In this paper we present an examination of (Al0.3Ga0.7)As layers grown by MOVPE at different temperatures over trenches etched into (100) GaAs in [0 1 1] direction. On the sidewalls of the trenches, the Al content is reduced compared to planar regions. High-resolution cathodoluminescence (CL) shows that the region with lower Al content is homogeneous along the sidewalls and that the Al content changes abruptly at the change of growth facets, To get information on the growth mechanism GaAs quantum wells are embedded within the (AlGa)As, Their CL emission wavelength is different for the sidewalls and planar regions indicating different thicknesses and thus growth rates. The GaAs growth rates are compared with the (AlGa)As growth rates and the change in Al content on the sidewalls, The results lead to a contradiction, Therefore, the growth of (AlGa)As over trenches cannot be treated as the sum of its binary components.