Alternative Route of Fracturing in GaN Films Formed by Nanowires Coalescence on Si Substrate

Self-assembled GaN nanowires (NWs) were used as buffer structures to grow GaN layers on Si(111). The coalescence of GaN NWs into a planar layer was studied by measuring the crystalline quality and mass density. While the mismatch strain was found eliminated by the GaN NWs, a pattern of cracks with the average spacing of 70 mu m developed in the coalesced 1.8 mu m thick GaN layer. The finite-element simulations indicated that the fracturing occurred during the postgrowth cooling of the sample due to the 55% mismatch of the thermal expansion coefficients of GaN and Si. Moreover, tensile stresses in the coalesced GaN layer originate from a pulling effect from the NWs, which bend during the cooling under the difference in displacement between the GaN layer and Si substrate. These results show a fundamentally distinct fracturing mechanism in planar GaN films on Si(111) intermediated by bending of NWs.

Automated summary

Self-assembled GaN nanowires were used as buffer structures for growing GaN layers on Si(111). The study found that while GaN nanowires eliminated mismatch strain, cracks appeared in the coalesced GaN layer. Finite-element simulations indicated that these cracks occurred during postgrowth cooling due to the mismatch of thermal expansion coefficients between GaN and Si. Moreover, the coalesced GaN layer experienced tensile stresses caused by the bending of the nanowires during cooling.