Exceptional thermal strain reduction by a tilting pillar architecture: Suspended Ge layers on Si (001)

In this paper we present the exceptional thermal strain release provided by micrometric Si pillar arrays to Ge epitaxial patches suspended on them, for different pillar aspect ratios and patch sizes. By combining 3D and 2D Finite Element Method simulations, low-energy plasma-enhanced chemical vapor deposition on patterned Si substrates, μ-Raman, μ-photoluminescence and XRD measurements, we provide a quantitative and consistent picture of this effect with the patch sizes. Strain relaxation up to 85% of the value for the corresponding planar films can be obtained for a squared patch 100 μm in size. Finally, the enhanced thermal strain relaxation is analytically explained in terms of the Si pillar lateral tilting, critically dependent on the pillar aspect ratio, very similarly to the well-known case of a deflected beam. Our results are transferable to any material deposited, or wafer bonded at high temperature, on any patterned substrate: wafer bowing can be controlled by micrometric patterned features well within the present capabilities of deep reactive ion etching.