Ferroelectrics everywhere: Ferroelectricity in magnesium substituted zinc oxide thin films

摘要

We demonstrate ferroelectricity in Mg-substituted ZnO thin films with the wurtzite structure. Zn_(1-x)Mg_xO films are grown by dual-cathode reactive magnetron sputtering on (111)-Pt// (0001)-Al_2O_3 substrates at temperatures ranging from 26 to 200°C for compositions spanning from x = 0 to x = 0.37. X-ray diffraction indicates a decrease in the c-lattice parameter and an increase in the a-lattice parameter with increasing Mg content, resulting in a nearly constant c/a axial ratio of 1.595 over this composition range. Transmission electron microscopy studies show abrupt interfaces between Zn_(1-x)Mg_xO films and the Pt electrode. When prepared at pO_2 = 0.025, film surfaces are populated by abnormally oriented grains as measured by atomic force microscopy for Mg concentrations ＞29%. Raising pO_2 to 0.25 eliminates the misoriented grains. Optical measurements show increasing bandgap values with increasing Mg content. When prepared on a 200°C substrate, films display ferroelectric switching with remanent polarizations exceeding 100μC cm~(-2) and coercive fields below 3MVcm~(-1) when the Mg content is between ～30% and ～37%. Substrate temperature can be lowered to ambient conditions, and when doing so, capacitor stacks show only minor sacrifices to crystal orientation and nearly identical remanent polarization values; however, coercive fields drop below 2 MV/cm. Using ambient temperature deposition, we demonstrate ferroelectric capacitor stacks integrated directly with polymer substrate surfaces.