Crystalline phase purity and twinning of Mg-doped zincblende GaN thin films

•Mg-doped zincblende GaN thin film samples were grown on GaAs substrates by plasma-assisted MBE.•Ga-rich regime conditions at the surface on the growth front yields nearly 100 % zincblende phase purity.•Higher Mg doping cell temperatures and N-rich conditions prompt wurtzite formation and twinning on the 〈111〉 faces.