The optical pumping spectra of InGaN/GaN single quantum well structures with high indium content after thermal annealing were analyzed at room temperature. Redshift of the peak position in the optical pumping spectra was observed after the samples were annealed for 0.5 h at 700 degreesC. However, blueshift of the peak position was shown after the samples were annealed for 1 and for 2 h. Three mechanisms of band structure deformation due to the thermal treatments have been proposed. The redshift of the peaks might be due to the reduction of the inhomogeneity of the indium content in quantum well. The blueshift of the peaks might originate from the strain relaxation resulting in the reduction of the piezoelectric field or/and the interfacial interdiffusion of In and Ga atoms between the well and barrier. (C) 2002 Elsevier Science Ltd. All rights reserved.