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In semi-insulating GaAs wafers the distribution of the total EL2, independent of its charge state, was determined by purely optical absorption measurements. In all (Czochralski-grown) wafers studied the radial distribution of the total EL2 is W-shaped and shows fourfold symmetry. So the fluctuations of the neutral EL2-concentration seen in the usual near infrared transmission mapping reflect essentially the inhomogeneity of the total EL2 distribution. The mean EL2+ concentration in all wafers was close to typical C- and Zn-concentrations in s.i. GaAs (≈ 2·1015 cm-3).
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Zach et al. (1989) studied this question.
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