GaN growth on high aspect ratio Si micro-pillar arrays prepared by deep reactive ion etching (DRIE) was investigated using Si(111), Si(110), Si(112), and Si(100) substrates. The sidewall roughness of the Si pillars resulting from DRIE was reduced by thermal annealing at 1100 degrees C in H-2 for 5 min. Conformal AlN buffer layers were grown at 1100 degrees C on the pillars. In contrast, GaN films grown at 1100 degrees C were always faceted since GaN preferentially nucleated and grew on the Si{1 (1) over bar0} and Su{1 (1) over bar1} sidewall planes of the Si pillars. The preferential growth lead to unique symmetries for the independent GaN fins depending on pillar orientation. Lateral growth of the GaN fins at 1100 degrees C was enhanced with increasing Will ratio up to 22710. However, the GaN fins did not coalesce seamlessly. A lower growth temperature of 800 degrees C gave rise to a conformal polycrystalline GaN shell. Post-annealing of the GaN shells at 1100 degrees C in H-2 for 15 min after growth at 800 degrees C induced highly textured GaN films although the GaN films became partly faceted. (C) 2012 Elsevier B.V. All rights reserved.