Defects in inorganic photorefractive materials and their investigations

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22 B. Briat et al. Extrinsic defects: Cr 2+ [96], Cr 3+ [110], Cr 5+ [111, 112], Mn 2+ [113], Mn 4+ [114], Fe 2+ [115], Fe 3+ [116, 112], Fe 4+ [108], Fe 5+ [117, 108], Fe 3+ -V O [117], Fe 4+ -V O [112], Co 2+ [118], Co 3+ -V O [112], Co 4+ -V O [112], Ni + Ba [119], Mo 5+ [120], Rh 2+ [112], Rh 3+ [108], Rh 4+ ,Rh 5+ [108] Ir 4+ [112], Nd 3+ [121], Gd 3+ [122], Er 3+ [25], Ce 3+ [123], Na + Ba -O− [124], K + Ba -O− [124], Al 3+ -O − [112] Intrinsic defects: Ti 3+ (bound to various unidentified defects [125] or free as a conduction band polaron [10]), Ti 3+ -Nb 5+ [112] Table 3. Defects identified in BaTiO 3. The underlined charge states are EPR-silent. Their presence was proved indirectly by combined EPR/optical absorption studies. Especially in these cases the related optical absorption bands and the charge transfer processes, in which these defects are involved, have been determined. temperatures. By doping with the shallow alkali acceptors, such as Na Ba ,and additional oxidation [107], the Fermi-level can be lowered. Table 3 represents a large toolbox of defects whose possible influence on the photorefractive performance can be assessed on a microscopic basis. In this context the dopings Co and Ce , in addition to Rh, have received high attention. With Co-doped BaTiO 3 , having a wide absorption band at 2.25 eV, larger two- beam coupling gains were observed, using 515 nm light, than with Fe, Cr, or Mn doping [128]. The presence of Co 3+ -V O and of Co 2+ was identified in the crystals used in this investigation. The Ce 3+/4+ level lies near midgap and holes trapped there thus have a rather long lifetime [123, 129]. Together with codoped Rh, introducing a Rh 3+/4+ level more shallow than Ce 3+/4+ , the material offers potential for nonvolatile holographic storage by using gated two-wave illumination [123]. 5.2 Ba 1−x Ca x TiO 3 (BCT) The major drawback of BaTiO 3 for its application as a photorefractive material is the fact, that its phase transition from the tetragonal to the orthorhombic phase near 8 ◦ C tends to deteriorate the optical quality of the crystals. The replacement of part of Ba by Ca forms the isostructural Ba 1−x Ca x TiO 3
Defects in inorganic photorefractive materials and their investigations 23 (BCT), with the congruently melting composition at x =0.23 [130]. BCT is tetragonal between 100 ◦ C and at least 50 K [131] and thus avoids the phase transition problem impeding the use of BT. The photorefractive properties of BCT as a host material are similarly favorable as those of BT [132]. The EPR detection of defects in BCT meets with difficulties: Because of the irregular lattice positions caused by the statistical replacement of Ba 2+ (161 pm) by the much smaller Ca 2+ (134 pm), the local crystal fields at the defect sites are rather non-uniform, leading to superpositions of rather diverse signals. The resulting wide and odd-shaped EPR patterns are hard to evaluate with respect to the underlying defect models. Furthermore, because of so far unknown reasons, the presence of Ca causes strong microwave losses. In spite of these difficulties, it has been possible to identify several defects in BCT:Rh by EPR [133]. Such crystals usually have sizeable background concentrations of iron. In all of the investigated BCT:Rh samples Fe 3+ , isolated as well as in various low symmetry associations, was identified. Besides BCT:Fe [134, 135], the system BCT:Rh has so far received the most attention in photorefractive studies, because also in BCT doping with Rh increases the infrared sensitivity. Initial investigations of BCT:Rh are described by Veenhuis et al. [136] and by Bernhardt et al. [137, 138]. Because of the similarity of BCT:Rh to BT:Rh, also the optical absorptions bands in both cases are quite similar. Based on the EPR information on defects in BT:Rh [139] it thus was possible to elucidate all photorefractively relevant light-induced charge transfer processes in BCT:Rh qualitatively [138] as well as quantitatively [106] in the same way as described above for BT:Rh. On the basis of the determined absorption cross-sections, the defect densities and the charge transfer parameters, again the photorefractive performance of the system is predicted. Bernhardt et al. [138] discovered that rather high pump light intensities - about hundred times higher than in BT:Rh - are necessary that the photoconductivity of the material outweighs the dark conductivity. Under this condition the space charge fields are saturated at their maxima. The authors attributed this fact to the higher relative background Fe content in BCT:Rh. Meyer et al. [106], however, could show that the Fe content in the samples is not as high as assumed by Bernhardt et al. [138]. It is proposed that the high dark conductivity of BCT might instead be caused by shallow hole traps induced by disorder [106]. With reduced BCT:Fe it was shown [135], using combined EPR/optical studies, that Fe 2+ has a wide absorption band peaked at 1.9 eV. 5.3 KNbO 3 (KN) Chapter 7 deals with the favorable photorefractive properties of this material in detail. In KNbO 3 , orthorhombic below ∼ 200 ◦ C, the defects Mn 2+ ,Fe 3+ , Fe 3+ -V O ,Co 2+ ,Co 2+ -V O ,Ir 4+ ,Ti 4+ -O − could be identified with EPR [25]. An extended EPR- and combined EPR/optical absorption study of KNbO 3 :Fe is included in Ref. [86]. This doping is known to improve the
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