Description: We propose the development of nanoscale superlattices (SLs) as the active elements of high efficiency thermoelectric coolers. Recent models predict that the thermoelectric figure of merit ZT can reach 4 for elements fabricated from HgCdTe-based SLs and could exceed 6 for related metal/semiconductor SLs. The feasibility of using HgCdTe and metal-based SL materials with embedded nanodefects for increased hot-carrier transport during thermoelectric cooling will be demonstrated in the proposed Phase 1 effort. We will perform calculations to optimize the material parameters of HgCdTe- and metal-based SL structures to maximize ZT. Next, we will use our extensive experience in molecular beam epitaxy to grow the designed structures. Finally, we will develop device structures and metallization methods appropriate for performing ZT measurements, measure the ZTs of fabricated devices and compare results with theory.

Benefits: High efficiency thermoelectric coolers possess a myriad of applications including portable cooling and precise temperature control for electronics, optics and medical systems. Present commercial thermoelectric devices operate at about 10% of the Carnot efficiency, whereas the efficiency of a compressor-based refrigerator varies from 30% to 90% depending on the refrigerator size. The temperature differences required in air conditioning are usually within the capacity of thermoelectric heat pumps, but their relatively poor coefficient of performance prohibits wide deployment. An increase of the thermoelectric figure of merit ZT above 3 is needed before thermoelectric technology can be used on large scales and potentially replace current refrigeration and air conditioning technologies. Our proposed coolers have a predicted ZT above 3 and hence can play a major role in next generation refrigerators and air conditioning systems.

Thermoelectric coolers have long contributed to NASA missions. For example, thermoelectric devices cool HgCdTe-based infrared imaging cameras such as those on the Hubble Space Telescope. They are employed as refrigerators in various space science experiments. Thermal loop spacecraft temperature control systems employ them as well. The same materials also hold great potential in thermionic energy conversion. All these applications will benefit from the improved efficiency of Hg1-xCdxTe- and metal-based SL thermoelectric coolers, together with the already realized benefits of solid state coolers, namely reliability, ruggedness and low cost. In particular, the possible monolithic integration of thermoelectric coolers and HgCdTe-based infrared focal plane arrays promises major reductions in the weight, size and cost of missions employing air- and space-borne high performance infrared imaging instruments