The coupled modes of plasmons and LO phonons appearing in the Raman spectra of highly doped GaAs layers have been investigated in order to study the local incorporation character of Si on different crystal facets. The Si-doped GaAs layers were grown by molecular beam epitaxy on patterned GaAs (100) substrates with etched ridges forming (111)A and (111)B facets with a lateral extension of a few micrometers. The local type and concentration of free charge carriers have been determined from an analysis of the coupled-mode Raman spectra. It is shown that Si acts as a donor in the material grown on the (111)B facets and as an acceptor in the material grown on the (111)A facets. ©1997 American Institute of Physics.