Thermal conductivity in nanoscale polysilicon structures with applications in sensors

Abstract

Thermal conductivity of polysilicon nanowires and thin films has been experimentally studied. Thermal conductivity has been measured with a microstructure fabricated alongside the polysilicon nanowires and thin films and a robust fabrication process has been developed. Results suggest that the in-plane and out-of-plane thermal conductivity are limited by phonon grain boundary and film boundary scattering respectively. The nanowires are fabricated by patterning a thin layer of low-pressure chemical vapor deposited polysilicon using e-beam lithography, consequently circumventing the prevalent process of pick-and-place process, in which metallic contacts are deposited to insure significant heat flow to the nanowire. By avoiding the need for metallic contacts a significant source of error in the calculation of the thermal conductivity is eliminated. Thermal conductivity of nanowires with a cross section of ˜60 nm × 100 nm and thin film with a cross-section of 3μm × 108 nm were measured to be both ˜3.5 W/m.K, suggeting that the thermal conductiviy is dominated by the out-of-plane phonon scattering in both cases. This is an almost 8× reduction from the thermal conductivity of bulk polysilicon at ˜30 W/m.K. The effect of grain size and doping concentration on the thermal conductivity of ˜100 nm polysilicon films is also experimentally studied.