Active solid-state devices (e.g. – transistors – solid-state diode – Thin active physical layer which is – Tunneling through region of reduced conductivity
Reexamination Certificate
1999-08-30
2003-04-01
Wilson, Allan R. (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Thin active physical layer which is
Tunneling through region of reduced conductivity
C257S031000, C257S035000
Reexamination Certificate
active
06541789
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a Josephson junction device which uses a high temperature oxide superconductor and has small spread in characteristic and a manufacturing method for the same.
2. Description of the Related Art
A high temperature oxide superconductor of a YBaCuO system has a high superconductor critical temperature (Tc) of about 90 K. Therefore, a non-expensive liquid-nitrogen having the boiling point of 77 K can be used as refrigerant, and a small and handy freezer that the low temperature of about 60 K can be easily achieved can be used as a cooling means. Also, the facilities for the maintenance of the low temperature can be simplified.
Conventionally, a superconductive electronic device and circuit with a high speed and low power consumption have been realized using a material such as Nb having a low superconductor critical temperature Tc. For the above reasons, if the device and circuit are possible to be realized using the high temperature superconductor, it contributes greatly to the industry.
A superconductor of a YBaCuO system is a main object for the study and development of application to the electronic device at present. The YBaCuO system high temperature superconductor thin film indicative of a good superconductive characteristic can be obtained through vapor-phase growth under the condition of the high substrate temperature of 600 to 800° C. and the high oxygen partial pressure in the order of 100 mTorr. One of the superconductive thin films having especially good characteristic has c-axis orientation in which a c-axis of its crystal structure is perpendicular to the substrate surface. On the other hand, depending on the growth condition, it is possible to form an a- or b-axis orientation film. However, the a- or b-axis orientation film is inferior in the superconductive characteristics such as the superconducting critical temperature Tc and the superconducting critical current value Ic, compared with the c-axis orientation film.
A substrate formed of SrTiO
3
, MgO, LaAlO
3
or NdGaO
3
is generally used for the growth of the superconductive thin film in consideration of matching with YBaCuO in lattice constant and thermal expansion coefficient and non-occurrence of solid phase reaction. Also, LaSrAlTaO is used as the substrate, since it has a good lattice matching with YBaCuO and has a low dielectric constant and a relatively large substrate can be formed, as shown in, for example, Journal of Crystal Growth, Vol. 109, pp.447-456, 1991.
The YBaCuO system has a strong anisotropic characteristic like the other high temperature superconductors. The superconductive coherence length is longer in the a- or b-axis direction than in the c-axis direction. The coherence length in the c-axis direction is as very short as about 0.3 nm. Therefore, it is desirable to flow a current in the a- or b-axis direction in wiring sections and Josephson junctions in the superconductor circuit using a high temperature superconductor.
For these reasons, when a high temperature superconductor electronic circuit is manufactured using the high quality c-axis orientation thin film, it is difficult to manufacture the Josephson junction of the high quality in a sandwich type Josephson junction in which it is necessary to flow current in the film thickness direction, i.e., in the c-axis direction of the high temperature superconductor crystal, unlike the conventional superconductor circuit using Nb. In this case, an.edge type Josephson junction device using an edge part of the superconductive thin film is suitable rather than the sandwich type Josephson junction device, as shown in “IEEE TRANSACTIONS ON MAGNETICS, VOL.27, NO.2, MARCH, 1991, pp.3062-3065”. Therefore, the study and development of the high temperature superconductor circuit is mainly performed using the edge type Josephson junction device.
In the high temperature superconductor Josephson junction device, the technique is not established for forming a very thin barrier layer as an Al
2
O
3
barrier layer used in the Nb system Josephson junction device having a low critical temperature Tc. Therefore, a non-superconductive oxide film such as a PrBaCuO system film and a superconductive oxide film such as a Co doped YBaCuO system film are deposited on a lower superconductive layer as a barrier layer. The PrBaCuO system crystal has a similar crystal structure to the YBaCuO system crystal and the PrBaCuO system non-superconductor oxide is easy to hetero-epitaxially grow on the YBaCuO layer. Also, the Co doped YBaCuO system superconductive film has a critical temperature Tc lower than YBaCuO superconductive film.
In the above, there is a problem in the spread of junction characteristics such as a superconductive critical current value Ic in a high temperature superconductor Josephson junction device. In the barrier layer forming method using the thin film growth method, it is difficult to grow the thin barrier layer sufficiently uniformly. As a result, the coverage of the lower superconductive layer by the barrier layer is low, so that the spread of the characteristics is large between the junctions. For this reason, the study of the high temperature superconductor Josephson junction device is accomplished by use of an interface control technique without using thin film growth technique, as shown in “APPLIED PHYSICS LETTERS, VOL.71, NO.17, OCTOBER, 1997, pp.2526-2528”. In this method, after the lower superconductive layer is etched to a predetermined shape, a combination of an annealing process in a vacuum and the irradiating process of accelerated ions is performed. Through the combination process, the surface portion of the lower superconductive layer is changed in the crystal structure to have a function as the barrier layer. However, there is the spread of critical current value Ic of (1&sgr;=±8%) even in the 10 samples of the high temperature superconductor Josephson junction device manufactured by the above method.
Also, as the superconductive material for the high temperature superconductor Josephson junction device, a copper oxide system material is used and is practicable in the high temperature region. In this case, however, it is especially difficult to form the barrier layer. Because the coherence length is as short as about 1 to 2 nm in the copper oxide system material, it is necessary that the barrier layer of the Josephson junction also has the film thickness approximately equal to the coherence length. When the film thickness of the barrier layer becomes thicker than the above coherence length, the Josephson current becomes difficult to flow and the quality of the Josephson junction is degraded. As a result, the function sometimes becomes not attained. However, in the conventional method, it is very difficult to form the barrier layer of such a thin film in actual.
In conjunction with the above description, a Josephson junction device and a manufacturing method for the same are disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 3-94486). In this reference, a high temperature oxide superconductor of LnBa
2
Cu
3
O
7-8
and an insulator of Ln
2
BaCuO
5
which is composed of the same elements as the superconductor are continuously formed in a sputtering apparatus. Then, an annealing process is carried out at a predetermined temperature in an oxygen atmosphere to produce a Josephson junction device.
Also, a method of manufacturing a barrier layer type electronic device is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 4-317381). In this reference, a YBaCuO oxide superconductor thin film is formed on a substrate. The forming process is carried out by a CVD method, a sputtering method or a vapor deposition method. Next, Fe ions are injected into the oxide superconductor thin film, and the injected ions are diffused in the internal direction of the thin film by a heating process to form a barrier layer. Next, the surface of the barrier layer which has received physical damage through the ion implantation is removed. Aft
Koshizuka Naoki
Sato Tetsuro
Tanaka Shoji
Wen Jian-Guo
NEC Corporation
Scully Scott Murphy & Presser
Wilson Allan R.
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