The surface morphology of Cl-2-etched GaAs(001) is measured as a function of etch time by atomic force microscopy and elastic light scattering. A flat surface is found to become rougher during the etch whereas a textured substrate becomes smoother. We have numerically simulated this behavior. It is found that the evolution of surface roughness at length scales between 50 nm and 5 mum can be described with excellent accuracy by a continuum equation for the surface height h((x) over right arrow ,t), which is given by dh/dt=nudel(2)h-lambda/2(delh)(2)-Kdel(4)h+eta, where eta is a random noise input.
