehiassarian, A. P., and Hovsepian, P. Eh.

Subjects
IRRADIATION, MAGNETRONS, IONS, AMORPHOUS carbon, X-ray diffraction, TRANSMISSION electron microscopy
Abstract

CrC with the fcc NaCl (B1) structure is a metastable phase that can be obtained under the non-equilibrium conditions of high ion irradiation. A nano-composite coating consisting of amorphous carbon embedded in a CrC matrix was prepared via the unbalanced magnetron sputtering of graphite and Cr metal targets in Ar gas with a high ionized flux (ion-to-neutral ratio Ji/Jn = 6). The nanoscale amorphous carbon clusters self-assembled into layers alternated by CrC, giving the composite a multilayer structure. The phase, microstructure, and composition of the coating were characterized using x-ray diffraction, transmission electron microscopy, and aberration corrected scanning transmission electron microscopy coupled with electron energy loss spectroscopy. The interpretation of the true coating structure, in particular the carbide type, is discussed. [ABSTRACT FROM AUTHOR]

Published
2011
Full Text
View/download PDF

8. Nanotopography of graphene

Author

Bangert, U., Gass, M. H., Bleloch, A. L., Nair, R. R., and Eccles, J.

Subjects
Surfaces and Interfaces, Condensed Matter Physics, Electronic, Optical and Magnetic Materials, Surfaces, Coatings and Films, Physics::Atomic and Molecular Clusters, Materials Chemistry, Physics::Atomic Physics, Electrical and Electronic Engineering
Abstract

Microscopically clean graphene surfaces are scrutinized via atomic resolution high angle dark field imaging and electron energy loss spectroscopy for presence of adsorbed atoms. Carbon ad-atoms can be observed in dark field images, whereas hydrogen is revealed in electron loss spectra. Spectrum images show the spatial distribution of hydrogen in pristine and deliberately H-dosed graphene; hydrogen is found on all graphene surfaces but the coverage is higher and more uniform in dosed samples. On clean surfaces the energy loss is characteristic of the ground level excitation in atomic hydrogen, whereas a spread in the hydrogen energy levels is found in hydrocarbon deposits. A rippling effect in the graphene sheets is revealed when lattice images are processed using fast Fourier transform (FFT) techniques. This contrast effect originates from changes in the bond length projection, due to inclinations of the sheet. It is suggested that point defects - vacancies and ad-atomes - influence the ripple patterns. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Published
2009
Full Text
View/download PDF

14. Properties of GaN Nanowires Grown by Molecular Beam Epitaxy

Author

Geelhaar, L., primary, Chèze, C., additional, Jenichen, B., additional, Brandt, O., additional, Pfüller, C., additional, Münch, S., additional, Rothemund, R., additional, Reitzenstein, S., additional, Forchel, A., additional, Kehagias, T., additional, Komninou, P., additional, Dimitrakopulos, G. P., additional, Karakostas, T., additional, Lari, L., additional, Chalker, P. R., additional, Gass, M. H., additional, and Riechert, H., additional

Published
2011
Full Text
View/download PDF

18. Structural and optical properties of high quality zinc-blende/wurtzite GaAs nanowire heterostructures

Author

Spirkoska, D., primary, Arbiol, J., additional, Gustafsson, A., additional, Conesa-Boj, S., additional, Glas, F., additional, Zardo, I., additional, Heigoldt, M., additional, Gass, M. H., additional, Bleloch, A. L., additional, Estrade, S., additional, Kaniber, M., additional, Rossler, J., additional, Peiro, F., additional, Morante, J. R., additional, Abstreiter, G., additional, Samuelson, L., additional, and Fontcuberta i Morral, A., additional

Published
2009
Full Text
View/download PDF

23. Defect characterization and analysis of III-V nanowires grown by Ni-promoted MBE

Author

Lari, L., primary, Murray, R. T., additional, Gass, M. H., additional, Bullough, T. J., additional, Chalker, P. R., additional, Kioseoglou, J., additional, Dimitrakopulos, G. P., additional, Kehagias, Th., additional, Komninou, Ph., additional, Karakostas, Th., additional, Chèze, C., additional, Geelhaar, L., additional, and Riechert, H., additional

Published
2008
Full Text
View/download PDF

28. Mapping of the effective electron mass in III-V semiconductors.

Author

Cullis, A. G., Hutchison, J. L., Gass, M. H., Sanchez, A. M., Papworth, A. J., Bullough, T. J., Beanland, R., and Chalker, P. R.

Abstract

The effective mass in semiconductors is related to the mobility of charge carriers as well as the density of states. As electron mobility inherently influences semiconductor device performance, knowledge of the effective electron mass (me*) is important, and significant effort is applied to obtain accurate values. In this work, low-loss electron energy loss spectroscopy is exploited to produce maps showing the variation of me* with nanometer scale resolution for a range of semiconductors. The alculated values of all systems have proven to be in agreement with the literature. [ABSTRACT FROM AUTHOR]

Published
2005
Full Text
View/download PDF

29. Nanoanalysis of InAs/GaAs quantum dots using low-loss EELS spectra.

Author

Cullis, A. G., Hutchison, J. L., Sánchez, A. M., Gass, M. H., Papworth, A. J., Beanland, R., Drouot, V., and Goodhew, P. J.

Abstract

The elemental distribution in InAs/GaAs quantum dots has been analysed using the d transition edges in the imaginary part of the dielectric function, ɛ2(E), obtained from the electron energy loss spectrum from a Kramers-Kronig analysis. Changes in the plasmon peak parameters have also been observed in areas of the InAs quantum dot with respect to the GaAs. [ABSTRACT FROM AUTHOR]

Published
2005
Full Text
View/download PDF

30. Changes in plasmon peak position in a GaAs/Tn0.2Ga0.8As structure.

Author

Cullis, A. G., Hutchison, J. L., Beanland, R., Sánchez, A. M., Papworth, A. J., Gass, M. H., and Goodhew, P. J.

Abstract

We have investigated changes in the plasmon loss peak seen in electron energy loss spectra from a 15 nm In0.2Ga0.8As layer in GaAs using a VG HB601 UX FEG-STEM. We observe a relative shift in plasmon peak position which is independent of sample thickness but varies slightly with diffraction condition. We interpret this behaviour as being primarily due to changes in lattice parameter, thus giving a new technique for the quantitative measurement of strain at the nm scale. The resolution of the technique is comparable to that of annular dark field images. [ABSTRACT FROM AUTHOR]

Published
2005
Full Text
View/download PDF

31. Manifestation of ripples in freestanding graphene in lattice images obtained in an aberrationcorrected scanning transmission electron microscope

Author

Bangert, U., Gass, M. H., Bleloch, A. L., Nair, R. R., and Geim, A. K.

Abstract

The shape of ripples in freestanding graphene is derived from bond length distributions in highangle annulardarkfield Zcontrast images, obtained in an aberrationcorrected scanning transmission electron microscope superSTEM. Local undulation patterns can readily be visualized by applying fast Fourier transform procedures to the lattice images, using a spatial frequency filter which passes the precise graphitic aplane spacing. Using aberrationcorrected STEM presents the unique opportunity to correlate atomic resolution imaging with the undulations. Values for projected bond lengths, and hence bond inclinations, can be obtained by adjusting the filter to the appropriate band pass. Ripples with amplitude ∼0.5 nm and width ∼5 nm were observed. Their patterns modify in repeated image scans, with prolonged scanning introducing vacancies. These, in conjunction with adatoms which are also revealed in Zcontrast, induce highly localized changes in the mechanical properties of graphene. Ripples in defected graphene remain dynamic, with changes occurring together with redistribution of defects. © 2009 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim

Published
2009
Full Text
View/download PDF

32. Defect characterization and analysis of IIIV nanowires grown by Nipromoted MBE

Author

Lari, L., Murray, R. T., Gass, M. H., Bullough, T. J., Chalker, P. R., Kioseoglou, J., Dimitrakopulos, G. P., Kehagias, Th., Komninou, Ph., Karakostas, Th., Chèze, C., Geelhaar, L., and Riechert, H.

Abstract

We report on transmission electron microscopy TEM and scanning transmission electron microscopy STEM analysis of GaN and Al0.2Ga0.8N nanowires NWs grown by radio frequency plasma assisted Molecular Beam Epitaxy PAMBE on csapphire substrates. Conventional TEM images of single nanowires removed from the substrate exhibit a high density of basal stacking faults. Further analyses were performed using the aberrationcorrected SuperSTEM facility. Geometric phase analysis GPA was applied to this data revealing the characteristic lattice displacement introduced by intrinsic basal stacking faults. The nickelbased “seeds”, which were employed to promote the NWtype growth, are found to be situated at the tip of each nanowire. Different types of analyses were applied to lattice images of these seeds including GPA, projection method analysis and Fast Fourier transform of the HREM images. All three approaches yielded lattice constants and displacements with good agreement. The measured lattice values in the GaN nanowire body were in good agreement with the lattice constants of relaxed bulk GaN. For the seed particles the resulting lattice spacing was attributable, within the experimental error, to either nickel oxide, Ni3Ga, or an intermediate alloy phase. The defect density was measured in the AlGaN nanowires and was found to be in the order of 106cm–1. The results are critically discussed in relation to the possible growth mechanisms of the NWs. © 2008 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim

Published
2008
Full Text
View/download PDF

33. Nanometer-scale strain measurements in semiconductors: An innovative approach using the plasmon peak in electron energy loss spectra.

Author

Sanchez, A. M., Beanland, R., Papworth, A. J., Goodhew, P. J., and Gass, M. H.

Subjects
NANOSTRUCTURED materials, SEMICONDUCTORS, PLASMONS (Physics), ELECTRON energy loss spectroscopy, ELECTRON spectroscopy, HETEROSTRUCTURES, SUPERLATTICES, PHYSICS
Abstract

We present an innovative technique for quantitative measurement of strain in semiconductor materials with high spatial resolution. The plasmon loss peak, seen in electron energy-loss spectra, has been considered following the Drude-Lorentz model, and we find that plasmon energy is extremely sensitive to lattice parameter. We have tested this model using a heterostructure of In0.2Ga0.8As and AlAs layers in GaAs. The experimental data are in excellent agreement with the model. We estimate that strains smaller than 0.036% can be detected, corresponding to a change of x=0.005 in InxGa1-xAs, at a spatial resolution better than 2.8 nm. [ABSTRACT FROM AUTHOR]

Published
2006
Full Text
View/download PDF

34. Structural and optical properties of high quality zinc-blende/wurtzite GaAs nanowire heterostructures

Author

Spirkoska, D., Arbiol i Cobos, Jordi, Gustafsson, A., Conesa Boj, Sònia, Glas, F., Zardo, I., Heigoldt, M., Gass, M. H., Bleloch, A. L., Estradé Albiol, Sònia, Kaniber, M., Rossler, J., Peiró Martínez, Francisca, Morante i Lleonart, Joan Ramon, Abstreiter, G., Samuelson, L., Fontcuberta i Morral, A., and Universitat de Barcelona

Subjects
Diamond-Type, III-V semiconductors, time resolved spectra, Growth, Condensed Matter::Materials Science, thermodynamics, Electronic-Properties, transmission electron microscopy, Nanotechnology, Twinning Superlattices, Gallium arsenide semiconductors, semiconductor heterojunctions, semiconductor quantum wells, Arsenur de gal·li, Optical properties, Nanotecnologia, Condensed Matter::Other, Zincblende-Type Semiconductors, semiconductor quantum wires, cathodoluminescence, Condensed Matter::Mesoscopic Systems and Quantum Hall Effect, gallium arsenide, Semiconductors, nanowires, Polytypism, photoluminescence, Raman spectra, Mechanism, Propietats òptiques, Stability
Abstract

The structural and optical properties of three different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented. A variety of shorter and longer segments of zinc-blende or wurtzite crystal phases are observed by transmission electron microscopy in the nanowires. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is increased. The downward shift of the emission peaks can be understood by carrier confinement at the interfaces, in quantum wells and in random short period superlattices existent in these nanowires, assuming a staggered band offset between wurtzite and zinc-blende GaAs. The latter is confirmed also by time-resolved measurements. The extremely local nature of these optical transitions is evidenced also by cathodoluminescence measurements. Raman spectroscopy on single wires shows different strain conditions, depending on the wurtzite content which affects also the band alignments. Finally, the occurrence of the two crystallographic phases is discussed in thermodynamic terms.

36. Structural and optical properties of high quality zinc-blende/wurtzite GaAs nanowire heterostructures

Author

Universitat de Barcelona, Spirkoska, D., Arbiol i Cobos, Jordi, Gustafsson, A., Conesa Boj, Sònia, Glas, F., Zardo, I., Heigoldt, M., Gass, M. H., Bleloch, A. L., Estradé Albiol, Sònia, Kaniber, M., Rossler, J., Peiró Martínez, Francisca, Morante i Lleonart, Joan Ramon, Abstreiter, G., Samuelson, L., Fontcuberta i Morral, A., Universitat de Barcelona, Spirkoska, D., Arbiol i Cobos, Jordi, Gustafsson, A., Conesa Boj, Sònia, Glas, F., Zardo, I., Heigoldt, M., Gass, M. H., Bleloch, A. L., Estradé Albiol, Sònia, Kaniber, M., Rossler, J., Peiró Martínez, Francisca, Morante i Lleonart, Joan Ramon, Abstreiter, G., Samuelson, L., and Fontcuberta i Morral, A.

Abstract

The structural and optical properties of three different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented. A variety of shorter and longer segments of zinc-blende or wurtzite crystal phases are observed by transmission electron microscopy in the nanowires. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is increased. The downward shift of the emission peaks can be understood by carrier confinement at the interfaces, in quantum wells and in random short period superlattices existent in these nanowires, assuming a staggered band offset between wurtzite and zinc-blende GaAs. The latter is confirmed also by time-resolved measurements. The extremely local nature of these optical transitions is evidenced also by cathodoluminescence measurements. Raman spectroscopy on single wires shows different strain conditions, depending on the wurtzite content which affects also the band alignments. Finally, the occurrence of the two crystallographic phases is discussed in thermodynamic terms.

37. Elemental mapping using the Ga 3d and In 4d transitions in the epsilon2 absorption spectra derived from EELS.

Author

Gass MH, Papworth AJ, Bullough TJ, and Chalker PR

Subjects
Microscopy, Electron, Scanning Transmission, Electron Probe Microanalysis, Gallium chemistry, Spectrum Analysis
Abstract

It is proposed that by using the valence-band states in electron energy loss spectroscopy, high-spatial resolution maps of quantitative elemental composition may be acquired with high acquisition rates. Further, it is shown that by using the epsilon(2) spectrum instead of single scattering data, the noise in the observed transitions and associated maps is significantly reduced. The epsilon(2) spectra are derived through a Kramers-Kronig transformation from electron energy loss spectra obtained in a scanning transmission electron microscope. Using transitions that occur in the epsilon(2) absorption spectrum (<40eV), quantitative elemental maps for III-V device structures have been produced. An example is provided using the Ga 3d transition to map a GaInNAs/GaAs laser structure. Weaker transitions such as In 4d have also been used to verify the Ga elemental distribution.

Published
2004
Full Text
View/download PDF

Catalog

Books, media, physical & digital resources
See catalog results