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| 3. |
- Latham, Chris, et al.
(författare)
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Electronic structure calculations for substitutional copper and monovacancies in silicon
- 2006
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Ingår i: Physica Scripta. - 0031-8949 .- 1402-4896. ; 2006:T126, s. 61-64
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Tidskriftsartikel (refereegranskat)abstract
- Two different computer program packages based on the self-consistent local-spin-density approximation-aimpro and vasp-are employed in this study of substitutional copper CuSi and monovacancies VSi in silicon, including the effects of their charge state. The programs differ in the types of basis sets and pseudopotentials they use, each with their own relative merits, while being similar in overall quality. This approach aims to reduce uncertainty in the results, particularly for small or subtle effects, where the risk is greatest that the conclusions are affected by artifacts specific to a particular implementation. The electronic structures of the two defects are closely related, hence they are expected to behave in a similar manner. For both defects structural distortions resulting in lower point group symmetries than Td (the highest possible) are found. This is in good agreement with the results of previous studies of VSi. Much less is known about symmetry-lowering effects for CuSi; however, the electronic levels of CuSi have been measured accurately, while those for VSi are less accessible. Calculating them is a challenging task for theory. The strategy we adopt, based purely on comparing total energies of supercells in different charge states, with and without model defects, reproduces the three known levels for CuSi reasonably well. Satisfactory results are also obtained for VSi in so far as can be judged for this more complex case.
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| 5. |
- Carvalho, A., et al.
(författare)
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Local-density-functional calculations of the vacancy-oxygen center in Ge
- 2007
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:11
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Tidskriftsartikel (refereegranskat)abstract
- We carry out a comprehensive density-functional study of the vacancy-oxygen (VO) center in germanium using large H-terminated Ge clusters. The importance of a nonlinear core correction to account for the involvement of the 3d electrons in Ge-O bonds is discussed. We calculate the electrical levels and the vibrational modes of VO0, VO-, and VO= finding close agreement with experiment. We also explore the reorientation, migration, and dissociation mechanisms of neutral and negatively charged VO and compare the calculated energy barriers with experimental data. We conclude that the defect is likely to anneal through both mechanisms.
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- Hounsome, L.S., et al.
(författare)
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Optical properties of vacancy related defects in diamond
- 2005
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Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley. - 1862-6300 .- 1862-6319. ; 202:11, s. 2182-2187
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Tidskriftsartikel (refereegranskat)abstract
- ‹110› vacancy chains, multi-vacancy clusters and vacancy discs have been modeled using density functional theory within the AIMPRO and DFTB codes. While a connection can be established between the results on vacancy chains and previous EPR experiments, no connection can be made between the point defects and the optical properties of natural type IIa brown diamonds. However, a vacancy disc consisting of a {111} double plane of vacancies is stable and possesses an absorption spectrum similar to that found in brown diamonds.
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- Hounsome, L.S., et al.
(författare)
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Origin of brown coloration in diamond
- 2006
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 73:12, s. 125203-1
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Tidskriftsartikel (refereegranskat)abstract
- Measurements of the absorption spectra of brown natural type IIa diamond as well as brown nitrogen-doped CVD diamond are reported. These are largely featureless and increase almost monotonically from about 1-5.5 eV. It is argued that the brown coloration is due to an extended defect and not to a point defect. First principles modeling studies demonstrate that the spectra could be attributed to vacancy disks lying on {111} planes. Such disks are unstable above about 200 vacancies and should relax to dislocation loops in natural diamond. Hydrogen is shown to passivate the optical activity of the disks.
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| 8. |
- Hounsome, L.S., et al.
(författare)
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Role of extended defects in brown colouration of diamond
- 2007
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Ingår i: Physica Status Solidi. C, Current topics in solid state physics. - : Wiley. - 1610-1634 .- 1610-1642. ; 4:8, s. 2950-2957
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Tidskriftsartikel (refereegranskat)abstract
- The absorption spectrum of brown diamond is broad and featureless, in both natural type IIa and CVD-grown material. It is argued that such an absorption is due to an extended, rather than a point, defect. Ab-initio modelling studies have been conducted on dislocations and extended vacancy-related defects. While certain dislocations could potentially explain the origin of colour, their density is too low to account for experimentally observed absorption magnitudes. It is demonstrated that a vacancy disk lying in the {111} plane has an absorption spectrum similar to that seen in natural and CVD brown diamond. Such disks are unstable above about 200 vacancies and should relax to dislocation loops in natural diamond. Hydrogen is shown to passivate the optical activity of the disks.
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- Hourahine, B., et al.
(författare)
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Evidence for H2* trapped by carbon impurities in silicon
- 2001
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Ingår i: Physica. B, Condensed matter. - 0921-4526 .- 1873-2135. ; 308, s. 197-201
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Tidskriftsartikel (refereegranskat)abstract
- Local mode spectroscopy and ab initio modelling are used to investigate two trigonal defects found in carbon-rich Si into which H had been in-diffused. Isotopic shifts with D and 13C are reported along with the effect of uniaxial stress. Ab initio modelling studies suggest that the two defects are two forms of the CH2* complex where one of the two hydrogen atoms lies at an anti-bonding site attached to C or Si, respectively. The two structures are nearly degenerate and possess vibrational modes in good agreement with those observed.
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