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.