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 × 103 is demonstrated at 20 K. The density of states for the gap states are in the range from the latter half of 1012 to 1013 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 ~1011 eV−1cm−2 by continual improvement of the gate stack makes bilayer graphene a promising candidate for future nanoelectronic device applications.
We demonstrate a considerable suppression of the low-field leakage through a Y2O3 topgate insulator on graphene by applying high-pressure O2 at 100 atm during post-deposition annealing (HP-PDA). Consequently, the quantum capacitance measurement for the monolayer graphene reveals the largest Fermi energy modulation (EF = 0.52 eV, i.e., the carrier density of 2×10 13 cm -2 ) in the solid-state topgate insulators reported so far. HP-PDA is the robust method to improve the electrical quality of high-k insulators on graphene.The deposition of ultrathin and reliable high-k dielectrics on graphene is required to realize the high transconductance in graphene field-effect transistors (FETs). Generally, two kinds of deposition methods are used: one is physical vapor deposition 1-4 (PVD) with low particle energies to avoid the defect introduction in graphene, 5 the other is atomic layer deposition (ALD) with buffer layers 6-8 to overcome the chemically inert surface of graphene. Recently, graphene FETs have been reported with ALD Al2O3 topgate insulators as thin as 2.6 nm. 9 Insulators fabricated by PVD and ALD, however, suffer from the dielectric breakdown at the voltage much lower than the complete breakdown voltage due to the fragility of dielectrics accumulated by the low-field leakage during the iterative measurements. Typical electrical field for this low-field dielectric breakdown is ~0.2 V/nm. This is a critical issue for the reliability of the topgate insulators. A common limitation for the insulators on graphene fabricated by both PVD and ALD is the lack of a robust methodology for post-deposition annealing (PDA) in an O2 atmosphere. Although PDA at a high temperature (e.g., ~500 C) is known to improve the electrical quality of the insulator considerably, 10 it introduces many defects in graphene by oxidation. 11 Here, from a thermodynamic viewpoint, 12 let's consider the Gibbs free energy change (G = G -RTlnPO2) for the oxidation reaction of M + O2 = MO2, where G is the standard Gibbs free energy change for the oxidation reaction, R is gas constant, PO2 is the oxygen partial pressure and M is the metal. G should be negative to facilitate the oxidation. The first term for G can be used for material selection. An appropriate rare-earth element for use in high-k insulators should be highly susceptible to oxidation. Figure 1(a) shows G of rare-earth, transition and representative elements at 300 C calculated using a thermodynamic database. 13 The G for yttrium is negatively largest among all of the metal oxides, even smaller than that of carbon. Therefore, Y2O3 can be obtained at relatively low oxidation temperatures and is thermodynamically stable on graphene. The band gap for Y2O3 is ~5.5 eV, which is almost identical to that of h-BN. 14 The dielectric constant of Y2O3 is ~12, 15 while it is ~3-4 for h-BN. 16 The second term for G is the process condition, which is derived from the oxygen potential. G should be further decreased to reduce the defects in the insulator such as oxygen vacancy. This c...
We demonstrate a considerable suppression of the low-field leakage through a Y 2 O 3 topgate insulator on graphene by applying high-pressure O 2 at 100 atm during post-deposition annealing. Then, two kinds of issues, band gap opening and inter-band scattering at high carrier density, are discussed through the density of states of bilayer graphene estimated by the quantum capacitance measurement.
In the bilayer graphene FETs, each upper sub-band of conduction and valence bands may contribute to the total current and it is asymmetric to each other. In this paper, its asymmetry is investigated from the quantum capacitance measurement in the dual-gated bilayer graphene FET.
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