Direct measurement of electric-field-screening length in thin graphite film

Electric-field-screening length in thin graphite film has been clarified by means of dual gating method. Sandwich type device structure which two gate electrodes are situated over and beneath a graphite film was constructed with Al top electrode. The Al electrode naturally generates thin gate insulator at graphite/Al interface, which enables extremely low voltage operation. Ambipolar charge conduction in a graphite film can be tuned by both top and back gate voltages. A scan of the top gate voltage ($V_{tg})$ generates a resistance peak in the ambiploar response. The back gate voltage ($V_{bg})$ shifts the ambipolar peak depending on the graphite thickness. The shift is larger in thinner film. The thickness-dependent peak shift is clarified in terms of the inter-layer screening length $\lambda $ to the electric field in the dual-gated graphite film. We assume that the gate-induced carriers decay exponentially from both surfaces, and that the conductivity in each layer increases proportionally to the induced carrier density. Then the condition for the ambipolar resistance peak in $V_{tg}$ scan is obtained as a function of $V_{bg}$, $\lambda $, and the graphite film thickness $d$. Applying this model to the thickness-dependence, we obtained a screening length of 1.2 nm experimentally.