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 3<br />
oxygen vacancy. If the charge of the defect is to be <strong>in</strong>dicated, a correspond<strong>in</strong>g<br />
superscript is added. Sometimes the charges are referred to that of the replaced<br />
ion, then Bi x Si is used for an antisite defect, if the replac<strong>in</strong>g <strong>and</strong> the replaced ion<br />
have the same charges. If the replac<strong>in</strong>g one is negative (positive), the notation<br />
is Bi ′ Si (Bi• Si ). Also the label Bi4+<br />
Si<br />
, <strong>in</strong>dicat<strong>in</strong>g the electronic configuration of the<br />
defect ion, may be used for the neutral case <strong>and</strong> Bi 3+<br />
Si<br />
(Bi 5+<br />
Si<br />
) for the negative<br />
(positive) one. For an overview on defect notations see Ref. [3].<br />
Fig. 1. Def<strong>in</strong>ition of defect levels,<br />
illustrated with the three basic<br />
models for charge transfer between<br />
defects <strong>in</strong> <strong>photorefractive</strong><br />
crystals [4]. The special cases are<br />
shown, where electrons are transferred<br />
from the valence b<strong>and</strong> to<br />
the defect levels. Electron transfer<br />
from defect levels to the conduction<br />
b<strong>and</strong> would lead to complementary<br />
schemes. Double arrows<br />
<strong>in</strong>dicate light-<strong>in</strong>duced transfer,<br />
s<strong>in</strong>gle arrows recomb<strong>in</strong>ation<br />
of a defect electron with a valence<br />
b<strong>and</strong> hole. a): one - center<br />
model: under illum<strong>in</strong>ation the total<br />
concentrations X 0 <strong>and</strong> X − are<br />
not changed. The model therefore<br />
does not lead to photochromicity.<br />
b, c): Here the shallow levels are<br />
metastably populated after optical<br />
excitation. The concentrations<br />
of the defect charge states change,<br />
lead<strong>in</strong>g to photochromicity.<br />
As a consequence of the broken translational symmetry, defects can <strong>in</strong>troduce<br />
levels <strong>in</strong> the gap between valence <strong>and</strong> conduction b<strong>and</strong>, which represent<br />
the eigenenergies of an ideal crystal. Depend<strong>in</strong>g on the position of the Fermilevel<br />
<strong>in</strong> the crystal, such defect levels may be occupied by electrons or be<br />
empty. For oxide <strong>materials</strong>, reduction <strong>and</strong> oxidation are convenient means to<br />
shift the Fermi-level. The <strong>photorefractive</strong> effect is based on the fact that the<br />
level population can also be changed by illum<strong>in</strong>ation, especially <strong>in</strong> oxide <strong>materials</strong><br />
often <strong>in</strong> a metastable manner. A level <strong>in</strong>troduced by a defect X, where<br />
the charge state ’0’ is assumed to coexist with the charge state ’-’ , is labelled<br />
X 0/− (Fig. 1), <strong>in</strong> analogy to the notation for redox-pairs <strong>in</strong> electrochemistry,<br />
see e.g. [5]. This means: if the level lies at an energy E X above the valence