PHONONS IN CDTE-HGTE SUPERLATTICES

We present a lattice dynamics study of CdTe-HgTe superlattices grown a long the (001) and (111) directions within the framework of the adiaba tic bond charge model. We find that the long-range Coulomb interaction between particles situated on the opposite sides of the interface nee ds to be handled carefully to get sensible frequencies and eigenvector s with the proper symmetry and continuity, CdTe-HgTe superlattices gro wn along the (001) direction show many propagating modes that travel w ith different wavevectors in CdTe and HgTe layers. The wavevector in H gTe differs from the bulk value, particularly at long wavelengths. For (lll) superlattices all of the optical modes are either confined or in terface modes. In both cases, the frequencies of the HgTe optical mode s 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 Ra man spectra of (001) superlattices can be explained in terms cf superl attice modes.