10 research outputs found
Physical properties of thermoelectric zinc antimonide using first-principles calculations
We report first principles calculations of the structural, electronic,
elastic and vibrational properties of the semiconducting orthorhombic ZnSb
compound. We study also the intrinsic point defects in order to eventually
improve the thermoelectric properties of this already very promising
thermoelectric material. Concerning the electronic properties, in addition to
the band structure, we show that the Zn (Sb) crystallographically equivalent
atoms are not exactly equivalent from the electronic point of view. Lattice
dynamics, elastic and thermodynamic properties are found to be in good
agreement with experiments and they confirm the non equivalency of the zinc and
antimony atoms from the vibrational point of view. The calculated elastic
properties show a relatively weak anisotropy and the hardest direction is the y
direction. We observe the presence of low energy modes involving both Zn and Sb
atoms at about 5-6 meV, similarly to what has been found in Zn4Sb3 and we
suggest that the interactions of these modes with acoustic phonons could
explain the relatively low thermal conductivity of ZnSb. Zinc vacancies are the
most stable defects and this explains the intrinsic p-type conductivity of
ZnSb.Comment: 33 pages, 8 figure
Influence of Uniaxial Stress on the Indirect Absorption Edge in Silicon and Germanium
The indirect optical absorption edge in silicon and germanium has been studied in the presence of shear strain. The splitting observed in the transmission spectrum is dependent on the direction and magnitude of the applied stress and on the polarization of the light with respect to the stress axis. The results have been interpreted in terms of changes in the valence- and conduction-band structure with strain. Neglecting strain dependence of phonon energies, various deformation potential constants have been determined from the experiments. The values are: Si, 80°K: Ξu=8.6±0.2 eV, |b|=2.4±0.2 eV, |d|=5.3±0.4 eV, Ξd+1 / 3Ξu-a=3.8±0.5 eV. Si, 295°K: Ξu=9.2±0.3 eV, |b|=2.2±0.3 eV, Ξd+1 / 3Ξu-a=3.1±0.5 eV. Ge, 80°K: Ξu=16.2±0.4 eV, b=-1.8±0.3 eV, d=-3.7±0.4 eV, Ξd+1 / 3Ξu-a=-2.0±0.5 eV. An observed nonlinear dependence of the splitting on stress has been interpreted as shifts of the exciton energies with uniaxial stress. A special experimental technique using a vibrating slit in the spectrometer was used in order to obtain an accurate determination of the fine structure in the absorption spectrum