This paper demonstrates the use of Sidewall Lateral Epitaxial Overgrowth (S-LEO) on a-plane GaN thick films on r-plane sapphire by hydride vapor phase epitaxy (HVPE). Comprehensive study of the extended defect microstructure of the a-plane GaN films was carried out using cross-sectional and plan-view transmission electron microscopy (TEM). A very low density of primary threading dislocations and a heterogeneous microstructure can be found in the GaN films. In the window region and N-face wing region, the extended defects included type I(1) and type I(2) basal stacking faults (BSFs), as well as prismatic stacking faults (PSFs) on a-planes. The density of type I(1) BSFs was in the order of similar to 2 x 10(5) cm(-1), type I(2) BSFs in the order of similar to 10(4) cm(-1), and corresponding localized partial dislocation density less than 1.5 x 10(9) cm(-2). PSFs on a-planes were connected to two neighboring type I(1) BSFs with an estimated density of 3 x 10(2) cm(-1) in the plan-view images. In the Ga-face overgrowth regions, the density of BSFs was lower than 10(4) cm(-1). However, inversion domains bounded by (1 (1) over bar 02), (1 (1) over bar0 (2) over bar), and (1 (1) over bar 00) planes were found in the Ga-face wing regions. The nature of inversion domain boundaries (IDB) on m-planes can be explained by the Auserman-Gehamn model using high-angle annular dark-field TEM images. Published by Elsevier B.V.