Gap state analysis in electric-field-induced band gap for bilayer graphene

TL;DR

This study systematically estimates the energy gap and gap state density in bilayer graphene under electric fields, revealing limitations due to gap states and potential for improved nanoelectronic devices.

Contribution

It provides the first quantitative estimation of gap states in bilayer graphene using quantum capacitance and transport measurements.

Findings

Energy gap of ~250 meV at high displacement field.

Gap state density ranges from 5×10^12 to 10^13 eV^-1cm^-2.

Current on/off ratio of ~3000 at 20 K.

Abstract

The origin of the low current on/off ratio at room temperature in dual-gated bilayer graphene field-effect transistors is considered to be the variable range hopping in gap states. However, the quantitative estimation of gap states has not been conducted. Here, we report the systematic estimation of the energy gap by both quantum capacitance and transport measurements and the density of states for gap states by the conductance method. An energy gap of ~250 meV is obtained at the maximum displacement field of ~3.1 V/nm, where the current on/off ratio of ~3*10^3 is demonstrated at 20 K. The density of states for the gap states are in the range from the latter half of 10^12 to 10^13 eV^-1cm^-2. Although the large amount of gap states at the interface of high-k oxide/bilayer graphene limits the current on/off ratio at present, our results suggest that the reduction of gap states below ~10^11 eV^-1cm^-2 by continual improvement of the gate stack makes bilayer graphene a promising candidate for future nanoelectronic device applications.