Study On The Electronic Structure And The Lattice Dynamical Properties Of ?-? Semiconductors

?-? semiconductors exhibit exotic properties in multiple aspects. PbTe and SnSe are important thermoelectric materials. Ge-Sb-Te phase-change alloys based on GeTe are applied in optical storage media. Lead-salts and their alloys are employed to fabricate mid-infrared optoelectronic devices working at or near room temperature. The devices based on ?-? semiconductors have widely applications in renewable energy,optical storage,environmental gas monitoring, medical diagnosing as well as military field. ?-? semiconductors also host unique electronic and lattice dynamical properties,which lead to fascinating physical properties. The work in this thesis is concerned with the physical properties of ?-? semiconductors and the main results are summarized as follows:1. We have investigated the electronic structure of group-? tellurides GeTe, SnTe,and PbTe using first-principles calculation. The abnormal trend in their electronic band structures, i.e., a band inversion in SnTe but not in GeTe and PbTe, is explained by symmetry analysis and the atomic energy levels and sizes. The highest cation s orbital energy of Sn among the three cation atoms, leading to the largest s-p coupling in SnTe with respect to GeTe and PbTe, makes SnTe band gap inverted at the L-point. We also demonstrate that the internal atom displacement in ?-GeTe and ?-SnTe induces strong admixture between conduction band minimum and valence band maximum and reduces s-p coupling, thus increases the band gap. The ?-SnTe is found to be a normal semiconductor because of reduced s-p coupling and admixture of p orbitals, which requires further experimental verification. Our study illustrates that the inter-level coupling could become an effective pathway to design topological insulators.2. Using angle-resolved photoemission spectroscopy (ARPES), we studied bulk and surface electronic band structures of PbTe (111) thin films grown by molecular beam epitaxy. The combination of ARPES and first-principles calculation clearly reveals the full 3D band structure, orbital characters, spin-orbit splitting energies, and surface states.The photon-energy-dependent spectra show the bulk character. Both the L and ?valence bands are observed and their energy difference is determined to be 0.2 eV. The spin-orbit splitting energies at L and T points as large as 0.62 eV and 0.88 eV are identified, respectively. The surface states below and close to the valence band maximum are unambiguously identified. By varying the photon polarization, the valence bands are observed to be composed of a mixture of Pb 6s and Te 5 pz orbitals with mostly in-plane even parity, which is attributed to the layered distortion in the vicinity of the PbTe (111) surface. These findings provide new insights into PbTe fundamental properties and shall benefit relevant thermoelectric and optoelectronic applications.3. We have investigated the lattice dynamics of PbTe using first-principles calculation. By applying biaxial epitaxial strain in (111) plane, we find that the incipient ferroelectric material PbTe is still paraelectric. We find that the enhancement of anharmonicity in PbTe by the (111) compressive strain. The results enrich our understandings of the lattice dynamical behavior of PbTe and indicate the strain as a potential pathway to engineer the thermoelectric performance of PbTe.