A semiconductor device having a planar III-N semiconductor layer includes a substrate including a wafer and a buffer layer of a buffer material different from a material of the wafer, the buffer layer having a growth surface, an array of nanostructures epitaxially grown from the growth surface, a continuous planar layer formed by coalescence of upper parts of the nanostructures at an elevated temperature T, where the number of lattice cells spanning a center distance between adjacent nanostructures are different at the growth surface and at the coalesced planar layer, and a growth layer epitaxially grown on the planar layer.