Electronic structure and optical properties of PbY and SnY (Y=S, Se, and Te)

Detta är en avhandling från Stockholm : KTH

Sammanfattning: Lead chalcogenides and tin chalcogenides and their alloys are IV?VI family semiconductors with unique material properties compared with similar semiconductors. For instance, PbY (Y = S, Se, and Te) are narrow-gap semiconductors with anomalous negative pressure coefficient and positive temperature coefficient. It is known that this behavior is related with the symmetry of wave functions in first Brillouin zone L-point, which moves the edges of valence band maximum and conduction band minimum towards each other with pressure increasing. SnTe has opposite behavior since its wavefunction symmetry is different from PbY. Therefore, by alloying PbTe and SnTe one can change and control the band gap energy and its pressure or temperature dependence. These chalcogenides alloys have therefore a huge potential in industrial low-wavelength applications and have been attracted the attention of researchers.This thesis comprises theoretical studies of PbY, SnY (Y = S, Se and Te) and the Pb1?xSnxTe alloys (x = 0.00, 0.25, 0.50, 0.75, and 1.00) by means of a first-principles calculation, using the full-potential linearized augmented plane waves method and the local density approximation.The optical properties of Pb1?xSnxTe alloys are investigated in terms of the dielectric function ?(?) = ?1(?) + i?2(?). We find strong optical response in the 0.5–2.0 eV region arising from optical absorption around the LW-line of the Brillouin zone. The calculated linear optical response functions agree well with measured spectra from ellipsometry spectroscopy performed by the Laboratory of Applied Optics, Linköping University. The calculations of the electronic band-edges of the binary PbY and SnY compounds, show similar electronic structure and density-of-states, but there are differences of the symmetry of the band-edge states at and near the Brillouin zone L-point. PbY have a band gap of Eg 0.15?0.30 eV. However, SnY are zero-gap semiconductors Eg = 0 if the spin-orbit interaction is excluded. The reason for this is that the lowest conduction band and the uppermost valence band cross along the LW line. When including in PbY. Although PbY and SnY have different band-edge physics at their respective equilibrium lattice constants, the change of the band-edges with respect to cell volume is qualitatively the same for all six chalcogenides. The calculations show that the symmetry of band edge at the L-point changes when lattice constant varies and this change affects the pressure coefficient. the spin-orbit interaction a gap Eg ? 0.2 eV is created, and hence this gap is induced by the spin-orbit interaction. At the L-point, the conduction-band state is a symmetric state and the valence-band state is antisymmetric thereby the L-point pressure coefficient +4L?4LpEg??/)L( in SnY is a positive quantity. In contrast to SnY, the PbY compounds have a band gap both when spin-orbit coupling is excluded and included; this gap is at the L-point, and the conduction-band state has and the valence-band state has symmetry, and thereby this band edge yields the characteristic negative pressure coefficient +4L?4LpEg??/)L(