ABSTRACTCubic Al_yGa_1−yN/GaN heterostructures on GaAs(001) substrates were grown by radio-frequency plasma-assisted molecular beam epitaxy. High resolution X-ray diffraction, spectroscopic ellipsometry and cathodoluminescence were used to characterize the structural and optical properties of the alloy epilayers. X-ray diffraction reciprocal space maps demonstrate the good crystal quality of the cubic Al_yGa_1−yN films. Both SE as well as room temperature CL of the Al_yGa_1−yN epilayer show a linear increase of the band gap with increasing Al-content. A pseudomorphically strained cubic 10 × (2.4 nm GaN/ 4.8 nm Al_0.12Ga_0.88N) multi-quantum well (MQW) structure has been realized. Cathodoluminescence clearly demonstrates strong radiative recombination due to quantized states in the GaN well layer at a photon energy of 3.323 eV.