High quality gallium nitride thin films have been grown on 6H-silicon carbide (0001) substrates at varying substrate temperatures using molecular beam epitaxy and characterized at low and high film thicknesses. The epitaxial layers show two regimes in temperature distinguishable by different morphology. For film thicknesses around the critical thickness, low temperature-growth is two dimensional while for higher temperatures, growth is in the form of three-dimensional columnar islands. At a thickness of about 200 nm, the low temperature films show a large density of spiral growths while high temperature films show a two-dimensional morphology. X-ray peak widths are seen to decrease with increasing substrate temperature. These results have been explained in terms of a model which proposes different misfit dislocation formation mechanisms in the two temperature regimes.