KR100216695B1 - Enhanced electron emitter - Google Patents

Abstract

The electron emitter formed of the diamond-like carbon layer has a diamond bonding structure having an electrically active defect at the emission position. Electrically active defects behave like very thin electron emitters with very low work function and improved current characteristics, including improved saturation current.

Description

Improved electron emitter

1 is a diagram showing a lattice structure of diamond-like carbon.

2 shows a lamination structure of carbon in a diamond-like material.

FIG. 3 shows a lattice structure of diamond-like carbon with a first type of authenticity that forms an electrically active defect.

4 shows a lattice structure of diamond-like carbon having a second type of dislocation that forms an electrically active defect.

5 is a schematic representation of screw defects in diamond bonds.

6 is an enlarged cross-sectional view of a diamond-like carbon layer having electrically active defects.

7 and 8 are graphs showing electron emission characteristics of the conventional field emission device and the device of FIG. 6, respectively.

9 is a graph comparing the electron emission of a device similar to that of FIG. 6 with a defect in the diamond-like carbon layer surface with a change in the radius of the emitter, and a conventional field emission device.

10 shows a lattice structure of the hydrotreated surface of diamond-like carbon.

11 is a cross-sectional view of a field emission device using a hydrotreated layer of diamond-like carbon with electrically active defects.

* Explanation of symbols for main parts of the drawings

10 lattice structure 20 annulus

50: field emission device

The present invention relates to improved electron emitters, in particular electron emitters having improved current characteristics in devices such as field emission devices.

[Background of invention]

Diamonds are known to have negative electron affinity. Diamonds are also known to emit electrons because of their negative electron affinity and emit electrons at much lower electric fields than other conventional electron emitters such as molybdenum or tungsten. This is not currently a controllable function. Emitter current is often much lower than expected and some samples that appear to have all the criteria for emission often emit no electrons at all.

Because of the large energy bandgap (5.5 eV) between the valence and conduction bands, the number of carriers in the diamond semiconductor is naturally small at room temperature. Currently known dopants have a very large ionization energy (about 1 eV) in diamond, so they have little effect on conduction below + 250 ° C. Therefore, even if the effective work function of diamond is positive and any work function between 0.2 eV and 0.7 eV (even if electron affinity is negative), its saturation current is low. Raising the saturation current is a major challenge to solve.

[Overview of invention]

It is an object of the present invention to provide an electron emitter having improved current characteristics.

Another object of the present invention is to provide a diamond or diamond-like carbon electron emitter having improved current characteristics.

Another object of the present invention is to provide a diamond or diamond shaped carbon electron emitter with improved saturation current.

It is another object of the present invention to provide a field emission device having a diamond or diamond shaped emitter having improved current characteristics.

In order to solve the above problems and achieve the above object, the present invention provides an electron emitter formed of a layer of material having a predetermined structure with a structurally electrically active defect at an emission position.

In order to solve the above problems and achieve the above object, the present invention provides an electron emitter formed of a material layer comprising diamond or diamond-like carbon having a diamond bonding structure having an electrically active defect at the emission position.

In order to solve the above problems and achieve the above object, the present invention provides a material comprising diamond or diamond-like carbon formed on the surface of the diamond or diamond-like carbon having a diamond bonding structure having an electrically active defect defining an electron emitter. A field emission device having a support substrate with layers is provided.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, tetrahedral bonding atoms in the lattice structure 10 of diamond-like carbon are shown. In the present specification, diamond-like carbon is defined as carbon in which defects are formed by carbon atoms bonded by diamond bonds generally referred to as abundance of SP ³ tetrahedral bonds, and diamonds as well as other diamond-containing carbon bonds. It includes any material. In the present specification, the graphite-like carbon is typically a graphite bonds is defined as the carbon in the crystal is a lattice structure formed, as well as the graphite by a carbon atom that is bonded in a known graphite bond, it referred to free the road for SP ² bond It includes any other material that contains.

The spatial lattice structure of carbon such as diamond is face-centered cubic fcc. The most basic cell of the grid is 0. 0 and 1/4. 1/4. Two identical carbon atoms at 1/4. This provides tetrahedral bonds, with the closest atom of each carbon atom being four and the next adjacent atom being twelve, with eight carbon atoms present in the unit cube. This structure is the result of covalent bonds.

In this covalent bond structure, there is a finite link between certain atoms, and the covalent electrons are mostly in the region between two covalent atoms (ie, the probability wave is the most dense between the atoms). Spend time. This creates bonds where the negative charge is concentrated, so adjacent bonds repel each other. When atoms such as carbon have several bonds (4 in diamond), the bonds occur at the same angle with respect to each other, which is 109 ° for diamond. The covalent bond is a directed bond and is very strong. The confinement energy of carbon atoms in diamond is 7.3 eV for isolated neutral atoms.

The diamond grating structure 10 shown in FIG. 1 is very important because the (111) plane in the structure is the same as the basic plane of the hexagonal closely packed (hcp) structure. In FIG. 2, a (111) layer (atoms denoted by A) is provided, with a second similar layer (atoms denoted by B) arranged above, the structure being no different from hcp. In other words, the structure becomes a face centered cubic or a close hexahedron. When the third layer (atomic indicated by C) is placed on the structure, it is determined whether it is an hcp structure or an fcc structure. If the third layer lies on a structure at the same position as the first layer, that is, if the C atoms are located directly above the A atoms without being displaced in the Z direction, the structure is a hcp structure or graphite. Layers of this structure may be represented by the ABABABAB structure. If the third layer is on a second possible position displaced from A and B atoms in the X, Y and Z directions (see also FIG. 2). The structure becomes an fcc structure or diamond. The layer of FIG. 2 may be represented by an ABCABCABC structure. In both structures (graphite and diamond structures of FIG. 2). The number of nearest neighbors is four. If the binding energy depends only on the closest near-field coupling, there is no difference between the fcc diamond structure and the hcp graphite structure. However, the atoms in the graphite layer are 1.4 Apart, bound by strong covalent bonds, but the interatomic distance between the layers is 3.3 Only weak van der Waals forces are present. Covalent bonds to graphite are planar bonds, ie bonds in a plane divided by 90 °.

The electrical properties of diamond and graphite are very different. Type IIb diamond naturally doped with boron is 10 ¹⁴ It has a resistivity of 10 ^{14 for} intrinsic diamonds. It has a much higher resistivity. Graphite 1375 10 ^-6 It is a metal conductor with a conductivity of. It has at least seven orders of magnitude difference in magnitude and about 20 orders of magnitude for intrinsic properties. Graphite About 5 per It is a semi-metal having a carrier of 10 ¹⁸ . The electrical conductivity of graphite is very large in the direction parallel to the hexagonal plane and small in the vertical direction (C axis). Incidentally, different orientations of covalent bonds with different energy levels act like effective conduction pathways. Therefore, the electrical properties can vary greatly for very small changes in the crystal structure between graphite and diamond.

There are several forms of defects that can occur in diamonds, which create useful properties of the present invention. The first defect is a screw dislocation, two embodiments of which are shown in FIGS. 3 and 4. There are also 60 ° dislocations and many other dislocations and variations that may easily form an extended network. The diamond grating has three slip faces, namely (001). (110). There is a (111) plane. The (111) plane is the most important slip plane, and it appears as the only slip plane that occurs in any situation except the most unusual ones.

In considering the lattice, the shortest transitional distance between any two carbon atoms in the diamond lattice is in the 110 direction (especially 1/2. 1 / 2.0 or the middle of the diagonal of the cube). Follow. The potential with Burgers vector in the 110 direction is the most stable (lowest free energy). Any direction in the grating can be regarded as the sum of consecutive 110 directions, so that a single potential has the same direction with respect to their axis. Three types of single potentials with Burgers vectors and axes along the 110 direction are spiral potentials, 60 ° potentials (with 60 ° Burgers vector to the potential axis) and edge type potentials with (100) sliding surfaces. All these potentials are used by electrically active defects.

5 schematically shows spiral defects in a diamond lattice. Spiral defects are generally the result of shearing, which occurs during the growth or deposition of diamond material. Like other dislocations, this dislocation creates an elastic strain field (stress field) in the surrounding crystal. To illustrate this, the radius r. If an annulus having a thickness dr and a unit length is arranged around the helical potential that causes the strength of the annulus 20 by b with an axial strength of b, then the average shear strain is b / 2π Strain stress

to be.

Where G is the strain rate. Note that the strain is long range since the stress is reduced to 1 / r. The strain energy of the annulus 20 per unit length

to be.

The strain energy of diamond crystals per unit potential length

Wherein R ₀ and R are the lower limit and the upper limit, respectively. R ₀ is a low level at which the lower limit for the integration, namely Hook's law, is not applied and the material behaves like an atom. The value for R ₀ is not critical because energy is its algebraic function. The upper level R is the boundary or other potential of the crystal that cancels the stress field. Since the energy of the strain field generated by the dislocation is a function of the square of the Burgers vector b, dividing the multiple potential into unit potential minimizes the free energy of the crystal. When two potentials with Burgers vectors b ₁ and b ₂ are combined into one potential with Burgers vector b ₃ , assuming that the change in irreversibility (TΔs) is not large, the increase in free energy is ΔE ΔE _e1 . This is a reasonable assumption in an elastic strain field without reorganization. ΔE _e1 is proportional to (b ₃ ² -b ₂ ² -b ₁ ² ). If ΔE _e1 is positive, the potentials are unstable, and potentials 1 and 2 repel each other. If ΔE _e1 is negative, the potential is stable and potentials 1 and 2 are attracted to each other. Due to the squared term of the Burgers vector size in the elastic energy, there are few multipotentials (eg, E _b3 (E _b2 + E _b1 )) at any location.

Typical values that can be substituted into the equation for strain energy are

G = 10 ⁸ psi (very conservative)

b = 2.5

Ro = 1b

R = 1 uM

The maximum radius R of the strain is optionally chosen to be 1 uM. The actual maximum radius is up to the boundary of the crystal. In practice, the range of crystal fields from strain defects is generally the distance from other strains that cancel the strain field with its strain field.

The energy of the strain field is relatively insensitive to R and R ₀ . The energy varies as a function of the ratio of the maximum field radius and the minimum field radius (before the material behaves like an atom). The above example using the aforementioned numbers is a calculation of the magnitude of energy used to estimate the possible behavior of the grating. Substituting this number, the strain energy is 44.4 eV or 17.8 eV / bond length Becomes This is enough energy to break the covalent bonds of the diamond lattice and regroup locally. Thus single bonds and even double bonds can be broken and regrouped. When the bond is reconstituted with covalent bonds remaining in the plane, a molecular film of graphite material is formed with its properties. The thin film of graphite structure is then added to the diamond structure so that an electrically active defect is formed.

6 shows a diamond-like material layer 30 having an electrically active defect 32. Generally, defects 32 in layer 30 are thinner (10 ) Tip of metal conductor coated with diamond (radius 10 It works similarly to an electron emitter formed of The superiority of this structure over a conventional field emission device is shown in FIGS. 7 and 8 are graphs showing electron emission characteristics of a conventional field emission device such as a tip, generally referred to as a Spindt emitter, and the device of FIG. 6, respectively. 7 is a graph of the emitted current (I) vs. the voltage or field potential applied to the tip. In Figure 7, the radius is 200 And a conventional conventional tip having a work function of 4.5 eV was used. In the graph of FIG. 8, the emitter of FIG. 6 has a radius of 10 And the work function of the material behaves like an emitter tip with 0.2 eV. Also, when the applied voltage or field potential is relatively low, the electron emission of the emitter of FIG. 6 is larger.

Alternative structures also exist because the structure of FIG. 6 appears to be a sharp tip-shaped emitter. If the electrically active defect 32 is located such that the free electrons in the defect 32 see the free space without the diamond layer (ie, on the surface of the layer 30), the defect 32 appears as a simple field emitter. 9 is a graph comparing electron emission of the above-described surface defects (curve 36) with respect to the conventional field emission device (curve 35). Curves 35 and 36 show electron emission for a free rod in the electric field as a function of tip radius, and for curve 35 molybdenum rods with a work function of 4.5 eV are used, curve 36 For the above, the above-described surface defect having a work function of 0.5 eV is used. The smaller the diameter, the slower the benefits of the lower work function of surface defects are lost at the sharp tip. If the mouthpiece is sharp enough, the work function becomes insignificant. Although a small work function is still desirable, reducing the emitter diameter reduces the need for improved emissions. The aforementioned defects (i.e. defects on the surface of the diamond) are sharper than any conventional field emitter tips, and therefore have substantial advantages in both work function and radius.

Lowering the tunnel barrier of the conducting element increases the emitted current. The change in work function is an important result, which links the behavior of diamond surface defects. In other words, if the surface of the diamond is contaminated or strain reconstituted into a non-diamond structure (except for the above example), the gain is lost. In order for the diamond layer to have a diamond bonding structure at the surface, a process known as hydrogenation is carried out on the exposed surface. In FIG. 10, the process is shown as a simplified diamond bond. Here, it can be seen that the non-hydrogenated carbon atoms 40, 41 are not bulk expanded, and thus are reconstituted into a stable low energy structure that does not have bulk characteristics. Stronger double bonds were formed between the carbon atoms 40, 41 than the surrounding single bonds, and thus the carbon atoms 40, 41 attract each other slightly closer. The low energy structure formed by the carbon atoms 40 and 41 is a poor electron emitter, which is undesirable for devices requiring such properties from the diamond.

The carbon atoms 42, 43, 44 are hydrogenated, that is, the hydrogen atoms 45, 46, 47 are each attached by a single bond. Therefore, the lattice structure formed of carbon atoms 42, 43, 44 appears the same on the surface, and thus appears as bulk expansion. The lattice structure of the carbon atoms 42, 43, 44 has bulk characteristics because it is bulk expansion, and thus a good electron emitter.

11 shows a cross section of a field emission device 50 using a hydrogenated layer 52 of diamond-like carbon with electrically active defects 53, 54, 55. Hydrogenation of layer 52 is shown by layer 56 on the surface. The electrically active defects 53, 54, 55 vary little by little at an angle and with slight differences in spacing, but generally appear almost perpendicular to the plane at regular intervals. For example, extended defects should be angled with respect to the surface of the diamond carbon to obtain the best results. It is also considered best if the extended defects make an angle in the range of 45 ° to 90 ° with respect to the surface.

The device 50 further comprises a support substrate 57, on which a conductive layer 58 is formed. Conductive layer 58 or conductive layers provide a means of electrically communicating with defects 53, 54, 55. Thus, as shown, current flow from a source (not shown) in conductive layer 58 is discharged into free space on layer 56 by defects 53, 54, 55.

Incomplete grids have empty vacancies. Interstitials. Impurities. Dislocations. Cellular and lineage structure. There are many kinds, such as grain boundaries and surfaces. The space in the lattice can actually lower the free energy of the crystal and therefore exists at equilibrium. The potential increases rather than lowers the free energy of the crystal. Thus, dislocations are defects of non-equilibrium type and can generally only be formed in an unequilibrium state during crystal growth. There are several types of errors in effectively causing dislocations. That is, (a) stress from mechanical sources applied externally, (b) thermally induced stress, (c) local stress due to increasing or decreasing concentrations of impurities, (d) sufficient space condensation, and (e) induced local Incorporation of stress (f) There are errors in the growth process. Externally applied mechanical stresses in the diamond bonds are generally removed due to the strength of the material. Thermally induced stresses and growth errors in the growth process are the two main causes of dislocations in the diamond material, which create the desired defects. Mistakes in growth are generally caused by multiple nucleation sites seeding growing and colliding crystal grains. If the two nucleation sites are sufficiently separated or different in orientation, eventually growing crystals meet and become different particles in the polycrystalline material. If the orientations of the two seeds are not identical but sufficiently similar, the growth lattice will meet and eventually form a spiral potential.

In the past, ion implantation of carbon C ⁺ was used to make conductive n-type diamond materials. The ion implantation can be used to create conductive defects due to the altered bonding structure in the crystal lattice. Note that although the present technology does not produce the best long conductive filament defects for electron emission, several advantages can be obtained from the technology, which are included within the scope of the present invention.

The diamond-like carbon electron emitter with improved current characteristics including improved saturation current has been described above. The improved current characteristics are realized by having timely active defects that locally enhance electron emission. In particular, the defects are formed of the same base material with different structures. Also described is a field emission device having a diamond-like emitter with improved current characteristics. Although carbon has been described herein, electron emitters made of materials such as aluminum nitride may be improved in a similar manner, i.e., with electrically active defects.

Although specific embodiments of the invention have been described above, changes and modifications may be made by those skilled in the art. Therefore, it is to be understood that the present invention is not limited to the above specific forms, and that all modifications without departing from the spirit and scope of the present invention are covered by the appended claims.

Claims (3)

An electron emitter formed of a material comprising a first phase portion characterized by a first chemical bond and a second phase portion characterized by a second chemical bond, wherein the first and second phase portions have a locally improved electron emission. And an electron emitter disposed adjacent to each other to define a non-isolated polyphase region in which the properties of the two phase portions are mixed to provide an improved electron emission structure. The electron emitter of claim 1, wherein the first and second chemical bonds are formed of aluminum nitride. An electron emitter formed of a material comprising a first phase portion characterized by a first chemical bond and a second phase portion characterized by a second chemical bond, wherein the first and second phase portions have a locally improved electron emission. An electron emitter disposed adjacent to each other to define a non-isolated polyphase region in which the properties of the two phase portions are mixed to provide an improved electron emission structure with the electron emitter disposed adjacent to the electron emitter, And a source connected to the conductive layer such that a conductive layer is in electrical communication with the emitting structure and a discharge current is generated from the current flow through the conductive layer and the improved electron emitting structure.