| About this Abstract |
| Meeting |
2011 Electronic Materials Conference
|
| Symposium
|
2011 Electronic Materials Conference
|
| Presentation Title |
CC5, Surface Adsorption and Charge Transport in Epitaxial Graphene on 6H-SiC |
| Author(s) |
Shamaita Shithi Shetu, MWK Nomani, Goutam Koley, MVS Chandrashekhar |
| On-Site Speaker (Planned) |
Shamaita Shithi Shetu |
| Abstract Scope |
Graphene has captured the attention of researchers as a prospective candidate for gas and vapor sensors with its remarkable properties including high electron mobility, high thermal conductivity and stiffness, room temperature quantum hall effect and ballistic transport. Our goal here is to obtain an understanding of charge carrier transport in sensing and reconcile the discrepancy between earlier measurements in [1]. Surface work function (SWF) change and conductance(%)change was observed due to adsorption of NO<sub>2</sub> and NH<sub>3</sub> in graphene layers on 6-H SiC. A reduction in SWF for NO<sub>2</sub> implies NO<sub>2</sub> dopant behaving as n-type while an increment in SWF implies NH<sub>3</sub> as p-type. This was opposite to the previous observations made on graphene on SiO<sub>2</sub> and free-standing graphite layers on SiO<sub>2</sub> indicating that the electron transfer behavior upon adsorption depends on the substrate. Another inconsistency was that conductance(%)change decreased with impurity concentration, even with increasing carrier concentration for NO<sub>2</sub> while it was opposite for NH<sub>3</sub> .These inconsistencies were reconciled using Boltzmann transport, by accounting for the influence of scattering from the adsorbed impurities. Graphene conductivity has been influenced by long range coulomb scattering from the screening of charged impurities on the surface of graphene and short range scattering arising due to electron-electron interaction and defects or dislocations [2]. Experimentally, it has been observed that long-range coulomb scattering from surface impurities dominates. We have interpreted total impurity density as n<sub>imp</sub>=n(NO<sub>2</sub> or NH<sub>3</sub>)+K where n(NO<sub>2</sub>or NH<sub>3</sub>) is the impurity concentration due to adsorption and K is the residual impurity density. Conductivity is calculated by considering both scattering mechanisms using the formulas in [2]. The carrier concentration was calculated from the measured SWF. We found excellent agreement with the experimental results approximating a residual impurity density ~4-5x10<sup>12</sup>/cm<sup>2</sup> and thickness of about 8-10ML. This is higher than that observed in exfoliated graphene ~10<sup>11</sup>/cm<sup>2</sup> without intentional impurities introduced. This discrepancy is currently not understood and is being investigated. For NO<sub>2</sub> adsorption SWF reduction implies electron donation by NO<sub>2</sub> to graphene layer and conductivity is reduced and the opposite for NH<sub>3</sub> adsorption. Finally, only the top ML of graphene was assumed to be modulated by the adsorbate impurities. This is due to 2 reasons: a) the electrostatic screening length in graphene is ~1ML and b) due to graphene’s small lattice constant, no molecules can diffuse through the layers. Thus, for a 10ML film, the effective %change in conductance is reduced by 10x compared to that for a single ML. In summary, residual impurity density and the thickness of layers play a significant role in charge carrier transport in graphene. |
| Proceedings Inclusion? |
Undecided |