The purpose of this study is to fabricate PZT films with a high linear electro-optic coefficient and a low propagation loss on glass substrate by using sputtering method. PZT films must be composed of pure perovskite phases in order to have good optical properties. The post-annealing process is needed for this purpose, which may induce cracks in the film. The crack formation during post-annealing process is a serious problem in fabricating PZT film on glass substrate. When the film was annealed by the rapid thermal annealing (RTA), crack was observed at the surface of films in the annealing condition of perovskite formation. From this research, the volume change induced by phase transformation is thought to be the main factor in crack formation, which is supported by the fact that cracking occurs only to the PZT films whose structures are completely transformed to the perovskite phases. When the film was annealed at high temperature for a long time by conventional furnace annealing (CFA), the pure perovskite phase could be obtained but cracks were readily formed under these conditions. Whereas, low temperature and short time annealing resulted in crack-free film but perovskite phases hardly formed. It was found from this study that crack-free PZT films with pure perovskite phase could be obtained by annealing at high temperature for a short time (650℃, 10min) regardless of film deposition temperature.
PZT films were deposited at the deposition temperatures ranging from room temperature to 500℃. $\alpha-PbO_2$ phase was observed for the films deposited at temperatures ranging from room temperature to 400℃. At 500℃, the highest temperature we used, there appeared intermediate phase peaks such as $PbTi_3O_7$ and $PbZrO_3$. The as-deposited PZT films were crystallized into randomly oriented polycrystalline perovskite phase when annealed at 650℃ for 10 min. The propagation loss measured by prism coupling method also decreased with increasing deposition temperature, and had the minimum value of 2dB/cm for the film deposited at 400℃. This loss value at 400℃, to the best of authors' knowledge, is the lowest value among the reported values for the PZT films deposited by sputtering, indicating that the low temperature deposition followed by post-annealing is the proper technique for optical application to give rise to smooth film surface. The r-E hysteresis loops were similar in shape to the P-E hysteresis loops and the linear electro-optic coefficient increased with increasing saturation polarization, indicating the linear electro-optic coefficient was affected by polarization characteristics. The linear electro-optic coefficient as expressed by $r_{max}=(｜r_{max}｜+｜-r_{max}｜)/2$ had the largest value of 80pm/V for the film deposited at 400℃. These changes of polarization and electro-optic properties of the PZT films with deposition temperature were attributed to the difference of the phase formed in the as-deposited films, which was expected to cause difference in the nucleation and growth of perovskite phase during post-annealing.
Seed layers were introduced to improve the electro-optical properties by fabricating PZT films with high degree of preferred orientation. In the as-deposited state, no crystalline phase appeared in the thin films with thicknesses of 17nm and 50nm, while Pb oxide phase appeared in the thin film with thickness of 85nm. When these thin films were annealed at 650℃ for 10 min, the 17nm-thick film showed no crystalline phase, but the 50nm-thick film had perovskite phase with (100) preferred orientation. The 85nm-thick film also had perovskite phase but the orientation was random. The PZT thin films with the thickness less than a certain value seem to crystallize into the perovskite phase with (100) plane that has a minimum surface energy among the PZT crystalline planes. PZT thin films showed different crystal orientations with varying the thickness after annealing process. These PZT thin films were used as seed layers to control the crystal orientations of the subsequently deposited PZT films. The PZT film deposited without seed layer had a randomly oriented perovskite phase. The PZT films fabricated on seed layers with amorphous phase and with randomly oriented phase both showed random crystal orientation. On the contrary, PZT film fabricated on (100) seed layer showed the (100) preferred orientation. PZT film oriented preferentially in (100) directions had superior dielectric constant and polarization properties to randomly oriented films. PZT film deposited without seed layer showed the linear electro-optic coefficient of 65pm/V, whereas PZT film deposited on the (100) seed layer showed 109pm/V as high as the value obtained from the PZT film epitaxially grown on single crystal substrate.
