The AlInGaN alloy has emerged as the material for potential use in UV light emitting diodes and lasers due to its higher emission quantum efficiency. However, surface defects like hexagonal pits appear on the surface of this quaternary alloy and formation of these pits can degrade the device performance. In this study, we have observed inverted hexagonal pits formation in AlInGaN epilayers grown on sapphire. The pits depth and diameter are found to increase with the epilayer thickness and pits agglomeration occurs when epilayer thickness is higher than 40 nm. Cross-sectional atomic force microscopy images of the inverted hexagonal pits show that these pits are truncated. This is probably due to the filling of pits associated with composition pulling effect in AlInGaN epilayers. An enhanced yellow luminescence emission is also detected from these inverted hexagonal pits. This is probably due to impurity decoration along the threading dislocations, which are terminated at the vertex of the pits. These impurities form deep levels and serve as effective transition levels for yellow luminescence emission. (C) 2004 Elsevier B.V. All rights reserved.