Near-infrared optical absorption in undoped bulk GaAs ingots is shown to be due essentially to the presence of the main deep donor EL2. Measurement of the corresponding absorption represents the first known method of quantitative determination of that level in semi-insulating material. Furthermore, complete quenching of the corresponding absorption is shown to occur under high intensity illumination. This strong effect, reported for the first time, can be directly related to the existence of a metastable state for the level EL2 presenting a strong lattice relaxation.
Semi-insulating GaAs materials, undoped or doped with concentration of chromium varying from 6×1015 to 4×1017 cm−3, have been studied using both Hall effect measurements and optical absorption measurements. It is definitively concluded that compensation comes from the presence of the deep donor EL2 in undoped materials, and from both this deep donor and the deep acceptor related to chromium in Cr-doped materials. Sets of curves are given which allow the determination of ND-NA, the concentration of shallow donors and acceptors, knowing the Hall mobility and the Cr concentration in a given sample. Such curves can be a working tool for assessing any piece of semi-insulating GaAs in a routine way.
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