A compact, ultra-high vacuum ion source for isotopically enriching and depositing ²⁸Si thin films

An ultrahigh vacuum (UHV) compatible Penning ion source for growing pure, highly enriched ²⁸Si epitaxial thin films is presented. Enriched ²⁸Si is a critical material for quantum information due to the elimination of nuclear spins. In some cases, the material must be grown by low temperature molecular beam epitaxy, e.g., scanning tunneling microscopy hydrogen lithography-based devices. Traditional high-purity physical vapor methods typically deliver a very small fraction of source material onto the target substrate, making the cost for use with highly enriched source materials very high. Thus, directed beam sources provide an efficient alternative. This UHV Penning source uses all metal or ceramic parts and a removable electromagnet to allow bake-out. The source gas is a commercial (natural isotope abundance) silane gas (SiH₄), an inexpensive source material. High enrichment levels up to 99.999 87% (8.32 × 10^-7 mol/mol ²⁹Si) and high chemical purity of 99.965% are shown without postprocessing. We present and discuss the discharge properties of this new source, the ion mass spectrum when coupled to our mass filter, and the secondary ion mass spectroscopy of the grown films.