Stress evolution during MOCVD growth of AlN buffer layers and GaN epilayers on (111) Si substrates was investigated using in situ wafer curvature measurements in order to understand the impact of growth conditions on thin film stress. AIN layers up to 400 nm thick were deposited over a temperature range of 600-1100degreesC at growth rates of 0.1-1 nm/s. In all cases, the growth stresses were tensile and the stress value did not change during growth. A sharp drop in the value of the tensile growth stress is observed between 900degreesC and 800degreesC from > 1 GPa to <0.4GPa. This drop is due a transition in the structure of the film from an epitaxially oriented crystalline film above 900degreesC to a highly defective polycrystalline film at 600degreesC. At a constant temperature however, the tensile stress in the film remained relatively constant with a change in growth rate. The origin of these stress differences and their impact on residual stress in GaN epitaxial layers grown on Si are discussed. (C) 2003 Elsevier B.V. All rights reserved.