Wurtzite surfaces

Dhesi S S, Stagarescu C B, Smith K E, Doppaiapudi D, Singh R and Moustakas T D 1997 Surface and buik eiectronic structure of thin-fiim wurtzite GaN Phys. Rev. B 56 10 271-5... 

By the use of mainly LEED and lately ion scattering techniques the location of many atomic adsorbates, their bond distances and bond angles from their nearest neighbor atoms have been determined. The substrates utilized in these investigations were low Miller Index surfaces of fee, hep and bcc metals in most cases, and low Miller Index surfaces of semiconductors that crystallize in the diamond, zincblende and wurtzite structures in some cases that could be cleaned and ordered with good reproducibility. 

ZnO(lOlO) Unreconstructed wurtzite structure, top Zn and 0 pulled into surface somewhat DLEED 115-117)... 

V. Oxides and Halides with the Wurtzite Structure Surface Structures, Reactivity, and Catalytic Activity... 

The chemisorption of water on ZnO has been investigated by Nagao and Morimoto (147). Dent and Kokes (148) explained H2 chemisorption and ethylene hydrogenation on the basis of a model in which the (0001), (0001), and non-close-packed faces such as (1010) planes of the wurtzite crystal structure are assumed to form the external crystal surface. Chemisorption of H2 was suggested to occur on Zn—O pair sites, since the Zn—H and O—H stretching... 

Fig. 3.1. (a) Primitive cell (heavy lines) of the wurtzite-structure lattice placed within a hexagonal prism, a and c are the lattice constants, (b) Schematic drawing of surfaces cut from a hexagonal single crystal with different crystallographic orientations (surface planes)... 

In the / -spectrum of the ZnO thin film, a similar plateau as in the 3 -spectrum of the ZnO bulk sample is present. However, the phonon modes of the sapphire substrate introduce additional features, for example atw 510, 630, and "-900 cm 1 [38,123]. The spectral feature at w 610 cm-1 is called the Berreman resonance, which is related to the excitation of surface polari-tons of transverse magnetic character at the boundary of two media [73]. In the spectral region of the Berreman resonance, IRSE provides high sensitivity to the A (LO)-mode parameters. For (OOOl)-oriented surfaces of crystals with wurtzite structure, linear-polarization-dependent spectroscopic... 

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... 

The variation of the ionization potential with surface orientation evident from the literature data corresponds well with a systematic study by Ranke using a cylindrical GaAs single crystal [91]. This revealed that the electron affinity of the cation terminated (111) surface of GaAs (corresponding to wurtzite (0001)) is 0.4-0.5eV lower than those of other surface terminations. The same variation is observed at single crystal surfaces of CdTe and... 

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