Novel buffer layer for the growth of GaN on c-sapphire

We propose a rocksalt (RS) structured chromium nitride (CrN) layer as a novel buffer for the growth of gallium nitride (GaN) films on c-sapphire substrate. A RS-CrN buffer has good advantages for growing GaN films on c-sapphire from the viewpoints of lattice mismatch and thermal expansion coefficient. The RS-CrN layer has mid value of both the lattice constant and the thermal expansion coefficient of GaN and c-sapphire. It is verified that the lattice mismatch between CrN and c-sapphire of 6.2–6.7%, which is experimentally estimated from reflection high-energy electron diffraction (RHEED) characterization, about 6.2–6.7%, agree well with the calculated value of 6.6%. By these advantages, N-polarity GaN epilayers grown on RS-CrN buffer layer show remarkably improved crystal quality when compared with those of grown on conventional low temperature (LT) GaN buffer using molecular beam epitaxy (MBE). For a further improvement of GaN crystallinity, we performed the surface treatment of Ga-exposure on CrN buffer layer. The Ga-exposure treatment on the RS-CrN buffer layer drastically improved the crystal quality of upper GaN epilayer, which was identified by measuring structural and optical properties using X-ray diffraction (XRD) and Photoluminescence (PL), respectively. The improved crystallinity is resulted not from the polarity conversion but from the stress relaxation effect by a metallic Ga interlayer. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)