The growth condition of GaAsN alloy films has been optimized for higher N contents, and the bandgap energy of the films and its temperature dependence have been investigated by photoreflectance spectroscopy. The GaAsN (N = 0-4.5%) alloy films were grown on GaAs (0 0 1) substrates by molecular beam epitaxy equipped with a radio frequency plasma nitrogen source. The As-4 flux was varied for the optimization of the growth. The films with a fairly flat surface and high crystal quality were obtained when the As-4 flux was close to the V/III ratio of 1. The bandgap energy of GaAsN with the N content of 4.5% was measured as low as 0.93 eV (1.33 mum). In the low (< 1%) N content region, the bowing parameter is estimated to be 22 eV. However, the deviation of the experimental data from the fitting curve becomes large as the N content increases due to the composition-dependent nature, of the bowing. The temperature dependence of the bandgap energy becomes significantly weak with increasing N-content. This suggests that the density of states around the band edge. of GaAsN is modified from that of GaAs due to the localized states possibly originated from N clusters. (C) 2002 Elsevier Science B.V. All rights reserved.