Electronic structure of non-polar surfaces

From both recent experimental and computational studies the Surface electronic structure of non-polar compound semiconductors comprises a filled valence band separated from an empty conduction band by a surface bandgap. The surface... 

It has been demonstrated through resort to published literature that bonding ionicity plays a dominant role in several important surface properties of all binary semiconductors with partial bonding ionicity. The following are included 1) The strong structural stability of non-polar surfaces. 2) The equally strong structural instability of polar surfaces. 3) The characteristic surface electronic... 

The electronic structure of polar surfaces does not follow the trends discussed at the beginning of the chapter for non-polar surfaces, primarily for two reasons. The first one comes from the number of broken bonds at the surface which may be large in some cases. For example, on the rocksalt lll faces, three in every six anion-cation bonds are broken. But the major reason is linked to the behaviour of the electrostatic potential discussed above, which induces large charge redistributions. In some cases, these charge redistributions may provide the compensation of the macroscopic electric field required to stabilize the surface. The electronic structure of three polar oxide surfaces has been calculated MgO lll, ZnO(OOOl) and (0001) and the weakly polar SrTiOsjOOl face. 

This section summarises some properties common to non-polar stoichiometric oxide surfaces and presents theoretical arguments to explain them. Their specificities come from the local environment of the surface sites, which have a lower coordination number than in the bulk. From this point of view, a close parallel with unsupported clusters or ultra-thin films can be established [3]. We will not explicitely consider here the properties associated to structural defects, such as steps or kinks, for the reason of space limitation. However, most of the time, the same concepts as those akin to terrace sites apply, but with an even larger strength since the local environment is more reduced. We will successively analyse structural characteristics, energetics, electron distribution, one-particle and two-particle excitations. 

A detailed study[81] of the solvent non-equilibrium response to electron transfer reactions at the interface between a model diatomic non-polar solvent and a diatomic polar solvent has shown that solvent relaxation at the liquid/liquid interface can be significantly slower than in the bulk of each liquid. In this model, the solvent response to the charge separation reaction A + D —> A + D+ is slow because large structural rearrangements of surface dipoles are needed to bring the products to their new equilibrium state. 

Conversion of electromagnetic wave (EW) polarization provides an efficient and powerful method for diagnostics of media a nd s tructures with reduced symmetry (e.g. anysotropic crystals, media with natural and artificial gyrotropy, periodic structures, solid-state surfaces and thin films). On the other hand, such media and structures can be used as polarization converters. The conversion of the polarization in surface layers and thin films is usually small [1,2] and achromatic because in this case the region of interaction of the EW with the polarization active medium is small and the interaction itself is non-resonant. However, the effect may increase substantially (resonantly) and the polarization converted radiation becomes colored when the external EW excites eigen-oscillations on optically active surface or in an optically active film. For example, under the non-uniform cyclotron resonance excitation in two-dimensional (2D) electron system, high conversion efficiency can be reached [3]. 

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