In this paper we show a detailed AFM observation of GaAs epilayers grown on GaAs (001) substrate patterned with SiO2 mask by low angle incidence microchannel epitaxy (LAIMCE). We have found that low index facets are formed as sidewalls of the epilayers grown in open window aligned along singular directions. We have also found that low index facets are formed as sidewalls for epilayers showing a zigzagging edge. In the case of the epilayers grown in an open window aligned 10 degrees off [010], for which lateral growth is maximum, we have found round shape sidewalls and (001) terraces at the boundary of the sidewall and the top surface. We concluded that the lateral growth proceeds when the sidewall is rough, while it stops when low index facets are formed as sidewalls.