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

Abstract

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 3V 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.

Gas Source Techniques for Molecular Beam Epitaxy of Highly Mismatched Ge Alloys

“…To examine the stability of the Ge-C core, crystals of 1 were dissolved in a solution of DMF, and ESMS clearly showed that the sp 3 hybridized carbon cluster core remained intact even after ionization (see Figure 2). In the positive ion mode, several cluster species were characterized by their isotope pattern [18,19,34,41,42]. The clusters retained the mass of the core Ge4C with several halides (m/z = CCl11Ge4 + (691), CBrCl10Ge4 + (737), CBr2Cl9Ge4 + (781), CBr3Cl8Ge4 + (825), and CBr4Cl7Ge4 + (870)), thus supporting the stability of compound 1.…”

“…Gas Phase FT-IR was carried out on a Bruker Tensor 27 (Bruker, Billerica, MA, USA) spectrometer using a Pike Tech 10 cm gas cell with KBr windows. 1D and 2D NMR were carried out for 1 H, and 13 C{ 1 H} using 2D homonuclear correlation spectroscopy (COSY) [35], and 13 C-HMBC [36,37] [18,19]. Previous reports indicated an initial reaction time of two hours at a 2.2 mmolar scale at room temperature with 79% yield [19].…”

“…Thermal dehydrogenation of the tetra-substituted E 4 C core in (H 3 E) 4 C (E = Si, Ge; see Figure 1) precursors was reported for Si 1−x−y Ge x C y alloys with bandgaps larger or smaller than Si 1−x Ge x [18][19][20][21][22][23][24][25][26]. The authors noted that these systems demonstrated a non-typical elongated Si-C and Ge-C bond length of 7% (1.89 Å (β-SiC)) and 5% (2.05 Å), respectively.…”

“…The first step of synthesis was similar to [18], producing (Cl 2 BrGe) 4 C (1) as follows. A Schlenk flask was charged with a 0.24 g (0.73 mmol) of CBr 4 , 8 mL of toluene and a stir bar resulting in a clear and colorless solution.…”

“…Part 2 describes the synthesis of our carbon precursor gas source, 4GeMe, including modifications to the previous reports of the synthesis. Undesirable carbon-carbon bonds are avoided using a carbon precursor based on a Ge4C core, in which each carbon atom is already fully surrounded by Ge [17,18]. This is distinct from the more common methylgermanes with Ge surrounded by multiple C atoms.…”