Crystallographic Structure of ZnO Surfaces

The wurtzite lattice of ZnO and its low-index surfaces are shown in Fig. 4.5. The basic low index surface terminations are (0001), (0001), (1010), and (1120). The (0001) and (0001) represent the zinc and oxygen-terminated surfaces of the polar 0001 direction, which corresponds to the 111 direction of the cubic zincblende lattice. In contrast to the polar (100) and (100) surfaces of the zincblende lattice, the surfaces with threefold symmetry (111) 

Surfaces of real crystals never adopt the bulk-truncated structures shown in Fig. 4.5. They reconstruct or relax (inwards or outward movement of the atoms) to minimize their surface energy [42]. Known surface structures of zincblende and wurtzite structure semiconductors are summarized in [43]. Nonpolar surfaces of wurtzite (1120) and (1010) surfaces show no lateral surface reconstructions and are supposed to have a structure similar to the well-known zincblende (110) surface, which is characterized by an inward relaxation of the surface cations and partial electron transfer from the surface cation dangling bond to the surface anion dangling bond [42,43]. 

The uncertainty concerning the identification of the stabilization mechanism on polar ZnO surfaces is partly due to the lack of atomically resolved STM images. Such images are possible for the nonpolar (1010) and (1120) surfaces [40,41] but have, to our knowledge, not been reported for polar surfaces. The polar cation terminated (111) surface of zincblende compounds typically displays a 2 x 2 reconstruction associated with removal of every fourth surface cation [43,50-52]. This structure is ideally suited to match the charging condition for surface stabilization for this particular surface orientation. The 2x2 reconstruction and the missing surface atoms can directly be observed by STM [52]. In contrast to literature [53], a 2 x 2 reconstruction is also frequently observed in our group for the (0001) surface of wurtzite CdS.4 The reconstruction on the anion terminated (III) surfaces of III—V and II-VI zincblende compounds are considerably more complex. These surfaces 

3 The factor of 1/4 results from the distance of the Zn and O lattice planes along the c-direction. 

Siepchen et al., Darmstadt University of Technology (unpublished results). 

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