In situ observation of coalescence-related tensile stresses during metalorganic chemical vapor deposition of GaN on sapphire

Surface roughness and stress evolution were monitored in situ during the growth of GaN on sapphire substrates using low-temperature AlN buffer layers of varying thickness. A reduction in buffer layer thickness decreases the concentration of GaN nucleation sites which in turn increases the time to nuclei coalescence, thus varying the temporal evolution of surface roughness. By monitoring the accompanying changes in stress evolution, it is shown that island coalescence consisting of initial contact followed by subsequent surface roughness reduction is a source of tensile stress during growth of GaN films on sapphire. Such delayed coalescence also leads to an improvement in the structural properties of the material.