Synthesis and optical band gaps of alloyed Si–Ge type II clathrates

Inorganic type II clathrates are low density, semiconducting allotropes of group IV elements with the potential for optoelectronic applications. This class of materials is predicted to have direct or nearly-direct band gaps, and, when Si and Ge are alloyed in the clathrate structure, the band gap is tunable in the range of 0.8–1.8 eV. In this work, we demonstrate for the first time the synthesis of alloyed Si–Ge type II clathrates. Within this alloy system, we find an amorphous region which is likely due to a miscibility gap. The optical absorptance spectra of the crystalline clathrate samples show the predicted band gap tuning with Ge content, and calculations find that the Si type II clathrate has a strong absorption coefficient for the direct interband transition. The findings in this work lay the foundation for the future use of type II clathrates in optoelectronic applications.