Photoconduction space-charge field

The most useful of the known photorefractives are LiNbC>3 and BaTiC>3. Both are ferroelectric materials. Light absorption, presumably by impurities, creates electron/hole pairs within the material which migrate anisotropically in the internal field of the polar crystal, to be trapped eventually with the creation of new, internal space charge fields which alter the local index of refraction of the material via the Pockels effect. If this mechanism is correct (and it appears established for the materials known to date), then only polar, photoconductive materials will be effective photorefractives. However, if more effective materials are to be discovered, a new mechanism will probably have to be discovered in order to increase the speed, now limited by the mobility of carriers in the materials, and sensitivity of the process. 

Soon after, photoconductivity experiments were interpreted totally differently. It was proposed that electrons have in fact very high low-field mobilities, on the order of 104 cm2/V s or more, their velocity saturating at the sound velocity as field is increased above a value of a few V/cm. The moving carriers were supposed to be polarons dressed with acoustic phonons [218,219]. This has not generally been accepted nor conclusively disproved. However, space-charge injection current experiments yielded electron mobilities on the order of 6 x 103 cm2V s [220]. 

When there is incomplete collection of charge, the primary photoconductivity is complicated by the presence of trapped space charge which distorts the electric field (see Section 10.1.2). Although the primary photoconductor is usually the best structure for a light detector, the recombination mechanisms are more commonly studied... 

In the photorefractive polymer composites described above, the value of the glass transition temperature is of great importance because it needs to be in a range where the electro-active chromophores can be reoriented by the photorefractive space-charge charge field. Adjustment of Tg is done though the addition of a plasticizer. Examples of plasticizers are shown in Fig. 23. As for the polymer matrixes, plasticizers can be photoconducting or inert. ECZ (molecule (a) in Fig. 23) is widely used with PVK. Inert plasticizers such as BBP (molecule (c) in Fig. 23) have also been combined with PVK [86]. 

Figure 19 Mobility of carriers in P-rhombohedral boron obtained by different methods and different authors. 1, From space-charge limited currents 2 and 3, (1h 4, field effect 6, thermally activated hopping O, from electrical conductivity and spin density , (Xh. > from electrical conductivity and ESR magnetoresistance , from ESR line width +, band mobility A, hopping mobUity A, from photoconductivity V, from high-field conductivity, I, Hall mobility and photoconductivity. (See Ref. 2 and references therein.)...

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