Homoepitaxial graphene tunnel barriers for spin transport

Abstract

Tunnel barriers are key elements for both charge-and spin-based electronics, offering devices with reduced power consumption and new paradigms for information processing. Such devices require mating dissimilar materials, raising issues of heteroepitaxy, interface stability, and electronic states that severely complicate fabrication and compromise performance. Graphene is the perfect tunnel barrier. It is an insulator out-of-plane, possesses a defect-free, linear habit, and is impervious to interdiffusion. Nonetheless, true tunneling between two stacked graphene layers is not possible in environmental conditions usable for electronics applications. However, two stacked graphene layers can be decoupled using chemical functionalization. Here, we demonstrate that hydrogenation or fluorination of graphene can be used to create a tunnel barrier. We demonstrate successful tunneling by measuring non-linear IV curves and a weakly temperature dependent zero-bias resistance. We demonstrate lateral transport of spin currents in non-local spin-valve structures, and determine spin lifetimes with the non-local Hanle effect. We compare the results for hydrogenated and fluorinated tunnel and we discuss the possibility that ferromagnetic moments in the hydrogenated graphene tunnel barrier affect the spin transport of our devices.

Colloquium : Spintronics in graphene and other two-dimensional materials

“…From the nonlocal spin valve measurements, they achieved spin injection efficiencies up to 45% and 17% at low temperatures for fluori-nated and hydrogenated graphene-based tunnel barriers, respectively. However calculated spin relaxation times in these devices are less than 0.2 ns and the created spin accumulation is very small or even absent at room temperature (Friedman et al, 2016). Recently, Gurram et al (2017) have observed bias-induced enhanced spin injection and detection efficiencies up to 100% in tunnel junctions with bilayer hBN barriers.…”

Graphene Spin Valves for Spin Logic Devices

“…For a defect-free barrier, spin injection of 26-30% and a large spin signal of 130 Ω have been reported. [52] The use of 2D materials such as hexagonal boron nitride (hBN) [74][75][76][77] and fluorinated graphene [78] as tunnel barriers exhibit high spin injection efficiency. The advantage of 2D materials is the absence of defects such as pinholes and an atomically flat surface.…”

Valley-polarized tunneling currents in bilayer graphene tunneling transistors

“…For hBN, experimental works [3,4,9,12,[32][33][34] suggest a value of the decay constant c(0) ≈ 5nm −1 . In the case of graphene, studies of its role as a barrier in magnetic tunnel junctions [35][36][37] and between metal contacts [38,39] showed that it behaves as a strong out-of-plane insulator. In fact, in experiments conducted in the absence of a magnetic field and in the presence of a field parallel to the graphene layers, the measured tunnelling current has been well described by assuming that all tunnelling from the further BLG layer is suppressed [10,12].…”