Origin of defect emission identified by polarized luminescence from aligned ZnO nanorods

It is found that both of the band-edge and defect emission from aligned ZnO nanorods are strongly polarized, and the intensities vary with the polarization angle by the relation of a square cosine function. The intensity of the UV emission has its maximum along the $c$ axis of a ZnO crystal. However, the intensity of the green emission is minimum in this direction. Namely, the two intensity curves are 90° out of phase. This unique characteristic of the polarization provides useful information to identify the fact that the green emission mostly occurs on the surface defects of the nanorods. This result is consistent with the currently accepted model that the green emission arises from the recombination between holes trapped at the surface defects and electrons trapped at the oxygen vacancy.