Novel methods for CVD of Ge₄C and (Ge₄C)(x)Si(y) diamond-like semiconductor heterostructures: Synthetic pathways and structures of trigermyl-(GeH₃)₃CH and tetragermyl-(GeH₃)₄C methanes

GeX₂·dioxane (X = Cl, Br) complexes insert completely into CBr₄ to afford the sterically crowded cluster compounds (BrCl₂Ge)₄C (1) and (Br₃Ge)₄C (2) in 80% and 95% yields, respectively. These display physical, spectroscopic, and structural properties that are indicative of highly symmetric molecules with a remarkably strained carbon center. Compounds 1 and 2 react with LiAlH₄ to produce the hydrides (H₃Ge)₃CH (3) and (H₃Ge)₄C (4) which are readily identified and characterized by spectroscopic methods and gas-phase electron diffraction. Compound 3 is also conveniently prepared from.the LiAlH₄ reduction of (GeBr₃)₃CH (5) which in turn is obtained by insertion of GeBr₂·dioxane into the C-Br bonds of bromoform. Refinement of the diffraction data for 3 confirmed a model of C₃ symmetry, with local C(3v) symmetry of the GeH₃ groups, and gave a Ge-C bond length of 1.96 Å. The structure refinement of 4 was based on a model of T symmetry and displayed a rather normal Ge-C bond distance of 1.97 Å, which is substantially shorter than that (2.049 Å) of the strained (Br₃Ge)₄C (2) compound. Density functional calculations closely reproduced the observed molecular structures for 3 and 4. The thermal dehydrogenation of 4 on (100) Si surfaces at 500°C resulted in the growth of a diamond-structured material with an approximate composition of Ge4C. Reactions of 4 with (SiH₃)₂ on Si yielded heteroepitaxial growth of metastable, monocrystalline (Ge₄C)(x)Si(y) alloy semiconductors that are intended to have band gaps wider than those of pure Si and Si(1-x)Ge(x) alloys and strained superlattices. The covalent cluster species described here not only are of intrinsic molecular interest but also provide a unique route to a new class of semiconductor materials and form a model for local carbon sites in Ge-C crystals and related electronic materials based on the diamond structure.