JP3000166B2 - Superconducting switching element - Google Patents

Description

The present invention relates to a switching element using a superconductor.

"Conventional technology" Conventional switching elements using superconductors include:
Current-injection type three-terminal device utilizing the unbalanced superconductivity effect (QU
ITERON) has been proposed. It has a Josephson junction structurally.

Research has been conducted on materials capable of forming a Josephson junction. For example, for an oxide high-temperature superconductor or the like, an element utilizing a weak grain boundary bond in polycrystal has been experimentally manufactured.

However, the grain boundaries are highly dependent on the data, and the characteristics of the device are greatly affected by the formed grain boundaries.If a Josephson junction based on the grain boundaries is used for the switching device, the reliability and mass productivity of the device will increase. There's a problem.

An object of the present invention is to provide a completely new type of switching element which is sufficiently reliable without using a Josephson junction which is difficult to manufacture.

Therefore, it is important not to use a Josephson junction using a grain boundary as is usually used. However, since it is very difficult to create an artificial Josephson junction, the present invention proposes a completely new type of superconducting switching element that has never been used before, without using a Josephson junction at all.

[Means for solving the problem]

An element having a laminated structure of a lower electrode, a lower insulating layer, a superconductor layer, a metal layer, an upper insulating layer, and an upper electrode. A superconducting switching element utilizing the fact that carriers in a layer are excessively injected into a superconductor, and thus the superconducting state is destroyed and becomes a resistance state.

FIG. 1 is a schematic sectional view of a switching element according to the present invention.
Shown in the figure.

That is, it has a laminated structure of a lower electrode (6), a lower insulating layer (5), a superconductor layer (4), a metal layer (3), an upper insulating layer (2), and an upper electrode (1). A bias current I is passed through the body layer. Superconducting state and applying a gate voltage V _in the above constant between upper and lower electrodes in the device is destroyed, the superconductor layer (4) is the output voltage Vout becomes resistance state occurs. As the superconductivity, a superconductor composed of yttrium (or a lanthanoid element) -barium-copper-oxygen, which is a low-carrier-concentration superconductor, or mainly bismuth-strontium-calcium-copper-
A superconductor composed of oxygen, or a superconductor composed mainly of thallium-barium-calcium-copper-oxygen or a superconductor composed of lanthanum-alkaline earth metal-copper-oxygen, or barium-potassium-bismuth-lead-oxygen Or a barium-bismuth-lead-
A superconductor made of oxygen or the like is used. The operation principle will be described.

The superconducting critical temperature (Tc) of a low-carrier-concentration superconductor strongly depends on the carrier concentration. When the number of carriers is small, it becomes a semiconductor and does not show superconductivity. It shows electrical conductivity, but loses superconductivity. To date, the cause is not clear, but it is clear that this phenomenon is not a non-equilibrium superconductivity effect because it occurs even in an equilibrium state.

In any case, superconductivity is lost in the presence of excess carriers.

FIG. 1 is an electronic phase diagram showing the principle of operation of the superconducting switching element in FIG. FIG. 3 (a) qualitatively shows the distribution of carriers, showing a state where no gate voltage is applied, and FIG. 3 (b) showing a state where a gate voltage is applied. In the figure, E _f indicates the Fermi surface, and the hatched portion indicates the presence of carriers.

Since the superconductor layer has a low carrier concentration, the potential is shallow. When a voltage is applied between the upper and lower electrodes, an electric field gradient is generated, and the shape of the potential is changed, as shown in FIG. 3 (b). At this time, the carrier concentration in the portion of the superconductor layer increases, but the superconductivity is lost due to the effect of the excess carriers described above, and the superconductor layer enters a normal conduction state. This superconducting-normal conducting transition is extremely fast (<about 10 ps), and if this is used for the switching operation, high-speed switching is possible. Although the gate voltage depends on the thickness of the superconductor layer, 0.1 V or more is sufficient, and is one order of magnitude lower than that of a semiconductor device.

The superconducting switching element according to the present invention is capable of high-speed operation, does not require a Josephson junction, is extremely simple to manufacture, and does not use uncertain factors such as grain boundaries, so that a highly reliable element can be obtained. And the reproducibility in the preparation is high.

 Examples are shown below to explain the contents of the present invention.

Example 1 FIG. 4 shows a manufacturing process of a superconducting switching element manufactured in this example.

First, as shown in FIG. 4 (a), oxide superconductor (8)
A Bi _{2 Sr 2} CaCu ₂ O ₈ thin film was formed by a known sputtering method on a magnesium oxide single crystal substrate (7) (100).
Created on the surface. The film thickness was about 0.1 μm, an extremely flat thin film, and the existence of grain boundaries was not recognized.

X-ray analysis showed that the film had its C-axis perpendicular to the substrate.

In addition, the measurement of the magnetic susceptibility revealed that the material exhibited superconductivity at a temperature of 90 K or less.

The oxide superconductor thin film was etched by a known photolithography technique to form a loop circuit having a width of 1 μm and a side length of 5 μm as shown in FIG. 2 (a).

First, a silver thin film having a thickness of 0.2 μm was formed as a metal layer (9) on an oxide superconductor thin film (8) using a metal mask.
It is selectively formed by a vacuum deposition method.

(FIG. 4 (b)) Further, an amorphous aluminum oxide film having a thickness of 0.5 μm was formed as the insulating layer (10) by a sputtering method.
(FIG. 4 (c)) Finally, a gold thin film (11) was formed to a thickness of about 0.5 μm on the lower front surface of the substrate (7) and the silver thin film by a vacuum evaporation method. (FIG. 4 (d)) In this case, the substrate (7) (magnesium oxide single crystal)
Is the lower insulating layer.

Thus, a superconducting switching element was formed between BEs shown in FIG. 2 (a).

Now, this circuit functions as a memory element, and its operation principle will be described below.

 The device is cooled with liquid nitrogen.

First, as shown in FIG. 2A, a gate voltage is applied to the switching element between BEs, and the switching element is in a resistance state. An electric current flows therethrough as shown in FIG. 2 (b). The current flows in the order of A->B->C->D->E-> F because it is in a superconducting state except between BE. Next, when the gate voltage of the switching element is reduced to zero, it appears that no current flows between BE, but in fact, as shown in FIG. 2 (c), B → E and E → B
That is, the opposite currents of the same magnitude are flowing.

Finally, when the external current is cut off as shown in FIG. 2 (d), there is no B → E current between BE, and only the E → B current.

Eventually, a closed permanent current loop of B → C → D → E → B remains, which is a memory state.

The above is the operating principle of the memory element using this switching element.

Reading memory uses a magnetic flux generated by a closed persistent current to transfer the magnetic flux to another superconducting device, such as a superconducting quantum interference device (SQUI
D).

[Effect] According to the present invention, a switching element using an oxide superconductor excellent in mass productivity and reproducibility can be produced. Since this element is a superconducting element that operates sufficiently even at the temperature of liquid nitrogen, the operating cost can be lower than that of a conventional superconducting element using a metal-based superconductor.

The switching element of the present invention can be operated at an extremely high switching speed, and it is extremely easy to use this to create a memory element.

Further, the reliability is remarkably superior to that of a device using a conventional grain boundary.

As is clear from the above, the present invention is an industrially useful invention.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view of the switching element of the present invention. FIG. 2 shows the operation principle when the switching element shown in FIG. 1 is applied to a superconductor memory. FIG. 3 is a diagram showing an operation state diagram of the superconducting switching element. FIG. 4 is a diagram schematically showing a method for producing a superconducting switching element. DESCRIPTION OF SYMBOLS 1 ... Upper electrode 2 ... Upper insulating layer 3 ... Metal layer 4 ... Superconductor layer 5 ... Lower insulating layer 6 ... Lower electrode 7 ... Substrate 8 ... Oxide superconductor 9 ... Silver Thin film 10… Insulating film 11… Gold thin film

Claims (3)

(57) [Claims] A lower electrode; a lower insulating layer on the lower electrode; a superconductor layer on the lower insulating layer; a metal layer on the superconductor layer; And an upper insulating layer,
A superconducting switching element, wherein an upper electrode is laminated on the upper insulating layer. 2. The superconductor layer according to claim 1, wherein the superconductor layer is made of a lanthanoid element-barium-copper-oxygen, and bismuth-strontium-calcium-
Superconductor layer composed of copper-oxygen, superconductor layer composed of thallium-barium-calcium-copper-oxygen, superconductor layer composed of lanthanum-alkaline earth metal-copper-oxygen, barium-potassium-bismuth-lead -A superconducting switching element characterized by being a superconductor layer composed of oxygen or a superconductor layer composed of barium-bismuth-lead-oxygen. 3. The method according to claim 1, wherein
A superconducting switching element, wherein the thickness of the superconductor layer is 500 nm or less.