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Spins in the `semiconductor vacuum' of silicon-28 ($^{28}$Si) are suitablequbit candidates due to their long coherence times. An isotopically purifiedsubstrate of $^{28}$Si is required to limit the decoherence pathway caused bymagnetic perturbations from surrounding $^{29}$Si nuclear spins (I=1/2),present in natural Si (nat Si) at an abundance of 4.67%. We isotopically enrichsurface layers of nat Si by sputtering using high fluence $^{28}$Si$^-$implantation. Phosphorus (P) donors implanted into one such $^{28}$Si layerwith ~3000 ppm $^{29}$Si, produced by implanting 30 keV $^{28}$Si$^-$ ions at afluence of 4x10^18 cm^-2, were measured with pulsed electron spin resonance,confirming successful donor activation upon annealing. The mono-exponentialdecay of the Hahn echo signal indicates a depletion of $^{29}$Si. A coherencetime of T2 = 285 +/- 14 us is extracted, which is longer than that obtained innat Si for similar doping concentrations and can be increased by reducing the Pconcentration in future. The isotopic enrichment was improved by employingone-for-one ion sputtering using 45 keV $^{28}$Si$^-$ implantation. A fluenceof 2.63x10^18 cm^-2 $^{28}$Si$^-$ ions were implanted at this energy into natSi, resulting in an isotopically enriched surface layer ~100 nm thick; suitablefor providing a sufficient volume of $^{28}$Si for donor qubits implanted intothe near-surface region. We observe a depletion of $^{29}$Si to 250 ppm asmeasured by secondary ion mass spectrometry. The impurity content and thecrystallization kinetics via solid phase epitaxy are discussed. The $^{28}$Silayer is confirmed to be a single crystal using transmission electronmicroscopy. This method of Si isotopic enrichment shows promise forincorporating into the fabrication process flow of Si spin qubit devices.

Isotopic enrichment of silicon by high fluence $^{28}$Si$^-$ ion implantation