JPH06216419A - Superconducting field-effect transistor - Google Patents

Abstract

PURPOSE:To realize a superconducting field-effect transistor capable of forming source-drain electrodes without damaging source-drain regions and capable of increasing an operating temperature. CONSTITUTION:A superconducting field-effect transistor 1 has a source region 3 and a drain region 4 consisting of an oxide high-temperature superconductor (YBa2Cu3O7-x) having film thickness of several dozen nm or more and a gate region 5 between these regions 3, 4. The gate region 5 is composed of the multilayer films of the oxide high-temperature superconductor layer 51 similarly made up of YBa2Cu3O7-x and a barrier layer 52 consisting of PrBa2Cu2O7-x (an oxide semiconductor). The film thickness of the oxide high-temperature superconductor layer 6 and the barrier layer 52 corresponds to the thickness of one unit lattice at that time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a field effect superconducting transistor, and more particularly to a technique for forming a gate region of a field effect superconducting transistor using an oxide superconductor.

[0002]

2. Description of the Related Art Since the electric resistance of a superconducting phenomenon becomes zero, it is expected to be applied to a superconducting element of high speed and low power consumption. Among them, since the oxide superconductor has a characteristic that the carrier density is low, development of a field effect superconducting transistor using the oxide superconductor has been advanced, and for example, a structure having a structure shown in FIG. 3 has been devised. ing. In this field effect superconducting transistor, the composition formula is YBa on the surface side of the substrate 11 or the like having a composition formula of MgO.
_{A 2} Cu ₃ O _7-x oxide high temperature superconductor layer 12 is laminated, and a source electrode 14, a drain electrode 16 and an insulating film 13 having a composition formula of SrTiO ₃ are laminated on the surface side thereof. Here, the gate electrode 15 is provided on the surface side of the insulating film 13, and the carrier density of the oxide high temperature superconductor layer 12 is modulated by the electric field applied from the gate electrode 15, so that the source electrode 14 and the drain electrode 16 are formed. By changing the conduction characteristics of the oxide high temperature superconductor layer 12 between the two, the gate voltage (V _g ) is −10 at solid lines B ₋₁₀ , B ₀ and B ₁₀ in FIG.
As shown in the relationship between the source-drain voltage V _SD and the source-drain current I _{SD at} V, 0 V, and +10 V, it operates as a field-effect transistor.

[0003]

However, in the conventional field effect type superconducting transistor, the electric field from the gate electrode 15 reaches the oxide high temperature superconducting layer 12 as the gate region and modulates its carrier density. Since the thickness is shallow, the film thickness of the oxide high temperature superconductor layer 12 is about 10 nm.
Since it needs to be set below, it has the following problems.

First, when the oxide high temperature superconductor layer 12 is thin, the oxide high temperature superconductor layer 12 is formed when the source electrode 14 and the drain electrode 16 are formed on the surface side of the oxide high temperature superconductor layer 12. It is that the superconductivity is impaired under the stress of time.

A second problem is that when the oxide high-temperature superconductor layer 12 is thin, the superconducting transition temperature at which its electric resistance value becomes zero tends to be low, and the field effect type superconducting transistor is operated. That is to set the temperature low. For example, even if the oxide high-temperature superconductor layer 12 has a thickness equivalent to one unit lattice, it exhibits superconductivity, but in the thin oxide high-temperature superconductor layer 12, unlike a bulk superconductor, The influence of fluctuations in the two-dimensional direction, that is, the thickness direction increases, the superconducting transition temperature decreases,
For example, X. X. Xi. et al, Phys. Rev.
Lett. Vol. 68, 1240 (1992),
In YBa ₂ Cu ₃ O _7-x , when the film thickness is 100 nm, the transition temperature is 90 K, while the film thickness is 2.
At 6 nm, the transition temperature is reported to be 5K.

In view of the above problems, the object of the present invention is to
Oxide source without affecting the high-temperature superconductor layer
It is to realize a field-effect superconducting transistor which can form a drain electrode and can improve the operating temperature.

[0007]

Means for Solving the Problems In order to solve the above-mentioned problems, means taken in the present invention are as follows: a source region and a drain region formed of an oxide high temperature superconductor on the surface side of a substrate; In a field effect superconducting transistor having a gate region provided between a region and a drain region, and a gate electrode capable of applying an electric field to the gate region through an insulating film, the gate region includes a source region and a drain region. An oxide high-temperature superconductor layer having the same composition as that of the oxide high-temperature superconductor, a low-critical-temperature oxide superconductor having a crystal structure similar to that of the oxide high-temperature superconductor layer, and an oxide high-temperature superconductor layer. A barrier film made of a composition of at least one of oxide semiconductors having a similar crystal structure is formed into a multilayer film.

Here, the oxide high-temperature superconductor layer and the barrier layer forming the gate region each have, for example, a thickness per layer within a range from a thickness corresponding to one unit lattice to several tens of nm. Set and the total number of layers is 3
While setting the barrier layer between the layers of the oxide high-temperature superconductor layer by setting the range from 30 to 30 layers, the thickness of the oxide high-temperature superconductor in the source region and the drain region is set to several tens nm or more. To do. When the total number of the high-temperature oxide superconductor layer and the barrier layer is two in the gate region formed of the multilayer film, the barrier layer is formed on the substrate side (lower layer side) and is formed on the upper layer side. By forming the oxide high temperature superconductor layer, the superconducting transition temperature of the oxide high temperature superconductor layer is increased.

Regarding the gate region, for example,
When 0 ≦ y ≦ 1, the composition formula is YBa ₂ Cu ₃ O.
_7-x oxide superconductor layer and composition formula Y _1-y Pr _y Ba ₂
Cu ₃ O _7-x , a multilayer film with a barrier layer in which y satisfies 0.3 ≦ y ≦ 1, an oxide superconductor layer having a composition formula of Bi ₂ Sr ₂ CaCu ₂ O _x and a composition formula of Bi ₂ Sr. ₂ Cu ₂ O _x barrier layer multilayer film, composition formula Bi ₂ Sr ₂ CaCu ₂ O _x oxide superconductor layer and composition formula Bi ₂ Sr ₂ Ca _1-y Y _y
Multilayer film with Cu ₂ O _x barrier layer, composition formula La _{2− x} Sr
_x CuO ₄ oxide superconductor layer and composition formula is La ₂ CuO
_{4 with} a barrier layer or a composition formula of La _2-x Ba _x
CuO composition formula of the oxide superconductor layer ₄ is La ₂ CuO ₄
It can be composed of a multilayer film together with the barrier layer.

[0010]

In the field effect superconducting transistor according to the present invention, when an electric field is applied to the gate region through the gate electrode with a potential applied between the source region and the drain region, the oxide high temperature superconductivity in the gate region Carrier modulation occurs due to the electric field effect in both the body layer and the barrier layer, and the source-drain current is controlled. here,
Since the gate region is a multi-layered film including an oxide high temperature superconductor layer having a low carrier density and a barrier layer, the modulation depth of the carrier density due to the electric field effect is about several tens nm. Therefore, even when the oxide high-temperature superconductor layer is relatively thick, a large change in the conduction characteristics can be obtained.

Further, since the gate region is separately formed between the source region and the drain region, the source region and the drain region can be formed thick regardless of the thickness of the gate region. Superconductivity can be obtained at a high temperature, and the source electrode and the drain electrode can be formed without impairing the superconductivity of the source region and the drain region.

Here, when the gate region is formed of a multilayer film having a barrier layer between the layers of the high-temperature oxide superconductor layer, the thickness of the high-temperature oxide superconductor layer and the barrier layer is changed from one unit lattice to several tens unit lattice. Even if the film thickness is equivalent, in the gate region, the proximity effect due to the oxide high-temperature superconductor layer occurs, the superconducting conductor exudes from the oxide high-temperature superconductor layer to the barrier layer, and the oxide high-temperature superconductor layers are barrier layers. Weakly coupled through. This proximity effect suppresses the effect of two-dimensional fluctuation in the oxide high-temperature superconductor layer, that is, the effect of fluctuation in the thickness direction of the film. The transition temperature at which the static resistance becomes zero is high. Further, in the multilayer film including the high temperature superconductor layer and the barrier layer, there is an electric field effect related to the influence of two-dimensional fluctuation. That is, when carrier modulation occurs in the oxide high-temperature superconductor layer and the barrier layer and the carrier density of the barrier layer increases, the high-temperature oxide superconductor layers are Josephson-bonded to each other, and the two-dimensional oxide high-temperature superconductor layer has a two-dimensional structure. The effect of fluctuation is suppressed. On the other hand, when the carrier density of the barrier layer decreases, the proximity effect between the oxide high-temperature superconductor layers decreases, and each oxide high-temperature superconductor layer is in a state similar to that of a single-layer film, that is, two-dimensional. The effect of fluctuation is expressed, the superconductivity is lost, and the state becomes normal. As a result, a large gain can be obtained.

[0013]

An embodiment of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a sectional view showing the structure of a field effect type superconducting transistor of this example.

In the figure, a field-effect superconducting transistor 1 of this example comprises a source region 3 and a drain region made of YBa ₂ Cu ₃ O _7-x as an oxide high temperature superconductor on the surface side of a substrate 2 made of SrTiO _3. 4 and a gate region 5 made of a multilayer film between them. A gate electrode 7 is formed on the surface side of the gate region 5 via an insulating film 6.
Is formed, and an electric field can be applied to the gate region 5 via the gate terminal G connected to the gate electrode 7. Further, a source electrode 31 is formed on the front surface side of the source region 3, and the source electrode S has a source terminal S.
While the drain electrode 41 is formed on the surface side of the drain region 4,
1 is connected to a drain terminal D, and a source-drain voltage can be applied between the source region 3 and the drain region 4 of the field effect superconducting transistor 1 via the source terminal S and the drain terminal D. ing.

Here, the gate region 5 is a three-layer oxide high-temperature superconductor layer 51 made of YBa ₂ Cu ₃ O _7-x, which is the same as the oxide high-temperature superconductor forming the source region 3 and the drain region 4. PrBa ₂ C as an oxide semiconductor having a crystal structure similar to that of the oxide high temperature superconductor layer 51.
In a structure in which the barrier layer 52 is interposed between the oxide high-temperature superconductor layers 51, the multilayer film is composed of two barrier layers 52 made of u ₂ O _7-x and the total number of layers is 5. Has become. Here, the thickness of the oxide high temperature superconductor layer 51 is YBa ₂ C
1.168n corresponding to the thickness of one unit cell of u ₃ O _7-x
m, and the thickness of the barrier layer 52 is PrBa ₂ Cu ₂ O.
It is 1.171 nm, which corresponds to the thickness of one unit lattice of _7-x . Further, the source region 3 and the drain region 4 are formed on both sides of the gate region 5 formed as a multilayer film so as to sandwich it, and the film thickness thereof is set to several tens nm or more. The barrier layer 52 is made of YBa ₂ Cu ₃ O.
It is formed as an oxide semiconductor by setting y of _7-x to 0.3 to 1.0. Instead of this, YB
In some cases, y of a ₂ Cu ₃ O _7-x is limited to 0.6 to 0.9 to form an oxide superconductor having a low transition temperature Tc.

In the field-effect superconducting transistor 1 having such a structure, the operating temperature is 77 K, and the source-drain voltage is applied between the source region 3 and the drain region 4 via the gate electrode 7. When an electric field is applied to the gate region 5 with a gate voltage (V _g ) of −10 in FIG.
The solid line A shows the relationship between the source-drain voltage V _SD and the source-drain current I _SD when V, 0 V, and +10 V are set.
_As indicated by ₋₁₀ , A ₀ , and A ₁₀ , the source-drain current I _SD becomes higher as the gate voltage (V _g ) becomes a negative potential from +10 V to 0 V and −10 V. That is, the gate voltage (V _g ) is changed from + 10V to 0V, −10V.
As the negative potential is applied to, the carrier modulation occurs in both the high-temperature oxide superconductor layer 51 and the barrier layer 52, and the source-drain current I _SD increases.
Here, the oxide high temperature superconductor layer 51 and the barrier layer 52
In all cases, since the carrier density is low, carrier modulation due to the electric field effect is sufficiently exerted. Moreover, the gate region 5 is
Since it has a multi-layer structure, the depth of carrier modulation corresponds to several tens nm when viewed from the entire gate region 5. Therefore, sufficient carrier modulation occurs in the gate region 5 of the field-effect superconducting transistor 1 of this example, and a large change in conduction characteristic occurs corresponding to the gate potential (V _g ).

Further, in the field effect superconducting transistor 1 of this example, since the gate region 5 is formed as a multilayer film of the oxide high temperature superconductor layer 51 and the barrier layer 52, the oxide high temperature superconductor layer 51 is formed. The superconducting transition temperature is high even if the film thickness per layer is thin. That is, as a proximity effect of the oxide high-temperature superconductor layer 51, when the superconducting conductors exude from the oxide high-temperature superconductor layer 51 into the barrier layer 52 and the carrier density of the barrier layer 52 increases, the oxide high-temperature superconductor layer 51. The two are weakly bonded to each other via the barrier layer 52.

Therefore, in the high-temperature oxide superconductor layer 51, the effect of the two-dimensional fluctuation, that is, the effect of the fluctuation in the thickness direction is suppressed, so that the temperature of transition to the superconducting state is high. Therefore, it is possible to operate even at a relatively high absolute temperature of about 77K.

Moreover, the negative gate voltage (V _g )
Is applied to increase the carrier density of the barrier layer 52,
Since the carrier density of the barrier layer 52 increases and the oxide high temperature superconductor layers 51 are Josephson-coupled to each other via the barrier layer 52, the influence of two-dimensional fluctuation is suppressed in the oxide high temperature superconductor layer 51. It On the other hand, when the positive side gate voltage (V _g ) is applied to lower the carrier density of the barrier layer 52, the same state as the single layer film used in the gate region of the conventional field effect superconducting transistor, That is, the two-dimensional fluctuation effect is exhibited in the high temperature oxide superconductor layer 51, and the superconducting state is changed to the normal conducting state. Also, in the high temperature oxide superconductor layer 51 itself, it changes between the superconducting state and the normal conducting state as the carrier density changes. Therefore, the conduction characteristics of the gate region 5 change significantly with the electric field applied from the gate electrode 6.

Therefore, in the field effect superconducting transistor 1 according to the present example, the electric field applied from the gate electrode 7 controls the carrier modulation as well as the effect of the two-dimensional fluctuation. A large gain can be obtained as compared with the field effect superconducting transistor. Moreover, in the field-effect superconducting transistor 1 according to the present example, the effect of two-dimensional fluctuation is suppressed and the superconducting transition temperature is high. Therefore, in the conventional field-effect superconducting transistor, the operating temperature is 8.2K. On the other hand, the field effect superconducting transistor 1 of this example has a high operating temperature of 77K.

Further, in the field effect superconducting transistor 1 of this example, the source region 3 and the drain region 4 are included.
Are formed separately from the gate region 5. Therefore, regardless of the thickness of the gate region 5, the source region 3 and the drain region 4 can be made of thick oxide high-temperature superconductors, so that the source region 3 and the drain region 4 can obtain superconductivity at high temperature. Moreover, since the source region 3 and the drain region 4 can be formed thick, the superconductivity of the source region 3 and the drain region 4 may be impaired even in the step of forming the source electrode 31 and the drain electrode 41 on the surface side of those regions. Absent.

In the field effect superconducting transistor 1 of this example, three oxide high temperature superconductor layers 51 made of YBa ₂ Cu ₃ O _7-x and P as an oxide semiconductor are used.
A five-layer multilayer film composed of two barrier layers 52 made of rBa ₂ Cu ₂ O _7-x was used, but the present invention is not limited to this, and the oxide high-temperature superconductor layer 51 and the barrier layer 52 having the same composition are used. 1
Even if a multilayer film of 30 layers is formed by laminating each layer at a thickness of about 1.17 nm and using this as the gate region 5, the source-drain voltage V _SD -source-drain current shown in FIG. A characteristic equivalent to the I _SD characteristic can be obtained. Further, an oxide high temperature superconductor layer 51 and a barrier layer 52 having the same composition as described above are laminated in a thickness of about several nm to form 5
Even if a multi-layered film of layers is formed and used as the gate region 5, the source-drain voltage V _SD -source-source shown in FIG.
A characteristic equivalent to the drain current I _SD characteristic can be obtained.

Further, in the field effect superconducting transistor of this example, the total number of layers of the multilayer film forming the gate region is 2
In the case of only the layer, the side in contact with the substrate 11 is the barrier layer 5
2 and the oxide high temperature superconductor layer 51 is laminated on the surface side. The reason is that when the oxide high-temperature superconductor layer 51 is laminated on the substrate 11 via the barrier layer 52, the transition temperature Tc of the oxide high-temperature superconductor layer 51 tends to increase,
A field effect superconducting transistor having a high operating temperature can be obtained. Moreover, since the source region 3 and the drain region 4 are formed separately from the gate region 5, the source region 3 and the drain region 4 are made of thick oxide high-temperature superconductor regardless of the thickness of the gate region 5. Since it can be configured, the source region 3 and the drain region 4 can obtain superconductivity at a high temperature, and even in the step of forming the source electrode 31 and the drain electrode 41 on the surface side of those regions, the source region 3 and the drain region 4 can be formed. 4 does not impair the superconductivity.

As the gate region, when 0 ≦ y ≦ 1, an oxide superconductor layer having a composition formula of YBa ₂ Cu ₃ O _7-x and a composition formula of Y _1-y Pr _y Ba _{2 are used.} Cu ₃ O _7-x
Even if a multilayer film with a barrier layer made of a superconducting film whose y is 0.60, 0.70, 0.80 or 0.90 is used, a field-effect superconducting device having the same effect as this example is obtained. A transistor can be realized. Furthermore, the composition formula is Bi ₂ Sr ₂
CaCu ₂ O _x oxide superconductor layer and composition formula Bi ₂
Sr ₂ Cu ₂ O _x or Bi ₂ Sr ₂ Ca _1-y Y _y Cu
A field effect superconducting transistor using a multi-layer film with a barrier layer of ₂ O _x as a gate region, having a composition formula of La _2−x Sr _x
Also in a field effect superconducting transistor using a multilayer film of an oxide superconductor layer of CuO ₄ or La _2−x Ba _x CuO ₄ and a barrier layer having a composition formula of La ₂ CuO ₄ as a gate region, The same effect as the field effect superconducting transistor is obtained.

Besides, as an oxide high temperature superconductor used for a component of a field effect superconducting transistor, C is used.
A perovskite structure having a u ₂ plane and a crystal structure similar to the perovskite structure is used as an oxide high-temperature superconductor in La-Sr-
Cu-O system, Ln-Ba-Cu-O system (as Ln,
Y, La, Nd, Sm, Eu, Gd, Tb, Dy, H
o, Er, Tm, Yb, Lu), Bi-Sr-Ca-C
u-O system, Tl-Ba-Cu-O system, Nd-Ce-Cu
-O type, Sr-Ca-Cu-O type, and the like, which have a high transition temperature, and which change into a superconducting phase, a metal phase, a semiconductor phase, or an insulator phase depending on the carrier concentration and have a low carrier concentration Superconductors can be used.

[0027]

As described above, in the field effect superconducting transistor according to the present invention, between the source region and the drain region, the oxide high temperature superconductor having the same composition as the oxide high temperature superconductor forming these regions is provided. It has a gate region composed of a multilayer film composed of a body layer and a barrier layer made of an oxide superconductor or an oxide semiconductor having a low critical temperature and a crystal structure similar to that of the oxide high temperature superconductor layer. It is characterized by Therefore, according to the present invention, since the source region and the drain region are formed separately from the gate region, the source region and the drain region can be formed thick regardless of the thickness of the gate region. The source electrode and the drain electrode can be formed without impairing the superconductivity. Here, in the portion where the barrier layer is arranged between the layers of the oxide high-temperature superconductor layer, carrier modulation can be caused in the oxide high-temperature superconductor layer and the barrier layer due to the electric field effect in the gate region. , Due to the proximity effect of the oxide high-temperature superconductor layer,
Since the two-dimensional fluctuation can be suppressed, the superconducting transition temperature can be increased. Moreover, since the carrier density of the barrier layer can be changed by the electric field effect to control the two-dimensional fluctuation, the conduction characteristic in the gate region is greatly changed, and a large gain can be obtained.

[Brief description of drawings]

FIG. 1 is a sectional view showing a structure of a field effect superconducting transistor according to an embodiment of the present invention.

FIG. 2 is a graph showing a relationship between a source-drain voltage and a source-drain current in the field effect superconducting transistor shown in FIG.

FIG. 3 is a cross-sectional view showing a configuration of a conventional field effect superconducting transistor.

FIG. 4 is a graph showing a relationship between a source-drain voltage and a source-drain current in the field effect superconducting transistor shown in FIG.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Field effect superconducting transistor 2, 11 ... Substrate 3 ... Source region 4 ... Drain region 5 ... Gate region 6, 13 ... Insulating film 7, 15 ... Gate electrode 12, 51 ... Oxide high temperature superconductor layer 14, 31 ... Source electrode 16, 41 ... Drain electrode 52 ... Barrier layer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Hiroshi Kimura 1-1, Tanabe Shinden, Kawasaki-ku, Kawasaki-shi, Kanagawa Fuji Electric Co., Ltd. No. 1 inside Fuji Electric Co., Ltd.

Claims (3)

[Claims] 1. A source region and a drain region formed of an oxide high-temperature superconductor, a gate region provided between the source region and the drain region, and an insulating film on the gate region on the surface side of the substrate. An oxide high temperature superconductor layer having the same composition as the oxide high temperature superconductor forming the source region and the drain region, and the gate electrode capable of applying an electric field via A barrier layer made of at least one composition of an oxide superconductor having a low critical temperature and a crystal structure similar to that of the high-temperature superconductor layer, and an oxide semiconductor having a crystal structure similar to that of the oxide high-temperature superconductor layer; A field-effect-type superconducting transistor, characterized in that the field-effect superconducting transistor is composed of a multi-layered film. 2. The oxide high temperature superconductor layer and the barrier layer forming the gate region according to claim 1,
In each case, the film thickness per layer is set in the range from the thickness corresponding to one unit lattice to several tens of nm, and the total number of layers is set in the range from 3 to 30 layers. The electric field is characterized in that the barrier layer is disposed between layers of the high temperature superconductor layer and the film thickness of the high temperature oxide superconductor in the source region and the drain region is set to several tens nm or more. Effective superconducting transistor. 3. The method according to claim 1 or 2, wherein 0 ≦
When y ≦ 1, the composition formula of the gate region is YB.
a ₂ Cu ₃ O _7-x oxide superconductor layer and composition formula Y _1-y
Pr _y Ba ₂ Cu ₃ O _7-x and a multilayer film with a barrier layer in which y satisfies 0.3 ≦ y ≦ 1, the composition formula is Bi ₂ Sr ₂ CaCu
₂ O _x oxide superconductor layer and composition formula Bi ₂ Sr ₂ Cu
A multilayer film with a barrier layer of ₂ O _x , a composition formula of Bi ₂ Sr ₂ Ca
Cu ₂ O _x oxide superconductor layer and composition formula Bi ₂ Sr ₂
Multi-layer film with a barrier layer of Ca _1-y Y _y Cu ₂ O _x , multi-layer film with an oxide superconductor layer having a composition formula of La _2-x Sr _x CuO ₄ and a barrier layer having a composition formula of La ₂ CuO ₄ And the composition formula is La
_2-x Ba _x CuO ₄ oxide superconductor layer and composition formula La
₂ A field-effect superconducting transistor comprising a multilayer film selected from the group consisting of a multilayer film with a barrier layer of ₂ CuO ₄ .