Charge and spin transport in graphene-based devices

Abstract

The field of spintronics offers new technologies and fundamental discoveries by using the spin degree of freedom of electron. Having low spin orbit coupling, negligible hyperfine interaction and extremely high electronic quality make graphene a promising material for spintronics studies. While the exceptionally long spin relaxation length was demonstrated experimentally in mechanically exfoliated graphene-based spin valve devices, the manipulation of spin current for the practical applications was missing. The experimental work presented in this thesis focuses on understanding the fundemantal spin transport properties of graphene to prepare it for future spintronics applications. In the first part of the thesis, I study the spin transport properties of CVD grown graphene. Spin injection, transport and detection in CVD single and bi-layer graphene are successfully demonstrated. I show that the CVD specific structural differences such as wrinkles, grain boundaries and residues do not limit spin transport properties of CVD graphene. The observation of long spin relaxation length comparable to the exfoliated graphene samples makes CVD graphene a promising material of choice for possible spintronics applications. The large scale CVD grown graphene also allows the batch-fabrication of large arrays of lateral spin valve devices with a fast-around time well suited for studying the device physics. In the second part of thesis, charge transport property of graphene is studied in heterostructure devices. While the graphene field effect transistors fabricated on various 2D substrates show enhanced electronic mobilities compared to conventional SiO2 substrate, BN and WS2 substrates appeared to be the most promising substrates to reach high electronic mobilities in graphene. Our results raise the importance of ideal choice of material for graphene-based heterostructure devices before building the complex heterostructures. The absence of significant spin orbit coupling in graphene is detrimental for the manipulation of spin current in graphene based devices. In the last part of thesis, I demonstrate that with the creation of an artificial interface between graphene and WS2 substrate, graphene acquires a SOC as high as 17meV with a proximity effect, three orders of magnitude higher than its intrinsic value. This proximity effect leads to the spin Hall effect even at room temperature. These results open the doors for the realization of Datta-Das type spin field effect transistors.