Stress control in GaN/sapphire templates for the fabrication of crack-free thick layers

The reduction of the tensile stress contained in GaN layers grown on sapphire by metalorganic vapor phase epitaxy (MOVPE) is achieved using a low density of initial GaN crystallites. The template layers exhibit a significant reduction in dislocation density, down to 5×107 cm−2 in ∼ 6 μm thick MOVPE GaN templates. The grain size and their density are controlled during the low temperature nucleation step of GaN on sapphire. Subsequently, very thick (up to ∼200 μm) crack-free GaN layers were successfully grown by hydride vapor phase epitaxy (HVPE). A direct correlation between the stress state of the GaN/sapphire template prepared by MOVPE and the critical thickness for crack appearance during HVPE growth thickening is presented.