Session V28: Focus Session: Graphene Device and Applications III

Graphene field-effect transistors for RF applicatoins

There has been growing interest in graphene as a replacement for III-V materials in MMIC applications because of its high mobility, its potential for high saturation velocity, and its nearly perfect two-dimensional electrostatics. We present results from the first experimental high-frequency measurements of graphene field-effect transistors (GFETs), demonstrating an f$_{T}$ of 14.7 GHz for a 0.5-$\mu $m-length device with a 30-nm-thick HfO$_{2}$ top-gate. Despite I$_{on}$/I$_{off} \quad \sim $7, high transconductances ($>$833 $\mu $S/$\mu $m) and current saturation are achieved. We present detailed measurement and analysis of velocity saturation in GFETs, demonstrating the potential for velocities approaching 10$^{8 }$cm/sec and the effect of an ambipolar channel on current-voltage characteristics. We find that the saturation velocity is sheet-carrier dependent and limited by interfacial phonon scattering from the SiO$_{2}$ substrate upon which the graphene is fabricated.   

Tunable spin-polarized terahertz excitations in graphene nanoribbons

Graphene nanoribbons have an energy gap that is tunable from zero to terahertz (THz) regime by an external gate field. The indirect energy gap in a nanoribbon of infinite length, however, is unsuitable for optical excitations. We report a theoretical investigation of such nanoribbons with a finite, nanoscale length. We show that such nanoribbons can be excited optically and exhibit unique electronic excitations in the THz regime. The results unveil THz radiation-induced edge standing spin waves with different wavelengths at the two edges and a resonant frequency that can be controlled by an external gate voltage, opening the possibility of THz ``opto-spintronic'' applications.   

THz Emission from Graphite Surfaces

Graphite formed by Van der Waals force between adjacent graphene sheets has been studied for more than six decades due to its relatively simple quasi-two-dimensional structure. Lately, because of its close relationship to carbon nanotubes and new physics originating from graphene's linear excitation spectrum and the 4-fold degeneracy graphitic materials have attracted much of research attention. However, still little is known about the high-freqency transport properties of these materials. In this work, we investigate graphite materials in THz regime by THz emission spectroscopy. Picosecond THz pulses were observed from a highly oriented pyrolytic graphite (HOPG) surface when it was illuminated by intenseultrafast optical pulses at an oblique angle. The emission was mostly p-polarized and increased linearly with pump fluence. Several potential mechanisms for the emission including surface nonlinearility will be discussed in the talk.   

Low - Frequency Noise in Graphene Transistors

We present the results of the experimental investigation of the low-frequency noise in three-terminal bilayer graphene devices. The quality of graphene layers has been verified with micro-Raman spectroscopy. Back-gated devices were fabricated using electron beam lithography and evaporation. The back-gate was used to adjust electrical conductivity through the graphene layer placed on top of Si/SiO$_{2}$ substrate. The charge neutrality point for examined devices was$\sim $10 V. The noise spectral density was rather low (on the order of $\sim $10E$^{-23}$--10E$^{-22}$ A$^{2}$/Hz at frequency of 1 kHz).The noise reveals generation-recombination (G-R) bulges. Presence of G-R bulges and deviation from the 1/f spectrum suggest that the noise is of carrier-number fluctuation origin due to carrier trapping by defects [1].The low values of the low-frequency noise add validity to the proposed electronic applications of graphene. [1] Q. Shao et al., IEEE EDL (2008).   

Environmental Effects on 1/f Noise in Graphene and CNT Based Devices

Graphite related materials such as carbon nanotubes, graphene and graphene oxide show promise for future electronic and chemical sensor applications. Nanotubes and graphene, in particular, have been shown to make exquisitely sensitive chemical sensors. Due to their low carrier density, the 1/f noise in these nanomaterials is very high. Understanding the cause of this noise is particularly important for chemical sensing applications, and the noise common to these materials may be one barrier to current practical success for graphitic sensors outside the lab. We have compared the noise power spectral density (PSD) of these three materials in different chemical environments and at different temperatures. This information should play a key role in guiding the development of future sensing devices as well as helping to illuminate the atomic scale interactions which lead to enhanced or suppressed 1/f noise in graphitic materials.   

Progress towards graphene as a quantum-limited electro-mechanical resonator

With its high stiffness, low density and relatively simple fabrication, graphene promises to be an ideal system for exploring the quantum limits of mechanical measurements. In particular, electronic transport through a graphene sheet suspended over an electrode can be strongly modulated by vibrations of the sheet -- whether through the standard field effect which changes the carrier density in the sheet, or through modulation of the Coulomb blockade in quantum dots formed in the sheet. In this work we present the novel fabrication scheme we are using for this work, which involves exfoliation and identification of single-layer graphene sheets on a PMMA layer above a silicon substrate, cross-linking of the PMMA to form supports for the graphene and metal electrodes, and subsequent lithography to form electrodes. Raman scattering measurements before and after patterning confirm the single-layer nature of the graphene, and preliminary low-temperature transport measurements show the feasibility of this system for quantum-limited sensitivity of resonant motion of the sheet.   

Developing resonant tunneling devices based on graphene

We present an \textit{ab-initio} study of the electronic properties of patterned graphene structures as candidate resonant tunneling devices. We consider graphene nanoribbons that have been modified with one or more narrow constrictions or patterned with periodic nanoscale antidotes[1]. Specifically, we focus on semi-metallic armchair nanoribbons with narrow semiconducting regions and semi-metallic zigzag nanoribbons patterned with antidots. Using a first-principles density functional theory (DFT) approach, we investigate the induced band-gap opening and transmission coefficients. We examine how varying the lengths of the constrictions, changing the separation between dots and their sizes affect transport properties. We will also discuss I-V characteristics of these graphene structures and evaluate the possibility of a negative differential resistance in these devices. [1] T. G. Pedersen \textit{et al.}, \textit{Physical Review Letters}, \textbf{100} 136804 (2008)   

Coherent Graphene Devices: Movable Mirror, Buffer and Memory

We theoretically report that, at a sharp electrostatic step potential in graphene, massless Dirac fermions can obtain a Goos-H\"{a}nchen-like shift under total internal reflection. Based on these results, we study the coherent propagation of the quasiparticles along a sharp graphene \emph{p-n-p} waveguide and derive novel dispersion relations for the guided modes. Consequently, coherent graphene devices (e.g. movable mirror, buffer and memory) induced only by the electric field effect can be proposed.   

Graphene interferometry

Ballistic transport calculations of graphene connected to two contacts are presented. The calculations are based on the nearest-neighbor, tight-binding approximation but are otherwise treated exactly within a Green function formalism. It is shown that under certain circumstances stable collective resonances emerge from a resonant structure that in general could be quite complicated. These collective resonances originate from a large number of non-equivalent conduction channels and are evenly spaced, except for a region close to the Fermi level. The separation between neighboring collective resonances depends to first order only on the contact separation. Their contrast, on the other hand, is affected by the width of the sample, temperature, and unevenness in the contact interfaces. Despite the existence of many potential sources that could degrade the collective resonances, these resonances could still prove to be observable experimentally.   

Nanotube Films and Their Application For Mode-Locked Lasers

Carbon nanotubes (CNTs) exhibit strong saturable absorption, i.e. they become transparent under sufficiently intense light. This has great potential for applications in photonics. By tuning the nanotube diameter it is easy to tune the saturable absorption in a broad optical range of interest for telecommunications, medicine and military applications. The performance of CNTs based saturable absorbers depends on concentration, bundle size, and transparency of the matrix where CNTs are dispersed. CNT saturable absorbers can be produced by cheap wet chemistry and can be easily integrated into polymer photonic systems. Here, we review the fabrication and characterization of saturable absorber based on CNT-polymer optical composites [1,2,3]. We use strong ultrasonication to obtain CNT solutions. Such solutions with different nanotube bundle sizes are then studied by photoluminescence excitation spectroscopy [4]. We find that exciton energy transfer between semiconducting CNTs is an efficient carrier relaxation channel in the bundles [4]. This fingerprints and quantifies the presence of small bundles and allows us to optimize the solutions used for composites preparation. We demonstrate picosecond pulse generation in a nanotube mode-locked waveguide laser [5], as well as 125 fs generation in an erbium doped fiber laser. We also report a novel SWNT- polycarbonate polymer composite, with a absorption maximum at 1550 nm and a bandwidth of about 300 nm [6]. This has strong saturable absorption with saturation intensity of 7 MW/cm$^{2}$. We demonstrate the first SWNT-mode-locked widely tunable fibre ring laser [7]. This is achieved through the control of amplification at the specific transitions of the Er$^{3+}$ gain medium by placing a band-pass filter in a laser cavity [7]. [1] A. G. Rozhin et al. Phys. Stat. Sol. (b) \textbf{243}, 3551 (2006). [2] V. Scardaci et al. Physica E \textbf{37}, 115 (2007) [3] T. Hasan et al. J. Phys. Chem C \textbf{111}, 12549 (2007) [4] P. H. Tan et al. Phys. Rev. Lett. \textbf{99}, 137402 (2007) [5] G. Della Valle et al., Appl. Phys. Lett. \textbf{89}, 231115 (2006) [6] V. Scardaci et. al. Adv. Mat. \textbf{20}, 4040 (2008 [7] F. Wang et. al. Nature Nano. Nov (2008).