Measurement and Theoretical Analysis of Temperature-Dependent Thermal Conductivity of Bi Thin Films

We report the effects of film thickness and grain size on the thermal transport properties of Bi thin films, which were investigated with both experimental studies and theoretical modeling. The cross-plane thermal conductivity of the films was measured over the temperature range of 30-300 K with the four-point-probe 3-omega method. For this study, 50-, 100-, 200-, 400-, and 650-nm-thick Bi thin films were prepared by thermal evaporation. The average thermal conductivities of the Bi thin films were determined to be 2.7-12.1 W/m.K in the temperature range of 30-300 K, indicating the thermal transport properties of the films are strongly dependent on film thickness and temperature. We suggest that this reduction in the thermal conductivity of the thin films compared to that of bulk Bi could be due to the enhanced phonon scattering from grain boundaries as the film thickness decreases to 50 nm. To investigate the effects of film thickness in more detail, the experimental results of the films were theoretically analyzed with a modified Callaway and Sondheimer models. The theoretical results agreed well with the measured values.