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Abstract:
Using scanning tunneling microscopy and spectroscopy, we have analyzed the growth of ZnO thin films on a Au(111) support. Because of the 12% lattice mismatch with the metal beneath, ZnO develops a (0001)-oriented coincidence lattice that gives rise to a well-ordered hexagonal Moiré pattern with 2.2 nm periodicity. This superstructure disappears at 4 ML film thickness, when wide, atomically flat terraces delimited by straight monatomic steps become detectable in the STM. The long-range order of the films is deduced from sharp hexagonal spot patterns in low-energy electron diffraction. STM-based luminescence and conductance spectroscopy reveals that the ZnO band gap approaches the bulk value in films thicker than 10 ML. Additional photon peaks with sub-band-gap energies indicate the presence of defects in the wurzite lattice. The intrinsic polarity of the ZnO(0001) surface is accounted for by a reduced Zn–O interlayer distance and an adsorption-mediated compensation scheme in thin and thick films, respectively.
