Diamond electron emission
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oduction Diamonds, with a wide bandgap of ∼5.4 eV, are electrical insulators and exhibit tremendous inertness to radiation as well as to chemical corrosion.1 Diamonds are known for the various properties these materials possess, such as extreme mechanical hardness, high bulk modulus (low compressibility) (1.2 × 1012 N/m2), high thermal conductivity (2 × 103 Wm–1 K–1), and low thermal expansion coefficient at room temperature (0.8 × 10–6 K–1 ) (see the Introductory article in this issue).2 Among these, the potential application to cold cathodes and field emission cathodes are of special interest. The versatility of the chemical vapor deposition (CVD) technique has enabled the development of diamond films with tailor-made properties.3,4 Ultrananocrystalline diamond (UNCD) films are a special category of diamond film with a grain size less than ∼10 nm,5,6 which is versatile for microstructural control and modification of physical properties.7,8 This article is aimed at illustrating the different methods that have been adopted to change the granular structure, the doping composition, and the surface properties of diamond films, thereby enhancing the electronic properties of diamond films for the development of various kinds of electrons sources, especially cold electron emission from negative electron
affinity (NEA) diamond and field emission/thermionic emission of electrons from UNCD films.
Cold electron emission from NEA diamond films The NEA nature of hydrogen-terminated diamond surfaces has been given attention for the realization of robust and highefficiency cold cathodes even though intrinsic diamond films are insulating in nature. There are two possible approaches to utilize the benefit of the NEA of the hydrogen-terminated diamond surface: (1) through the fabrication of diamond p–n junction and (2) the secondary electron emission from NEA diamond.
Diamond p–n junction NEA cathode From recent studies based on total photoelectron yield spectroscopy, p-type and intrinsic diamond surfaces show good electron emission characteristics when they are terminated by hydrogen.9 However, these diamond films do not have conducting electrons to supply for electron emission from their surfaces. In the case of n-type diamond, which also shows NEA when its surface is hydrogen terminated, electrons are thermally excited to the conduction band from which they are expected to easily escape into vacuum. Still, a low electron emission yield was observed.9 In subsequent studies, this was
I-Nan Lin, Department of Physics, Tamkang University, Taiwan; [email protected] Satoshi Koizumi, National Institute for Materials Science, Japan; [email protected] Joan Yater, Naval Research Laboratory, Washington, DC, USA; [email protected] Franz Koeck, Department of Physics, Arizona State University, USA; [email protected] DOI: 10.1557/mrs.2014.101
© 2014 Materials Research Society
MRS BULLETIN • VOLUME 39 • JUNE 2014 • www.mrs.org/bulletin
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DIAMOND ELECTRON EMISSION
attributed to charge transfer at the surface, which leads to upw
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