We have investigated the effect of using As-2 source instead of a more commonly used As-4 source on the crystalline quality of GaNxAs1-x thin films grown by atomic hydrogen-assisted molecular beam epitaxy. Improved structural and optical properties of GaNAs thin films were obtained by using As-2 source. The nitrogen atoms were incorporated into GaAs at a more stable rate under As-2 flux than As-4 flux, and a two-dimensional nucleation growth mode was promoted for growth of GaNxAs1-x with As-2 source. As a consequence, the surface roughness measured for a 500 nm-thick Ga0.008N0.992As sample grown with As-2 flux was 1-2 monolayers, which was three times more smoother than that for As-4 sample. The photoluminescense measurements showed an improved potential fluctuation of 78.1 meV and twice the intensity at room temperature for As-2 sample. (c) 2008 Elsevier B.V. All rights reserved.