Cubic AlyGa1-yN/GaN heterostructures on GaAS(0 0 1) substrates were grown by radio-frequency plasma-assisted molecular beam epitaxy. High resolution X-ray diffraction, micro-Raman, spectroscopic ellipsometry, and cathodoluminescence measurements were used to characterize the structural, optical and vibrational properties of the AlyGa1-yN epilayers. The AlN mole fraction y of the alloy was varied between 0.07 <y<0.20. X-ray diffraction reciprocal space maps demonstrate the good crystal quality of the cubic AlyGa1-yN films. The measured Raman shift of the phonon modes of the AlyGa1-yN alloy was in excellent agreement with theoretical calculations. Both SE and CL of the AlyGa1-yN epilayer showed a linear increase of the band gap with increasing Al-content. <(c)> 2001 Elsevier Science B.V. All rights reserved.