| 1. |
- Aavikko, R., et al.
(författare)
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Clustering of vacancy defects in high-purity semi-insulating SiC
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:8, s. 085208-
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Tidskriftsartikel (refereegranskat)abstract
- Positron lifetime spectroscopy was used to study native vacancy defects in semi-insulating silicon carbide. The material is shown to contain (i) vacancy clusters consisting of four to five missing atoms and (ii) Si-vacancy-related negatively charged defects. The total open volume bound to the clusters anticorrelates with the electrical resistivity in both as-grown and annealed materials. Our results suggest that Si-vacancy-related complexes electrically compensate the as-grown material, but migrate to increase the size of the clusters during annealing, leading to loss of resistivity. © 2007 The American Physical Society.
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| 2. |
- 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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| 3. |
- Carlsson, Patrick, 1975-, et al.
(författare)
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EPR and ab initio calculation study on the EI4 center in 4H and 6H-SiC
- 2010
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 82:23, s. 235203-
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Tidskriftsartikel (refereegranskat)abstract
- We present new results from electron paramagnetic resonance (EPR) studies of the EI4 EPR center in 4H- and 6H-SiC. The EPR signal of the EI4 center was found to be drastically enhanced in electron-irradiated high-purity semi-insulating materials after annealing at 700-750°C. Strong EPR signals of the EI4 center with minimal interferences from other radiation-induced defects in irradiated high-purity semiinsulating materials allowed our more detailed study of the hyperfine (hf) structures. An additional large-splitting 29Si hf structure and 13C hf lines of the EI4 defect were observed. Comparing the data on the defect formation, the hf interactions and the annealing behavior obtained from EPR experiments and from ab initio supercell calculations of different carbon-vacancy related complexes, we suggest a complex between a carbon vacancy-carbon antisite and a carbon vacancy at the third neighbor site of the antisite in the neutral charge state, (VC-CSiVC)0, as a new defect model for the EI4 center.
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| 4. |
- 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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| 5. |
- Egilsson, T., et al.
(författare)
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Excitation properties of hydrogen-related photoluminescence in 6H-SiC
- 2000
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 62:11, s. 7162-7168
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Tidskriftsartikel (refereegranskat)abstract
- We have studied the excitation properties of a well-known hydrogen-related bound exciton (H-BE) photoluminescence (PL) in 6H-SiC. In the case of the so-called primary H-BE's, photoluminescence excitation (PLE) spectroscopy reveals several excited states that have not been reported previously. In order to explain these states we propose a pseudodonor model. The primary H-BE's are thus regarded as donors where strongly localized holes serve as the positive cores. From a comparison between the PLE spectra of the three different primary H-BE's corresponding to the three inequivalent substitutional lattice sites in 6H-SiC, we attempt to distinguish between the hexagonal and cubic lattice sites. We have also investigated the dependence of the optically induced quenching of the H-BE PL on the energy of the exciting light. We observe that the quenching of the H-BE PL is only efficient when the exciting light has energy above the threshold for phonon-assisted free-exciton (FE) formation or when its energy coincides with the energy needed for resonant absorption into the H-BE states. When creating FE's, we observe different types of behavior depending on the initial conditions. We argue that our results are best explained by the existence of two configurations of the same charge state of the H defect, namely a stable one: A (giving rise to the H-BE PL), and a metastable one: B (not revealed in the PL spectrum). The recombination of excitons bound at these two configurations can give rise to the transformations A?B and B?A. The existence of the B configuration is revealed through the effect of the B?A process on the temporal changes of the H-BE PL.
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| 6. |
- Gali, A., et al.
(författare)
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Ab initio supercell calculations on aluminum-related defects in SiC
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:4
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio supercell calculations of the binding energies predict complex formation between aluminum and carbon interstitials in SiC. In high-energy implanted SiC aluminum acceptor can form very stable complexes with two carbon interstitials. We also show that carbon vacancy can be attached to shallow aluminum acceptor. All of these defects produce deep levels in the band gap. The possible relation of these defects to the recently found aluminum-related deep-level transient spectroscopy centers is discussed. © 2007 The American Physical Society.
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| 7. |
- Gali, Adam, et al.
(författare)
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Aggregation of carbon interstitials in silicon carbide : A theoretical study
- 2003
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 68:12
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Tidskriftsartikel (refereegranskat)abstract
- Ab initio supercell calculations have been carried out to investigate clusters of carbon interstitials in 3C- and 4H-SiC. Based on the calculated formation energies, the complex formation of carbon interstitials or their aggregation to carbon antisites is energetically favored in SiC. The electronic and vibronic properties of the carbon interstitials and their aggregates depends strongly on the polytype. Using the calculated hyperfine constants and local vibrational modes of carbon clusters the possible relation to known carbon-related centers will be discussed.
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| 8. |
- Gali, Adam, et al.
(författare)
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Correlation between the antisite pair and the D-I center in SiC
- 2003
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 67:15
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Tidskriftsartikel (refereegranskat)abstract
- The D-I low temperature photoluminescence center is a well-known defect stable up to 1700 degreesC annealing in SiC, still its structure is not yet known. Combining experimental and theoretical studies, in this paper we will show that the properties of an antisite pair can reproduce the measured one-electron level position and local vibration modes of the D-I center, and are consistent with other experimental findings as well. We give theoretical values of the hyperfine constants of the antisite pair in its paramagnetic state as a means to confirm a model.
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| 9. |
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| 10. |
- Gällström, Andreas, et al.
(författare)
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Optical properties and Zeeman spectroscopy of niobium in silicon carbide
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 92:7, s. 1-14
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Tidskriftsartikel (refereegranskat)abstract
- The optical signature of niobium in the low-temperature photoluminescence spectra of three common polytypes of SiC (4H, 6H, and 15R) is observed and confirms the previously suggested concept that Nb occupies preferably the Si-C divacancy with both Si and C at hexagonal sites. Using this concept we propose a model considering a Nb-bound exciton, the recombination of which is responsible for the observed luminescence. The exciton energy is estimated using first-principles calculation and the result is in very good agreement with the experimentally observed photon energy in 4H SiC at low temperature. The appearance of six Nb-related lines in the spectra of the hexagonal 4H and 6H polytypes at higher temperatures is tentatively explained on the grounds of the proposed model and the concept that the Nb center can exist in both C1h and C3v symmetries. The Zeeman splitting of the photoluminescence lines is also recorded in two different experimental geometries and the results are compared with theory based on phenomenological Hamiltonians. Our results show that Nb occupying the divacancy at the hexagonal site in the studied SiC polytypes behaves like a deep acceptor.
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