(Invited) Quantum Capacitance Measurement of Bilayer Graphene

The ability to electrostatically tune the band gap in bilayer graphene attracts great attention. Although the theory predicts the band gap of ~250meV, the off-current is considerably reduced only at the temperature lower than 1 mK and generally high at room temperature. This is explained by the variable range hopping in gap states. However, there should be intrinsically no interface states in graphene because there is no dangling bonds on the basal plane. The origin for the gap states is still open question. Moreover, in the actual device operation, the high on-current as well as the low off-current are crucial. There are four sub-bands in bilayer graphene; two in the conduction bands and two in the valence bands. However, the contribution of the sub-bands to the electrical transport for on-current is quite limited due to the unreliable quality of the high-ktop gate insulator on bilayer graphene.

Recently, we demonstrate the ultra-high displacement of ~8 V/nm (n=~4×10¹³ cm^-2) in bilayer graphene using the solid state Y₂O₃ top gate [1], which has been reached only by the ion gating so far. The ultra-high displacement provides the access to both the carrier response issue in the largely-opened band gap and the inter-band scattering issue at the high carrier density. In this study, we focus on the quantum capacitance (C_Q) measurements [2] for bilayer graphene because the density of states (DOS) can be extracted through C_Q=e²DOSand the scattering issues can be excluded. The frequency dispersion in C-V curve reveals that the carriers in bilayer graphene electrically communicate with trap sites within the band gap. The local breakdown of A-B stacking, which results in the local conduction sites, might be the origin for the gap states. Moreover, the systematic comparison of I-V and C-V curves reveals that the filling of carriers in the high energy sub-bands results in the reduction of the conductivity due to the inter-band scattering.

[1] K. Nagashio, K. Kanayama, T. Nishimura, A. Toriumi, IEDM Tech. 2013.

[2] K. Nagashio, T. Nishimura, A. Toriumi, APL, 2013, 102, 173507.