Mechanism of Field Emission in Diamond and Diamondlike Carbon
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0,18
because of its conducting grain boundaries and it is found to vary inversely with the grain size [3]. The lowest threshold fields
-c
02
were 0.05
0.1
0.15
02
found
by Talin
et
al
[6]
for
0.2 nanocrystalline diamond, and by Zhu [7] for
1/E (pV Fig. I Fowler-Nordheim plot of field emission currents from ta-C, showing effective barriers heights of 0.04 - 0.1 eV [9].
sintered diamond grits. Boron-doped diamond has a high threshold field for emission, and this is lowered by (nominal) phosphorus doping and more strikingly by nitrogen doping [4] to fields as low as 0.5
V/pIm.
83 Mat. Res. Soc. Symp. Proc. Vol. 509 © 1998 Materials Research Society
strikingly by nitrogen doping [4] to fields as low as 0.5 VWpm. Diamond-like carbon (DLC) is a semiconductor whose band gap varies from 1 eV to 4 eV according to its sp3 bonding fraction. Field emission from DLC is generally easier than from diamond. Tested in parallel plate geometries, the typical threshold emission field for DLC is 20-40 V/ILtm [8], which can be lowered t o about 10 VWtm for more optimised films, and to 5 VWLm for nitrogen doped films [9]. For tetrahedral amorphous carbon or 'ta-C' deposited by filtered cathode vacuum arc, the threshold field decreases to from 25 to about 10 V/tm for the highest sp 3 content [9]. It is recognised that emission in the parallel plate geometry often comes from a series of high intensity spots, rather than as uniform emission from the whole surface [10]. Some other types of carbon can make good emitters. As points enhance the local field, emission is quite easy from carbon nanotubes. deHeer et al [11] found emission at 15 V/ýtm, while Wang et al [12] obtained emission at 0.8 V/IAm. For displays, it is now recognised that the threshold field parameter has limited value. Because of the exponential character of the Fowler-Nordheim equation, emission often occurs from particular sites on the film. A critical parameter is the emission site density, which should approach 106 cmi2 for a display quality cathode. The development by Coil [10] of a nano-structured carbon with a 'coral-like' surface is of importance. This carbon has a surface with numerous coral-like protrusions with radius of 3-7 nm, and gives emission at 5-10 V/ptm with a very high emission site density. EMISSION MECHANISM In view of the NEA of diamond, it was first assumed that the easy field emission from carbon arose from a low electron affinity. However, closer inspection of the data shows that this is not correct. Field emission is a tunneling process and the current density J (A.m-2) obeys the Fowler-Nordheim equation
J = aE 2 .exp(Band diagr m
Electron energy distnbution
Emission from valence band
Hot electron emission from conduction band
Fig 2. (a) Electron energy distributions for field emission mainly from the valence band, with a steep high energy side and a width proportional to the local field. (b) Emission due to hot electrons, which shows a high energy tail.
.-
(1) ,OE'
where 4)(eV) is the barrier height, E is the field (V/m),
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