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Sökning: WFRF:(Paskova Tanja 1961 )

  • Resultat 11-20 av 34
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11.
  • Darakchieva, Vanya, 1971-, et al. (författare)
  • Phonons in strained AlGaN/GaN superlattices
  • 2007
  • Ingår i: 6th International Symposium on Blue Laser and Light Emitting Diodes,2006. - Physica Status Solidi, vol C4 : WILEYVCH Verlag GmbH & Co. KGaA. ; , s. 170-
  • Konferensbidrag (refereegranskat)
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12.
  • Darakchieva, Vanya, 1971-, et al. (författare)
  • Strain-related structural and vibrational properties of thin epitaxial AIN layers
  • 2004
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 70:4, s. 045411-
  • Tidskriftsartikel (refereegranskat)abstract
    • The effect of film thickness on the strain and structural properties of thin epitaxial AIN films has been investigated by high resolution x-ray diffraction techniques and transmission electron microscopy. As a result a sublayer model of the degree of strain and related defects for all films is proposed. A sublayer with low defect density and a strain gradient is found to be present in all films and it reaches a maximum thickness of 65 nm. The films are compressively strained and the strain relaxation after a thickness of 65 nm is shown to be accompanied by misfit dislocation generation and increase of the mosaic tilt. The vibrational properties of the films have been studied by generalized infrared spectroscopic ellipsometry. The proposed sublayer model has been successfully applied to the analysis of the ellipsometry data through model calculations of the infrared dielectric function which confirm the sublayer model. It is found that the strain gradient results in a gradient of the phonon mode frequencies and broadening parameter. The initial strain relaxation in the films leads to narrowing of the observable infrared modes, while further strain relaxation broadens the modes when substantial defect generation occurs.
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13.
  • Darakchieva, Vanya, 1971-, et al. (författare)
  • Structural characteristics and lattice parameters of hydride vapor phase epitaxial GaN free-standing quasisubstrates
  • 2005
  • Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 97:1, s. 013517-
  • Tidskriftsartikel (refereegranskat)abstract
    • We have studied the lattice parameters of hydride vapor phase epitaxy (HVPE)-GaN quasisubstrates in relation to their structural properties. Layers grown on single-layer metalorganic vapor phase epitaxy (MOVPE) templates and on epitaxial lateral overgrown MOVPE templates are characterized by Raman scattering, high-resolution x-ray diffraction, and reciprocal space mapping. The strain relaxation in the films versus their thickness was found to proceed similarly in the GaN samples grown using the two types of templates but the strain saturates at different nonzero levels. The lattice parameters of relatively thin HVPE-GaN free-standing quasisubstrates indicate that no total strain relaxation is achieved after the sapphire removal. The lattice parameters of the thick quasisubstrates grown on different templates are not affected by the separation process and are found to have values very close to the reference strain-free lattice parameters of GaN powder. © 2005 American Institute of Physics.
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14.
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15.
  • Monemar, Bo, 1942-, et al. (författare)
  • Growth of thick GaN layers by hydride vapor phase epitaxy
  • 2005
  • Ingår i: Journal of Ceramic Processing Research. - 1229-9162. ; 6:2, s. 153-162
  • Tidskriftsartikel (refereegranskat)abstract
    • In this paper we describe recent experimental work on the growth of thick GaN layers (up to >300 μm) on sapphire with hydride vapour phase epitaxy (HVPE), the removal of the sapphire substrate by the laser liftoff technique, and the properties of these thick GaN layers. Two different growth setups were used, one horizontal and one vertical system. Specific conditions in the growth procedure, like gas flow pattern, growth rate and the use of buffer layers, strongly influence the properties of the grown layers. Important defect problems are cracking (both during and after growth), and the generation of dislocations and surface pits. A large bowing is also observed for thick layers, depending very much on the properties of the initially grown material. For growth of thick layers excessive parasitic growth of GaN upstream of the substrate has to be avoided. Laser liftoff is demonstrated to be a feasible process to remove the sapphire substrate, causing the GaN surface bowing to decrease and revert from convex to concave. The threading dislocation density of 300 μm thick GaN layers is found to be about 107 cm-2, rather independent of the type of buffer layer employed. It reduces further in thicker layers. The pit density varies with growth conditions, it can be reduced if the parasitic growth is avoided. The bowing is a serious problem, since the layers have to be polished to make the surface epi-ready. The XRD rocking curve widths measured seem to correlate with the bowing of the layers, a reduction by about a factor two is often observed when the substrate is removed. Optical characterisation like photoluminescence (PL) and ir spectroscopic ellipsometry (IRSE) is very useful to monitor strain in the layers, as well as impurities and point defects. Residual shallow donors are related to O and Si, shallow acceptors are mainly of intrinsic origin, i.e. complexes with the Ga vacancy.
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16.
  • Paskov, Plamen, 1959-, et al. (författare)
  • Anisotropy of the free exciton emission in GaN grown on a-plane sapphire
  • 2002
  • Ingår i: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 190:1, s. 75-79
  • Tidskriftsartikel (refereegranskat)abstract
    • The influence of the anisotropic in-plane strain on the optical response of GaN layers grown on a-plane sapphire is investigated. A splitting of the G5 states of the A and B free excitons into two components, polarized parallel and perpendicular to the strain direction, is observed. The experimental results are discussed in the framework of the effective exciton Hamiltonian formalism.
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17.
  • Paskov, Plamen, 1959-, et al. (författare)
  • Anisotropy of the in-plane strain in GaN grown on A-plane sapphire
  • 2002
  • Ingår i: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 234:3, s. 892-896
  • Tidskriftsartikel (refereegranskat)abstract
    • A comparative study of GaN layer grown by hydride vapour phase epitaxy on A-plane sapphire before and after removal of the substrate is presented. A large anisotropy of the in-plane strain in the as-grown sample is revealed by X-ray diffraction measurements and polarized photoluminescence. The strain anisotropy is found to modify the selection rules for the transitions leading to a splitting of the optically active states of the A and B excitons. Almost complete strain relaxation and recovery of the optical isotropy in the (0001) plane is observed in the free-standing layer.
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18.
  • Paskov, Plamen, 1959-, et al. (författare)
  • Internal structure of free excitons in GaN
  • 2001
  • Ingår i: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 228:2, s. 467-470
  • Tidskriftsartikel (refereegranskat)abstract
    • Polarized photoluminescence is used to study the fine structure of free excitons in thick GaN layers grown on differently oriented sapphire substrates. The singlet-triplet splitting of the A exciton is measured and the exchange interaction constant in GaN is determined. For the samples grown on the a-plane sapphire, splitting of the A and B excitons induced by the uniaxial in-plane stress is also observed.
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19.
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20.
  • Paskov, Plamen, 1959-, et al. (författare)
  • Optical properties of nonpolar a-plane GaN layers
  • 2006
  • Ingår i: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 40:4-6 SPEC. ISS., s. 253-261
  • Tidskriftsartikel (refereegranskat)abstract
    • We have studied optical properties of nonpolar a-plane GaN layers grown on r-plane sapphire by metalorganic chemical vapor deposition and hydride vapor phase epitaxy using different nucleation schemes. Several emission bands, which are not typical for c-plane GaN, are observed in the photoluminescence spectra and their excitation-intensity, temperature, and polarization dependencies are examined. In addition, the spatial distribution of the emissions was examined by cathodoluminescence imaging and relations of the different emissions with particular structural features in the layers are revealed. The results are discussed with emphasis on the origin of the emission line and particular recombination mechanisms. © 2006 Elsevier Ltd. All rights reserved.
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