Towards experimental realisation of spin filtering at the graphene-nickel(111) interface

This thesis presents the results of a master assignment aimed to verify the theoretical prediction of perfect spin filtering at the interface between graphene and the (111) surface of nickel by means of Tedrow-Meservey measurements. Karpan et al. predicted perfect spin filtering as a result of the almost perfect lattice match of graphene and the close-packed surface of nickel's fcc crystal, and the electronic properties of these materials. To verify this prediction devices are fabricated that are based on a vertical tunnel junction, comprising the graphene-nickel(111) interface, a tunnel barrier (aluminium oxide) and a superconductor (aluminium). A magnetic field is applied to spin-split the characteristic density of states for this superconductor as a result of the Zeeman effect, such that it can be used to measure the spin polarisation of electrons tunnelling from the graphene-nickel interface. This measurement technique was invented by Tedrow and Meservey. Both the spin filtering effect itself and the used measurement technique impose strict requirements on the used substrate, i.e. graphene grown on nickel by means of chemical vapour deposition, the device design and the fabrication. The main results comprise the structural and magnetic properties of the used nickel substrate as well as the characteristics of the graphene layer. A purchased substrate of CVD grown graphene on nickel is shown to fulfil the minimal requirements for the intended purpose. Following earlier research Raman spectroscopy is shown to be a useful technique to characterise graphene. In combination with cluster analysis it is a powerful tool to determine, amongst other properties, the thickness of graphene layers. Furthermore, a clear correspondence between optical images, scanning electron microscopy images and Raman measurements is shown, which allows for the use of an optical microscope to estimate the graphene thickness. A marker pattern makes it possible to directly relate images made by an optical microscope to Raman measurements. Cluster analysis again proofs to be an extremely powerful method to correlate the measurements based on the markers and subsequently analyse the results. Based on these findings, a method for lithographic alignment of the junction area based on optical images and AFM images is proposed, which employs the shown alignment of atomic force microscopy images on optical images. Placement of contact holes with this method takes into account the properties of both nickel and graphene. A non-destructive process to fabricate the junctions has been developed and tunnelling is verified to be the main transport mechanism. However, device fabrication revealed challenges, in particular related to the roughness of the nickel substrate, which results from the high temperature in the CVD process of graphene growth. As a results the thin layers of the top electrode probably are not continuous. Further optimisation of the fabrication process is needed. Nevertheless, junctions that show interesting and reproducible results in low-temperature (magneto) transport studies are successfully fabricated. Defect assisted tunnelling and, although rare, features consistent with superconductivity are demonstrated in addition to presently unexplained features, which require further research.