Correlation of p-doping in CVD Graphene with Substrate Surface Charges

Since its discovery in 2004 by A. K. Geim and K. Novoselov1, graphene has become one of the most promising materials for future micro- and nano-electronic devices such as field effect transistors (gFETs), gas sensors, ultra-capacitors and many other electronic applications where optical transparency, high mobility, and tuneability play crucial roles. Importantly, it brings promise of scaling FETs in accordance with Moore’s law2 without encountering performance degradation and short channel effects that are seen with Si devices at similar geometries3,4. However, the theoretical performance of graphene is not yet achieved in real materials prepared by wafer scalable deposition techniques such as CVD. The ability to locally modulate the charge carrier density of graphene has enabled the creation of gFET devices. Theoretically the ambipolar field effect in gFETs exhibits a symmetric drain-source current voltage characteristic (I–V curve) about the Dirac point (the point of minimum conductivity σmin) which resides at zero applied gate voltage (Vg = 0). However, real world devices almost always have an intrinsic hole concentration, biasing Vg at the Dirac point to a finite positive value. It is well known that the carrier concentration in graphene is strongly affected by adsorbates in contact with graphene and this is considered to be the most likely cause of the intrinsic carrier density bias. For brevity Vg at σmin is referred to as VD from here onwards. In this study we investigate how typical SiO2 cleaning techniques for graphene devices affects graphene’s transport properties by measuring VD in surface treated gFET devices. We report that the aggressiveness of each surface treatment induces a greater positive biasing of VD and hence a greater hole carrier concentration in graphene. The cause of this effect is attributed to varying surface charge densities trapped within/near the graphene-dielectric interface induced by each treatment. We reason that a charge density residing in the graphene/substrate interface alters the hydrophobicity of the graphene. Using no applied gate voltage we scale the degree of induced surface charge density by measuring the contact angles (θ) of a water droplet placed on the graphene and the bare substrate of each treated gFET device. This finding quantifies substrate treatment as a subtle and often overlooked source of doping in graphene. We understandably go on to report on a correlation between θ and VD in graphene and present an accompanying electrowetting model in the hope of developing a facile technique for quick estimation of the p-doping level.