Compositional and thickness dependence of ferroelectric Zn(Ce,Mn)O epitaxial thin films

In this study, we investigated the composition and thickness dependence of ferroelectricity in c-axis-oriented Zn1−x(Ce,Mn)xO (Zn(Ce,Mn)O) epitaxial thin films co-doped with Ce and Mn in an approximately 1:1 ratio. With increasing (Ce,Mn) doping concentration, the c-axis length decreased while the a-axis length increased, and the c/a ratio decreased to 1.581 with 7% (Ce,Mn) doping. Meanwhile, (Ce,Mn) doping reduced leakage current and dielectric loss, confirming the contribution of Mn doping. In the Zn(Ce,Mn)O thin films doped with 7%–15% (Ce,Mn), clear ferroelectricity was confirmed. The 7% and 11% (Ce,Mn)-doped Zn(Ce,Mn)O thin films exhibited high remanent polarization (Pr) exceeding 80 μC/cm2, while the coercive electric field (Ec) was approximately 2.5 MV/cm, independent of the doping concentration. Notably, a high Pr exceeding 100 μC/cm2 was also observed in the ZnMnO epitaxial thin films without Ce doping, and Pr decreased with decreasing Ce concentration. For the 11% (Ce,Mn)-doped Zn(Ce,Mn)O thin film, the thickness dependence of ferroelectricity was investigated. A decrease in Pr was observed for film thicknesses below 50 nm. However, ferroelectricity with Pr of 15 μC/cm2 was confirmed even at a film thickness of 30 nm. At this thickness, the polarization switching voltage can be significantly reduced to approximately 9 V. The ferroelectric nature of the 30 nm-thick Zn(Ce,Mn)O thin films was also confirmed by observing polarization switching via piezoresponse force microscopy.