| AVS 59th Annual International Symposium and Exhibition | |
| Graphene and Related Materials Focus Topic | Friday Sessions |
| Session GR+EM+ET+MS+NS-FrM |
| Session: | Graphene Device Physics and Applications |
| Presenter: | V.D. 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 D.J. Meyer, U.S. Naval Research Laboratory A. Nath, George Mason University C.R. Eddy, Jr., U.S. Naval Research Laboratory D.K. Gaskill, U.S. Naval Research Laboratory |
| Correspondent: | Click to Email |
Scalable high-κ dielectric integration is needed to realize graphene-based THz transistors. Yet, the inert nature of graphene inhibits direct application of high-quality uniform atomic layer deposition (ALD) dielectrics. While several methods have rendered the surface more susceptible to ALD[1], they often degrade mobility and/or shift the Dirac voltage due to charges within the gate stack. Recently, we developed a dry chemical functionalization approach using XeF2 that results in conformal, thin high-κ ALD oxide films with a 10-25% improvement in graphene mobility[2], high dielectric constants (HfO2=18.5, Al2O3=8.9), and small Dirac voltage shifts (HfO2=2V, Al2O3=0.1V), indicating the effectiveness of F functionalization. We will present in-depth details of our fluorination process, discuss its advantages and limitations with respect to other methods used to enhance ALD reactivity with graphene, and provide future directions for this field of study.
Fluorination of EG surfaces was performed in a Xactix X3 XeF2 etcher operating in pulse mode. Results show that 15 nm pinhole-free Al2O3 and HfO2 films are obtained with an optimized XeF2 surface treatment prior to ALD consisting of six, 20s pulses (XeF2=1 torr, N2=35 torr). This optimal treatment resulted in ~6% fluorine surface coverage, as semi-ionic C-F bonds (F1s ~687eV) only, which provided additional ALD reaction sites needed to obtain uniform oxide films. This unique semi-ionic nature of the C-F bond allows the graphene lattice to maintain planarity and minimize degradation to transport properties.
Theoretical studies suggest that the semi-ionic nature of the C-F bond is related to the graphene electron sheet carrier density (ne), requiring at least 1013 cm-2 to form[3]. To test this, EG samples with similar thickness but varying ne (2x1012-1.3x1013 cm-2) were fluorinated simultaneously using the optimal conditions above. Samples with ne >1x1013 cm-2 had only semi-ionic C-F bonding, but those with ne <1x1013 cm-2 had both covalent and semi-ionic bonding – verifying the theoretical calculations. The amount of covalent bonding increased as ne decreased, and an increased pinhole density was seen in subsequent Al2O3 films. This implies that the underlying EG properties can impact the effectiveness of this fluorination method. Yet, by adjusting the pulse conditions one can tailor this method to still obtain uniform ALD oxides on low carrier density and even p-type EG. To this end, results of our XeF2 approach on p-type H2 intercalated EG samples will be shown.
1. Garces,et.al. JVST B 30(3) 03D104 (2012)
2. Wheeler,et.al. Carbon50 2307 (2012)
3. Sofo,et.al. Phys Rev B 83(8) 081411(R) (2011)