The accuracy of near-infrared absorption coefficients for EL2 concentration determination by transmittance measurement in thin GaAs wafers is investigated. Using wafers having the same absorption coefficients but varying in thickness, we found that the error in absorption coefficient is larger than 30% in 0.4 mm-thick wafers when reported reflectance derived from refractive index is used in the calculation. We therefore propose to use an appropriate reflectance which is determined by measuring the transmittance of different thickness wafers. Adopting the appropriate reflectance, the error is reduced to less than 2%.
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