A silicon spin vacuum: isotopically enriched $^{28}$silicon-on-insulator and $^{28}$silicon from ultra-high fluence ion implantation
Isotopically enriched silicon (Si) can greatly enhance qubit coherence times by minimizing naturally occurring $^{29}$Si which has a non-zero nuclear spin. Ultra-high fluence $^{28}$Si ion implantation of bulk natural Si substrates was recently demonstrated as an attractive technique to ultra-high $^{28}$Si isotopic purity. In this work, we apply this $^{28}$Si enrichment process to produce $^{28}$Si and $^{28}$Si-on-insulator (SOI) samples. Experimentally, we produced a $^{28}$Si sample on natural Si substrate with $^{29}$Si depleted to 7~ppm (limited by measurement noise floor), that is at least 100 nm thick. This is achieved with an ion energy that results in a sputter yield of less than one and an ultra-high ion fluence, as supported by our improved computational model that is based on fitting a large number of experiments. Further, our model predicts the $^{29}$Si and $^{30}$Si depletion in our sample to be less than 1~ppm. In the case of SOI, ion implantation conditions are found to be more stringent than those of bulk natural Si in terms of minimizing threading dislocations upon subsequent solid phase epitaxy annealing. Finally, we do not observe open volume defects in our $^{28}$SOI and $^{28}$Si samples after SPE annealing (620°C, 10 minutes).