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| 2. |
- Labit, B., et al.
(författare)
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Dependence on plasma shape and plasma fueling for small edge-localized mode regimes in TCV and ASDEX Upgrade
- 2019
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 59:8
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Tidskriftsartikel (refereegranskat)abstract
- © 2019 Institute of Physics Publishing. All rights reserved. Within the EUROfusion MST1 work package, a series of experiments has been conducted on AUG and TCV devices to disentangle the role of plasma fueling and plasma shape for the onset of small ELM regimes. On both devices, small ELM regimes with high confinement are achieved if and only if two conditions are fulfilled at the same time. Firstly, the plasma density at the separatrix must be large enough (ne,sep/nG ∼ 0.3), leading to a pressure profile flattening at the separatrix, which stabilizes type-I ELMs. Secondly, the magnetic configuration has to be close to a double null (DN), leading to a reduction of the magnetic shear in the extreme vicinity of the separatrix. As a consequence, its stabilizing effect on ballooning modes is weakened.
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| 3. |
- Aradi, B., et al.
(författare)
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Ab initio density-functional supercell calculations of hydrogen defects in cubic SiC
- 2001
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 63:245202
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Tidskriftsartikel (refereegranskat)abstract
- Based on ab initio density-functional calculations in supercells of 3C-SiC, the stable configurations of hydrogen and dihydrogen defects have been established. The calculated formation energies are used to give semiquantitative estimates for the concentration of hydrogen in SiC after chemical vapor deposition, low temperature H-plasma anneal, or heat treatment in high temperature hydrogen gas. Vibration frequencies, spin distributions, and occupation levels were also calculated in order to facilitate spectroscopic identification of these defects. (V+nH) complexes are suggested as the origin of some of the signals assigned earlier to pure vacancies. Qualitative extrapolation of our results to hexagonal polytypes explains observed electrical passivation effects of hydrogen.
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| 4. |
- Aradi, B, et al.
(författare)
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Boron centers in 4H-SiC
- 2001
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Ingår i: Materials science Forum, Vols. 353-356. ; , s. 455-458
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Konferensbidrag (refereegranskat)abstract
- The origin of the "deep boron related acceptor level" in SIC is subject to a lot of controversy. Based on ENDOR investigations, a B-Si+V-C model was suggested, while PL studies indicated the acceptor on the carbon sublattice. Our former ab initio LDA molecular cluster calculation showed that in the B-Si+V-C complex the carbon vacancy acts as the acceptor. Now, ah initio LDA supercell calculations have been carried out for boron-related complexes to calculate the occupation levels in 4H-SiC. It has been found that the 0/- level for the B-Si+V-C complex lies in the upper half of the gap, therefore it can be disregarded as the origin of the "deep boron-related acceptor level". Investigating other feasible boron-related complexes, B-Si+Si-C appears to be the best candidate.
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| 5. |
- Aradi, B., et al.
(författare)
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Impurity-controlled dopant activation : Hydrogen-determined site selection of boron in silicon carbide
- 2001
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 79:17, s. 2746-2748
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Tidskriftsartikel (refereegranskat)abstract
- The geometry and formation energy of substitutional B and Al dopants as well as their complexes with hydrogen have been calculated in 4H-SiC using first-principles methods. Our results show that boron selecting the silicon site and, therefore, getting activated as a shallow acceptor depends on the presence of hydrogen which is promoted into the crystal by boron itself. Without hydrogen, boron would mostly be incorporated at the carbon site. Aluminum does not show this behavior: it always selects the silicon site and is incorporated independently of hydrogen. © 2001 American Institute of Physics.
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| 6. |
- Aradi, B, et al.
(författare)
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Impurity-controlled dopant activation - The role of hydrogen in p-type doping of SiC
- 2002
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Ingår i: Materials Science Forum, Vols. 389-393. ; , s. 561-564
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Konferensbidrag (refereegranskat)abstract
- Hydrogen is a natural contaminant of SiC growth processes, and may influence the doping efficiency. Hydrogen incorporation proportional to that of boron was observed during CVD growth while the amount of hydrogen was two orders of magnitude less than the aluminum concentration. Passivation by complex formation with hydrogen has been proven both for Al and B. The experimentally observed reactivation energy of these complexes differ by 0.9 eV. Our ab initio supercell calculations in 4H-SiC indicate, that in the absence of hydrogen, boron is incorporated as isolated substitutional and prefers the carbon site, while under typical CVD conditions boron is incorporated together with hydrogen (in equal amounts), favoring the silicon site. Therefore, the presence of H is advantageous for the activation of B as a shallow acceptor. In contrast to boron, aluminum is incorporated independently of the presence of hydrogen as isolated substitutional at the silicon site. The calculated difference between the dissociation of the stable dopant plus hydrogen complexes agrees very well with experiments. Vibration frequencies for the dopant complexes have been also calculated.
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| 7. |
- Aradi, B., et al.
(författare)
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Passivation of p-type dopants in 4H-SiC by hydrogen
- 2001
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Ingår i: Physica B, Vols. 308-310. ; , s. 722-725
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Konferensbidrag (refereegranskat)abstract
- Experimental investigations showed passivation of the p-type dopants B and Al in 4H-SiC by the formation of B+H and Al+H complexes. The dissociation energies of these complexes differed by 0.9 eV. Ab initio supercell calculations have been performed to investigate the interaction of H with B and Al in hexagonal 4H-SiC. The total energy, geometry and electronic structure of the possible complexes have been determined. Site dependencies have also been investigated. The most stable configurations were found with H at a bond center site next to B at the Si site, and with H at the antibonding site of a carbon atom which is first neighbor to Al at a Si site. Both the BSi+HBC and the AlSi+HAB(C) complexes turned out to be electrically inactive. The different structure of the passivated complexes explains the observed difference in their dissociation energy: the calculated difference of the binding energies of these complexes is 0.9 eV, which agrees well with the experimental finding. © 2001 Elsevier Science B.V. All rights reserved.
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| 8. |
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| 9. |
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| 10. |
- Deak, P, et al.
(författare)
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Vacancies and their complexes with H in SiC
- 2000
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Ingår i: Materials Science Forum, Vols. 338-343. - Stafa-Zurich, Switzerland : Trans Tech Publications Inc.. ; , s. 817-820
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Konferensbidrag (refereegranskat)abstract
- Ab initio calculations (LDA and MCSF) have been carried out for vacancies (V-Si and V-C) and interstitial H, as well as for V+H complexes in 3C SiC. Relative stability of different charge-states/configurations and occupation levels were determined in supercells with plane wave basis sets while vibration frequencies and spin distributions were calculated in clusters with localized basis functions. Both types of vacancies show amphoteric electrical activity. In equilibrium, atomic He is at the AB(C), and H is at the T-Si site, while H-0 does not appear to be stable with respect to them, so H can also act both as a deep donor and an electron trap. Hydrogen can passivate the V-Si acceptor but not the V-C donor. Conditions for the formation of the possible V+H centers and their properties are given and used to discuss experimental information (or the lack of them) about H in SiC.
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