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Title:Probing the upper limits of current flow in one-dimensional carbon conductors
Author(s):Liao, Albert
Director of Research:Pop, Eric
Doctoral Committee Chair(s):Pop, Eric
Doctoral Committee Member(s):Coleman, James J.; King, William P.; Leburton, Jean-Pierre; Lyding, Joseph W.
Department / Program:Electrical & Computer Eng
Discipline:Electrical & Computer Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Carbon Nanotubes
Graphene Nanoribbons
Joule Breakdown
Current Density
Impact Ionization
Zener Tunneling
Subband Transport
Abstract:We use breakdown thermometry to study carbon nanotube (CNT) devices and graphene nanoribbons (GNRs) on SiO2 substrates. Experiments and modeling find the CNT-substrate thermal coupling scales proportionally to CNT diameter. Diffuse mismatch modeling (DMM) reveals the upper limit of thermal coupling ~0.7 WK 1m 1 for the largest diameter (3-4 nm) CNTs. Similarly, we extracted the GNR thermal conductivity (TC), ~80 (130) Wm 1K 1 at 20 (600) oC across our samples, dominated by phonons, with estimated <10% electronic contribution. The TC of GNRs is an order of magnitude lower than that of micron-sized graphene on SiO2, suggesting strong roles of edge and defect scattering, and the importance of thermal dissipation in small GNR devices. We also compare the peak current density of metallic single-walled CNTs with GNRs. We find that as the “footprint” (width) between such a device and the underlying substrate decreases, heat dissipation becomes more efficient (for a given width), allowing for higher current densities. Because of their smaller dimensions and lack of edges, CNTs can carry larger current densities than GNRs, up to ~16 mA/μm for an m-SWNT with a diameter of ~0.7 nm versus ~3 mA/μm for a GNR having a width of ~15 nm. Such cur-rent densities are the highest possible in any diffusive conductor, to our knowledge. We also study semiconducting and metallic single-walled CNTs under vacuum. Sem-iconducting single-wall CNTs under high electric field stress (~10 V/µm) display a re-markable current increase due to avalanche generation of free electrons and holes. Unlike in other materials, the avalanche process in such 1D quantum wires involves access to the third subband and is insensitive to temperature, but strongly dependent on diameter ~exp( 1/d 2). Comparison with a theoretical model yields a novel approach to obtain the inelastic optical phonon emission length, λOP,ems ≈ 15d nm. We find that current in metallic single-walled CNTs does not typically saturate, unlike previous observations which suggested a maximum current of ~25 μA. In fact, at very high fields (>10 V/μm) the current continues to increase with a slope ~0.5–1 μA/V, allowing m-CNTs to reach currents well in excess of 25 μA. Subsequent modeling sug-gests that carriers tunnel from the contacts into higher subbands. This allows currents to reach ~30–35 μA, which correspond to a current density of ~9 mA/μm for diameters of ~1.2 nm.
Issue Date:2013-02-03
Rights Information:Copyright 2012 Albert Liao
Date Available in IDEALS:2013-02-03
Date Deposited:2012-12

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