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- Darakchieva, Vanya, 1971-, et al.
(författare)
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Anisotropic strain and phonon deformation potentials in GaN
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:19, s. 195217-
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Tidskriftsartikel (refereegranskat)abstract
- We report optical phonon frequency studies in anisotropically strained c -plane- and a -plane-oriented GaN films by generalized infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The anisotropic strain in the films is obtained from high-resolution x-ray diffraction measurements. Experimental evidence for splitting of the GaN E1 (TO), E1 (LO), and E2 phonons under anisotropic strain in the basal plane is presented, and their phonon deformation potentials c E1 (TO), c E1 (LO), and c E2 are determined. A distinct correlation between anisotropic strain and the A1 (TO) and E1 (LO) frequencies of a -plane GaN films reveals the a A1 (TO), b A1 (TO), a E1 (LO), and b E1 (LO) phonon deformation potentials. The a A1 (TO) and b A1 (TO) are found to be in very good agreement with previous results from Raman experiments. Our a A1 (TO) and a E1 (LO) phonon deformation potentials agree well with recently reported theoretical estimations, while b A1 (TO) and b E1 (LO) are found to be significantly larger than the theoretical values. A discussion of the observed differences is presented. © 2007 The American Physical Society.
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- Darakchieva, Vanya, et al.
(författare)
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Assessment of phonon mode characteristics via infrared spectroscopic ellipsometry on a-plane GaN
- 2006
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Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 243:7, s. 1594-1598
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Tidskriftsartikel (refereegranskat)abstract
- Generalized infrared spectroscopic ellipsometry was applied to study the vibrational properties of anisotropically strained a-plane GaN films with different thicknesses. We have established a correlation between the phonon mode parameters and the strain, which allows the determination of the deformation potentials and strain-free frequency of the GaN A,(TO) mode. These results are compared with previous theoretical and experimental findings and discussed.
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- Darakchieva, Vanya, 1971-, et al.
(författare)
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Optical phonons in a-plane GaN under anisotropic strain
- 2008. - 1
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Ingår i: Group-III nitrides with nonpolar surfaces: growth, properties and devices. - : Wiley. - 9783527407682 ; , s. 219-253
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- This is the first monograph to discuss in detail the current stage of development of nonpolar nitrides, with specific emphasis on the three main topics of crystal growth, properties and device studies. World–class researchers summarize their own recent achievements in their respective fields of expertise, covering both nonpolar and semipolar nitride materials. The bulk of the discussion in each chapter is related to the physical properties of the material obtained by the respective technique, in particular, defect density and properties of the defects in nonpolar nitrides. In addiiton, the optical and vibrational properties are also addressed in several chapters, as is progress in heterostructures, quantum wells and dots based on the AlGaN/GaN and the InGaN/GaN systems. Finally, an outlook of the application areas of the differently grown materials is presented in most chapters, together with the capabilities and limitations of the respective growth approaches used.
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- Darakchieva, Vanya, 1971-, et al.
(författare)
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Phonon mode behavior in strained wurtzite AlN/GaN superlattices
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 71:11, s. 115329-
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Tidskriftsartikel (refereegranskat)abstract
- We have studied phonons in AlN/GaN superlattices with different periods but a constant well-to-barrier ratio using a combination of infrared spectroscopic ellipsometry and Raman scattering spectroscopy. The strain evolution in the superlattice structures is assessed by high-resolution x-ray diffraction and reciprocal space mapping. We have identified E1(TO), A 1(LO) and E2 localized, and E1(LO) and A 1(TO) delocalized superlattice modes. The dependencies of their frequencies on in-plane strain are analyzed and discussed, and the strain-free frequencies of the superlattice modes are estimated. A good agreement between theory and experiment is found in the case of GaN localized modes, while large deviations between theoretically estimated and experimentally determined frequency shifts are observed for the AlN localized modes. The delocalization effect on the A1(TO) and E1(LO) phonons, as well as the free-carrier effect on the E1(LO) phonon are also discussed. ©2005 The American Physical Society.
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- Darakchieva, Vanya, 1971-, et al.
(författare)
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Phonons in strained AlGaN/GaN superlattices
- 2007
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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-
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Konferensbidrag (refereegranskat)
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- Darakchieva, Vanya, 1971-, et al.
(författare)
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Structural characteristics and lattice parameters of hydride vapor phase epitaxial GaN free-standing quasisubstrates
- 2005
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 97:1, s. 013517-
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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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- Figge, S, et al.
(författare)
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Optoelectronic devices on bulk GaN
- 2005
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 281:1, s. 101-106
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Tidskriftsartikel (refereegranskat)abstract
- The homoeptaxial fabrication of GaN-based devices has advantages against heteroepitaxial realization on substrates such as sapphire or SiC, since heteroepitaxy implies a lot of problems like lattice mismatch, different thermal expansion coefficients, and needs an extensive optimization of the growth at the heterointerface. In this paper we will discuss GaN-based light-emitting devices grown by homoepitaxy in comparison to devices grown on sapphire. A special emphasis is laid on the pretreatment of the GaN substrate and the device characteristics on different substrates. In detail will be discussed the advantages of the higher thermal conductivity of GaN and how this effects the device performance. (c) 2005 Elsevier B.V. All rights reserved.
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