Jen-Ru Chen

Control of Schottky Barriers in Single Layer MoS2 Transistors with Ferromagnetic Contacts

MoS2 and related metal dichalcogenides (MoSe2, WS2, WSe2) are layered two-dimensional materials that are promising for nanoelectronics and spintronics. For instance, large spin-orbit coupling and spin splitting in the valence band of single layer (SL) MoS2 could lead to enhanced spin lifetimes and large spin Hall angles. Understanding the nature of the contacts is a critical first step for realizing spin injection and spin transport in MoS2. Here, we have investigated Co contacts to SL MoS2 and find that the Schottky barrier height can be significantly decreased with the addition of a thin oxide barrier (MgO). Further, we show that the barrier height can be reduced to zero by tuning the carrier density with back gate. Therefore, the MgO could simultaneously provide a tunnel barrier to alleviate conductance mismatch while minimizing carrier depletion near the contacts. Such control over the barrier height should allow for careful engineering of the contacts to realize spin injection in these materials.

Spin Relaxation in Single-Layer Graphene with Tunable Mobility

Graphene is an attractive material for spintronics due to theoretical predictions of long spin lifetimes arising from low spin-orbit and hyperfine couplings. In experiments, however, spin lifetimes in single-layer graphene (SLG) measured via Hanle effects are much shorter than expected theoretically. Thus, the origin of spin relaxation in SLG is a major issue for graphene spintronics. Despite extensive theoretical and experimental work addressing this question, there is still little clarity on the microscopic origin of spin relaxation. By using organic ligand-bound nanoparticles as charge reservoirs to tune the mobility between 2700 and 12 000 cm(2)/(V s), we successfully isolate the effect of charged impurity scattering on spin relaxation in SLG. Our results demonstrate that, while charged impurities can greatly affect mobility, the spin lifetimes are not affected by charged impurity scattering.

Integrating MBE materials with graphene to induce novel spin-based phenomena

Magnetism in graphene is an emerging field that has received much theoretical attention. In particular, there have been exciting predictions for induced magnetism through proximity to a ferromagnetic insulator as well as through localized dopants and defects. Here, the authors discuss their experimental work using molecular beam epitaxy to modify the surface of graphene and induce novel spin-dependent phenomena. First, they investigate the epitaxial growth of the ferromagnetic insulator EuO on graphene and discuss possible scenarios for realizing exchange splitting and exchange fields by ferromagnetic insulators. Second, they investigate the properties of magnetic moments in graphene originating from localized pz -orbital defects (i.e., adsorbed hydrogen atoms). The behavior of these magnetic moments is studied using nonlocal spin transport to directly probe the spin-degree of freedom of the defect-induced states. They also report the presence of enhanced electron g-factors caused by the exchange fields present in the system. Importantly, the exchange field is found to be highly gate dependent, with decreasing g-factors with increasing carrier densities.

Effect of in situ deposition of Mg adatoms on spin relaxation in graphene

We have systematically introduced charged impurity scatterers in the form of Mg adsorbates to exfoliated single-layer graphene and observe little variation of the spin relaxation times despite pronounced changes in the charge transport behavior. All measurements are performed on nonlocal graphene tunneling spin valves exposed in situ to Mg adatoms, thus systematically introducing atomic-scale charged impurity scattering. While charge transport properties exhibit decreased mobility and decreased momentum scattering times, the observed spin lifetimes are not significantly affected, indicating that charged impurity scattering is inconsequential in the present regime of spin relaxation times (

Enhanced spin injection efficiency and extended spin lifetimes in graphene spin valves

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Spin transport and relaxation in graphene

1 ns).

Facile growth of monolayer MoS2 film areas on SiO2

Enhanced spin injection efficiency and extended spin lifetimes are achieved in graphene spin valves. Spin injection efficiency is enhanced via tunneling spin injection into graphene through an MgO barrier. A large nonlocal magetoresistance of 130 Ω is observed for a single layer graphene (SLG) spin valve at room temperature (RT) with spin injection efficiency of ~ 26-30%. Extended spin lifetimes are observed using tunneling contact to suppress the contact induced spin relaxation. In SLG, spin lifetime as long as 771 ps is observed at RT. In bilayer graphene (BLG), we observe the spin lifetime of 6.2 ns at 20 K, which is the longest value reported in any graphene spin valve. Furthermore, contrasting spin relaxation behaviors are observed in SLG and BLG, which suggests that Elliot-Yafet spin relaxation dominates in SLG at low temperatures, while Dyakonov-Perel spin relaxation dominates in BLG at low temperatures.6