Epitaxial growth of$^{28}$Si enriched \textit{in situ} to 99.9998{\%} for quantum information

In support of quantum information devices, we epitaxially deposit \textgreater 100 nm $^{28}$Si films enriched \textit{in situ} to \textgreater 99.9998 {\%} isotope fraction at high temperature. Using our silicon enrichment ion beam deposition source, we explore electrical and structural properties of our $^{28}$Si films using \textit{in situ }reflection high energy electron diffraction (RHEED), transmission electron microscopy (TEM) and electrical measurements including capacitance--voltage profiling. Secondary ion mass spectrometry (SIMS) is used to show $^{28}$Si films have residual $^{29}$Si isotope fractions \textless 1 ppm (40 times less than previously reported $^{28}$Si sources). We also demonstrate the ability to produce isotope heterostructures with applications including $^{28}$Si/$^{28}$Si$^{74}$Ge quantum wells. $^{28}$Si is a critical material for quantum computing as removal of $^{29}$Si spins means qubits such as phosphorous atoms can have nuclear coherence (T$_{2})$ times of minutes even up to room temperature and can be addressed optically due to hyperfine transitions not normally resolvable in natural Si. Despite these advantages, $^{28}$Si is quite scarce making it clear that an alternate source such as the one we demonstrate is needed.