Superconductivity in hyperdoped Ge by molecular beam epitaxy

Superconducting germanium films are an intriguing material for possible applications in fields such as cryogenic electronics and quantum bits. Recently, there has been a great deal of progress in hyperdoping of Ga doped Ge using ion implantation. Thin film growth of such a material would be advantageous, allowing homoepitaxy of doped and undoped Ge films and opening possibilities for vertical Josephson junctions. Here, we present our studies on the growth of one layer of hyperdoped superconducting germanium thin film via molecular beam epitaxy. We observe a fragile superconducting phase, which is extremely sensitive to processing conditions and can easily phase-segregate, forming a percolated network of pure gallium metal. By suppressing phase segregation through temperature control, we find a superconducting phase that is unique and appears coherent to the underlying Ge substrate.