Wurtzite nonpolar surfaces

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]. 

In summary, on nonpolar compound surfaces, the surface energy is minimized by transferring electronic charge from the cation to the anion, thus yielding empty dangling bonds at the cation versus occupied dangling bonds at the anion. As we will see in Section 13.4, this is a general mechanism of the compound semiconductors, which holds also for the nonpolar surfaces of wurtzite crystals. 

Most surfaces of compound semiconductors are polar, that is, the number of anions and cations per surface unit cell is not balanced. While for the zinc blende materials there is only one nonpolar exception, the (110) face, for the wurtzite structures, there are two nonpolar surfaces, the m-plane (1100) and a-plane (1120) [98]. In wurtzite materials, a (110) surface does not exist because of the different crystal structure. 

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... 

One of the most important aspects of the films and heterostructures with nonpolar and semipolar surfaces is related to the polarization dependence of their optical properties. The polarization anisotropy has been studied both theoretically and experimentally in nonpolar GaN [87, 88], as well as in InN [105]. The optical polarization anisotropy in wurtzite nitrides originates from their valence band structure, which can be significantly modified by the anisotropic in-plane strain in the films. 

Figure 13.32 Drawings of relaxed nonpolar cleavage surfaces of wurtzite (a) the (1010) surface and (b) the (1120) surface. (From Ref. [98].)...

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