Reconstruction polar semiconductors

LEED patterns do not tell the nature of the reconstruction, only its symmetry. Thus a scries of workers have speculated a variety of patterns of distortion or missing atoms. It has not been possible to choose between these experimentally (except possibly in the case of (110) surfaces of polar semiconductors, to be discussed), so the field has been left in an unsatisfactory state. 

We can also expect reconstruction on the surfaces of polar semiconductors. In particular, the distortions on the (110) cleavage plane may be expected to be of the same form as on the (110) surfaces of homopolar semiconductors, as shown in Fig. 10-5, with the nonmetallic atom displaced outward since its hybrid is doubly occupied the metallic atom is displaced inward with its purely p-like hybrid unoccupied. This is the distortion proposed by MacRac and Gobcli (1966) for essentially the same reasons described here. This appears to have been confirmed by recent analysis of LEED data (Lubinsky, Duke, Lee, and Mark, 1976). 

Figures 5.2-7-5.2-17 give the accepted reconstruction models for a selection of covalent and polar semiconductors, together with STM (scanning tunneling microscopy) images of some of the surfaces. Tables 5.2-6, 5.2-7, and 5.2-8 give the positions of the atoms in reconstructed Si(lll) 2x1 and Si (111)7x7 surfaces and the parameters of the rotation/relaxation model of polar semiconductors.

It is interesting to compare the surface structure of these polar faces with the structurally equivalent Si (111) face. As silicon is a purely covalent material, no effects of bonding ionicity should be apparent. It has been established that the Si (111) surface is always reconstructed and never faceted.The details of the reconstruction depend on how the surface is prepared, that is, whether it is cleaved (2x1 reconstruction) or ordered by the bombard-anneal technique (7x7 reconstruction) and to what temperature.it is annealed. Suck comparisons have revealed that the. observed Si CIU) surface reconstruction for clean surfaces is never the same as that observed with the polar compound semiconductor faces. It may, therefore, be concluded that bonding ionicity plays at least an important, if not a dominant, role in determining the observed reconstructions of the polar compound semiconductor surfaces. As Si (111) does not form facets when annealed to the highest temperatures, the existence of facets on polar compound semiconductor faces annealed to the highest possible temperatures can be attributed to bonding ionicity. 

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