Layers of GaN0.1As0.9 were grown by molecular beam epitaxy (MBE) on GaAs(001). By high-resolution scanning electron microscopy (SEM), 3-4 mu m surface defects were observed in 4-mu m-thick layers. Their origin is a combination of nitrogen reaction with the substrate and the lattice mismatch. These defects had a conical shape and their walls consisted of GaN nano-crystallites with solved arsenic as detected by Auger-electron spectroscopy (AES).