The influence of AlN, Si3N4 and SiO2 surface-passivation dielectrics on high-temperature strain relaxation of AlGaN in AlGaN/GaN heterostructures was investigated in the range from room temperature to 600 degrees C by means of x-ray diffraction. The Si3N4 and SiO2 films produce an additional biaxial tensile stress to the underlying AlGaN barrier layer, whereas the AlN film produces an additional biaxial compressive stress. The authors found that the process of long-time heating and cooling results in a nonreversible lattice relaxation in the AlGaN layer. The passivation dielectrics used in this work can mitigate this high-temperature strain relaxation. The AlN dielectric manifested a better effect than the Si3N4 and SiO2 dielectrics. (c) 2007 American Vacuum Society.