Why Crystal Engineering of Oxides?
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LLETIN/SEPTEMBER 1994
ventional semiconductor transistors. The difficulty in fabricating SIS tunnel junctions originates, ^evidently, from the very short (~2 to 15 A) coherence lengths of high Tc superconductors. This necessitates not only high-quality but also atomicscale engineering of both superconducting and insulating films. Such thin-film technology is almost established in semiconductors, but similar methods cannot be simply applied to high Tc oxides, due to some specific features originating from the ionic bond character, the multicomponent nature of the material, and mobile oxygen in the crystals. Other problems include the availability of high-quality single-crystal substrates and maturity of supporting technologies such as substrate cleaning. Until recently, we had neither a strong need for atomically engineered oxide films nor a theoretical background for predicting characteristic properties or practical applications
Perovskite: ABO3 High Resistivity I Metallic I Superconductivity
of such films. Under these circumstances, few people were interested in developing the technology for fabricating them. The discovery of high Tc oxide superconductors changed the situation. Besides their extremely short coherence lengths, high Tc oxides have a tendency to increase their Tc with increases in the number of CuC>2 layers in a layered perovskite unit cell structure, represented in Figure 1. Thus, atomically controlled epitaxy of oxides is also significant for the exploration of new superconducting materials.4 Layered lattices related to high Tc cuprates could be reconstructed and/or modified if we established the technology of assembling two-dimensional lattices of one to several atomic layers with designed compositions. Key factors and process parameters should include lattice matching between the substrate and growth layers, Coulomb interactions at the heterointerface to be formed, oxygen control under the film deposition conditions, and condition settings to produce surface migration to achieve two-dimensional growth. This crystal engineering technology is more advanced than the conventional sequential deposition by multitarget sputtering or pulsed laser deposition. In situ diagnosis of the surface reactions and interface characterization are indispensable for quantitative control and evaluation of the film growth. Once we have developed this new material technology, we can even use it to search for new properties and physical phenomena. The technology can be combined with semiconductor microelectronics to fabricate ferroelectric oxide films to be used as
High-Tc Superconductor B'O
AO
A'O
BO 2
CuO 2
f CuO, TIO, HgO I Bi2O2, TI 2 O 2 A' = Ba, Sr
Ferroelectricity
A = Y, Ca n = 1,2,..
Piezoelectricity _CuO 2 _ n-1
Ferrimagnetism Optical function
AO
A'O
Figure 1. Unified layered lattice structure of perovskite-based functional oxides.
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Why Crystal Engineering of Oxides?
Table 1: Several Approaches to Crystal Engineering of High Tc Oxide Films. Article by Growth mode
Kawasaki and Nantoh Continuou
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