The Thermoelectric Properties of Rare Earths as Dopants in InGaAs Films
Current energy technologies lose over half of the energy input to waste heat. Thermoelectric materials can recover some of this waste heat by converting it into electricity. Thermoelectric devices have no moving parts, so they are low noise and highly reliable, making them particularly suitable for extreme environments. A good thermoelectric has low thermal conductivity to maintain large temperature gradients and high electrical conductivity to effectively transport carriers across that temperature gradient. One of the major challenges in engineering such thermoelectrics is effectively decoupling these parameters. These relationships are quantified in the dimensionless thermoelectric figure of merit, ZT, where a ZT of 1 is considered commercially viable.
Doping MBE grown InGaAs films with rare earths forms embedded nanoparticles that have been shown to improve thermoelectric efficiency of InGaAs. Rare earth doping effectively overcomes the problematic relationship between electrical and thermal conductivities. These embedded particles effectively decouple thermal and electrical properties by contributing carriers to increase electrical conductivity as well as forming scattering centers for mid to long wavelength phonons to decrease thermal conductivity. However, the mechanism for carrier generation from rare earths is poorly understood. Comparing different rare earths as dopants in InGaAs, we find a positive correlation with the electrical activation efficiency as the rare earth arsenide nanoparticles are more closely lattice matched to the host matrix. This is in contrast to traditional Si doped InGaAs, which is fully ionized at room temperature. The high doping efficiency of Si leads it to be as good or better of a dopant for thermoelectrics compared to the best rare earths studied. We observe that rare earth doped InGaAs has thermal activation of carriers at high temperature, giving it the potential to be a more efficient thermoelectric in this regime than traditionally doped InGaAs.
A method was developed to determine the thermoelectric efficiency of a material system over a range of conductivities using only a few experimental data points. This allows for more efficient mapping of a material system for thermoelectrics. Using this analysis, high temperature measurements show that carrier scattering from rare earth impurities compensates the enhancement from thermally generated carriers, giving Si the potential to be a better thermoelectric dopant in InGaAs at high temperature. Extrapolating temperature dependent measurements to higher temperatures shows that a ZT greater than 3 should be theoretically possible for Gd or Si doped InGaAs at 700C.