The　electrical　and　structural　properties　of　Mg　delta-doped　GaN　epilayers　grown　by　MOCVD　were　investigated.　Compared　to　uniform　Mg-doping　GaN　layers,　it　has　been　shown　that　the　delta-doping　(delta-doping)　process　could　suppress　the　dislocation　density　and　enhance　the　p-type　performance.　The　influence　of　pre-purge　step　on　the　structural　properties　of　GaN　was　also　investigated.　The　hole　concentration　of　p-GaN　decreases　when　using　a　pre-purge　step.　These　results　can　be　explained　convincingly　using　a　simple　model　of　impurity　incorporation　under　Ga-free　growth　condition.　(C)　2007　Elsevier　B.V.　All　rights　reserved.