We have quantitatively compared the evolution of threading dislocations (TDs) in GaN films grown on (1 1 1) Si substrates using different buffer/interlayer structures: a compositionally graded AlxGa1-xN (0 <= x <= 1) buffer layer, a thin. high-temperature (HT) AlN interlayer (IL), and an AlN/GaN/AlxGa1-xN multilayer. Plan-view transmission electron microscopy (TEM) shows a reduction in TD density in GaN films grown on graded AlxGa1-xN buffer layers, and an increase in TD density in GaN films grown on HT AlN ILs, in comparison with those grown directly on an AlN buffer layer. Cross-sectional TEM reveals bending and annihilation of TDs within the graded AlxGa1-xN buffer layer, which lead to a decrease of TD density in the overgrown GaN films. On the other hand, a high density of TDs forms at the GaN/AlN IL interface, resulting in an increase in TD density in the GaN film. In addition, growing a thin AlN + GaN bilayer before growing the compositionally graded AlxGa1-xN buffer layer significantly reduces the TD density in the AlxGa1-xN buffer layer, which subsequently further reduces the TD density in the overgrown GaN film. (c) 2006 Elsevier B.V. All rights reserved.