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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18 B. Briat et al.<br />
[99, 100]. EPR/optical <strong>in</strong>vestigations on this system [21, 101] showed that a<br />
ma<strong>in</strong> part among the occurr<strong>in</strong>g photo-<strong>in</strong>duced charge transfers <strong>in</strong>volves the<br />
three defects Rh 3+ ,Rh 4+ <strong>and</strong> Rh 5+ , fulfill<strong>in</strong>g the ’3-valence model’ (Fig. 1)<br />
[102]. On this basis <strong>and</strong> rely<strong>in</strong>g on experimentally determ<strong>in</strong>ed values of the<br />
effective trap density N eff , Huot et al. [103], <strong>and</strong> Corner et al. [104] analyzed<br />
the charge transfer properties of the system quantitatively. However, because<br />
the available experimental <strong>in</strong>formation was not sufficient to determ<strong>in</strong>e all relevant<br />
parameters, a simplified theoretical basis was employed. Later, us<strong>in</strong>g<br />
the EPR/optical method, a complete solution of the problem was possible<br />
[105, 106].<br />
The procedure starts with <strong>in</strong>vestigat<strong>in</strong>g the wavelength dependence of the<br />
photochromic coloration of a BT:Rh crystal [98], <strong>in</strong>duced by a series of ris<strong>in</strong>g<br />
pump light energies. The result is plotted over the field of the pump light,<br />
E pump , <strong>and</strong> probe light, E probe , energies (Fig. 9c). Here <strong>and</strong> <strong>in</strong> the follow<strong>in</strong>g<br />
only a rather brief sketch of the method is given. For further details see Refs.<br />
[105, 106]. The ma<strong>in</strong> features <strong>in</strong> Fig. 9c are a strong light <strong>in</strong>duced transparency<br />
at E probe =1.9 eV <strong>and</strong> pronounced absorption <strong>in</strong>creases at 1.6 eV <strong>and</strong> 3.0 eV.<br />
Simultaneous measurements of the EPR of Rh 4+ , the only EPR-active Rh<br />
charge state <strong>in</strong> ’as grown’ BT:Rh - observable at T ≤ 20 K, show that the<br />
<strong>in</strong>tensity of this EPR signal has an identical dependence on E pump as that of<br />
the transparency along E probe =1.9 eV. This assigns the b<strong>and</strong> at 1.9 eV to<br />
Rh 4+ . Further EPR studies [107] <strong>in</strong>dicate that the Rh 4+ <strong>in</strong>tensity is decreased<br />
by the transfer of a valence b<strong>and</strong> electron to Rh 4+ . In this way the EPR-silent<br />
charge state Rh 3+ is created. The b<strong>and</strong> at 3.0 eV is attributed to Rh 3+ ,ly<strong>in</strong>g<br />
higher than Rh 4+ because of its lower charge. The hole created <strong>in</strong> the valence<br />
b<strong>and</strong> by the electron transfer to Rh 4+ is expected to be captured by another<br />
Rh 4+ ,caus<strong>in</strong>gRh 5+ , which is also EPR-silent. Because of its higher charge,<br />
less energy is needed to excite a valence b<strong>and</strong> electron to Rh 5+ . Therefore the<br />
other strong feature <strong>in</strong> Fig. 9c, at 1.6 eV, is assigned to Rh 5+ . In a similar way<br />
the further structures <strong>in</strong> Fig. 9c are attributed to various charge states of Fe;<br />
this element is usually present <strong>in</strong> BT as an un<strong>in</strong>tended background impurity.<br />
These assignments fulfill systematic topological constra<strong>in</strong>ts typical for plots<br />
of the type of Fig. 9c [108].<br />
Summariz<strong>in</strong>g this part: By comb<strong>in</strong>ed EPR/optical absorption studies,<br />
based on the photochromic behavior of BT, the optical absorption b<strong>and</strong>s <strong>in</strong>dicated<br />
by vertical dashed l<strong>in</strong>es <strong>in</strong> Fig. 9c, have been identified. Among these<br />
only the defects Rh 4+ ,Fe 5+ <strong>and</strong> Fe 3+ are EPR-active. The position of the<br />
Fe 3+ b<strong>and</strong>, covered by the fundamental absorption, has been <strong>in</strong>ferred from<br />
its peak energy <strong>in</strong> KTaO 3 , Fig. 10. The other optical b<strong>and</strong>s, belong<strong>in</strong>g to the<br />
EPR-silent defects Rh 3+ ,Rh 5+ <strong>and</strong> Fe 4+ , are assigned by consistency arguments<br />
of the type as forwarded for Rh 3+ <strong>and</strong> Rh 5+ . This identification of the<br />
optical absorption b<strong>and</strong>s allows that they can be used as ’f<strong>in</strong>gerpr<strong>in</strong>ts’ of the<br />
correspond<strong>in</strong>g defects. They can be employed likewise at room temperature;<br />
EPR-measurements, on the other h<strong>and</strong>, usually require low temperatures. As<br />
shown, also absorption b<strong>and</strong>s correspond<strong>in</strong>g to EPR-silent defects could be