We have studied the role of aluminum in the formation of an etch barrier at the GaAs/ AlxGa1−xAs interface during reactive ion etching in CCl2F2plasma. The minimum AlxGa1−xAs thickness needed to form the barrier is Al mole fraction dependent and was determined with etching experiments monitored by optical emission spectroscopy. Effective AlxGa1−xAs layers for forming an etch barrier are 275 Å forx=0.02, 22 Å forx=0.10, 15 Å forx=0.15, 12 Å forx=0.20, and 9 Å forx=0.30. For all Al mole fractions exceptx=0.02, these thicknesses correspond to a sheet dose equivalent to 3/4 of a monolayer of Al in the original AlxGa1−xAs layer. Barrier layers forx=0.02, 0.10, 0.25, and 0.30 were examined without air exposure by angle‐dependent x‐ray photoelectron spectroscopy. For samples that are not overetched, the surface is covered with ∼20 Å of AlF3intermixed with a gallium halide containing chlorine and fluorine and is depleted of arsenic. For substantially overetched barriers, a 30 Å layer is formed with gallium halide present at the surface, AlF3found farther in, and arsenic depletion throughout the barrier. During extreme overetch, barrier layers on the order of tens of Å in thickness were not etched away and yet did not completely prevent very slow etching of underlying GaAs. Barrier layers on the order of 60 Å in thickness did prevent etching of underlying GaAs. Collectively the data suggest that the role of Al is formation of AlF3exclusively and that only this compound is responsible for stopping the GaAs etch.