A ternary type-I Si clathrate K(8)Ga(8)Si(38) has been revealed to be an indirect band gap semiconducting material with an energy gap (E(g)) of approximately 0.10 eV, which is much smaller than the calculated E(g) value that is 0.15 eV wider than E(g) of elemental Si with the diamond-type structure.

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We have investigated the melt growth of Mg2Si crystal and its electrical and optical properties. Progress in Mg source purity and stoichiometric control during the growth enabled the development of a high purity Mg2Si crystal with low carrier density and a high stable Mg2Si with good doping controllability. The Mg2Si crystal grown by the pressure controlled Bridgman method using 5N purity or 6N purity of Mg source and purified PG crucible showed low electron density (∼1015 cm−3) and high electron mobility (485 cm2 V−1 s−1 at 300 K and 21900 cm2 V−1 s−1 at 40 K). Silver doping in the high purity crystals performed the low-hole density of p-type Mg2Si (∼3 × 1016 cm−3). Ionization energy of residual Al donor in the high purity crystal and Ag acceptor in the Ag doped crystals was determined as 8–9 meV and 26 meV, respectively. Indirect band gap energy Eg of approximately 0.61 eV at 300 K and 0.69 eV at 4 K were estimated by the optical transmission measurements on the high purity crystals. It is also found that the Sb-doped melt grown crystal had good power factor around room temperature (26 µW cm−1 K−2 at 270 K).

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The periodic nanostructuring of inner part of semiconductors can be successfully accomplished by the infrared ultrashort pulse laser with a double pulse configuration. Self‐organized nanostructures inside semiconductor could be induced empirically only if it is indirect band gap semiconductor. The strained silicon regions with a width of about 100 nm are self‐aligned parallel to the polarization direction of the first arriving pulses, despite of the polarization direction of the secondly arriving pulses. AFM inspections reveal that such strained silicon nanostructures exhibit high electric conductivity and low thermal conductivity. The formation mechanisms would be interpreted in terms of the electrostrictive force through the interaction between electron‐hole plasma and phonon. Apart from the basic understanding, such nanostructured silicon will open the door to the fabrication of the self‐contained thermoelectric devices.

We investigated optical absorption spectra near the fundamental absorption edge of β-FeSi 2 single crystals by transmission measurements. The phonon structure corresponding to the emission and absorption component was clearly observed in the low-temperature absorption spectra. Assuming exciton state in the indirect allowed transition, we determined a phonon energy of 0.031 ± 0.004 eV. A value of 0.814 eV was obtained for the exciton transition energy at 4K. * Electronic address: udono@ee.ibaraki.ac.jp 1

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.