Nanostructuring of devices for nanoscience applications

Résumé

Having the ability of physically accessing the realm of molecules and atoms, ofnanoparticles and nanotubes, leads to constant advances in our fundamental scientificknowledge. Moreover, it sets the ground for developing new technologies in many differentfields, such as biomedicine, energy, materials science or food industry. The words¿nanoscience¿ and ¿nanotechnology¿ englobe every aspect of all this interdisciplinaryresearch.Lithographic techniques allow patterning nanostructures and eventually, building devicesthat constitute our means for exploring the world at the nanometre scale. Fabricatingdevices for nanoscience applications typically requires patterning nanostructures on surfaceswith different characteristics (concerning their chemical, electrical or mechanical properties).Apart from that, the proposed experiments often aim at probing objects with sizes that gobeyond the resolution capabilities of the state-of-the-art lithographic techniques.This thesis is dedicated to the study of electron beam lithography (EBL), its practicallimits and complementary strategies for fabricating devices and nanostructures fornanoscience research. The working pace and activities carried out during the developmentof this thesis have partly been conditioned by the needs of the newly establishedlaboratories of CIC nanoGUNE, principally, regarding the optimisation of a few basiclithographic processes. Setting up new equipment and preparing protocols for thefabrication and characterisation of nanodevices also required special attention.To begin with, we optimised the lithographic process on a range of substrates with verydifferent conductivity and thicknesses, such as insulating glass and CaF2, highly doped siliconchips or electron-transparent Si3N4 membranes. Each type of substrate had a differentresponse upon exposure to the electron beam and the process had to be accordinglyadapted to each case. Apart from that, we explored the resolution capability of our inhouseEBL resources (working at moderate beam energies <20 kV), with which we couldachieve minimum feature sizes of around 20 nm. Among the obtained results we couldhighlight plasmonic nanostructures on insulating substrates and electrodes with high aspectratio nanogaps aimed for molecular electronics. On the other hand, we had theopportunity of experimenting with a high resolution electron beam writer (working at 100kV) for patterning nanostructures on graphene flakes. This work was carried out in thefacilities of the Technical University of Denmark (Kgs. Lyngby, Denmark) during an externalstay of 3 months. In this case, the lithographic process was carried out at a whole waferscale, starting from the preparation of substrates by graphene exfoliation. Nanostructureswith a minimum feature size of around 10 nm were patterned onto the graphene flakesemploying a high resolution negative resist.We also studied electromigration as a complementary technique for obtaining deviceswith elements that were beyond the limits achievable by EBL. The method consisted onpatterning metallic wires with nanometric cross-section and breaking them by electricalfatigue under controlled conditions. Electromigration tests were carried out in differentmetals and alloys, obtaining different types of structures depending on the employedprocedure. The devices obtained by this method showed distinct features in their chargetransport characteristics. We could thus identify tunnel junctions, metallic nanoconstrictionsand single electron transistors.We then focused our attention on studying palladium-based single electron transistors inmore detail, since electromigration proved to be a convenient method for the fabrication ofthese devices. The obtained devices showed single electron charging effects andoccasionally, other features in their charge transport, such as the Kondo resonance. Apartfrom that, we could observe some peculiar features, which were interpreted as being partof the characteristic resonances of one dimensional nanostructures that had formed in thedevices as a result of electromigration.Finally, we utilised the combination of EBL and electromigration for obtaining devices forspintronic research. More specifically, we intended to achieve a platform for studying purespin transport in low dimensional systems. Thus, we performed electromigration in lateralspin valves (LSVs) with lithographically pre-defined nanoconstrictions. We proceeded togradually narrowing the constrictions by electromigration and measuring the spin transportin the devices at different stages. Although time constraints have prevented concluding theproject, we have proposed further experiments for achieving our initial objectives. On onehand, we had the prospect of studying the transport of pure spin currents when reachingsizes at which conductance quantisation starts to appear. On the other hand, we wanted todetermine if it was possible to observe the tunnelling of pure spin currents through breakjunctions.In conclusion, throughout this thesis we achieved the fabrication of devices by EBL ondifferent types of substrates. The obtained minimum feature sizes were approximately atthe resolution limit of our EBL equipment. On the other hand, we obtained several types ofdevices beyond the capabilities of EBL by complementing the fabrication withelectromigration. The obtained nanostructures were aimed for research applications in thefields of electronics, spintronics and plasmonics.