1. |
- Carvahlo, A., et al.
(författare)
-
Studies of the VO centre in Ge using first principles cluster calculations
- 2006
-
Ingår i: Materials Science in Semiconductor Processing. - : Elsevier BV. - 1369-8001 .- 1873-4081. ; 9:4-5, s. 489-493
-
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.
|
|
2. |
- Carvalho, A., et al.
(författare)
-
Local-density-functional calculations of the vacancy-oxygen center in Ge
- 2007
-
Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 75:11
-
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.
|
|
3. |
- Carvalho, A., et al.
(författare)
-
Oxygen defects in irradiated germanium
- 2007
-
Ingår i: Journal of materials science. Materials in electronics. - : Springer Science and Business Media LLC. - 0957-4522 .- 1573-482X. ; 18:7, s. 781-786
-
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.
|
|
4. |
- Coutinho, J., et al.
(författare)
-
Ab initio modeling of defect levels in Ge clusters and supercells
- 2006
-
Ingår i: Materials Science in Semiconductor Processing. - : Elsevier BV. - 1369-8001 .- 1873-4081. ; 9:4-5, s. 477-483
-
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
|
|
5. |
- Coutinho, J., et al.
(författare)
-
Calculation of deep carrier traps in a divacancy in germanium crystals
- 2006
-
Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 88:9, s. 91919-
-
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.
|
|
6. |
- Coutinho, J., et al.
(författare)
-
Electronic structure and Jahn-Teller instabilities in a single vacancy in Ge
- 2005
-
Ingår i: Journal of Physics. - 0953-8984 .- 1361-648X. ; 17:48, s. L521-L527
-
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.
|
|
7. |
- Coutinho, J., et al.
(författare)
-
Electronic structure of divacancy-hydrogen complexes in silicon
- 2003
-
Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 15:39, s. S2809-S2814
-
Tidskriftsartikel (refereegranskat)abstract
- Divacancy-hydrogen complexes (V2H and V2H2) in Si are studied by ab initio modelling using large supercells. Here we pay special attention to their electronic structure, showing that these defects produce deep carrier traps. Calculated electrical gap levels indicate that V2H2 is an acceptor, whereas V2H is amphoteric, with levels close to those of the well known divacancy. Finally our results are compared with the available data from deep level transient spectroscopy and electron paramagnetic resonance experiments.
|
|
8. |
- Coutinho, J., et al.
(författare)
-
Limits to n-type doping in Ge : formation of donor-vacancy complexes
- 2008
-
Ingår i: Diffusion and defect data, solid state data. Part A, Defect and diffusion forum. - 1012-0386 .- 1662-9507. ; 273-276, s. 93-98
-
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.
|
|
9. |
- Coutinho, J., et al.
(författare)
-
Local vibrations on hydrogen dimers in dilute SiGe crystalline solutions
- 2005
-
Ingår i: Materials Science & Engineering. - : Elsevier BV. - 0921-5107 .- 1873-4944. ; 124/125:Suppl., s. 363-367
-
Tidskriftsartikel (refereegranskat)abstract
- Atomic hydrogen is a concomitant impurity in semiconductors. Its presence in Si, Ge and SiGe alloys has been established by means of paramagnetic resonance, optical, electrical and theoretical modeling studies. Hydrogen self-trapping is known to occur in Si and Ge, resulting in the formation of molecular hydrogen and H2* interstitial dimers. Here we report on the properties of H22* complexes in dilute SiGe alloys, by using an ab initio density functional method. It is found that these complexes form preferentially within Si-rich regions. H2* dimers in Si-rich alloys show vibrational properties similar to those in pure Si. On the other hand, in Ge-rich material the minority Si atoms act as nucleation sites, with the consequent formation of at least one preferential H2*- Si defect variant, showing a distinct vibrational activity.
|
|
10. |
- Coutinho, J., et al.
(författare)
-
Strong compensation of n-type Ge via formation of donor-vacancy complexes
- 2007
-
Ingår i: Physica. B, Condensed matter. - : Elsevier BV. - 0921-4526 .- 1873-2135. ; 401-402, s. 179-183
-
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
|
|