A detailed analysis is presented on relaxation mechanism of low temperature Si1-xGex layers deposited on Si(0 0 1) by low pressure chemical vapor deposition. While Si1-xGex layers with x = 0.02-0.20 deposited above 680 degreesC relax by formation of 60degrees dislocations characterized by fast propagation, at lower growth temperature the relaxation mechanism changes. The critical thickness is reduced markedly, relaxation is fast being governed by dislocations with slow propagation. This indicates the presence of a high density of point defects in the low temperature layers. We used these features to realize relaxed buffer layers with a low density of threading dislocations. (C) 2004 Elsevier Ltd. All rights reserved.