  The dewetting of thin Pt films on different surfaces is investigated as a
means to provide the patterning for the top-down fabrication of GaN nanowire
ensembles. The transformation from a thin film to an ensemble of nanoislands
upon annealing proceeds in good agreement with the void growth model. With
increasing annealing duration, the size and shape uniformity of the nanoislands
improves. This improvement speeds up for higher annealing temperature. After an
optimum annealing duration, the size uniformity deteriorates due to the
coalescence of neighboring islands. By changing the Pt film thickness, the
nanoisland diameter and density can be quantitatively controlled in a way
predicted by a simple thermodynamic model. We demonstrate the uniformity of the
nanoisland ensembles for an area larger than 1 cm$^2$. GaN nanowires are
fabricated by a sequence of dry and wet etching steps, and these nanowires
inherit the diameters and density of the Pt nanoisland ensemble used as a mask.
Our study achieves advancements in size uniformity and range of obtainable
diameters compared to previous works. This simple, economical, and scalable
approach to the top-down fabrication of nanowires is useful for applications
requiring large and uniform nanowire ensembles with controllable dimensions.
