RESUMO
It has been highly debated whether the thermal conductivity κ of a disordered SiGe alloy can be lowered by redistributing its constituent species so as to form an ordered superlattice. By ab initio calculations backed by systematic experiments, we show that Ge segregation occurring during epitaxial growth can lead to κ values not only lower than the alloy's, but also lower than the perfect superlattice values. Thus we theoretically demonstrate that κ does not monotonically decrease as the Si- and Ge-rich regions become more sharply defined. Instead, an intermediate concentration profile is able to lower κ below both the alloy limit (total intermixing) and also the abrupt interface limit (zero intermixing). This unexpected result is attributed to the peculiar behavior of the phonon mean free path in realistic Si/Ge superlattices, which shows a crossover from abrupt-interface- to alloylike values at intermediate phonon frequencies of â¼3 THz. Our calculated κ's quantitatively agree with the measurements when the realistic, partially intermixed profiles produced by segregation are considered.
RESUMO
In this work we investigate the microscopic structure and dynamics of the molten equimolar alloy, Se(50)Te(50) using a combination of neutron and x-ray diffraction experiments, reverse Monte Carlo analysis, and first principles molecular dynamics. The range of temperatures studied covers the semiconductor/metal transition. From our results it can be seen that the latter is associated with an increase in coordination numbers and a reduced tendency to heterocoordination. In agreement with previous inelastic neutron scattering experiments, our molecular dynamics calculation predict a certain widening of the stretching vibrational modes band in connection with the increase of coordination and the presence of longer bonds in the metallic phase.