Abstract
Correlations between the level of p-doping exhibited in large area chemical vapour deposition (CVD) graphene field effect transistor structures (gFETs) and residual charges created by a variety of surface treatments to the silicon dioxide (SiO2) substrates prior to CVD graphene transfer are measured. Beginning with graphene on untreated thermal oxidised silicon, a minimum conductivity (σmin) occurring at gate voltage Vg = 15 V (Dirac Point) is measured. It was found that more aggressive treatments (O2 plasma and UV Ozone treatments) further increase the gate voltage of the Dirac point up to 65 V, corresponding to a significant increase of the level of p-doping displayed in the graphene. An electrowetting model describing the measured relationship between the contact angle (θ) of a water droplet applied to the treated substrate/graphene surface and an effective gate voltage from a surface charge density is proposed to describe biasing of Vg at σmin and was found to fit the measurements with multiplication of a correction factor, allowing effective non-destructive approximation of substrate added charge carrier density using contact angle measurements.
Highlights
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 graphene field effect transistor structures (gFETs) device
The average contact angle of the ing garmapohreenhey(dθrLo+2pθhRili=c sθu,rsfaeceeF. iTgh. 4e inset) effect ranges from 43°–92°, with of monolayer graphene on more aggressive substrate treatments creatthe contact angle compared to the underlying substrate can usually be ignored because it is transparent to wetting effects in most cases[8]
We found that gFET substrates treated with ultra-pure water sonication have a significant contact angle difference with measurements made on the graphene being more hydrophobic than bare treated SiO2, with an increased contact angle of ≈ 15°
Summary
We report that the aggressiveness of each surface treatment induces a greater positive biasing of VD and 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. 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
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