Superconductor-Semiconductor Epitaxy for Integrated Quantum Electronics

Superconductor-semiconductor (S-Sm) circuit elements have become very popular in recent years for their potential in creating small foot-print devices and low-power voltage controlled Josephson junctions. These S-Sm elements can be used in quantum-limited amplifiers, tunable couplers, qubits or switches. One attractive choice of materials system includes group IV materials, as they can integrate monolithically with existing mature foundry technologies making for a seamless transition to useful applications. A major challenge which must necessarily be resolved in the use of group IV materials for S-Sm circuit elements is stabilizing a superconducting phase in group IV. This experimental realization of superconducting semiconductors has been elusive in experiments over the past three decades. Nevertheless, there have been numerous efforts to create superconducting phases in Si and Ge. Here, we report the epitaxial growth of hyperdoped Ga:Ge films by molecular beam epitaxy with extreme hole concentrations 5x10²¹ cm^-3 that yield superconductivity with a critical temperature of T_C = 3.5 K and an out-of-plane critical field of 1~T at 270~mK. More importantly the homoepitaxy of doped and undoped layers allow creating vertical Josephson junction structures allowing for scaling to millions of Josepshon junctions in one process.