Light-induced charge-transport properties of photorefractive barium-calcium-titanate crystals doped with iron

Abstract

Nominally pure and iron doped, as-grown, and thermally annealed photorefractive barium-calcium-titanate crystals of the congruently melting composition Ba0.23Ca0.77TiO3 (BCT) are investigated by holographic and conventional electrical techniques. Refractive-index changes, two-beam-coupling gains, photoconductivities, dark conductivities, and bulk-photovoltaic current densities are measured. As-grown and oxidized crystals are hole conductive and at usual illumination conditions (light wavelength 514.5 nm, light intensity between 0.1 and 1 W/cm(2)) all measured properties are excellently described by an one-center charge-transport model. The effective electrooptic coefficient r(333) is only about 30 pm/V and thus much smaller than the value obtained from interferometric measurements. Two-beam-coupling gains as high as 7 cm(-1) are achieved. Doping with iron increases considerably the effective trap density, and bulk-photovoltaic fields of the order of some kilovolts per centimeter are observed in iron-doped crystals. Typical response times of iron-doped, as-grown, or oxidized crystals are about 0.5 s at 1 W/cm(2). Reduction yields electron-conductive BCT. The dark storage time increases from 6 min in the as-grown state to 3 h upon a slight reduction treatment, but decreases for strongly reduced samples. The investigation reveals that BCT will become a very promising alternative to barium-titanate crystals (BaTiO3) for many applications. (C) 2000 American Institute of Physics. [S0021-8979(00)04910-0].