We fabricated high-quality strain-relaxed thin SiGe layers by Ar ion implantation into Si substrates before epitaxial growth. The surface of 100-nm-thick Si0.8Ge0.2 layers, the relaxation ratio of which was more than 80%, was found to be very smooth, with a rms roughness of 0.34 nm. Cross-sectional transmission electron microscopy analysis confirmed that strain-relieving dislocations were effectively generated due to the ion-implantation-induced defects and confined in the vicinity of the heterointerface, resulting in a dislocation-free SiGe surface. Moreover, in-plane strain-field fluctuation was found to be largely reduced by this ion implantation method.

The BaSi 2 semiconductor is a promising candidate for an earth-abundant solar cell absorber. In this study, we have realized a crack-free BaSi 2 film by a simple thermal evaporation technique on a CaF 2 substrate at a growth temperature of 500 • C for electrical characterization. A Si layer preliminarily formed on the substrate by sputtering is a key to obtain stoichiometric BaSi 2 film. Detailed structural characterization of the evaporated films with different Si layer thicknesses by X-ray diffraction, scanning and transmission electron microscopy demonstrates that a crack-free 370-nm-thick BaSi 2 film is formed by consuming the Si layer. It is observed that the 90-nm-thick bottom part is microcrystalline and contains Ar atoms, which come from Si deposition atmosphere. The surface of the BaSi 2 layer is found to be covered by an amorphous Si layer due to Si-rich vapor at the last stage of evaporation. Electrical properties of the BaSi 2 film is revealed by Hall measurement and the electron density and mobility are found to be 6×10 20 cm −3 and 0.04 cm 2 /V·s, respectively. Owing to a better crystalline quality contributed by the preliminarily-deposited Si layer, minority-carrier lifetime of the evaporated film (0.6 µs) is twenty times longer than previous films on glass substrates.

In order to control the electrical properties of an evaporated BaSi 2 film, which is an emerging candidate for the absorber-layer material of earth-abundant thin-film solar cells, we have investigated the effects of deposition rate on the produced phases, microstructure, and carrier density of the thin films grown by thermal evaporation of BaSi 2. X-ray diffraction results show that a high substrate temperature is necessary for BaSi 2 formation at a high deposition rate, which is discussed from viewpoints of vapor composition and diffusion time. Microstructural characteristics such as grain size of 30-120 nm, oxide particle arrays present around the interface, and partial oxidation at a low substrate temperature are revealed by cross-sectional transmission electron microscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy combined with an energy-dispersive X-ray spectroscopy. With increasing deposition rate, the crystalline quality of BaSi 2 is found to improve, as evidenced by a decrease in full-width at half maximum of a [Si 4 ] 4À vibration band in Raman spectra. At the same time, electron density, which is determined by Hall measurement, decreases with deposition rate. The variation of electron density is discussed on the basis of microstructural characteristics and BaSi 2 formation mechanism. The most probable reason is concluded to be composition deviation from stoichiometry.

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A selective-ion-implantation technique was developed for introducing uniaxial strain into Si/Ge heterostructures. Laterally selective ion implantation with a stripe pattern was carried out into a Si substrate, followed by SiGe overgrowth in the whole region. Large strain relaxation of SiGe occurred selectively only in the ion-implanted area. This largely relaxed SiGe was found to considerably affect the strain state of the neighboring strained SiGe in the unimplanted area, resulting in the realization of a highly asymmetric strain state, that is, uniaxial strain. This result indicates that this technique has a high potential to realize high-mobility Si/Ge channels with uniaxial strain.