Novel Methods for CVD of Ge4C and (Ge4C)xSiy Diamond-like Semiconductor Heterostructures: Synthetic Pathways and Structures of Trigermyl-(GeH3)3CH and Tetragermyl-(GeH3)4C Methanes

GeX 2 ·dioxane (X = Cl, Br) complexes insert completely into CBr 4 to afford the sterically crowded cluster compounds (BrCl 2 Ge) 4 C ( 1 ) and (Br 3 Ge) 4 C ( 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 LiAlH 4 to produce the hydrides (H 3 Ge) 3 CH ( 3 ) and (H 3 Ge) 4 C ( 4 ) which are readily identified and characterized by spectroscopic methods and gas-phase electron diffraction. Compound 3 is also conveniently prepared from the LiAlH 4 reduction of (GeBr 3 ) 3 CH ( 5 ) which in turn is obtained by insertion of GeBr 2 ·dioxane into the C−Br bonds of bromoform. Refinement of the diffraction data for 3 confirmed a model of C 3 symmetry, with local C 3 v symmetry of the GeH 3 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 (Br 3 Ge) 4 C ( 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 Ge 4 C. Reactions of 4 with (SiH 3 ) 2 on Si yielded heteroepitaxial growth of metastable, monocrystalline (Ge 4 C) 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.