Synthesis of New Composition of Matter (H3Ge)2SiH2 and (H3Ge)3SiH: Novel Pathways to Ge-rich Ge(1-x)(Si)x Virtual Substrates and Active Layers for Optical Electronic Devices Based on Si

This disclosure describes a synthetic route that provides the entire family of (H3Ge)4-xSiHx(x=0-3) silicon-germanium hydrides with molecular formulas H3Ge-SiH3, (H3Ge)2SiH2, (H3Ge)3SiH, (H3Ge)4Si. As expected these compounds posses simple tetrahedral structures in which a single Si atom is bonded directly to H and GeH3 groups. The (H3Ge)2SiH2, (H3Ge)3SiH and (H3Ge)4Si species have been prepared for the first time and are isolated as volatile colorless liquids that display the necessary physical and chemical properties to be viable vapor deposition sources. The previously known H3Ge-SiH3 analog has been synthesized in practical yields to be considered a viable alternative to the commercially available derivatives such as disilane [(SiH3)2] and digermane [(GeH3)2] in industrial applications. Potential uses of the disclosed compounds include synthesis and development of electronic optical materials and devices based on the Si-Ge and Si-Ge-Sn semiconductor systems. Targeted deposition experiments have been conducted via gas-source molecular beam epitaxy (MBE) and UHV-CVD to delineate the parameter space for growth of device quality films directly on silicon substrates. The synthetic routes of the molecules utilize high-yield single-step substitution reactions. A complete characterization of the final products was conducted via a range of spectroscopic and analytical methods such as multinuclear NMR experiments, gas source IR, mass spectrometry and elemental analysis. The data collectively confirm the proposed molecular compositions and structures. The experimental results compare extremely well with first principles calculations data of the spectroscopic and bonding properties of the molecules. A detail investigation of the physical and chemical properties have shown that the compounds can be purified to yield semiconductor grade products which are highly suitable for industrial application in Si-based technologies and manufacturing processes. MBE and UHV-CVD growth experiments yielded single crystal Si-Ge films with high Ge concentrations. The film compositions obtained from the unimolecular depositions of H3Ge-SiH3, (H3Ge)2SiH2, (H3Ge)3SiH and (H3Ge)4Si are SiGe, SiGe2, SiGe3 and SiGe4 indicating that the entire Si/Ge molecular framework of the precursor molecule is incorporated into the film. This indicates that our new method enables precise control of the composition and structure at the atomic scale. The films are of much higher quality than those previously deposited utilizing conventional sources under similar conditions, and grow strain free on Si without the need for graded compositions or lift-off technologies. The films display atomically flat surface morphologies and relatively low defect densities with the bulk of the defects concentrated at the Si interface. The highly coherent and smooth layers fulfill the requirements to be suitable candidates for development of lattice engineered "virtual substrates" for subsequent growth of strained Si and Ge channel devices extremely high mobilities. The devices have important applications in state-of-the-art high performance field-effect transistors (FET) and bipolar junction transistors.Our method offers crucial advantages to conventional methods for manufacturing device-quality strain-free Si1-xGex "virtual substrates" and buffer layers with Ge-rich concentrations. It is well established that the quality of the Si1-xGex buffer layers is crucially important for effective device fabrication and circuit performance. The buffer layers must display low dislocation densities, low surface roughness and high uniformity of layer thickness. Current strategies are based on growth of thick compositionally graded buffer layers, in which the misfit strain between the SiGe epilayer and Si substrate is gradually relieved with increasing film thickness. Via the graded composition method a layer thickness of 5-10 mm is required to achieve material that posses dislocation densities of 6106 cm-2 and surface roughness with RMS values of ~30 nm, for 50% Ge concentration. For Ge contents higher than 50% the defect densities and film roughness become much worse due to the increase in the lattice mismatch between the film and the substrate. This requires an even greater film thickness to achieve acceptable defect densities and a chemical-mechanical polishing (CMP) step to smoothen the surface before growing additional device structures. The extreme film thickness and the CMP step makes processing of the devices very expensive and in some cases creates additional problems and degradation of key film properties.In summary, we have demonstrated the preparation of (H3Ge)2SiH2, (H3Ge)3SiH and (H3Ge)4Si as new compositions of matter in the of (H3Ge)4-xSiHx(x=0-3) family of compounds. Our methodology also affords the previously reported SiH3GeH3 analog in viable yields to be a useful as a reagent in semiconductor research and development.