Anisotropy of charge and heat transport in zinc oxide at high temperatures

Over a century ago, zinc oxide (ZnO) found a niche as a pigment for white paint. More recently, scientists discovered that the inorganic compound harbored unique properties that could be promising for electronic and optoelectronic device applications. For example, it has a wide bandgap, is biocompatible and remains stable in the presence of high-energy radiation.

In such devices, performance depends on charge and heat transport processes. A new article investigates whether electron and phonon transport exhibits anisotropy in crystalline ZnO above room temperature.

The researchers observed noticeable anisotropy between the c-axis and ab-plane directions from room temperature up to 750 K. The results can have significant implications for the design of certain ZnO-based devices, especially for nanoscale single crystal ZnO whose transport anisotropy can be very high.

After synthesizing ZnO samples with a strong crystallographic texture, the authors measured the charge and heat transport properties at and above room temperature, up to 750 K. The experiments revealed remarkable differences in electrical conductivity, the Hall coefficient, the Seebeck coefficient and electron mobility between the ZnO c-axis and ab-plane directions.

With the aid of physical model analysis and first-principles density functional theory calculations, they also explored the origins of electron and phonon transport anisotropy. The findings suggest that the electron mobility anisotropy is due to the anisotropic piezoelectric scattering of electrons. The lattice thermal conductivity anisotropy, which is only weakly dependent on temperature, is attributed to the difference in phonon group velocities and Umklapp phonon scattering rates.

Source: “Electron and phonon transport anisotropy of ZnO at and above room temperature,” by Xin Liang and Changan Wang, Applied Physics Letters (2020). The article can be accessed at http://doi.org/10.1063/1.5139563 .