Epitaxy-driven synthesis of elemental Ge/Si strain-engineered materials and device structures via designer molecular chemistry

We describe the systematic epitaxial engineering of device-quality elemental structures in the Ge/Si system. By introducing small concentrations of (GeH ₃) ₂CH ₂ or GeH ₃CH ₃ organometallic additives into conventional Ge ₂H ₆, we have developed several new low-temperature CVD growth strategies that permit heteroepitaxy of highly dissimilar materials and provide unprecedented control of film microstructure, morphology, composition, and tuning of optical properties. Optimized molecular mixtures of these compounds have enabled layer-by-layer growth via facile elimination of extremely stable CH ₄ and H ₂ byproducts, consistent with calculated chemisorption energies and surface reactivities. Collectively, our experiments indicate that the additives confer unique pseudosurfactant behavior that profoundly alters the classic Stranski-Krastanov growth mechanism of epitaxial Ge on Si surfaces. Using this approach, we have produced atomically smooth, carbon-free Ge layers directly on Si with dislocations densities less than 1 × 10 ⁵ cm ^-2 (significantly less than those attainable from the best competing processes) at unprecedented low temperatures (350-420°C) compatible with selective area growth applications. Full relaxation of the film is readily achieved via formation of Lomer dislocations confined to the Ge/Si interface, which should, in principle, allow film dimensions approaching bulk values to be achieved on a Si substrate. Here, films with thicknesses up to several micrometers have been grown for use as passive/active heterostructure components. The practical utility of the approach is demonstrated for the first time by growing pure Ge seamlessly, conformally, and selectively in the "source/drain" regions of prototypical device structures. This innovation represents an ultimate extension of uniaxial strain techniques using group IV materials and is likely to have applications in the integration of microelectronics with optical components (photodiodes) into a single chip. As an additional example for high-mobility device template application, we have grown tensile Si films on the Ge buffers via decomposition of SiH ₃SiH ₂SiH ₃. The new Ge growth processes also provide a unique route to extend the utility of elemental Ge into the wider IR optoelectronic domain by tuning its fundamental optical properties using tensile strain as a main parameter. In this study, we use the metal-organic additives to circumvent traditional surface-energy limitations and produce for the first time high-quality, thermally stable, tensile strained Ge layers at low temperature (350-380°C) on Ge _1-ySn _y-buffered Si(100). The precise strain state of the epilayers is controlled by varying the Sn content of the buffer, yielding tunable record-high tensile strains as high as 0.43%. This strain-tuning strategy may offer the prospect of producing direct optical gaps in elemental Ge.