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.