Under the tight-binding formalism, this paper suggests that effect of strain can be considered in terms of Harrison's interatomic matrix elements, taking acount of the homogeneous bond stretching and bond bending in ultrathin layer (GaP)1/(InP)1(lll) systems. Using recursion method, we have calculated the bulk and the surface electronic structures of the stable (GaP)1/(InP)1(lll) superlattice determined by the Keating model. Our result shows that its band gap is 1.88 eV, which is smaller by 0.31 eV than the average of the calculated gaps of bulk GaP(2.91 eV) and lnP(1.48 eV). As compared with the calculated auxiliary systems, we find that the band gaps of the superlattices become larger and the Fermi levels become lower with the increasing of the strain of the bond length. Furthermore, the obtained number of the occupied electrons on different sites clearly shows that InP is more ionic than GaP. Finally, we show the existence of an approxiamte SPz+Px-Py dehybrid on (111) surface of this strained-layer superlattice and point out that the contribution of S or P partial orbital of a (111) surface atom to this dehybrid is different from the case of a (001) surface atom of GaAs/AlAs (001) superlattice.
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