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| 2. |
- Balducci, C, et al.
(författare)
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Negative impact and positive value in caregiving : Validation of the COPE index in a six-country sample of carers
- 2008
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Ingår i: The Gerontologist. - 0016-9013 .- 1758-5341. ; 48:3, s. 276-286
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: The present study attempts to further validate the COPE Index on a large sample of carers drawn from six European countries. Design and Methods: We used a cross-sectional survey, with approximately 1,000 carers recruited in each of six countries by means of a common standard evaluation protocol. Our saturation recruitment of a designated quota of carers occurred by means of several channels, in identified geographical zones within countries. Interviews were carried out with primary informal carers by use of a common assessment tool. We subjected items of the COPE Index to principal component analysis and we assessed emergent components through the use of Cronbach's alpha reliability procedures. We examined factor components as summative scales for confirmatory correlations with caregiving and psychological variables. Results: Three components emerged, which we identified as the negative impact of caregiving, the positive value of caregiving, and the quality of support for caregiving. Internal consistency was good for negative impact and satisfactory for positive value and quality of support. Negative value was most consistently and strongly correlated with caregiving and psychological variables, although we did find diverse associations between these variables and the COPE Index subscales. Implications: The COPE Index is a brief, first-stage assessment of some sophistication that can enable health and social care professionals to develop appropriately targeted interventions to enhance the positive aspects of the caregiving experience and quality of support, as well as reduce the negative impacts of caregiving. Copyright 2008 by The Gerontological Society of America.
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| 3. |
- Carvahlo, A., et al.
(författare)
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Studies of the VO centre in Ge using first principles cluster calculations
- 2006
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Ingår i: Materials Science in Semiconductor Processing. - : Elsevier BV. - 1369-8001 .- 1873-4081. ; 9:4-5, s. 489-493
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Tidskriftsartikel (refereegranskat)abstract
- The small or near-zero band gap of germanium found by supercell local density functional treatments causes difficulties in the study of the properties of charged defects in Ge. Here, we use large H-terminated Ge clusters together with a non-linear core corrected (NLCC) Ge pseudopotential to explore the structure, vibrational and electrical properties of the vacancy-oxygen complex (VO). The cluster possesses a gap sufficiently wide to contain defect related energy levels, thus allowing us to model the three charge states of the defect. The local vibrational modes (LVM) calculated for the neutral (VO0), negatively charged (VO-) and double negative (VO=) defect are 602, 684 and 694 cm- 1, and can be favourably compared with experimental values of 621, 669 and 716 cm- 1, respectively [Markevich VP, et al. Physica B 2003; 340-2, 844-8]. Using substitutional gold (Aus) as a marker defect, electric levels of VO are found at E (-/ 0) = Ev + 0.30 eV and E (= /-) = Ec - 0.29 eV, in excellent agreement with the respective experimental enthalpies for hole and electron emission Δ Hh (-/ 0) ≃ 0.32 eV and Δ He (= /-) = 0.26 eV, respectively. Finally, the migration, dissociation and reorientation energies of the defect are also reported.
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| 4. |
- 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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| 5. |
- Carvalho, A., et al.
(författare)
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Oxygen defects in irradiated germanium
- 2007
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Ingår i: 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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Tidskriftsartikel (refereegranskat)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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| 6. |
- Coutinho, J., et al.
(författare)
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Ab initio modeling of defect levels in Ge clusters and supercells
- 2006
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Ingår i: Materials Science in Semiconductor Processing. - : Elsevier BV. - 1369-8001 .- 1873-4081. ; 9:4-5, s. 477-483
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Tidskriftsartikel (refereegranskat)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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| 7. |
- Coutinho, J., et al.
(författare)
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Calculation of deep carrier traps in a divacancy in germanium crystals
- 2006
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 88:9, s. 91919-
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Tidskriftsartikel (refereegranskat)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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| 8. |
- Coutinho, Jose, et al.
(författare)
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Early stage donor-vacancy clusters in germanium
- 2007
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Ingår i: Journal of materials science. Materials in electronics. - : Springer Science and Business Media LLC. - 0957-4522 .- 1573-482X. ; 18:7, s. 769-773
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Tidskriftsartikel (refereegranskat)abstract
- There is considerable experimental evidence that vacancies in Ge dominate several solid state reactions that range from self-diffusivity to metal and dopant transport. It is therefore vital that we fully understand how vacancies interact with other point defects in Ge. Here we have a look at the properties of small donor-vacancy (Sb n V m with m,n ≤ 2) complexes in Ge by ab-initio density functional modeling. Particular attention has been payed to binding energies and to the electronic activity of the complexes. We found that all aggregates may contribute to the n→ p type conversion that is typically observed under prolonged MeV irradiation conditions. In general, Sb n V m defects are double acceptors. It is also suggested that spontaneous formation of Sb3V complexes may limit the activation level of donors introduced by ion implantation.
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| 9. |
- Coutinho, J., et al.
(författare)
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Electronic structure and Jahn-Teller instabilities in a single vacancy in Ge
- 2005
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Ingår i: Journal of Physics. - 0953-8984 .- 1361-648X. ; 17:48, s. L521-L527
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Tidskriftsartikel (refereegranskat)abstract
- Density functional modelling studies of the single vacancy in large Ge clusters are presented. We take a careful look at the origin of Jahn-Teller instabilities as a function of the vacancy net charge, resulting in a variety of structural relaxations. By comparing electron affinities of the vacancy with those from defects with well established gap states, we were able to estimate three acceptor states for the vacancy at E(-/0) ≤ Ev+0.2 eV, E(≤/-) ≤ Ec-0.5 eV and eV. As opposed to the Si vacancy, the defect in Ge is not a donor. We also show that these dissimilarities have fundamental consequences for the electronic/atomic picture of other centres, such as transition metals in germanium crystals.
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| 10. |
- Coutinho, J., et al.
(författare)
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Limits to n-type doping in Ge : formation of donor-vacancy complexes
- 2008
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Ingår i: 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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Tidskriftsartikel (refereegranskat)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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