Interface States in Bilayer Graphene Encapsulated by Hexagonal Boron Nitride

The threshold voltages at the onset of conduction for electron and hole branches can provide information on band gap values or interface states in a gap. We measured conductivity of bilayer graphene encapsulated by hexagonal boron nitride as a function of back and top gates, where another bilayer graphene is used as a top gate. From the measured conductivity the transport gap values were extracted assuming zero interface trap states, and they are close to the theoretically expected gap values. From a little discrepancy an average density of interface states per energy within a band gap ( D_it) is also estimated. The data clearly show that D_it decreases as a bilayer graphene band gap increases rather than being constant. Despite the decreasing trend of D_it, interestingly the total interface states within a gap increases linearly as a band gap increases. This is because of ∼2 × 10¹⁰ cm^-2 interface states localized at band edges even without a band gap, and other gap states are equally spread over the gap.