(i) Field of the Invention
This invention relates to sandwich-type Josephson junction elements, and more particularly to a sandwich-type Josephson junction element which is free from any deterioration due to a thermal cycle and which has a high operating temperature.
(ii) Brief Description of the Prior Art
A superconducting tunneling junction having the Josephson effect can give rise to the transition between the zero voltage and a finite voltage by controlling a magnetic field. Since the switching time in this case lies in a range of 10-100 p.s., the Josephson junction is extraordinarily excellent as a switching element. Therefore, the utilization as an element for operation or storage in an electronic computer has been expected of the Josephson junction element. Besides, many uses including the application as an electromagnetic wave detector etc. have been revealed.
The shape of the Josephson junction has several types. Among them, the sandwich-type junction in which a voltage in the normal state is independent of currents or magnetic fields is suitable for use in a logic circuit. Pb. and a Pb.-based alloy have heretofore been employed for the superconductive electrodes of the sandwich-type junctions. The materials Pb. and Pb.-based alloy have the advantage that the junctions are readily formed. The Josephson elements employing them, however, have the difficulty that a hillock appears in a base electrode due to the thermal cycle between a room temperature and a liquid helium temperature and tears an oxide barrier layer to deteriorate the Josephson voltage-current characteristics. Moreover, since the critical temperature of the material forming the electrode is as low as about 7.degree. K., it is difficult to make the operating temperature of the element about 5.degree. K. or higher. At the operating temperature of about 5.degree. K., the Josephson junction element cannot be operated by cooling it with a closed cycle cryogenic refrigerator, and it needs to be immersed in liquid helium in order to operate it.
Nb. has also been known as a material for the superconductive electrode of the sandwich-type Josephson junction element. The material Nb. does not undergo the hillock unlike the materials Pb. and Pb.-based alloy, and the critical temperature thereof is about 2.degree. K. higher than that of Pb. and is about 9.degree. K., so that it is more excellent than Pb. and the Pb.-based alloy as the material for the electrode of the Josephson junction element. However, even the Josephson junction element employing Nb. for the electrode has an operating temperature of about 6.degree. K. It cannot be operated by cooling it with the closed cycle cryogenic refrigerator, and still requires the cooling with liquid helium.
As superconductive materials exhibiting high critical temperatures, there have been known a large number of materials including Nb.sub.3 Sn., Nb.sub.3 Ge., V.sub.3 Si. etc., all of which have critical temperatures higher than that of Nb. It seems that, if electrodes are made of these materials, Josephson junction elements which can be operated by the cooling with the closed cycle cryogenic refrigerator will be produced. In actuality, however, in case where such a material is evaporated onto a barrier layer as a counter electrode, the substrate temperature or the temperature of a composite body consisting of a base electrode and the barrier layer needs to be set at several hundred .degree.C. or above. When the counter electrode material is deposited at such high temperatures, the barrier layer and the deposited electrode material react, or pinholes appear in the barrier layer, with the result that a good Josephson junction effect as expected is not attained. On the other hand, even when the material such as Nb.sub.3 Sn. is evaporated onto the barrier layer at a substrate temperature lower than about 500.degree. C., an electrode of high critical temperature is not produced. Accordingly, even when these materials are applied to the counter electrodes of the sandwich-type Josephson junction elements, the produced elements cannot be operated by the cooling with the closed cycle cryogenic refrigerator.
In this manner, it can be said that any sandwich-type Josephson junction element capable of operating by cooling it with the closed cycle cryogenic refrigerator without the use of liquid helium has not heretofore been known. The performance of the present-day closed cycle cryogenic refrigerator is approximately 6.5.degree.-9.degree. K. in terms of the cold end temperature at the zero refrigeration capacity, approximately 8.2.degree.-9.8.degree. K. at a refrigeration capacity of 0.5 W, and approximately 9.degree.-10.degree. K. at a refrigeration capacity of 1 W. It turns out that the Josephson junction element operable by the cooling with the closed cycle cryogenic refrigerator must have an operating temperature of about 8.5.degree. K. or above.
If elements whose operating temperatures are below 8.5.degree. K. and which require the cooling with liquid helium have operating temperatures higher than those of the conventional elements, the cooling thereof will be facilitated to that extent. Needless to say, therefore, such Josephson junction elements are also useful.
The following references are cited to show the state of the art; (i) Japanese Patent Application Laid-open Specification No. 47-13274, (ii) Japanese Patent Application Laid-open Specification No. 52-97695, (iii) Japanese Patent Application Laid-open Specification No. 53-53298.