Operation of multi-finger graphene quantum capacitance varactors using planarized local bottom gate electrodes

Abstract

The operation of multi-finger graphene quantum capacitance varactors fabricated using a planarized local bottom gate electrode, HfO 2 gate dielectric, and large-area graphene is described. As a function of the gate bias, the devices show a room-temperature capacitance tuning range of 1.22-1 over a voltage range of 62 V. An excellent theoretical fit of the temperature-dependent capacitance-voltage characteristics is obtained when random potential fluctuations with standard deviation of 65 mV are included. The results represent a first step in realizing graphene quantum capacitance varactors for wireless sensing applications. V C 2012 American Institute of Physics.Graphene, a two-dimensional sp 2 -bonded allotrope of carbon, has many unique properties that make it an interesting material for electronic device applications. 1-6 A particular characteristic that has been of interest lately is the quantum capacitance effect, which is especially prominent in graphene due to its very low density of states (DOS). This property, while being studied by numerous groups, 7-19 has only recently been proposed as the operational mechanism of an electronic device. 19 In Ref. 19, it was proposed that a metal-oxide-graphene capacitor could be utilized as the variable capacitor in a passive LC sensing circuit, where the high mobility in graphene would enable high quality factor (Q) to be achieved. Such sensors have the potential to be much smaller and have higher tuning range than resonators based upon micro-electro-mechanical systems (MEMS). 20,21 In order to create a practical graphene varactor, large-area graphene 22 must be transferred onto a local metal gate electrode, upon which thin, high-j dielectric material has been deposited. In addition, a multi-finger geometry must be utilized in order to allow high capacitances to be achieved while minimizing series resistance. In this paper, we report the fabrication and operation of a graphene quantum capacitance varactor fabricated using a planarized local bottom electrode, HfO 2 gate dielectric, and single-layer graphene grown by chemical vapor deposition (CVD).The device structure is shown in Fig. 1(a). The fabrication started by growing 90 nm of thermal SiO 2 on an n-type Si wafer. After patterning the gate resist using optical contact lithography, the SiO 2 was recessed using a buffered oxide etch, followed by evaporation and lift-off of Ti/Pd (10/40 nm). On top of the resulting planarized gate electrode, 20 nm of HfO 2 was deposited by atomic-layer deposition (ALD) at 300 C. Single-layer graphene grown by CVD on a Cu foil was then coated with PMMA and the Cu removed using FeCl 3 . The graphene was then transferred onto the wafer with the local bottom gate electrode using an aqueous transfer process, and the PMMA removed using solvent cleaning. The graphene was then patterned and etched using an O 2 plasma. Finally, contact electrodes to the graphene consisting of Ti/Au (10/100 nm) were patterned and lifted off completing the fabrication process. An optical micrograph of a compl...

Graphene-Based Quantum Capacitance Wireless Vapor Sensors

“…HE quantum capacitance effect is a direct, observable manifestation of the Pauli exclusion principle. While this effect is particularly prominent in the two-dimensional material graphene [1][2][3][4][5][6][7][8][9][10][11][12][13] due to its low density of states, few if any practical uses for this effect have been demonstrated to date. It has recently been proposed that the quantum capacitance effect could be utilized to realize wireless sensors due to graphene's energy-dependent density of states and excellent surface sensitivity [14].…”

Capacitive Sensing of Intercalated H2O Molecules Using Graphene

“…15 These capacitors use a local buried gate electrode over which a thin layer of HfO2 is deposited, followed by CVD-grown graphene on top. 16 The total capacitance, CTOT of such a device equals (COX 1 + CQ 1 ) 1 , where COX is the oxide capacitance and CQ is the quantum capacitance of graphene. 17 If COX is sufficiently large, the capacitance varies vs. applied voltage and has a minimum when the Fermi level is at the Dirac point.…”

A numerical model of electrical characteristics for the monolayer graphene field effect transistors

“…Many researchers investigated the drain-source current model of MLG based on the linear dispersion and provided proper reproduction of current-voltage characteristics for experimentally fabricated graphene field effect transistors (GFETs) [5,6], but with the carrier saturation velocity (y sat ) significantly overestimated when the carrier density is small. Meanwhile, the inevitablely presented impurities lead to the formation of inhomogeneous electron-hole puddles [7][8][9], therefore the linear Dirac carrier dispersion breaks up into a disordered graphene system in MLG [10][11][12][13][14][15]. The effects of disorder on the quantum capacitance and carrier density of graphene devices have been studied for the first time using a Gaussian distribution of the potential fluctuations [13][14][15].…”

“…Meanwhile, the inevitablely presented impurities lead to the formation of inhomogeneous electron-hole puddles [7][8][9], therefore the linear Dirac carrier dispersion breaks up into a disordered graphene system in MLG [10][11][12][13][14][15]. The effects of disorder on the quantum capacitance and carrier density of graphene devices have been studied for the first time using a Gaussian distribution of the potential fluctuations [13][14][15]. The exponential distribution model is an approximation of the physics-based Gaussian model which leads to analytical expression for quantum capacitance and drain-source current [10][11][12].…”