Charge carrier transport asymmetry in monolayer graphene

TL;DR

This study investigates charge transport asymmetry in monolayer graphene, revealing differences in electron and hole mobilities, and discrepancies in carrier density measurements from Hall effect and SdH oscillations.

Contribution

It provides new insights into charge carrier asymmetry and measurement discrepancies in monolayer graphene at low temperatures and high magnetic fields.

Findings

Mobility of holes is less than that of electrons at low charge densities.

Carrier density from Hall effect is underestimated compared to SdH oscillations.

Charge neutrality point shifts and hysteresis diminish at low temperatures.

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.