Defects in inorganic photorefractive materials and their investigations
Defects in inorganic photorefractive materials and their investigations
Defects in inorganic photorefractive materials and their investigations
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26 B. Briat et al.<br />
’1 - 3’) <strong>in</strong> the visible region <strong>and</strong> at least two components (’4,5’) <strong>in</strong> the near<br />
IR are created. Qualitatively similar results concern<strong>in</strong>g this photochromic behavior<br />
<strong>in</strong> BGO <strong>and</strong> BSO were reported by another group [154, 155]. Strong<br />
MCD signals are associated with b<strong>and</strong>s ’1 - 5’, demonstrat<strong>in</strong>g the paramagnetic<br />
character of the related defect(s), <strong>in</strong> contrast to the situation observed<br />
<strong>in</strong> the bleached state.<br />
O<br />
O 2 O 2<br />
Bi 3+<br />
O 2<br />
Fig. 12. Model of the Bi 4+<br />
M<br />
antisite defect. ODMR measurements show that the<br />
hole is delocalized to an O 2− neighbor of Bi 3+ . There it is stabilized by lattice<br />
distortion, caused by the decreased attraction between O − <strong>and</strong> Bi 3+ . Because of<br />
the <strong>in</strong>itial equivalence of the tetrahedrally arranged oxygen ions, the system can be<br />
called a bound small hole polaron. The process of optical absorption is <strong>in</strong>dicated<br />
by the dashed l<strong>in</strong>e: the hole is transferred under Franck-Condon conditions from its<br />
<strong>in</strong>itial O − site to a neighbor<strong>in</strong>g O 2− -ion. Because of the equivalence of the three<br />
f<strong>in</strong>al O 2− -ions, a tunnel<strong>in</strong>g of the excited hole among them is expected, lead<strong>in</strong>g to<br />
two optical absorption b<strong>and</strong>s of nearly equal <strong>in</strong>tensities <strong>and</strong> widths [11]. The dashed<br />
hole transition can alternatively be viewed as a transition of an electron from the<br />
O 2− -ions, be<strong>in</strong>g part of the valence b<strong>and</strong>, to O − . This expla<strong>in</strong>s the transitions 2 <strong>and</strong><br />
3 <strong>in</strong> Fig. 13.<br />
The ODMR [151, 149], performed with the MCD signals ’1 - 3’, could be<br />
attributed to one unpaired sp<strong>in</strong> (S =1/2) with a strong hyperf<strong>in</strong>e coupl<strong>in</strong>g<br />
to the nuclear sp<strong>in</strong> (9/2) of the 100% abundant 209 Bi. All evidence led to<br />
the assignment of the MCD signals to an antisite defect [156] Bi 4+ or - <strong>in</strong><br />
M<br />
more detail - Bi 3+<br />
M<br />
’dressed’ by a hole, delocalized on one of the four tetrahedral<br />
oxygen neighbors. Although the observed symmetry of the defect was<br />
essentially isotropic, trigonal distortion was not excluded [157]. As concluded<br />
from the ODMR-MCD studies, all b<strong>and</strong>s ’1 - 3’ <strong>in</strong> Fig. 6 have the same Bi 4+<br />
M<br />
<strong>in</strong>itial state. B<strong>and</strong>s ’4 <strong>and</strong> 5’, <strong>in</strong> contrast, have to be assigned to different paramagnetic<br />
species [146] show<strong>in</strong>g a very broad ODMR signal for microwaves at<br />
35 GHz [153]. The magnetic <strong>and</strong> optical properties of Al 4+<br />
Si<br />
[150] <strong>and</strong> Ga 4+<br />
Si<br />
[153] were also established for the first time by ODMR.<br />
The transitions lead<strong>in</strong>g to the observed optical properties of all these defects<br />
are shown <strong>in</strong> Fig. 13. In the bleached state only the charge state Bi 3+<br />
M<br />
is<br />
present. The CD b<strong>and</strong> (’0’) associated with the absorption shoulder reflects<br />
the spectral dependence of the photoionization of an electron from Bi 3+<br />
M<br />
to<br />
the conduction b<strong>and</strong>; its onset around 2.4 eV at 77 K or lower <strong>and</strong> around