Metallo-organic Chemical Vapor Deposition Application of Metallo-organic Calix[4]arene-base Barium Precursor

Abstract

There is considerable interest in the deposition of ®lms containing barium because of their technologically important properties. These materials include perovskite phase ferroelectric ceramics such as the BaO±TiO2 family [1] and Ba1yxSrx TiO3 [2], ceramic superconductors such as YBa2Cu3O7yx (YBCO) [3] and Tl2Ba2Ca2Cu3O7yx [4], magnetic materials such as BaFe12O19 [5] and display materials such as Ce-doped BaS [6]. Films of these materials have been deposited by a variety of vaporphase chemical and physical methods. The potential bene®ts of metallo-organic chemical vapor deposition (MOCVD) over other ®lm deposition techniques such as laser ablation, sputtering, molecular beam epitaxy, etc. are that MOCVD-derived ®lms can be deposited under conditions that provide conformation coverage and can be deposited at low temperatures. 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 ®lms because it is particularly dif®cult to prepare molecular 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 dif®cultly 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-vaporpressure 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 wellknown 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 ®rst application of this Ba±calixarene precursor (see Fig. 1) for the BTO thin ®lms by dipcoating 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 compared with a traditional barium â-dekitonate sample. The barium calixarene precursor was primarily used for YBCO thin-®lm deposition. Ba±calix[4]arene has an extended chemical formula 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 re ux for 90 h [13]. Barium â-dekitonate precursor used for comparison of transport properties was prepared by re ux 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 Yand