Photoconductivity in LiNbO_3 crystals codoped with MgO and Cr_2O_3

摘要

The photoconductivity of LiNbO_3 crystals codoped with Cr_2O_3 and different concentrations of MgO from 0 up to 6 mol % is reported. The spectral excitation of photocurrent presents an intense band corresponding to the Cr~(3+) induced blue absorption edge, but illuminating at the ~4A_2 → ~4T_1 ionic transition band a weak photocurrent is also excited. The photoconductivity is interpreted in terms of Cr~(3+) ionization and the formation of Nb_(Nb)~(4+) small polarons as charge carriers. Photoconductivity below about 140 K is suppressed. The analysis of the photocurrent temperature dependence results in a 0.14 ± 0.02 eV binding energy of the polaron at low temperature. Photoconductivity induced by excitation in the intraionic Cr~(3+) can be explained by a tunnelling process between the excited Cr~(3+) and the polaron state at the next-nearest-neighbour Nb_(Nb)~(5+).