In Situ Stress Measurements During GaN Growth on Ion-Implanted AlN/Si Substrates

In situ wafer curvature measurements were used in combination with postgrowth structural characterization to study the evolution of film stress and microstructure in GaN layers grown by metalorganic chemical vapor deposition on N+ ion-implanted AlN/Si (111) substrates. The results were compared with growth on identical unimplanted substrates. In situ stress measurements revealed that, for the unimplanted sample, the GaN initiated growth under compressive stress of −1.41 GPa which arose due to lattice mismatch with the AlN buffer layer. In contrast, GaN growth on the ion-implanted sample began at lower compressive stress of −0.84 GPa, suggesting a reduction in epitaxial stress. In both cases, the compressive growth stress was fully relaxed after ~0.7 μm and minimal tensile stress was generated during growth. During post-growth cooling, tensile stress was introduced in the GaN layer of both samples due to thermal expansion mismatch. Post-growth optical microscopy characterization, however, demonstrated that the ion-implanted sample had lower density of channeling cracks compared with the unimplanted sample. Cross-sectional transmission electron microscopy images of the sample grown on ion-implanted Si with no post-implantation nitrogen annealing revealed the formation of horizontal cracks in the implanted region beneath the AlN buffer layer. The weakened layer acts to decouple the GaN film from the Si substrate and thereby reduces the density of channeling cracks in the film after growth.