We demonstrate, for the first time, very strong blue emission at room temperature in GaNAs grown by molecular beam epitaxy (MBE). We have studied the MBE growth of GaN at approximately 800 degreesC in the presence of a wide range of arsenic fluxes from 10(-9) to 10(-5) mbar using both As, and As,. We have shown that the surface reconstructions in the absence and presence of arsenic are significantly different. Using atomic force microscopy, we have shown that arsenic acts as a surfactant, at low fluxes we obtain large flat islands showing atomic steps. In addition the surface roughness improves with increasing arsenic overpressure, Using Auger electron spectroscopy, we have shown that the surface concentration of As is independent of the As flux for a wide range of arsenic fluxes from 10(-9) to 10(-5) mbar and is practically the same for both As, and As,. Our photoluminescence studies have allowed us to propose a growth mechanism for As-doped GaN in which cubic GaN is formed from GaAs by N atoms replacing As. This study suggests that GaNAs may replace InGaN in MBE grown opto-electronic devices.