High-resolution x-ray diffractometry was employed to investigate crystallinity and interface properties of InGaN/GaN multiple quantum wells (MQWs) heterostructures grown by metalorganic vapor phase epitaxy on (0001) sapphire substrates. To obtain a complimentary picture, coherent-grown and relaxed InGaN/GaN MQW structures were investigated. In addition, thermal annealing at 1000degreesC was subjected to samples to estimate their thermal stability. A structural variation by thermal process was observed only for the coherent-grown InGaN/GaN MQWs due to an interdiffusion of indium and gallium atoms at InGaN/GaN interfaces. Photoluminescence (PL) experiment at room temperature revealed that a postgrowth annealing also affected the optical properties. For relaxed InGaN/GaN MQWs, a reduction of PL intensity was confirmed with a factor of 1/2 while a slight decrease of PL intensity was shown by coherent-grown InGaN/GaN MQWs. The effects of thermal annealing on the structural and optical properties were depended on the strain conditions that the thermal annealing was made the interface profile unclear for coherent-grown InGaN/GaN MQWs, and degraded the emission efficiency for relaxed InGaN/GaN MQWs. (C) 2003 American Vacuum. Society.