Shot noise and conductivity at high bias in bilayer graphene: Signatures of electron-optical phonon coupling

Abstract

We have studied electronic conductivity and shot noise of bilayer graphene (BLG) sheets at high bias voltages and low bath temperature ${T}_{0}=4.2$ K. As a function of bias, we find initially an increase of the differential conductivity, which we attribute to self-heating. At higher bias, the conductivity saturates and even decreases due to backscattering from optical phonons. The electron-phonon interactions are also responsible for the decay of the Fano factor at bias voltages $V>0.1$ V. The high bias electronic temperature has been calculated from shot-noise measurements, and it goes up to $\ensuremath{\sim}$1200 K at $V=0.75$ V. Using the theoretical temperature dependence of BLG conductivity, we extract an effective electron-optical phonon scattering time ${\ensuremath{\tau}}_{\mathrm{e}--\mathrm{op}}$. In a 230-nm-long BLG sample of mobility $\ensuremath{\mu}=3600$ cm${}^{2}\phantom{\rule{0.16em}{0ex}}$V${}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}$s${}^{\ensuremath{-}1}$, we find that ${\ensuremath{\tau}}_{\mathrm{e}--\mathrm{op}}$ decreases with increasing voltage and is close to the charged impurity scattering time ${\ensuremath{\tau}}_{\mathrm{imp}}=60$ fs at $V=0.6$ V.