Optical transmittance and reflectance studies and evidence of weak electron-phonon interaction in Type-I Ge clathrate Ba_8Ga_(16)Ge_(30)

摘要

The optical properties of ternary type-I Ge clathrate Ba_8Ga_(16)Ge_(30) are investigated by optical reflectance and transmittance measurements. The refractive index and extinction coefficient are calculated from the reflectance spectrum via the modified Kramers-Kronig analysis method between 0.5 and 3.2 eV. The photon energy dependence of the optical absorption coefficient reveals that BagGai_6Ge_(30) is an indirect band gap semiconductor with a gap energy E_g of 0.69 eV at 5.7 K and 0.66 eV at 285 K. The temperature dependence of E_g can be satisfactorily described by an equation based on the Bose-Einstein statistics model. When compared with those of common elemental, III-V, and II-VI semiconductors, the small temperature coefficient dE_g/dT of the Ba_8Ga_(16)Ge_(30) can be considered to represent the weak electron-phonon interaction in the Ba_8Ga_(16)Ge_(30) clathrate.