AVS 58th Annual International Symposium and Exhibition
    Graphene and Related Materials Focus Topic Tuesday Sessions
       Session GR-TuA

Paper GR-TuA8
Fluorine Functionalization of Epitaxial Graphene for Uniform Deposition of Ultrathin High-κ Dielectrics

Tuesday, November 1, 2011, 4:20 pm, Room 209

Session: Graphene on Dielectrics, Graphene Transfer to Novel Substrates
Presenter: Virginia Wheeler, U.S. Naval Research Laboratory
Authors: V.D. Wheeler, U.S. Naval Research Laboratory
N.Y. Garces, U.S. Naval Research Laboratory
L.O. Nyakiti, U.S. Naval Research Laboratory
R.L. Myers-Ward, U.S. Naval Research Laboratory
J. Culbertson, U.S. Naval Research Laboratory
C.R. Eddy Jr., U.S. Naval Research Laboratory
D.K. Gaskill, U.S. Naval Research Laboratory
Correspondent: Click to Email
Thermal atomic layer deposition (ALD) is a viable approach to attain high-quality ultrathin dielectric films needed for graphene devices, but the hydrophobic nature of the graphene surface inhibits direct application of thermal ALD oxides. Several methods have been explored to render the surface more susceptible to ALD[1-3], but these techniques often result in graphene mobility degradation and/or shifts in the Dirac voltage due to charge in the gate stack. In this work, we investigated a simple dry chemical approach using XeF2to functionalize the graphene surface prior to ALD which results in conformal ultrathin high-κ oxides without degradation of the underlying graphene electrical properties. Epitaxial graphene samples were grown on semi-insulating, on-axis (0001) 6H-SiC substrates using an Aixtron VP508 SiC reactor at 1650°C for 120 min. Fluorination of the graphene was performed in a Xactix X3 etcher operating in pulse mode. Optimum fluorine exposure conditions consisted of six, 20s pulses with constant XeF2 and N2 carrier gas partial pressures of 1 and 35 torr, respectively. X-ray photoelectron spectroscopy (XPS) was used to chemically analyze the functionalized surface prior to oxide deposition. ALD Al2O3 and HfO2 films (≤ 15 nm) were deposited at temperatures between 150 - 225 °C using TMA or TEMAHf and deionized (DI) water precursors. Growth was initiated with 20 DI water pulses. Oxide coverage was characterized with atomic force microscopy and scanning electron microscopy, while graphene mobility changes were observed with van der Pauw Hall measurements. Capacitance-voltage (C-V) measurements were conducted on Ti/Au C-V dots to extract the dielectric constant and electrical quality of the oxide. Initial results show that 15 nm conformal, uniform Al2O3 and HfO2 films are obtained with an optimized XeF2 surface treatment prior to ALD. XPS showed that the optimum XeF2 treatment resulted in ~6% fluorine on the surface and the presence of only C-F bonds which provide ALD reaction sites needed for uniform oxide deposition. Graphene mobilities were maintained, and occasionally increased, implying little impact of the XeF2 treatment or ALD oxide on the underlying graphene properties. Raman spectroscopy reveals no change in the D/G ratio after XeF2 and oxide deposition, verifying that the graphene lattice quality is maintained. The viability of the fluorination method for achieving ultrathin films (<10 nm) will be presented along with electrical C-V data to show the electronic quality of the ALD oxides.

1. Robinson, et.al. ACSNano 4(5) 2667 (2010)

2. Farmer, et. al. Nano Letters 9(12) 4474 (2009)

3. Lee, et.al. ECS Transactions 19(5) 225 (2009)