Polar Wurtzite Surfaces

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... 

ZnO surfaces are more complex than those of the rock-salt type oxides Uke MgO and NiO. ZnO crystalhzes in the wurtzite structure in which each Zn cation is tetrahedrally coordinated to four O anions and vice versa [105]. This crystal structure has no inversion center. The most important low-index surface planes are two polar planes, the Zn-terminated ZnO(OOOl) and 0-termi-nated ZnO(OOO-l) plane, and two neutral planes, ZnO(lO-lO) and ZnO(l 1-20). According to Nosker et al. [106] and Tasker [107], the two polar surfaces are thermodynamically unstable, however, they can be easily prepared and characterized experimentally, and do even show rather regular (1x1) LEED patterns [108]. This indicates that they are not stabilized by major reconstructions or other modifications. Therefore, it was believed for a long time that both polar surfaces exist in an unreconstructed bulk-Hke trimcation. Several contradicting proposals have been made to explain how the stability of the polar un-... 

Type 3 surfaces In the third family, the layers are not electrically neutral and the repeat unit bears a non-zero dipole moment. These are polar surfaces. The 111 faces of rocksalt crystals, the wurtzite 0001 faces, the fluorite 110 faces, etc., are polar. They deserve special attention due to their electric peculiarities. We postpone their analysis to Section 3.3. 

Nosker et al. (1970) have proposed models of polar surfaces for the wurtzite(0001 face and the blende 111 face, in which a macroscopic... 

We have seen that although the 111 surfaces are polar, similar to the 001 siufaces, the way they reconstruct differs significantly from the 001 surfaces because of the different bond configuration of the single atoms in the (111) direction with respect to the (001) direction. In the case of 111 surfaces, the atoms are threefoldbonding configuration is also present on wurtzite 0001 surfaces. In the following section, we will see that the reconstructions of these surfaces resemble some of the structural aspects just discussed for the cubic 111 surfaces. 

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)... 

There exist three polymorphs of ZnO, with zinc-blende, wurtzite and rocksalt structures. Atoms are tetra-coordinated in the two first forms, while in the latter, which is stabilized at higher pressure, the local environment is octahedral. The most thoroughly studied phase is the wurtzite one. Non polar (1010) and (1120) as well as polar (0001) or (OOOT) planar surfaces can be prepared. 

ZnO adopts the wurtzite structure in which Zn and O are both tetrahedrally coordinated to their counter ions. The 0-terminated (000 1) and Zn-terminated (0001) basal faces, as well as the non-polar (1010) prism face have all been the subject of adsorbate structure determinations. As regards their clean structures, LEED-IV was utilised as a probe over twenty years ago [117,118], and very recently they have been reexamined with SXRD [119-121]. From these measurements it was found that neither polar face relaxes greatly, a feature ascribed to surface enhanced covalency [121], For the (1010) face, SXRD [120] indicates only small displacements of the surface Zn and O away from their bulk terminated positions. 

Wurtzite GaN is a polar material. Therefore, along the c-axis, there are N-face (N-polar) or Ga-face (Ga-polar) orientations on the GaN surface. 

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. 

The polarization-related effects in wurtzite heterostructures can be entirely avoided by growing devices on alternative orientations of GaN crystals, such as the 1100 m planes or the 1120 a planes, in which the polar c-axis is parallel to the free surface and any planar hetero-interfaces. Since polarization fields exist entirely within the plane of the device structure, heterostructures grown on these planes exhibit flat-band conditions under zero bias. 

In the wurtzite structure, the polar faces are the (0001) and (OOOT) faces. In ZnO, they are terminated by a zinc and an oxygen plane, respectively. It seems that their surface energies are lower than in the rocksalt crystals. The experiments performed on these materials do not mention impurity or faceting problems, and the Zn-terminated surface was found to be unrelaxed (Sambi et al., 1994). 

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. 

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