Optical transmittance and reflectance studies and evidence of weak electron-phonon interaction in Type-I Ge clathrate Ba8Ga16Ge30

The optical properties of ternary type-I Ge clathrate Ba8Ga16Ge30 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 Ba8Ga16Ge30 is an indirect band gap semiconductor with a gap energy E-g of 0.69 eV at 5.7K 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 Ba8Ga16Ge30 can be considered to represent the weak electron-phonon interaction in the Ba8Ga16Ge30 clathrate. Published by AIP Publishing.