Defects in Rutile. III. Optical and Electrical Properties of Impurities and Charge Carriers

Abstract

Optical absorption spectra of various OH (OD) complexes in rutile have been studied as a function of light polarization, impurity concentration, and temperature. It is shown that the ${\mathrm{H}}^{+}$ (${\mathrm{D}}^{+}$) occupies sites in the basal plane, probably displaced somewhat from \textonehalf{}00, and can associate with various +3 substitutional cations, causing a shift of the OH (OD) stretching vibration to higher frequencies. Harmonic and combination-mode absorption has also been observed. Detailed absorption spectra and their polarization dependence for conduction-band electrons and for electrons in an Fe impurity band are presented, and the strength of the absorptions is related to the carrier concentration. These spectra apparently result from intra- or interband transitions of electrons. In neither case is a simple Drude-type absorption (as in a metal) adequate to account for the spectra. Preliminary data on the Fe impurity band indicate conductivity >0.1 ${\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ at room temperature for a carrier density of \ensuremath{\sim}7\ifmmode\times\else\texttimes\fi{}${10}^{19}$/${\mathrm{cm}}^{3}$. Qualitative information on diffusion and solubility of various cation impurities is given. These data and results presented in the preceding papers permit a fairly complete, though not yet quantitative, description of the electronic defect structure of rutile. In particular, it is concluded that rutile does not normally exhibit $p$-type conductivity, because of the ease of formation of ${\mathrm{Ti}}^{4+}$ interstitials; and that the donor defect in "reduced" rutile may be either ${\mathrm{Ti}}^{4+}$ or ${\mathrm{H}}^{+}$, depending on reduction temperature and ambient atmosphere, or may be any of a wide variety of metal impurities if these are present.