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Surface reconstructions of II-VI(001) surfaces
(page mainly designed by Thorsten Volkmann)


Many II-VI semiconductos like e.g. CdTe, ZnTe or ZnSe crystallize in the zinc-blende structure. If a CdTe crystal is viewed parallel to the [001] direction, one observes alternating Cd and Te layers which form a square lattice. However, at the surface the positions of the atoms may differ from those in the bulk because the crystal tries to minimize its surface free energy. The rearrangement of the surface atoms is called surface reconstruction. In general it leads to a decreased symmetry at the surface compared to the bulk material.
In contrast to III-V semiconductors, where a large number of complicated surface reconstructions is possible, only relatively few and simple reconstructions are observed on II-VI(001) surfaces. These include vacancy structures (i.e. structures where only a fraction of the lattice sites at the surface is occupied) or the formation of dimers.

The following picture shows the relevant surface reconstructions for a CdTe(001) surface, which serves as an example system in the following. These are the c(2x2) and (2x1) vacancy structures for a Cd terminated surface (left and middle), and the Te terminated surface with Te dimers arranged in rows leading to a (2x1) symmetry (right). Red crosses mark missing atoms at the surface, and the shaded areas indicate the surface unit cells.

Surface reconstructions of a CdTe(001) surface

In an MBE environment, a c(2x2) or a (2x1) (local) surface structure can dominate the Cd-terminated surface, depending on temperature and deposition fluxes. Experimentally, one observes for instance, a temperature driven re-ordering of the surface in absence of deposition, i.e. under sublimation conditions.
We have modelled quasi-static configurations of the system in terms of an equilbrium lattice gas description of the surface. Variants of the model are presented and discussed in, e.g., the following publications:

M. Biehl, M. Ahr, W. Kinzel, M. Sokolowski, T. Volkmann
A lattice gas model of II-VI(001) semiconductor surfaces   
preprint version of Europhys. Lett. 53, 169 (2001)

(PS version)     (PDF version)

M. Ahr and M. Biehl
Flat (001) surfaces of II-VI semiconductors: A lattice gas model
preprint version of Surface Science 505, 124 (2002)

(PS version)    (PDF version)

Models which allow to also simulate sublimation and growth and their interplay with the competing surface reconstructions are, for instance, discussed here:

M. Ahr and M. Biehl
Modelling sublimation and atomic layer epitaxy in the presence of competing surface reconstructions
preprint version of Surface Science (Letters) 488 , L553 (2001)

(PS version)

T. Volkmann, M. Ahr, and M. Biehl
Kinetic model of II-VI(001) semiconductor surfaces: growth rates in Atomic Layer Epitaxy
preprint version of an article in Phys. Rev. B 69 (2004) 165303
(PS version)    (PDF version)

The fact that the c(2x2) and (2x1) vacancy structures permit a surface coverage of at most 1/2 ML Cd has a direct impact on the achievable growth rates in Atomic Layer Epitaxy. For a more detailed discussion, illustrations and animations, click here. -->