Thermoelectric properties of ultrathin silicon nanowires

• Published in: Physical review. B, Condensed matter and materials physics Vol. 86; no. 11
• Main Authors: Ramayya, E. B., Maurer, L. N., Davoody, A. H., Knezevic, I.
• Format: Journal Article
• Language: English
• Published: 24-09-2012
• Subjects: Boltzmann transport equation; Degradation; Electronics; Mathematical analysis; Nanowires; Resistivity; Thermal conductivity; Thermoelectricity
• ISSN: 1098-0121; 1550-235X
• DOI: 10.1103/PhysRevB.86.115328

We calculate the room-temperature thermoelectric properties of highly doped ultrathin silicon nanowires (SiNW) of square cross section (3 x 3 to 8 x 8 nm super(2)) by solving the Boltzmann transport equations for electrons and phonons on an equal footing, using the ensemble Monte Carlo technique for each. We account for the two-dimensional confinement of both electrons and phonons and all the relevant scattering mechanisms, and present data for the dependence of electrical conductivity, the electronic and phononic thermal conductivities, the electronic and phonon-drag Seebeck coefficients, as well as the thermoelectric figure of merit (ZT) on the SiNW rms roughness and thickness. ZT in ultrascaled SiNWs does not increase as drastically with decreasing wire cross section as suggested by earlier studies. The reason is surface roughness, which (beneficially) degrades thermal conductivity, but also (adversely) degrades electrical conductivity and offsets the Seebeck coefficient enhancement that comes from confinement. Overall, room-temperature ZT of ultrathin SiNWs varies slowly with thickness, having a soft maximum of about 0.4 at the nanowire thickness of 4 nm.