As-grown carbon nanotube quantum dots with superconducting contacts

The progress in fabrication technology and the miniaturization of nanostructured devices in the recent past has attracted a lot of interest in the field of electronic circuits on the nanoscale where the system's spatial dimensions allow for the investigation of quantum phenomena. Since their first identification by S. Iijima in 1991, carbon nanotubes (CNTs) have been implemented in electronic junctions making use of their extraordinary electronic and mechanical properties. The investigation of CNT quantum dots (QDs) has given insight into their intrinsic electronic properties, but since certain intrinsic electronic features of CNTs are often masked by disorder that is introduced during the device preparation process, the need for very clean CNTs without any defects has lead to the fabrication of as-grown CNT devices by J. Cao et al. in 2005. Since their introduction, such devices have been shown to be nearly defect-free and have lead to a considerable number of interesting results. In contrast to conventionally prepared CNT devices, the basis of the fabrication scheme is the synthesis of the CNT being the very last production step. This prevents residues of post-fabrication processes from sticking to the CNT and the electron radiation applied to the device for imaging and localization is avoided. However, this process needs the electrodes to be pre-fabricated which drastically limits the choice of contact materials, mainly by the CNT growth process at high temperatures. Therefore, platinum (Pt) is the main candidate for normal metal contacts at the moment.
Going a step further by using superconductors as contacts to such clean, as-grown devices is a promising goal and starting point for both, the fabrication of as-grown CNTs with S contacts and the prospect of new effects and observations, since the combination of CNT QDs with superconducting (S) electrodes has already been shown to be a nice tool for several applications. This project is about the road from the fabrication of such devices to their characterization by electronic transport measurements at both, room temperature and cryogenic temperatures. In the end, despite functional S contacts, clear signs of the S proximity effect in as-grown CNT devices could not be observed. However, the fabrication scheme itself was established and resulted in clean devices that lead to some other interesting results in the field of inelastic cotunneling and electron-phonon coupling.
The initial steps were first, to select S materials suitable for this special fabrication scheme and second, to check them for the S proximity effect on CNT QDs. In the beginning, test with rhenium (Re) contacts on a conventionally prepared CNT device worked quite well, meaning it was possible to observe the S gap in the density of states of the leads in transport measurements. Therefore, in the next step Re was applied as contact material for as-grown CNT devices, since it provides the important feature of surviving the CNT growth process unaltered in its S properties. It was possible to measure devices of good cleanliness, but unfortunately, it was not possible to observe any S proximity effect on as-grown CNT devices. Therefore, much effort was put into finding alternative S materials and the improvement of the CNT-S interface.
The first concept was to add a thin contact layer of tungsten (W) on top of the Re contacts where the idea was that W is supposed to form much stronger bonds to CNTs. This results in a better electrical coupling and should increase the chance for Cooper pair injection. Additional investigations were done on Re/W alloys and niobium nitride (NbN) contacts. However, no Cooper pair injection was observed. Hence, it is suggested that some effects at the interface cause a dephasing and a breaking of the Cooper pairs. Nevertheless, on the road to the actual goal of this thesis, some interesting results were observed:
For example, as-grown CNTs contacted to NbN/Pt double layer systems showed clean and stable QDs. On one of them, tilted resonances within the Coulomb blockaded region were observed and initially considered as renormalized inelastic cotunneling thresholds. This first assumption was disproved by several features that could not be explained by means of cotunneling events. Therefore, this interpretation was discarded and the picture of a parallel double QD system was taken into account. This offered a stability diagram consisting of two superimposed Coulomb diamond patterns. In addition, one of them was assumed to be accompanied by a series of excited state lines. Due to their equidistance and combined appearance with negative differential conductance, they were interpreted as originating from tunneling induced vibrational excitations of a suspended part of the CNT. This is affirmed by the fact that the energy separation matches the excitation energy of the longitudinal stretching mode for the length of a suspended part of the CNT.