Ge-Si superlattice nanowires for high efficiency thermoelectric devices

ABSTRACT: The widespread use of thermoelectric devices to scavenge waste heat is currently limited due to two major reasons low ZT and the incompatibility of the material or process to form high-density thermojunctions needed for high-efficiency thermoelectric power generators. In this program, Agiltron Inc. in collaboration with Marquette University proposes to develop germanium-silicon mechano-electronic superlattice nanowire as a material with high ZT, based on Ge nanodots on Si nanowires, which forms a superlattice electronic band structure without the need for compositional modulation. Our proposed approach would improve ZT over that of currently demonstrated Si nanowires by two mechanisms. First, the Ge quantum dots on Si nanowires act as local stressors as they are grown in directions orthogonal to the nanowires"long direction, causing phonon scattering and reducing the thermal conductivity in nanowires. Second, the electronic superlattice results in miniband formation, increasing the density of states, thereby increasing both the Seebeck coefficient in the nanowires, holding potential for ZT>2 at room temperatures, and ZT>4 at or near 600K. Additionally, the Ge-Si superlattice nanowires are fabricated using conventional silicon micromachining, including optical lithography and anisotropic wet etch, allowing wide latitude in device design and robust and inexpensive manufacturing. BENEFIT: Our proposed method is based on conventional bulk micromachining and optical projection lithography, allowing for dense arrays of junctions with high fill factor, enabling high efficiency thermoelectric power generators and heat pumps that can be manufactured inexpensively. The development of thermoelectric materials with ZT>4 have potential widespread application in both civil and defense applications such as for efficient power scavenging from waste heat generated in jet turbine engines, combustion engines, nuclear reactors and hot water supplies. Exploiting temperature differences available in nature and in/on artificial objects can be used to power autonomous devices, and thermoelectric power generators that use body heat as an energy source are appropriate for portable low-power applications. Additionally, at such high ZT, thermoelectric pump cooling efficiencies compare favorably to those of conventional two-stage Freon-based cooling in refrigeration, potentially saving cost and harmful greenhouse emissions.