Highly crystalline superconducting hyperdoped germanium thin films

Superconducting group IV materials are highly promising for quantum information due to the homoepitaxial alignment with the underlying substrate, reducing material disorder at the film/substrate interface. Furthermore, increasing interest in germanium systems for both spin qubits, gate-tunable superconducting qubits, and topological phases has put a spotlight on the necessity for thin film superconductors that readily interface with group IV systems. However, the hyperdoped phase is thought to require dopant incorporation above typical thermodynamical solubility limits and thus most efforts have been focused on non-equilibrium techniques. Very recent work has shown that superconductivity is observed in Ga-doped germanium system using molecular beam epitaxy. In this talk we will present an expanded study towards illuminating the atomic fine structure of superconducting germanium thin films grown via MBE. Through a combination of synchrotron x-ray scattering and cross-sectional STEM, we observe that our superconducting MBE-grown films exhibit well-dispersed Ga-dopants throughout the film as substitutional defects. The homoepitaxial interface between the Ge substrate and the superconducting Ge film is well-defined, the films are of high crystalline quality, and no Ga clustering is found.