To determine accurately the absorption coefficient of GaAs n-doped crystals, we developed an apparatus which permits simultaneous measurement of the coefficients of absorption K and reflection R12 of samples. We have determined with high precision that the absorption edge is exponential, and thus brought to light that not only the concentration of the n impurities, but also their nature changes the slope of the curve log K plotted as a function of the energy. We attribute this to stresses produced in the lattice by impurity atoms. Finally, we note that the reflection coefficients vary greatly from one sample to another.
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