Thermionic Cascade in Graphene-Boron Nitride Heterostructures

We investigate the inter- and intra-layer photocurrent of a single pixel consisting of two graphene layers separated by an ultrathin hexagonal boron nitride barrier. The basic device response displays an interlayer photocurrent which increases super-linearly with excitation power and has a voltage-tunable time constant (~1 ps) that increases linearly with the inverse interlayer voltage. This is consistent with the thermally driven drift of hot electronic charge carriers across the interlayer barrier. In contrast to this ordinary behavior, we also observed an extraordinarily strong enhancement of the interlayer photoconductance near the Dirac point. This Dirac point photoconductance is sensitive to even a weak intralayer excitation voltage and can be strongly quenched at temperatures near 70K and below. This photoconductance can be further suppressed into negative differential photoconductance as the intralayer excitation voltage increases. We attribute this remarkable behavior to ultrafast thermionic cascade triggered by enhanced electronic temperatures near the Dirac point. Furthermore, its efficient quenching achieved through intralayer excitation voltage indicates non-conventional and efficient cooling pathways that may serve as a hallmark of Dirac fluid in graphene.