Improved High Frequency Performance and Scaling Effects of Graphene Transistors on Diamond-like Carbon

Graphene has attracted significant attention in recent years as a result of its high carrier mobility and saturation velocity, along with the ability to synthesize large-scale, high quality graphene sheets utilizing low-cost chemical vapor deposition (CVD) methods. Recently, Wu et.al. from IBM investigated the use of a diamond-like carbon (DLC) film grown on SiO2 as a substrate for transferring the graphene films.

Reviewed by Jeff Morse, PhD, National Nanomanufacturing Network

• Wu Y, Lin Y, Bol AA, Jenkins KA, Xia F, Farmer DB, Zhu Y, Avouris P. 2011. High-frequency, Scaled Graphene Transistors on Diamond-like Carbon. Nature 472:74-78. doi:10.1038/nature09979

Graphene has attracted significant attention in recent years as a result of its high carrier mobility and saturation velocity, along with the ability to synthesize large-scale, high quality graphene sheets utilizing low-cost chemical vapor deposition (CVD) methods. Since graphene does not have a bandgap, a remaining drawback for transistor switching applications is the low On/Off ratio. While bandgap engineering techniques developed to address this issue have proven successful, challenges remain to create a sizable electronic bandgap without degrading the electronic transport properties of the material. In contrast, many radio frequency (RF) applications, such as amplifiers or mixers, do not require large On/Off ratios. In order to achieve their potential for commercialization, graphene transistor and integrated circuit designs for such high frequency applications must exploit low-cost, scalable graphene synthesis approaches. While high frequency RF transistors have been realized using exfoliated graphene or silicon carbide substrates, these approaches are not scalable. Transistors fabricated from graphene grown by CVD on copper (Cu) substrates then transferred to silicon dioxide (SiO₂) substrates perform less favorably due to enhanced scattering by phonons and defects at the SiO₂ surface.

Recently, Wu et.al. from IBM investigated the use of a diamond-like carbon (DLC) film grown on SiO₂ as a substrate for transferring the graphene films. DLC has the benefits of higher phonon energy and lower surface trap density, and therefore should have less impact on the electronic transport properties of the graphene. Using this approach, the authors further reported on the scaling of graphene transistor gate length and its impact on RF performance. Single layer graphene was first grown on a Cu foil at 1000°C, then lifted from the Cu using a polymethylmethacrylate (PMMA) coating as a protective carrier while dissolving the Cu substrate. The graphene was then transferred to the DLC substrate and the PMMA coating was removed in acetone. Raman spectroscopy measurements confirmed the single layer structure and uniformity of the graphene after the transfer process. Standard top-down processing was utilized to fabricate the high-frequency graphene RF transistors. Electron beam lithography was used to pattern the source and drain contacts, along with the gate electrodes. Plasma etching was used to remove graphene outside the channel region, and the gate dielectric was Al₂O₃ formed in two-step process by first oxidizing an electron beam deposited aluminum film, followed by  an additional layer of Al₂O₃ grown by atomic layer deposition (ALD). Arrays of transistors were fabricated having gate lengths of 550 nm, 140 nm, and 40 nm, with the 40 nm devices representing the shortest gate length devices demonstrated to date.

Measurement of transistor current-voltage (I-V) characteristics demonstrated excellent transconductance for drain voltages V_d=0-1.5 V and gate voltage V_g=0 to -8 V. The gate voltage modulation of the short gate length devices was much less than the longer gate devices, suggesting that contact resistance dominated the I-V characteristics. At this point, contact resistance for graphene transistors remains a key challenge. High frequency scattering parameters were used to measure the cut-off frequency (ft) of the devices, the frequency at which the current gain becomes unity. Transistors having gate length L_g=550 nm exhibited f_t=26 GHz, whereas the devices having L_g=40 nm had f_t=155 GHz. Most notably, the ft did not exhibit the typical inverse dependence on gate length (1/L_g) for the L_g=40 nm devices. Thus for the short gate length transistors, high-frequency characteristics are dominated by device contact resistance. Further optimization of RF design through minimizing gate capacitance and contact resistance, and increasing gate voltage modulation of the I-V characteristics will result in ultimate device performance.

Figure reprinted by permission from Macmillan Publishers Ltd: Wu Y, Lin Y, Bol AA, Jenkins KA, Xia F, Farmer DB, Zhu Y, Avouris P. 2011. High-frequency, Scaled Graphene Transistors on Diamond-like Carbon. Nature 472:74-78. doi:10.1038/nature09979