Photorefractive Properties of Cobalt-Doped Barium Titanate
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PHOTOREFRACTIVE PROPERTIES OF COBALT-DOPED BARIUM TITANATE J. Y. CHANG, M. H. GARRET1t, H. P. JENSSENt and C. WARDE*
Department of Materials Science and Engineering, tCenter for Materials Science and Engineering, *Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, 77 Massachusetts Ave., Rm 10-372F, Cambridge, MA 02139 and P. TAYEBATI Foster Miller, Inc., 350 Second Ave., Waltham, MA 02154
ABSTRACT A series of cobalt-doped barium titanate crystals were grown that have high beam-coupling gain. Our photorefractive characterization shows that with increasing cobalt concentration in the crystal a deep level is formed and the relative effect of shallow levels on the space-charge field amplitude diminishes. This gives the crystal a long dark storage time. We find that cobalt-doping in barium titanate increases the two-beam coupling gain by reducing the dark conductivity and increasing the total effective trap density.
INTRODUCTION Barium titanate is a photorefractive ferroelectric crystal that has a variety of applications in optical information processing [1,2]. These include for example optical memories, self-pumped phase conjugate mirrors, neural networks and associative memories. Recently, Rytz et al [1] examined undoped and transition-metal-ion doped barium titanate, BaTiO 3 , and found that cobalt-doping reproducibly increased barium titanates' beam-coupling gain in the as-grown state relative to other dopants. They suggested that an increase in gain arises from higher effective trap densities and higher relative photoconductivity factors. From their EPR analysis, they also suggested that cobalt-doping in barium titanate forms a cobalt-oxygen vacancy complex that is a deep level with energies in the range of 2.0-2.3 eV above the valence band. Similarly, we have grown a series of Co-doped BaTiO 3 and have also obtained high beam coupling gain. Our photorefractive characterization indicates that the high gain in our Co-doped crystals is due to their higher total effective trap density and from the reduction of their dark conductivity due to the diminishing effects of shallow traps as the cobalt concentration is increased. Characterization shows that the cobalt-doped series of barium titanate crystals has a sublinear dependence of the response time with intensity which cannot be explained by a single-carrier or two-carrier single-species deep level model. Analysis by Mahgerefteh and Feinberg [2], and Brost and Motes [4] explain the sublinear dependence of the photoconductivity or response time on intensity by a model that includes both deep and shallow levels. Mahgerefteh and Feinberg categorized their barium titanate crystals as type A (low dark conductivity) or type B (high dark conductivity) depending on the relative density of photo-active donors and acceptors (or the deep and shallow levels). In this paper, we use the deep and shallow trap model of Tayebati and Mahgerefteh [3] to interpret our data. Our results show that cobalt-doping produces a level that is weakl
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