Electron and phonon transport anisotropy of ZnO at and above room temperature

Due to the lack of experimental evidence, it is not clearly known if the charge and heat transport of ZnO exhibit noticeable anisotropy at and above room temperature. Here, we measure the charge and heat transport properties of strongly crystallographically textured ZnO polycrystals at and above room temperature, up to 750 K. Our observations reveal a remarkable difference of the electrical conductivity, Hall coefficient, Seebeck coefficient, and electron mobility between the ZnO c-axis and ab-plane directions. The origin of electron transport anisotropy is discussed. We experimentally show that the lattice thermal conductivity for the c-axis direction can be at least 20% larger than that for the ab-plane direction, and the anisotropy ratio is only weakly dependent on temperature, which qualitatively agrees well with our first-principles density functional theory (DFT) calculations. Our DFT calculations also reveal that the heat transport difference between the c-axis and ab-plane directions is due to the anisotropy in phonon group velocities and Umklapp phonon scattering rates. The present work fills in the knowledge gap about ZnO. These findings can provide important implications for designing ZnO crystals to optimize the material or device performance in numerous applications where charge and/or heat transport properties are important.