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 great deal of progress in hyperdoping of Ga doped Ge using ion implantation. The thin film growths would be advantageous allowing homoepitaxy of doped and undoped Ge films 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

Molecular beam epitaxy growth of superconducting tantalum germanide

Developing new material platforms for use in superconductor-semiconductor hybrid structures is desirable due to limitations caused by intrinsic microwave losses present in commonly used III/V material systems. With the recent reports that tantalum provides drastic improvements when implemented in superconducting circuit elements over traditional Nb and Al films, exploring Ta as an alternative superconductor in hybrid material systems seems necessary. Here, we present our study on the growth of Ta on semiconducting Ge (001) substrates grown via molecular beam epitaxy. We show that the Ta diffuses into the Ge matrix in a self-limiting nature resulting in extremely smooth and abrupt surfaces and interfaces that are ideal for future cQED device fabrication. The films have a nominal composition of TaGe$_{2}$ and form a native oxide of nominal composition Ta$_{2}$Ge$_{2}$O$_{5}$ that also forms a sharp interface with the underlying film. These films are superconducting with a $T_{C}\sim 1.8-2$K and $H_{C}^{\perp} \sim 1.88T$, $H_{C}^{\parallel} \sim 5.1T$

Characterizing losses in InAs two-dimensional electron gas-based gatemon qubits

The tunnelling of cooper pairs across a Josephson junction (JJ) allow for the nonlinear inductance necessary to construct superconducting qubits, amplifiers, and various other quantum circuits. An alternative approach using hybrid superconductor-semiconductor JJs can enable superconducting qubit architectures with all electric control. Here we present continuous-wave and time-domain characterization of gatemon qubits and coplanar waveguide resonators based on an InAs two-dimensional electron gas. We show that the qubit undergoes a vacuum Rabi splitting with a readout cavity and we drive coherent Rabi oscillations between the qubit ground and first excited states. We measure qubit relaxation times to be $T_1 =$ 100 ns over a 1.5 GHz tunable band. We detail the loss mechanisms present in these materials through a systematic study of the quality factors of coplanar waveguide resonators. While various loss mechanisms are present in III-V gatemon circuits we detail future directions in enhancing the relaxation times of qubit devices on this platform

Pinhole-seeded lateral epitaxy and exfoliation of GaSb films on graphene-terminated surfaces

Remote epitaxy represents a promising method for the synthesis of thin films on lattice-mismatched substrates, but its atomic-scale mechanisms are still unclear. Here, the authors demonstrate the growth of exfoliatable GaSb films on graphene-terminated GaSb (001) via seeded lateral epitaxy, showing that pinhole defects in graphene serve as selective nucleation sites