Calculated polarizations and piezoelectric constants of wurtzite ZnO, CdO and MgO

The macroscopic polarization in wurtzite crystal structures such as GaN and ZnO has a strong influence on the electrical and optical properties. This property has been exploited in GaN/AlGaN heterostructures, where the difference in the polarizations between layers induces an electric field at the interface forming a high mobility two-dimensional electron gas (2DEG)~\footnote[1]{U. K. Mishra,Y. Wu, B.P. Keller, S. Keller and S.P. Denbaars \textit{IEEE Transactions on Microwave theory and Techniques}, \textbf{46}, 6 (1998)}. . Here we calculate the polarization and the piezoelectric properties of the corresponding wurtzite-structure binary oxides ZnO, MgO and CdO. The knowledge of these properties is essential to explore the possibility of creating similar high mobility polarization induced 2DEGs in the ZnO-based material system. However, since wurtzite-structure MgO and CdO are not experimentally accessible, the values can be obtained computationally. We use the recently developed self-interaction corrected pseudopotential (pseudo-SIC) implementation~\footnote[2]{ A. Filippetti and N.A. Spaldin \textit{Phys. Rev. B} \textbf{67}, 125109 (2003).} of the density functional theory and the widely used Berry phase method~\footnote[3]{R. D. Kingsmith and D. Vanderbilt \textit{Phys. Rev. B} \textbf{49}, 5828 (1994).} for obtaining the polarization and piezoelectric constants. We find that the polarization gradients between the end-point compounds in the MgO-ZnO-CdO are larger~\footnote[4]{P. Gopal and N.A. Spaldin \textit{submitted}} than in the GaN analogues.