Quaternary AlInGaN multiple quantum well structures with (Al)InGaN wells were grown on sapphire substrates by metalorganic chemical vapor deposition. The structural and optical properties of the samples were investigated by means of high-resolution X-ray diffraction (HRXRD) and photoluminescence (PL). The shift of satellite peaks in HRXRD shows the fact that the Al composition increases and In composition decreases with increasing Al precursor flow rate during the growth process of quaternary AlInGaN wells or barriers. The PL intensity could be enhanced by optimizing Al composition in the barrier layers and further improved by adding Al atoms into InGaN wells due to the reduction in the quantum-confined Stark effect, even if the surface morphology is degraded when using AlInGaN as well layers observed from atomic force microscopy images. The reasons for the improvement are attributed to the reduced polarization mismatch and the enhanced carrier localization effect induced by Al composition increasing. (C) 2014 Elsevier B.V. All rights reserved.