Enhanced stress relaxation of GaN epilayers on silicon (1 1 1) substrates with a unique AlN/GaN multilayer design

Hetero-epitaxial growth of gallium nitride (GaN) on silicon (Si) substrates remains a challenging task due to the significant mismatch of the thermal expansion coefficient between both materials. Such hetero-epitaxial growth typically results in high tensile stress for the GaN layer and hence micro-cracks upon cooling down. The present study attempts to mitigate this problem by introducing a unique aluminum nitride/gallium nitride multilayer (AlN/GaN ML) design as a new alternative method to promote stress relaxation in the overgrown GaN, hence reducing the cracks formation. Unlike conventional designs, the proposed AlN/GaN ML consists of AlN layers which are thicker than the GaN layers, giving a thickness ratio of 57%. Additionally, the impact of AlN/GaN pairs, specifically at 20, 40, 60, and 80 pairs on stress relaxation, dislocation density, and surface quality, was investigated. The results indicate that the AlN/GaN ML is effective in reducing the tensile thermal stress in the overgrown GaN layer, especially with 60 pairs. Accordingly, a low dislocation density with crack-free and smoother surface can be achieved. Increase in the AlN/GaN ML pairs to 80 resulted in micro-cracks on the surface. The GaN layer on Si substrate with the optimized AlN/GaN ML can serve as a beneficial template for realizing cracks-free III-nitrides-based semiconductor devices.