Defects in inorganic photorefractive materials and their investigations

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