Thermal conductivity of hetero-epitaxial ZnO thin films on c - And r -plane sapphire substrates: Thickness and grain size effect

The thermal conductivities of c- and a-axis-oriented zinc oxide (ZnO) thin films with nominal thicknesses of 100, 200, and 300 nm are investigated. The c- and a-axis-oriented ZnO thin films were synthesized by radio frequency magnetron sputtering on the c- and r-plane sapphire substrates, respectively. The epitaxial relationship between the ZnO thin film and the c-plane sapphire substrate is (0001) [1 1 00] || (0001) [11 2 0], and that between the ZnO thin film and the r-plane sapphire substrate is (11 2 0) [1 1 00] || (01 1 2) [11 2 0]. The c-axis-oriented ZnO thin film has a columnar structure, whereas the a-axis-oriented ZnO thin film has a single domain-like structure and a significantly flat surface. The thermal conductivity of the c-axis-oriented ZnO thin film is in the range of 18-24 W m ^-1 K ^-1 , whereas for the a-axis-oriented ZnO thin film, it is in the range of 24-29 W m ^-1 K ^-1 . For the c-axis-oriented ZnO thin films, the phonon scattering on both the out-of-plane and in-plane grain boundaries affects the thermal conductivity. In contrast, the thermal conductivity of the a-axis-oriented ZnO thin films decreases with the decrease of the film thickness. The distribution of the normalized cumulative thermal conductivity of the a-axis-oriented ZnO thin films suggests that the heat transport carrier mostly consists of phonons with the mean free paths between 100 nm and 1 μm.