The heteroepitaxy of GaN directly grown directly on Si-face 4H-SiC(0 0 0 1) by molecular beam epitaxy is explored and characterized using in situ spectroscopic ellipsometry. Critical steps of the process, including SiC substrate cleaning, substrate termination through nitridation, GaN nucleation and growth, are monitored in real time. Some key relationships between growth mode, as observed by ellipsometry, and material properties are given. (c) 2005 Elsevier B.V. All rights reserved.