Increasing spin qubit coherence times via the isotopic enrichment of silicon by high fluence ion implantation

Spins in the ‘semiconductor vacuum’ of silicon-28 (²⁸Si) are excellent qubit candidates due to their long coherence times [1]. An isotopically purified qubit environment of ²⁸Si is required to limit the decoherence pathway caused by magnetic perturbations from surrounding ²⁹Si nuclear spins (I = 1/2), present in natural Si (^natSi) at an abundance of 4.67%. With charge noise from the dielectric environment being pushed down, the operational errors of state-of-the-art silicon spin qubits are becoming increasingly dominated by residual ²⁹Si spins [2]. A promising method to produce high-purity ²⁸Si material using only natural sources and standard laboratory equipment is to isotopically enrich surface layers of ^natSi by sputtering using high fluence ²⁸Si^- implantation. This method has been demonstrated to increase the coherence time of an ensemble of implanted phosphorus (P) donors [3]. In the latest work, we produce a large area (6 mm x 6 mm) of isotopically enriched ²⁸Si by implanting ²⁸Si^- ions (60 keV, 1e19 cm^-2), suitable for hosting many spin qubits. This enriched material is then processed to fabricate donor spin qubit devices. After completion of a typical fabrication process, a ~150 nm-thick layer of ²⁸Si remains, with a residual concentration of <1 ppm ²⁹Si, as measured by secondary ion mass spectrometry. This constitutes an ideal environment to host silicon spin qubits, with around 800 times fewer ²⁹Si nuclear spins than present in typical spin qubit devices [1]. Near-surface implanted donor spin qubits will be measured in this high-purity ²⁸Si material, with hopes to push the records for qubit coherence and control fidelities in silicon.



[1] J. T. Muhonen et al., Nat. Nanotechnol. 9, 986-991 (2014)

[2] P. Steinacker et al., arXiv:2410.15590 (2024)

[3] D. Holmes et al., Phys. Rev. Mat. 5, 014601 (2021)