Relaxation of thin SiGe layers (similar to 90 nm) grown by molecular beam epitaxy using a low temperature growth step (120-200 degreesC) has been investigated using two-dimensional reciprocal space mapping of X-ray diffraction. The samples studied have bt en divided in two groups, depending on the substrate cooling process during the growth of the low temperature layer. II has been found that a higher degree of relaxation was easily achieved for the sample group without growth interruption. A process window for full relaxation of the Si0.74Ge0.26 layer has been observed in the range of 140-150 degreesC.