We investigated the optical and crystal quality of the AlInGaN quaternary epilayer, grown by metalorganic chemical vapor deposition (MOCVD), used to suppress the thermal damage of InGaN multiple quantum wells (MQWs). AlxIn0.02Ga0.98-xN showed a strong band edge emission in photoluminescence (PL) and high-resolution X-ray diffraction (HR-XRD) measurements confirmed the high crystal quality. Considering the lattice mismatch and thermal stability, an AlInGaN quaternary layer was introduced to protect the InGaN active layer. The surface morphology of InGaN QWS with the InGaN cap layer significantly changed from spiral surface to damaged-step like structure, whereas that of InGaN QWs with AlInGaN cap layer was still retained a spiral surface structure. Because of higher thermal stability, the optical quality of InGaN MQWs was significantly enhanced by introducing an AlInGaN quaternary protective layer during the high-temperature ramp-up process. (C) 2008 Elsevier B.V. All rights reserved.