Abstract:
A field emission cold cathode utilizes a film of carbon flake field emitters deposited thereon. The carbon flakes may exhibit rolled edges, but are still sufficient to provide improved field emission characteristics. A cold cathode using such carbon flake field emitters can be utilized to produce afield emission flat panel display, which can be implemented for use with a computer system.

Description:
TECHNICAL FIELD 
     The present invention relates in general to field emission devices and in particular, to cold cathodes used in field emission devices. 
     BACKGROUND INFORMATION 
     Carbon flake is a carbon material with a graphitic structure. It can be as thin as one or more layers of sp 2 -bonded carbon atoms (graphite layers), and can be very long in two other dimensions. The length of a flake can be on the order of microns, whereas the thickness is on the order of nanometer or tens of nanometers. Thus, the aspect ratio for this material is very high. A flake, by its nature, is a system of ordered or turbostratic graphite layers. Carbon flakes fall into a class of nanostructured carbon materials. The flakes can be grown by several methods that fall into the following categories: 
     1. DC Glow Discharge. This method involves a direct current glow discharge between two electrodes in a gas environment. The plasma between the two electrodes is of the order of 1000° C. or higher. This method produces carbon flakes along with other types of carbon materials such as carbon nanotubes. This method is used for depositing directly onto a substrate. 
     2. Thermal CVD (Chemical Vapor Deposition) Method. In this method, a carbon precursor gas and a substrate are heated to a temperature of 600° C. and higher while thermal decomposition of the precursor is observed. The substrate has a catalyst on the working surface, which gives rise to growing carbon structures like carbon nanotubes and carbon flakes. A bias voltage can be used to make carbon nanostructures grow straight. This method is used for depositing directly onto a substrate. 
     Carbon nanostructures are believed to be good field emitters due to their chemical stability, sufficient electric conductivity, and low value of electron affinity. Carbon nanostructures fall into two categories with respect to their phase composition and morphology, namely, carbon films with low electron affinity, and those with a high aspect ratio: 
     1. Diamond films, diamond-like films, and nanocrystalline diamond fall into the first category where the low or negative values of electron affinity are essential, as described in F. J. Himspel, J. A. Knapp, et al. Phys. Rev. B, vol. 20, p. 624, 1979. 
     2. Carbon nanotubes and filaments fall into the second category where high values of electric field enhancement factors are essential. 
     These two groups of carbon materials specify two different approaches to making low-field carbon cold cathodes. Cold cathode materials from the second category are proven to have better field emission performance than those materials from the first category. The general problem with these materials is that the field emission properties of these carbon nanostructures are not always good. 
     The films from the first category have a rather poor defect-induced conductivity and low field enhancement factors, and, hence, lower density of emission sites and higher extraction fields. Along with susceptibility to ion bombardment, these films are sensitive to a chemical nature of bombarding species since the adsorbed atoms can significantly increase the electron affinity of these materials. 
     The films from the second category are also susceptible to degradation of the sharp tips of the carbon fibers and nanotubes. The increase of ultimate emission current requires higher density of nanotubes. However, too densely packaged nanotubes or nanofibers need higher extraction fields due to the screening effect that lowers the local electric field near the tips. Electrostatic calculations show that the optimal distance between the nanotubes should be on the order of the nanotube average height (See L. Nilsson, O. Groening, C. Emmenegger, O. Kuettel, E. Schaller, L. Schlapbach, H. Kind, J. M. Bonard, K. Kern, Applied Physics Letters, vol. 76, p. 2071, 10, Apr. 2000). These materials have higher electron affinity due to the graphitic nature of the carbon that they are formed of. 
     The problems with these materials lead to arcing and poor operation of field emission devices. One wishes to obtain good-performance cold cathodes that are robust, considerably less susceptible to ion bombardment, have high aspect ratio, and chemically inert. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the foregoing needs by providing a carbon flake field emitter that is grown by a plasma-assisted chemical vapor deposition method on a semiconductor or metal substrate. A mixture of hydrogen (H 2 ) and methane (CH 4 ) (or other hydrocarbon gasses or liquids) is used as a carbon precursor. The substrate is heated to a high temperature above 600 C so that decomposition of carbon containing gas species occurs near the surface of the substrate. 
     The average distance between the emitting edges of the flakes is on the order of their size, which provides optimal field emission conditions. 
     Carbon flakes preferably originate from an interfacial layer of carbon material initially formed on the substrate at the beginning of the deposition. Since it is a carbon-to-carbon chemical bonding, the flakes have good adhesion to this layer. 
     Carbon flake field emitters differ from the other carbon-based cold cathodes in the following: 
     1. Better electric conductivity since the carbon flakes have a graphitic structure. As well as in graphite, the conductivity along the basal plane of the flake is almost metallic. 
     2. More robust than nanofibers since they are a two-dimensional structure rather than one-dimensional like the materials from the second category. 
     3. An increasingly extended emission area over the flake edge that enables higher emission currents. 
     4. Less susceptible to spoiling the field enhancement factor since this factor is a square root function of a length-to-thickness ratio for edge emission unlike a linear function of this ratio for tip emission from carbon fibers and nanotubes. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates an SEM image of a carbon flake field emitter; 
     FIG. 2 illustrates an SEM image of an as-cleaved cross-section of a carbon flake field emitter; 
     FIG. 3 illustrates a graph of a comparison of emission properties between different carbon emitters; 
     FIG. 4 illustrates a cathode utilizing carbon flakes in accordance with the present invention; 
     FIG. 5 illustrates one embodiment of a grown carbon flake; 
     FIG. 6 illustrates another embodiment of a grown carbon flake; 
     FIG. 7 illustrates a cross section of a display device configured in accordance with the present invention; and 
     FIG. 8 illustrates a data processing system configured in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth such as display configurations, etc. to provide a thorough understanding of the present invention. However, it will be obvious to those skilled in the art that the present invention may be practiced without such specific details. In other instances, well-known circuits have been shown in block diagram form in order not to obscure the present invention in unnecessary detail. For the most part, details concerning timing considerations and the like have been omitted in as much as such details are not necessary to obtain a complete understanding of the present invention and are within the skills of persons of ordinary skill in the relevant art. 
     Refer now to the drawings wherein depicted elements are not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
     Referring to FIG. 4, there is illustrated a cold cathode configured in accordance with the present invention. A substrate  101  has a conductor material  102  deposited thereon. Then carbon flakes  103 - 105  are grown on top of the conductor material  102 . The carbon flake field emitters  103 - 105  can be grown by a plasma-assisted chemical vapor deposition (“CVD”) method using a mixture of hydrogen (H 2 ) and methane (CH 4 ) or other hydrocarbon gas as a carbon precursor. The substrate  102  has a temperature of at least 400° C., and is heated by a heater, or by adjacent plasma, or by hot carbon containing gas. The substrate  102  is cooled if its temperature is too high to form the flakes  103 - 105 . During decomposition of carbon containing gas, the carbon atoms, or bonded carbon atoms, or carbon radicals assemble and form an sp 2 -bonded carbon structure initiated on the substrate. The carbon radicals can further decompose leaving the carbon atoms bonded to the growing flake. The growth process occurs on the edges of the flakes  103 - 105  provided that flake is growing in lateral dimensions as new carbon atoms are bonded. This carbon structure  103 - 105  comprises the layers of sp 2 -bonded atoms of carbon. The layers can be stacked together to form thicker flakes. During decomposition, gas species other than carbon may incorporate as defects in carbon structure. Such defects, as well as intrinsic defects of carbon structure may cause irregularities in this structure and, in turn, cause the flake bending. 
     Note that in an alternative embodiment, the carbon flake emitters  103 - 105  can be grown directly onto a semiconductor or metal substrate. Such a substrate may also be comprised of a semi-metal. A morphology of the carbon flake film containing the carbon flake field emitters  103 - 105  is shown in FIG.  1 . 
     The average distance between the emitting edges of the flakes is on the order of their size, which provides the optimal field emissions conditions. Carbon flakes preferably originate from an interfacial layer of carbon material initially formed on the substrate at the beginning of the deposition. Since it is a carbon-to-carbon chemical bonding, the flakes have good adhesion to this layer, as seen in the FEM image illustrated in FIG.  2 . 
     FIG. 3 illustrates the dramatically improved field emission characteristics of the carbon flake film as opposed to film grown in a tube furnace, and a diamond-like film. 
     Returning to FIG. 4, it is seen that the carbon flakes often have rolled edges as illustrated by carbon flakes  103  and  104 . Carbon flakes  103  are rolled so significantly that they essentially form a loop on top of the substrate surface. FIGS. 5 and 6 further illustrate carbon flakes  104  and  105 , respectively, which have less significant rolled edges as illustrated by the carbon flake  104  in FIG. 5, and almost no rolling of the edges as illustrated by carbon flake  105  in FIG.  6 . Both FIGS. 5 and 6 further illustrate arrows  501  and  601  shown solely as lying along a surfaces of the carbon flakes from the base of the carbon flake at the substrate towards the edge of the carbon flake. The arrows designated with the director {overscore (M)} are perpendicular to the arrows  501  and  506  at each point along such vectors. As can be seen in FIG. 5, when a carbon flake  104  has a rolled edge, the director {overscore (M)} varies along the surface from the base  502  to the edge  503  thus resulting in the director {overscore (M)} having a varying direction as it moves along the vector  501 . Despite such rolled edges, such carbon flakes still exhibit very good field emission characteristics as evidenced in FIG.  3 . 
     FIG. 7 illustrates a cross-section of a partial display device  707  using the cold cathode  100  illustrated in FIG. 4, where the layer of carbon flake film is designated by the numeral  701 , which will contain carbon flakes  103 - 105 . A predetermined distance away from the cold cathode  100 , an anode  700  is positioned with a glass substrate  702 , an indium tin oxide layer  703 , and phosphor layer  704 . A voltage bias is placed between the anode  700  and the cathode  100  to set up an electric field to cause the emission of electrons from the carbon flake from  701  towards the phosphor layer  704 , so that it will result in the emission of protons through substrate  702  to form an image on the display  707 . Note that one or more gate electrode  705  may also be utilized to enhance the field emission and switching characteristics of the display  707 . 
     A representative hardware environment for practicing the present invention is depicted in FIG. 8, which illustrates a typical hardware configuration of data processing system  813  in accordance with the subject invention having central processing unit (CPU)  810 , such as a conventional microprocessor, and a number of other units interconnected via system bus  812 . Data processing system  813  includes random access memory (RAM)  814 , read only memory (ROM)  816 , and input/output (I/O) adapter  818  for connecting peripheral devices such as disk units  820  and tape drives  840  to bus  812 , user interface adapter  822  for connecting keyboard  824 , mouse  826 , and/or other user interface devices such as a touch screen device (not shown) to bus  812 , communication adapter  834  for connecting data processing system  813  to a data processing network, and display adapter  836  for connecting bus  812  to display device  838 . CPU  810  may include other circuitry not shown herein, which will include circuitry commonly found within a microprocessor, e.g., execution unit, bus interface unit, arithmetic logic unit, etc. CPU  810  may also reside on a single integrated circuit. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims.