The structural, magnetic and electrical properties of a series of GdN thin films grown by molecular beam epitaxy on AlN on silicon substrates have been studied. In situ scanning tunneling microscopy and reflection high energy electron diffraction evidence the formation of three-dimensional GdN triangle-shaped islands in the early stage of the growth which coalesce as growth develops. It is then shown by X-ray diffraction that the thickness and the growth temperature play a key role in determining the structural properties of the GdN films and optimum growth conditions are found. A significant structural dependence of the magnetic properties is observed with results suggesting superparamagnetism in very thin films while thicker films show uniform ferromagnetism. (C) 2014 Elsevier B.V. All rights reserved.