Structurally Controlled Large-Area 10 nm Pitch Graphene Nanomesh

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Structurally Controlled Large-Area 10-nm-Pitch Graphene Nanomesh by Focused Helium Ion Beam Milling Marek Edward Schmidt, Takuya Iwasaki, Manoharan Muruganathan, Mayeesha Haque, Huynh Van Ngoc, Shinichi Ogawa, and Hiroshi Mizuta ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00427 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 27, 2018

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ACS Applied Materials & Interfaces

Structurally Controlled Large-Area 10-nm-Pitch Graphene Nanomesh by Focused Helium Ion Beam Milling Marek Edward Schmidt*1, Takuya Iwasaki1, Manoharan Muruganathan1, Mayeesha Haque1, Huynh Van Ngoc1, Shinichi Ogawa1, Hiroshi Mizuta1,3 1

School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1

Asahidai, Nomi, Ishikawa, 923-1292, Japan 2

Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and

Technology (AIST), 16-1 Onogawa, Tsukuba 305-8569, Japan 3

Hitachi Cambridge Laboratory, Hitachi Europe Ltd., J. J. Thomson Avenue, CB3 0HE

Cambridge, United Kingdom KEYWORDS: suspended graphene nanomesh, graphene antidot lattice, helium ion beam milling, energy gap, sub-10-nm

Graphene nanomesh (GNM) is formed by patterning graphene with nanometer-scale pores separated by narrow necks. GNMs are of interest due to their potential semiconducting characteristics when quantum confinement in the necks leads to an energy gap opening. GNMs also have potential for use in phonon control and water filtration. Furthermore, physical phenomena, such as spin qubit, are predicted at pitches below 10-nm fabricated with precise structural control. Current GNM patterning techniques suffer from either large

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dimensions or a lack of structural control. This work establishes reliable GNM patterning with a sub-10-nm pitch and a < 4-nm pore diameter by the direct helium ion beam milling of suspended monolayer graphene. Due to the simplicity of the method, no post-patterning processing is required. Electrical transport measurements reveal an effective energy gap opening of up to ~450 meV. The reported technique combines the highest resolution with structural control and opens a path toward GNM-based, room-temperature semiconducting applications.

1. INTRODUCTION After the demonstration of graphene’s superior properties, such as mechanical stability and mobility,1 it was quickly realized that without opening a sizable energy gap, this material could not be used in the field of electronics.2,3 In particular, while graphene is an excellent conductor with an extraordinarily high carrier mobility, current through the material cannot be suppressed to turn a graphene-based device off. Currently, the formation of a narrow constriction, which opens an energy gap due to quantum confinement, is the most promising method for overcoming this challenge.4–7 Graphene nanoribbons (GNRs) are prepared by various methods, such as masked plasma etching,6 direct electron-beam cutting,8 or helium ion beam milling (HIBM).9 The low driving currents and transconductances of GNRs can be addressed by the recently proposed graphene nanomeshes (GNMs, sometimes called graphene antidot lattices), which form mesh-like networks of short GNRs.10–17 Energy gap opening of up to 140 meV caused by quantum confinement has been reported.11 An extraordinarily high energy gap value of ~1.2 eV was determined by XPS measurements in a reduced graphene oxide nanomesh.16 However, the substantiation of this value by electrical measurements has not been reported. Apart from being an interesting material for semiconducting and tunneling magnetoresistance applications,18–22 nanoporous graphene is attractive for use in water filtration and23,24 as an electrode material in batteries,25 as well as


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in gas sensing26 and phonon control.27–29 In the latter regard, the high Young’s modulus of graphene (~1 GPa) helps to increase the wavelength of room-temperature phonons into the sub-10-nm scale, and is thus preferred over silicon. Furthermore, GNM patterning with structural control is envisioned to enable fabrication of electronic waveguides or spin qubits in graphene.30–32 Despite all these extraordinary properties, there is a wide gap between the scale at which these physical phenomena are observable and the currently obtainable smallest dimensions and structural controllability. Fundamentally, nanopores can be patterned in graphene with high resolution by electronbeam irradiation, allowing simultaneous observation.33,34 However, the slow speed (~5 s for an individual 3.5-nm-diameter pore in multilayer graphene) is the most likely reason why large-area GNMs have not been reported.33 Consequently, a wide range of techniques – mostly relying on masked or direct etching – have been demonstrated,35 such as electronbeam lithography,17,36,37 block copolymer lithography,10,38,39,39 nanoparticles,40,41 selfassembled nanosphere lithography,12,13,42–44 interferometric lithography,15 anodic aluminum oxide templates,14,22,45 and nanoimprint lithography,11,46 among others. Although most of these methods can be applied to arbitrarily large areas and are well suited for large-scale fabrication, plasma etching is well known to cause significant edge defects, and once the etching mask is realized, individual width control is not possible. Neck widths down to 5-7 nm have been realized using such techniques; however, the variability is large.10,39 Among the discussed methods, electron-beam lithography (EBL) is the most versatile in terms of structural control; however, it is not competitive with the other approaches in terms of pitch and neck size (see detailed comparison in Section 1, supplementary information). Thus, an alternative technique that combines high resolution and structural control is required to further increase the energy gap opening for room-temperature semiconductor applications,


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explore the new physics of graphene waveguides and quantum qubits set out by Pedersen et al. and others,30,32 and promote the use of GNMs for phonon control and water filtration. Here, we present the formation of a GNM in monolayer graphene by direct HIBM with superior resolution and structural control. The tightly focused beam of helium ions interacts with the suspended graphene in precisely controllable locations (sub-nm beam positioning control), and physical nanopores are formed through the removal of carbon atoms.47,48 Excellent scalability of the approach and pore diameter tunability by beam dwell time is demonstrated. Furthermore, the opening of an effective energy gap, Eg, of ~450 meV is observed at room temperature in a suspended GNM with a pitch of 18 nm. Remarkably, this gap vanishes when some pores are missing, which demonstrates the possibility of GNM formation with negligible dimensional tolerances. 2. EXPERIMENTAL SECTION The suspended graphene devices are based on commercial chemical vapor deposition (CVD) monolayer graphene on a SiO2 (290 nm)/Si substrate. First, the substrate is annealed in forming gas (H2:Ar = 1:9) at 250°C for 3 hours to thoroughly remove any organic contaminants that might be left after sample preparation. Next, EBL with a PMMA/MMA copolymer resist stack is used to pattern large anchor electrodes, as shown in Figure 1a. Before electron-beam evaporation of 5/90 nm of Cr/Au, the exposed graphene is removed by reactive ion etching (RIE), followed by lift-off in acetone. In this way, the Cr/Au is directly in contact with the SiO2 substrate, and peeling of the metal is effectively avoided. By using a weak radio frequency power of only 30 Watts at an O2 pressure of 4 Pa, the monolayer graphene is reliably removed within


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Figure 1. (a) Schematic of the process of fabricating a GNM. After the contacting and patterning of monolayer CVD graphene, BHF etching is used to suspend the graphene. Focused HIBM is used to directly pattern pores. (b) Scanning electron micrograph of typical suspended graphene. SE micrograph showing the results of a pore charge development series for (c) pitch P = 12 nm and (d) P = 9 nm. (e) SE brightness profile along the arrow in (d). The three pores with P = 9 nm are clearly defined with a pore diameter of ~ 3.5 nm. Scale bars are 50 nm if no value is given.

10 seconds without affecting the resist. Next, a second EBL step is used to pattern smaller contact electrodes that overlap the CVD graphene and the contact electrodes (same conditions). Then, hydrogen silsesquioxane (HSQ) high-resolution negative resist is used to define the shape of the suspended graphene films. HSQ is converted to SiO2 and acts as a


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hard mask during the subsequent RIE etching, and this mask is readily removed during buffered hydrofluoric acid (BHF) etching. Finally, the devices are released in BHF for 60 sec (short devices) or ~180 sec (Section 2, supplementary information). To avoid collapse due to surface tension, the devices are dried in a critical point drier. Finally, the devices are annealed for a second time at 250°C for 3 hours in forming gas to remove residues from the resist stack. Figure 1b shows a scanning electron micrograph of a suspended graphene device ~200 nm long and ~100 nm wide. Next, the devices are loaded into the helium ion microscope (Zeiss Orion Plus) at a pressure of

Structurally Controlled Large-Area 10 nm Pitch Graphene Nanomesh

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