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Patrick Torresani

Hole quantum spintronics in strained germanium heterostructures

Published on 14 June 2017


Thesis presented June 14, 2017

Abstract:
This thesis focuses on low temperature experiments in germanium based heterostructure in the scope of quantum spintronic. First, theoretical advantages of Ge for quantum spintronic are detailed, specifically the low hyper fine interaction and strong spin orbit coupling expected in Ge. In a second chapter, the theory behind quantum dots and double dots systems is explained, focusing on the aspects necessary to understand the experiments described thereafter, that is to say charging effects in quantum dots and double dots and Pauli spin blockade. The third chapter focuses on spin orbit interaction. Its origin and its effect on energy band diagrams are detailed. This chapter then focuses on consequences of the spin orbit interaction specific to two dimensional germanium heterostructure, that is to say Rashba spin orbit interaction, D’Yakonov Perel spin relaxation mechanism and weak antilocalization. In the fourth chapter are depicted experiments in Ge/Si core shell nanowires. In these nanowire, a quantum dot form naturally due to contact Schottky barriers and is studied. By the use of electrostatic gates, a double dot system is formed and Pauli spin blockade is revealed. The fifth chapter reports magneto-transport measurements of a two-dimensional hole gas in a strained Ge/SiGe heterostructure with the quantum well laying at the surface, revealing weak antilocalization. By fitting quantum correction to magneto-conductivity characteristic transport times and spin splitting energy of 2D holes are extracted. Additionally, suppression of weak antilocalization by a magnetic field parallel to the quantum well is reported and this effect is attributed to surface roughness and virtual occupation of unoccupied subbands. Finally, chapter number six reports measurements of quantization of conductance in strained Ge/SiGe heterostructure with a buried quantum well. First the heterostructure is characterized by means of magneto-conductance measurements in a Hall bar device. Then another device engineered specifically as a quantum point contact is measured and displays steps of conductance. Magnetic field dependence of these steps is measured and an estimation of the g-factor for heavy holes in germanium is extracted.

Keywords:
Quantum dots, Germanium, Nanowires, Spintronics, Holes

On-line thesis.