LETTER pubs.acs.org/NanoLett
Epitaxial Graphene Transistors: Enhancing Performance via Hydrogen Intercalation Joshua A. Robinson,*,†,‡ Matthew Hollander,†,‡ Michael LaBella, III,‡ Kathleen A. Trumbull,‡ Randall Cavalero,‡ and David W. Snyder‡,§ †
Materials Science and Engineering, ‡Electro-Optics Center, and §Chemical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
bS Supporting Information ABSTRACT: We directly demonstrate the importance of buffer elimination at the graphene/SiC(0001) interface for high frequency applications. Upon successful buffer elimination, carrier mobility increases from an average of 800 cm2/(V s) to >2000 cm2/(V s). Additionally, graphene transistor current saturation increases from 750 to >1300 mA/mm, and transconductance improves from 175 mS/mm to >400 mS. Finally, we report a 10 improvement in the extrinsic current gain response of graphene transistors with optimal extrinsic current gain cutoff frequencies of 24 GHz. KEYWORDS: Epitaxial graphene, hydrogenation, transistor, cutoff frequency, carrier mobility, buffer layer
wing to its excellent intrinsic properties,1,2 graphene has the potential for operating in the terahertz regime.3 In fact, recent reports have shown the potential for exfoliated,4 chemical vapor deposited,5 and epitaxial graphene6 to exceed frequency performance of Si-MOSFET technology. Graphene derived from silicon carbide (SiC), referred to as epitaxial graphene, has proven to be an excellent material system for high-frequency electronic applications.6 9 Graphene research has progressed at a rapid pace and within only a few years, graphene transistors have reached an intrinsic current gain cutoff frequency (fT) of 300 GHz3 with the potential for going much higher. Considering fT µ gm/Cg and gm µ μn,p where gm is the transistor transconductance, Cg is gate capacitance, and μn,p charge carrier mobility, it is clear that μn,p plays an important role in one’s ability to achieve high frequency operation. Epitaxial graphene grown on SiC(0001) (EGSi) often suffers from low charge carrier mobility and significant temperature dependence10 compared to exfoliated11 and chemical vapor deposited (CVD) graphene.12,13 Additionally, while epitaxial graphene grown on SiC(0001) has proven to be comparable in carrier mobility to exfoliated and CVD graphene,23 the lack of thickness control on the micrometer scale limits it is applicability in electronics where uniformity on the wafer scale is paramount.14 Therefore, even with degraded mobility, growth of EGSi is still a very attractive route for wafer scale graphene electronics. In order to bring EGSi to the forefront of analog electronics, one must address the presence of an interfacial layer between graphene and SiC(0001).15,16 This interface layer is at least partially covalently bound to the SiC substrate and is not considered to be electronically active,18 yet it has been shown to be a primary source of carrier doping and
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scattering.14,15 Therefore, it is highly attractive to develop a route to remove the interfacial layer between graphene and SiC(0001). Recently, successful “cleaving” of the buffer layer in EGSi was achieved via hydrogen intercalation.17 22 This process, which in its simplest form amounts to exposing EGSi to molecular hydrogen at elevated temperatures, results in what is referred to as quasi-free-standing epitaxial graphene (QF-EGSi).16 Successful passivation of the graphene/SiC(0001) buffer layer is evidenced via X-ray photoelectron spectroscopy,18,23 where the elimination of spectral signatures directly related to the buffer layer (S1 and S2, Figure 2a) occurs. Additionally, a shift in carrier type from electrons to holes occurs, accompanied by a substantial increase in carrier mobility.20 Therefore, it is reasonable to expect that the elimination of the buffer layer will enhance transistor performance. Here we present work that directly demonstrates the importance of buffer elimination at the graphene/SiC(0001) interface to achieving superior transistor performance. Our work correlates hydrogenation of monolayer graphene with the structural (Raman) and electronic (Hall) properties of the resultant QF-EGSi. Low-temperature hydrogenation results in only partial elimination of the buffer, as evidenced by XPS, and is correlated with significant degradation of the graphene structure and carrier mobility. Upon full elimination of the buffer layer at elevated temperatures (>900 °C) the buffer layer signatures in XPS (S1 and S2 peaks) are no longer present, there is nearly complete elimination of the Raman D-peak, and there is a Received: June 13, 2011 Revised: July 23, 2011 Published: August 01, 2011 3875
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Figure 1. Scanning electron micrographs (a e) of completed graphene radio frequency transistors, demonstrating gate lengths from 75 nm (b) to 1000 nm (e). Graphene transistors are fabricated on 100 mm wafers (f) and utilize a 10 nm HfO2 seeded gate dielectric to isolate the gate, drain, and source (e).
significant increase in the carrier mobility from 800 cm2/(V s) to >2000 cm2/(V s). Finally, we demonstrate significant improvements in direct-current (DC) and radio frequency (RF) device performance of QF-EGSi compared to as-grown EGSi. Epitaxial graphene is synthesized on 15 15 mm squares and 100 mm wafers via silicon sublimation from SiC(0001) at 1625 °C for 15 min at 1 Torr.24 Following synthesis, epitaxial graphene is characterized via Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and Lehighton noncontact mobility and sheet resistance (LEI, Inc.). Raman confirms the successful growth of monolayer EGSi on the SiC(0001) terrace and bilayer EGSi on the terrace step edge.23,25,26 Subsequently, samples undergo exposure to molecular hydrogen (H2) at 600 1200 °C, 600 Torr for 30 120 min (referred to as hydrogenation). Following hydrogenation, peak fitting of the 2D Raman spectra indicates the presence of bilayer QF-EGSi on the SiC(0001) terrace center, and multilayer QF-EGSi on the terrace step edge. Finally, Hall cross, transfer-length method (TLM), and transistor structures are fabricated as described elsewhere.27 Figure 1a e consists of scanning electron micrographs of finished transistor structures utilized in this work. Ohmic contacts of Ti/Au exhibit specific contact resistance values of 8 10 8 to 3 10 7 Ohm-cm2. Hafnium oxide (HfO2) is utilized as the gate dielectric. Eight nanometers of HfO2 is deposited via seeded atomic layer deposition, where the seed material is a nonreactively evaporated, high purity ultrathin layer of HfO2 oxide.26 Carrier mobility is measured using 5 5 um Hall crosses in ambient air at 5000 kG, 100 uA, a minimum of 20 Hall crosses are measured per sample. We note that
measurement in air results in p-type doping of graphene by approximately 2 1012 cm 2, attributed to p-doping via a thin water overlayer.28 This results in a reduction in carrier concentration of as-grown epitaxial graphene from approximately 7 1012 cm 2 to 3 6 1012 cm 2. Alternatively, hydrogenated graphene with a majority carrier of holes experiences an increase in carrier concentration of similar magnitude. Transistor characterization is completed using an integrated DC/RF semiconductor parameter analyzer (Keithley Instruments) and Anritsu vector network analyzer. The RF measurement system was calibrated using the short-open-load-through method. Direct current measurements include drain current versus voltage (Ids/Vds), drain current versu gate voltage (Ids/Vg), transconductance (gm = δIds/δVg), and extrinsic current gain (H21) versus frequency. The drain voltage (Vds) for gm and H21 measurements is set to 1 V, while Vg is swept from 3 to 3 V. Data presented for H21 versus frequency is collected at Vds = 1 V, and Vg is set such that gm is maximized. Saturation current (Idss) is reported as Ids at Vds = 1 V, Vg = ( 3 V, where Vg = +3 V for asgrown epitaxial graphene and 3 V for hydrogenated epitaxial graphene. Figure 1f is a photograph of a 100 mm (inset, 75 mm) graphene wafer with >75 000 test structures utilized in this work. Figure 1g schematically illustrates the layer structure of a finished FET. Hydrogenation of EGSi results in elimination of the graphene/ SiC(0001) buffer layer. Evidenced by XPS, the S1 and S2 components of the C1s spectra present in as-grown graphene are eliminated upon full hydrogenation (Figure 2a), indicating successful passivation of the graphene/SiC(0001) buffer layer.18 3876
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Figure 2. Hydrogenation of epitaxial graphene on SiC(0001) results in the elimination of the buffer layer at the graphene/SiC(0001) interface (a); additionally, (b) during low temperature hydrogenation, there is a significant increase in the Raman D peak that is attributed to partial elimination of the buffer layer. Once the hydrogenation temperature increases to g900 °C, nearly complete elimination of the Raman D peak occurs, which is correlated well with significant decrease in graphene sheet resistance (c,d), enhancement in carrier mobility (e) and a change in carrier type from electrons to holes.
Additionally, we find that Raman spectroscopy directly correlates with the degree of hydrogenation. Figure 2b summarizes the evolution of the Raman spectra as a function of hydrogenation temperature and time. We note that as-grown graphene on SiC(0001) can be difficult to cleanly deconvolute from the SiC(0001) spectra, however as the level of hydrogen intercalation increases, deconvolution of the spectra becomes significantly easier. Additionally, as evidenced in Figure 1b, at low temperatures (600 750 °C) and short times (∼30 min) we find an increase in the defect level (Raman D peak, 1360 cm 1), resulting in a D/G ratio degradation from 1000 Ohm/sq (Figure 2c); however, upon hydrogenation,
the graphene sheet resistance is reduced to e200 Ohm/sq (Figure 2d). This phenomenon is attributed to the addition of a graphene layer upon hydrogenation, as well as passivation of the buffer layer. Additionally, we find that low-temperature hydrogenation (600 750 °C) often results in significant degradation of the carrier mobility, as well as significant carrier doping (Figure 2e). This is strongly correlated with the increased D peak in Raman, as well as residual S1 and S2 buffer layer components in XPS (Figure 2a) that suggests a mixture of regions of EGSi and QF-EGSi. Confirming the presence of mixed EGSi/QF-EGSi regions, Hall measurements indicate hydrogenation at 600 and 750 °C for 30 min results in a mixture of n- and p-type graphene within the same sample. Hall, Raman, and XPS thus strongly indicate that partial hydrogenation of epitaxial graphene leads not only to an intermediate degradation of structure (Figure 2b) but also to significant degradation of transport properties in EGSi (Figure 2e). Once total buffer elimination is achieved, all Hall structures exhibit p-type behavior, additionally there is a 200 300% increase in carrier mobility from 700 to 900 cm2/(V s) to an average of 2050 cm2/(V s) when hydrogenated at 1050 °C for 60 min (Figure 2e). In addition to enhancing transport, the carrier concentration of the hydrogenated graphene increases by nearly 200% to 1.3 1013 cm2/(V s). While the absolute carrier concentration is approximately 4 1012 cm 2 higher than previously reported, we note that Hall measurements presented in this work are accomplished in air. Measurement of a subset of the same hall structures under vacuum (10 8 Torr) results in an increase in carrier mobility to an average of 2375 cm2/(V s) and reduction of carrier concentration to 8.0 ((0.5) 1012 cm 2, indicating environmental doping can significantly impact transport.28 Additionally, graphene grown for our work is single layer plus buffer. Upon hydrogenation, this system is transformed into bilayer graphene.16 Therefore, the combination of environmental doping and bilayer graphene results in higher carrier concentration and lower carrier mobility, than reported by Speck et al.20 even though our graphene appears less defective as measured via Raman spectroscopy. The combination of large area, high 3877
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Figure 3. Hydrogenation of EGSi to form QF-EGSi results in high quality graphene-based transistors that exhibit a dirac point near Vg = 0 (a). Additionally, the process of hydrogenation results in a 200% increase in saturation current and transconductance of graphene transistors (b). While there is significant reduction in graphene sheet resistance (Figure 2d), short channel effects still persist as evidenced by a reduction in charge modulation when the gate length is reduced from 750 nm (c) to 75 nm (d). Finally, hydrogenation of EGSi is a scalable process that improves saturation current (e), yields gm,max values of 100 mS/mm (f), significantly enhances the magnitude of gm,min values to >300mS/mm (g), and provides easy access to the Dirac point (h) for ambipolar applications. Legend is equivalent for (c) and (d).
mobility, and low sheet resistance opens up graphene to not only high speed applications, but also to high current density applications. Graphene transistors utilizing EGSi significantly benefit from hydrogenation. Transistors utilizing as-grown EGSi exhibit on/ off ratios of 3 5, saturation currents (Idss) of 800 1000 mA/mm, and a maximum transconductance (gm) of 150 200 mS/mm.26 While these values are quite reasonable for epitaxial graphene transistors,5,31,32 the use of high-k dielectrics deposited via ALD often results in an inability to access the dirac point in Id/Vg curves,5,26 ultimately limiting the transistors performance where ambipolar applications are of interest. The process of hydrogenation results in a significant shift of the dirac point from Vg < 3 V to near Vg = 0 V (Figure 3a). This is thought to be the result of combining highly p-type graphene (Figure 2e) and the n-type doping that occurs from the dielectric overlayer,26 which ultimately yields a minimum conduction point near zero volts for graphene FETs utilizing a 1 μm gate. Additionally, we find asymmetric behavior in the Ids/Vg curves (Figure 3a). This phenomenon is well documented in the literature,5,31,33,34 and can be related to Klein tunneling at the channel/metal (source and drain) interface since it is known that the metal contacts dope the graphene n-type.35 While hydrogenation leads to top-gated graphene transistors with a dirac point near zero volts, it also significantly improves transistor performance. Considering gm = μn,p*Cox*(Wg/Lg), where Cox, Wg, and Lg is the oxide capacitance, gate length, and gate width, respectively, one expects that as carrier mobility increases, gm will increase. This is evidenced in Figure 3b, where we find Idss and the magnitude of maximum gm increases by >200% compared to as-grown graphene. Transistor saturation current increases from an average of 775 to 1400 mA/mm with minimum conduction increasing from 200 to 400 mA/mm. Transconductance values also increase significantly from ∼180 to >400 mS/mm (Figure 3b) at Vg = 1 V, providing direct
evidence that elimination of the buffer between graphene and SiC(0001) is beneficial not only to carrier transport, but also transistor characteristics. We note that improved transistor performance is evident across full-scale 100 mm wafers. Figure 3e h provides wafer maps of saturation current (e), maximum transconductance (gm,max; Figure 3f), minimum transconductance (gm,min; Figure 3g), and Dirac point (h). Here we define gm,max and gm,min as the peak gm values for the n-branch and p-branch of the Id versus Vg curve (Figure 3a), respectively. Evident in the maps is a slight nonuniformity of FET performance from top left to bottom right on the wafer with superior performance of the graphene transistors (evidenced by enhanced gm,min in Figure 3g) located in the bottom right. While we find variation in FET performance, we note that each of the >1000 evaluated transistors, out perform their nonhydrogenated counter parts in all categories. Hydrogenated graphene transistors still suffer from “short channel” effects. This is evidenced in Figure 3c,d, where we have plotted Id versus Vd for a 750 and 75 nm gate length device with source drain spacing of 750 nm. Similar to previous reports,4 we find that as the gate length is reduced, there is a loss of current modulation in the graphene channel. This phenomenon may be attributed to an increasing role that contact resistance plays in highly scaled devices,4,36,37 or the increased source and drain resistance contribution that results from ungated graphene (Figure 3d).38 In this work, the source drain spacing is fixed while the gate length is reduced by 10, thus we would not expect that contact resistance is responsible for the loss of gate modulation when scaling of the FET gate. As a result, current devices (with scaled gates) will benefit by reducing the source/ drain spacing or going to a self-aligned process.37 In addition to enhanced DC performance, the frequency response of EGSi-based transistors benefits from the process of hydrogen intercalation and buffer elimination. Previous works have shown graphene-based intrinsic-fT values as high as 3878
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improves extrinsic-fT from approximately 2.3 GHz to an average of 7.3 GHz at Lg = 1 um (Figure 4a). This represents a 2.5 improvement in extrinsic-fT without any change in transistor design or fabrication process. Therefore it is reasonable to conclude that the improved frequency response, at least to the first order, is attributed to the elimination of the buffer at the graphene/SiC(0001) interface, a known source of carrier scattering.15 Upon scaling the gate from 1 μm to 250 nm and 125 nm, the extrinsic-fT increases from 7.3 to 19.1 and 23.9 GHz, respectively (Figure 4b). This represents a 3.5 increase in extrinsic-fT for an 8 reduction in Lg, constituting an ∼1/(2Lg) behavior. However, while our work marks a significant improvement in graphene transistor technology, the 2 deviation from 1/Lg indicates that significant work must continue to reduce access resistance, improve the oxide/graphene interface, and enhance gate/channel electrostatic coupling to close the gap between reported intrinsic-fT and measured extrinsic-fT before graphene transistors can truly be considered for ultrahigh frequency operation. We have presented research that directly shows the importance of introducing a hydrogen intercalation step following epitaxial graphene synthesis on SiC(0001). Hydrogenation results in the elimination of the graphene/SiC(0001) buffer layer and provides a means for significant improvements in carrier mobility from approximately 800 cm2/(V s) to >2000 cm2/(V s). Additionally, the process of hydrogenation results in a change in carrier type that offsets the n-type doping that occurs when utilizing atomic layer deposition for the formation of top-gate dielectrics and yields easily assessable dirac points in the graphene transistors. Furthermore, graphene transistor performance benefits significantly, with >200% increase in Idss and gm that ultimately translates into superior extrinsic current gain frequency performance.
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional information addressing environmental effects on Hall measurements of graphene. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Figure 4. Hydrogenation of EGSi results in significant improvements in the extrinsic frequency response of RF transistors (a), where RF transistors utilizing a 1μm gate length experiences a increase in extrinsic-fT from 2.3 to 7.3 GHz. Additionally, when scaling to gate lengths of 250 and 125 nm (b), there is again a significant increase in extrinsic-fT from 8 to 19.1 and 23.9 GHz, respectively. As a result, through the process of hydrogenation, there is significant improvement in the frequency response, which provides a route for superior extrinsic transistor performance.
300 GHz,3 with nearly ideal 1/Lg behavior;4 however, little attention has been given to the extrinsic (measured) current gain frequency response of graphene transistors. Typical extrinsic-fT values, including our own work on EGSi,26 range from 2 to 9 GHz,3,4,26,31,39,40 with one report as high as 20 GHz for 150 nm gate lengths.41 However, there appears to be no extrinsic-fT trending with 1/Lg. This absence of 1/Lg behavior is likely the result of increased parasitic resistances that arise from significant contact resistance and regions of ungated graphene in the channel.3,4,37 Using identical device fabrication processes as our previous work,26 we find that the hydrogenation process
Corresponding Author
*E-mail:
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’ ACKNOWLEDGMENT The authors would like to thank H. Madan and S. Datta for fruitful discussions on RF devices. This work was supported by the Naval Surface Warfare Center Crane, Contract No. N0016409-C-GR34. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the Naval Surface Warfare Center Crane Division. Support for the Cambridge ALD System, WiteC Raman system, and JEOL 2010F TEM was provided by the National Nanotechnology Infrastructure Network at Penn State. ’ REFERENCES (1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183–191. (2) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666–669. 3879
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