Local vibrational mode absorption was used to investigate the electrically active fraction of C in semi-insulating GaAs. A strictly linear relationship to the total chemical C concentration measured by spark source mass spectrometry is found. Using charged particle activation analysis as a reference method, a new calibration factor f/sub 77/=(7.2/spl plusmn/0.2)/spl times/10/sup 15/ cm/sup -1/ for the absorption integral at 77 K is derived. Based on the temperature dependence of the absorption a calibration factor f/sub 300/=(7.5/spl plusmn/0.5)/spl times/10/sup 15/ cm/sup -1/ is suggested for the room temperature measurement.
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