Phonons in CdTe-HgTe superlattices
We present a lattice dynamics study of CdTe-HgTe superlattices grown along the (001) and (111) directions within the framework of the adiabatic bond charge model. We find that the long-range Coulomb interaction between particles situated on the opposite sides of the interface needs to be handled carefully to get sensible frequencies and eigenvectors with the proper symmetry and continuity. CdTe-HgTe superlattices grown along the (001) direction show many propagating modes that travel with different wavevectors in CdTe and HgTe layers. The wavevector in HgTe differs from the bulk value, particularly at long wavelengths. For (111) superlattices all of the optical modes are either confined or interface modes. In both cases, the frequencies of the HgTe optical modes do not map well to the bulk LO branch, and the highest HgTe LO mode consistently remains below the bulk value even for 27 layers of HgTe. Our results indicate that all of the unidentified peaks seen in the Raman spectra of (001) superlattices can be explained in terms of superlattice modes.