1D Edge Contacts to 2D Material Heterostructures

Graphene has been in the focus of research in many ﬁelds of applications due to its unique properties. In particular, the 2D nature, low charge carrier concentration and high mobility of carriers are promising properties for the use in magnetic Hall sensors. At the same time, low spin-orbit coupling and negligible hyperﬁne interactions make it interesting for spin-polarized electron transport. However, single graphene layer, if unprotected, is prone to defects introduced during fabrication processes and also defects due to interfaces with other insulators or contact materials. These factors would inevitably lead to decrease of graphene device performance. By encapsulating graphene in hexagonal boron nitride (h-BN), another insulating atomically ﬂat two-dimensional (2D) material with superior interface properties with graphene, one can fabricate heterostructures for robust and high-performance devices. Utilizing one-dimensional (1D) edge contacts to graphene sheet in such devices based on 2D materials would also allow to minimize contacts-induced degradation of channel properties. The graphene/h-BN heterostructures for studied devices were prepared both by exfoliation from bulk crystals and by transfer of CVD-grown materials over large area. After patterning the 2D heterostructures, 1D edge contacts were fabricated by means of electron or laser beam lithography and electron beam evaporation of metals. In these devices, proof-of-concept for batch fabrication of Hall elements on large area all-CVD h-BN/graphene/h-BN heterostructures is demonstrated. Such 1D edge contacts of ferromagnetic materials to graphene/h-BN heterostructures are also explored for spin injection into graphene in devices with novel design. The ﬁndings described in this thesis allow to advance the graphene Hall elements fabrication technology towards large-scale, industry-compatible manufacturing and lay basis for understanding and further optimization of the phenomena that drive and inﬂuence the operation of graphene spin-based devices with novel design involving 1D edge contacts. Graphene is known as an eﬀective spin transport medium owing to its low spin-orbit coupling and negligible hyperﬁne interactions. The conventional spin valve devices for the study of spin communication in graphene utilize metallization of its surface for fabrication of contacts which include tunnel barrier essential for improved spin injection. However, such tunnel barriers usually suﬀer from pin-holes, surface roughness and other defects which contribute to additional spin dephasing at the graphene/contacts interfaces. This chapter investigates the novel nonlocal spin valve devices with CVD graphene/h-BN heterostructures and 1D edge contacts, which are expected to reduce the contact area as well as the mentioned interface problems related to it. The nonlocal spin valve measurements demonstrate extraneous magnetoresistance switching behaviour which is shown to be dominated by the local Hall eﬀect in the presence of fringe ﬁelds from ferromagnetic contacts, which have more prominent inﬂuence on device operation in edge contact geometry. Systematic measurements of the signal were carried out at diﬀerent current biases, temperatures and gate voltages. Only one switching of the magnetoresistance signal was observed supporting the argument in favor of the nature of observed behaviour due to local Hall eﬀect as well as the gate voltage dependence of nonlocal device response, where a change of sign of the signal is observed on both sides of the Dirac curve due to electron or hole conduction in the channel. These ﬁndings open the platform for further understanding the novel device concept of spin injection through 1D edge contact and for improvements of such device performance. investigation of Hall sensors fabricated on large area commercially available CVD graphene with protection by exfoliated hexagonal boron nitride (h-BN). The prepared graphene/h-BN heterostructures show good materials properties as characterized by Raman spectroscopy. To connect the graphene active regions of Hall samples to the outputs the 1D edge contacts were used which show reliable and stable electrical properties. The operation of Hall sensors shows the current-related sensitivity up to 366 V/AT − 1 and the measurements of magnetic ﬁeld down to millitesla range. The ability to change carrier concentration and type of charge carriers (n- or p-type) in graphene by the gate voltage is demonstrated by the gate tunability of the Hall sensitivity. due its high mobility and low carrier concentration, highly promising atomically thin material that could potentially replace widely used today’s Si-based Hall sensors. In this chapter, the scalable fabrication of Hall sensors on large area graphene encapsulated in hexagonal boron nitride (h-BN) using only commercially available CVD materials is demonstrated. The h-BN/graphene/h-BN stakes were prepared by layer transfer technique batch-fabricated by utilizing 1D edge contacts to graphene. The devices showed robust performance over the chip scale with tunneling characteristics of the edge contacts. Magnetic Hall sensitivity down to millitesla range is observed at room temperature with current-related sensitivities up to 97 V/AT -1 . This work opens avenues for more mature technology of fabrication of all-CVD h-BN/graphene/h-BN heterostructures-based Hall elements for industrial implementation.