In this work we studied the impact of the growth mode on structural and optical properties of thick GaAsN layers grown on GaAs (100) substrates using plasma-source molecular beam epitaxy. The investigated GaAsN layers had similar average nitrogen content (N ∼ 1.4-1.6%), but were formed using different growth approaches. We found that both structural and optical properties of GaAsN layers crucially depend on the growth mode. We observed the formation of an ultrashort-period monolayer GaAsN-GaAs superlattice upon pulsed GaAsN deposition with laterally modulated nitrogen composition. We attribute this phenomenon to self-organization of nitrogen-rich and nitrogen-poor domains due to phase separation. We demonstrate the possibility of forming short-period GaAsN/GaAs superlattices using multi-cycle GaAsN deposition in order to fabricate high planarity lateral domain structures suitable for fundamental research and practical applications.