(Invited) The Thermoelectric Properties of Ge/SiGe Based Superlattices: from Materials to Energy Harvesting Modules

We report recent progresses in the fabrication & characterization of multilayered Si & Ge –based thermoelectric materials & generators. Thermoelectric modules are used in a variety of applications to both harvest heat to generate electricity or to provide cooling by Peltier effect under electrical excitation. The performance of a thermoelectric material is estimated using, two main figures of merit: ZT=α²σ/κ & the power factor PF= α²σ, where α is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity & T is the temperature of operation.

The choice of Si & Ge is driven by the low cost and process compatibility of these materials with the CMOS industry, with a final vision of the integration of thermoelectric modules, power management unit and sensor on a stand alone platform. So far, SiGe alloys have been employed for high temperature application, close to room temperature the figures of merit are quite poor. The design, for bulk materials, is challenging as σ and κ are proportionally correlated from the Wiedmann Franz law - the optimization of one parameter impairs the other. However, the presence of low dimensional structures introduces an extra degree of freedom in the design of the material. Mature Si/SiGe and Ge/SiGe heteroepitaxial Low Energy Plasma Enhaced Chemical Vapour Deposition (LEPECVD) growth technology have been employed to realize complex multilayered structures for the investigation. Micro and nano-fabrication processes have been used to realize metrology structures fully integrated with the specimen. A complete module using both p-type and n-type is presented and the optimization for a series of designs using low dimensional structures is reported.