JOURNAL OF MATERIALS SCIENCE LETTERS 17 (1998) 1491–1493 Metallo-organic chemical vapor deposition application of metallo- organic calix[4]arene-base barium precursor V. BURTMAN Department of Analytical and Inorganic Chemistry, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel E-mail: burtman@vms.huji.ac.il There is considerable interest in the deposition of films containing barium because of their technologi- cally important properties. These materials include perovskite phase ferroelectric ceramics such as the BaO–TiO 2 family [1] and Ba 1x Sr x TiO 3 [2], ceramic superconductors such as YBa 2 Cu 3 O 7x (YBCO) [3] and Tl 2 Ba 2 Ca 2 Cu 3 O 7x [4], magnetic materials such as BaFe 12 O 19 [5] and display materials such as Ce-doped BaS [6]. Films of these materials have been deposited by a variety of vapor- phase chemical and physical methods. The potential benefits of metallo-organic chemical vapor deposi- tion (MOCVD) over other film deposition techniques such as laser ablation, sputtering, molecular beam epitaxy, etc. are that MOCVD-derived films can be deposited under conditions that provide conforma- tion coverage and can be deposited at low tempera- tures. Moreover, there can be a high level of compositional control with the technique scaled to coat large areas uniformly. There is also the possibility of area-selected deposition [7]. Because the MOCVD method is based on the chemical nature of this process, it critically depends on the chemistry of starting metallo-organic materials known as precursors. Taking into account the low stability and sometimes the volatility of some traditional starting materials [8], the adequate choice of appropriate metallo-organic precursors is one of the main problems to be solved in order to reach reproducible results. This is exacerbated in the case of the deposition of Group 2 metal-containing films because it is particularly difficult to prepare mole- cular compounds of the heavier elements as precursors with all the appropriate characteristics. The availability of suitable precursors for the transport and deposition of Group 2 elements is the major difficultly in the MOCVD technique [9]. The origin of this problem lies in the tendency of the Group 2 elements to form oligomeric species as a result of their high coordination number (barium, for example, prefers a coordination number of 8–12 [10]), and a resulting small charge-to-size ratio, which is not conducive to formation of high-vapor- pressure species. It was also shown that in the Group 2 element range, the conventional â-dekitonate Ba precursor is the most problematic to handle [11] due to its higher crystal ionic radius than Ca and Sr. One of the alternative ways to overcome the well- known unsuitability of the traditional â-dekitonate barium precusor should be the design and synthesis of new metallo-organic materials with improved structural stability through the use of more suitable organic ligands. Calixarene ligand seems to exhibit an appropriate spatial structure for forming stable metallo-organic compounds with barium [12]. The syntheses and first application of this Ba–calixarene precursor (see Fig. 1) for the BTO thin films by dip- coating technique have been reported [13]. In this letter, we study the Ba-metallo-organic compound based on a calixarene ligand as a potential MOCVD barium precursor. The transport properties of the barium calixarene precursor was studied and com- pared with a traditional barium â-dekitonate sample. The barium calixarene precursor was primarily used for YBCO thin-film deposition. Ba–calix[4]arene has an extended chemical for- mula of barium salt of the p-tert-butylcalix[4]arene or schematically Ba(C[4]A) (shown in Fig. 1, Ba (C[4]A)) was obtained through the reaction of barium with p-tert-butylcalix[4]arene solution in dry tetrahydrofurane (THF) by reflux for 90 h [13]. Barium â-dekitonate precursor used for comparison of transport properties was prepared by reflux of barium metal in THF with â-dekitonate ligand under anhydrous conditions [14] following recrystallization from methanol. The barium precursor was kept in a glass container (not tightly enclosed) and stored in a drawer exposed to air for different periods of time prior to re-measuring its structural and transport properties. We labeled this material ‘‘aged barium precursor.’’ We used Ba(C[4]A) and regular Y- and 0261-8028 # 1998 Kluwer Academic Publishers t-Bu t-Bu t-Bu t-Bu HO OH O O Ba Figure 1 Structure of Ba–calix[4]arene and â-dekitonate MOCVD precursors. 1491