ORIENTED FILM GROWTH, NOT EPITAXY IN HTSC FILM GROWTH

A survey of the literature reveals that the following materials have b een used as the substrates for ''epitaxial'' deposition of YBCO high T (c) superconducting (HTSC) thin films: SrTiO3, ZrO2 (or YSZ), MgO, LaA lO3, Si and Al2O3 (with or without buffer layers), SiO2, Pt, MgAl2O4, even noncrystalline substrates. An analysis of the structural relation s between the substrate and HTSC phase is presented. It is obvious tha t the term ''epitaxy'' is misapplied in many of these cases. We analyz e the evidence for actual crystal structural control from the substrat e influencing the oriented growth of the desired phase and find it non existent in many cases, very weak in others, and persuasive in a few c ases. For the superconductor quality what matters is the quality of th e HTSC film, not what it is sitting on. This quality can be quantified in four steps, the first three describing the degree of orientation i n none (random-orientation), one (growth direction), or two (in film p lane) crystallographic directions. The fourth is the degree of continu ity of the film or absence of grain boundaries (i.e., a 100% perfectly oriented set of separated islands with grain boundaries will clearly be considerably poorer than a continuous (single) phase). We propose t hat instead of the term epitaxial one should focus only on the degree of orientation and continuity of the ''oriented film.'' The phase imme diately contiguous to the film (including many so-called buffer layers ) should be designated substrate. Any phase below such a substrate is structurally and chemically insignificant (to the YBCO growth), althou gh it may be necessary to obtain the oriented ''buffer layer.'' Such p hases should be designated as carriers, not substrates.