Observation and Analysis of Ordered and Disordered Structures on the ZnO(0001) Polar Surface

Abstract

Using low temperature scanning tunneling microscopy (STM), we discovered locally ordered patches of O adatoms and single Zn vacancies on the Zn-terminated ZnO(0001) polar surface. Such patches are determined to be metastable ordered structures on the surface. Density functional theory (DFT) calculations show that Zn atoms bonded to an O adatom encounter a larger reaction barrier for leaving lattice sites, explaining the observed general disordered nature of the Zn-terminated surface that is populated by cavities of different shapes and sizes and disordered distribution of adatoms. The interplay among different driving mechanisms provides valuable insight as to how a polar surface of an ionic crystal achieves its lowest energy reconstructed surface structure. Comparisons between the charge on surface vs bulk layers for a relaxed (1 × 1) slab and a slab bounded on two ends by reconstructed surfaces with stoichiometric changes reveal that in a neutral environment, the polar surface of an ionic crystal tends ...