Crystal growth method for thin films of BiSrCaCuO oxides

Details Crystal growth method for thin films of BiSrCaCuO oxides Crystal growth method for thin films of BiSrCaCuO oxides

1999-03-08

2001-02-06

Utech, Benjamin L. (Department: 1765)

Single-crystal, oriented-crystal, and epitaxy growth processes;

Processes of growth with a subsequent step of heat treating...

C117S089000, C117S092000, C117S103000, C117S108000, C117S944000

Reexamination Certificate

active

06183552

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a crystal growth method for thin films of oxides wherein vaporphase deposition is used to grow crystals for thin films of the oxides Bi
2
Sr
2
Ca
n
Cu
n+1
O
6+2n
, where n is an integer equal to 1 or greater.
2. Description of the Background
The oxides Bi
2
Sr
2
Ca
n
Cu
n+1
O
6+2n
, where n is an integer equal to 1 or greater can be produced by vapor-phase deposition methods, and those in which n is 1 to 4 are known as materials that exhibit superconductivity.
Methods of producing thin films of Bi
2
Sr
2
Ca
n
Cu
n+1
O
6+2n
oxides that have been developed and utilized include sputtering, laser ablation, molecular beam epitaxy, chemical vapor deposition (CVD) and others. Note that in the following description, regarding Bi
2
Sr
2
Ca
n
Cu
n+1
O
6+2n
, n is always an integer equal to 1 or greater if not stated specifically.
When thin films of the oxides Bi
2
Sr
2
Ca
n
Cu
n+1
O
6+2n
are produced according to the prior art, the fraction of the element bismuth that is incorporated into the thin-film crystals is small and sensitive to variations in the growth temperature. For this reason, the optimal growth environment is limited to small regions, and moreover, the bismuth in the thin film is often deficient compared to the ideal oxide composition.
This is a major problem in the crystal growth of thin films of the oxides Bi
2
Sr
2
Ca
n
Cu
n+1
O
6+2n
which contain bismuth as a constituent element, becoming a major impediment to improving the quality of these thin films. Moreover, even if it were possible to find growth temperatures and oxidation gas conditions that minimize this problem, these would be limited to extremely narrow conditions and reproduction of thin films having the same level of quality would be difficult.
As a method of solving this problem, a crystal growth method for thin films of multi-element oxides containing bismuth as a constituent element is known, having been disclosed in the Specification of published examined Japanese patent application JP-A-10-158094, for example. This method is a crystal growth method for thin films of multi-element oxides containing bismuth as a constituent element using a vapor-phase deposition method, characterized in that it is:
a crystal growth method for thin films of multi-element oxides containing bismuth as a constituent element wherein the growth environment is set to conditions in which oxides of bismuth alone are not formed, but the intended multi-element oxide is formed, and an excess of bismuth compared to the other elements is supplied in said growth environment, thereby preventing the deficiency of bismuth and also evaporating excess bismuth from the thin film.
By means of this method, the crystal growth of thin films of the oxide Bi
2
Sr
2
CuO
6
with no excess or deficiency of bismuth can be performed.
However, the application of this method to the oxides Bi
2
Sr
2
Ca
n
Cu
n+1
O
6+2n
is difficult.
FIG. 8
shows one example of the results of an attempt at the crystal growth of thin films the oxide Bi
2
Sr
2
CaCu
2
O
8
by means of the aforementioned conventional crystal growth method. To wit, in the case of the oxide Bi
2
Sr
2
CuO
6
, even if the amount of bismuth supplied is increased (to a supply ratio x), the bismuth content ratio z constantly maintains a value near the target value of 2, but in the case of the oxide Bi
2
Sr
2
CaCu
2
O
8
, as is clear from
FIG. 8
, the bismuth content ratio z does not remain at 2 but changes toward
3
together with the increase in the amount of bismuth supplied.
This is based on the fact that in a growth environment in which an excess of bismuth is supplied, a chemical change occurs according to the following chemical formula (1):
Bi+Bi
2
Sr
2
CaCu
2
O
8
+1.50→1.5[Bi
2
(Sr
2/3
Ca
1/3
)
2
CuO
6
]+0.5CuO  (1)
This chemical formula (1) means that since Bi
2
Sr
2
CaCu
2
O
8
and Bi
2
Sr
2
CuO
6
have similar crystal structures and the chemical properties of Sr and Ca are similar, a Bi
2
Sr
2
CuO
6
analog represented by Bi
2
(Sr
2/3
Ca
1/3
)
2
Cu
2
O
6
is generated by some of the strontium sites in the Bi
2
Sr
2
CuO
6
being occupied by calcium.
In this manner, if one attempts to grow Bi
2
Sr
2
Ca
n
Cu
n+1
O
6+2n
oxides by means of the aforementioned conventional crystal growth method, there are problems in that the bismuth composition diverges from the target composition, leading to the generation of different phases or the precipitation of impurities, so the growth of high-quality thin films is difficult.
The present invention was proposed in light of the aforementioned problems, and its object is to provide a crystal growth method for thin films of oxides that is able to grow high-quality thin films of Bi
2
Sr
2
Ca
n
Cu
n+1
O
6+2n
oxides and prevent excess or deficiency of bismuth, which is one of its constituent elements.
SUMMARY OF THE INVENTION
In order to achieve the aforementioned object, this invention provides a crystal growth method for thin films of oxides wherein a vapor-phase deposition method is used to grow crystals for Bi
2
Sr
2
Ca
n
Cu
n+1
O
6+2n
oxide thin film, where n is an integer equal to 1 or greater, comprising a first step of growing a Bi
2
Sr
2
CuO
6
oxide thin film to an arbitrary number of molecular layers by setting a growth environment to conditions in which oxides of bismuth alone are not formed, but intended multi-element oxide is formed, and supplying said growth environment with an excess of bismuth compared with other elements, thereby preventing deficiency of bismuth and also evaporating excess bismuth from the thin film, a second step of causing a layer containing calcium atoms and copper atoms each in the amount of n/2 of the number of strontium atoms contained in said Bi
2
Sr
2
CuO
6
oxide thin film to accumulate upon said Bi
2
Sr
2
CuO
6
oxide thin film, and a third step of, in a state in which environmental temperature is set higher than the environmental temperature in the first step, causing said Bi
2
Sr
2
CuO
6
oxide thin film and the accumulated calcium atoms and copper atoms to react to grow crystals for a thin film of an oxide Bi
2
Sr
2
Ca
n
Cu
n+1
O
6+2n
, where n is an integer equal to 1 or greater.
The crystal growth method according to the present invention also includes the case wherein said first step, second step and third step are repeated to build up at least two or more layers of said thin film of the oxide Bi
2
Sr
2
Ca
n
Cu
n+1
O
6+2n
, where n is an integer equal to 1 or greater.
Moreover, the crystal growth method according to the present invention also includes the case wherein among said two or more layers of the thin film of the oxide Bi
2
Sr
2
Ca
n
Cu
n+1
O
6+2n
built up, at least one of the layers has a different value of n than the other layers.
As described above, with this invention, upon a Bi
2
Sr
2
CuO
6
thin film is accumulated a layer containing calcium and copper atoms each in the amount of n/2 of the number of strontium atoms contained in this Bi
2
Sr
2
CuO
6
, and these atoms are caused to react while preventing excess or deficiency of bismuth, and thus the generation of different phases or precipitation of impurities is suppressed, so it is possible to obtain a high-quality Bi
2
Sr
2
Ca
n
Cu
n+1
O
6+2n
oxide thin film.


REFERENCES:
patent: 5296455 (1994-03-01), Harada et al.
patent: 05009100 (1993-01-01), None
patent: 10-158094 (1998-06-01), None

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