Electron Field Emission from Boron Nitride Thin Films.

Abstract

A systematic study of electron field emission from boron nitride thin films is presented, establishing nanostructured thin film cubic boron nitride (cBN) as a robust and chemically inert material with a low effective workfunction, able to sustain electron emission in a space plasma environment. RHEED data shows the films as polycrystalline, composed of partially oriented crystallites of cBN with predominantly (001) crystallographic texture relative to the Si substrate. FTIR data showed our films to be overwhelmingly cBN, with a volume fraction greater than 75%. AFM images show nanostructures relevant to field enhancement, with a mean feature height of 79 nm, mean RMS roughness of 19 nm, average grain size of 155 nm2 ±84 nm2, and a mean feature radius of ~7 nm. The results are discussed in the light of current theoretical models for electron field emission, including particulars relevant to semiconductors and nanostructured surfaces. Electron emission thresholds were measured from under 1 V/μm up to just under 20 V/μm in vacuum. Voltage sweep measurements were made both in vacuo and in various gas environments relevant to space applications. Repeatability of emission results was demonstrated, albeit with indications of threshold shifts, possibly due to desorption of adsorbate impurities. Time dependence measurements at constant extraction field show stable field emission over periods of extended operation. An effective barrier height Φw of approximately 9.3meV for the as-grown cBN thin films is measured, based on the application of the generalised Fowler-Nordheim theory to the electron field emission measurements, and employing a model of the film surface as an ensemble of self assembled protruberances in the shape of prolate half ellipsoids of revolution on a flat surface. To our knowledge, this is the first experimental determination of this important parameter for cBN films. It appears that the low value of Φw measured for cBN is a direct consequence of the wide gap nature of the band structure, and is evidence in favor of an NEA-type of emission mechanism in cBN. Overall, the results in this study provide ample motivation for further investigations of cubic boron nitride as a promising field emission material.