Epitaxial Ge0.81Sn0.19 nanowires for nanoscale mid-infrared emitters

Highly oriented Ge _0.81Sn _0.19 nanowires have been synthesized by a low-temperature chemical vapor deposition growth technique. The nanostructures form by a self-seeded vapor-liquid-solid mechanism. In this process, liquid metallic Sn seeds enable the anisotropic crystal growth and act as a sole source of Sn for the formation of the metastable Ge _1-xSn _x semiconductor material. The strain relaxation for a lattice mismatch of ϵ = 2.94% between the Ge (111) substrate and the constant Ge _0.81Sn _0.19 composition of nanowires is confined to a transition zone of <100 nm. In contrast, Ge _1-xSn _x structures with diameters in the micrometer range show a 5-fold longer compositional gradient very similar to epitaxial thin-film growth. Effects of the Sn growth promoters' dimensions on the morphological and compositional evolution of Ge _1-xSn _x are described. The temperature- and laser power-dependent photoluminescence analyses verify the formation of a direct band gap material with emission in the mid-infrared region and values expected for unstrained Ge _0.81Sn _0.19 (e.g., band gap of 0.3 eV at room temperature). These materials hold promise in applications such as thermal imaging and photodetection as well as building blocks for group IV-based mid- to near-IR photonics.