Thermoelectric performance in n-type bulk silicon: The influence of dopant concentration and dopant species

Silicon (Si) has received recent interest for thermoelectric (TE) applications. For all TE materials, accurately tuning the doping concentration remains the easiest way to maximise the thermoelectric figure-of-merit (ZT). This study investigates the thermoelectric properties at 300 K of n-type Si as a function of both dopant concentration (N ∼10¹⁹–10²⁰cm⁻³) and dopant species (P, As and Sb), including measurements of electrical resistivity, thermal conductivity, Seebeck coefficient and Hall mobility. All properties were found to vary as a function of both doping concentration and species, leading to impacts on the ZT. The electrical resistivity was lowest for P-doped Si and highest in Sb-doped Si. For the Seebeck coefficient, the situation was reversed. The thermal conductivity was lowest for Sb-doping and highest in P-doped Si. In all cases As-doping was the intermediate dopant. An optimum doping concentration was realized at a value of ∼6–7 × 10¹⁹cm⁻³, and is similar for both As- and P-doped Si. For Sb-doping, the optimum value is likely to be similar, but the highest doping in commercially available wafers was ∼4 × 10¹⁹cm⁻³. At 300 K, ZT ∼0.010 is achieved for P-doped bulk Si, however the best overall value was for As-doped Si, at ∼0.013. For Sb doping the best value is ∼0.012, though a higher value is likely to be possible, but only if doping levels approximately double the concentrations available for this current study can be achieved in starting substrates. These results provide a useful insight for researchers who are selecting a starting substrate for top-down nano-structuring approaches to Si thermoelectrics, where a wafer with optimised ZT is required.