Optical and electrical properties of β-FeSi_2 under pressure

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Fundamental questions of environmentally friendly semiconductor β-FeSi_2 is that photoluminescence has been observed in sub-micron ball samples but not in thin-film sample. For the first step to try to make this question clear it is important to elucidate basic properties of both samples. Our continued high-pressure studies are summarized in the following.We have investigated the high-pressure optical absorption of β-FeSi_2, thin films prepared from Si/Fe multilayers on Si(001) with template and SiO_2 capping. It is found that the experimental absorption coefficient in the range of photon energy of about 0.3 eV beyond the band gap is a few orders magnitude larger than the first-prinsicples calculated absorption coefficient. A possible explanation for this large absorption coefficient is the saddle-point exciton effect by the calculated band structure. The effect of pressure on high interband transitions up to 3 eV inβ-FeSi_2 is studied by measuring reflectivity spectra up to 16 GPa. Th … More e evaluated linear pressure coefficients for the interband transitions are all positive and about 8 meV/GPa for the transitions below 1.6 eV and about 16 meV/GPa for those beyond 1.6 eV. This characteristic indicates that the density of states near the energy gap consists of two different bands, with large- and small-pressure coefficients.The hole mobility of intentionally undoped p-typeβ-FeSi_2 thin films was investigated. With increasing annealing temperature and time, the hole mobility increased to approximately 450 cm^2/Vs at room temperature. The observed hole mobility was analyzed by considering various carrier scatterings such as acoustic-phonon and polar-optical-phonon scattering, intervalley scattering, ionized impurity scattering, and grain-boundary scattering. The nice fit of the mobility to the experimental results reveals that the polar-optical-phonon scattering determines the hole mobility at room temperature.Various optical measurements confirm that optical absorption in uniform thin films made from nanocrystalline iron disilicide(nc-FeSi_2) with a 3-5nm radius is larger by about 10 % than that of single crystalline □-FeSi_2. It is also found that the hydrogenation of nc-FeSi_2 changes strongly its optical characteristic energies. The nanocrystalline state appears characteristically in the imaginary part of dielectric constants of □-FeSi_2 around 2 to 3 eV. Less