A giant electromechanical d33 coefficient 110pC∕N is obtained in ferroelectric V-doped ZnO films, which is nearly one order of magnitude higher than that of undoped samples. It is considered that the switchable spontaneous polarization induced by V dopants and the accompanying relatively high permittivity should be responsible for the enhancement of piezoelectric response. Moreover, from another point of view, an easier rotation of V–O bonds which are noncollinear with c axis under electric field might be the microscopic origin of this anomaly. The improved piezoelectric properties could make V-doped ZnO a promising candidate for piezoelectric devices.

(0001) oriented polycrystalline Cr-doped ZnO films have been prepared on n-Si(111) single-crystal substrates by nonequilibrium reactive magnetron cosputtering. The c-axis texture of the films weakens and a transformation of doping mechanism from CrZn to CrZn+Cri is indicated as the doping concentration increases. The Cr dopants are demonstrated to exist as Cr3+ ions in the films. Ferroelectric measurements show that the Ag∕Zn0.94Cr0.06O∕n-Si heterostructure displays well-defined hysteresis loop with a remanent polarization ∼0.2μC∕cm2 and a coercive field ∼50kV∕cm at room temperature. The capacitance-voltage curves with clockwise traces show typical memory windows, which symmetrically widen as the sweep amplitude increases. Ferroelectricity in Cr-doped ZnO was also established by a displacement-voltage “butterfly” loop. The observed ferroelectric behavior is attributed to the partial replacement of host Zn2+ ions by smaller Cr3+ ions, which occupy off-center positions and thereby induce permanent electric dipoles. Moreover, electrical transport studies reveal that the conduction mechanism in Cr-doped ZnO is a combination of field-assisted ionic conduction and trap-controlled space-charge-limited conduction, which prevail in lower and higher voltage regions, respectively. A higher leakage occurs as the doping concentration increases, which may originate from a higher density of defects. Besides, a high piezoelectric d33 coefficient ∼120pm∕V is also achieved by Cr substitutions, which could make Cr-doped ZnO a promising material in piezoelectric devices.