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Semiconductor superlattices has proven to be a unique system to study the effect of new periodic and non-periodic modulations on the lattice dynamics of crystalline compounds [1]. On the theoretical side, the very simple symmetry of the superlattice constituents, being of diamond type for Silicon and Germanium and of zinc-blende type for most of III-V and II-VI materials, allows to obtain with simple approaches a full theoretical description of the effect of the composition modulation and to preserve an excellent physical insight into the specific behaviors of phonons in superlattices, such as zone folding or confinement. The same conclusion applies to the coupling between light and superlattice folded acoustic phonons discussed in terms of the photoelastic tensor.
On the experimental side, Raman scattering, though being restricted to the small light wavevectors, provides a large amount of information on the eigenfrequencies and eigendisplacements of superlattice phonons. Last but not least, Molecular Beam Epitaxy has reached such a level of perfection, particularly for the GaAs/AlAs system, that almost any arbitrary structure can be fabricated with a high degree of control and perfection. I will give in this lecture a detailed description of the basic properties of acoustic vibrations in superlattices and of their coupling to light, as measured in Raman scattering. The objective is to demonstrate that our understanding and control has reached such a high level of perfection that the available tools can be used with confidence to design new structures with predefined vibrationnal properties.
I will also to point the limitations of the standard photoelastic approach and emphasize some approximations and their possible consequences for the correct description of the new phononic devices discussed in other lectures.
[1] Raman spectroscopy of vibrations in superlattices, B. Jusserand and M. Cardona, in Light Scattering in Solids V: Superlattices and Other Microstructures, edited by M. Cardona and G. Güntherodt, Topics in Applied Physics 66, p49 (Springer-Verlag, Berlin, Heidelberg 1989) |
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