GaAs evaporation during molecular beam epitaxy (MBE) is measured using reflection high-energy electron diffraction (RHEED). On the (001) surface there is a first-order phase transition from a 2 × 4 to 1 × 1 reconstruction. Upon crossing the phase boundary into the 1 × 1 structure, layer-by-layer evaporation of GaAs is observed. This evaporation affects the rate of growth of GaAs and AlxGa1-xAs by MBE. The dependence of the growth rate on substrate temperature and on As, Al, and Ga fluxes is followed by measuring RHEED intensity oscillations. The results agree quantitatively with the mass-action analysis of Heckingbottom [R. Heckingbottom, J. Vac. Sci. Technol. B 3, 572 (1985)]. At substrate temperatures above 850 K no differences between As2 and As4 incident fluxes are observed.
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