A patterned sapphire substrate with shallow U-shaped stripe grooves along the < 11 (2) over bar0 > direction prepared by chemical wet etching was used for the growth of high-quality gallium-nitride-based epilayers without air gap formation. From characterization by scanning electron microscopy, transmission electron microscopy, and micro-photoluminescence, it was determined that the crystalline and optical qualities of gallium-nitride-based epilayers grown on the groove region are better than those on the ridge region. In addition, the qualities of epilayers near the edge area are better than those near the center area on the groove region examined by cathodoluminescence. The growth evolution deduced from the orientations of threading dislocations on the ridge and groove regions was proposed. (C) 2008 The Electrochemical Society.