Gallium nitride and related materials: Challenges in materials processing
Boules and large wafers of the III-Nitrides of AlN, GaN and InN having a low density of dislocations are not available. As such, essentially all nitride films and device structures are grown on either sapphire or silicon carbide substrates containing a previously deposited buffer layer of GaN, AlN or AlGaN. These films grow via complex thermodynamically- and kinetically-controlled mechanisms and contain significant residual stresses and densities of defects that affect the properties of all optoelectronic and microelectronic devices produced in this materials system. It is the purpose of this paper to describe the challenges presented by the growth of these heteroepitaxial films and some of the recent results of research to understand the complex relationships between film growth, and stress and defect generation as well as the reduction in these mechanical and microstructural problems. Films of GaN grow on AlN/SiC substrates via the formation of a 1-1.5 nm thick wetting layer and the subsequent growth and coalescence of islands. These films are biaxially stressed. Increasing their thickness causes a gradual change in their average strain and their local strain from compression to tension due to the mismatch in lattice parameters between GaN and AlN and the mismatch in the coefficients of thermal expansion between GaN and SiC. A portion of the compressive stress is relieved within the first 20 nm due to the formation of misfit dislocations. Copious threading dislocations are also generated. Additional microstructural problems are presented by the SiC substrates that contain domains with varying size and tilt that are mimicked in the GaN films and that mask most variations in the FWHM of their X-ray rockingcurves. Reduction in both the residual stresses and the dislocation density has been achieved via lateral overgrowth techniques. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.