AlGaN and Si-doped GaN layers were implemented as marker layers during GaN epitaxial growth in order to get a deeper scientific understanding of the growth process on structured substrates. Both types of marker layers show a good visibility without any parasitic influences on the crystal structure or growth behavior. These marker layers were used to study the development of the different crystal facets as growth proceeds. This technique provided access to a precise control of a multistep growth sequence, enabling a thorough optimization of our facet assisted epitaxial lateral overgrowth (FACELO) process by inserting additional properly designed growth steps. (C) 2004 Elsevier B.V. All rights reserved.