Inanc Meric

Boron nitride substrates for high-quality graphene electronics

Graphene devices on standard SiO(2) substrates are highly disordered, exhibiting characteristics that are far inferior to the expected intrinsic properties of graphene. Although suspending the graphene above the substrate leads to a substantial improvement in device quality, this geometry imposes severe limitations on device architecture and functionality. There is a growing need, therefore, to identify dielectrics that allow a substrate-supported geometry while retaining the quality achieved with a suspended sample. Hexagonal boron nitride (h-BN) is an appealing substrate, because it has an atomically smooth surface that is relatively free of dangling bonds and charge traps. It also has a lattice constant similar to that of graphite, and has large optical phonon modes and a large electrical bandgap. Here we report the fabrication and characterization of high-quality exfoliated mono- and bilayer graphene devices on single-crystal h-BN substrates, by using a mechanical transfer process. Graphene devices on h-BN substrates have mobilities and carrier inhomogeneities that are almost an order of magnitude better than devices on SiO(2). These devices also show reduced roughness, intrinsic doping and chemical reactivity. The ability to assemble crystalline layered materials in a controlled way permits the fabrication of graphene devices on other promising dielectrics and allows for the realization of more complex graphene heterostructures.

Current saturation in zero-bandgap, top-gated graphene field-effect transistors

The novel electronic properties of graphene, including a linear energy dispersion relation and purely two-dimensional structure, have led to intense research into possible applications of this material in nanoscale devices. Here we report the first observation of saturating transistor characteristics in a graphene field-effect transistor. The saturation velocity depends on the charge-carrier concentration and we attribute this to scattering by interfacial phonons in the SiO2 layer supporting the graphene channels. Unusual features in the current-voltage characteristic are explained by a field-effect model and diffusive carrier transport in the presence of a singular point in the density of states. The electrostatic modulation of the channel through an efficiently coupled top gate yields transconductances as high as 150 microS microm-1 despite low on-off current ratios. These results demonstrate the feasibility of two-dimensional graphene devices for analogue and radio-frequency circuit applications without the need for bandgap engineering.

One-dimensional electrical contact to a two-dimensional material.

Better Contact Along the Edge Electrical contact to graphene is normally done with metal contacts on its flat face, where there are few strong bonding sites for the metal. Wang et al. (p. 614) encapsulated graphene with hexagonal boron nitride sheets and made metal contacts along its edge, where bonding orbitals are exposed. The resulting heterostructures had high electronic performance, with room-temperature carrier mobilities near the theoretical phonon-scattering limit. Metal contacts to graphene along its edge improve bonding and, in turn, electronic performance. Heterostructures based on layering of two-dimensional (2D) materials such as graphene and hexagonal boron nitride represent a new class of electronic devices. Realizing this potential, however, depends critically on the ability to make high-quality electrical contact. Here, we report a contact geometry in which we metalize only the 1D edge of a 2D graphene layer. In addition to outperforming conventional surface contacts, the edge-contact geometry allows a complete separation of the layer assembly and contact metallization processes. In graphene heterostructures, this enables high electronic performance, including low-temperature ballistic transport over distances longer than 15 micrometers, and room-temperature mobility comparable to the theoretical phonon-scattering limit. The edge-contact geometry provides new design possibilities for multilayered structures of complimentary 2D materials.

Chemical Vapor Deposition-Derived Graphene with Electrical Performance of Exfoliated Graphene

While chemical vapor deposition (CVD) promises a scalable method to produce large-area graphene, CVD-grown graphene has heretofore exhibited inferior electronic properties in comparison with exfoliated samples. Here we test the electrical transport properties of CVD-grown graphene in which two important sources of disorder, namely grain boundaries and processing-induced contamination, are substantially reduced. We grow CVD graphene with grain sizes up to 250 μm to abate grain boundaries, and we transfer graphene utilizing a novel, dry-transfer method to minimize chemical contamination. We fabricate devices on both silicon dioxide and hexagonal boron nitride (h-BN) dielectrics to probe the effects of substrate-induced disorder. On both substrate types, the large-grain CVD graphene samples are comparable in quality to the best reported exfoliated samples, as determined by low-temperature electrical transport and magnetotransport measurements. Small-grain samples exhibit much greater variation in quality and inferior performance by multiple measures, even in samples exhibiting high field-effect mobility. These results confirm the possibility of achieving high-performance graphene devices based on a scalable synthesis process.

Channel length scaling in graphene field-effect transistors studied with pulsed current-voltage measurements.

We investigate current saturation at short channel lengths in graphene field-effect transistors (GFETs). Saturation is necessary to achieve low-output conductance required for device power gain. Dual-channel pulsed current-voltage measurements are performed to eliminate the significant effects of trapped charge in the gate dielectric, a problem common to all oxide-based dielectric films on graphene. With pulsed measurements, graphene transistors with channel lengths as small as 130 nm achieve output conductance as low as 0.3 mS/μm in saturation. The transconductance of the devices is independent of channel length, consistent with a velocity saturation model of high-field transport. Saturation velocities have a density dependence consistent with diffusive transport limited by optical phonon emission.

RF performance of top-gated, zero-bandgap graphene field-effect transistors

We present the first experimental high-frequency measurements of graphene field-effect transistors (GFETs), demonstrating an fT of 14.7 GHz for a 500-nm-length device. We also present detailed measurement and analysis of velocity saturation in GFETs, demonstrating the potential for velocities approaching 108 cm/sec and the effect of an ambipolar channel on current-voltage characteristics.

Graphene field-effect transistors based on boron nitride gate dielectrics

Graphene field-effect transistors are fabricated utilizing single-crystal hexagonal boron nitride (h-BN), an insulating isomorph of graphene, as the gate dielectric. The devices exhibit mobility values exceeding 10,000 cm2/V-sec and current saturation down to 500 nm channel lengths with intrinsic transconductance values above 400 mS/mm. The work demonstrates the favorable properties of using h-BNas a gate di-electric for graphene FETs.

Graphene Field-Effect Transistors with Gigahertz-Frequency Power Gain on Flexible Substrates

The development of flexible electronics operating at radio-frequencies (RF) requires materials that combine excellent electronic performance and the ability to withstand high levels of strain. In this work, we fabricate graphene field-effect transistors (GFETs) on flexible substrates from graphene grown by chemical vapor deposition (CVD). Our devices demonstrate unity-current-gain frequencies, f(T), and unity-power-gain frequencies, f(max), up to 10.7 GHz and 3.7 GHz, respectively, with strain limits of 1.75%. These devices represent the only reported technology to achieve gigahertz-frequency power gain at strain levels above 0.5%. As such, they demonstrate the potential of CVD graphene to enable a broad range of flexible electronic technologies which require both high flexibility and RF operation.

Graphene Field-Effect Transistors Based on Boron–Nitride Dielectrics

Two-dimensional atomic sheets of graphene represent a new class of nanoscale materials with potential applications in electronics. However, exploiting the intrinsic characteristics of graphene devices has been problematic due to impurities and disorder in the surrounding dielectric and graphene/dielectric interfaces. Recent advancements in fabricating graphene heterostructures by alternately layering graphene with crystalline hexagonal boron nitride (hBN), its insulating isomorph, have led to an order of magnitude improvement in graphene device quality. Here, recent developments in graphene devices utilizing boron-nitride dielectrics are reviewed. Field-effect transistor (FET) characteristics of these systems at high bias are examined. Additionally, existing challenges in material synthesis and fabrication and the potential of graphene/BN heterostructures for novel electronic applications are discussed.

Electronic compressibility of layer-polarized bilayer graphene

We report on a capacitance study of dual gated bilayer graphene. The measured capacitance allows us to probe the electronic compressibility as a function of carrier density, temperature, and applied perpendicular electrical displacement D. As a band gap is induced with increasing D, the compressibility minimum at charge neutrality becomes deeper but remains finite, suggesting the presence of localized states within the energy gap. Temperature dependent capacitance measurements show that compressibility is sensitive to the intrinsic band gap. For large displacements, an additional peak appears in the compressibility as a function of density, corresponding to the presence of a 1-dimensional van Hove singularity (vHs) at the band edge arising from the quartic bilayer graphene band structure. For D > 0, the additional peak is observed only for electrons, while D < 0 the peak appears only for holes. This asymmetry that can be understood in terms of the finite interlayer separation and may be useful as a direct probe of the layer polarization.