Charge carrier transport asymmetry in monolayer graphene

Abstract

The conductivity and Hall effect were measured in CVD-grown monolayer graphene as a function of the gate voltage, $V_{g}$, at temperatures down to $T$ = 2 K and in magnetic fields up to $B$ = 8 T. The minimal conductivity was observed at positive $V_{g}$ which shows the position of the charge neutrality point, $V_{NP}$. With decreasing $T$, $V_{NP}$ first decreases, but stop to decrease at low $T$. Hysteresis of conductivity shows similar behavior: it decreases with decreasing $T$ and disappears at low $T$. A significant asymmetry was observed at low density of charge carriers $|n|=(n,p)$: mobility of holes was less than mobility of electrons. The asymmetry decreases with increasing $|n|$. It was observed that the value of $|n|$ determined from the Hall effect is less than the full value induced by $V_{g}$. In strong perpendicular $B$, Shubnikov-de Haas (SdH) oscillations were observed in the longitudinal conductivity, $\sigma_{xx}$, together with half-integer quantum Hall plateaus. It was found that $|n|$ determined from SdH oscillations is equal to the full value induced by $V_{g}$ as opposed to the Hall effect. Explanatory models for all observed phenomena are discussed.