Defects in inorganic photorefractive materials and their investigations

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18 B. Briat et al. [99, 100]. EPR/optical investigations on this system [21, 101] showed that a main part among the occurring photo-induced charge transfers involves the three defects Rh 3+ ,Rh 4+ and Rh 5+ , fulfilling the ’3-valence model’ (Fig. 1) [102]. On this basis and relying on experimentally determined values of the effective trap density N eff , Huot et al. [103], and Corner et al. [104] analyzed the charge transfer properties of the system quantitatively. However, because the available experimental information was not sufficient to determine all relevant parameters, a simplified theoretical basis was employed. Later, using the EPR/optical method, a complete solution of the problem was possible [105, 106]. The procedure starts with investigating the wavelength dependence of the photochromic coloration of a BT:Rh crystal [98], induced by a series of rising pump light energies. The result is plotted over the field of the pump light, E pump , and probe light, E probe , energies (Fig. 9c). Here and in the following only a rather brief sketch of the method is given. For further details see Refs. [105, 106]. The main features in Fig. 9c are a strong light induced transparency at E probe =1.9 eV and pronounced absorption increases at 1.6 eV and 3.0 eV. Simultaneous measurements of the EPR of Rh 4+ , the only EPR-active Rh charge state in ’as grown’ BT:Rh - observable at T ≤ 20 K, show that the intensity of this EPR signal has an identical dependence on E pump as that of the transparency along E probe =1.9 eV. This assigns the band at 1.9 eV to Rh 4+ . Further EPR studies [107] indicate that the Rh 4+ intensity is decreased by the transfer of a valence band electron to Rh 4+ . In this way the EPR-silent charge state Rh 3+ is created. The band at 3.0 eV is attributed to Rh 3+ ,lying higher than Rh 4+ because of its lower charge. The hole created in the valence band by the electron transfer to Rh 4+ is expected to be captured by another Rh 4+ ,causingRh 5+ , which is also EPR-silent. Because of its higher charge, less energy is needed to excite a valence band electron to Rh 5+ . Therefore the other strong feature in Fig. 9c, at 1.6 eV, is assigned to Rh 5+ . In a similar way the further structures in Fig. 9c are attributed to various charge states of Fe; this element is usually present in BT as an unintended background impurity. These assignments fulfill systematic topological constraints typical for plots of the type of Fig. 9c [108]. Summarizing this part: By combined EPR/optical absorption studies, based on the photochromic behavior of BT, the optical absorption bands indicated by vertical dashed lines in Fig. 9c, have been identified. Among these only the defects Rh 4+ ,Fe 5+ and Fe 3+ are EPR-active. The position of the Fe 3+ band, covered by the fundamental absorption, has been inferred from its peak energy in KTaO 3 , Fig. 10. The other optical bands, belonging to the EPR-silent defects Rh 3+ ,Rh 5+ and Fe 4+ , are assigned by consistency arguments of the type as forwarded for Rh 3+ and Rh 5+ . This identification of the optical absorption bands allows that they can be used as ’fingerprints’ of the corresponding defects. They can be employed likewise at room temperature; EPR-measurements, on the other hand, usually require low temperatures. As shown, also absorption bands corresponding to EPR-silent defects could be
Defects in inorganic photorefractive materials and their investigations 19 identified. The shapes of the absorption bands are derived from a deconvolution of the photochromic absorption in the plot of Fig. 9c into five Gaussian bands [106] (Fig. 9d). This is possible for all values of E pump with a maximal deviation between experiment and model of 10 −3 cm −1 . Fig. 9. a: Schematic representation of the four levels in BT:Rh, defined by the three charge states of Rh and by three ones of the background contamination Fe. The chosen energies are not to scale. The levels exchange holes with the valence band by optical excitation (double arrows), thermal excitation (wavy arrows) and recombination (single arrows). The corresponding rates are indicated beside the arrows. Here the parameters have the following meanings: S, absorption cross-sections; q, quantum efficiencies for ionization after absorption; β, thermal transition rates; γ, recombination parameters. b: Discrimination between optical level (end of optical excitation in sketched configuration diagram) and thermal level, both for the special case of Rh 3+/4+ level. For simplicity this distinction is not included in Fig. 9a. c: Dependence of the absorption changes (grey scale) induced by pump light (vertical energy scale). The probe light energy is given as abscissa. Monitoring pump light-induced EPR changes allows to assign the absorption features to specific defects, indicated by vertical dashed lines. d: The deconvolution of the absorption changes in c, compared with the defect densities derived from EPR, leads to the shown absorption cross-sections.
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