Thin films of guest-free type-II Si clathrate (Si136) were fabricated on Si(111) wafers in two steps: NaxSi136 thin-film formation by thermal decomposition of NaSi precursor films and Na removal from the NaxSi136 film by a heat treatment with iodine. Cross-sectional TEM observation and XRD and Raman measurements verified the formation of 1-µm-thick Si136 films on the Si wafer. Since the prepared films showed n-type conduction, pn junction devices were developed by a Si136/p-type Si structure. This device showed a photovoltaic (PV) response under white light illumination. The thin film formation and the PV response of Si136 indicated this Si allotrope to be the next-generation platform for semiconductor technology.

Recent studies on type-II Si/Ge clathrates for photovoltaic (PV) applications are reviewed. The band-gap energies (Eg) experimentally estimated for Si and Ge clathrates are discussed on the basis of the comparison with theoretical calculation. For the Si and Ge clathrates, Eg = 1.7–1.9 and 0.6–0.8 eV are acceptable values, respectively. Thin films of type-II Si clathrates have been successfully synthesized on Si wafers. A device was fabricated using a Si clathrate thin film, and its PV response was observed. For the Ge clathrate, a single-crystal-like thin film was identified to epitaxially grow on a Ge(111) wafer. The PV response and epitaxial growth of group IV clathrates found in recent studies strongly suggest that these new materials will be the next-generation platform for semiconductor technology.

In this study, we prepared Si clathrate films (Na 8 Si 46 and Na x Si 136) 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 vapor 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 micrometers. Furthermore, this technique enabled the selective growth of Na 8 Si 46 and Na x Si 136 using the appropriate crystalline orientation of Si substrates.

In this study, we attempted to prepare films of Si clathrates (Na8Si46 (Type I) and Nax Si136(Type II)) and NaSi by using Si single crystalline substrates as the Si source. NaSi, which is the precursor of the Si clathrate, was successfully generated as a highly orientated film on the Si substrate, by exposing Na vapor under Ar atmosphere. The NaSi film was then transformed into Si clathrate film of several µm in thickness, by heat treatment (400 °C, 3 h) under vacuum (∼ 10‐4 Pa). Type I and II clathrates were selectively synthesized by using the crystal orientation (100) and (111) of Si substrates, respectively. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A thin film of a type II Ge clathrate, Na x Ge 136 , was epitaxially grown on a (111) substrate of Ge with a diamond structure (α-Ge). A Zintl phase NaGe film was synthesized in advance by a reaction of the substrate surface with Na vapor under an Ar atmosphere, and was highly oriented such that the NaGe(100) planes were parallel to the Ge(111) surface. The NaGe film was transformed to the Na x Ge 136 film by heat treatment under dynamic vacuum. XRD measurements demonstrated that the prepared film consisting of twin crystals with a (111) twin plane was epitaxially grown with the <111> direction normal to the substrate surface. It was also suggested that the lattice mismatch between Na x Ge 136 and the Ge substrate is relaxed by a buffer layer of α-Ge having a triple-period superlattice. The electrical resistivity of the Na x Ge 136 film was estimated from the I-V measurements to be in the order of 10 1 -10 2 Ω m. ExperimentalFrequently, type II clathrates are synthesized from Zintl alloys of alkali elements and Si or Ge. 1,3,12,16,20,21 Fig. 1(b) shows 5630