JPH069261B2 - Semiconductor coupled superconducting device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a superconducting device having a semiconductor at a junction, that is, a superconductor-semiconductor-superconductor coupling device.

Many attempts have been made for a superconducting device using a semiconductor as a barrier because the barrier length can be increased because the energy barrier for electrons is low and a superconducting three-terminal device can be expressed by electrical control of the semiconductor. However, the one for practical use has not been obtained.

[Conventional technology]

For example, FIG. 1 shows an example of a semiconductor coupled superconducting device. In the figure, 1 is a semiconductor substrate, 2 is a superconductor (superconducting electrode), and 3 is a semiconductor-superconductor interface. In the device realized so far, single crystal silicon is used as the semiconductor 1, and the p-type concentration is increased by diffusion or ion implantation, and a superconducting current is obtained.

However, in this case, the carrier concentration is 1 × 10 ²⁰ cm ⁻³ or more, and the superconducting electrode interval L is only about 0.1 μm. When the carrier is 10 ^{20 20} cm ^-3 , it is no longer a semiconductor but a metallic one, and the characteristics as a semiconductor such as a transistor or a FET element cannot be utilized. The carrier concentration should be at least 5 × 10 ¹⁹ cm ^-3 or less. Further, when the superconducting distance is 0.1 μm, it is very difficult to form the third terminal on the semiconductor.

The characteristics of the semiconductor-coupled superconducting device are closely related to the superconducting diffusion length ξ _N in the semiconductor. According to the superconducting proximity effect theory, ξ _N is the carrier density in a semiconductor n (cm ^-3 ).
When its mobility is μ (cm ² / V · S), it is proportional to n ^1/3 μ ^1/2 . For example, taking high-mobility compound semiconductor n-InAs as an example, 4
In cryogenic near ^{K n = 2 × 10 18 cm} -3, μ = 10,000cm 2
/ V · S, ξ _N = 0.22 μm, n = 2 × 10 ¹⁷ cm ^-3 , μ = 20,
At 000 cm ² / V · S, ξ _N = 0.15 μm (effective electron mass is 0.028). In the case of an ordinary superconductor-metal-superconductor element, a superconducting current flows even when L / ξ _N is about 1 to 5, so in the case of the above InAs, L is 0.2 to
A superconducting current should be obtained even in an element of 0.5 μm or more. However, in reality, no superconducting current was obtained. This shows the importance of the electrical properties of the semiconductor-superconducting interface at very low temperatures.

[Problems to be solved by the invention]

The present invention has a problem in the above-mentioned conventional superconductor-semiconductor-superconductor coupling element, that is, in order to obtain a superconducting current, the interval between the superconducting electrodes must be extremely short, and the carrier concentration of the semiconductor can be reduced. The present invention intends to solve the problem of having to make it extremely high and realize a superconducting two-terminal or three-terminal element that can be put to practical use.

[Means for solving problems]

The present invention solves the above problems by making the interface electrical characteristics of a semiconductor and a superconductor ohmic.

Here, as a comparative example for understanding the present invention, a conventionally known super shock absorber device will be shown. This is because the superconductor and the semiconductor form a contact barrier and are in contact with each other. The IV characteristics of the superconductor and the semiconductor are shown in Fig. 2B, and the energy band diagram is shown in Fig. 3A. In this case, when the voltage is near 0, a tunnel current determined by the barrier height Eb and w in the depletion tank (w is inversely proportional to the parallel root of the carrier concentration of the semiconductor) flows. Is not linear and its quantity is very small. A large current flows when the voltage is near the superconducting gap energy Δ (Δ is 1.3 to 1.5 meV for commonly used Nb and Pb). The Super Shoutoki device is located near I
This utilizes the non-linearity of the −V characteristic. The above is the description of the conventional super shock absorber element, but also for the element whose barrier height is said to be negative, such as n-type InAs,
In the case of joining with a metal without removing the "dirt" such as a natural oxide film formed on the surface regardless of the present invention, the IV characteristic is as shown in Fig. 2 (a) b due to the potential barrier. become. In any case, the IV characteristic has the second
For the one shown in Fig. (A) b, the second part of the enlarged view is shown.
It was found that the superconducting current Ic cannot be obtained as shown in Fig. (B) -B.

On the other hand, in the case of the present invention, for example, when n-type InAs is used as the semiconductor 1 and the superconducting electrode 2 is formed by, for example, the vapor deposition method, a natural oxide film or the like left on the surface immediately before the sputtering by argon sputtering or the like. In the case where the dirt is removed, as shown in Fig. 2 (a) -i, the IV characteristic becomes completely linear (so-called ohmic property), and the superconducting current Ic is obtained only in this case. I knew it. This is related to the fact that the natural oxide film and the like were removed by cleaning the surface and an ideal interface was obtained as shown in FIG. 3 (b). This surface cleaning is necessary even for the surface subjected to chemical etching, and is best performed in the same vacuum chamber immediately before forming the superconductor. As a cleaning method, there is a method using an ion beam, ozone, light irradiation, or a reactive gas in addition to the high frequency sputter cleaning, and the optimum method is selected depending on the semiconductor used.

Such IV characteristics and superconducting current are possible even when the barrier height is positive and the magnitude thereof is small and the depletion tank width is also small. As the former semiconductor having negative via height, in addition to the n-type InAs, P-type GaSb,
P-type InSb and the like are known. Also, a ternary or quaternary mixed crystal containing these semiconductors, for example, In _x Ga _1-x As, In _x Al _1-x As, (where x is close to 1), etc., should have a negative and barrier height. You can In order to obtain good ohmic characteristics, particularly in the latter case where the via height is positive, a high concentration layer on the interface side with the semiconductor superconductor 2 may be used, that is, an n ⁺ -nn ⁺ structure, if necessary (( In this case, w in the energy band diagram
Becomes smaller). An example is shown in FIG. 4, and 4 is a high concentration layer. This is because it is preferable that the internal semiconductor 1 has a low concentration for carrier control and that an ohmic characteristic is obtained. In order to make full use of its characteristics as a semiconductor, it is necessary to keep the internal carrier concentration below 5 × 10 ¹⁹ cm ^-3 . It is important for the semiconductor-coupled superconducting three-terminal device to bring about ohmic characteristics within this carrier concentration range. In this case, ohmic characteristics can be obtained by injecting a carrier into the semiconductor, and a superconducting current can be passed.

By the way, let us estimate the contact resistance required to obtain the superconducting current. There is no theory for Ic in semiconductor-coupled superconducting devices, but according to the theory of tunnel junctions, the maximum value of the IcR product is determined by the superconducting energy gap of the superconductor used, and it is on the order of 2 mV for Nb and Pb. R is a normal conduction resistance, which is the resistance of the ohmic portion in this case. For simplicity, the resistance of the semiconductor itself is ignored, and the element resistance is only the contact resistance between the semiconductor and the superconductor. Considering thermal fluctuation, the maximum required superconducting current is 10 μA.
Therefore, the element resistance is required to be 100Ω or less.
If the electrode area where the superconducting current can be effectively obtained is about 0.5 μm from the end of the superconducting electrode and the element width is 100 μm, the contact resistance is required to be 5 × 10 ⁻⁵ Ω · cm ² or less. .
According to the present invention, such a low contact resistance can be sufficiently obtained. Further, according to the present invention, in the superconductor-semiconductor-superconducting pair coupling element, the superconducting electrode interval required to obtain the superconducting current can be made relatively long, and the carrier concentration of the semiconductor can be made relatively large. It is possible to lower it.

The above is described by obtaining the superconducting current between the two terminals of the semiconductor-coupled superconducting device, but it goes without saying that the present invention can also be applied to a semiconductor-coupling superconducting three-terminal device.

〔The invention's effect〕

According to the present invention, as apparent from the above, the superconductor-
In the semiconductor-superconductor coupling element, it is possible to make the superconducting electrode interval required to obtain a superconducting current relatively long, and to make the carrier concentration of the semiconductor relatively low. This greatly contributes to the practical application of superconducting two-terminal elements or three-terminal elements.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a semiconductor-coupled superconducting device, FIG. 2 (a) is an IV characteristic diagram of the semiconductor-coupling superconducting device, (b) is an enlarged view near the origin of (a), and FIG. 3 is Each of the semiconductor-coupled superconducting devices (a) has a finite barrier (Eb <0), and (b) the barrier is negative (Eb <0).
FIG. 4 is an energy band diagram in the case of (0) (both are n-type semiconductors), and FIG. 4 is a cross-sectional view of an example in which a high-concentration layer of the semiconductor coupled superconducting element of the present invention is formed. DESCRIPTION OF SYMBOLS 1 ... Semiconductor substrate 2 ... Superconducting electrode 3 ... Semiconductor-superconductor interface 4 ... High concentration layer

Claims (3)

[Claims] 1. A superconductor-semiconductor-superconductor coupling element, wherein a semiconductor and a superconductor are in ohmic contact with each other, and a barrier height of electrons or holes at a contact portion with the superconductor is negative as the semiconductor. 2. A semiconductor-coupled superconducting device comprising a semiconductor, a compound semiconductor, or a mixed crystal thereof, wherein the mother oxide film of the semiconductor is removed at the contact portion. 2. The carrier concentration of the semiconductor is 5 × 10 ¹⁹ c
The semiconductor-coupled superconducting device according to claim 1, wherein the superconducting device is m ⁻³ or less. 3. The internal structure of the semiconductor is defined as n ⁺ , -n-n ^+.
The semiconductor-coupled superconducting device according to claim 1, wherein the superconductor is in contact with the n ⁺ layer.