Reproducible Growth of Highly Oriented (OO1) YSZ Films on Amorphous SiO 2 Substrates by MOCVD
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ABSTRACT The possibility of growing crystallographically aligned films of good quality on polycrystalline or amorphous substrates is of great interest, both for fundamental understanding of film growth mechanisms, and for potential applications such as buffer layers for silicon-on-insulator devices and an epitaxial transition layer for cuprate superconductor thin films grown on substrates with large misfits. Deposition conditions for yttria-stabilized zirconia (YSZ) thin films were investigated and optimized, and highly (001) oriented YSZ films were reproducibly prepared on amorphous substrates by MOCVD. We believe that the formation of a highly oriented crystalline film on an amorphous substrate can be interpreted in terms of the inherent features of MOCVD process, and a working model of this process is suggested. INTRODUCTION The possibility of growing crystallographically aligned films of good quality on polycrystalline or amorphous substrates is of great interest, both for fundamental understanding of film growth mechanisms, and for other potential applications such as buffer layers for silicon-on-insulator devices and an epitaxial transition layer for cuprate superconductor thin films grown on substrates with large misfits. In this work, a space cryogenic application was considered. The goal of this application is to reduce conduction of heat into liquid helium dewars while providing good electrical signal passthrough from the sensors immersed in these dewars. HTSC thin films can satisfy these contradictory requirements since at cryogenic temperatures they have both very low thermal conductivities and zero electrical resistance. High quality HTSC films have been produced by MOCVD in recent years, thus allowing potential fabrication of a high density electrical connection array with a two orders of magnitude reduction in heat flow compared to previously existing technology. (Table I).
Table I. Heat flows through electrical connection arrays of different constructions (in WattM). Material Manganin Screen-printed High density TBCCO YBCO
Qb
1.8xlO-4
Qc 7.5xlO -4 4.5x 10-4
Q 7.5xi0-4 6.3x10-4
7.2x 10- 6 7.2x 10- 6
1.2x10-7 1.8x10-6
3x10- 7 3x10- 7
7.6x 10-6 9.3 x 10-6
Qs
129
Mat. Res. Soc. Symp. Proc. Vol. 415 01996 Materials Research Society
Fused silica is the best substrate to support these films since it has one of the lowest known thermal conductivities at those temperatures. However, an amorphous silica substrate requires an intermediate buffer layer between the substrate and the HTSC film to produce an appropriate lattice match and to prevent chemical interaction between them. In our case the thermal conductivity of the buffer layer has also to be considered. A number of materials have been investigated as buffer layers for HTSC films, including LaAIO3, MgO, YSZ, Y203, and CeO2. Of these, yttria stabilized zirconia (YSZ) is the best candidate since its thermal conductivity is among the lowest known at 30 K (k=10- 2 W/cm-K). YSZ is chemically compatible with HTSC materials and has a cubic fluori
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