Thin-film formation of Si clathrates on Si wafers

• Exposure of Na vapour on the surface of Si wafers formed NaSi precursor films.
• The NaSi films have high crystalline orientations.
• Si clathrate thin films were formed by annealing under vacuum.
• Type I and II Si clathrates were selectively obtained on the Si(100) and (111) wafers, respectively.

In this study, we prepared Si clathrate films (Na8Si46 and NaxSi136) using a single-crystalline Si substrate. Highly oriented film growth of Zintl-phase sodium silicide, which is a precursor of Si clathrate, was achieved by exposing Na vapour to Si substrates under an Ar atmosphere. Subsequent heat treatment of the NaSi film at 400 °C (3 h) under vacuum (<10−2 Pa) resulted in a film of Si clathrates having a thickness of several micrometres. Furthermore, this technique enabled the selective growth of Na8Si46 and NaxSi136 using the appropriate crystalline orientation of Si substrates.