MOCVD Routes to Thin Films for Superconducting Applications. Precursor Synthesis and Film Processing Issues
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R
CF 3
triglyme
-L
M R
n
CF3
L=
or tetraglyme
n
R = t-Bu (dpm) or CF 3 (hfa) - Diketonates (1)
-
Diketonate Adducts (2)
Figure 1. Schematic structural diagrams of typical MOCVD precursors for use in the fabrication of HTS or dielectric thin films. suitable for HTS MOCVD (Figure 1, 1).8 In addition, these molecular species bring with them a strong history of successful reaction chemistry applicable to virtually any metal in the periodic chart. This allows for great flexibility in choosing/synthesizing novel P-diketonates capable of encapsulating a metal center, thereby preventing intermolecular interactions within the crystal lattice. Such lattice interactions would obviously decrease the precursor volatility, and the overall efficiency of the CVD system. Despite the success of the 13-diketonate (dpm) systems, in some instances the steric bulk of these ligands do not provide sufficient stabilization of the cationic metal center. This problem is particularly noteworthy in the formation of stable, volatile alkaline earth (Ca+2 , Sr+2 , and Ba+2 ) species, in which their small charge-to-radius ratio and kinetic lability have made encapsulation a daunting challenge to the synthetic chemist. 9 In addition, when coordination is accomplished using dpm, the metal centers act either as Lewis acids, rapidly absorbing ambient water to fill the coordination sphere, or form oligomeric species. 10a,b An evolutionary step forward that solved these shortcomings was the marriage of the M(hfa) 2 species with a stabilizing, polydentate Lewis base (Figure 1, 2). 1lab In these molecules, the choice of the electron-withdrawing hfa ligand is required to stabilize the polyether (glyme) adduct. Another tunable parameter built into these systems is the size of the glyme used to coordinately saturate the metal center. This property can be adjusted to the preferred coordination sphere for a given metal ion, thus offering an inexpensive, intuitive, and simple pathway into many new volatile MOCVD precursors. Similady, modification of the polyether symmetry can have the beneficial effect of further lowering12the melting point as seen in species such as Ba(hfa)2omep (mep = An advantage of these liquid precursors is the constant evaporation methylethylpentaglyme). surface area (as opposed to powders which sinter) which aids by assuring reproducible precursor transport in the MOCVD environment. Indeed, we have found the chemistry for this generation of precursors (M(hfa)noglyme) to be applicable in many instances outside of Group II, a prime in this study, being the superior volatility of La(hfa)3 etri when compared to example, found 13 La(dpm)3. The synthesis of new precursors is a fundamental concern in MOCVD, but equally as important is a reliable, general method for assessing the volatility of potential new precursors. To address this latter objective we have developed a simple, general method for screening CVD precursors before synthetic scale-up or placing them in our reactors. 14 This process is based on the vacuum TGA analysis of
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