We have found that ZnO films consisting of epitaxially ordered arrays of closely packed hexagonal microcrystallites grown on vicinal (1000) sapphire exhibit biaxial in-plane optical anisotropy. The optical anisotropy resonance occurs near the band gap energy of ZnO. The line shape of the resonance is consistent with that induced by an in-plane anisotropic strain. The direction of the anisotropy coincides with the projection of the miscut direction of the (0001) sapphire substrates onto the sample surface plane. The magnitude of the anisotropy is generally larger for films with high crystalline quality, and on substrates with larger miscut angles. A possible origin of the strain anisotropy due to the miscut angle and the difference in thermal expansion rate of sapphire along its c and a axes is proposed.
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