| 1. |
- Monemar, Bo, 1942-, et al.
(författare)
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Defects in Gallium Nitride
- 1999
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Ingår i: International Workshop on Materials Science,1999. - Proc. of the International Workshop on Materials Science 99, ed. by F. F. Bekker et al., Vol. 1 : Hanoi National University Publishing House. ; , s. 28-
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Konferensbidrag (refereegranskat)
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| 2. |
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| 3. |
- Monemar, Bo, 1942-, et al.
(författare)
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The 3.466 eV Bound Exciton in GaN
- 2001
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Ingår i: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 228:2, s. 489-492
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Tidskriftsartikel (refereegranskat)abstract
- We discuss the available optical data for the 3.466 eV bound exciton in GaN, which has been a controversial issue in the recent literature. We conclude that the experimental results are only consistent with the identification as an exciton bound at a neutral acceptor with a spin-like bound hole. The chemical identity is still not clear.
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| 4. |
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| 5. |
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| 6. |
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| 7. |
- 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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| 8. |
- 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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| 9. |
- 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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| 10. |
- 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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| 11. |
- 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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| 12. |
- Darakchieva, Vanya, 1971-, et al.
(författare)
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Strain-related structural and vibrational properties of thin epitaxial AIN layers
- 2004
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 70:4, s. 045411-
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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)
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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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| 14. |
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| 15. |
- Monemar, Bo, 1942-, et al.
(författare)
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Growth of thick GaN layers by hydride vapor phase epitaxy
- 2005
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Ingår i: Journal of Ceramic Processing Research. - 1229-9162. ; 6:2, s. 153-162
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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)
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Anisotropy of the free exciton emission in GaN grown on a-plane sapphire
- 2002
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Ingår i: Physica status solidi. A, Applied research. - 0031-8965 .- 1521-396X. ; 190:1, s. 75-79
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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)
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Anisotropy of the in-plane strain in GaN grown on A-plane sapphire
- 2002
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Ingår i: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 234:3, s. 892-896
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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)
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Internal structure of free excitons in GaN
- 2001
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Ingår i: Physica status solidi. B, Basic research. - 0370-1972 .- 1521-3951. ; 228:2, s. 467-470
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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)
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Optical properties of nonpolar a-plane GaN layers
- 2006
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Ingår i: Superlattices and Microstructures. - : Elsevier BV. - 0749-6036 .- 1096-3677. ; 40:4-6 SPEC. ISS., s. 253-261
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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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| 21. |
- Paskov, Plamen, 1959-, et al.
(författare)
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Photoluminescence study of near-surface GaN/AlN superlattices
- 2008
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. ; , s. 68940G1-
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Konferensbidrag (refereegranskat)abstract
- We report on the emission properties of GaN/AlN superlattices (SLs) grown by metalorganic chemical vapor deposition on a thick GaN layer. Nominally undoped and Si-doped SL structures with the well/barrier thickness ratio 3:1 and different SL periods are investigated. It is found that in these SLs without capping layer the energy position, intensity and linewidth of the emission are determined by the interplay of the built-in polarization field, the depletion field arising from the pinning of the Fermi level at the surface, and the screening of the electric field in the quantum well due to the both the polarization-induced two-dimension electron gas (2DEG) and the photo-generated carriers. A non-uniform equilibrium electron distribution and an electron accumulation at the bottom AlN/GaN interface are evidenced by the observed recombination of the 2DEG with the photo-excited holes occurring below the GaN bandgap.
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| 22. |
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| 23. |
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| 24. |
- Paskova, Tanja, 1961-, et al.
(författare)
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Characterization of mass-transport grown GaN by hydride vapour-phase epitaxy
- 2004
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 273:1-2, s. 118-128
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Tidskriftsartikel (refereegranskat)abstract
- A comprehensive study of the morphological, optical and microstructural properties of mass-transport (MT) and conventionally grown GaN by hydride vapour-phase epitaxy is presented. Spatially resolved techniques have been utilized to reveal in a comparative way, the characteristics of the material grown either in predominant vertical or lateral growth modes. A strong donor-acceptor pair (DAP) emission is observed from the MT regions with a distinctive intensity contrast between the exciton and DAP emission bands from MT and nontransport regions. Secondary ion mass spectroscopy and imaging were employed to investigate the impurity incorporation into different regions. An increase of residual oxygen and aluminium impurity concentrations was found in the MT areas. In addition, positron annihilation spectroscopy showed a strong signal of Ga vacancy clusters in the MT grown material. The increase of the point defect concentrations of both Ga vacancy and oxygen impurity, most likely forming defect complexes, is related to the enhancement of the DAP emission.
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| 25. |
- Paskova, Tanja, 1961-, et al.
(författare)
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Defect and emission distributions in bulk GaN grown in polar and nonpolar directions : a comparative analysis
- 2008
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. ; , s. 68940D1-
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Konferensbidrag (refereegranskat)abstract
- We have investigated bulk GaN material grown by HVPE either in the conventional polar [0001] direction and subsequently sliced with nonpolar surfaces or grown in the nonpolar [11-20] direction. Spatially resolved techniques such as cathodoluminescence imaging and transmission electron microscopy, as well as profile measuring techniques such as positron annihilation spectroscopy and secondary ion mass spectroscopy were employed to directly visualize the extended structural defects, and point defect (impurity and vacancy) distributions along the growth axes. A comparative analysis of the results shows a distinctive difference in the distribution of all kind of defects along the growth axes. A significant decrease in the defect density in material grown along the polar direction, in contrast to the constant behavior of the high defect density in material grown along the nonpolar direction points out the low-defect superior quality of the former material and indicates the preferable way of producing high-quality GaN substrates with nonpolar surfaces.
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| 26. |
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| 27. |
- Paskova, Tanja, 1961-, et al.
(författare)
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Effect of sapphire-substrate thickness on the curvature of thick GaN films grown by hydride vapor phase epitaxy
- 2007
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 102:12
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Tidskriftsartikel (refereegranskat)abstract
- The effect of sapphire-substrate thickness on the curvature and stress in thick hydride vapor phase epitaxial GaN films was studied by high-resolution x-ray diffraction at variable temperatures. The curvature was found to have the maximum value for comparable thicknesses of the film and the substrate, while the stress at the film surface decreases with increasing film thickness and increases with increasing substrate thickness, which is in very good agreement with the simulation results. The curvature at the growth temperature was found to be strongly influenced by the value of the intrinsic tensile strain, which is determined by the film/substrate thickness ratio. © 2007 American Institute of Physics.
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| 28. |
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| 29. |
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| 30. |
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| 31. |
- Paskova, Tanja, 1961-, et al.
(författare)
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Strain-free low-defect-density bulk GaN with nonpolar orientation
- 2006
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Ingår i: MRS Proceedings 2006 MRS Fall Meeting. - New York, NY, United States : Materials Research Society. ; , s. I3.4-
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Konferensbidrag (refereegranskat)abstract
- Bulk GaN sliced in bars along (11-20) and (1-100) planes from a boule grown in the [0001] direction by HVPE was confirmed as strain free material with a low dislocation density by using several characterization techniques. The high-structural quality of the material allows photoluminescence studies of free excitons, principal donor bound excitons and their two-electron satellites with regard to the optical selection rules. Raman scattering study of the bulk GaN with nonpolar orientations allows a direct access to the active phonon modes and a direct determination of their strain-free positions.
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| 32. |
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| 33. |
- Strocov, V.N., et al.
(författare)
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Band structure effects in nitrogen K-edge resonant inelastic X-ray scattering from GaN
- 2004
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Ingår i: Physica status solidi. B, Basic research. - : Wiley. - 0370-1972 .- 1521-3951. ; 241:7
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Tidskriftsartikel (refereegranskat)abstract
- Systematic experimental data on resonant inelastic X-ray scattering (RIXS) in GaN near the N K-edge are presented for the first time. Excitation energy dependence of the spectral structures manifests the band structure effects originating from momentum selectivity of the RIXS process. This finding allows obtaining k-resolved band structure information for GaN crystals and nanostructures. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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| 34. |
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