X-ray diffraction is used to characterize residual stresses in GaN/AlN/6H-SiC(0 0 0 1) structures grown by molecular beam epitaxy. The measurements in the temperature range of 25-600degreesC document a change of stresses in GaN and AlN layers, as interpreted by the specific mismatch of thermal expansion coefficients. Primarily, the influence of AlN buffer thickness on the stress state of GaN is evaluated. With the increase of AIN buffer thickness from 20 to 200 nm the room-temperature stress in GaN thin films changes from 0.015 to -0.833GPa. Likewise, the effect of stress on the energy band gap is further shown by photoluminescence measurements, indicating a shift of the near band gap emission in the GaN layers with varying stress. The results thus suggest a possibility for stress engineering in GaN thin films on 6H-SiC(0 0 0 1) substrates. (C) 2002 Elsevier Science B.V. All rights reserved.