JPH03149882A - Field effect transistor - Google Patents

Abstract

PURPOSE:To prevent critical current density from being lowered by providing a gate insulating film which does not cause mutual diffusion and reaction with a superconductor, and realizing and making a single crystal high quality oxide superconductor film by epitaxially growing the oxide superconductor film on the insulating film. CONSTITUTION:As a gate electrode 3 comprising an oxide superconductor, one among materials of LnBaCuOy (Ln is at least one among yttrium or lanthanoid elements), BiSrCaCuOy, TlBaCaCuOy or NdCeCuOy, is employed and epitaxially grown on a gate insulating film 2. In GaAs single crystal growth on Si1 a mismatching rate of the lattice is set 1% or lower. LnXO3 (X is any one of Fe, Co, and Ni) or LaSrMO4 (M is any one of Cr, Mn, Fe, Co, Ni, Ga, Rh, and Al), which includes as one of constituting elements a rare earth element is employed, as a perovskite composite oxide being a gate insulating film mate rial.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to a field effect transistor using a superconductor as a gate electrode.

[-Conventional technology J A so-called MOS structure in which an oxide film or other insulating film is interposed between a gate electrode and a semiconductor active layer, or M
In a field effect transistor with an IS structure, the element that governs its high frequency characteristics is the ratio of gate-to-toe capacitance ca* to mutual conductance g#, Cgs/gg. The smaller this value is, the higher the performance is. To improve, Cj! It is necessary to reduce s and increase g. The mutual conductance g can be expressed as gllll=gm,/(l+gm, (Rs+Rg)). Here, g+++ is the intrinsic mutual conductance, R1 is the gate-to-gate resistance, and R8 is the gate resistance. As can be seen from this equation, g depends on the gate resistance, even if we try to make the gate electrode thinner to improve performance. The gate resistance increases and g no longer increases. In order to solve the problem of gate resistance, a field effect transistor using a superconductor as the gate electrode was proposed. In this case, the gate resistance becomes almost zero, and a dramatically high g can be obtained. When a metal superconductor such as Nb or Pb is used as a superconductor, the superconducting transition temperature of these metals is the absolute temperature 1
Since the temperature is below 0, an extremely low temperature environment is required, and the equipment and cost required for cooling were a problem, but with the discovery of so-called oxide high-temperature superconductors whose superconducting transition temperature exceeds the liquid nitrogen temperature, the cooling problem was solved. has been significantly reduced. [Problems to be Solved by the Invention] However, when using an oxide superconductor as a gate electrode, if SiO, which is commonly used in Si semiconductors, is used as the gate insulating film, the The superconducting properties of oxide superconductors, such as the superconducting transition temperature and critical current density, deteriorate, resulting in problems such as lowering the operating temperature of the device and making it impossible to obtain the required gate current. . This is because impurities are incorporated into the superconductor film due to mutual diffusion and reactions at the interface in the high-temperature environment during formation of the oxide superconductor thin film, and the formed superconductor thin film is polycrystalline. This is due to the presence of many grain boundaries. Therefore, a field effect transistor using SrTiO, which has excellent crystal structure similarity and lattice matching with the oxide superconductor film as the gate insulating film, has been proposed, but the lattice constant of SrTiO and Si SrTiO on Si due to the poor lattice match of SrTiO. did not grow epitaxially and became polycrystalline, and the oxide superconducting thin film formed on it had interlayers that had many grain boundaries and reduced the critical current density. . An object of the present invention is to provide a gate insulating film interposed between a semiconductor and a superconductor in which mutual diffusion and reaction with the superconductor do not occur, and in which an oxide superconducting film is epitaxially grown on this insulating film. The aim is to realize a high-quality oxide superconducting film and to suppress the decrease in critical current density due to the presence of grain boundaries by making it a single crystal.

[Means to solve the problem]

In order to achieve the above object, a field effect transistor of the present invention has a gate electrode on an active layer of a semiconductor substrate with an insulating film interposed therebetween. LnBaCuO, system (Ln is yttrium or at least one of the lanthanide elements, the same applies hereinafter)
, B i S rcacuo, system, TIBaCaCuO
An oxide superconductor based on a perowskite structure consisting of at least one of y-based or N d Ce Cu O, and as the gate insulating film, LnXO, (X
are Fe, Go, NiAl. at least one of Ga), or LaSrMO
, (M is at least one of Cr, Mn%Fe, Co.Ni, Ga1Rh%Al). v, the semiconductor substrate is made of Si, the composition of the gate insulating film is formed to vary in the thickness direction, and the lattice mismatch rate at the interface between the gate insulating film and the active layer and the gate A special feature characterized in that the lattice mismatch rate at the interface between the insulating film and the oxide superconductor is 1% or less, respectively.1 A gate insulating film is heteroepitaxially grown on a Si substrate, and further thereon. For heteroepitaxial growth of oxide superconductor thin films. When selecting a gate insulating film material, it is necessary to satisfy several conditions. Firstly, its lattice constant is approximately equal to that of Si, secondly, its lattice constant is approximately equal to that of oxide superconductor, and thirdly, its crystal structure is similar to that of oxide superconductor. It is similar to the body. Furthermore, it is important that the reactivity with Si is low and that there is little deterioration of superconductivity due to diffusion of the insulating film material. The present invention uses an oxygen octahedron as a gate insulating film material, which has a lattice constant that almost matches the lattice constant of an oxide superconductor (lattice mismatch rate of 4% or less), and which is the basic skeleton of the crystal structure of an oxide superconductor. The most important feature is the selection of composite oxides with a perowskite structure. Furthermore, it is a supplementary feature that a rare earth element, which is a constituent element of the oxide superconductor, is included as one of the constituent elements of the composite oxide. This is to realize an oxide superconductor gate MIS type field effect transistor in which all structures including the gate insulating film and the oxide superconductor gate electrode are made into single crystals, which is difficult to achieve with conventional technology.

Embodiment 1 FIG. 1 is a schematic cross-section showing the basic structure of an oxide superconductor gate MIS type electric field transistor according to the present invention. Reference numeral 1 denotes an St substrate, which is made of a single crystal. Reference numeral 2 denotes a gate insulating film, which is epitaxially formed on a Si substrate. , 3 is a gate electrode made of an oxide superconductor. LnBaCuOy system (Ln represents yttrium (Y) or at least one of the lanthanide elements, the same applies hereinafter), BiSrcacuO system, TlBaCaCuOy system, or NdCeCuO system.
, is a base material and is epitaxially formed on the gate insulating film. 4.5 are source and drain electrodes, respectively. To form such a structure, as is well known, Ln, Ba, and C having superconducting transition temperatures Tc ~ 90K are required.
The basic structure of the u, Or system is a perowskite structure in which oxygen octahedrons centered around Cull atoms are regularly arranged. Bi Srcac with transition temperature ~8OK, ~IIOK
Uoy-based superconductors (B i, S r for transition temperature ~80
, (,a, CullOy, transition temperature ~ IIO has B i
, Sr, Ca, Cu, Oy), or TlBaCaCuOy superconductors with a transition temperature of ~120°C can be considered to have a similar structure. Therefore, the lower layer film in which such a material is grown by heteroepitaxial growth is preferably a material with a perowskite structure and lattice matching. Ln, Ba, Cu, Oy-based oxide superconductors are basically orthorhombic with lattice constants of a~3.82A and b~3°88A.
However, this crystal structure has a pseudo lattice constant a, and
In the BiSrCaCuO system, both the ~80 phase and the ~IIOK phase are almost tetragonal with a and ~5.4A. In general, when considering lattice structures, it is common practice in crystallography to consider diagonals as sublattices. Therefore, a,×rIr/2 to 3J2A can be taken as the diagonal sublattice. The TlBaCaCuOy system is a tetragonal system with a, ~3,86A. Next, we will discuss the degree of lattice mismatch for realizing a single crystal. In the most technologically advanced field of semiconductors, the mismatch rate is allowed to be 1% or less. Furthermore, in GaAs single crystal growth on Si, which is a representative example of heteroepitaxial growth, a lattice mismatch of ~4% is allowed. High-quality single crystals of both have now been realized and put into practical use. In the present invention, the lattice constant mismatch condition is set to 4% or less based on such general scientific and technological concepts. Although a large number of composite oxide materials having the above-mentioned oxygen octahedral perouskite structure have been reported, those having a lattice mismatch with the oxide superconductor of 4% or less are limited. Furthermore, materials that are less likely to react thermally or chemically with the oxide superconductor are considered to be more suitable for the gate insulating film (therefore, materials with as high a melting point as possible are desirable). Oxide superconductors contain rare earth elements, and have the characteristic that their superconducting properties do not deteriorate even when rare earth elements are mixed. Focusing on these, it is considered that a compound system containing a rare earth element as one of the constituent elements is suitable for the perowskite composite oxide serving as the gate insulating film material. Matching of lattice parameters, perouskite structure,
As a rare earth (Ln) oxide system, LnXO with an orthorhombic structure (X is Fe, Co1Ni
), LnAlO, LaGaO, - (A group), or LaSrMO, which has a tetragonal structure (
M is C", Mn, Fe, Co, Ni%Ga%Rh%A
(any of l) - (i group), etc. The structure of group A can be pseudotetragonal. E7 times these lattice constants, including B group materials, are S
It is very close to the lattice constant of i, 5.43 A, and is easily epitaxially grown on Si. Table 1 shows the lattice constants of typical perouskite oxides that satisfy the above conditions, and for group A, the lattice constants when pseudotetragonal are shown. Even the combination of these deviations, which also shows the lattice constant, is within the range of the general heteroepitaxial lattice matching condition (mismatch degree of 4% or less). Therefore,
It is possible to epitaxially form a gate insulating film made of the above-mentioned material on the Si active layer, and furthermore, a gate electrode film made of an oxide superconductor thereon. A superconductor having a current density can be constructed, and a high-performance field-effect transistor with zero gate resistance and high g can be realized. Table 1 Material Lattice constant (A) Gate insulating film YA 10. 3.68GdAlO,3
,71 EuA 10. 3,725NdA 10
．． 3.752SmA 10.
3.734P rA 10. 3,7
57LaAl0. 3,778NdGaO
, 3,851 PrGa0. 3-863LaGa0.
3.875CeGaO, 3,879 GdGaO, :t840 LaSrCr0. 3.84 LaSrMnO, 3,88 LaSrFe0. 3.86 LaSrCo0. 3.80 LaSrNi0. 3.80 Material Lattice constant (A) LaSrGaO, 3,84 LaSrRh0. 3.92 LaSrAl0. 3.75 Gate electrode (La, -xMJ *Cu0. 3.78 (M=
Ba, Sr, Ce) YBa, Cu, 0.
3.82 3.88 (Orthorhombic) BiSrcac
uO system 5.4 (/(T = 3.82)TI
BaCaCuO system 3,86 NdCeCuO system 3.95 The following is a margin. In the next example, the gate insulating film is formed using a mixture of two or more of the above-mentioned selected materials.
) is turned on. Since the above-selected material has a perouskite structure, it is uniformly mixed in all of
More precise lattice matching becomes possible. For example, the oxide superconducting gate electrode is made of B i Srcacuo, the gate insulating film is made of Nd (A
1. ．． ．． If Ga, , )O, the lattice constant is 3.82
A becomes Bi Srcacuo, which has a perfect lattice match (mismatch rate θ%) with the sublattice of the system, making it optimal for so-called epitaxial growth. It goes without saying that the combination of the two can be arbitrary in Table 1. At this time, lattice matching is also achieved with Si within 1%. In order to realize even stricter lattice matching, the value of X is changed continuously in the thickness direction of the gate insulating film. For example, an oxide superconducting gate electrode can be used as an oxide superconducting gate electrode. The gate insulating film is L a (A I , -xG aJ O
, then the value of X at the interface on the Si side is 0.63, and the value of X at the interface on the oxide superconductor gate electrode side is 0.85,
It is sufficient to continuously change X from 0.63 to 0.85 in the thickness direction of the gate isolation g film. It goes without saying that the combination with the oxide superconductor gate electrode material and the value of X can be determined with reference to the data in Table 1. As a method of controlling the lattice constant of the gate insulating film, the lanthanoid element part of L n Ga Om is made into multiple compositions, and the composition ratio is changed to S on the interface side with Si.
It is determined to have lattice matching with i, and from there it is gradually changed to the interface side with the oxide superconductor. A method of achieving lattice matching with the oxide superconductor at the interface with the oxide superconductor is also possible. in particular. By gradually changing the composition in the gate oxide film so that the composition on the Si interface side becomes GdGaO and the composition on the oxide superconductor interface side becomes CeGaO, Strict lattice matching can be achieved. At this time, regarding the composition change in the film, change the composition to G
d, −X Cex Ga O, as Gd
, the composition of Ce may be changed, and it is also possible to adjust the lattice constant by further mixing other lanthanide elements. As yet another example, a B group material is used as the gate insulating film, and the composition on the interface side with Si is changed to LaSrCr.
O, or LaSrGaO, and the composition is changed in the film to form LaSrFeO on the oxide superconductor gate electrode interface side.
, and TlBaCaCuO as the superconductor gate electrode.
If a y-based oxide superconductor is selected, the lattice matching at both interfaces will be extremely good. In this way, in the above embodiment, the gate insulating film of the oxide superconductor gate field effect transistor is lattice matched (mismatch rate of 4% or less) to the superconducting oxide gate electrode and is nXO, (X is Fe% Go
, Ni), LnAlO,, LnGa
O,, (Ln is a lanthanoid element or Y)-(A group), or LaSrMO, (with a tetragonal structure)
M is Cr1Mn, Fe1Co, Ni%Ga%Rh. Since Al curry (group B) or the like was selected, heteroepitaxial growth is possible from the Si semiconductor substrate to the gate insulating film and the gate electrode, and the entire device can be grown as a single crystal. Therefore, it is possible to completely eliminate the Josephson junction that occurs at the grain boundaries in the oxide superconductor gate electrode, and it is possible to achieve a high superconducting transition temperature and high current density with little deterioration, and to reduce the gate resistance. Therefore, it is possible to realize a high-performance MIS type field effect transistor. It goes without saying that in addition to the embodiments described above, combinations satisfying the lattice matching conditions can be selected from Table 1. [Effects of the Invention] As explained above, according to the present invention, Josephson junctions occurring at grain boundaries in oxide superconductor gate electrodes can be completely eliminated, resulting in less deterioration and higher superconductivity. It is possible to realize a transition temperature and a high current density, and it is possible to reduce gate resistance, and it is possible to realize a high-performance MIS type field effect transistor.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of the basic structure of an oxide superconductor gate MIS type field effect transistor according to the present invention. l...Si substrate which is the main part of the field effect transistor 2...Gate insulating film

Claims (1)

[Claims] 1. In a field effect transistor having a gate electrode on an active layer of a semiconductor substrate with an insulating film interposed therebetween, the gate electrode is an LnBaCuO_y system (Ln is at least one of yttrium and lanthanide elements). ), BiSrCaCuO_y system, TlBaC
An oxide superconductor based on a perowskite structure consisting of at least one of aCuO_y system or NdCeCuO_y system is used, and the gate insulating film is LnXO, (X is Fe, Co, Ni, Al, Ga
or LaSrMO, (M is Cr, Mn, Fe, Co, Ni
, Ga, Rh, and Al). 2. The semiconductor substrate is made of Si, and the gate insulating film is formed so that its composition changes in the film thickness direction, and the lattice mismatch rate at the interface between the gate insulating film and the active layer and the gate insulating film are 2. The field effect transistor according to claim 1, wherein a lattice mismatch rate at the interface between the film and the oxide superconductor is 1% or less.