High-Performance Charge Transport in Semiconducting Armchair

Sep 7, 2017 - Here, we show that sub-10 nm nanoribbons grown on Ge(001) can simultaneously achieve a high on/off conductance ratio of 2 × 104 and a h...
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High Performance Charge Transport in Semiconducting Armchair Graphene Nanoribbons Grown Directly on Germanium Robert M. Jacobberger, and Michael S. Arnold ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b03220 • Publication Date (Web): 07 Sep 2017 Downloaded from http://pubs.acs.org on September 10, 2017

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High Performance Charge Transport in Semiconducting Armchair Graphene Nanoribbons Grown Directly on Germanium

Robert M. Jacobberger and Michael S. Arnold*

Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States

*E-mail: [email protected].

Abstract: The growth of graphene on Ge(001) via chemical vapor deposition can be highly anisotropic, affording the facile synthesis of crystallographically controlled, narrow, long, oriented nanoribbons of graphene that are semiconducting, whereas unpatterned continuous graphene is semimetallic. This bottom-up growth overcomes long-standing challenges that have limited top-down ribbon fabrication (e.g., inadequate resolution and disordered edges) and yields ribbons with long segments of smooth armchair edges. The charge transport characteristics of sub-10 nm ribbons synthesized by this technique (which are expected to have bandgaps sufficiently large for semiconductor electronics applications) have not yet been characterized. Here, we show that sub-10 nm nanoribbons grown on Ge(001) can simultaneously achieve high on/off conductance ratio of 2×104 and high on-state conductance of 5 µS in field-effect transistors, favorably comparing to or exceeding the performance of nanoribbons fabricated by other methods. These promising results demonstrate that the direct synthesis of nanoribbons on Ge(001) could provide a scalable pathway towards the practical realization of high performance semiconducting graphene electronics, provided that the width uniformity and positioning of the nanoribbons are improved.

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Keywords: graphene nanoribbon; semiconductor; field-effect transistor; charge transport; chemical vapor deposition; germanium

Unlike unpatterned two-dimensional graphene, which is semimetallic, quasi-one-dimensional nanoribbons of graphene can be semiconducting, with the largest bandgaps occurring for ribbons that are narrower than 10 nm and that have smooth, well-defined armchair edges (i.e., C – C bonds are parallel to the long-axis of the ribbon).1 Semiconducting nanoribbons are promising candidates for field-effect transistors (FETs) because of their potential for achieving high charge carrier mobility,2 carrier velocity,3 current carrying capacity,4 and electrical3 and thermal5 conductivity and because of their atomically thin bodies, which allow for excellent electrostatic control. Consequently, nanoribbons may enable nextgeneration semiconductor electronics technologies including logic gates, high frequency communication devices, optoelectronics, photonics, and sensors.3,

6-14

Practically, however, the properties and

applicability of nanoribbons have been limited by difficulties in their synthesis. One of the most successful synthetic approaches (with regards to producing nanoribbons with excellent charge transport properties) has been the sonochemical unzipping of exfoliated graphite in solution, which yields nanoribbons as narrow as 2 nm with nearly atomically smooth edges.15-17 In FETs, these ribbons have exhibited on-state conductance (Gon) of 2–6 µS and on/off conductance ratio (Gon/Goff) of 2×104 – 5×106, which are the highest performance metrics reported in literature. The widespread applicability of sonochemical unzipping is limited, however, as it does not offer control over the edge orientation of the ribbons, results in poor yield, and does not allow for the deposition of ribbons from solution onto substrates with controlled placement and alignment. Duplicating the high-degree of structural precision and excellent charge transport properties afforded by sonochemical unzipping using more controllable and scalable methods has proven difficult. For example, patterning and etching nanoribbons from continuous sheets of graphene via top-down lithography yields relatively wide ribbons that have rough, defective edges,18-20 limiting Gon/Goff to 7×102 with a correspondingly low Gon of 7×10-2 µS in FETs.21 Wide ribbons can be narrowed to < 5 nm via mild 2 ACS Paragon Plus Environment

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gas-phase oxidation22 or hydrogen plasma etching23-25 to increase Gon/Goff to 1×103 – 5×105; however, ribbons generally become discontinuous when narrowing to sub-8 nm widths due to pre-existing width non-uniformity.22 In contrast, bottom-up polymerization has been exploited to create nanoribbons with nearly monodisperse and atomically precise width and edge structure.26-30 However, it has been difficult to form low-impedance contacts to these ribbons in FETs because the ribbons are short (~20 nm), leading to insufficient contact lengths (Lc), and because the bandgaps of the ribbons are large (as a result of their especially narrow widths), leading to Schottky barriers at the contacts. For example, armchair nanoribbons that are 0.74 nm in width have yielded FETs with low Gon of 7.5×10-5 µS, which also limits Gon/Goff to 103.31 Implementing wider ribbons that are 0.98 and 1.48 nm in width has improved performance, but Gon has still been limited to 1×10-2 – 1×10-1 at Gon/Goff of 103 – 104.32 Furthermore, similar to sonochemical unzipping, the positioning of individual nanoribbons via bottom-up polymerization is a major challenge. It is clear that in order to exploit graphene nanoribbons in technology, significant advances in synthesis are needed that yield large-area, densely packed arrays of aligned and crystallographically controlled, semiconducting nanoribbons with smooth edges and narrow widths. Towards this end, we have recently reported a scalable bottom-up approach for growing self-defined nanoribbons via chemical vapor deposition (CVD).33 We discovered that the CVD of CH4 on Ge(001) can lead to highly anisotropic growth of graphene, directly yielding oriented nanoribbons (aligned roughly along Ge〈110〉) with predominantly smooth armchair edges and widths that are tunable down to virtually zero by controlling the growth rate and time.33 For example, ribbons as narrow as 2 nm have been observed via scanning tunneling microscopy (STM), which also shows that the ribbon edges consist of smooth armchair segments with roughness that can vary by only 1–2 lattice constants of graphene over ribbon lengths of tens of nanometers.33, 34 Nanoribbons grown on Ge(001) do not nucleate at the same time, leading to polydispersity in their width and length. Moreover, ribbons nucleate at random positions on the substrate. Nonetheless, if 3 ACS Paragon Plus Environment

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these aspects can be controlled, for example via the use of nanoscale seeds, then this synthetic route could be a promising approach for realizing high fidelity semiconducting graphene-based materials for technology because of its ability to yield nanoribbons with smooth edges, long lengths, and narrow widths. One open question, however, pertains to the charge transport properties of these ribbons. In our previous work, we were only been able to characterize the charge transport properties of relatively wide ribbons (> 10 nm) because of limitations in the ribbon transfer methodology utilized and in the FET architecture employed. As a consequence of the relatively large widths of the ribbons measured, Gon/Goff was limited to 101 – 102. In this work, we use an optimized nanoribbon transfer protocol and shorter channel lengths (Lch) to ensure that nanoribbons narrower than 10 nm can be accessed in FET measurements. We grow a polydisperse mixture of nanoribbons with widths varying from approximately 5 to 30 nm and measure their charge transport characteristics. We show that nanoribbons grown on Ge(001) via CVD can achieve both a high Gon of 5 µS and a high Gon/Goff of 2×104 (at room temperature) at the same time, favorably comparing to or exceeding the performance of nanoribbons fabricated by other methods and motivating future research into this promising approach for nanoribbon synthesis.

Results and Discussion Arrays of randomly-distributed graphene nanoribbons aligned roughly along Ge〈110〉 are grown on Ge(001) via CVD. The nanoribbons are transferred onto SiO2 (15 nm) on Si or HfO2 (15 nm) on Si substrates using a dry transfer method adapted from Lee et al.,35 which is schematically shown in Figure 1a-e. Briefly, the nanoribbons are peeled off of the Ge(001) surface using a sacrificial multilayer stack of Au / poly(methyl methacrylate) (PMMA) / thermal release tape (Figure 1a-c) and are stamped onto the target substrate (Figure 1d). The thermal release tape, PMMA, and Au are sequentially removed, resulting in only ribbons on the substrate (Figure 1e). The alignment and position of the ribbons after growth on Ge(001) (Figure 1f) are preserved after transfer (Figure 1g) with high yield over large areas (> 1 cm2), resulting in arrays of ribbons with minimal structural distortion. Thus, this synthesis and subsequent dry 4 ACS Paragon Plus Environment

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transfer offer a simple and reproducible method to fabricate arrays of aligned nanoribbons on arbitrary substrates, including technologically-relevant dielectrics. After transfer of the nanoribbons, FETs are fabricated. Source and drain electrodes with Lch of 25–120 nm are defined with electron-beam lithography and Cr/Pd/Au (0.7/10/8.3 nm) contacts are thermally evaporated. The contacts are patterned at random locations across the substrate so that the ribbons are perpendicular to the source and drain electrodes. This random placement of devices results in FETs containing nanoribbons of varying width in which the contact lengths (Lc) between the ribbon and the metal electrodes are of varying distance. Figure 1h shows a schematic of the FET architecture, and Figure 1i shows a scanning electron microscopy (SEM) image of an FET with a nanoribbon channel with apparent width of ~7 nm and Lch of ~25 nm. The width distribution (Figure 2a) of the nanoribbons is characterized via SEM. Most of the nanoribbons vary in width from 5 to 30 nm. Approximately 1.5, 12, and 59% of the ribbons have widths < 7.5, 10, and 15 nm, respectively. The length of each ribbon is also measured to determine the aspect ratio. Most nanoribbons have an aspect ratio in the range of 10 to 40, but the aspect ratio generally increases with decreasing width (Figure 2b). The narrowest ribbons are typically the shortest. For example, based on the measured data in Figure 2a-b, ribbons with width of 7.5, 10, and 15 nm are expected to have lengths of roughly 220, 245, and 285 nm, respectively. As a result of the random positioning of the FET channels with respect to the nanoribbons, FETs are more likely to spatially register with longer ribbons. The probability of an FET registering with a nanoribbon of length, L, such that the nanoribbon spans the channel from the source to the drain electrode, scales as L - Lch. This relationship is applied to the measured width (Figure 2a) and length (Figure 2b) distributions of nanoribbons to generate Figure 2c, which characterizes the probability of occurrence of FETs registered to nanoribbons as a function of width. From Figure 2c, we expect that approximately 1.0, 11, and 53% of FETs will contain ribbons of widths < 7.5, 10, and 15 nm, respectively. Note that the width of ribbons in FET channels cannot be accurately measured because of screening from the electrodes; and, thus, we use Figure 2c to guide interpretation of the FET results, below. 5 ACS Paragon Plus Environment

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The current-voltage characteristics of the nanoribbon FETs are measured in ambient laboratory conditions at room temperature. Figure 3a-c compares the Ids versus Vgs characteristics of three FETs with Gon/Goff of 2×102, 1×103, and 2×104, respectively, and Gon of 13, 4, and 5 µS, respectively, measured on 15 nm of SiO2 at Vds of 0.1 V. With increasing Vds, Gon/Goff decreases (see Figure S1), which can possibly be attributed to band-to-band tunneling in which electrons are injected from the drain into the conduction band of the ribbons.8,

36

There is hysteresis in the Ids versus Vgs data, which is expected for graphene

nanoribbon and carbon nanotube FETs on oxide substrates measured in ambient conditions. This hysteresis has been attributed to charge traps on the surface of the oxide dielectric37 as well as from adsorbed water and oxygen molecules,38 and can be reduced or eliminated via encapsulation,39 surface treatment,40 or by using hydrophobic substrates.41 The threshold voltage in the forward (negative to positive) Vgs sweep is likely more representative of the threshold voltage in the absence of hysteresis than in the reverse (positive to negative) Vgs sweep.41 The Ids versus Vgs characteristics often display small features, such as local maxima and minima. Some of these features are reproducible in sequential Vgs sweeps of the same device, while others are not (see Figure S2 and Supporting Information). Figure 3d plots the Ids versus Vgs characteristics of a nanoribbon with Gon/Goff of 1×104 on 15 nm of HfO2, which can be compared to the FET on 15 nm of SiO2 with similar Gon/Goff in Figure 3c. The HfO2 gate dielectric increases electrostatic control over the ribbon channel due to its higher dielectric constant of ~16, making it a more promising dielectric for more aggressively scaled, high-performance FETs. The Igs versus Vgs curves for the FETs on SiO2 and HfO2 in Figure 3 are plotted in Figure S3. Figure 4a compares Gon versus Gon/Goff for 178 FETs with Gon/Goff > 40 measured at Vds of 0.1 V on both SiO2 and HfO2 substrates at room temperature. For FETs with Gon/Goff > 103, which are obtained with Lch of 25–65 nm, Gon is relatively high and varies from 0.3–8 µS. Similarly, for FETs with Gon/Goff < 103, which are obtained with Lch of 25–120 nm, Gon is also high and varies from 0.4–27 µS. The nanoribbons exhibit large variability in both Gon/Goff and Gon. The variability in Gon/Goff can be largely attributed to polydispersity in width. We analyze the expected versus the measured variability in Gon/Goff in Figure 4b. The expected variability is determined from the distribution in Figure 2c, in which the 6 ACS Paragon Plus Environment

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expected Gon/Goff is determined as a function of nanoribbon width, w, according to an empirical relationship found by Li et al.,15 Gon/Goff = a·exp(Eg/kBT), where Eg = b/w is the bandgap of the ribbon, a and b are constants, kB is Boltzmann’s constant, and T is the temperature of 300 K. Li et al. justify this relationship because Goff is thermally activated over a Schottky barrier to the conduction band on the order of ~Eg15 and first-principles calculations have predicted a similar dependence of Eg on w.1 Based on data from Li et al., we use a = 0.24 and b = 1.6 eV nm, which estimates Gon/Goff of 1, 5, 15, 102, 103, and 106 for widths of 50, 20, 15, 10, 7.5, and 4 nm, respectively. The resulting expected distribution in Gon/Goff closely matches the measured distribution (Figure 4b). We expect roughly 1.0, 11, and 53% of the FETs to have Gon/Goff > 103, 102, and 15, respectively, which agrees well with our measured yield of 1.2, 10, and 30%, respectively. Figure 4a also shows that Gon at a given Gon/Goff can vary by over an order of magnitude. We attribute this variation, in large part, to variations in Lc between the nanoribbon and the source and drain electrodes. If Lc is less than the transfer length (~100 nm for graphene),42-44 the contact resistance is expected to increase roughly linearly with decreasing Lc.45 The length of the source and drain electrodes used here varies from 30–500 nm. In FETs in which the length of the source and drain electrodes is < 100 nm, Lc is guaranteed to be less than the transfer length. Even when the length of the source and drain electrodes is much greater than the transfer length, it is still likely that Lc is less than the transfer length due to the short lengths of the ribbons and the random registry of ribbons in the FET channels. In Figure 4c, we compare Gon versus Gon/Goff at room temperature of nanoribbons synthesized on Ge(001) via CVD with those reported in literature that have Gon/Goff > 40, which are fabricated via lithographic patterning (orange diamonds),21, upward triangles),22,

23, 25

46-48

narrowing of wider ribbons via edge etching (green

polymerization (blue squares),31,

32

sonochemical unzipping of graphite (red

circles),15-17 and unzipping of carbon nanotubes (magenta downward triangles).49, 50 In addition to CVD on Ge(001), the only methods that yield ribbons with Gon/Goff > 103 in FETs are polymerization,31,

32

sonochemical unzipping of graphite,15-17 and edge etching of wider ribbons.22, 23 Importantly, the Gon of nanoribbons grown on Ge(001) at a given Gon/Goff are among the highest reported in literature. For the 7 ACS Paragon Plus Environment

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nanoribbons with the highest Gon/Goff, Gon is likely at least partially limited by a Schottky barrier at the nanoribbon/contact interface, which is indicated by the non-linear Ids versus Vds characteristics at low Vds (see Figure S5 and Supporting Information).51 From the relationship of Li et al.,15 we can estimate that the ribbons with Gon/Goff of 102, 103, 104 have bandgaps of 0.16, 0.22, and 0.28 eV, respectively. These data confirm that nanoribbons grown on Ge(001) have large bandgaps >> kBT at room temperature and indicate that the crystallinity and edge morphology of the ribbons33,

34

are sufficiently well ordered to

realize high on-state conductance and promising charge transport characteristics.

Conclusions In conclusion, the development of nanoribbon technologies has been hindered by difficulties in the scalable synthesis of ribbons with sub-10 nm widths, controlled crystallographic orientation, and smooth armchair edges, making it difficult to realize both high Gon/Goff and high Gon in FETs. The direct synthesis of aligned, narrow, semiconducting graphene nanoribbons with predominantly smooth armchair edges on Ge(001) via CVD overcomes many of these synthetic challenges. Consequently, nanoribbons synthesized by this technique can simultaneously exhibit high Gon/Goff of 2×104 and high Gon of 5 µS in FETs, favorably comparing to or exceeding the performance of ribbons fabricated by other techniques and validating the high-quality of nanoribbons grown on Ge(001). If the polydispersity in ribbon width and length is reduced and if the location of the ribbons can be controlled, large-area arrays of narrow semiconducting nanoribbons grown using CVD may enable significant advances in state-of-the-art semiconductor electronics.

Methods Nanoribbon Synthesis. Ge(001) (Wafer World) substrates are loaded into a horizontal quartz tube furnace with inner diameter of 34 mm and the system is evacuated to < 10-5 torr. The system is then filled to atmospheric pressure with a mixture of 200 sccm Ar (99.999%) and 100 sccm H2 (99.999%). The Ge(001) substrates are annealed for 3 h at 910 °C. The samples are removed from and then reloaded into 8 ACS Paragon Plus Environment

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the furnace. The system is again pumped to < 10-5 torr and filled to atmospheric pressure with 200 sccm Ar and 100 sccm H2. The Ge(001) samples are annealed for 30 min at 910 °C and then 2.0 sccm CH4 (99.99%) is introduced to begin nanoribbon synthesis. Growth is terminated after ~2 h by sliding the furnace away from the growth region. The ribbons are characterized using SEM (LEO 1530). Nanoribbon Transfer. After synthesis, the nanoribbons are transferred onto SiO2 (15 nm) on Si or HfO2 (15 nm) on Si wafers. 60 nm of Au is deposited on the nanoribbon/Ge(001) surface. 4% PMMA (950 kg/mol) in anisole is spin-coated on the Au film and the sample is baked at 185 °C for 10 min in an N2 environment (< 1 ppm O2 and < 1 ppm H2O). Thermal release tape (Nitto Denko, # 3198MS) is used to

separate

the

PMMA/Au/ribbons

from

the

Ge(001)

surface.

The

thermal

release

tape/PMMA/Au/ribbons is stamped onto the desired substrate and the tape is released by baking at 120 °C in air. The sample is then baked at 185 °C for 10 min in an N2 environment. PMMA is removed in acetone at 120 °C followed by an oxygen reactive ion etch at 50 W and an O2 pressure of 10 mtorr for 20 s. Finally, Au is removed using a KI/I2/H2O solution followed by rinsing in H2O at 90 °C, yielding graphene nanoribbons on SiO2 or HfO2. Device Fabrication. Electron-beam lithography is used to define the source and drain contacts. Thermal evaporation is used to deposit 0.7 nm Cr / 10 nm Pd / 8.3 nm Au. The Si substrate is used as a back gate and 15 nm of SiO2 or 15 nm of HfO2 is used as the gate dielectric. All devices are measured at room temperature in ambient laboratory conditions using a Keithley 2636A SourceMeter. The assigned off-state current in Figure 4, and for all devices discussed in the manuscript, is determined from measured Ids versus Vgs characteristics. The gate leakage current measured in FETs with the same architecture (but without a ribbon spanning the channel) is added to the off-state current. The gate leakage current arises from the transient capacitance current.

Associated Content Supporting Information. Effect of Vds on Gon/Goff and Gon, reproducibility of small features in the Ids versus Vgs characteristics, Igs versus Vgs characteristics, effect of annealing the nanoribbons on Gon/Goff and 9 ACS Paragon Plus Environment

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Gon, and Ids versus Vds output characteristics. This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Author *E-mail: [email protected].

Acknowledgements This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award # DE-SC0016007.

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13. Yan, Q. M.; Huang, B.; Yu, J.; Zheng, F. W.; Zang, J.; Wu, J.; Gu, B. L.; Liu, F.; Duan, W. H. Intrinsic Current-Voltage Characteristics of Graphene Nanoribbon Transistors and Effect of Edge Doping. Nano Lett. 2007, 7, 1469-1473. 14. Liang, G. C.; Neophytou, N.; Lundstrom, M. S.; Nikonov, D. E. Ballistic Graphene Nanoribbon Metal-Oxide-Semiconductor Field-Effect Transistors: A Full Real-Space Quantum Transport Simulation. J. Appl. Phys. 2007, 102, 054307. 15. Li, X. L.; Wang, X. R.; Zhang, L.; Lee, S. W.; Dai, H. J. Chemically Derived, Ultrasmooth Graphene Nanoribbon Semiconductors. Science 2008, 319, 1229-1232. 16. Wang, X. R.; Ouyang, Y. J.; Li, X. L.; Wang, H. L.; Guo, J.; Dai, H. J. Room-Temperature AllSemiconducting Sub-10-nm Graphene Nanoribbon Field-Effect Transistors. Phys. Rev. Lett. 2008, 100, 206803. 17. Wang, X. R.; Li, X. L.; Zhang, L.; Yoon, Y.; Weber, P. K.; Wang, H. L.; Guo, J.; Dai, H. J. NDoping of Graphene Through Electrothermal Reactions with Ammonia. Science 2009, 324, 768-771. 18. Han, M. Y.; Ozyilmaz, B.; Zhang, Y. B.; Kim, P. Energy Band-Gap Engineering of Graphene Nanoribbons. Phys. Rev. Lett. 2007, 98, 206805. 19. Chen, Z. H.; Lin, Y. M.; Rooks, M. J.; Avouris, P. Graphene Nano-Ribbon Electronics. Phys. E 2007, 40, 228-232. 20. Han, M. Y.; Brant, J. C.; Kim, P. Electron Transport in Disordered Graphene Nanoribbons. Phys. Rev. Lett. 2010, 104, 056801. 21. Ponomarenko, L. A.; Schedin, F.; Katsnelson, M. I.; Yang, R.; Hill, E. W.; Novoselov, K. S.; Geim, A. K. Chaotic Dirac Billiard in Graphene Quantum Dots. Science 2008, 320, 356-358. 22. Wang, X. R.; Dai, H. J. Etching and Narrowing of Graphene from the Edges. Nat. Chem. 2010, 2, 661-665. 23. Xie, L. M.; Jiao, L. Y.; Dai, H. J. Selective Etching of Graphene Edges by Hydrogen Plasma. J. Am. Chem. Soc. 2010, 132, 14751-14753. 24. Zhang, X. W.; Yazyev, O. V.; Feng, J. J.; Xie, L. M.; Tao, C. G.; Chen, Y. C.; Jiao, L. Y.; Pedramrazi, Z.; Zettl, A.; Louie, S. G.; Dai, H. J. Experimentally Engineering the Edge Termination of Graphene Nanoribbons. ACS Nano 2013, 7, 198-202. 25. Yang, R.; Zhang, L. C.; Wang, Y.; Shi, Z. W.; Shi, D. X.; Gao, H. J.; Wang, E. G.; Zhang, G. Y. An Anisotropic Etching Effect in the Graphene Basal Plane. Adv. Mater. 2010, 22, 4014-4019. 26. Cai, J. M.; Ruffieux, P.; Jaafar, R.; Bieri, M.; Braun, T.; Blankenburg, S.; Muoth, M.; Seitsonen, A. P.; Saleh, M.; Feng, X. L.; Mullen, K.; Fasel, R. Atomically Precise Bottom-Up Fabrication of Graphene Nanoribbons. Nature 2010, 466, 470-473. 27. Huang, H.; Wei, D. C.; Sun, J. T.; Wong, S. L.; Feng, Y. P.; Castro Neto, A. H.; Wee, A. T. S. Spatially Resolved Electronic Structures of Atomically Precise Armchair Graphene Nanoribbons. Sci. Rep. 2012, 2, 983. 28. Chen, Y. C.; de Oteyza, D. G.; Pedramrazi, Z.; Chen, C.; Fischer, F. R.; Crommie, M. F. Tuning the Band Gap of Graphene Nanoribbons Synthesized from Molecular Precursors. ACS Nano 2013, 7, 6123-6128. 29. Vo, T. H.; Shekhirev, M.; Kunkel, D. A.; Morton, M. D.; Berglund, E.; Kong, L. M.; Wilson, P. M.; Dowben, P. A.; Enders, A.; Sinitskii, A. Large-Scale Solution Synthesis of Narrow Graphene Nanoribbons. Nat. Commun. 2014, 5, 3189. 30. Narita, A.; Feng, X. L.; Hernandez, Y.; Jensen, S. A.; Bonn, M.; Yang, H. F.; Verzhbitskiy, I. A.; Casiraghi, C.; Hansen, M. R.; Koch, A. H. R.; Fytas, G.; Ivasenko, O.; Li, B.; Mali, K. S.; Balandina, T.; Mahesh, S.; De Feyter, S.; Mullen, K. Synthesis of Structurally Well-Defined and Liquid-PhaseProcessable Graphene Nanoribbons. Nat. Chem. 2014, 6, 126-132. 31. Bennett, P. B.; Pedramrazi, Z.; Madani, A.; Chen, Y. C.; de Oteyza, D. G.; Chen, C.; Fischer, F. R.; Crommie, M. F.; Bokor, J. Bottom-Up Graphene Nanoribbon Field-Effect Transistors. Appl. Phys. Lett. 2013, 103, 253114. 32. Llinas, J. P.; Fairbrother, A.; Barin, G.; Ruffieux, P.; Shi, W.; Lee, K.; Choi, B. Y.; Braganza, R.; Kau, N.; Choi, W.; Chen, C.; Pedramrazi, Z.; Dumslaff, T.; Narita, A.; Feng, X.; Müllen, K.; Fischer, F.; 11 ACS Paragon Plus Environment

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Zettl, A.; Crommie, M.; Fasel, R.; Bokor, J. Short-Channel Field Effect Transistors with 9-Atom and 13Atom Wide Graphene Nanoribbons. arXiv:1605.06730 [cond-mat.mes-hall] 2016. 33. Jacobberger, R. M.; Kiraly, B.; Fortin-Deschenes, M.; Levesque, P. L.; McElhinny, K. M.; Brady, G. J.; Delgado, R. R.; Roy, S. S.; Mannix, A.; Lagally, M. G.; Evans, P. G.; Desjardins, P.; Martel, R.; Hersam, M. C.; Guisinger, N. P.; Arnold, M. S. Direct Oriented Growth of Armchair Graphene Nanoribbons on Germanium. Nat. Commun. 2015, 6, 8006. 34. Kiraly, B.; Mannix, A. J.; Jacobberger, R. M.; Fisher, B. L.; Arnold, M. S.; Hersam, M. C.; Guisinger, N. P. Sub-5 nm, Globally Aligned Graphene Nanoribbons on Ge(001). Appl. Phys. Lett. 2016, 108, 213101. 35. Lee, J. H.; Lee, E. K.; Joo, W. J.; Jang, Y.; Kim, B. S.; Lim, J. Y.; Choi, S. H.; Ahn, S. J.; Ahn, J. R.; Park, M. H.; Yang, C. W.; Choi, B. L.; Hwang, S. W.; Whang, D. Wafer-Scale Growth of SingleCrystal Monolayer Graphene on Reusable Hydrogen-Terminated Germanium. Science 2014, 344, 286289. 36. Radosavljevic, M.; Heinze, S.; Tersoff, J.; Avouris, P. Drain Voltage Scaling in Carbon Nanotube Transistors. Appl. Phys. Lett. 2003, 83, 2435-2437. 37. Lee, J. S.; Ryu, S.; Yoo, K.; Choi, I. S.; Yun, W. S.; Kim, J. Origin of Gate Hysteresis in Carbon Nanotube Field-Effect Transistors. J. Phys. Chem. C 2007, 111, 12504-12507. 38. Kim, W.; Javey, A.; Vermesh, O.; Wang, O.; Li, Y. M.; Dai, H. J. Hysteresis Caused by Water Molecules in Carbon Nanotube Field-Effect Transistors. Nano Lett. 2003, 3, 193-198. 39. Ha, T. J.; Kiriya, D.; Chen, K.; Javey, A. Highly Stable Hysteresis-Free Carbon Nanotube ThinFilm Transistors by Fluorocarbon Polymer Encapsulation. ACS Appl. Mater. Interfaces 2014, 6, 84418446. 40. Franklin, A. D.; Tulevski, G. S.; Han, S. J.; Shahrjerdi, D.; Cao, Q.; Chen, H. Y.; Wong, H. S. P.; Haensch, W. Variability in Carbon Nanotube Transistors: Improving Device-to-Device Consistency. ACS Nano 2012, 6, 1109-1115. 41. Lefebvre, J.; Ding, J.; Li, Z.; Cheng, F.; Du, N.; Malenfant, P. R. L. Hysteresis Free Carbon Nanotube Thin Film Transistors Comprising Hydrophobic Dielectrics. Appl. Phys. Lett. 2015, 107, 243301. 42. Xia, F. N.; Perebeinos, V.; Lin, Y. M.; Wu, Y. Q.; Avouris, P. The Origins and Limits of MetalGraphene Junction Resistance. Nat. Nanotechnol. 2011, 6, 179-184. 43. Grosse, K. L.; Bae, M. H.; Lian, F. F.; Pop, E.; King, W. P. Nanoscale Joule Heating, Peltier Cooling and Current Crowding at Graphene-Metal Contacts. Nat. Nanotechnol. 2011, 6, 287-290. 44. Huang, B. C.; Zhang, M.; Wang, Y. J.; Woo, J. Contact Resistance in Top-Gated Graphene FieldEffect Transistors. Appl. Phys. Lett. 2011, 99, 032107. 45. Murrmann, H.; Widmann, D. Current Crowding on Metal Contacts to Planar Devices. IEEE Trans. Electron Devices 1969, ED16, 1022-1024. 46. Bai, J. W.; Duan, X. F.; Huang, Y. Rational Fabrication of Graphene Nanoribbons Using a Nanowire Etch Mask. Nano Lett. 2009, 9, 2083-2087. 47. Liao, L.; Bai, J. W.; Cheng, R.; Lin, Y. C.; Jiang, S.; Huang, Y.; Duan, X. F. Top-Gated Graphene Nanoribbon Transistors with Ultrathin High-k Dielectrics. Nano Lett. 2010, 10, 1917-1921. 48. Pan, Z. H.; Liu, N.; Fu, L.; Liu, Z. F. Wrinkle Engineering: A New Approach to Massive Graphene Nanoribbon Arrays. J. Am. Chem. Soc. 2011, 133, 17578-17581. 49. Jiao, L. Y.; Zhang, L.; Wang, X. R.; Diankov, G.; Dai, H. J. Narrow Graphene Nanoribbons from Carbon Nanotubes. Nature 2009, 458, 877-880. 50. Jiao, L. Y.; Zhang, L.; Ding, L.; Liu, J. E.; Dai, H. J. Aligned Graphene Nanoribbons and Crossbars from Unzipped Carbon Nanotubes. Nano Res. 2010, 3, 387-394. 51. Hazeghi, A.; Krishnamohan, T.; Wong, H. S. P. Schottky-Barrier Carbon Nanotube Field-Effect Transistor Modeling. IEEE Trans. Electron Devices 2007, 54, 439-445.

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Figure 1. (a-e) Schematic of the nanoribbon transfer. Graphene nanoribbons with predominately smooth armchair edges and that are aligned roughly along Ge〈110〉 are grown on Ge(001) via CVD (a). The ribbons are coated with 60 nm of Au, 300 nm of PMMA, and thermal release tape (b). The thermal release tape is lifted to separate the ribbons from Ge(001) (c) and the ribbon array is stamped onto the target substrate (d). The thermal release tape is released by applying heat, followed by removal of PMMA in acetone and etching of Au in KI/I2/H2O (e). (f-g) SEM images of nanoribbons after growth on Ge(001) (f) and after transfer to SiO2 (g). (h) Schematic of the nanoribbon FET architecture in which the nanoribbon channel (with channel length of Lch) is contacted by Cr/Pd/Au source and drain electrodes (with contact length of Lc), the Si substrate serves as the back gate, and SiO2 or HfO2 serves as the gate dielectric. (i) SEM image of an FET with nanoribbon channel with apparent width of ~7 nm and Lch of ~25 nm. Scale bars in f, g, and i are 1 µm, 1 µm, and 100 nm, respectively.

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Figure 2. (a-b) Histogram of width (a) and aspect ratio plotted against width (b) measured by SEM for 327 ribbons from a representative sample used to fabricate FETs. (c) Probability of occurrence of ribbons in FETs is plotted against ribbon width. Top axis provides the expected Gon/Goff based on data from Li et al.15

Figure 3. (a-c) Plot of Ids versus Vgs for nanoribbon FETs with Gon/Goff of 2×102 (a), 1×103 (b), and 2×104 (c) at Vds of 0.1 V on 15 nm of SiO2 on Si. (d) Plot of Ids versus Vgs for a nanoribbon FET with Gon/Goff of 1×104 at Vds of 0.1 V on 15 nm of HfO2 on Si. All FETs are measured at room temperature in ambient laboratory conditions. The forward and reverse Vgs sweeps are indicated with solid and dashed arrows, respectively.

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Figure 4. (a) Plot of Gon versus Gon/Goff for 178 FETs with Gon/Goff > 40 at Vds of 0.1 V on 15 nm of SiO2 on Si (black circles) and 15 nm of HfO2 on Si (orange diamonds). Before patterning the source and drain electrodes, the ribbons are annealed using various conditions, which does not significantly impact the FET performance (see Figure S4 and Supporting Information). (b) Expected (black squares) and measured (red circles) cumulative probability of occurrence of nanoribbons in FETs is plotted against Gon/Goff. (c) Comparison of Gon versus Gon/Goff at room temperature for nanoribbons synthesized on Ge(001) via CVD (black and gray stars) compared to those reported in literature with Gon/Goff > 30. The plot does not include an FET with an armchair nanoribbon channel synthesized via polymerization in Bennett et al.,31 in which Gon/Goff is 103 and Gon is 7.5×10-5 µS.

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