Transport Spectroscopy of Ultraclean Tunable Band Gaps in Bilayer Graphene

The importance of controlling both the charge carrier density and the band gap of a semiconductor cannot be overstated, as it opens the doors to a wide range of applications, including, for example, highly‐tunable transistors, photodetectors, and lasers. Bernal‐stacked bilayer graphene is a unique van‐der‐Waals material that allows tuning of the band gap by an out‐of‐plane electric field. Although the first evidence of the tunable gap is already found 10 years ago, it took until recent to fabricate sufficiently clean heterostructures where the electrically induced gap can be used to fully suppress transport or confine charge carriers. Here, a detailed study of the tunable band gap in gated bilayer graphene characterized by temperature‐activated transport and finite‐bias spectroscopy measurements is presented. The latter method allows comparing different gate materials and device technologies, which directly affects the disorder potential in bilayer graphene. It is shown that graphite‐gated bilayer graphene exhibits extremely low disorder and as good as no subgap states resulting in ultraclean tunable band gaps up to 120 meV. The size of the band gaps are in good agreement with theory and allow complete current suppression making a wide range of semiconductor applications possible. The tunable band gap of bilayer graphene (BLG) is investigated using finite bias transport spectroscopy. This technique enables the study of disorder‐induced subgap states. By comparing different gating technologies, it is found that only graphite‐gated BLG devices do not suffer from subgap states, leading to ultraclean tunable band gaps of up to 120 meV and demonstrating a truly semiconducting behavior.