Electronics with 2D Materials

Realizing continued performance gains in Si CMOS technology by channel length scaling is becoming increasingly challenging due to the growing importance of short-channel effects such as fringe capacitance parasitics, degraded electrostatics, and gate leakage. To overcome these effects recent efforts in Silicon technology include the development of a double-gate or finFET architecture to improve device parasitics and boost device transconductance, integration of high-k gate dielectrics to reduce gate leakage, and uniaxial strained silicon channels to boost ballistic velocities. Alternatively there has been intense interest in pursuing carbon-based electronics, i.e. carbon nanotubes and graphene, in an effort to beat the performance limitations intrinsic to silicon.

Graphene is a 2-D sheet of carbon atoms and has only very recently been explored as an electronic material. It is a zero-bandgap semiconductor with a linear E-k dispersion relationship, making it a very unique material. The advantages here, similar to carbon nanotubes, are very high mobilities and saturation velocities and the potential for nearly perfect two-dimensional electrostatics in field-effect devices. Despite the zero-bandgap nature of graphene, field-effect devices with Ion/Ioff ratios of approximately 10 can be constructed and many analog/RF applications of these devices can be pursued.

We maintain an interest in pursuing foundational research into the DC and high-frequency characteristics of graphene FETs. For example, we have demonstrated the first realization of saturating graphene-FET device characteristics with larger-than-unity current gains well up to the gigahertz regime; we developed a device model for these novel FETs under high bias; and performed the first study of channel-length scaling effects down to 100 nm channel lengths, demonstrating the viability of graphene-based electronics to compete with existing silicon technology.

illustrationOur recent focus has been on further improving graphene-based technologies by developing techniques to engineer ultra-high performance devices. We developed a novel graphene/dielectric heterostructure that resulted in device performance with almost an order of magnitude improvement over previous graphene-FETs. Our dielectric, hexagonal boron nitride (an insulating isomorph of graphene), proves to be an ideal choice for graphene electronics and a successful result in quest of finding a complimentary dielectric for carbon electronics.

We are also actively exploring new ways to utilize the many unique electronic and physical properties of graphene for new device applications. For example, we are pushing the limitations of dual-gated bilayer graphene where a field tunable bandgap allows unprecedented in-situ engineering of device performance for both digital and analogue applications; by engineering novel graphene/dielectric heterostructures we are studying strongly interacting multi-layer graphene heterostructures as a possible new route towards quantum switching; and we are exploring the integration of graphene-FETs with unconventional substrates that may allow the utilization of carbon-based electronics with systems where it has been traditionally difficult to apply Silicon technology.

Related Publications

Yoonhee Lee, Scott M. Trocchia, Steven B. Warren, Erik F. Young, Sefi Vernick, and Kenneth L. Shepard, “Electrically Controllable Single-Point Covalent Functionalization of Spin-Cast Carbon-Nanotube Field-Effect Transistor Arrays“. ACS Nano Publication Date: September 27, 2018, DOI: 10.1021/acsnano.8b03073.

Single-point-functionalized carbon-nanotube field-effect transistors (CNTFETs) have been used to sense conformational changes and binding events in protein and nucleic acid structures from intrinsic molecular charge. The key to utilizing these devices as single-molecule sensors is the ability to attach a single probe molecule to an individual device. In contrast, with noncovalent attachment approaches such as those based on van der Waals interactions, covalent attachment approaches generally deliver higher stability but have traditionally been more difficult to control, resulting in low yield. Here, we present a single-point-functionalization method for CNTFET arrays based on electrochemical control of a diazonium reaction to create sp3 defects, combined with a scalable spin-casting method for fabricating large arrays of devices on arbitrary substrates. Attachment of probe DNA to the functionalized device enables single-molecule detection of DNA hybridization with complementary target, verifying the single-point functionalization. Overall, this method enables single-point defect generation with 80% yield.

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Tarun Chari, Rebeca Ribeiro-Palau, Cory R. Dean, and Kenneth Shepard Resistivity of Rotated Graphite−Graphene Contacts NanoLetters DOI: 10.1021/acs.nanolett.6b01657

Robust electrical contact of bulk conductors to two-dimensional (2D) material, such as graphene, is critical to the use of these 2D materials in practical electronic devices. Typical metallic contacts to graphene, whether edge or areal, yield a resistivity of no better than 100 Ω μm but are typically >10 kΩ μm. In this Letter, we employ single-crystal graphite for the bulk contact to graphene instead of conventional metals. The graphite contacts exhibit a transfer length up to four-times longer than in conventional metallic contacts. Furthermore, we are able to drive the contact resistivity to as little as 6.6 Ω μm2 by tuning the relative orientation of the graphite and graphene crystals. We find that the contact resistivity exhibits a 60° periodicity corresponding to crystal symmetry with additional sharp decreases around 22° and 39°, which are among the commensurate angles of twisted bilayer graphene.

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We present the characterization of ballistic graphene field-effect transistors (GFETs) with an effective oxide thickness of 3.5 nm. Graphene channels are fully encapsulated within hexagonal boron nitride, and self-aligned contacts are formed to the edge of the single-layer graphene. Devices of channel lengths (LG) down to 67 nm are fabricated, and a virtual-source transport model is used to model the resulting current–voltage characteristics. The mobility and sourceinjection velocity as a function of LG yields a mean-free-path, ballistic velocity, and effective mobility of 850 nm, 9.3×107 cm/s, and 13 700 cm2/Vs, respectively, which are among the highest velocities and mobilities reported for GFETs. Despite these bestin- class attributes, these devices achieve transconductance (gm) and output conductance (gds) of only 600 and 300 μS/μm, respectively, due to the fundamental limitations of graphene’s quantum capacitance and zero-bandgap. gm values, which are less than those of comparable ballistic silicon devices, benefit from the high ballistic velocity in graphene but are degraded by an effective gate capacitance reduced by the quantum capacitance. The gds values, which limit the effective power gain achievable in these devices, are significantly worse than comparable silicon devices due to the properties of the zero-bandgap graphene channel.

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I. Meric, C. Dean, S. J. Han, L. Wang, K. A. Jenking, J. Hone, and K. L. Shepard, “High-frequency performance of graphene field effect transistors with saturating IV-characteristics,” International Electron Devices Meeting, 2011, pp. 2.1.1-2.1.4. [ Data Download ]

High-frequency performance of graphene field-effect transistors (GFETs) with boron-nitride gate dielectrics is investigated. Devices show saturating IV characteristics and fmax values as high as 34 GHz at 600-nm channel length. Bias dependence of fT and fmax and the effect of the ambipolar channel on transconductance and output resistance are also examined.

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C. R. Dean, A. F. Young, P. Cadden-Zimansky, L. Wang, H. Ren, K. Watanabe, T. Taniguchi, P. Kim, J. Hone, and K. L. Shepard, “Multicomponent fractional quantum Hall effect in graphene,” Nature Physics 7, pp. 693-696, 2011.

The fractional quantum Hall effect1–4 (FQHE) in an electron gas with multiple internal degrees of freedom provides a model system to study the interplay between symmetry breaking and emergent topological order5. In graphene, the structure of the honeycomb lattice endows the electron wave functions with an additional quantum number, termed valley isospin, which, combined with the usual electron spin, yields fourfold degenerate Landau levels (LLs; refs 6,7). This additional symmetry modifies the FQHE and is conjectured to produce new incompressible ground states in graphene8–17. Here we report multiterminal measurements of the FQHE in high mobility graphene devices fabricated on hexagonal boron nitride substrates18. The measured energy gaps are large, particularly in the second Landau level, where they are up to 10 times larger than those reported in the cleanest conventional systems. In the lowest Landau level the hierarchy of FQH states reflects the additional valley degeneracy.

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Inanc Meric, Cory R. Dean, Andrea F. Young, Natalia Baklitskaya, Noah J. Tremblay, Colin Nuckolls, Philip Kim, and Kenneth L. Shepard, “Channel Length Scaling in Graphene Field-Effect Transistors Studied with Pulsed Current−Voltage Measurements,” Nano Letters 11(3), pp. 1093-1097, 2011.

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.

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I. Meric, C. Dean, A. F. Young, J. Hone, P. Kim, and K. L. Shepard, “Graphene field-effect transistors based on boron nitride gate dielectrics,” International Electron Devices Meeting, 2010, pp. 23.2.1-23.2.4.

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 dielectric for graphene FETs.

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C. R. Dean, A. F. Young, I. Meric, C. Lee, L. Wang, S. Sorgenfrei, K. Watanabe, T. Taniguchi, P. Kim, K. L. Shepard, J. Hone “Boron nitride substrate for high-quality graphene electronics,” Nature Nanotechnology 5, 722-726, 22 August 2010.

Graphene devices on standard SiO2 substrates are highly disordered, exhibiting characteristics that are far inferior to the expected intrinsic properties of graphene1–12. Although suspending the graphene above the substrate leads to a substantial improvement in device quality13,14, 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 SiO2. 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 dielectrics15 and allows for the realization of more complex graphene heterostructures.

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I. Meric, N. Baklitskaya, P. Kim, and K. L. Shepard, “RF performance of top-gated, zero-bandgap graphene field-effect transistors,” International Electron Devices Meeting, 2008.

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.

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I. Meric, M. Y. Han, A. F. Young, B. Ozyilmaz, P. Kim, K. L. Shepard, “Current saturation in zero-bandgap, top-gated graphene field-effect transistors,” Nature Nanotechnology 3, pp. 654-59, 2008.

The novel electronic properties of graphene1–4, 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 channels5,6. 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 mS mm21 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.

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