Thin-film barristor: A gate-tunable vertical graphene-pentacene device

We fabricate a vertical thin-film barristor device consisting of highly doped silicon (gate), 300 nm SiO${}_2$ (gate dielectric), monolayer graphene, pentacene, and a gold top electrode. We show that the current across the device is modulated by the Fermi energy level of graphene, tuned with an external gate voltage. We interpret the device current within the thermionic emission theory, showing a modulation of the energy barrier between graphene and pentacene as large as 300 meV.

Figure 1

Band diagram for the graphene pentacene vertical device with zero gate voltage (a) and with a negative gate voltage (b) which decreases the Fermi energy of graphene, and because pentacene is a $p$-type semiconductor, reduces the energy barrier between graphene and pentacene. (c) Schematic of the device.

Figure 2

Current I through the graphite (a) and graphene (b) pentacene vertical device as a function of the bias voltage $V$ at different gate voltages $V_g$. Insets show the respective optical images of the devices before (left) and after (right) deposition of top electrode. The bar corresponds to 50 $\mu$m. The arrows indicate the window opened on a 150 nm thick HSQ layer (left) and the two electrodes used for transport (right).

Figure 3

Semilog plot of I/T${}^2$ as a function of 1/T for the graphene-pentacene current at different gate voltages. Current was measured at $V_{DC}=4$ V. Straight lines are fits to the Richardson-Dushman thermionic emission theory [Eq. (1)]. Fits are done in two different temperature ranges: 288–300 K (in blue) and 270–288 K (in red).

Figure 4

Energy barrier at different gate voltages for graphene extracted from the fits to the thermionic emission theory in Fig. 3 in the temperature range 270–288 K (a) (black squares) and in the temperature range 288–300 K (b). In (b) the barrier energy is shown for different bias voltages 4, 8, and 12 V. The continuous line in (a) corresponds to the modulation of the Fermi energy of graphene with the gate voltage assuming that the graphene neutrality point is at 50 V. The continuous lines in (b) correspond to the expected changes in energy barrier due to the Frenkel-Poole correction at different bias voltages (see text). The energy barrier at different gate voltages for graphite (extracted at bias voltage 5 V and $240<T<252$) is shown for comparison, red circles in (a).

Figure 5

Semilog plot of the conductance as a function of the square root of the applied voltage for $V_{\mathrm{DC}}>7$ V from $T=267$ K to $T=300$ K every 3 K for $V_g=50$ V.