Control of Graphene Field‐Effect Transistors by Interfacial Hydrophobic Self‐Assembled Monolayers

• DOI: 10.1002/adma.201101340
• Publication Date: 2011-07-04T18:58:55Z
• Authors: Wi Hyoung Lee; Jaesung Park; Youngsoo Kim; Kwang S. Kim; Byung Hee Hong; Kilwon Cho
• Keywords: DIELECTRICS; Transistors, Electronic; PHASE; SCATTERING; GROWTH; Graphite; 02 engineering and technology; FILMS; Silicon Dioxide; 0210 nano-technology; Hydrophobic and Hydrophilic Interactions

Hydrophobic self-assembled monolayers (SAMs) with alkyl chains of various lengths were inserted between CVD-grown graphene layers and their SiO2 substrates (figure). As the SAM chain length increased, substrate-induced doping was suppressed by ordered close-packed structure SAMs long chains. Accordingly, transistors constructed on exhibited higher electron/hole mobilities lower Dirac point voltages. Graphene has received considerable attention as a potential alternative material for use in next-generation semiconductor technology.1, 2 Since its discovery, graphene-based electronic devices have been fabricated SiO2/Si to take advantage optical contrast monolayer because ease fabrication metal-oxide-semiconductor (MOS) structures.3 The performances field-effect (FETs) substrates, however, are limited charged impurity scattering,4, 5 extrinsic scattering surface phonons,6 resonant from atomic scale defects,7 corrugation or residual adsorbates.8, 9 To overcome these performance limits, buffer layer may be gate dielectric modify properties SiO2. This approach provides an method reducing adsorbate-limited doping. For example, hexamethyldisilazane (HMDS),10 phenyl-terminated organosilane,11 polymeric layers12 used reduce intrinsic levels, thereby enhancing electrical FETs. However, mechanism which FETs affected not elucidated systematic studies effects needed. Previous experiments mainly tested mechanically exfoliated flakes. Because exfoliation process depends substrate, large variations samples expected this source graphene. end, recent chemical vapor deposition (CVD) methods enabled synthesis large-area uniform high-quality benefits CVD employed study reported here.13, 14 A conducted using physical graphene/dielectric (SiO2) interfaces controlled treating hydrophobic two organoalkyl silanes, octyltrichlorosilane (OTS) octadecyltrichlorosilane (ODTS), varying (C8 C18, respectively) well previously described HMDS C1. These modified nature interface graphene, modulated response Thus, significantly improved presence layer. simple straightforward manner. OTS ODTS-SAMs spontaneously gate-dielectric surfaces via reaction involving hydrolysis condensation (see Experimental Section details).15 prepared spin-coating.16 characteristics wettability SAM-modified listed Table 1. expected, thickness increased increasing length. packing density evaluated X-ray photoemission spectroscopy (XPS), shown Figure 1a,b. intensity single C1s peak at 284.6 eV (HMDS < ODTS). In parallel, Si2p peaks 99.3 (Si0) 103.3 (Si4+) decreased > ODTS) due substrate. C1s/Si2p ratio, normalized SAMs, measure relative density.17-19 high (10%) densities OTS-SAMs. observation investigated characterizing structures OTS- Fourier transform infrared (FTIR) spectroscopy. number C atoms 8 18 positions corresponding symmetric asymmetric CH2 vibrations shifted wavenumbers, 2855 2851 cm−1 2927 2920 cm−1. shifts indicated that ordering changed disordered (ODTS). Trans conformations favored van der Waals forces longer chains.18, 20 prevalence trans turn ODTS-SAMs. On other hand, HMDS-modified did display (Figure 1d). only reacted silanol groups engage self-condensation (Si-O-Si bonds). Owing sites single-chain HMDS, energy much than energies ODTS-SAMs-modified 1).16 XPS spectra representing a) b) S2p binding SAMs-modified substrates. c) FTIR methylene (CH2-) vibration different (C) (OTS: C8, ODTS: C18). d) Chemical (left) organosilane-modified (right) (OTS n = 7 ODTS 17). transferred copper foil substrate evaluate interactions Section). Raman spectroscopy, 2a. 2D band (untreated Furthermore, G bands clearly red-shifted untreated ratio band, full width half maximum (FWHM) sensitive levels sample.21 Whereas 2D-band/G-band systematically length, showed opposite trend. changes demonstrated effectively reduced inset shows magnified bands. Comparative plots I(2D)/I(G) ratios 2D-band widths. Ultraviolet (at secondary electron emission region) comparative plot work functions. films examined depositing Au source/drain electrodes directly onto without photolithography processes 3a). 3b conductivities function voltage VG. VGDirac, majority carrier population one type another, gradually 52 V 24 ODTS-modified transition determined excess charge carriers remaining it correlated magnitude films.23 3c, VGDirac further VG 0 (i.e., blocking efficiency enhanced). results indicate hole screened chains, 2. positive value even supported SiO2, attributed unintentional example poly(methyl methacrylate) (PMMA) residues surface.8 Schematic diagram showing having conductivity formed (VG). Dirac-point mobilities. e) Detailed view density-dependent near graph (b). f) Transconductance gm conclusion, we report decreased, thus hole/electron could explained scattering, indicating impurities tuned Our arises quality appropriate chemistries essential Materials device fabrication: following procedure literature.19 spin-coated clean baked 150 °C 1 h. synthesized literature.14, 27 As-grown covered PMMA (Mw 240 kg mol−1) floated aqueous solution 0.1 M ammonium persulfate ((NH4)2S2O8) solution. After all had etched away, film support (300 nm, capacitance 10.8 nF cm−2)/Si remained silicon after removed acetone. define FETs, thermally evaporated through shadow mask (channel length: 30 μm, channel width: 300 μm). Characterization: measuring contact angles probe liquids (deionized water diiodomethane).28 ellipsometer (M-2000V, J. A. Woollam), root-mean-square (rms) roughness force microscopy (AFM, Digital Instruments). characterized photoelectron ESCALAB 220i spectrometer Al Kα line (1486.6 eV). Chain conformation (Bruker IFS 66v). obtained (Renishaw, RM-1000 Invia) excitation 2.41 (514 Ar+ ion laser). functions cutoff ultraviolet (UPS; He I source, hν 21.2 eV) 4B1 beamline Pohang Accelerator Laboratory Korea. current–voltage analyzed Keithley 2636A parameter analyzers. W. H. Lee Park contributed equally work. research National Research Foundation (NRF) grant (No. 2009-0093485), NRF (National Honor Scientist Program, 2010-0020414), Converging Center Program (2010K001066), Basic Science (2011-0006270) Korea (NRF). authors thank providing synchrotron radiation sources 8A2 beam lines study.