JP2000268709A - Ferroelectrtic electron emission cold cathode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To ensure a stable and large emission amount of electrons with a long service life by forming a protection film of high electron emission material on a main surface to emit electrons. SOLUTION: A ferroelectric PZT thin plate of bulk shape with sizes of 10×15 mm and a thickness of 50 μm and an Ir electrode having a film thickness of 500 nm layed over the whole rear surface of the plate are formed by spattering as a lower surface electrode 13. On the front surface thereof, as an upper surface electrode 14, an Ir electrode having a film thickness of 100 nm is formed in a stripe shape with a line of 500 μm and a space of 300 μm. On the whole front surface including the upper surface electrode 14, an MgO film serving as a protection film 15 is formed with a film thickness of 100 nm by an EB depostion method to thereby form a ferroelectric electron emission cathode 11. By forming the protection film 15 in this way, electrons are emitted similarly with the initinal case of starting application of voltage even if the number of application of pulse voltage exceeds 1000, and the amount of emission is made sufficiently stable. Also, by forming the film 15, the amount of emission itself is increased by about 30%.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron emission cold cathode used as an electron source, and more particularly to a structure of an electron emission cold cathode using a ferroelectric material.

[0002]

2. Description of the Related Art A ferroelectric has spontaneous polarization caused by permanent dipoles arranged in parallel or antiparallel.
It is possible to change this by applying an external perturbation. For example, a polarization change due to an electric field is observed as a polarization hysteresis (hysteresis), and when a stress or a temperature change is given, it appears as a piezoelectric effect or a pyroelectric effect, respectively. These phenomena are observed by the transfer of charges to and from the electrodes formed on the ferroelectric, but by appropriately selecting the shape or position of the electrodes, the film thickness, etc.
It is also possible to emit electrons from the ferroelectric.

[0003] Various researches have been conducted on the above-mentioned phenomenon of electron emission from a ferroelectric substance.
Various experiments such as inducing a polarization change by applying a perturbation such as application of an electric field, a temperature change, and light irradiation to a typical ferroelectric single crystal such as TiO ₃ from the outside have been performed. However, in these experiments, a relatively gradual change in polarization was used, and the current density of emitted electrons was extremely small at 10 ^-9 A / cm ² , so that application to practical devices was difficult. Was thought to be. However, recently, lead zirconate titanate (hereinafter abbreviated as PZT),
By applying a high-speed pulse electric field to a ferroelectric ceramic such as PLZT to which a small amount of La is added,
It is known that electron emission of about 10 ² A / cm ² is performed.
Since the report by Gundel et al., There has been increasing motivation to apply this phenomenon to electronic devices.

[0004] For example, H. The ferroelectric electron-emitting cold cathode 1 reported by Gundel et al., As shown in FIG. As shown in FIG. 2, the ferroelectric electron emission cold cathode 5 described in the publication includes an insulating film 6 and an auxiliary electrode 7 as main components in addition to a lower electrode 2, a ferroelectric 3, and an upper electrode 4. I have. The ferroelectric electron-emitting cold cathode having the structure shown in FIGS. 1 and 2 applies an alternating electric field between the lower electrode and the upper electrode, thereby causing a sudden change in the electric field to cause the ferroelectric internal cold cathode. This causes a change in polarization (polarization reversal), and at that time, electrons existing near the upper surface electrode are repelled by Coulomb force to emit electrons.

[0005]

However, the above-mentioned ferroelectric electron-emitting cold cathode has the following problems.

That is, in the conventional cold cathode, when an operation of applying an alternating electric field between the lower electrode and the upper electrode to cause polarization inversion in the ferroelectric material is repeated, the surface and / or the upper surface of the ferroelectric material are reduced. Fatigue and deterioration occur inside the electrode,
The emission of electrons gradually stops. This degradation is usually
Since this occurs when a pulse voltage is applied several to several tens of times, it is impossible to use the present cold cathode as it is.
Although it is conceivable to reduce the absolute value of the pulse voltage applied in order to suppress this deterioration, in this case, a new problem arises in that the current density of the emitted electrons decreases. Further, the conventional cold cathode has a problem that a sufficiently satisfactory electron emission amount has not yet been obtained, and the emission amount is unstable.

Due to these problems, at present, an electron emission cold cathode using a ferroelectric has not been put to practical use yet. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a ferroelectric electron-emitting cold cathode which has a long life and can stably obtain a large amount of emitted electrons, overcoming the above technical problems.

[0008]

In order to solve the above-mentioned problems, a ferroelectric electron-emitting cold cathode according to the present invention has a back electrode and an upper electrode on both main surfaces of a ferroelectric. In the ferroelectric electron-emitting cold cathode, a material having a high electron-emitting ability, such as Mg, is formed on the other main surface of the ferroelectric on which the upper electrode is formed.
O, ZnO, CeO ₂ , Y ₂ O ₃ , BaO, CaO, Sr
A protective film made of O or the like was formed.

As described above, by forming the protective film made of a material having a high electron-emitting ability on the main surface from which electrons are emitted, the conventional cold-cooling method can be used in both aspects of the life and the stability of the amount of emitted electrons. The present inventors have found that a ferroelectric electron-emitting cold cathode that is much better than the cathode can be obtained, and have completed the present invention. The detailed mechanism by which the above-described effects can be obtained by employing the configuration of the present invention is not clear at present, but the action that has been concentrated on the ferroelectric surface due to the sudden reversal of the polarization is likely to be caused by the protective film. It is considered that the electron emission from the protective film having a high electron emission ability is performed while being dispersed and relaxed on the side, thereby prolonging the life of the cold cathode and stabilizing the emission amount.

[0010] The ferroelectric used in the present invention is P
Ceramic ferroelectrics such as ZT, PLZT, and BaTiO ₃ , and polymer ferroelectrics such as PVDF can be used. The thickness of the ferroelectric is preferably 20 nm to 2000 μm, more preferably 100 nm to 200 μm. This depends on the conditions of use, but if the thickness of the ferroelectric material is less than 20 nm, there is a high possibility that the electrodes formed on both surfaces of the ferroelectric material will be short-circuited. It is necessary to do so. The ferroelectric used may be a bulk material or a thin film formed.

As the electrode material used in the present invention, any general electrode material can be used. Specifically, Pt, Au, Cu, Al, Ni, Ir, Cs, C
Metals such as r and W, and alloys thereof. Among them, an electrode material having a low work function, such as Ir or Cs, is desirable from the viewpoint of easy electron emission. Although any film formation method can be used as a method for forming the electrode, it is preferable to use a film formation technique such as an evaporation method or a sputtering method. This is because, as described later, in the cold cathode having this structure, when the thickness of the electrode is large, electrons are less likely to be emitted.

The thickness of the electrode (particularly, the upper electrode) formed by the cold cathode of the present invention is preferably 5000 ° or less, more preferably 500 ° or less. The electrode thickness is 5000mm
If the thickness is larger than that, emission of electrons is hindered due to the thickness. Although this value slightly varies depending on the electrode material used, for example, when Pt is used as the electrode material, it is most preferably about 100 to 500 °.

The operating electric field applied in the present invention may be either positive or negative, and in any case, its absolute value is preferably in the range of about 300 kV / cm or less. 3
This is because, when an electric field exceeding about 00 kV / cm is applied, a problem arises in that the electrode and the ferroelectric are broken by the high electric field. The pulse time of the operating electric field is
Both positive and negative are preferably in the range of 0.01 to 1000 μsec, more preferably 5 to 200 μsec. 0.01
If the time is less than μ seconds, the pulse time is short, so that a sufficient amount of electrons cannot be obtained, and even if an electric field is applied for more than 1000 μ seconds, the amount of electrons emitted is almost the same as the amount of electrons emitted in a pulse time of 1000 μ seconds or less. .

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A ferroelectric electron emission cold cathode 11 according to an embodiment of the present invention is mainly formed on a thin ferroelectric material 12 and on the entire back surface of the ferroelectric material 12, as shown in FIG. A lower electrode 13 to be formed, an upper electrode 14 formed in a stripe shape on the surface of the ferroelectric 12, and a protective film 15 formed on the entire front surface including the upper electrode 14. I have. Although FIG. 3 shows only the cross-sectional shape of the cold cathode 11, the upper electrode 14 and the protective film 15 are formed to extend in the depth direction.

Specifically, the ferroelectric material 12 is 10 × 1
A 5 mm square, 50 μm thick bulk PZT thin plate is used. The lower electrode 13 is formed by a sputtering method, and the upper electrode 14 is formed by a sputtering method. The stripe-shaped upper electrode 14 has a line / space of 500 μm / 3.
It is formed to a thickness of 00 μm. The protective film 15 is made of an MgO film and is formed to a thickness of 100 nm by the EB evaporation method.

Here, the cold cathode 11 of the present invention having the above-described structure is used.
And a conventional cold cathode (not shown) in which an MgO protective film is not formed (other structures are the same as the cold cathode 11 of the present invention) using a measuring apparatus shown in FIG. , Its lifetime and the stability of the amount of emitted electrons. The ferroelectrics constituting these two types of cold cathodes are electrically polarized in advance. When measuring,
Only one stripe of the top electrode is used.

Hereinafter, the measuring method will be specifically described. First, as shown in FIG. 4, the sample 20 is fixed to the holder 22 in the vacuum chamber 21 and then the upper electrode of the sample 20 is grounded. Next, positive and negative pulse voltages generated by the pulse applying unit 23 are applied to the lower electrode 13. At this time, the applied pulse is ± 200 V and the pulse width is 10 μsec.
Electrons emitted by the application of the pulse voltage are collected by a collector 24 arranged 10 mm above the sample 20. Here, the peak value of the emission current is measured by observing a change in potential caused by the current passing through the resistor R using the oscilloscope 25, and time integration is performed based on the peak value using the integration operation function of the oscilloscope. By doing so, the amount of emitted electrons is calculated.

FIG. 5 shows a comparison between the cold cathode 11 of the present invention and the conventional cold cathode obtained according to the above-described measuring method. As can be seen from FIG. 5, in the conventional cold cathode, the amount of emitted electrons is almost zero by application of a pulse voltage of about 10 times. It is considered that this is because the ferroelectric and / or the top electrode deteriorated due to the repetition of rapid polarization reversal, as described in the section of the prior art. On the other hand, in the cold cathode 11 of the present invention on which the protective film 15 is formed, the pulse voltage application frequency is 1000 times.
Even after the number of times, electrons are emitted as in the beginning of the voltage application, and the amount of emitted electrons is also sufficiently stable.
Further, it can be confirmed that the emission amount itself is increased by about 30% as compared with the conventional cold cathode by forming the protective film 15.

In this embodiment, MgO is used as the material of the protective film 15. However, similarly, ZnO having a high secondary electron emission ability is used.
Measurement of each material of O, CeO ₂ , Y ₂ O ₃ , BaO, CaO, and SrO confirmed that the same effect as when the MgO film was formed was obtained. Further, in the present embodiment, the protective film 15 is formed on the entire surface of the ferroelectric material 12, but, for example, as shown in FIG. 6, the protective film is formed near three boundaries between the ferroelectric material 12, the upper electrode 13, and the air. The same effect can be obtained by forming. In addition, in this example, a bulk PZT thin plate was used as the ferroelectric,
For example, a thin film P formed by a plasma CVD method or the like.
A ZT thin film may be used as the ferroelectric of the present invention.

[0020]

As described above, by forming the protective film made of a material having a high electron emission ability on the main surface from which electrons are emitted, the polarization is suddenly reversed, so that the concentration is concentrated inside the ferroelectric. Electrons are emitted from the protective film while dispersing and mitigating the effect that has occurred on the protective film side, so that both the lifetime and the stability of the amount of emitted electrons are significantly better than conventional cold cathodes. A ferroelectric electron emission cold cathode can be obtained.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a conventional ferroelectric electron emission cold cathode.

FIG. 2 is a cross-sectional view showing another conventional ferroelectric electron emission cold cathode.

FIG. 3 is a sectional view showing a ferroelectric electron emission cold cathode according to the present invention.

FIG. 4 is a schematic sectional view showing an apparatus for measuring the characteristics of a ferroelectric electron emission cold cathode.

FIG. 5 is a comparison diagram comparing the electron emission characteristics of the cold cathode of the present invention and a conventional cold cathode.

FIG. 6 is a sectional view showing a cold cathode according to another embodiment of the present invention.

[Explanation of symbols]

 11: Ferroelectric electron emission cold cathode 12: Ferroelectric 13: Lower electrode 14: Upper electrode 15: Protective film

Claims (3)

[Claims] 1. A ferroelectric comprising: a ferroelectric having spontaneous polarization; a back electrode formed on one main surface of the ferroelectric; and an upper electrode formed on the other main surface of the ferroelectric. And a protective film made of a material having a high electron emission ability is formed on the other main surface of the ferroelectric on which the upper electrode is formed. cathode. 2. The material of said protective film is MgO, Zn
_{O, CeO 2, Y 2 O} 3, BaO, CaO, ferroelectric electron emission cold cathode as claimed in claim 1, characterized in that using at least one selected from the group consisting of SrO. 3. The ferroelectric electron emission cold cathode according to claim 1, wherein the upper surface electrode is formed in a stripe shape, a mesh shape, or an array shape.