| 1. |
- Carvalho, A., et al.
(author)
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Local-density-functional calculations of the vacancy-oxygen center in Ge
- 2007
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In: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:11
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Journal article (peer-reviewed)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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| 2. |
- Carvalho, A., et al.
(author)
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Oxygen defects in irradiated germanium
- 2007
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In: Journal of materials science. Materials in electronics. - : Springer Science and Business Media LLC. - 0957-4522 .- 1573-482X. ; 18:7, s. 781-786
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Journal article (peer-reviewed)abstract
- The defects present in oxygen-rich irradiated germanium have been extensively characterised from an experimental point of view. Here, we summarise recent theoretical findings obtained using the cluster method and discuss their relation with the experimental data. In order to find a microscopic interpretation of the reactions taking place in this material upon annealing up to 400°C, we performed nudged elastic band (NEB) calculations of the migration and dissociation paths of VO, as well as a modelling of other oxygen-related complexes that are expected to form in this temperature range. Energy barriers of 1.5 (1.1) eV and 1.2 (0.9) eV for the dissociation and migration of the neutral (negatively charged) VO defect are found. We compare these with the activation energies estimated from the analysis of Hall effect, deep level transient spectroscopy (DLTS) and infra-red (IR) spectroscopy annealing data reported in literature.
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| 3. |
- Coutinho, J., et al.
(author)
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Ab initio modeling of defect levels in Ge clusters and supercells
- 2006
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In: Materials Science in Semiconductor Processing. - : Elsevier BV. - 1369-8001 .- 1873-4081. ; 9:4-5, s. 477-483
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Journal article (peer-reviewed)abstract
- Most density-functional studies of defects in semiconductors invariably use (i) a supercell that imitates the host crystal, as well as (ii) a local treatment of the exchange-correlation potential. The first approximation has had an enormous success in many materials, provided that the size of cell is large enough to minimize long-range interactions between the infinite lattice of defects. For instance, these may arise from strain fields or from the overlap between shallow defect states. The second approximation, when combined with the periodic boundary conditions, leads to an essentially metallic density of states in Ge, which can compromise any investigation of electronic transitions involving gap levels. Here, we report on two approaches to surmount these difficulties, namely (i) to open the gap by reducing the host to a Ge cluster of atoms whose states are confined by a surface potential and (ii) to use supercells, but choosing carefully the Brillouin zone sampling scheme, taking k-points that minimize the admixture between defect-related gap states and the host density of states. These methods are utilized in the calculation of the electronic structure of the vacancy, divacancy, and vacancy-donor pairs in Ge
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| 4. |
- Coutinho, J., et al.
(author)
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Calculation of deep carrier traps in a divacancy in germanium crystals
- 2006
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 88:9, s. 91919-
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Journal article (peer-reviewed)abstract
- We present an ab initio density functional study on the electronic structure and electrical properties of divacancies in Ge. Although suffering essentially different Jahn-Teller distortions when compared to the analogous defect in Si, the relative location of the electrical levels in the gap does not differ radically in both materials. We propose a V2 model that is responsible for a donor level at Ev+0.03 eV, a first acceptor state at Ev+0.3 eV, and a second acceptor level at Ec-0.4 eV. The latter is only 0.1 eV deeper than an electron trap that has been recently linked to a divacancy in proton implanted material.
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| 5. |
- Coutinho, J., et al.
(author)
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Limits to n-type doping in Ge : formation of donor-vacancy complexes
- 2008
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In: Diffusion and defect data, solid state data. Part A, Defect and diffusion forum. - 1012-0386 .- 1662-9507. ; 273-276, s. 93-98
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Journal article (peer-reviewed)abstract
- Vacancies and interstitials in semiconductors play a fundamental role in both high temperature diffusion and low temperature radiation and implantation damage. In Ge, a serious contender material for high-speed electronics applications, vacancies have historically been believed to dominate most diffusion related phenomena such as self-diffusivity or impurity migration. This is to be contrasted with silicon, where self-interstitials also play decisive roles, despite the similarities in the chemical nature of both materials. We report on density functional calculations of the formation and properties of vacancy-donor complexes in germanium. We predict that most vacancy-donor aggregates are deep acceptors, and together with their high solubilities, we conclude that they strongly contribute for inhibiting donor activation levels in germanium.
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| 6. |
- Coutinho, J., et al.
(author)
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Strong compensation of n-type Ge via formation of donor-vacancy complexes
- 2007
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In: Physica. B, Condensed matter. - : Elsevier BV. - 0921-4526 .- 1873-2135. ; 401-402, s. 179-183
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Journal article (peer-reviewed)abstract
- Vacancies and interstitials in semiconductors play a fundamental role in both high-temperature diffusion and low-temperature radiation and implantation damage. In Ge, a serious contender material for high-speed electronics applications, vacancies have historically been believed to dominate most diffusion related phenomena such as self-diffusivity or impurity migration. This is to be contrasted with silicon, where self-interstitials also play decisive roles, despite the similarities in the chemical nature of both materials. We report on density functional calculations of the formation and properties of vacancy-donor complexes in germanium. We predict that most vacancy-donor aggregates are deep acceptors, and together with their high solubilities, we conclude that they strongly contribute for inhibiting donor activation levels in germanium
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| 7. |
- Jones, R., et al.
(author)
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Theoretical investigations of the energy levels of defects in germanium
- 2005
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In: Solid State Phenomena. - 1012-0394 .- 1662-9779. ; 108-109, s. 697-702
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Journal article (peer-reviewed)abstract
- The donor and acceptor levels of defects in Ge as well as in Si are found using a local density functional method applied to large H-terminated defective clusters. The surfaces of the clusters are modified so that their band gaps are aligned with experimental values. It is shown that the resulting energies of the first donor and acceptor levels are within about 0.2 eV of the experimental values
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