High Resolution EELS Study of Ge_1-ySn_y and Ge_1-x-ySi_xSn_y Alloys

Abstract

Germanium (Ge) is a promising material for photonic device applications. Direct band gap photoluminescence is seen from elemental Ge due to the small energy separation between the Land Γvalleys [1]. This separation is further reduced by alloying Ge with Sn [2]. Unfortunately, the solid solubility of Sn in Ge is very low (<1.1%) limiting the thermal stability of Ge 1-y Sn y alloys with high Sn contents (y>0.011) [3]. Ternary Ge 1-x-y Si x Sn y alloys exhibit an enhanced thermodynamic stability relative to Ge 1-y Sn y analogs due to increased mixing entropy afforded by the incorporation of Si in the Ge 1-y Sn y lattice [4]. The incorporation of Si not only improves the thermal stability but changes the electronic structure [5]. The atomic distribution in these alloys at the sub-nanometer scale is of paramount concern, since even slight deviations from randomness can have a dramatic impact on the electronic structure. In this study Ge 1-y Sn y and Ge 1-x-y Si x Sn y were grown on Ge buffered Si(100) and were extensively characterized by cross-section transmission electron microscopy (XTEM) and "element-selective" electron energy loss spectral (EELS) mapping. The TEM samples were prepared using mechanical thinning and dimple polishing followed by argon-ion-milling. High-angle annulardark-field (HAADF) images and EELS spectra were collected on JEOL ARM 200F operated at 200kV with spot size 0.13nm in aberration corrected STEM mode. Atom-selective EELS mapping of lattice columns was performed using a GATAN Enfinium TM spectrometer.