Defects in inorganic photorefractive materials and their investigations

Defects in inorganic photorefractive materials and their investigations 15
4.2 Extrinsic defects
At first the dopings are treated which prevent or reduce the formation of Nb Li
during crystal growth from Li-deficient melts, such as Mg or Zn with concentrations
in the percent range. Further information on such ’optical damage
resistant’ dopings is given in Chapter 6. As has been mentioned above, the reduction
of crystals of this type leads to the formation of almost free electrons,
transformed to Nb 4+
Nb
polarons by coupling to the lattice. This is an indication
for the absence of Nb Li as trapping centers. Several additional dopings in such
crystals were investigated by EPR. The corresponding signals are characterized
by their large width, resulting from the addition of the doping-induced
disorder to the intrinsic one. Table 1 contains also information on the extrinsic
defects investigated in such crystals. It was found that additional extrinsic
ions tend to be incorporated at the Nb-sites of the lattice; apparently it is
more favorable that the Mg ions, rather abundant in the melt, replace Li ions.
Fig. 7. Comparison of EPR signals of LN:Cr and LN:Fe in congruent and stoichiometric
LN. Top: ’New’ Cr 3+
Nb
signals (a), characterized by a weak axial crystal
field typical for Nb replacement, as compared to those (b) of ’old’ Cr 3+
Li
with typical
strong axial crystal field [88]. Bottom: In congruent LN the ’old’ Fe 3+
Li
(d) is observed.
In stoichiometric material two additional types of Fe 3+ spectra arise (c) and
all lines become more narrow [64].
Here it has to be noted that it is rather difficult to determine from basic arguments
at which site, Li or Nb, cation dopings will enter LN. Both sites have