Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J28: Focus Session: Graphene Device and Applications I |
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Sponsoring Units: FIAP Chair: James Hannon, IBM Room: 330 |
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J28.00001: Electronic properties of graphene and its operation at GHz frequencies Invited Speaker: Graphene, a two-dimensional carbon crystal, possesses great potential for applications in nanoelectronics because of its high intrinsic carrier mobility and the possibility of being processed using the well-established planar top-down technology in semiconductor industries. The former makes graphene an ideal candidate for electronic devices operating at high frequencies, while the latter allows us to tailor the transport properties of graphene devices by controlling their channel geometry. For example, it is, in principle, possible to create metallic and/or semiconducting graphene nanostructures if a precise edge termination can be achieved. In this talk, I will present our recent experimental studies on transport properties of graphene nanoribbons and high-frequency characteristics of graphene transistors operated at GHz frequencies. High-quality graphene nanoribbons with widths down to 30 nm are fabricated by e-beam lithography. In these graphene nanoribbon devices, clear plateau features are observed in the measured conductance as a function of gate voltage at T < 80K, indicating the formation of subbands due to quantum confinement in nanoribbons. This conductance quantization behavior is observed in both metallic and semiconducting nanoribbons, and provides the direct experimental evidence of quantum size confinement effects and the formation of subbands for 1D graphene nanostructures. To explore the high-frequency transport in graphene, top-gated graphene field-effect transistors are fabricated and S- parameter measurements are performed to obtain their transport properties at microwave frequencies. In these graphene transistors, we found that the measured intrinsic current gain shows the ideal 1/f frequency dependence, indicating an FET- like behavior in these devices. The cutoff frequency $f_T$ at which the current gain becomes unity is proportional to the dc transconductance $g_m$ of the device, and is consistent with the relation $f_T=g_m/(2\pi C_G)$. The peak $f_T$ was found to increase with a reducing gate length, and a cut-off frequency beyond 20GHz was measured in a graphene transistor with a gate length of 150 nm. This work is done in collaboration with Ph. Avouris, D. Farmer, K. Jenkins, A. Valdes-Garcia, V. Perebeinos, J. Small. [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:03PM |
J28.00002: Graphene Frequency Multipliers Han Wang, Daniel Nezich, Jing Kong, Tomas Palacios In this paper we demonstrate a new application for graphene: full-wave signal rectification and frequency doubling. Due to its ambipolar transport properties, graphene field-effect transistors (GFET) show a ``V''-shaped transfer characteristic about the minimum conduction point. Frequency doubling can be realized with a single GFET by biasing the gate to the minimum conduction point and superimposing a sinusoidal input signal to the gate. Electrons and holes will conduct in alternative half cycles to produce an output signal at the drain, whose fundamental frequency is twice that of the input. Sub-linear IV characteristics of the GFET near minimum conduction point help improve the spectrum purity of the output signal. In our experiments, for an input frequency of 10 KHz, the output signal showed excellent spectrum purity (94{\%} of RF power at 20 KHz) in the absence of any filtering elements. Given the extremely high electron mobility in graphene ($>$100,000 cm$^{2}$/Vs at room-temperature), such ambipolar devices have the potential to operate at very high frequencies and allow the fabrication of new THz sources and sensors, as well as high speed transmitters and receivers. [Preview Abstract] |
Tuesday, March 17, 2009 12:03PM - 12:15PM |
J28.00003: Chemically derived graphene nanoribbons: physical properties and electronics Xinran Wang, Yijian Ouyang, Xiaolin Li, Li Zhang, Hailiang Wang, Jing Guo, Hongjie Dai Graphene electronics is a promising field of graphene research due to extremely high carrier mobility and the ability to fabricate true nanometer scale devices. We show that sub-10nm graphene nanoribbons, which are semiconductors with suitable bandgap for nano-electronics, can be synthesized via chemistry. Electrical transport measurements show that GNRs have finite bandgaps which are inversely proportional to widths. Unlike carbon nanotubes, all the sub-10nm GNRs are semiconductors and afford graphene field-effect transistors (FETs) with on-off ratio higher than 10$^{5}$ at room temperature. The performance of individual GNRFET is assessed and compared with CNTFETs. The scattering mean free path of carriers in GNRs is estimated, and the limiting factors are discussed. The performance of chemical GNRs and plasma etched GNRs are also compared and discussed. [Preview Abstract] |
Tuesday, March 17, 2009 12:15PM - 12:51PM |
J28.00004: Non-volatile memory devices using graphene and ferroelectric thin films Invited Speaker: The unique linear energy band dispersion and its purely 2D crystalline structure have made graphene a rising star not only for fundamental research but also for nanoscale device applications. Here we demonstrate a novel non-volatile memory device using a combination of graphene and a ferroelectric thin film. The binary information, i.e. ``1'' and ``0'', is represented by the high and low resistance states of the graphene working channels and is switched by the polarization directions of the ferroelectric thin film. A highly reproducible resistance change exceeding 300\% is achieved in our graphene-ferroelectric hybrid devices under ambient conditions. The experimental observations are explained by the electrostatic doping of graphene by the remnant electrical field at the ferroelectric/graphene interface. [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:27PM |
J28.00005: Tuning Disorder and Interactions in Graphene Invited Speaker: One (of many) unique aspects of graphene is that it is an atomically-thin two-dimensional electron system, open to manipulation and study using surface science techniques. This aspect of graphene has allowed us to tune both the disorder strength the interaction strength, allowing unprecedented control over a condensed matter system. Experiments are performed on atomically-clean graphene on SiO$_{2}$ in ultra-high vacuum. Addition of potassium to graphene is used to study the dependence of the mobility and minimum conductivity point on charged impurity density. Tuning the dielectric environment through addition of an ice overlayer has two effects: charged impurity scattering is reduced, due to reduced Coulomb interaction between impurities and carriers, while short-range scattering is increased, due to reduced screening. In sharp contrast to graphene with charged impurity disorder, which remains metallic at low temperature, even a small amount of irradiation-induced point disorder produces a divergence of the resistivity and insulating behavior at low temperature. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J28.00006: Graphene electronics: joule heat and charge density in active devices Marcus Freitag, Mathias Steiner, Yves Martin, Vasili Perebeinos, Zhihong Chen, James C. Tsang, Phaedon Avouris We use Raman scattering microscopy to measure the shifts of the 2D and G-bands resulting from the electronic power dissipation in the graphene sheet. Extracted images of the temperature distribution show peak temperatures of up to 1000K in the middle of the graphene device. The metallic contacts act as the dominant heat sinks, because the thermal conductivity of graphene is far greater than the gate-oxide thermal conductivity. We model thermal transport and obtain excellent agreement in peak temperature and functional form. Velocity saturation due to phonons with 50meV energy is observed, suggesting that substrate polar phonons limit the high-bias conduction in graphene. Trapped charges are also detected and we find that application of a high current is associated with drain-induced barrier lowering in back-gated graphene devices. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J28.00007: n-Type Behavior of Graphene Supported on Si/SiO2 Substrates Humberto Gutierrez, Hugo Romero, Ning Sheng, Jorge Sofo, Peter Eklund, Parsoon Joshi, Srinivas Tadigadapa Results are presented from an experimental and theoretical study of electronic properties of back-gated graphene field effect transistors (FETs) on Si/SiO$_{2}$ substrates. The excess charge on the graphene was observed by sweeping the gate voltage to determine the charge neutrality point in the graphene. Devices exposed to laboratory environment for several days were always found to be initially p-type. After $\sim $20 h at 200 \r{ }C in $\sim $5$\times $10$^{-7}$ Torr vacuum, the FET slowly evolved to n-type behavior with a final excess electron density on the graphene of 4$\times $10$^{12}$ electrons/cm$^{2}$. This value is in excellent agreement with our theoretical calculations on SiO$_{2}$, where we have used molecular dynamics to build the SiO$_{2}$ structure and then density functional theory to compute the electronic structure. The essential theoretical result is that SiO$_{2}$ has a significant surface state density just below the conduction band edge that donates electrons to the graphene to balance the chemical potential at the interface. An electrostatic model for the FET is also presented that produces an expression for the gate bias dependence of the carrier density. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J28.00008: Photocurrent imaging and efficient photon detection in a graphene transistor Fengnian Xia, Thomas Mueller, Roksana Golizadeh-mojarad, Marcus Freitag, Yu-ming Lin, James Tsang, Vasili Perebeinos, Phaedon Avouris We measure the channel potential of a graphene transistor using a scanning photocurrent imaging technique. In this approach, the photon-induced current between the source and drain is measured when the excitation laser beam is scanned across device at various gate biases. Potential profiles are then inferred from photocurrent measurements. We show that at a certain gate bias, the impact of the metal on the channel potential profile extends into the channel for more than 1/3 of the total channel length from both source and drain sides, hence most of the channel is affected by the metal. The barrier height between the metal and graphene interface is experimentally determined to be around 95 meV from transport and photocurrent measurements. As the gate bias exceeds the Dirac point voltage, V$_{Dirac}$, the original p-type graphene channel turns into a p-n-p channel. When laser beam from He-Ne laser with a wavelength of 632.8 nm is focused on the p-n junctions, an impressive external responsivity of $\sim $0.001 A/W is achieved, given that only a single layer of atoms are involved in photon detection. The possibility of using graphene p-n junctions in high-bandwidth photonic applications is discussed. [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J28.00009: Quantum capacitance measurement in high performance graphene field-effect devices Zhihong Chen, Joerg Appenzeller We demonstrate first quantum capacitance measurements on single-layer graphene devices and elucidate on their relevance for the extraction of mobility for scaled graphene FETs. We also show experimentally that multi-layer graphene devices can be easily distinguished from single-layer graphene within our capacitance measurement approach, a previously unnoticed fact. [Preview Abstract] |
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