From Diamond to Carbon Nanotube Field Emitter
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From Diamond to Carbon Nanotube Field Emitter O. Gröning, L-O. Nilsson, P. Gröning and L. Schlapbach Gruppe für Festkörperphysik Physik Departement der Universität Fribourg, Chemin du musee 3, CH-1700 Fribourg (Switzerland) Abstract In this paper we review the physics and the expectations that were put into the negative electron affinity (NEA) mediated field emission of chemical vapor deposition CVD diamond films and how the emitter technology made possible by this mechanism could have challenged the classical metal micro-tip field emitter arrays. We discuss the dependency between emitter performance of micro-tip emitter arrays and feature size (size of the field enhancing tip) and due to this to the connection between emitter performance and fabrication costs. We introduce the concept of the field enhancement distribution function f(β) for a useful characterization of the field emission properties of thin film emitter and show how this distribution function can be measured by scanning anode field emission microscopy. Using f(β) measured on a thin film of randomly oriented multiwalled carbon nanotubes we show that even these kinds of low cost emitters can show a field emission performance comparable to micro-tip arrays, yet that the large spread in field enhancement values between the individual emitter prevent this performance to be fully exploited. This because the field range in which such thin film emitters can be operated is limited due to emitter disruption and triggering of vacuum arcs. Simulations show how resistor-limited emission can solve these limitations. Introduction In recent years various kinds of carbon thin films have been recognized as interesting materials for field emission cathodes. The development of the field emission flat panel display in the same time has further boosted this interest. Therefore investigation of the field emission properties and mechanisms of carbon based materials as single crystalline and chemical vapor deposition (CVD) diamond, diamond like carbon, nanocrystalline graphite and carbon nanotubes has become a domain of very active research. Though field electron emission from a perfectly flat metal surface requires electric field on the order of 2500 Vµm-1, for the carbon based materials mentioned above field emission currents could be observed for applied electric fields below 10 Vµm-1. Classical Fowler-Nordheim like field emission describes the tunneling of electrons close to the Fermi energy through a narrow surface potential barrier. When an electric field of the order of 2500 Vµm-1 is present at a typical metal surface with a work function of 5 eV the surface potential step confining the electrons to the solid, becomes a triangular shaped surface potential barrier. As the width of the barrier at the Fermi energy approaches 2 nm, the electrons have a non-negligible probability of tunneling from the solid into vacuum. Fields of the order of 2500 Vµm-1 can practically only be created when the field enhancing effect of conducting tiplike structures is exploited. The field
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