Quantum transport in self-assembled Ge nanowires

There is an intense effort in information technology to find solutions to the problems emerging from the miniaturization of conventional complementary metal oxide semiconductor devices. While Ge was the material used for the realization of the first transistor in 1947 it was not considered any more for decades. In an effort to increase the performance of the ever smaller transistors, Ge has moved back into focus because of its lower effective mass and higher mobility for holes in p-type transistor. Also in the field of spin qubits, SiGe has emerged as a promising material. SiGe nanostructures are expected to have long coherence times due to the absence of hyperfine interaction (in isotopically purified crystals). For electrons spin coherence times of up to 3 microseconds have been measured in two dimensional electron gases; the spin coherence time is expected to be further enhanced by additional confinement. Indeed, just few weeks ago electrons localized in donors in pure 28Si have shown coherence times up to 2s. Finally, very recently, it has been suggested that ultrathin strained Ge nanowires show an unusually large SO interaction, larger than those present in InAs and InSb nanowires, suggesting that such one- dimensional wires should hold great potential for the realization of Majorana bound states. In view of the above facts one could speak of a new era for Ge in information technology. The applicant aims to investigate such new physical concepts in a Ge based material system: SiGe self-assembled nanostructures grown in the Stranski-Krastanow mode. The self-assembled growth of crystalline SiGe nanostructures on Si was reported for the first time in 1990. This created great expectations that such nanostructures could provide a valid route towards innovative, scalable and CMOS-compatible nanodevices. Two decades later the applicant was able to realize the first devices based on such structures, and to investigate for the first time their electronic properties. His results indicate that SiGe self-assembled quantum dots display a rather unique combination of properties, i.e. low hyperfine interaction and strong spin-orbit coupling. The aim of the present project is to move a step further and explore ultrathin strained one dimensional SiGe self-assembled quantum structures, which are expected to possess even more promising properties, both for spintronic applications as well as for spin basedopological quantum computation. The objective of the present proposal is above all to: a) investigate the electronic properties of these structures b) study spin-injection by means superconducting contacts, c) investigate whether SiGe quantum wires can support helical states, which would make them thus an interesting alternative, to III-V nanowires, system for studying the physics of Majorana bound states.

Since many decades, researchers have the dream of realizing a quantum computer. In condensed matter physics there are intense efforts all around the world to identify which are the most promising systems which could be used as a quantum bit, the fundamental element from which such a quantum computer could be built. In this line we have investigated the potential of a newly developed material system: Germanium self-assembled nanowires which are directly grown on silicon. Within this project we have studied their fundamental properties. By applying a magnetic field we were able to split the degenerate orbital level of the confined holes into two. Such a two level system can be used as a qubit. We have found out that depending on the direction of the applied magnetic field the separation of the two levels varies. Such measurements will allow us to obtain deeper understanding of the properties of the confined carriers. At higher temperatures our devices show signatures of electronic transport via one dimensional subbands. This opens up the possibility to study whether, as has been theoretically predicted, devices made out of Germanium can support helical states, i.e. states in which opposite spins flow in the opposite directions. Finally we have done the first attempts to couple our semiconductor structures with superconductors which can stand magnetic fields in the Tesla regime. Such experiments are the first step in order to investigate whether hole type systems can support exotic quasiparticles, known as Majorana fermions. If such can be realized, one can investigated also the potential of Majorana fermions for the realization of a slightly different type of a quantum computer.