Germanium based quantum electronics

Germannium based quantum electronics

It is widely believed that next-generation electronic devices will exploit the quantum mechanical nature of electrons, as opposed to the classical operating principles of current CMOS electronics. This is one of the leading paradigms at the origin of quantum spintronics, a developing field whose goal is to create electronic devices with functionalities originating from spin degrees of freedom. Typical examples of quantum spintronic devices are spin qubits, the elementary building blocks of a spin-based quantum processor, and spin transistors, i.e. transistors where current depends on the gate-tunable spin state of the carriers.

Gate control of an electronic spin state can be readily obtained in the presence of strong spin-orbit interaction, mixing (gate-sensitive) charge and spin degrees of freedom.

In collaboration with the group of Prof. Myronov (Warwick University, UK), we are investigating the potential offered by p-type Ge/SiGe heterostructures embedding a high-mobility two-dimensional hole gas confined to a thin Ge layer. Preliminary studies, at both experimental [1-4] and theoretical level [5,6], have revealed that in this material system spin-orbit interaction can be even larger than in most n-type III-V semiconductors. Hence it should enable fast (nanosecond regime) coherent manipulation of hole spins by means of externally applied microwave-frequency electric fields [3]. According to recent predictions, the strong spin-orbit coupling of SiGe nanostructures should also enable the important possibility to couple individual spins on relatively long distances by means of interposed floating gates [5]. Moreover, in Ge-rich one-dimensional nanowires, a large spin-orbit coupling together with experimentally accessible transverse electric fields could induce robust helical conduction modes in which spin and momentum are “locked” to each other [6].

In the presence of superconducting proximity effect, these hole-type helical modes should result in a 1D topological superconducting state with Majorana quansiparticles at the edges. Besides having a strong spin-orbit interaction in the valence band, Ge can form very good (Schottky-barrrier-free) contacts with metals and superconductors. This stems from the fact that the metal (or superconductor) Fermi energy tends to pin very close to the Ge valence band edge.

Our group has already a long-standing experience with Ge-based devices. Our experimental work initially focused on SiGe self-assembled nanocrystals grown by molecular-beam epitaxy. We showed that such nanocrystals can be individually contacted by nanofabricated metal electrodes to form single-hole transistors[1-3].  Using aluminum as contact metal, devices behaving as resonant supercurrent transistors were obtained below the superconducting critical temperature of the aluminium contacts [1].

1.     G. Katsaros, P. Spathis, M. Stoffel, F. Fournel, M. Mongillo, V. Bouchiat, F. Lefloch, A. Rastelli, O.G. Schmidt, S. De Franceschi,                 Nature Nanotech. 5, 458 (2010).
2.     G. Katsaros, V. N. Golovach, P. Spathis, N. Ares, M. Stoffel, F. Fournel, O. G. Schmidt, L. I. Glazman, and S. De Franceschi., Phys. Rev. Lett. 107, 246601 (2011).
3.     N. Ares, G. Katsaros, V.N. Golovach, J.J. Zhang, A. Prager, L.I. Glazman, O.G. Schmidt, S. De Franceschi, Appl. Phys. Lett. 103,263113 (2013).
4.     Matthias Brauns, Joost Ridderbos, Ang Li, Erik P. A. M. Bakkers, and Floris A. Zwanenburg, Phys. Rev. B 93, 121408(R) (2016)
5.     M. Trif, V. N. Golovach, and D. Loss, Phys. Rev. B 77, 045434 (2008).
6.     C. Kloeffel, M. Trif, and D. Loss, Phys. Rev. B 84, 195314 (2011).