Free-Carrier Absorption in Reduced SrTiO3

Optical absorption in stoichiometric and reduced, semiconducting single crystals of SrTiO₃ has been measured in the region 0.4-5 μ at room temperature and 77°K. Reduced crystals show a rapid increase of absorption with increasing wavelength beyond 0.6 μ, characteristic of free-carrier absorption. The absorption is proportional to electron concentration and nearly independent of temperature. For wavelengths between 0.7 and 2.5 μ the absorption varies as λ^2.5, suggesting that longitudinal-optical-mode scattering is the dominant electron scattering mechanism at these temperatures. The magnitude of the observed free-carrier absorption is in poor agreement with that predicted from weak-coupling polaron theory but does agree with optical constants determined from recent reflectivity measurements. The validity of a weak-coupling theory is questionable in SrTiO₃, since the electron-longitudinal-optical-phonon coupling constant is greater than 3.