Comparison of characteristics of thin PZT films on Si-on-sapphire and Si substrates

The properties of ferroelectric sol-gel PZT films deposited on a silicon-on-sapphire (SOS) substrate are compared with those of PZT films formed on a Si substrate. The crystalline structure, asymmetry of the hysteresis loops, polarization dependences of the transient current, short-circuit photocurrent, and open-circuit photovoltage, as well as the substrate bending have been studied. The PZT on SOS films are textured in a single (111) direction and exhibit symmetric hysteresis loops with a strong remanent polarization. The PZT films on Si are textured in the main (111) and weaker (100) directions, have weaker polarization, and exhibit an asymmetry of the hysteresis loops, which is reflected in the magnitude of the transient current and photocurrent. It is shown that the sapphire substrate has a convex bending that causes a compressive stress in the film plane, which weakens the effect of the lattice mismatch between PZT and Pt. By contrast, the Si substrate has a concave curvature, which causes the film to stretch. The deformations and mechanical stresses within the films were estimated. For PZT on Si, an estimate of the strain gradient along the (111) axis was obtained, which makes it possible to relate the asymmetry of the hysteresis loops to the flexoelectric polarization, with the flexoelectric coefficient for sol-gel PZT films found to be 0.0154 $\mu$C/cm. The results obtained show that the sapphire substrate provides a better quality of thin PZT films.