JP2000114508A - Single electron tunneling element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a single electron tunneling element which has stable fundamental operations as an element, is highly stable against fluctuations in the ambient temperature, can store data for a long time when the element is used as a memory, and can obtain a high S/N(signal/noise ratio). SOLUTION: A single electron tunneling element 10 is provided with a first electrode 13 and a second electrode 14 both of which are constituted of electric conductors, a third electrode 15 which is connected to the first and second electrodes 13 and 14 through a single electron tunneling junction, and a fourth electrode 20 which is connected to the third electrode 15 through an insulating film 19 and constitutes a capacitor together with the electrode 15. The third electrode (intermediate electrode) 15 is constituted to contain at least two or more very small independent areas composed of a ferromagnetic material in an insulator 21 by scattering the particles 22 of the ferromagnetic material in the insulator 21.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a single electron tunneling device, and more particularly, to a single electron tunneling device capable of utilizing both charge and spin as attributes of electrons.

[0002]

2. Description of the Related Art In recent years, advances in science and technology related to physics and electronics, and on the other hand, advances in microfabrication technology in the field of semiconductors, etc., have deepened understanding of electron tunneling phenomena. Has been observed experimentally. That is, the effect competing with the tunnel effect has an electrostatic effect. When electrons jump into the metal due to the tunnel effect, an imbalance of electric charges occurs and the electrostatic energy increases. Therefore, the tunnel effect is suppressed by the electrostatic effect. This is called the Coulomb blockade phenomenon. More specifically, when the capacity of the junction becomes extremely small, a difference occurs in the electrostatic energy depending on whether or not the tunnel junction stores one electron, and the energy of the difference is e ² / 2C [ e is the elementary charge (e = 1.6
X10 ^-19 coulomb), and C is the junction capacitance]. When C becomes very small, the energy of the difference rapidly increases and becomes experimentally observable. Attempts to fabricate functional elements such as transistors and memories called so-called single electron tunnel elements using the Coulomb blockade phenomenon have been made recently. This single-electron tunneling device is well-suited to miniaturization and high-density devices, and has features that are suitable for low power consumption. It has been studied as a next-generation device, and the "Introduction to SingleChage Tnn"
nling.Coulomb Blockade Phenomena in Nanostructure
s ", Proc. NATO Advanced Study Inst. Les Houchs, Franc
e, March 1991, eds.MHDevort and H. Grabert (Plenum, 1
992) 1-19 or "Room-Temperature Single-Elect
ron Memory ", K.Yano, T.Ishii, T.Hashimoto, T.Kobayash
i, F.Murai, and K.Seki: IEEE TRANSACTIONS ON ELECTRON
DEVICES, VOL. 41, No. 9, P. 1628, 1994 and the like.

[0003] However, the conventional single-electron tunneling device disclosed in the above-mentioned literature, which is based solely on the Coulomb blockade phenomenon, has the following disadvantages. In other words, since the Coulomb blockade phenomenon puts the operating principle (basic of operation) only on the so-called capacitance and charging phenomena, a single-electron tunneling element based solely on the Coulomb blockade phenomenon has a large effect of leakage current. There is a disadvantage that the basic operation becomes unstable. Specifically, there is a drawback that a practically sufficient S / N ratio (signal / noise ratio) cannot be obtained (that is, the S / N ratio is low). In addition, due to the property of easily leaking electric charge and the leakage current, there is a disadvantage that a storage time (storage holding time) cannot be sufficiently secured when applied to, for example, a memory element. Further, there is a drawback that stability against changes in ambient temperature and instability of current caused by the probability of a tunnel phenomenon cannot be improved.

Japanese Patent Application Laid-Open No. 7-94759 discloses a single-electron tunneling element in which the degree of freedom of electron spin is included in one of its characteristics. This publication describes that the object is to include the degree of freedom of electron spin in one of the characteristics in the conventional single-electron tunneling device described above. However, in the single-electron tunneling device described here, the ferromagnetic material having spins is formed in a continuous single structure, and according to this structure, the electron spins take an ordered arrangement, and The spin directions are always always the same. Further, the structure is such that a completely irregular arrangement of spins cannot be obtained. Therefore,
In this structure, the arrangement of spins can be changed from a completely disordered arrangement to an ordered arrangement. Therefore, the degree of freedom of electron spin, which controls electron conduction (electron transfer) depending on the degree of spin orientation, is characteristic. It is clear that the principle or basic structure suitable for the purpose of reflecting the above has not been achieved, and depending on the configuration of the single electron tunneling element described in this publication, the degree of freedom of electron spin can be set to a characteristic. It cannot be reflected, and no remarkable improvement from the conventional single-electron tunneling device described in each of the above documents is seen. In addition, the characteristics and reliability were insufficient.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned drawbacks of the prior art, and an object of the present invention is to provide a device having a stable basic operation as a device and a high S / O ratio.
An object of the present invention is to provide a single electron tunneling device capable of obtaining an N ratio (signal / noise ratio). Further objects of the present invention are:
An object of the present invention is to provide a single-electron tunneling device having high current stability and high stability against changes in ambient temperature. It is still another object of the present invention to provide a single electron tunneling element having a sufficiently long storage time (storage retention time) when used as a memory.

[0006]

The single electron tunneling device of the present invention capable of achieving the above object of the present invention comprises a first electrode and a second electrode, and a single electrode with respect to the first and second electrodes. A third electrode connected via a one-electron tunnel junction, and connected to the third electrode via an insulating film;
A single-electron tunneling device including a third electrode and a fourth electrode constituting a capacitor, wherein the third electrode has at least two minute regions made of a ferromagnetic material in its structure. Here, the minute region composed of at least two ferromagnetic materials may be formed by ferromagnetic particles dispersed in an insulator, or at least two layers of ferromagnetic materials alternately stacked with the insulator. And the third electrode may be an artificial lattice made of an insulator and a ferromagnetic material.

[0007] The ferromagnetic material in the third electrode of the present invention is:
It is made of one or more materials selected from the group consisting of iron, cobalt, nickel, and alloys thereof, and the insulator is selected from the group consisting of aluminum oxide, silicon oxide, and magnesium oxide. It is preferable that it is composed of one or more materials. Further, a mutual distance between at least two small regions made of ferromagnetic material in the third electrode is in a range exceeding zero and not more than a diffusion length of electron spin. Furthermore, it is preferable that the single-electron tunneling device of the present invention includes a means for controlling the arrangement and / or direction of the spins of the electrons in at least two ferromagnetic materials in the third electrode.

The operation of the single-electron tunneling device of the present invention will be described. In the conventional single-electron tunneling device based on a change in Coulomb energy between electrons, the increase in Coulomb energy due to single-electron tunneling is limited by the single-electron tunneling. It determines the characteristics of the tunnel element when it is created, and does not depend at all on spin, which is another degree of freedom of electrons. However, in the present invention, the intermediate electrode (third electrode) is an electrode including at least two or more minute regions made of a ferromagnetic material, and two independent minute regions made of a ferromagnetic material are provided in the electron propagation path. With the introduction described above, the effect of the magnetic field, that is, the effect of the spin blockade phenomenon can be added to the single-electron tunneling element.

In the single electron tunneling device of the present invention, electrons tunnel through the insulator while maintaining their spin directions. This principle will be described with reference to the conceptual diagram of FIG. When electrons tunnel, for example, when there is no application of an external magnetic field and the spin directions of all the small regions are not uniform and irregular (random) (FIG. 2A), the tunnel electrons are easily scattered, The resistance of the electron to the tunnel is large, making it difficult for the electron to tunnel (move). Conversely, when the spin directions of a plurality of independently existing micro regions are aligned (FIG. 2B) due to the application of an external magnetic field (indicated by the arrow H in FIG. 2), the tunnel Electrons are not easily scattered, the resistance of the electrons to the tunnel is small, and the electrons easily tunnel (move) (spin-dependent tunneling). Furthermore, the irregularity (or regularity) can be controlled by arbitrarily controlling the degree of alignment of the spin direction between the micro regions.
The tunnel difficulty can be controlled according to the change of the tunnel.
This conceptual diagram illustrates the importance of having at least two independent ferromagnetic regions. That is, the change in energy when only the change in Coulomb energy is considered is represented by Uc (which indicates an effective increase in electrostatic energy when two are connected in series in a single junction) and the flip of spin ( For example, Us adds a change in energy accompanying a change in upward spin to a downward spin), and changes in characteristics (for example, IV characteristics) due to the magnitude relationship between Uc and Us when tunneling from one electrode to the intermediate electrode. The controllable novel single electron tunneling device of the present invention one electron can be realized. This makes it possible to realize a single tunneling element based on a new principle combining the effect of spin and the effect of electrostatic energy.

The effect of the magnetic field (the effect of the spin-dependent tunneling) facilitates the control of electron tunneling, and provides a high S / N ratio (signal / noise ratio) such as an output current. Furthermore, by switching the spin orientation between regular and irregular by controlling the external magnetic field, the tunneling process of electrons can be more efficiently controlled, and the instability of electron movement (current) due to the stochastic nature of tunneling. Is to supplement. Furthermore, the action of the magnetic field is based on the magnetization phenomenon of the ferromagnetic material, and is of a non-capacitive type, that is, a non-charge type, which is basically free from the influence of leakage current, thereby further increasing the S / N ratio. It will improve.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to basic embodiments. FIG. 1 is a schematic sectional view showing one embodiment of a single electron tunneling device 10 according to the present invention. An element is formed on a silicon oxide film 12 on a silicon substrate 11, and a first electrode 13 shown in FIG.
And the second electrode 14 are composed of electric conductors. Known conductors such as aluminum and gold can be used for these conductors. The third electrode (intermediate electrode) 15 has a structure including at least two or more independent minute regions made of a ferromagnetic material. Specifically, as shown in FIG. 1, a structure in which ferromagnetic particles 22 are dispersed in an insulator 21 (granular structure)
Alternatively, a structure formed by an artificial lattice including a ferromagnetic material and an insulator may be used. As the ferromagnetic material in the intermediate electrode 15, a known ferromagnetic material can be used, and at least one of iron, cobalt, nickel, and an alloy thereof is used.
It is preferable to use seeds in order to obtain a clearer spin arrangement effect. As the insulator in the third electrode 15, a known insulator can be used, but it is preferable to use at least one of aluminum oxide, silicon oxide, and magnesium oxide in order to obtain a better tunnel effect. Is preferred.

The present invention is characterized in that the third electrode 15 has a structure including at least two or more minute regions made of a ferromagnetic material as described above. FIG. 3 shows the insulator 21
It is a conceptual diagram showing the spin direction of ferromagnetic particles 22 in third electrode 15 having a structure (granular structure) in which ferromagnetic particles 22 are dispersed therein. FIG. 3A shows a case where there is no application of an external magnetic field in the granular structure, and the spin directions of all the small regions are not uniform and are random (random). FIG. Middle arrow H
) Indicates that all the spin directions of the plurality of independently existing micro regions are aligned. FIG.
FIG. 4 is a conceptual diagram showing the spin direction of the ferromagnetic material 22 in the third electrode 15 formed by the artificial lattice including the ferromagnetic material 22 and the insulator 21. FIG. 4A shows a case where no external magnetic field is applied in the artificial lattice structure, and the spin directions of all the small regions are irregular and random (random). FIG. 4 indicates the case where all the spin directions of the plurality of independently existing micro regions are aligned.

In the embodiment shown in FIG. 1, the third electrode 15 has a structure in which ferromagnetic particles 22 are dispersed in an insulator 21, but the ferromagnetic region of the third electrode 15 is
In the case of a structure in which the electrodes 13 and 14 are in contact with each other, the electrodes 13 and 14
It is necessary to provide an insulating layer such as an insulating film between the first electrode 15 and the third electrode 15. Similarly, the insulating layer can be composed of an oxide such as aluminum oxide, silicon oxide, and magnesium oxide. The fourth electrode 20 can be made of aluminum, gold, or the like. FIG. 5 is a schematic cross-sectional view showing a state in which a third electrode 15 having a granular structure is arranged between the first electrode 13 and the second electrode 14, and FIG. 5A is a partially enlarged view of the embodiment of FIG. In this case, the third electrode 15 and the electrodes 13 and 14 are in close contact with each other. However, a predetermined insulating region 16 can be formed between the electrodes as shown in FIG.

FIG. 6 is a schematic sectional view showing a state in which a third electrode 15 having an artificial lattice structure is arranged between the first electrode 13 and the second electrode 14, and FIG. Since the ferromagnetic body 22 is in contact with the electrodes 13 and 14 due to the direction of the above, a predetermined insulating region 16 is provided between the two electrodes. As shown in FIG. 6B, when the insulators 21 are located at both ends of the third electrode 15 due to the directionality of the artificial lattice, the third electrode 15 and the electrodes 13 and 14 are in close contact with each other. Is also good.

Here, the insulator 21 in the third electrode 15
The mutual distance between the microregions 22 made of ferromagnetic material separated by the above is within a range of more than zero and less than or equal to the diffusion distance of the spin of electrons. Is important. The mutual distance between the minute regions depends on the combination of the ferromagnetic material and the insulator of the third electrode constituting material, but is desirably between 5 Å and 2000 Å. Below 5 Å, device fabrication becomes difficult, and above 2000 Å, spin-dependent tunneling is no longer possible. More preferably, it is desirable to be between 10 angstroms and 1500 angstroms. If it is less than 10 angstroms, device fabrication becomes increasingly difficult, and if it is more than 1500 angstroms, the preservation of the spin direction in the tunnel becomes increasingly poor.

The granular structure and artificial lattice structure comprising a ferromagnetic material and an insulator in the third electrode of the present invention can be formed by a reactive sputtering method, a sputtering method, an ion plating method, a plasma CVD (plasma chemical vapor deposition).
Method, CVD (chemical vapor deposition), molecular beam epitaxy (MBE), electron beam evaporation (EB), ionization evaporation, vacuum evaporation, etc., or electroplating,
It can be manufactured by appropriately using a known method such as a wet method such as an anodizing method. Further, by controlling these manufacturing conditions, the mutual distance between the ferromagnetic minute regions can be adjusted. In the case of the dry method, a so-called multiple evaporation source mechanism is preferably used so that a plurality of types of raw material can be set.

In the single electron tunneling device of the present invention, in addition to the above-mentioned electrodes and the insulating film, there may be provided a means for changing the arrangement (orientation) of the spins of the electrons in the ferromagnetic material at the third electrode. It is desirable from the viewpoint of improving the effect of reflecting the degree of freedom of electron spin on the characteristics.
As means for controlling the arrangement of electron spins that can be used here, means such as magnetization by a word line current or magnetization by a minute magnet can be considered. On the other hand, when this type of magnetization is not applied, the spins are in a random arrangement (at a granular structure) or an antiferromagnetic arrangement (at an artificial lattice structure), and the spin arrangement (orientation) is in an irregular state (random orientation). Is achieved.

[0018]

EXAMPLES Hereinafter, the present invention will be described specifically with reference to Examples, but the present invention is not limited thereto. (Embodiment 1) A method for manufacturing an element structure shown in a schematic sectional view in FIG. 1 used in this embodiment will be described below. In the first embodiment, the third electrode has a structure (granular structure) in which ferromagnetic particles are dispersed in an insulator. First, a 220 nm silicon oxide film 12 is formed on a silicon substrate 11. An aluminum film having a thickness of 20 nm is formed thereon. Subsequently, a resist is formed on the aluminum, and the resist is patterned by using an electron beam lithography technique. Thereafter, the aluminum is processed into an aluminum line having a width of 100 nm by using a reactive ion etching (RIE) method. After processing, the resist is removed.

Next, after a silicon oxide film 18 is formed to a thickness of 20 nm by a chemical vapor deposition method, electron beam exposure and RI
Drill a groove at right angles to the aluminum wire using the E method. At this time, the underlying aluminum is also etched together so that the etching slightly extends to the underlying silicon oxide film. Although not provided in this embodiment, when an insulating film is formed as necessary, the cross section of the cut aluminum wire is thermally oxidized at 500 ° C. to form an insulating film having a thickness of several nm. I just need. Then, Al and Co, which is a ferromagnetic material, are simultaneously sputtered in an oxygen atmosphere by using a reactive sputtering method, so that Al ₂ O
_{3 A} 45 nm-thick third electrode having a granular structure in which Co particles (ferromagnetic particles) having a particle size of about 2 nm are dispersed in (insulator) 21 is deposited, and the dispersion deposited on the upper surface is etched. When measured with a transmission electron microscope, the distance between adjacent particles 22 was about 1 nm on average, which was the diffusion distance of electron spin (about 20 to 50 n).
m). Subsequently, after a silicon oxide film 19 and an aluminum film 20 were deposited, patterning was performed by an ordinary method to form a single electron tunneling element having the fourth electrode 20 and having the shape shown in FIG.

In the single electron tunneling device thus manufactured, after applying a magnetic field of about +5 kOe to align the spin direction of Co particles in the third electrode in one direction,
When the tunnel current-gate voltage characteristics were measured, excellent output characteristics with good responsiveness were obtained, and the S / N ratio (signal /
The noise ratio was as high as 54 db (decibel). Further, it was confirmed that the same characteristics were exhibited even when the environmental temperature was changed to 10 to 40 ° C., and the S / N ratio was stable even when the ambient temperature changed. Further, it was confirmed that the tunnel current was changed by changing the magnitude of the applied magnetic field from 0 to +5 kOe to change the degree of regularity of the spin arrangement of the ferromagnetic material (Co), and it was possible to control the tunnel current. Was.

Example 2 An element structure shown in FIG. 1 was manufactured in the same manner as in Example 1. However, in the second embodiment, the third electrode has an artificial lattice structure including a thin-film laminated structure of an insulator and a ferromagnetic material. Except for the third electrode, it was formed in the same manner as in Example 1, and the third electrode was formed as follows. First
A ferromagnetic Fe layer is formed by the EB method. Next, Al
By sputtering in an oxygen atmosphere,
_{A 2} O ₃ layer is formed. Each layer thickness is between 20 and 40 angstroms. This was repeated 12 times to obtain a third electrode having an artificial lattice structure of Fe and Al ₂ O ₃ . In this case, as shown in FIG. 6A, a current (electrons) has a parallel magnetoresistive structure flowing in parallel to the artificial lattice plane.

In the single-electron tunneling device thus manufactured, the Fe in the third electrode is actuated by the action of the magnetic field.
After aligning the spin directions of the layers, the tunnel current-gate voltage characteristics were measured in the same manner as in Example 1. As a result, excellent output characteristics with good responsiveness similar to those in Example 1 were obtained.
The S / N ratio (signal / noise ratio) was high, and the output stability against ambient temperature changes was high. Similarly to Example 1, the tunnel current was controllable by changing the degree of regularity of the spin arrangement of the ferromagnetic material (Fe). In the second embodiment, the current (electrons) has a parallel magnetoresistive structure (FIG. 6A) flowing in parallel to the artificial lattice plane. However, originally, the current (electrons) is perpendicular to the artificial lattice plane. It is more preferable to use a flowing perpendicular magnetoresistive structure (FIG. 6B). To approach this, it is effective to use a so-called oblique evaporation method in which the substrate surface is inclined with respect to the evaporation source.

The materials and methods used in Examples 1 and 2 are merely examples, and various materials, methods, and element structures can be modified and implemented without departing from the gist of the present invention. It is. The device can be applied to various applications such as transistors, memories, and logic circuits using them.

Comparative Example 1 An element structure similar to that of FIG. 1 was manufactured in the same manner. However, in Comparative Example 1, the third electrode was made of a non-magnetic material and had a structure having no spin effect. Except for the third electrode, it was formed in the same manner as in Example 1, and the third electrode was formed as follows. A third electrode consisting only of Al was obtained by sputtering. Tunnel current-gate voltage characteristics of the single electron tunneling device thus manufactured were measured in the same manner as in Example 1. As a result, output characteristics with insufficient response were obtained, and S / N
The ratio (signal / noise ratio) is as low as 38 db,
Further, the output stability was low with respect to the ambient temperature change. Further, it was impossible to control the tunnel current using the change in the spin arrangement as seen in Example 1.

Comparative Example 2 An element structure similar to that shown in FIG. 1 was manufactured in the same manner. However, in Comparative Example 2, the third electrode was a ferromagnetic material having a continuous single structure. Except for the third electrode, the electrode was formed in the same manner as in Example 1. As the third electrode, a third electrode made of only ferromagnetic Fe was obtained by the EB method. When the tunnel current-gate voltage characteristics of the single electron tunneling device thus manufactured were measured, the output characteristics were as low as 40 / db in S / N ratio (signal / noise ratio). The output stability against temperature changes was low. Further, it was impossible to control the tunnel current using the change in the spin arrangement as seen in Example 1.

[0026]

As described above, according to the present invention, the behavior of a single-electron tunneling element due to only the effect of the Coulomb energy of a single electron, the so-called Coulomb blockade effect, and one more degree of freedom of electrons. Spin behavior, that is, the effect of the so-called spin blockade, can be added, and the basic operation as an element is stable,
It is possible to realize a single electron tunneling device capable of obtaining a high S / N ratio (signal / noise ratio). Further, it is possible to provide a single-electron tunneling element having high current stability and high stability against a change in ambient temperature. Furthermore, it is possible to provide a single electron tunneling element memory having a sufficiently long storage retention time.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a single electron tunnel device of the present invention.

FIGS. 2A and 2B are conceptual diagrams showing the spin directions of a minute region made of a ferromagnetic material in a tunnel element. FIG. 2A shows a random state in which no external magnetic field is applied, and FIG. And the spin directions are aligned.

FIG. 3 is a conceptual diagram showing the spin direction of ferromagnetic particles in a third electrode having a structure in which ferromagnetic particles are dispersed in an insulator. FIG. (B) shows a case where the spin directions are aligned by application of an external magnetic field.

FIG. 4 is a conceptual diagram showing a spin direction of ferromagnetic particles in a third electrode having an artificial lattice structure of an insulator and a ferromagnetic material. FIG. (B) shows the case where the spin directions are aligned by application of an external magnetic field.

FIG. 5 is a schematic cross-sectional view showing a state in which a third electrode having a granular structure is arranged between the first electrode and the second electrode, where (A) is directly arranged, and (B) is via an insulating layer. Shows the case where they are arranged.

FIG. 6 is a schematic cross-sectional view showing a state in which a third electrode having an artificial lattice structure is arranged between the first electrode and the second electrode, and FIG. (B) shows the case of the magnetoresistive structure, which is directly arranged, and shows the case of the perpendicular magnetoresistive structure.

[Explanation of symbols]

 Reference Signs List 10 single electron tunneling element 11 silicon substrate 12 silicon oxide film 13 first electrode 14 second electrode 15 third electrode 16 insulating film (insulating region) 18 silicon oxide film (insulating film) 19 silicon oxide film (insulating film) 20th Four electrodes 21 Insulator 22 Ferromagnetic material (micro area composed of ferromagnetic material)

Claims (7)

[Claims] 1. A first electrode and a second electrode, a third electrode connected to the first and second electrodes via a single electron tunnel junction, and a third electrode In a single-electron tunneling element having a fourth electrode connected to an insulating film and forming a capacitor together with a third electrode, the third electrode has a small region made of at least two ferromagnetic materials in its structure. A single-electron tunneling device comprising: 2. The single-electron tunneling device according to claim 1, wherein the minute region composed of at least two ferromagnetic materials is formed by ferromagnetic particles dispersed in an insulator. 3. An artificial structure comprising at least two ferromagnetic micro-regions formed by at least two layers of ferromagnetic materials alternately laminated with an insulator, and a third electrode comprising an insulator and a ferromagnetic material. The single electron tunneling device according to claim 1, comprising a lattice. 4. The ferromagnetic material of the third electrode comprises iron, cobalt,
The single-electron tunneling device according to any one of claims 1 to 3, comprising one or more materials selected from the group consisting of nickel and alloys thereof. 5. The insulator of the third electrode, which is made of one or more materials selected from the group consisting of aluminum oxide, silicon oxide and magnesium oxide. 5. The single electron tunneling device according to any one of 4. 6. The method according to claim 1, wherein a mutual distance between at least two small regions made of a ferromagnetic material in the third electrode is in a range of more than zero and less than a diffusion length of electron spin. 6. The single-electron tunneling device according to any one of 5. 7. The unit according to claim 1, further comprising: means for controlling the arrangement and / or direction of spins of electrons in at least two ferromagnetic materials in the third electrode. One electron tunneling element.