Long-period superlattices

Superlattices are of great interest for the study of phenomena which arise at interfaces of media. These artificial layered crystals are systems with condensed interfaces since the total area of the interfaces in them is proportional to the volume. In these conditions specific surface and quasi-two-dimensional effects must make an important contribution to the bulk crystal optics. Some of them are analyzed in the present chapter. 

Since E( is continuous at the interfaces and the variation of Et inside each layer at t], Co -C A can be neglected, then e[1 1 = e[2 1 Et, where Et is the value of the electric field strength averaged over the superlattice. Hence we obtain 

This means that the dielectric tensor of the superlattice consisting of layers with optically isotropic material has the form characteristic of a uniaxial crystal. Assuming that the layers are parallel to the (x, y) plane and normal to the z-axis, then the dielectric tensor of the superlattice has components 

If the wavevector of light propagating in the superlattice k lies in the (x, z) plane, then 

Analogously, for extraordinary waves with vector E lying in the same plane as the wavevector k and the optical axis of the superlattice, the dispersion relation is 

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McLaren, A. C., MacKenzie, W. S. (1976). The spatial coherence of x-ray and electron beams and its influence on the diffraction patterns from materials with long-period superlattices, phys. stat. sol., (a)33, 491-5. 

Abstract. We describe the state-of-the-art in the creation of ordered superlattices of adsorbed atoms, molecules, semiconductor quantum dots, and metallic islands, by means of self-assembly during atomic-beam growth on single crystal surfaces. These surfaces often have long-period reconstructions or strain relief patterns which are used as template for heterogeneous nucleation. However, repulsive adsorbate-adsorbate interactions may also stabilize ordered superlattices, and vertical correlations of growth sequences of buried islands will be discussed in the case of semiconductor quantum dots. We also present new template surfaces considered as particularly promising for the creation of novel island superlattices. 

By varying the degree of polymerization, the periodic length scale of the superlattice can be adjusted over a range from some nanometers up to several hundred nanometers. Figure 8 shows with the example of PB-PEO how the lamellar long period d is connected with the degree of polymerization N. The... 

The major attention is given to the conqjensated GaAs superlattices with the layer thickness of 40 nm and concentration of donors and acceptors of 10 cm. The superlattice named No. 4i contains i-layers n-i-p-I stracture) and No. 4 means no i-layers n-p-n-p stmcture). Both structures belong to the long-period doping superlattices and their photoluminescence properties were measured in a wide temperature range. Pronounced effects of a-irradiation were observed [7]. 

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