JP4114264B2 - Ferroelectric electron emission cold cathode - Google Patents

Description

[0001]
BACKGROUND 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.
[0002]
[Prior art]
Ferroelectrics have spontaneous polarization caused by permanent dipoles arranged in parallel or antiparallel, and this can be changed by applying a perturbation from the outside. For example, a polarization change due to an electric field is observed as a polarization history phenomenon (hysteresis), and appears as a piezoelectric effect or a pyroelectric effect when a stress or temperature change is applied. These phenomena are observed by the entrance and exit of electric charges to and from the electrodes formed on the ferroelectric. By appropriately selecting the shape or formation position of the electrode, the film thickness, etc., electrons from the ferroelectric can be observed. Release is also possible.
[0003]
Various studies have been conducted on the electron emission phenomenon from the above-described ferroelectrics. For example, a typical ferroelectric single crystal such as BaTiO ₃ is applied with an electric field, temperature change, and light irradiation from the outside. Various experiments have been conducted to induce polarization changes by applying perturbations such as. However, these experiments use a relatively gradual polarization change, and the current density of emitted electrons is as small as 10 ^-9 A / cm ² , so that it is difficult to apply to practical devices. It was thought to be. However, in recent years, by applying a high-speed pulse electric field to a ferroelectric ceramic such as lead zirconate titanate (hereinafter abbreviated as PZT) or PLZT to which a small amount of La is added, 10 to 10 ² A That the electron emission of about / cm ² is performed. Since being reported by Gundel et al., There is a growing momentum to apply this phenomenon to electronic devices.
[0004]
For example, H.M. As shown in FIG. 1, a ferroelectric electron emission cold cathode 1 reported by Gundel et al. Has a lower surface electrode 2, a ferroelectric material 3, and an upper surface electrode 4 as its main components, and for example, Japanese Patent Laid-Open No. 5-325777. As shown in FIG. 2, the ferroelectric electron emission cold cathode 5 described in the publication has an insulating film 6 and an auxiliary electrode 7 in addition to the lower surface electrode 2, the ferroelectric material 3, and the upper surface electrode 4, as main components. Yes. The ferroelectric electron emission cold cathode having the structure shown in FIGS. 1 and 2 applies an alternating electric field between the lower surface electrode and the upper surface electrode, so that the internal electric field of the ferroelectric material changes with a rapid change in the electric field. It is considered that a change in polarization (polarization reversal) occurs, and electrons existing in the vicinity of the upper surface electrode are repelled by Coulomb force to emit electrons.
[0005]
[Problems to be solved by the invention]
However, the ferroelectric electron emission cold cathode described above has the following problems.
[0006]
That is, in the conventional cold cathode, when an alternating electric field is applied between the lower surface electrode and the upper surface electrode and the operation of causing polarization inversion in the ferroelectric body is repeated, the surface of the ferroelectric and / or the upper surface electrode is repeated. Fatigue and deterioration occur, and electrons are no longer emitted. Since this deterioration usually occurs when a pulse voltage is applied several times to several tens of times, it has been impossible to put the cold cathode into practical use as it is. In order to suppress this deterioration, it is conceivable to reduce the absolute value of the applied pulse voltage. However, in this case, there arises a new problem that the current density of emitted electrons is reduced. In addition, the conventional cold cathode has not yet obtained a sufficiently satisfactory amount of electron emission, and has a problem that the amount of emission is unstable.
[0007]
In view of these problems, an electron emission cold cathode using a ferroelectric material has not yet been put into practical use. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a ferroelectric electron emission cold cathode which can overcome the above technical problems, has a long life, and can stably obtain a large electron emission amount.
[0008]
[Means for Solving the Problems]
In order to solve the above-described problems, the ferroelectric electron emission cold cathode of the present invention is a ferroelectric electron emission cold cathode having a back electrode and an upper electrode on both main surfaces of the ferroelectric, on the other main surface of the formed ferroelectric of the upper electrode, a material having high electron emission ability, e.g. _{MgO, ZnO, CeO 2, Y} 2 O 3, BaO, CaO, to form a protective film made of SrO or the like.
[0009]
In this way, by forming a protective film made of a material having a high electron emission capability on the main surface from which electrons are emitted, in terms of both life and stability of the amount of emitted electrons, compared to conventional cold cathodes. The present inventors have found that a ferroelectric electron emission cold cathode that is remarkably excellent can be obtained, and have completed the present invention. Although the detailed mechanism by which the above-described effect can be obtained by employing the configuration of the present invention is not clear at present, the protective film is probably caused by the action concentrated on the ferroelectric surface by abruptly reversing the polarization. It is considered that the lifetime of the cold cathode and the emission amount are stabilized by emitting electrons from the protective film having high electron emission ability while being dispersed and relaxed on the side.
[0010]
As the ferroelectric used in the present invention, ceramic ferroelectrics such as PZT, PLZT, and BaTiO ₃ , polymer ferroelectrics such as PVDF, and the like can be used. The thickness of the ferroelectric is preferably 20 nm to 2000 μm, more preferably 100 nm to 200 μm. Although this depends on the conditions of use, if the thickness of the ferroelectric material is less than 20 nm, the electrodes formed on both surfaces are more likely to short-circuit, and if the thickness is greater than 2000 μm, the operating electric field becomes very large. It is necessary to do this. In addition, the ferroelectric material used may be a bulk material or a thin film formed.
[0011]
Any general electrode material can be used as the electrode material used in the present invention. Specific examples include metals such as Pt, Au, Cu, Al, Ni, Ir, Cs, Cr, W, and alloys thereof. In particular, an electrode material having a low work function such as Ir or Cs is desirable in terms of the ease of electron emission. Although any film forming method can be used as the method for forming the electrode, it is preferable to use a film forming technique such as vapor deposition or sputtering. This is because, as will be described later, in the cold cathode of this structure, electrons tend to be less likely to be emitted as the electrode thickness increases.
[0012]
The thickness of the electrode (particularly the upper surface electrode) formed by the cold cathode of the present invention is preferably 5000 mm or less, more preferably 500 mm or less. This is because when the thickness of the electrode is greater than 5000 mm, electron emission is hindered due to the thickness. Although this value varies slightly depending on the electrode material used, for example, when Pt is used as the electrode material, about 100 to 500 mm is most preferable.
[0013]
The operating electric field applied in the present invention may be either positive or negative, and in either case, the absolute value is preferably in the range of about 300 kV / cm or less. This is because, when an electric field exceeding about 300 kV / cm is applied, the high electric field causes a problem that the electrode and the ferroelectric material are destroyed. The pulse time of the operating electric field is preferably 0.01 to 1000 μsec, more preferably 5 to 200 μsec, both positive and negative. If the pulse time is less than 0.01 μs, a sufficient amount of electron emission cannot be obtained because the pulse time is short, and even if an electric field is applied over 1000 μs, the amount of electron emission emitted in a pulse time of 1000 μs or less is almost the same. It is.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
As shown in FIG. 3, a ferroelectric electron emission cold cathode 11 according to an embodiment of the present invention mainly includes a thin plate-shaped ferroelectric 12, a bottom electrode 13 formed on the entire back surface of the ferroelectric 12, and The upper surface electrode 14 formed in a stripe pattern on the surface of the ferroelectric 12 and a protective film 15 formed on the entire surface including the upper surface electrode 14. FIG. 3 shows only the cross-sectional shape of the cold cathode 11, but the upper surface electrode 14 and the protective film 15 are formed so as to extend in the depth direction.
[0015]
Specifically, a bulk PZT thin plate of 10 × 15 mm square and 50 μm thickness is used as the ferroelectric body 12. Further, an Ir electrode having a thickness of 500 nm is formed on the lower surface electrode 13 and an Ir electrode having a thickness of 100 nm is formed on the upper surface electrode 14 by a sputtering method. The striped top electrode 14 is formed with a line / space of 500 μm / 300 μm. The protective film 15 is made of an MgO film and is formed to a thickness of 100 nm by EB vapor deposition.
[0016]
Here, the cold cathode 11 of the present invention having the above-described configuration and a conventional cold cathode (not shown) in which an MgO protective film is not formed (other configurations are the same as those of the cold cathode 11 of the present invention). The lifetime of the sample and the stability of the amount of emitted electrons are measured using the measuring apparatus shown in FIG. The ferroelectrics constituting these two types of cold cathodes are electrically polarized in advance. In the measurement, only one stripe of the upper surface electrode is used.
[0017]
Hereinafter, the measurement 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 the upper electrode of the sample 20 is grounded. Next, positive and negative pulse voltages generated by the pulse applying means 23 are applied to the lower surface 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 disposed 10 mm above the sample 20. Here, a change in potential due to the current passing through the resistor R is observed using an oscilloscope 25 to measure the peak value of the emission current, and time integration is performed using the integration calculation function of the oscilloscope based on this peak value. As a result, the amount of emitted electrons is calculated.
[0018]
Here, with respect to the cold cathode 11 of the present invention and the conventional cold cathode, the measurement results obtained according to the above-described measurement method are shown in FIG. As can be seen from FIG. 5, in the conventional cold cathode, the amount of emitted electrons is almost zero by applying the pulse voltage about 10 times. This is presumably because the ferroelectric and / or the upper surface electrode deteriorated due to repeated repeated polarization inversion as described in the prior art section. On the other hand, in the cold cathode 11 of the present invention in which the protective film 15 is formed, even when the number of pulse voltage applications exceeds 1000, electrons are emitted as in the beginning of voltage application, and the emission amount is sufficiently stable. have. Further, it can be confirmed that by forming the protective film 15, the emission amount itself is increased by about 30% compared to the conventional cold cathode.
[0019]
In this example, MgO was used as the material of the protective film 15, but similarly measured for each material of ZnO, CeO ₂ , Y ₂ O ₃ , BaO, CaO, SrO having a high secondary electron emission ability, It was confirmed that the same effect as that obtained when the MgO film was formed was obtained. In this embodiment, the protective film 15 is formed on the entire surface of the ferroelectric 12. However, as shown in FIG. 6, for example, a protective film is formed near the three boundary portions of the ferroelectric 12, the upper surface electrode 13, and air. The same kind of effect can be obtained by forming. In addition, in this embodiment, a bulk PZT thin plate is used as the ferroelectric, but a thin PZT thin film formed by, for example, a plasma CVD method may be used as the ferroelectric of the present invention.
[0020]
【The invention's effect】
In this way, by forming a protective film made of a material having a high electron emission capacity on the main surface from which electrons are emitted, the action that has been concentrated inside the ferroelectric substance by abruptly reversing the polarization can be obtained. Since electrons are emitted from the protective film while being dispersed and relaxed on the protective film side, ferroelectric electron emission is far superior to conventional cold cathodes in both aspects of life and stability of the amount of emitted electrons. A 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 a characteristic measuring apparatus of a ferroelectric electron emission cold cathode.
FIG. 5 is a comparative diagram comparing electron emission characteristics of a cold cathode of the present invention and a conventional cold cathode.
FIG. 6 is a cross-sectional view showing a cold cathode according to another embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 11 ... Ferroelectric electron emission cold cathode 12 ... Ferroelectric 13 ... Lower surface electrode 14 ... Upper surface electrode 15 ... Protective film

Claims (2)

A ferroelectric electron emission cold cathode comprising a ferroelectric having spontaneous polarization, a back electrode formed on one main surface of the ferroelectric, and a top electrode formed on the other main surface of the ferroelectric Because
A protective film made of at least one material selected from MgO, ZnO, CeO ₂ , Y ₂ O ₃ , BaO, CaO, and SrO is formed on the other main surface of the ferroelectric on which the upper surface electrode is formed. A ferroelectric electron emission cold cathode.  2. 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.

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