Bulk InAsN films and monolayer-InAsN/GaAs single quantum wells (SQWs) have been grown on GaAs(001) substrates at 500° C or below by RF-plasma-assisted molecular beam epitaxy (RF-MBE). Bulk InAsN films with fairly uniform compositions as revealed by X-ray diffraction have been successfully obtained up to the N concentration as high as 5.48%. The N incorporation is enhanced with decreasing growth temperature as expected for the metastable alloy. The Burstein-Moss effect is dominant for the optical properties near the band edge in the bulk InAsN films, giving a blue-shift of absorption edge due to the degenerate electrons in the conduction band. In the InAsN/GaAs SQWs, however, the BM effect is well suppressed to show the red-shift of photoluminescence (PL) peak energy due to the bandgap narrowing with the N incorporation, which is the manifestation of the huge bandgap bowing commonly found in the III-V-N-type alloys. This interpretation is justified when the difference in the shape of the density-of-states (DOS) function between the bulk and SQW is properly taken into account. © 2005 Elsevier B.V. All rights reserved.