Strain-driven anomalous elastic properties of GeSn thin films

Elastic strain engineering in the GeSn bandgap structure is an attractive area for designing novel material properties. The linear interpolation of the elastic constants of Ge and Sn is commonly used to estimate their respective values for Ge1-xSnx alloys. This work reveals that Young's modulus of Ge1-xSnx epitaxial layers has a non-monotonic dependence on Sn composition. It is shown that the decrease in the elastic modulus correlates with the increase in Sn content in pseudomorphically grown Ge1-xSnx-epilayers with Sn concentration in the range of 1-5 at. % and subcritical thicknesses. An anomalous increase in the elastic modulus is observed with the further increase in Sn content (12 at. %), which is also accompanied by an increase in in-plane tensile strain. Phase separation and a decrease in the elastic modulus are observed for Ge1-xSnx-epilayers grown above the critical thickness with Sn concentration = 12 at. %. A correlation between the experimental elastic moduli and calculated elastic energies explains the complexity of strain-driven anomalous elastic properties of Ge1-xSnx-epilayers. The observed anomalous behavior of the Young's modulus for these GeSn epitaxial layers appears to be related to their recently predicted and observed short-range atomic order.