| 1. |
- Deak, Peter, et al.
(författare)
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Accurate defect levels obtained from the HSE06 range-separated hybrid functional
- 2010
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Ingår i: PHYSICAL REVIEW B. - : American Physical Society. - 1098-0121. ; 81:15, s. 153203-
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Tidskriftsartikel (refereegranskat)abstract
- Defect levels are a problem for standard implementations of density-functional theory and the error also influences the energetics. We demonstrate that the HSE06 functional, which describes the electronic structure of all group-IV semiconductors well (including Ge), gives highly accurate charge transition levels, too, if the defect wave function is host related-independent of localization. The degree of fulfilling the generalized Koopmans theorem shows the reliability of the results and the highest-occupied eigenvalue always seems to give the correct vertical ionization energy.
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| 2. |
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| 3. |
- Gueorguiev Ivanov, Ivan, et al.
(författare)
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Optical properties of the niobium centre in 4H, 6H, and 15R SiC
- 2013
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Ingår i: SILICON CARBIDE AND RELATED MATERIALS 2012. - : Trans Tech Publications. ; , s. 405-408
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Konferensbidrag (refereegranskat)abstract
- A set of lines in the photoluminescence spectra of 4H-, 6H-, and 15R-SiC in the near-infrared are attributed to Nb-related defects on the ground of doping experiments conducted with 4H-SiC. A model based on a an exciton bound at the Nb-centre in an asymmetric split vacancy configuration at a hexagonal site is proposed, which explains the structure of the luminescence spectrum and the observed Zeeman splitting of the lines.
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| 4. |
- Gällström, Andreas, et al.
(författare)
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Electronic Configuration of Tungsten in 4H-, 6H-, and 15R-SiC
- 2012
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Ingår i: Materials Science Forum Vols 717 - 720. - : Trans Tech Publications Inc.. - 9783037854198 ; , s. 211-216
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Konferensbidrag (refereegranskat)abstract
- A commonly observed unidentified photoluminescence center in SiC is UD-1. In this report, the UD-1 center is identified to be tungsten related. The identification is based on (i) a W-doping study, the confirmation of W in the samples was made using deep level transient spectroscopy (DLTS), (ii) the optical activation energy of the absorption of UD-1 in weakly n-type samples corresponds to the activation energy of the deep tungsten center observed using DLTS. The tungsten-related optical centers are reported in 4H-, 6H-, and 15R-SiC. Further, a crystal field model for a tungsten atom occupying a Si-site is suggested. This crystal field model is in agreement with the experimental data available: polarization, temperature dependence and magnetic field splitting.
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| 5. |
- Gällström, Andreas, et al.
(författare)
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Optical identification and electronic configuration of tungsten in 4H-and 6H-SiC
- 2012
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Ingår i: Physica. B, Condensed matter. - : Elsevier. - 0921-4526 .- 1873-2135. ; 407:10, s. 1462-1466
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Tidskriftsartikel (refereegranskat)abstract
- Several optically observed deep level defects in SiC are still unidentified and little is published on their behavior. One of the commonly observed deep level defects in semi-insulating SiC is UD-1. less thanbrgreater than less thanbrgreater thanThis report suggests that UD-1 is Tungsten related, based on a doping study and previously reported deep level transient spectroscopy data, as well as photo-induced absorption measurements. The electronic levels involved in the optical transitions of UD-1 are also deduced. The transitions observed in the photoluminescence of UD-1 are from a Gamma(C3v)(4), to two different final states, which transform according to Gamma(C3v)(5)circle plus Gamma(C3v)(6) and Gamma(C3v)(4), respectively.
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| 6. |
- Ivady, Viktor, et al.
(författare)
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Asymmetric Split-Vacancy Defects in SiC Polytypes: A Combined Theoretical and Electron Spin Resonance Study
- 2011
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Ingår i: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 107:19, s. 195501-
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Tidskriftsartikel (refereegranskat)abstract
- Transition metal defects were studied in different polytypes of silicon carbide (SiC) by ab initio supercell calculations. We found asymmetric split-vacancy (ASV) complexes for these defects that preferentially form at only one site in hexagonal polytypes, and they may not be detectable at all in cubic polytype. Electron spin resonance study demonstrates the existence of ASV complex in niobium doped 4H polytype of SiC.
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| 7. |
- Ivady, V., et al.
(författare)
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Transition Metal Defects in Cubic and Hexagonal Polytypes of SiC: Site Selection, Magnetic and Optical Properties from ab initio Calculations
- 2012
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Ingår i: Materials Science Forum Vol 717 - 720. - : Trans Tech Publications Inc.. ; , s. 205-210
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Konferensbidrag (refereegranskat)abstract
- Relatively little is known about the transition metal defects in silicon carbide (SiC). In this study we applied highly convergent and sophisticated density functional theory (DFT) based methods to investigate important transition metal impurities including titanium (Ti), vanadium (V), niobium (Nb), chromium (Cr), molybdenum (Mo) and tungsten (W) in cubic 3C and hexagonal 4H and 6H polytypes of SiC. We found two classes among the considered transition metal impurities: Ti, V and Cr clearly prefer the Si-substituting configuration while W, Nb, and Mo may form a complex with a carbon vacancy in hexagonal SiC even under thermal equilibrium with similar concentration. If the metal impurity is implanted into SiC or when many carbon impurities exist during the growth of SiC then complex formation between the Si-substituting metal impurity and the carbon vacancy should be considered. This complex pair configuration exclusively prefers the hexagonal-hexagonal sites in hexagonal polytypes and may be absent in the cubic polytype. We also studied transition metal doped nano 3C-SiC crystals in order to check the effect of the crystal field on the d-orbitals of the metal impurity.
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| 8. |
- Son, Nguyen Tien, et al.
(författare)
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Identification of Niobium in 4H-SiC by EPR and ab Initio Studies
- 2012
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Ingår i: Materials Science Forum Vols 717 - 720. - : Trans Tech Publications Inc.. ; , s. 217-220
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Konferensbidrag (refereegranskat)abstract
- In unintentionally Nb-doped 4H-SiC grown by high-temperature chemical vapor deposition (HTCVD), an electron paramagnetic resonance (EPR) center with C-lh symmetry and an electron spin S=1/2 was observed. The spectrum shows a hyperfine structure consisting of ten equal-intensity hyperfine (hf) lines which is identified as due to the hf interaction between the electron spin and the nuclear spin of Nb-93. An additional hf structure due to the interaction with two equivalent Si neighbors was also observed. Ab initio supercell calculations of Nb in 4H-SiC suggest that Nb may form a complex with a C-vacancy (V-C) resulting in an asymmetric split-vacancy (ASV) defect, Nb-Si-V-C. Combining results from EPR and supercell calculations, we assign the observed Nb-related EPR center to the hexagonal-hexagonal configuration of the AVS defect in the neutral charge state, (Nb-Si-V-C)(0).
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| 9. |
- Son Tien, Nguyen, et al.
(författare)
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Electron paramagnetic resonance and theoretical studies of Nb in 4H- and 6H-SiC
- 2012
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Ingår i: Journal of Applied Physics. - : American Institute of Physics (AIP). - 0021-8979 .- 1089-7550. ; 112:8, s. 083711-
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Tidskriftsartikel (refereegranskat)abstract
- High purity silicon carbide (SiC) materials are of interest from high-power high temperature applications across recent photo-voltaic cells to hosting solid state quantum bits, where the tight control of electrically, optically, and magnetically active point defects is pivotal in these areas. 4H- and 6H-SiC substrates are grown at high temperatures and the incorporation of transition metal impurities is common. In unintentionally Nb-doped 4H- and 6H-SiC substrates grown by high-temperature chemical vapor deposition, an electron paramagnetic resonance (EPR) spectrum with C-1h symmetry and a clear hyperfine (hf) structure consisting of ten equal intensity hf lines was observed. The hf structure can be identified as due to the interaction between the electron spin S - 1/2 and the nuclear spin of Nb-93. Additional hf structures due to the interaction with three Si neighbors were also detected. In 4H-SiC, a considerable spin density of similar to 37.4% was found on three Si neighbors, suggesting the defect to be a complex between Nb and a nearby carbon vacancy (V-C). Calculations of the Nb-93 and Si-29 hf constants of the neutral Nb on Si site, Nb-Si(0), and the Nb-vacancy defect, NbSiVC0, support previous reported results that Nb preferentially forms an asymmetric split-vacancy (ASV) defect. In both 4H- and 6H-SiC, only one Nb-related EPR spectrum has been observed, supporting the prediction from calculations that the hexagonal-hexagonal defect configuration of the ASV complex is more stable than others.
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| 10. |
- Szabo, Aron, et al.
(författare)
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Group-II acceptors in wurtzite AlN: A screened hybrid density functional study
- 2010
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Ingår i: APPLIED PHYSICS LETTERS. - : American Institute of Physics. - 0003-6951 .- 1077-3118. ; 96:19, s. 192110-
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Tidskriftsartikel (refereegranskat)abstract
- We systematically studied the group-II acceptors in wurtzite AlN by screened hybrid density functional calculations. We show that the shallowest isolated group-II substitutional defect is Mg, while codoping of Mg and O may yield even shallower acceptor level.
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