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

To investigate the relationship between the electromechanical response, d₃₃, 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 d₃₃ of more than 110 pC/N is obtained in Fe-ZnO films when Fe³⁺ with an ionic radius of 0.64 substitutes for the Zn²⁺ (0.74 ) site. The d₃₃ (less than 7 pC/N) is smaller than that of undoped ZnO films (11.6 pC/N) when Fe²⁺ (0.76 ) substitutes for Zn²⁺. The enhanced electromechanical response is ascribed to polarization rotation induced by the external electric field. The microscopic origin is considered. Substitution of Fe³⁺ with smaller ionic size for Zn²⁺ 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 Fe²⁺ substitutes for Zn²⁺, rotation of Fe-O1 bonds becomes more difficult and thus the d₃₃ 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.