Date of Award
Doctor of Philosophy (PhD)
Dana A. Browne
I present an extension of the adiabatic Bond Charge model to study lattice dynamics of highly polar II-VI semi-conductors and semi-metals and CdTe-HgTe superlattices grown along (001) and (111) directions. This is the first attempt to study phonons in CdTe-HgTe superlattices. My results for phonon dispersion, specific heat and elastic constants for the six bulk materials are in good agreement with experimental observations. The best parameter sets for the six compounds show trends that are consistent with the parameters for group IV elemental semi-conductors and III-V compounds. For CdTe-HgTe superlattices, I find that the long range Coulomb interaction between particles situated on the opposite sides of the interface needs to be handled carefully to get positive eigenvalues and eigenvectors with proper symmetry and continuity. For CdTe-HgTe superlattices grown along (001) direction, many propagating modes are seen which travel with different wavevectors in CdTe and HgTe layers. The wavevectors in HgTe are different from the corresponding bulk values. This difference is significant for long wavelength modes. For (111) superlattices all 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. Also, in both cases the frequency of the highest HgTe LO mode does not reach the bulk limit even for 27 monolayer thick HgTe layers. These effects are interpreted in terms of the lowering of HgTe-frequencies from their bulk values due to the presence of higher charges in CdTe layers. My results for (001) superlattices indicate that all the un-identified peaks seen in the Raman spectra of these structures can be explained in terms of superlattice modes.
Rajput, Bhagwan Das, "An Adiabatic Bond Charge Model for the Phonon Properties of II-VI Materials and Superlattices." (1997). LSU Historical Dissertations and Theses. 6442.