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, 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 Fe3+ with an ionic radius of 0.64 angstrom substitutes for the Zn2+ (0.74 angstrom) site. The d(33) (less than 7 pC/N) is smaller than that of undoped ZnO films (11.6 pC/N) when Fe2+ (0.76 angstrom) 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 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.