Photo-conductivity decay properties of Fe-doped congruent lithium niobate crystals

Photo-conductivity transient processes of Fe:LiNbO3 congruent crystals are investigated by electro-chemical analyzer. The experiments are executed with Fe:LiNbO3 crystals of different Fe concentrations in the conditions of different laser intensities. The results show that the transient photo-conductivity of the Fe-doped lithium niobate crystal is formed through a complex process of electron transport; the decay of photo-conductivity can be fitted to an exponential function and a stretched-exponential function. The dependences of the fitting parameters on laser intensity and iron-doped concentration are measured. The values of amplitudes sigma(max)(1), sigma(max)(2), time constant tau(2) and stretching factor beta increase strongly at low intensities, and tau(2) and beta reach their saturation value for higher intensities; with the increase of the concentration of Fe ions, the values of sigma(max)(1), sigma(max)(2) and tau(2) incerease, but beta decreases. With experimental results, we propose a charge transfer model which includes the migration of electrons in the conduction band and the jumping of electrons between small-polarons. The model better explains the main features of photo-conductivity decay for Fe-doped congruent lithium niobate crystals.