Over half of the energy from non-sustainable sources such as coal, natural gas, and petroleum that is converted for electricity and transportation is lost as waste heat. In 2012 alone, the U.S. rejected ∼58.1 quadrillion BTUs as unusable energy everywhere from power plants to automobile exhaust pipes to laptop CPUs. Thermoelectric materials, which convert heat into electricity within a solid-state device, offer a viable technology to capture some of this widely-spread waste heat and convert it directly into usable electricity.

Molecular beam epitaxy (MBE) provides a means to explore new ideas through fine control of doping and heterostructures, and opens the door to new nanostructures accessed only in a kinetically-limited growth regime. ErAs:InGaAlAs nanocomposites grown by MBE have previously shown a high thermoelectric figure of merit, ZT∼1.33 at 800 K, which make them promising for thermoelectric applications. But those results have raised several key questions: why does an isovalent atom like Er cause n-type conductivity in InGa(Al)As, and would other rare earth elements or other III-V semiconductors be even better suited for thermoelectric applications? Beyond rare earth doping, are there new and better heterojunction engineering schemes to improve a variety of thermoelectric composites?

Working with researchers in Arthur C. Gossard and John E. Bower's groups, ErAs semimetal nanoparticle - InGaAs semiconductor composites were optimized and measured, yielding a ZT of ∼1.7 at 850 K. A solubility limit of Er in InGaAs was identified, and along with researchers in Chris G. Van de Walle's group, it was shown that Er, which is isovalent with In and Ga, is a n-type dopant when positioned on an interstitial crystallographic site. Furthermore, it was found that the position of Er atoms and formation of ErAs particles can be controlled in the kinetically-limited growth regime by changing growth temperature and growth rate. Hong Lu and I explored other rare earth elements, such as Ce, Sm, Yb, and Eu and other semiconductors, such as InGa(Al)Sb and InAsSb, offering a wide range of thermal and electrical properties. ErSb:InGaSb was shown to have improved p-type thermoelectric properties, and independent control of electrical conductivity and thermal conductivity was demonstrated by codoping Ce and Si in InGaAs. Additionally and unrelated to rare earth doping, John E. Bowers and I conceived and demonstrated a new strategy for improving Seebeck coefficient using heterojunctions.