Nanostructured materials have been demonstrated with great success in improving the thermoelectric figure of merit (ZT=S2σ/κ) owing to their remarkably reduced thermal conductivity κ. Recently, with κ being engineered close to their low limits, further boosting the dimensionless ZT over unity in nanostructures relies more and more on enhancing the power factor (S2σ). However, due to the overwhelming surface effects, traditional strategies that aim to decouple S and σ for optimizing the power factor, i.e. by resonant scattering or energy filtering, can hardly be applied in nanostructured materials, such as quasi-1D nanowires.
Here, we demonstrate a modulation of thermoelectric power factor in Si nanowires by increasing the hole transport mobility via rationally designed radial dopant inhomogeneity. The nanowires were B-doped and were configured to exhibited a heavily doped surface but lightly doped core. Such modulated dopant distribution facilitated surface-to-core diffusion of mobile carriers, and hence decoupled the axial/longitudinal carrier transport from the dominant surface and dopant impurity scatterings. Tailoring the radial dopant profile under this strategy to be δ-doping-like allowed us to achieve apparently increased power factors over that of homogeneously doped systems. As evidenced by field-effect measurements, the enhancement was clearly related to the increased hole mobility in such nanowires. In the rationally designed δ-doped Si nanowires, the extracted hole mobility at the relatively heavily doped concentration of ~1019 cm-3 was even 4 times higher that of bulk Si, showing the great potential in optimizing the power factor under this strategy.
In distinct from the earlier heterostructured nanocomposites or nanowires, the present strategy induces dopant modulation while without creating heterointerfaces. Thus, the possible mobility degradation by interface related mid-band trapping states can be avoided. Regarding to such advantage, this strategy may find universal prospects in many thermoelectric materials.
This work was supported by NEXT Project. T.K. was supported by FIRST program. T.Y and K.U thank the financial support of CREST.
The Institute of Scientific and Industrial Research, Osaka University
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