Tunable graphene doping by modulating the nanopore geometry on a SiO₂/Si substrate

A tunable graphene doping method utilizing a SiO₂/Si substrate with nanopores (NP) was introduced. Laser interference lithography (LIL) using a He-Cd laser (λ = 325 nm) was used to prepare pore size- and pitch-controllable NP SiO₂/Si substrates. Then, bottom-contact graphene field effect transistors (G-FETs) were fabricated on the NP SiO₂/Si substrate to measure the transfer curves. The graphene transferred onto the NP SiO₂/Si substrate showed relatively n-doped behavior compared to the graphene transferred onto a flat SiO₂/Si substrate, as evidenced by the blue-shift of the 2D peak position (∼2700 cm^-1) in the Raman spectra due to contact doping. As the porosity increased within the substrate, the Dirac voltage shifted to a more positive or negative value, depending on the initial doping type (p- or n-type, respectively) of the contact doping. The Dirac voltage shifts with porosity were ascribed mainly to the compensation for the reduced capacitance owing to the SiO₂-air hetero-structured dielectric layer within the periodically aligned nanopores capped by the suspended graphene (electrostatic doping). The hysteresis (Dirac voltage difference during the forward and backward scans) was reduced when utilizing an NP SiO₂/Si substrate with smaller pores and/or a low porosity because fewer H₂O or O₂ molecules could be trapped inside the smaller pores.