electron emission from diamond

Lothar Ley, Jing-biao Cui, Juergen Ristein, and Markus Stammler

Institute of Technical Physics, University of Erlangen, Erwin-Rommel-Str. 1, D-91058 Erlangen, Germany

Abstract

Diamond in the form of thin films prepared by chemical vapor deposition (CVD) from the gas phase or in the form of nanocrystals exhibits excellent field emission properties with onset fields of the order 10 V/mm or less and a reasonable density of emission sites. This makes diamond a potential candidate for cold cathode field emitters with the advantage that no special structuring is required as in the case of Spindt tips. The mechanism for field emission from diamond is, however, rather elusive. Initially it was thought that the negative electron affinity (NEA) of hydrogen terminated diamond surfaces is the crucial property because it removes any surface barrier for electron emission from the conduction band. However, since diamond is an insulator with a band gap of 5.5 eV that can at best be doped p-type the problem remains of how to get a sufficient concentration of electrons into the conduction band. Further experimental evidence let to the conclusion that it is not diamond but rather the graphitic component present in all CVD diamond films that is responsible for the electron emission. This scenario solves some problems but raises the question why hydrogenation lowers the emission threshold of CVD diamond and why hydrogenation is a necessary requirement to observe any field emission from diamond nanocrystals at all.

We have addressed these questions by investigating the field emission properties of diamond-graphite composites as a function of composition both for oxidized and hydrogen covered diamond. The composites consist of mixtures of nanocrystalline diamond and graphite particles and they serve as model systems for films consisting of a mixture of diamond and graphitic phases. The crucial advantage of the composites is that the ratio of diamond to graphite can be adjusted at will while the average field enhancement factor of each component remains unchanged.

The measurements prove that graphite is indeed the phase responsible for field emission. The apparent field emission threshold is strongly influenced by the conductivity of the composites, which undergo something of an insulator-metal transition at the percolation threshold for transport through the graphitic grains embedded in the insulating diamond matrix. Hydrogenation of the composites has two effects. It removes the percolation threshold by providing a conducting path to the emission sites via the hydrogen induced surface conductivity of diamond. It also lowers the effective emission threshold of graphite in contact with diamond that exhibits NEA after hydrogenation despite the fact that the work function of graphite itself is not affected by hydrogenation.

Corresponding author: Lothar Ley
Tel: ++49-9131-8527090
Institute of Technical Physics
Fax: ++49-9131-85227889
University of Erlangen
lothar.ley@physik.uni-erlangen.de
Erwin-Rommel-Str. 1
D-91058 Erlangen, Germany

.

Field emission from material with mixed diamond-graphitic composition proceeds via emission from the graphitic phase. Hydrogenation affects the diamond phase only. The field emission properties improve nevertheless because hydrogenated diamond lowers the resistivity of the mixed phase by the surface conductivity of diamond and it lowers the effective emission threshold of the graphitic phase in contact with diamond through the NEA property of diamond.

Keywords: diamond-graphite composites, mechanism of field emission, effect of hydrogenation on field emission, effective work function of graphite in contact with diamond, surface conductivity