Highly ${ }^{28} \mathrm{Si}$ Enriched Silicon by Localised Focused Ion Beam Implantation

TL;DR

This paper demonstrates a method to locally deplete ${ }^{29} ext{Si}$ in silicon wafers using focused ion beam implantation, achieving ultra-high enrichment levels suitable for quantum computing applications.

Contribution

It introduces a novel technique for localized silicon isotope enrichment with unprecedented purity, enabling scalable quantum device fabrication.

Findings

Achieved residual ${ }^{29} ext{Si}$ levels of 2.3 ppm in enriched regions.

Confirmed solid phase epitaxial re-crystallization after annealing.

Method is compatible with existing silicon fabrication processes.

Abstract

Solid-state spin qubits within silicon crystals at mK temperatures show great promise in the realisation of a fully scalable quantum computation platform. Qubit coherence times are limited in natural silicon owing to coupling to the isotope ${ }^{29} \mathrm{Si}$ which has a non-zero nuclear spin. This work presents a method for the depletion of ${ }^{29} \mathrm{Si}$ in localised volumes of natural silicon wafers by irradiation using a 45 keV ${ }^{28} \mathrm{Si}$ focused ion beam with fluences above $1 \times 10^{19} \, \mathrm{ions} \, \mathrm{cm}^{-2}$. Nanoscale secondary ion mass spectrometry analysis of the irradiated volumes shows unprecedented quality enriched silicon that reaches a minimal residual ${ }^{29} \mathrm{Si}$ value of 2.3 $\pm$ 0.7 ppm and with residual C and O comparable to the background concentration in the unimplanted wafer. Transmission electron microscopy lattice images confirm the solid phase epitaxial re-crystallization of the as-implanted amorphous enriched volume extending over 200 nm in depth upon annealing. The ease of fabrication, requiring only commercially available natural silicon wafers and ion sources, opens the possibility for co-integration of qubits in localised highly enriched volumes with control circuitry in the surrounding natural silicon for large-scale devices.