Hole quantum dots in planar silicon and in GeSi nanowires*

Ge/Si core/shell nanowires are suitable candidates for electrically driven spin qubits, and for the creation of Majorana fermions [1]. In highly tuneable hole quantum dots [2, 3], we observe shell filling of new orbitals and corresponding Pauli spin blockade [4]. In nanowires with superconducting Al leads we create a Josephson junction via proximity-induced superconductivity. A gate-tuneable supercurrent is observed with a maximum of ~60 nA [5]. We identify two different regimes: Cooper pair tunnelling via multiple subbands in the open regime the device [6], while near depletion the supercurrent is carried by single-particle levels of a quantum dot operating in the few-hole regime [5].

Secondly, we create ambipolar quantum dots in silicon nanoMOSFETs. We investigate the conformity of Al, Ti and Pd nanoscale gates by means of transmission electron microscopy [7]. We define low-disorder electron quantum dots with Pd gates [8], and depletion-mode hole quantum dots in undoped silicon [9]. For the latter we use fixed charge in a SiO₂/Al₂O₃ dielectric stack to induce a 2DHG at the Si/SiO₂ interface. The depletion-mode design avoids complex multilayer architectures requiring precision alignment and allows directly adopting best practices already developed for depletion dots in other material systems.

[1] C. Kloeffel et al., Phys. Rev. B 84, 195314 (2011).
[2] M. Brauns et al., Applied Physics Letters 109, p. 143113 (2016).
[3] F. Froning et al., Applied Physics Letters 113, 073102 (2018).
[4] M. Brauns et al., Phys. Rev. B 94, 041441(R) (2016).
[5] J. Ridderbos et al., Advanced Materials, 1802257, (2018).
[6] J. Xiang et al., Nature Nanotechnology 1, 3 (2006).
[7] P. C. Spruijtenburg et al., Nanotechnology, (2018).
[8] M. Brauns et al., Scientific Reports 8, 5690, (2018).
[9] S. V. Amitonov et al., Applied Physics Letters 112, p. 023102 (2018).