Multilayer thermoelectric refrigeration in Hg1−xCdxTe superlattices

The thermoelectric figure of merit ZT of ${\mathrm{Hg}}_{\text{1−x}}{\mathrm{Cd}}_x\mathrm{Te}$ superlattices (SLs) having currents along the growth axis is computed using a realistic SL band structure and the multisubband Boltzmann equation. For a narrow well and wide barrier, a heavy $\text{C1}$ and higher-lying light $\text{C2}$ subband combine to form a (nonoptimal) carrier-energy filter, enhancing the thermopower. The multilayer thermionic emission model accounts for this effect qualitatively but not quantitatively. However, for a narrow well and narrow-barrier system, ZT is 20% larger than that in the wide-barrier structure, indicating that devices based on carrier-energy filter/thermionic processes are not necessarily advantageous. ZT is almost three times larger than that in ${\mathrm{Bi}}_{\mathrm{2}}{\mathrm{Te}}_{\mathrm{3}}$ and is four times larger than that in an alloy with the average composition of the SL. This effect is associated with reduced lattice thermal conductivity in the SL rather than improved electronic transport.