Development of Ge/SiGe semiconducting quantum dot devices for hole-based spin qubits.

Confining few electrons in silicon-based heterostructures via lithographically-defined gated quantum dots (QD) enables the manipulation of the spin degree of freedom for quantum information processing. In recent years, the hole-based QDs based on strained Ge/SiGe semiconducting quantum wells have rapidly advanced as a compelling platform for spin qubits [1]. Some of the appealing features of the Ge-based spin qubits are their coherent properties achievable by solid-state all-electrical control readout enabled by their intrinsic spin-orbit interaction [2], and their prospects of scalability resulting from long-range coupling via superconducting circuits [3]. From a fabrication standpoint, two key components for the formation of the Ge-QDs is the electrical contacts to the Ge-well and the gate dielectric interface quality over the active dot area. On the forementioned aspect, we explore the use of metallic germano-silicide contacts to the Ge quantum well due to their lower temperature requirements for the fabrication and ability to be proximitized to the quantum dot active area avoiding structural damage caused by a high fluence ion implantation. We estimate the contact resistance via low temperature transport measurements on gated transfer line method devices (TLMs) with contacts formed by platinum germano-silicides (Pt-Ge-Si). On the later aspect, the gate dielectric interface, we focus on optimizing the quality of the ALD-grown Al2O3/SiGe interfaces. We present the results from a series of X-ray photoemission spectroscopy (XPS) measurements on the Al₂O₃/SiGe and Al₂O₃/Pt-Ge-Si interfaces. Our XPS data shows the presence of unwanted Si in the dielectric layer presenting a medium for bulk and interface charge traps that contribute to a significant source of charge noise in the device. We present methods to mitigate the Si presence in the gate stack and near the Al₂O₃/Pt-Ge-Si interface. Lastly, we will present device modeling of the electrostatic gate response on the Ge-QW using the MaSQE (Modeling and Simulation for Quantum Exploration) Schrödinger-Poisson solver assuming a single-band hole model.

References:

[1] Scappucci, G., et al. Nat Rev Mater 6, 926–943 (2021).

[2] Hendrickx, N.W., et al. Nat. Mater. 23, 920–927 (2024).

[3] Sagi, O., et al. Nat Commun 15, 6400 (2024).