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rely on the availability of high-purity metal-organic precursors that display the appropriate reactivity for film growth [3a, 5]. This is particularly important in oxide materials, since the volatile species must survive oxidizing and/or moisture-rich background environments to selectively decompose at the hot-zone. Previously, our efforts [6] focused on the challenges of small charge-to-radius ratio metals (i.e., Ca2 , Srý , Ba2 ), but recently we have begun to evaluate lanthanide (La3 +, Ce +, Nd +, Er 3+) and main group (Sn2+) precursors, which offer many of the same challenges. Our need for highly volatile and reliable lanthanide sources stems from a continued interest in YBa 2Cu 307.8 (YBCO) lattice matched insulators (or "epi" layers), particularly CeO 2 [7]. CeO 2 , with a cubic fluorite structure (a = 5.411 A), excellent lattice match, and compatible thermal expansion coefficient to YBCO has been widely investigated as a buffer/intermediate layer, and recent work has shown it to be a crucial component in coated conductor technology when heteroepitaxially deposited with yttria-stabilized zirconia (YSZ) [8]. Although CeO 2 has previously been grown by a variety of physical and chemical vapor deposition techniques, MOCVD routes remain problematic, typically requiring high substrate (> 680 0 C) and high precursor temperatures (> 200'C) [7]. The latter is especially detrimental, since the common 0diketonate source, Ce(dpm) 4 (dpm = 2,2,6,6-tetramethyl-3,5-heptanedionato) is always a solid that decomposes above - 150'C [7a, 7c-e]. Therefore, developing new cerium precursors, with improved thermal stability and lower melting points (liquids present a constant surface area for 37

Mat. Res. Soc. Symp. Proc. Vol. 574 © 1999 Materials Research Society

evaporation, unlike solids which typically sinter or shift-pack), became a prerequisite for this study. With the success of the homoleptic (all ligands are identical), tris[2,2-dimethyl-5-N-2methoxyethylimino-3-hexanonato] lanthanide(s), Ln(miki) 3 (the organic ligand is hereafter referred to as "miki"), we sought to expand the synthetic chemistry to potentially include main group elements. Specifically, we initiated a program for volatile, low melting (< 25'C, vide infra) Sn2+ sources for use in the MOCVD-mediated growth of n-type transparent conducting oxides (e.g., ln 203-SnO2; Ga 20 3 -ln2 0 3-SnO 2 ; Ga 3 .,In5+,Sn2O]6, for 0.2 _