The high-temperature characteristics of strain in Al0.22Ga0.78N layers, with and without a Si3N4 passivation layer, were investigated at temperatures from room temperature to 813 K by means of high-resolution X-ray diffraction. The results show that a small temperature-dependent strain relaxation occurs in the unpassivated Al0.22Ga0.78N layers when the temperature exceeds 523 K. After passivating, an additional tensile strain and an initial increase of the in-plane tensile strain with increasing temperature were observed in Al0.22Ga0.78N layers, and at higher temperatures the in-plane tensile strain only decreases slightly in the 100-nm-thick Al0.22Ga0.78N layer, but a pronounced temperature-dependent strain relaxation occurs in the 50-nm-thick one due to the fact that the thickness of the Al0.22Ga0.78N layer is close to the critical thickness, and hence the increase of tensile strain induced by passivation Will result in partial strain relaxation via the formation of cracks or the glide motion and multiplication of dislocations. (c) 2006 Elsevier B.V. All rights reserved.