Elektronentransport und Spindynamik in selbstorganisierten SiGe Nanostrukturen
Final Report Abstract
During my DFG Forschungsstipendium I have studied the electronic properties of Ge-based nanostructures. I have focused on a particular type of SiGe nanostructures epitaxially grown on silicon-on-insulator substrates: SiGe self-assembled islands. Since their discovery in 1990, these nanocrystals have been heavily investigated from the viewpoint of their structural properties and their growth mechanism. Much of this research activity was motivated by the general belief that SiGe selfassembled nanocrystals could provide a valid route towards scalable and CMOS-compatible nanodevices. Yet no device realization had ever been reported. During these two years we believe that we made a first important step in this direction. We have recently reported the first realisation of single-hole transistors based on such individual SiGe nanocrystals. A variety of low-temperature transport regimes depending on the strength of the tunnel coupling to the leads were observed. Transport spectroscopy reveals largely anisotropic hole g-factors. By changing the number of holes localized within the SiGe QDs a clear modulation of the g-factor is observed indicating that the g-factors are linked to the corresponding orbital wavefunctions. For studying the effect of an external electric field on the value of the g-factor for the same orbital wave function, i.e. for the same number of holes, we have fabricated dual gate devices. Our measurements demonstrate that the g-factor can be changed by a factor of ~ 50% when changing the value of a perpendicular electric field. The observed tunable g-factors make SiGe self-assembled QDs an interesting material system for performing all-electrical spin coherent manipulations. The g-factor deviations from the free-particle value constitute indirect evidence of a strong coupling between orbital and spin degrees of freedom. Quantitative evidence of a strong spin-orbit coupling is obtained from the observation of field-induced avoided crossings between energy levels with different spin quantum number. Interestingly, the coupling strength is found to vary (and even vanish) with the field direction. Our experiments have furthermore demonstrated that it is possible to form low-resistance contacts to superconducting electrodes, and have therefore demonstrated the first example of a singlehole supercurrent transistor based on SiGe. In summary, self-assembled SiGe nanocrystals provide a new versatile playground for investigating a variety of quantum phenomena in condensed matter physics. In particular, access to the strong-coupling limit could open up new opportunities to explore spin–orbit physics and other spinrelated phenomena, such as the Kondo effect in combination with superconducting and possibly ferromagnetic correlations.
Publications
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Transistor a effet de Champ sur Ilot de Materiau Semiconducteur Auto-assemble, French Patent : number N°10 58246 (Field effect transistors based on self assembled semiconductor islands)
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Hybrid superconductor-semiconductor devices made from self-assembled SiGe nanocrystals on silicon. Nature Nanotechnology 5, 458 (2010)
G. Katsaros, P. Spathis, M. Stoffel, F. Fournel, M. Mongillo, V. Bouchiat, F. Lefloch, A. Rastelli, O. G. Schmidt and S. De Franceschi
