Thermoelectric cross-plane properties on p- and n-Ge/SixGe1-x superlattices

Abstract

-Silicon and germanium materials have demonstrated an increasing attraction for energy harvesting, due to their sustainability and integrability with complementary metal oxide semiconductor and micro-electro-mechanical-systems technology. The thermoelectric efficiencies for these materials, however, are very poor at room temperature and so it is necessary to engineer them in order to compete with telluride based materials, which have demonstrated at room temperature the highest performances in literature [1].Micro-fabricated devices consisting of mesa structures with integrated heaters, thermometers and Ohmic contacts were used to extract the cross-plane values of the Seebeck coefficient and the thermal conductivity from p-and n-Ge/Si x Ge 1-x superlattices. A second device consisting in a modified circular transfer line method structure was used to extract the electrical conductivity of the materials. A range of p-Ge/Si 0.5 Ge 0.5 superlattices with different doping levels was investigated in detail to determine the role of the doping density in dictating the thermoelectric properties. A second set of n-Ge/Si 0.3 Ge 0.7 superlattices was fabricated to study the impact that quantum well thickness might have on the two thermoelectric figures of merit, and also to demonstrate a further reduction of the thermal conductivity by scattering phonons at different wavelengths. This technique has demonstrated to lower the thermal conductivity by a 25% by adding different barrier thicknesses per period.

High Thermoelectric Power Factor Realization in Si-Rich SiGe/Si Superlattices by Super-Controlled Interfaces

“…The high S 2 σ is related to the high S originating from a high conduction band degeneracy of six in Si. Therefore, nanostructuring Si has drawn much attention to reducing inherently high κ of Si. − ,− ,,,,,− ,− However, the concomitant S 2 σ reduction is a crucial issue in nanostructured systems , because of the uncontrolled factors, such as interface carrier scattering, degeneracy lifting, and effective electron mass variation. In the case of Si-based superlattice (SL) films, the interface with large atomic size difference (defined as D atom ) such as that of strained epitaxial Ge/Si SLs , enhanced interface phonon scattering, leading to low κ .…”