A method of making a semiconductor device, comprising: forming a plurality of semiconductor seeds of a first III-nitride material through a mask provided over a substrate; growing a second III-nitride semiconductor material; planarizing the grown second semiconductor material to form a plurality of discrete base elements having a substantially planar upper surface. Preferably the step of planarizing involves performing atomic distribution of III type atoms of the grown second semiconductor material under heating to form the planar upper surface, and without supply of III type atoms is carried out during the step of planarization.