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Room Temperature Strong Emission and Excitonic Enhancement in Multiple‐Stacked Nano‐Porous ZnO Thin Film

Electronic and structural properties of ZnO films before and after H2 annealing treatment are studied. The initial system is smooth amorphous ZnO films that are intentionally grown at room temperature. A dramatic change is found in the crystallinity and the surface transforms to a multiple‐stacked nanoporous structure upon annealing as observed by scanning electron microscopy and X‐ray diffraction. Photoluminescence spectroscopy shows the presence of a new mid‐gap state below the first optical transition, and the annealing treatment transforms the ZnO system to more transparent film. Interestingly, it is found that the structural evolution changes the dielectric function measured by high‐resolution, angle‐dependent spectroscopic ellipsometry and this generates strong emission at ≈2.5 eV. These results show the importance of structural modification via H2 annealing to control the optical properties and electronic structure of ZnO films.