Photoinduced phenomena in Sr1-xCaxTiO₃, 0 ≤ x ≤ 0.12

Photoinduced changes of the spectral, temperature and time dependences of the conductivity, the Cr3+ R-luminescence, the electron paramagnetic resonance, the refractive index and of the spatial variation of the linear birefringence are measured in nominally pure SrTiO3 and ferroelectric Sr1-xCaxTiO3 (x=0.007 and 0.12). In both materials a large, long time persisting increase of the photoconductivity for light with energy less than 2.1 eV was found as a result of previous illumination with higher photon energies. This memory threshold coincides with the low energy edge of the strongest extrinsic band in the excitation spectra. Basic mechanisms of extrinsic photoelectronic processes in both materials are proposed. They involve a parallel action of (i) photoinduced charge transfer from the valence to the conduction band via deep energy levels within the gap, (ii) recombination due to Cr3+-Cr4+ centers, and (iii) trapping and light-induced emission processes via relatively shallow traps.