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