JPS6142178A - Semiconductor-coupled superconductive element and manufacture thereof - Google Patents

Abstract

PURPOSE:To relatively lengthen the interval between superconductive electrodes, and to relatively reduce the carrier concentration, by a method wherein a semiconductor is brought into ohmic contact with a superconductor. CONSTITUTION:When an ideal interface can be obtained by removal of a spontaneous oxide film and the like due to the cleaning of the surface, a superconductive current with entirely linear I-V characteristics can be obtained. The I-V characteristics and the superconductive current are small in size even when the barrier height is positive, and the width of a depletion layer is possible if small. When the barrier height is positive, if necessary this element can be put in a structure that the semiconductor is changed into a high cencentration layer on the side of the interface between the superconductor 2, i.e. an n<+>-n-n<+> structure. It is necessary to keep the inner carrier concentration at 5X10<19>cm<-3> or less in order to make a great use of characteristics as the semiconductor.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a superconducting device having a semiconductor at a junction, that is, a superconductor-semiconductor-superconductor coupling device.

Superconducting devices using semiconductors as a barrier have been the subject of many attempts because the barrier length can be made longer due to the low energy barrier to electron C, and there is the possibility of realizing superconducting three-terminal devices by electrically controlling the semiconductor. has been done, but it is not practical (= what is offered is not obtained.

[Conventional technology]

For example, FIG. 1 shows an example of a semiconductor-coupled superconducting element. In the figure, 1 is a semiconductor substrate, 2 is a superconductor (superconducting electrode), and 3 is a semiconductor-superconductor interface. In the devices realized so far, a superconducting current has been obtained by using single crystal silicon as the semiconductor 1 and increasing the concentration of P type by diffusion or ion implantation.

However, in this case, the carrier concentration is ×1020. ,−
5 or more, and superconducting electrode spacing of only about 0.1 μm has been realized. If the carrier is 1001n, it can no longer be called a semiconductor, but is metallic, and the characteristics of a semiconductor such as a transistor or FET element cannot be utilized. At least carrier concentration is 5x101
It must be less than 9cm-5. Also, the superconducting interval is 0
．． At 1 μm, it is very difficult to form a third terminal on a semiconductor.

By the way, the characteristics of semiconductor-coupled superconducting elements are closely related to the superconducting diffusion length ξ in the semiconductor. According to the superconducting proximity effect theory, ξI represents the carrier density in the semiconductor as 3 (Cm
-5) If its mobility is μ (cm'/V-8), then le 1
/3μm/2. For example, if we take the high mobility compound semiconductor n-1nAz as an example, the cryogenic temperature near 4 (n = 2x '+018cm-', p = 10,000
cm”/V-:S.

ξ, v=0.22μm, nw2 x 10”cm-
', μ''20t000 Cm2/VeS and ξ-0,
It is about 15 μm (here, the effective mass of the electron is 0.02
8). In the case of a normal superconductor-metal-superconductor element, superconducting current flows even when L/ξN is about 1 to 5, so in the case of fnAz mentioned above, the probability is 0.2 to 0.
A superconducting current should be obtained even in a device with a diameter of 5 μm or more. However, in reality, superconducting current could not be obtained. This shows the importance of the electrical properties of semiconductor-superconducting transitions at extremely low temperatures.

[Problem that the invention seeks to solve]

The present invention solves the problems in the conventional superconductor-semiconductor-superconductor coupling device f2 mentioned above, namely, in order to obtain a superconducting current, the distance between the superconducting electrodes must be extremely short; The aim is to solve the problem of having to make the carrier m degree extremely high and to realize a superconducting two-terminal or three-terminal device that can be put to practical use.

[Means for solving problems]

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

Here, a conventionally known super Schottky element will be shown as a comparative example for understanding the present invention.

In this case, a superconductor and a semiconductor are in contact by forming a Schottky barrier, and the IV characteristic thereof is shown at the beginning of FIG. 2, and the energy band diagram is shown in FIG. 5 (α). In this case, when the voltage is around O, a tunnel current flows that is determined by the palyanite Ek and w in the depletion tank (w is inversely proportional to the square root of the carrier concentration of the semiconductor), but in what is called a super Schottky element, I-V The characteristics are not linear and the quantities are very small. The voltage is the gap energy of the superconductor △ (△ is 1.5 to 1 for commonly used Nh and Pb
．． 5rrhgV), a large current flows. The super Schottky element utilizes the nonlinearity of the IV characteristic in this vicinity. The above is an explanation of the conventional suit (Schottky element), but even for devices with a negative barrier height such as the Le-type 1nAz, "dirt" such as a natural oxide film formed on the surface, regardless of the present invention. When bonded to metal without removing it, the l-Tl characteristic becomes as shown in Figure 2 (α) due to the potential barrier.In both cases, the I-V characteristic becomes the second factor (α). α)
For things to be shown by mouth, see Figure 2 (b), which is a partially enlarged view.
) As stated above, it was found that two superconducting currents IC cannot be obtained.

(2) In contrast, in the case of the present invention, for example, when a Le-type 1nA, P is used as a semiconductor and the superconducting electrode 1 is formed by, for example, a vapor deposition method, immediately before that, argon sputter cleaning, etc. (2) When contaminants such as oxide films are removed, the I-V characteristics become completely linear (so-called ohmic characteristics) as shown in Figure 2 (α) A, and in this case, for the first time, the superconducting current 1c It turns out that it is possible to obtain

This is related to the fact that natural oxide films and the like were removed by cleaning the surface and an ideal interface was obtained as shown in FIG. 6(A). This surface cleaning is necessary even if the surface has been chemically etched, and it is best to perform it in the same vacuum chamber immediately before forming the superconductor. In addition, there are methods using ion beams, ozone, light irradiation, reactive gases, etc., and the most suitable method is selected depending on the semiconductor used.

Such I-V characteristics and superconducting current are possible when the barrier 8ite is positive (in both cases, its size is small and the depletion layer is also small).

The former negative barrier eight? In addition to the above-mentioned L-type 1nAz, P-type Ga5h, P-type Inch, and the like are known as semiconductors having a P-type. Also, ternary or quaternary mixed crystals containing these semiconductors, such as In, Ga1-acAs, In, Al1
-xAl p (in both cases, X is close to 1), etc., can also have a negative barrier height. In order to obtain good ohmic characteristics, especially when the latter barrier 8ite is positive (if necessary, the interface side of the superconducting i of the semiconductor may be made into a high concentration layer, that is, a rule-rule structure).
In this case, W becomes smaller in the energy band diagram). An example is shown in FIG. 4, where 4 is the alpine concentration layer. This is because the internal semiconductor is for carrier control (lower concentration is better, and ohmic characteristics can be obtained.In order to take advantage of the characteristics of a semiconductor, the internal carrier concentration must be 5xl. 0”c
It is necessary to keep m= or less. It is important for a semiconductor-coupled superconducting three-terminal device to have ohmic characteristics within this range of carrier concentration. In this case, it is also possible to obtain ohmic characteristics by injecting carriers into the semiconductor, and to flow a superconducting current.

By the way, let's estimate the contact resistance required to obtain superconducting current. There is no theory for Ic in semiconductor-coupled superconducting devices, but according to the theory of tunnel junctions, Ic
The maximum value of 1 is determined by the superconducting energy gap of the superconductor used, and for Nb, Ph, etc., it is 2rnV.
It is of the order of. R is a normal conduction resistance, which in this case is the resistance of the ohmic part.

For simplicity, we ignore the resistance of the semiconductor itself and assume that the element resistance is only the contact resistance between the semiconductor and the superconductor. Considering thermal fluctuations, the maximum superconducting current actually required is 10 μA or more, so the element resistance is required to be 100 Ω or less. If the electrode region where a superconducting current can be effectively obtained is about 0.5 μm from the edge of the superconducting electrode, and if the inside of the device is 100 μm, then the contact resistance needs to be 5×10 −5 Ω·C♂ or less. According to the present invention (2), it is possible to sufficiently obtain a contact resistance as low as this level. According to the present invention, a superconducting current can be obtained in a superconductor-semiconductor-superconductor coupled device. The distance between the electrodes can be made relatively long, and the carrier concentration of the semiconductor can be made relatively low.

Although the above explanation has been made by obtaining two superconducting currents between two terminals of a semiconductor-coupled superconducting element, it goes without saying that it can also be applied to a semiconductor-coupled superconducting three-terminal element.

〔Effect of the invention〕

According to the present invention, as is clear from the above, superconductors -
In a semiconductor-superconductor coupled device, increasing the superconducting electrode spacing required to obtain a superconducting current by relatively (2)
Furthermore, since it is possible to reduce the carrier concentration of the semiconductor by a relatively low level, it will greatly contribute to the practical application of semiconductor-coupled superconducting two-terminal devices or three-terminal devices.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view of a semiconductor coupled superconducting device, Figure 2 (α)
is an IV characteristic diagram of a semiconductor-coupled superconducting device, and (b) is an (α
), and Figure 3 shows an enlarged view of the vicinity of the origin of the semiconductor-coupled superconducting element (α), which has a finite barrier (AI'h>0). (Energy band diagram for Al when the barrier is negative (Eb<o) (all for square semiconductors). Figure 4 is a cross-sectional view of an example of forming a high concentration layer of the semiconductor coupled superconducting element of the present invention. 1...Semiconductor substrate 2...Superconducting electrode 5...Semiconductor-superconductor interface 4...High concentration layer Fig. 1s6 (cL) Fig. 2 μV Fig. 4

Claims (5)

[Claims] (1) In a superconductor-semiconductor-superconductor coupled device,
A semiconductor-coupled superconducting element characterized by ohmic contact between a semiconductor and a superconductor. (2) The semiconductor is comprised of a semiconductor having a negative barrier height for electrons at a contact portion with the superconductor, a compound semiconductor, or a mixed crystal thereof. The semiconductor-coupled superconducting device described above. (3) The carrier concentration of the semiconductor is 5×10^1^9c
The semiconductor-coupled superconducting device according to claim 1, characterized in that m^-^3 or less. (4) The internal structure of the semiconductor is n^+-n-n^+, and the superconductor is in contact with the n^+ layer. Semiconductor-coupled superconducting device. (5) In the method for manufacturing a superconductor-semiconductor-superconductor coupled device, immediately before forming a superconducting electrode on the semiconductor, high-frequency sputter cleaning, ion beam cleaning, light irradiation cleaning, or ozone, reactive gas 1. A method for manufacturing a semiconductor-coupled superconducting element, which comprises performing surface cleaning such as cleaning using a solvent, etc., and forming a superconducting electrode without removing it from a vacuum chamber.