A composite semiconductor electrode with the structure "n-Si/p-CuI/ITO/n-i-p a-Si/n-p GaP/ITO/RuO 2 " was fabricated for the purpose of achieving efficient solar water splitting. The electrode showed a stable photoanodic current due to oxygen evolution with a large negative photoshift (V p ) of about 2.2 V from an anodic current at a RuO 2 electrode. The photoshift was large enough for full water splitting. A photoelectrochemical (PEC) cell, composed of the composite electrode, a Pt counter electrode, and 0.10 M Na 2 SO 4 (pH 6.3), generated a photocurrent density of 1.88 mA cm -2 under simulated solar illumination (AM 1.5 G, 100 mW cm -2 ), yielding a solar to chemical conversion efficiency of 2.3% as calculated from the photocurrent value. The result has shown that the combination of "crystalline Si/a-Si/GaP" is suitable for efficient solar water splitting. It is shown that the efficiency can be increased by use of GaP with a well-regulated p-n junction.

Photoinduced structural change in hydrogenated amorphous silicon (a-Si:H) has been studied by a sensitive bending detection method using an optical lever. We observed that a-Si:H films show not only thermal expansion due to a photothermal effect but also residual and persistent expansion after light soaking. The volume change is recovered by thermal annealing at 200 °C. A dehydrogenated sample annealed at 550 °C and a microcrystalline sample, in which photoinduced defects are not created, show little photoinduced expansion. The photoinduced expansion and photoinduced defect density show identical time evolution. These results suggest that the photoinduced expansion is related to the photoinduced defect creation. A quantitative evaluation of the photoinduced expansion indicates that the photoinduced structural change is spread over several molecular volumes around a photocreated defect.

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.

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Photothermal deflection spectroscopy has been applied to optical absorption measurements in As 2 S 3 , Se, and As-S glasses. The spectroscopic method is demonstrated to be suitable for evaluation of small absorption in thin samples, while its accuracy is affected by light scattering. For As 2 S 3 , as-evaporated films give higher residual absorption than those of the bulk. In the As-S system, As 2 S 3 shows the most prominent weak absorption tail, and As 17 S 83 exhibits a peculiar spectrum. The origins of these features are discussed.

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.

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 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)