Case II Controlled Stoichiometry and Designed Ligand Elimination

This class of precursors is based on a controlled ligand elimination pathway. With the use of the principles of organometallic chemistry, the ligands or combination of lig-ands in the precursor is judiciously chosen so that stripping of the organic periphery, which is usually the source of contamination in MDN, is simple and clean (Eq. (3)).

In the case of Mg-Al heterometal alkoxide precursors discussed here, the modification of the precursor by the introduction of hydride or alkyl ligands uses the ^-elimination pathway on the metal centers. For example, Kim et al. have synthesized alkyl-modified derivatives of the general formula [Mg(^-OR)2AlMe2]2 (R = Pr', Bu') and successfully used them as single sources to deposit stoichiomet-ric MgAl2O4 films on Si(001) substrates [263]. Both of the precursors are much more volatile than MgAl2(OR)8 compounds; Mg[(^-O'Pr)2AlMe2]2 can be vapor-transported at room temperature, whereas Mg[(^-O'Bu)2AlMe2]2 should be heated to 60 °C. Veith et al. have designed a more elegant strategy in which the terminal alkoxide ligands on Al centers were replaced by hydride ligands [320] to obtain a hydride-modified Mg-Al ferf-butoxide, [MgAl2H4(OBu()4]. As a consequence of the drastic reduction in the molecular weight, an enhanced volatility is observed for the modified precursors ([MgAl2(OBu()8], 100 °C; [MgAl2Me4(OBu')4], 60 °C; [MgA^OBuOJ, 45 °C).

Both modification schemes are based on the abstraction of ^-hydrogen (by an incipient carbanion) from the alkoxy group to eliminate methane ([MgAl2Me4(OBu()4]) or dihy-drogen ([MgAl2H4(OBu()4]) and isobutene (Eq. (4)). This phenomenon is well established in organometallic chemistry and has been exploited for the deposition of ZnO and MgO from methyl zinc alkoxide and methyl magnesium alkoxide, respectively [233]. Figure 3 shows the ligand elimination mechanism operative in the alkyl- and hydride-modified Mg-Al alkoxides. The on-line mass spectral analysis performed during the chemical vapor deposition of [MgA!2H4(OBu')4] revealed isobutylene and dihydrogen to be the only volatile by-products. Veith et al. verified further the proposed mechanism by investigating the thermal decomposition of deuterated compound, [MgAl2D4(OBu()4] [320]. The observation of a new peak at m/z = 3 instead of the peak due to molecular hydrogen (m/z = 2) confirmed the abstraction of ^-hydrogen by the terminal deuteride lig-ands, resulting in the formation of HD. The above example illustrates that a careful selection of ligands can be used to design the decomposition behavior of the precursor compound. Since all of the thermolysis products are volatile organic compounds, "organic-free" nanomaterials can be obtained at low temperatures.

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