Cr-doped semi-insulating GaN templates were grown using radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE). The samples were characterized by high resolution x-ray diffraction (HRXRD), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The Cr incorporation was strongly dependent on the growth condition, which was primarily influenced by Cr cell temperature. The activation energy of dark conductivity was about 0.48 eV which corresponds to the depth of the dominant electron traps pinning the Fermi level. The experimental results indicated that Cr doping did not affect the crystalline orientation of the GaN film but introduced more threading dislocations, and step-graded GaN/AlxGa1-xN superlattices (SLs) played an important role in hindering the penetration of the threading dislocations. (C) 2012 Elsevier B.V. All rights reserved.