In view of the above, an alternative monohydride precursor, (HClGaN3)3, was synthesized [136, 141], which is a moderately volatile molecular system consisting of interconnected tetrameric Ga4N4 units (Fig. 31).

The compound decomposes easily at low temperatures by elimination of HCl and N2 to grow GaN on Si and sapphire (Eq. (19)).

An obvious extension of the above approach suggests the simpler dihydride analog of (Cl2GaN3)3 to be a potential precursor, as it could be expected to be more volatile (low molecular weight and increased covalent character). Indeed, [H2GaN3]3 is an air-sensitive liquid, volatile at room temperature, that can be distilled readily at 40 0C (0.2 torr). This simple hydride analog is particularly desirable as a CVD source [467] because it eliminates benign H2 and N2 byproducts to yield stoichiometric GaN at temperatures as low as 150 0C (Eq. (20)).

[H2GaN3]3 exists as a trimer of C3„ symmetry in the gas phase, as indicated by ab initio calculations. The Ga3N3 cyclic core of the structure (Fig. 32) represents the basic building block that is structurally consistent with the tetrahedral structures of the group III nitrides, including those of GaN.

The dihydrido compound was successfully used to obtain high-purity GaN films free of carbon, oxygen, or hydrogen

Figure 31. Representation of the Ga4N4 cyclic core of (HClGaN3)4. Reprinted with permission from [136], J. McMurran et al., Inorg. Chem. 37, 6638 (1998). © 1998, American Chemical Society.
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