GaN layers on bulk m-plane, , and GaN substrates were grown by metal organic vapor phase epitaxy. XRD rocking curves have a FWHM of less than 150″, indicating excellent crystalline quality. However in many cases surface morphology exhibits hillocks with a height of 1–2 µm and a lateral extension of 50–200 µm whereas a smooth surface would be desirable for optoelectronic devices. The influence of growth parameters on the surface morphology was studied. The goal was, to constrain the material redistribution, that is necessary to form large hillocks. This was achieved by lowering the adatom diffusion length by a reduction of temperature and an increased reactor pressure. In the case of the and semipolar planes a reduction of the adatom diffusion length leads to a reduction of hillock density, hillock size and a smoother surface between hillocks. However, the m-plane surface does not react to a reduction of adatom mobility. Even at 890°C and 400 mbar rectangular pyramids cover the surface. In contrast to the other planes, the becomes instable, when the adatom diffusion length is reduced.