P. J. Zomer

A transfer technique for high mobility graphene devices on commercially available hexagonal boron nitride

We present electronic transport measurements of single and bilayer graphene on commercially available hexagonal boron nitride. We extract mobilities as high as 125 000 cm2 V−1 s−1 at room temperature and 275 000 cm2 V−1 s−1 at 4.2 K. The excellent quality is supported by the early development of the ν = 1 quantum Hall plateau at a magnetic field of 5 T and temperature of 4.2 K. We also present a fast, simple, and accurate transfer technique of graphene to hexagonal boron nitride crystals. This technique yields atomically flat graphene on boron nitride which is almost completely free of bubbles or wrinkles. The potential of commercially available boron nitride combined with our transfer technique makes high mobility graphene devices more accessible.

Electronic spin transport in graphene field-effect transistors

Spin transport experiments in graphene, a single layer of carbon atoms ordered in a honeycomb lattice, indicate spin-relaxation times that are significantly shorter than the theoretical predictions. We investigate experimentally whether these short spin-relaxation times are due to extrinsic factors, such as spin relaxation caused by low impedance contacts, enhanced spin-flip processes at the device edges, or the presence of an aluminum oxide layer on top of graphene in some samples. Lateral spin valve devices using a field-effect transistor geometry allowed for the investigation of the spin relaxation as a function of the charge density, going continuously from metallic hole to electron conduction (charge densities of n similar to 10(12) cm(-2)) via the Dirac charge neutrality point (n similar to 0). The results are quantitatively described by a one-dimensional spin-diffusion model where the spin relaxation via the contacts is taken into account. Spin valve experiments for various injector-detector separations and spin precession experiments reveal that the longitudinal (T-1) and the transversal (T-2) relaxation times are similar. The anisotropy of the spin-relaxation times tau and tau(perpendicular to), when the spins are injected parallel or perpendicular to the graphene plane, indicates that the effective spin-orbit fields do not lie exclusively in the two-dimensional graphene plane. Furthermore, the proportionality between the spin-relaxation time and the momentum-relaxation time indicates that the spin-relaxation mechanism is of the Elliott-Yafet type. For carrier mobilities of 2x10(3)-5x10(3) cm(2)/V s and for graphene flakes of 0.1-2 mu m in width, we found spin-relaxation times on the order of 50-200 ps, times which appear not to be determined by the extrinsic factors mentioned above.

Linear scaling between momentum- and spin scattering in graphene

Spin transport in graphene carries the potential of a long spin-diffusion length at room temperature. However, extrinsic relaxation processes limit the current experimental values to 1-2 mu m. We present Hanle spin precession measurements in gated lateral spin valve devices in the low to high (up to 10(13) cm(-2)) carrier density range of graphene. A linear scaling between the spin-diffusion length and the diffusion coefficient is observed. We measure nearly identical spin- and charge diffusion coefficients indicating that electron-electron interactions are relatively weak and transport is limited by impurity potential scattering. When extrapolated to the maximum carrier mobilities of 2x10(5) cm(2)/Vs, our results predict that a considerable increase in the spin-diffusion length should be possible.

Controlling spin relaxation in hexagonal BN-encapsulated graphene with a transverse electric field.

We experimentally study the electronic spin transport in hexagonal BN encapsulated single layer graphene nonlocal spin valves. The use of top and bottom gates allows us to control the carrier density and the electric field independently. The spin relaxation times in our devices range up to 2 ns with spin relaxation lengths exceeding 12 μm even at room temperature. We obtain that the ratio of the spin relaxation time for spins pointing out-of-plane to spins in-plane is τ(⊥)/τ(||) ≈ 0.75 for zero applied perpendicular electric field. By tuning the electric field, this anisotropy changes to ≈ 0.65 at 0.7 V/nm, in agreement with an electric field tunable in-plane Rashba spin-orbit coupling.

Fast pick up technique for high quality heterostructures of bilayer graphene and hexagonal boron nitride

We present a fast method to fabricate high quality heterostructure devices by picking up crystals of arbitrary sizes. Bilayer graphene is encapsulated with hexagonal boron nitride to demonstrate this approach, showing good electronic quality with mobilities ranging from 17 000 cm2 V−1 s−1 at room temperature to 49 000 cm2 V−1 s−1 at 4.2 K, and entering the quantum Hall regime below 0.5 T. This method provides a strong and useful tool for the fabrication of future high quality layered crystal devices.

Spin Transport in High-Quality Suspended Graphene Devices

We measure spin transport in high mobility suspended graphene (μ ≈ 10(5)cm(2)/(V s)), obtaining a (spin) diffusion coefficient of 0.1 m(2)/s and giving a lower bound on the spin relaxation time (τ(s) ≈ 150 ps) and spin relaxation length (λ(s) = 4.7 μm) for intrinsic graphene. We develop a theoretical model considering the different graphene regions of our devices that explains our experimental data.

Large yield production of high mobility freely suspended graphene electronic devices on a polydimethylglutarimide based organic polymer

The recent observation of a fractional quantum Hall effect in high mobility suspended graphene devices introduced a new direction in graphene physics, the field of electron–electron interaction dynamics. However, the technique used currently for the fabrication of such high mobility devices has several drawbacks. The most important is that the contact materials available for electronic devices are limited to only a few metals (Au, Pd, Pt, Cr, and Nb) because only those are not attacked by the reactive acid etching fabrication step. Here we show a new technique that leads to mechanically stable suspended high mobility graphene devices and is compatible with almost any type of contact material. The graphene devices prepared on a polydimethylglutarimide based organic resist show mobilities as high as 600.000 cm2/Vs at an electron carrier density of n = 5.0 × 109 cm−2 at 77 K. This technique paves the way toward complex suspended graphene based spintronic, superconducting, and other types of devices.

24-mu m spin relaxation length in boron nitride encapsulated bilayer graphene

We have performed spin and charge transport measurements in dual gated high mobility bilayer graphene encapsulated in hexagonal boron nitride. Our results show spin relaxation lengths

Relating hysteresis and electrochemistry in graphene field effect transistors

{\ensuremath{\lambda}}_{s}

High Gain Hybrid Graphene–Organic Semiconductor Phototransistors

up to