| AVS 56th International Symposium & Exhibition | |
| Graphene Topical Conference | Tuesday Sessions |
| Session GR+EM+MS-TuM |
| Session: | Graphene and Carbon-based Electronics |
| Presenter: | D.K. Gaskill, U.S. Naval Research Laboratory |
| Authors: | D.K. Gaskill, U.S. Naval Research Laboratory J. Moon, HRL Laboratories, LLC J.L. Tedesco, U.S. Naval Research Laboratory J.A. Robinson, The Pennsylvania State University A.L. Friedman, U.S. Naval Research Laboratory P.M. Campbell, U.S. Naval Research Laboratory G.G. Jernigan, U.S. Naval Research Laboratory J.K. Hite, U.S. Naval Research Laboratory R.L. Myers-Ward, U.S. Naval Research Laboratory C.R. Eddy, Jr., U.S. Naval Research Laboratory M.A. Fanton, The Pennsylvania State University |
| Correspondent: | Click to Email |
Recently, the world’s first graphene RF field effect transistors (FETs) have been fabricated using photolithography on epitaxial graphene (EG) grown on 50 mm SiC semi-insulating wafers. The RF FETs had fmax of 14 GHz at 5 Vds for 2 μm gate widths and results are expected to improve as gate widths are scaled down. Continued research addressing key materials issues is needed to push the performance metrics for devices fabricated on wafer-scale EG significantly higher. Some of these key materials issues are morphology and thickness control, enhanced mobility, uniformity of sheet carrier density and resistivity, and optimizing atomic layer deposition (ALD) of high dielectric constant oxides on EG. In this presentation, we will present details on our approach using Si sublimation from SiC substrates for growing EG on 50.8 and 76.2 mm diameter SiC wafers and discuss the impact of key material issues on RF device performance.
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Epitaxial graphene was synthesized using a commercial Aixtron VP508 SiC epitaxial growth reactor on the Si- and C-faces of 4H- and 6H-SiC semi-insulating 0° oriented substrates at temperatures from approximately 1225 to 1700°C and for times ranging from 10 to 300 min. Substrates were 16 x 16 mm2 coupons and 50.8 and 76.2 mm diameter wafers. Both in-vacuo (10-6 to 10-4 mbar) and Ar ambient (50-200 mbar) Si sublimation synthesis conditions were investigated. Dielectrics were deposited using a Cambridge NanoTech Savannah 200 ALD system. EG was characterized by a wide array of tools including atomic force, Nomarksi and scanning tunneling microscopies, Raman spectroscopy, Hall effect, and Lehighton contactless resistivity and mobility wafer probe.
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The growth of EG on 50.8 mm Si-face wafers using the in-vacuo process was optimized and excellent relative resistivity uniformity of 2.8% and record 300 K Hall mobilities up to 2700 cm2V-1s-1 were found. Raman spectroscopy mapping of the 2D peak on the wafers determined: (1) the majority of the film was monolayer EG, (2) two layers of EG could be found at step edges and (3) the EG was continuous across the wafer. RF FETs fabricated using the latest optimized wafer-scale EG exhibited state-of-the-art ambipolar behavior, Ion/Ioff ratios and peak transconductances. Frequency performance metrics were established for these devices such as fT●Lg products of 10 GHz●μm and fmax of 14 GHz. To our knowledge, the highest field effect mobilities reported to-date have been measured using these devices. We will discuss the impact of key material parameters associated with EG for these record results as well as the impact of Ar ambient controlled graphenization on future RF devices.