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Natalia Ares

Electronic transport and spin control in SiGe self-assembled quantum dots

Published on 24 October 2013

Thesis presented October 24, 2013

Quantum mechanics displays all its exciting strangeness already by considering the Schrödinger equation in a one-dimensional square well potential; tunneling events put this statement in evidence. To recreate this situation in a given material system is an inspiring playground and a big step towards taking control of quantum mechanisms. For instance, it is now possible to confine electrons in solid state devices enabling amore efficient solar-cell technology. Confining individual electron spins has in fact been suggested as a possible approach to the realization of a quantum computer. Each electron spin forms a natural two-level systems encoding an elementary bit of quantum information (a socalled spin qubit). This proposal, by Loss and Di Vincenzo, has contributed to the opening of an active research field referred to as quantum spintronics. Spin qubits rely on the fact that spin states can preserve their coherence on much longer time scales than charge (i.e. orbital) states. A confinement potential can be created artificially in many different ways; producing constant magnetic fields and spatially inhomogeneous electric fields, applying oscillating electric fields, using conductive oxide layers, etc. To take advantage of the band-alignment of different semiconductors is among these. The relevant dimensions of the considered system should still be smaller than the phase coherence length of the confined particles in order that their quantum behavior is preserved. So far, most of the progress has been achieved using GaAs-based semiconductor heterostructures. In such layered systems the motion of carriers is confined to a plane and further confinement is achieved by means of lithographic techniques, which allow lateral confinement to be achieved on a sub-100 nm length scale. In this way, quasi-zero-dimensional systems whose electronic states are completely quantized, i.e. quantum dots (QDs), can be devised. Various time-resolved techniques involving high-frequency electrical signals have been developed to manipulate and read-out the spin state of confined electrons in GaAs QDs, and several years ago the first spin qubits were reported. In GaAs-based QDs, however, the quantum coherence of electron spins is lost on relatively short time scales due to the hyperfine interaction with the nuclear spins (both Ga and As have non-zero nuclear spin moments). In spite of significant advances on controlling the nuclear polarization [3, 4], this problem remains unsolved. In the past few years an increasing effort is concentrating on alternative material systems in which hyperfine interaction is naturally absent or at least very weak and, in principle, controllable by isotope purification. While Si fulfils this requirement and it is the dominant material in modern microelectronics, it suffers from low mobility compared to III-V semiconductors, which obstructs its application for quantum spintronics. SiGe structures offer a way to circumvent this problem that is still compatible with standard silicon processes. I have focused mainly on the study of the electronic properties of SiGe self-assembled islands, also called SiGe nanocrystals. This work, which condensates the main points of this study, is organized in six chapters. In the first chapter, I describe the basics of the growth of SiGe self-assembled islands and the properties of the quasi-zero-dimensional confinement potential that they define. Chapter 2 is devoted to the basics of electronic transport in these structures. Chapter 3 deals with the electric modulation of the hole g-factor in SiGe islands, which would enable a fast manipulation of the spin states. In Chapter 4 I present theoretical and experimental findings related to spin selectivity in SiGe QDs and Chapter 5 is dedicated to the realization of an electron pump in InAs nanowires based on this effect. Finally, Chapter 6 exhibits our progress towards the study of coupled SiGe QD devices.

Nanocrystals, Quantum dots, Silicon, Quantum transport

On-line thesis.