JPS61171180A - Semiconductor coupled superconductive element - Google Patents

Abstract

PURPOSE:To increase the electrode gap much more than conventional, and further to realize superconductive two-terminal or three-terminal elements, by a method wherein a p-type semiconductor substrate with a two dimensional electron gas formed in the surface inversion layer and two superconductive electrodes in ohmic contact with the surface inversion layer of the substrate are included. CONSTITUTION:The surface of a p-type InAs is in inversion to n-type regardless of carrier concentration, and a two-dimensional electron gas (2DEG) is formed in this inversion layer. The ohmic contact of superconductive electrodes 2 with the semiconductor 1 is required. In this respect, the Schottky barrier height to the metal of the n-type InAs layer 5 or inversion in the surface of the p-type InAs substrate 1 is always negative and forms the ohmic contact. Therefore, the electron tunnel probability Tj at the semiconductor/superconductor interface is much larger than that of p-type Si or the like. From this point, the electrode gap L is taken long, about 0.5mum; thereby, the preparation of a three-terminal structure as well as a two-terminal structure is enabled, and the two-terminal action that superconductive current flows through the 2DEG is enabled.

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 coupled device.

[Summary of the invention]

The present invention provides a semiconductor-coupled superconducting element in which two superconducting electrodes are coupled by a two-dimensional electron gas (2DEG) formed in an n-type inversion layer on the other side of a P-type semiconductor.

[Conventional technology]

Since the invention of tunnel-type Josephson devices, much research has been conducted on superconducting three-terminal devices corresponding to transistors and FETs in semiconductors. Among these, many attempts have been made to develop semiconductor-coupled superconducting devices because of their low barrier hydride and wide electrode spacing, and the possibility of three-terminal operation through electrical control of the semiconductor. However, we have not obtained anything that can be put to practical use.

Figure 13 shows the cross-sectional structure of a conventional semiconductor coupling device.
In what has been realized so far, single-crystal Sl is used as the semiconductor of the substrate 1, and superconducting electrodes 2 are formed close to each other as shown in FIG. Superconducting current was obtained with this structure. Regarding this, R, C, Ruby & T, Van Duze
r: IEIJ Trana, Elect, De
Vice.

It is reported in ED-28, 1394, ('81).

By the way, the characteristics of a semiconductor-coupled superconducting element are closely related to the superconducting diffusion length ξN in the semiconductor and the surface characteristics of the superconductor and the semiconductor. Superconducting proximity effect theory J, 5et
o & T, Van Duzer: Low Te
mpera-ture Physies-LT-13+
328. Nevr York+ Plenum.

('74), the maximum superconducting current Ic is Iccg
Tj”exP(-L/ξN)/ξN (Formula 1).Here, Tj is the tunneling probability of electrons on the superconductor/semiconductor surface, and in order to obtain a larger Ic than the above equation, Tj must be Generally, a Schottky barrier is formed on a metal/semiconductor surface, but Tj increases as the barrier height decreases and the barrier width increases.

FIG. 14 is an energy band diagram of a superconductor-P-type silicon-superconductor element (6'), where EF is the Fermi level #Ec and Ey are the lower and upper energy levels of the conduction band and the double valence band, respectively. In the case of P-type silicon, the barrier height Eb is 0.2 eV. The barrier J width W depends on the carrier concentration n, and the larger n is, the thinner it becomes. Therefore, in the case of nine elements using P-type silicon, it was necessary to make n as large as possible, i.e., 10"am7". On the other hand, if ξN is the mobility of the semiconductor as p (am/VS), then ξNcg p 34 n 3 ((Formula 2)% Formula % In the case of the P-type Si described above, p at T = 4.2 K6
00m"/VB, and ξN is short, about 0.011rm. In this way, in an element using P-type silicon, n is 10
Despite the large am, ξN was short, and only an element with an element length L of around 0.1 μm could be realized. When the carrier concentration is 10!08 m'', it can no longer be called a semiconductor and is metallic.
Even if an ES (current injection driven) structure is realized, it will be extremely insensitive to changes in the applied voltage to the gate and the inflow current, and it will not be possible to take advantage of the characteristics of a semiconductor such as a transistor or FIT element. Can not. If the superconducting electrode spacing is 0.1 μm, it is very difficult to form the third terminal on the semiconductor.

[Problem that the invention seeks to solve]

The present invention solves the problems with the conventional superconductor-semiconductor-superconductor coupling device mentioned above, namely, the distance between superconducting electrodes must be extremely short in order to obtain superconducting current, and the carrier concentration of the semiconductor must be reduced. The aim is to solve the problem of having to increase the height extremely high and to realize a superconducting two-terminal or three-terminal device with excellent characteristics.

[Means for solving problems]

The present invention is based on the fact that a two-dimensional electron gas (referred to as 2DEC) is formed in the surface inversion layer of a P-type semiconductor, and that the superconducting diffusion length ξN in two dimensions is calculated using the formula for the three-dimensional (bulk semiconductor) described above. This was done by paying attention to the analysis result that 8w CL p becomes n for the ξ do μ hat 3 of 2, as described later.

In the present invention, 2DIG in the surface inversion layer of a P-type semiconductor
K thus provides a semiconductor-coupled superconducting element in which two superconducting electrodes are coupled.

In the configuration of the present invention, an n-type inversion layer is formed on the narrow surface of the P-type semiconductor, and a 2DEC is formed therein, and the semiconductor layer on which the 2DEC is formed is in contact with the two superconducting electrodes. It needs to be ohmic.

The present invention will be explained in detail below along with its operation. [Operation] According to the proximity effect theory, the superconducting diffusion length ξN in a semiconductor is given by ξN=(fID/2πkBT). Here, when 1=-, h is a blank constant, D is a diffusion coefficient 2π number, kB is a Boltzmann coefficient, and T is a temperature.

In the three-dimensional case, D is 1) = T VFII (V, where V is the Fermi velocity and l is the mean free path), but in the two-dimensional case, kp = (2gn,)y2. vF=fIk,/m
*, so ξN of the two-dimensional system is ξN=(5"μ/4rkBTem*)'r(2rns)
¥ Cp in 3 ¥ - (Equation 3) is found. Compared to ξN (CpKn) in the three-dimensional case, for example, when both n and nB increase by one digit, the effect is greater in the two-dimensional case, and it can be said that controllability improves accordingly.Also, increasing ξN For n + n
It is the same that B must be increased, but as mentioned above, in the case of a three-dimensional (bulk) semiconductor, if a large amount of dopant is doped to increase n, it becomes a scatterer and p decreases. As a result, ξN becomes smaller. However, in the case of two dimensions, this is not the case for the following reasons.

Generally, in order to increase the carrier concentration n of a bulk semiconductor,
It is necessary to dope the same amount of dopant, and as a result, the mobility decreases due to scattering by this dopant (in the case of P-type silicon mentioned above*
n=10 (7 m = 60 am”/VS
), but the surface inversion of the P-type semiconductor 2D in the 4 layers
In the case of EC, even if the carrier concentration of the substrate is originally small, it is
Carriers gather in a narrow region of 10 nm naturally or by an electric field. Therefore, the surface density n of the carrier 17) is 10'~10"am-" (1018 when converted to n2)
cm-'' or more), and the mobility μ is also large because there are originally few scatterers in the substrate.Furthermore, nB can be controlled by an externally applied electric field.

Next, in the present invention, it is required that the contact between the superconducting electrode and the semiconductor layer in which the 2DKG is formed is ohmic, and the meaning of this ohmic in the present invention will be explained.

An ohmic contact between a semiconductor and a metal means that the current-voltage characteristics of the contact conform to Ohm's law.

Generally, a Schottky barrier as shown in Fig. 14, an oxide barrier as shown in Fig. 15, etc. are used at the contact area between a metal and a semiconductor.
A barrier or the like is formed by the combination of the two. At extremely low temperatures such as liquid helium temperature (4.2 K at 1 atm), the current flowing through this barrier is mainly due to the tunnel effect, and its current-voltage characteristics are Ohmink in the sense mentioned above, and due to this Contact resistance occurs.

This resistance is inversely proportional to the tunneling probability T. Since Tj strongly depends on the height and width of the barrier, especially the width, it is necessary to make the barrier width thinner in order to lower the contact resistance and allow more current to flow. Let's apply this to the element. Superconducting critical current (maximum superconducting current) c and normal resistance R,
If a superconducting metal such as Nb or Pb is used for the electrode 2, the product of is required. Temporarily RN
Considering only the above contact resistance, the contact area between the superconducting electrode 2 and the 2DEC of the inversion layer 5 is 5 nm x 110
If 0p (thickness of 2DKG x element width), the contact resistance needs to be 9 x 10-gΩam'' (, (xlo 0m) or less).

The ohmic characteristics referred to in the present invention refer to barrier characteristics having such a small contact resistance.

〔Example〕

(Example 1) FIG. 1 shows a cross-sectional structure of an example of the present invention. In the figure, 1 is a P-type InAs substrate, 2 is a superconducting electrode, 5 is an inversion layer, and 6 is an S layer formed by sputtering or vapor deposition.
An insulating film such as iO or 8102 is used, and 7 is a third electrode made of gold (Au) or the like.

The surface of P-type InAs is inverted to n-type regardless of the carrier concentration9, and 2DEC is formed in this inversion layer. FIG. 2 shows a band diagram at the third electrode (gate) portion of the embodiment shown in FIG. As shown in the figure, the second surface of P-InAiO is inverted to n-type, and 2
DKG is occurring. Carrier concentration n of said 2 DEG
is controlled by the voltage vf applied to the third electrode 7, thereby controlling the superconducting current flowing in the 2DKG between the two superconducting electrodes 2.

As mentioned above, the surface of P-InAs is inverted to the n-type regardless of the carrier concentration, and 2DEC is formed in this inversion layer, but there is a leakage current due to tunneling from the n-inversion layer to the P layer. Considering that, the carrier concentration is 2 to 3 x
Io "am-" or less is desirable, in this case n in 4.2
B = 2.5 x 1G"cm," p = 50
00 am”/VS has been realized (E, Yam
aguchi: ExtendedAbstract
of the 1984 International
ional Conference 5olid
5tal Devices and Materi
als, Kobe.

('84), 371), ξN is also 0.19 pm
and long.

Furthermore, in this embodiment, the superconducting electrode 2 and the semiconductor 1 are required to be in ohmic contact as described above (the tunneling probability Tj of electrons on the semiconductor/superconductor surface is large); Regarding the Schottky barrier height of the inversion layer on the surface of the P-type InAs substrate 10 with respect to the metal of the n-type InAs layer, as shown in FIG.
Form a contact. Therefore, the electron tunneling probability Tj on the semiconductor/superconductor surface is much larger than that of P-type silicon or the like.

In this way, in this example, since ξN is long and the tunneling probability Tj is large, a large nIc (maximum superconducting current) can be obtained from Equation 2 shown above (holds true regardless of two-dimensional or three-dimensional). That should be obvious.

The relationship between electrode spacing and ξN is I) JE TRANSA
C-JTION ON ELKCTRON DE
VICES, VOL, 10-28.

NO, 11, NOVEMBER1981, pp 139
4 to 1397 show that L can range from several times ξN to IO times.

In this example, for this reason, the electrode spacing can be set as long as about 0.5 μm, and therefore not only a two-terminal structure but also a three-terminal structure can be created. operation is possible, and furthermore, like a NO8FET in a semiconductor, the voltage Vf applied to the electrode 7
Then, n8 is changed as described above, thereby controlling the two-terminal characteristics. So-called three-terminal operation can be performed.

(Example 2) As shown in FIG. 4, a metal electrode (third electrode) 7t-
In this example, a metal electrode 7 is formed in the opening of the insulating film 6. The superconducting electrode 2 is the same as in the first embodiment.

FIG. 5 shows an energy band diagram for the gate portion (third electrode 7). In this case, the two-terminal operation is the same as in the first embodiment, but the three-terminal operation is different. That is, in this case, MESFIT-like or three-terminal operation based on current injection is performed.

(Example 3) Fig. 6 shows that a ridge (protrusion) 8 is formed on the surface of a P-InAs substrate 1, and superconducting electrodes 2ONl are formed on both sides of the ridge.
This is an example of a two-terminal structure in which layers are formed.

FIG. 9 shows its IV characteristics. In the figure, (a
) is the IV characteristic when direct current is applied, superconducting current flows in the region (1), and Ql) is the voltage state. 0) is the characteristic of irradiation with 10 GHz microwave. Note that the origins in (a) and (b) are shown shifted for ease of viewing.

(Example 4) FIG. 7 shows a MIS structure three-terminal element that is the same as Example 1 except that a ridge 8 is formed.

FIG. 10 shows how the superconducting current changes when the gate voltage is changed in this example (Ic in the figure is the maximum superconducting current and RN is the normal resistance).

(Example 5) The example shown in FIG. 8 is a three-terminal element with an MES structure, and the ridge
The structure is the same as that in FIG. 4 of the second embodiment except for the structure.

FIG. 11 shows Ic when the injection current Ij is changed in this example.
As shown in the figure, the superconducting current is controlled by Ij.

(Example 6) Figure n shows P-type InA on the back surface of the P-type InAs substrate 1! 1
In this embodiment, the electrode 10 is formed of a metal that forms an ohmic contact with the metal, such as an alloy of Au and Zn (Ausop+znto%). The electrode 10 corresponds to a junction gate, and the width of the depletion layer in the p-n junction (in this case, the n layer is an inversion layer) is controlled by the voltage applied between the electrode 10 and the electrode 2 or 7, thereby controlling n4. Perform three-terminal operation to change. Note that when applying a voltage between the electrodes 10 and 2, the electrode 7 and the insulating film 6 are not necessary.

Although the embodiments have been described above, the present invention is not limited thereto. For example, the present invention can include a two-dimensional electron gas (2DKG
) can be used, and various superconducting materials other than Nb and Pb can be used as the superconducting electrode material.

〔Effect of the invention〕

As described above, the present invention provides a semiconductor-coupled superconducting element in which two superconducting electrodes are coupled by 2DEC formed in an n-type inversion layer of a P-type semiconductor with different surfaces.
, two-terminal operation in which a superconducting current flows through the 2DEG is possible, and the electrode spacing can be made wider than in conventional elements. Furthermore, it is possible to perform so-called three-terminal operation in which the multi-terminal characteristics are controlled by the third electrode, and according to the present invention, a two-terminal or three-terminal element with excellent characteristics can be realized.

[Brief explanation of drawings]

1 to 3 are a sectional view of a first embodiment of the present invention, a band diagram for a gate region, and a band diagram for a superconducting electrode region, and FIGS. 4 and 5 are a sectional view of a first embodiment of the present invention, respectively. The cross-sectional view and energy band diagram of the second embodiment of the present invention, FIGS. 6 to 8 are cross-sectional views of the third to fifth embodiments of the present invention, and FIGS. I of the element of the example of
-V characteristic diagram, diagram showing changes in superconducting current due to gate voltage, diagram showing dependence of IC injection current, Figure n is a sectional view of the sixth embodiment of the present invention, Figures 13 and 14 15 is a cross-sectional view and band diagram of a conventional element, and FIG. 15 is an energy band diagram showing an oxide barrier. 1...P-type InAs substrate 2...Superconducting electrode 5...(n-type) inversion layer 6...Insulating film 7...Third electrode 8...Ridghi L-1 of Example 1 Cross-sectional view 1g1 Figure energy band 2 Day 2 Figure 3 Cross-sectional view of Example 2 Figure 4 Energy band diagram Figure 5 Cross-sectional view of Example 3 Figure 6 Cross-sectional view of R example 4 Figure 7 Example 5 1Fr side view of 80a Voltage V (5hV/div) IV characteristic diagram 9th figure Injection current Ij (mA) IC injection current dependence fI Ho f pot 11th # side view of Example 6 FIG. 12 Cross-sectional view of the conventional example Energy band diagram of the conventional example Fig. 13 Fig. 14 Side barrier energy band diagram Fig. 15

Claims (4)

[Claims] (1) Contains a P-type semiconductor substrate in which a two-dimensional electron gas is formed in a surface inversion layer, and two superconducting electrodes that are in ohmic contact with the surface inversion layer of the P-type semiconductor substrate. Features of semiconductor-coupled superconducting devices. (2) The semiconductor-coupled superconducting device according to claim 1, wherein the P-type semiconductor substrate is a P-type InAs substrate. (3) a P-type semiconductor substrate in which a two-dimensional electron gas is formed in a surface inversion layer; two superconducting electrodes in ohmic contact with the surface inversion layer of the P-type semiconductor substrate; MIS type, ME equipped between conductive electrodes
A semiconductor-coupled superconducting device characterized by having an S-type or junction-type gate. (4) a P-type semiconductor substrate in which a two-dimensional electron gas is formed in a surface inversion layer; two superconducting electrodes in ohmic contact with the surface inversion layer of the P-type semiconductor substrate; and the P-type semiconductor. 1. A semiconductor-coupled superconducting element comprising an ohmic electrode formed on a substrate side.