The shape of the spiral steps was used to experimentally estimate the surface supersaturation of GaAs microchannel epitaxy, whose atomically-flat surface is suitable for the step observation. In the Si-doped sample, the vertical growth rate was found to have a strong relation to the surface supersaturation. The vertical growth is greatly suppressed below a critical value of 0.17%, and it linearly increases with the supersaturation above the value. The mechanism is ascribed to the pinning effects of the dopants on the surface. Reference of the non-doped sample shows that the vertical growth rate is simply proportional to the surface supersaturation, and that the pinning effect disappears. (C) 2016 Elsevier B.V. All rights reserved.