Enhanced electromechanical response of Fe-doped ZnO films by modulating the chemical state and ionic size of the Fe dopant

Abstract

To investigate the relationship between the electromechanical response, ${d}_{33}$, of doped ZnO and the dopant ionic size, the chemical state and ionic size of Fe in Fe-ZnO films were modulated through doping with various Fe concentrations and postannealing. We found that an enhanced ${d}_{33}$ of more than 110 pC/N is obtained in Fe-ZnO films when ${\text{Fe}}^{3+}$ with an ionic radius of $0.64\text{ }\text{\AA{}}$ substitutes for the ${\text{Zn}}^{2+}$ $(0.74\text{ }\text{\AA{}})$ site. The ${d}_{33}$ (less than 7 pC/N) is smaller than that of undoped ZnO films (11.6 pC/N) when ${\text{Fe}}^{2+}$ $(0.76\text{ }\text{\AA{}})$ substitutes for ${\text{Zn}}^{2+}$. The enhanced electromechanical response is ascribed to polarization rotation induced by the external electric field. The microscopic origin is considered. Substitution of ${\text{Fe}}^{3+}$ with smaller ionic size for ${\text{Zn}}^{2+}$ results to the easier rotation of noncollinear Fe-O1 bonds along the $c$ axis under the applied field. Thus, the external electric field needed for polarization rotation is smaller. When bigger ${\text{Fe}}^{2+}$ substitutes for ${\text{Zn}}^{2+}$, rotation of Fe-O1 bonds becomes more difficult and thus the ${d}_{33}$ is smaller. A general mechanism is derived. Doping ZnO with a small ion produces enhanced electromechanical responses whereas doping ZnO with a big ion results in decreased electromechanical responses. This mechanism is useful for guiding the design of new wurtzite semiconductors with enhanced electromechanical responses.