Charge Transfer and Defects in Ultrathin Metal Oxide – Graphene Spintronic Interfaces*

The way metal oxides layers interface with graphene determines the performance of graphene spintronic devices. Titanium and Aluminum-based oxides are two widely used tunnel barriers in such devices at the contacts, and the impact of ultrathin metal oxide layers on graphene in electrical and spin transport properties and spin relaxation in graphene remains unclear. While it is believed that Al-oxide exhibits pinholes that lead to spin relaxation at contact interfaces, using Ti-based tunnel barriers, ns spin lifetimes are observed^1,2.  In this work, we investigate the influence of TiO_x and AlO_x ultrathin layers on graphene via electrical measurement, structural and spectroscopic techniques,  and theory. We observe that both oxides layers lead to p-type doping in graphene, with atomic force microscopy revealing distinct coverage and topographic features. While surface charge transfer occurs in both cases, in sharp contrast to TiO_x, the AlO_x|graphene samples show the emergence of sp³ defects as revealed by Raman spectroscopy and confirmed by X-ray photoelectron spectroscopy. Our electronic structure calculations suggest interface configurations that match the charge transfer and defect emergence at these metal-oxide|graphene interfaces. This study concludes that while ultrathin TiO_x on the top of graphene involves charge transfer doping, however, at the AlO_x|Graphene interface, a combination of charge transfer and defect doping occurs that can explicate the implications to electrical and spin transport across such interfaces.