Thermal-Field Electron Emission from Nanostructured CVD Diamond Films

Abstract

This work reports the behavior of electron field emission from nanostructured CVD diamond films at elevated temperatures. The study is motivated by the possibility of using these structures in high-temperature electronics or direct energy conversion processes. Four nanostructured CVD diamond films were tested: nanocrystalline diamond, nitrogen-doped nanocrystalline diamond, peaked diamond microtips, and truncated diamond microtips. All samples displayed reasonably efficient field emission characteristics. For each, the onset of field emission decreased as the sample temperature increased. Truncated diamond microtips were found to provide the largest current-carrying capacity and effective emission current density. These values were 90μA and 12 1 1010A/cm2 respectively. At a temperature of 700 K, peaked diamond microtips showed the lowest turn-on field recorded at 38V μm. The highest turn-on field, at this same temperature, was found from the undoped nanocrystalline diamond and recorded at 90V μm. Address all correspondence to this author. NOMENCLATURE A effective emission area D tunneling transmission coefficient, see Eq. 2 E electron energy Ea conduction band energy EF Fermi level Ex axial electron energy F applied electric field h Planck constant h reduced Planck constant J current density kB Boltzmann constant m electron rest mass N electron supply function, see Eq. 5 p electron momentum q electron charge T temperature V voltage x axial coordinate β field enhancement factor φ work function μ chemical potential 1 American Institute of Aeronautics and Astronautics 8th AIAA/ASME Joint Thermophysics and Heat Transfer Conference 24-26 June 2002, St. Louis, Missouri AIAA 2002-3024 Copyright © 2002 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. INTRODUCTION Much attention has been paid to diamond thin films because of their potential applications in optics, high-speed electronics and thermal management devices. Diamond is chemically inert and has superior mechanical stability over typical materials used as electron field emission devices. Diamond’s sp3 tetragonal configuration consists of a central carbon atom with linear covalent bonds to neighboring carbon atoms 10. This particular arrangement facilitates phonon transport through the lattice of the structure, giving diamond a thermal conductivity four times that of pure copper 6. Furthermore, diamond is a wide band gap semiconductor. This material property results in diamond being an excellent electrical insulator. Diamond’s unique characteristic of being an excellent thermal conductor and electrical insulator make it ideal for nanoeletromechanical devices and electronic packaging. Nanocrystalline diamond films show strongly enhanced field emission properties that can be demonstrated reproducibly 9. The success of doping these films coupled with the absence of microscale features (i.e., using as-deposited films) make fabrication and implementation of nanocrystalline diamond relatively straightforward 13. Diamond tip arrays appear to be promising candidates for electron field emission devices (FEDs) because of the field enhancement produced by their geometry 1;8. This geometric enhancement effect occurs due to a deformation of the potential field. The altered potential field enhances the local electric field, enabling emission of electrons into vacuum at lower applied voltages. However, unlike nanocrystalline diamond, manufacturing diamond tips and gated tip arrays often requires many laborious post-fabrication etching processes. The present work reports the thermal effects of field emission for both nanocrystalline diamond and diamond tip arrays. High-temperature field emission experiments were performed on each sample. Current-voltage and FowlerNordheim plots are analyzed and discussed with corresponding theoretical formulations. Results show substantial differences between the electron field emission characteristics of the four samples. THEORETICAL FORMULATION A detailed discussion on the physics of field emission can be found from 5, and 4. The following provides an outline of the major physical concepts. The emission current density is defined as