(Invited) Quantum Dots and Superconductivity in Silicon Systems

Abstract

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 [5,7,8].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 [9]. We define low-disorder electron quantum dots with Pd gates [10], and depletion-mode hole quantum dots in undoped silicon [11]. 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. Finally, we have realized ambipolar charge sensing: we have fabricated a single-electron transistor next to a single-hole transistor, and tuned both quantum dots to sense charge transitions of the other quantum dot. Using active charge sensing the single-electron transistor can detect the few-charge regime in the hole quantum dot [12].[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, p. 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] J. Ridderbos et al., Physical Review Materials, 3, 084803 (2019). [8] J. Ridderbos et al., Nano Letters 20, 1, p. 122 (2020). [9] P. C. Spruijtenburg et al., Nanotechnology, (2018). [10] M. Brauns et al., Scientific Reports 8, 5690, (2018). [11] S. V. Amitonov et al., Applied Physics Letters 112, p. 023102 (2018). [12] A. J. Sousa de Almeida et al., Phys. Rev. B 101, 201301(R) (2020). Figure 1