Atom probe tomography studies on highly Mg-doped homoepitaxial GaN (0001) layers with concentrations of 5 × 10(19) cm(-3) and 1 × 10(20) cm(-3) were performed. Mg cluster formation was observed only in the higher doped sample whereas in the lower doped sample the Mg distribution was homogeneous. CL measurements have shown that the emission normally attributed to stacking faults was only present in the lower doped layers (with Mg concentration of ∼5 × 10(19) cm(-3) or less), but absent in the higher doped layer, where Mg clusters were detected. Mg clusters are proposed to produce a screening effect, thereby destroying the exciton binding on the SFs and thus rendering them optically inactive.