The cause of carrier removal in GaAs due to plasma processing is found
to consist of two mechanisms from a photoreflectance depth-profiling
study. In the close vicinity of the surface, the Fermi level is pinned
by the defects created by ion bombardment. In a much deeper region, e
lectrical passivation of doped impurities themselves is playing the do
minant role. Secondary-ion mass analysis confirmed that hydrogen diffu
sion is indeed responsible for the latter process. We suggest that the
physically damaged surface layer acts as a sponge for even a trace am
ount of hydrogen in the processing environment.