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Fabrication and Electron Transport Study of Nanowire based Quantum Devices

Final Report Summary - NANOQUANTUMDEVICES (Fabrication and Electron Transport Study of Nanowire based Quantum Devices)

The central aim of the present ERC project was to start a new research direction at the host institute: the fabrication and transport characterization of nanoelectronic devices. Since device fabrication facilities were not available before, an electron beam lithography (EBL) system has been installed. It consist a second-hand scanning electron microscope (Jeol 848) combined with Raith lithography software (see Fig. 1a). After optimizing the system sub 50nm writing resolution has been achieved (see Fig. 1b) which opened the way to fabricate various nanostructures. It was widely used to define electric leads to nanoobjects, like semiconductor nanowires, graphene. Furthermore it was also used for other purposes as well, like generating optical lithography masks or template for nanowire growth. The cryogenic infrastructure of the host laboratory has also been extended to enable low temperate electric characterization of the fabricated nanocircuits (see Fig. 1c-d): sample holders and computer controlled transport measurement setup has been developed (see Fig. 1c-d).

Beside the EBL, several fabrication steps are required to produce an InAs Nanowire (NW) based electric circuits (e.g. plasma cleaning, passivation, metallization, bonding), which were all optimized using the available equipments in the home surrounding. Various InAs nanowire based devices were fabricated. We have extensively investigated the possibility to electrically tune the spin-orbit interaction in these circuits by weak localization measurements (see Fig. 2a). Quantum dots have also been formed in such InAs wires by different fabrication techniques, e.g. local chemical etching of the NW or high resolution bottom gate structures predefined bellow the NW (see. Fig. 2b) [1]. If an InAs NW based quantum dot coupled to a ferromagnetic (F) lead the spin ground state of the dot can be gate tunable [2]. We have shown, that this configuration is very promising for efficient spin injector devices [3]. First the quantum dot at the ferromagnet normal interface greatly enhances the spin polarization of the current (see Fig. 2c), secondly it allows the change of the orientation of the polarization by electrical means. The quantum dot system is even more exciting, when the F - quantum dot system is coupled to a superconducting lead in addition [4], where the interplay of ferromagnetism and superconductivity provides a special subgap feature.

Beside InAs NW based devices we also stared to develop graphene nanocircuits. Particularly, we explored the fabrication of graphene nanoribbons (see Fig. 2) with the combination of a special carbo thermal etching process and EBL [5]. Atomic sized junctions provides an other interesting superconducting hybrid system, when a single atom contact is placed between two superconducting electrode. From the nonlinear subgap I-V characteristics the full mesoscopic pin code of the single atom contact can be extracted (see Fig. 2e). We have performed detailed pin code analysis for In nanojunctions [6]. The direct visualization of atomic and single molecular junctions is not possible for most of the experiments, therefore the identification of the precise atomic configuration is based on the transport data and its comparison to simulation. We proposed a new statistical method to analyze the transport data, which provides significantly more information than the commonly used conductance histogram technique [7].

In the framework of the present grant a new research group has been established at the host institute focusing on quantum transport in hybrid nanocircuits ( Beside the PI, 4 master students and 1 BSc has been involved in the research activity. Since 2010, the PI also organizes a new Nanophysics seminar & Journal Club on weekly basis, which provides a unique opportunity for the young researchers and students at the host university to get familiar with the recent research results of the field ( In addition a new course has been organized for physics graduate and undergraduate students with the title of Transport in Complex Nanostructures.

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