FAR-INFRARED SPECTROSCOPY OF PHONONS AND PLASMONS IN SEMICONDUCTOR SUPERLATTICES

Experimental far-infrared spectra of semiconductor superlattices and the related theory are reviewed. An account is given of Fourier-transform spectroscopy in various forms. In discussing superlattice spectra, a distinction is made between long- and short-period superlattices. In the former the constituent layers are many monolayers thick and can be described as slabs of the corresponding bulk material. A fairly simple account of the far-infrared optics can be based on this description, which is usually applied by treating the superlattice as a uniaxial effective medium in which the dielectric-tensor components are given by averages over the dielectric constants of the layer materials. Examples of spectra are shown for all the main experimental techniques. In short-period superlattices the effect of confinement on the dielectric function must be included; this review concentrates on the effect on phonon modes. Experimental measurements of confined-mode frequencies show significant shifts from values predicted by theories that assume perfect interfaces. This indicates that the effect of interface roughness must be included into phonon-confinement models. It is shown how this can be incorporated into a 1D lattice-dynamic model which leads to a generalised effective-medium description. Comparison with experimental results is presented. Possible future developments are discussed in a final section.