The symmetry of the twin boundaries of ZnO epitaxial film was detected with reflective second harmonic generation. The twin boundaries exhibited mirror

symmetry with a polar configuration across the boundary plane and yielded a non-vanishing polar contribution to reflective second harmonic generation. The non-vanishing second-order susceptibility supported the idea that the measured reflective second harmonic generation originated from the planar defect, which depended upon the residual stress in the thin film. The reflective second harmonic generation results were analyzed by correlating macroscopic data, from optical probes, with microscopic data arising from tunnelling electron microscopy.

Optical Second Harmonic Generation from the Twin Boundary of ZnO Thin Films Grown on Silicon. Lo, K.Y., Lo, S.C., Yu, C.F., Tite, T., Huang, J.Y., Huang, Y.J., Chang, R.C., Chu, S.Y.: Applied Physics Letters, 92[9], 091909