Optical band gap of semiconductive type II Si clathrate purified by centrifugation

Abstract The optical properties of Si clathrate, which is believed to have potential as a new Si-based material for optical devices, were investigated in this study. Si clathrates with type II structure (Na x Si 136 ) with low Na content ( x = 1.3, 2.0) were synthesized by thermal decomposition of NaSi. The synthesized samples were purified by centrifugation using a solution of CH 2 Br 2 –C 2 Cl 4 to remove impurities. Using the obtained high purity samples (Na 1.3 Si 136 , 93 wt.%; Na 2.0 Si 136 , 90 wt.%), the optical absorption spectra of Na x Si 136 were clarified, for the first time, from diffuse reflection measurements. The inclusion of Na in Si 136 was found to cause free carrier absorption in the infrared region but had little effect on the fundamental absorption edge found in the visible region.