Free-Carrier Absorption in Reduced SrTiO3

Optical absorption in stoichiometric and reduced, semiconducting single crystals of SrTi${\mathrm{O}}_{3}$ has been measured in the region 0.4-5 \ensuremath{\mu} at room temperature and 77\ifmmode^\circ\else\textdegree\fi{}K. Reduced crystals show a rapid increase of absorption with increasing wavelength beyond 0.6 \ensuremath{\mu}, 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 \ensuremath{\mu} the absorption varies as ${\ensuremath{\lambda}}^{2.5}$, 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 SrTi${\mathrm{O}}_{3}$, since the electron-longitudinal-optical-phonon coupling constant is greater than 3.