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

Natalia Ares 1 
1 LaTEQS - Laboratoire de Transport Electronique Quantique et Supraconductivité
PHELIQS - PHotonique, ELectronique et Ingénierie QuantiqueS : DRF/IRIG/PHELIQS
Abstract : Quantum mechanics displays all its exciting strangeness already by considering the Schrödingerequation in a one-dimensional square well potential; tunnelling events put this statement in evidence.To recreate this situation in a given material system is an inspiring playground and a big step towardstaking control of quantum mechanisms. For instance, it is now possible to confine electrons in solidstatedevices enabling amore efficient solar-cell technology. Confining individual electron spins has infact been suggested as a possible approach to the realization of a quantum computer. Each electronspin forms a natural two-level systems encoding an elementary bit of quantum information (a socalledspin qubit). This proposal, by Loss and DiVincenzo, has contributed to the opening of an activeresearch field referred to as quantum spintronics. Spin qubits rely on the fact that spin states canpreserve 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 constantmagnetic fields and spatially inhomogeneous electric fields, applying oscillating electric fields, usingconductive oxide layers, etc. To take advantage of the band-alignment of different semiconductors isamong these. The relevant dimensions of the considered system should still be smaller than the phasecoherence length of the confined particles in order that their quantum behaviour is preserved.So far, most of the progress has been achieved using GaAs-based semiconductor heterostructures. Insuch layered systems themotion of carriers is confined to a plane and further confinement is achievedbymeans of lithographic techniques, which allow lateral confinement to be achieved on a sub-100 nmlength scale. In this way, quasi-zero-dimensional systems whose electronic states are completelyquantized, i.e. quantum dots (QDs), can be devised.Various time-resolved techniques involving high-frequency electrical signals have been developed tomanipulate and read-out the spin state of confined electrons in GaAs QDs, and several years ago thefirst spin qubits were reported. In GaAs-based QDs, however, the quantum coherence of electronspins is lost on relatively short time scales due to the hyperfine interactionwith the nuclear spins (bothGa and As have non-zero nuclear spin moments). In spite of significant advances on controlling thenuclear polarization [3, 4], this problem remains unsolved.In the past few years an increasing effort is concentrating on alternative material systems in whichhyperfine interaction is naturally absent or at least very weak and, in principle, controllable by isotopepurification. While Si fulfils this requirement and it is the dominant material in modernmicroelectronics, it suffers from low mobility compared to III-V semiconductors, which obstructs itsapplication for quantum spintronics. SiGe structures offer a way to circumvent this problem that isstill compatible with standard silicon processes.I have focused mainly on the study of the electronic properties of SiGe self-assembled islands, alsocalled SiGe nanocrystals. This work, which condensates the main points of this study, is organized insix chapters. In the first chapter, I describe the basics of the growth of SiGe self-assembled islands andthe properties of the quasi-zero-dimensional confinement potential that they define. Chapter 2 isdevoted to the basics of electronic transport in these structures. Chapter 3 deals with the electricmodulation of the hole g-factor in SiGe islands, which would enable a fast manipulation of the spinstates. In Chapter 4 I present theoretical and experimental findings related to spin selectivity in SiGeQDs and Chapter 5 is dedicated to the realization of an electron pump in InAs nanowires based on thiseffect. Finally, Chapter 6 exhibits our progress towards the study of coupled SiGe QD devices.
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Natalia Ares. Electronic transport and spin control in SiGe self-assembled quantum dots. Quantum Physics [quant-ph]. Université de Grenoble, 2013. English. ⟨NNT : 2013GRENY060⟩. ⟨tel-01499920⟩

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