Klein tunneling, contact doping, and pn-interfaces in gated suspended graphene

Abstract

The inherent asymmetry of the electric transport in graphene is attributed to Klein tunneling across barriers defined by pn-interfaces between positively and negatively charged regions. By combining conductance and shot noise experiments we determine the structure of the tunneling barrier in a high-quality suspended sample with Au/Cr/Au contacts. We observe an asymmetric resistance Rodd=100−70 Ω across the Dirac point at carrier density |n|=0.3−4⋅1011 cm−2, while the Fano factor displays a non-monotonic asymmetry in the range Fodd∼0.03−0.1. Our findings agree with Klein tunneling calculations based on the Dirac equation with a trapezoidal barrier. Comparison between the model and the data yields the barrier height for tunneling, an estimate of the thickness of the interface d<20 nm, and the contact region doping corresponding to a Fermi level offset of ∼−18 meV. Strong pinning of the Fermi level under the metallic contact is observed due to large contact capacitance.