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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<strong>Defects</strong> <strong>in</strong> <strong>in</strong>organic <strong>photorefractive</strong> <strong>materials</strong> <strong>and</strong> <strong>their</strong> <strong>in</strong>vestigations 27<br />
Fig. 13. Spectroscopic model<br />
for sillenite crystals. The notation<br />
0-4 for the various absorption<br />
processes is the same as that used<br />
<strong>in</strong> figure 11.<br />
2.2 eV at room temperature <strong>in</strong>dicates the thermal energy of the Bi 3+/4+<br />
M<br />
level<br />
with respect to the conduction b<strong>and</strong>. The ionized electron is then trapped at<br />
one of the donor states D + , which are all empty before, <strong>and</strong> the paramagnetic<br />
D 0 is formed. Photoabsorption <strong>and</strong> photo-MCD experiments have shown that<br />
the b<strong>and</strong>s <strong>in</strong> the visible region can be partly bleached at low temperature by<br />
a further illum<strong>in</strong>ation either at 1.5 eV (800 nm) or, more effectively, at 2.2 eV<br />
(560 nm). In the first <strong>in</strong>stance, the electron ionized from the D 0/+ level is<br />
trapped at the Bi 3+/4+<br />
M<br />
level. In the second process, electrons are excited<br />
from the valence b<strong>and</strong> to the Bi 3+/4+<br />
M<br />
level.<br />
B<strong>and</strong> ’1’ <strong>in</strong> the MCD <strong>and</strong> absorption spectra (colored state) is assigned<br />
[153] to the ionization of an electron from the Bi 4+/5+<br />
M<br />
level to the conduction<br />
b<strong>and</strong>, i.e. to the second ionization of the antisite defect. This is supported<br />
by the follow<strong>in</strong>g observations: 1) as shown by ODMR, the <strong>in</strong>itial state of the<br />
transition is the Bi 4+<br />
M<br />
state; 2) the b<strong>and</strong> does not occur under strong acceptor<br />
(e.g. Al) dop<strong>in</strong>g. This removes all Bi 4+<br />
M<br />
charge states. S<strong>in</strong>ce the Al3+/4+<br />
Si<br />
level<br />
was shown earlier, us<strong>in</strong>g thermal bleach<strong>in</strong>g studies [150], to lie at most 0.5 eV<br />
above the valence b<strong>and</strong>, one has to conclude that the Al 3+/4+<br />
Si<br />
level lies below<br />
the Bi 4+/5+<br />
M<br />
level, as shown <strong>in</strong> Fig. 13. The position of the former at about E C<br />
- 2.7 eV at 4.2 K is taken from the onset of b<strong>and</strong> ’1’. F<strong>in</strong>ally, the broad MCD<br />
<strong>and</strong> optical absorption features ’4’ <strong>and</strong> ’5’ were tentatively assigned [146] to<br />
oxygen vacancies.<br />
Fig. 11 shows that b<strong>and</strong>s ’2’ <strong>and</strong> ’3’ dom<strong>in</strong>ate the optical absorption <strong>in</strong><br />
the colored state. As shown above, they are caused by electron excitation<br />
from the valence b<strong>and</strong> to Bi 4+<br />
M<br />
or, alternatively, to the excitation of the Bi4+<br />
M<br />
hole to the valence b<strong>and</strong>. A hole at one of the tetrahedral oxygen ions next<br />
to Bi 3+<br />
M<br />
represents a small hole polaron bound <strong>in</strong> tetrahedral symmetry [11].<br />
By the self-localization at one oxygen site the symmetry is lowered to trigonal.<br />
It is a natural consequence [11, 158] of such a system that its optical<br />
absorption consists of two strong, equally wide b<strong>and</strong>s of nearly identical <strong>in</strong>tensities,<br />
separated by some tenths of an eV. One of the two excited states is