It has been known that $Pb(Zr,Ti)O_3$(PZT) thin films are promising not only for non-volatile memories using their remanent polarization, but also for conventional dynamic random access memories(DRAM) using their high dielectrics. However, it has been clarified that several problems must be overcome before the application.
Recently, some research groups reported that buffer layers played a very important role in the growth of high-quality ferroelectric thin films. In the present study, we firstly deposited $PbTiO_3$ thin films by MOCVD and investigated optimum conditions for buffer layer, and then fabricated PZT thin films with and without buffer layer on Pt-base electrode($Pt/SiO_2/Si)$. The effects of buffer layer on phase formation and electrical properties such as dielectric constant, polarization-field(P-E) and current-voltage(I-V) were also studied.
$PbTiO_3$ and PZT films were deposited at 340℃ and 380℃, respectively and annealed by RTA system. $PbTiO_3$ thin films with Pb/Ti=0.8-1.2 ratio deposited at 340℃ showed homogeneous microstructure and smooth morphology which were essential conditions as a role of buffer layer. On the contrary, $PbTiO_3$ thin films with more excess Pb content or higher deposition temperature showed porous and rough morphology.
PZT thin films which were annealed at 650℃ were examined by X-ray diffraction pattern and scanning electron microscopy(SEM) method. With increasing the Pb/(Zr+Ti) ratio from deficient to stoichiometric composition, crystallinity was relatively improved. It was shown that PZT films without buffer layer have low crystallinity because it was not sufficient for annealing temperature of 650℃ to form stable perovskite phase. From the SEM analysis, we could observe that the films phase consisted of perovskite phase and amorphous phase.
Whereas, PZT films with buffer layer showed good crystallinity and amorphous phase couldn't be found by SEM analysis. Specially, grain size of PZT thin films was decreased with increasing of Pb/(Zr+Ti) ratio.
Based on these results, we could infer that $PbTiO_3$ buffer layer acted as nucleation sites and lowered the activation energy for nucleation. Therefore, temperature of phase formation would be reduced and perovskite phase could be easily formed at low annealing temperature. Besides, grain size also decreased because of more site of nucleation.
With increasing the Pb/(Zr+Ti) ratio from deficient to stoichiometric composition, dielectric constant, remanent polarization and leakage current were increased, regardless of use of buffer layer. The PZT films with buffer layer had high dielectric constant and large remanent polarization
Compared with PZT films without buffer layer. These improvement of ferroelectric properties could be resulted from increasing of perovskite phase content. The use of buffer layer increased leakage current of PZT films at the condition of same annealing temperature.
The insertion of buffer layer increased leakage current at same annealing temperature. These phenomena could be explained by reduction of amorphous phase. With decreasing the content of amorphous phase because of buffer layer effects, leakage current of films was increased.. But, because good ferroelectric properties could be obtained at low annealing temperature by using of buffer layer, lower leakage current could be obtained with similar ferroelectric properties at low annealing temperature.
The results of phase analysis and electrical properties suggested the optimum thickness for good ferroelectric properties would be existed.
