Method for preparing low-resistant p-type SrTiO3

Details Method for preparing low-resistant p-type SrTiO3 Method for preparing low-resistant p-type SrTiO3

2002-12-23

2004-09-14

Kunemund, Robert (Department: 1765)

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

Forming from vapor or gaseous state

With decomposition of a precursor

C117S104000, C117S105000, C117S108000, C117S949000

Reexamination Certificate

active

06790278

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a method for reducing the resistance of a perovskite-type transition-metal oxide SrTiO
3
.
BACKGROUND ART
A perovskite-type transition-metal oxide SrTiO
3
has a wide bandgap energy of 3.3 eV. Therefore, SrTiO
3
gets attention as a base material of other perovskite-type transition-metal oxides, a nonlinear resistive element, a ferroelectric material or the like, and a SrTiO
3
film having a high dielectric constant are actively developing.
A high dielectric film consisting of SrTiO
3
is typically prepared through a spattering method using a SrTiO
3
sintered body as a target (e.g. Japanese Patent Laid-Open Publication Nos. 6-140385 and 9-153598).
On the other hand, it has been difficult to prepare a p-type SrTiO
3
having a desirably lowered resistance even by adopting any conventional technologies. Specifically, a conventional method of doping a p-type dopant N (nitrogen) by itself has been able to increase carrier concentration only up to about 10
14
cm
−3
due to the deep impurity level and resulting low activation rate of N acceptor. Further, the doping of N as an acceptor causes a compensation effect, which inevitably leads to oxygen deficiency serving as a donor. Such negative factors obstruct to achieve a desired low-resistance p-type SrTiO
3
.
DISCLOSURE OF INVENTION
The inventors have found a new technical concept of simultaneously doping or co-doping an acceptor with a donor having a high chemical bonding strength with the acceptor intentionally to provide a shallowed accepter level and suppressed compensation effect, and have verified that the above problem could be solved by the developed technique.
According to a first aspect of the present invention, there is provided a method for preparing a low-resistance p-type SrTiO
3
, wherein an acceptor and a donor are co-doped into a perovskite-type transition-metal oxide SrTiO
3
during crystal growth.
According to a second aspect of the present invention, there is provided a method for preparing a low-resistance p-type SrTiO
3
, wherein an acceptor (A) and a donor (D) are co-doped into a perovskite-type transition-metal oxide SrTiO
3
during crystal growth to form an impurity complex (A-D-A) of the acceptor and the donor through a crystal growth method in a thermal nonequilibrium state, whereby an impurity formation energy is reduced by less than that in a method of doping the acceptor by itself, so as to provide an increased concentration of the acceptor.
According to a third aspect of the present invention, there is provided a method for preparing a low-resistance p-type SrTiO
3
, wherein an acceptor (A) and a donor (D) are co-doped into a perovskite-type transition-metal oxide SrTiO
3
during crystal growth to form an impurity complex (A-D-A) of the acceptor and the donor through a crystal growth method in a thermal nonequilibrium state, whereby a Coulomb scattering mechanism having a long range force is converted into a dipole or multipole scattering mechanism having a short range force, so as to provide an increased mobility of the impurities.
According to a fourth aspect of the present invention, there is provided a method for preparing a low-resistance p-type SrTiO
3
, wherein an acceptor (A) and a donor (D) are co-doped into a perovskite-type transition-metal oxide SrTiO
3
during crystal growth to form an impurity complex (A-D-A) of the acceptor and the donor through a crystal growth method in a thermal nonequilibrium state, whereby the acceptor has a shallower impurity level than that in a method of doping N by itself, so as to provide an increased activation rate of the acceptor.
In the present invention, in order to prevent oxygen deficiency, the donor may be Nb oxide, and the acceptor may be N oxide.
Further, the donor may be an n-type dopant of Nb (niobium), and the acceptor may be a p-type dopant of N (nitrogen). In this case, the n-type dopant of Nb and the p-type dopant of N are co-doped at a rate of 1:(1+x) where 0<x<100, so as to provide p-type carriers doped in a high concentration of 10
19
cm
−3
to 10
21
cm
−3
. In the ratio of Nb (niobium) to N (nitrogen), the amount of N should be greater than that of Nb to obtain a p-type SrTiO
3
, and a significant co-doping effect can be observed at the ratio of up to about 1:100.
In the method of the present invention, the SrTiO
3
crystal can be grown on a substrate such as Si, SrTiO
3
or &bgr;Al
2
O
3
by employing an MBE (Molecular Beam Epitaxy) method, laser-MBE method, CVD (Chemical Vapor Deposition) method or spattering method.
The CVD method promotes crystal growth under hydrogen atmosphere, and a hydrogen donor will remain in the resulting crystal because hydrogen acts as a donor. However, the remaining donor can be removed by subjecting the resulting crystal to annealing (at about 500° C.), and thereby an acceptor acts to provide a desirable low-resistance p-type crystal.


REFERENCES:
Chiang et a., “Grain-boundary chemistry of barium titanate and strontuin titanate. I. High temperature equilibrium space change”, Journal of the American Ceramic Society vol. 73, No. 11 Nov. 1990 pp. 3278-3285 abstact only.*
Glinchuk et al., “ESR study of impurities in strontium titanate films”, Physics of the Solid State, vol. 43 No. 5 May 2001 pp. 809-812 abstract only.*
Patent Abstract of Japan No. 07-273220, dated Oct. 20, 1995. See PCT search rpt.
Patent Abstract of Japan No. 2000-026194, dated Jan. 25, 2000. See PCT search report.
Patent Abstract of Japan No. 06-140385, dated May 20, 1994.
Patent Abstract of Japan No. 09-153598, dated Jun. 10, 1997.
Jong-Won Yoon et al.; Applied Physics Letters, vol. 74, No. 5, pp. 738-740, Feb. 1, 1999.
Meeting Abstracts of the Physical Society of Japan, vol. 55. Issue 1. Part 4, p. 842, Sectional Meeting, Mar. 22-25, 2000.

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