Doping effect on the Raman spectrum and photorefractive properties of LiNbO3 crystals

Abstract

Raman scattering spectroscopy is used to assess the cation ordering and photorefractive effect in nominally undoped and doped (Mg2+ , Gd3+ , and Y3+) LiNbO3 crystals with the stoichiometric and congruent compositions. The results demonstrate that, in a certain range of (low) Mg2+ and Gd3+ concentrations, doping increases the degree of cation ordering and notably reduces the magnitude of the photorefractive effect. Higher Mg2+ and Gd3+ concentrations lead to gradual cation disordering, as evidenced by the broadening of Raman bands and the increase in the magnitude of the photorefractive effect. Stoichiometric single crystals exhibit the strongest photorefractive effect because, owing to the lower defect density, they contain a lower concentration of shallow electron traps (sticking levels near the bottom of the conduction band). As a consequence, the efficiency of the alternative radiative-recombination channel due to the sticking levels drops markedly, and most of the photoexcited electrons are captured by deeper traps. It is shown for the first time that the intensity of the band corresponding to the stretching mode of the bridging oxygens in the NbO6 octahedra is sensitive to dipole ordering in the cation sublattice: an increase in the degree of dipole ordering (in spontaneous polarization) in response to compositional changes is accompanied by an increase in the intensity of the band in question.