Field electron emission materials and devices

Details Field electron emission materials and devices Field electron emission materials and devices

2001-01-30

2004-02-03

McCall, Eric S. (Department: 2855)

Electric lamp and discharge devices

Discharge devices having a multipointed or serrated edge...

C445S003000

Reexamination Certificate

active

06686679

ABSTRACT:

This invention relates to field electron emission materials, and devices using such materials.
There have been many proposals for broad-area field electron mission materials, many or most of which concentrate on the use of diamond or amorphous carbon as an emitting material of special significance. In the context of this definition, a broad-area field emitter is any material that by virtue of its composition, micro-structure, work function or other property emits useable electronic currents at macroscopic electrical fields that might be reasonably generated at a planar or near-planar surface.
The reader is referred to UK Patent 2 304 989 (Tuck, Taylor & Latham) for examples of emitting materials, including many other than diamond. The present application relates particularly to field electron emission materials involving a primary interface region between a conductive surface, or an electrically conductive particle on it, and an insulating layer, and a secondary interface region between that insulating layer and the environment in which the field electron emission material is disposed.
A critical issue in insulator-based field emitting systems is the injection of electrons from a substrate (often a metal) into the conduction band of the insulator.
FIG. 1
a
is a reasonable representation of the current state of knowledge of such systems, although this still falls short of an exact description. In particular the sharp cut off in the density of states at the band edges is unlikely in highly heterogeneous amorphous materials. However, with these caveats in mind, such a diagram is a useful representation. Electron emission through a dielectric coating is effectively controlled by three factors: injection of the electrons
1503
into the dielectric from the conducting substrate
1500
; transport through the dielectric to the surface as indicated by line
1511
; and subsequent escape through or over the surface barrier
1506
into the vacuum
1502
. A practical insulating layer will have both donor
1507
and acceptor defect sites
1509
in the band gap. The most notable effect is when there are donor states in the band gap relatively close to the bottom of the conduction band. In this case electrons from the donor states
1507
tunnel back into the metal and a Schottky barrier
1510
is formed, see also FIG.
1
(
b
), which enables electrons to tunnel through it from the metal into the conduction band. Bayliss and Latham (K. H. Bayliss and R. V.
Latham, Proc. Roy. Soc. Lond. A
403 (1986) 285-311) have described the conditions required for forming such a Schottky barrier and its significance to electron emission into the dielectric. The Schottky barrier has an associated forward voltage drop. This becomes a particular issue as the particle size is reduced in the metal-insulator-metal-insulator-vacuum (MIMIV) emitters described by Tuck, Taylor and Latham (UK Patent 2304989) to enable them to be used in gated structures such as those described in our patent application GB 2 330 687. Whilst the electric field across the MIM region of a MIMIV emitter can be maintained by reducing the insulator thickness, the absolute voltage will fall to values below the forward voltage drop of the Schottky barrier thus stopping injection of electrons into the insulator.
A more general discussion of the metal-insulator contact in the case of diamond and diamond-like carbon is given by Robertson (J. Robertson,
Mat. Res. Soc. Symp. Proc
. 471 (1997) 217-229).
Transport through the dielectric depends critically on its nature. For relatively defect-free material, transport will be in the conduction band, with lattice scattering limiting conduction. Electrons may become ballistic rather than staying close to the bottom of the conduction band (D. J. DiMaria and M. V. Fischetti,
Excess electrons in dielectric media, eds Ferradini and Jay
-
Gerin
, p315-348, (CRC Princetoun:1991) ISBN 0849369622). By contrast, in a glassy material, with many donor and trapping sites, conduction will be dominated by the Poole-Frenkel effect, field-assisted ionisation of donors and traps, and the electrons will remain close to the Fermi level. In general conduction is non-ohmic with evidence of saturation effects, presumably due to space charge in some cases.
The final step is the emission of electrons from the dielectric surface into vacuum. In the case of hydrogen terminated diamond which has a negative electron affinity, and with the electron transport in the conduction band, there is no barrier to overcome and all electrons arriving at the surface will be emitted. In the case of a low positive electron affinity, such as an un-terminated diamond surface, there is usually sufficient electron heating in the transport to the surface to allow emission through thermionic and thermally enhanced tunnelling. For higher electron affinities, either the field at the surface must be high enough to enable tunnelling or there must be sufficient ballistic electrons that can pass over the barrier. Otherwise the surface must be modified to lower the effective electron affinity. Two possible means of achieving this lowering of the surface barrier are either modifying the surface composition e.g. by caesiating the surface or emptying surface donor states to leave a positively charged surface. The latter is the basis of the forming mechanism proposed by Bayliss and Latham.
An emitter of this type has initially to undergo a forming process. A relatively high switch-on field has to be applied to the device to obtain emission, but after removing this field, a much lower threshold field is required for emission. The actual mechanisms responsible for this behaviour are very difficult to establish because of the small dimensions of the conducting channels. Dearnaley et al. (G. Dearnaley, A. M. Stoneham and D. V. Morgan,
Rep. Prog. Phys
., 33, (1970) 1129-1191) suggest the formation of conducting filaments in the films for MIM (metal-insulator-metal) structures, while Bayliss and Latham suggest that a positive space charge is established in the insulator and at its surface.
Many papers on diamond and diamond-like-carbon field emitters make no mention of any forming process. However, a forming process is described for diamond emitters both by Xu et al. (N. S. Xu, Y. Tzeng, and R. V. Latham,
J. Phys. D
26 (1993) 1776-1780) and by Givargizov et al. (E. I. Givargizov, V. V. Zhirnov, A. V. Kuznetsov and P. S. Plekhanov,
J. Vac. Sci. Technol, B
14 (1996) 2030-31). It seems probable that other workers in this area concentrate on the reversible I-V characteristics of the emitters and may overlook the initial forming process.
It is probable that no one mechanism is appropriate to all situations and that a combination may apply in many cases.
For diamond films, the limiting factor to emission has been found by many workers to be the metal-diamond back contact (e.g. M. W. Geis, J. C. Twichell and T. M. Lyszczarz,
J. Vac. Sci. Technol. B
14, (1996) 2060-67) and U.S. Pat. No. 5,713,775. However, no systematic method of overcoming this problem has been described.
Examples of ad hoc solutions are as follows.
Geis et al. showed that emission thresholds could be greatly reduced by introducing nitrogen into the diamond. The nitrogen defects are close enough to the conduction band to allow a Schottky barrier to be formed, reducing the field necessary to inject electrons into the diamond conduction band. Geis et al. considered also that “roughening” of the surfaces between metal and diamond was of considerable importance, roughening being of the order of 10 nm.
In fact it is likely that many examples of diamond and carbon-based films have an interface roughness of this order without intentional treatments. What is really needed is a more general strategy that can be applied to interfaces whether they are rough or smooth.
Schlesser et al reported improved emission for an annealed molybdenum-diamond interface (R. Schlesser, M. T. McClure, W. B. Choi, J. J. Hren and Z. Sitar,
Appl. Phys Lett
. 70 (1997) 1596-98)
Chuang et al reported improved emission for diamond

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