Effect of stopping-layer-assisted boron-ion implantation on the electrical properties of graphene: Interplay between strain and charge doping

We report a new method of ion implantation for hole doping of graphene in which a layer of polymethyl methacrylate (PMMA) is used as a stopping layer to control the B-ion distribution in the graphene layer. This method is very useful for graphene doping in many aspects because it employs the ion energy comparable to what is commercially used in the semiconductor processes and strongly enhances the doping effect in contrast to the previous studies, resulting from B-ions-induced gating effect. PMMA/graphene/Cu-foil stacks were implanted with 35 keV B⁻ ions to nominal fluences (ϕ_B) of 0.5–50 × 10¹⁰ cm⁻² at room temperature. The electron/hole mobilities are sharply reduced by doping at ϕ_B = 0.5 × 10¹⁰ cm⁻², but above this, they increase with increasing ϕ_B, as estimated from the Dirac curves. The Raman data and theoretical considerations suggest that the electrical properties of the B-doped graphene are governed by strain effect at low ϕ_B, but by charge-doping effect at high ϕ_B.