Rehybridization and charge transfer

In order to shift electrons from surface cations to anions, the cation changes its sp into the planar sp configuration  

The empty and filled dangUng bond states of the relaxed surface move out of the band gap of the semiconductor, that is, in the valence and conduction band, respectively. Consequently, the band gap is free of surface states. This holds for all III-V compounds, except GaP(llO), where the dangling bond states derived from the P atom are still above the valence band maximum [13-20]. The electronic band structure of the (110) surfaces is discussed in more detail in Section 13.5. 

The inequivalence of the two surface atomic sites can experimentally be imaged by STM (scanning tunneling microscopy) using different bias voltages. Feenstra and coworkers studied the GaAs( 110) surface and showed that STM does not display 

The (110) surfaces of compound semiconductors are very well understood systems. Owing to the relatively simple relaxation of the surface, in contrast to 

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. 

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