Spins in the “semiconductor vacuum” of silicon-28 (Si-28) are suitable qubit candidates due to their long coherence times. An isotopically purified substrate or epilayer of Si-28 is required to limit the decoherence pathway caused by magnetic perturbations from surrounding Si-29 nuclear spins (I = 1/2), present in natural Si (Si-nat) at an abundance of 4.67%. We isotopically enrich surface layers of Si-nat by sputtering using high fluence Si-28(-) implantation. Phosphorus (P) donors implanted into one such Si-28 layer with similar to 3000 ppm Si-29, produced by implanting 30 keV Si-28(-) ions at a fluence of 4 x 10(18) cm(-2), were measured with pulsed electron spin resonance, confirming successful donor activation upon annealing. The monoexponential decay of the Hahn echo signal indicates a depletion of Si-29. A coherence time of T-2 = 285 +/- 14 mu s is extracted, which is longer than that obtained in Si-nat for similar doping concentrations and can be increased by reducing the P concentration in the future. Guided by simulations, the isotopic enrichment was improved by employing one-for-one ion sputtering using 45 keV Si-28(-) implanted with a fluence of 2.63 x 10(18) cm(-2) into Si-nat. This resulted in an isotopically enriched surface layer similar to 100 nm thick, suitable for providing a sufficient volume of Si-28 for donor qubits implanted into the near-surface region. We observe a depletion of Si-29 to 250 ppm as measured by secondary ion mass spectrometry. The impurity content and the crystallization kinetics via solid phase epitaxy are discussed. The Si-28 layer is confirmed to be a single crystal using transmission electron microscopy. This method of Si isotopic enrichment shows promise for incorporation into the fabrication process flow of Si spin-qubit devices.
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