Is gold in silicon a negative-U centre?

CIC nanoGUNE Seminars

Fabiano Corsetti, nanoGUNE
nanoGUNE seminar room, Tolosa Hiribidea 76, Donostia - San Sebastian
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Is gold in silicon a negative-U centre? It is well-known that the simulation of defects in bulk material presents a challenge for first-principles methods, as the disruption caused by the defect centre on the crystalline lattice can be extremely long-ranged. We first investigate and systematically quantify the various contributions to the total finite size error for the silicon vacancy in density-functional theory, focussing on spurious electrostatic interactions, elastic interactions and wavefunction orthogonality constraints between periodic images of the defect centre; our calculations demonstrate that the defect formation energy and stable charge state transition levels exhibit different convergence behaviour with respect to supercell size, depending on the Brillouin zone sampling used. We then apply the insights obtained from the vacancy to the study of gold in silicon, a defect of long-standing technological importance, which has not yet been well-characterised by modern first-principles methods. We argue that careful consideration of finite size effects is crucial for obtaining the correct behaviour of the system, and show that in the dilute limit the defect becomes a non-spin-polarised negative-U centre due to the effect of Jahn- Teller distortion. The resulting stable charge state transition levels are in excellent agreement with the donor and acceptor levels measured experimentally, and therefore allow for a clear identification of these levels; furthermore, our results suggest that the assumption used in previous studies on the nature of the two experimental levels is incorrect.