Improvement of Phase Stability and Accurate Determination of Optical Constants of SnO Thin Films by Using Al2O3 Capping Layer

In this letter, it is proposed that the usage of Al2O3 capping layer can tremendously improve the phase stability of SnO thin films, which allows the accurate determination of the optical constants of the SnO films without the perturbation arising from impurity phases. For the SnO films, the refraction index and extinction coefficient are significantly influenced by the crystallinity. The nondirect optical bandgap of the amorphous SnO films is determined to be 2.27 eV, whereas two nondirect optical transitions are observed in the polycrystalline SnO films and the corresponding gap energies are estimated to be 0.50 and 2.45 eV, respectively.