Isotopic enrichment of silicon by high fluence 28Si- ion implantation

Spins in the "semiconductor vacuum"of silicon-28 (Si28) are suitable qubit candidates due to their long coherence times. An isotopically purified substrate or epilayer of Si28 is required to limit the decoherence pathway caused by magnetic perturbations from surrounding Si29 nuclear spins (I=1/2), present in natural Si (Sinat) at an abundance of 4.67%. We isotopically enrich surface layers of Sinat by sputtering using high fluence 28Si- implantation. Phosphorus (P) donors implanted into one such Si28 layer with ∼3000 ppm Si29, produced by implanting 30 keV 28Si- ions at a fluence of 4×1018cm-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 Si29. A coherence time of T2=285±14μs is extracted, which is longer than that obtained in natSi 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 28Si- implanted with a fluence of 2.63×1018cm-2 into natSi. This resulted in an isotopically enriched surface layer ∼100 nm thick, suitable for providing a sufficient volume of Si28 for donor qubits implanted into the near-surface region. We observe a depletion of Si29 to 250 ppm as measured by secondary ion mass spectrometry. The impurity content and the crystallization kinetics via solid phase epitaxy are discussed. The Si28 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.