In this paper, a study of germanium as n-type dopant in plasma-assisted molecular beam epitaxy of GaN is presented. The germanium incorporation is studied as a function of the germanium effusion cell temperature and growth temperature of the GaN layer. The influence of the doping concentration on the electrical, structural and morphological properties of the GaN layer will be studied using Hall and high-resolution X-ray measurements (rocking curve and theta - 2theta) measurements. Optical examination of the surface morphology was performed with differential interference contrast microscopy, a scanning electron microscope or transmission electron microscope. Doping of GaN with germanium results in crack-free n-type material up to values of n = 4 x 10(20) cm(-3) with a 1:1 relation between carrier concentration and vapor pressure. Even higher carrier concentrations can be obtained, n = 3.6 x 10(21) cm(-3), but there are indications that the solubility limit of germanium in GaN of these layers is exceeded, thereby deteriorating the morphology of the surface of the layer and changing the electrical and structural properties. TEM measurements reveal that secondary phases are indeed formed. (C) 2004 Elsevier B.V. All rights reserved.