Photogeneration of charge carriers in titanium oxides

We have measured action spectra of the photoconductivity of rutile crystal at ∼4 K. The photoconductivity spectrum shows a keen rise at 3.0 eV and shows a peak at 3.2 eV and a shoulder around 3.7 eV. The threshold energy of the photoconductivity excitation well agrees with the onset of the fundamental optical absorption. This result indicates that the carriers contributing to the photoconductivity are generated by the fundamental excitation of the crystal. We have found that the photoconductivity shows remarkable field dependence. Below 5 V/mm, the photoconductivity was almost field independent. On the other hand, increasing the field strength in the range from 5 to 25 V/mm strongly enhanced the photoconductivity and varied the shape of the action spectrum. The origin of the field dependence is discussed