Selective epitaxial growth of Ge dot structures is investigated to obtain highly regular dot arrays with excellent size uniformity in the nanoscale. The dot structures are grown in patterned fine windows ranging in diameter from 650 to 90 nm on Si(100) substrates covered with SiO 2 masks. The dimensions and number of the dots grown in a window significantly change depending on window size, growth temperature and time and the thickness of the Si buffer layer. These growth characteristics are considered to be induced by the Stranski-Krastanov growth mode and migration mechanism of Ge atoms on Si substrates. It is noted that Ge dots whose diameter is much smaller than the pattern size are formed with high uniformity and that the position is precisely controlled by SEG. The Ge dot structures are found to give rise to prominent luminescence with well separated phonon replicas and the energy position systematically changes when the growth conditions are varied.

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BaSi 2 is a promising absorber material for nextgeneration thin-film solar cells (TFSCs). For high-efficiency TFSCs, a suitable interlayer should be found for every light absorber. However, such an interlayer has not been studied for BaSi 2 . In this study, we investigated amorphous Zn 1−x Ge x O y films as interlayers for BaSi 2 . The Zn/Ge atomic ratio in the Zn 1−x Ge x O y film and the optical band gap depend on the substrate temperature during sputtering deposition. A suitable i-Zn 1−x Ge x O y /BaSi 2 heterointerface with spike-type conduction band offset was achieved when Zn 1−x Ge x O y was deposited on BaSi 2 at 50 °C. Furthermore, photoresponsivity measurements revealed that Zn 1−x Ge x O y has an excellent surface passivation effect on BaSi 2 . When the thickness of Zn 1−x Ge x O y was 2 nm, a high photoresponsivity of 0.9 A/W was obtained for a 500 nm thick BaSi 2 layer at a wavelength of 780 nm under an applied bias voltage of 0.5 V between the front and rear electrodes, where the photoresponsivity in the short-wavelength region was significantly improved compared with that of BaSi 2 capped with an amorphous Si layer. X-ray photoelectron spectroscopy revealed that the Zn 1−x Ge x O y films suppressed the oxidation of the BaSi 2 surface, decreasing the carrier recombination rate. This is the first demonstration of passivation interlayers for BaSi 2 with suitable band alignment for carrier transport and surface passivation effects.

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We have grown undoped n-BaSi2 epitaxial films with different grain sizes on Si(111) and characterized their minority-carrier lifetime, τ. We found that τ value in undoped n-BaSi2 did not depend on average grain area, but on surface condition. The samples with mirror surfaces had large τ of about 0.4 μs and those with cloudy surface small τ of about 8 μs. We tried to cap the sample surface in situ with a 3 nm Ba or Si layer in order to control the surface of BaSi2, and succeeded to intentionally form BaSi2 with large τ.