Cathodes can emit electrons by photoemission, thermionic emission, and field emission, or as the result of negative electron affinity. A field-emission cathode (or field emitter) provides electrons when subjected to an electric field of sufficient strength. The electric field is created by applying a suitable voltage between the cathode and an electrode, typically referred to as the anode or gate electrode, situated a short distance away from the cathode.
Various techniques have been explored for creating field emitters. Chason, U.S. Pat. No. 5,019,003, fabricates a field emitter by depositing preformed electron-emissive objects on a substrate consisting of dielectric and/or electrically conductive material. The preformed objects, which have sharp edges, can consist entirely of electron-emissive material such as molybdenum or titanium carbide. Alternatively, the preformed objects can consist of electrically insulating cores with thin electron-emissive coatings over the insulating cores. The longest dimension of the objects is approximately 1 .mu.m. A bonding layer is employed to bond the objects to the substrate.
Jaskie et al ("Jaskie I"), U.S. Pat. No. 5,141,460, discloses a technique in which diamond is used in fabricating a field emitter. Kane et al ("Kane I"), U.S. Pat. No. 5,129,850, discloses a related technique for manufacturing a field emitter that utilizes diamond. The fabrication techniques in Jaskie I and Kane I generally entail implanting carbon into a substrate to create diamond nucleation sites and then growing diamond crystallites at the diamond nucleation sites. The resulting regions of diamond crystallites appear to be electron emissive.
Use of diamond to provide electrons is desirable for a number of reasons. Depending on how it is produced, diamond can have a low work function. This is advantageous because the electric field needed to emit electrons decreases as the work function decreases. Diamond has a low chemical reactivity. In particular, the gases typically present in a sealed vacuum device such as a CRT have little effect on diamond. Also, changes in temperature affect diamond less than most materials used as electron emitters.
In Jaskie I and Kane I, the diamond crystallites are grown by chemical vapor deposition ("CVD"). While CVD is economically suitable for depositing many materials, diamond CVD is costly because the diamond CVD growth rate is low and a high CVD temperature is needed. The diamond CVD in Jaskie I and Kane I appears too expensive for low-cost volume production of CRTs in flat-panel televisions.
Jaskie et al ("Jaskie II"), U.S. Pat. No. 5,278,475, produces a gated field emitter that utilizes diamond crystallites as electron sources. The diamond crystallites are deposited across the upper surface of a supporting structure consisting of a substrate or a patterned layer of conductive/semiconductive material formed on an electrically insulating substrate. A dielectric layer is deposited over the diamond crystallites. A gate (or control) electrode layer, likewise consisting of conductive/semiconductive material, is deposited on the dielectric layer. Openings are formed through the gate electrode and dielectric layer to expose diamond crystallites at selected areas of the supporting structure.
Kane et al ("Kane II"), U.S. Pat. No. 5,252,833, discloses a similar gated field emitter in which diamond crystallites provide electrons. The diamond crystallites in Kane II are situated on conductive/semiconductive paths at the bottoms of openings through a dielectric layer and an overlying gate electrode. The diamond crystallites consist of polycrystalline diamond. Taking note of the fact that the (positive) affinity of a material to retain electrons increases the surface work function and thus increases the electric field needed for an electron to escape the material, Kane II indicates that polycrystalline diamond with a (111) crystallographic orientation is particularly useful as an electron source because (111) polycrystalline diamond has a negative electron affinity.
Electron affinity is an important consideration in choosing an electron source. However, maintaining a negative electron affinity during volume field-emitter production requires special steps. Also, it is not clear that the diamond crystallites in Jaskie II and Kane II will be securely fixed to the underlying material in a manner that permits a control voltage to be suitably impressed on the diamond crystallites. As a result, the gated field emitters of Jaskie II and Kane II may not perform well. It would be advantageous to have an electron-emitting device in which diamond or a related carbon-containing material can be utilized as an electron source and which can be fabricated in a manner that avoids the above-mentioned disadvantages of the prior art.