The effect of growth parameters on impurity incorporation, surface morphology, crystal quality, and the strain state has been studied for ammonia molecular beam epitaxy (MBE) growth of GaN on SiC substrates. Study of the growth mechanisms has identified specific growth regimes ideal for growth of high quality and low strain, crack-free GaN/AlGaN HFETs on SiC wafers. A 2-D growth regime was found at moderate temperatures around 780 degrees C and high ammonia flow of 200 sccm, which yields smooth interface and low oxygen impurity level, but quite high tensile strain. Combined with a strain relief GaN/AlN (0.2 mu m/0.2 mu m) buffer structure, this approach produced excellent HFETs with mobilities of 1700-1800 cm(2)/Vs and near zero or compressive strain. A 3-D growth regime was found at high temperatures around 880 degrees C and moderate ammonia flow of 100 sccm. Although 3-D growth results in rougher surface, this approach yields low tensile strain( < 0.1%) and does not require multilayered strain relief structure. Electron mobility exceeding 1600 cm(2)/Vs has been achieved using the 3-D growth approach. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.