Zinc oxide (ZnO) is one of the compounds that are widely used in science and technology, due to their unique physico-chemical properties. It is an n-type direct-gap semiconductor with band gap 3.35 eV for E[up tack]Lc transitions and 3.023 cV for E[parallel to]c transitions. Its optical anisotropy is studied by normal incidence transmitance difference spectroscopy (TDS). ZnO has rhombohedral lattice structure but it cannot be revealed by TDS. Instead, biaxial symmetry is obtained and it is believed to be due to the strains developed along two orthogonal directions within the lattice in the film. Resonance occurs at 3.29 eV, very closed to the band gap. By using an ambient-film-substrate with Fresnel optics, the line shape of resonance changing with thickness is successfully simulated an...[ Read more ]

Zinc oxide (ZnO) is one of the compounds that are widely used in science and technology, due to their unique physico-chemical properties. It is an n-type direct-gap semiconductor with band gap 3.35 eV for E[up tack]Lc transitions and 3.023 cV for E[parallel to]c transitions. Its optical anisotropy is studied by normal incidence transmitance difference spectroscopy (TDS). ZnO has rhombohedral lattice structure but it cannot be revealed by TDS. Instead, biaxial symmetry is obtained and it is believed to be due to the strains developed along two orthogonal directions within the lattice in the film. Resonance occurs at 3.29 eV, very closed to the band gap. By using an ambient-film-substrate with Fresnel optics, the line shape of resonance changing with thickness is successfully simulated and the origin of the anisotropy is well explained. The intrinsic anisotropy of ZnO epilayer is not due to the off-plane anisotropy from the mis-oriented substrate but the anisotropic strains along two orthogonal directions, which leads to the shift in the band gap energy along those directions through the deformation potential. With the value of 2.5x10[to the power of negative five]eV shift in the band gap, the difference in the strains is calculated as 6.54x10[to the power of negative six]. These anisotropic strains are found to be dependent on the mis-orientation of the substrate and the film quality. The greater the mis-orientation, the greater the anisotropic strains are. In the meantime, better film quality will also enhance the anisotropic strains.