Letter pubs.acs.org/NanoLett
Controlled n‑Type Doping of Carbon Nanotube Transistors by an Organorhodium Dimer Michael L. Geier,† Karttikay Moudgil,‡ Stephen Barlow,‡ Seth R. Marder,‡ and Mark C. Hersam*,†,§,∥ †
Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States School of Chemistry and Biochemistry, Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia 30332, United States § Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States ∥ Department of Electrical Engineering and Computer Science, Northwestern University, Evanston, Illinois 60208, United States ‡
S Supporting Information *
ABSTRACT: Single-walled carbon nanotube (SWCNT) transistors are among the most developed nanoelectronic devices for high-performance computing applications. While p-type SWCNT transistors are easily achieved through adventitious adsorption of atmospheric oxygen, n-type SWCNT transistors require extrinsic doping schemes. Existing n-type doping strategies for SWCNT transistors suffer from one or more issues including environmental instability, limited carrier concentration modulation, undesirable threshold voltage control, and/or poor morphology. In particular, commonly employed benzyl viologen n-type doping layers possess large thicknesses, which preclude top-gate transistor designs that underlie high-density integrated circuit layouts. To overcome these limitations, we report here the controlled n-type doping of SWCNT thin-film transistors with a solution-processed pentamethylrhodocene dimer. The charge transport properties of organorhodium-treated SWCNT thin films show consistent ntype behavior when characterized in both Hall effect and thin-film transistor geometries. Due to the molecular-scale thickness of the organorhodium adlayer, large-area arrays of top-gated, n-type SWCNT transistors are fabricated with high yield. This work will thus facilitate ongoing efforts to realize high-density SWCNT integrated circuits. KEYWORDS: Single-walled carbon nanotubes, n-type, chemical doping, thin-film transistors, top-gate
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shown limited ability to tune the electronic properties of SWCNT-based devices. However, recent progress in benzyl viologen dopant and hybrid organic/inorganic encapsulation layers has enabled the fabrication of complementary metaloxide-semiconductor (CMOS) static random access memory circuits based on SWCNT TFTs.35 Although this result demonstrates the suitability of SWCNTs for sophisticated electronic circuits, the tuning of the n-type SWCNT TFT electronic properties is not easily achieved. Furthermore, the thick layer required for benzyl viologen doping does not allow for top-gate device structures, ultimately leading to nonoptimal circuit layout and resulting limitations in integrated circuit density and complexity. As a step toward overcoming these issues, we introduce here the use of a molecular n-type dopant, pentamethylrhodocene dimer ((RhCp*Cp)2), for producing ntype SWCNT TFTs that can be controllably doped by solution-processing methods. From both Hall effect and TFT measurements, the resulting SWCNT thin-film electronic
ingle-walled carbon nanotubes (SWCNTs) have shown exceptional promise for electronic applications due to a complementary set of desirable properties including solution processability, high carrier mobility, and chemical stability.1−5 In particular, the development of solution-processed highpurity semiconducting SWCNTs has allowed large-area deposition of SWCNT films,6 leading to extensive studies of SWCNT thin-film transistors (TFTs) in a variety of device structures.7−10 Due to adventitious atmospheric p-type doping of SWCNT TFTs under ambient conditions, p-type unipolar devices are easily attained and have been the focus of extensive optimization and integration.11−13 However, n-type doping of SWCNTs has proven to be considerably more challenging, resulting in an extensive list of n-type doping strategies including substitutional doping with nitrogen,14 and deposition of potassium metal,15,16 polyethylene imine,17 low workfunction contact metals,18−23 metal oxides/nitrides,24−28 and several strongly reducing small molecules.29−34 These existing SWCNT n-type doping approaches have processing and stability limitations. For instance, substitutional doping decreases carrier mobility, low work-function metals oxidize in ambient conditions, and metal oxides/nitrides have © XXXX American Chemical Society
Received: April 4, 2016 Revised: May 15, 2016
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DOI: 10.1021/acs.nanolett.6b01393 Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. Top-gated SWCNT TFT structure. (A) Optical micrograph of the local top-gated SWCNT TFT structure with channel length of 50 μm and width of 150 μm. (B) Schematic cross-section of the local top-gated SWCNT TFT structure that is fabricated on a SiO2/Si substrate, Cr/Au source and drain bottom contacts, >99% semiconducting SWCNT thin film, n-type dopant (RhCp*Cp)2, ALD seeding layer (PTCDA), 50 nm Al2O3 gate dielectric, and 30 nm Ni top-gate. (C) Close-up schematic of the top-gated SWCNT TFT materials. (D) The molecular structure of PTCDA that serves as an ALD seeding layer. (E) The molecular structure of the n-type dopant (RhCp*Cp)2.
Figure 2. Hall effect characteristics. (A) Cross-sectional schematic of the Hall effect samples fabricated on a glass substrate, 100 nm of Au bottom contacts, >99% semiconducting SWCNT thin film, n-dopant (RhCp*Cp)2, and 50 nm Al2O3 encapsulant. (B) Schematic of the square Van der Pauw geometry for the Hall effect samples. (C) Hall voltage at room temperature under an applied 0.88 T magnetic field. (D) Sheet resistance as a function of the exposure time of the SWCNT sample to (RhCp*Cp)2. Error bars are the standard deviation of three measurements.
conditions due to coupling of their redox activity to C−C bond cleavage.36,38 Recent studies have shown that this class of dimeric molecules also effectively n-type dope low-dimensional semiconductor films.39,40 Here, we explore the organorhodium dimeric dopant, (RhCp*Cp)2, for controlled n-type doping of top-gated SWCNT TFTs. The top-gate, bottom-contact SWCNT TFT layout is described in Figure 1. Specifically, Figure 1A shows an optical micrograph of the local top-gate TFT, and Figure 1B,C contains the cross-sectional schematics of all the materials comprising the TFTs. The TFTs were fabricated on Si/SiO2 substrates, with bottom source/drain (S/D) contacts patterned by photolithography and deposited by thermal evaporation of Cr/Au (1 nm/30 nm), followed by liftoff in acetone. The ∼99% semiconducting SWCNTs, synthesized by the arc discharge method (P2, Carbon Solutions) with an average diameter of 1.4 nm, were sorted by previously reported density gradient ultracentrifugation (DGU) methods.10,35 Semiconducting arc discharge SWCNTs possess a relatively broad range of diameters and chiralities, which maximizes DGU sorting yields, and have been widely employed in SWCNT-based electronic devices and circuits.10,35 Furthermore, fabrication
characteristics show consistent and tunable n-type behavior. Importantly, the resulting n-type SWCNT TFTs possess complete p-type carrier suppression analogous to the n-type carrier suppression induced by adsorbed atmospheric dopants in p-type SWCNT TFTs. Since the organorhodium adlayer is molecularly thin, this approach also enables the demonstration of high-yield n-type SWCNT TFTs fabricated in a top-gate device structure, thus creating future opportunities for improved SWCNT integrated circuit layout and function. The ability to control semiconductor carrier concentration and type through controlled doping underlies the success of CMOS device design and integration. While doping is routinely achieved in traditional bulk semiconductors, it is considerably less well understood and optimized in emerging nanoelectronic materials. With exceptionally high surface-to-volume ratios, the adsorption of small-molecule dopants with reducing (electrondonating) or oxidizing (electron-accepting) characteristics to the surfaces of low-dimensional semiconductors provides a pathway to doping control.36,37 In particular, a subset of dimers formed by 19-electron organometallic sandwich compounds and organic radicals can act as strongly reducing n-type dopants for organic semiconductors with high stability under ambient B
DOI: 10.1021/acs.nanolett.6b01393 Nano Lett. XXXX, XXX, XXX−XXX
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in Figure 3. In particular, Figure 3A contains the SWCNT TFT transfer curves as a function of dopant exposure time. With no
processes that employ semiconducting arc discharge SWCNTs and show a high degree of reproducibility and homogeneity can thus be deemed to be insensitive to diameter and chirality.10,35 The DGU-sorted SWCNTs were deposited by vacuum filtration from solution onto a 50 nm pore-size mixed cellulose-ester membrane and then transferred onto the substrate by dissolving the filter membrane in acetone. The active-channel SWCNTs containing a linear density of ∼10 SWCNTs/μm (Figure S1) were defined by patterning with photolithography and oxygen reactive ion etching, followed by liftoff of the resist in acetone. Residual adsorbed atmospheric dopants were removed by vacuum annealing the devices at 230 °C for 1 h. To n-type dope the TFTs, a 2.5 mM solution of (RhCp*Cp)2 (synthesized as previously reported41,42 and also commercially available (Sigma-Aldrich, Product #795615)) in anhydrous toluene was prepared in a nitrogen glovebox. The SWCNT TFTs were then exposed to the (RhCp*Cp)2 solution for various lengths of time ranging from quickly dipping the sample (∼1 s) to 10 min. The devices were subsequently rinsed with toluene to remove excess unbound (RhCp*Cp)2. To enable top-gated devices, high-quality, pinhole-free dielectric must be deposited on top of the resulting heterogeneous substrate. To homogenize the substrate surface chemistry and seed uniform dielectric growth, 0.6 nm of perylene-3,4,9,10tetracarboxylic dianhydride (PTCDA) (structure shown in Figure 1D) was deposited by thermal evaporation.43−45 The devices were then transported under nitrogen atmosphere with minimal ambient exposure (
