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Ultrathin C-doped Ge (Ge:C) buffers are used as effective buffer layers to grow thin and smooth Si1−xGex relaxed virtual substrates on Si over a wide range of Ge content (x = 0.23, 0.38, 0.50, 0.65, 1) by ultrahigh vacuum chemical vapor deposition. High degree of relaxation (≥90%) in thin Si1−xGex films (∼50 nm thick) is demonstrated using this approach without additional annealing. Raman data suggest that the Ge:C buffers are still under compressive stress with the subsequent Si1−xGex layer growth. A low threading dislocation density of the order of ∼6 × 104 cm−2 is obtained from the relaxed 500-nm Si0.77Ge0.23 film grown using this method.

A SiGe-based n-channel tunnel field-effect transistor design employing a strained-Si/strained-Ge staggered-gap heterojunction with a small effective band-gap (122 meV) at the interface is investigated via numerical simulations using a semi-classical quantum correction obtained from the density-gradient model. A gate-normal tunneling geometry is used to increase tunneling area and reduce subthreshold swing. The strain leads to degeneracy breaking among the silicon conduction band valleys, reducing the density of states and associated quantum capacitance with better gate-to-tunnel barrier coupling. Performance evaluation using a figure-of-merit “I60,” where the drain current corresponds to a subthreshold slope of 60 mV/decade, suggests that the device has the potential to be competitive with modern metal-oxide-semiconductor field-effect transistors.

Here, we present work on epitaxial Ge films grown on a thin buffer layer of C doped Ge (Ge:C). The growth rate of Ge:C is found to slow over time and is thus unsuitable for thick (>20 nm) layers. We demonstrate Ge films from 10 nm to >150 nm are possible by growing pure Ge on a thin Ge:C buffer. It is shown that this stack yields exceedingly low roughness levels (comparable to bulk Si wafers) and contains fewer defects and higher Hall mobility compared to traditional heteroepitaxial Ge. The addition of C at the interface helps reduce strain by its smaller atomic radius and its ability to pin defects within the thin buffer layer that do not thread to the top Ge layer.