Electron Emission Properties of Diamond and III-V Nitrides
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rn1-V NITRIDES
R. J. Nemanich, P.K. Baumann, M.J. Benjamin, S.L. English, J.D. Hartman, A.T. Sowers, B.L. Ward, P.C. Yang Department of Physics and Department of Materials Science and Engineering North Carolina State University, Raleigh, NC 27695-8202
ABSTRACT Wide bandgap semiconductors such as diamond and the III-V nitrides of GaN and AIN exhibit small or even negative electron affinities. Recent results have shown that surface treatments will modify the electron affinity of diamond to cause a negative electron affinity (NEA). Results are presented which correlate the field emission from single crystal p-type (boron doped) diamond with the electron affinity of the surfaces. The field emission is explored for nitrogen doped polycrystalline films. The threshold for field emission is significantly higher than from p-type diamond, and in fact, most surfaces are severely damaged during the emission measurement. High resolution photo-electron emission microscopy (PEEM) and field emission electron microscopy (FEEM) are employed to determine the relation of the emission to the surface morphology. PEEM results presented for diamond indicate relatively uniform emission with increased intensity at protruding crystallite edges. Results are presented for cold cathodes fabricated from epitaxial nitrides grown on 6H-SiC.
INTRODUCTION The materials of diamond, boron nitride, SiC, Al-Ga nitrides and their alloys are often considered in the group of diamond-type materials because of their strong bonding leading to hard materials. These materials are stable at relatively high temperatures, and they resist defect formation and dopant diffusion. With these characteristics, they have been considered as a group suitable for high power applications. Similarly, for field emission applications, these same properties will be important for high current emission and resistance to sputtering induced degradation. Electron emission from wide bandgap semiconductors offers the potential for cold cathode structures which would have a wide range of applications. Common semiconductors are based on sp 3 type bonding, and the incorporation of impurities can lead to p- or n-type doping characteristics. The discovery of a high quantum yield for photo-electron emission from diamond (111) surfaces indicated the potential for diamond as an electron emitter [1]. It was later concluded that the diamond (111) surface, when terminated with hydrogen exhibits a negative electron affinity [2]. The property of a negative electron affinity will allow conduction band electrons to be freely emitted from a surface into vacuum without a barrier. The electron affinity of a semiconductor has been discussed in the same terms as the work function of a metal. The work
35 Mat. Res. Soc. Symp. Proc. Vol. 509 01998 Materials Research Society
function of an ideal metal surface is ascribed to aspects related to the electronic levels of the atoms involved in the material and the surface dipole. In the most fundamental terms, the surface dipole of a metal is related to the f
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