Coupling of coherent misfit strain and composition distributions in core–shell Ge/Ge1-xSnx nanowire light emitters

Abstract

Core–shell Ge/Ge1-xSnx nanowires exhibit strong room temperature light emission under optical pumping in photoluminescence compared with Ge as a consequence of the elastic strain distribution across the coherent core–shell interface. We examine the interplay between strain and Sn composition distributions to gain insight into how these affect optical properties. There is a two-fold synergistic effect on the optical properties produced by the core–shell nanowire geometry. First, the Ge core acts as an elastically compliant substrate for growth of an axially lattice-matched epitaxial Ge1-xSnx shell, which facilitates growth of high-quality single-crystal Ge1-xSnx having intense photoluminescence. At the same time, tensile misfit strain in the Ge core serves to decrease the direct gap transition energy with respect to the indirect gap transition energy, thus enhancing its optical emission. Although characterization of strain in core–shell nanowire cross sections is complicated, choice of thinning orientation allows accurate, spatially localized strain analysis in the electron microscope. Phase field simulations provide an estimate of the expected strain, which matches well with experimental results, and they explain the stability of six Sn-poor spokes that form approximately 60° apart during growth of the Ge1-xSnx shell by considering the elastic strain energy. The evolution of the strain distribution during shell growth is also simulated using the phase field model. The coupling of core–shell strain arising from Sn incorporation in the shell and the resulting enhancement of optical properties makes this core–shell nanowire architecture promising for group IV semiconductor nanophotonics.