Cr-substitution-induced ferroelectric and improved piezoelectric properties of Zn1-xCrxO films

(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 Cr-Zn to Cr-Zn+Cr-i 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 similar to 0.2 mu C/cm(2) and a coercive field similar to 50 kV/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 d(33) coefficient similar to 120 pm/V is also achieved by Cr substitutions, which could make Cr-doped ZnO a promising material in piezoelectric devices. (C) 2008 American Institute of Physics.