| 1. |
- Ashkenov, N., et al.
(författare)
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Infrared dielectric functions and phonon modes of high-quality ZnO films
- 2003
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 93:1, s. 126-133
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Tidskriftsartikel (refereegranskat)abstract
- A study was performed on the phonon modes and infrared dielectric functions of high-quality ZnO thin films. The pulsed laser deposition technique was used to deposit the ZnO films on c-plane sapphire substrates and were investigated by high-resolution transmission electron microscopy, high-resolution x-ray diffraction and Rutherford backscattering experiments. The accurate long-wavelength dielectric constant limits of the films were also obtained and were compared with near-band-gap index-of-refraction data upon the Lyddane-Sachs-Teller relation for both film and bulk samples. It was found that the phonon modes of the film were highly consistent with those of the bulk sample.
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| 2. |
- Atanassov, A, et al.
(författare)
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Grazing incident asymmetric X-ray diffraction of beta-FeSi2 layers, produced by ion beam synthesis
- 2004
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Ingår i: Vacuum. - : Elsevier BV. - 0042-207X .- 1879-2715. ; 76:02-Mar, s. 277-280
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Tidskriftsartikel (refereegranskat)abstract
- The crystal structure of beta-FeSi2 phase, prepared by ion beam synthesis (IBS) method, followed by rapid thermal annealing (RTA) is investigated by grazing incident asymmetric X-ray diffraction (GIAXRD). The X-ray spectra, obtained at different grazing angles, indicated that the beta-FeSi2 phase is formed in the whole implantation range. From the comparison of the reflections intensities ratios, it is found that in the metal-deficient regions, where the beta-FeSi2 phase is present in the form of precipitates, the crystallites orientation is influenced by the one of the silicon substrates, while the orientation in the metal-rich region is different and depends on the annealing temperature. (C) 2004 Elsevier Ltd. All rights reserved.
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| 3. |
- Darakchieva, Vanya, et al.
(författare)
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Deformation potentials of the E-1(TO) mode in AlN
- 2002
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 80:13, s. 2302-2304
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Tidskriftsartikel (refereegranskat)abstract
- The deformation potentials of the E-1(TO) mode in AlN are experimentally determined by combining infrared reflection spectroscopy and x-ray diffraction measurements and using a reported value of the Raman-stress factor for hydrostatically stressed bulk AlN. The deformation potentials are found to strongly depend on published stiffness constants of AlN. A comparison with earlier theoretically calculated values of the deformation potentials is made. (C) 2002 American Institute of Physics.
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| 4. |
- Darakchieva, Vanya, 1971-, et al.
(författare)
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Deformation potentials of the E1 (TO) and E2 modes of InN
- 2004
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 84:18, s. 3636-3638
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Tidskriftsartikel (refereegranskat)abstract
- The determination of deformation potentials of E1(TO) and E 2 modes of InN were discussed. The deformation potentials were evaluated for two sets of stiffness constants using x-ray diffraction, IR spectroscopic ellipsometry (IRSE), Raman scattering, and Grüneisen parameter values. The InN layer were grown on GaN buffer layers on (0001) sapphire by molecular beam epitaxy. It was found that the strain-free values of the InN E1(TO) mode was 477.9 cm-1 and 491.9 cm -1 for the E2 modes.
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| 5. |
- Darakchieva, Vanya, et al.
(författare)
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Generalized infrared ellipsometry study of thin epitaxial AlN layers with complex strain behavior
- 2003
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Konferensbidrag (refereegranskat)abstract
- The effect of film thickness on the strain and structural properties of thin epitaxial AlN films has been investigated, and a sub-layer model of the degree of strain and related defects for all films is suggested. The vibrational properties of the films have been studied by generalized infrared spectroscopic ellipsometry. The proposed sub-layer model has been successfully applied to the analysis of the ellipsometry data trough model calculations of the infrared dielectric function. © 2003 Elsevier B.V. All rights reserved.
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| 6. |
- Darakchieva, Vanya, et al.
(författare)
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Infrared ellipsometry and Raman studies of hexagonal InN films : Correlation between strain and vibrational properties
- 2004
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Ingår i: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 36:4-6, s. 573-580
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Tidskriftsartikel (refereegranskat)abstract
- The vibrational properties of InN films with different strain have been studied using Infrared ellipsometry and Raman scattering spectroscopy. We have established a correlation between the phonon mode parameters and the strain, which allows the determination of the deformation potentials and the strain-free frequencies of the InN E1(TO) and E2 modes. The LO phonons and their coupling to the free-carrier plasmon excitations are also discussed in relation to the carrier concentration in the films. © 2004 Elsevier Ltd. All rights reserved.
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| 7. |
- Darakchieva, Vanya, et al.
(författare)
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Lattice parameters of GaN layers grown on a-plane sapphire : Effect of in-plane strain anisotropy
- 2003
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 82:5, s. 703-705
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Tidskriftsartikel (refereegranskat)abstract
- The lattice parameters of GaN layers grown on a-plane sapphire were investigated. The hydride vapor phase epitaxy and metalorganic vapor phase epitaxy were used for the determination of parameters. The strain anisotropy was found to have different values in the films and obtained values of parameters were grouped around two values.
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| 8. |
- Darakchieva, Vanya, et al.
(författare)
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Residual strain in HVPE GaN free-standing and re-grown homoepitaxial layers
- 2003
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Ingår i: Physica status solidi. A, Applied research. - : Wiley. - 0031-8965 .- 1521-396X. ; 195:3, s. 516-522
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Tidskriftsartikel (refereegranskat)abstract
- The lattice parameters of as-grown hydride vapor phase epitaxy GaN layers on sapphire, free-standing layers after the substrate lift-off, and homoepitaxial layers grown on the free-standing layers are measured. The in-plane and out-of-plane strains are calculated. It is found that the substrate removal leads to strain relaxation in the crack-free GaN free-standing layers to a certain extent. A small increase of the strain in the GaN homoepitaxial layers compared to the free-standing layers is observed. Cathodoluminescence (CL) spectroscopy and imaging, photoluminescence (PL) and Raman measurements are used as complementary tools in the residual strain analysis.
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| 9. |
- Darakchieva, Vanya, et al.
(författare)
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Strain evolution in high temperature AlN buffer layers for HVPE-GaN growth
- 2002
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Ingår i: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 190:1, s. 59-64
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Tidskriftsartikel (refereegranskat)abstract
- High temperature AlN buffer layers are deposited on a-plane sapphire by reactive magnetron sputtering. The effect of the buffer thickness on the AlN structural properties and surface morphology are studied in correlation with the subsequent hydride vapour phase epitaxy of GaN. A minimum degree of mosaicity and screw dislocation density is determined for a 50 nm thick AlN buffer. With increasing the AlN thickness, a strain relaxation occurs as a result of misfit dislocation generation and higher degree of mosaicity. A blue shift of the E-1(TO) frequency evaluated by means of infrared reflection spectroscopy is linearly correlated with an increase in biaxial compressive stress in the films through the IR stress factor k(E1)(b) = 2.57 +/- 0.26 cm(-1) GPa(-1).
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| 10. |
- Darakchieva, Vanya, 1971-
(författare)
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Strain-related structural and vibrational properties of group-III nitride layers and superlattices
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This PhD thesis is focused on strain-related phenomena in group-III nitride layers and heterostructures. Key issues in material properties as phonon mode behavior, structure and lattice parameters of AlN, InN and GaN, as well as of AlN/GaN superlattices are addressed in order to give answers to some open questions. The research is motivated by the enormous technological application potential of the group-III nitride materials for optoelectronic devices and high temperature, and high power devices. Due to the lack of native substrates, the group-III nitrides are typically grown on foreign substrates resulting inbuilt-in strain in the films, as well as, in misorientation and defect formation. Substantial efforts have been concentrated on the research of strain-related fundamental properties of group-III nitrides. However, some of the strain-related structural and vibrational properties of these materials remain unclear and this gap has to be filled in order to fully explore the possibilities for applications.The thesis contributes to the knowledge of the strain-related phenomena ingroup-III nitrides by concentrating on the following: i) lattice parameters and strain relaxation in GaN, AlN, InN films with different thicknesses, as well as free-standing GaN quasi-substrates; ii) phonon mode behavior and deformation potentials of AlN and InN; iii) structural evolution in AlN and GaN; iv)phonon mode behavior and strain evolution in AlN/GaN superlattices. The layers studied were grown by a variety of growth techniques and nucleation schemes aiming at establishing of a comprehensive understanding of the material properties.The thesis is organized as follows: In the first part a general description of the basic properties of group-III nitrides is given with a special emphasis on the structural and vibrational properties. The vibrational properties of lowdimensional group-III nitrides are also reviewed. After that, basic concepts of strain phenomenon in group-III nitrides, as well as the strain effects on phonons and on structural properties are discussed. Finally, the experimental techniques used are introduced.The second part of the thesis consists of six papers providing informationon specific strain-related structural and vibrational properties of III-nitride layers and superlattices.The lattice parameters of GaN films grown by two different techniques on a-plane sapphire are the subject of Paper I. We have determined two different values of the in-plane lattice parameter of the GaN layers in contrast to only one in the case of films grown on c-plane sapphire. We suggest that the observed distortion of the hexagonal symmetry can be attributed to the presence of anisotropic in-plane strain in the films. A new approach to determination of lattice parameters has been proposed.Paper II deals with the lattice parameters of GaN quasi-substrates grown by hydride vapor phase epitaxy using two nucleation schemes. The lattice parameters of both, Ga- and N-polar faces of the quasi-substrates were determined and compared. The strain relaxation phenomena in such free-standing layers and the concept of strain-free lattice parameters are discussed.Papers III and IV are devoted to the deformation potentials of the E 1 (TO)mode in AlN, and the E1 (TO) and E2 modes in InN, where a combination of infraredellipsometry, infrared reflection, Raman scattering and x-ray diffractionstudies was used. The reported results in Paper III present the first experimentalvalues for the E1 (TO) deformation potentials of AlN and the results for the InN deformation potentials in Paper IV are the first ever reported.The strain evolution and its effect on the structural and vibrational properties of thin epitaxial AlN layers with different thicknesses have been studied in Paper V. A model of the strain evolution with film thickness and related defect structure is suggested and it is successfully used in the infrared ellipsometry data analysis.Paper VI is focused on the strain evolution in AlN/GaN superlattices withdifferent periods and its effect on their phonon properties. The presence of phonons originating from the superlattice constituents was revealed and their nature is discussed.
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