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and the films obtained from the Ni-Ga precursor contained NiGa, NiGa4, Ni2Ga3, and NiGa4. Deposition from (CO)4-CoGaCl2(THF) in the temperature range 250-350 °C gave nearly stoichiometric CoGa films with some chlorine contamination. A mass spectral study (EI-MS) of the precursor showed the abundance (15%) of the stripped heter-obimetallic species [CoGa]+ corresponding to the stability of the Co-Ga bond [537]. The chlorine content in the films decreased at higher temperatures (350-400 ° C), but the film became cobalt-rich, indicating that a decrease in the chlorine contamination is associated with a loss of gallium. This is probably due to the formation of volatile GaCl3, which is favored by the dissociation of Co-Ga bonds occurring above 350 ° C. This example shows the relevance of investigating the gas-phase behavior of precursors, particularly in view of the different fragmentation mechanisms operative in the thermal degradation of the molecule, depending on the decomposition temperature.

Fischer et al. have performed extensive studies on the CVD of intermetallics from molecular precursors containing metal-metal bonds between d and p block elements [539-542]. Volatile heterodinuclear organometallic compounds L(CO)3Co-GaR2(D) and L(CO)3Co-Ga[(CH2)y NR](R) (R = H, CH3, C2H5, CH2Bu', CH2SiMe3; D = THF, NMe3, NQH^; L = CO, PMe3, PPh3; R1 = CH3, C2H5) were studied as single molecular precursors for the deposition of binary CoGa alloy thin films. The films, grown in a hot wall reactor, had Co/Ga atomic ratios in the range of 1 to 3, depending upon the substrate temperature and the type of substituents at the gallium atom. The molecular structure of a prototype of the compound series L(CO)3- Co-GaR2(D) is shown in Figure 36. The deviation in the Co:Ga ratio when compared with that in the precursor was observed when the deposition was performed at low temperatures. The formation of thermally stable gallium alkyl complexes was suggested as a probable reason for the Ga deficiency in the deposits [539].

Figure 36. Molecular structure of (CO)4Co-Ga(CH2'Bu)2(C4H8O). Reprinted with permission from [539], R. A. Fischer and A. Miehr, Chem. Mater. 8, 497 (1996). © 1996, American Chemical Society.

Similarly, a series of complexes based on Fe-Ga bonds [540] was used to deposit films with different Fe:Ga ratios. [(CO)4FeGa{(CH2)3NMe2}]2, a tetranuclear species with a preformed Fe2Ga2 ring, and the gallium-rich complex [(CO)4Fe{Ga(CH2)3NMe2}(Bu')}2] (Scheme 26) produced Fe-Ga films with strict control of the stoichiometry. The efficient elimination of ligands due to fi-H elimination was proposed to be the reason for excellent film quality and low deposition temperatures (200-250 °C). Shore et al. have used HFeCo3(CO)12 and H2FeRu3(CO)12 as single sources for the CVD of FeCo3 and FeRu3 alloy films [408]. When occurring in cold wall reactors, partial decomposition of the HFeCo3(CO)12 into volatile organo-iron compounds and less volatile cobalt species was observed, resulting in iron enrichment of the films. Nevertheless, pure heterobimetallic films could be obtained from photochemical vapor deposition by irradiation with a mercury arc lamp, with CO as a carrier gas. Chi et al. have used the Fe-Sn compounds (C5H5)Fe(CO)2(SnMe3) and cis-Fe(CO)4(SnMe3)2 based on different Fe:Sn ratios to grow FeSn and FeSn2 films [131]. Pure FeSn films were obtained with (C5H5)Fe(CO)2(SnMe3), whereas films composed of the FeSn2 phase with a minor constituent of FeSn were obtained with cis-Fe(CO)4(SnMe3)2. Similarly, CoSn films with small amounts of a-Co3Sn2 were prepared by CVD of Me3SnCo(CO)4 and Ph3SnCo(CO)4 [119].

Scheme 26.

Figure 36. Molecular structure of (CO)4Co-Ga(CH2'Bu)2(C4H8O). Reprinted with permission from [539], R. A. Fischer and A. Miehr, Chem. Mater. 8, 497 (1996). © 1996, American Chemical Society.

Scheme 26.

The formation of intermetallic compounds from organo-metallic precursors is highly sensitive to the experimental conditions and requires an optimization of the deposition process for a phase-selective synthesis, especially in those cases where a large number of phases coexist in the binary phase diagrams.

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