Density functional investigation of boron incorporation in silicon-vacancy complexes
Graphical abstract
Introduction
Silicon-vacancy complexes (SiV) in diamond have superior optical properties relative to other diamond single photon sources, including the nitrogen-vacancy (NV) centre [1]. It exhibits no spectral diffusion [1], has a high spectral overlap [1], strong zero-phonon line (ZPL) polarization [2], a Debye-Waller factor of around 0.8 [3] and 70% emission into the 738 nm ZPL compared to 2–3% in the case of the NV centre [1]. For a long time SiV− was proffered for quantum applications, but its utility is limited by a short lifetime arising from orbital coupling and the fact that its polarization wavelengths also polarize any nearby NV− centres [3]. SiV0 has a polarization wavelength independent of NV− manipulation [3] and recent research has demonstrated optical spin polarization of SiV0 with a similar Debye factor to SiV− [3].
Since different charge states of SiV are growing in interest for quantum information and communication networks [3] employing Fermi-level control via co-doping diamond with boron and nitrogen has been suggested [4] and used [5] to achieve SiV0 or SiV− respectively, but one must then consider the formation of by-products such as complexes of SiV with the dopants. Silicon defects have been found decorated by hydrogen [6,7] or nitrogen [8,9], and other silicon-related centres have been proposed but their structure not unambiguously identified [10,11]. Whilst there has been research into the likely nitrogen by-products when attempting to form SiV− [8] there is yet to be a similar characterisation of boron-containing by-products which may form when using boron to form the neutral charge state of SiV. This report presents an examination of a range of such defects. Hydrogen, due its abundance during CVD diamond growth [6,12,13], has also been considered as a component.
Section snippets
Method
Density functional theory applied within the supercell approach was employed using the AIMPRO software package [14], with defects modelled using bulk-diamond supercells made up from 2 × 2 × 2, 3 × 3 × 3 and 4 × 4 × 4 conventional unit-cells comprised from 64, 216 and 512 carbon atoms, respectively. All results in this work use 512 supercells, unless stated otherwise.
Calculations employed spin-polarized local density (LDA) [15] and the generalized gradient approximation by Perdew-Burke-Ernzerhof
Results
We have optimised and analysed complexes in diamond related to silicon, boron, lattice vacancies and hydrogen. These structures are seen as the potential by-products when doping silicon-containing diamond with boron, in attempting to manipulate SiV charge states. We comment on the structure of these defects in comparison to their component parts, discuss the viability of them forming in diamond, and examine possible routes to detection for each complex.
Conclusion
DFT was used to model complexes that may form in Si and B co-doped diamond. Of those modelled, SiVB and SiVBH were found to be energetically stable for a range of charge states, whereas a substitutional SiB pair proposed as the origin for both EPR and optical centres from experiment [10,11] are unlikely to be bound under growth or annealing conditions. We therefore conclude that the experimentally observed centres are not simple SiB substitutional pairs. The identification of the microscopic
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This research made use of the Rocket High Performance Computing service at Newcastle University. Authors gratefully acknowledge funding received from the EPSRC Centre for Doctoral Training in Diamond Science & Technology.
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