Improved placement precision of donor spin qubits in silicon using molecule ion implantation*

Arrays of donor spin qubits have the potential to form the basis for powerful silicon quantum processors and can be engineered using deterministic single ion implantation [1]. The donor placement precision, crucial for qubit readout and coupling, can be improved by implanting molecule ions instead of atomic donor ions. Here, we investigate the suitability of phosphorus difluoride (PF₂) molecule ions to fabricate high-quality ³¹P donor qubits. Since ¹⁹F nuclei have a spin of I = 1/2, it is imperative to ensure that they do not couple to ³¹P donor electrons as they would contribute to decoherence. Using secondary ion mass spectrometry, we confirm that ¹⁹F diffuses out of the active region of qubit devices while the P donors remain close to their original location during a donor activation anneal. PF₂-implanted qubit devices were then fabricated and electron spin resonance measurements were performed on the ³¹P donor electron. We measure coherence times for the ³¹P donor electron comparable to those found in conventional ³¹P ion-implanted qubit devices and find no evidence of ¹⁹F nuclei coupled to the ³¹P donor electron. Molecule ions therefore show great promise for producing high-precision deterministically-implanted arrays of donor spin qubits.

[1] A. M. Jakob et al., Advanced Materials 34 (3), 2103235 (2021)