The effective surface diffusion coefficient of Ga along the  direction on vicinal GaAs(001)2×4 surfaces during molecular-beam epitaxy was measured using specular ion current measurements. In this technique, 3 keV Ar ions were impinged upon the surface at a glancing angle (typically 3°), and the specularly scattered ion current was measured. Since specular reflections require a locally flat surface, adatoms cause a decrease in the measured current, allowing an average adatom density measurement. The time dependence of the Ga adatom population was measured during and after Ga deposition. Diffusion coefficients, obtained from the adatom lifetimes using a simple model of diffusion to the step edges, were fit well by the expression D = 2×10−9 exp(−0.73 eV/kT)cm2/s from 400 to 600 °C. © 1999 American Institute of Physics.