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Chemically enhanced double-gate bilayer graphene field-effect transistor with neutral channel for logic applications

This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Nanotechnology 25 345203 (http://iopscience.iop.org/0957-4484/25/34/345203) View the table of contents for this issue, or go to the journal homepage for more

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Nanotechnology Nanotechnology 25 (2014) 345203 (8pp)

doi:10.1088/0957-4484/25/34/345203

Chemically enhanced double-gate bilayer graphene field-effect transistor with neutral channel for logic applications Amirhasan Nourbakhsh1,2, Tarun K Agarwal1,3, Alexander Klekachev1,5, Inge Asselberghs1,2, Mirco Cantoro1,6, Cedric Huyghebaert1, Marc Heyns1,4, Marian Verhelst3, Aaron Thean1 and Stefan De Gendt1,2 1

imec, Kapeldreef 75, B-3001 Leuven, Belgium Department of Chemistry, KULeuven, Celestijnenlaan 200f, B-3001 Leuven, Belgium 3 Department of Electrical Engineering, KULeuven, Kasteelpark Arenberg 10, B-3001 Leuven, Belgium 4 Department of Metallurgy and Materials Engineering, KULeuven, Kasteelpark Arenberg 44, B-3001 Leuven, Belgium 2

E-mail: [email protected] Received 11 June 2014, revised 4 July 2014 Accepted for publication 8 July 2014 Published 7 August 2014 Abstract

In this article, we present the simulation, fabrication, and characterization of a novel bilayer graphene field-effect transistor exhibiting electron mobility up to ∼1600 cm2 V−1 s−1, a room temperature Ion/Ioff ≈ 60, and the lowest total charge (∼1011 cm−2) reported to date. This is achieved by combined electrostatic and chemical doping of bilayer graphene, which enables one to switch off the device at zero top-gate voltage. Using density functional theory and atomistic simulations, we obtain physical insight into the impact of chemical and electrostatic doping on bandgap opening of bilayer graphene and the effect of metal contacts on the operation of the device. Our results represent a step forward in the use of bilayer graphene for high-performance logic devices in the beyond-complementary metal−oxide−semiconductor (CMOS) technology paradigm. Keywords: graphene, bilayer graphene, band gap, chemical functionalization 1. Introduction

environments and is mechanically very rigid [7]. Its electronic transport properties have been found to be greatly superior to those of materials traditionally used in microelectronics. Therefore, graphene is one of the most promising candidates as a material for post-complementary metal−oxide−semiconductor (CMOS) applications [8, 9]. Single-layer graphene (SLG) is a gapless semimetal. As a consequence, transistors using SLG as the active channel exhibit a poor Ion/Ioff ratio (generally ∼10) and cannot be switched off. This is one of the main limitations hindering the use of graphene in microelectronics for logic applications. To solve this problem, the creation of a bandgap in SLG is required. A number of approaches have been used to induce a bandgap opening in SLG. For example, when SLG is tailored in a few nanometer-wide ribbons, a quantum confinementinduced bandgap appears [10–12].

Graphene is a two-dimensional (2D), semimetallic, atomically thin film in which carbon atoms are structurally arranged in a sp2 honeycomb lattice relying on in-plane, covalent σ-bonds. It was successfully isolated for the first time in 2004 by Geim et al by micromechanically exfoliating graphite crystals on top of Si/SiO2 film stacks [1]. Significant breakthroughs have been achieved in graphene applied research: the switching behavior with fT in excess of hundreds GHz [2], high frequency photo-detectors [3], optical communications [3], single molecule detectors [4], and high-mobility transistors [5, 6]. Graphene is also chemically stable in nonoxidizing 5

With imec at the time of this work; currently with Institut de Science et d’Ingénierie Supramoléculaires, Strasbourg, France. 6 With imec at the time of this work; currently with Samsung, Seoul, Korea. 0957-4484/14/345203+08$33.00

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© 2014 IOP Publishing Ltd Printed in the UK

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greater than 3.0 V nm−1. However, when such a high electric field is applied across BLG, the charge density in BLG exceeds 1013 cm−2. Therefore, to switch off the transistor, the primary gate must generate a very high electric displacement field to compensate for the excess charge and position the Fermi level in the bandgap, which may reach the breakdown voltage of conventional dielectric materials such as SiO2. Another unfavorable aspect of this approach is that the topgate stack should include a high-k dielectric material in direct contact with the BLG to induce the required magnitude of the electric field. Atomic layer deposition (ALD) has been used to deposit high-k dielectric materials (e.g., Al2O3) on graphene, and electric fields as high as 2.5 V nm−1 have been achieved [18–20]. However, the direct growth of ALD high-k dielectrics on graphene induces disorder of the graphene lattice, which can result in the formation of impurity bands close to the edges of the conductance and valence bands. This, in turn, causes a decrease of about two orders of magnitude in the charge transport bandgap compared with the optical bandgap [19]. The second approach to achieve bandgap opening in BLG involves the gating of BLG using chemical species adsorbed on the top surface of graphene. The species physisorb to graphene by weak van der Waals forces, and therefore the chemical and structural integrity of the pristine graphene lattice is preserved, as is its electronic structure. Being a strictly 2D material, graphene is extremely sensitive to adsorbates and other molecules in direct contact with its surface. This property can be exploited to tailor the electronic properties of graphene. In fact, an effective electric field can be induced by placing excess charge on the top layer, resulting in charge redistribution and asymmetry between the top and bottom layers. Various examples of adsorbed species have been reported, including metals and adatoms [21], organic compounds [22, 23], inorganic salts [24], and gases [25]. All of these species result in either n- or p-type doping of graphene, depending on the difference in the electronegativity between graphene and the adsorbate. Doping BLG by chemical physisorption resembles the effect of external gating. However, the doping approaches mentioned above are not easily controlled and not compatible with typical CMOS process flows. Moreover, doping BLG only on the top side results in a high charge density in the graphene, and the electric fields achievable with the doping technique are insufficient to open a bandgap large enough for efficient device switching. In general, three main issues have to be addressed when designing a practical BLG field effect transistor (BLG-FET): (1) inducing an electric displacement field larger than 3 V nm−1 across the gate, (2) keeping the total charge density (close to) zero, and (3) preserving the structural integrity of graphene.

Figure 1. (a) Schematic representation of AB-stacked BLG and (b)

electronic band structure of both pristine BLG (in the absence of the gate) and gated BLG.

A radically different approach involves using bilayer graphene (BLG) [13, 14]. BLG consists of two layers of SLG stacked vertically in the Bernal (AB) stacking arrangement and interacting via their π-electrons (see figure 1(a)). It is a fascinating and complex system in its own right, and distinct from both the monolayer and the traditional 2D electron gas (2DEG), even though it shares some characteristics with both forms. Similar to SLG, BLG also has a zero bandgap, and is therefore a semimetal. However, a bandgap can be introduced in BLG if the inversion symmetry of the two stacked layers is broken by the use of an electric field gate applied perpendicular to the BLG plane, as shown in figure 1(b). BLG then becomes a semiconductor with a bandgap that depends on the strength of the applied electric field and can be tuned from 0 to 300 meV [13, 15–17]. The maximum value of the bandgap that can be induced depends only on the interlayer coupling energy. So far, the bandgap opening in BLG by vertical symmetry breaking, as previously described, has been achieved using two approaches. The first approach involves the use of an external electrostatic gate in direct contact with the BLG. For example, a top-gate stack has been used to establish an electric displacement field perpendicular to the BLG plane. The field induces two different excess charge densities on the two layers of BLG, and thus induces charge density asymmetry between the two layers [13, 16]. The Coulomb interaction between the two asymmetric charges causes the opening of a bandgap between the conductance and valence energy bands in the BLG band diagram. An optical bandgap of 250 meV has been measured by infrared spectroscopy [18]. To achieve the maximum bandgap, the applied electric field has to be

2. Results and discussion The present work describes a novel way to overcome the three issues outlined above. We report a double-gate BLGFET architecture in which control of the BLG active channel 2

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Nanotechnology 25 (2014) 345203

Figure 3. Effect of transverse electric field, E, on the bandgap opening of BLG. The cases of the pristine BLG and F4TCNQ-doped BLG are plotted together.

shows the total density of states (DOS) of F4TCNQ–SLG and the projection of the DOS (PDOS) onto the SLG and F4TCNQ components within 3 eV of the Fermi energy (EF). In this plot, the Dirac energy point (ED, where the DOS is zero) is clearly visible, located approximately 0.45 eV above the EF, which indicates electron withdrawal from graphene by the F4TCNQ molecule. The adsorption of F4TCNQ on SLG leads to a net perpendicular moment of 2.3 Debye towards the graphene surface. Therefore, the combination of SLG and the ad-molecule film resembles a uniform dipolar slab of infinite extension with built-in electric field normal to the surface, and could be capable of producing the same response in graphene as an external electric field. Figure 3 shows the cumulative effect of ‘chemical gating’ by F4TCNQ and external transverse electric field (E) on the bandgap opening in BLG. The bandgap in both pristine and F4TCNQ-doped BLG monotonically increases with increasing E. However, F4TCNQ doping significantly improves the bandgap. For example, with an external E = 3 V nm−1 in vacuum, which corresponds to E = 0.77 V nm−1 if SiO2 (εr = 3.9) is used as the gate dielectric, the bandgap magnitude in BLG with additional F4TCNQ functionalization is ∼300 meV, which is ∼20% higher than can be achieved in pristine BLG. The built-in electric field arising from this charge redistribution can be calculated by the macroscopic average of the electrostatic potential,

Figure 2. (a) Optimized structure of F4TCNQ on a 5 × 5 unit cell of SLG and (b) total and projected DOS of F4TCNQ-doped SLG.

is achieved by a combination of electrostatic and chemical doping. In our design, the top gate is the primary gate that controls the switching of the BLG channel, while the back gate (doped Si) governs the electrostatics of the channel and the BLG/metal contacts. Chemical doping is achieved by placing a thin layer of tetrafluoro-tetracyanoquinodimethane (F4TCNQ) in the top-gate stack, in direct contact with the top layer of the BLG. F4TCNQ is a strong electron acceptor with high electron affinity (5.2 eV) that can significantly p-dope carbon nanomaterials [22, 26]. In this configuration, the top layer is more hole-populated than the bottom layer as a result of close interaction with F4TCNQ molecules. To understand the role of F4TCNQ dopant on the electronic structure of BLG, we performed density functional theory calculations for a specific configuration. The calculations were carried out using the self-consistent pseudopotential method as implemented in the SIESTA code [27, 28] with the generalized gradient approximation. Atomic geometries were relaxed until the forces were