Selective Area Flow Modulation Epitaxial growth of GaN is carried out in a low pressure Organometallic Vapor Phase Epitaxy reactor. This process is known to enhance reactant surface migration lengths on patterned group III-arsenide and phosphide growth surfaces. With this process, high quality laterally overgrown GaN epitaxial materials result. Under the ammonia rich growth conditions used, enhanced migration (by flux modulation) across masked regions of the substrate has not been observed. The mask materials were silicon dioxide and silicon nitride, both deposited on GaN/AlGaN buffer structures on sapphire and SiC substrates. Window stripes were patterned parallel and perpendicular to the (1100) crystal directions to observe the orientation dependence of the lateral growth rate. Structures exhibited heights above the mask surface as large as 30 microns and atomically smooth surfaces. With a periodic array of stripe window openings in the mask, planarized laterally overgrown surfaces are achieved after roughly 4 microns of overgrowth. Chemical assisted ion beam etching with chlorine gas was used to delineate defects in the selectively grown layers. Additional evidence on the defect reduction is given by Atomic Force and Scanning Transmission Electron Microscopies.