The structural properties of a-plane AlGaN/GaN multiple quantum wells grown on the r-plane sapphire substrate have been characterized. The pentagonal and inverted octagonal surface pits, consisting of several non-polar and semi-polar crystalline facets, are clearly observed and distinguished. The Al incorporation efficiency of the non-polar and semi-polar facets of these special inverted octagonal surface pits has been verified in the order of (11 (2) under bar2) < (10<(12)under bar>) < (1<(1)under bar>00) approximate to (11 (2) under bar0) < (20<(2)under bar>1) by cathodoluminescence measurements at room temperature. The evolution of these inverted octagonal surface pits could be due to the results of interaction between different stacking faults. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3610990] All rights reserved.