Trapping levels in hydrothermal and solution grown bismuth titanium oxide
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Trapping levels in hydrothermal and solution grown bismuth titanium oxide D. Eirug Davies and M. T. Harris Rome Laboratory, Hanscom Air Force Base, Massachusetts 01731 (Received 15 April 1996; accepted 14 October 1996)
Deep trapping levels in Bi12 TiO20 obtained by top seeded solution growth and by the hydrothermal technique have been compared. This was undertaken as such levels directly influence the photorefractive behavior of the material. It is found that the most predominant of the peaks revealed by thermally stimulated conductivity measurements represents two rather than a single defect level and that the deeper of the two becomes more significant in hydrothermally grown material. One defect found in the solution pulled material is notably absent from that produced hydrothermally. The consequence of adding phosphorus doping and the manner in which it affects the deep levels has also been examined.
Certain members of the sillenite family of materials are often grown for consideration in photorefractive applications.1 The most commonly encountered members of the group are BSO (Bi12 SiO20 ) and BGO (Bi12 GeO20 ), both of which can readily be pulled from a congruent melt. The phase diagram of BTO (Bi12 TiO20 ) makes it clear from the outset that its growth cannot be implemented in such a straightforward manner. Two of the more viable growth alternatives, pulling from solution (as opposed to from a congruent melt) and the more complex hydrothermal growth technique, are currently being used within the laboratory. The growth of BTO from solution has been previously investigated by Bruton et al.2 With some modifications that are primarily related to the degree of Bi richness in the melt, crystals up to , 200 g are currently produced that are dark brown in coloration. By contrast, the inclusion-free crystals provided by the hydrothermal technique are pale yellow to green in color. The hydrothermal work evolved from earlier success in its implementation within the laboratory for providing BSO.3 The deep trapping levels that arise in the as-grown crystals are of more than just a passing interest. It is the ionization of some such levels, when the material is illuminated nonuniformly by an input signal beam, that provides the basis for the photorefractive effect. The carriers so photogenerated tend to diffuse toward lesser illuminated regions where they subsequently become retrapped. This gives rise to space-charge regions with accompanying electric fields that, in turn, through the Pockels Effect, cause the localized refractive index variations utilized in the photorefractive effect. In the present work a comparison is made for the first time between the deep lying trapping levels found in material produced by the two different growth methods
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cited. The presence of the levels was determined through TSC (thermal stimulated conductivity) measurements. An unfiltered filament lamp is used for trap filling at cryogenic temperatures, and the s
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